1,5-Sigmatropic formyl migration in 1,3-dimethylbenz[f]indene-1-carbaldehyde; evidence for product-like transition states

Abstract

The title compound 1 has been prepared in an optically active form via a convenient seven-step sequence. The migratory ability of the formyl group allows the 1,5-shift 1(arrows) involving disruption of naphthalene aromaticity to proceed with modest activation [ΔG^‡= 31.84 ± 0.6 kcal mol^–1(145 °C)]. Activation enthalpies for 1,5-formyl migration in 1, 3, 4 and 5 correlate well with changes in π-electron energy in going from these compounds to their respective rearrangement products. Product-like transition states involving important cleavage of the bond between the migration origin and the formyl group are suggested for these rearrangements.