Mechanistic Elucidation of the 2-Norbornyl to 1,3-Dimethylcyclopentenyl Cation Isomerization

Extremely complicated isomerization mechanisms for the isomerization of the 2‐norbornyl to 1,3‐dimethylcyclopentenyl cation observed in the gas phase have been elucidated through Born–Oppenheimer molecular dynamics simulations and ab initio computations. These quantum chemical explorations revealed that there are two viable complex rearrangement pathways. The longer pathway involves consecutive ring‐openings with the formation of acyclic allylic cation intermediates. Although these are avoided in the shorter pathway, both routes are feasible, because the energy barriers of their first steps differ by only 1.0 kcal/mol. Both pathways eventually converge to the same methylcyclohexenyl cation, which undergoes ring contraction and methyl and hydride shifts to yield the C7H11+ global minimum, namely the 1,3‐dimethylcyclopentenyl cation. The mechanism for the 2‐norbornyl (1) to 1,3‐dimethylcyclopentenyl cation (16) isomerization has been elucidated by employing ab initio computations. Two viable complex pathways eventually converge to the same methylcyclohexenyl cation 11, which undergoes ring contraction, and methyl and hydride shifts to the target global minimum 16.