When a very fast radical probe cyclization hints at a carbanionic intermediate rather than at a radical one: a further proof of a polar mechanism for the aryl bromide–alkyllithium exchange reaction

Abstract

Dedicated to Professor Marcial Moreno-Mañas on the occasion of his AbstractA new, very fast aryl radical clock, the 2-bromophenyl 3-phenyl-2-propenyl ether 1, was reacted with n-BuLi in THF. A large amount of cyclized product was formed. Such a product could have arisen from an efficient cyclization of a radical formed by dissociation of the radical anion yielded by an ET to 1. Evolution with time, temperature effects and deuteration results, strongly hint, however, that the mechanism of this exchange reaction is rather of the polar type (S N 2 or "ate complex") in agreement with previous reports from the literature based on evidences gained by other types of approaches. The carbanionic cyclization described here is one of the fastest ever reported.

Role of Organolithium Aggregates and Mixed Aggregates in Organolithium Mechanisms

“…The Li/Br exchange in secondary alkyl systems may involve radical intermediates, but aryl, , vinyl, and cyclopropyl halides do not, by the stereochemical and/or cyclizable probe criteria.…”