Regioselectivity of the tri-pi-methane rearrangement: mechanistic and exploratory organic photochemistry.
The di-pi-methane rearrangement with two pi-groups attached to the central "methane carbon" of the reactant and which leads to a pi-substituted cyclopropane has been studied intensively. Our present research had the goal of elucidating the regioselectivity of the tri-pi-methane counterpart. The reactants with three pi groups attached to the central carbon mechanistically are capable of affording both di-pi-methane and tri-pi-methane photoproducts. In common with the di-pi-methane system, bridging of two of the pi-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 pi-pi-bridging step and the three-ring opening of the cyclopropyldicarbinyl diradical exhibit regioselectivity parallel to that of the di-pi counterpart. Both three-ring and five-ring photoproducts were formed with the ratio varying with conversion. Since the three-ring (i.e. di-pi-methane) photoproducts were found to ring expand to the five-ring (i.e. tri-pi-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.
Zimmerman
Howard E
HE
Chemistry Department, University of Wisconsin, Madison, Wisconsin 53706, USA.
Novak
Tibor
T