New Insights on the Mechanism of Thermal Cleavage of Unsaturated Bicyclic Diaziridines: A DFT Study

A DFT calculations are carried out at UB3LYP/6-311++G (3df, 2p) levels of theory to study electrocyclic thermal cleavage of four (R) derivatives of unsaturated bicyclic diaziridines, 1(X-R), to produce corresponding (Z) and (E) azomethine imides (2(X-Z), 2(X-E), 3(X-Z) and 3(X-E)), where X=H, Me, t-Bu and Ph. Cleavage of 1(X-R) series to form the most stable 3(X-Z) product, (path 2) is found the favored procedure because of delocalized negative charge on five atoms and lower steric effect in related transition state. According to IRC calculations in paths 1 and 2, C-6-N-1 bond is cleaved before the rate determinating step (transition state). The stability of unsaturated bicyclic diaziridines and their corresponding (Z) and (E) azomethine imides is in the following order in gas phase and chloroform, tetrahydrofuran, and acetone solvents: 3(X-Z)<3(X-E)<2(X-Z)<2(X-E)<1(X-R)<1(X-S).