Oxidation of Aldehydes into Carboxylic Acids by a Mononuclear Manganese(III) Iodosylbenzene Complex through Electrophilic C–H Bond Activation

Abstract

The oxidation of aldehyde is one of the fundamental reactions in the biological system. Various synthetic procedures and catalysts have been developed to convert aldehydes into corresponding carboxylic acids efficiently under ambient conditions. In this work, we report the oxidation of aldehydes by a mononuclear manganese(III) iodosylbenzene complex, [MnIII(TBDAP)(OIPh)(OH)]2+ (1), with kinetic and mechanistic studies in detail. The reaction of 1 with aldehydes resulted in the formation of corresponding carboxylic acids via a pre-equilibrium state. Hammett plot and reaction rates of 1 with 1°-, 2°-, and 3°-aldehydes revealed the electrophilicity of 1 in the aldehyde oxidation. A kinetic isotope effect experiment and reactivity of 1 toward cyclohexane­carboxaldehyde (CCA) analogues indicate that the reaction of 1 with aldehyde occurs through the rate-determining C–H bond activation at the formyl group. The reaction rate of 1 with CCA is correlated to the bond dissociation energy of the formyl group plotting a linear correlation with other aliphatic C–H bonds. Density functional theory calculations found that 1 electrostatically interacts with CCA at the pre-equilibrium state in which the C–H bond activation of the formyl group is performed as the most feasible pathway. Surprisingly, the rate-determining step is characterized as hydride transfer from CCA to 1, affording an (oxo)methylium intermediate. At the fundamental level, it is revealed that the hydride transfer is composed of H atom abstraction followed by a fast electron transfer. Catalytic reactions of aldehydes by 1 are also presented with a broad substrate scope. This novel mechanistic study gives better insights into the metal oxygen chemistry and would be prominently valuable for development of transition metal catalysts.

Industry-oriented method for the aqueous phase oxidation of crude 5-hydroxymethyl furfural (HMF) to 2,5-furandicarboxylic acid (FDCA)

“…At the fundamental level, it is revealed that the hydride transfer is composed of H atom abstraction, followed by a fast electron transfer. 54,55 Cu-Mn mixed metal oxide is the concordant pair reported for the oxidation of p-cresol and toluene. 56,57 The selective oxidation of benzyl alcohol to benzaldehyde has been investigated using molecular oxygen over copper-manganese mixed oxide, which showed improved catalytic activity and product selectivity.…”

Recent Advances in Nonprecious Metal Catalysis

“…Synthesis of aryl and alkyl carboxylic acids via oxidation of the corresponding aldehydes under mild conditions using manganese catalysis was recently reported by the Cho group (Scheme 13). 47 This provides an alternate approach to the Pinnick oxidation for carboxylic acid synthesis. Mechanistic studies revealed the reaction to proceed via a net hydride transfer from the aldehyde to the catalyst, forming an intermediate oxonium species which reacts further to form the carboxylic acid.…”

Synthetic Advances for Mechanistic Insights: Metal–Oxygen Intermediates with a Macrocyclic Pyridinophane System

“…After we reported the first example of a mononuclear nonheme manganese(III)−iodosylbenzene complex, we found that 9 is reactive with aldehydes, affording corresponding carboxylic acids. 52 The addition of cyclohexanecarboxaldehyde (CCA) to the solution of 9 resulted in the decay of 9 to [Mn III (TBDAP)-(OH)(A)] n+ (A = OTf − or CH 3 CN) and the formation of cyclohexanecarboxylic acid (66%) as a product. A Hammett plot obtained from the oxidation of benzaldehyde derivatives afforded a negative slope of −1.9, indicating an electrophilic pathway.…”