Stereochemistry in the Knoevenagel reaction of methyl arylsulphinylacetate and aldehydes

Abstract

The stereochemistry of the mechanism of the amine-catalyzed Knoevenagel reaction has been studied. Simple treatment of methyl arylsulphinylacetate (1) and aldehydes (2) with a catalytic amount of a secondary amine produced thermodynamically stable E-alkenes (E)-(3), and the intermediary diastereoisomeric amino compounds (8) were isolated. In acetic acid (8) underwent anti-elimination to yield (E)-(3) and (Z)-(3), while under the Knoevenagel reaction conditions only compounds (E)-(3) were obtained from (8). Both condensation and elimination were found to be reversible, and the stereochemistry of products was determined in the elimination step via stable planar carbanions (9), in which the small difference in the steric requirements of sulphinyl and carbonyl group is effective for the formation of (E)-(3).