Synthetic organic electrosynthesis has grown in the past few decades by achieving many valuable transformations for synthetic chemists. Although electrocatalysis has been popular for improving selectivity and efficiency in a wide variety of energy-related applications, in the last two decades, there has been much interest in electrocatalysis to develop conceptually novel transformations, selective functionalization, and sustainable reactions. This review discusses recent advances in the combination of electrochemistry and homogeneous transition-metal catalysis for organic synthesis. The enabling transformations, synthetic applications, and mechanistic studies are presented alongside advantages as well as future directions to address the challenges of metal-catalyzed electrosynthesis.

One of the questions not yet elucidated in the electrocatalytic oxidation of glucose is whether the first step of dehydrogenating proton-coupled electron transfer (PCET) concerns the hydrogen directly bound to an anomeric carbon (β-anomer) or that bound to oxygen of the anomeric carbon (α-anomer). The knowledge is necessary for renewable-energy-powered electrosynthesis of chemicals/fuels. To decipher that, we have used α-d-, β-d-, and d-glucose models to interrogate the electrocatalysis of the glucose anomers in neutral and alkaline pHs. We have also optimized a pulse methodology to directly grow surfactant- and binder-free gold particles onto the gas diffusion electrode (GDE) as free-standing electrocatalysts to bridge the scales between fundamental and applied research in fuel cells and electrolysis. Cyclic voltammetry measurements show that the electrooxidation of all of the glucose anomers starts at a potential region, where the gold surface is not yet fully oxidized and is dominated by the dehydrogenating adsorption of glucose, which rules out the hypothesis that glucose first adsorbs on the hydroxylated gold surface. The results in neutral pHs highlight the better electrocatalytic reactivity of the α-anomer over the β-anomer and the opposite in alkaline pHs, which invalidates the traditional thoughts that the β-anomer would always be the most reactive. Potential-dependent energy profiles computed by density functional theory (DFT) mainly confirm the promoted approach by the OH of the anomeric carbon (α-anomer). The deciphering of the electrocatalytic reactivity of glucose anomers at GDE-Au electrocatalysts, where gluconate is the main oxidation product at high selectivity and faradaic efficiency (>80%), opens opportunities to stimulate the electrosynthesis of renewable platform chemicals from the cellulosic biomass. The high selectivity and faradaic efficiency toward gluconate, a commodity renewable chemical, open opportunities to stimulate the biomass-fueled electrosynthesis.

The kinetics of many proton-coupled electron transfer (PCET) reactions cannot be adequately described by stepwise proton and electron transfer. Concerted electron− proton transfer (CPET) is another possibility, but examples exist where stepwise mechanisms are not viable yet there is no compelling evidence for CPET. This study investigates such a reaction, the oxidation of an NH-containing phenylenediamine radical cation, H 2 PD + , in the presence of pyridines in acetonitrile, using CV and UV/vis spectroelectrochemistry. As observed previously, the E 1/2 for the radical oxidation jumps to a considerably more negative potential upon addition of 1 equiv of pyridine. The CV wave broadens but stays chemically reversible. Further addition of pyridine leads to smaller E 1/2 shifts with continued reversibility. Different explanations have been put forth for this behavior; however, this study provides strong evidence that the E 1/2 shift can be completely explained by the overall reaction being H 2 PD + + pyr − e − → HPD + + Hpyr + . Classic stepwise proton−electron transfer cannot explain the reversibility, but it can be explained by a "wedge" scheme mechanism in which electron and proton transfer occurs in a stepwise fashion within the H-bond complex formed as an intermediate in proton transfer. This result points to the important role H-bonding may play in PCET even without CPET.

