Intramolecular Activation of C–O Bond by an o-Boryl Group in o-(Alkoxysilyl)(diarylboryl)benzenes
- Tomomi ShimizuTomomi ShimizuDepartment of Applied Chemistry, Graduate School of Science and Engineering, Hosei University, 3-7-2 Kajino-cho, Koganei, Tokyo 184-8584, JapanMore by Tomomi Shimizu,
- Shogo MorisakoShogo MorisakoDepartment of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, JapanMore by Shogo Morisako,
- Yohsuke YamamotoYohsuke YamamotoDepartment of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, JapanMore by Yohsuke Yamamoto, and
- Atsushi Kawachi*Atsushi Kawachi*E-mail: [email protected]Department of Chemical Science and Technology, Faculty of Bioscience and Applied Chemistry, Hosei University, 3-7-2 Kajino-cho, Koganei, Tokyo 184-8584, JapanMore by Atsushi Kawachi
Copyright © 2019 American Chemical Society
Abstract
Halogen–lithium exchange reaction of o-(silyl)bromobenzene 5 with tert-BuLi afforded o-(silyl)lithiobenzene 6, which was reacted with (alkoxy)diarylboranes 7 to form borate intermediates 8. Treatment of 8 with chlorotrimethylsilane formed o-(alkoxysilyl)(diarylboryl)benzenes 4. The C–O bond in 4 was activated by intramolecular interaction between the oxygen atom and the boron atom. 4a readily reacted with MeOH and EtOH to afford the corresponding alkoxysilanes 10 and 11, respectively. Treatment of 10 with 1,4-diazabicyclo[2.2.2]octane (DABCO) afforded the silyloxyborate complex 13.
Introduction
ARTICLE SECTIONS
Lewis acids have been recognized as useful catalysts in synthetic organic chemistry as well as main group chemistry. For example, Lewis acids such as BF3, AlCl3, and SnCl4 activate the C–O bond in carbonyls, ethers, and epoxides by coordinating to the oxygen atom and lead to its cleavage.(1) In recent years, arylboranes have attracted much attention as Lewis acids. Piers and Oestreich reported that B(C6F5)3 can activate the Si–H bond in the hydrosilylation of aromatic aldehydes, ketones, and esters.(2,3) In contrast to the well-studied Si–H bond activation, the silicon–heteroatom bond activation has been less investigated. Wrackmeyer reported that 2-boryl-1-(aminosilyl)alkene (I) readily reacted with nucleophiles to afford 2-boryl-1-(alkoxysilyl)alkene (II) owing to the intramolecular Si–N bond activation by the boryl group (Scheme 1).(4)
Scheme 1
Scheme 1. Intramolecular Si–N Bond Activation
Recently, we synthesized o-(hydrosilyl)(diarylboryl)benzene, 1, in which the dimesitylboryl and hydrosilyl groups were linked through an o-phenylene skeleton.(5) The Si–H bond was activated by the boryl group for several types of reactions (Scheme 2): (i) dehydrogenative condensation with alcohols to form 2 (R = Me (a); i-Pr (b)),(5a) (ii) nucleophilic displacement by a fluoride ion to form 3,(5b) and (iii) H-Mes exchange between the hydrogen atom on the Si atom and mesityl (Mes) group on the boron atom.(5c)
Scheme 2
Scheme 2. Si–H Bond Activation by an o-Boryl Group in 1
Herein, we report the preparation of o-(alkoxysilyl)(boryl)benzenes bearing less sterically demanding aryl groups (Ar = p-tolyl, p-t-butylphenyl) on the boron atom, and the activation of C–O bond by diarylboryl groups.
Results and Discussion
ARTICLE SECTIONS
o-[(Isopropoxy)dimethylsilyl](diarylboryl)benzenes 4 were prepared as shown in Scheme 3. The Br–Li exchange reaction of o-(dimethylsilyl)bromobenzene (5) with n-BuLi produced o-silyl(lithio)benzene 6,(5a) which reacted with diaryl(isopropoxy)borane 7 to form lithium [(isopropoxysilyl)phenyl]hydroborate 8 in 56% yield. The 11B nuclear magnetic resonance (NMR) spectrum of 8 showed a doublet at δ = −9.7 because of the coupling of boron to one hydride (1JB–H = 66 Hz) in the typical region for tetracoordinate borates. The 29Si NMR spectra of 8 was observed as a singlet at δ = 13.4. It is plausible that the initially formed lithium [(hydrosilyl)phenyl](isopropoxy)borate 9 underwent intramolecular hydride–isopropoxide exchange to form 8. Treatment of 8 with chlorotrimethylsilane in situ afforded o-[(isopropoxy)silyl](diarylboryl)benzenes 4.(6) Compound 4a was isolated by distillation as a colorless oil in 70% yield, whereas 4b was isolated by recrystallization from toluene as colorless crystals in 58% yield. Density functional theory (DFT) calculations at B3PW91/6-31G(d) level of theory showed that anion part of 8 was more stable than that of 9 by 9.2–12.3 kcal/mol.(7)
Scheme 3
Scheme 3. Preparations of 4 via Hydride–Isopropoxide Exchange in 9
The reaction of 4a with MeOH and EtOH in a J. Young NMR tube resulted in the formation of methoxysilane 10 and ethoxysilane 11 in 96 and 92% yield, respectively, as shown in Scheme 4. In contrast, 4a did not react with tert-BuOH at all. When the same reaction was performed in a reaction flask, 10 and 11 were isolated as colorless crystals in 77 and 59% yield, respectively.
Scheme 4
Scheme 4. Reactions of 4a with Alcohols (NMR Yields Are Given in Parentheses)
The 11B NMR signal of 4a (δ = 31) was shifted upfield, whereas the 29Si NMR signal of 4a (δ = 20.0) was shifted downfield as compared to the corresponding values of 2b (δ(11B) = 73; δ(29Si) = 4.6). With decreasing steric bulkiness of the alkoxy groups (4a > 11 > 10), the 11B NMR signal was shifted upfield and the 29Si NMR signal was shifted downfield, as shown in Table 1. These chemical shifts were also compared to those of the corresponding dimethylphenyl(alkoxy)silanes 12 (R = Me, Et, i-Pr), showing that the electronic effect of the alkoxy groups is not significant. The intramolecular coordination of the oxygen atom to the boron atom increased the electron density on the boron atom and decreased that on the silicon atom.
Table 1. 11B and 29Si NMR Shifts of o-(Silyl)(diarylboryl)benzenes
| compounds | R | δ(11B) | δ(29Si) |
|---|---|---|---|
| 2b | i-Pr | 73 | 4.6 |
| 4a | i-Pr | 31 | 20.0 |
| 10 | Me | 17 | 33.1 |
| 11 | Et | 20 | 29.5 |
| 12 | Me | 8.5 | |
| 12 | Et | 6.1 | |
| 12 | i-Pr | 4.0 |
Molecular structure of 10 was finally determined by X-ray crystallographic analysis (Figure 1).(8) The B···O interatomic distance (1.652(2) Å) was much shorter than the sum of the van der Waals radii of the two elements (B: 1.85 Å; O: 1.52 Å)(9) and only 7–8% longer than the sum of their covalent bond radii (B: 0.84 Å; O: 0.66 Å).(9) The boron atom adopted an intermediate geometry between tetrahedral and trigonal planar with the sum of the bond angles around the boron atom (∑(C–B–C) = 343°). The five-membered ring consisting of C1, Si1, O1, B1, and C2 atoms was almost planar and coplanar with the phenylene ring. The oxygen atom adopted a trigonal planar geometry (∑(O) = 359°) and the tetrahedral geometry of the boron atom was distorted because of the small endocyclic bond angle (O1–B1–C2 = 99.78(9)°). The Si–O (1.714(1) Å) and C–O (1.448(1) Å) bonds were longer than the typical Si–O (1.51 Å) and C–O (1.43 Å) bonds,(9) respectively, but slightly shorter than those of silylated oxonium ions.(10) These structural parameters support the intramolecular coordination of the oxygen atom to the boron atom as mentioned above.
Figure 1
Figure 1. Molecular structure of 10 at 30% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): B1–O1, 1.652(2); Si1–O1, 1.714(1); B1–O1, 1.652(2), C9–O1; 1.448(1); C1–Si1–O1, 93.79(5); Si1–O1–B1, 116.68(6); and O1–B1–C2, 99.78(9).
To gain further insight into the electronic structure of the (alkoxysilyl)(boryl)benzenes, DFT calculations of 10 and 2 (R = Me) were performed at B3PW91/6-31++G(d,p) level of theory (Figure 2 and Supporting Information).(7) The lone pair of electrons on the oxygen atom in 10 contributed to highest occupied molecular orbital (HOMO) – 1 (−6.27 eV). The HOMO (−6.46 eV) of 10 is the π orbital of the tolyl groups, and the lowest unoccupied molecular orbital (LUMO, −0.59 eV) is mainly the π* orbital of the phenylene skeleton. The vacant 2p orbital on the boron atom in 10 was involved in LUMO + 2 (−0.40 eV). The charge distribution was revealed by natural bond orbital (NBO) analysis (Mulliken charge in parentheses), as shown in Figure 3.(11) Compared to 2 (R = Me), 10 exhibited more positive charges on the oxygen atom (−0.84 vs −0.92) and the methyl carbon atoms (+0.39 vs +0.31) bonded to it, while the boron atom had lesser positive charges (+0.74 vs +1.97).
Figure 2
Figure 2. Optimized structure of 10 at the B3PW91/6-31++G(d,p) level of theory with overlay of HOMO – 1 (left) and LUMO + 2 (right) (isosurface value = 0.04).
Figure 3
Figure 3. NBO charge and Mulliken charge (in parentheses) distributions in 10 (left) and 2 (R = Me) (right).
The perturbation theory energy analysis in NBO basis reveals delocalization from the donor LPO to the acceptor LP*B with the occupancy of 0.306:(12) the stabilization energy E(2) was calculated to be 16.2 kcal/mol (see Supporting Information).
