Selectivity Origin of Organic Electrosynthesis Controlled by Electrode Materials: A Case Study on Pinacols

Abstract

Unveiling the origin of an electrode's ability to control the reaction outcome and identifying key factors to explore a promising electrode for selective synthesis of value-added chemicals are highly desirable for minimizing the reliance on the dominant trial-and-error screening mode of electrode materials. Here, the electroreductive pinacol coupling of aromatic carbonyl compounds in an alkaline solution was selected as a model; a hydrogen adsorption free energy (ΔG H* ) far from 0, the specific aryl ring adsorption of substrates, and the facile desorption of products were proposed factors that make a material ideal for pinacol synthesis. These factors made carbon fiber paper (CP) optimal for hydrobenzoin synthesis with 99% selectivity, 96% Faraday efficiency, and the reaction rate of 0.6 mmol cm −2 h −1 . The aryl ring adsorbed on the CP surface promoted electron transfer and endowed the pinacol production with high selectivity over a wide range of potentials and current densities. Furthermore, hydrogen bonding was also crucial in reducing the reaction energy for generating the ketyl radical, a key intermediate for pinacols, in the alkaline solution. The CP could be reused for 10 cycles while maintaining 99% selectivity for pinacol. This electrochemical method showcased a wide substrate scope, which could be developed for the paired anodic fabrication of benzaldehyde and cathodic production of pinacol using the CP as bifunctional electrodes, and gram-scale synthesis of pinacol in a membrane reactor, highlighting its great promise.

Electrochemical Hydrogenation, Hydrogenolysis, and Dehydrogenation for Reductive and Oxidative Biomass Upgrading Using 5-Hydroxymethylfurfural as a Model System

“…One such example is the reductive dimerization of furfural or HMF to form pinacol-like products (Figure ). It is believed that this reaction involves the formation of a ketyl radical intermediate. ,,− Both outer sphere and inner sphere reaction mechanisms have been proposed. , …”

“…Several recent studies have found that the initial H-atom addition to generate the ketyl radical on route to pinacol formation does not involve the formation and transfer of H ads , unlike hydrogenation of the aldehyde group to form an alcohol. ,, This agrees well with the observations that pinacol formation is favored in a low overpotential ,, region where H ads cannot form and with electrodes that are poor at forming H ads such as C, ,− Pb, ,, In, and Au . It has also been reported that pinacol formation is favored when a higher concentration of HMF or furfural is used because this has the effect of promoting the formation of the ketyl radical over H ads . ,, …”

Electrochemical formal homocoupling of sec-alcohols

“…Electrochemical pinacol coupling would be a promising alternative to avoid the use of low-valent metal reductants. The reported methods commonly carried out in a divided cell [29][30][31][32][33][34] or an undivided cell with sacrificial anodes [35], such as Al, Mg, and Sn, to prevent undesired oxidative reactions (Scheme 1b) [36][37][38][39]. While sacrificial anodes enable the reactions to be performed with a simple and user-friendly undivided cell set-up, consuming the anode material with generating stoichiometric metal waste may be a serious issue in terms of green and sustainable chemistry.…”

Electrochemical Deoxygenative Barbier-Type Reaction

“…Under these conditions, cross-coupling product 3a was isolated in 81% yield (Table , entry 1). As a result of GCMS analysis of the reaction solution, deoxyhydrogenation of 1b (4-methyl-1,1′-biphenyl) and self-coupling of 2a (1,2-diphenylethane-1,2-diol) were found as the side reactions. The Faradaic efficiency of this reaction was calculated to be 27% (3.7 F).…”