Brønsted Acid Catalyzed Asymmetric Silylation of Biaryl Diols

Abstract

We report a Brønsted acid catalyzed enantioselective silylation of biaryl diols with an allylsilane as a silicon source. This process enables facile access to enantioenriched biaryl silyl ethers with an axial stereogenicity. A control experiment supports a mechanism proceeding by desymmetrization followed by kinetic resolution.

Key words

Brønsted acid - imidodiphosphorimidate - silylation - axial chirality - desymmetrization - kinetic resolution

• References and Notes

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• 9 Silylation of Biaryl Diols; General Procedure A GC vial equipped with a Teflon-coated magnetic stir bar was charged with catalyst 4 (2.5 mol%), biaryl diol 1 (0.1 mmol) and solvent (CHCl₃ or CH₂Cl₂), and the resulting mixture was cooled to –50 or –30 °C in a cryostat. After 10 min, allylsilane 2 (2 or 1.5 equiv.) was slowly added and the reaction mixture was stirred for 3–5 d at the same temperature. After complete conversion as indicated by TLC, the reaction was quenched with trimethylamine. The solvent was removed in vacuo and the mixture was purified by column chromatography on silica gel to afford the desired silyl ether 3. (S)-6-((tert-Butyldimethylsilyl)oxy)-2′-methyl-[1,1′-biphenyl]-2-ol [(S)-3a] Yield: 12.6 mg (40%); white solid; [α]_D ²⁵ –25.5 (c 0.53, CHCl₃. ¹H NMR (501 MHz, CD₂Cl₂): δ = 7.34–7.24 (m, 3 H), 7.16 (dd, J = 7.4, 1.7 Hz, 1 H), 7.12 (t, J = 8.2 Hz, 1 H), 6.60 (dd, J = 8.2, 1.0 Hz, 1 H), 6.50 (dd, J = 8.1, 1.0 Hz, 1 H), 4.75 (s, 1 H), 2.12 (s, 3 H), 0.65 (s, 9 H), 0.07 (s, 3 H), –0.06 (s, 3 H). ¹³C NMR (126 MHz, CD₂Cl₂): δ = 154.4, 154.0, 139.2, 132.7, 131.5, 130.9, 129.1, 128.8, 126.5, 120.2, 111.7, 108.5, 25.3, 19.8, 18.0, –4.3, –4.6. EI-HRMS: m/z [M]^+• calcd for C₁₉H₂₆O₂Si: 314.1695; found: 314.1697. HPLC (IA-3, heptane/isopropanol = 95:5, 0.5 mL/min, 298 K, 220 nm): t_R1 = 8.6 min, t_R2 = 10.8 min; er = 95:5.
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