Homolytic rearrangements of bicyclo[2.2.0]hexane and bicyclo[3.2.0]heptane

Abstract

Free radicals abstract hydrogen from both the bridge and bridgehead sites in bicyclo[2.2.0]hexane (4). The bicyclo[2.2.0]hexane-1-yl radical was observed by e.p.r. spectroscopy. The bicyclo[2.2.0]hexan-2-yl radical rearranges by stereoelectronically forbidden β-scission to give cyclohex-3-enyl radicals. Unlike other cyclobutanes, compound (4) undergoes an S_H2 reaction with bromine atoms. Free radicals abstract hydrogen only from the methylene groups of the C₅ ring in bicyclo[3.2.0]heptane (15a). The bicyclo[3.2.0]heptan-2-yl radicals were observed by e.p.r. spectroscopy, as was their rearrangement, by stereoelectronically allowed β-scission, to 2-(cyclopent-2-enyl)ethyl radicals. Bromine atoms abstract hydrogen from (15a) and no S_H2 reaction was detected. The radicals and their rearrangements were studied by semi-empirical MINDO/3 and MNDO methods.