The photochemical nucleophile–olefin combination, aromatic substitution reaction (Part 2): methanol – cyclic olefins, 1,4-dicyanobenzene

Abstract

Direct irradiation of acetonitrile–methanol (3:1) solutions of 1,4-dicyanobenzene and the cyclic olefins, cyclohexene, 1-methylcyclohexene, norbornene, and 2-methylnorbornene, leads to formation of regio- and stereoisomers of the 1:1:1 (alcohol:olefin:aromatic) adducts. This reaction can be photosensitized by electron transfer; addition of electron donors, biphenyl or phenanthrene, to the irradiation mixture generally increases the efficiency and yield of adduct formation. The efficiency of the reaction and the ratio of isomeric adducts are also affected by the addition of salts, particularly magnesium perchlorate. All of the possible regio- and stereoisomers from cyclohexene and 1-methylcyclohexene have been identified, two from cyclohexene and four from 1-methylcyclohexene. Three of the four possible isomers from norbornene were characterized; the endo,endo isomer was not detected. There are eight possible isomers from 2-methylnorbornene; six were detected and five have been isolated and identified. The two sterically hindered isomers, those having both the 4-cyanophenyl and the methoxy groups in the endo position, and exo-3-(4-cyanophenyl)-endo-2-methoxy-exo-2-methylnorbornane, were not characterized. The structures of the products were established largely on the basis of the 1H and 13C nuclear magnetic resonance spectra. The mechanism of the reaction is discussed, with emphasis on those factors that may affect the product ratio. The most striking observation is that the reaction is regioselective when magnesium perchlorate is added to the irradiation mixture.