Radical Addition of Silanes to Alkenes Followed by Oxidation

Abstract

Phenyldimethylsilane and trichlorosilane are shown to undergo efficient radical hydrosilylation reactions, on reaction with various alkenes, using triethylborane as the initiator. Adducts from the trichlorosilane reactions can be oxidised to afford alcohols in good yields. This two-step process leads to the anti-Markovnikov hydration of alkenes.

Key words

addition reaction - alcohol - alkene - radical reaction - ­silane

References and Notes

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6

All new compounds gave spectroscopic and HRMS data in accord with their structures.

7

Typical Procedure for the Addition of Phenyldimethylsilane to Alkenes To a stirred solution of the alkene (10 mmol, 1 equiv), phenyldimethylsilane (2.0 g, 15 mmol, 1.5 equiv) in THF (3 mL) was added Et₃B in THF (0.5 mL, 1 M solution, 5 mmol, 0.5 equiv) and, shortly after, triisopropylsilane thiol (105 µL, 0.5 mmol, 5 mol%, care required due to noxious smell). After stirring at r.t. for 1 h, a further portion of Et₃B in THF (0.5 mL, 1 M solution 5 mmol, 0.5 equiv) was added and the mixture was left stirring overnight. Removal of the solvent under reduced pressure afforded the crude product which was purified by flash silica chromatography (elution gradient PE to PE-EtOAc = 5:1) to afford the silane addition products 1a-j (81-97%).
Representative Analytical Data[3-(4-Methoxyphenyl)propyl]dimethyl(phenyl)silane (1a) R _f  = 0.45 (PE-EtOAc = 10:1). IR (thin film): ν_max = 3010 (w), 2988 (w), 2959 (w), 2837 (w), 1743 (m), 1503 (w), 1512 (s), 1466 (m) cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.57-7.53 (2 H, m, ^ArCH), 7.42-7.38 (3 H, m, ^ArCH), 7.11 (2 H, app dd, J = 8.6, 2.1 Hz, ^ArCH), 6.87 (2 H, app dd, J = 8.6, 2.1 Hz, ^ArCH), 3.81 (3 H, s, OCH₃), 2.61 (2 H, t, J = 7.7 Hz, ^ArCH₂), 1.71-1.63 (2 H, m, CH₂), 0.86-0.79 (2 H, m, SiCH₂), 0.29 [6 H, s, Si(CH₃)₂]. ^¹³C NMR (100 MHz, CDCl₃): δ = 157.9 (^ArCO), 139.6 (^ArC), 134.4 (^ArC), 133.7 (2 × ^ArCH), 129.5 (2 × ^ArCH), 128.9 (^ArCH), 127.9 (2 × ^ArCH), 113.7 (2 × ^ArCH), 55.0 (OCH₃), 38.5 (^ArCH₂), 25.8 (CH₂), 15.0 (SiCH₂), -3.5 [2 × Si(CH₃)₂]. ESI-MS: m/z (%) = 285 (20), 284 (100) [M⁺], 207 (10).
Octyl(dimethyl)(phenyl)silane (1b) R _f = 0.80 (PE). IR (thin film): ν_max = 3058 (w), 2918 (s), 2857 (s), 2120 (w), 1470 (m), 1431 (m) cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.55-7.45 (2 H, m, ^ArCH), 7.38-7.30 (3 H, m, ^ArCH), 1.33-1.20 (12 H, m, CH₂), 0.91 (3 H, t, J = 7.0 Hz, CH₃), 0.77 (2 H, t, J = 8.1 Hz, SiCH₂), 0.29 [6 H, s, Si(CH₃)₂]. ^¹³C NMR (100 MHz, CDCl₃): δ = 139.7 (^ArC), 133.5 (2 × ^ArCH), 128.7 (^ArCH), 127.7 (2 × ^ArCH), 33.6 (CH₂), 31.9 (CH₂), 29.3 (2 × CH₂), 23.8 (CH₂), 22.6 (CH₂), 18.2 (CH₂), 15.7 (CH₂), 14.1 (CH₃), -3.0 [Si(CH₃)₂]. ESI-MS: m/z (%) = 250 (20), 249 (100) [MH⁺], 233 (30), 171 (20)
[3-(3,4-Dimethoxyphenyl)propyl]dimethyl(phenyl)-silane (1j) R _f = 0.30 (PE-EtOAc = 10:1). IR (thin film): ν_max = 3004 (w), 2990 (w), 2952 (w), 2929 (w), 2833 (w), 1739 (m), 1509 (w), 1514 (s), 1464 (m) cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.53-7.44 (2 H, m, ^ArCH), 7.37-7.34 (3 H, m, ^ArCH), 6.79 (1 H, d, J = 8.0 Hz, ^ArCH), 6.69 (1 H, dd, J = 8.0, 1.9 Hz, ^ArCH), 6.66 (1 H, d, J = 1.9 Hz, ^ArCH), 3.86 (6 H, s, OCH₃), 2.57 (2 H, t, J = 7.6 Hz, ^ArCH₂), 1.68-1.56 (2 H, m, CH₂), 0.83-0.77 (2 H, m, SiCH₂), 0.26 [6 H, s, Si(CH₃)₂]. ^¹³C NMR (100 MHz, CDCl₃): δ = 148.7 (^ArCO), 147.1 (^ArCO), 139.3 (^ArC), 135.3 (^ArC), 133.7 (2 × ^ArCH), 128.9 (^ArCH), 127.8 (2 × ^ArCH), 120.4 (^ArCH), 111.7 (^ArCH), 111.1 (^ArCH), 56.0 (OCH₃), 55.9 (OCH₃), 39.3 (^ArCH₂), 26.1 (CH₂), 15.4 (SiCH₂), -3.0 [2 × Si(CH₃)₂]. ESI-MS: m/z (%) = 315 (20), 314 (100) [M⁺], 283 (30), 237 (15).

