Bis-Phenoxo-Cu Complexes: Formal Aromatic Hydroxylation via Aryl-Cu Intermediate Species.

Ullmann-type copper-mediated arylC-O bond formation has attracted the attention of the catalysis and organometallic communities, although the mechanism of these copper-catalyzed coupling reactions remains a subject of debate. We have designed well-defined triazamacrocyclic-based aryl-Cu complexes as an ideal platform to study the C-heteroatom reductive elimination step with all kinds of nucleophiles, and in this work we focus our efforts on the straightforward synthesis of phenols by using HO as nucleophile. Seven well-defined aryl-Cu complexes featuring different ring size and different electronic properties have been reacted with water in basic conditions to produce final bis-phenoxo-Cu complexes, all of which are characterized by XRD. Mechanistic investigations indicate that the reaction takes place by an initial deprotonation of the NH group coordinated to Cu center, subsequent reductive elimination with HO as nucleophile to form phenoxo products, and finally air oxidation of the Cu produced to form the final bis-phenoxo-Cu complexes, whose enhanced stability acts as a thermodynamic sink and pushes the reaction forward. Furthermore, the corresponding triazamacrocyclic-Cu complexes react with O to undergo 1e oxidation to Cu and subsequent C-H activation to form aryl-Cu species, which follow the same fate towards bis-phenoxo-Cu complexes. This work further highlights the ability of the triazamacrocyclic-Cu platform to undergo aryl-OH formation by reductive elimination with basic water, and also shows the facile formation of rare bis-phenoxo-Cu complexes.