Alkane Hydroxylation by a Nonheme Iron Catalyst that Challenges the Heme Paradigm for Oxygenase Action

Abstract

A nonheme iron catalyst catalyzed stereoselective oxidation of alkanes with H2O2 with remarkable efficiency and exhibiting an unprecedented high incorporation of water into the oxidized products. The present results challenge the canonical description of oxygenases, the standard oxo-hydroxo tautomerism that applies to heme systems and serves as a precedent for alternative pathways for the oxidation of hydrocarbons at nonheme iron oxygenases.

One-electron oxidation of an oxoiron(IV) complex to form an [O═Fe V ═NR] + center

“…Examples of nitridoiron(V) and nitridoiron(VI) complexes have also been reported, demonstrating the ability of the N 3− ligand to allow access to high-valent iron complexes, even with supporting polydentate ligands that are neutral or monoanionic (25)(26)(27)(28)(29). Lastly, the characterization of 2 and 2-H þ demonstrates the possibility of a neutral nonheme ligand to support an Fe(V) center and lends credence to mechanistic proposals for Fe V ðOÞðOHÞ oxidants in arene cis-dihydroxylation by Rieske dioxygenases (5) and in alkane, olefin, and water oxidation by a series of bioinspired synthetic iron catalysts (6)(7)(8)(9)11). For the latter, evidence for the fleeting iron(V) intermediates has been obtained from low 6.…”

“…However, unlike the porphyrin ligand in cytochrome P450, none of the ligands in the nonheme iron active site appears likely to undergo one-electron oxidation to stabilize the high-valent state. Fe V ═O oxidants are also implicated in alkane hydroxylation (6,7), olefin epoxidation and cis-dihydroxylation (8)(9)(10), and water oxidation (11) by bioinspired nonheme iron catalysts supported by neutral tetradentate ligands, and direct evidence for the formation of oxoiron(V) oxidants has been obtained in two cases by mass spectrometry (10,12). Despite the wealth of synthetic oxoiron(IV) complexes identified during the last decade (13), to date there is only one spectroscopically well-characterized example of an oxoiron(V) complex, ½Fe V ðOÞðTAMLÞ − (where TAML is tetraamido macrocyclic ligand), which is stabilized by the tetraanionic nature of the TAML ligand (14).…”

Computational Insight into the Mechanism of Alkane Hydroxylation by Non-heme Fe(PyTACN) Iron Complexes. Effects of the Substrate and Solvent

“…It is has been experimentally proved on the basis of isotopic labeling and product analyses that the radical is short living and does not diffuse freely in the alkane hydroxylation reaction catalyzed by the [Fe V (O)(OH)(PyTACN)] 2+ complex. [22][23][24] .…”

Iron Catalysis in Synthetic Chemistry

“…Later, Que and co-workers (117) reported another N 4 -tetradentate ligated iron complex [Fe II (CF 3 SO 3 ) 2 ( Me 2 PyTACN)], PyTACN = bis(2-pyridylmethyl)-1,4,7-triazacyclononane, with a distorted octahedral center. These complexes showed high catalytic activity for the oxidation of cyclohexane, as well as other tertiary C-H bonds of cis-1,2-dimethylcyclohexane, adamentane, and 2,3-dimethylbutane (Scheme 91).…”