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Strain-Releasing Ring-Opening Diphosphinations for the Synthesis of Diphosphine Ligands with Cyclic Backbones
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  • Chandu G. Krishnan
    Chandu G. Krishnan
    Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
    JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
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    Orcidhttps://orcid.org/0000-0002-1302-9975
  • Hideaki Takano
    Hideaki Takano
    Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
    JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
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    Orcidhttps://orcid.org/0000-0003-0744-9292
  • Hitomi Katsuyama
    Hitomi Katsuyama
    Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
    JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
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    Orcidhttps://orcid.org/0000-0002-2753-0402
  • Wataru Kanna
    Wataru Kanna
    Department of Chemistry, Faculty of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
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    Orcidhttps://orcid.org/0000-0002-2931-6661
  • Hiroki Hayashi
    Hiroki Hayashi
    Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
    JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
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    Orcidhttps://orcid.org/0000-0003-2081-6886
  • Tsuyoshi Mita*
    Tsuyoshi Mita
    Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan
    JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-6655-3439
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JACS Au

Cite this: JACS Au 2024, 4, 10, 3777–3787
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https://doi.org/10.1021/jacsau.4c00347
Published July 29, 2024

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Abstract

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Diphosphine ligands based on cyclobutane, bicyclo[3.1.1]heptane, and bicyclo[4.1.1]octane were synthesized from the corresponding highly strained, small, cyclic organic molecules, i.e., bicyclo[1.1.0]butane, [3.1.1]propellane, and [4.1.1]propellane, employing a ring-opening diphosphination. Under photocatalytic conditions, the three-component reaction of a diarylphosphine oxide, one of the aforementioned strained molecules, and a diarylchlorophosphine results in the smooth formation of the corresponding diphosphines in high yield. The obtained diphosphines can be expected to find applications in functional molecules due to their unique structural characteristics, which likely impart specific properties on their associated metal complexes and coordination polymers (e.g., a zigzag-type structure). The feasibility of the initial radical addition can be estimated using density-functional-theory calculations using the artificial force induced reaction (AFIR) method. This study focuses on the importance of integrating experimental and computational methods for the design and synthesis of new diphosphination reactions that involve strained, small, cyclic organic molecules.

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Introduction

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Significant advancements in the development of novel functionalized materials, such as phosphine ligands, have recently been reported. (1,2) At present, one fascinating avenue of exploration in this area is the synthesis of diphosphine ligands that coordinate to metal centers in either a bidentate or monodentate fashion. Diphosphine ligands with a bidentate coordination mode allow the formation of metallacycles, which are stabilized by chelation effects, (3,4) while those with a monodentate mode serve as molecular linkers, resulting in the formation of supramolecular assemblies. (5−12) The reactivity of diphosphine ligands is primarily contingent upon two crucial parameters: the cone angle (13−15) and the bite angle. (3,4) These angles can be precisely tailored by leveraging the steric effects stemming from the substituents on the phosphorus atoms and the backbone linkers that connect the two phosphorus atoms (Scheme 1a). In the case of the substituents on the phosphorus atoms, electronic effects are also pivotal in orchestrating the reactivity of the metal complex. Unsymmetric diphosphine ligands that carry different substituents on each phosphorus atom, may have, through a judicious selection of substituents, potentially exceptional reactivity profiles. Consider, for instance, a diphosphine ligand where one phosphorus atom bears electron-donating groups, while its counterpart bears electron-withdrawing moieties.

Scheme 1

Scheme 1. Importance of Bidentate Phosphine Ligands
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This configuration can be expected to endow the ligand with distinctive reactivity and properties compared to its symmetric counterparts. Such modified reactivity and properties arise from the push–pull effect, which contributes to the precise modulation of electron density on the central metal atom. Indeed, unsymmetric diphosphine ligands have, on occasion, demonstrated an enhanced capacity to augment the reactivity, regioselectivity, and enantioselectivity of transition-metal-catalyzed reactions when compared to their symmetrical counterparts. (16−21) Consequently, both the architecture of the backbone linkers and the nature of the substituents attached to the phosphorus atoms wield substantial influence over the reactivity of diphosphine metal complexes. This underscores the pressing need for facile and universally applicable methods for the synthesis of both symmetric and unsymmetric diphosphine ligands with a diverse array of carbon frameworks.
Small cyclic hydrocarbons can exhibit substantial ring strain, that primarily stems from the unconventional bond angles adopted by the sp3-hybridized carbon atoms. (22−24) In recent years, considerable research attention has been devoted to polycyclic hydrocarbons, bicyclo[1.1.0]butane, (25−32) and propellanes. (33−49) These intriguing compounds are characterized by two or three interconnected rings, each sharing a common C–C bond. [1.1.1]Propellane (32−44) has garnered significant interest due to its high reactivity with cations, anions, and radicals. This heightened reactivity can be attributed to the exceptionally strained C–C bonds between the bridgehead carbons (charge-shift bonds). The reaction of [1.1.1]propellane with an anion or a radical leads to a bicyclo[1.1.1]pentane (BCP) skeleton, which represents a biological isostere of benzene rings, alkynes, or tert-butyl groups (Scheme 1b). (50−54) Notably, our research group has previously reported a three-component reaction (3CR) for the synthesis of symmetric/unsymmetric diphosphine ligands, (55,56) and one of these ligands is a straight-shaped diphosphine derived from [1.1.1]propellane (Scheme 1c). (56) The key to this significant synthetic advancement lies in the 3CR strategy, where a diarylphosphine oxide, a highly strained hydrocarbon scaffold, and a diarylchlorophosphine can be assembled under photocatalytic conditions.
Taking into consideration the reactivity of highly strained small-ring compounds and the utility of the three-dimensional hydrocarbon structures generated from their reactions, the development of a 3CR involving small-ring compounds would facilitate the creation of novel functional diphosphine ligands. In the present study, we have determined the activation barriers for the addition of electron-withdrawing phosphoryl radicals (Ph2P(═O)·) to seven strained hydrocarbon molecules based on cyclopropane, bicyclo[1.1.0]butane, [n.1.1]propellane, and [n.n.n]propellane scaffolds (Scheme 2). Based on these calculated results, we successfully synthesized several diphosphine ligands via a 3CR using the strained hydrocarbons bicyclo[1.1.0]butane, (25−32) [3.1.1]propellane, (43,45,46) and [4.1.1]propellane. (47−49)

Scheme 2

Scheme 2. 3CR for the Synthesis of Diphosphine Ligands
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Results and Discussion

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Prior to conducting experimental trials of the diphosphination procedures, our investigation commenced with density-functional-theory (DFT) calculations using the artificial force induced reaction (AFIR) method (57−60) at the UωB97X-D/Def2-SVP level in dichloromethane (SMD model). Subsequently, we directed our attention to simulating the addition of the phosphoryl radical (Ph2P(═O)·, 1′) to different highly strained molecules (Table 1). We calculated the energies of the two species independently, assuming that the phosphoryl radical and the substrate are infinitely far apart. We then combined these energies, which serves as the reference for drawing an energy diagram. The phosphoryl radical exhibits greater reactivity toward hydrocarbons than the phosphinyl radical (Ph2P·) because the electron-withdrawing radical prefers to react with electron-rich hydrocarbon substrates (for details, see the Supporting Information (S37)). The phosphoryl radical can be readily generated via photoirradiation from the in situ-formed diphosphine oxide (Ph2P(═O)–PPh2) derived from the phosphine oxide (Ph2P(═O)H) and the chlorophosphine (Ph2PCl). (55,56)
Table 1. DFT-Calculated Activation Barriers for the Addition of a Phosphoryl Radical to Strained Molecules
We first performed calculations for the addition of a phosphoryl radical to the simple ring-strained molecule cyclopropane, which was found to have an extremely high activation barrier (ΔG‡ = 30.3 kcal/mol; entry 1). In contrast, bicyclo[1.1.0]butane (25−32) has a much lower activation barrier (ΔG‡ = 14.0 kcal/mol; entry 2). As calculated in our previous study, [1.1.1]propellane (33−44) has a reasonably low activation barrier (ΔG‡ = 8.5 kcal/mol; entry 3). The calculations performed for the reaction with [2.1.1]propellane (61) revealed the lowest activation barrier (ΔG‡ = 5.7 kcal/mol) of the compounds shown in Table 1 (entry 4). Unsymmetric propellanes, such as [3.1.1]propellane (43,45,46) and [4.1.1]propellane, (47−49) have two geometries that the phosphoryl radical can access, with a large difference in activation barriers between the two paths. One geometry has a low barrier while the other has a high barrier (entries 5–8). The geometries where the bulky cyclopentane or cyclohexane faces the same direction as the oxygen atom of the phosphoryl radical have higher activation barriers (ΔG‡ = 13.4 and 12.0 kcal/mol; entries 5 and 7). Conversely, when the cyclic hydrocarbon is orientated in the opposite direction to the oxygen atom of the phosphoryl radical, a much lower activation barrier is observed (ΔG‡ = 8.1 and 9.2 kcal/mol; entries 6 and 8). We also performed calculations for the reactions of [2.2.2]propellane (62,63) and [3.3.3]propellane, (64,65) which revealed that the former exhibits a reasonable activation barrier (ΔG‡ = 12.2 kcal/mol), while the latter has a very high activation barrier (ΔG‡ = 54.9 kcal/mol) with an endergonic reaction profile (entries 9 and 10). Based on these calculations, all listed strained compounds, except cyclopropane and [3.3.3]propellane, exhibit reasonable activation barriers. However, [2.1.1]propellane (61) and [2.2.2]propellane (62,63) have been reported to be insufficiently stable toward isolation (vide infra), and the diphosphination of [1.1.1]propellane has been already reported by our research group. (56) Therefore, we focused on experimentally verifying the diphosphination of bicyclo[1.1.0]butane, (25−32) [3.1.1]propellane, (43,45,46) and [4.1.1]propellane, (47−49) in addition to the confirmation of the nonreactivity of cyclopropane.

Cyclopropane

As expected, given the formidable activation barrier (ΔG‡ = 30.3 kcal/mol) estimated by DFT calculations (vide supra), several attempts at the 3CR involving the opening of cyclopropane (10% in N2 gas under 10 atm pressure) using phosphine oxide 1 and chlorophosphine 2 were unsuccessful (Figure 1). Instead of the targeted 1,2-bis(diarylphosphino)propane derivatives, unsymmetric and symmetric diphosphines 3aa and 3bb were obtained in a ca. 20% yield.

Figure 1

Figure 1. 3CR of cyclopropane with 1 and 2.

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Bicyclo[1.1.0]butane

Next, we examined the 3CR involving bicyclo[1.1.0]butane because its calculated activation barrier is reasonably low (ΔG‡ = 14.0 kcal/mol). Bicyclo[1.1.0]butane was prepared as a Et2O solution according to a slightly modified literature procedure (28) (for details, see the Supporting Information (S3)). Unfortunately, when using (p-NMe2-C6H4)2P(═O)H (1a) and (p-CF3–C6H4)2PCl (2a) as substrates for the 3CR with bicyclo[1.1.0]butane, several unidentified compounds were formed, while the formation of the targeted unsymmetric 4aa was not observed. Instead of the generation of 4aa, diphosphine 3aa was obtained in 37% yield (Figure 2). However, when Ph2P(═O)H (1b) and Ph2PCl (2b) were used as substrates, symmetric derivative 4bb was obtained in 12% yield with a trans/cis ratio of 3.8/1 together with diphosphine 3bb in 41% yield. The structure of the minor 4bb-cis isomer was determined by an nOe experiment with an 8% correlation (for details, see the Supporting Information (S17)).

Figure 2

Figure 2. 3CR of bicyclo[1.1.0]butane.

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Despite obtaining a small amount of 4bb, a substantial quantity of unidentified byproducts were formed, probably due to the generation of rather reactive secondary radicals after the addition of the phosphoryl radical. Moreover, as bicyclo[1.1.0]butane is a gaseous substance that could potentially evaporate during light irradiation (the reported bp is 8 °C (27)), it was decided to use a solid bicyclobutane, which does not evaporate and is easier to handle. Benzensulfonyl-substituted bicyclo[1.1.0]butane (5), which is readily prepared from inexpensive benzenesulfinic acid sodium salt and 4-bromobut-1-ene, (34) was subsequently employed for the 3CR. The sulfonyl group can be readily removed under reductive conditions. The activation barrier of the addition of phosphoryl radical 1′ to 5 (ΔG‡ = 15.2 kcal/mol) is sufficient to allow experiments to be conducted. As a result, the product yields increased compared to when bicyclo[1.1.0]butane was used, and unsymmetric 6aa was obtained in 23% yield (cis/trans = 1.6/1) and symmetric derivative 6bb in 53% yield (cis/trans = 4/1) (Figure 3). A 0.7 mmol-scale reaction also worked well, resulting in the formation of 6bb in 57% yield with a cis/trans ratio of 3.4/1. The major diastereomer of 6bb was unambiguously determined to have a cis configuration via X-ray diffraction (XRD) analysis of a suitable single crystal. (66)

Figure 3

Figure 3. 3CR of sulfonylated bicyclo[1.1.0]butane. a0.7 mmol-scale synthesis.

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The sulfonylated compound 6bb (cis/trans mixture) was smoothly desulfonylated using Mg powder in the presence of NiBr2 (20 mol %) to afford the target molecule 4bb in good yield (Figure 4). In this process, the stereochemistry of 6bb was not maintained, affording 4bb with a trans/cis ratio that was different to the original ratio of 6bb. As the desired Mg-mediated process involves a radical process, the resulting secondary radial formed after the desulfonylation undergoes epimerization to give 4bb in an almost 1:1 trans/cis ratio. In the case of unsymmetric 6aa, a significantly simultaneous defluorination of the CF3 group occurred during the desulfonylation, and the reductive desulfonylation with Mg was therefore deemed to be unsuitable for CF3-containing compounds.

Figure 4

Figure 4. Removal of the sulfonyl group under Mg-mediated conditions.

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We then investigated the use of 4bb-cis in the synthesis of a cis-coordinate bidentate ligand (Figure 5). After oxidative exchange of the S atom on the phosphine sulfide moiety of 4bb-cis with an O atom via treatment with mCPBA, the corresponding dioxide 7bb-cis was obtained in 70% yield. 7bb-cis was readily reduced using air-stable 1,3-diphenyl-disiloxane (PhH2Si–O–SiH2Ph; DPDS) (67,68) in toluene under reflux conditions without the need for a glovebox to afford 8bb-cis in 36% yield. The obtained 8bb-cis reacts with NiCl2(dme) to form a red powder of NiCl2(8bb-cis). Recrystallization from CH2Cl2/hexane yielded red/orange single crystals that contain one molecule of CH2Cl2 per unit cell. A single-crystal X-ray diffraction analysis revealed a bidentate chelating coordination of the diphosphine to Ni with a cyclobutane backbone. (66) It should be noted here that the cyclobutane ring is slightly distorted, most likely as a result of the propensity of the ligand to form a chelate. This type of chelating diphosphine-metal complex has not been reported so far; interestingly, while the number of bridging carbon atoms is the same as in dppp (three carbons), the bite angle is larger. Specifically, the X-ray structure reveals that its bite angle is 94.95 °, thus exceeding the reported value for dppp in NiCl2(dppp) (91.77 °). (69) This difference should most likely be attributed to the reduced flexibility of the carbon chain caused by the constrained cyclobutane ring, suggesting potential for future applications in catalysis. (1−4)

Figure 5

Figure 5. Conversion of 4bb-cis to cis-coordinate Ni-complex NiCl2(8bb-cis). Bottom right: crystal structure of NiCl2(8bb-cis) with thermal ellipsoids at 50% probability; all hydrogen atoms as well as one molecule of CH2Cl2 have been omitted for clarity.

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Subsequently, we conducted a theoretical examination of the mechanism of the light-promoted radical reaction in order to investigate the trans-selectivity. For that purpose, we carried out DFT calculations using Ph2P(═O)–PPh2 (3bb′), which should be initially generated from 1b and 2b in the presence of a base (Figure 6). When using 3bb′ for the reaction with 5, 6bb was obtained in similar yield (48% yield), which indicates 3bb′ is an intermediate of this radical reaction. The calculations were again conducted using the AFIR method (57−60) at the UωB97X-D/Def2-SVP level in dichloromethane (SMD model) (for details, see the Supporting Information (S36)). Under blue LED irradiation, [Ir{dF(CF3)ppy}2(dtbbpy)]PF6 absorbs light at 440 nm and is excited to its triplet state (ΔET = 61.8 kcal/mol), (70) which then transfers energy to 3bb′, which in turn is excited to the triplet state (ΔG = 44.7 kcal/mol). (56) Simultaneous dissociation of the P(═O)–P single bond occurs from the triplet state of 3bb′, and the resulting phosphoryl radical (Ph2P(═O)·) reacts with bicyclo[1.1.0]butane to generate a secondary-alkyl radical (ΔG‡ = 14.0 kcal/mol). When radical addition occurs with the phosphinyl radical (Ph2P·) instead of with the phosphoryl radical, the resulting activation barrier is higher (ΔG‡ = 19.2 kcal/mol vs 14.0 kcal/mol). The thus obtained secondary-alkyl radical may react associatively with the remaining phosphinyl radical (radical–radical coupling) to afford the target molecule 4bb′.

Figure 6

Figure 6. Plausible reaction mechanism based on DFT calculations for the 3CR with bicyclo[1.1.0]butane. The reaction diagrams were calculated at the UωB97X-D/Def2-SVP/SMD(DCM) level.

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However, both the secondary-alkyl radical and the phosphinyl radical exhibit transient character, (71) and they seem to be present at low concentrations. Therefore, we calculated another possibility involving a radical-chain mechanism, which revealed an activation barrier of 6.4 kcal/mol for the radical addition of the secondary-alkyl radical to another molecule of diphosphine 3bb′ in a trans-selective fashion through I-trans. However, the cis-selective phosphination via I-cis proceeds with a bit higher activation energy (8.6 kcal/mol). Accordingly, both paths could give the products with an exergonic energy profile. Based on the activation barrier for the radical-chain addition, which can be reasonably overcome at room temperature, we currently consider that the radical-chain process may be operative, even though the termination process with radical–radical coupling can also be expected to be operative in the system, similar to the case of [1.1.1]propellane. (56) We also conducted calculations for the reaction of the secondary-alkyl radical with another molecule of bicyclo[1.1.0]butane. Our calculations indicate that the activation barriers for the radical dimerization of bicyclo[1.1.0]butane via II-trans and II-cis are higher than that of the anticipated radical-chain pathway (ΔG‡ = 19.2/19.5 kcal/mol (trans/cis for dimerization) vs 6.4/8.6 kcal/mol (trans/cis for product formation)), effectively ruling out the possibility of product polymerization.

[1.1.1]Propellane and [2.1.1]Propellane

Our research group has previously reported the diphosphination of [1.1.1]propellane. (56) In contrast, [2.1.1]propellane, distinguished by a central carbon atom bonded to one cyclobutane ring and two cyclopropane rings, is known to be unstable and difficult to isolate, rendering its synthesis exceedingly difficult. (48,61)

[3.1.1]Propellane

Subsequently, our investigation shifted toward [3.1.1]propellane as a substrate for the 3CR given its suitably low activation barrier (ΔG‡ = 8.1 kcal/mol). We investigated the reaction of 1 with [3.1.1]propellane and 2 and were able to successfully synthesize several diphosphines (Figure 7). This synthetic process yielded both unsymmetric and symmetric diphosphine derivatives 9 in yields of up to 74%. A variety of electron-donating phosphine oxides and electron-withdrawing chlorophosphines can be used in this 3CR. We first examined the electron-donating phosphine oxide (p-NMe2-C6H4)2P(═O)H (1a) and electron-deficient chlorophosphine (p-CF3–C6H4)2PCl (2a) under photochemical reaction conditions. Under photocatalytic conditions using blue LED irradiation, the unsymmetric diphosphine derivative 9aa was obtained in 74% yield. The 3CR also proceeded without the need for a photocatalyst under white LED irradiation for 24 h, which is similar to reaction conditions that we have developed previously. (55,56) Without protection using S8, 9aa′ was obtained in a similar yield (69%). Symmetric derivative 9bb was prepared from 1b and 2b in 60% yield. Without sulfur protection, 9bb′ was obtained in 64% yield. We then continued to extend the substrate scope. For that purpose, we screened various electron-donating phosphine oxides in the reaction with the electron-withdrawing chlorophosphine 2a. The 3CR between electron-donating (p-OMe-C6H4)2P(═O)H (1c) and 2a proceeded well under otherwise identical conditions. Furthermore, the 3CR with electron-withdrawing (p–Cl-C6H4)2P(═O)H (1d) and bulky (1-naphthyl)2P(═O)H (1e) proceeded smoothly. Next, electron-donating 1a was used as the phosphine oxide and several chlorophosphines were screened. Electron-withdrawing (p–Cl-C6H4)2PCl (2c) afforded the desired product 9ac in moderate yield. Furthermore, the use of electron-donating (p-OMe-C6H4)2PCl (2d) provided 9ad, albeit that the yield was somewhat low (25%).

