Regioselectivity in the meta photocycloaddition of substituted benzenes to alkenes and 1,3-dienes

Abstract

The cause of the difference in regioselectivity between the meta photocycloaddition of substituted benzenes to alkenes and 1,3-dienes is investigated. The degeneracy of the two highest occupied molecular orbitals (HOMOs) and the two lowest unoccupied molecular orbitals (LUMOs) in benzene implies that the first excited state of benzene has to be described by at least two electron configurations. From molecular orbital (MO) correlation diagrams, it is clear that one of these electron configurations is involved in the meta photocycloaddition to alkenes and the other in the addition to 1,3-dienes. The interaction at the start of the reaction between the electron configurations of the addends and the charge transfer configurations is examined using zero-order wavefunctions. From these theoretical inquiries, the existence of a (slightly) polarized complex at the beginning of both addition processes is deduced. However, the polarization at the start of the addition to alkenes differs from the polarization in the addition to 1,3-dienes. The influence of substituents on the polarization is considered and the regioselectivity can be rationalized. The correspondence between our method, the exciplex mechanism invoked by Mattay and the frontier orbital method used by Houk is discussed.