Kinetics and mechanisms of the oxidation of phenols by a trans-dioxoruthenium(VI) complex

The kinetics of the oxidation of phenols by trans-[Ru^VI(L)(O)₂]²⁺ (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in aqueous acidic solutions and in acetonitrile. In H₂O the oxidation of phenol produces the unstable 4,4′-biphenoquinone, as evidenced by a rapid increase and then a slow decrease in absorbance at 398 nm. The first step is first-order in both Ru^VI and phenol, and rate constants are dependent on [H⁺] according to the relationship k_f = k_x + (k_yK_a/[H⁺]), where k_x and k_y are the rate constants for the oxidation of PhOH and PhO^-, respectively. At 298 K and I = 0.1 M, k_x = 12.5 M^-1 s^-1 and k_y = 8.0 × 10⁸ M^-1 s^-1. At I = 0.1 M and pH = 2.98, the kinetic isotope effects are k(H₂O)/k(D₂O) = 4.8 and 0.74 for k_x and k_y, respectively, and k_f(C₆H₅OH)/k_f (C₆D₅OH) = 1.1. It is proposed that the k_x step occurs by a hydrogen atom abstraction mechanism, while the k_y step occurs by an electron-transfer mechanism. In both steps the phenoxy radical is produced, which then undergoes two rapid concurrent reactions. The first is a further three-electron oxidation by Ru^VI and Ru^V to give p-benzoquinone and other organic products. The second is a coupling and oxidation process to give 4,4′-biphenoquinone, followed by the decay step, k_s. A similar mechanism is proposed for reactions in CH₃CN. A plot of log k_x vs O-H bond dissociation enthalpies (BDE) of the phenols separates those phenols with bulky tert-butyl substituents in the ortho positions from those with no 2,6-di-tert -butyl groups into two separate lines. This arises because there is steric crowding of the hydroxylic groups in 2,6-di-tert-butyl phenols, which react more slowly than phenols of similar O-H BDE but no 2,6-tert-butyl groups. This is as expected if hydrogen atom abstraction but not electron transfer is occurring.