Copper-Catalyzed Monooxygenation of Phenols: Evidence for a Mononuclear Reaction Mechanism

The Cu^I salts [Cu(CH₃ CN)₄ ]PF and [Cu(oDFB)₂ ]PF with the very weakly coordinating anion Al(OC(CF₃ )₃ )₄^- (PF) as well as [Cu(NEt₃ )₂ ]PF comprising the unique, linear bis-triethylamine complex [Cu(NEt₃ )₂ ]⁺ were synthesized and examined as catalysts for the conversion of monophenols to o-quinones. The activities of these Cu^I salts towards monooxygenation of 2,4-di-tert-butylphenol (DTBP-H) were compared to those of [Cu(CH₃ CN)₄ ]X salts with "classic" anions (BF₄^- , OTf^- , PF₆^- ), revealing an anion effect on the activity of the catalyst and a ligand effect on the reaction rate. The reaction is drastically accelerated by employing Cu^II -semiquinone complexes as catalysts, indicating that formation of a Cu^II complex precedes the actual catalytic cycle. This result and other experimental observations show that with these systems the oxygenation of monophenols does not follow a dinuclear, but a mononuclear pathway analogous to that of topaquinone cofactor biosynthesis in amine oxidase.