Structure-selectivity relationship in the cleavage of spirocyclopropyl oxindoles: An experimental and theoretical investigation

Heterogeneous catalytic hydrogenation of strained nitrile substituted spirocyclopropyl oxindoles in acetic anhydride, allowed to the regioselective formation of ring-opened 2-oxohomotryptamines accompanied by the ring-retained spirocyclopropyl acetamides as by products. The C3[sbnd]C9 bond fission would be induced by H atom attack via the plausible intermediacy of a stabilized benzolactam carbon-centered radical. The substituent effects on the stability of such radicals were analyzed in terms of the energy of SOMO orbitals, showing good agreement with σ_m Hammett constants. The theoretical results reflect experimental findings on the reactivity of the analyzed compounds.