(198b) Adjusting Ligand Characteristics Of Rhodium Complexes To Improve Reactivity Of Aromatic Hydrocarbon Oxidative Carbonylation To Aromatic Acid

In the presence of trifluoroacetic acid (TFAH), trifluoroacetic anhydride (TFAA), and carbon monoxide, rhodium catalyzes the oxidative carbonylation of toluene to yield toluic acid. The reaction is thought to proceed via an electrophilic mechanism in which a rhodium trifluoroacetate complex binds to and activates the C-H bond in the para position: Rh(TFA)3 + C7H8 --> Rh(C7H7)(TFA)2 + TFAH. CO insertion into the Rh-C bond of the aryl complex followed by reductive elimination of the resulting acetyl with trifluoroacetate yields a mixed anhydride of toluic and trifluoroacetic acids. Subsequent hydrolysis produces the desired p-toluic acid. Re-oxidation of the rhodium by vanadium and oxygen completes the catalytic cycle. The ligands on the rhodium metal center play an integral role in establishing the rate at which each step in the mechanism occurs. We report here a study of the influence of various halogenated acetate ligands in rhodium complexes on the oxidative carbonylation of aromatic hydrocarbons to their corresponding aromatic acids. Rhodium in the presence of trifluoroacetate resulted in 8.6% conversion of toluene to toluic acid (330 turnovers) after four hours. Although replacing trifluoroacetate with trichloroacetate or pentafluoropriopioniate resulted in reduced activity resulted in decreased activity, the use of chlorodifluoroacetate improved rhodium activity to 13.0% conversion (500 turnovers) under similar conditions. This improvement in activity was observed with a variety of aromatic hydrocarbons including benzene, ethylbenzene, propylbenzene, cumene, and tert-butyl benzene. Interestingly, both chlorodifluoroacetate and trichloroacetate complexes outperformed trifluoroacetate ligands for xylene oxidative carbonylation. Trichloroacetate complexes yielded 250 turnovers and chlorodifluorocomplexes resulted in 400 turnovers relative of o-xylene to 2,5-dimethylbenzoic acid relative to 120 turnovers provided by trifluoroacetate complexes. These changes indicate the possibility of precisely tuning the ligands in the catalytic complex to favor the reaction. The electron-withdrawing effects and proton abstraction ability of each halogenated acetate ligand will be discussed in detail with regards to the catalytic mechanism. The discussion will also entail the applicability of Gutmann donor and acceptor numbers as a metric for assessing the ability of a ligand to affect catalytic activity.