8

Typical Procedure for the Oxidation of Phenyldimethylsilanes 1a-c To a stirred solution of the dimethylphenylsilane 1a-c (5.0 mmol, 1 equiv) in dry CH₂Cl₂ (15 mL) at r.t. was added BF₃-AcOH complex (1.4 mL, 10.0 mmol, 2 equiv), and the resulting solution was stirred for 6 h, during which time the solution turned orange. The reaction mixture was quenched by being poured slowly into a stirred solution of 1 M NaHCO₃ (100 mL), the aqueous layer was extracted with CH₂Cl₂ (2 × 75 mL), the combined organic extracts were dried over MgSO₄ and evaporated under reduced pressure to afford the fluorosilanes 2a-c as a pale yellow oil (0.71-0.93 g, 75-88%). No further purification was carried out, and the resulting oil was subjected to the oxidation conditions.

9

Typical Procedure for the Oxidation of Fluorosilanes 2a-c To a stirred solution of the unpurified fluorosilane 2a-c (3.4-4.0 mmol, 1 equiv) and anhyd KF (0.39-0.46 g, 6.8-8.0 mmol, 2 equiv) in dry DMF (5 mL) at r.t. was added drop-wise a solution of MCPBA (1.38-1.62 g, 85%, 6.8-8.0 mmol, 2 equiv) in dry DMF (10 mL). The resulting solution was stirred for 4 h at r.t. The reaction mixture was diluted with CH₂Cl₂ (75 mL) and washed successively with aq Na₂S₂O₃ (2 × 50 mL), aq Na₂CO₃ (2 × 50 mL), brine (50 mL), then dried over MgSO₄ and purified by flash chroma-tography (PE-Et₂O = 10:1) to afford alcohols 3a-c as colourless oils (0.11-0.18 g, 25-31%).

11

Typical Procedure for the Addition of Trichlorosilane to Alkenes 1a-c To a stirred solution of the alkene (5.0 mmol, 1 equiv) in THF (5 mL) at 0 ˚C, under air, was added Cl₃SiH (1.0 mL, 10.0 mmol, 2 equiv) followed by the slow dropwise addition of Et₃B (2.0 mL, 1 M solution in THF, 2.0 mmol, 0.4 equiv). The resulting solution was stirred at 0 ˚C for 1 h, after which a further portion of Et₃B (2.0 mL, 1 M solution in THF, 2.0 mmol, 0.4 equiv) was added, and the mixture was stirred for a further 1 h at 0 ˚C followed by addition of a further portion of Et₃B (2.0 mL, 1 M solution in THF, 2.0 mmol, 0.4 equiv). The resulting solution was stirred at 0 ˚C for 1 h then warmed to r.t. and stirred for a further 4 h. Removal of the solvent under reduced pressure afforded the crude trichlorosilane addition product, as an oil.

12

Typical Procedure for the Oxidation of Trichlorosilanes to Give Alcohols 3a-f The crude trichlorosilane addition product was taken up in THF (75 mL), and the solution was stirred at r.t. (under air) while MeOH (75 mL) was slowly added, after which KF (2.6 g, 45.0 mmol, 9 equiv) and KHCO₃ (9.00 g, 90.0 mmol 18 equiv) were added, and the suspension was stirred for 1 h. To the resulting white suspension was added H₂O₂ (5.1 mL, 30% solution, 45.0 mmol, 9 equiv), and the reaction mixture was vigorously stirred for 24 h; after which Na₂S₂O₃˙5H₂O (7.4 g, 30.0 mmol, 6 equiv) was added, and the mixture was stirred for 1 h. The mixture was filtered through a Celite plug, and the filter cake was rinsed with Et₂O (50 mL). The filtrate was concentrated under vacuum, the resulting residue was dissolved in CH₂Cl₂ (50 mL), dried over MgSO₄, and the solvent was removed in vacuo to afford the crude product. This was purified by flash silica chromatography (elution gradient PE to PE-EtOAc = 2:1) to afford alcohols 3a-f (39-51%).