Ab initio study of 1,4-pentadienyl electrocyclic reactions

The thermochemistry and transition states of the electrocyclic ring closures of the resonance-stabilized 1,4-pentadienyl radical to cyclopenten-3-yl, cyclobut-2-enylmethyl, and 2-vinylcyclopropyl are investigated at Hartree-Fock and coupled-cluster levels of theory. The CCSD(T)//QCISD/cc-pVDZ calculations predict activation barriers of 130, 169, and 236 kJ/mol, respectively, and DeltaH values of -60, 115, and 155 kJ/mol. Experimental evidence for the appearance of vinylcyclopropyl following photolytic generation of pentadienyl is more likely the result of a distinct electrocyclic reaction than quenching of a two-step mechanism for formation of cyclopentenyl. Higher energy pathways for formation of polycyclic structures are also briefly examined.