Nucleophilic substitution and elimination in 2-halogenovinyl ketones

Abstract

The reactions of cis- and trans-p-anisyl 2-chloro- and 2-bromo-vinyl ketones (I) and (II) with benzenethiolate and methoxide ions in methanol have been studied and their rates measured. Both cis- and trans-isomers react with benzenethiolate ion via direct substitution to give cis- and trans-p-anisyl 2-phenylthiovinyl ketones (III) and (IV) with retention of configuration. However, trans-sulphide (IV) isomerises into (III) in the reaction conditions. cis-Chloro-(Ia) and cis-bromo-(Ib) derivatives react with methoxide ion via elimination–addition, whereas the corresponding trans-isomers (IIa,b) undergo, very likely, a process of direct substitution. In both cases the solvent adds to the first reaction product, vinyl ether (VI), yielding the same keto-acetal (VII). The mechanism of elimination has been studied by means of isotopic techniques.