The photocycloaddition reaction of 8-methoxypsoralen with olefins

Abstract

The photocycloaddition reactions of 8-methoxypsoralen (8-MOP) to the π-electron-rich olefin 2,3-dimethyl-2-butene (DMB) and the π-electron-deficient olefins fumaronitrile (FN) and dimethylfumarate (DF) were studied as a model for the photosensitizing 8-MOPpyrimidine base photocycloaddition reaction. The direct and sensitized photoreactions were carried out in a liquid methanol solution and a frozen benzene solution. The major photoadducts 8-MOP<⪢DMB, 8-MOP<⪢FN and 8-MOP<⪢DF were monitored by silica gel thin layer chromatography (TLC) and were isolated by preparative TLC and silica gel column chromatography. The structures of the photoadducts were determined by physical methods to be 1:1 C 4 cyclo-adducts formed through the pyrone double bond of excited 8-MOP. The same photoadducts were obtained on irradiation of the frozen solution. The fluorescence of 8-MOP was quenched by DMB, FN, DF and tetracyano-ethylenr. Linear Stern-Volmer plots with k q values of 7.27 × 10 8 1 mol −1 s −1 2.36 × 10 9 1 mol −1 s −1, 8.65 × 10 8 1 mol −1 and 2.91 × 10 9 1 mol −1 s −1 respectively were obtained. A singlet exciplex formation mechanism is proposed for the photocycloaddition reaction of 8-MOP to olefins. However, the triplet mechanism cannot be ruled out, particularly for 8-MOP<⪢DMB.