Intramolecular Activation of C–O Bond by an o-Boryl Group in o-(Alkoxysilyl)(diarylboryl)benzenes

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

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. 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. 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

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 (¹J_B–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. 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. 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.

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.

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.

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 LP_O 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. C–O Bond Cleavage in 10 with DABCO.

Scheme 6. C–O Bond Cleavage in 10 with KF/18-crown-6.

Conclusions

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

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, ¹J_B–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 (C₆D_6, 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 (C₆D_6, δ) 20.02 (br). ²⁹Si{¹H} NMR (C₆D_6, 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) ų, Z = 8, D_calcd = 1.161 g/cm³, R(I > 2σ(I)) = 0.0386, R_w (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.

Supporting Information Available

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)

The authors declare no competing financial interest.