Abstract
Species specificity and regioselectivities in the photocycloadditions of some conjugated enones and dienones were confirmed to be controlled by the interaction between frontier molecular orbitals (FMO’s), calculated using PM3-CI level. The peri-([2+2] or [4+2]), site-(3, 4- or 5, 6-addition) and regioselective photoadducts in the triplet photoreactions of 2-pyrones with electron-deficient or electron-rich alkenes were considered to be formed through the regioselective 3-β or 6-β two-center interactions at the first-steps, and periselective ring-closures from the biradical intermediates. Regio-, site- and stereoselective [2+2]cycloadducts from the excited singlet or triplet reactions of 2-pyridones with proper electron-deficient alkenes were also confirmed to be caused mainly through the orbital and static four-center interactions, or biradical formation followed by ring-closure.
The regioselectivity difference between the photocycloadditions of two cyclic enones, such as 2-cyclohexenone and 2-cyclopentenone were similarly analysed by the FMO method. The regioselectivity of the enones was concluded to be controlled mainly by the FMO interactions at the first-steps and by the relative reactivity of the following ring closure of the biradical intermediates from cyclopentenone.
Relationship between the species specificity and site-, regio- and periselectivities in the photocycloadditions of cyclic conjugated enones and dienones could be interpreted from their FMO properties.
