Configurational isomerism in crystalline forms of benzophenone anils

Abstract

4-Methyl-N-(4-nitro-α-phenylbenzylidene)aniline, which in solution exists as a mixture of rapidly interconverting stereoisomers with Z and E configurations, is found in the solid state to have three crystalline forms. The A form consists of three independent molecules, all with the Z configuration, whereas the B(with two independent molecules) and C forms contain only molecules of the E configuration. The B and C forms at ambient temperature are converted into the A form, thus providing an example of rearrangement of the E to Z imine, which, because of the rapid approach to an equilibrium mixture in solution, can only be observed in the solid state as a controlled transition between isolable starting material and product.