EFFECT OF SUBSTITUENTS ON THE COMPETITION BETWEEN SEVERAL MECHANISMS OF NUCLEOPHILIC VINYLIC SUBSTITUTION

In an attempt to encourage vinylic substrates to undertake an S(RN)1-l ike nucleophilic substitution route, a process that requires acquisiti on of one electron, a series of stilbene halides bearing one electron- withdrawing substituent on the double bond has been synthesized. Deter mination of the reduction potential of these substrates confirmed an i ncreased tendency to acquire electrons. However, this effect was so st rong that in the reaction of substrates 1-3 with the pinacolone enolat e ion under typical S(RN)1 conditions, two consecutive electron-transf er steps occurred, ultimately producing diphenylacetylene 6 without an y S(RN)1 products. Evidence was also gathered for a relevant contribut ion from an ionic halophilic route to the formation of 6. The structur ally similar propenal derivative 4 gave instead a novel ionic deformyl ation process, presumably initiated by a nucleophilic addition of the enolate ion to the aldehydic carbonyl and followed by bromide loss. Th e 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(RN)1 process and comparison with cases of other vinyl halides analogously undergoing m ultiple reaction pathways is made.