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CommunicationDecember 17, 2015

Bifunctional Iminophosphorane Catalyzed Enantioselective Sulfa-Michael Addition to Unactivated α-Substituted Acrylate Esters
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Journal of the American Chemical Society

Cite this: J. Am. Chem. Soc. 2015, 137, 51, 15992–15995

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https://doi.org/10.1021/jacs.5b10226

Published December 17, 2015

Abstract

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The highly enantioselective sulfa-Michael addition of alkyl thiols to unactivated α-substituted acrylate esters catalyzed by a bifunctional iminophosphorane organocatalyst under mild conditions is described. The strong Brønsted basicity of the iminophosphorane moiety of the catalyst provides the necessary activation of the alkyl thiol pro-nucleophile, while the two tert-leucine residues flanking a central thiourea hydrogen-bond donor facilitate high enantiofacial selectivity in the protonation of the transient enolate intermediate. The reaction is broad in scope with respect to the alkyl thiol, the ester moiety, and the α-substituent of the α,β-unsaturated ester, affords sulfa-Michael adducts in excellent yields (up to >99%) and enantioselectivities (up to 96% ee), and is amenable to decagram scale-up using catalyst loadings as low as 0.05 mol %.

Subjects

The Michael addition of a carbon-centered (C–H) or heteroatom-centered (X–H) acid to a conjugated electron-deficient alkene is a fundamental reaction in organic chemistry that allows the direct and efficient construction of C–C or C–X bonds with perfect atom economy. (1) In this union, the creation of stereogenic centers, either directly at the β-carbon or indirectly through protonation at the α-carbon, is common and controlling the enantioselectivity with both metal-rich and metal-free catalysts has been the subject of intense research activity over the decades. (2) Recently, much activity has focused on organocatalytic methods, (3) and enantioselective additions of a wide range of pro-nucleophiles under iminium, enamine, or tertiary amine Brønsted base/H-bond donor catalysis to various conjugated electron deficient acceptors including enals, enones, nitroolefins, and other reactive Michael acceptors have been successfully achieved. (3g, 3h)

To date, however, one class of Michael acceptor, α,β-unsaturated esters substituted with alkyl or aryl groups at either the α- or β-position, has remained a persistent challenge in enantioselective organocatalysis due to their low inherent electrophilicity (4) and low propensity for catalyst activation. Both subclasses are problematic in their own way, and each one requires a solution. In this paper, we chose to tackle α-substituted acrylate esters. (5) Our primary aim was to realize reactivity and enantiocontrol with substrates possessing simple alkyl groups at the α-position. To the best of our knowledge, organocatalytic enantioselective Michael additions to methacrylate esters has been limited to NHC catalysis of aldehydes in the Stetter reaction, and examples under Brønsted base catalysis have yet to be reported. (6)

An effective approach to overcoming low inherent reagent electrophilicity in Brønsted base catalyzed addition reactions of pronucleophiles is to raise the Brønsted basicity of the catalyst. (7) Under a fast acid/base proton transfer regime, augmented Brønsted basicity in the catalyst increases the concentration of the nucleophilic conjugate base and, as a consequence, the rate of the bimolecular addition step. To this end, we recently developed a new family of modular bifunctional iminophosphorane (BIMP) superbase organocatalysts for the first general enantioselective organocatalytic nitro-Mannich reaction to unactivated ketimines; (8) a reaction where an organosuperbase was essential for reactivity. (9) In the same vein, we postulated that the poor reactivity of unactivated methacrylate esters toward nucleophilic addition may be overcome using our BIMP organosuperbase family (Scheme 1). We chose the SMA addition of alkyl thiols as this is an important reaction for the asymmetric construction of chiral sulfides, (10, 11) and no catalytic enantioselective version to unactivated α-substituted acrylate esters under metal-free catalysis has previously been reported. (6b) Related literature examples have employed activated derivatives including imides (11f, 11o) and oxazolidinones (10h, 11m, 11q, 11r) as the Michael acceptor or possess activating α-substituents. (11b, 11d, 11g, 11q) Additionally the use of simple aliphatic thiols (11h, 11k, 11n, 11o, 11q, 11u) (pK_a(DMSO) = 17) (12) in organocatalytic asymmetric sulfa-Michael additions is much more challenging than the use of more acidic thiol pro-nucleophiles such as aryl thiols or thiocarboxylic acids. (11) Our hope was that the strong Brønsted basicity of the BIMP would surmount the low inherent electrophilicity of the Michael acceptor by increasing the concentration of the thiol conjugate base. Following C–S bond formation, selective enantiofacial protonation of the transient enolate intermediate would deliver the enantioenriched Michael adduct and release the catalyst back into the cycle (Scheme 1).

Scheme 1. Proposed BIMP Catalyzed Enantioselective Sulfa-Michael Addition to Methyl Methacrylate

1-Propanethiol was chosen to test reactivity in a model reaction using the inexpensive feedstock chemical methyl methacrylate (2a) as the Michael acceptor. A promising reactivity profile was initially established using 10 mol % of our previously reported first generation tert-leucine derived BIMP catalyst 1a derived from triphenylphosphine (Figure 1 and Table 1, entry 1). (13) After just 3 h at rt, 49% yield of product 4a was afforded with an encouraging ee of 66%. However, switching to the analogous but more basic catalyst 1b derived from tris(p-methoxyphenyl)phosphine gave rise to a significant boost in reactivity, and good levels of enantiocontrol were also witnessed; adduct 4a was afforded in quantitative yield and with 72% ee (Table 1, entry 2). Poor reactivity was observed with less basic BIMP catalyst 1c (Table 1, entry 3), and a drop in enantioselectivity was witnessed using tributylphosphine derived BIMP 1d (Table 1, entry 4).

Figure 1. Bifunctional iminophosphorane (BIMP) organocatalysts screened for performance in the sulfa-Michael addition reaction. PMP = p-methoxyphenyl.

Table 1. Catalyst Screen in the Sulfa-Michael Addition of 1-Propanethiol to Methyl Methacrylate^a

entry	catalyst	yield^b (%)	ee^c (%)
1	1a	49	66
2	1b	>99	72
3	1c	9	68
4	1d	91	35
5	1e	>99	57
6^d	1f	57	37
7	1g	40	4
8	1h	91	9
9	1i	40	6
10	1j	97	76
11	1k	91	29
12	1l	>99	84
13	1m	>99	87
14^e	1m	86	89
15^f	1m	97	94

Reactions were carried out with 0.20 mmol of 3a and 1.0 mmol of 2a.

Isolated yield.

Enantiomeric excess (ee) determined by HPLC analysis on a chiral stationary phase.

Catalyst 1f used at 5 mol %.

Reaction performed in 0.5 M Et₂O.

Reaction conducted in 4.0 mL of Et₂O at rt using 5 mol % 1m and quenched after 24 h.

Simple modification of the thiourea hydrogen-bond donor group of the first generation BIMP organocatalysts led to no improvement in the level of enantiocontrol (Table 1, entries 5–7, and Supporting Information), and accordingly alternative second generation BIMP organocatalyst designs were considered. Drawing inspiration from the cyclohexanediamine-derived H-bond donor organocatalysts pioneered by Jacobsen (14) and Takemoto (15) and their co-workers, we synthesized the corresponding BIMPs 1h and 1i and assessed them in the model reaction (Table 1, entries 8 and 9). Although catalytically active, both afforded 4a with low levels of enantiocontrol, and we postulated that the trans-1,2-diaminocyclohexane motif was having a detrimental effect on the enantioselectivity. This was confirmed when hybrid catalyst 1j, arising from a fusion of the amide/thiourea unit of 1h with the tert-leucine residue of our original BIMP catalysts 1a–g, was synthesized; we were delighted to observe enhanced enantioselectivity (76% ee, entry 10), while maintaining the excellent reactivity.

Further elaboration of this second generation BIMP catalyst design revealed that both stereocenters were contributing to enantiocontrol in the formation of 4a. When the diastereomeric catalyst 1k was tried, the enantioselectivity was reduced to 29% ee (Table 1, entry 11), while maintaining the same absolute configuration, indicating that the stereogenic center of the amide/thiourea unit was less influential on enantiofacial control than the stereogenic center proximal to the iminophosphorane. Fine-tuning of the amide moiety of the catalyst (16) revealed 1m, which afforded 4a in 87% ee (Table 1, entry 13). A reoptimization of the reaction conditions to 0.05 M in diethyl ether resulted in a marked improvement in the enantioselectivity to 94% ee while maintaining near quantitative yields (Table 1, entries 14 and 15 and Supporting Information).

With optimized reaction conditions established, we investigated the scope of the transformation (Table 2). An initial screen using 1-propanethiol showed good performance across a range of small, medium, and large ester types (89–96% ee, entries 1–6), although the more sterically bulky esters (e.g., R¹ = t-Bu, entry 6) substantially retarded the reaction rate. Subsequent scope with respect to the thiol was investigated using methyl methacrylate and minimal variation to the high enantioselectivity was observed irrespective of alkyl chain length, branching in the α- or β-positions, or cyclic substituents (Table 2, entries 7–12). (17)

Table 2. Scope with Respect to the Ester Group and Thiol Pro-nucleophiles^a

entry	R¹	R²	product	yield^b (%)	ee^c (%)
1	Me (2a)	n-Pr (3a)	4a	97	94
2^d	Ph (2b)	n-Pr (3a)	4b	>99	95
3	Et (2c)	n-Pr (3a)	4c	87	92
4^d	Bn (2d)	n-Pr (3a)	4d	78	89
5	i-Pr (2e)	n-Pr (3a)	4e	71	93
6^e	t-Bu (2f)	n-Pr (3a)	4f	36	96
7	Me (2a)	n-Pent (3b)	4g	>99	93
8	Me (2a)	i-Pent (3c)	4h	86	92
9	Me (2a)	i-Pr (3d)	4i	>99	92
10	Me (2a)	c-Hex (3e)	4j	81	90
11	Me (2a)	Bn (3f)	4k	85	90
12	Me (2a)	PMB (3g)	4l	89	89

Reactions were carried out with 0.20 mmol of thiol (3) and 1.0 mmol of methacrylate ester (2).

Isolated yield.

Determined by HPLC or GC analysis on a chiral stationary phase.

