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

The reactivity of MCl3(η(1)-TEMPO) (M = Fe, 1; Al, 2; TEMPO = 2,2,6,6-tetramethylpiperidine-N-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 1 was effective in cleanly converting these substrates to the corresponding aldehyde or ketone. Complex 2 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 1 or 2 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(η(1)-TEMPOH) (23) and [FeBr2(η(1)-TEMPOH)]2(μ-O) (24), via oxidation of the solvent, Et2O

Influence of connector groups on the interactions of substituents with carbon-centered radicals

High-level G3X(MP2)-RAD calculations have been carried out to examine the effect of interposing a “connector” group (W) on the interaction between a substituent (X) and the radical center in carbon-centered radicals (•CH₂–W–X). The connector groups include −CH₂–, −CH═CH–, −C≡C–, −p-C₆H₄–, −m-C₆H₄–, and −o-C₆H₄–, and the substituents include H, CF₃, CH₃, CH═O, NH₂, and CH═CH₂. Analysis of the results is facilitated by introducing two new quantities termed radical connector energies and molecule connector energies. We find that the −CH₂– connector effectively turns off π-electron effects but allows the transmission of σ-electron effects, albeit at a reduced level. The effect of a substituent X attached to the −CH═CH– and −C≡C– connector groups is to represent a perturbation of the effect of the connector groups themselves (i.e., CH═CH₂ and C≡CH)