The carbanionic mechanism in the prototropy of 1,3-diphenylpropenes and 2-methyl-1,3-diphenylpropenes

Abstract

The base-catalysed isomerization of 1,3-diphenylpropenes and 2-methyl-1,3-diphenylpropenes has been studied. A detailed mechanism based on the intermediacy of carbanions is suggested. The effect of geometrical isomerism on carbanion stability has also been studied. The free energy of formation for the carbanions generated from 1,3-diphenylpropene decreases in the order cis–cis > trans–cis > trans–trans indicating that the [1,3] interaction across the top of the carbanion is dominant. The intramolecularity of the reaction has also been studied. It was found that the more stable the carbanion, the more intramolecular is the reaction. This result is explained by consideration of the particular structure of the carbanionic intermediates.