Theoretical study of the mechanism of dimerization of N,C-disubstituted carbodiimides

Semiempirical calculations (AM1) have been carried out on the formation of 1,3-diazetidinediimines by dimerization of carbodiimides. The model cases of carbodiimide itself, HN=C=NH and N-amino-N′-methylcarbodiimide, H 2N-N=C=N-CH3, have been studied. For the second compound, eight isomeric diazetidines can be obtained by cyclodimerization. The two most stable isomers are the bis(hydrazono)-1,3-dimethyldiazetidines of Z,Z and E,E configuration which correspond to the kind of structures actually formed. For the study of the kinetic aspects of the dimerization, the complete two-dimensional surface has been determined for carbodiimide itself. It appears that the asynchronous reaction is favoured over the synchronous approach. In the case of N-amino-N′-methylcarbodiimide, twelve asynchronous transition states have been calculated. The lowest transition state corresponds to the formation of 1-amino-3-methyl-2,4-bis(hydrazono)diazetidine, analogous to the diazetidine isomer whose existence has been postulated to explain the mass spectra of bis(heteroarylimino)-1,3-diaryldiazetidines.