Mechanistic Insights into the Aerobic Oxidation of Aldehydes: Evidence of Multiple Reaction Pathways during the Liquid Phase Oxidation of 2-Ethylhexanal

Unveiling the Hidden Reactivity in the N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aldehydes: Unlocking Its Powerful Catalytic Performance

An innovative solution that overcomes the long-standing inherently low efficiency in N-heterocyclic carbene-catalyzed aerobic oxidation of aldehydes is reported. This solution included the design and synthesis of a novel polymerized catalyst and the utilization of a flow reactor. The unprecedentedly high efficiency achieved via this protocol makes it synthetically applicable. A total turnover number (TON) of 26,300 was achieved based on recycling experiments (runs). The highest TON in a single run could be up to 2475 with a turnover frequency (TOF) of 208 h-1, far superior to its traditional counterpart, in which a typical TON ranges from 20 to 100 with a TOF of less than 10 h-1. The catalyst has been recycled over 50 times and is still fully active. The success was attributed to the discovery of hidden reactivity, which was observed for the first time as an autoacceleration in the reaction rate during kinetic investigations. The research also provided concrete evidence supporting the conclusion that radical intermediates played crucial roles in the catalytic cycle by having a determinative impact on the overall reaction rate.

Solvent Effect on Product Distribution in the Aerobic Autoxidation of 2-Ethylhexanal: Critical Role of Polarity

In the aerobic oxidation of aldehydes to acids, how the solvent affect the reaction remains unclear. Herein, the solvent effect in the oxidation of 2-ethylhexanal (2-ETH) to 2-ethylhexanoic acid (2-ETA) was systematically investigated. The vastly different product distributions were observed which could be ascribed to the dominant intermolecular forces. Though strong intermolecular forces in protic solvents limit the oxidation, the optimal 2-ETA yield (96%) was obtained in ipropanol via gradually evaporating the solvent to remove the interactions. Theoretical calculations further revealed that the hydrogen bonds between reactant and protic solvent increase the C-H bond energy (-CHO in 2-ETH). Meanwhile, the hydrogen bonds may improve 2-ETA selectivity by promoting H transfer in the oxidation rearrangement step. Our work discloses the critical role of polarity in determining the reactivity and selectivity of 2-ETH oxidation, and could guide the rational design of more desirable reaction processes with solvent effect.