Figure 7

Figure 7. 3CR of [3.1.1]propellane. aWhite LED irradiation for 24 h without a photocatalyst.

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Next, we embarked on an in-depth exploration of the transformation of both 9aa and 9bb into their corresponding diphosphine dioxide derivatives, i.e., 10aa and 10bb. We also examined the subsequent production of diphosphine ligands 11aa and 11bb from 10aa and 10bb (Figure 8). The reaction process began with the oxidative substitution of the sulfur atoms on the phosphine sulfide moiety with oxygen atoms, achieved via treatment with mCPBA, which led to the synthesis of the targeted diphosphine dioxide compounds 10 in reasonable yield. Subsequently, the thus obtained diphosphine dioxides 10 underwent reduction by air-stable DPDS (67,68) in toluene under reflux conditions. This reduction proved efficient, yielding unsymmetric and symmetric diphosphine ligands 11 in good yield, thus underscoring the robustness and versatility of the synthetic methodology employed.

Figure 8

Figure 8. Synthesis of diphosphine ligands and metal complexes. Bottom right: crystal structures of 12 and 12′ with thermal ellipsoids at 30% probability; fluorine atoms on the hfa ligands and all hydrogen atoms are omitted for clarity.

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Another notable result is the possibility to subsequently using the unsymmetric dioxide 10aa in the synthesis of a novel Eu-based coordination polymer 12 via the reaction with Eu(hfa)3(H2O)2 (hfa = hexafluoroacetylacetonate). The resulting coordination polymer 12 was obtained in 39% yield after crystallization. A detailed single-crystal X-ray diffraction (XRD) analysis unambiguously confirmed the coordination mode of the europium ions within polymer 12. (66) In 12, each europium ion is intricately coordinated by three hfa ligands and two phosphine-oxide ligands. Notably, each phosphine-oxide moiety in the ligand exhibits different electronic characteristics, i.e., one is electron-donating and the other electron-withdrawing. An octacoordinated metal center, which is similar to the coordination polymer 12′ derived from [1.1.1]propellane, (56) is formed. However, the packing structure of polymer 12 differs significantly from that of the polymer 12′. While 12′ forms an approximately straight-shaped wire, 12 exhibits a wire-shaped structure with zigzag bends on the same plane (Figure 8), wherein the bends to ∼98° occur every three Eu centers.
This structural difference arises from variances in the linker structures. Two C–P bonds are positioned in nearly the same straight line (174°) due to the linkage of the BCP skeleton. Conversely, the [3.1.1]bicycloheptane skeleton introduces two C–P bonds at a specific angle (117°), which most likely influences the zigzag structure in the crystal packing. However, the intrinsic origin of the bent structure for every three Eu units remains unexplained at present. The resulting polymeric architecture, dictated by this unique coordination pattern and the packing structure, showcases exceptional alignment, thereby establishing a promising framework with potential applications in the realm of luminescent photonic materials. Our ongoing research in this area is focused on exploring the optical properties of this intriguing polymeric complex.
We subsequently conducted a theoretical examination of the mechanism of the light-promoted radical reaction of [3.1.1]propellane, supported by DFT calculations (Figure 9). In this substrate, not only blue LED light but also white LED light without any photocatalyst proves suitable for promoting the reaction (cf. 9aa in Figure 7). (55,56) Under white LED irradiation, 3bb′ directly absorbs light to facilitate its radical-dissociation reaction. After the homolytic cleavage of the P(═O)–P single bond, phosphoryl radical (Ph2P(═O)·) reacts with [3.1.1]propellane to generate a tertiary-alkyl radical (ΔG‡ = 8.1 kcal/mol). When radical addition occurs with the phosphinyl radical (Ph2P·) instead of the phosphoryl radical, the resulting activation barrier is higher (ΔG‡ = 13.8 kcal/mol vs 8.1 kcal/mol). The tertiary-alkyl radical thus obtained may undergo an associative reaction with the remaining phosphinyl radical (radical–radical coupling) to yield the target molecule 9bb′. We also explored another possibility involving a radical-chain mechanism, which revealed an activation barrier of 5.9 kcal/mol for the radical addition of the tertiary-alkyl radical to another molecule of diphosphine 3bb′. Considering the low likelihood of an encounter between the two radical species discussed above, in this case we also believe that the radical-chain process may be operative together with radical–radical coupling during the termination process. (56) We also conducted calculations for the reaction of the tertiary-alkyl radical with another molecule of [3.1.1]propellane, indicating that the activation barrier for the radical dimerization of [3.1.1]propellane is higher than that of the anticipated radical-chain pathway (ΔG‡ = 13.0 kcal/mol vs 5.9 kcal/mol), which prevents the dimerization with another [3.1.1]propellane.

Figure 9

Figure 9. Plausible reaction mechanism based on DFT calculations for the 3CR with [3.1.1]propellane. The reaction diagrams were calculated at the UωB97X-D/Def2-SVP/SMD(DCM) level.

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[4.1.1]Propellane

[4.1.1]Propellane is a fused compound that has already been synthesized. (47−49) However, our slightly modified method using PhLi-mediated cyclization leads to a robust and reproducible synthetic procedure for the synthesis of [4.1.1]propellane (for details, see the Supporting Information (S10)). Due to the reasonably favorable activation barrier for the process (ΔG‡ = 9.2 kcal/mol), we mixed phosphine oxide 1 with [4.1.1]propellane and chlorophosphine 2, which resulted in the formation of diphosphines 13aa and 13bb in good yield (Figure 10). Furthermore, similar to the [3.1.1]propellane synthesis, S to O exchange by mCPBA followed by reduction using air-stable DPDS (67,68) afforded unsymmetric and symmetric free diphosphine ligands 15aa and 15bb in good yields (Figure 11).

Figure 10

Figure 10. 3CR of [4.1.1]propellane.

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Figure 11

Figure 11. Conversion to free diphosphine ligands.

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We further calculated the pathway of the reaction of 3bb′ with [4.1.1]propellane (Figure 12). The generated phosphoryl radical (Ph2P(═O)·) reacts with [4.1.1]propellane to generate the tertiary-alkyl radical (ΔG‡ = 9.2 kcal/mol). The activation energy for the reaction with the phosphinyl radical (Ph2P·) is higher than that for the reaction with the phosphoryl radical (ΔG‡ = 17.2 kcal/mol). Compared to the radical addition to [3.1.1]propellane, the activation barriers for the reaction with [4.1.1]propellane are slightly higher. The radical-chain mechanism, whereby an additional molecule of 3bb′ reacts with the tertiary-alkyl radical, which leads to the formation of 13bb′, has a reasonable activation barrier of 8.4 kcal/mol. However, the activation barrier for the pathway where the tertiary-alkyl radical reacts with another molecule of [4.1.1]propellane is higher than that for the generation of 13bb′ (ΔG‡ = 16.8 kcal/mol vs 8.4 kcal/mol), which is similar to the case of [3.1.1]propellane. This result indicates that selective product formation occurs along the possible reaction pathways examined here.

Figure 12

Figure 12. Plausible reaction mechanism based on DFT calculations for the 3CR with [4.1.1]propellane. The reaction diagrams were calculated at the UωB97X-D/Def2-SVP/SMD(DCM) level.

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[2.2.2]Propellane and [3.3.3]Propellane

[2.2.2]Propellane, characterized by a central carbon atom bonded to three cyclobutane rings, is notoriously unstable, making its synthesis formidably challenging. Numerous attempts to synthesize [2.2.2]propellane have been made to date, but regrettably, all of these endeavors have met with failure due to the thermal instability of [2.2.2]propellane. (62,63) On the other hand, the synthesis of [3.3.3]propellane, which features a central carbon atom connected to three cyclopentane rings in a slightly different arrangement, has been shown to be possible, (64,65) and the existing literature provides a viable synthetic route to [3.3.3]propellane. (64,65) However, the calculated activation barriers shown in Table 1 revealed that achieving diphosphination for this molecule would be highly unlikely due to a high activation barrier and an endergonic reaction pathway. Consequently, we have decided to forego the synthesis of [3.3.3]propellane due to the unfavorable energy profile.

Conclusions

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In conclusion, this research represents a remarkable achievement in the synthesis of strained hydrocarbon-based diphosphine ligands, including diphosphinated cyclobutane, bicyclo[3.1.1]heptane, and bicyclo[4.1.1]octane, as evident by our rigorous DFT calculations. Furthermore, the results presented here hold immense promise for various potential applications in functional molecules. For example, a bicyclo[1.1.0]butane-derived cis-coordinated diphosphine was transformed into the corresponding Ni-complex, wherein the bite angle is wider than that in the structurally closely related complex NiCl2(dppp). Moreover, a [3.1.1]propellane-derived diphosphine dioxide was transformed into a unique Eu-based coordination polymer that exhibits a zigzag-type polymeric structure. These results underscore the profound impact of our work within this field of chemistry, reaching far beyond the confines of the laboratory. Furthermore, the synergy between our experimental research and computational calculations is pivotal for the strategic design of radical difunctionalization processes for highly strained molecules. This amalgamation of experimental and theoretical work not only advances our understanding of chemical transformations, but also offers valuable insights into the manipulation of high-energy substrates, paving the way for groundbreaking advancements in the realm of chemical synthesis.

Methods

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General Procedure for Diphosphination of Small Ring Molecules.
In an oven-dried round-bottom flask were placed phosphine oxide 1 (0.20 mmol, 1.0 equiv) and [Ir{dF(CF3) ppy}2(dtbbpy)]PF6 (0.4 mg, 0.4 μmol, 0.20 mol %). DCE (1.5 mL), chlorophosphine 2 (0.20 mmol, 1.0 equiv) and Et3N (28 μL, 0.20 mmol, 1.0 equiv) were added under nitrogen. After the resulting mixture was stirred at room temperature for 15 min, then 0 °C for 5 min, a substrate (0.20 mmol, 1.0 equiv) was added. The reaction mixture was stirred at 0 °C for 4 h under the irradiation of blue LED lights (PR160L-440 nm Kessil light ×2). The test tube was positioned between the two lamps, approximately 2 cm away from one of them. Sulfur (7.7 mg, 0.24 mmol, 1.2 equiv of S) was added into the reaction mixture, and the mixture was stirred for 1 h. After the solvent was evaporated to give the crude mixture, the crude product was purified by silica-gel column chromatography to afford the product.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.4c00347.

  • Additional experimental details, materials, and methods, including photographs of the experimental setup (PDF)

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Author Information

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  • Corresponding Author
    • Tsuyoshi Mita - Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan;  JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan;  Orcidhttps://orcid.org/0000-0002-6655-3439;  Email: [email protected]
  • Authors
    • Chandu G. Krishnan - Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan;  JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan;  Orcidhttps://orcid.org/0000-0002-1302-9975
    • Hideaki Takano - Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan;  JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan;  Orcidhttps://orcid.org/0000-0003-0744-9292
    • Hitomi Katsuyama - Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan;  JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan;  Orcidhttps://orcid.org/0000-0002-2753-0402
    • Wataru Kanna - Department of Chemistry, Faculty of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan;  Orcidhttps://orcid.org/0000-0002-2931-6661
    • Hiroki Hayashi - Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan;  JST, ERATO Maeda Artificial Intelligence in Chemical Reaction Design and Discovery Project, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan;  Orcidhttps://orcid.org/0000-0003-2081-6886
  • Author Contributions

    The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Dr. Mingoo Jin and Kosuke Higashida are gratefully acknowledged for performing the X-ray crystallographic analyses. Miyu Harukawa, Makoto Tsurui, and Prof. Yasuchika Hasegawa of the Faculty of Engineering at Hokkaido University are greatly acknowledged for their contributions to preparing the Eu-based coordination polymer and conducting its crystallographic analysis. This work was financially supported by JST-ERATO (JPMJER1903), JSPS-WPI, and Grants-in-Aid for Scientific Research (B) (22H02069), Transformative Research Areas (A) (Digitalization-driven Transformative Organic Synthesis (Digi-TOS)) (22H05330), and Young Scientists (22K14673). T.M. thanks the Naito Foundation for financial support.

