THERMAL REARRANGEMENTS .25. THE AUTOMERIZATION OF BENZENE AS A RADICAL-INITIATED REACTION

The thermal isomerization of [1,4-D-2]- (3a) and [1,2-C-13(2)]benzene (1a) has been studied in excess hydrogen at 750-850 degrees C, with co ntact time less than 1.2 s and very low partial pressure in a quartz f low system. In both cases, the main isomerization products are the cor responding meta isomers. The data suggest a radical intramolecular int erchange of the benzene carbon atoms by 1,2-C shifts. The multistep re action cascade is initiated by H addition to the benzene ring followed by transannular homoallyl rearrangements involving the intermediate f ormation of bicyclo[3.1.0]hexenyl and cyclopentadienylmethyl radicals. This pathway constitutes a side reaction competing with the direct st abilization of the cyclohexadienyl radicals formed preferentially at h igh temperature.