The B···O interaction was expected to render the carbon atom more electropositive. Thus, treatment of 10 with 1,4-diazabicyclo[2.2.2]octane (DABCO) afforded silyloxyborate–(DABCO-Me)+ complex 13, as shown in Scheme 5. Compound 13 was isolated as colorless crystals by recrystallization from dimethyl sulfoxide (DMSO) in 79% yield. The structure of 13 was characterized by its 11B NMR (δ = 3.0) and 29Si NMR (δ = 9.1) signals, which were consistent with those of oxasilaboratacyclopentene (δ(11B) = 6.0; δ(29Si) = 10.5)(5d) and benzosiloxaborole (δ(11B) = 31.0; δ(29Si) = 22.3).(13) The reaction of 10 with NEt3 in tetrahydrofuran (THF) did not occur even after heating at 80 °C for a day. It is worth noting that compound 10 was regenerated upon treatment of 13 with MeI.(4a) The reaction of 10 with KF in the presence of 18-crown-6 led to the Me–O bond cleavage to form 14 in 70% yield as a white precipitate from THF–hexane (Scheme 6). The reason why the fluoride ion attacks the methyl carbon rather than the boron atom may be that formation of the stable 5-membered ring is more favorable than formation of an acyclic fluoroborate.
Scheme 5
Scheme 5. C–O Bond Cleavage in 10 with DABCO
Scheme 6
Scheme 6. C–O Bond Cleavage in 10 with KF/18-crown-6
Conclusions
ARTICLE SECTIONS
o-[(Isopropoxy)silyl](diarylboryl)benzenes 4 bearing less sterically demanding aryl groups were prepared. The interaction between the oxygen atom and boron atom was confirmed by NMR spectra, X-ray crystal structure analysis, and DFT calculations. The B···O interaction led the C–O bond activation.
Experimental Section
ARTICLE SECTIONS
1H (400 MHz), 13C (100 MHz), 11B (128.3 MHz), and 29Si (79.5 MHz) NMR spectra were recorded using a Bruker Avance III 400 spectrometer. 1H and 13C chemical shifts were referenced to the residual solvent signals CDCl3 (δ(1H) = 7.26, δ(13C) = 77.00); C6D6 (δ(1H) = 7.20, δ(13C) = 128.00), and DMSO-d6 (δ(1H) = 2.50; δ(13C) = 39.52). 11B, and 29Si chemical shifts were referenced to external standards BF3·OEt2 (δ = 0), and tetramethylsilane (δ = 0), respectively. The mass spectra (EI) were recorded 70 eV using a JEOL JMS-Q1000GC Mk II mass spectrometer, and the elemental analyses were performed using the JSL MICRO CORDER JM10 elemental analyzer.
Triisopropyl borate (Tokyo Chemical Industry Co., Ltd.) was distilled under a nitrogen atmosphere over calcium hydride. Chlorotrimethylsilane (Tokyo Chemical Industry Co., Ltd.) was treated with small pieces of sodium under a nitrogen atmosphere to remove dissolved HCl, and the supernatant was used. tert-Butyllithium in pentane (Kanto Chemical Co., Inc.) and DABCO (Tokyo Chemical Industry Co., Ltd.) were used as received. KF (Wako Pure Chemical Industries, Ltd.) was dried in vacuo at 100 °C, 18-crown-6 (Wako Pure Chemical Industries, Ltd.) was recrystallized from CH3CN, and o-(dimethylsilyl)bromobenzene (5) was prepared according to the literature methods.(5a)
THF and Et2O were distilled under a nitrogen atmosphere over sodium benzophenone ketyl. Hexane was distilled under a nitrogen atmosphere over sodium. All reactions were carried out under an inert gas atmosphere.
(Isopropoxy)di(p-tolyl)borane (7a)
To a mixture of Mg turnings (972 mg, 40.0 mmol) and one crystal of I2 in Et2O (5 mL), a few drops of a solution of 4-bromotoluene (4.9 mL, 40.0 mmol) in Et2O (15 mL) were added at room temperature. After the reaction started, Et2O (20 mL) was added and the remaining solution was added dropwise at a rate that maintained a steady reflux. When the addition was complete, the reaction mixture was cooled to room temperature, added to triisopropyl borate (4.50 mL, 19.6 mmol) in Et2O (20 mL) at 0 °C, and stirred overnight at room temperature. Chlorotrimethylsilane (5.3 mL, 40.0 mmol) was then added dropwise at room temperature and the reaction mixture was stirred for 6 h. Subsequently, the solvents were removed in vacuo and the residue was diluted with hexane (40 mL) and filtered. The filtrate was subjected to bulb-to-bulb distillation (90–110 °C/0.85 mmHg) to obtain 7a (2.77 g, 56% yield) as a colorless viscous oil. 1H NMR (C6D6, δ) 1.19 (d, J = 6 Hz, 6H), 2.20 (s, 6H), 4.66 (sept, J = 6 Hz, 1H), 7.15–7.17 (m, 4H), 7.74–7.76 (m, 4H). 13C{1H} NMR (C6D6, δ) 21.54, 24.88, 69.50, 128.69, 134.55, 135.32, 139.82. 11B NMR (C6D6, δ) 44.95 (br). MS(EI) m/z 252 (M+, 9), 160 (M+-p-tolyl, 16), 119 (M+-p-tolyl-i-Pr, 100). Anal. Calcd for C17H21BO: C, 80.97; H, 8.39; Found: C, 80.61; H, 8.68.
(Isopropoxy)di(p-tert-butylphenyl)borane (7b)
To a mixture of Mg turnings (972 mg, 40.0 mmol) and one crystal of I2 in Et2O (5 mL), a few drops of a solution of 1-bromo-4-tert-butylbenzene (6.8 mL, 40.0 mmol) in Et2O (15 mL) were added at room temperature. After the reaction started, Et2O (20 mL) was added and the remaining solution was added dropwise at a rate that maintained a steady reflux. When addition was complete, the reaction mixture was cooled to room temperature, added to triisopropyl borate (4.50 mL, 19.6 mmol) in Et2O (20 mL) at 0 °C, and stirred overnight at room temperature. Next, chlorotrimethylsilane (5.3 mL, 40.0 mmol) was added dropwise at room temperature and the reaction mixture was stirred for 6 h. The solvents were subsequently removed in vacuo, and the residue was diluted with hexane (40 mL) and filtered. The filtrate was subjected to bulb-to-bulb distillation (150–170 °C/0.85 mmHg) to obtain 7b (3.16 g, 48% yield) as a colorless viscous oil. 1H NMR (C6D6, δ) 1.21 (d, J = 6 Hz, 6H), 1.29 (s, 18H), 4.68 (sept, J = 6 Hz, 1H), 7.42–7.45 (m, 4H), 7.81–7.83 (m, 4H). 13C{1H} NMR (C6D6, δ) 24.93, 31.33, 34.74, 69.58, 124.81, 126.02, 127.12, 134.51. 11B NMR (C6D6, δ) 45.29 (br). MS(EI) m/z 266 (M+-i-Pr-2Me, 31), 251 (M+-i-Pr-3Me, 100). Anal. Calcd for C23H33BO: C, 82.14; H, 9.89; Found: C, 82.44; H, 9.78.
o-[(Isopropoxy)dimethylsilyl][di(p-tolyl)boryl]benzene (4a)
A solution of tert-BuLi in pentane (1.56 mol/L, 3.8 mL, 6.00 mmol) was added to a solution of 5 (645 mg, 3.00 mmol) in Et2O (6 mL) at −78 °C over 4 min. After the reaction mixture was stirred at the same temperature for 2 h, 7a (756 mg, 3.00 mmol) in Et2O (3 mL) was added over 3 min. The reaction mixture was stirred at the same temperature for 30 min and then warmed to room temperature. Next, chlorotrimethylsilane (0.56 mL, 4.50 mmol) was added and the mixture was stirred for 2 h. After the solvents were removed in vacuo, the residue was dissolved in hexane (20 mL) and filtered. The filtrate was subjected to bulb-to-bulb distillation (170–180 °C/0.90 mmHg) to obtain 4a (811 mg, 70% yield) as a colorless oil. 1H NMR (C6D6, δ) 0.30 (s, 6H), 0.77 (d, J = 6 Hz, 6H), 2.27 (s, 6H), 4.43 (sept, J = 6 Hz, 1H), 7.17–7.19 (m, 4H), 7.23 (dd, J = 7 Hz, J = 2 Hz, 1H), 7.26 (ddd, J = 7 Hz, J = 7 Hz, J = 2 Hz, 1H), 7.43–7.45 (m, 2H), 7.72–7.74 (m, 4H). 13C{1H} NMR (C6D6, δ) 2.75, 21.39, 24.14, 72.57, 125.69, 128.45, 128.67, 129.65, 130.07, 130.61, 136.01, 137.00 (signals corresponding to the ipso carbons in the two p-tolyl groups and the ipso carbon in the phenyl group were not observed). 11B NMR (C6D6, δ) 30.94 (br). 29Si{1H} NMR (C6D6, δ) 19.99. Anal. Calcd for C25H31BOSi: C, 77.71; H, 8.09; Found: C, 77.50; H, 8.32.
o-[(Isopropoxy)dimethylsilyl][di(p-tert-butylphenyl)boryl]benzene (4b)
A solution of tert-BuLi in pentane (1.56 mol/L, 3.8 mL, 6.00 mmol) was added to a solution of 5 (645 mg, 3.00 mmol) in Et2O (6 mL) at −78 °C over 4 min. After the reaction mixture was stirred at this temperature for 2 h, 7b (1.01 g, 3.00 mmol) in Et2O (3 mL) was added over 3 min. The reaction mixture was stirred at the same temperature for 30 min and then allowed to warm to room temperature. Next, chlorotrimethylsilane (0.56 mL, 4.50 mmol) was added and the mixture was stirred for 2 h. After the solvents were removed in vacuo, the residue was dissolved in hexane (20 mL) and filtered. The filtrate was concentrated in vacuo to give a white solid, which was recrystallized from toluene at −18 °C to obtain 4b (821 mg, 58% yield) as a colorless crystal. 1H NMR (C6D6, δ) 0.33 (s, 6H), 0.74 (d, J = 6 Hz, 6H), 1.35 (s, 18H), 4.48 (sept, J = 6 Hz, 1H), 7.23 (ddd, J = 7 Hz, J = 7 Hz, J = 2 Hz, 1H), 7.27 (ddd, J = 7 Hz, J = 7 Hz, J = 2 Hz, 1H), 7.42–7.47 (m, 6H), 7.76–7.78 (m, 4H). 13C{1H} NMR (C6D6): δ 2.91, 24.00, 31.57, 34. 51, 73.14, 124.50, 125.64, 128.28, 129.81, 129.87, 130.67, 135.58, 149.95 (signals corresponding to the ipso carbons in the two p-tert-butylphenyl groups and the ipso carbon in the phenyl group were not observed). 11B NMR (C6D6, δ) 27.96 (br). 29Si{1H} NMR (C6D6, δ) 21.31. MS(EI) m/z 294 (M+-i-Pr-p-t-BuPh, 20), 279 (M+-i-Pr-p-t-BuPh-Me, 100). Anal. Calcd for C31H43BOSi: C, 79.12; H, 9.21; Found: C, 78.81; H, 9.03.
