posted on 2007-06-08, 00:00authored byWilliam F. Bailey, James M. Bobbitt, Kenneth B. Wiberg
The mechanism of the oxidation of primary and secondary alcohols by the oxoammonium cation derived
from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has been investigated computationally at the B3LYP/6-31+G* level, along with free energies of solvation, using a reaction field model. In basic solution, the
reaction involves formation of a complex between the alkoxide anion and the oxoammonium cation in
a pre-oxidation equilibrium wherein methoxide leads to a much larger formation constant than isopropoxide.
The differences in free energy of activation for the rate-determining hydrogen transfer within the pre-oxidation complexes were small; the differences in complex formation constants lead to a larger rate of
reaction for the primary alcohol, as is observed experimentally. In acidic solution, rate-determining
hydrogen atom transfer from the alcohol to the oxoammonium cation had a large unfavorable free energy
change and would proceed more slowly than is observed. A more likely path involves a hydride transfer
that would be more rapid with a secondary alcohol than primary, as is observed. Transition states for this
process were located.