References

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    A review, with 298 refs.
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  14. 14
    Bunten, K. A.; Chen, L.; Fernandez, A. L.; Poë, A. J. Cone Angles: Tolman’s and Plato’s. Coord. Chem. Rev. 2002, 233–234, 41– 51,  DOI: 10.1016/S0010-8545(02)00099-1
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    Cone angles: Tolman's and Plato's
    Bunten, Kevin A.; Chen, Lezhan; Fernandez, Anthony L.; Poe, Anthony J.
    Coordination Chemistry Reviews (2002), 233-234 (), 41-51CODEN: CCHRAM; ISSN:0010-8545. (Elsevier Science B.V.)
    A review discusses the application of Tolman's cone angles for phosphites and other phosphorus ligands for quant. account of steric effects on substitution reaction enthalpies. Correlations of reaction enthalpy are presented which include the account for π-acidity parameters. The successful application of Tolman cone angles for P-donor and other ligands in accounting quant. for steric effects in a wide variety of physicochem. processes is contrasted with the variability of cone angles obtained from crystallog. studies. It is maintained that the latter are not relevant in describing steric effects for reactions in soln. Problems with cone angles for ligands with conformational uncertainties are best dealt with by systematic measurement of deviations of data for those ligands from trends defined by ligands with less ambiguous cone angles. In fact a body of cone angles for all ligands could be obtained by adjusting cone angles to give perfect fits to individual steric profiles and then averaging the values obtained from a large no. of such studies. In this way a set of cone angles could be obtained that are divorced from their origins in Tolman's models and justified solely by their successful quant. application. Consideration of data sets which include P(OMe)3 and P(OiPr)3 as a ligands suggests that the former should have its Tolman cone angle increased by ca. 10°, and the Tolman cone angle for the latter should be decreased by ca. 5°. Thus the deviant behavior of conformationally ambiguous ligands may be systematic, and could depend on whether the P-donors are acting as ligands or nucleophiles. However, the concept of a Platonically perfect set of cone angles is probably justified for many ligands that are conformationally unambiguous and these ideal cone angles may be essentially identical with Tolman's values. In the course of these analyses a new set of π-acidity parameters was developed and values are tabulated.
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    Bilbrey, J. A.; Kazez, A. H.; Locklin, J.; Allen, W. D. Exact Ligand Cone Angles. J. Comput. Chem. 2013, 34, 1189– 1197,  DOI: 10.1002/jcc.23217
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    Exact ligand cone angles
    Bilbrey, Jenna A.; Kazez, Arianna H.; Locklin, Jason; Allen, Wesley D.
    Journal of Computational Chemistry (2013), 34 (14), 1189-1197CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
    Many properties of transition-metal complexes depend on the steric bulk of bound ligands, usually quantified by the Tolman (θ) and solid (Θ) cone angles, which have proven utility but suffer from various limitations and coarse approxns. Here, we present an improved, math. rigorous method to det. an exact cone angle (θ°) by solving for the most acute right circular cone that contains the entire ligand. The procedure is applicable to any ligand, planar or nonplanar, monodentate or polydentate, bound to any metal center in any environment, and it is ideal for analyzing structures from quantum chem. computations as well as X-ray crystallog. expts. Exact cone angles were evaluated for a wide array of phosphine and amine ligands bound to palladium, nickel, or platinum by optimizing structures using B3LYP/6-31G* d. functional theory with effective core potentials for the transition metals. The mean abs. deviations of the std. θ and Θ parameters from the exact cone angles were 15-25°, mostly caused by distortions from the assumed idealized structures. © 2013 Wiley Periodicals, Inc.
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  16. 16
    Casey, C. P.; Paulsen, E. L.; Beuttenmueller, E. W.; Proft, B. R.; Matter, B. A.; Powell, D. R. Electronically Dissymmetric DIPHOS Derivatives Give Higher n:i Regioselectivity in Rhodium-Catalyzed Hydroformylation Than Either of Their Symmetric Counterparts. J. Am. Chem. Soc. 1999, 121, 63– 70,  DOI: 10.1021/ja982117h
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    Electronically Dissymmetric DIPHOS Derivatives Give Higher n:i Regioselectivity in Rhodium-Catalyzed Hydroformylation Than Either of Their Symmetric Counterparts
    Casey, Charles P.; Paulsen, Evelyn Lin; Beuttenmueller, Eckart W.; Proft, Bernd R.; Matter, Brock A.; Powell, Douglas R.
    Journal of the American Chemical Society (1999), 121 (1), 63-70CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Electronic effects on rhodium-catalyzed hydroformylation of 1-hexene with electronically dissym. DIPHOS derivs. [3,5-(CF3)2C6H3]2PCH2CH2PPh2 = [DIPHOS-(3,5-CF3,H)] (1), [2-(CF3)C6H4]2PCH2CH2PPh2 = [DIPHOS-(2-CF3,H)] (2), [3,5-(CF3)2C6H3]2PCH2CH2P[2-(CH3)C6H4]2 = [DIPHOS-(3,5-CF3,2-CH3)] (3), and [2-(CF3)C6H4]2PCH2CH2P[2-(CH3)C6H4]2 = [DIPHOS-(2-CF3,2-CH3)] (4) were investigated. Two apical-equatorial chelate isomers were obsd. for model (diphosphine)Ir(CO)2H complexes of dissym. diphosphines 1-4. In each case, the equatorial phosphine of the major isomer (96-60%) had electron-withdrawing aryl substituents. These dissym. DIPHOS derivs. were used to test the hypothesis that an electron-withdrawing substituent on an equatorial phosphine increases the hydroformylation n:i ratio while an electron-withdrawing substituent on an apical phosphine decreases the n:i ratio. In agreement with the predictions of this hypothesis, hydroformylation with the dissym. diphosphine ligand DIPHOS-(3,5-CF3,H) (1), gave an n:i ratio of 4.2:1, higher than either of the sym. ligands DIPHOS, 2.6:1, and DIPHOS-(3,5-CF3), 1.3:1. Similar observations were made for hydroformylations with 2-4.
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    Carraz, C.-A.; Ditzel, E. J.; Orpen, A. G.; Ellis, D. D.; Pringle, P. G.; Sunley, G. J. Rhodium(I) Complexes of Unsymmetrical Diphosphines: Efficient and Stable Methanol Carbonylation Catalysts. Chem. Commun. 2000, 1277– 1278,  DOI: 10.1039/b002802i
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    Rhodium(I) complexes of unsymmetrical diphosphines: efficient and stable methanol carbonylation catalysts
    Carraz, Charles-Antoine; Orpen, A. Guy; Ellis, Dianne D.; Pringle, Paul G.; Ditzel, Evert J.; Sunley, Glenn J.
    Chemical Communications (Cambridge) (2000), (14), 1277-1278CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Rhodium complexes of unsym. diphosphines of the type Ph2PCH2CH2PAr2, Ar = F substituted Ph groups, are efficient catalysts for carbonylation of methanol and have extended service life compared with ligand-modified catalyst under temps. of 150-200° and pressure of 10-60 bar. The diphosphines were prepd. and fully characterized. The catalysts were prepd. by addn. of fluoro-diphosphines to [Rh2(μ-Cl)2(CO)4] in MeOH; some of the I analogs were also prepd. Several features of the catalysis are reminiscent of iridium Cativa carbonylation catalysts.
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    Yue, W.-J.; Xiao, J.-Z.; Zhang, S.; Yin, L. Rapid Synthesis of Chiral 1,2-Bisphosphine Derivatives through Copper(I)-Catalyzed Asymmetric Conjugate Hydrophosphination. Angew. Chem., Int. Ed. 2020, 59, 7057– 7062,  DOI: 10.1002/anie.201916076
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    Rapid Synthesis of Chiral 1,2-Bisphosphine Derivatives through Copper(I)-Catalyzed Asymmetric Conjugate Hydrophosphination
    Yue, Wen-Jun; Xiao, Jun-Zhao; Zhang, Shuai; Yin, Liang
    Angewandte Chemie, International Edition (2020), 59 (18), 7057-7062CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    1,2-Bisphosphines have been identified as one class of important and powerful chiral ligands in asym. catalysis with transition metals. Herein, a copper(I)-catalyzed asym. hydrophosphination of α,β-unsatd. phosphine sulfides was developed with the assistance of "soft-soft" interaction between copper(I)-catalyst and the phosphine sulfide moiety, which afforded 1,2-bisphosphine derivs. with diversified electronic nature and steric hindrance in high to excellent yields with high to excellent enantioselectivity. Moreover, the challenging catalytic asym. hydrophosphination/protonation reaction was achieved with excellent enantioselectivity. Strikingly, the dynamic kinetic resoln. of racemic diarylphosphines was also successfully carried out with high to excellent diastereo- and enantioselectivities. Interestingly, the nucleophilic copper(I)-diphenylphosphide species was characterized by 31P NMR spectrum and mass spectrum. At last, three products were transformed to chiral 1,2-bisphosphines, which were employed as ligands in Rh-catalyzed asym. hydrogenation of α-amino-α,β-unsatd. ester. The α-amino acid deriv. was produced in high enantioselectivity, which demonstrated the utility of the present methodol.
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    RajanBabu, T. V.; Casalnuovo, A. L. Role of Electronic Asymmetry in the Design of New Ligands: The Asymmetric Hydrocyanation Reaction. J. Am. Chem. Soc. 1996, 118, 6325– 6326,  DOI: 10.1021/ja9609112
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    Role of Electronic Asymmetry in the Design of New Ligands: The Asymmetric Hydrocyanation Reaction
    RajanBabu, T. V.; Casalnuovo, Albert L.
    Journal of the American Chemical Society (1996), 118 (26), 6325-6326CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Enantioselectivity of Ni(0)-catalyzed asym. hydrocyanation of a vinylarene can be enhanced by the use of C2- or pseudo-C2-sym. ligands in which the two chelating atoms are electronically different. Thus 3,4-di-O-diarylphosphino-α-fructofuranoside with an electron-withdrawing P at C4 and an electron-rich P at C3 gave the highest enantioselectivity ever recorded for asym. hydrocyanation of a vinylarene. The role of electronic asymmetry is further confirmed with the use of a simpler 1,2-bis-phosphinite ligand derived from (S,S)-tartranil. Stereoelectronic effects in the formation or decompn. of a penultimate intermediate involved in the reaction maybe responsible for this unusual effect.
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    Nozaki, K. Unsymmetric Bidentate Ligands in Metal-Catalyzed Carbonylation of Alkenes. Chem. Rec. 2005, 5, 376– 384,  DOI: 10.1002/tcr.20046
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    Unsymmetric bidentate ligands in metal-catalyzed carbonylation of alkenes
    Nozaki, Kyoko
    Chemical Record (2005), 5 (6), 376-384CODEN: CRHEAK; ISSN:1527-8999. (John Wiley & Sons, Inc.)
    A review. Characteristic features of unsym. bidentate ligands, in which the two coordination atoms are not equiv., are reviewed with a focus on their use in metal-catalyzed olefin carbonylations. High enantioselectivity for a variety of substrates was achieved using (R,S)-BINAPHOS in Rh-catalyzed hydroformylation. An unsym. chiral bis(phosphino)ferrocene ligand shows high productivity accompanied by high regio- and enantioselectivities in the Pd-catalyzed alternating copolymn. of 1-alkene with CO. The advantages of electronic unsymmetry are demonstrated esp. in the spectroscopic observation of single steps involved in catalytic cycles.
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    Thomas, A. A.; Speck, K.; Kevlishvili, I.; Lu, Z.; Liu, P.; Buchwald, S. L. Mechanistically Guided Design of Ligands That Significantly Improve the Efficiency of CuH-Catalyzed Hydroamination Reactions. J. Am. Chem. Soc. 2018, 140, 13976– 13984,  DOI: 10.1021/jacs.8b09565
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    Mechanistically Guided Design of Ligands That Significantly Improve the Efficiency of CuH-Catalyzed Hydroamination Reactions
    Thomas, Andy A.; Speck, Klaus; Kevlishvili, Ilia; Lu, Zhaohong; Liu, Peng; Buchwald, Stephen L.
    Journal of the American Chemical Society (2018), 140 (42), 13976-13984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Using a mech. guided ligand design approach, a new ligand (SEGFAST) for the CuH-catalyzed hydroamination reaction of unactivated terminal olefins has been developed, providing a 62-fold rate increase over reactions compared to DTBM-SEGPHOS, the previous optimal ligand. Combining the resp. strengths of computational chem. and exptl. kinetic measurements, we were able to quickly identify potential modifications that lead to more effective ligands, thus avoiding synthesizing and testing a large library of ligands. By optimizing the combination of attractive, noncovalent ligand-substrate interactions and the stability of the catalyst under the reaction conditions, we were able to identify a finely tuned hybrid ligand that greatly enables accelerated hydrocupration rates with unactivated alkenes. Moreover, a modular and robust synthetic sequence was devised, which allowed for the practical, gram-scale synthesis of these novel hybrid ligand structures.
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    Wiberg, K. B. The Concept of Strain in Organic Chemistry. Angew. Chem., Int. Ed. 1986, 25, 312– 322,  DOI: 10.1002/anie.198603121
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    Turkowska, J.; Durka, J.; Gryko, D. Strain Release – An Old Tool for New Transformations. Chem. Commun. 2020, 56, 5718– 5734,  DOI: 10.1039/D0CC01771J
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    Strain release - an old tool for new transformations
    Turkowska, Joanna; Durka, Jakub; Gryko, Dorota
    Chemical Communications (Cambridge, United Kingdom) (2020), 56 (43), 5718-5734CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. Strain-release driven transformations give access to attractive bioisosteric motifs highly prized by medicinal chemists and they are characteristic of mols. possessing distorted bond lengths and angles. By broadening the chem. space in drug discovery, recently, these compds. have attracted a lot of interest. Their reactivity stems mainly from an increased energy and destabilization. As a result, the opening of the bridging bond occurs under the action of both nucleophiles and electrophiles as well as radical species and transition metals. Though the bridge bond dominates their reactivity, it is also influenced by the substitution pattern. This feature article focuses on strain-release driven strategies paying particular attention to the most recent (year > 2010) advances.
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    Bellotti, P.; Glorius, F. Strain-Release Photocatalysis. J. Am. Chem. Soc. 2023, 145, 20716– 20732,  DOI: 10.1021/jacs.3c08206
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    Strain-Release Photocatalysis
    Bellotti, Peter; Glorius, Frank
    Journal of the American Chemical Society (2023), 145 (38), 20716-20732CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A review. The concept of strain in org. compds. is as old as modern org. chem. and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate-with significant driving force-rigid aliph. systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an addnl. dimension to strain-release processes by leveraging the energy of photons to create chem. complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis-with emphases on mechanisms, catalytic cycles, and current limitations-the unique chem. architectures that can be produced, and possible future directions. strain release.
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    Blanchard, E. P., Jr.; Cairncross, A. Bicyclo[1.1.0]butane Chemistry. I. The Synthesis and Reactions of 3-Methylbicyclo[1.1.0]butanecarbonitriles. J. Am. Chem. Soc. 1966, 88, 487– 495,  DOI: 10.1021/ja00955a020
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    Bicyclo[1.1.0]butane chemistry. I. The synthesis and reactions of 3-methylbicyclo[1.1.0]butanecarbonitriles
    Blanchard, E. P., Jr.; Cairnross, A.
    Journal of the American Chemical Society (1966), 88 (3), 487-95CODEN: JACSAT; ISSN:0002-7863.
    The hydrohalogenation-dehydrohalogenation of 3-methylenecyclobutanecarbonitriles provides a facile, high-yield synthesis of 3-methylbicyclo[1.1.0]butanecarbonitriles. The latter react with a broad spectrum of reagents including acids, electron-deficient multiple bonds, nucleophiles, free radicals, and halogens to give cyclobutanes and cyclobutenes. The bicyclobutane ring system is thermally labile leading to a synthesis of substituted butadienes. Catalytic hydrogenation of bicyclobutanes consumes 2 moles of H leading to open-chain structures. The bicyclobutane ring system is stable to certain transformations of the nitrile group. These reactions are discussed in terms of the scope, conditions and mechanistic implications.
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    Cairncross, A.; Blanchard, E. P., Jr. Bicyclo[1.1.0]butane Chemistry. II. Cycloaddition Reactions of 3-Methylbicyclo[1.1.0]butanecarbonitriles. The Formation of Bicyclo[2.1.1]hexanes. J. Am. Chem. Soc. 1966, 88, 496– 504,  DOI: 10.1021/ja00955a021
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    Bicyclo[1.1.0]butane chemistry. II. Cycloaddition reactions of 3-methylbicyclo[1.1.0]butanecarbonitriles. The formation of bicyclo[2.1.1]hexanes
    Cairncross, A.; Blanchard, E. P., Jr.
    Journal of the American Chemical Society (1966), 88 (3), 496-504CODEN: JACSAT; ISSN:0002-7863.
    cf. preceding abstr. 3-Methylbicyclo[1.1.0]butanecarbonitrile (I) reacts with butadiene, acrylonitrile, maleonitrile, fumaronitrile, C2H4, styrene, p-methoxystyrene, and 1-(N,N-dimethylamino)cyclopentene to form 1:1 adducts. In all cases, derivs. of 4-methylbicyclo[2.1.1]-hexanecarbonitrile were formed along with other products. Evidence is presented which strongly suggests diradical intermediates in the cycloaddn. reaction as well as in the formation of monocyclic products from C2H4. Thermolysis of I in the liquid phase at 150° produces several dimers, two of which have been partially identified as cycloadducts between I and its pyrolysis products. Brief attempts to observe thermally activated inversion of derivs. of bicyclo [1.1.0]butane failed because of competitive decompn.
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    Lampman, G. M.; Aumiller, J. C. Bicyclo[1.1.0]butane. Org. Synth. 1971, 51, 55,  DOI: 10.1002/0471264180.os051.13
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    D’yachenko, A. I.; Abramova, N. M.; Zotova, S. V.; Nesmeyanova, O. A.; Bragin, O. V. New Synthesis of Bicyclo[1.1.0]butane Hydrocarbons. Russ. Chem. Bull. 1985, 34, 1885– 1889,  DOI: 10.1007/BF00953929
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    Kelly, C. B.; Milligan, J. A.; Tilley, L. J.; Sodano, T. M. Bicyclobutanes: From Curiosities to Versatile Reagents and Covalent Warheads. Chem. Sci. 2022, 13, 11721– 11737,  DOI: 10.1039/D2SC03948F
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    Bicyclobutanes: from curiosities to versatile reagents and covalent warheads
    Kelly, Christopher B.; Milligan, John A.; Tilley, Leon J.; Sodano, Taylor M.
    Chemical Science (2022), 13 (40), 11721-11737CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    A review. The unique chem. of small, strained carbocyclic systems has long captivated org. chemists from a theor. and fundamental standpoint. A resurgence of interest in strained carbocyclic species has been prompted by their potential as bioisosteres, high fraction of sp3 carbons, and limited appearance in the patent literature. Among strained ring systems, bicyclo[1.1.0]butane (BCB) stands apart as the smallest bicyclic carbocycle and is amongst the most strained carbocycles known. Despite the fact that BCBs have been synthesized and studied for well over 50 years, they have long been regarded as lab. curiosities. However, new approaches for prepg., functionalizing, and using BCBs in "strain-release" transformations have positioned BCBs to be powerful synthetic workhorses. Further, the olefinic character of the bridgehead bond enables BCBs to be elaborated into various other ring systems and function as covalent warheads for bioconjugation. This review will discuss the recent developments in the synthesis and functionalization of BCBs as well as the applications of these strained rings in synthesis and drug discovery. An overview of the properties and the historical context of this interesting structure will be provided.
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    Golfmann, M.; Walker, J. C. L. Bicyclobutanes as Unusual Building Blocks for Complexity Generation in Organic Synthesis. Commun. Chem. 2023, 6, 9,  DOI: 10.1038/s42004-022-00811-3
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    Bicyclobutanes as unusual building blocks for complexity generation in organic synthesis
    Golfmann, Maxim; Walker, Johannes C. L.
    Communications Chemistry (2023), 6 (1), 9CODEN: CCOHCT; ISSN:2399-3669. (Nature Portfolio)
    A review. Bicyclobutanes are among the most highly strained isolable org. compds. and their assocd. low activation barriers to reactivity make them intriguing building-blocks in org. chem. In recent years, numerous creative synthetic strategies exploiting their heightened reactivity have been presented and these discoveries have often gone hand-in-hand with the development of more practical routes for their synthesis. Their proclivity as strain-release reagents through their weak central C-C bond has been harnessed in a variety of addn., rearrangement and insertion reactions, providing rapid access to a rich tapestry of complex mol. scaffolds. This review will provide an overview of the different options available for bicyclobutane synthesis, the main classes of compds. that can be prepd. from bicyclobutanes, and the assocd. modes of reactivity used.
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    Tyler, J. L.; Aggarwal, V. K. Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics. Chem.─Eur. J. 2023, 29, e202300008  DOI: 10.1002/chem.202300008
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    Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics
    Tyler, Jasper L.; Aggarwal, Varinder K.
    Chemistry - A European Journal (2023), 29 (29), e202300008CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The use of metalated (aza)bicyclo[1.1.0]butanes in synthesis is currently experiencing a renaissance, as evidenced by the numerous reports in the last 5 years that have relied on such intermediates to undergo unique transformations or generate novel fragments. Since their discovery, these species have been demonstrated to participate in a wide range of reactions with carbon and heteroatom electrophiles, as well as metal complexes, to facilitate the rapid diversification of (aza)bicyclo[1.1.0]butane-contg. compds. Key to this is the relative acidity of the bridgehead C-H bonds which promotes facile deprotonation and subsequent functionalization of an unsubstituted position on the carbon framework via the intermediacy of a metalated (aza)bicyclo[1.1.0]butane. Addnl., the late-stage incorporation of deuterium atoms in strained fragments has led to the elucidation of numerous reaction mechanisms that involve strained bicycles. The continued investigation into the inimitable reactivity of metalated bicycles will cement their importance within the field of organometallic chem.
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  32. 32
    Kelly, C. B.; Colthart, A. M.; Constant, B. D.; Corning, S. R.; Dubois, L. N. E.; Genovese, J. T.; Radziewicz, J. L.; Sletten, E. M.; Whitaker, K. R.; Tilley, L. J. Enabling the Synthesis of Perfluoroalkyl Bicyclobutanes via 1,3 γ-Silyl Elimination. Org. Lett. 2011, 13, 1646– 1649,  DOI: 10.1021/ol200121f
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    Enabling the Synthesis of Perfluoroalkyl Bicyclobutanes via 1,3 γ-Silyl Elimination
    Kelly, Christopher B.; Colthart, Allison M.; Constant, Brad D.; Corning, Sean R.; Dubois, Lily N. E.; Genovese, Jacqueline T.; Radziewicz, Julie L.; Sletten, Ellen M.; Whitaker, Katherine R.; Tilley, Leon J.
    Organic Letters (2011), 13 (7), 1646-1649CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    Two new bicyclobutanes were prepd. from cyclobutyl systems by a novel, solvolytic, carbocation-based methodol. An electron-withdrawing perfluoroalkyl group at the incipient cationic center enhances neighboring-group participation of the γ-silyl group, inducing facile, remarkably selective 1,3-elimination yielding only bicyclobutanes. The method unlocks potential access to a host of EWG-substituted strained rings and a potential new method for the synthesis of trifluoromethylcyclopropanes.
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  33. 33
    Wiberg, K. B.; Taddell, S. T. Reactions of [1.1.1]Propellane. J. Am. Chem. Soc. 1990, 112, 2194– 2216,  DOI: 10.1021/ja00162a022
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    Reactions of [1.1.1]propellane
    Wiberg, Kenneth B.; Waddell, Sherman T.
    Journal of the American Chemical Society (1990), 112 (6), 2194-216CODEN: JACSAT; ISSN:0002-7863.
    The free-radical addn. reactions of [1.1.1]propellane (I) are described in some detail and allowed the prepn. of a wide variety of 1,3-disubstituted bicyclo[1.1.1]pentanes. The reaction of I with free radicals was more rapid than that of bicyclo[1.1.1]butane (II), whereas bicyclo[2.1.0]pentane (III) was relatively inert. In some cases the free-radical addns. led to oligomers, and in the case of THF addn. the chain-transfer const. was measured. The addn. of thiophenol to I followed by redn. with the lithium radical anion from 4,4'-di-tert-butylbiphenyl gave 1-lithiobicyclo[1.1.1]pentane, from which a variety of 1-substituted bicyclo[1.1.1]pentanes may be prepd. In the Baeyer-Villiger oxidn. of 1-benzoylbicyclo[1.1.1]pentane, the tert-Bu group migrated in preference to the bicyclopentyl group. Conversion of the ketone to the tosylhydrazone followed by base treatment gave products of the type expected from the corresponding carbene. The reaction of I with NO in carbon disulfide gave a unique reaction in which nitro and thiocyano groups were introduced. The reactions of I-III with NO2 also were examd. Whereas I gave 1,3-dinitrobicyclo[1.1.1]pentane, the other hydrocarbons followed different reaction paths. The reaction of I with electron-deficient alkenes and alkynes are described in some detail and are compared with the corresponding reactions of II and III. Here, the relative reactivities of I and II were often comparable but varied considerably with the reagent used. Again, III was relatively unreactive. The reaction of I with Rh(I) gave a dimer, and evidence is presented for a metallocarbene intermediate. The authors counsel safety in the prepn. of trifluoroperacetic acid with 70% H2O2.
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  34. 34
    Gianatassio, R.; Lopchuk, J. M.; Wang, J.; Pan, C.-M.; Malins, L. R.; Prieto, L.; Brandt, T. A.; Collins, M. R.; Gallego, G. M.; Sach, N. W.; Spangler, J. E.; Zhu, H.; Zhu, J.; Baran, P. S. Strain-release Amination. Science 2016, 351, 241– 246,  DOI: 10.1126/science.aad6252
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    34
    Strain-release amination
    Gianatassio, Ryan; Lopchuk, Justin M.; Wang, Jie; Pan, Chung-Mao; Malins, Lara R.; Prieto, Liher; Brandt, Thomas A.; Collins, Michael R.; Gallego, Gary M.; Sach, Neal W.; Spangler, Jillian E.; Zhu, Huichin; Zhu, Jinjiang; Baran, Phil S.
    Science (Washington, DC, United States) (2016), 351 (6270), 241-246CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    To optimize drug candidates, modern medicinal chemists are increasingly turning to an unconventional structural motif: small, strained ring systems. However, the difficulty of introducing substituents such as bicyclo[1.1.1]pentanes, azetidines, or cyclobutanes often outweighs the challenge of synthesizing the parent scaffold itself. Thus, there is an urgent need for general methods to rapidly and directly append such groups onto core scaffolds. Here we report a general strategy to harness the embedded potential energy of effectively spring-loaded C-C and C-N bonds with the most oft-encountered nucleophiles in pharmaceutical chem., amines. Strain-release amination can diversify a range of substrates with a multitude of desirable bioisosteres at both the early and late stages of a synthesis. The technique has also been applied to peptide labeling and bioconjugation.
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    Shelp, R. A.; Walsh, P. J. Synthesis of BCP Benzylamines from 2-Azaallyl Anions and [1.1.1]Propellane. Angew. Chem., Int. Ed. 2018, 57, 15857– 15861,  DOI: 10.1002/anie.201810061
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    35
    Synthesis of BCP Benzylamines From 2-Azaallyl Anions and [1.1.1]Propellane
    Shelp, Russell A.; Walsh, Patrick J.
    Angewandte Chemie, International Edition (2018), 57 (48), 15857-15861CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    For bioactive mols., bicyclo[1.1.1]pentanes (BCPs) are an emerging isostere of rigid spacer groups that have shown potential to improve drug-like qualities. As BCPs become an increasingly popular motif for evaluation in drug candidates, org. chemists must meet the demand to reliably incorporate them into new targets. To provide access to BCP analogs of diaryl methanamines, a ubiquitous scaffold in medicinal chem., we report the synthesis of BCP benzylamines through reactivity of [1.1.1]propellane with 2-azaallyl anions, which are generated in situ from N-benzyl ketimines. The reaction proceeds rapidly at room temp. and tolerates a broad substrate scope, providing straightforward access to 23 new BCP benzylamine derivs. Initial expts. support the intermediacy of a BCP anion. Addnl., the reaction can be promoted by substoichiometric loadings of base, highlighting an unusual reactivity of both 2-azaallyls and [1.1.1]propellane.
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  36. 36
    Yu, S.; Noble, A.; Bedford, R. B.; Aggarwal, V. K. Methylenespiro[2.3]hexanes via Nickel-Catalyzed Cyclopropanations with [1.1.1]Propellane. J. Am. Chem. Soc. 2019, 141, 20325– 20334,  DOI: 10.1021/jacs.9b10689
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    36
    Methylenespiro[2.3]hexanes via Nickel-Catalyzed Cyclopropanations with [1.1.1]Propellane
    Yu, Songjie; Noble, Adam; Bedford, Robin B.; Aggarwal, Varinder K.
    Journal of the American Chemical Society (2019), 141 (51), 20325-20334CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    [1.1.1]Propellane is a highly strained tricyclic hydrocarbon whose reactivity is dominated by addn. reactions across the central inverted bond to provide bicyclo[1.1.1]pentane derivs. These reactions proceed under both radical and two-electron pathways, hence providing access to a diverse array of products. Conversely, transition metal-catalyzed reactions of [1.1.1]propellane are underdeveloped and lack synthetic utility, with reported examples generally yielding mixts. of ring-opened structural isomers, dimers, and trimers, often with poor selectivity. Herein, authors report that Ni(0) catalysis enables the use of [1.1.1]propellane as a carbene precursor in cyclopropanations of a range of functionalized alkenes to give methylenespiro[2.3]hexane products I (R1 = H, Me, CH2OTBS, etc.; R1 = H, Me, Ph, etc.; R3 = Ph, 4-CF3C6H4, 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl, etc. ). Computational studies provide support for initial formation of a Ni(0)-[1.1.1]propellane complex followed by concerted double C-C bond activation to give the key 3-methylenecyclobutylidene-nickel intermediate.
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  37. 37
    Lasányi, D.; Tolnai, G. L. Copper-Catalyzed Ring Opening of [1.1.1]Propellane with Alkynes: Synthesis of Exocyclic Allenic Cyclobutanes. Org. Lett. 2019, 21, 10057– 10062,  DOI: 10.1021/acs.orglett.9b03999
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    Copper-Catalyzed Ring Opening of [1.1.1]Propellane with Alkynes: Synthesis of Exocyclic Allenic Cyclobutanes
    Lasanyi, Daniel; Tolnai, Gergely L.
    Organic Letters (2019), 21 (24), 10057-10062CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    Despite the long history and interesting properties of propellanes, these compds. still have tremendous potential to be exploited in synthetic org. chem. Herein author disclose an exptl. simple procedure to achieve cyclobutane-contg. allenes and alkynes through a copper-catalyzed ring opening of [1.1.1]propellane and subsequent reaction with ethynes.
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    Kim, J. H.; Ruffoni, A.; Al-Faiyz, Y. S. S.; Sheikh, N. S.; Leonori, D. Divergent Strain-Release Amino-Functionalization of [1.1.1]Propellane with Electrophilic Nitrogen-Radicals. Angew. Chem., Int. Ed. 2020, 59, 8225– 8231,  DOI: 10.1002/anie.202000140
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    38
    Divergent Strain-Release Amino-Functionalization of [1.1.1]Propellane with Electrophilic Nitrogen-Radicals
    Kim, Ji Hye; Ruffoni, Alessandro; Al-Faiyz, Yasair S. S.; Sheikh, Nadeem S.; Leonori, Daniele
    Angewandte Chemie, International Edition (2020), 59 (21), 8225-8231CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Herein the authors report the development of a photocatalytic strategy for the divergent prepn. of functionalized bicyclo[1.1.1]pentylamines. This approach exploits, for the first time, the ability of nitrogen-radicals to undergo strain-release reaction with [1.1.1]propellane. This reactivity is facilitated by the electrophilic nature of these open-shell intermediates and the presence of strong polar effects in the transition-state for C-N bond formation/ring-opening. With the aid of a simple reductive quenching photoredox cycle, the authors have successfully harnessed this novel radical strain-release amination as part of a multicomponent cascade compatible with several external trapping agents. Overall, this radical strategy enables the rapid construction of novel amino-functionalized building blocks with potential application in medicinal chem. programs as p-substituted aniline bioisosteres.
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  39. 39
    Zhang, X.; Smith, R. T.; Le, C.; McCarver, S. J.; Shireman, B. T.; Carruthers, N. I.; MacMillan, D. W. C. Copper-mediated Synthesis of Drug-like Bicyclopentanes. Nature 2020, 580, 220– 226,  DOI: 10.1038/s41586-020-2060-z
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    Copper-mediated synthesis of drug-like bicyclopentanes
    Zhang, Xiaheng; Smith, Russell T.; Le, Chip; McCarver, Stefan J.; Shireman, Brock T.; Carruthers, Nicholas I.; MacMillan, David W. C.
    Nature (London, United Kingdom) (2020), 580 (7802), 220-226CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Multicomponent reactions are relied on in both academic and industrial synthetic org. chem. owing to their step- and atom-economy advantages over traditional synthetic sequences1. Recently, bicyclo[1.1.1]pentane (BCP) motifs have become valuable as pharmaceutical bioisosteres of benzene rings, and in particular 1,3-disubstituted BCP moieties have become widely adopted in medicinal chem. as para-Ph ring replacements2. These structures are often generated from [1.1.1]propellane via opening of the internal C-C bond through the addn. of either radicals or metal-based nucleophiles3-13. The resulting propellane-addn. adducts are then transformed to the requisite polysubstituted BCP compds. via a range of synthetic sequences that traditionally involve multiple chem. steps. Although this approach was effective so far, a multicomponent reaction that enables single-step access to complex and diverse polysubstituted drug-like BCP products would be more time efficient compared to current stepwise approaches. Here the authors report a one-step three-component radical coupling of [1.1.1]propellane to afford diverse functionalized bicyclopentanes using various radical precursors and heteroatom nucleophiles via a metallaphotoredox catalysis protocol. This copper-mediated reaction operates on short timescales (five minutes to one hour) across multiple (more than ten) nucleophile classes and can accommodate a diverse array of radical precursors, including those that generate alkyl, α-acyl, trifluoromethyl and sulfonyl radicals. This method was used to rapidly prep. BCP analogs of known pharmaceuticals, one of which is substantially more metabolically stable than its com. progenitor.
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  40. 40
    Sterling, A. J.; Dürr, A. B.; Smith, R. C.; Anderson, E. A.; Duarte, F. Rationalizing the Diverse Reactivity of [1.1.1]Propellane through σ–π-Delocalization. Chem. Sci. 2020, 11, 4895– 4903,  DOI: 10.1039/D0SC01386B
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    40
    Rationalizing the diverse reactivity of [1.1.1]propellane through σ-π-delocalization
    Sterling, Alistair J.; Durr, Alexander B.; Smith, Russell C.; Anderson, Edward A.; Duarte, Fernanda
    Chemical Science (2020), 11 (19), 4895-4903CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    [1.1.1]Propellane is the ubiquitous precursor to bicyclo[1.1.1]pentanes (BCPs), motifs of high value in pharmaceutical and materials research. The classical Lewis representation of this mol. places an inter-bridgehead C-C bond along its central axis; 'strain relief'-driven cleavage of this bond is commonly thought to enable reactions with nucleophiles, radicals and electrophiles. We propose that this broad reactivity profile instead derives from σ-π-delocalization of electron d. in [1.1.1]propellane. Using ab initio and DFT calcns., we show that its reactions with anions and radicals are facilitated by increased delocalization of electron d. over the propellane cage during addn., while reactions with cations involve charge transfer that relieves repulsion inside the cage. These results provide a unified framework to rationalize exptl. observations of propellane reactivity, opening up opportunities for the exploration of new chem. of [1.1.1]propellane and related strained systems that are useful building blocks in org. synthesis.
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  41. 41
    Huang, W.; Keess, S.; Molander, G. A. Dicarbofunctionalization of [1.1.1]Propellane Enabled by Nickel/Photoredox Dual Catalysis: One-Step Multicomponent Strategy for the Synthesis of BCP-Aryl Derivatives. J. Am. Chem. Soc. 2022, 144, 12961– 12969,  DOI: 10.1021/jacs.2c05304
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    Dicarbofunctionalization of [1.1.1]Propellane Enabled by Nickel/Photoredox Dual Catalysis: One-Step Multicomponent Strategy for the Synthesis of BCP-Aryl Derivatives
    Huang, Weichen; Keess, Sebastian; Molander, Gary A.
    Journal of the American Chemical Society (2022), 144 (28), 12961-12969CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Bicyclo[1.1.1]pentane (BCP) motifs as para-disubstituted aryl bioisosteres are playing an emerging role in pharmaceutical, agrochem., and materials chem. The vast majority of these structures are obtained from a BCP electrophile or nucleophile, which are themselves derived from [1.1.1]propellane via cleavage of the internal C-C bond through the addn. of either radicals or metal-based nucleophiles. Compared with the current stepwise approaches, a multicomponent reaction that provides direct access to complex and diverse disubstituted BCP products would be more attractive. Herein, authors report a single-step, multicomponent approach to synthetically versatile arylated BCP products via nickel/photoredox catalysis. Importantly, this three-component process allows two C-C bonds to be formed in a single step and sets three quaternary centers, unprecedented in any previously reported methods. The method has been demonstrated to allow access to complex BCP architectures from aryl halide and radical precursor substrates.
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  42. 42
    Kraemer, Y.; Ghiazza, C.; Ragan, A. N.; Ni, S.; Lutz, S.; Neumann, E. K.; Fettinger, J. C.; Nöthling, N.; Goddard, R.; Cornella, J.; Pitts, C. R. Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight. Angew. Chem., Int. Ed. 2022, 61, e202211892  DOI: 10.1002/anie.202211892
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    Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight
    Kraemer, Yannick; Ghiazza, Clement; Ragan, Abbey N.; Ni, Shengyang; Lutz, Sigrid; Neumann, Elizabeth K.; Fettinger, James C.; Nothling, Nils; Goddard, Richard; Cornella, Josep; Pitts, Cody Ross
    Angewandte Chemie, International Edition (2022), 61 (48), e202211892CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Authors leveraged the recent increase in synthetic accessibility of SF5Cl and Ar-SF4Cl compds. to combine chem. of the SF5 and SF4Ar groups with strain-release functionalization. By effectively adding SF5 and SF4Ar radicals across [1.1.1]propellane, author's accessed structurally unique bicyclopentanes, bearing two distinct elements of bioisosterism. Upon evaluating these "hybrid isostere" motifs in the solid state, authors measured exceptionally short transannular distances; in one case, the distance rivals the shortest nonbonding C···C contact reported to date. This prompted SC-XRD and DFT analyses that support the notion that a donor-acceptor interaction involving the "wing" C-C bonds is playing an important role in stabilization. Thus, these heretofore unknown structures expand the palette for highly coveted three-dimensional fluorinated building blocks and provide insight to a more general effect obsd. in bicyclopentanes.
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    Pickford, H. D.; Ripenko, V.; McNamee, R. E.; Holovchuk, S.; Thompson, A. L.; Smith, R. C.; Mykhailiuk, P. K.; Anderson, E. A. Rapid and Scalable Halosulfonylation of Strain-Release Reagents. Angew. Chem., Int. Ed. 2023, 62, e202213508  DOI: 10.1002/anie.202213508
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    Rapid and Scalable Halosulfonylation of Strain-Release Reagents
    Pickford, Helena D.; Ripenko, Vasyl; McNamee, Ryan E.; Holovchuk, Serhii; Thompson, Amber L.; Smith, Russell C.; Mykhailiuk, Pavel K.; Anderson, Edward A.
    Angewandte Chemie, International Edition (2023), 62 (3), e202213508CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A one-pot halosulfonylation of [1.1.1]propellane, [3.1.1]propellane and bicyclo[1.1.0]butanes I (R = N,N-bis(propan-2-yl)carbamoyl, S(O)2Ph; R1 = H, Me) proceeds under practical, scalable and mild conditions. The sulfonyl halides R2S(O)2X (R2 = Me, cyclopropyl, Ph, 5-bromothiophen-2-yl, etc.; X = I, Br, Cl) used in this chem. feature aryl, heteroaryl and alkyl substituents, and are conveniently generated in situ from readily available sulfinate salts R2S(O)OM (M = Na, Li) and halogen atom sources. This methodol. enables the synthesis of an array of pharmaceutically and agrochem. relevant halogen/sulfonyl-substituted bioisosteres II, III and cyclobutanes IV, on up to multidecagram scale.
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    Nassir, M.; Ociepa, M.; Zhang, H.-J.; Grant, L. N.; Simmons, B. J.; Oderinde, M. S.; Kawamata, Y.; Cauley, A. N.; Schmidt, M. A.; Eastgate, M. D.; Baran, P. S. Stereocontrolled Radical Thiophosphorylation. J. Am. Chem. Soc. 2023, 145, 15088– 15093,  DOI: 10.1021/jacs.3c05655
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    Frank, N.; Nugent, J.; Shire, B. R.; Pickford, H. D.; Rabe, P.; Sterling, A. J.; Zarganes-Tzitzikas, T.; Grimes, T.; Thompson, A. L.; Smith, R. C.; Schofield, C. J.; Brennan, P. E.; Duarte, F.; Anderson, E. A. Synthesis of meta-Substituted Arene Bioisosteres from [3.1.1]Propellane. Nature 2022, 611, 721– 726,  DOI: 10.1038/s41586-022-05290-z
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    45
    Synthesis of meta-substituted arene bioisosteres from [3.1.1]propellane
    Frank, Nils; Nugent, Jeremy; Shire, Bethany R.; Pickford, Helena D.; Rabe, Patrick; Sterling, Alistair J.; Zarganes-Tzitzikas, Tryfon; Grimes, Thomas; Thompson, Amber L.; Smith, Russell C.; Schofield, Christopher J.; Brennan, Paul E.; Duarte, Fernanda; Anderson, Edward A.
    Nature (London, United Kingdom) (2022), 611 (7937), 721-726CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
    It was found that [3.1.1]propellane can be synthesized on a multigram scale, and readily undergoes a range of radical-based transformations to generate medicinally relevant carbon- and heteroatom-substituted BCHeps, including pharmaceutical analogs. Comparison of the absorption, distribution, metab. and excretion (ADME) properties of these analogs reveals enhanced metabolic stability relative to their parent arene-contg. drugs, validating the potential of this meta-arene analog as an sp3-rich motif in drug design. Collectively, these results show that bicyclo[3.1.1]heptanes (BCHeps) can be prepd. on useful scales using a variety of methods, offering a new surrogate for meta-substituted benzene rings for implementation in drug discovery programs.
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    Iida, T.; Kanazawa, J.; Matsunaga, T.; Miyamoto, K.; Hirano, K.; Uchiyama, M. Practical and Facile Access to Bicyclo[3.1.1]heptanes: Potent Bioisosteres of meta-Substituted Benzenes. J. Am. Chem. Soc. 2022, 144, 21848– 21852,  DOI: 10.1021/jacs.2c09733
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    Practical and Facile Access to Bicyclo[3.1.1]heptanes: Potent Bioisosteres of meta-Substituted Benzenes
    Iida, Toranosuke; Kanazawa, Junichiro; Matsunaga, Tadafumi; Miyamoto, Kazunori; Hirano, Keiichi; Uchiyama, Masanobu
    Journal of the American Chemical Society (2022), 144 (48), 21848-21852CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Focused on the bicyclo[3.1.1]heptane (BCH) scaffold as a novel bioisostere of meta-substituted benzenes, anticipating that [3.1.1]propellane was a versatile precursor of diversely functionalized BCHs. Described a practical preparative method for [3.1.1]propellane from newly developed 1,5-diiodobicyclo[3.1.1]heptane, as well as difunctionalization reactions of [3.1.1]propellane leading to functionalized BCHs. Also reported postfunctionalization reactions of these products.
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    Hamon, D. P. G.; Trenerry, V. C. Carbenoid Insertion Reactions: Formation of [4.1.1]Propellane. J. Am. Chem. Soc. 1981, 103, 4962– 4965,  DOI: 10.1021/ja00406a059
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  • Abstract

