Photochemistry of epoxyquinones. Part 7. Photo-induced cycloaddition of 2,3-dimethyl-2,3-dihydro-2,3-epoxy-1,4-naphthoquinone to olefins
Irradiation of 2,3-dimethyl-2,3-dihydro-2,3-epoxy-1,4-naphthoquinone (1a) in benzene gave the carbonyl ylide (2a) or the 1,3-diradical (3a), which was trapped with a variety of olefins, including simple alkenes, cycloalkenes, aromatic olefins, allyl alcohols, electron-rich olefins, and electron-deficient olefins. The excited state of (1a) was quenched by electron-rich olefins presumably via charge-transfer, while (2a) or (3a) react most readily with electron-deficient olefins. Cycloaddition of (2a) or (3a) to cis- and trans-but-2-ene proceeded stereospecifically. Further irradiation of the primary adducts gave the spirophthalide (5) and the alkylidenephthalides Z-(7) and E-(7)via preferential α-cleavage at the more crowded side. On the basis of the evidence of quantum yields and triplet lifetimes, this preference is believed to arise from the different efficiencies of the radical recombination of possible biradicals.