Brønsted Acid Catalyzed Asymmetric Silylation of Alcohols

Abstract

We report a catalytic enantioselective desymmetrization of meso-1,2-diols by monosilylation using a chiral enantiopure Brønsted acid as catalyst and hexamethyldisilazane (HMDS) as silyl source.

Key words

enantioselective Brønsted acid catalysis - silyl transfer - meso-diols - desymmetrization - asymmetric counteranion-directed catalysis (ACDC)

• References and Notes

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• 7 For further screening of reaction conditions and catalysts, see the Supporting Information.
• 8 The following procedure is representative: To a solution of catalyst 5 (3.6 mg, 5.00 μmol) in cyclohexane (1.0 mL) HMDS (2g, 12.5 μL, 0.0600 mmol) was added at r.t. and the resulting mixture was stirred for 2 h, after which, meso-diol 1a (21.4 mg, 0.100 mmol) was added. The reaction was judged to be complete when the reaction mixture turned into a clear, homogeneous solution. The solvent was removed by evaporation and the crude mixture was purified by silica gel column chromatography (hexane–EtOAc, 95:5), giving 3a (24.0 mg, 84%) as a colorless solid. ¹H NMR (500 MHz, CD₂Cl₂): δ = 7.22–7.30 (m, 10 H), 4.74 (d, J = 5.0 Hz, 1 H), 4.71 (d, J = 5.0 Hz, 1 H), 2.39 (br, 1 H), –0.09 (s, 9 H). ¹³C NMR (125 MHz, CD₂Cl₂): δ = 141.6, 141.5, 128.3, 128.2, 128.1, 128.0, 127.9, 127.7, 79.6, 78.9, 0.00. HRMS (ESI, +ve): m/z [M + Na] calcd for C₁₇H₂₂NaO₂Si: 309.1287; found: 309.1281. HPLC (Chiralcel OJ-H, heptane–i-PrOH = 95:5, flow rate = 0.5 mL/min, λ = 206 nm): t _{R (major)} = 12.6 min, t _{R (minor)} = 16.1 min.