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

Spectroscopic and DFT Characterization of a Highly Reactive Nonheme Fe^V–Oxo Intermediate

The reaction of [(PyNMe₃)­Fe^II(CF₃SO₃)₂], <b>1</b>, with excess peracetic acid at −40 °C generates a highly reactive intermediate, <b>2b</b>(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an <i>S</i> = 1/2 EPR signal having <i>g</i>-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second <i>S</i> = 1/2 species, <b>2a</b>(PAA), assigned to a low-spin acylperoxoiron­(III) center. Unfortunately, contaminants accompanying the <b>2</b>(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe₃ (an electron-enriched version of PyNMe₃) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate <b>3b</b>(CPCA) with a Mössbauer isomer shift δ = −0.08 mm/s that indicates an iron­(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of <b>3b</b>(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe₃)­Fe^V(O)­(OC­(O)­R)]²⁺ (∼75%) with some Fe^IV(O)­(^•OC­(O)­R) and Fe^III(OOC­(O)­R) character. DFT studies of <b>3b</b>(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1–O2 bond of 0.35. This unusual geometry facilitates reversible O1–O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe^III(OC­(O)­R) ferric acyl peroxides and Fe^IV(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe–O1 Raman frequency (815 cm^–1) relative to the 903 cm^–1 value predicted for the hypothetical [(MeO-PyNMe₃)­Fe^V(O)­(NCMe)]³⁺ complex