ABSTRACT
Up to about 1950 nucleophilic displacement reactions at unsaturated carbon were considered to possess concerted or stepwise mechanisms. Theoretical considerations have been invoked to exclude the possibility of concerted mechanisms in nucleophilic substitution reactions at trigonal carbon. Nucleophilic displacements at the vinyl centre are analogous to those at the carbonyl centre and have been the subject of considerable scrutiny. The mechanisms available for this process follow those of displacements at the carbonyl centre. A substantial body of stereochemical data for substitution at the vinyl centre has accumulated and it was originally thought that the highly stereoselective reactions were good evidence for concerted displacements. Early studies of nucleophilic substitution at an acetylenic carbon showed that the reaction was very sluggish compared with other substitution reactions. However, since 1962 there have been many facile syntheses effected according to this route and this has raised the possibility of a concerted mechanism.
