Impacts of the Regeneration Pathways of the Oxoammonium Cation on Electrochemical Nitroxyl Radical-Mediated Alcohol Oxidation

Abstract

Recent studies investigating the electrocatalytic properties of nitroxyl radicals (e.g. 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)) for electrochemical alcohol oxidation have demonstrated that the thermodynamic equilibrium oxidation potential and the steric environments unique to each nitroxyl radical have significant implications for their electrocatalytic abilities for alcohol oxidation. However, as the catalytic cycle of the nitroxyl radical involves not only the production and use of the oxoammonium cation for alcohol oxidation but also the regeneration of the oxoammonium cation from the hydroxylamine, the mechanism and rate of the regeneration of the oxoammonium cation critically affect the overall rate of electrochemical nitroxyl radical-mediated alcohol oxidation. Currently, a detailed understanding of this important aspect is lacking. Herein, the factors that affect the regeneration pathways of the oxoammonium cation are systematically investigated, and the impact that the regeneration pathways have on the overall catalytic activity for electrochemical alcohol oxidation is demonstrated. In this study, 4-acetamido TEMPO and 2-azaadamantane-N-oxyl, which are on the opposite ends of the spectrum of nitroxyl radical derivatives in terms of thermodynamic driving force and steric effects, are used as example nitroxyl radicals. As such, the results and discussion provided in this study can be used as comprehensive guidelines to understand the catalytic activity and regeneration pathways of any TEMPO derivative or polycyclic nitroxyl radical during electrochemical alcohol oxidation.

Redox‐Mediated Alcohol Oxidation Coupled to Hydrogen Gas Formation in a Dye‐Sensitized Photosynthesis Cell

“…We employed TEMPO 0/ + -couple in solution as ar edox mediator as 1) it has been successfully utilized as ar edox mediatori nD SSCs, indicating TEMPO + production in situ [22][23][24][25][26] and 2) having the oxidation reactioni ns olution instead of at the surface bounds pecies may lead to less charge recombinationa nd therefore higher photocurrents, 3) TEMPO + is used as as acrificialo xidant for many relevant oxidation reactions in organic synthesis so the device could be used for differentr eactions. [27][28][29][30][31] To properly study such DSPEC we have designedanew reactor that allows in situ monitoringo fr eaction progress.…”

Molecular Electrocatalytic Hydrogenation of Carbonyls and Dehydrogenation of Alcohols

“…[63] The regeneration of the oxoammonium species generally follows a comproportionation pathway, but at high potentials and fast kinetics (in the case of ACT), the 2e À / 1H + is observed (Scheme 10). [66] Therefore, depending on the applied potential and mediator-type, rate-limiting events might be either the alcohol oxidation step or the regeneration step.…”

Comparison of homogeneous and heterogeneous catalysts in dye-sensitised photoelectrochemical cells for alcohol oxidation coupled to dihydrogen formation

“…Firstly in a catalytic sense, TEMPO + is known as a sacrificial oxidant for many relevant oxidation reactions in solution organic syntheses. [34][35][36][37][38] Secondly, TEMPO is active as a homogeneous and a heterogeneous electrocatalyst, i.e. both when in solution and when bound to the electrode surface.…”