Ionic Liquid Stabilized 2,2,6,6-Tetramethylpiperidine 1-Oxyl Catalysis for Alcohol Oxidation

Abstract

N-Oxyl reagents, particularly 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), have been extensively used for alcohol oxidations. While TEMPOmediated oxidations are kinetically and thermodynamically favorable in high-pH electrolytes, base-induced degradation often results in significant loss of catalytic activity. Herein, we demonstrate enhanced alkaline stability of a TEMPO derivative in ionic liquids (ILs). By incorporating TEMPO in an imidazoleanchored IL, no loss of current was observed at pH 10.0 after 2.0 h during the oxidation of butanol and glycerol, while TEMPO in polycaprolactone (PCL), a patternable binder material, degraded 58.5% and 67.1%, respectively. The stability enhancement was further demonstrated by analyzing the conversion of glycerol in an 800 μL electrochemical cell using bulk chemical analysis techniques. Successive cycles of glycerol oxidation indicated 14-fold stability enhancement by applying IL in a TEMPO electrode composite in comparison to PCL. The strategy demonstrated here provides an opportunity to prepare catalytic systems with enhanced stability. Further, this method provides the ability to convert what are typically homogeneous catalysts to heterogeneous systems.

2,2,6,6‐Tetramethylpiperidinyloxyl (TEMPO) Radical Mediated Electro‐Oxidation Reactions: A Review

“…This property, when merged with various TEMPO derivatives, will aid in the transformation of primary alcohols. [47] Several investigations have been conducted to understand the mechanism of TEMPO-mediated electrochemical oxidations of alcohols. The electrochemical active catalyst for the electrooxidation of alcohols is the oxoammonium ion which is formed by the single-electron oxidation of TEMPO at the anode.…”

Electrochemical Decarboxylative Oxygenation of Carboxylic Acids

“…Isotope labeling experiments using H 2 18 O further suggest that water is incorporated into 2a (Scheme D,E). TEMPO can act as a redox catalyst − in the electrochemical oxidation of alcohol III to yield the final product 2a , which is supported by the cyclic voltammogram analysis of TEMPO in the presence and absence of III (Scheme H).…”

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis

“…In contrast, organic nitroxyls often exhibit good reactivity with a turnover rate of 1–2 s –1 . Particularly, TEMPO (2,2,6,6-tetramethyl-1-piperidine N -oxyl) has been extensively studied for electrocatalytic alcohol oxidation. , Typically, TEMPO is electrochemically oxidized to generate the oxidant oxoammonium species (TEMPO + ), followed by the formation of TEMPO + /alkoxide adduct, which yields aldehyde or ketone products via intramolecular hydrogen transfer (Figure A). This electrochemical generation of TEMPO + by one-electron oxidation of TEMPO is simple and clean as compared to chemical oxidation.…”