Redox-photosensitised reactions. Part 6. Stereospecific ring cleavage of 1-phenoxy-1,2,2a,3,4,8b-hexahydrocyclobuta[a]naphthalene-8b-carbonitrile and its 2-methyl derivatives by redox-photosensitisation

Abstract

Ring cleavage of endo-1-phenoxy-1,2,2a,3,4,8b-hexahydrocyclobuta[a]naphthalene-8b-carbonitrile (1a) occurs to give phenyl vinyl ether and 3,4-dihydronaphthalene-1-carbonitrile in a 1 : 1 ratio upon the selective photoexcitation of phenanthrene, naphthalene, or triphenylene at 313 nm in the presence of p-dicyanobenzene in acetonitrile. With the cis- and trans-2-methyl derivatives of (1a), the photosensitised ring cleavage is stereospecific. The key mechanistic pathway is suggested to be the formation of a π-complex between the cyclobutanes and the cation radicals of the arenes, in which the phenoxy-group of the cyclobutanes plays important roles. It is speculated that the ring cleavage reaction proceeds by means of a concerted-like mechanism via the π-complex without complete hole transfer.