Singlet and triplet photocycloaddition reactions of 2‐pyridones with propenoate and 2,4‐pentadienotes, and the frontier molecular orbital analysis

Abstract

AbstractRegioselective photocycloadditions of 2‐pyridones (1) with 2,4‐pentadienoates (3) were analysed and compared with the reactions with propenoate (2), and origins of the different regioselectivities were inferred from frontier molecular orbital properties by the use of PM3‐CI method.Direct photoreactions of 1 with 3 being α,β,γ,δ‐unsaturated carboxylates gave four types of regio‐selective [2+2]cycloadducts, 3‐β:4‐α‐4, 5, 3‐δ:4‐γ‐6,7,5‐δ: 6‐γ‐8, 9 and 5‐δ:6‐γ‐[2+2]cycloadducts 10,11. Sensitized photoreactions of 1 with 3 did not occur, and fluorescence of 1 was weakly quenched by 3. Otherwise, main cycloadducts in the sensitized reactions of 1 with 2 have been obtained as 5‐α:6‐β‐[2+2]cycloadducts, which were different from main 3‐β:4‐α‐[2+2]cycloadducts of the direct reactions, and these site‐and regio‐selectivities were inferred from the different frontier molecular orbital coefficients at the. 3‐and 6‐positions of 1 for the triplet and singlet states. Formations of 4–7 were also interpreted by both effects of frontier orbital HSOMO‐LUMO interactions and of the electrostatic interaction between 1 and 3. The other site‐and regioselective adducts 8–11 were inferred to be formed by the electrostatic interactions similar to 2‐pyridone photodimerization.