Free radical reactions of bicyclo[2.1.1]hexane and bicyclo[2.2.1]heptane

Abstract

t-Butoxyl radicals abstract hydrogen from C(2) of bicyclo[2.1.1]hexane (3) to give bicyclo[2.1.1]hexane-2-yl radicals (6). At T > 250 K radicals (6) rearrange by β-scission to cyclopent-3-enylmethyl radicals; both species were observed by e.p.r. spectroscopy. In spite of the much greater ring strain the activation energy for rearrangement of (6) is about the same as that of cyclobutylmethyl radicals. Bromine atoms abstract hydrogen from C(2) of (3), but bis(trimethylsilyl)aminyl radicals abstract the bridgehead methine hydrogen at C(1) as well as the methylene hydrogens at C(2). There is very little abstraction of the bridgehead hydrogen in bicyclo[2.2.1]heptane by bis(trimethylsilyl)aminyl radicals. Thus, the greater bridgehead reactivity of (3) as compared with the bridge positions is not simply due to the lower selectivity of the bis(trimethylsilyl)aminyl radicals, but is an intrinsic property of the bicycloalkane.