Regioselectivity of the tri-π-methane rearrangement: Mechanistic and exploratory organic photochemistry

Article abstract of DOI:10.1021/jo034189j

The di-π-methane rearrangement with two π-groups attached to the central "methane carbon" of the reactant and which leads to a π-substituted cyclopropane has been studied intensively. Our present research had the goal of elucidating the regioselectivity of the tri-π-methane counterpart. The reactants with three π groups attached to the central carbon mechanistically are capable of affording both di-π-methane and tri-π-methane photoproducts. In common with the di-π-methane system, bridging of two of the π-systems affords a cyclopropyldicarbinyl diradical intermediate that opens to an allyl carbinyl diradical. This diradical has the option of closing to a three-membered or a five-membered ring. It was found that the regioselectivity of the initial π-π-bridging step and the three-ring opening of the cyclopropyldicarbinyl diradical exhibit regioselectivity parallel to that of the di-π counterpart. Both three-ring and five-ring photoproducts were formed with the ratio varying with conversion. Since the three-ring (i.e. di-π-methane) photoproducts were found to ring expand to the five-ring (i.e. tri-π-methane) products, kinetics were employed to determine to what extent the reaction proceeds in a two-step versus direct formation of the five-ring product. It was found that the direct route was the major one.