Xylylenes, radical cations

The electron transfer photochemistry of benzocyclobutenes in the presence of dienophiles exhibits another interesting feature of CIP reactions [93]. For example, charge transfer irradiation of the EDA complex composed of 1,2-diphenylbenzocy-clobutene (36) and TCNE (IS) leads to the CIP (36 15 ), of which 36 spontaneously ring-opens to the o-xylylene (37) radical cation, followed by the collapse of the new CIP (Eqs. (20, 21)), leading to the Diels-Alder adduct 39 in high yield and with high efficiency. 

The PET reactions of isobutylene(2-methylpropene) (21) in the absence of methanol have yielded a novel photochemical nucleophile-alkene combination aromatic substitution reaction. The solvent, acetonitrile, was found to act as a nucleophile and add to the alkene radical cation to give a distonic radical cation that subsequently adds to the 1,4-dicyanobenzene radical anion. Cyclization to the ortho position of the phenyl group leads to product formation. Two-colour, two-laser techniques have been used to study the photolysis of a, a -dichloro-o- or -p-xylene to yield xylylenes. ... 

The lack of stereospecificity in equation (72) (as compared to equations (70) and (71)) is readily accommodated by the long lifetime of the triplet ion pair produced by the sensitized irradiation method. The latter allows the facile ring opening of DBC+ cation radical followed by isomerization of the resulting xylylene cation radical, i.e.,... 

The long-lived isomeric xylylene cation radical then undergoes either coupling to the adducts in equation (72) or back electron transfer followed by Diels-Alder reaction of the resulting neutral xylylenes and chloranil. 

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