Reaction performed with 0.40 mmol of 2.

Reaction quenched after 48 h.

Next, variation of the α-substituent of α-substituted α,β-unsaturated methyl or phenyl esters (2g–r) was studied using 1-propanethiol as the S-centered nucleophile (Table 3). Using methyl ester Michael acceptors, the reaction performed well with substituted α-methyl substituents possessing electron withdrawing groups such as vinyl, ester, or phenyl moieties or with an α-phenyl substituent (83–93% ee, Table 3, entries 1–4). With alkyl substituted methyl substituents or branched substituents at the α-position, it was advantageous to use the slightly more active phenyl ester to compensate for the reduction in reaction rate while maintaining high levels of enantiocontrol (85–92% ee, Table 3, entries 6–9). Pleasingly, in addition to the phenyl substituted substrate 2j, the reaction was also applicable to electron rich and deficient aryl substituents (84–94% ee, entries 10–12).

Table 3. Scope with Respect to the α-Substituent on the Michael Acceptor^a

entry	R¹	R³	product	yield^b (%)	ee^c (%)
1	Me	allyl (2g)	4m	94	93
2	Me	CH₂CO₂Me (2h)	4n	85	90
3^d	Me	Bn (2i)	4o^e	99	86
4	Me	Ph (2j)	4p	84	83
5^d	Me	Et (2k)	4q	47	92
6	Ph	Et (2l)	4r	85	92
7	Ph	i-Pr (2m)	4s	98	88
8	Ph	c-Hex (2n)	4t	93	85
9	Ph	c-Pent (2o)	4u	96	90
10^f	Me	4-OMe(C₆H₄) (2p)	4v	94	86
11	Me	2-OMe(C₆H₄) (2q)	4w	89	94
12	Me	2-NO₂(C₆H₄) (2r)	4x	98	84

Reactions were carried out with 0.20 mmol of 3a and 0.40 mmol of 2.

Isolated yield.

Determined by HPLC analysis on a chiral stationary phase.

Reaction quenched after 48 h.

Absolute configuration of 4o determined by chemical correlation (see Supporting Information).

Compound 2p (0.20 mmol) was used.

Although our methodological study was routinely carried out at 5 mol % catalyst loading, all indications suggested that our optimal BIMP catalyst 1m was highly active in the SMA and that the loadings could be significantly reduced. (18) Indeed, in the reaction of 2b with 3a, after re-optimization of the reaction parameters, we were delighted to find the catalyst loading of 1m could be readily reduced down to 0.05 mol % while performing the reaction on a 100 mmol scale (Scheme 2). Near full conversion was achieved after 24 h using only ∼40 mg of in situ generated 1m whereupon the reaction was quenched, a crude ee of 90% was measured, and the product was purified by distillation to afford 4b in 84.5% isolated yield with no loss in ee. To demonstrate the synthetic utility of the β-mercapto ester products 4 resulting from the reaction, 4b was subsequently oxidized to the sulfone 5a, hydrolyzed to the acid 5b, reduced to the alcohol 5c, and converted quantitatively to amides 5d and 5e by direct aminolysis with benzylamine and allylamine, respectively.

Scheme 2. Decagram Scale Sulfa-Michael Addition and Subsequent Derivatization^a

In summary, the first highly enantioselective organocatalytic SMA of alkyl thiol pro-nucleophiles to unactivated α-substituted acrylate esters has been developed. Good levels of reactivity, and excellent enantioselectivities were achieved across a diverse range of alkyl thiols and unactivated α-substituted acrylate esters using a new second generation bifunctional iminophosphorane (BIMP) superbase organocatalyst. The ability of BIMP organocatalysts to enable unactivated α-substituted acrylate esters to undergo asymmetric conjugate additions is, we believe, a significant advancement in the field, and further work to uncover the breadth of their use is ongoing in our laboratories and will be disclosed in due course.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.5b10226.

Experimental details and characterization data (PDF)

ja5b10226_si_001.pdf (6.61 MB)

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Author Information

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Corresponding Author
- Darren J. Dixon - The Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.; Email: [email protected]
Authors
- Alistair J. M. Farley - The Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.
- Christopher Sandford - The Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.
Author Contributions
A.J.M.F. and C.S. contributed equally.
Notes
The authors declare no competing financial interest.

Acknowledgment

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This work was supported by the EPSRC (Studentship and Doctoral Training Grant [EP/M50659X/1] to A.J.M.F.), AstraZeneca (Studentship to A.J.M.F.), and the SCI (Postgraduate Scholarship to A.J.M.F.).

