Electrophilic arene hydroxylation and phenol O-H oxidations performed by an unsymmetric μ-η¹:η¹-O ₂-peroxo dicopper(II) complex

Reactions of the unsymmetric dicopper(II) peroxide complex [Cu ^II ₂(μ-η¹: η¹-O₂)(m-XYL^N3N4)]²⁺ (1O₂, where m-XYL is a heptadentate N-based ligand), with phenolates and phenols are described. Complex 1O₂ reacts with p-X-PhONa (X=MeO, Cl, H, or Me) at -90°C performing tyrosinase-like ortho-hydroxylation of the aromatic ring to afford the corresponding catechol products. Mechanistic studies demonstrate that reactions occur through initial reversible formation of metastable association complexes [Cu^II ₂(μ- η¹:η¹-O₂)(p-X-PhO) (m-XYL^N3N4)]⁺ (1O₂·X-PhO) that then undergo ortho-hydroxylation of the aromatic ring by the peroxide moiety. Complex 1O₂ also reacts with 4-X-substituted phenols p-X-PhOH (X=MeO, Me, F, H, or Cl) and with 2,4-di-tert-butylphenol at -90°C causing rapid decay of 1O₂ and affording biphenol coupling products, which is indicative that reactions occur through formation of phenoxyl radicals that then undergo radical C-C coupling. Spectroscopic UV/Vis monitoring and kinetic analysis show that reactions take place through reversible formation of ground-state association complexes [Cu^II ₂(μ- η¹:η¹-O₂)(X-PhOH) (m-XYL^N3N4)]²⁺ (1O₂·X-PhOH) that then evolve through an irreversible rate-determining step. Mechanistic studies indicate that 1O₂ reacts with phenols through initial phenol binding to the Cu₂O₂ core, followed by a proton-coupled electron transfer (PCET) at the rate-determining step. Results disclosed in this work provide experimental evidence that the unsymmetric 1O₂ complex can mediate electrophilic arene hydroxylation and PCET reactions commonly associated with electrophilic Cu₂O₂ cores, and strongly suggest that the ability to form substrate·Cu₂O₂ association complexes may provide paths to overcome the inherent reactivity of the O ₂-binding mode. This work provides experimental evidence that the presence of a H⁺ completely determines the fate of the association complex [Cu^II ₂(μ-η¹: η¹-O₂)(X-PhO(H))(m-XYL^N3N4)] ⁿ⁺ between a PCET and an arene hydroxylation reaction, and may provide clues to help understand enzymatic reactions at dicopper sites.