A COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION OF FREE TERT-BUTYLMETHYLENE, A CARBENE STABILIZED BY SIGMA-BOND PARTICIPATION

The reactivity of tert-butylmethylene, 1, has been explored computatio nally and experimentally by C atom deoxygenation of 2,2-dimethyl-propa nal, 5. Ab initio calculations ([QCISD(T)/6-31+G(2d,p)]// MP2/6-31 G(d )) indicate that intramolecular C-H insertion to generate 1,1-dimethyl cyclopropane, 3, is more favorable than intramolecular C-C insertion t o produce 2-methyl-2-butene, 4 Delta H-C-C(double dagger) - Delta H-C- H(double dagger) = 3.7 kcal/mol (1 atm, 298 K) and Delta S-C-C(double dagger) - Delta(C-H)(double dagger) = 0.5 eu). In agreement with these theoretical predictions, deoxygenation of 5 by C atoms at 77, 158, an d 195 K yields only the C-H insertion product 3. The MP2 geometry of 1 reveals that this carbene is stabilized by participation of the neigh boring C-C and C-H bonds (C-1-C-2-C-3 angle = 79.2(degrees)). The enth alpy barrier to the loss of N-2 from 2,2-dimethyl-1-diazopropane, 2, t o generate 1 at the MP2/6-31G(d)+ZPC level was 32.7 kcal/mol in a reac tion exothermic by 30.5 kcal/mol. This decomposition of 2 did not show any tendency to bypass the free carbene in going to 3 and/or 4.