Hydroborate 8b
A solution of tert-BuLi in pentane (1.56 mol/L, 3.8 mL, 6.00 mmol) was added to a solution of 5 (645 mg, 3.00 mmol) in Et2O (6 mL) at −78 °C. After stirring at the same temperature for 2 h, 7b (1.01 g, 3.00 mmol) in Et2O (3 mL) was added. The reaction mixture was stirred at this temperature for 30 min and then allowed to warm to room temperature. The solvents were subsequently removed in vacuo, and the resulting white solid was dissolved in THF (5 mL). The solvent was removed in vacuo, and the residue was dissolved in hexane (10 mL) and filtered. The filtrate was concentrated in vacuo to obtain a white solid, which was recrystallized from hexane at −18 °C to obtain 8b (1.05 g, 61% yield) as a colorless crystal. 1H NMR (C6D6, δ) 0.66 (s, 6H), 0.68 (d, J = 6 Hz, 6H), 1.11 (m, 8H, THF), 1.35 (s, 18H), 3.06 (m, 8H, THF), 3.80 (sept, J = 6 Hz, 1H), 7.25 (ddd, J = 7 Hz, J = 7 Hz, J = 1 Hz, 1H), 7.37–7.39 (m, 5H), 7.65 (dd, J = 7 Hz, J = 1 Hz, 1H), 7.68 (d, J = 8 Hz, 4H), 8.10 (d, J = 8 Hz, 1H). 13C{1H} NMR (C6D6, δ) 1.50, 24.53, 25.09 (THF), 31.76, 34.25, 67.37 (THF), 68.10, 123.65, 124.58, 129.74, 133.08, 135.83, 137.40, 139.89, 146.67 (signals corresponding to the ipso carbons in the two p-tert-butylphenyl groups and the ipso carbon in the phenyl group were not found). 11B NMR (C6D6, δ) −9.66 (d, 1JB–H = 66 Hz). 29Si{1H} NMR (C6D6, δ) 13.43. Anal. Calcd for C39H60BLiO3Si: C, 75.22; H, 9.71; Found: C, 75.02; H, 9.88.
o-[(Methoxy)dimethylsilyl][di(p-tolyl)boryl]benzene (10)
To a solution of 4a (772 mg, 2.00 mmol) in THF (4.0 mL), MeOH (90 μL, 2.20 mmol) was added via a syringe at room temperature. The reaction mixture was stirred at the same temperature for 10 min and then concentrated in vacuo to afford a white solid. Recrystallization from toluene at −18 °C gave 10 (710 mg, 66% yield) as a colorless crystal. 1H NMR (C6D6, δ) 0.08 (s, 6H), 2.26 (s, 6H), 2.93 (s, 3H), 7.13–7.19 (m, 5H), 7.24 (ddd, J = 8 Hz, J = 8 Hz, J = 1 Hz, 1H), 7.33 (ddd, J = 8 Hz, J = 1 Hz, J = 1 Hz, 1H), 7.50–7.53 (m, 5H). 13C{1H} NMR (C6D6, δ) −1.36, 21.35, 50.58, 125.45, 128.54, 129.76, 130.47, 131.05, 133.46, 134.74, 135.71 (signals corresponding to the ipso carbons in the two p-tolyl groups and the ipso carbon in the phenyl group were not observed). 11B NMR (C6D6, δ) 17.35 (br). 29Si{1H} NMR (C6D6, δ) 33.12. MS(EI) m/z 252 (M+-p-tolyl-Me, 36), 237 (M+-p-tolyl-2Me, 100). Anal. Calcd for C23H27BOSi: C, 77.09; H, 7.59; Found: C, 76.82; H, 7.56.
o-[(Ethoxy)dimethylsilyl][di(p-tolyl)boryl]benzene (11)
To a solution of 4a (772 mg, 2.00 mmol) in THF (4.0 mL), EtOH (0.13 mL, 2.20 mmol) was added via a syringe at room temperature. The reaction mixture was stirred at the same temperature for 10 min and then concentrated in vacuo to afford a white solid. Recrystallization from hexane at −18 °C gave 11 (439 mg, 59% yield) as a colorless crystal. 1H NMR (C6D6, δ) 0.22 (s, 6H), 0.62 (t, J = 7 Hz, 4H), 2.29 (s, 6H), 3.64 (q, J = 7 Hz, 2H), 7.20–7.23 (m, 5H), 7.27 (ddd, J = 8 Hz, J = 8 Hz, J = 1 Hz, 1H), 7.37 (ddd, J = 7 Hz, J = 2 Hz, J = 1 Hz, 1H), 7.52 (ddd, J = 7 Hz, J = 2 Hz, J = 1 Hz, 1H), 7.63 (d, J = 8 Hz, 4H). 13C{1H} NMR (C6D6, δ) 0.46, 16.14, 21.34, 63.29, 125.45, 128.48, 129.62, 130.31, 130.78, 134.11, 134.89, 135.81 (signals corresponding to the ipso carbons in the two p-tolyl groups and the ipso carbon in the phenyl group were not observed). 11B NMR (C6D6, δ) 20.02 (br). 29Si{1H} NMR (C6D6, δ) 29.50. Anal. Calcd for C24H29BOSi: C, 77.41; H, 7.85; Found: C, 77.03; H, 7.94.
Silyloxyborate-[(Me-DABCO)+] Complex 13
A solution of 10 (179 mg, 0.50 mmol) and DABCO (56 mg, 0.50 mmol) in THF (1 mL) was stirred at room temperature for 2 days. The solvent was removed in vacuo and the residue was recrystallized from DMSO at room temperature to obtain 13 (186 mg, 79% yield) as a colorless crystal. 1H NMR (DMSO-d6, δ) 0.13 (s, 6H), 2.12 (s, 6H), 2.75 (s, 3H), 2.87 (t, J = 8 Hz, 6H), 3.01 (t, J = 8 Hz, 6H), 6.75 (d, J = 8 Hz, 4H), 6.81 (t, J = 7 Hz, 1H), 6.95 (dd, J = 7 Hz, J = 1 Hz, 1H), 7.18 (d, J = 7 Hz, 1H), 7.35 (d, J = 8 Hz, 4H), 7.39 (d, J = 7 Hz, 1H). 13C{1H} NMR (DMSO-d6, δ) 2.82, 20.84, 44.61, 50.62 (t, J = 4 Hz), 53.14 (t, J = 3 Hz), 122.38, 126.30, 126.35, 128.49, 129.20, 130.10, 132.14, 144.31, 158.92 (br) (signals corresponding to the ipso carbons in the two p-tolyl groups were not observed). 11B NMR (DMSO-d6, δ) 2.54 (br). 29Si{1H} NMR (DMSO-d6, δ) 9.12. Anal. Calcd for C29H39BN2OSi: C, 74.03; H, 8.35; N, 5.95 Found: C, 73.82; H, 8.30; N, 6.12.
Silyloxyborate-[K(18-crown-6)+] Complex 14
A solution of 10 (71 mg, 0.20 mmol), 18-crown-6 (53 mg, 0.20 mmol), and KF (12 mg, 0.20 mmol) in toluene (0.6 mL) was stirred at room temperature for 12 h. Subsequently, the solvent was removed in vacuo. The resulting white solid was dissolved in THF (0.5 mL) and toluene (1 mL) was slowly added to the solution. The resulting two-layer solution was allowed to stand at room temperature for a day to obtain 14 (108 mg, 70% yield) as a colorless crystal. 1H NMR (CDCl3, δ) 0.35 (s, 6H), 2.22 (s, 6H), 3.40 (s, 24H, crown), 6.91 (d, J = 8 Hz, 4H), 6.96 (ddd, J = 8 Hz, J = 8 Hz, J = 1 Hz, 1H), 7.04 (ddd, J = 7 Hz, J = 7 Hz, J = 1 Hz, 1H), 7.34 (d, J = 8 Hz, 1H), 7.36 (d, J = 7 Hz, 1H), 7.50 (d, J = 8 Hz, 4H). 13C{1H} NMR (CDCl3, δ). 2.93, 21.17, 69.51 (crown), 122.86, 127.11, 127.44, 128.37, 129.50, 131.31, 133.39, 142.99 (signals corresponding to the ipso carbons in the two p-tolyl groups and the ipso carbon in the phenyl group were not observed). 11B NMR (CDCl3, δ) 2.79 (br). 29Si{1H} NMR (CDCl3, δ) 11.26. Anal. Calcd for C42H64BKO9Si: C, 67.10; H, 8.58; Found: C, 66.76; H, 8.60.
Reaction of 13 with MeI: Formation of 10
To a solution of 13 (141 mg, 0.30 mmol) in THF (1 mL), MeI (56 μL, 0.90 mmol) was added dropwise via a syringe at room temperature and the reaction mixture was stirred at the same temperature for 10 min. After the solvents were removed in vacuo, the residue was dissolved in toluene (1 mL) and filtered. The filtrate was cooled to −18 °C to obtain colorless crystals of 10 (72 mg, 67% yield).