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Highly Enantioselective Intermolecular Stetter Reaction of Simple Acrylates: Synthesis of α-Chiral γ-Ketoesters
Wurz, Nathalie E.; Daniliuc, Constantin G.; Glorius, Frank
Chemistry - A European Journal (2012), 18 (51), 16297-16301CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
A novel N-heterocyclic carbene (NHC) I was designed by combining an electron-rich 2,6-dimethoxy substituent and underestimated yet promising chiral motif. With this NHC in hand, a highly enantioselective intermol. Stetter reaction of aldehydes R1CHO (R1 = Ph, 4-ClC6H4, 2-FC6H4, PhCH2, 2-furyl, etc.) with simple acrylates, e.g. CH2:CR2CO2R3 (R2 = Me, Et, n-Bu, Ph, PhCH2, AcNH, etc.; R3 = Me, Et, i-Pr, n-Bu), yielding versatile chiral γ-keto esters, e.g. II, has been developed.
7
(a) Ishikawa, T. Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts; Wiley: New York, 2009.
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Chiral guanidine-catalyzed enantioselective reactions
Leow, Dasheng; Tan, Choon-Hong
Chemistry - An Asian Journal (2009), 4 (4), 488-507CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Chiral guanidine catalysts share common characteristics such as high pKa values and dual hydrogen bonding modes of activation, and high catalytic activities and enantioselectivities can often be achieved. The utilization of guanidines as catalysts was growing at a steady pace. In the past few years, it has attracted tremendous attention through several landmark achievements. This article highlights the development of chiral guanidine catalysis in asym. synthesis.
(d) Leow, D.; Tan, C.-H. Synlett 2010, 2010, 1589 DOI: 10.1055/s-0029-1219937
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Guanidine organocatalysis
Selig, Philipp
Synthesis (2013), 45 (6), 703-718CODEN: SYNTBF; ISSN:0039-7881. (Georg Thieme Verlag)
A review. An overview of the most commonly used guanidine organocatalysts and their applications in org. synthesis is presented. Privileged structures of open, monocyclic and bicyclic guanidines and guanidinium salts are showcased with prominent examples from the literature. Free guanidines have found widespread use as strong Bronsted base catalysts in asym. synthesis. Guanidinium salts are employed as weak Bronsted acids or hydrogen-bond-donor catalysts and chiral counterions. The nucleophilic and Lewis basic properties of guanidines are still rarely exploited, but, as of late, have been gaining increasing recognition.
(h) Krawczyk, H.; Dzięgielewski, M.; Deredas, D.; Albrecht, A.; Albrecht, Ł. Chem. - Eur. J. 2015, 21, 10268 DOI: 10.1002/chem.201500481
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Chiral Iminophosphoranes-An Emerging Class of Superbase Organocatalysts
Krawczyk, Henryk; Dziegielewski, Marek; Deredas, Dariusz; Albrecht, Anna; Albrecht, Lukasz
Chemistry - A European Journal (2015), 21 (29), 10268-10277CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Chiral Bronsted base catalysis is a fascinating and highly explored field of research. For many years catalysts based on chincona alkaloid chiral scaffolds have constituted privileged systems widely employed in numerous base-promoted org. transformations. Recently, a novel group of chiral base catalysts has been successfully introduced. The application of organosuperbases, namely cyclopropenimines, guanidines, and iminophosphoranes, as chiral catalysts is receiving increasing attention. The aim of this Concept article is to summarize recent progress in the field of chiral iminophosphorane superbase organocatalysis. Catalysts design, different approaches to their synthesis, and applications in asym. synthesis are outlined and discussed in detail.
For selected examples, see:
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Enantioselective Synthesis of α-Amino Nitriles from N-Benzhydryl Imines and HCN with a Chiral Bicyclic Guanidine as Catalyst
Corey, E. J.; Grogan, Michael J.
Organic Letters (1999), 1 (1), 157-160CODEN: ORLEF7; ISSN:1523-7060. (American Chemical Society)
A novel catalytic enantioselective Strecker synthesis of chiral α-amino nitriles and α-amino acids is described and analyzed with regard to the possible mechanistic basis for stereoselectivity. Key features of the enantioselective process include (1) the use of the chiral bicyclic guanidine I as catalyst and (2) the use of the N-benzhydryl substituent on the imine substrate.
(j) Ishikawa, T.; Araki, Y.; Kumamoto, T.; Seki, H.; Fukuda, K.; Isobe, T. Chem. Commun. 2001, 245 DOI: 10.1039/b009193f
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Chiral Proton Catalysis: A Catalytic Enantioselective Direct Aza-Henry Reaction
Nugent, Benjamin M.; Yoder, Ryan A.; Johnston, Jeffrey N.
Journal of the American Chemical Society (2004), 126 (11), 3418-3419CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Nonracemic β-aryl-β-aminonitroalkanes I (R = H, Me; R1 = H, 2-O2N, 3-O2N, 4-O2N, 4-Cl, 4-F3C, 4-F3CO) are prepd. in 50-69% yields, 7:1-19:1 diastereoselectivities (for R = Me), and in 59-95% ee by the stereoselective aza-Henry reaction of nitroalkanes RCH2NO2 (R = H, Me) to the N-Boc imines RC6H4CH:NBoc (Boc = tert-butoxycarbonyl) in the presence of nonracemic di(quinolinylamino)cyclohexane triflic acid salt II•F3CSO3H. II is prepd. by amination of 2-chloroquinoline with (1R-trans)-1,2-cyclohexanediamine in the presence of Pd(dba)2, racemic BINAP, and sodium tert-butoxide; II•F3CSO3H is prepd. as a bench-stable white solid by addn. of triflic acid to II in methylene chloride. The free base II does not act as a catalyst for enantioselective Henry reactions in the absence of acid. II•F3CSO3H is proposed to act as a catalyst using polar ionic hydrogen bonds to accelerate the reaction while controlling its stereoselectivity; the catalyst is effective without either a Bronsted base additive or preactivation of the nitroalkane.
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7l
Axially Chiral Guanidine as Enantioselective Base Catalyst for 1,4-Addition Reaction of 1,3-Dicarbonyl Compounds with Conjugated Nitroalkenes
Terada, Masahiro; Ube, Hitoshi; Yaguchi, Yusuke
Journal of the American Chemical Society (2006), 128 (5), 1454-1455CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A new strategy for designing chiral guanidine mols. I [R1 = Me, n-Pr; R2 = Ph, 4-Me3CC6H4, 3,5-(F3C)2C6H3, etc.] is presented, which features the introduction of an axially chiral binaphthyl backbone. The axially chiral guanidine catalysts thus developed facilitated the highly enantioselective 1,4-addn. reaction of 1,3-dicarbonyl compds. with a broad range of conjugated nitroalkenes and showed extremely high catalytic activity.
(m) Uraguchi, D.; Sakaki, S.; Ooi, T. J. Am. Chem. Soc. 2007, 129, 12392 DOI: 10.1021/ja075152+
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Chiral Tetraaminophosphonium Salt-Mediated Asymmetric Direct Henry Reaction
Uraguchi, Daisuke; Sakaki, Sawako; Ooi, Takashi
Journal of the American Chemical Society (2007), 129 (41), 12392-12393CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Chiral tetraaminophosphonium salts I (R = Ph, m-xylyl, 4-tolyl, 4-F3CC6H4) possessing the phosphorus-centered [5.5]-spirocyclic core have been designed and synthesized in a single step from L-valine-derived diamine. The three-dimensional mol. structure was successfully verified by the single-crystal X-ray diffraction anal., which also identified a secondary interaction between the phosphonium cation and chloride ion via double hydrogen-bonding. The potential of this novel onium salt as a chiral org. mol. catalyst has been demonstrated in an application to asym. direct Henry reaction of R2CHO (R2 = Ph, 1-naphthyl, 1-octyl, etc.) with R1CH2NO2 (R1 = H, Me, Et).
(n) Davis, T. A.; Wilt, J. C.; Johnston, J. N. J. Am. Chem. Soc. 2010, 132, 2880 DOI: 10.1021/ja908814h
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Bifunctional Asymmetric Catalysis: Amplification of Bronsted Basicity Can Orthogonally Increase the Reactivity of a Chiral Bronsted Acid
Davis, Tyler A.; Wilt, Jeremy C.; Johnston, Jeffrey N.
Journal of the American Chemical Society (2010), 132 (9), 2880-2882CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The reactivity of a series of sym. chiral Bronsted acids (polar ionic hydrogen-bond donors) follows the counterintuitive trend wherein the more Bronsted basic member is a more effective catalyst for the aza-Henry (nitro-Mannich) reaction. This new design element leads to a substantially more reactive catalyst for the aza-Henry reaction, one that can promote the addn. of a secondary nitroalkane. Addnl., when an achiral Bronsted acid (TfOH) is used in slight excess of the neutral, chiral bisamidine ligand, diastereoselection can be optimized to levels generally greater than 15:1 while the enantioselection remains unchanged at generally > 90% ee.
(o) Sohtome, Y.; Shin, B.; Horitsugi, N.; Takagi, R.; Noguchi, K.; Nagasawa, K. Angew. Chem., Int. Ed. 2010, 49, 7299 DOI: 10.1002/anie.201003172
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Entropy-controlled catalytic asymmetric 1,4-type Friedel-Crafts reaction of phenols using conformationally flexible guanidine/bisthiourea organocatalyst
Sohtome, Yoshihiro; Shin, Bongki; Horitsugi, Natsuko; Takagi, Rika; Noguchi, Keiichi; Nagasawa, Kazuo
Angewandte Chemie, International Edition (2010), 49 (40), 7299-7303, S7299/1-S7299/55CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Conformationally flexible org. compds. were found to promote the title transformation. These soft organocatalysts, which are able to control processes through the differential activation entropies (ΔΔS⧺S-R) of the reactive intermediates, lead to high stereoselectivities without the requirement of fine-tuning the reaction temps.
(p) Davis, T. A.; Johnston, J. N. Chem. Sci. 2011, 2, 1076 DOI: 10.1039/c1sc00061f
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Catalytic, enantioselective synthesis of stilbene cis-diamines: A concise preparation of (-)-Nutlin-3, a potent p53/MDM2 inhibitor
Davis, Tyler A.; Johnston, Jeffrey N.
Chemical Science (2011), 2 (6), 1076-1079CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
The first highly diastereo- and enantioselective addns. of aryl nitromethane pronucleophiles to aryl aldimines are described. Identification of an electron rich chiral Bis(AMidine) catalyst for this aza-Henry variant was key to this development, leading ultimately to differentially protected cis-stilbene diamines in two steps. This method then became the lynchpin for an enantioselective synthesis of (-)-Nutlin-3 (Hoffmann-La Roche), a potent cis-imidazoline small mol. inhibitor of p53-MDM2 used extensively as a probe of cell biol. and currently in drug development.
(q) Uraguchi, D.; Yoshioka, K.; Ueki, Y.; Ooi, T. J. Am. Chem. Soc. 2012, 134, 19370 DOI: 10.1021/ja310209g
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Highly Regio-, Diastereo-, and Enantioselective 1,6- and 1,8-Additions of Azlactones to Di- and Trienyl N-Acylpyrroles
Uraguchi, Daisuke; Yoshioka, Ken; Ueki, Yusuke; Ooi, Takashi
