Comparison of hydrogen abstraction and homolytic substitution in pentacyclo[4.n.0.0^2,5.0^3,8.0^4,7]alkanes

The rate of hydrogen atom abstraction from basketane (pentacyclo[4.4.0.0(2,5).0(3,8).0(4,7)]decane) by tert-butoxyl radicals to produce 9-basketyl radicals was shown by EPR spectroscopy to be ca. 50 mol(-1) dm(3) s(-1) at 165 K. A similar study with homocubane (pentacyclo[ 4.3.0.0(2,5).0(3,8).0(4,7)]nonane) showed that the rate constant was even smaller (< 4 mol(-1) dm(3) s(-1) at 165 K). Photobromination of basketane gave a mixture of 9-bromobasketane, bromochlorotricyclodecenes, dibromotricyclodecenes and tetrabromotricyclodecanes. These products were accounted for by a mechanism involving competition between the initial bromine atom abstracting a methylene hydrogen, or homolytically substituting at one or other of the three different cube bridgehead C-atoms. Photobromination of homocubane was also studied but gave only dihalotricycloalkenes and tetrabromotricycloalkanes from homolytic substitution. The two pentacycloalkanes furnish two more examples of the rare homolytic cleavage of carbon-carbon bonds shared by two cyclobutane rings.