o-Quinonoid compounds. Part 13. 1,5-Acyl shifts in substituted indenes; conversion of 1-acyl- into 2-acyl-indenes and orienting mechanistic experiments

Abstract

The 1-acyl-1,3-diphenylindenes (3; R = Ph) with acyl(X)= CHO, COMe, COPh, CONHMe, or CO₂Me have been prepared and shown to undergo thermal rearrangement to the corresponding 2-acylindenes (5; R = Ph). The 1-acyl-1,3-dimethylindenes (3; R = Me) with X = CHO, COMe, COPh, CO₂·;C₆H₄·NO₂(p), COPh, or CO₂Me were prepared similarly and shown to undergo slower thermal rearrangement to 2-acylindenes (5; R = Me). A mechanism for the rearrangement involving formation of the 2H-indenes (4) is supported by the results of trapping experiments with N-phenylmaleimide. These trapping experiments, together with evidence for basic catalysis of the rearragement for (3; R = Ph, X = CHO or COMe) and experiments on transfer of non-equilibrium spin with (3; R = Me, X = CHO), reveal the occurrence of rapid degenerate formyl and acetyl migration in these indenes. More rapid migration of acetyl than of hydrogen in the 2H-indene intermediate (9) is consistent with thermal conversion of 1-acetyl-3-methyl-1-phenylindene (8) into the 1-methyl-3-phenyl isomer (10). In contrast to formyl, acetyl, and benzoyl groups, ester groups (CO₂R) migrate more slowly than hydrogen in the 2H-indene intermediates (4); accordingly the rates of rearrangement of 1- to 2-substituted esters provide migratory aptitude data for these groups. Migratory aptitude increases in the order CO₂Me < CO₂Ph < CO₂·C₆H₄NO₂(p) for the indenes (3; R = Me). Preliminary tests support a concerted mechanism for the 1,5-shift of the acyl groups.