Electrochemical studies on β-lactams. Part 1. Electroreduction of 3-halogeno-β-lactams

Abstract

The electrochemical reduction in aprotic solvents of 3-halogeno-β-lactams (1a–c) and (2a–c), with or without added proton donors or/and electrophiles, has been investigated. Without added substrates, the carbanion arising from the cleavage of the carbon–halogen bond undergoes protonation (mainly from the ‘parent’ molecule) and, competitively, ring-opening reactions yielding the corresponding dehalogenated β-lactam and α,β-unsaturated amide. In the presence of proton donors (CH₃CO₂H) or electrophiles (CO₂), the protonation and coupling reactions, respectively, become largely predominant, and the dehalogenated or carboxylated β-lactams are the main products. In the presence of BrCH₂CH₂CN the protonation reaction is preferred, and the dehalogenated β-lactam predominates over the substitution product. The high yields of 3-carboxy-β-lactams, a class of compounds not easily accessible by chemical methods, are noteworthy from a synthetic point of view.