Electrochemical hydrogenation of α-ketoesters and benzoxazinones using carbon electrodes and a sustainable Brønsted acid

A convenient electrochemical methodology for the hydrogenation of benzoxazinones and aryl-substituted α-ketoester substrates is presented, using carbon electrodes and sustainable Brønsted acids.

Renewable Hydride Donors for the Catalytic Reduction of CO2: A Thermodynamic and Kinetic Study

Increasing atmospheric CO₂ concentration and dwindling fossil fuel supply necessitate the search for efficient methods for CO₂ conversion to fuels. Assorted studies have shown pyridine and its derivatives capable of (photo)electrochemically reducing CO₂ to methanol, and some mechanistic interpretations have been proposed. Here, we analyze the thermodynamic and kinetic aspects of the efficacy of pyridines as hydride-donating catalytic reagents that transfer hydrides via their dihydropyridinic form. We investigate both the effects of functionalizing pyridinic derivatives with electron-donating and electron-withdrawing groups on hydride-transfer catalyst strength, assessed via their hydricity (thermodynamic ability) and nucleophilicity (kinetic ability), and catalyst recyclability, assessed via their reduction potential. We find that pyridines substituted with electron-donating groups have stronger hydride-donating ability (having lower hydricity and larger nucleophilicity values), but are less efficiently recycled (having more negative reduction potentials). In contrast, pyridines substituted with electron-withdrawing groups are more efficiently recycled, but are weaker hydride donors. Functional group modification favorably tunes hydride strength or efficiency, but not both. We attribute this problematic coupling between the strength and recyclability of pyridinic hydrides to their aromatic nature and suggest several avenues for overcoming this difficulty.

The True Fate of Pyridinium in the Reportedly Pyridinium-Catalyzed Carbon Dioxide Electroreduction on Platinum

Protonated pyridine (PyH⁺ ) has been reported to act as a peculiar and promising catalyst for the direct electroreduction of CO₂ to methanol and/or formate. Because of recent strong incentives to turn CO₂ into valuable products, this claim triggered great interest, prompting many experiments and DFT simulations. However, when performing the electrolysis in near-neutral pH electrolyte, the local pH around the platinum electrode can easily increase, leading to Py and HCO₃ ^- being the predominant species next to the Pt electrode instead of PyH⁺ and CO₂ . Using a carefully designed electrolysis setup which overcomes the local pH shift issue, we demonstrate that protonated pyridine undergoes a complete hydrogenation into piperidine upon mild reductive conditions (near 0 V vs. RHE). The reduction of the PyH⁺ ring occurs with and without the presence of CO₂ in the electrolyte, and no sign of CO₂ electroreduction products was observed, strongly questioning that PyH⁺ acts as a catalyst for CO₂ electroreduction.

Cocatalysis: Role of Organic Cations in Oxygen Evolution Reaction on Oxide Electrodes

Cocatalysis is a promising approach toward enhanced electrocatalytic activity. We report such synergic catalysis involving organic xanthylium-based catalyst, Xan²⁺, and oxides formed on the electrode surface. The oxygen evolution reaction (OER) was observed on some working electrodes (gold, platinum, glassy carbon, boron-doped diamond), while others (titanium and fluorine-doped tin oxide) exhibited no OER activity. On the basis of experimental data and supported by calculations, we propose a mechanism in which oxidized Xan²⁺ activates electrode toward the rate-determining O-O bond formation. In light of our findings, efficient OER electrocatalysis can be achieved using materials that strongly bind oxygen species and electron-deficient organic cations.

