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Synlett 2011(19): 2811-2814  
DOI: 10.1055/s-0031-1289568
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Radical Addition of Silanes to Alkenes Followed by Oxidation

Matthew J. Palframana, Andrew F. Parsons*a, Paul Johnsonb
a Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
Fax: +44(1904)322516; e-Mail: andy.parsons@york.ac.uk;
b AstraZeneca, Alderley Park, Macclesfield, Cheshire, SK10 4TF, UK
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Publication History

Received 8 September 2011
Publication Date:
09 November 2011 (online)

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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): νmax = 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): νmax = 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): νmax = 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%).