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    Scheme 1

    Scheme 1. Importance of Bidentate Phosphine Ligands
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    Scheme 2

    Scheme 2. 3CR for the Synthesis of Diphosphine Ligands
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    Figure 1

    Figure 1. 3CR of cyclopropane with 1 and 2.

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    Figure 2

    Figure 2. 3CR of bicyclo[1.1.0]butane.

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    Figure 3

    Figure 3. 3CR of sulfonylated bicyclo[1.1.0]butane. a0.7 mmol-scale synthesis.

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    Figure 4

    Figure 4. Removal of the sulfonyl group under Mg-mediated conditions.

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    Figure 5

    Figure 5. Conversion of 4bb-cis to cis-coordinate Ni-complex NiCl2(8bb-cis). Bottom right: crystal structure of NiCl2(8bb-cis) with thermal ellipsoids at 50% probability; all hydrogen atoms as well as one molecule of CH2Cl2 have been omitted for clarity.

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    Figure 6

    Figure 6. Plausible reaction mechanism based on DFT calculations for the 3CR with bicyclo[1.1.0]butane. The reaction diagrams were calculated at the UωB97X-D/Def2-SVP/SMD(DCM) level.

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    Figure 7

    Figure 7. 3CR of [3.1.1]propellane. aWhite LED irradiation for 24 h without a photocatalyst.

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    Figure 8

    Figure 8. Synthesis of diphosphine ligands and metal complexes. Bottom right: crystal structures of 12 and 12′ with thermal ellipsoids at 30% probability; fluorine atoms on the hfa ligands and all hydrogen atoms are omitted for clarity.

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    Figure 9

    Figure 9. Plausible reaction mechanism based on DFT calculations for the 3CR with [3.1.1]propellane. The reaction diagrams were calculated at the UωB97X-D/Def2-SVP/SMD(DCM) level.

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    Figure 10

    Figure 10. 3CR of [4.1.1]propellane.

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    Figure 11

    Figure 11. Conversion to free diphosphine ligands.

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    Figure 12

    Figure 12. Plausible reaction mechanism based on DFT calculations for the 3CR with [4.1.1]propellane. The reaction diagrams were calculated at the UωB97X-D/Def2-SVP/SMD(DCM) level.

