Reaction Rate Acceleration Enabled by Tethered Lewis Acid-Lewis Base Bifunctional Catalysis: A Catalytic, Enantioselective [2+2] Ketene Aldehyde Cycloaddition Reaction

Abstract

A tethered Lewis acid-Lewis base (LA*-LB*) bifunctional catalyst promotes the asymmetric [2+2] cycloaddition reaction between ketene and aldehydes rapidly. The LA*-LB* bifunctional catalyst, a quinine tethered Co(III)-salen complex (5 mol%) catalyzes the [2+2] cycloaddition reaction to produce the C4-substituted β-lactones in uniformly >99% ee and high isolated yields (71-97%). The dramatic rate acceleration, the hallmark of cooperative intramolecular bifunctional catalysis, is achieved for the catalytic, enantioselective [2+2] cycloaddition reaction between aldehydes and unsubstituted ketene.

Key words

Lewis acids - Lewis bases - bifunctional catalysts - rate acceleration - enantioselectivity

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For examples of generating ketene from acyl halides and Hünig’s base in situ, see ref 2a and 5.