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Synlett 2007(1): 0075-0078  
DOI: 10.1055/s-2006-958436
LETTER
© Georg Thieme Verlag Stuttgart · New York

A Convenient Synthesis of Z-Allylsilanes with Good Stereoselectivity Promoted by Samarium Diiodide

José M. Concellón*, Humberto Rodríguez-Solla, Carmen Simal, Cecilia Gómez
Departamento Química Orgánica e Inorgánica, Universidad de Oviedo, C/ Julián Clavería, 8, 33006, Oviedo, Spain
Fax: jmcg@fq.uniovi.es;
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Publication History

Received 9 October 2006
Publication Date:
20 December 2006 (online)

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Abstract

Synthesis of Z-allylsilanes in high or good yields and with good stereoselectivity is achieved from O-acetylated 1-silyl-3-chloro alcohols promoted by SmI2. The starting compounds were easily prepared from 2-chloroaldehydes and a mechanism is proposed to explain the stereoselectivity of the β-elimination reaction.

Key words

Allylsilanes - diastereoselectivity - eliminations - samarium

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General Procedure for the Synthesis of Compounds 3.
Trimethylsilylmethyllithium (12 mL, 1.0 M solution in pentane) was added at -85 °C to a solution of the corresponding α-chloroaldehyde 1 (10 mmol) in THF (10 mL). After stirring for 2 h, the reaction mixture was quenched by addition of sat. aq NH4Cl (10 mL). Standard work-up provided crude 3-chloro-1-(trimethylsilyl)alkan-2-ols 2. The O-acetylation reaction was carried out by treatment of the corresponding crude 3-chloro-1-(trimethylsilyl)alkan-2-ols 2 (1 mmol) with Et3N (10 mL), Ac2O (10 mL) and a catalytic amount of DMAP (5 mg). The reaction mixture was stirred for 12 h at r.t., then the reaction was quenched with ice-cold H2O (30 mL). The organic material was extracted with CH2Cl2. The combined extracts were dried over Na2SO4 and the solvent was removed under reduced pressure affording compounds 3 which were utilized without further purification. Compounds 3 were obtained as a mixture of diastereoisomers (roughly 1:1), after column chromatography (hexane-EtOAc, 5:1).

