Structural effects on the reactivity of carbon radicals in homolytic aromatic substitution. Part 4. The nucleophilicity of bridgehead radicals

Abstract

The reactions of 1-adamantyl (1), 3,3-dimethylbicyclo[2.2.2]octan-1-yl (2), and 7,7-dimethylbicyclo[2.2.1]heptan-1-yl (3) radicals with protonated pyridines and quinoline have been investigated and the relative rates of substitution have been determined. The results obtained showed that the three bridgehead radicals have nucleophilic properties which decrease in the order: (1) > (2) > (3). This behaviour is explained on the basis of a different degree of charge development in the transition state of the addition step of the radicals to the protonated substrates, as a consequence of different ring strain in the three polycyclic systems. A comparison of the present results with those already available for other alkyl radicals is also reported, in order to evaluate the effect of electronic configuration on the reactivity of carbon radicals in homolytic aromatic substitution.