Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

The reactivity of MCl₃(η¹-TEMPO) (M = Fe, <b>1</b>; Al, <b>2</b>; TEMPO = 2,2,6,6-tetramethylpiperidine-<i>N</i>-oxyl) with a variety of alcohols, including 3,4-dimethoxybenzyl alcohol, 1-phenyl-2-phenoxyethanol, and 1,2-diphenyl-2-methoxyethanol, was investigated using NMR spectroscopy and mass spectrometry. Complex <b>1</b> was effective in cleanly converting these substrates to the corresponding aldehyde or ketone. Complex <b>2</b> was also able to oxidize these substrates; however, in a few instances the products of overoxidation were also observed. Oxidation of activated alkanes, such as xanthene, by <b>1</b> or <b>2</b> suggests that the reactions proceed via an initial 1-electron concerted proton–electron transfer (CPET) event. Finally, reaction of TEMPO with FeBr₃ in Et₂O results in the formation of a mixture of FeBr₃(η¹-TEMPOH) (<b>23</b>) and [FeBr₂(η¹-TEMPOH)]₂(μ-O) (<b>24</b>), via oxidation of the solvent, Et₂O