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.