Hydrodynamic Rocking Disc Electrode Study of the TEMPO‐mediated Catalytic Oxidation of Primary Alcohols

Abstract

The hydrodynamically thinned diffusion layer at a sinusoidally rocking disc is approximately uniform and can be expressed in terms of a diffusion layer thickness δRoDE=9.0D13ν16Θf with D, the diffusion coefficient of the redox active species, v, the kinematic viscosity, Θ, the total rocking angle (here 90 degrees), and f, the rocking frequency (ranging here from 0.83 to 25 Hz). For the one‐electron oxidation of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) in aqueous carbonate buffer pH 9.5, it is shown that there is quantitative agreement between the expression for the diffusion layer thickness and experimental data. Next, for seven primary alcohols, the catalytic TEMPO‐mediated oxidation mechanism is investigated under rocking disc conditions. Chemical rate constants are evaluated (by varying the diffusion layer thickness) employing the DigiElch4.F simulation package. Trends in the chemical rate constants (compared with DFT calculations) suggest enhanced reactivity for methanol versus higher primary alcohols and for aromatic versus non‐aromatic primary alcohols. Rocking disc voltammetry allows quantitative mechanistic analysis in the laminar flow regime.

Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions

“…A report by Marken and co-workers in 2016 utilized a hydrodynamic rocking disc electrode technique to further examine the TEMPO-mediated oxidation of primary alcohols. 102 This and other mechanistic studies for aminoxyl-catalyzed alcohol electrooxidation corroborate the mechanistic findings and trends discussed above. 103–105 The majority of these mechanistic studies have focused on basic conditions, owing to the enhanced rates of aminoxyl-mediated alcohol oxidation at higher pH.…”

Integrated Electro‐Biocatalysis for Amine Alkylation with Alcohols

“…The system allows the presence of halogen groups in the aromatic ring, although the reaction should be stopped earlier to avoid the generation of several by‐products by halogen group reaction at the cathode in our undivided cell. ( 6 d ) Primary aliphatic alcohols such as 1‐hexanol provided lower conversions ( 6 e ), which is consistent with the literature reports [18] . Our system also allowed the electro‐oxidation of secondary aliphatic alcohols, although the addition of 0.2 equivalents of TEMPO were needed ( 6 f – 6 j ).…”

Integrating TEMPO and Its Analogues with Visible‐Light Photocatalysis

“…Two techniques are generally used to investigate the reaction mechanism: DFT calculations and electron paramagnetic resonance (EPR) spin‐trapping experiments. More recently, rocking disc electrode voltammetry has been applied to investigate the complicated mechanism for the TEMPO‐mediated catalytic oxidation of alcohols . Cooperative electrocatalytic oxidation of alcohols with direct oxidation of TEMPO to TEMPO + , a proton‐coupled electron‐transfer (PCET) redox process, as a critical step has been disclosed .…”

“…More recently,r ocking disc electrode voltammetry has been applied to investigate the complicatedm echanism for the TEMPO-mediated catalytic oxidation of alcohols. [60] Cooperativee lectrocatalytic oxidation of alcohols with direct oxidation of TEMPO to TEMPO + ,aproton-coupled electron-transfer( PCET) redox process, as ac riticals tep has been disclosed. [61] Thus, the mechanism for TEMPO-mediated oxidation reactions should become clearer,w hichs hould provide the opportunity for constructing cooperative catalytic systems with TEMPO.…”