Comparison of hydrogen abstraction and homolytic substitution in pentacyclo[4.n.0.02,5.03,8.04,7]alkanes

Abstract

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 dm³ 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 dm³ 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.