A Thioether-Ligated Cupric Superoxide Model with Hydrogen Atom Abstraction Reactivity.

The central role of cupric superoxide intermediates proposed in hormone and neurotransmitter biosynthesis by noncoupled binuclear copper monooxygenases like dopamine-β-monooxygenase has drawn significant attention to the unusual methionine ligation of the Cu ("Cu") active site characteristic of this class of enzymes. The copper-sulfur interaction has proven critical for turnover, raising still-unresolved questions concerning Nature's selection of an oxidizable Met residue to facilitate C-H oxygenation. We describe herein a model for Cu, [(NS)Cu] ([]), and its O-bound analog [(NS)Cu(O)] ([·O]). The latter is the first reported cupric superoxide with an experimentally proven Cu-S bond which also possesses demonstrated hydrogen atom abstraction (HAA) reactivity. Introduction of O to a precooled solution of the cuprous precursor []B(CF) (-135 °C, 2-methyltetrahydrofuran (2-MeTHF)) reversibly forms [·O]B(CF) (UV/vis spectroscopy: λ 442, 642, 742 nm). Resonance Raman studies (413 nm) using O [O] corroborated the identity of [·O] by revealing Cu-O (446 [425] cm) and O-O (1105 [1042] cm) stretches, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed a Cu-S interatomic distance of 2.55 Å. HAA reactivity between [·O] and TEMPO-H proceeds rapidly (1.28 × 10 M s, -135 °C, 2-MeTHF) with a primary kinetic isotope effect of / = 5.4. Comparisons of the O-binding behavior and redox activity of [] vs [], the latter a close analog of [] but with all N atom ligation (i.e., NS vs N), are presented.