Mechanism of the catalytic oxidation of tertiary alcohols by the water-soluble Mn-TMPyP/KHSO5 system: beta-fragmentation versus O-neophyl rearrangement

Oxidation of 4-(1-hydroxy-1-phenylethyl)benzoic acid HPEBA with a water-sol uble metalloporphyrin as catalyst and KHSO5 as oxygen atom donor gives the major products. acetophenone AC and acetylbenzoic acid ABA, by a C-aliph-C- Ar bond cleavage, but a minor product, benzoyloxybenzoic acid BOBA, require s the insertion of an oxygen atom to form the ester. This compound becomes the main oxidation product on increasing the amount of acetonitrile in the reaction medium. and its formation is oxygen-dependent. The conversion is d rastically lowered by using D2O instead of H2O, suggesting that an alkoxyl radical is formed in the rate-determining step. Labeling experiments using O-18(2) or (H2O)-O-18 under different reaction conditions show that the car bonyl oxygen atoms of AC and ABA originate either from substrate, water or dioxygen. However, the carbonyl oxygen atom in the ester group of BOBA orig inates from dioxygen while the other oxygen atom of the ester remains unlab eled. These results can be explained by an O-neophyl rearrangement of the i nitial alkoxyl radical to afford a carbon-based radical which then reacts w ith dioxygen or Mn-IV-OH/water. In a competitive reaction pathway, direct b eta-scission of the alkoxyl radical leads to unlabeled products. The oxidat ion of other tertiary diaryl alcohols is also discussed.