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General Procedure for the Synthesis of Allylsilanes 4. A solution of SmI2 (1.2 mmol) in THF (12 mL) was added dropwise, under a nitrogen atmosphere, to a stirred solution of the corresponding starting material 3 (0.4 mmol) at r.t. The reaction mixture was then refluxed for 8 h. After this time, it was quenched with aq HCl (0.1 M, 10 mL). The organic material was extracted with Et2O. The combined extracts were dried over Na2SO4 and the solvent was removed under reduced pressure affording crude compounds 4 which were purified by short-column chromatography (silica gel, pentane as eluent). Spectroscopical data for the compounds 4 which are not described in the literature are given here.
(Z)-Dodeca-2,11-dienyltrimethylsilane (4d): R f = 0.83 (pentane). 1H NMR (300 MHz, CDCl3): δ = 5.80 (ddt, J = 17.0, 10.2, 6.7 Hz, 1 H), 5.40-5.32 (m, 1 H), 5.28-5.21 (m, 1 H), 4.98 (ddt, J = 17.0, 2.3, 1.6 Hz, 1 H), 4.91 (ddt, J = 10.2, 2.3, 1.1 Hz, 1 H), 2.08-1.97 (m, 4 H), 1.47 (d, J = 8.5 Hz, 2 H), 1.43-1.25 (m, 10 H), 0.01 (s, 9 H). 13C NMR (75 MHz, CDCl3): δ = 139.1 (CH), 127.6 (CH), 125.1 (CH), 114.0 (CH2), 33.7 (CH2), 29.7 (CH2), 29.3 (CH2), 29.3 (CH2), 29.0 (CH2), 28.8 (CH2), 27.0 (CH2), 18.3 (CH2), -1.9 (3 × CH3). MS (70 eV): m/z (%): 238 (3) [M+], 73 (100), 59 (11), 41 (11). IR (neat): 2925, 2854, 1465, 1378 cm-1. Anal. Calcd for C15H30Si: C, 75.54; H, 12.68. Found: C, 75.22; H, 12.90.
Trimethyl[(2Z,8Z)-undeca-2,8-dienyl]silane (4e): R f = 0.85 (pentane). 1H NMR (300 MHz, CDCl3): δ = 5.44-5.22 (m, 4 H), 2.06-1.97 (m, 6 H), 1.47 (d, J = 8.3 Hz, 2 H), 1.39-1.34 (m, 4 H), 0.96 (t, J = 7.5 Hz, 3 H), 0.00 (s, 9 H). 13C NMR (75 MHz, CDCl3): δ = 131.5 (CH), 129.2 (CH), 127.5 (CH), 125.3 (CH), 29.5 (CH2), 29.4 (CH2), 27.0 (CH2), 26.9 (CH2), 20.5 (CH2), 18.3 (CH2), 14.3 (CH3), -1.9 (3 × CH3). MS (70 eV): m/z (%) = 224 (2) [M+], 150 (18), 142 (21), 121 (16), 73 (100), 59 (36), 45 (38). HRMS: m/z calcd for C13H25Si [M+ - Me]: 209.1726; found: 209.1729. IR (neat): 3007, 2929, 1462, 1248, 855 cm-1.
Trimethyl[(E/Z)-4-phenylpent-2-enyl]silane (4g): R f = 0.65, 0.60 (hexane). 1H NMR (300 MHz, CDCl3): δ = 7.37-7.21 (m, 10 H), 5.55-5.43 (m, 4 H), 3.75 (q, J = 7.5 Hz, 1 H), 3.51-3.46 (m, 1 H), 1.68-1.49 (m, 4 H), 1.40 (t, J = 6.0 Hz, 3 H), 1.38 (t, J = 6.8 Hz, 3 H), 0.06 (s, 9 H), 0.05 (s, 9 H). 13C NMR (75 MHz, CDCl3): δ = 147.0 (C), 146.8 (C), 133.6 (CH), 132.6 (CH), 128.3 (2 × CH), 128.2 (2 × CH), 127.1 (2 × CH), 126.9 (2 × CH), 125.7 (CH), 125.7 (CH), 125.1 (CH), 124.6 (CH), 42.4 (CH), 36.8 (CH), 22.5 (CH3), 21.7 (CH3), 18.5 (2 × CH2), -1.80 (3 × CH3), -1.90 (3 × CH3). MS (70 eV): m/z (%) = 218 (41) [M+], 144 (20), 135 (23), 129 (19), 105 (16), 73 (100), 59 (16), 45 (25). HRMS: m/z calcd for C14H22Si [M+]: 218.1491; found: 218.1487. IR (neat): 3004, 2957, 1248, 856, 698 cm-1.
Trimethyl[(E/Z)-3-methyldodec-2-enyl]silane (4i): R f = 0.90 (pentane). 1H NMR (300 MHz, CDCl3): δ = 5.17-5.08 (m, 2 H), 2.06-2.03 (m, 2 H), 1.99-1.86 (m, 4 H), 1.84-1.82 (m, 2 H), 1.67 (s, 3 H), 1.52 (s, 3 H), 1.39-1.12 (m, 28 H), 0.89-0.85 (m, 6 H), -0.02 (s, 9 H). 13C NMR (75 MHz, CDCl3): δ = 133.1 (C), 132.6 (C), 119.9 (CH), 119.8 (CH), 39.8 (CH2), 31.9 (2 × CH), 31.4 (CH2), 29.6 (2 × CH2), 29.5 (2 × CH2), 29.3 (2 × CH2), 29.2 (CH2), 28.2 (CH2), 27.9 (CH2), 25.5 (CH2), 23.3 (CH3), 22.6 (2 × CH2), 18.4 (CH2), 18.2 (CH2), 15.6 (CH3), 14.0 (2 × CH3), -1.80 (3 × CH3), -1.80 (3 × CH3). MS (70 eV): m/z (%) = 254 (6) [M+], 180 (2), 73 (100), 59 (5), 41 (6). HRMS: m/z calcd for C16H34Si [M+]: 254.2430; found: 254.2426. IR (neat): 2925, 2855, 1458, 1247, 857 cm-1.

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This is consistent with other previously reported SmI2-mediated β-elimination methodologies with Z-selectivity (see ref. 7-9). So, when aryl-substituted olefins were obtained an enhancement of the E-stereoisomer was observed.

19

An isomerization process of the Z-phenyl alkene to the more stable E-isomer cannot be discarded under this reaction conditions.