Ring expansion in methylene pyrrole radicals. Quantum chemical calculations

Ring expansions in W-methylene pyrrole, 2-methylene pyrrole, and 3-methylene pyrrole radicals 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 ξ (cc-pVDZ) and augmented aug-cc-pVDZ basis sets. The potential energy surfaces for ring expansion in methylene pyrrole contain several intermediates and transition states. The process, which takes place by insertion of the methylene group into the pyrrole ring in the three isomers, occurs via two principal mechanisms. One mechanism is associated with cleavage of the C-N or C-C bonds of the pyrrole ring already in the first step. In the second mechanism, the transition states of the first stage consist of a new three-membered ring and the original pyrrole ring fused together. In all the three isomers of methylene pyrrole, the reaction pathways leading to ring expansion include intermediates that, via additional transition states, lead to the production of hydropyridyl radical. The latter, by a very fast H-atom ejection, produces pyridine. Ring expansion in the molecule N-methylpyrrole does take place, but the energy level of the transition state is very high. The reaction coordinate in the process is cleavage of the C-N bond and a C-H bond of the methyl group, which from a kinetic viewpoint is equivalent to ejection of a H atom from the molecule and ring expansion in N-methylene pyrrole. The structure and energetics of the various pathways are shown.