Intramolecular Photoarylation of Alkenes by Phenyl Cations

Acetone-sensitized irradiation of various o-chlorophenyl allyl ethers in polar solvents led to either (dihydro)benzofurans or chromanes. The reaction appeared to involve photoheterolysis of the aryl-Cl bond followed by phenyl cation addition onto the tethered double bond either in 5-exo or 6-endo modes. The adduct cation gave the end products by deprotonation; addition of chloride anion or of the solvent, depending on the structure; and the conditions used. Preference for the 5-exo mode increased in passing from medium polarity (methylene chloride, ethyl acetate) to high polarity solvents (aqueous acetonitrile, methanol, 2,2,2-trifluoroethanol), for which this was often the exclusive path. The same compounds underwent photohomolysis when irradiated in cyclohexane, and radical cyclization was one of the process occurring. Substitution of a methylene group for the ether oxygen atom made 6-endo cyclization by far the main path in a related o-chlorophenylbutene. Again, the selectivity was higher in polar protic solvents. The results are discussed in terms of in cage ion pair versus free phenyl cation reactions.