Polarity of the Aromatic Substrate

Cydopropyl radicals show a selectivity substantially identical to that of phenyl radical in benzene series (Table 21) the results of Table 22 indicate that in protonated p3uidine series, the selectivity of cyclopropyl radical is much higher than 

This dramatic difference of behavior has been interpreted in terms of transition state 81). The benzene series has not enough electron-deficient character to determine a transition state similar to a charge-transfer complex, so that the reaction rates would be affected more by the stability of the intermediate cyclo-hexadienyl radicals (23, 24) than by polar effects 

In protonated pyridine series, a transition state similar to a charge-transfer complex (27) would allow the clear distinguishing even of carbon free radicals differing little in nucleophilicity, such as methyl, cyclopropyl and phenyl radicals. 

The polarity of the substrate is therefore not less important than the polarity of the free radical in determining the sensitivity to polar effects and the consequent selectivity and synthetic interest. 

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