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

Abstract

The CuI salts [Cu(CH3CN)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 CuI salts towards monooxygenation of 2,4‐di‐tert‐butylphenol (DTBP‐H) were compared to those of [Cu(CH3CN)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 CuII‐semiquinone complexes as catalysts, indicating that formation of a CuII 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.