An ab initio study of a retro-Wolff rearrangement: from diazafulvenone to cyanovinyl isocyanate without a singlet .alpha.-oxocarbene intermediate

Ab initio molecular orbital calculations have been used to study the unimolecular rearrangement of diazafulvenone leading to cyanovinyl isocyanate. The geometries of stationary points were optimized at HF/6-31G** while relative energies were obtained from multireference configuration interaction (MRCISDQ) calculations with the 6-31G** basis set. A singlet oxocarbene structure was not found to be a local minimum but rather a transition structure connecting diazafulvenone to isocyanovinyl isocyanate, the latter molecule being a distinct intermediate during the rearrangement to cyanovinyl isocyanate. Diazafulvenone is calculated to lie 14 and 32 kcal/mol, respectively, above isocyano- and cyanovinyl isocyanate (Z conformers) and to be separated from the isocyanide form by an energy barrier of 44 kcal/mol. The diazafulvenone --> isocyanovinyl isocyanate interconversion, which can be regarded as a retro-Wolff rearrangement, is thus a concerted reaction without a discrete singlet oxocarbene intermediate. It also constitutes the rate-determining step of the entire rearrangement process, in agreement with experimental observation. As shown by an analysis of localized orbitals, the diazafulvenone --> isocyanovinyl isocyanate interconversion involves six electron pairs that circulate following a cyclic motion.