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  • References

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      Orton, G. R. F.; Pilgrim, B. S.; Champness, N. R. The Chemistry of Phosphines in Constrained, Well-defined Microenvironments. Chem. Soc. Rev. 2021, 50, 4411– 4431,  DOI: 10.1039/D0CS01556C
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      The chemistry of phosphines in constrained, well-defined microenvironments
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      Chemical Society Reviews (2021), 50 (7), 4411-4431CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Developments in the confinement of phosphines within micro- or nano-environments are explored. Phosphines are ubiquitous across metal coordination chem. and underpin some of the most famous homogeneous transition metal catalysts. Constraining phosphines within confined environments influences not only their behavior but also that of their metal complexes. Notable examples include the use of metal-org. frameworks (MOFs) or metal-org. cages (MOCs) to support phosphines which demonstrate how the microenvironment within such constructs leads to reactivity modification. The development of phosphine confinement is explored and parallels are drawn with related constrained macrocyclic systems and mech. interlocked mols. The review concludes by identifying areas that remain a challenge and those that will provide new avenues for research.
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      Dierkes, P.; van Leeuwen, P. W. N. M. The Bite Angle Makes the Difference: A Practical Ligand Parameter for Diphosphine Ligands. J. Chem. Soc., Dalton Trans. 1999, 1519– 1530,  DOI: 10.1039/a807799a
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      The bite angle makes the difference: a practical ligand parameter for diphosphine ligands
      Dierkes, Peter; van Leeuwen, Piet W. N. M.
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1999), (10), 1519-1530CODEN: JCDTBI; ISSN:0300-9246. (Royal Society of Chemistry)
      A review with 72 refs.; over the past twenty years, a correlation between the P-M-P bite angle in diphosphine complexes and selectivity has been obsd. in various catalytic reactions such as hydroformylation, hydrocyanation and cross coupling. The large no. of examples indicates that this correlation is not fortuitous. In order better to understand the underlying principles of the bite angle effect, we have first analyzed crystal structures available in the Cambridge Crystallog. Database. Systematic searches indicate that for many bidentate diphosphine ligands the P-M-P angles conc. in surprisingly small ranges, even if complexes of different metals in various oxidn. states are considered. Several examples in the literature show that continuous electronic changes assocd. with changing bite angles cannot only be verified by different spectroscopic techniques, but also explained on a theor. level (Walsh diagrams). The ligand bite angle is a useful parameter for the explanation of obsd. rates and selectivities and likewise for the design of ligands for new catalytic reactions.
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      van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Ligand Bite Angle Effects in Metal-Catalyzed C–C Bond Formation. Chem. Rev. 2000, 100, 2741– 2770,  DOI: 10.1021/cr9902704
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      Ligand Bite Angle Effects in Metal-catalyzed C-C Bond Formation
      van Leeuwen, Piet W. N. M.; Kamer, Paul C. J.; Reek, Joost N. H.; Dierkes, Peter
      Chemical Reviews (Washington, D. C.) (2000), 100 (8), 2741-2769CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review with 215 refs. summarizing the effect of the bite angle of bidentate ligands on the catalytic formation of C-C bonds, including alternating copolymn. of alkenes and carbon monoxide, hydroxycarbonylation of styrene, hydrocyanation of alkenes, cross-coupling reactions, allylic alkylation, Diels-Alder reaction, hydroformylation, and amination of aryl halides.
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      Jia, G.; Puddephatt, R. J.; Scott, J. D.; Vittal, J. J. Organometallic Polymers with Gold(I) Centers Bridged by Diphosphines and Diacetylides. Organometallics 1993, 12, 3565– 3574,  DOI: 10.1021/om00033a032
      5
      Organometallic polymers with gold(I) centers bridged by diphosphines and diacetylides
      Jia, Guochen; Puddephatt, Richard J.; Scott, John D.; Vittal, Jagadese J.
      Organometallics (1993), 12 (9), 3565-74CODEN: ORGND7; ISSN:0276-7333.
      The new ligand, 4,4'-iso-Pr2PC6H4C6H4P-iso-Pr2 and new digold(I) diacetylides [(AuC≡CAC≡CAu)x], A = 1,4-C6H4, 4,4'-C6H4C6H4, 1,4-C6H2-2,5-Me2, were synthesized. Model digold diacetylides were prepd. in the following ways: reaction of [ClAu(μ-X)AuCl], X = 4,4'-iso-Pr2PC6H4C6H4P-iso-Pr2 or 1,4-Ph2PC6H4PPh2, with PhC≡CH and base or reaction of [(AuC≡CPh)x] with X gave [PhC≡CAu(μ-X)AuC≡CPh] while reaction of [(AuC≡CAC≡CAu)x] with PMe3 gave [Me3PAuC≡CAC≡CAuPMe3]. The structure of [PhC≡CAu(μ-P)AuC≡CPh] (X = 4,4'-iso-Pr2PC6H4C6H4P-iso-Pr2) was detd. by single-crystal X-ray diffraction and shown to adopt a conformation with (phenylethynyl)gold(I) units mutually anti. Polymers were prepd. in ways similar to those of the model compds. Thus, polymers [(C≡CAC≡CAuXAu)x] were prepd. by reaction of [(AuC≡CAC≡CAu)x] with X or by reaction of [ClAu(μ-X)AuCl] with HC≡CAC≡CH and base. The latter synthetic method gave polymers with AuCl end groups when X = 4,4'-iso-Pr2PC6H4C6H4P-iso-Pr2 but only the derivs. [ClAu(μ-X)AuC≡CAC≡CAu(μ-X)AuCl] when X = 1,4-Ph2PC6H4PPh2, the difference being attributed to the lower soly. of the phenylphosphine deriv. This work showed that kinked, linear polymers could be prepd. successfully with diphosphine bridging ligands and that sol. polymers could be prepd. if bulky alkyl substituents were present on P.
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    6. 6
      Imhof, D.; Burckhardt, U.; Dahmen, K.-H.; Joho, F.; Nesper, R. Synthesis and Crystal Structure Determination of Bifunctional Phosphine-Linked Triplatinum Double-Cluster Complexes. Inorg. Chem. 1997, 36, 1813– 1820,  DOI: 10.1021/ic960846x
      6
      Synthesis and Crystal Structure Determination of Bifunctional Phosphine-Linked Triplatinum Double-Cluster Complexes
      Imhof, Daniel; Burckhardt, Urs; Dahmen, Klaus-Hermann; Joho, Felix; Nesper, Reinhard
      Inorganic Chemistry (1997), 36 (9), 1813-1820CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Reactions of [Pt3(μ-CO)3(PCy3)3] (1) and [Pt3(μ-CNXyl)2(μ-CO)(CNXyl)(PCy3)2] (2) (Cy = C6H11, Xyl = (CH2)2C6H4) with 1/2 equiv of a bifunctional metal phosphine cation [(MPR'2)2(R)]2+ (M = Cu, Ag, Au; R = C6H4, (CH2)2C6H4, Fe(C5H5)2; R' = C6H5, C6H11) yielded quant. [{Pt3(μ-CO)3(PCy3)3}2{(MPR'2)2 (R)}]2+ and [{Pt3(μ-CNXyl)2(μ-CO)(CNXyl)(PCy3)2}2{(MPR'2)2(R)}]2+, resp. The compds. were characterized by IR-, MS-, and 31P-NMR spectroscopy. The x-ray structure is given for [{Pt3(μ-CO)3(PCy3)3}2{(AuPPh2)2(CH2)2C6H4}][PF6]2 (14), which crystallizes in the triclinic space group P‾1 with Z = 1, a 15.350, b 17.150, c 20.446 Å, α 84.54, β 84.84, and γ 64.56°. The structure was refined to R = 0.0435 for the 8430 obsd. reflections (I > 3σ(I)).
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    7. 7
      Van Calcar, P. M.; Olmstead, M. M.; Balch, A. L. Ligand Connected Metal Clusters. The Molecular Structures and Solid State Packing of {Ru3(CO)11}2(bis(diphenylphosphino)ethane) and {Ru3(CO)11}2(1,4-bis(diphenylphosphino)benzene). Inorg. Chim. Acta 1998, 270, 28– 33,  DOI: 10.1016/S0020-1693(97)05820-9
      7
      Ligand connected metal clusters. The molecular structures and solid state packing of {Ru3(CO)11}2(bis(diphenylphosphino)ethane) and {Ru3(CO)11}2(1,4-bis(diphenylphosphino)benzene)
      Van Calcar, Pamela M.; Olmstead, Marilyn M.; Balch, Alan L.
      Inorganica Chimica Acta (1998), 270 (1,2), 28-33CODEN: ICHAA3; ISSN:0020-1693. (Elsevier Science S.A.)
      The prepn. and structural characterization of {Ru3(CO)11}2(1,4-bis(diphenylphosphino)benzene), a modified synthesis of 1,4-bis(diphenylphosphino)benzene, and the structural characterization of {Ru3(CO)11}2(bis(diphenylphosphino)ethane) are reported. In both compds. two metal cluster units are connected through ditertiary-phosphine ligands. Both mols. consist of centrosym. units in which the diphosphine ligands are largely covered by the triangular Ru clusters. No direct interaction between the two cluster units occurs within individual mols. Mol. packing in the solid state is dominated by interactions between sets of CO ligands in motifs that were previously identified in the solid state structure of the parent cluster, Ru3(CO)12.
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    8. 8
      Li, D.; Feng, Q.; Feng, X.-L.; Cai, J.-W. A Photoluminescent Metallophane [Cu2(μ-dppb)2(CH3CN)4](BF4)2 with a Chair Conformation: Synthesis, Structural and Spectroscopic Studies. Inorg. Chem. Commun. 2003, 6, 361– 364,  DOI: 10.1016/S1387-7003(02)00777-3
      There is no corresponding record for this reference.
    9. 9
      Koshevoy, I. O.; Karttunen, A. J.; Tunik, S. P.; Haukka, M.; Selivanov, S. I.; Melnikov, A. S.; Serdobintsev, P. Y.; Khodorkovskiy, M. A.; Pakkanen, T. A. Supramolecular Luminescent Gold(I)–Copper(I) Complexes: Self-Assembly of the AuxCuy Clusters inside the [Au3(diphosphine)3]3+ Triangles. Inorg. Chem. 2008, 47, 9478– 9488,  DOI: 10.1021/ic801073k
      9
      Supramolecular Luminescent Gold(I)-Copper(I) Complexes: Self-Assembly of the AuxCuy Clusters inside the [Au3(diphosphine)3]3+ Triangles
      Koshevoy, Igor O.; Karttunen, Antti J.; Tunik, Sergey P.; Haukka, Matti; Selivanov, Stanislav I.; Melnikov, Alexei S.; Serdobintsev, Pavel Yu.; Khodorkovskiy, Mikhail A.; Pakkanen, Tapani A.
      Inorganic Chemistry (2008), 47 (20), 9478-9488CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The reactions between diphosphine-alkynyl Au complexes (PhC2Au)PPh2(C6H4)nPPh2(AuC2Ph) (n = 1, 2, 3) with Cu+ give the heterometallic aggregates, the compn. of which may be described by a general formula [{AuxCuy(C2Ph)2x}Au3{PPh2(C6H4)nPPh2}3]3+(y-x) (n = 1, 2, 3; x = (n + 1)(n + 2)/2; y = n(n + 1)). These compds. display very similar structural patterns and consist of the [AuxCuy(C2Ph)2x]y-x alkynyl clusters wrapped in the [Au3(diphosphine)3]3+ triangles. The complex for n = 1 was characterized crystallog. and spectrally, the larger ones (n = 2, 3) were studied in detail by NMR spectroscopy. Their luminescence behavior was studied, and a remarkably efficient emission with a max. quantum yield of 0.92 (n = 1) was detected. Photophys. expts. demonstrate that an increase of the size of the aggregates decreases photostability and photoefficiency. Computational studies were performed to provide addnl. insight into the structural and electronic properties of these supramol. complexes. The theor. results obtained are in good agreement with the exptl. data, supporting the proposed structural motif. These studies also suggest that the obsd. efficient long-wavelength luminescence originates from metal-centered transitions within the heterometallic Au-Cu core.
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    10. 10
      Koshevoy, I. O.; Karttunen, A. J.; Lin, Y.-C.; Lin, C.-C.; Chou, P.-T.; Tunik, S. P.; Haukka, M.; Pakkanen, T. A. Synthesis, Photophysical and Theoretical Studies of Luminescent Silver(I)–Copper(I) Alkynyl-Diphosphine Complexes. Dalton Trans. 2010, 39, 2395– 2403,  DOI: 10.1039/b920856a
      There is no corresponding record for this reference.
    11. 11
      Rohacova, J.; Sekine, A.; Kawano, T.; Tamari, S.; Ishitani, O. Trinuclear and Tetranuclear Re(I) Rings Connected with Phenylene, Vinylene, and Ethynylene Chains: Synthesis, Photophysics, and Redox Properties. Inorg. Chem. 2015, 54, 8769– 8777,  DOI: 10.1021/acs.inorgchem.5b01397
      11
      Trinuclear and Tetranuclear Re(I) Rings Connected with Phenylene, Vinylene, and Ethynylene Chains: Synthesis, Photophysics, and Redox Properties
      Rohacova, Jana; Sekine, Akiko; Kawano, Tsubasa; Tamari, Sho; Ishitani, Osamu
      Inorganic Chemistry (2015), 54 (17), 8769-8777CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Highly luminescent trinuclear and tetranuclear ring-shaped Re(I) complexes wherein the Re units are linked with rigid bidentate phosphine ligands, namely, bis(diphenylphosphino)-p-phenylene, -trans-vinylene, and -ethynylene, were synthesized and fully characterized. Their strong emissive properties and the long lifetimes of their triplet metal-to-ligand charge transfer excited states originate primarily from enhanced, rigidity-induced interligand interactions between the 2,2'-bipyridine (bpy) ligand and the Ph groups of the phosphine ligands. Another type of interligand interaction was also obsd. between the bpy ligand and the phosphine-bridging group; this interaction also strongly affected the photophys. and redox properties of the Re-rings.
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    12. 12
      Fernández-Moreira, V.; Cámara, J.; Smirnova, E. S.; Koshevoy, I. O.; Laguna, A.; Tunik, S. P.; Blanco, M. C.; Gimeno, M. C. Tuning the Energy Emission from Violet to Yellow with Bidentate Phosphine Gold(III) Complexes. Organometallics 2016, 35, 1141– 1150,  DOI: 10.1021/acs.organomet.6b00135
      12
      Tuning the Energy Emission from Violet to Yellow with Bidentate Phosphine Gold(III) Complexes
      Fernandez-Moreira, Vanesa; Camara, Jessica; Smirnova, Ekaterina S.; Koshevoy, Igor O.; Laguna, Antonio; Tunik, Sergey P.; Blanco, M. Carmen; Gimeno, M. Concepcion
      Organometallics (2016), 35 (8), 1141-1150CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The synthesis and characterization of luminescent gold(III) compds., obtained by coordination of the metal center to different phosphines, is described. To avoid deactivation of luminescence by the presence of low-energy d-d ligand field states in the gold(III) center, the ligands bonded to the metallic center have been carefully chosen, among which we used bidentate phosphines with different nos. of phenylene or alkynyl-phenylene spacers and pentafluorophenyl groups. The reaction of [Au(C6F5)3(tht)] (tht = tetrahydrothiophene) with the corresponding diphosphines gave the complexes [{Au(C6F5)3}2(1,4-PPh2(C6H4)nPPh2)] (n = 1-3) and [{Au(C6F5)3}2(PPh2C≡C(C6H4)nC≡CPPh2)] (n = 0-2). The study of their optical behavior reveals emission color variations from violet to yellow for the compds. contg. the phosphines with one, two, and three phenylene spacers, resp., and much more fine-tuning, from deep blue to brilliant blue for those intercalating alkynyl and phenylene spacers. Four of the new complexes were also characterized by X-ray diffraction crystallog., showing supramol. structures formed through hydrogen bonding.
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    13. 13
      Tolman, C. A. Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis. Chem. Rev. 1977, 77, 313– 348,  DOI: 10.1021/cr60307a002
      13
      Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis
      Tolman, Chadwick A.
      Chemical Reviews (Washington, DC, United States) (1977), 77 (3), 313-48CODEN: CHREAY; ISSN:0009-2665.
      A review, with 298 refs.
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    14. 14
      Bunten, K. A.; Chen, L.; Fernandez, A. L.; Poë, A. J. Cone Angles: Tolman’s and Plato’s. Coord. Chem. Rev. 2002, 233–234, 41– 51,  DOI: 10.1016/S0010-8545(02)00099-1
      14
      Cone angles: Tolman's and Plato's
      Bunten, Kevin A.; Chen, Lezhan; Fernandez, Anthony L.; Poe, Anthony J.
      Coordination Chemistry Reviews (2002), 233-234 (), 41-51CODEN: CCHRAM; ISSN:0010-8545. (Elsevier Science B.V.)
      A review discusses the application of Tolman's cone angles for phosphites and other phosphorus ligands for quant. account of steric effects on substitution reaction enthalpies. Correlations of reaction enthalpy are presented which include the account for π-acidity parameters. The successful application of Tolman cone angles for P-donor and other ligands in accounting quant. for steric effects in a wide variety of physicochem. processes is contrasted with the variability of cone angles obtained from crystallog. studies. It is maintained that the latter are not relevant in describing steric effects for reactions in soln. Problems with cone angles for ligands with conformational uncertainties are best dealt with by systematic measurement of deviations of data for those ligands from trends defined by ligands with less ambiguous cone angles. In fact a body of cone angles for all ligands could be obtained by adjusting cone angles to give perfect fits to individual steric profiles and then averaging the values obtained from a large no. of such studies. In this way a set of cone angles could be obtained that are divorced from their origins in Tolman's models and justified solely by their successful quant. application. Consideration of data sets which include P(OMe)3 and P(OiPr)3 as a ligands suggests that the former should have its Tolman cone angle increased by ca. 10°, and the Tolman cone angle for the latter should be decreased by ca. 5°. Thus the deviant behavior of conformationally ambiguous ligands may be systematic, and could depend on whether the P-donors are acting as ligands or nucleophiles. However, the concept of a Platonically perfect set of cone angles is probably justified for many ligands that are conformationally unambiguous and these ideal cone angles may be essentially identical with Tolman's values. In the course of these analyses a new set of π-acidity parameters was developed and values are tabulated.
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    15. 15
      Bilbrey, J. A.; Kazez, A. H.; Locklin, J.; Allen, W. D. Exact Ligand Cone Angles. J. Comput. Chem. 2013, 34, 1189– 1197,  DOI: 10.1002/jcc.23217
      15
      Exact ligand cone angles
      Bilbrey, Jenna A.; Kazez, Arianna H.; Locklin, Jason; Allen, Wesley D.
      Journal of Computational Chemistry (2013), 34 (14), 1189-1197CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
      Many properties of transition-metal complexes depend on the steric bulk of bound ligands, usually quantified by the Tolman (θ) and solid (Θ) cone angles, which have proven utility but suffer from various limitations and coarse approxns. Here, we present an improved, math. rigorous method to det. an exact cone angle (θ°) by solving for the most acute right circular cone that contains the entire ligand. The procedure is applicable to any ligand, planar or nonplanar, monodentate or polydentate, bound to any metal center in any environment, and it is ideal for analyzing structures from quantum chem. computations as well as X-ray crystallog. expts. Exact cone angles were evaluated for a wide array of phosphine and amine ligands bound to palladium, nickel, or platinum by optimizing structures using B3LYP/6-31G* d. functional theory with effective core potentials for the transition metals. The mean abs. deviations of the std. θ and Θ parameters from the exact cone angles were 15-25°, mostly caused by distortions from the assumed idealized structures. © 2013 Wiley Periodicals, Inc.
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    16. 16
      Casey, C. P.; Paulsen, E. L.; Beuttenmueller, E. W.; Proft, B. R.; Matter, B. A.; Powell, D. R. Electronically Dissymmetric DIPHOS Derivatives Give Higher n:i Regioselectivity in Rhodium-Catalyzed Hydroformylation Than Either of Their Symmetric Counterparts. J. Am. Chem. Soc. 1999, 121, 63– 70,  DOI: 10.1021/ja982117h
      16
      Electronically Dissymmetric DIPHOS Derivatives Give Higher n:i Regioselectivity in Rhodium-Catalyzed Hydroformylation Than Either of Their Symmetric Counterparts
      Casey, Charles P.; Paulsen, Evelyn Lin; Beuttenmueller, Eckart W.; Proft, Bernd R.; Matter, Brock A.; Powell, Douglas R.
      Journal of the American Chemical Society (1999), 121 (1), 63-70CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Electronic effects on rhodium-catalyzed hydroformylation of 1-hexene with electronically dissym. DIPHOS derivs. [3,5-(CF3)2C6H3]2PCH2CH2PPh2 = [DIPHOS-(3,5-CF3,H)] (1), [2-(CF3)C6H4]2PCH2CH2PPh2 = [DIPHOS-(2-CF3,H)] (2), [3,5-(CF3)2C6H3]2PCH2CH2P[2-(CH3)C6H4]2 = [DIPHOS-(3,5-CF3,2-CH3)] (3), and [2-(CF3)C6H4]2PCH2CH2P[2-(CH3)C6H4]2 = [DIPHOS-(2-CF3,2-CH3)] (4) were investigated. Two apical-equatorial chelate isomers were obsd. for model (diphosphine)Ir(CO)2H complexes of dissym. diphosphines 1-4. In each case, the equatorial phosphine of the major isomer (96-60%) had electron-withdrawing aryl substituents. These dissym. DIPHOS derivs. were used to test the hypothesis that an electron-withdrawing substituent on an equatorial phosphine increases the hydroformylation n:i ratio while an electron-withdrawing substituent on an apical phosphine decreases the n:i ratio. In agreement with the predictions of this hypothesis, hydroformylation with the dissym. diphosphine ligand DIPHOS-(3,5-CF3,H) (1), gave an n:i ratio of 4.2:1, higher than either of the sym. ligands DIPHOS, 2.6:1, and DIPHOS-(3,5-CF3), 1.3:1. Similar observations were made for hydroformylations with 2-4.
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    17. 17
      Carraz, C.-A.; Ditzel, E. J.; Orpen, A. G.; Ellis, D. D.; Pringle, P. G.; Sunley, G. J. Rhodium(I) Complexes of Unsymmetrical Diphosphines: Efficient and Stable Methanol Carbonylation Catalysts. Chem. Commun. 2000, 1277– 1278,  DOI: 10.1039/b002802i
      17
      Rhodium(I) complexes of unsymmetrical diphosphines: efficient and stable methanol carbonylation catalysts
      Carraz, Charles-Antoine; Orpen, A. Guy; Ellis, Dianne D.; Pringle, Paul G.; Ditzel, Evert J.; Sunley, Glenn J.
      Chemical Communications (Cambridge) (2000), (14), 1277-1278CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Rhodium complexes of unsym. diphosphines of the type Ph2PCH2CH2PAr2, Ar = F substituted Ph groups, are efficient catalysts for carbonylation of methanol and have extended service life compared with ligand-modified catalyst under temps. of 150-200° and pressure of 10-60 bar. The diphosphines were prepd. and fully characterized. The catalysts were prepd. by addn. of fluoro-diphosphines to [Rh2(μ-Cl)2(CO)4] in MeOH; some of the I analogs were also prepd. Several features of the catalysis are reminiscent of iridium Cativa carbonylation catalysts.
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    18. 18
      Yue, W.-J.; Xiao, J.-Z.; Zhang, S.; Yin, L. Rapid Synthesis of Chiral 1,2-Bisphosphine Derivatives through Copper(I)-Catalyzed Asymmetric Conjugate Hydrophosphination. Angew. Chem., Int. Ed. 2020, 59, 7057– 7062,  DOI: 10.1002/anie.201916076
      18
      Rapid Synthesis of Chiral 1,2-Bisphosphine Derivatives through Copper(I)-Catalyzed Asymmetric Conjugate Hydrophosphination
      Yue, Wen-Jun; Xiao, Jun-Zhao; Zhang, Shuai; Yin, Liang
      Angewandte Chemie, International Edition (2020), 59 (18), 7057-7062CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      1,2-Bisphosphines have been identified as one class of important and powerful chiral ligands in asym. catalysis with transition metals. Herein, a copper(I)-catalyzed asym. hydrophosphination of α,β-unsatd. phosphine sulfides was developed with the assistance of "soft-soft" interaction between copper(I)-catalyst and the phosphine sulfide moiety, which afforded 1,2-bisphosphine derivs. with diversified electronic nature and steric hindrance in high to excellent yields with high to excellent enantioselectivity. Moreover, the challenging catalytic asym. hydrophosphination/protonation reaction was achieved with excellent enantioselectivity. Strikingly, the dynamic kinetic resoln. of racemic diarylphosphines was also successfully carried out with high to excellent diastereo- and enantioselectivities. Interestingly, the nucleophilic copper(I)-diphenylphosphide species was characterized by 31P NMR spectrum and mass spectrum. At last, three products were transformed to chiral 1,2-bisphosphines, which were employed as ligands in Rh-catalyzed asym. hydrogenation of α-amino-α,β-unsatd. ester. The α-amino acid deriv. was produced in high enantioselectivity, which demonstrated the utility of the present methodol.
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    19. 19
      RajanBabu, T. V.; Casalnuovo, A. L. Role of Electronic Asymmetry in the Design of New Ligands: The Asymmetric Hydrocyanation Reaction. J. Am. Chem. Soc. 1996, 118, 6325– 6326,  DOI: 10.1021/ja9609112
      19
      Role of Electronic Asymmetry in the Design of New Ligands: The Asymmetric Hydrocyanation Reaction
      RajanBabu, T. V.; Casalnuovo, Albert L.
      Journal of the American Chemical Society (1996), 118 (26), 6325-6326CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Enantioselectivity of Ni(0)-catalyzed asym. hydrocyanation of a vinylarene can be enhanced by the use of C2- or pseudo-C2-sym. ligands in which the two chelating atoms are electronically different. Thus 3,4-di-O-diarylphosphino-α-fructofuranoside with an electron-withdrawing P at C4 and an electron-rich P at C3 gave the highest enantioselectivity ever recorded for asym. hydrocyanation of a vinylarene. The role of electronic asymmetry is further confirmed with the use of a simpler 1,2-bis-phosphinite ligand derived from (S,S)-tartranil. Stereoelectronic effects in the formation or decompn. of a penultimate intermediate involved in the reaction maybe responsible for this unusual effect.
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    20. 20
