(301e) Effects of Ligand Composition On the Activity of Rh(III) for the Oxidative Carbonylation of Toluene to p-Toluic Acid

Rh-based catalysts are known to activate C-H bonds for the oxidative carbonylation of toluene to form p-toluic acid in the presence of carbon monoxide [1-2]. The reaction is thought to proceed through an electrophilic mechanism [3], which is consistent with the observation that the oxidative carbonylation of various hydrocarbons proceeds faster in the presence of trifluoroacetic acid than in the presence of acetic acid [4-5]. The superior performance of trifluoroacetic acid to acetic acid has been attributed to the higher electrophilicity that it imparts to the metal cation [6-7]. The principle objective of the present study was to develop a systematic understanding of the effects of ligand composition on Rh catalysts for the oxidative carbonylation of toluene to p-toluic acid, especially why methanesulfonic acid derivatives are not effective. During typical reactions, anhydrous toluene, the designated acid, either CF3SO2OH, CH3SO2OH, CF3CF2COOH, CF3COOH, CClF2COOH, CCl3COOH, CCl2HCOOH, CClH2COOH, or CH3COOH, the corresponding anhydride, Rh(acac)3, and NH4VO3 were placed into an autoclave, and pressurized with O2 and CO. The autoclave was heated to 353K and allowed to react for 4 h, before quenching with ice water. The resulting products of each reaction were analyzed by gas chromotography. The peak highest catalyst activity was observed for chlorodifluoroacetic acid, while activity decreased monotonically for acids both more and less acidic than chlorodifluoroacetic acid. Density functional theory calculations indicate that reaction kinetics are described by two significant steps. The first step is coordination of toluene to the catalyst, and the second step is activation of a C-H bond in toluene. The toluene binding step is equilibrated and tends to favor the bound state as the pKa acid associated with the anion ligand decreases in pKa, consistent with the notion of increasing Rh electrophilicity when more electron-withdrawing ligands are present. In contrast, the C-H bond activation step tends to become slower as the acid pKa decreases, due to the inability of less basic anions to accept the proton that leaves the toluene. By quantifying the relative effects of these two steps, it is found that it is possible to predict that CClF2COO- anions represent an optimal balance of properties, in accordance with experimental observations.

References

1. Fujiwara, Y., Kawata, I., Sugimoto, H., Taniguchi, H. J. Organometal. Chem. 256, C35-C36 (1983).

2. Sugimoto, H., Kawata, I., Taniguchi, H., Fujiwara, Y. J. Organometal. Chem. 266, C44-C46 (1984).

3. Jintoko, T., Fujiwara, Y., Kawata, I., Kawauchi, T., Taniguchi, H. J. Organometal. Chem. 385, 297-306 (1990).

4. Fujiwara, Y., Takaki, K., Taniguchi, Y. Synlett 7, 591-599 (1996).

5. Grushin, V. V., Thorn, D. L., Marshall, W. J., Petrov, V. ACS Symp. Ser. 885, 393-406 (2004).

6. Taniguchi, Y. Yamaoka, Y., Nakata, K., Takaki, K., Fujiwara, Y. Chem. Lett. 5, 345-346 (1995).

7. Kalinovskii, I. O., Gel'bshetin, A. I., Pogorelov, V. V. Russ. J. Gen. Chem. 71 No. 9., 1643-1466 (2001)..