Journal of the American Chemical Society (2012), 134 (47), 19370-19373CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A vinylog of Michael addn. (1,6-addn.) of azlactones to δ-substituted dienyl N-acylpyrroles has been developed with virtually complete 1,6-, diastereo-, and enantioselectivities by means of chiral P-spiro triaminoiminophosphorane as a catalyst. This system has been successfully extended to an unprecedented bis-vinylog of Michael addn. (1,8-addn.) of azlactones to ζ-substituted trienyl N-acylpyrroles with high levels of regio- and stereocontrol.
(r) Corbett, M. T.; Uraguchi, D.; Ooi, T.; Johnson, J. S. Angew. Chem., Int. Ed. 2012, 51, 4685 DOI: 10.1002/anie.201200559
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(s) Bandar, J. S.; Lambert, T. H. J. Am. Chem. Soc. 2012, 134, 5552 DOI: 10.1021/ja3015764
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Enantioselective Bronsted Base Catalysis with Chiral Cyclopropenimines
Bandar, Jeffrey S.; Lambert, Tristan H.
Journal of the American Chemical Society (2012), 134 (12), 5552-5555CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Cyclopropenimines are a highly effective new class of enantioselective Broensted base catalysts. A chiral 2,3-bis(dialkylamino)cyclopropenimine catalyzes the rapid Michael reaction of a glycine imine substrate with high levels of enantioselectivity. A preparative scale reaction to deliver 25 g of product is demonstrated, and a trivial large scale synthesis of the optimal catalyst is shown. The basicity of a 2,3-bis(dialkylamino)cyclopropenimine is measured for the first time and is approx. equiv. to the P1-tBu phosphazene base. An x-ray crystal structure of the protonated catalyst is shown along with a proposed mechanistic and stereochem. rationale.
(t) Misaki, T.; Jin, N.; Kawano, K.; Sugimura, T. Chem. Lett. 2012, 41, 1675 DOI: 10.1246/cl.2012.1675
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(u) Shubina, T. E.; Freund, M.; Schenker, S.; Clark, T.; Tsogoeva, S. B. Beilstein J. Org. Chem. 2012, 8, 1485 DOI: 10.3762/bjoc.8.168
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Synthesis and evaluation of new guanidine-thiourea organocatalyst for the nitro-Michael reaction: theoretical studies on mechanism and enantioselectivity
Shubina, Tatyana E.; Freund, Matthias; Schenker, Sebastian; Clark, Timothy; Tsogoeva, Svetlana B.
Beilstein Journal of Organic Chemistry (2012), 8 (), 1485-1498, No. 168CODEN: BJOCBH; ISSN:1860-5397. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)
A new guanidine-thiourea organocatalyst has been developed and applied as bifunctional organocatalyst in the Michael addn. reaction of di-Et malonate to trans-β-nitrostyrene. Extensive DFT calcns., including solvent effects and dispersion corrections, as well as ab initio calcns. provide a plausible description of the reaction mechanism.
(v) Takeda, T.; Terada, M. J. Am. Chem. Soc. 2013, 135, 15306 DOI: 10.1021/ja408296h
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(w) Bandar, J. S.; Lambert, T. H. J. Am. Chem. Soc. 2013, 135, 11799 DOI: 10.1021/ja407277a
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Cyclopropenimine-Catalyzed Enantioselective Mannich Reactions of tert-Butyl Glycinates with N-Boc-Imines
Bandar, Jeffrey S.; Lambert, Tristan H.
Journal of the American Chemical Society (2013), 135 (32), 11799-11802CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Cyclopropenimine is shown to catalyze Mannich reactions between glycine imines and N-Boc-aldimines with high levels of enantio- and diastereocontrol. The reactivity of cyclopropenimine is shown to be substantially greater than that of a widely used thiourea cinchona alkaloid-derived catalyst. A variety of aryl and aliph. N-Boc-aldimines are effective substrates for this transformation. A preparative-scale reaction to deliver >90 mmol of product is shown using 1 mol % catalyst. The products of this transformation can be converted into several useful derivs.
(x) Goldys, A. M.; Núñez, M. G.; Dixon, D. J. Org. Lett. 2014, 16, 6294 DOI: 10.1021/ol5029942
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Structure-activity relationship studies of cyclopropenimines as enantioselective Bronsted base catalysts
Bandar, Jeffrey S.; Barthelme, Alexandre; Mazori, Alon Y.; Lambert, Tristan H.
Chemical Science (2015), 6 (2), 1537-1547CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A series of structure-activity relationship studies investigated that provided an enhanced understanding of the effectiveness of certain cyclopropenimines as enantioselective Bronsted base catalysts. These studies showed the crucial importance of dicyclohexylamino substituents in mediating both reaction rate and enantioselectivity. In addn., an unusual catalyst CH···O interaction, which provided both ground state and transition state organization, was discussed. Cyclopropenimine stability studies had led to the identification of new catalysts with greatly improved stability. Finally, addnl. demonstrations of substrate scope and current limitations were provided.
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Bifunctional Iminophosphorane Organocatalysts for Enantioselective Synthesis: Application to the Ketimine Nitro-Mannich Reaction
Nunez, Marta G.; Farley, Alistair J. M.; Dixon, Darren J.
Journal of the American Chemical Society (2013), 135 (44), 16348-16351CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The design, synthesis, and development of a new class of modular, strongly basic, and tunable bifunctional Bronsted base/H-bond-donor organocatalysts are reported. These catalysts incorporate a triaryliminophosphorane as the Bronsted basic moiety and are readily synthesized via a last step Staudinger reaction of a chiral organoazide and a triarylphosphine. Their application to the first general enantioselective organocatalytic nitro-Mannich reaction of nitromethane to unactivated ketone-derived imines allows the enantioselective construction of β-nitroamines possessing a fully substituted carbon atom [e.g., I-catalyzed nitro-Mannich reaction of ketenimine II with MeNO2 afforded III with 98% conversion and in 85% ee at room temp.]. The reaction is amenable to multigram scale-up, and the products are useful for the synthesis of enantiopure 1,2-diamine and α-amino acid derivs.
9
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Organocatalytic Carbon-Sulfur Bond-Forming Reactions
Chauhan, Pankaj; Mahajan, Suruchi; Enders, Dieter
Chemical Reviews (Washington, DC, United States) (2014), 114 (18), 8807-8864CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Organocatalysis, which uses small org. mols. to catalyze org. transformations, is a relatively new and rapidly growing field within the domain of catalytic asym. and nonenantioselective synthesis. The use of small org. mols. as catalysts offers several fundamental advantages over the metal- and biocatalysts, as they can easily be obtained from readily available materials, they are insensitive to air and moisture, they are robust, less toxic, and they can also provide both enantiomers of bioactive compds. such as drugs and natural products, with high enantioselectivity. The organocatalysts not only promote simple C-C, C-N, C-O, C-P, and C-S bond formations, but they also facilitate more complex domino/cascade reactions via multicomponent one-pot protocols. The organocatalytic C-C and carbon-heteroatom bond formations have been reviewed from time to time. Although several examples of organocatalyzed C-S bond formations existed in the literature even before the renaissance of organocatalysis in the year 2000 and a significant growth has been witnessed in recent years, however, this topic has not been reviewed so far in a general way.
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Pracejus, H.; Wilcke, F. W.; Hanemann, K.
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Leow, Dasheng; Lin, Shishi; Chittimalla, Santhosh Kumar; Fu, Xiao; Tan, Choon-Hong
Angewandte Chemie, International Edition (2008), 47 (30), 5641-5645CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
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The first example of a catalytic enantioselective addn. to and nitronate protonation of trisubstituted nitroalkenes R1R2C:CR3NO2 [R1 = R2 = Me; R1R2 = CH2OCH2, CH2NBocCH2, CH2CH2OCH2CH2, (CH2)5; R3 = Me, Et, i-Pr, PhCH2, MeO2CCH2CH2] to produce highly enantioenriched products R4C(O)SCR1R2CHR3NO2 (R4 = Me, Ph) with a tetrasubstituted carbon is reported. Thioacids R4C(O)SH added in excellent yields and with high enantioselectivities to both activated and unactivated nitroalkenes. The 1,2-nitrothioacetate products can be readily converted in two steps to biomedically relevant 1,2-aminosulfonic acids without loss of enantiopurity.
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16
For selected examples demonstrating the variation to and optimization of the amide group, see:
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Asym., catalytic reactions of oxocarbenium ions are reported. Simple, chiral urea and thiourea derivs., e.g. I, are shown to catalyze the enantioselective substitution of silyl ketene acetals, e.g. II [X = (CH2)n, n = 1, 2], onto 1-chloroisochromans III (R = H, 6-F, 6-MeO, 5-Me, etc.). A mechanism involving anion binding by the chiral catalyst to generate a reactive oxocarbenium ion is invoked. Catalysts bearing tertiary benzylic amide groups afforded highest enantioselectivities, with the optimal structure being derived from enantioenriched 2-arylpyrrolidine derivs.
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An exptl. and computational investigation of amido-thiourea promoted imine hydrocyanation has revealed a new and unexpected mechanism of catalysis. Rather than direct activation of the imine by the thiourea, as had been proposed previously in related systems, the data are consistent with a mechanism involving catalyst-promoted proton transfer from hydrogen isocyanide to imine to generate diastereomeric iminium/cyanide ion pairs that are bound to catalyst through multiple noncovalent interactions; these ion pairs collapse to form the enantiomeric α-aminonitrile products. This mechanistic proposal is supported by the observation of a statistically significant correlation between exptl. and calcd. enantioselectivities induced by eight different catalysts (P « 0.01). The computed models reveal a basis for enantioselectivity that involves multiple stabilizing and destabilizing interactions between substrate and catalyst, including thiourea-cyanide and amide-iminium interactions.
17
The reaction with PhSH proceeded in 90% yield and 27% ee.
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Highly active organocatalysts that allow very low catalyst loadings (<0.5 mol%) are still relatively rare. For a review, see:
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Low-loading asymmetric organocatalysis
Giacalone, Francesco; Gruttadauria, Michelangelo; Agrigento, Paola; Noto, Renato
Chemical Society Reviews (2012), 41 (6), 2406-2447CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
This crit. review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Bronsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, and phase-transfer and N-heterocyclic carbene catalyzes.

