Electrophilic Arene Hydroxylation and Phenol OH Oxidations Performed by an Unsymmetric μ-η1:η1-O2-Peroxo Dicopper(II) Complex

Abstract

Reactions 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 CC 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