Enantioselective Protonation of Silyl Enol Ethers Catalyzed by a Chiral Pentacarboxycyclopentadiene-Based Brønsted Acid

Abstract

The enantioselective protonation of silyl enol ethers was realized in the presence of a pentacarboxycyclopenta-1,3-diene-based chiral Brønsted acid catalyst with water as an achiral proton source to give the corresponding α-aryl ketones in good yields and up to 75% ee.

Key words

protonation - Brønsted acids - asymmetric catalysis - organo­catalysis - carbonyl compounds - silyl enol ethers

• References and Notes

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• 9 (2R)-2-Phenylcyclohexanone (3a); Typical Procedure PCCP catalyst 1b (19.3 mg, 0.02 mmol, 10 mol%) was weighed into a cryogenic vial. The vial was cooled to –10 °C, and H₂O (4 μL 1.1 equiv) and anhyd xylenes (2 mL) were added. After 15 min at –10 °C, silyl enol ether 2a (49 mg, 0.2 mmol, 1.0 equiv) was added dropwise to the stirred mixture. After 8 h, the reaction was quenched by the addition of sat. aq NaHCO₃ (3 mL) and the mixture was extracted with CH₂Cl₂ (25 mL). The organic layer was washed with brine (5 mL), dried (NaSO₄), filtered, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, hexanes–EtOAc (100:1 to 10:1)] to give a white solid; yield: 35 mg (99%, 74% ee); [α]_D ²⁵ +50.7 (c = 0.4, CHCl₃). HPLC: Chiralpak Lux 5u cellulose-1 [hexanes–i-ProH (99:1), flow rate = 1.0 mL/min; λ = 215 nm], t _R (minor, S) = 14.8 min; t _R (major, R) = 16.2 min. ¹H NMR (400 MHz, CDCl₃): δ = 7.33 (t, J = 7.4 Hz, 2 H), 7.25 (t, J = 7.4 Hz, 1 H), 7.14 (d, J = 7.4 Hz, 2 H), 3.60 (dd, J = 12.1, 5.4 Hz, 1 H), 2.63–2.36 (m, 2 H), 2.26–2.30 (m, 1 H), 2.21–2.09 (m, 1 H), 2.09–1.90 (m, 2 H), 1.90–1.71 (m, 2 H).