Effects of Ligand Composition on the Oxidative Carbonylation of Toluene to Toluic Acid Catalyzed by Rh(III) Complexes

Abstract

Experimental and theoretical studies were conducted to investigate the influence of anionic ligands (e.g., CF(3)COO(-), CH(3)SO(3)(-)) on the catalytic activity and selectivity of Rh(III) in the oxidative carbonylation of toluene to toluic acid. The catalyst activity was found to pass through a maximum as the pK(a) of the conjugate Brønsted acid decreases from 4.63 to -2.00, with the maximum activity occurring at pK(a) = 0.35, corresponding to CClF(2)COOH. The theoretical analysis showed that the strength of toluene coordination increases with decreasing basicity of the anion (i.e., decreasing pK(a) of the corresponding acid). In contrast, the activation barrier for C-H activation increases with decreasing ligand pK(a). The experimentally observed effect of anion composition on catalyst activity can be explained using calculations of the apparent rate coefficient for toluene activation based on density functional theory and transition state theory. The ratio of p- to m-toluic acid formed increases with decreasing acid pK(a) and passes through a maximum for pK(a) = 0, corresponding to CF(3)COOH. The effect of anion composition on the isomer selectivity is attributed to changes in the charge density on the Rh(III) cation, which in turn affect the distribution of charge on the para and meta carbon atoms of the aromatic ring.

Oxidative C−H/C−H Cross‐Coupling Reactions between N‐Acylanilines and Benzamides Enabled by a Cp*‐Free RhCl3/TFA Catalytic System

“…In addition to the yield, the chemoselectivity of the reaction—to favor cross‐coupling over homo‐coupling—could be efficiently improved by increasing the amount of TFA. Considering that CF 3 COO − (OTFA − ) as the anion enhances the electrophilicity of the Rh III center, the intermediates of this reaction were next investigated by NMR and HRMS analysis (Figure and Section IX in the Supporting Information). ESI‐HRMS analysis of a mixture of [Cp*RhCl 2 ] 2 , AgSbF 6 , Ag 2 CO 3 , and TFA in toluene implied the formation of [Cp*Rh(OTFA)] + ( I ; calcd: 351.0079, found: 351.0074) [Eq.…”

Insight into Regioselective Control in Aerobic Oxidative C–H/C–H Coupling for C3-Arylation of Benzothiophenes: Toward Structurally Nontraditional OLED Materials

“…During our initial optimization campaign, the reaction was performed in the presence of [RhCp*Cl 2 ] 2 as the catalyst, AgSbF 6 as the additive, and Ag 2 CO 3 as the oxidant under a N 2 atmosphere, affording only the C2-arylated product ( 3a′ ) in 58% yield (Scheme a, entry 1). Subsequently, trifluoromethanesulfonate (OTf – ) was explored as a counteranion to enhance the electrophilicity of the Rh­(III) center. , To our delight, the addition of TfOH led to a mixture of C3/C2-regioisomeric products in 25% yield with a ratio of 1:1 (Scheme a, entry 2). This encouraging observation clearly indicated that the reaction sites of benzothiophene could be switched practically through the modification of the electrophilicity of the metal center.…”

Oxidative C−H/C−H Cross‐Coupling Reactions between N‐Acylanilines and Benzamides Enabled by a Cp*‐Free RhCl3/TFA Catalytic System

“…To our delight, the desired biaryl 3a was obtained in 10 %yield (Table S1, entry 1). Considering that CF 3 COO À (OTFA À )a st he anion enhances the electrophilicity of the Rh III center, [12] the intermediates of this reaction were next investigated by NMR and HRMS analysis ( (Figure 1b,c). In addition to the yield, the chemoselectivity of the reaction-to favor cross-coupling over homo-coupling-couldb ee fficiently improved by increasing the amount of TFA.…”

“…In addition to the yield, the chemoselectivity of the reaction-to favor cross-coupling over homo-coupling-couldb ee fficiently improved by increasing the amount of TFA. Considering that CF 3 COO À (OTFA À )a st he anion enhances the electrophilicity of the Rh III center, [12] the intermediates of this reaction were next investigated by NMR and HRMS analysis ( Figure 1a nd Section IX in the Supporting Information). ESI-HRMS analysis of am ixture of [Cp*RhCl 2 ] 2 ,A gSbF 6 , Ag 2 CO 3 ,a nd TFAi nt oluene implied the formation of [Cp*Rh(OTFA)] + (I;c alcd:3 51.0079, found:3 51.0074) [Eq.…”