Hydride Shuttle Formation and Reaction with CO2 on GaP(110)

Adsorbed hydrogenated N-heterocycles have been proposed as co-catalysts in the mechanism of pyridine (Py)-catalyzed CO₂ reduction over semiconductor photoelectrodes. Initially, adsorbed dihydropyridine (DHP*) was hypothesized to catalyze CO₂ reduction through hydride and proton transfer. Formation of DHP* itself, by surface hydride transfer, indeed any hydride transfer away from the surface, was found to be kinetically hindered. Consequently, adsorbed deprotonated dihydropyridine (2-PyH^- *) was then proposed as a more likely catalytic intermediate because its formation, by transfer of a solvated proton and two electrons from the surface to adsorbed Py, is predicted to be thermodynamically favored on various semiconductor electrode surfaces active for CO₂ reduction, namely GaP(111), CdTe(111), and CuInS₂ (112). Furthermore, this species was found to be a better hydride donor for CO₂ reduction than is DHP*. Density functional theory was used to investigate various aspects of 2-PyH^- * formation and its reaction with CO₂ on GaP(110), a surface found experimentally to be more active than GaP(111). 2-PyH^- * formation was established to also be thermodynamically viable on this surface under illumination. The full energetics of CO₂ reduction through hydride transfer from 2-PyH^- * were then investigated and compared to the analogous hydride transfer from DHP*. 2-PyH^- * was again found to be a better hydride donor for CO₂ reduction. Because of these positive results, full energetics of 2-PyH^- * formation were investigated and this process was found to be kinetically feasible on the illuminated GaP(110) surface. Overall, the results presented in this contribution support the hypothesis of 2-PyH^- *-catalyzed CO₂ reduction on p-GaP electrodes.

Electrochemical Behavior of Pyridinium and N-Methyl Pyridinium Cations in Aqueous Electrolytes for CO2 Reduction

The electrochemical reduction of aqueous pyridinium and N-methyl pyridinium ions is investigated in the absence and presence of CO₂ and electrolysis reaction products on glassy carbon, Au, and Pt electrodes are studied. Unlike pyridinium, N-methyl pyridinium is not electroactive at the Pt electrode. The electrochemical reduction of the two pyridine derivatives was found to be irreversible on glassy carbon. These results confirmed the essential role of the N-H bond of the pyridinium cation. In contrast, the electrochemical response of N-methyl pyridinium ion at the glassy carbon electrode suggests that a specific interaction occurs between the glassy carbon surface and the aromatic ring of the pyridinium derivative. For all electrodes, an enhancement of current was observed in the presence of CO₂ . However, NMR spectroscopy of the solutions following electrolysis showed no formation of methanol or other possible byproducts of the reduction of CO₂ in the presence of either pyridinium derivative ion.

Thermodynamic and kinetic hydricities of metal-free hydrides

Thermodynamic and kinetic hydricities provide useful guidelines for the design of hydride donors with desirable properties for catalytic chemical reductions.

Electrochemical hydrogenation of a benzannulated pyridine to a dihydropyridine in acidic solution

Electrochemistry is used to demonstrate the selective hydrogenation of a benzannulated pyridine to a biomimetic dihydropyridine using sustainable Brønsted acids.

Dihydropteridine/Pteridine as a 2H+/2e- Redox Mediator for the Reduction of CO2 to Methanol: A Computational Study

Conflicting experimental results for the electrocatalytic reduction of CO₂ to CH₃OH on a glassy carbon electrode by the 6,7-dimethyl-4-hydroxy-2-mercaptopteridine have been recently reported [ J. Am. Chem. Soc. 2014 , 136 , 14007 - 14010 , J. Am. Chem. Soc. 2016 , 138 , 1017 - 1021 ]. In this connection, we have used computational chemistry to examine the issue of this molecule's ability to act as a hydride donor to reduce CO₂. We first determined that the most thermodynamically stable tautomer of this aqueous compound is its oxothione form, termed here PTE. It is argued that this species electrochemically undergoes concerted 2H⁺/2e^- transfers to first form the kinetic product 5,8-dihydropteridine, followed by acid-catalyzed tautomerization to the thermodynamically more stable 7,8-dihydropteridine PTEH₂. While the overall conversion of CO₂ to CH₃OH by three successive hydride and proton transfers from this most stable tautomer is computed to be exergonic by 5.1 kcal/mol, we predict high activation free energies (ΔG^‡_HT) of 29.0 and 29.7 kcal/mol for the homogeneous reductions of CO₂ and its intermediary formic acid product by PTE/PTEH₂, respectively. These high barriers imply that PTE/PTEH₂ is unable, by this mechanism, to homogeneously reduce CO₂ on a time scale of hours at room temperature.