Electrophilic Arene Hydroxylation and Phenol OH Oxidations Performed by an Unsymmetric μ‐η1:η1‐O2‐Peroxo Dicopper(II) Complex

AbstractReactions of the unsymmetric dicopper(II) peroxide complex [CuII2(μ‐η1:η1‐O2)(m‐XYLN3N4)]2+ (1 O2, where m‐XYL is a heptadentate N‐based ligand), with phenolates and phenols are described. Complex 1 O2 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 [CuII2(μ‐η1:η1‐O2)(p‐X‐PhO)(m‐XYLN3N4)]+ (1 O2⋅X‐PhO) that then undergo ortho‐hydroxylation of the aromatic ring by the peroxide moiety. Complex 1 O2 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 1 O2 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 [CuII2(μ‐η1:η1‐O2)(X‐PhOH)(m‐XYLN3N4)]2+ (1 O2⋅X‐PhOH) that then evolve through an irreversible rate‐determining step. Mechanistic studies indicate that 1 O2 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 1 O2 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 O2‐binding mode. This work provides experimental evidence that the presence of a H+ completely determines the fate of the association complex [CuII2(μ‐η1:η1‐O2)(X‐PhO(H))(m‐XYLN3N4)]n+ between a PCET and an arene hydroxylation reaction, and may provide clues to help understand enzymatic reactions at dicopper sites.