Nucleophilic reactions in ethylenic derivatives. Part XI. Substitution and elimination in β-halogenostyrenes

Abstract

The reactions of cis- and trans-β-chloro-4-nitrostyrenes with benzenethiolate and methoxide ions in methanol show the usual features of vinylic reactivity : i.e., only in the case of the cis-isomer with methoxide ion does elimination compete favourably with direct substitution. Particular care was devoted to the investigation of the elimination mechanism, which was also studied with the unsubstituted cis-β-chloro- and -β-bromo-styrenes. The presence of a primary isotope effect (K_H/K_D= 1·6–3) and the absence of any H/D exchange proved that the elimination followed an E2-like mechanism with a large carbanionic character.