X-ray Crystallographic Analysis
X-ray crystallographic data for 10 was collected using a SMART APEX-II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 173 K at the Department of Chemistry, Graduate School of Science, Hiroshima University. The structure was solved by direct methods using SIR 97 and refined by a full-matrix least-squares procedure based on F2 with SHELX-97.(8) All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were located at the expected positions by geometrical calculations and refined isotropically or found on the difference Fourier map and refined isotropically.
Crystal data for 10: C23H27BOSi, fw 358.34, orthorhombic, Pbca (No. 61), a = 17.568(2) Å, b = 12.8876(16) Å, c = 18.107(2) Å, V = 4099.6(9) Å3, Z = 8, Dcalcd = 1.161 g/cm3, R(I > 2σ(I)) = 0.0386, Rw (all data) = 0.1159, GOF = 1.043, T = 173 K.
CCDC-1953122 (10) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Computational Methods
Computations were executed with the Gaussian 09 program package at the Research Center for Computing and Multimedia Studies, Hosei University.(7,11) The structures of 2 and anionic parts of 8 and 9, and 10 were optimized at the B3PW91/6-31G(d) and B3PW91/6-31++G(d,p) level of theory.
ARTICLE SECTIONS
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03784.
Table of crystallographic data for 10 (PDF/CIF) and computational work for 2 (R = Me) and anionic parts of 8 and 9, and 10 (PDF)
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Author Information
ARTICLE SECTIONS
- Atsushi Kawachi - Department of Chemical Science and Technology, Faculty of Bioscience and Applied Chemistry and , Hosei University, 3-7-2 Kajino-cho, Koganei, Tokyo 184-8584, Japan;
http://orcid.org/0000-0001-7306-7669;
The authors declare no competing financial interest.
Acknowledgments
ARTICLE SECTIONS
This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” (15H00961) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors thank Dr. S. Hosokawa (Hosei University) for the measurement of the elemental analysis.
References
ARTICLE SECTIONS
This article references 13 other publications.
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Soc. 1996, 118, 9440– 9441, DOI: 10.1021/ja961536g[ACS Full Text.], [CAS], Google Scholar2ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XmtF2hs7g%253D&md5=6f0ab4c8403f544dcd9384fc623b77beTris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and EstersParks, Daniel J.; Piers, Warren E.Journal of the American Chemical Society (1996), 118 (39), 9440-9441CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Tris(pentafluorophenyl)borane (1-4 mol %) catalyzes the addn. of Ph3SiH to carbonyl functions of arom. substrates p-XC6H4COR (X = Me, H, Cl, NO2; R = H, Me, OEt). Turnover frequencies for X = H substrates are 19, 45 and 637 h for R = H, Me and OEt, resp. and rates of hydrosilation increase as X becomes more electron withdrawing. Mechanistic studies, including substrate/borane binding equil., structural studies on substrate/borane adducts and kinetic measurements support a hydrosilation mechanism characterized by borane activation of the silane reagent rather than the carbonyl substrate. Key observations in support of this unusual nucleophilic/electrophilic hydrosilation mechanism were the obsd. inhibition of the reaction by increased concns. of substrate and the obsd. order of reactivity which indicates that the weaker binding substrates are reduced faster.(b) Parks, D. J.; Blackwell, J. M.; Piers, W. E. Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions. J. Org. Chem. 2000, 65, 3090– 3098, DOI: 10.1021/jo991828a[ACS Full Text.], [CAS], Google Scholar2bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXisFalurc%253D&md5=2a7fbd998708513d02b270026b9beb2fStudies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl FunctionsParks, Daniel J.; Blackwell, James M.; Piers, Warren E.Journal of Organic Chemistry (2000), 65 (10), 3090-3098CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)The strong organoborane Lewis acid B(C6F5)3 catalyzes the hydrosilation (using R3SiH) of arom. and aliph. carbonyl functions at convenient rates with loadings of 1-4%. For aldehydes and ketones, the product silyl ethers were isolated in 75-96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45-70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quant. kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concns. of substrate have an inhibitory effect on the obsd. reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition expts. The borane thus must dissoc. from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C6F5)3]-. In addn. to the kinetic data, this mechanistic proposal is supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C6F5)3 catalyzes H/D and H/H scrambling in silanes in the absence of substrate, computational studies, the synthesis of models for proposed intermediates, and other isotope labeling and crossover expts.(c) Blackwell, J. M.; Morrison, D. J.; Piers, W. E. B(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethers. Tetrahedron 2002, 58, 8247– 8254, DOI: 10.1016/S0040-4020(02)00974-2[Crossref], [CAS], Google Scholar2chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XnsVagsbw%253D&md5=73947fd0293d19aa00cef292caac4fcdB(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethersBlackwell, James M.; Morrison, Darryl J.; Piers, Warren E.Tetrahedron (2002), 58 (41), 8247-8254CODEN: TETRAB; ISSN:0040-4020. (Elsevier Science Ltd.)The 1,4-hydrosilylation of a variety of simple α,β-unsatd. enones as catalyzed by B(C6F5)3 (1-2%) is described. 2-Cyclopentenone, 2-methyl- and 2,3,4,5-tetramethyl-2-cyclopentenones, as well as 2-cyclohexenone, 4,4-dimethylcyclohexenone (6), 2-methyl-5-(2-propenyl)-2-cyclohexenone and PhCH:CHCOR (R = Ph, Me, H) were hydrosilylated by Ph2MeSiH in the presence of B(C6F5)3 to give the corresponding enol silyl ethers. Cyclohexenone 6 was hydrosilylated by Et3SiH, Ph3SiH and R1Me2SiH (R1 = tBu, Ph) in the same conditions. For substrates with no steric hindrance near the β-carbon, 1,4-addn. of silane is very clean; in other instances, 1,2-hydrosilylation is competitive. The reaction is facile with five com. available silane reagents. Prolonged reaction times and excess of PhMe2SiH gave double-hydrosilylation products for 2-cyclopentenone and 6, the corresponding 1,2-siloxy-silyl derivs. The net trans stereochem. of H-Si addn. to the silyl enol ether C:C bond was established and points to a stepwise mechanism for this reaction. This was supported by the observation and full spectroscopic characterization of a silylcarboxonium ion intermediate with an [HB(C6F5)3]- counter-anion in the hydrosilylation of the silyl-enol ether derived from 6 and PhMe2SiH.
- 3(a) Rendler, S.; Oestreich, M. Conclusive Evidence for an SN2-Si Mechanism in the B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Compounds: Implications for the Related Hydrogenation. Angew. Chem., Int. Ed. 2008, 47, 5997– 6000, DOI: 10.1002/anie.200801675[Crossref], [CAS], Google Scholar.3ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXpsleitL0%253D&md5=1c4cc074df92b2a5d6988af89f83ee91Conclusive evidence for an SN2-Si mechanism in the B(C6F5)3-catalyzed hydrosilylation of carbonyl compounds: implications for the related hydrogenationRendler, Sebastien; Oestreich, MartinAngewandte Chemie, International Edition (2008), 47 (32), 5997-6000, S5997/1-S5997/31CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)Effective Walden-type anal. showcases the usefulness of silanes with a stereogenic Si center as stereochem. probes. The B(C6F5)3-catalyzed hydrosilylation of acetophenone with chiral (SiR)-tetrahydro(isopropyl)silanaphthalene and likely the related hydrogenation proceed through linear B-H-Si-O transition states, as verified by flawless inversion of the abs. configuration at Si to give chiral silyl ether (SiR,R)-I which subsequently undergoes DIBAL-redn. to a chiral (R)-1-phenylethanol.(b) Hog, D. T.; Oestreich, M. B(C6F5)3-Catalyzed Reduction of Ketones and Imines Using Silicon-Stereogenic Silanes: Stereoinduction by Single-Point Binding. Eur. J. Org. Chem. 2009, 2009, 5047– 5056, DOI: 10.1002/ejoc.200900796 .(c) Mewald, M.; Oestreich, M. Illuminating the Mechanism of the Borane-Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and Silane. Chem. – Eur. J. 2012, 18, 14079– 14084, DOI: 10.1002/chem.201202693[Crossref], [PubMed], [CAS], Google Scholar3chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xhtlylt7nI&md5=3bbd0549d67f69ee34664a1162dde332Illuminating the Mechanism of the Borane-Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and SilaneMewald, Marius; Oestreich, MartinChemistry - A European Journal (2012), 18 (44), 14079-14084, S14079/1-S14079/16CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)The redn. of C:O groups with silanes catalyzed by electron-deficient boranes follows a counterintuitive mechanism in which the Si-H bond is activated by the B Lewis acid prior to nucleophilic attack of the carbonyl O atom at the Si atom. The borohydride thus formed is the actual reductant. These steps were elucidated by using a Si-stereogenic silane, but applying the same technique to the related redn. of C:N groups was inconclusive due to racemization of the Si atom. The present study now proves by the deliberate combination of the authors' axially chiral borane catalyst and axially chiral silane reagents (in both enantiomeric forms) that the mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochem. outcomes for the borane/silane pairs show that both participate in the enantioselectivity-detg. hydride-transfer step. These expts. became possible after the discovery that the authors' axially chiral C6F5-substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.