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Cite this: J. Am. Chem. Soc. 2015, 137, 51, 15992–15995

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https://doi.org/10.1021/jacs.5b10226

Published December 17, 2015

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Abstract
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Scheme 1
Scheme 1. Proposed BIMP Catalyzed Enantioselective Sulfa-Michael Addition to Methyl Methacrylate
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Figure 1
Figure 1. Bifunctional iminophosphorane (BIMP) organocatalysts screened for performance in the sulfa-Michael addition reaction. PMP = p-methoxyphenyl.
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Scheme 2
Scheme 2. Decagram Scale Sulfa-Michael Addition and Subsequent Derivatization^a
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Scheme aReagents and conditions: (a) catalyst 1m (0.05 mol %), MTBE, 55 °C, 24 h; (b) H₂O₂/TFA, rt, 4 h, >99% yield, 89% ee; (c) LiOH, THF/H₂O, rt, 2 h, 82% yield, 86% ee; (d) DIBAL-H, THF, −78 °C, 1 h, 64% yield, 90% ee; (e) benzylamine (3 equiv), neat, 0 °C, 12 h, 99% yield, 89% ee; (f) allylamine (3 equiv), neat, 0 °C, 2 h, 99% yield, 91% ee.
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  (d) Mohr, J. T.; Hong, A. Y.; Stoltz, B. M. Nat. Chem. 2009, 1, 359 DOI: 10.1038/nchem.297
  5d
  Enantioselective protonation
  Mohr, Justin T.; Hong, Allen Y.; Stoltz, Brian M.
  Nature Chemistry (2009), 1 (5), 359-369CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
  A review. Enantioselective protonation is a common process in biosynthetic sequences. The decarboxylase and esterase enzymes that effect this valuable transformation are able to control both the steric environment around the proton acceptor (typically an enolate) and the proton donor (typically a thiol). Recently, several chem. methods for achieving enantioselective protonation have been developed by exploiting various means of enantio-control in different mechanisms. These lab. transformations have proved useful for the prepn. of a no. of valuable org. compds. Here, recent reports of enantioselective protonations are reviewed, classifying them according to mechanism, with a discussion of how a deeper understanding of the processes can lead to improved methods for effecting this most fundamental method of obtaining enantiopure compds.
  (e) Leow, D.; Shen, J.; Su, Y.; Peh, G. Mini-Rev. Org. Chem. 2014, 11, 410 DOI: 10.2174/1570193X1104140926165654
  There is no corresponding record for this reference.
6. 6
  For metal catalyzed enantioselective conjugate additions to methyl methacrylate, see:
  (a) Belokon’, Y. N.; Kochetkov, K. A.; Churkina, T. y. D.; Ikonnikov, N. S.; Orlova, S. A.; Smirnov, V. V.; Chesnokov, A. A. Mendeleev Commun. 1997, 7, 137 DOI: 10.1070/MC1997v007n04ABEH000790
  There is no corresponding record for this reference.
  (b) Emori, E.; Arai, T.; Sasai, H.; Shibasaki, M. J. Am. Chem. Soc. 1998, 120, 4043 DOI: 10.1021/ja980397v
  There is no corresponding record for this reference.
  (c) Saito, S.; Tsubogo, T.; Kobayashi, S. J. Am. Chem. Soc. 2007, 129, 5364 DOI: 10.1021/ja0709730
  6c
  Chiral Calcium Complexes as Bronsted Base Catalysts for Asymmetric Addition of α-Amino Acid Derivatives to α,β-Unsaturated Carbonyl Compounds
  Saito, Susumu; Tsubogo, Tetsu; Kobayashi, Shu
  Journal of the American Chemical Society (2007), 129 (17), 5364-5365CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A novel catalyst system based on complexes of calcium which promote the catalytic asym. 1,4-addn. reactions and [3+2] cycloaddn. reactions of α-amino acid derivs. with α,β-unsatd. carbonyl compds. have been developed. The reactions proceeded smoothly in the presence of 5-10 mol % of the chiral calcium catalyst to afford the desired adducts in high yields with high diastereo- and enantioselectivities. A wide range of α,β-unsatd. esters and amides were applicable, and other glycine and even dl-alanine derivs. reacted with several α,β-unsatd. carbonyl compds. to afford the corresponding substituted pyrrolidine derivs. in high yields with excellent diastereo- and enantioselectivities. In the reactions with dl-alanine derivs., quaternary asym. carbons were constructed efficiently.
  (d) Tsubogo, T.; Saito, S.; Seki, K.; Yamashita, Y.; Kobayashi, S. J. Am. Chem. Soc. 2008, 130, 13321 DOI: 10.1021/ja8032058
  There is no corresponding record for this reference.
  (e) Wang, X.; Buchwald, S. L. J. Am. Chem. Soc. 2011, 133, 19080 DOI: 10.1021/ja2092689
  6e
  Rh-Catalyzed Asymmetric Hydroformylation of Functionalized 1,1-Disubstituted Olefins
  Wang, Xiao; Buchwald, Stephen L.
  Journal of the American Chemical Society (2011), 133 (47), 19080-19083CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The first method for the highly enantioselective rhodium-catalyzed hydroformylation of 1,1-disubstituted olefins has been developed. By employing either of the P-chirogenic phosphine ligands BenzP* I, and QuinoxP* II, linear aldehydes, e.g., III, with β-chirality can be prepd. in a highly enantioselective fashion with good chemo- and regioselectivities.
  (f) Maleev, V. I.; North, M.; Larionov, V. A.; Fedyanin, I. V.; Savel’yeva, T. F.; Moscalenko, M. A.; Smolyakov, A. F.; Belokon, Y. N. Adv. Synth. Catal. 2014, 356, 1803 DOI: 10.1002/adsc.201400091
  There is no corresponding record for this reference.
  (g) Filloux, C. M.; Rovis, T. J. Am. Chem. Soc. 2015, 137, 508 DOI: 10.1021/ja511445x
  There is no corresponding record for this reference.
  For organocatalyzed additions, see:
  (h) Wurz, N. E.; Daniliuc, C. G.; Glorius, F. Chem. - Eur. J. 2012, 18, 16297 DOI: 10.1002/chem.201202432
  6h
  Highly Enantioselective Intermolecular Stetter Reaction of Simple Acrylates: Synthesis of α-Chiral γ-Ketoesters
  Wurz, Nathalie E.; Daniliuc, Constantin G.; Glorius, Frank
  Chemistry - A European Journal (2012), 18 (51), 16297-16301CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A novel N-heterocyclic carbene (NHC) I was designed by combining an electron-rich 2,6-dimethoxy substituent and underestimated yet promising chiral motif. With this NHC in hand, a highly enantioselective intermol. Stetter reaction of aldehydes R1CHO (R1 = Ph, 4-ClC6H4, 2-FC6H4, PhCH2, 2-furyl, etc.) with simple acrylates, e.g. CH2:CR2CO2R3 (R2 = Me, Et, n-Bu, Ph, PhCH2, AcNH, etc.; R3 = Me, Et, i-Pr, n-Bu), yielding versatile chiral γ-keto esters, e.g. II, has been developed.
7. 7
  (a) Ishikawa, T. Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts; Wiley: New York, 2009.
  For a review on chiral organosuperbases, see:
  There is no corresponding record for this reference.
  (b) Ishikawa, T.; Kumamoto, T. Synthesis 2006, 2006, 737 DOI: 10.1055/s-2006-926325
  There is no corresponding record for this reference.
  (c) Leow, D.; Tan, C.-H. Chem. - Asian J. 2009, 4, 488 DOI: 10.1002/asia.200800361
  7c
  Chiral guanidine-catalyzed enantioselective reactions
  Leow, Dasheng; Tan, Choon-Hong
  Chemistry - An Asian Journal (2009), 4 (4), 488-507CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Chiral guanidine catalysts share common characteristics such as high pKa values and dual hydrogen bonding modes of activation, and high catalytic activities and enantioselectivities can often be achieved. The utilization of guanidines as catalysts was growing at a steady pace. In the past few years, it has attracted tremendous attention through several landmark achievements. This article highlights the development of chiral guanidine catalysis in asym. synthesis.
  (d) Leow, D.; Tan, C.-H. Synlett 2010, 2010, 1589 DOI: 10.1055/s-0029-1219937
  There is no corresponding record for this reference.
  (e) Ishikawa, T. Chem. Pharm. Bull. 2010, 58, 1555 DOI: 10.1248/cpb.58.1555
  There is no corresponding record for this reference.
  (f) Fu, X.; Tan, C.-H. Chem. Commun. 2011, 47, 8210 DOI: 10.1039/c0cc03691a
  There is no corresponding record for this reference.
  (g) Selig, P. Synthesis 2013, 45, 703 DOI: 10.1055/s-0032-1318154
  7g
  Guanidine organocatalysis
  Selig, Philipp
  Synthesis (2013), 45 (6), 703-718CODEN: SYNTBF; ISSN:0039-7881. (Georg Thieme Verlag)
  A review. An overview of the most commonly used guanidine organocatalysts and their applications in org. synthesis is presented. Privileged structures of open, monocyclic and bicyclic guanidines and guanidinium salts are showcased with prominent examples from the literature. Free guanidines have found widespread use as strong Bronsted base catalysts in asym. synthesis. Guanidinium salts are employed as weak Bronsted acids or hydrogen-bond-donor catalysts and chiral counterions. The nucleophilic and Lewis basic properties of guanidines are still rarely exploited, but, as of late, have been gaining increasing recognition.
  (h) Krawczyk, H.; Dzięgielewski, M.; Deredas, D.; Albrecht, A.; Albrecht, Ł. Chem. - Eur. J. 2015, 21, 10268 DOI: 10.1002/chem.201500481
  7h
  Chiral Iminophosphoranes-An Emerging Class of Superbase Organocatalysts
  Krawczyk, Henryk; Dziegielewski, Marek; Deredas, Dariusz; Albrecht, Anna; Albrecht, Lukasz
  Chemistry - A European Journal (2015), 21 (29), 10268-10277CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Chiral Bronsted base catalysis is a fascinating and highly explored field of research. For many years catalysts based on chincona alkaloid chiral scaffolds have constituted privileged systems widely employed in numerous base-promoted org. transformations. Recently, a novel group of chiral base catalysts has been successfully introduced. The application of organosuperbases, namely cyclopropenimines, guanidines, and iminophosphoranes, as chiral catalysts is receiving increasing attention. The aim of this Concept article is to summarize recent progress in the field of chiral iminophosphorane superbase organocatalysis. Catalysts design, different approaches to their synthesis, and applications in asym. synthesis are outlined and discussed in detail.
  For selected examples, see:
  (i) Corey, E. J.; Grogan, M. J. Org. Lett. 1999, 1, 157 DOI: 10.1021/ol990623l
  7i
  Enantioselective Synthesis of α-Amino Nitriles from N-Benzhydryl Imines and HCN with a Chiral Bicyclic Guanidine as Catalyst
  Corey, E. J.; Grogan, Michael J.
  Organic Letters (1999), 1 (1), 157-160CODEN: ORLEF7; ISSN:1523-7060. (American Chemical Society)
