Side-Chain Oxidation of α-Substituted 4-Methoxytoluenes by Potassium 12-Tungstocobalt(III)ate. The Effect of α-Substituents on the Formation and Deprotonation of the Intermediate Cation Radicals

Article abstract of DOI:10.1021/jo00077a036

A kinetic study of the side-chain oxidation of α-substituted 4-methoxytoluenes (4-MeOPhCH2R, R = H, Me, OH, OMe, OAc, CN) by K5Co(III)W12O40*11H2O (Co(III)W) in AcOH/H2O (55:45) has been carried out.The reactions follow complex kinetics, suggesting that both the electron transfer and the radical cation deprotonation steps influence the reaction rate.A careful kinetic analysis has allowed the determination of the values of k₁, the rate constant for the electron transfer step, and of k_-1/k₂, the ratio between the rate constant of the back electron transfer and the one of the deprotonation of the cation radical.A satisfactory Marcus correlation between k₁ and the free energy changes for the electron-transfer step is obtained, affording a value of 41 kcal mol^-1 for the intrinsic barrier of the process.Therefrom a value of 57 kcal mol^-1 for the reorganization energy pertaining to the conversion of 4-MeOPhCH2R into 4-MeOPhCH2R(1+) can be calculated.By assuming that k_-1 is constant (the back electron transfer should be diffusion controlled for all substrates), the relative rate constants for the deprotonation of the radical cations, k₂(R)/k₂(H), can be obtained by the (k_-1/k₂)_H/(k_-1/k₂)_R ratios.It has been observed that k₂(R)/k₂(H) increases by increasing the oxidation potential of the substrate, whereas the reverse occurs with k₁.The effect on k₁ is, however, much larger than on k₂(R)/k₂(H), which allows us to conclude that α-substituents influence the oxidation rate of 4-methoxytoluene mainly through the effect exerted on the rate of the electron-transfer step.