      Nozaki, K. Unsymmetric Bidentate Ligands in Metal-Catalyzed Carbonylation of Alkenes. Chem. Rec. 2005, 5, 376– 384,  DOI: 10.1002/tcr.20046
      20
      Unsymmetric bidentate ligands in metal-catalyzed carbonylation of alkenes
      Nozaki, Kyoko
      Chemical Record (2005), 5 (6), 376-384CODEN: CRHEAK; ISSN:1527-8999. (John Wiley & Sons, Inc.)
      A review. Characteristic features of unsym. bidentate ligands, in which the two coordination atoms are not equiv., are reviewed with a focus on their use in metal-catalyzed olefin carbonylations. High enantioselectivity for a variety of substrates was achieved using (R,S)-BINAPHOS in Rh-catalyzed hydroformylation. An unsym. chiral bis(phosphino)ferrocene ligand shows high productivity accompanied by high regio- and enantioselectivities in the Pd-catalyzed alternating copolymn. of 1-alkene with CO. The advantages of electronic unsymmetry are demonstrated esp. in the spectroscopic observation of single steps involved in catalytic cycles.
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    21. 21
      Thomas, A. A.; Speck, K.; Kevlishvili, I.; Lu, Z.; Liu, P.; Buchwald, S. L. Mechanistically Guided Design of Ligands That Significantly Improve the Efficiency of CuH-Catalyzed Hydroamination Reactions. J. Am. Chem. Soc. 2018, 140, 13976– 13984,  DOI: 10.1021/jacs.8b09565
      21
      Mechanistically Guided Design of Ligands That Significantly Improve the Efficiency of CuH-Catalyzed Hydroamination Reactions
      Thomas, Andy A.; Speck, Klaus; Kevlishvili, Ilia; Lu, Zhaohong; Liu, Peng; Buchwald, Stephen L.
      Journal of the American Chemical Society (2018), 140 (42), 13976-13984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Using a mech. guided ligand design approach, a new ligand (SEGFAST) for the CuH-catalyzed hydroamination reaction of unactivated terminal olefins has been developed, providing a 62-fold rate increase over reactions compared to DTBM-SEGPHOS, the previous optimal ligand. Combining the resp. strengths of computational chem. and exptl. kinetic measurements, we were able to quickly identify potential modifications that lead to more effective ligands, thus avoiding synthesizing and testing a large library of ligands. By optimizing the combination of attractive, noncovalent ligand-substrate interactions and the stability of the catalyst under the reaction conditions, we were able to identify a finely tuned hybrid ligand that greatly enables accelerated hydrocupration rates with unactivated alkenes. Moreover, a modular and robust synthetic sequence was devised, which allowed for the practical, gram-scale synthesis of these novel hybrid ligand structures.
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    22. 22
      Wiberg, K. B. The Concept of Strain in Organic Chemistry. Angew. Chem., Int. Ed. 1986, 25, 312– 322,  DOI: 10.1002/anie.198603121
      There is no corresponding record for this reference.
    23. 23
      Turkowska, J.; Durka, J.; Gryko, D. Strain Release – An Old Tool for New Transformations. Chem. Commun. 2020, 56, 5718– 5734,  DOI: 10.1039/D0CC01771J
      23
      Strain release - an old tool for new transformations
      Turkowska, Joanna; Durka, Jakub; Gryko, Dorota
      Chemical Communications (Cambridge, United Kingdom) (2020), 56 (43), 5718-5734CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. Strain-release driven transformations give access to attractive bioisosteric motifs highly prized by medicinal chemists and they are characteristic of mols. possessing distorted bond lengths and angles. By broadening the chem. space in drug discovery, recently, these compds. have attracted a lot of interest. Their reactivity stems mainly from an increased energy and destabilization. As a result, the opening of the bridging bond occurs under the action of both nucleophiles and electrophiles as well as radical species and transition metals. Though the bridge bond dominates their reactivity, it is also influenced by the substitution pattern. This feature article focuses on strain-release driven strategies paying particular attention to the most recent (year > 2010) advances.
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    24. 24
      Bellotti, P.; Glorius, F. Strain-Release Photocatalysis. J. Am. Chem. Soc. 2023, 145, 20716– 20732,  DOI: 10.1021/jacs.3c08206
      24
      Strain-Release Photocatalysis
      Bellotti, Peter; Glorius, Frank
      Journal of the American Chemical Society (2023), 145 (38), 20716-20732CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A review. The concept of strain in org. compds. is as old as modern org. chem. and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate-with significant driving force-rigid aliph. systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an addnl. dimension to strain-release processes by leveraging the energy of photons to create chem. complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis-with emphases on mechanisms, catalytic cycles, and current limitations-the unique chem. architectures that can be produced, and possible future directions. strain release.
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    25. 25
      Blanchard, E. P., Jr.; Cairncross, A. Bicyclo[1.1.0]butane Chemistry. I. The Synthesis and Reactions of 3-Methylbicyclo[1.1.0]butanecarbonitriles. J. Am. Chem. Soc. 1966, 88, 487– 495,  DOI: 10.1021/ja00955a020
      25
      Bicyclo[1.1.0]butane chemistry. I. The synthesis and reactions of 3-methylbicyclo[1.1.0]butanecarbonitriles
      Blanchard, E. P., Jr.; Cairnross, A.
      Journal of the American Chemical Society (1966), 88 (3), 487-95CODEN: JACSAT; ISSN:0002-7863.
      The hydrohalogenation-dehydrohalogenation of 3-methylenecyclobutanecarbonitriles provides a facile, high-yield synthesis of 3-methylbicyclo[1.1.0]butanecarbonitriles. The latter react with a broad spectrum of reagents including acids, electron-deficient multiple bonds, nucleophiles, free radicals, and halogens to give cyclobutanes and cyclobutenes. The bicyclobutane ring system is thermally labile leading to a synthesis of substituted butadienes. Catalytic hydrogenation of bicyclobutanes consumes 2 moles of H leading to open-chain structures. The bicyclobutane ring system is stable to certain transformations of the nitrile group. These reactions are discussed in terms of the scope, conditions and mechanistic implications.
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    26. 26
      Cairncross, A.; Blanchard, E. P., Jr. Bicyclo[1.1.0]butane Chemistry. II. Cycloaddition Reactions of 3-Methylbicyclo[1.1.0]butanecarbonitriles. The Formation of Bicyclo[2.1.1]hexanes. J. Am. Chem. Soc. 1966, 88, 496– 504,  DOI: 10.1021/ja00955a021
      26
      Bicyclo[1.1.0]butane chemistry. II. Cycloaddition reactions of 3-methylbicyclo[1.1.0]butanecarbonitriles. The formation of bicyclo[2.1.1]hexanes
      Cairncross, A.; Blanchard, E. P., Jr.
      Journal of the American Chemical Society (1966), 88 (3), 496-504CODEN: JACSAT; ISSN:0002-7863.
      cf. preceding abstr. 3-Methylbicyclo[1.1.0]butanecarbonitrile (I) reacts with butadiene, acrylonitrile, maleonitrile, fumaronitrile, C2H4, styrene, p-methoxystyrene, and 1-(N,N-dimethylamino)cyclopentene to form 1:1 adducts. In all cases, derivs. of 4-methylbicyclo[2.1.1]-hexanecarbonitrile were formed along with other products. Evidence is presented which strongly suggests diradical intermediates in the cycloaddn. reaction as well as in the formation of monocyclic products from C2H4. Thermolysis of I in the liquid phase at 150° produces several dimers, two of which have been partially identified as cycloadducts between I and its pyrolysis products. Brief attempts to observe thermally activated inversion of derivs. of bicyclo [1.1.0]butane failed because of competitive decompn.
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    27. 27
      Lampman, G. M.; Aumiller, J. C. Bicyclo[1.1.0]butane. Org. Synth. 1971, 51, 55,  DOI: 10.1002/0471264180.os051.13
      There is no corresponding record for this reference.
    28. 28
      D’yachenko, A. I.; Abramova, N. M.; Zotova, S. V.; Nesmeyanova, O. A.; Bragin, O. V. New Synthesis of Bicyclo[1.1.0]butane Hydrocarbons. Russ. Chem. Bull. 1985, 34, 1885– 1889,  DOI: 10.1007/BF00953929
      There is no corresponding record for this reference.
    29. 29
      Kelly, C. B.; Milligan, J. A.; Tilley, L. J.; Sodano, T. M. Bicyclobutanes: From Curiosities to Versatile Reagents and Covalent Warheads. Chem. Sci. 2022, 13, 11721– 11737,  DOI: 10.1039/D2SC03948F
      29
      Bicyclobutanes: from curiosities to versatile reagents and covalent warheads
      Kelly, Christopher B.; Milligan, John A.; Tilley, Leon J.; Sodano, Taylor M.
      Chemical Science (2022), 13 (40), 11721-11737CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      A review. The unique chem. of small, strained carbocyclic systems has long captivated org. chemists from a theor. and fundamental standpoint. A resurgence of interest in strained carbocyclic species has been prompted by their potential as bioisosteres, high fraction of sp3 carbons, and limited appearance in the patent literature. Among strained ring systems, bicyclo[1.1.0]butane (BCB) stands apart as the smallest bicyclic carbocycle and is amongst the most strained carbocycles known. Despite the fact that BCBs have been synthesized and studied for well over 50 years, they have long been regarded as lab. curiosities. However, new approaches for prepg., functionalizing, and using BCBs in "strain-release" transformations have positioned BCBs to be powerful synthetic workhorses. Further, the olefinic character of the bridgehead bond enables BCBs to be elaborated into various other ring systems and function as covalent warheads for bioconjugation. This review will discuss the recent developments in the synthesis and functionalization of BCBs as well as the applications of these strained rings in synthesis and drug discovery. An overview of the properties and the historical context of this interesting structure will be provided.
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    30. 30
      Golfmann, M.; Walker, J. C. L. Bicyclobutanes as Unusual Building Blocks for Complexity Generation in Organic Synthesis. Commun. Chem. 2023, 6, 9,  DOI: 10.1038/s42004-022-00811-3
      30
      Bicyclobutanes as unusual building blocks for complexity generation in organic synthesis
      Golfmann, Maxim; Walker, Johannes C. L.
      Communications Chemistry (2023), 6 (1), 9CODEN: CCOHCT; ISSN:2399-3669. (Nature Portfolio)
      A review. Bicyclobutanes are among the most highly strained isolable org. compds. and their assocd. low activation barriers to reactivity make them intriguing building-blocks in org. chem. In recent years, numerous creative synthetic strategies exploiting their heightened reactivity have been presented and these discoveries have often gone hand-in-hand with the development of more practical routes for their synthesis. Their proclivity as strain-release reagents through their weak central C-C bond has been harnessed in a variety of addn., rearrangement and insertion reactions, providing rapid access to a rich tapestry of complex mol. scaffolds. This review will provide an overview of the different options available for bicyclobutane synthesis, the main classes of compds. that can be prepd. from bicyclobutanes, and the assocd. modes of reactivity used.
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    31. 31
      Tyler, J. L.; Aggarwal, V. K. Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics. Chem.─Eur. J. 2023, 29, e202300008  DOI: 10.1002/chem.202300008
      31
      Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics
      Tyler, Jasper L.; Aggarwal, Varinder K.
      Chemistry - A European Journal (2023), 29 (29), e202300008CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The use of metalated (aza)bicyclo[1.1.0]butanes in synthesis is currently experiencing a renaissance, as evidenced by the numerous reports in the last 5 years that have relied on such intermediates to undergo unique transformations or generate novel fragments. Since their discovery, these species have been demonstrated to participate in a wide range of reactions with carbon and heteroatom electrophiles, as well as metal complexes, to facilitate the rapid diversification of (aza)bicyclo[1.1.0]butane-contg. compds. Key to this is the relative acidity of the bridgehead C-H bonds which promotes facile deprotonation and subsequent functionalization of an unsubstituted position on the carbon framework via the intermediacy of a metalated (aza)bicyclo[1.1.0]butane. Addnl., the late-stage incorporation of deuterium atoms in strained fragments has led to the elucidation of numerous reaction mechanisms that involve strained bicycles. The continued investigation into the inimitable reactivity of metalated bicycles will cement their importance within the field of organometallic chem.
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    32. 32
      Kelly, C. B.; Colthart, A. M.; Constant, B. D.; Corning, S. R.; Dubois, L. N. E.; Genovese, J. T.; Radziewicz, J. L.; Sletten, E. M.; Whitaker, K. R.; Tilley, L. J. Enabling the Synthesis of Perfluoroalkyl Bicyclobutanes via 1,3 γ-Silyl Elimination. Org. Lett. 2011, 13, 1646– 1649,  DOI: 10.1021/ol200121f
      32
      Enabling the Synthesis of Perfluoroalkyl Bicyclobutanes via 1,3 γ-Silyl Elimination
      Kelly, Christopher B.; Colthart, Allison M.; Constant, Brad D.; Corning, Sean R.; Dubois, Lily N. E.; Genovese, Jacqueline T.; Radziewicz, Julie L.; Sletten, Ellen M.; Whitaker, Katherine R.; Tilley, Leon J.
      Organic Letters (2011), 13 (7), 1646-1649CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
      Two new bicyclobutanes were prepd. from cyclobutyl systems by a novel, solvolytic, carbocation-based methodol. An electron-withdrawing perfluoroalkyl group at the incipient cationic center enhances neighboring-group participation of the γ-silyl group, inducing facile, remarkably selective 1,3-elimination yielding only bicyclobutanes. The method unlocks potential access to a host of EWG-substituted strained rings and a potential new method for the synthesis of trifluoromethylcyclopropanes.
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    33. 33
      Wiberg, K. B.; Taddell, S. T. Reactions of [1.1.1]Propellane. J. Am. Chem. Soc. 1990, 112, 2194– 2216,  DOI: 10.1021/ja00162a022
      33
      Reactions of [1.1.1]propellane
      Wiberg, Kenneth B.; Waddell, Sherman T.
      Journal of the American Chemical Society (1990), 112 (6), 2194-216CODEN: JACSAT; ISSN:0002-7863.
      The free-radical addn. reactions of [1.1.1]propellane (I) are described in some detail and allowed the prepn. of a wide variety of 1,3-disubstituted bicyclo[1.1.1]pentanes. The reaction of I with free radicals was more rapid than that of bicyclo[1.1.1]butane (II), whereas bicyclo[2.1.0]pentane (III) was relatively inert. In some cases the free-radical addns. led to oligomers, and in the case of THF addn. the chain-transfer const. was measured. The addn. of thiophenol to I followed by redn. with the lithium radical anion from 4,4'-di-tert-butylbiphenyl gave 1-lithiobicyclo[1.1.1]pentane, from which a variety of 1-substituted bicyclo[1.1.1]pentanes may be prepd. In the Baeyer-Villiger oxidn. of 1-benzoylbicyclo[1.1.1]pentane, the tert-Bu group migrated in preference to the bicyclopentyl group. Conversion of the ketone to the tosylhydrazone followed by base treatment gave products of the type expected from the corresponding carbene. The reaction of I with NO in carbon disulfide gave a unique reaction in which nitro and thiocyano groups were introduced. The reactions of I-III with NO2 also were examd. Whereas I gave 1,3-dinitrobicyclo[1.1.1]pentane, the other hydrocarbons followed different reaction paths. The reaction of I with electron-deficient alkenes and alkynes are described in some detail and are compared with the corresponding reactions of II and III. Here, the relative reactivities of I and II were often comparable but varied considerably with the reagent used. Again, III was relatively unreactive. The reaction of I with Rh(I) gave a dimer, and evidence is presented for a metallocarbene intermediate. The authors counsel safety in the prepn. of trifluoroperacetic acid with 70% H2O2.
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    34. 34
      Gianatassio, R.; Lopchuk, J. M.; Wang, J.; Pan, C.-M.; Malins, L. R.; Prieto, L.; Brandt, T. A.; Collins, M. R.; Gallego, G. M.; Sach, N. W.; Spangler, J. E.; Zhu, H.; Zhu, J.; Baran, P. S. Strain-release Amination. Science 2016, 351, 241– 246,  DOI: 10.1126/science.aad6252
      34
      Strain-release amination
      Gianatassio, Ryan; Lopchuk, Justin M.; Wang, Jie; Pan, Chung-Mao; Malins, Lara R.; Prieto, Liher; Brandt, Thomas A.; Collins, Michael R.; Gallego, Gary M.; Sach, Neal W.; Spangler, Jillian E.; Zhu, Huichin; Zhu, Jinjiang; Baran, Phil S.
      Science (Washington, DC, United States) (2016), 351 (6270), 241-246CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      To optimize drug candidates, modern medicinal chemists are increasingly turning to an unconventional structural motif: small, strained ring systems. However, the difficulty of introducing substituents such as bicyclo[1.1.1]pentanes, azetidines, or cyclobutanes often outweighs the challenge of synthesizing the parent scaffold itself. Thus, there is an urgent need for general methods to rapidly and directly append such groups onto core scaffolds. Here we report a general strategy to harness the embedded potential energy of effectively spring-loaded C-C and C-N bonds with the most oft-encountered nucleophiles in pharmaceutical chem., amines. Strain-release amination can diversify a range of substrates with a multitude of desirable bioisosteres at both the early and late stages of a synthesis. The technique has also been applied to peptide labeling and bioconjugation.
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    35. 35
      Shelp, R. A.; Walsh, P. J. Synthesis of BCP Benzylamines from 2-Azaallyl Anions and [1.1.1]Propellane. Angew. Chem., Int. Ed. 2018, 57, 15857– 15861,  DOI: 10.1002/anie.201810061
      35
      Synthesis of BCP Benzylamines From 2-Azaallyl Anions and [1.1.1]Propellane
      Shelp, Russell A.; Walsh, Patrick J.
      Angewandte Chemie, International Edition (2018), 57 (48), 15857-15861CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      For bioactive mols., bicyclo[1.1.1]pentanes (BCPs) are an emerging isostere of rigid spacer groups that have shown potential to improve drug-like qualities. As BCPs become an increasingly popular motif for evaluation in drug candidates, org. chemists must meet the demand to reliably incorporate them into new targets. To provide access to BCP analogs of diaryl methanamines, a ubiquitous scaffold in medicinal chem., we report the synthesis of BCP benzylamines through reactivity of [1.1.1]propellane with 2-azaallyl anions, which are generated in situ from N-benzyl ketimines. The reaction proceeds rapidly at room temp. and tolerates a broad substrate scope, providing straightforward access to 23 new BCP benzylamine derivs. Initial expts. support the intermediacy of a BCP anion. Addnl., the reaction can be promoted by substoichiometric loadings of base, highlighting an unusual reactivity of both 2-azaallyls and [1.1.1]propellane.
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    36. 36
      Yu, S.; Noble, A.; Bedford, R. B.; Aggarwal, V. K. Methylenespiro[2.3]hexanes via Nickel-Catalyzed Cyclopropanations with [1.1.1]Propellane. J. Am. Chem. Soc. 2019, 141, 20325– 20334,  DOI: 10.1021/jacs.9b10689
      36
      Methylenespiro[2.3]hexanes via Nickel-Catalyzed Cyclopropanations with [1.1.1]Propellane
      Yu, Songjie; Noble, Adam; Bedford, Robin B.; Aggarwal, Varinder K.
      Journal of the American Chemical Society (2019), 141 (51), 20325-20334CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      [1.1.1]Propellane is a highly strained tricyclic hydrocarbon whose reactivity is dominated by addn. reactions across the central inverted bond to provide bicyclo[1.1.1]pentane derivs. These reactions proceed under both radical and two-electron pathways, hence providing access to a diverse array of products. Conversely, transition metal-catalyzed reactions of [1.1.1]propellane are underdeveloped and lack synthetic utility, with reported examples generally yielding mixts. of ring-opened structural isomers, dimers, and trimers, often with poor selectivity. Herein, authors report that Ni(0) catalysis enables the use of [1.1.1]propellane as a carbene precursor in cyclopropanations of a range of functionalized alkenes to give methylenespiro[2.3]hexane products I (R1 = H, Me, CH2OTBS, etc.; R1 = H, Me, Ph, etc.; R3 = Ph, 4-CF3C6H4, 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl, etc. ). Computational studies provide support for initial formation of a Ni(0)-[1.1.1]propellane complex followed by concerted double C-C bond activation to give the key 3-methylenecyclobutylidene-nickel intermediate.
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    37. 37
      Lasányi, D.; Tolnai, G. L. Copper-Catalyzed Ring Opening of [1.1.1]Propellane with Alkynes: Synthesis of Exocyclic Allenic Cyclobutanes. Org. Lett. 2019, 21, 10057– 10062,  DOI: 10.1021/acs.orglett.9b03999
      37
      Copper-Catalyzed Ring Opening of [1.1.1]Propellane with Alkynes: Synthesis of Exocyclic Allenic Cyclobutanes
      Lasanyi, Daniel; Tolnai, Gergely L.
      Organic Letters (2019), 21 (24), 10057-10062CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
      Despite the long history and interesting properties of propellanes, these compds. still have tremendous potential to be exploited in synthetic org. chem. Herein author disclose an exptl. simple procedure to achieve cyclobutane-contg. allenes and alkynes through a copper-catalyzed ring opening of [1.1.1]propellane and subsequent reaction with ethynes.
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    38. 38
      Kim, J. H.; Ruffoni, A.; Al-Faiyz, Y. S. S.; Sheikh, N. S.; Leonori, D. Divergent Strain-Release Amino-Functionalization of [1.1.1]Propellane with Electrophilic Nitrogen-Radicals. Angew. Chem., Int. Ed. 2020, 59, 8225– 8231,  DOI: 10.1002/anie.202000140
      38
      Divergent Strain-Release Amino-Functionalization of [1.1.1]Propellane with Electrophilic Nitrogen-Radicals
      Kim, Ji Hye; Ruffoni, Alessandro; Al-Faiyz, Yasair S. S.; Sheikh, Nadeem S.; Leonori, Daniele
      Angewandte Chemie, International Edition (2020), 59 (21), 8225-8231CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Herein the authors report the development of a photocatalytic strategy for the divergent prepn. of functionalized bicyclo[1.1.1]pentylamines. This approach exploits, for the first time, the ability of nitrogen-radicals to undergo strain-release reaction with [1.1.1]propellane. This reactivity is facilitated by the electrophilic nature of these open-shell intermediates and the presence of strong polar effects in the transition-state for C-N bond formation/ring-opening. With the aid of a simple reductive quenching photoredox cycle, the authors have successfully harnessed this novel radical strain-release amination as part of a multicomponent cascade compatible with several external trapping agents. Overall, this radical strategy enables the rapid construction of novel amino-functionalized building blocks with potential application in medicinal chem. programs as p-substituted aniline bioisosteres.
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    39. 39
      Zhang, X.; Smith, R. T.; Le, C.; McCarver, S. J.; Shireman, B. T.; Carruthers, N. I.; MacMillan, D. W. C. Copper-mediated Synthesis of Drug-like Bicyclopentanes. Nature 2020, 580, 220– 226,  DOI: 10.1038/s41586-020-2060-z
      39
      Copper-mediated synthesis of drug-like bicyclopentanes
      Zhang, Xiaheng; Smith, Russell T.; Le, Chip; McCarver, Stefan J.; Shireman, Brock T.; Carruthers, Nicholas I.; MacMillan, David W. C.
      Nature (London, United Kingdom) (2020), 580 (7802), 220-226CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Multicomponent reactions are relied on in both academic and industrial synthetic org. chem. owing to their step- and atom-economy advantages over traditional synthetic sequences1. Recently, bicyclo[1.1.1]pentane (BCP) motifs have become valuable as pharmaceutical bioisosteres of benzene rings, and in particular 1,3-disubstituted BCP moieties have become widely adopted in medicinal chem. as para-Ph ring replacements2. These structures are often generated from [1.1.1]propellane via opening of the internal C-C bond through the addn. of either radicals or metal-based nucleophiles3-13. The resulting propellane-addn. adducts are then transformed to the requisite polysubstituted BCP compds. via a range of synthetic sequences that traditionally involve multiple chem. steps. Although this approach was effective so far, a multicomponent reaction that enables single-step access to complex and diverse polysubstituted drug-like BCP products would be more time efficient compared to current stepwise approaches. Here the authors report a one-step three-component radical coupling of [1.1.1]propellane to afford diverse functionalized bicyclopentanes using various radical precursors and heteroatom nucleophiles via a metallaphotoredox catalysis protocol. This copper-mediated reaction operates on short timescales (five minutes to one hour) across multiple (more than ten) nucleophile classes and can accommodate a diverse array of radical precursors, including those that generate alkyl, α-acyl, trifluoromethyl and sulfonyl radicals. This method was used to rapidly prep. BCP analogs of known pharmaceuticals, one of which is substantially more metabolically stable than its com. progenitor.