- 4(a) Köster, R.; Seidel, G.; Wrackmeyer, B. Organosubstituierte 2,5-Dihydro-1,2,5-oxoniasilaboratole Charakterisierung und Reaktivität. Chem. Ber. 1991, 124, 1003– 1016, DOI: 10.1002/cber.19911240506 .(b) Wrackmeyer, B.; Suβ, J.; Milius, W. 2,5-Dihydro-1,2,5-azoniasilaboratole Derivatives – Useful Starting Materials in Heterocyclic Synthesis. Chem. Ber. 1996, 129, 147– 153, DOI: 10.1002/cber.19961290207[Crossref], [CAS], Google Scholar.4bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XpsFKltg%253D%253D&md5=f17cee8dd392843a2643265cac8ef1fe2,5-dihydro-1,2,5-azoniasilaboratole derivatives: useful starting materials in heterocyclic synthesisWrackmeyer, Bernd; Suess, Juergen; Milius, WolfgangChemische Berichte (1996), 129 (2), 147-53CODEN: CHBEAM; ISSN:0009-2940. (VCH)Me2Si(NEt2)C≡CR (R = Me, Bu, SiMe3) reacts with Et3B stereoselectively by 1,1-ethyloboration to give the alkenes with the boryl and silyl group in cis-positions at the C:C bond. Owing to the strongly intramol. coordinative N-B bond, these products are azoniasilaboratoles I (R = Me, Bu, SiMe3, X = NEt2). Protic reagents such as azoles react with I (X = NEt2, R = Me) to give Et2NH and the resp. N-azolyl derivs. such as II (R = Me, X = CH, N) which contain tetracoordinate boron. II (X = CH), derived from indazole, was characterized by x-ray anal. With EtOH, the corresponding oxoniasilaboratoles I (R = Me, SiMe3, X = OEt) are obtained. Treatment of I with H2O affords 1,2,5-oxasilaborolanes, presumably via an intermediate with the structure of a 2,5-dihydro-1,2,5-oxoniasilaboratole. All products were characterized by their 1H, 11B, 13C, 15N, and 29Si NMR data.(c) Wrackmeyer, B.; Milius, W.; Tok, O. L. Reaction of Alkyn-1-yl(diorganyl)silanes with 1-Boraadamantane: Si-H-B Bridges Confirmed by the Molecular Structure in the Solid State and in Solution. Chem. – Eur. J. 2003, 9, 4732– 4738, DOI: 10.1002/chem.200304961[Crossref], [PubMed], [CAS], Google Scholar4chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXot1yksLc%253D&md5=45a8710f018f3b578a0a1045377a09c9Reaction of alkyn-1-yl(diorganyl)silanes with 1-boraadamantane: Si-H-B bridges confirmed by the molecular structure in the solid state and in solutionWrackmeyer, Bernd; Milius, Wolfgang; Tok, Oleg L.Chemistry - A European Journal (2003), 9 (19), 4732-4738CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)1-Boraadamantane 1 reacted with 1-alkynylsilanes contg. one or two Si-H functions adjacent to triple bond, giving 4-methylene-3-borahomoadamantane derivs. with Si-H-B bridging hydrogens. Reaction of R1C≡CSiHR2 (2a-f, R1 = SiMe3, Ph, Bu, SiHPh2, ferrocenyl; R = Me, iPr, Ph) proceeds under mild conditions, giving 4-methylene-3-borahomoadamantane derivs. I (4a-f) quant. and selectively by 1,1-organoboration. An electron deficient Si-H-B bridge was present in the product. The analogous reaction of 1 with an 1-alkynyldisilane BuC≡CSiMe2SiHMe2 (3) gave the corresponding alkene deriv. 5 without the Si-H-B bridge. Evidence for the Si-H-B bridge in 4 was given by IR data, an extensive set of NMR spectroscopical data (1H, 11B, 13C, 29Si NMR) including various unusual isotope effects on chem. shifts and coupling consts., as well as from the crystal structure of 4e in the solid state. The crystal structure of precursor of 4e, Ph2Si(H)C≡CSi(H)Ph2 (2e) is also reported.
- 5(a) Reaction with alcohols:Kawachi, A.; Zaima, M.; Tani, A.; Yamamoto, Y. Dehydrogenative Condensation of (o-Borylphenyl)hydrosilane with Alcohols and Amines. Chem. Lett. 2007, 36, 362– 363, DOI: 10.1246/cl.2007.362[Crossref], [CAS], Google Scholar.5ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXjsVShtrs%253D&md5=89b1fa9ac8b9fccc082e785515d42d10Dehydrogenative condensation of (o-borylphenyl)hydrosilane with alcohols and aminesKawachi, Atsushi; Zaima, Masatoshi; Tani, Atsushi; Yamamoto, YohsukeChemistry Letters (2007), 36 (3), 362-363CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)O-[(Dimesitylboryl)phenyl]dimethylsilane (1a) undergoes dehydrogenative condensation with alcs. and amines, giving the corresponding alkoxysilanes and aminosilanes in moderate to high yields. It is plausible that the ortho-boryl group in 1a electrophilically activates the Si-H bond. An X-ray crystal structure of 1a is presented and discussed.(b) Reaction with KFKawachi, A.; Morisaki, H.; Tani, A.; Zaima, M.; Yamamoto, Y. Reaction of o-(HSiR2)(BMes2)C6H4 with a fluoride ion: Fluoride attack at silicon and hydride transfer from silicon to boron to form F-Si···H-B interaction. Heteroatom. Chem. 2011, 22, 471– 475, DOI: 10.1002/hc.20709[Crossref], [CAS], Google Scholar.5bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXnvFamtL8%253D&md5=3544a5015ba7e5d5cf82769995fd0743Reaction of o-(HSiR2)(BMes2)C6H4 with a fluoride ion: Fluoride attack at silicon and hydride transfer from silicon to boron to form F-Si···H-B interactionKawachi, Atsushi; Morisaki, Hiroshi; Tani, Atsushi; Zaima, Masatoshi; Yamamoto, YohsukeHeteroatom Chemistry (2011), 22 (3-4), 471-475CODEN: HETCE8; ISSN:1042-7163. (John Wiley & Sons, Inc.)Reactions of o-(HSiR2)(BMes2)C6H4 (R = Me (3a), Ph (3b)) with a F- ion afford [o-(FSiR2)(HBMes2)C6H4]- (R = Me (5a), Ph (5b)), which involve F-Si···H-B interactions to render the Si atom pentacoordinate. The F-Si···H-B interaction in 5a was confirmed by x-ray crystallog. anal. and atoms-in-mols. (AIM) anal. © 2011 Wiley Periodicals, Inc.(c) H-Ar Ligand Exchange:Kawachi, A.; Morisaki, H.; Nishioka, N.; Yamamoto, Y. Intramolecular H-Ar Ligand Exchange between Silicon and Boron: Functionality Transfer of Si-H to B-H. Chem. – Asian J. 2012, 7, 546– 553, DOI: 10.1002/asia.201100678[Crossref], [PubMed], [CAS], Google Scholar.5chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhs12gsLrN&md5=4d65305124299339680b64f6af1d5e63Intramolecular H-Ar Ligand Exchange between Silicon and Boron: Functionality Transfer of Si-H to B-HKawachi, Atsushi; Morisaki, Hiroshi; Nishioka, Norimasa; Yamamoto, YohsukeChemistry - An Asian Journal (2012), 7 (3), 546-553CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)Ortho-C6H4(SiR3-nHn)(BMes2) (Mes = mesityl; R = Me, Ph; n = 1, 2) underwent Mes-H ligand exchange between the Si atom and the B atom to form o-C6H4(SiMesR3-nHn-1)(BMesH) upon heating. The resulting hydroborane intermediates immediately react with benzaldehyde to afford the corresponding benzyloxyboranes. A DFT study of model compds. reveals the transition states of the ligand exchange. A hydride abstraction from the Si atom by the B center is key to reaching the transition states, which include the tricoordinate silyl-cation moiety and the tetracoordinate hydridoborate moiety. The mol. structures of two benzyloxyboranes were detd. by x-ray crystallog.(d) Formation of Si-O-B linkage:Kawachi, A.; Zaima, M.; Yamamoto, Y. Intramolecular Reaction of Silanol and Triarylborane: Boron-Aryl Bond Cleavage and Formation of a Si-O-B Heterocyle. Organometallics 2008, 27, 4691– 4696, DOI: 10.1021/om8004405[ACS Full Text], [CAS], Google Scholar5dhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtValurbN&md5=f942de13542a821ca976b8539095e950Intramolecular Reaction of Silanol and Triarylborane: Boron-Aryl Bond Cleavage and Formation of a Si-O-B HeterocycleKawachi, Atsushi; Zaima, Masatoshi; Yamamoto, YohsukeOrganometallics (2008), 27 (18), 4691-4696CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)[2-(Hydroxydimethylsilyl)phenyl]dimesitylborane (1) underwent intramol. cyclization to give 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboracyclopentene (shown as I) and mesitylene by B-aryl bond cleavage. Whereas NEt3 as an additive accelerated the cyclization, DBU stabilized a silyloxyborate complex, 1,8-diazabicyclo[5.4.0]-7-undecenium 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboratacyclopentene, which was characterized by x-ray crystallog. anal. and HF calcns. The D-labeling expts. revealed that the H migrated from the hydroxyl group in 1 to the leaving mesitylene.
