2,2,6,6-Tetramethyl piperidine-1-oxyl (TEMPO)-mediated catalytic oxidation of benzyl alcohol in acetonitrile and ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate [BMIm][PF 6]: Kinetic analysis

Abstract

TEMPO (2,2,6,6-tetramethyl piperidine-1-oxyl) is electrochemically oxidized to a stable form of the cation (TEMPO +) in acetonitrile (CH 3CN) or 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIm][PF 6]) media. Cyclic voltammograms were characterized by a well-defined one-electron reversible redox couple in both media at low scan rates. The reduced form of TEMPO + is catalytically regenerated in a follow-up chemical reaction with benzyl alcohol (BA) in the presence of 2,6-lutidine. It was observed that in [BMIm][PF 6], the redox currents are largely suppressed compared to that in CH 3CN. The apparent heterogeneous electron-transfer rate constant ( k app 0 ) of the quasi-reversible redox reaction of TEMPO was determined at a Pt electrode and found to be 1.9 × 10 −3 cm s −1 and 4.5 × 10 −2 cm s −1 in [BMIm][PF 6] and CH 3CN, respectively. With the aid of chronoamperometry (CA), the homogeneous rate constant for the catalytic oxidation of benzyl alcohol by TEMPO, in the presence of 2,6-lutidine in CH 3CN was estimated to be 5.53 × 10 1 M −1 s −1 which is approximately double, relative to the value of 2.91 × 10 1 M −1 s −1 determined in [BMIm][PF 6].