Computational investigation of the mechanism of addition of singlet carbenes to bicyclobutanes.

Singlet carbenes are known to react with bicyclobutanes to yield 1,4-diene products, as in the addition of dichlorocarbene to bicyclobutane to yield 1,1-dichloro-1,4-pentadiene. At least two mechanisms have been proposed to explain this unusual reaction: (1) a concerted process and (2) a stepwise process involving a zwitterionic intermediate. Ab initio electronic structure calculations have been performed in order to help distinguish between these two mechanistic possibilities. In the parent system, the concerted pathway and the corresponding transition structure are readily located. On the other hand, the hypothesized zwitterionic intermediate does not correspond to a minimum at most levels of theory, even in the presence of a polar medium representing the solvent. Instead, this structure corresponds to a transition state or, at best, an extremely shallow minimum. The two pathways--one unambiguously concerted, the other possibly leading through an extremely shallow minimum (intermediate)--have very similar barriers and are expected to be competitive. In the substituted 1,2,2-trimethylbicyclobutane system, five regioisomeric concerted pathways exist and lead to four different diene products. Two of these pathways lie well below the others in energy, and they alone are expected to play a significant role at ordinary temperatures. Of these two pathways, the one calculated to have the slightly lower barrier leads to the only product that is reported experimentally. In addition, a sixth geometry of approach exists, leading over a transition structure of comparable energy to a shallow minimum that corresponds to a zwitterionic intermediate. The calculated potential energy surface suggests that the reaction can proceed through this intermediate both to the observed diene product and to one of the other isomers. It therefore appears that the concerted and stepwise mechanisms are competitive in the substituted system. Taken together, the calculated pathways and barriers do not adequately account for the very pronounced regioselectivity observed experimentally; only modest regioselectivity would be predicted at best. Examination of a calculated potential energy surface defined over two relevant internal coordinates sheds further light on the reaction and suggests that the experimentally observed regioselectivity might derive in considerable part from dynamic effects.