Mechanistically Driven Development of an Iron Catalyst for Selective <i>Syn</i>-Dihydroxylation of Alkenes with Aqueous Hydrogen Peroxide

Product release is the rate-determining step in the arene <i>syn</i>-dihydroxylation reaction taking place at Rieske oxygenase enzymes and is regarded as a difficult problem to be resolved in the design of iron catalysts for olefin <i>syn</i>-dihydroxylation with potential utility in organic synthesis. Toward this end, in this work a novel catalyst bearing a sterically encumbered tetradentate ligand based in the tpa (tpa = tris­(2-methylpyridyl)­amine) scaffold, [Fe^II(CF₃SO₃)₂(^5‑tips3tpa)], <b>1</b> has been designed. The steric demand of the ligand was envisioned as a key element to support a high catalytic activity by isolating the metal center, preventing bimolecular decomposition paths and facilitating product release. In synergistic combination with a Lewis acid that helps sequestering the product, <b>1</b> provides good to excellent yields of diol products (up to 97% isolated yield), in short reaction times under mild experimental conditions using a slight excess (1.5 equiv) of aqueous hydrogen peroxide, from the oxidation of a broad range of olefins. Predictable site selective <i>syn</i>-dihydroxylation of diolefins is shown. The encumbered nature of the ligand also provides a unique tool that has been used in combination with isotopic analysis to define the nature of the active species and the mechanism of activation of H₂O₂. Furthermore, <b>1</b> is shown to be a competent synthetic tool for preparing O-labeled diols using water as oxygen source