o-Quinonoid compounds. Part 19. Relative migratory aptitudes for doubly and triply bonded groups in 1,3-dimethyldibenz[e,g]indenes

Abstract

The 1,5-sigmatropic shift of unsaturated groups [CHO, CO₂Me, CN, (E)-CHCHCO₂Et, (E)-CHCHCOPh, CCCOPh] is ca. 10²-times faster in the 1,3-dimethyldibenz[e,g]indenes (cyclo-penta[I]phenanthrenes)(5; X = unsaturated group) than in the simple indenes (1). This permits observation of relatively clean rearrangement of triply-bonded migrating groups to give the 2-substituted dibenzindenes (16). The CN group migrates ca. 10⁸-times more slowly than the CHO group, and the CCCOPh group migrates at least 500-times more slowly than the (E)-CHCHCOPh group. The slow migration of triply bonded groups compared with doubly bonded groups in cyclopentadiene systems may be associated with increased strain in bridged rearrangement transition states.