Mechanisms of ring contraction,C—H insertion and 1,2-hydrogen migration of cyclobutylidene

Isomerizations of cyclobutylidene 1 to methylenecyclopropane 2 , cyclobutene 3 and bicyclobutane 4 have been studied using ab initio method. Equilibrium and transition structures are fully optimized with 3–21G basis set. The reaction barriers are 78, 75 and 110 kJ/mol at 6–31 G^**, respectively, 1→2 occurs via the outward disrotation of two end C-C bonds, 1→3 via the attack of an exo hydrogen to the carbene p AO before the transition state, and 1→4 is unable to compete with 1→2 and 1→3 for its higher barrier.