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Electrophilic Arene Hydroxylation and Phenol OH Oxidations Performed by an Unsymmetric μ‐η 1 :η 1 ‐O 2 ‐Peroxo Dicopper(II) Complex
Author(s) -
GarciaBosch Isaac,
Ribas Xavi,
Costas Miquel
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201102372
Subject(s) - chemistry , hydroxylation , catechol , electrophile , moiety , phenols , ligand (biochemistry) , radical , medicinal chemistry , peroxide , phenol , proton coupled electron transfer , stereochemistry , electron transfer , photochemistry , organic chemistry , biochemistry , receptor , enzyme , catalysis
Reactions of the unsymmetric dicopper(II) peroxide complex [Cu II 2 (μ‐η 1 :η 1 ‐O 2 )( m‐ XYL N3N4 )] 2+ ( 1 O 2 , where m ‐XYL is a heptadentate N‐based ligand), with phenolates and phenols are described. Complex 1 O 2 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‐η 1 :η 1 ‐O 2 )( p ‐X‐PhO)( m‐ XYL N3N4 )] + ( 1 O 2 ⋅X‐PhO) that then undergo ortho ‐hydroxylation of the aromatic ring by the peroxide moiety. Complex 1 O 2 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 O 2 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 II 2 (μ‐η 1 :η 1 ‐O 2 )(X‐PhOH)( m‐ XYL N3N4 )] 2+ ( 1 O 2 ⋅X‐PhOH) that then evolve through an irreversible rate‐determining step. Mechanistic studies indicate that 1 O 2 reacts with phenols through initial phenol binding to the Cu 2 O 2 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 O 2 complex can mediate electrophilic arene hydroxylation and PCET reactions commonly associated with electrophilic Cu 2 O 2 cores, and strongly suggest that the ability to form substrate⋅Cu 2 O 2 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‐η 1 :η 1 ‐O 2 )(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.