  A novel catalytic enantioselective Strecker synthesis of chiral α-amino nitriles and α-amino acids is described and analyzed with regard to the possible mechanistic basis for stereoselectivity. Key features of the enantioselective process include (1) the use of the chiral bicyclic guanidine I as catalyst and (2) the use of the N-benzhydryl substituent on the imine substrate.
  (j) Ishikawa, T.; Araki, Y.; Kumamoto, T.; Seki, H.; Fukuda, K.; Isobe, T. Chem. Commun. 2001, 245 DOI: 10.1039/b009193f
  There is no corresponding record for this reference.
  (k) Nugent, B. M.; Yoder, R. A.; Johnston, J. N. J. Am. Chem. Soc. 2004, 126, 3418 DOI: 10.1021/ja031906i
  7k
  Chiral Proton Catalysis: A Catalytic Enantioselective Direct Aza-Henry Reaction
  Nugent, Benjamin M.; Yoder, Ryan A.; Johnston, Jeffrey N.
  Journal of the American Chemical Society (2004), 126 (11), 3418-3419CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Nonracemic β-aryl-β-aminonitroalkanes I (R = H, Me; R1 = H, 2-O2N, 3-O2N, 4-O2N, 4-Cl, 4-F3C, 4-F3CO) are prepd. in 50-69% yields, 7:1-19:1 diastereoselectivities (for R = Me), and in 59-95% ee by the stereoselective aza-Henry reaction of nitroalkanes RCH2NO2 (R = H, Me) to the N-Boc imines RC6H4CH:NBoc (Boc = tert-butoxycarbonyl) in the presence of nonracemic di(quinolinylamino)cyclohexane triflic acid salt II•F3CSO3H. II is prepd. by amination of 2-chloroquinoline with (1R-trans)-1,2-cyclohexanediamine in the presence of Pd(dba)2, racemic BINAP, and sodium tert-butoxide; II•F3CSO3H is prepd. as a bench-stable white solid by addn. of triflic acid to II in methylene chloride. The free base II does not act as a catalyst for enantioselective Henry reactions in the absence of acid. II•F3CSO3H is proposed to act as a catalyst using polar ionic hydrogen bonds to accelerate the reaction while controlling its stereoselectivity; the catalyst is effective without either a Bronsted base additive or preactivation of the nitroalkane.
  (l) Terada, M.; Ube, H.; Yaguchi, Y. J. Am. Chem. Soc. 2006, 128, 1454 DOI: 10.1021/ja057848d
  7l
  Axially Chiral Guanidine as Enantioselective Base Catalyst for 1,4-Addition Reaction of 1,3-Dicarbonyl Compounds with Conjugated Nitroalkenes
  Terada, Masahiro; Ube, Hitoshi; Yaguchi, Yusuke
  Journal of the American Chemical Society (2006), 128 (5), 1454-1455CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A new strategy for designing chiral guanidine mols. I [R1 = Me, n-Pr; R2 = Ph, 4-Me3CC6H4, 3,5-(F3C)2C6H3, etc.] is presented, which features the introduction of an axially chiral binaphthyl backbone. The axially chiral guanidine catalysts thus developed facilitated the highly enantioselective 1,4-addn. reaction of 1,3-dicarbonyl compds. with a broad range of conjugated nitroalkenes and showed extremely high catalytic activity.
  (m) Uraguchi, D.; Sakaki, S.; Ooi, T. J. Am. Chem. Soc. 2007, 129, 12392 DOI: 10.1021/ja075152+
  7m
  Chiral Tetraaminophosphonium Salt-Mediated Asymmetric Direct Henry Reaction
  Uraguchi, Daisuke; Sakaki, Sawako; Ooi, Takashi
  Journal of the American Chemical Society (2007), 129 (41), 12392-12393CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Chiral tetraaminophosphonium salts I (R = Ph, m-xylyl, 4-tolyl, 4-F3CC6H4) possessing the phosphorus-centered [5.5]-spirocyclic core have been designed and synthesized in a single step from L-valine-derived diamine. The three-dimensional mol. structure was successfully verified by the single-crystal X-ray diffraction anal., which also identified a secondary interaction between the phosphonium cation and chloride ion via double hydrogen-bonding. The potential of this novel onium salt as a chiral org. mol. catalyst has been demonstrated in an application to asym. direct Henry reaction of R2CHO (R2 = Ph, 1-naphthyl, 1-octyl, etc.) with R1CH2NO2 (R1 = H, Me, Et).
  (n) Davis, T. A.; Wilt, J. C.; Johnston, J. N. J. Am. Chem. Soc. 2010, 132, 2880 DOI: 10.1021/ja908814h
  7n
  Bifunctional Asymmetric Catalysis: Amplification of Bronsted Basicity Can Orthogonally Increase the Reactivity of a Chiral Bronsted Acid
  Davis, Tyler A.; Wilt, Jeremy C.; Johnston, Jeffrey N.
  Journal of the American Chemical Society (2010), 132 (9), 2880-2882CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The reactivity of a series of sym. chiral Bronsted acids (polar ionic hydrogen-bond donors) follows the counterintuitive trend wherein the more Bronsted basic member is a more effective catalyst for the aza-Henry (nitro-Mannich) reaction. This new design element leads to a substantially more reactive catalyst for the aza-Henry reaction, one that can promote the addn. of a secondary nitroalkane. Addnl., when an achiral Bronsted acid (TfOH) is used in slight excess of the neutral, chiral bisamidine ligand, diastereoselection can be optimized to levels generally greater than 15:1 while the enantioselection remains unchanged at generally > 90% ee.
  (o) Sohtome, Y.; Shin, B.; Horitsugi, N.; Takagi, R.; Noguchi, K.; Nagasawa, K. Angew. Chem., Int. Ed. 2010, 49, 7299 DOI: 10.1002/anie.201003172
  7o
  Entropy-controlled catalytic asymmetric 1,4-type Friedel-Crafts reaction of phenols using conformationally flexible guanidine/bisthiourea organocatalyst
  Sohtome, Yoshihiro; Shin, Bongki; Horitsugi, Natsuko; Takagi, Rika; Noguchi, Keiichi; Nagasawa, Kazuo
  Angewandte Chemie, International Edition (2010), 49 (40), 7299-7303, S7299/1-S7299/55CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Conformationally flexible org. compds. were found to promote the title transformation. These soft organocatalysts, which are able to control processes through the differential activation entropies (ΔΔS⧺S-R) of the reactive intermediates, lead to high stereoselectivities without the requirement of fine-tuning the reaction temps.
  (p) Davis, T. A.; Johnston, J. N. Chem. Sci. 2011, 2, 1076 DOI: 10.1039/c1sc00061f
  7p
  Catalytic, enantioselective synthesis of stilbene cis-diamines: A concise preparation of (-)-Nutlin-3, a potent p53/MDM2 inhibitor
  Davis, Tyler A.; Johnston, Jeffrey N.
  Chemical Science (2011), 2 (6), 1076-1079CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  The first highly diastereo- and enantioselective addns. of aryl nitromethane pronucleophiles to aryl aldimines are described. Identification of an electron rich chiral Bis(AMidine) catalyst for this aza-Henry variant was key to this development, leading ultimately to differentially protected cis-stilbene diamines in two steps. This method then became the lynchpin for an enantioselective synthesis of (-)-Nutlin-3 (Hoffmann-La Roche), a potent cis-imidazoline small mol. inhibitor of p53-MDM2 used extensively as a probe of cell biol. and currently in drug development.
  (q) Uraguchi, D.; Yoshioka, K.; Ueki, Y.; Ooi, T. J. Am. Chem. Soc. 2012, 134, 19370 DOI: 10.1021/ja310209g
  7q
  Highly Regio-, Diastereo-, and Enantioselective 1,6- and 1,8-Additions of Azlactones to Di- and Trienyl N-Acylpyrroles
  Uraguchi, Daisuke; Yoshioka, Ken; Ueki, Yusuke; Ooi, Takashi
  Journal of the American Chemical Society (2012), 134 (47), 19370-19373CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A vinylog of Michael addn. (1,6-addn.) of azlactones to δ-substituted dienyl N-acylpyrroles has been developed with virtually complete 1,6-, diastereo-, and enantioselectivities by means of chiral P-spiro triaminoiminophosphorane as a catalyst. This system has been successfully extended to an unprecedented bis-vinylog of Michael addn. (1,8-addn.) of azlactones to ζ-substituted trienyl N-acylpyrroles with high levels of regio- and stereocontrol.
  (r) Corbett, M. T.; Uraguchi, D.; Ooi, T.; Johnson, J. S. Angew. Chem., Int. Ed. 2012, 51, 4685 DOI: 10.1002/anie.201200559
  There is no corresponding record for this reference.
  (s) Bandar, J. S.; Lambert, T. H. J. Am. Chem. Soc. 2012, 134, 5552 DOI: 10.1021/ja3015764
  7s
  Enantioselective Bronsted Base Catalysis with Chiral Cyclopropenimines
  Bandar, Jeffrey S.; Lambert, Tristan H.
  Journal of the American Chemical Society (2012), 134 (12), 5552-5555CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Cyclopropenimines are a highly effective new class of enantioselective Broensted base catalysts. A chiral 2,3-bis(dialkylamino)cyclopropenimine catalyzes the rapid Michael reaction of a glycine imine substrate with high levels of enantioselectivity. A preparative scale reaction to deliver 25 g of product is demonstrated, and a trivial large scale synthesis of the optimal catalyst is shown. The basicity of a 2,3-bis(dialkylamino)cyclopropenimine is measured for the first time and is approx. equiv. to the P1-tBu phosphazene base. An x-ray crystal structure of the protonated catalyst is shown along with a proposed mechanistic and stereochem. rationale.
  (t) Misaki, T.; Jin, N.; Kawano, K.; Sugimura, T. Chem. Lett. 2012, 41, 1675 DOI: 10.1246/cl.2012.1675
  There is no corresponding record for this reference.
  (u) Shubina, T. E.; Freund, M.; Schenker, S.; Clark, T.; Tsogoeva, S. B. Beilstein J. Org. Chem. 2012, 8, 1485 DOI: 10.3762/bjoc.8.168
  7u
  Synthesis and evaluation of new guanidine-thiourea organocatalyst for the nitro-Michael reaction: theoretical studies on mechanism and enantioselectivity
  Shubina, Tatyana E.; Freund, Matthias; Schenker, Sebastian; Clark, Timothy; Tsogoeva, Svetlana B.
  Beilstein Journal of Organic Chemistry (2012), 8 (), 1485-1498, No. 168CODEN: BJOCBH; ISSN:1860-5397. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)
  A new guanidine-thiourea organocatalyst has been developed and applied as bifunctional organocatalyst in the Michael addn. reaction of di-Et malonate to trans-β-nitrostyrene. Extensive DFT calcns., including solvent effects and dispersion corrections, as well as ab initio calcns. provide a plausible description of the reaction mechanism.
  (v) Takeda, T.; Terada, M. J. Am. Chem. Soc. 2013, 135, 15306 DOI: 10.1021/ja408296h
  There is no corresponding record for this reference.
  (w) Bandar, J. S.; Lambert, T. H. J. Am. Chem. Soc. 2013, 135, 11799 DOI: 10.1021/ja407277a
  7w
  Cyclopropenimine-Catalyzed Enantioselective Mannich Reactions of tert-Butyl Glycinates with N-Boc-Imines
  Bandar, Jeffrey S.; Lambert, Tristan H.
