CHLORIDE-ION EXCHANGE AND E-REVERSIBLE-Z ISOMERIZATION OF AN ELECTROPHILIC VINYL-CHLORIDE

The exchange (ex) and the accompanying isomerization (isom) in the reaction of methyl (E)- and (Z)-β-chloro-α-cyano-p-nitrocinnamates [(E)-5 and (Z)-5] with 36Cl- n acetonitrile were investigated. Exchange was much faster than isomerization, kex/kisom = 54 [(E)-5] and 123 [(Z)-5]. Both exchange and isomerization were faster for (Z)-5 than for (E)-5. The exchange was a relatively fast process, and extrapolation suggested that kex(Cl-)/ksubstitution (piperidine) of 5 is 2.7. The results are discussed in terms of formation of intermediate carbanion, which exchanged the chloride with highly preferred retention. The modes of rotations leading to inversion (isomerization) were analyzed, and it was shown that isomerizations via 120° counterclockwise rotation and 180° clockwise rotation around the Cα-Cβ bond in the carbanion are distinguishable in principle, although we failed to determine the mode of rotation in our system. The relatively high nucleophilicity of Cl- was ascribed to lower hidden reversal of the nucleophilic attack by Cl- than in less activated systems. The differences in reactivity of (Z)-5 and (E)-5 were analyzed in terms of steric interactions in the transition states for the 60° and 120° internal rotations. © 1989 American Chemical Society.