Ring expansion and vinylic nucleophilic substitution competing for (tert-alkyl)2C[dbnd]C(Li)–Cl in carbenoid chain processes

Abstract

Vinylic nucleophilic substitution (SNV) reactions of unactivated, cyclic α,α-dichloroalkenes [(tert-alkyl)2CCCl2] with aryllithiums (RLi) to give (tert-alkyl)2CC(Cl)–R are presented to be carbenoid chain reactions, which involve the unsaturated Cl,Li-carbenoids (tert-alkyl)2CC(Li)–Cl as transient intermediates. The chains are longer and the overall reactions much slower in tert-butyl methyl ether (t-BuOMe) than in THF as the solvent. In competition with the fast SNV step of these Cl,Li-carbenoids, the Fritsch–Buttenberg–Wiechell (FBW) ring expansion in t-BuOMe (but less so in THF) generates short-lived cyclohexyne species, which are trapped by the accompanying RLi species to yield chlorocyclohexene derivatives [(tert-alkyl)–(Cl)CC(R)–(tert-alkyl)] as the FBW chain products.