Ring Expansion and Isomerization in N-Methylindole and N-Methyleneindole Radical. Quantum Chemical and Kinetics Calculations

Abstract

Ring expansion and isomerization in N-methylindole and N-methyleneindole were studied by the Becke three-parameter hybrid method with Lee−Yang−Parr correlation functional approximation (B3LYP). Structure, energy, and frequency calculations were carried out with the Dunning correlation-consistent polarized double-ζ (cc-pVDZ) basis set. The reaction leading to ring expansion in N-methyleneindole proceeds via an intermediate that with an additional transition state produces hydroquinoline radical. The latter, by a fast H-atom ejection forms quinoline. The intermediate consists of a newly formed three-membered ring fused to the five-membered pyrrole ring. Isoquinoline production proceeds via an intermediate containing three rings fused together that increases the energetics of the process in comparison with quinoline production. Several transition states and intermediates are common to both ring expansion and isomerization of N-methyleneindole so that there are competing parallel pathways that determine the final distribution among the isomerization and ring expansion products. Potential energy surfaces for interisomerization among the various isomers of methylindole were calculated, and several reaction pathways are suggested. Ring expansion in the molecule of N-methylindole does take place, but its energy barrier is very high. The reaction coordinate in this process is cleavage of the C−N bond and one of the C−H bonds of the methyl group that from a kinetics viewpoint is equivalent to ejection of a H-atom from the molecule and ring expansion from N-methyleneindole radical. Rate constants for isomerization and ring expansion were obtained by transition-state theory, applying multiwell calculations with computer modeling. The structure, energetics, and additional parameters on the potential energy surfaces and the rate constants for the various processes are reported.