Nuclear magnetic resonance investigation of intermediates formed by the action of nucleophiles on substituted stilbenes

Abstract

A nuclear magnetic resonance investigation of the colored species formed by the action of bases on a series of α-cyano-4-nitro-4′-X-stilbenes characterizes these as the intermediate carbanions formed by the addition of base to the β-carbon atom of the double bond. The nuclear magnetic resonance spectra reveal that, whereas there is free rotation about the β-phenyl—Cβ and Cβ—Cα bonds in these adducts, there is restricted rotation around the α-phenyl—Cα bond due to very substantial delocalization of the negative charge into this ring. It has been verified that the same intermediates are formed by the action of bases on either the cis or trans isomers of α-cyano-4-nitrostilbene. The relation of these observations to the theory of alkene nucleophilic substitution reactions is discussed.