Evidence for thermal 3,5-sigmatropy of 7-vinylnorcaradienes

7-Phenyl-7-vinylcycloheptatrienes rearrange via their endo-norcaradiene valence tautomers (1; a–d) to give the dihydroindenes (2; a–d) and the Cope products (3; a–c); dihydroindene formation is stereospecific, and strongly inhibited in the benzonorcaradienes (7; b–d) in agreement with 3,5-sigmatropy rather than vinylcyclopropane–cyclopentene rearrangement.