MO Analysis of Transition States in Some Selective Photocycloadditions of 2-Pyridones

Abstract

Regioselective photocycloadditions of conjugated cyclic enones are very effective for synthetic applications, but understanding of the selectivity control in the excited states is not clear so far. Contribution of the back-reactions may be not disregarded. 2-Pyridones(1), one of heterocyclic dienones mainly give regio-, site- and stereoselective endo-3,4-[2+2]cycloadducts(3d) and novel azocinones(5) from the excited singlet state, and exo-5,6-[2+2]cycloadducts(4x) from the triplet state by photoreactions with proper electron-dificient alkenes, such as methyl acrylate(2a) or acrylonitrile(2b). In this paper, we quantitatively propose two types of concerted-like two-step reaction mechanisms to products 3d and 5, and also biradical mechanism to product 4x, by performing the transition state(TS) analysis using MOPAC. Each TS for back-reactions of 3d, 4x, and 5 to the reactants 1 and 2 was also estimated.