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    40. 40
      Sterling, A. J.; Dürr, A. B.; Smith, R. C.; Anderson, E. A.; Duarte, F. Rationalizing the Diverse Reactivity of [1.1.1]Propellane through σ–π-Delocalization. Chem. Sci. 2020, 11, 4895– 4903,  DOI: 10.1039/D0SC01386B
      40
      Rationalizing the diverse reactivity of [1.1.1]propellane through σ-π-delocalization
      Sterling, Alistair J.; Durr, Alexander B.; Smith, Russell C.; Anderson, Edward A.; Duarte, Fernanda
      Chemical Science (2020), 11 (19), 4895-4903CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      [1.1.1]Propellane is the ubiquitous precursor to bicyclo[1.1.1]pentanes (BCPs), motifs of high value in pharmaceutical and materials research. The classical Lewis representation of this mol. places an inter-bridgehead C-C bond along its central axis; 'strain relief'-driven cleavage of this bond is commonly thought to enable reactions with nucleophiles, radicals and electrophiles. We propose that this broad reactivity profile instead derives from σ-π-delocalization of electron d. in [1.1.1]propellane. Using ab initio and DFT calcns., we show that its reactions with anions and radicals are facilitated by increased delocalization of electron d. over the propellane cage during addn., while reactions with cations involve charge transfer that relieves repulsion inside the cage. These results provide a unified framework to rationalize exptl. observations of propellane reactivity, opening up opportunities for the exploration of new chem. of [1.1.1]propellane and related strained systems that are useful building blocks in org. synthesis.
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    41. 41
      Huang, W.; Keess, S.; Molander, G. A. Dicarbofunctionalization of [1.1.1]Propellane Enabled by Nickel/Photoredox Dual Catalysis: One-Step Multicomponent Strategy for the Synthesis of BCP-Aryl Derivatives. J. Am. Chem. Soc. 2022, 144, 12961– 12969,  DOI: 10.1021/jacs.2c05304
      41
      Dicarbofunctionalization of [1.1.1]Propellane Enabled by Nickel/Photoredox Dual Catalysis: One-Step Multicomponent Strategy for the Synthesis of BCP-Aryl Derivatives
      Huang, Weichen; Keess, Sebastian; Molander, Gary A.
      Journal of the American Chemical Society (2022), 144 (28), 12961-12969CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Bicyclo[1.1.1]pentane (BCP) motifs as para-disubstituted aryl bioisosteres are playing an emerging role in pharmaceutical, agrochem., and materials chem. The vast majority of these structures are obtained from a BCP electrophile or nucleophile, which are themselves derived from [1.1.1]propellane via cleavage of the internal C-C bond through the addn. of either radicals or metal-based nucleophiles. Compared with the current stepwise approaches, a multicomponent reaction that provides direct access to complex and diverse disubstituted BCP products would be more attractive. Herein, authors report a single-step, multicomponent approach to synthetically versatile arylated BCP products via nickel/photoredox catalysis. Importantly, this three-component process allows two C-C bonds to be formed in a single step and sets three quaternary centers, unprecedented in any previously reported methods. The method has been demonstrated to allow access to complex BCP architectures from aryl halide and radical precursor substrates.
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    42. 42
      Kraemer, Y.; Ghiazza, C.; Ragan, A. N.; Ni, S.; Lutz, S.; Neumann, E. K.; Fettinger, J. C.; Nöthling, N.; Goddard, R.; Cornella, J.; Pitts, C. R. Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight. Angew. Chem., Int. Ed. 2022, 61, e202211892  DOI: 10.1002/anie.202211892
      42
      Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight
      Kraemer, Yannick; Ghiazza, Clement; Ragan, Abbey N.; Ni, Shengyang; Lutz, Sigrid; Neumann, Elizabeth K.; Fettinger, James C.; Nothling, Nils; Goddard, Richard; Cornella, Josep; Pitts, Cody Ross
      Angewandte Chemie, International Edition (2022), 61 (48), e202211892CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Authors leveraged the recent increase in synthetic accessibility of SF5Cl and Ar-SF4Cl compds. to combine chem. of the SF5 and SF4Ar groups with strain-release functionalization. By effectively adding SF5 and SF4Ar radicals across [1.1.1]propellane, author's accessed structurally unique bicyclopentanes, bearing two distinct elements of bioisosterism. Upon evaluating these "hybrid isostere" motifs in the solid state, authors measured exceptionally short transannular distances; in one case, the distance rivals the shortest nonbonding C···C contact reported to date. This prompted SC-XRD and DFT analyses that support the notion that a donor-acceptor interaction involving the "wing" C-C bonds is playing an important role in stabilization. Thus, these heretofore unknown structures expand the palette for highly coveted three-dimensional fluorinated building blocks and provide insight to a more general effect obsd. in bicyclopentanes.
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    43. 43
      Pickford, H. D.; Ripenko, V.; McNamee, R. E.; Holovchuk, S.; Thompson, A. L.; Smith, R. C.; Mykhailiuk, P. K.; Anderson, E. A. Rapid and Scalable Halosulfonylation of Strain-Release Reagents. Angew. Chem., Int. Ed. 2023, 62, e202213508  DOI: 10.1002/anie.202213508
      43
      Rapid and Scalable Halosulfonylation of Strain-Release Reagents
      Pickford, Helena D.; Ripenko, Vasyl; McNamee, Ryan E.; Holovchuk, Serhii; Thompson, Amber L.; Smith, Russell C.; Mykhailiuk, Pavel K.; Anderson, Edward A.
      Angewandte Chemie, International Edition (2023), 62 (3), e202213508CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A one-pot halosulfonylation of [1.1.1]propellane, [3.1.1]propellane and bicyclo[1.1.0]butanes I (R = N,N-bis(propan-2-yl)carbamoyl, S(O)2Ph; R1 = H, Me) proceeds under practical, scalable and mild conditions. The sulfonyl halides R2S(O)2X (R2 = Me, cyclopropyl, Ph, 5-bromothiophen-2-yl, etc.; X = I, Br, Cl) used in this chem. feature aryl, heteroaryl and alkyl substituents, and are conveniently generated in situ from readily available sulfinate salts R2S(O)OM (M = Na, Li) and halogen atom sources. This methodol. enables the synthesis of an array of pharmaceutically and agrochem. relevant halogen/sulfonyl-substituted bioisosteres II, III and cyclobutanes IV, on up to multidecagram scale.
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    44. 44
      Nassir, M.; Ociepa, M.; Zhang, H.-J.; Grant, L. N.; Simmons, B. J.; Oderinde, M. S.; Kawamata, Y.; Cauley, A. N.; Schmidt, M. A.; Eastgate, M. D.; Baran, P. S. Stereocontrolled Radical Thiophosphorylation. J. Am. Chem. Soc. 2023, 145, 15088– 15093,  DOI: 10.1021/jacs.3c05655
      There is no corresponding record for this reference.
    45. 45
      Frank, N.; Nugent, J.; Shire, B. R.; Pickford, H. D.; Rabe, P.; Sterling, A. J.; Zarganes-Tzitzikas, T.; Grimes, T.; Thompson, A. L.; Smith, R. C.; Schofield, C. J.; Brennan, P. E.; Duarte, F.; Anderson, E. A. Synthesis of meta-Substituted Arene Bioisosteres from [3.1.1]Propellane. Nature 2022, 611, 721– 726,  DOI: 10.1038/s41586-022-05290-z
      45
      Synthesis of meta-substituted arene bioisosteres from [3.1.1]propellane
      Frank, Nils; Nugent, Jeremy; Shire, Bethany R.; Pickford, Helena D.; Rabe, Patrick; Sterling, Alistair J.; Zarganes-Tzitzikas, Tryfon; Grimes, Thomas; Thompson, Amber L.; Smith, Russell C.; Schofield, Christopher J.; Brennan, Paul E.; Duarte, Fernanda; Anderson, Edward A.
      Nature (London, United Kingdom) (2022), 611 (7937), 721-726CODEN: NATUAS; ISSN:1476-4687. (Nature Portfolio)
      It was found that [3.1.1]propellane can be synthesized on a multigram scale, and readily undergoes a range of radical-based transformations to generate medicinally relevant carbon- and heteroatom-substituted BCHeps, including pharmaceutical analogs. Comparison of the absorption, distribution, metab. and excretion (ADME) properties of these analogs reveals enhanced metabolic stability relative to their parent arene-contg. drugs, validating the potential of this meta-arene analog as an sp3-rich motif in drug design. Collectively, these results show that bicyclo[3.1.1]heptanes (BCHeps) can be prepd. on useful scales using a variety of methods, offering a new surrogate for meta-substituted benzene rings for implementation in drug discovery programs.
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    46. 46
      Iida, T.; Kanazawa, J.; Matsunaga, T.; Miyamoto, K.; Hirano, K.; Uchiyama, M. Practical and Facile Access to Bicyclo[3.1.1]heptanes: Potent Bioisosteres of meta-Substituted Benzenes. J. Am. Chem. Soc. 2022, 144, 21848– 21852,  DOI: 10.1021/jacs.2c09733
      46
      Practical and Facile Access to Bicyclo[3.1.1]heptanes: Potent Bioisosteres of meta-Substituted Benzenes
      Iida, Toranosuke; Kanazawa, Junichiro; Matsunaga, Tadafumi; Miyamoto, Kazunori; Hirano, Keiichi; Uchiyama, Masanobu
      Journal of the American Chemical Society (2022), 144 (48), 21848-21852CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Focused on the bicyclo[3.1.1]heptane (BCH) scaffold as a novel bioisostere of meta-substituted benzenes, anticipating that [3.1.1]propellane was a versatile precursor of diversely functionalized BCHs. Described a practical preparative method for [3.1.1]propellane from newly developed 1,5-diiodobicyclo[3.1.1]heptane, as well as difunctionalization reactions of [3.1.1]propellane leading to functionalized BCHs. Also reported postfunctionalization reactions of these products.
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    47. 47
      Hamon, D. P. G.; Trenerry, V. C. Carbenoid Insertion Reactions: Formation of [4.1.1]Propellane. J. Am. Chem. Soc. 1981, 103, 4962– 4965,  DOI: 10.1021/ja00406a059
      There is no corresponding record for this reference.
    48. 48
      Fuchs, J.; Szeimies, G. Synthese von [n.l.l]Propellanen (n = 2, 3, 4). Chem. Ber. 1992, 125, 2517– 2522,  DOI: 10.1002/cber.19921251126
      There is no corresponding record for this reference.
    49. 49
      Chen, M.; Cui, Y.; Chen, X.; Shang, R.; Zhang, X. C-F Bond Activation Enables Synthesis of Aryl Difluoromethyl Bicyclopentanes as Benzophenone-Type Bioisosteres. Nat. Commun. 2024, 15, 419,  DOI: 10.1038/s41467-023-44653-6
      There is no corresponding record for this reference.
    50. 50
      Locke, G. M.; Bernhard, S. S. R.; Senge, M. O. Nonconjugated Hydrocarbons as Rigid-Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry. Chem.─Eur. J. 2019, 25, 4590– 4647,  DOI: 10.1002/chem.201804225
      50
      Nonconjugated Hydrocarbons as Rigid-Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry
      Locke, Gemma M.; Bernhard, Stefan S. R.; Senge, Mathias O.
      Chemistry - A European Journal (2019), 25 (18), 4590-4647CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Nonconjugated hydrocarbons, like bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, triptycene, and cubane are a unique class of rigid linkers. Due to their similarity in size and shape they are useful mimics of classic benzene moieties in drugs, so-called bioisosteres. Moreover, they also fulfill an important role in material sciences as linear linkers, in order to arrange various functionalities in a defined spatial manner. In this Review article, recent developments and usages of these special, rectilinear systems are discussed. Furthermore, we focus on covalently linked, nonconjugated linear arrangements and discuss the phys. and chem. properties and differences of individual linkers, as well as their application in material and medicinal sciences.
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    51. 51
      Ma, X.; Pham, L. N. Selected Topics in the Syntheses of Bicyclo[1.1.1]Pentane (BCP) Analogues. Asian J. Org. Chem. 2020, 9, 8– 22,  DOI: 10.1002/ajoc.201900589
      51
      Selected Topics in the Syntheses of Bicyclo[1.1.1]Pentane (BCP) Analogues
      Ma, Xiaoshen; Nhat Pham, Luu
      Asian Journal of Organic Chemistry (2020), 9 (1), 8-22CODEN: AJOCC7; ISSN:2193-5807. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. This Mini focuses on selected topics in the syntheses of bicyclo[1.1.1]pentane (BCP) analogs. A brief historical introduction was included. The content covers various synthetic routes of 1-substituted and 1,3-disubstituted BCPs. Selective examples of synthetically useful building blocks are summarized for each category for synthetic org. chemists' and medicinal chemists' ref. A compare-and-contrast anal. is also applied to evaluate these routes and to demonstrate the progress of strategic functional group transformations using modern org. methodologies. By systematically analyzing the synthetic strategies to access these strained mols., author also hope that this Mini will provide inspirations for future developments in this area.
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    52. 52
      Mykhailiuk, P. K. Saturated Bioisosteres of Benzene: Where to Go Next?. Org. Biomol. Chem. 2019, 17, 2839– 2849,  DOI: 10.1039/C8OB02812E
      52
      Saturated bioisosteres of benzene: where to go next?
      Mykhailiuk, Pavel K.
      Organic & Biomolecular Chemistry (2019), 17 (11), 2839-2849CODEN: OBCRAK; ISSN:1477-0520. (Royal Society of Chemistry)
      A review. The replacement of para-substituted benzenes with satd. bi- and polycyclic bioisosteres - bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane and cubane, - often increases the potency, selectivity and metabolic stability of bioactive compds. The currently remaining challenge for chemists, however, is to rationally design, synthesize and validate the satd. bioisosteres for ortho- and meta-substituted benzenes.
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    53. 53
      Anderson, J. M.; Measom, N. D.; Murphy, J. A.; Poole, D. L. Bridge Functionalisation of Bicyclo[1.1.1]pentane Derivatives. Angew. Chem., Int. Ed. 2021, 60, 24754– 24769,  DOI: 10.1002/anie.202106352
      53
      Bridge Functionalisation of Bicyclo[1.1.1]pentane Derivatives
      Anderson, Joseph M.; Measom, Nicholas D.; Murphy, John A.; Poole, Darren L.
      Angewandte Chemie, International Edition (2021), 60 (47), 24754-24769CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. ''Escaping from flatland'', by increasing the satn. level and three-dimensionality of drug-like compds., can enhance their potency, selectivity and pharmacokinetic profile. One approach that has attracted considerable recent attention is the bioisosteric replacement of arom. rings, internal alkynes and tert-Bu groups with bicyclo[1.1.1]pentane (BCP) units. While functionalisation of the tertiary bridgehead positions of BCP derivs. is well-documented, functionalization of the three concyclic secondary bridge positions remains an emerging field. The unique properties of the BCP core present considerable synthetic challenges to the development of such transformations. However, the bridge positions provide novel vectors for drug discovery and applications in materials science, providing entry to novel chem. and intellectual property space. This Minireview aims to consolidate the major advances in the field, serving as a useful ref. to guide further work that is expected in the coming years.
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    54. 54
      Shire, B. R.; Anderson, E. A. Conquering the Synthesis and Functionalization of Bicyclo[1.1.1]pentanes. JACS Au 2023, 3, 1539– 1553,  DOI: 10.1021/jacsau.3c00014
      54
      Conquering the Synthesis and Functionalization of Bicyclo[1.1.1]pentanes
      Shire, Bethany R.; Anderson, Edward A.
      JACS Au (2023), 3 (6), 1539-1553CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)
      A review. Bicyclo[1.1.1]pentanes (BCPs) have become established as attractive bioisosteres for para-substituted benzene rings in drug design. Conferring various beneficial properties compared with their arom. "parents," BCPs featuring a wide array of bridgehead substituents can now be accessed by an equiv. variety of methods. In this perspective, the evolution of this field and focus on the most enabling and general methods for BCPs synthesis, considering both scope and limitation was discussed. Recent breakthroughs on the synthesis of bridge-substituted BCPs are described, as well as methodologies for postsynthesis functionalization. New challenges and directions for the field, such as the emergence of other rigid small ring hydrocarbons and heterocycles possessing unique substituent exit vectors were further explored.
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    55. 55
      Takano, H.; Katsuyama, H.; Hayashi, H.; Kanna, W.; Harabuchi, Y.; Maeda, S.; Mita, T. A Theory-driven Synthesis of Symmetric and Unsymmetric 1,2-Bis(diphenylphosphino)ethane Analogues via Radical Difunctionalization of Ethylene. Nat. Commun. 2022, 13, 7034,  DOI: 10.1038/s41467-022-34546-5
      There is no corresponding record for this reference.
    56. 56
      Takano, H.; Katsuyama, H.; Hayashi, H.; Harukawa, M.; Tsurui, M.; Shoji, S.; Hasegawa, Y.; Maeda, S.; Mita, T. Synthesis of Bicyclo [1.1.1] pentane (BCP)-Based Straight-Shaped Diphosphine Ligands. Angew. Chem., Int. Ed. 2023, 62, e202303435  DOI: 10.1002/anie.202303435
      There is no corresponding record for this reference.
    57. 57
      Maeda, S.; Ohno, K.; Morokuma, K. Systematic Exploration of the Mechanism of Chemical Reactions: The Global Reaction Route Mapping (GRRM) Strategy Using the ADDF and AFIR Methods. Phys. Chem. Chem. Phys. 2013, 15, 3683– 3701,  DOI: 10.1039/c3cp44063j
      57
      Systematic exploration of the mechanism of chemical reactions: the global reaction route mapping (GRRM) strategy using the ADDF and AFIR methods
      Maeda, Satoshi; Ohno, Koichi; Morokuma, Keiji
      Physical Chemistry Chemical Physics (2013), 15 (11), 3683-3701CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Global reaction route mapping (GRRM), a fully-automated search for all important reaction pathways relevant to a given purpose, from quantum chem. calcns. enables systematic elucidation of complex chem. reaction mechanisms. However, GRRM had previously been limited to very simple systems. This is mainly because such calcns. are highly demanding even in small systems when a brute-force sampling is considered. Hence, the authors have developed two independent but complementary methods: anharmonic downward distortion following (ADDF) and artificial force induced reaction (AFIR) methods. ADDF can follow reaction pathways starting from local min. on the potential energy surface (PES) toward transition structures (TSs) and dissocn. channels. AFIR can find pathways starting from two or more reactants toward TSs for their associative reactions. ADDF searches for A X type isomerization and A X + Y type dissocn. pathways, whereas AFIR finds A + B X (+ Y) type associative pathways. Both follow special paths called the ADDF path and the AFIR path, and these tend to pass through near TSs of corresponding reaction pathways, giving approx. TSs. Such approx. TSs can easily be reoptimized to corresponding true TSs by std. geometry optimizations. From these two methods, the authors proposed practical strategies of GRRM. The GRRM strategies were applied to a variety of chem. systems ranging from thermal- and photochem.-reactions in small systems to organometallic- and enzyme-catalysis, from quantum chem. calcns. In this perspective, the authors present an overview of the GRRM strategies and some results of applications. Their practical usage for systematic prediction is also discussed.
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    58. 58
      Maeda, S.; Taketsugu, T.; Morokuma, K. Exploring Transition State Structures for Intramolecular Pathways by the Artificial Force Induced Reaction Method. J. Comput. Chem. 2014, 35, 166– 173,  DOI: 10.1002/jcc.23481
      58
      Exploring transition state structures for intramolecular pathways by the artificial force induced reaction method
      Maeda, Satoshi; Taketsugu, Tetsuya; Morokuma, Keiji
      Journal of Computational Chemistry (2014), 35 (2), 166-173CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
      Finding all required transition state (TS) structures is an important but hard task in theor. study of complex reaction mechanisms. An efficient automated TS search method, artificial force induced reaction (AFIR), was extended to intramol. reactions. The AFIR method was developed for intermol. associative pathways between two or more reactants. Although it also was applied to intramol. reactions by dividing mols. manually into fragments, the fragmentation scheme was not automated. The authors propose an automated fragmentation scheme. Using this fragmentation scheme and the AFIR method, a fully automated search algorithm for intramol. pathways is introduced. This version for intramol. reactions is called single-component AFIR (SC-AFIR), to distinguish it from multicomponent AFIR for intermol. reactions. SC-AFIR was tested with two reactions, the Claisen rearrangement and the first step of cobalt-catalyzed hydroformylation, and successfully located all important pathways reported in the literature. © 2013 Wiley Periodicals, Inc.
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    59. 59
      Maeda, S.; Harabuchi, Y.; Takagi, M.; Saita, K.; Suzuki, K.; Ichino, T.; Sumiya, Y.; Sugiyama, K.; Ono, Y. Implementation and Performance of the Artificial Force Induced Reaction Method in the GRRM17 Program. J. Comput. Chem. 2018, 39, 233– 251,  DOI: 10.1002/jcc.25106
      59
      Implementation and performance of the artificial force induced reaction method in the GRRM17 program
      Maeda, Satoshi; Harabuchi, Yu; Takagi, Makito; Saita, Kenichiro; Suzuki, Kimichi; Ichino, Tomoya; Sumiya, Yosuke; Sugiyama, Kanami; Ono, Yuriko
      Journal of Computational Chemistry (2018), 39 (4), 233-251CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
      This article reports implementation and performance of the artificial force induced reaction (AFIR) method in the upcoming 2017 version of GRRM program (GRRM17). The AFIR method, which is one of automated reaction path search methods, induces geometrical deformations in a system by pushing or pulling fragments defined in the system by an artificial force. In GRRM17, three different algorithms, i.e., multicomponent algorithm (MC-AFIR), single-component algorithm (SC-AFIR), and double-sphere algorithm (DS-AFIR), are available, where the MC-AFIR was the only algorithm which has been available in the previous 2014 version. The MC-AFIR does automated sampling of reaction pathways between two or more reactant mols. The SC-AFIR performs automated generation of global or semiglobal reaction path network. The DS-AFIR finds a single path between given two structures. Exploration of min. energy structures within the hypersurface in which two different electronic states degenerate, and an interface with the quantum mechanics/mol. mechanics method, are also described. A code termed SAFIRE will also be available, as a visualization software for complicated reaction path networks. © 2017 The Authors Journal of Computational Chem. Published by Wiley Periodicals, Inc.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhvVSnu7jF&md5=139418f640e8df87009356cbb30eb57e
    60. 60
      Maeda, S.; Harabuchi, Y. Exploring Paths of Chemical Transformations in Molecular and Periodic Systems: An Approach Utilizing Force. WIREs Comput. Mol. Sci. 2021, 11, e1538  DOI: 10.1002/wcms.1538
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      Wiberg, K. B.; Walker, F. H.; Pratt, W. E.; Michl, J. [2.1.1]Propellane. Reaction of 1,4-Diiodobicyclo[2.1.1]hexane with tert-Butyllithium and with Potassium Atoms. J. Am. Chem. Soc. 1983, 105, 3638– 3641,  DOI: 10.1021/ja00349a048
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      Eaton, P. E.; Temme, G. H. [2.2.2]Propellane System. J. Am. Chem. Soc. 1973, 95, 7508– 7510,  DOI: 10.1021/ja00803a052
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      [2.2.2]Propellane system
      Eaton, Philip E.; Temme, George H., III
      Journal of the American Chemical Society (1973), 95 (22), 7508-10CODEN: JACSAT; ISSN:0002-7863.
      The synthesis, isolation and characterization of the first known member of the [2.2.2]propellane family are described.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2cXhsVagsA%253D%253D&md5=f94110bae2a97efb6ffa01bfb70985ac
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      Wiberg, K. B.; Pratt, W. E.; Bailey, W. F. Reaction of 1,4-Diiodonorbornane, 1,4-Diiodobicyclo[2.2.2]octane, and 1,5-Diiodobicyclo[3.2.1]octane with Butyllithium. Convenient Preparative Routes to the [2.2.2]- and [3.2.1]Propellanes. J. Am. Chem. Soc. 1977, 99, 2297– 2302,  DOI: 10.1021/ja00449a045
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      Weber, R. W.; Cook, J. M. General Method for the Synthesis of [n.3.3]Propellanes, n ≥ 3. Can. J. Chem. 1978, 56, 189– 192,  DOI: 10.1139/v78-030
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      Wender, P. A.; Dreyer, G. B. Synthetic Studies on Arene-Olefin Cycloadditions 4. Total Synthesis of (±)-Modhephene. J. Am. Chem. Soc. 1982, 104, 5805– 5807,  DOI: 10.1021/ja00385a051
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    66. 66

