Electrocatalytic Alcohol Oxidation with TEMPO and Bicyclic Nitroxyl Derivatives: Driving Force Trumps Steric Effects

Abstract

Bicyclic nitroxyl derivatives, such as 2-azaadamantane N-oxyl (AZADO) and 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO), have emerged as highly effective alternatives to TEMPO-based catalysts for selective oxidation reactions (TEMPO = 2,2,6,6-tetramethyl-1-piperidine N-oxyl). Their efficacy is widely attributed to their smaller steric profile; however, electrocatalysis studies described herein show that the catalytic activity of nitroxyls is more strongly affected by the nitroxyl/oxoammonium redox potential than by steric effects. The inexpensive, high-potential TEMPO derivative, 4-acetamido-TEMPO (ACT), exhibits higher electrocatalytic activity than AZADO and ABNO for the oxidation of 1° and 2° alcohols. Mechanistic studies provide insights into the origin of these unexpected reactivity trends. The superior activity of ACT is especially noteworthy at high pH, where bicyclic nitroxyls are inhibited by formation of an oxoammonium-hydroxide adduct.

Electrochemical Alcohol Oxidation Mediated by TEMPO-like Nitroxyl Radicals

“…Most recently, Stahl and co‐workers have shown that the best catalytic rates are obtained with TEMPO derivatives bearing an electron‐withdrawing substituent in the 4‐position, such as inexpensive 4‐acetamido‐TEMPO. In detail, the reaction rate is positively correlated with the mid‐point potential ( E mp , Figure ) of the one‐electron nitroxyl/oxoammonium couple, providing evidence that the oxoammonium reduction potential is more significant than steric effects in controlling the reaction rate …”

Facile syntheses of 3,4-disubstituted pyrroline nitroxides and their further synthetic applications

“…Please check this box if you have no corrections to make to the PDF file 6 co-oxidants, 7 and tools for controlled radical polymerization. 8 The best studied stable radicals are nitroxide free radicals, however, their chemistry faces challenges, including carbon-carbon bond formation in the presence of ambiphilic nitroxide moieties or the synthesis of hybrid molecules, for example, incorporating nitroxide moieties into bio-50 molecules.…”

Spin Density Distribution in a Nitroxide Biradical Containing 13C-Enriched Acetylene Groups in the Bridge: DFT Calculations and EPR Investigation

“…Stable radicals of different types are presently uzed in a variety of fields [2] including spin labeling [3][4][5][6][7][8][9][10][11][12], the construction of organic magnetic materials [13], MRI contrast agents [14], redox-active components of organic radical batteries [15], co-oxidants [16] and tools for controlled radical polymerization [17]. Some biradicals were found to be promissing polarizing agents for dynamic nuclear polarization [18,19].…”