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(American Chemical Society)Trisilyloxonium ions, key intermediates in understanding the mechanism of cationic polymn. of cyclosiloxanes, were for the first time prepd. by reacting Me3SiH with trityl tetrakis(pentafluorophenyl)borate (TPFPB) in the presence of siloxanes and directly obsd. by NMR spectroscopy at -70°. Polymn. or oligomerization of siloxanes can be initiated by in-situ formed trisilyloxonium ions. The mechanism of the reactions is discussed. Structural parameters and 29Si NMR chem. shifts of a no. of silyloxonium ions were calcd. by ab initio/IGLO methods. The results are in good agreement with the obtained exptl. data.(b) Kira, M.; Hino, T.; Sakurai, H. Chemistry of organosilicon compounds. 292. An NMR Study of the Formation of Silyloxonium Ions by Using Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as Counteranion. J. Am. Chem. 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The evidence for the formation of cyclic silyloxonium ions was obtained by monitoring the reaction of 3-ethoxypropylsilanes with trityl-TFPB by NMR spectroscopy. The use of TFPB as a nonnucleophilic counteranion was crucial for the formation of the silyloxonium ions; silyl perchlorates did not show significant interaction with ethers.(c) Kira, M.; Hino, T.; Sakurai, H. Siloxycarbenium Tetrakis[3,5-bis(trifluoromethyl)phenyl]borates and Their Role in Reactions of Ketones with Nucleophiles. Chem. Lett. 1992, 21, 555– 558, DOI: 10.1246/cl.1992.555 .(d) Driess, M.; Barmeyer, R.; Monsé, C.; Merz, K. E(SiMe3)4+ Ions (E=P, As): Persilylated Phosphonium and Arsonium Ions. Angew. Chem., Int. 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NMR investigations of the selenium- and tellurium-contg. precursor silanes 3d-f and 8d,f revealed a significant through-space J-coupling between the chalcogen nuclei and the Me2SiH group. Exptl. and computational results typify the synthesized cations 9 and 10 as chalconium ions. The imposed ring strain weakens the Si-Ch linkage compared to acyclic chalconium ions. This attenuation of the Si-Ch bond strength is more pronounced in the acenaphthene series. Surprisingly, the Si-O bonds in oxonium ions 9a and 10a are the weakest Si-Ch linkage in both series. The synthesized silyl chalconium borates are active in hydrodefluorination reactions of alkyl fluorides with silanes. A cooperative activation of the silane by the Lewis acidic (silicon) and by the Lewis basic side (chalcogen) is suggested.(f) Bläsing, K.; Labbow, R.; Michalik, D.; Reiß, F.; Schulz, A.; Villinger, A.; Walker, S. On Silylated Oxonium and Sulfonium Ions and Their Interaction with Weakly Coordinating Borate Anions. Chem. – Eur. J. 2019, DOI: 10.1002/chem.201904403
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- Tomomi Shimizu, Atsushi Kawachi. Synthesis, reactions, and photophysical properties of o-(alkoxysilyl)(borafluorenyl)benzenes. Journal of Organometallic Chemistry 2020, 912 , 121179. https://doi.org/10.1016/j.jorganchem.2020.121179
Abstract
Scheme 1
Scheme 1. Intramolecular Si–N Bond ActivationScheme 2
Scheme 2. Si–H Bond Activation by an o-Boryl Group in 1Scheme 3
Scheme 3. Preparations of 4 via Hydride–Isopropoxide Exchange in 9Scheme 4
Scheme 4. Reactions of 4a with Alcohols (NMR Yields Are Given in Parentheses)Figure 1
Figure 1. Molecular structure of 10 at 30% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): B1–O1, 1.652(2); Si1–O1, 1.714(1); B1–O1, 1.652(2), C9–O1; 1.448(1); C1–Si1–O1, 93.79(5); Si1–O1–B1, 116.68(6); and O1–B1–C2, 99.78(9).
Figure 2
Figure 2. Optimized structure of 10 at the B3PW91/6-31++G(d,p) level of theory with overlay of HOMO – 1 (left) and LUMO + 2 (right) (isosurface value = 0.04).
Figure 3
Figure 3. NBO charge and Mulliken charge (in parentheses) distributions in 10 (left) and 2 (R = Me) (right).
Scheme 5
Scheme 5. C–O Bond Cleavage in 10 with DABCOScheme 6
Scheme 6. C–O Bond Cleavage in 10 with KF/18-crown-6References
ARTICLE SECTIONSThis article references 13 other publications.
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- 2(a) Parks, D. J.; Piers, W. E. Tris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and Esters. J. Am. Chem. Soc. 1996, 118, 9440– 9441, DOI: 10.1021/ja961536g[ACS Full Text.], [CAS], Google Scholar2ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XmtF2hs7g%253D&md5=6f0ab4c8403f544dcd9384fc623b77beTris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and EstersParks, Daniel J.; Piers, Warren E.Journal of the American Chemical Society (1996), 118 (39), 9440-9441CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Tris(pentafluorophenyl)borane (1-4 mol %) catalyzes the addn. of Ph3SiH to carbonyl functions of arom. substrates p-XC6H4COR (X = Me, H, Cl, NO2; R = H, Me, OEt). Turnover frequencies for X = H substrates are 19, 45 and 637 h for R = H, Me and OEt, resp. and rates of hydrosilation increase as X becomes more electron withdrawing. Mechanistic studies, including substrate/borane binding equil., structural studies on substrate/borane adducts and kinetic measurements support a hydrosilation mechanism characterized by borane activation of the silane reagent rather than the carbonyl substrate. Key observations in support of this unusual nucleophilic/electrophilic hydrosilation mechanism were the obsd. inhibition of the reaction by increased concns. of substrate and the obsd. order of reactivity which indicates that the weaker binding substrates are reduced faster.(b) Parks, D. J.; Blackwell, J. M.; Piers, W. E. Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions. J. Org. Chem. 2000, 65, 3090– 3098, DOI: 10.1021/jo991828a[ACS Full Text.], [CAS], Google Scholar2bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXisFalurc%253D&md5=2a7fbd998708513d02b270026b9beb2fStudies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl FunctionsParks, Daniel J.; Blackwell, James M.; Piers, Warren E.Journal of Organic Chemistry (2000), 65 (10), 3090-3098CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)The strong organoborane Lewis acid B(C6F5)3 catalyzes the hydrosilation (using R3SiH) of arom. and aliph. carbonyl functions at convenient rates with loadings of 1-4%. For aldehydes and ketones, the product silyl ethers were isolated in 75-96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45-70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quant. kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concns. of substrate have an inhibitory effect on the obsd. reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition expts. The borane thus must dissoc. from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C6F5)3]-. In addn. to the kinetic data, this mechanistic proposal is supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C6F5)3 catalyzes H/D and H/H scrambling in silanes in the absence of substrate, computational studies, the synthesis of models for proposed intermediates, and other isotope labeling and crossover expts.(c) Blackwell, J. M.; Morrison, D. J.; Piers, W. E. B(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethers. Tetrahedron 2002, 58, 8247– 8254, DOI: 10.1016/S0040-4020(02)00974-2[Crossref], [CAS], Google Scholar2chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XnsVagsbw%253D&md5=73947fd0293d19aa00cef292caac4fcdB(C6F5)3 catalyzed hydrosilation of enones and silyl enol ethersBlackwell, James M.; Morrison, Darryl J.; Piers, Warren E.Tetrahedron (2002), 58 (41), 8247-8254CODEN: TETRAB; ISSN:0040-4020. (Elsevier Science Ltd.)The 1,4-hydrosilylation of a variety of simple α,β-unsatd. enones as catalyzed by B(C6F5)3 (1-2%) is described. 2-Cyclopentenone, 2-methyl- and 2,3,4,5-tetramethyl-2-cyclopentenones, as well as 2-cyclohexenone, 4,4-dimethylcyclohexenone (6), 2-methyl-5-(2-propenyl)-2-cyclohexenone and PhCH:CHCOR (R = Ph, Me, H) were hydrosilylated by Ph2MeSiH in the presence of B(C6F5)3 to give the corresponding enol silyl ethers. Cyclohexenone 6 was hydrosilylated by Et3SiH, Ph3SiH and R1Me2SiH (R1 = tBu, Ph) in the same conditions. For substrates with no steric hindrance near the β-carbon, 1,4-addn. of silane is very clean; in other instances, 1,2-hydrosilylation is competitive. The reaction is facile with five com. available silane reagents. Prolonged reaction times and excess of PhMe2SiH gave double-hydrosilylation products for 2-cyclopentenone and 6, the corresponding 1,2-siloxy-silyl derivs. The net trans stereochem. of H-Si addn. to the silyl enol ether C:C bond was established and points to a stepwise mechanism for this reaction. This was supported by the observation and full spectroscopic characterization of a silylcarboxonium ion intermediate with an [HB(C6F5)3]- counter-anion in the hydrosilylation of the silyl-enol ether derived from 6 and PhMe2SiH.
- 3(a) Rendler, S.; Oestreich, M. Conclusive Evidence for an SN2-Si Mechanism in the B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Compounds: Implications for the Related Hydrogenation. Angew. Chem., Int. Ed. 2008, 47, 5997– 6000, DOI: 10.1002/anie.200801675[Crossref], [CAS], Google Scholar.3ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXpsleitL0%253D&md5=1c4cc074df92b2a5d6988af89f83ee91Conclusive evidence for an SN2-Si mechanism in the B(C6F5)3-catalyzed hydrosilylation of carbonyl compounds: implications for the related hydrogenationRendler, Sebastien; Oestreich, MartinAngewandte Chemie, International Edition (2008), 47 (32), 5997-6000, S5997/1-S5997/31CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)Effective Walden-type anal. showcases the usefulness of silanes with a stereogenic Si center as stereochem. probes. The B(C6F5)3-catalyzed hydrosilylation of acetophenone with chiral (SiR)-tetrahydro(isopropyl)silanaphthalene and likely the related hydrogenation proceed through linear B-H-Si-O transition states, as verified by flawless inversion of the abs. configuration at Si to give chiral silyl ether (SiR,R)-I which subsequently undergoes DIBAL-redn. to a chiral (R)-1-phenylethanol.(b) Hog, D. T.; Oestreich, M. B(C6F5)3-Catalyzed Reduction of Ketones and Imines Using Silicon-Stereogenic Silanes: Stereoinduction by Single-Point Binding. Eur. J. Org. Chem. 2009, 2009, 5047– 5056, DOI: 10.1002/ejoc.200900796 .(c) Mewald, M.; Oestreich, M. Illuminating the Mechanism of the Borane-Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and Silane. Chem. – Eur. J. 2012, 18, 14079– 14084, DOI: 10.1002/chem.201202693[Crossref], [PubMed], [CAS], Google Scholar3chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xhtlylt7nI&md5=3bbd0549d67f69ee34664a1162dde332Illuminating the Mechanism of the Borane-Catalyzed Hydrosilylation of Imines with Both an Axially Chiral Borane and SilaneMewald, Marius; Oestreich, MartinChemistry - A European Journal (2012), 18 (44), 14079-14084, S14079/1-S14079/16CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)The redn. of C:O groups with silanes catalyzed by electron-deficient boranes follows a counterintuitive mechanism in which the Si-H bond is activated by the B Lewis acid prior to nucleophilic attack of the carbonyl O atom at the Si atom. The borohydride thus formed is the actual reductant. These steps were elucidated by using a Si-stereogenic silane, but applying the same technique to the related redn. of C:N groups was inconclusive due to racemization of the Si atom. The present study now proves by the deliberate combination of the authors' axially chiral borane catalyst and axially chiral silane reagents (in both enantiomeric forms) that the mechanisms of these hydrosilylations are essentially identical. Unmistakable stereochem. outcomes for the borane/silane pairs show that both participate in the enantioselectivity-detg. hydride-transfer step. These expts. became possible after the discovery that the authors' axially chiral C6F5-substituted borane induces appreciable levels of enantioinduction in the imine hydrosilylation.
