Proton-transfer reactions of alkylaromatic cation radicals. The effect of alpha-substituents on the kinetic acidity of p-methoxytoluene cation radicals

The ratesof deprotonation for a number of a-substitutedp-methoxytolueneca tion radicals (4-MeOPhCH2X*+ with X = H, Me, OAc, OH, OMe, C1, CN, Ph, 4-MeOPh, and 4-MeOPhCD3'+) have been determined by a laser photolysis technique in MeCN. The cation radicals have been generated from the corresponding neutral substrates either by biphenyl/9,10-dicyanoanthracene photosensitized oxidation or by reaction with NO3* generated by photolysis of cerium(1V) ammonium nitrate. The deprotonations have been induced by 2,6-lutidine in the former case and by NO3- in the latter. It has been found that all a-substituents increase the deprotonation rate, with the rate constant (kzob) almost reaching the diffusion-controlled limit when X = CN and the base is NOj-. The kzob values appear to parallel the thermodynamic acidity of the cation radicals, and good Bronsted correlations have been obtained with a values of 0.24 for both the bases, which indicates a reactant-like transition state, with very little buildup of positive charge at the a-carbon, a conclusion also supported by the small (2.1 with 2,6-lutidine and 2.0 with NO3-) deuterium kinetic isotope effects. The data have also been treated in terms of the Marcus equation, and intrinsic barriers of 12.2 and 15.2 kcal mol-' for the proton-transfer reactions induced by N03-and 2,6-lutidine, respectively, have been calculated. These values are very close to those generally found in acid-base reactions of carbon acids. Comparison of these deprotonation rates with those of a-substituted p-xylene cation radicals, has, however, revaled that a-substituents of +R type (OH, OMe, and, to a less extent, Me) have a much larger rate-enhancing effect in the deprotonation of 4-MePhCHzXo+ than in that of 4-MeOPhCH2X0+; the contrary is instead observed with the a-CN substituent (-I or -R). It is suggested that the kinetic effect of an a-substituent on the deprotonation rate of an alkylaromatic cation radical is controlled by its capacity to change the homolytic C-H bond dissociation energy as well as by the extent of positive charge which develops at the a-carbon in the transition state. The latter in turn may be influenced by the nature of ring substituents, depending on their relative ability to delocalize a positive charge.