  Journal of the American Chemical Society (2013), 135 (32), 11799-11802CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Cyclopropenimine is shown to catalyze Mannich reactions between glycine imines and N-Boc-aldimines with high levels of enantio- and diastereocontrol. The reactivity of cyclopropenimine is shown to be substantially greater than that of a widely used thiourea cinchona alkaloid-derived catalyst. A variety of aryl and aliph. N-Boc-aldimines are effective substrates for this transformation. A preparative-scale reaction to deliver >90 mmol of product is shown using 1 mol % catalyst. The products of this transformation can be converted into several useful derivs.
  (x) Goldys, A. M.; Núñez, M. G.; Dixon, D. J. Org. Lett. 2014, 16, 6294 DOI: 10.1021/ol5029942
  There is no corresponding record for this reference.
  (y) Bandar, J. S.; Barthelme, A.; Mazori, A. Y.; Lambert, T. H. Chem. Sci. 2015, 6, 1537 DOI: 10.1039/C4SC02402H
  7y
  Structure-activity relationship studies of cyclopropenimines as enantioselective Bronsted base catalysts
  Bandar, Jeffrey S.; Barthelme, Alexandre; Mazori, Alon Y.; Lambert, Tristan H.
  Chemical Science (2015), 6 (2), 1537-1547CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A series of structure-activity relationship studies investigated that provided an enhanced understanding of the effectiveness of certain cyclopropenimines as enantioselective Bronsted base catalysts. These studies showed the crucial importance of dicyclohexylamino substituents in mediating both reaction rate and enantioselectivity. In addn., an unusual catalyst CH···O interaction, which provided both ground state and transition state organization, was discussed. Cyclopropenimine stability studies had led to the identification of new catalysts with greatly improved stability. Finally, addnl. demonstrations of substrate scope and current limitations were provided.
  (z) Uraguchi, D.; Yamada, K.; Ooi, T. Angew. Chem., Int. Ed. 2015, 54, 9954 DOI: 10.1002/anie.201503928
  There is no corresponding record for this reference.
  (aa) Işik, M.; Unver, M. Y.; Tanyeli, C. J. Org. Chem. 2015, 80, 828 DOI: 10.1021/jo5022597
  There is no corresponding record for this reference.
  (ab) Gao, X.; Han, J.; Wang, L. Org. Lett. 2015, 17, 4596 DOI: 10.1021/acs.orglett.5b02323
  There is no corresponding record for this reference.
8. 8
  Núñez, M. G.; Farley, A. J. M.; Dixon, D. J. J. Am. Chem. Soc. 2013, 135, 16348 DOI: 10.1021/ja409121s
  8
  Bifunctional Iminophosphorane Organocatalysts for Enantioselective Synthesis: Application to the Ketimine Nitro-Mannich Reaction
  Nunez, Marta G.; Farley, Alistair J. M.; Dixon, Darren J.
  Journal of the American Chemical Society (2013), 135 (44), 16348-16351CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The design, synthesis, and development of a new class of modular, strongly basic, and tunable bifunctional Bronsted base/H-bond-donor organocatalysts are reported. These catalysts incorporate a triaryliminophosphorane as the Bronsted basic moiety and are readily synthesized via a last step Staudinger reaction of a chiral organoazide and a triarylphosphine. Their application to the first general enantioselective organocatalytic nitro-Mannich reaction of nitromethane to unactivated ketone-derived imines allows the enantioselective construction of β-nitroamines possessing a fully substituted carbon atom [e.g., I-catalyzed nitro-Mannich reaction of ketenimine II with MeNO2 afforded III with 98% conversion and in 85% ee at room temp.]. The reaction is amenable to multigram scale-up, and the products are useful for the synthesis of enantiopure 1,2-diamine and α-amino acid derivs.
9. 9
  Pahadi, N. K.; Ube, H.; Terada, M. Tetrahedron Lett. 2007, 48, 8700 DOI: 10.1016/j.tetlet.2007.10.016
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10. 10
  For a review on asymmetric sulfa-Michael additions, see:
  (a) Enders, D.; Lüttgen, K.; Narine, A. A. Synthesis 2007, 2007, 959 DOI: 10.1055/s-2007-965968
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  For selected examples using metals, see:
  (b) Nishimura, K.; Ono, M.; Nagaoka, Y.; Tomioka, K. J. Am. Chem. Soc. 1997, 119, 12974 DOI: 10.1021/ja9729950
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  (c) Kanemasa, S.; Oderaotoshi, Y.; Wada, E. J. Am. Chem. Soc. 1999, 121, 8675 DOI: 10.1021/ja991064g
  There is no corresponding record for this reference.
  (d) Nishimura, K.; Ono, M.; Nagaoka, Y.; Tomioka, K. Angew. Chem., Int. Ed. 2001, 40, 440 DOI: 10.1002/1521-3773(20010119)40:2<440::AID-ANIE440>3.0.CO;2-A
  There is no corresponding record for this reference.
  (e) Hui, Y.; Jiang, J.; Wang, W.; Chen, W.; Cai, Y.; Lin, L.; Liu, X.; Feng, X. Angew. Chem., Int. Ed. 2010, 49, 4290 DOI: 10.1002/anie.201000105
  There is no corresponding record for this reference.
  (f) Bonollo, S.; Lanari, D.; Pizzo, F.; Vaccaro, L. Org. Lett. 2011, 13, 2150 DOI: 10.1021/ol200379r
  There is no corresponding record for this reference.
  (g) Kitanosono, T.; Sakai, M.; Ueno, M.; Kobayashi, S. Org. Biomol. Chem. 2012, 10, 7134 DOI: 10.1039/c2ob26264a
  There is no corresponding record for this reference.
  (h) Ogawa, T.; Kumagai, N.; Shibasaki, M. Angew. Chem., Int. Ed. 2012, 51, 8551 DOI: 10.1002/anie.201204365
  There is no corresponding record for this reference.
  See also ref 6b.
11. 11
  For a review on organocatalytic asymmetric SMA reactions, see:
  (a) Chauhan, P.; Mahajan, S.; Enders, D. Chem. Rev. 2014, 114, 8807 DOI: 10.1021/cr500235v
  11a
  Organocatalytic Carbon-Sulfur Bond-Forming Reactions
  Chauhan, Pankaj; Mahajan, Suruchi; Enders, Dieter
  Chemical Reviews (Washington, DC, United States) (2014), 114 (18), 8807-8864CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Organocatalysis, which uses small org. mols. to catalyze org. transformations, is a relatively new and rapidly growing field within the domain of catalytic asym. and nonenantioselective synthesis. The use of small org. mols. as catalysts offers several fundamental advantages over the metal- and biocatalysts, as they can easily be obtained from readily available materials, they are insensitive to air and moisture, they are robust, less toxic, and they can also provide both enantiomers of bioactive compds. such as drugs and natural products, with high enantioselectivity. The organocatalysts not only promote simple C-C, C-N, C-O, C-P, and C-S bond formations, but they also facilitate more complex domino/cascade reactions via multicomponent one-pot protocols. The organocatalytic C-C and carbon-heteroatom bond formations have been reviewed from time to time. Although several examples of organocatalyzed C-S bond formations existed in the literature even before the renaissance of organocatalysis in the year 2000 and a significant growth has been witnessed in recent years, however, this topic has not been reviewed so far in a general way.
  For selected examples, see:
  (b) Pracejus, H.; Wilcke, F. W.; Hanemann, K. J. Prakt. Chem. 1977, 319, 219 DOI: 10.1002/prac.19773190208
  11b
  Asymmetrically catalyzed additions of thiols to derivatives of α-aminoacrylic acid and nitroolefins
  Pracejus, H.; Wilcke, F. W.; Hanemann, K.
  Journal fuer Praktische Chemie (Leipzig) (1977), 319 (2), 219-29CODEN: JPCEAO; ISSN:0021-8383.
  Optically-active cysteine derivs. in optical yields ≤54% were prepd. by the asym. addn. of thiols RSH (R = PhCH2, Ph2CH, Ph3C) to Me α-phthalimidoacrylate or 2-ethyl-4-methylene-1,3-oxazolin-5-one in the presence of optically-active base catalysts, preferably cinchona alkaloids. Similar asym. addn. of PhCR2SH (R = H or Ph) to PhCH:CR1NO2 (R = H or Me) was also effected.
  (c) Hiemstra, H.; Wynberg, H. J. Am. Chem. Soc. 1981, 103, 417 DOI: 10.1021/ja00392a029
  There is no corresponding record for this reference.
  (d) Kumar, A.; Salunkhe, R. V.; Rane, R. A.; Dike, S. Y. J. Chem. Soc., Chem. Commun. 1991, 485 DOI: 10.1039/c39910000485
  There is no corresponding record for this reference.
  (e) McDaid, P.; Chen, Y.; Deng, L. Angew. Chem., Int. Ed. 2002, 41, 338 DOI: 10.1002/1521-3773(20020118)41:2<338::AID-ANIE338>3.0.CO;2-M
  There is no corresponding record for this reference.
  (f) Li, B.-J.; Jiang, L.; Liu, M.; Chen, Y.-C.; Ding, L.-S.; Wu, Y. Synlett 2005, 603 DOI: 10.1055/s-2005-863710
  There is no corresponding record for this reference.
  (g) Leow, D.; Shishi, L.; Chittimalla, S. K.; Fu, X.; Tan, C.-H. Angew. Chem., Int. Ed. 2008, 47, 5641 DOI: 10.1002/anie.200801378
  11g
  Enantioselective protonation catalyzed by a chiral bicyclic guanidine derivative
  Leow, Dasheng; Lin, Shishi; Chittimalla, Santhosh Kumar; Fu, Xiao; Tan, Choon-Hong
  Angewandte Chemie, International Edition (2008), 47 (30), 5641-5645CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The guanidine deriv. I catalyzes a tandem conjugate addn.-enantioselective protonation reaction of phthalimidoacrylates with thiols and itaconimides with phosphine oxides. Optically pure analogs of cysteine and cystine were obtained in this way. In highly enantioselective deuteration reactions, a small but significant kinetic isotope effect was obsd.
  (h) Liu, Y.; Sun, B.; Wang, B.; Wakem, M.; Deng, L. J. Am. Chem. Soc. 2009, 131, 418 DOI: 10.1021/ja8085092
  There is no corresponding record for this reference.
  (i) Kimmel, K. L.; Robak, M. T.; Ellman, J. A. J. Am. Chem. Soc. 2009, 131, 8754 DOI: 10.1021/ja903351a
  There is no corresponding record for this reference.
  (j) Rana, N. K.; Selvakumar, S.; Singh, V. K. J. Org. Chem. 2010, 75, 2089 DOI: 10.1021/jo902634a
  There is no corresponding record for this reference.
  (k) Dai, L.; Wang, S.-X.; Chen, F.-E. Adv. Synth. Catal. 2010, 352, 2137 DOI: 10.1002/adsc.201000334
  There is no corresponding record for this reference.
  (l) Dai, L.; Yang, H.; Chen, F. Eur. J. Org. Chem. 2011, 2011, 5071 DOI: 10.1002/ejoc.201100403
  There is no corresponding record for this reference.
  (m) Rana, N. K.; Singh, V. K. Org. Lett. 2011, 13, 6520 DOI: 10.1021/ol202808n
  There is no corresponding record for this reference.
  (n) Palacio, C.; Connon, S. J. Chem. Commun. 2012, 48, 2849 DOI: 10.1039/c2cc17965b
  There is no corresponding record for this reference.
  (o) Dai, L.; Yang, H.; Niu, J.; Chen, F. Synlett 2012, 2012, 314 DOI: 10.1055/s-0031-1290113
  There is no corresponding record for this reference.
  (p) Uraguchi, D.; Kinoshita, N.; Nakashima, D.; Ooi, T. Chem. Sci. 2012, 3, 3161 DOI: 10.1039/c2sc20698f
  11p
  Chiral ionic Bronsted acid-achiral Bronsted base synergistic catalysis for asymmetric sulfa-Michael addition to nitroolefins
  Uraguchi, Daisuke; Kinoshita, Natsuko; Nakashima, Daisuke; Ooi, Takashi
