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] [PF6]:: Kinetic analysis

TEMPO (2,2,6,6-tetramethyl piperidine-1-oxyl) is electrochemically oxidized to a stable form of the cation (TEMPO+) in acetonitrile (CH3CN) or 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIm][PF6]) 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][PF6], the redox currents are largely suppressed compared to that in CH3CN. 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 x 10(-3) cm s(-1) and 4.5 x 10(-2)cm s(-1) in [BMImj[PF6] and CH3CN, 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 CH3CN was estimated to be 5.53 x 10(1) M-1 s(-1) which is approximately double, relative to the value of 2.91 x 10(1) M-1 s(-1) determined in [BMIm][PF6]. (c) 2008 Elsevier Ltd. All rights reserved.