Potential Nonadiabatic Reactions: Ring-Opening 4,6-Dimethylidenebicyclo[3.1.0]hex-2-ene Derivatives to Aromatic Reactive Intermediates

Abstract

Potential singlet-triplet surface crossings for the ring opening of 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives were explored using density functional theory (DFT) and complete active space self-consistent field (CASSCF) methods. Since these ring openings involve relatively high energy species that lead to relatively stable aromatic species, a good scenario for potential nonadiabatic events, we posited that the reaction paths of these ring openings might come close to or cross excited state surfaces. At the DFT level of theory, all reaction paths exhibited characteristics suggestive of singlet-triplet intersections along their paths. 6-Methylidenebicyclo[3.1.0]hex-3-en-2-one and a closely related derivative (4-methylidenebicyclo[3.1.0]hex-2-en-6-one) were explored at the CASSCF level of theory; CASSCF results were qualitatively similar to DFT results and yielded spin-orbit couplings of 1.1-1.4 cm(-1) at the singlet-triplet crossing points.

When To Let Go—Diradical Intermediates from Zwitterionic Transition State Structures?

“…In a follow-up to their 1999 study, Carpenter and co-workers concluded in 2005 that allene-ene-yne cyclization also involves a post-transition state conical intersection that allows access to both zwitterion and the diradical intermediates . In 2013, Stumetz, Nadeau, and Cremeens described calculations (CASSCF and DFT) on the ring-opening of 4,6-dimethylidenebicyclo[3.1.0]­hex-2-ene derivatives to form bis-benzyl diradical intermediates (Scheme , bottom), predicting that post-transition state crossings between the singlet and triplet surfaces occur for these reactions . Related “two-state reactivity” has been described for inorganic reactions involving transition metals .…”

“…14 In 2013, Stumetz, Nadeau, and Cremeens described calculations (CASSCF and DFT) on the ring-opening of 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives to form bis-benzyl diradical intermediates (Scheme 2, bottom), predicting that post-transition state crossings between the singlet and triplet surfaces occur for these reactions. 15 Related "two-state reactivity" has been described for inorganic reactions involving transition metals. 16 We wondered whether similar post-transition state surface crossings were occurring in the Brummond−Chen cyclizations we had examined previously (Scheme 1d).…”