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Mechanism of the Catalytic Oxidation of Tertiary Alcohols by the Water‐Soluble Mn‐TMPyP/KHSO 5 System: β‐Fragmentation versus O ‐Neophyl Rearrangement
Author(s) -
Wietzerbin Karine,
Bernadou Jean,
Meunier Bernard
Publication year - 1999
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199909)1999:9<1467::aid-ejic1467>3.0.co;2-c
Subject(s) - chemistry , alkoxy group , catalysis , benzoic acid , photochemistry , acetophenone , bond cleavage , oxygen , medicinal chemistry , hydroxyl radical , catalytic cycle , reaction mechanism , autoxidation , acetonitrile , organic chemistry , radical , alkyl
Oxidation of 4‐(1‐hydroxy‐1‐phenylethyl)benzoic acid HPEBA with a water‐soluble metalloporphyrin as catalyst and KHSO 5 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, requires 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 drastically lowered by using D 2 O instead of H 2 O, suggesting that an alkoxyl radical is formed in the rate‐determining step. Labeling experiments using 18 O 2 or H 2 18 O under different reaction conditions show that the carbonyl oxygen atoms of AC and ABA originate either from substrate, water or dioxygen. However, the carbonyl oxygen atom in the ester group of BOBA originates from dioxygen while the other oxygen atom of the ester remains unlabeled. These results can be explained by an O ‐neophyl rearrangement of the initial alkoxyl radical to afford a carbon‐based radical which then reacts with dioxygen or Mn IV ‐OH/water. In a competitive reaction pathway, direct β‐scission of the alkoxyl radical leads to unlabeled products. The oxidation of other tertiary diaryl alcohols is also discussed.