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)-CH
CHCO2Et, (E)-CH
CHCOPh, C
CCOPh] 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 C
N group migrates ca. 108-times more slowly than the CHO group, and the C
CCOPh group migrates at least 500-times more slowly than the (E)-CH
CHCOPh 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.