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

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.

Allylsilanes - diastereoselectivity - eliminations - samarium

References and Notes

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<A NAME="RG29906ST-2A">2a</A> For reviews concerning the synthesis of allylsilanes, see: Sarkar TK. Synthesis  1990,  969 

<A NAME="RG29906ST-2B">2b</A> Sarkar TK. Synthesis  1990,  1101 

<A NAME="RG29906ST-2C">2c</A> To see some examples of the synthesis of E-allylsilanes: Tanigawa Y. Fuse Y. Murahashi S.-I. Tetrahedron Lett.  1982,  23:  557 

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<A NAME="RG29906ST-10A">10a</A> α-Chloroaldehydes can be easily obtained, see: Halland N. Braunton A. Bachmann S. Marigo M. Jørgensen KA. J. Am. Chem. Soc.  2004,  126:  4790 

<A NAME="RG29906ST-10B">10b</A> α-Chlorination of α-alkylated aldehydes was carried out using sulfuryl chloride: Stevens CL. Farkas E. Gillis B. J. Am. Chem. Soc.  1954,  76:  2695 

<A NAME="RG29906ST-11">11</A>

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).

<A NAME="RG29906ST-12">12</A> The solution of SmI₂ in THF was rapidly obtained by reaction of diiodomethane with samarium powder in the presence of sonic waves: Concellón JM. Rodríguez-Solla H. Bardales E. Huerta M. Eur. J. Org. Chem.  2003,  1775 

<A NAME="RG29906ST-13">13</A>

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.

<A NAME="RG29906ST-14">14</A> Hsiao C.-N. Shechter H. Tetrahedron Lett.  1982,  23:  1963 

<A NAME="RG29906ST-15">15</A> Shiragami H. Kawamoto T. Imi K. Matsubara S. Utimoto K. Nozaki H. Tetrahedron  1988,  44:  4009 

Other six-membered-ring transition-state models have been proposed to explain the selectivity in other reactions of SmI₂:

<A NAME="RG29906ST-16A">16a</A> Molander GA. Etter JB. Zinke PW. J. Am. Chem. Soc.  1987,  109:  453 

<A NAME="RG29906ST-16B">16b</A> Urban D. Skrydstrup T. Beau JM. J. Org. Chem.  1998,  63:  2507 

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<A NAME="RG29906ST-16D">16d</A> Concellón JM. Pérez-Andrés JA. Rodríguez-Solla H. Chem. Eur. J.  2001,  7:  3062 

<A NAME="RG29906ST-16E">16e</A> Concellón JM. Rodríguez-Solla H. Chem. Soc. Rev.  2004,  33:  599 ; and references 7-9

To see previous works about the γ-effect of the silicon atom, see:

<A NAME="RG29906ST-17A">17a</A> Sommer LH. Dorfman E. Goldberg GM. Whitmore FC. J. Am. Chem. Soc.  1946,  68:  488 

<A NAME="RG29906ST-17B">17b</A> Shiner VJ. Ensinger MW. J. Am. Chem. Soc.  1986,  108:  842 

<A NAME="RG29906ST-17C">17c</A> Davidson ER. Shiner VJ. J. Am. Chem. Soc.  1986,  108:  3135 

<A NAME="RG29906ST-18">18</A>

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.

<A NAME="RG29906ST-19">19</A>

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

<A NAME="RG29906ST-20">20</A> A similar behavior has been observed in other SmI₂-promoted processes when benzylic radicals are generated: Davies SG. Rodríguez-Solla H. Tamayo JA. Cowley AR. Concellón C. Garner AC. Parkes AL. Smith AD. Org. Biomol. Chem.  2005,  3:  1435