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We present a sacrificial anode-free approach to reductive homocoupling of organohalides that does not require a co-catalyst. In this approach, a divided electrochemical cell with aprotic and aqueous compartments separated by an anion exchange membrane enables coupling of the cathodic homocoupling reaction with anodic oxidation of urea. We show that...
Citations
... [57] While sacrificial anodes offer some benefits in organic electrosynthesis, [60,61] the dissolution of the anode can complicate electrolysis, altering the inter-electrode gap as reaction progresses, [62] and the generation of stoichiometric metal waste can complicate waste disposal on a larger scale. [63] A few methods in the literature have avoided the use of sacrificial anodes in nickel-mediated electrohomocouplings by utilizing water [56] and urea oxidations [64] as counter-reactions in a divided cell. However, the use of cell separators increases cell resistivity and adds complexity to the system, therefore, undivided electrosynthesis without a sacrificial anode in nickelcatalyzed electrohomocouplings would be desirable. ...
Bifuran motifs can be accessed with nickel‐bipyridine electrocatalyzed homocouplings of bromine‐substituted methyl furancarboxylates, which, in turn, can be prepared from hemicellulose‐derived furfural. The described protocol uses sustainable carbon‐based graphite electrodes in the simplest setup – an undivided cell with constant current electrolysis. The reported method avoids using a sacrificial anode by employing triethanolamine as an electron donor.
Electrochemical pinacol coupling of carbonyl compounds in an undivided cell with a sacrificial anode would be a promising approach toward synthetically valuable vic -1,2-diol scaffolds without using low-valent metal reductants. However, sacrificial anodes produce an equimolar amount of metal waste, which may be a major issue in terms of sustainable chemistry. Herein, we report a sacrificial anode-free electrochemical protocol for the synthesis of pinacol-type vic -1,2-diols from sec -alcohols, namely benzyl alcohol derivatives and ethyl lactate. The corresponding vic -1,2-diols are obtained in moderate to good yields, and good to high levels of stereoselectivity are observed for sec -benzyl alcohol derivatives. The present transformations smoothly proceed in a simple undivided cell under constant current conditions without the use of external chemical oxidants/reductants, and transition-metal catalysts.
A novel functional composite material comprising a polymeric layer of the nickel complex with the Schiff base and silver nanoelectrodes embedded into pores of the nickel complex was synthesized. The poly[Ni(SaltmEn)]/Ag composite was studied by cyclic voltammetry, electrochemical quartz crystal microbalance method, and scanning electron microscopy with X-ray microanalysis. The synthesized poly[Ni(SaltmEn)]/Ag composite material was found to exhibit catalytic activity for the electrochemical reduction of trichloromethane.
Although electrocarboxylation reactions use CO2 as a renewable synthon and can incorporate renewable electricity as a driving force, the overall sustainability and practicality of this process is limited by the use of sacrificial anodes such as magnesium and aluminum. Replacing these anodes for the carboxylation of organic halides is not trivial because the cations produced from their oxidation inhibit a variety of undesired nucleophilic reactions that form esters, carbonates, and alcohols. Herein, a strategy to maintain selectivity without a sacrificial anode is developed by adding a salt with an inorganic cation that blocks nucleophilic reactions. Using anhydrous MgBr2 as a low-cost, soluble source of Mg2+ cations, carboxylation of a variety of aliphatic, benzylic, and aromatic halides was achieved with moderate to good (34-78%) yields without a sacrificial anode. Moreover, the yields from the sacrificial-anode-free process were often comparable or better than those from a traditional sacrificial-anode process. Examining a wide variety of substrates shows a correlation between known nucleophilic susceptibilities of carbon-halide bonds and selectivity loss in the absence of a Mg2+ source. The carboxylate anion product was also discovered to mitigate cathodic passivation by insoluble carbonates produced as byproducts from concomitant CO2 reduction to CO, although this protection can eventually become insufficient when sacrificial anodes are used. These results are a key step toward sustainable and practical carboxylation by providing an electrolyte design guideline to obviate the need for sacrificial anodes.
Electrochemical reduction of benzylic halides represents a convenient route to generating carbanions for their subsequent coupling with CO2 to obtain various carboxylic acids. Despite the industrial prospects of this synthetic process, it still lacks systematic studies of the efficient catalysts and reaction media design. In this work, we performed a detailed analysis of the catalytic activity of a series of different metal electrodes towards electroreduction of benzylic halides to corresponding radicals and carbanions using cyclic voltammetry. Specifically, we screened and summarized the performance of 12 bulk metal cathodes (Ag, Au, Cu, Pd, Pt, Ni, Ti, Zn, Fe, Al, Sn, and Pb) and 3 carbon-based materials (glassy carbon, carbon cloth, and carbon paper) towards electrocarboxylation of eight different benzylic halides and compare it to direct CO2 reduction in acetonitrile. Extensive experimental studies along with a detailed analysis of the results allowed us to map specific electrochemical properties of different metal electrodes, i.e., the potential zones related to the one- and two-electron reduction of organic halides as well as the potential windows where the electrochemical activation of CO2 does not occur. The reported systematic analysis should facilitate the development of nanostructured electrodes based on group 10 and 11 transition metals to further optimize the efficiency of electrocarboxylation of halides bearing specific substituents and make this technology competitive to current synthetic methods for the synthesis of carboxylic acids.
Sustainable electrosynthesis technologies are rapidly developing stimulated by the drive for sustainable chemical manufacturing and the increasingly accessible renewable electricity prices. The electrochemical utilization of easily available and renewable feedstock, such as CO2 and nitrogen, has attracted significant attention as it can additionally help closing disrupted carbon and nitrogen cycles. While direct CO2 reduction has benefited from recent advancements in the catalyst, electrolyte, reactor and system design, developments in electrochemical coupling of CO2 with organic precursors to yield value‐added chemicals have been lagging behind due to the apparent disconnect between the direct CO2 reduction and the organic electrosynthesis communities. Currently, electrocarboxylation reactions require high operating voltages, show low current densities, limited selectivity towards target products and are associated with low atom economy due to the reliance on sacrificial anode dissolution as a counter electrode process. Advancing this indirect electrochemical CO2 utilization strategy will enable a sustainable synthetic route to valuable chemicals including non‐steroidal anti‐inflammatory drugs and precursors for plasticizers and commercially‐relevant polymers – all of which are currently produced with high carbon footprint and low atom economy. This perspective briefly discusses the current state‐of‐the‐art in electroorganic synthesis with CO2 as a C1 synthon and suggests several strategies that can be borrowed from direct CO2 reduction breakthroughs to advance electrocarboxylation technology and bring it closer to industrial implementation.
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