Toward a Unified Mechanism for Oxoammonium Salt-Mediated Oxidation Reactions: A Theoretical and Experimental Study Using a Hydride Transfer Model

Abstract

A range of oxoammonium salt-based oxidation reactions have been explored computationally using density functional theory (DFT), and the results have been correlated with experimentally derived trends in reactivity. Mechanistically, most reactions involve a formal hydride transfer from an activated C-H bond to the oxygen atom of the oxoammonium cation. Several new potential modes of reactivity have been uncovered and validated experimentally.

Solvent‐Promoted Oxidation of Aromatic Alcohols/Aldehydes to Carboxylic Acids by a Laccase‐TEMPO System: Efficient Access to 2,5‐Furandicarboxylic Acid and 5‐Methyl‐2‐Pyrazinecarboxylic Acid

“…Also, much better results in both substrate conversion and product yield were obtained in acetate buffer (50 mm, pH 5) than in phosphate buffer (50 mm, pH 6) (Figure 1B), despite comparable final pH values in the two media (pH 3.4 vs pH 3.6). Based on the results obtained in this work and previous work, [28,29] the possible roles that citrate played in catalytic oxidation of HMF may be discussed as follows: (1) to act as buffer to avoid great pH changes of the reaction mixtures; (2) to aid the FFCA hydration; (3) to assist rate-determining hydride transfer from alcohols to oxoammonium. Unlike aldehydes/ketones, the formation of carboxylic acids led to the reduced pH of the reaction mixtures, which might have a deleterious effect on both the activities of oxoammonium and TraL.…”

“…Previously Bailey and Leadbeater proposed a mechanism of oxoammonium-mediated oxidation of alcohols under acidic conditions using computational methods, [28,29] involving a ratedetermining hydride transfer from an activated CH bond of alcohols to the electrophilic oxygen of the oxoammonium cation (Scheme 2). Presumably, the multi-carboxyl compound citrate may serve as a base to assist the hydride transfer process, [29] thus accelerating the HMF oxidation.…”

Oxoammonium‐Mediated Allylsilane–Ether Coupling Reaction

“…Treatment of cinnamyl acetate 2 e under the above optimized reactions left the substrate unchanged. This absence of reactivity is in line with the fact that the oxcarbenium ion intermediate is destabilized by the acetate function [14a] . Substitution at the aromatic ring was then evaluated (Scheme 4, bottom left and top right boxes).…”

Selective TEMPO‐Oxidation of Alcohols to Aldehydes in Alternative Organic Solvents

“…In the case of the “classical” TEMPO‐catalysed oxidation using bleach as oxidation agent the oxidation of primary alcohols to the aldehyde was found to proceed much faster than the oxidation of secondary alcohols as shown for the oxidation of a mixture of n ‐nonan‐1‐ol and n ‐nonan‐2‐ol . Generally, TEMPO‐catalyzed oxidations can proceed in two different ways, following different mechanisms . Under acidic conditions, a hydride or proton transfer between catalyst and alcohol substrate can occur, while under basic conditions a pre‐oxidation complex is formed via an alkoxide attack on the electrophilic nitrogen of the oxammonium cation.…”