Reaction mechanism of di-π-methane rearrangement of 4-phenyl-4H-pyran: A CASSCF/MRMP2 study

The potential energy surfaces (PESs) were investigated for di-π-methane rearrangement of 4-phenyl-4H-pyran at CASSCF(12,12)/6-31G(d) and multireference second-order perturbation theory. The three-step mechanism proposed by Zimmerman was confirmed for the reaction from the excited triplet state. The minimum energy path indicated that the initial σ-bond formation takes place from the 3(π,π*) state localized at the C2C3 olefinic moiety to give a primary phenyl-bridged biradical (3BR1). This biradical is immediately converted to a 1,3-biradical (3BR2). The intersystem crossing to 1BR2 can take place in the vicinity of the minimum energy crossing point due to the spin-orbit coupling interaction. 1BR2 is rather different from 3BR2 in geometrical and electronic structures and is regarded as a biradicaloid with zwitterionic character. The subsequent ring-closure of 1BR2 is almost barrierless and yields the expected product, 6-endo-phenyl-2-oxabicyclo[3.1.0]hexene. These results provide deeper insight into the previous experimental observations regarding the photochemical reactions of related compounds.