Electrochemical Pinacol Coupling of Acetophenone Using Boron‐Doped Diamond Electrode

Nakahara, Kenshin; Naba, Keisuke; Saitoh, Tohru; Sugai, Tomoya; Obata, Rika; Nishiyama, Shigeru; Einaga, Yasuaki; Yamamoto, Takashi

doi:10.1002/celc.201900202

Cited by 22 publications

(14 citation statements)

References 54 publications

(21 reference statements)

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“…The Yamamoto group recently reported an electrochemical pinacol coupling of ketones using boron-doped diamond (BDD) as the electrode. 63 However, low conversion yields (10−79%) were obtained, which might be due to the lack of effective adsorption of aryl rings on the BDD surface. These results suggested that the adsorption of the aryl ring was favorable to the formation of pinacol 2a.…”

Section: Resultsmentioning

confidence: 99%

“…This result further confirmed the importance of the adsorption of the aryl ring in this electrochemical pinacol coupling. The Yamamoto group recently reported an electrochemical pinacol coupling of ketones using boron-doped diamond (BDD) as the electrode . However, low conversion yields (10–79%) were obtained, which might be due to the lack of effective adsorption of aryl rings on the BDD surface.…”

Section: Resultsmentioning

confidence: 99%

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

Liu

Zhou

et al. 2021

ACS Catal.

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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.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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

Liu

Zhou

et al. 2021

ACS Catal.

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“…Thus, the C–O ether cleavage on the skeletal Ni electrode surface is qualitatively different from the electron transfer process. Meanwhile, as reported by Yamamoto and co-workers, carbon electrodes are capable of electron transfer to the benzylic ketone of acetophenone, leading to pinacol coupling of two ketyl radicals to generate 2,3-diphenylbutane-2,3-diol as the stereoisomers …”

Section: Resultsmentioning

confidence: 85%

“…Meanwhile, as reported by Yamamoto and co-workers, carbon electrodes are capable of electron transfer to the benzylic ketone of acetophenone, leading to pinacol coupling of two ketyl radicals to generate 2,3-diphenylbutane-2,3-diol as the stereoisomers. 42 With the RVC at high current, the amount of acetophenone dropped quickly within 1 h of cathodic treatment (see S30). After 9 h of reduction, only about 21% of 1-phenylethanol was observed (Table 1, Entry 1).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…After 9 h of reduction, only about 21% of 1-phenylethanol was observed (Table , Entry 1). Instead, the pinacol product from dimerization of the radical anions was detected (see S31); such dimerization is hard to avoid in electron transfer reductions of ketones. , Importantly, no such dimerization products were found in the skeletal Ni catalyzed processes.…”

Section: Resultsmentioning

confidence: 99%

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Multiple Mechanisms Mapped in Aryl Alkyl Ether Cleavage via Aqueous Electrocatalytic Hydrogenation over Skeletal Nickel

et al. 2020

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We present here detailed mechanistic studies of electrocatalytic hydrogenation (ECH) in aqueous solution over skeletal nickel cathodes to probe the various paths of reductive catalytic C–O bond cleavage among functionalized aryl ethers relevant to energy science. Heterogeneous catalytic hydrogenolysis of aryl ethers is important both in hydrodeoxygenation of fossil fuels and in upgrading of lignin from biomass. The presence or absence of simple functionalities such as carbonyl, hydroxyl, methyl, or methoxyl groups is known to cause dramatic shifts in reactivity and cleavage selectivity between sp3 C–O and sp2 C–O bonds. Specifically, reported hydrogenolysis studies with Ni and other catalysts have hinted at different cleavage mechanisms for the C–O ether bonds in α-keto and α-hydroxy β-O-4 type aryl ether linkages of lignin. Our new rate, selectivity, and isotopic labeling results from ECH reactions confirm that these aryl ethers undergo C–O cleavage via distinct paths. For the simple 2-phenoxy-1-phenylethane or its alcohol congener, 2-phenoxy-1-phenylethanol, the benzylic site is activated via Ni C–H insertion, followed by beta elimination of the phenoxide leaving group. But in the case of the ketone, 2-phenoxyacetophenone, the polarized carbonyl π system apparently binds directly with the electron rich Ni cathode surface without breaking the aromaticity of the neighboring phenyl ring, leading to rapid cleavage. Substituent steric and electronic perturbations across a broad range of β-O-4 type ethers create a hierarchy of cleavage rates that supports these mechanistic ideas while offering guidance to allow rational design of the catalytic method. On the basis of the new insights, the usage of cosolvent acetone is shown to enable control of product selectivity.

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Electrocatalytic Hydrogenation and Reductive Coupling of Aryl Ketones: Highly efficient Straightforward Metal‐Free Access to Alcohols and Pinacols

2022

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An electrocatalytic pathway is disclosed for the hydrogenation and reductive pinacol coupling of aryl ketones. The operationally simple protocol neither requires an external catalyst nor any sacrificial anode and proceeds efficiently under the aerobic condition at an ambient temperature and pressure. Under the optimized condition, diaryl ketones transform completely to the corresponding diarylmethanols whereas aryl ketones undergo reductive pinacol coupling leading to the respective 1,2‐diol compounds in good to excellent yields. The protocol showed a high degree of substrate compatibility with moderate to very good diastereoselectivity depending on the substrates. The scope of the developed method was also extended to the different bioactive molecules opening up the possibility for their late‐stage synthetic modification.

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Electrochemical Pinacol Coupling of Acetophenone Using Boron‐Doped Diamond Electrode

Cited by 22 publications

References 54 publications

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

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

Multiple Mechanisms Mapped in Aryl Alkyl Ether Cleavage via Aqueous Electrocatalytic Hydrogenation over Skeletal Nickel

Electrocatalytic Hydrogenation and Reductive Coupling of Aryl Ketones: Highly efficient Straightforward Metal‐Free Access to Alcohols and Pinacols

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