Stereochemistry of nucleophilic substitution of (E)- and (Z)-3-chloro-2-phenylpropenonitriles; criticism of a paper by Rappoport and Topol

Abstract

Contrary to a report by Rappoport and Topol nucleophilic substitution of (E)- and (Z)-3-chloro-2-phenylpropenonitriles does not always proceed with retention of configuration. Strong bases, such as sodium ethoxide in ethanol and sodium phenoxide in tetrahydrofuran, cause complete ‘racemisation,’ whereas reactions with weak bases, such as morpholine, piperidine, and phenol–triethylamine, proceed with retention of configuration. Moreover the (E)- and (Z)-vinylic ethers thus obtained do not undergo isomerisation; however the (E)-enaminonitriles do isomerise to the stable Z-form. The choice of ethanol as solvent for kinetic measurements is unfortunate, since it is itself involved in such substitution reactions. Physical and spectroscopic data reported by Rappoport and Topol are corrected, and the assignment of E- and Z-configurations to the 3-ethoxy-2-phenylpropenonitrile isomers is reversed.