UDFT and MCSCF descriptions of the photochemical Bergman cyclization of enediynes

Several singlet and triplet potential energy surfaces (PES) for the Bergman cyclization of cis-1,5-hexadiyne-3-ene (1a) have been computed by UDFT, CI , CASCI, CASSCF, and CASMP2 methods. It is found that the first six excited states of la can be qualitatively described as linear combinations of the configurations of weakly interacting ethylene and acetylene units. Although the symmetry relaxation from C-2 nu to C-2 makes cyclization of the 1(3)B state Woodward-Hoffmann allowed, it also increases the probability of compe ting cis-trans isomerization. Hydrogen atom abstraction is another plausibl e pathway because the terminal alkyne carbons possess a large radical chara cter. In view of the competing processes, we conclude that the Bergman cycl ization along the 1(3)B path is unlikely despite its exothermicity (DeltaH( rxn)(CASMP2) = -42 kcal/mol). Calculations on cyclic analogues of la lead t o similar conclusions. A less exothermic, but more plausible pathway for ph otochemical cyclization lies on the 2(1)A PES (DeltaH(rxn)(CASMP2) = -18 kc al/mol). Compared to the 1(1)A(1) and 1(3)B states, the 2(1)A state has les s in-plane electron repulsion which may facilitate cyclization. The resulti ng p-benzyne intermediate has an unusual electronic structure combining sin glet carbene and open-shell diradical features. Deactivation of the 2(1)A s tate of 1a is a competing pathway.