Species Specificity and Regio-selectivities in the Photocycloadditions of Cyclic Conjugated Enones and Heterocyclic Conjugated Dienones, and Their Frontier MO Analysis

Takaaki SUISHU, Toru OBATA, Tetsuro SHIMO and Kenichi SOMEKAWA*

Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University; Korimoto, 1-21-40, Kagoshima-shi 890-0065 Japan
Present address: Research Center at Chisso Petrochemical Corporation; Goi-Kaigan 5-1, Ichihara-shi 290-0058 Japan

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.

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