Cation effect on the course of anionic reactions in the presence of a solid base

Abstract

The formation of cyclopropanes by the MacCoy reaction is reinvestigated in the presence solid sodium or potassium hydroxide, with or without a quaternary ammonium salt, in dioxanne: ▪ With NaOH alone, the major reaction is the saponification of the α-chloro-ester 1; the cyclopropanes formation is the only reaction observed using either KOH, or the two bases with NBu + 4Br −. The saponification by NaOH is attributed to the complexation of the carbonyl group by Na +, which favours the nucleophilic attack of the hydroxide anion: ▪ The complexation is weaker with K + than with Na +, and then, the proton abstraction of the α-chloroester is predominant. These results show that metallic cations have a similar behaviour in a crystalline network and in solution. The stereoselectivity is known to be determined during the cyclisation step. The predominant formation of the Z isomer is explained by chelate formation of metal cation with the two carbonyl groups, while the E isomer is favoured when the size of the cation is too large to allow such a chelate formation. ▪ The different stereochemical results obtained by addition of the ammonium salt involve the association of the intermediate anion 2A with NBu + 4Br −.Furthermore, the condensation should occur in the organic phase with the phase transfer reagent, and on the solid surface when the bases are used alone.