Computational insight into the thermal reactivity of N-methyl-3-cyanomethyl-2-vinylindole. Competition between two pericyclic reactions

The direct cycloaddition of a N-methyl-3-cyanomethyl-2-vinylindole (1) with a N-methylmaleimide (2) is not observed. The in situ formation of an indolo-2,3-quinodimethane intermediate (3) leads instead of the normal cycloadduct to an 1,2,3,4-tetrahydrocarbazole (5). To help our understanding of this reaction, we performed a DFT study. The formation of both, the direct cycloadduct (4) and the intermediate (3) are found to be not thermodynamically favorable. However, the small amount of 3 formed in the medium reacts with the dienophile in this way explaining the regio-selectivity of the reaction. The diastereo-selectivity is finally explained by a Curtin-Hammett-type energy profile.