Experimental and Theoretical Investigations of Ring-Expansion in 1-Methylcyclopropylcarbene

1-Methylcyclopropylcarbene, generated by photolysis of two isomeric hydrocarbon precursors, undergoes ring-expansion readily to give 1-methylcyclobutene. Experimentally, intramolecular carbon−hydrogen insertions are not observed. Trapping studies with TME demonstrates the formation of the expected cyclopropane adduct, and via a double-reciprocal analysis, the lifetime of 1-methylcyclopropylcarbene was determined to be 12 ns in 1,1,2-trichlorotrifluoroethane. Computational studies show that the barrier to ring-expansion is significantly smaller in 1-methylcyclopropylcarbene than in cyclopropylcarbene. The origin of the increased rate of ring-expansion is due to stabilization of the positive charge that occurs at the incipient tertiary carbon that is attached to the migrating carbon center.