Effect of substituents on the competition between several mechanisms of nucleophilic vinylic substitution

Abstract

In an attempt to encourage vinylic substrates to undertake an S_RN1-like nucleophilic substitution route, a process that requires acquisition of one electron, a series of stilbene halides bearing one electron-withdrawing substituent on the double bond has been synthesized. Determination of the reduction potential of these substrates confirmed an increased tendency to acquire electrons. However, this effect was so strong that in the reaction of substrates 1–3 with the pinacolone enolate ion under typical S_RN1 conditions, two consecutive electron-transfer steps occurred, ultimately producing diphenylacetylene 6 without any S_RN1 products. Evidence was also gathered for a relevant contribution from an ionic halophilic route to the formation of 6. The structurally similar propenal derivative 4 gave instead a novel ionic deformylation process, presumably initiated by a nucleophilic addition of the enolate ion to the aldehydic carbonyl and followed by bromide loss. The fluorenylidene derivative 5 although it has a very favourable redox potential, reacted by an ionic addition-elimination route. Competition by other routes thus hampers the occurrence of the S_RN1 process and comparison with cases of other vinyl halides analogously undergoing multiple reaction pathways is made.