Radical stabilization energy advantage

Enough substituted cyclopropanes have now been subjected to careful kinetic studies so that a characteristic pattern of reactivity and stereochemical preferences has emerged. Substituents facilitate stereomutations in proportion to their ability to stabilize 1,3-trimethylene diradical structures. The values for both k 2 and (k + k2) stereomutation rate constants relate linearly with consistent measures of substituent radical stabilization energies with equal sensitivities. Experimentally determined (A , + / 2)- i2 ratios do not vary widely they range from 1.4 to 2.5 over a fair diversity of substituents. Neither do kf.kj ratios vary widely. The majority fall between 1 1 and 2.5 1 the largest yet reported gives 2(CHD) a symmetry corrected kinetic advantage over A i(CDPh) in 1-phenyl-1,2,3-d3-cyclo-propanes of 5 1. 

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. 

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