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

Validation and Mechanistic Studies of the Headspace Effect in MOF Synthesis

The influence of headspace volume and composition on metal-organic framework (MOF) synthesis has been largely overlooked with no mechanisms currently invoking these factors. Herein, we reveal that an increased volume of oxygen in the headspace during the synthesis of zinc carboxylate-based MOFs (MOF-5, MOF-177, and IRMOF-8) as well as UiO-66 and ZJU-28 accelerates crystal formation. Investigation into the underlying mechanism of this phenomenon elucidates the critical roles played by all reagents, and some of their byproducts, in MOF synthesis. These insights are the key to developing more efficient and sustainable synthetic strategies for MOF production.

C(sp2)-H Acylation of Benzo[h]quinolines at Room Temperature via Aldehyde Autoxidation and Palladium Catalysis Cooperation

Herein, we report Pd-catalyzed C(sp2)-H acylation using aldehyde as acyl source and O2 as the green oxidant at room temperature. A selective association of acyl radical formed in-situ during aldehyde autoxidation with Pd-catalysis is the key to the process. The reaction afforded products in good yields (up to 82 %) and it is scalable. The proposed PdII/PdIII-catalytic cycle is supported by a series of control studies and DFT calculations.

Reactive Intermediate Confinement in Beta Zeolites for the Efficient Aerobic Epoxidation of α-Olefins

The efficient conversion of long-chain linear α-olefins (LAOs) into industrially useful epoxides is of pivotal importance. Mukaiyama epoxidation based on the use of molecular oxygen as the sole oxidant and aldehyde as the cosubstrate offers a promising route for LAOs epoxidation. However, challenges associated with epoxide forming selectivity and aldehyde coupling efficiency have long impeded the adoption of Mukaiyama epoxidation in large-scale applications. Herein, we show that confinement of key intermediates involved in the parallel epoxidation pathways within a Beta zeolite unlocks a selectivity of greater than 95 % towards the epoxides at the expense of minimal consumption of only 1.5 equivalents of the aldehyde, achieving an efficiency better than the state-of-the-art homogeneous and heterogeneous catalysts. Moreover, the incorporation of Sn sites into the Beta zeolite framework further facilitates the adsorption activation process of the aldehyde cosubstrate, thereby increasing the concentration of acylperoxy radicals and accelerating the kinetic process of the epoxidation step. Consequently, this work not only provides an efficient and green epoxidation route over zeolite catalysts with easily available O2 as the oxidant, but also systematically reveals the fundamental understanding of the zeolite confinement effects on steering the reaction pathway, which benefits the further development of valorization of LAOs.

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.