PROTON-TRANSFER REACTIONS OF ALKYLAROMATIC CATION RADICALS - THE EFFECT OF ALPHA-SUBSTITUENTS ON THE KINETIC ACIDITY OF P-METHOXYTOLUENE CATION RADICALS

The rates of deprotonation for a number of alpha-substituted p-methoxytoluene cation radicals (4-MeOPhCH2X.+ with X = H, Me, OAc, OH, OMe, Cl, 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(IV) 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 alpha-substituents increase the deprotonation rate, with the rate constant (k2obs) almost reaching the diffusion-controlled limit when X = CN and the base is NO3-. The k2obs 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 alpha-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-1 for the proton-transfer reactions induced by NO3-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 alpha-substituted p-xylene cation radicals, has, however, revaled that alpha-substituents of +R type (OH, OMe, and, to a less extent, Me) have a much larger rate-enhancing effect in the deprotonation of 4-MePhCH2X.+ than in that of 4-MeOPhCH2X.+; the contrary is instead observed with the alpha-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 alpha-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.