Computational explorations of vinylcyclopropane-cyclopentene rearrangements and competing diradical stereoisomerizations

The rearrangements and stereoisomerizations of four systems, vinylcyclopropane, 4-tert-butylvinylcyclopropane, 5-methylvinylcyclopropane, and 2,5-dimethylvinylcyclopropane, as well as a variety of deuterated derivatives and 1- and 2-methyl-, methoxy-, difluoro-, and amino-substituted species, were studied by density functional theory calculations using the B3LYP functional and the 6-31G* basis set. Energies were evaluated with CASSCF(4,4)/6-31G* single point calculations. The major product is obtained by the si pathway. Structures on this path are essentially pure diradical in character. Higher energy diradical species and intermediates are responsible for the scrambling of the stereochemistry. The stereoselectivity of the reaction is increased by substituents which increase the relative energy of the species involved in competing stereoselectivities. The computed secondary kinetic isotope effects reproduce the experimental values reported in the literature.