DISSOCIATION AND AROMATIZATION OF A SEMIBENZENE - REACTIONS OF TRIPHENYLMETHYL AND METHYL ISOBUTYRYL RADICALS

Semibenzene 4, which can be regarded as the recombination product of triphenylmethyl and methyl isobutyryl radicals (5), affords exactly these intermediates on thermolysis or inefficiently on direct photolysis. Recombination and disproportionation of these dissimilar radicals proceeds with a much lower barrier than the dimerization of triphenylmethyl. Whereas thiophenol or triplet 9-fluorenone aromatize 4, thermolysis in the presence of 1,4-cyclohexadiene allows trapping of 5 and oligomeric radical 13. From the measured heat of aromatization (22.0 kcal/mol), the C-H bond dissociation enthalpy of 4 and its analogue lacking the side chain (''p-isotriphenylmethane'') is calculated to be 54 kcal/mol, the lowest value known for any closed-shell, neutral hydrocarbon. Exposure of 4 to the atmosphere causes rapid autoxidation to hydroperoxide 19, which thermolyzes in the GC to aromatic ketones and phenols instead of undergoing a 1,2-aryl shift.