Abstract: Isomerizations of methylenecyclopropylidene were studied using ab initio gradient method. The ring opening occurs via a Huckel aromatic transition state. It is a 2 e reaction before the transition state and a 4 e one thereafter, obeying Huckel 4n+2 and 4n rules, respectively. The 1,2 Hmigration occurs by the attack of the carbene p orbital on the shift hydrogen. The ring opening barriers is 24 kJ/ mol, and the hydrogen shift barrier is 118 kJ/mol, at the RHF/6-31G^**//STO-3G level. Relative to the ring opening of cyclopropylidene, the substitution results in a larger reactivity but does not change the type and the mechanism of the reaction.

Key words: Huckel type transition state, Disrotatory, Conrotatory