Solid-State Chemistry at an Isolated Copper(I) Center with O 2 (original) (raw)

2009, Angewandte Chemie International Edition

The controlled oxygenation of organic compounds at room temperature by O 2 remains a challenge of the most importance. In that context, metalloenzymes are fascinating natural factories that perform regio-and stereoselective oxygenation of organic substrates. Among these, copper monooxygenases such as PHM (peptidylglycine a-hydroxylating monooxygenase), DbH (dopamine b-hydroxylase), and TbH (tyramine b-hydroxylase) catalyze a two-electron oxidation process corresponding to the insertion of an oxygen atom into a CÀH bond with O 2 and two electrons provided by ascorbate, thus releasing water. Quercetinase, in contrast, catalyzes a four-electron oxidation process with the oxygenation-decarbonylation of quercetine. For these enzymes, it has been demonstrated that the reactive chamber contains a single copper center. In the case of monooxygenases, however, a second copper center is present approximately 10 away, which allows sequential transfer of electrons from ascorbate to the catalytic site. In analogy to the well-known cytochrome P450 chemistry, it has been long proposed that two electrons are required to activate O 2 at a copper center, leading to the hypothesis that Cu II OOH (or more recently suggested CuO) was the reactive species in copper monooxygenases. Relatively recently, however, a series of studies, either biochemical, chemical, or theoretical, have suggested that Cu II OOC, corresponding to an only oneelectron-reduced O 2 species, might well be the reactive species responsible for the C À H bond breaking process in these enzymes. More recently, while exploring the properties of a copper center isolated in the tris(aminoethyl)amine (tren) cap of our calix[6]arene-based model compounds, we discovered that the Cu I complex reacts with O 2 to produce a species that leads to the oxygenation of the ligand, in which the insertion of one and two oxygen atoms was detected. [14c] In view of the geometrical constraints of the system, this study has provided evidence that a mononuclear Cu I center can activate O 2 for breaking a C À H bond. Herein, we report a study related to a system of the same family of calixarenebased ligands, but with a tris(2-pyridylmethyl)amine (tmpa) cap in place of the tren unit.