Thermal equilibration of Z- and E-isomers of 5-arylmethylenehydantoins. Evidence for non-bonded aromatic π⋯ methyl attractions

Abstract

Standard free energy differences between the Z- and E-isomers of 14 5-arylmethylenehydantoins have been estimated by thermal equilibration in dimethyl sulphoxide. The Z-configuration is more stable for the N-unsubstituted compounds while the E-configuration is preferred for those N-1methyl derivatives with only para-substituents in the aryl group. The ΔG° values vary with the nature of the para-substituents. For those N-1 methyl derivatives with two ortho-substituents in the aryl group, the configurational preference is dependent on the electronic effects of these aryl substituents, suggesting the presence of non-bonded aromatic π⋯ Me interactions. The factors influencing the relative stabilities of Z- and E-isomers are discussed.