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

The Cu(I) salts [Cu(CH(3)CN)(4)]PF and [Cu(oDFB)(2)]PF with the very weakly coordinating anion Al(OC(CF(3))(3))(4) (−) (PF) as well as [Cu(NEt(3))(2)]PF comprising the unique, linear bis‐triethylamine complex [Cu(NEt(3))(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(CH(3)CN)(4)]X salts with “classic” anions (BF(4) (−), OTf(−), PF(6) (−)), 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.