Experimental and theoretical investigations of ring-expansion in 1-methylcyclopropylcarbene

1-Methylcyclopropylcarbene, generated by photolysis of two isomeric hydroca rbon precursors, undergoes ring-expansion readily to give 1-methylcyclobute ne. Experimentally, intramolecular carbon-hydrogen insertions are not obser ved. Trapping studies with TME demonstrates the formation of the expected c yclopropane adduct, and via a double-reciprocal analysis, the lifetime of 1 -methylcyclopropylcarbene was determined to be 12 ns in 1,1,2-trichlorotrif luoroethane. Computational studies show that the barrier to ring-expansion is significantly smaller in 1-methylcyclopropylcarbene than in cyclopropylc arbene. The origin of the increased rate of ring-expansion is due to stabil ization of the positive charge that occurs at the incipient tertiary carbon that is attached to the migrating carbon center.