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

Electrophilic Arene Hydroxylation and Phenol O<img src="http://onlinelibrarystatic.wiley.com/undisplayable_characters/00f8ff.gif" alt="[BOND]">H Oxidations Performed by an Unsymmetric μ-η1:η1-O2-Peroxo Dicopper(II) Complex:

Abstract

Reactions of the unsymmetric dicopper(II) peroxide complex [Cu^II₂(μ-η¹:η¹-O₂)(m-XYL^N3N4)]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 [Cu^II₂(μ-η¹:η¹-O₂)(p-X-PhO)(m-XYL^N3N4)]⁺ (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 [Cu^II₂(μ-η¹:η¹-O₂)(X-PhOH)(m-XYL^N3N4)]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 [Cu^II₂(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.

The way it goes: Reactions of complex [Cu^II₂(μ-η¹:η¹-O₂)(L)]²⁺ (1 O₂) with phenolates and phenols are described (see scheme). Whereas 1 O₂ reacts with PhONa performing tyrosinaselike ortho-hydroxylation of the aromatic ring, its reaction with phenols affords CC coupling products. Mechanistic studies reveal that the presence of a H⁺ completely determines the fate of the association complex [1 O₂-(PhO(H))] between a proton-coupled electron transfer (PCET) and an arene hydroxylation reaction.