Theoretical Analysis of the Mechanism for the Oxidative Carbonylation of Toluene to <i>p</i>-Toluic Acid by Rhodium Complexes

Density functional theory has been used to investigate the mechanism and kinetics of the liquid-phase, oxidative carbonylation of toluene to p-toluic acid (C7H8 + CO + 1/2O2 → p-C7H6COOH + H2O) catalyzed by Rh(III) cations. In toluene solution containing trifluoroacetic acid and dissolved CO, Rh(III) is coordinated to three trifluoroacetate (TFA) anions and two CO molecules as Rh(CO)2(TFA)3. The oxidative carbonylation of toluene is initiated by the addition of toluene across one of the Rh−O bonds of Rh(CO)2(TFA)3 to form (C7H7)Rh(CO)2(TFAH)(TFA)2. The latter species undergoes isomerization and CO migration to produce (C7H7CO)Rh(CO)(TFAH)(TFA)2, which then coordinates another molecule of CO. The mixed anhydride of toluic and tirfluoroacetic acid, C7H7C(O)O(O)CCF3 and Rh(CO)3(TFA), are produced by reductive elimination from (C7H7CO)Rh(CO)2(TFAH)(TFA)2. Para-toluic acid is then formed by hydrolysis of C7H7C(O)O(O)CCF3. The proposed reaction mechanism explains many of the observations reported in our previous experimental work (Zakzeski, J. J.; Bell, A. T. J. Mol. Catal. A 2007, 276, 8) and, in particular, the effect of temperature on the ratio of p- to m-toluic acid, the effects of H2O and the partial pressure of CO on the loss of catalyst activity, and the effect of Rh concentration on the formation of a catalytically inactive Rh dimer species.