The quest for a triplet ground-state alkene: Highly twisted CC double bonds

摘要

Density functional theory and extrapolated CCSD(T) computations of several "anti-Bredt" alkenes were carried to explore possible 1,2-diyl "alkene" candidates with a triplet ground state. Ten candidates containing twisted double bonds at the bridgehead positions of bicyclic structures (1-6) or adamantene (7-10) derivatives were studied. Based on a combination of ring strain, rigid scaffolding, and steric crowding, four species were identified to have surprisingly low singlet-triplet energy gaps (lower than 4 kcal/mol). A tert-butyl substituted bicyclic structure (4) was identified to have a near-zero singlet-triplet energy gap, but no triplet ground-state alkene was found. Ring strain energy (RSE) calculations, pi-orbital axis vector (POAV) analyses, and multiple linear regression models were performed to elucidate the geometric and energetic effects of double bond twisting in 1-10. Based on our computational exploration, it appears unlikely that there is a ground-state triplet olefin.