      CCDC 2324260 (6bb-cis), 2333566 (NiCl2(8bb-cis)), and 2324261 (12) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the Cambridge Crystallographic Data Centre; for details, see the Supporting Information (S34).

      There is no corresponding record for this reference.
    67. 67
      Buonomo, J. A.; Eiden, C. G.; Aldrich, C. C. Scalable Synthesis of Hydrido-Disiloxanes from Silanes: A One-Pot Preparation of 1,3-Diphenyldisiloxane from Phenylsilane. Synthesis 2018, 50, 278– 281,  DOI: 10.1055/s-0036-1588580
      67
      Scalable Synthesis of Hydrido-Disiloxanes from Silanes: A One-Pot Preparation of 1,3-Diphenyldisiloxane from Phenylsilane
      Buonomo, Joseph A.; Eiden, Carter G.; Aldrich, Courtney C.
      Synthesis (2018), 50 (2), 278-281CODEN: SYNTBF; ISSN:1437-210X. (Georg Thieme Verlag)
      A simple, 1-pot, and high-yielding synthesis of 1,3-diphenyldisiloxane is presented. The prepn. of similar sym. disiloxane materials is also accomplished with this same protocol. This mechano-chem. procedure is efficient and highly scalable, furnishing a convenient route to hydrido-disiloxanes from widely accessible com. available silanes.
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      Buonomo, J. A.; Eiden, C. G.; Aldrich, C. C. Chemoselective Reduction of Phosphine Oxides by 1,3-Diphenyl-Disiloxane. Chem.─Eur. J. 2017, 23, 14434– 14438,  DOI: 10.1002/chem.201703875
      68
      Chemoselective Reduction of Phosphine Oxides by 1,3-Diphenyl-Disiloxane
      Buonomo, Joseph A.; Eiden, Carter G.; Aldrich, Courtney C.
      Chemistry - A European Journal (2017), 23 (58), 14434-14438CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Redn. of phosphine oxides to the corresponding phosphines represents the most straightforward method to prep. these valuable reagents. However, existing methods to reduce phosphine oxides suffer from inadequate chemoselectivity due to the strength of the P:O bond and/or poor atom economy. Herein, the authors report the discovery of the most powerful chemoselective reductant for this transformation to date, 1,3-diphenyl-disiloxane (DPDS). Additive-free DPDS selectively reduces both secondary and tertiary phosphine oxides with retention of configuration even in the presence of aldehyde, nitro, ester, α,β-unsatd. carbonyls, azocarboxylates, and cyano functional groups. Arrhenius anal. indicates that the activation barrier for redn. by DPDS is significantly lower than any previously calcd. silane redn. system. Inclusion of a catalytic Bronsted acid further reduced the activation barrier and led to the 1st silane-mediated redn. of acyclic phosphine oxides at room temp.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhsFWgt7fP&md5=d99c2cd714caa81687213d7bc3b92373
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      Bomfim, J. A. S.; de Souza, F. P.; Filgueiras, C. A. L.; de Sousa, A. G.; Gambardella, M. T. P. Diphosphine Complexes of Nickel: Analogies in Molecular Structures and Variety in Crystalline Arrangement. Polyhedron 2003, 22, 1567– 1573,  DOI: 10.1016/S0277-5387(03)00263-8
      69
      Diphosphine complexes of nickel: analogies in molecular structures and variety in crystalline arrangement
      Bomfim, Joao A. S.; de Souza, Fabio P.; Filgueiras, Carlos A. L.; de Sousa, Alexsandro G.; Gambardella, Maria Teresa P.
      Polyhedron (2003), 22 (12), 1567-1573CODEN: PLYHDE; ISSN:0277-5387. (Elsevier Science Ltd.)
      This work comprised the prepn. of six complexes of Ni(II) and their study by spectroscopy and x-ray diffraction. These complexes are: [Ni(dppe)Cl2]CH2Cl2 (I), [Ni(dppen)Cl2] (II), [Ni(dppm)Br2]CH2Cl2 (III), [Ni(dppe)(NCS)2] (IV), [Ni(dppen)I2] (V), [Ni(dppp)Cl2]CH2Cl2 (VI), where dppe = 1,2-bis(diphenylphosphine)ethane, dppen = cis-1,2-bis(diphenylphosphine)ethene, dppm = bis(diphenylphosphine)methane, dppp = 1,3-bis(diphenylphosphine)propane. The structures of complexes III-VI are unknown in the literature. Although the mol. structures of the six complexes present many similarities, their cryst. arrangements vary considerably. This is a feature not presented hitherto in previous work involving complexes of this type. A systematic spectroscopic study (IR, UV-visible, 31P{1H} NMR) was undertaken on all six complexes.
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      Strieth-Kalthoff, F.; James, M. J.; Teders, M.; Pitzer, L.; Glorius, F. Energy Transfer Catalysis Mediated by Visible Light: Principles, Applications, Directions. Chem. Soc. Rev. 2018, 47, 7190– 7202,  DOI: 10.1039/C8CS00054A
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      Energy transfer catalysis mediated by visible light: principles, applications, directions
      Strieth-Kalthoff, Felix; James, Michael J.; Teders, Michael; Pitzer, Lena; Glorius, Frank
      Chemical Society Reviews (2018), 47 (19), 7190-7202CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Harnessing visible light to access excited (triplet) states of org. compds. can enable impressive reactivity modes. This tutorial review covers the photophys. fundamentals and most significant advances in the field of visible-light-mediated energy transfer catalysis within the last decade. Methods to det. excited triplet state energies and to characterize the underlying Dexter energy transfer are discussed. Synthetic applications of this field, divided into four main categories (cyclization reactions, double bond isomerizations, bond dissocns. and sensitization of metal complexes), are also examd.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsVGrtrbL&md5=e348c7e11a61ef190dfcb1240cdedd28
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      Wong, S. K.; Sytnyk, W.; Wan, J. K. S. The Flash Photolysis of Tetraphenyldiphosphine, Triphenylphosphine, and Diphenylphosphine in Alcohols. Can. J. Chem. 1971, 49, 994– 1000,  DOI: 10.1139/v71-165
      There is no corresponding record for this reference.

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