Thermal solid-state Z/E isomerization of 2-alkylidene-4-oxothiazolidines: effects of non-covalent interactions

Abstract

Configurational isomerization of stereo-defined 5-substituted and un-substituted 2-alkylidene-4-oxothiazolidines (1) in the solid state, giving the Z/E mixtures in various ratios, was investigated by H-1-NMR spectroscopy, X-ray powder crystallography and differential scanning calorimetry (DSC). The Z/E composition can be rationalized in terms of non-covalent interactions, involving intermolecular and intramolccular hydrogen bonding and directional non-bonded 1,5-type S center dot center dot center dot O interactions. X-Ray powder crystallography, using selected crystalline (Z)-4-oxothiazolidine substrate, revealed transformation to the amorphous state during the irreversible Z - gt E process. A correlation between previous results on the Z/E isomerization in solution and now in the solid state was established.