The photochemistry of 4-halobenzonitriles and 4-haloanisoles with 1,1-diphenylethene in methanol. Homolytic cleavage versus electron-transfer pathways

The photochemical reactivity of a series of 4-halobenzonitriles and 4-haloanisoles with 1,1-diphenylethene in a nucleophilic solvent (methanol) has been investigated. Analysis of the photochemical reactions involving the 4-halobenzonitriles revealed formation of alkene-methanol adducts, such as 1-methoxy-2,2-diphenylethane, 1-methoxy- 2,2-diphenylethene, and 1,1-dimethoxy-2,2-diphenylethane, indicative of a photochemical electron-transfer mechanism. These products were not significant in the photochemical reactions involving the 4-haloanisoles. Both the 4- halobenzonitriles and the 4-haloanisoles produced an arene-alkene-methanol Markovnikov adduct, 1-aryl-2-methoxy- 2,2-diphenylethane (aryl = 4-cyanophenyl or 4-methoxyphenyl). This compound was shown to undergo an acid- catalysed elimination to 1-aryl-2,2-diphenylethene under the reaction conditions, which subsequently underwent a 6π-electrocyclization to the 3-substituted(cyano or methoxy)-9-phenylphenanthrene. Possible mechanisms for the observed reactivity are discussed and evaluated.