The C₇H₆ energy surface: phenylcarbene and 2-Methylene-3,5-cyclohexadienylidene–A duality of mechanism

Abstract

The conversion of phenylcarbene (1) into fulvenallene (17) and ethynylcyclo-pentadiene (18) on gas-phase pyrolysis has been examined by ¹³C-tracer techniques. Complete randomization of all seven carbon atoms has been shown to precede ring contraction. The result is consistent with a pool of intermediates [cycloheptatrienylidene (19) and bicyclo[4,1,0]hepta-2,4,6-triene (21)], interconverting rapidly through tautomerism and concomitant H-shifts. Dimerization, intramolecular trapping, and ring contraction constitute the major exits from this pool.     Gas-phase thermolysis of indazole generates the same products (17)/(18) and in this case the mechanism has been investigated by the intramolecular trapping in 1-deutero-3-methylindazole to yield deuterostyrenes. Two pathways are revealed, proceeding respectively through phenylcarbene and 2-methylene-3,5-cyclohexadienyl-idene (36); factors affecting the latter pathway are discussed.