  Chemical Science (2012), 3 (11), 3161-3164CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A highly enantioselective sulfa-Michael addn. to arom. and aliph. nitro olefins is reported by a synergistic catalysis of a chiral ionic Bronsted acid homo-1b·HBArF and 2,6-lutidine. The potential utility of this new method is clearly demonstrated by its application to the syntheses of a novel, optically active taurine deriv. and β-sultam. The synthesis of the target compds. was achieved using a spirocyclic phosphonium compd. (I) as most efficient catalyst.
  (q) Breman, A. C.; Smits, J. M. M.; de Gelder, R.; van Maarseveen, J. H.; Ingemann, S.; Hiemstra, H. Synlett 2012, 23, 2195 DOI: 10.1055/s-0032-1317081
  There is no corresponding record for this reference.
  (r) Unhale, R. A.; Rana, N. K.; Singh, V. K. Tetrahedron Lett. 2013, 54, 1911 DOI: 10.1016/j.tetlet.2013.01.004
  11r
  Organocatalytic enantioselective transient enolate protonation in conjugate addition of thioacetic acid to α-substituted N-acryloyloxazolidinones
  Unhale, Rajshekhar A.; Rana, Nirmal K.; Singh, Vinod K.
  Tetrahedron Letters (2013), 54 (15), 1911-1915CODEN: TELEAY; ISSN:0040-4039. (Elsevier Ltd.)
  Organocatalytic conjugate addn. of thioacetic acid to a series of α-substituted N-acryloyloxazolidin-2-ones followed by enantioselective protonation has been studied in the presence of thiourea catalysts derived from cinchona alkaloids. Conjugate addn./protonation adducts have been obtained up to 97% ee and high yields. The methodol. could serve as an easy and practical route to the syntheses of useful biol. active mols.
  (s) Phelan, J. P.; Patel, E. J.; Ellman, J. A. Angew. Chem., Int. Ed. 2014, 53, 11329 DOI: 10.1002/anie.201406971
  11s
  Catalytic Enantioselective Addition of Thioacids to Trisubstituted Nitroalkenes
  Phelan, James P.; Patel, Evan J.; Ellman, Jonathan A.
  Angewandte Chemie, International Edition (2014), 53 (42), 11329-11332CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The first example of a catalytic enantioselective addn. to and nitronate protonation of trisubstituted nitroalkenes R1R2C:CR3NO2 [R1 = R2 = Me; R1R2 = CH2OCH2, CH2NBocCH2, CH2CH2OCH2CH2, (CH2)5; R3 = Me, Et, i-Pr, PhCH2, MeO2CCH2CH2] to produce highly enantioenriched products R4C(O)SCR1R2CHR3NO2 (R4 = Me, Ph) with a tetrasubstituted carbon is reported. Thioacids R4C(O)SH added in excellent yields and with high enantioselectivities to both activated and unactivated nitroalkenes. The 1,2-nitrothioacetate products can be readily converted in two steps to biomedically relevant 1,2-aminosulfonic acids without loss of enantiopurity.
  (t) Wang, R.; Liu, J.; Xu, J. Adv. Synth. Catal. 2015, 357, 159 DOI: 10.1002/adsc.201400664
  11t
  Organocatalytic Enantioselective Sulfur-Michael Addition of Thioacetic Acid to Arylmethylidenemalonates
  Wang, Renchao; Liu, Jing; Xu, Jiaxi
  Advanced Synthesis & Catalysis (2015), 357 (1), 159-167CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)
  An organocatalytic enantioselective sulfur-Michael addn. of thioacetic acid to arylmethylidenemalonates was developed with high yields and up to 97% enantiomeric excess. Both enantiomers of the products were accessible with two different organocatalysts, I and II. The current method provides the first, practical, and convenient prepn. of enantiomerically enriched acetylthiomethylmalonate derivs.
  (u) Fu, N. K.; Zhang, L.; Luo, S. Z.; Cheng, J. P. Org. Lett. 2014, 16, 4626 DOI: 10.1021/ol5022178
  There is no corresponding record for this reference.
12. 12
  Bordwell, F. G.; Hughes, D. L. J. Org. Chem. 1982, 47, 3224 DOI: 10.1021/jo00138a005
  There is no corresponding record for this reference.
13. 13
  The conversion to 4a was <5% after 14 days using a cinchonine derived bifunctional thiourea catalyst [890044-38-9].
  There is no corresponding record for this reference.
14. 14
  (a) Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 4901 DOI: 10.1021/ja980139y
  14a
  Schiff Base Catalysts for the Asymmetric Strecker Reaction Identified and Optimized from Parallel Synthetic Libraries
  Sigman, Matthew S.; Jacobsen, Eric N.
  Journal of the American Chemical Society (1998), 120 (19), 4901-4902CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  This paper outlines the application of parallel combinatorial library synthesis to the discovery and optimization of a chiral catalyst for the formal addn. of hydrogen cyanide to imines (the Strecker reaction). Through an iterative sequence involving the prepn. and evaluation of 3 solid-phase libraries I [P = polystyrene resin support; R1 = (S)-CH2CHMe2, (R)-CH2CHMe2, (S)-4-imidazolylmethyl, (S)-Ph,, (S)-CMe3; R2 = Ph; R2R2 = (CH2)4, CHN(CH2Ph)CH2; R3 = H, Br, CMe3; R4 = H, Br, OMe, NO2, CMe3; X = O, S] contg. a total of 192 compds., optimization of reaction enantioselectivity was achieved from an initial lead result of 19% ee up to 91% ee. The catalyst identified through optimization for the hydrocyanation of N-allylbenzaldimine proved effective for a range of imine substrates. In particular, >80% ee was achieved for the first time with any catalyst system for the Strecker reaction of aliph. imines. The structural features that lead to high enantioselectivity are quite unanticipated, with nonintuitive synergistic effects displayed between catalyst components.
  (b) Lalonde, M. P.; Chen, Y.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2006, 45, 6366 DOI: 10.1002/anie.200602221
  There is no corresponding record for this reference.
  For an example of a tertiary amine Brønsted base bifunctional organocatalyst incorporating the amide-thiourea moiety, see:
  (c) Berkessel, A.; Mukherjee, S.; Cleemann, F.; Müller, T. N.; Lex, J. Chem. Commun. 2005, 1898 DOI: 10.1039/b418666d
  There is no corresponding record for this reference.
15. 15
  Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am. Chem. Soc. 2003, 125, 12672 DOI: 10.1021/ja036972z
  15
  Enantioselective Michael Reaction of Malonates to Nitroolefins Catalyzed by Bifunctional Organocatalysts
  Okino, Tomotaka; Hoashi, Yasutaka; Takemoto, Yoshiji
  Journal of the American Chemical Society (2003), 125 (42), 12672-12673CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Michael reaction of malonates to nitroolefins with chiral bifunctional organocatalysts, bearing both a thiourea and tertiary amino group, e.g., I, afforded Michael adducts with high yields and enantioselectivities (up to 95%, up to 93% ee).
16. 16
  For selected examples demonstrating the variation to and optimization of the amide group, see:
  (a) Vachal, P.; Jacobsen, E. N. J. Am. Chem. Soc. 2002, 124, 10012 DOI: 10.1021/ja027246j
  There is no corresponding record for this reference.
  (b) Berkessel, A.; Mukherjee, S.; Müller, T. N.; Cleemann, F.; Roland, K.; Brandenburg, M.; Neudörfl, J.-M.; Lex, J. Org. Biomol. Chem. 2006, 4, 4319 DOI: 10.1039/b607574f
  There is no corresponding record for this reference.
  (c) Reisman, S. E.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2008, 130, 7198 DOI: 10.1021/ja801514m
  16c
  Enantioselective Thiourea-Catalyzed Additions to Oxocarbenium Ions
  Reisman, Sarah E.; Doyle, Abigail G.; Jacobsen, Eric N.
  Journal of the American Chemical Society (2008), 130 (23), 7198-7199CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Asym., catalytic reactions of oxocarbenium ions are reported. Simple, chiral urea and thiourea derivs., e.g. I, are shown to catalyze the enantioselective substitution of silyl ketene acetals, e.g. II [X = (CH2)n, n = 1, 2], onto 1-chloroisochromans III (R = H, 6-F, 6-MeO, 5-Me, etc.). A mechanism involving anion binding by the chiral catalyst to generate a reactive oxocarbenium ion is invoked. Catalysts bearing tertiary benzylic amide groups afforded highest enantioselectivities, with the optimal structure being derived from enantioenriched 2-arylpyrrolidine derivs.
  (d) Zuend, S. J.; Jacobsen, E. N. J. Am. Chem. Soc. 2009, 131, 15358 DOI: 10.1021/ja9058958
  16d
  Mechanism of Amido-Thiourea Catalyzed Enantioselective Imine Hydrocyanation: Transition State Stabilization via Multiple Non-Covalent Interactions
  Zuend, Stephan J.; Jacobsen, Eric N.
  Journal of the American Chemical Society (2009), 131 (42), 15358-15374CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  An exptl. and computational investigation of amido-thiourea promoted imine hydrocyanation has revealed a new and unexpected mechanism of catalysis. Rather than direct activation of the imine by the thiourea, as had been proposed previously in related systems, the data are consistent with a mechanism involving catalyst-promoted proton transfer from hydrogen isocyanide to imine to generate diastereomeric iminium/cyanide ion pairs that are bound to catalyst through multiple noncovalent interactions; these ion pairs collapse to form the enantiomeric α-aminonitrile products. This mechanistic proposal is supported by the observation of a statistically significant correlation between exptl. and calcd. enantioselectivities induced by eight different catalysts (P « 0.01). The computed models reveal a basis for enantioselectivity that involves multiple stabilizing and destabilizing interactions between substrate and catalyst, including thiourea-cyanide and amide-iminium interactions.
17. 17
  The reaction with PhSH proceeded in 90% yield and 27% ee.
  There is no corresponding record for this reference.
18. 18
  Highly active organocatalysts that allow very low catalyst loadings (<0.5 mol%) are still relatively rare. For a review, see:
  Giacalone, F.; Gruttadauria, M.; Agrigento, P.; Noto, R. Chem. Soc. Rev. 2012, 41, 2406 DOI: 10.1039/C1CS15206H
  18
  Low-loading asymmetric organocatalysis
  Giacalone, Francesco; Gruttadauria, Michelangelo; Agrigento, Paola; Noto, Renato
  Chemical Society Reviews (2012), 41 (6), 2406-2447CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  This crit. review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Bronsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, and phase-transfer and N-heterocyclic carbene catalyzes.
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Bifunctional Iminophosphorane Catalyzed Enantioselective Sulfa-Michael Addition to Unactivated α-Substituted Acrylate Esters
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Bifunctional Iminophosphorane Catalyzed Enantioselective Sulfa-Michael Addition to Unactivated α-Substituted Acrylate Esters
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