o-Quinonoid compounds. Part 14. 1,5-Acyl shifts in substituted indenes: migratory aptitudes from racemisation rates

Abstract

Racemisation rates for the indenes (1) provide migratory aptitudes for acyl, vinyl, ethynyl, nitrile, and alkyl groups, as well as hydrogen. Migratory aptitude decreases in the order: HCO > Bz ∼ Ac > H > vinyl > CONHMe > CO2Ph > CO2Me > CN ∼ CCH > alkyl. It is suggested that a similar order may apply to all 1,5-sigmatropic shifts. It is proposed that the fast migration of formyl, acetyl, and benzoly groups is, at least in part, due to a secondary interaction involving the carbonyl π* orbital. Small substituent effects on the rearrangement of the p-substituted benzoylindenes, and small solvent rate effects suggest little increased polarity in the rearrangement transition states. Racemisation of (1; X = Ac) and (1; X = Bz) is accelerated in the presence of boron trifluoride–ether.