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-haloa nisoles with 1,1-diphenylethene in a nucleophilic solvent (methanol) has be en investigated. Analysis of the photochemical reactions involving the 4-ha lobenzonitriles revealed formation of alkene-methanol adducts, such as 1-me thoxy-2,2-diphenylethane, 1-methoxy-2,2-diphenylethene, and 1,1-dimethoxy-2 ,2-diphenylethane, indicative of a photochemical electron-transfer mechanis m. These products were not significant in the photochemical reactions invol ving the 4-haloanisoles. Both the 4-halobenzonitriles and the 4-haloanisole s produced an arene-alkene-methanol Markovnikov adduct, 1-aryl-2-methoxy-2, 2-diphenylethane (aryl = 4-cyanophenyl or 4-methoxyphenyl). This compound w as shown to undergo an acid-catalysed elimination to 1-aryl-2,2-diphenyleth ene under the reaction conditions, which subsequently underwent a 6 pi-elec trocyclization to the 3-substituted(cyano or methoxy)-9-phenylphenanthrene. Possible mechanisms for the observed reactivity are discussed and evaluate d.