The electron transfer photochemistry of allenes with cyanoarenes. Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) and related reactions

Abstract

The photochemical reactions of 1,2,4,5-tetracyanobenzene (1), 1,4-dicyanobenzene (2) and 1,4-dicyanonaphthalene (3) with tetramethylallene (4) and 1,1-dimethylallene (5) in acetonitrile–methanol solution have been investigated. Both 1 and 2 give 1∶1∶1 arene–allene–methanol products (7–12) in which addition of methanol occurs exclusively at the central allene carbon with aromatic substitution at the termini. The formation of these products is rationalised on the basis of a photoinduced electron transfer mechanism described as the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Cyanoarene 3 undergoes photochemical reactions initially similar to the photo-NOCAS reaction, but with addition to the arene occurring instead of substitution. The primary product (example, 22) was not isolated since it undergoes further photochemical reactions. Compound 22 and its dimethyl analogues react via a photoinduced intramolecular [2π + 2π] cycloaddition across the 3,4-double bond to give tricyclo 1∶1∶1 arene–allene–methanol adducts (14, 17 and 18) or add methanol and cyclise to bicyclo 1∶1∶2 arene–allene–methanol adducts (15, 19 and 20). The use of biphenyl (6) as a co-donor enhances the efficiency of all the photoinduced electron transfer reactions studied. In fact, removing 6 from the reaction mixture containing 3 and 5 diverts the reaction from the electron transfer pathway and instead gives an exciplex-mediated [4π + 2π] cycloadduct, 21. The mechanisms, with particular reference to the regiochemical selectivities observed in the photoreactions involving 5, are discussed.