Thermal Rearrangements, XXV . The Automerization of Benzene as A Radical‐Initiated Reaction

The thermal isomerization of [1,4‐D 2 ]‐ ( 3a ) and [1,2‐ 13 C 2 ]benzene ( 1a ) has been studied in excess hydrogen at 750–850°C with contact time less than 1.2 s and very low partial pressure in a quartz flow system. In both cases, the main isomerization products are the corresponding meta isomers. The data suggest a radical intramolecular interchange of the benzene carbon atoms by 1,2‐C shifts. The multistep reaction cascade is initiated by H addition to the benzene ring followed by transannular homoallyl rearrangements involving the intermediate formation of bicyclo[3.1.0]hexenyl and cyclopentadienylmethyl radicals. This pathway constitutes a side reaction competing with the direct stabilization of the cyclohexadienyl radicals formed preferentially at high temperature.