Effect of substituents on the competition between several mechanisms of nucleophilic vinylic substitution
Abstract
In an attempt to encourage vinylic substrates to undertake an SRN1-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 SRN1 conditions, two consecutive electron-transfer steps occurred, ultimately producing diphenylacetylene 6 without any SRN1 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 SRN1 process and comparison with cases of other vinyl halides analogously undergoing multiple reaction pathways is made.