Cyclodextrin‐Encapsulated <i>N</i> ‐Heterocyclic Carbene–Copper(I) Complexes for Homogeneous Catalysis in Pure Water: Anaerobic Oxidation of Arylboronic Acids with Water as the Formal Oxidant

Homogeneous copper‐based systems for air‐mediated oxidation of arylboronic acids in water remain scarce. Herein, it is shown that water‐soluble cyclodextrin(CD)‐encapsulated N ‐heterocyclic carbene (NHC)–copper(I) complexes based on modified permethylated α or β CDs, catalyze the ipso ‐hydroxylation‐deborylation reaction of arylboronic acids in pure water at low copper loading. Optimum conditions for the formation of phenols included the presence of stoichiometric triethylamine, which renders the catalytic system totally homogeneous. Although the reaction is relatively slow, it is compatible with a wide range of substrates, including substrates which are usually inert to oxidation by H 2 O 2 . The efficiency of the α and β‐CD‐based catalysts is compared, revealing a significant cavity effect on the reaction rate. The effect of the nature and position of the arylboronic acid substituents on the conversion is studied, showing a significant “ ortho ” effect in many cases. An experimental study of the mechanism shows that water acts as a formal oxidant and that the reaction does not requires dioxygen. These results, combined with previous knowledge of the constraints imposed by the CD cavity on metal coordination, led us to propose a copper(I)‐based mechanism involving CuOH‐type active species.