SINGLET PHOTOCHEMISTRY OF VINYLCYCLOPROPENES - REGIOSELECTIVITY AND MECHANISM - MECHANISTIC AND EXPLORATORY ORGANIC-PHOTOCHEMISTRY

A study of the direct photochemistry of 1-anisyl-2-phenyl-3-methyl-3-isobutenylcyclopropene (5) was pursued with the objective of elucidating the reaction regioselectivity and casting light on alternative reaction mechanisms for this reaction known to afford cyclopentadienes. Two mechanisms had previously been advanced-one proceeding by an initial three-ring bond fission to afford an intermediate vinyl carbene and the other by vinyl-vinyl bonding to afford a housane diradical intermediate. The two reaction mechanisms predict different reaction regiochemistry. The experimental observation in the case of vinylcyclopropene 5 was a preference for formation of that product in which the C1-C3 three-ring single bond is severed to give an anisyl-substituted carbene. Then bonding occurs between the isobutenyl moiety and the carbene center. CASSCF computations mere run on S-1 of the reactant cyclopropene as well as S-0 and S-1 of the two regioisomeric carbenes. The preferred carbene, both in the ground state and the first excited singlet, was that having the anisyl group on the carbene center. The S-1 carbenes were Linear at the carbene center while the S-0 carbenes were bent. The aryl group at the carbene center of S-1 preferred to overlap with a pentadienyl pi-system. The lower energy of the excited singlet carbenes with anisyl at the carbene center accounts for the regioselectivity of opening with internal conversion occurring during or subsequent to bond stretching.