Mechanism of the oxidation of alcohols by oxoammonium cations.
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.
Bailey
William F
WF
Department of Chemistry, The University of Connecticut, Storrs, Connecticut 06269-3060, USA. william.bailey@uconn.edu
Bobbitt
James M
JM
Wiberg
Kenneth B
KB
D000438:Alcohols / Q000737:chemistry*; D003497:Cyclic N-Oxides / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007659:Ketones / Q000138:chemical synthesis*; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D012996:Solutions; D013816:Thermodynamics
D000438:Alcohols; D003497:Cyclic N-Oxides; D007659:Ketones; D012996:Solutions; C003959:TEMPO