Reactivity of 1-arylnitrosoethylenes towards indole derivatives

Abstract

The reactivity of 1-arylnitrosoalkenes toward indole, 1-methylindole, and 3-methylindole is described. In contrast with the previously observed chemical behaviour of 1-(p-bromophenyl)nitrosoethylene towards pyrrole, the studied heterodienes reacted with indole and 1-methylindole to afford E-oximes via hetero-Diels–Alder reactions. The reaction with 3-methylindole also proceeds via cycloaddition giving the corresponding 1,2-oxazine. Quantum chemical calculations, at the DFT level, indicate that the energy barriers associated with the reactions between 1-(p-bromophenyl)nitrosoethylene and indole and its derivatives are similar to those observed for the reaction between this nitrosoalkene and pyrrole. However, the calculated energy of the theoretical Diels–Alder cycloadducts involving indole and pyrrole clearly suggests that the Diels–Alder reaction is privileged in the case of indole. Furthermore, in the case of the indole, the energy difference between reactants and products clearly favors the regiochemistry observed experimentally.