- 4(a) Köster, R.; Seidel, G.; Wrackmeyer, B. Organosubstituierte 2,5-Dihydro-1,2,5-oxoniasilaboratole Charakterisierung und Reaktivität. Chem. Ber. 1991, 124, 1003– 1016, DOI: 10.1002/cber.19911240506 .(b) Wrackmeyer, B.; Suβ, J.; Milius, W. 2,5-Dihydro-1,2,5-azoniasilaboratole Derivatives – Useful Starting Materials in Heterocyclic Synthesis. Chem. Ber. 1996, 129, 147– 153, DOI: 10.1002/cber.19961290207[Crossref], [CAS], Google Scholar.4bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XpsFKltg%253D%253D&md5=f17cee8dd392843a2643265cac8ef1fe2,5-dihydro-1,2,5-azoniasilaboratole derivatives: useful starting materials in heterocyclic synthesisWrackmeyer, Bernd; Suess, Juergen; Milius, WolfgangChemische Berichte (1996), 129 (2), 147-53CODEN: CHBEAM; ISSN:0009-2940. (VCH)Me2Si(NEt2)C≡CR (R = Me, Bu, SiMe3) reacts with Et3B stereoselectively by 1,1-ethyloboration to give the alkenes with the boryl and silyl group in cis-positions at the C:C bond. Owing to the strongly intramol. coordinative N-B bond, these products are azoniasilaboratoles I (R = Me, Bu, SiMe3, X = NEt2). Protic reagents such as azoles react with I (X = NEt2, R = Me) to give Et2NH and the resp. N-azolyl derivs. such as II (R = Me, X = CH, N) which contain tetracoordinate boron. II (X = CH), derived from indazole, was characterized by x-ray anal. With EtOH, the corresponding oxoniasilaboratoles I (R = Me, SiMe3, X = OEt) are obtained. Treatment of I with H2O affords 1,2,5-oxasilaborolanes, presumably via an intermediate with the structure of a 2,5-dihydro-1,2,5-oxoniasilaboratole. All products were characterized by their 1H, 11B, 13C, 15N, and 29Si NMR data.(c) Wrackmeyer, B.; Milius, W.; Tok, O. L. Reaction of Alkyn-1-yl(diorganyl)silanes with 1-Boraadamantane: Si-H-B Bridges Confirmed by the Molecular Structure in the Solid State and in Solution. Chem. – Eur. J. 2003, 9, 4732– 4738, DOI: 10.1002/chem.200304961[Crossref], [PubMed], [CAS], Google Scholar4chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXot1yksLc%253D&md5=45a8710f018f3b578a0a1045377a09c9Reaction of alkyn-1-yl(diorganyl)silanes with 1-boraadamantane: Si-H-B bridges confirmed by the molecular structure in the solid state and in solutionWrackmeyer, Bernd; Milius, Wolfgang; Tok, Oleg L.Chemistry - A European Journal (2003), 9 (19), 4732-4738CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)1-Boraadamantane 1 reacted with 1-alkynylsilanes contg. one or two Si-H functions adjacent to triple bond, giving 4-methylene-3-borahomoadamantane derivs. with Si-H-B bridging hydrogens. Reaction of R1C≡CSiHR2 (2a-f, R1 = SiMe3, Ph, Bu, SiHPh2, ferrocenyl; R = Me, iPr, Ph) proceeds under mild conditions, giving 4-methylene-3-borahomoadamantane derivs. I (4a-f) quant. and selectively by 1,1-organoboration. An electron deficient Si-H-B bridge was present in the product. The analogous reaction of 1 with an 1-alkynyldisilane BuC≡CSiMe2SiHMe2 (3) gave the corresponding alkene deriv. 5 without the Si-H-B bridge. Evidence for the Si-H-B bridge in 4 was given by IR data, an extensive set of NMR spectroscopical data (1H, 11B, 13C, 29Si NMR) including various unusual isotope effects on chem. shifts and coupling consts., as well as from the crystal structure of 4e in the solid state. The crystal structure of precursor of 4e, Ph2Si(H)C≡CSi(H)Ph2 (2e) is also reported.
- 5(a) Reaction with alcohols:Kawachi, A.; Zaima, M.; Tani, A.; Yamamoto, Y. Dehydrogenative Condensation of (o-Borylphenyl)hydrosilane with Alcohols and Amines. Chem. Lett. 2007, 36, 362– 363, DOI: 10.1246/cl.2007.362[Crossref], [CAS], Google Scholar.5ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXjsVShtrs%253D&md5=89b1fa9ac8b9fccc082e785515d42d10Dehydrogenative condensation of (o-borylphenyl)hydrosilane with alcohols and aminesKawachi, Atsushi; Zaima, Masatoshi; Tani, Atsushi; Yamamoto, YohsukeChemistry Letters (2007), 36 (3), 362-363CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)O-[(Dimesitylboryl)phenyl]dimethylsilane (1a) undergoes dehydrogenative condensation with alcs. and amines, giving the corresponding alkoxysilanes and aminosilanes in moderate to high yields. It is plausible that the ortho-boryl group in 1a electrophilically activates the Si-H bond. An X-ray crystal structure of 1a is presented and discussed.(b) Reaction with KFKawachi, A.; Morisaki, H.; Tani, A.; Zaima, M.; Yamamoto, Y. Reaction of o-(HSiR2)(BMes2)C6H4 with a fluoride ion: Fluoride attack at silicon and hydride transfer from silicon to boron to form F-Si···H-B interaction. Heteroatom. Chem. 2011, 22, 471– 475, DOI: 10.1002/hc.20709[Crossref], [CAS], Google Scholar.5bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXnvFamtL8%253D&md5=3544a5015ba7e5d5cf82769995fd0743Reaction of o-(HSiR2)(BMes2)C6H4 with a fluoride ion: Fluoride attack at silicon and hydride transfer from silicon to boron to form F-Si···H-B interactionKawachi, Atsushi; Morisaki, Hiroshi; Tani, Atsushi; Zaima, Masatoshi; Yamamoto, YohsukeHeteroatom Chemistry (2011), 22 (3-4), 471-475CODEN: HETCE8; ISSN:1042-7163. (John Wiley & Sons, Inc.)Reactions of o-(HSiR2)(BMes2)C6H4 (R = Me (3a), Ph (3b)) with a F- ion afford [o-(FSiR2)(HBMes2)C6H4]- (R = Me (5a), Ph (5b)), which involve F-Si···H-B interactions to render the Si atom pentacoordinate. The F-Si···H-B interaction in 5a was confirmed by x-ray crystallog. anal. and atoms-in-mols. (AIM) anal. © 2011 Wiley Periodicals, Inc.(c) H-Ar Ligand Exchange:Kawachi, A.; Morisaki, H.; Nishioka, N.; Yamamoto, Y. Intramolecular H-Ar Ligand Exchange between Silicon and Boron: Functionality Transfer of Si-H to B-H. Chem. – Asian J. 2012, 7, 546– 553, DOI: 10.1002/asia.201100678[Crossref], [PubMed], [CAS], Google Scholar.5chttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhs12gsLrN&md5=4d65305124299339680b64f6af1d5e63Intramolecular H-Ar Ligand Exchange between Silicon and Boron: Functionality Transfer of Si-H to B-HKawachi, Atsushi; Morisaki, Hiroshi; Nishioka, Norimasa; Yamamoto, YohsukeChemistry - An Asian Journal (2012), 7 (3), 546-553CODEN: CAAJBI; ISSN:1861-4728. (Wiley-VCH Verlag GmbH & Co. KGaA)Ortho-C6H4(SiR3-nHn)(BMes2) (Mes = mesityl; R = Me, Ph; n = 1, 2) underwent Mes-H ligand exchange between the Si atom and the B atom to form o-C6H4(SiMesR3-nHn-1)(BMesH) upon heating. The resulting hydroborane intermediates immediately react with benzaldehyde to afford the corresponding benzyloxyboranes. A DFT study of model compds. reveals the transition states of the ligand exchange. A hydride abstraction from the Si atom by the B center is key to reaching the transition states, which include the tricoordinate silyl-cation moiety and the tetracoordinate hydridoborate moiety. The mol. structures of two benzyloxyboranes were detd. by x-ray crystallog.(d) Formation of Si-O-B linkage:Kawachi, A.; Zaima, M.; Yamamoto, Y. Intramolecular Reaction of Silanol and Triarylborane: Boron-Aryl Bond Cleavage and Formation of a Si-O-B Heterocyle. Organometallics 2008, 27, 4691– 4696, DOI: 10.1021/om8004405[ACS Full Text], [CAS], Google Scholar5dhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtValurbN&md5=f942de13542a821ca976b8539095e950Intramolecular Reaction of Silanol and Triarylborane: Boron-Aryl Bond Cleavage and Formation of a Si-O-B HeterocycleKawachi, Atsushi; Zaima, Masatoshi; Yamamoto, YohsukeOrganometallics (2008), 27 (18), 4691-4696CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)[2-(Hydroxydimethylsilyl)phenyl]dimesitylborane (1) underwent intramol. cyclization to give 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboracyclopentene (shown as I) and mesitylene by B-aryl bond cleavage. Whereas NEt3 as an additive accelerated the cyclization, DBU stabilized a silyloxyborate complex, 1,8-diazabicyclo[5.4.0]-7-undecenium 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboratacyclopentene, which was characterized by x-ray crystallog. anal. and HF calcns. The D-labeling expts. revealed that the H migrated from the hydroxyl group in 1 to the leaving mesitylene.
