Rearrangement of azirine intermediates to nitriles: Theoretical study of cleavage of 3, 4-dihydro-1aH-azirine[2, 3-c]pyrrol-2-one to cyanoketene-formaldimine complex

A detailed investigation of the reaction path for the thermal w x rearrangement of 3,4-dihydro-1a H-azirine 2,3-c pyrrol-2-one to yield a cyanoketene]formaldimine complex is carried out at the MP2r6-31G* and B3LYPr6-31G* levels of theory. The ring opening of the five-membered pyrrolinone ring and the formation of the nitrile group takes place in a concerted manner, presenting a significant strain energy release and allowing for an Ž . electronic stabilization by coarctate conjugation of the transition structure TS . These two factors make possible a moderate energy barrier. Although the structural features B3LYPr6-31G* theoretical levels, it is found that the MP2 Ž Ž . Ž . energy barrier 28.8 CCSD T r6-31G*rrMP2r6-31G* value 17.1 kcalrmol . The complex electronic rearrangement can be rationalized using the theory of coarctate transition structures developed by Herges as the evolution of an azirine structure without referring to a hypothetical vinyl nitrene intermediate. Q 1998 John Wiley & Sons, Inc. J Comput Chem 19: 912]922, 1998