Homolytic ring fission reactions of bicyclo[n.1.0]alkanes and bicyclo[n.1.0]alk-2-yl radicals: electron spin resonance study of cycloalkenylmethyl radicals

Abstract

Hydrogen abstraction from bicyclo[n.1.0]alkanes (n= 3–6) by t-butoxyl radicals was examined by an e.s.r. technique. The main site of attack was C(2) giving bicyclo[n.1.0]alk-2-yl radicals which rearranged by β-scission of the outer cyclopropane bonds to give cycloalkenylmethyl radicals. This is in contrast to the bicycyclo[n.1.0]alk-2-yl radicals (n= 1,2) which rearranged by fission of the inter-ring bonds to give cycloalkenyl radicals. β-Scission in bicyclo[n.1.0]alk-2-yl radicals was examined by semi-empirical SCF MO calculations. The conformations and barriers to internal rotation of the cycloalkenylmethyl radicals were determined from the variation in the β-H hyperfine splitting constants with temperature. Photobromination of bicyclo[n.1.0]alkanes (n= 3,4) was also investigated in CCl₄ solution. The main process was bimolecular homolytic substitution (S_H2) by bromine atoms at the cyclopropane carbons, but there was an increase in hydrogen abstraction with ring size. The S_H2 reactions parallel the β-scission reactions of the bicyclo[n.1.0]alk-2-yl radicals in that the main bond undergoing fission changes from the inter-ring bond to the outer cyclopropane bond as the ring size increases.