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

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.

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

References and Notes

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11

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.

12

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.

13

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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17

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).

18

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%.

19 For a review on selective deprotection of silyl ether, see: Crouch RD. Tetrahedron  2004,  60:  5833