Catalytic Sulfoxidation by Dinuclear Copper Complexes

Abstract

The ability of the copper enzyme tyrosinase to promote oxygen atom insertion into the aromatic C! H bond of phenolic substrates, and also the simpler oxidation of o-diphenols, by O2 [1] has stimulated an enormous number of biomimetic studies based on copper complexes. [2] The enzymatic reactions occur through a key oxygenated intermediate that has been thoroughly characterized spectroscopically [3] and, more recently, also structurally. [4] Synthetic copper‐dioxygen complexes can be formed with different stoichiometries and binding modes depending on the ligand, temperature, solvent, counterion, etc. [2] The enzymatic oxygen atom transfer reaction to exogenous phenols has been reproduced by using two types of copper‐dioxygen complexes, namely, the m-h 2 :h 2 -peroxodicopper(II) and bisAm-oxo)dicopperA species. [2] Recently, we reported a new reactivity for tyrosinase: The asymmetric oxidation of sulfides to sulfoxides, in which the enzyme behaves as an external monooxygenase, with a co-substrate to support the catalytic reaction. [5] The only precedent for the oxidation of an exogenous sulfide by a synthetic copper complex is the report by Itoh et al. on the stoichiometric sulfoxidation reaction promoted by a bisAm-oxo)dicopperA complex. [6] They found that the reaction proceeds with fast substrate binding to the copper followed by oxygen transfer, with the latter process occurring through a direct oxygen transfer mechanism. Other examples are known in which the sulfide group undergoing oxidation is part of the ligand supporting the Cu I complex. [7] Herein we report that m-h 2 :h 2 -peroxodicopper(II) complexes derived from diaminetetrabenzimidazole ligands can also support a sulfoxidation reaction with O2 and that with a suitable sacrificial co-substrate the reaction can be carried out catalytically, similarly to reactions promoted by tyrosinase. The catalytic oxidation of sulfides is a process of considerable interest because organic sulfoxides are synthetic intermediates for the preparation of biologically active molecules. [8a] However, the methods based on metal catalysis in general make use of peroxides, [8b‐e] whereas use of molecular oxygen is less frequent. [8f] In any case, the nature of the catalyst oxidizing species is often unknown. Of the catalytic processes reported, only a few that concern the oxidation of sulfides by Cu II complexes in the presence of hydrogen peroxide have appeared, [9] and the catalytic reaction occurs with high efficiency in only one case. [9c]