- 6Katz, H. E. Anion complexation and migration in (8-silyl-1-naphthyl)boranes. Participation of hypervalent silicon. J. Am. Chem. Soc. 1986, 108, 7640– 7645, DOI: 10.1021/ja00284a031[ACS Full Text], [CAS], Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2sXhvVKmsA%253D%253D&md5=18b9d59d379dca4078ce206b5981ff59Anion complexation and migration in (8-silyl-1-naphthyl)boranes. Participation of hypervalent siliconKatz, Howard EdanJournal of the American Chemical Society (1986), 108 (24), 7640-5CODEN: JACSAT; ISSN:0002-7863.Title compds. I [R = Me (II), OEt (III), F (IV)] were prepd. and their complexes with F- were examd. The structure of II·F- was detd. by multinuclear NMR and x-ray crystallog., revealing an interaction between F- and pentacoordinate Si. Syntheses of III from dimethyl(8-iodo-1-naphthyl)silane and of IV from III proceeded via intramol. anion migrations and hypervalent Si intermediates. III is the only reported example of a silyl ether coordinated to a borane. The complex IV·F- was more stable than II·F- because of increased participation of FMe2Si in F- bonding compared to Me3Si. This work represents the first consideration of Si as a participant in bidentate anion complexation and the first exploration of bidentate Lewis acids in which the two active functional groups are different.
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- 10(a) Olah, G. A.; Li, X.-Y.; Wang, Q.-J.; Rusul, G.; Prakash, G. K. S. Trisilyloxonium Ions: Preparation, NMR Spectroscopy, Ab Initio/IGLO Studies, and Their Role in Cationic Polymerization of Cyclosiloxanes. J. Am. Chem. Soc. 1995, 117, 8962– 8966, DOI: 10.1021/ja00140a010[ACS Full Text.], [CAS], Google Scholar10ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2MXnsFWjur0%253D&md5=3e9e94dfbd352fbea09c630a7e39c3a2Trisilyloxonium Ions: Preparation, NMR Spectroscopy, Ab Initio/IGLO Studies, and Their Role in Cationic Polymerization of CyclosiloxanesOlah, George A.; Li, Xing-Ya; Wang, Qunjie; Rasul, Golam; Prakash, G. K. SuryaJournal of the American Chemical Society (1995), 117 (35), 8962-6CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Trisilyloxonium ions, key intermediates in understanding the mechanism of cationic polymn. of cyclosiloxanes, were for the first time prepd. by reacting Me3SiH with trityl tetrakis(pentafluorophenyl)borate (TPFPB) in the presence of siloxanes and directly obsd. by NMR spectroscopy at -70°. Polymn. or oligomerization of siloxanes can be initiated by in-situ formed trisilyloxonium ions. The mechanism of the reactions is discussed. Structural parameters and 29Si NMR chem. shifts of a no. of silyloxonium ions were calcd. by ab initio/IGLO methods. The results are in good agreement with the obtained exptl. data.(b) Kira, M.; Hino, T.; Sakurai, H. Chemistry of organosilicon compounds. 292. An NMR Study of the Formation of Silyloxonium Ions by Using Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as Counteranion. J. Am. Chem. Soc. 1992, 114, 6697– 6700, DOI: 10.1021/ja00043a013[ACS Full Text.], [CAS], Google Scholar10bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XltFWlur0%253D&md5=5312d041740f7c0204ce7d2f5106e69aChemistry of organosilicon compounds. 292. An NMR study of the formation of silyloxonium ions by using tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as counteranionKira, Mitsuo; Hino, Takakazu; Sakurai, HidekiJournal of the American Chemical Society (1992), 114 (17), 6697-700CODEN: JACSAT; ISSN:0002-7863.The capability of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB) as a counteranion for organosilicenium ions was investigated by NMR spectroscopy. Although reactions of hydrosilanes with trityl-TFPB did not give the corresponding silicenium ions as long-lived species in dichloromethane-d2, the reactions produced rather stable silyloxonium ions in the presence of ethers at low temps. The evidence for the formation of cyclic silyloxonium ions was obtained by monitoring the reaction of 3-ethoxypropylsilanes with trityl-TFPB by NMR spectroscopy. The use of TFPB as a nonnucleophilic counteranion was crucial for the formation of the silyloxonium ions; silyl perchlorates did not show significant interaction with ethers.(c) Kira, M.; Hino, T.; Sakurai, H. Siloxycarbenium Tetrakis[3,5-bis(trifluoromethyl)phenyl]borates and Their Role in Reactions of Ketones with Nucleophiles. Chem. Lett. 1992, 21, 555– 558, DOI: 10.1246/cl.1992.555 .(d) Driess, M.; Barmeyer, R.; Monsé, C.; Merz, K. E(SiMe3)4+ Ions (E=P, As): Persilylated Phosphonium and Arsonium Ions. Angew. Chem., Int. Ed. 2001, 40, 2308– 2310, DOI: 10.1002/1521-3773(20010618)40:12<2308::AID-ANIE2308>3.0.CO;2-Q[Crossref], [CAS], Google Scholar.10dhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXkvVWks7o%253D&md5=6ad21b8b4dee115523a2876acea44c78E(SiMe3)4+ ions (E = P, As): persilylated phosphonium and arsonium ionsDriess, Matthias; Barmeyer, Rainer; Monse, Christian; Merz, KlausAngewandte Chemie, International Edition (2001), 40 (12), 2308-2310CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)Reaction of silylium salts, [Me3Si(L)]+ X- [L = C6H6, X = B(C6F5)4], with trissilylphosphine and -arsine, E(SiMe3)3 (E = P, As), gave persilylated phosphonium and arsonium ions, [(Me3Si)4E]+ X-, which were characterized by x-ray crystallog. along with reaction byproduct [Et2OSiMe3]+ X-.(e) Kordts, N.; Künzler, S.; Rathjen, S.; Sieling, T.; Großekappenberg, H.; Schmidtmann, M.; Müller, T. Silyl Chalconium Ions: Synthesis, Structure and Application in Hydrodefluorination Reactions. Chem. – Eur. J. 2017, 23, 10068– 10079, DOI: 10.1002/chem.201700995[Crossref], [PubMed], [CAS], Google Scholar.10ehttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXos1Ciu7k%253D&md5=5a6221d1f24a489f09195f586b22f40eSilyl Chalconium Ions: Synthesis, Structure and Application in Hydrodefluorination ReactionsKordts, Natalie; Kuenzler, Sandra; Rathjen, Saskia; Sieling, Thorben; Grossekappenberg, Henning; Schmidtmann, Marc; Mueller, ThomasChemistry - A European Journal (2017), 23 (42), 10068-10079CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)The synthesis of two series of silylated chalconium borates, 9 and 10, which are based on the peri-naphthyl and peri-acenaphthyl framework, is reported (chalcogen (Ch): O, S, Se, Te). NMR investigations of the selenium- and tellurium-contg. precursor silanes 3d-f and 8d,f revealed a significant through-space J-coupling between the chalcogen nuclei and the Me2SiH group. Exptl. and computational results typify the synthesized cations 9 and 10 as chalconium ions. The imposed ring strain weakens the Si-Ch linkage compared to acyclic chalconium ions. This attenuation of the Si-Ch bond strength is more pronounced in the acenaphthene series. Surprisingly, the Si-O bonds in oxonium ions 9a and 10a are the weakest Si-Ch linkage in both series. The synthesized silyl chalconium borates are active in hydrodefluorination reactions of alkyl fluorides with silanes. A cooperative activation of the silane by the Lewis acidic (silicon) and by the Lewis basic side (chalcogen) is suggested.(f) Bläsing, K.; Labbow, R.; Michalik, D.; Reiß, F.; Schulz, A.; Villinger, A.; Walker, S. On Silylated Oxonium and Sulfonium Ions and Their Interaction with Weakly Coordinating Borate Anions. Chem. – Eur. J. 2019, DOI: 10.1002/chem.201904403
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- 13Formation of Si-O-B linkage:Brzozowska, A.; Ćwik, P.; Durka, K.; Kli, T.; Laudy, A. E.; Luliński, S.; Serwatowski, J.; Tyski, S.; Urban, M.; Wróblewski, W. Benzosiloxaboroles: Silicon Benzoxaborole Congeners with Improved Lewis Acidity, High Diol Affinity, and Potent Bioactivity. Organometallics 2015, 34, 2924– 2932, DOI: 10.1021/acs.organomet.5b00265[ACS Full Text], [CAS], Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVSjtb%252FL&md5=a2f3dacbf9c201ff27957ccad3151689Benzosiloxaboroles: Silicon Benzoxaborole Congeners with Improved Lewis Acidity, High Diol Affinity, and Potent BioactivityBrzozowska, Aleksandra; Cwik, Pawel; Durka, Krzysztof; Klis, Tomasz; Laudy, Agnieszka E.; Lulinski, Sergiusz; Serwatowski, Janusz; Tyski, Stefan; Urban, Mateusz; Wroblewski, WojciechOrganometallics (2015), 34 (12), 2924-2932CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)The synthesis and physicochem. properties of benzosiloxaboroles, the Si analogs of an important class of heterocyclic compds.-benzoxaboroles-is presented. They were prepd. by halogen-Li exchange reactions of (2-bromophenyl)boronates with BuLi followed by the silylation or boronation of (2-lithiophenyl)dimethylsilanes. The cyclization of the resulting 2-(dimethylsilyl)phenylboronates apparently occurs through intramol. dehydrogenative cyclization reaction in the presence of H2O. Unlike the case for benzosiloxaborole, the formation of its analog contg. a thiophene ring is thermodynamically unfavorable, which was confirmed by theor. calcns. The presence of a B-O-Si linkage results in increased Lewis acidity with respect to the analogous benzoxaboroles. The acidity is strongly enhanced by fluorination or introduction of Ph groups at the Si atom. Selected compds. show good antifungal activity, and thus they are potential small-mol. therapeutic agents. They can also serve as effective receptors for biol. relevant diols under neutral pH conditions.
Supporting Information
ARTICLE SECTIONSThe Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03784.
Table of crystallographic data for 10 (PDF/CIF) and computational work for 2 (R = Me) and anionic parts of 8 and 9, and 10 (PDF)
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