Oxidation or reduction of organic redox couples typically leads to large changes in acidity or basicity, with the result that proton transfer often accompanies electron transfer, particularly in aqueous solution. In less polar organic solvents, H-bonding also can play an important role. While it is generally appreciated that proton transfer will have a greater effect on the overall reaction than H-bonding, it is not always straight forward to distinguish between the two, and, despite a considerable amount of research, a complete, quantitative understanding of the relative roles that the two play in the voltammetry that is observed upon addition of acids or bases to organic redox couples in non-aqueous solution remains elusive. Most of the research in this area has been done with quinones in the presence of H-donors. In contrast, this study focuses on p-tetramethylphenylenediamine, H2PD, in the presence of H-acceptors. While quinones undergo two successive reductions to form the increasingly basic radical anion, then dianion, phenylenediamines, which are weakly basic to begin with, undergo two successive oxidations to form an increasingly acidic radical cation, then quinoidal dication. Upon addition of DMF, a weak H-acceptor, to H2PD in acetonitrile, small negative shifts in potential of the second oxidation are observed in the cyclic voltammetry (CV), with little change in wave shape. This is consistent with H-bonding of the DMF to the quinoidal dication, H2PD2+. Somewhat similar behavior is observed when slightly more basic guests are added such as cyanopyridine or trifluoromethylpyridine, but, unlike with DMF, with just 1 equivalent pyridine guest, a significant shift in the potential of the second oxidation is observed, followed by smaller shifts with additional equivalents. We believe that this behavior signals proton transfer between the pyridine, pyr, and the H2PD2+, so that the overall reaction occurring in the second oxidation corresponds to H2PD+ + pyr = HPD+ + Hpyr+ + e- . In this case, the observed E1/2 should depend on the pKa of the Hpyr+. To test this hypothesis, the voltammetry of H2PD is currently being studied with different pyridines that cover a range of pKa values. If correct, the explanation for the continued shift in potential with increasing concentrations of pyridine is well-accounted for simply by applying the Nernst equation to the overall reaction. However, while proton transfer can explain the potentials of the CV waves in the presence of added pyridine, simulations of the voltammetry show that proton transfer by itself cannot explain the observed reversibility of the second oxidation wave in the presence of increasing amounts of added pyridine. This is where H-bonding can play a role. By including H-bonding steps, and allowing electron transfer to occur through the H-bond complex formed between H2PD2+ and pyr (the intermediate in the proton transfer), the simulations can nicely explain both the observed potential shifts and the reversibility of the waves.

The kinetics of many proton-coupled electron transfer (PCET) reactions cannot be adequately described by step-wise proton and electron transfer. Concerted electron-proton transfer (CPET) is another possibility, but examples exist where step-wise mechanisms are not viable yet there is no compelling evidence for CPET. This study investigates such a reaction, the oxidation of an NH-containing phenylenediamine radical cation, H2PD+, in the presence of pyridines in acetonitrile. H2PD+ is formed by a net one electron oxidation of 2,3,5,6-tetramethylphenylenediamine in acetonitrile via a rather complicated mechanism that likely proceeds through a H-bonded dimer. Once formed, it can be further oxidized at more positive potentials to the quinoidal dication, H2PD2+, which will be considerably more acidic than the radical cation. Indeed, the E1/2 for the radical oxidation jumps to a considerably more negative potential upon addition of 1 equivalent of the weak base pyridine. The CV wave broadens but stays chemically reversible. Further addition of pyridine leads to smaller E1/2 shifts with continued reversibility. This behavior could possibly be explained by stabilization of H2PD2+ through H-bonding or proton transfer to pyridine, however, UV-vis spectroelectrochemical experiments provide definitive evidence for proton transfer. Furthermore, CV studies with 4-substituted pyridine derivatives of weaker basicity show that the observed E1/2 with 1 equivalent of the pyridine depends in a Nernstian fashion on the pKa of the conjugate acid of the pyridine, indicating that all the pyridines studied deprotonate H2PD2+ to give the quinoidal cation, HPD+. The continued E1/2 shift observed upon further addition of the pyridines can then be completely explained by application of the Nernst equation to the overall reaction H2PD+ + pyr → HPD+ + Hpyr+ + e−. However, while this explains the thermodynamics, the classic step-wise proton-electron transfer mechanism for this reaction cannot explain the observed reversibility at high base concentration. In contrast, the reversibility can be explained by a “wedge” scheme mechanism in which electron and proton transfer occur within the H-bond complex formed as an intermediate in proton transfer. In this case, the electron-proton transfer could be concerted, however, the kinetics for the second oxidation in the presence of pyridine show no significant isotope effect. Furthermore, a new reduction peak that appears at faster scan rates suggests the presence of an intermediate in the electron-proton transfer. Both results strongly suggest that the electron-proton transfer is step-wise, not concerted, within the H-bond complex. This result points to the important role H-bonding may play in PCET even without CPET.