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Synlett 2006(5): 0771-0775  
DOI: 10.1055/s-2006-933129
LETTER
© Georg Thieme Verlag Stuttgart · New York

Rh(I)-Catalyzed O-Silylation of Alcohol with Vinylsilane

Jung-Woo Park, Hoon-Jo Chang, Chul-Ho Jun*
Department of Chemistry and Centre for Bioactive Molecular Hybrid (CBMH), Yonsei University, Seoul 120-749, Korea
Fax: +82(2)31472644; e-Mail: junch@yonsei.ac.kr;
Further Information

Publication History

Received 4 January 2006
Publication Date:
09 March 2006 (online)

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Abstract

Silyl ethers can be produced from alcohols and vinyl­silanes under a rhodium(I) catalyst. The reaction is believed to proceed through an O-H bond cleavage of alcohol by rhodium(I) complex and a subsequent hydride insertion into vinylsilane ­followed by β-silyl elimination of the resulting β-silylethyl ­rhodium(III) complex.

Key words

silyl ether - alcohol - vinylsilane - Wilkinson’s complex - O-H bond cleavage

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The Reaction of 1a and 2a with 3, 4a and Benzoic Acid (Equation 1) A screw-capped pressure vial (1 mL) was charged with 36.6 mg (0.300 mmol) of 4-hydroxybenzaldehyde (1a), 213.8 mg (1.200 mmol) of triethylvinylsilane (2a), 13.9 mg (0.015 mol) of (Ph3P)3RhCl(3), 13.0 mg (0.120 mmol) of 2-amino-3-picoline (4a), 3.6 mg (0.030 mmol) of benzoic acid, 120.0 mg of THF, and 60.0 mg of toluene. The reaction mixture was stirred for 12 h in an oil bath that was preheated to 160 °C. After cooling to r.t., the vial was opened carefully, and a crude mixture was purified by column chromato-graphy (SiO2, n-hexane-EtOAc = 5:2). For a GC analysis, the crude mixture was filtered through a small plug of silica gel to remove the catalyst. The ratio of 5a and 5b was determined by GC analysis.
Compound 5a: 1H NMR (250 MHz, CDCl3): δ = 7.92-7.88 (d, J = 8.7 Hz, 2 H), 6.91-6.87 (d, J = 8.7 Hz, 2 H), 5.86 (br s, 1 H), 2.91-2.84 (t, J = 8.3 Hz, 2 H), 0.99-0.88 (m, 11 H), 0.61-0.51 (q, J = 8.0 Hz, 6 H). 13C NMR (62.9 MHz, CDCl3): δ = 202.2, 161.2, 131.1, 129.2, 115.8, 32.9, 7.6, 6.7, 3.4. MS (EI, 70eV): m/z (rel. intensity) = 263 (1) [M - 1], 235 (100), 161 (11), 121 (10), 103 (9), 89 (13), 75 (9). IR (KBr): 3357, 2952, 2902, 2867, 1662, 1605, 1578, 1512, 1285, 1239, 1216, 1169, 1016, 973, 838, 750 cm-1. HRMS (CI): m/z calcd for C15H25O2Si [M - H+]: 265.1624; found: 265.1624. Anal. Calcd for C15H24O2Si: C, 68.13; H, 9.15. Found: C, 68.38; H, 9.19. Compound 5b: 1H NMR (250 MHz, CDCl3): δ = 7.90-7.86 (d, J = 8.8 Hz, 2 H), 6.90-6.87 (d, J = 8.8 Hz, 2 H), 2.91-2.84 (t, J = 8.2 Hz, 2 H), 1.00-0.93 (m, 20 H), 0.82-0.72 (q, J = 7.9 Hz, 6 H), 0.62-0.55 (q, J = 8.0 Hz, 6 H). 13C NMR (62.9 MHz, CDCl3): δ = 200.4, 160.2, 130.4, 119.9, 32.9, 7.6, 6.7, 6.1, 5.2, 3.4. MS (EI, 70eV): m/z (rel. intensity) = 377 (2) [M - 1], 349 (100), 235 (19), 146 (12), 132 (20), 87 (17), 59 (8). IR (neat): 2956, 2910, 2879, 1682, 1605, 1508, 1462, 1420, 1277, 1235, 1166, 1012, 973, 904, 838, 746 cm-1. HRMS (EI): m/z calcd for C21H39O2Si2 [M - H+]: 379.2489; found: 379.2488. Anal. Calcd for C15H24O2Si: C, 66.60; H, 10.11.ound: C, 66.20; H, 10.17.

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Only the hydroacylated product 5a was obtained and gave 70% isolated yield when the reaction of 4-hydroxybenz-aldehyde (1a) and triethylvinylsilane (2a) was carried out at 130 °C for 5 h under the same catalytic system.

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When the reaction of benzyl alcohol (1b) and trimethyl-vinylsilane (2b) was carried out in toluene at 110 °C for
1.5 h in the presence of bis[cyclooctene]iridium(I) chloride dimer (1.5 mol%) and PPh3 (6 mol%) instead of (Ph3P)3RhCl(3), a quantitative yield of benzaldehyde was determined by GCMS. This result implies that the iridium catalyst favors β-hydrogen elimination over β-silyl elimination on the contrary to the rhodium catalyst.

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Typical Procedure for the Catalytic Reaction of 1b and 2b with 3 (Table 1, Entry 1) A screw-capped pressure vial (1 mL) was charged with 27.0 mg (0.250 mmol) of benzyl alcohol (1b), 30.1 mg (0.300 mmol) of trimethylvinylsilane (2b), 4.6 mg (0.005 mol) of (Ph3P)3RhCl(3), and 50.0 mg of toluene. The reaction mixture was stirred in an oil bath that was preheated to 100 °C for 2 h. After cooling to r.t., the vial was opened carefully and the crude mixture was purified by column chromatography (SiO2, n-hexane-EtOAc = 15:1) to afford 41.8 mg (93%) of benzyloxytrimethylsilane (6a).

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Typical Procedure for the Silyl Group Exchange of Silyl Ether 6a with 2a in the Presence of H2O, 3 and Trifluoroacetic Acid (Table 2, Entry 1)
A screw-capped pressure vial (1 mL) was charged with 45.0 mg (0.250 mmol) of benzyloxytrimethylsilane (6a), 6.9 mg (0.0075 mmol) of (Ph3P)3RhCl(3), 4.5 mg (0.250 mmol) of H2O, 1.7 mg (0.0125 mmol) of TFA, 124.5 mg (0.875 mmol) of triethylvinylsilane (2a), and 75.0 mg of THF. The reaction mixture was stirred in an oil bath that was preheated to 130 °C for 1 h. After cooling to r.t., the vial was opened carefully and H2O was removed by anhyd CaCl2. For a GC analysis, the crude mixture was filtered through a small plug of silica gel to remove the catalyst. The conversion yield, as determined by a GC analysis, was 100%.