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

The Cu-I 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 Cu-I 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 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.

Automated summary

In this study, Cu-I salts were synthesized and used as catalysts for the conversion of monophenols to o-quinones. The effects of anions and ligands on the catalyst activity were investigated, and it was found that Cu-II-semiquinone complexes can significantly accelerate the reaction, indicating a mononuclear pathway in the oxygenation process.