Substituent-directed regioselectivity in the photocycloaddition of 2,3-dimethylbuta-1,3-diene to the benzene ring

Abstract

The reaction mode selectivity and regiochemistry of the photocycloaddition of 2,3-dimethylbuta-1,3-diene (DMBD) to the benzene ring are markedly dependent on the arene substituent. Mixtures of 1 : 1 adducts and large proportions of diene dimers result from parent benzene and Ph-R for R = Me, OMe, CF₃, CO₂Me or F, and (2π+ 2π) cycloadducts are formed between styrene and phenylacetylene and DMBD. In marked contrast, the (4 + 4) photocycloadduct 4 is essentially the sole product from benzonitrile and the diene. The exceptional directing influence of the cyano group towards 2,5-, 1′,4′-attack of the diene is little perturbed by the presence or position of Me, OMe or F substituents on the benzonitrile. The role in these (4 + 4) cycloadditions of polarisation in the S₁ arene on approach of the diene is discussed.