Hydroxylation versus Halogenation of Aliphatic C−H Bonds by a Dioxygen-Derived Iron-Oxygen Oxidant: Functional Mimicking of Iron Halogenases

Mechanistic Study of Chemoselectivity for Carbon Radical Hydroxylation versus Chlorination with FeIII (OH)(Cl) Complexes

The Fe^III (OH)(Cl) complex resembles the key intermediate proposed for the non-heme iron halogenases. Goldberg and co-workers reported that the Fe^III (OH)(Cl) RC reacts with triphenylmethyl radical 1 to give an exclusive hydroxylation product. To understand the chemoselectivity of the reaction of RC with 1, density functional theory (DFT) calculations have been conducted. From RC, the competing pathways were identified as the OH-transfer, Cl-transfer, and isomerization pathways. The direct Cl-transfer is more favorable than direct OH-transfer by 2.8 kcal/mol. The hydrogen bonding interactions between the hydroxyl group and the pendent amine ligand impede the direct OH-transfer from RC. Compared with the direct Cl-transfer pathway, the isomerization pathways require lower barriers. In isomer RC_iso2 , the equatorial hydroxyl group, which has smaller diabatic bond dissociation energy, prefers to transfer to form the hydroxylation product. In Fe^III (Cl)₂ RC2 and RC2_iso , the equatorial chloride group also prefers to transfer to give the chlorination product.

Non-Heme-Iron-Mediated Selective Halogenation of Unactivated Carbon-Hydrogen Bonds

Oxidation of the iron(II) precursor [(L¹)Fe^IICl₂], where L¹ is a tetradentate bispidine, with soluble iodosylbenzene (^sPhIO) leads to the extremely reactive ferryl oxidant [(L¹)(Cl)Fe^IV=O]⁺ with a cis disposition of the chlorido and oxido coligands, as observed in non‐heme halogenase enzymes. Experimental data indicate that, with cyclohexane as substrate, there is selective formation of chlorocyclohexane, the halogenation being initiated by C−H abstraction and the result of a rebound of the ensuing radical to an iron‐bound Cl⁻. The time‐resolved formation of the halogenation product indicates that this primarily results from ^sPhIO oxidation of an initially formed oxido‐bridged diiron(III) resting state. The high yield of up to >70 % (stoichiometric reaction) as well as the differing reactivities of free Fe²⁺ and Fe³⁺ in comparison with [(L¹)Fe^IICl₂] indicate a high complex stability of the bispidine‐iron complexes. DFT analysis shows that, due to a large driving force and small triplet‐quintet gap, [(L¹)(Cl)Fe^IV=O]⁺ is the most reactive small‐molecule halogenase model, that the Fe^III/radical rebound intermediate has a relatively long lifetime (as supported by experimentally observed cage escape), and that this intermediate has, as observed experimentally, a lower energy barrier to the halogenation than the hydroxylation product; this is shown to primarily be due to steric effects.

Oxo-Free Hydrocarbon Oxidation by an Iron(III)-Isoporphyrin Complex

Metal-halides that perform proton coupled electron-transfer (PCET) oxidation are an important new class of high-valent oxidant. In investigating metal-dihalides, we reacted [Fe^III(Cl)(T(OMe)PP)] (1, T(OMe)PP = meso-tetra(4-methoxyphenyl)porphyrinyl) with (dichloroiodo)benzene. An Fe^III-meso-chloro-isoporphyrin complex [Fe^III(Cl)₂(T(OMe)PP-Cl)] (2) was obtained. 2 was characterized by electronic absorption, ¹H NMR, EPR, and X-ray absorption spectroscopies and mass spectrometry with support from computational analyses. 2 was reacted with a series of hydrocarbon substrates. The measured kinetic data exhibited a nonlinear behavior, whereby the oxidation followed a hydrogen-atom-transfer (HAT) PCET mechanism. The meso-chlorine atom was identified as the HAT agent. In one case, a halogenated product was identified by mass spectrometry. Our findings demonstrate that oxo-free hydrocarbon oxidation with heme systems is possible and show the potential for iron-dihalides in oxidative hydrocarbon halogenation.

Aliphatic C-C Bond Cleavage in α-Hydroxy Ketones by a Dioxygen-Derived Nucleophilic Iron-Oxygen Oxidant

A nucleophilic iron-oxygen oxidant, formed in situ in the reaction between an iron(II)-benzilate complex and O₂ , oxidatively cleaves the aliphatic C-C bonds of α-hydroxy ketones. In the cleavage reaction, α-hydroxy ketones without any α-C-H bond afford a 1:1 mixture of carboxylic acid and ketone. Isotope labeling studies established that one of the oxygen atoms from dioxygen is incorporated into the carboxylic acid product. Furthermore, the iron(II) complex cleaves an aliphatic C-C bond of 17-α-hydroxyprogesterone affording androstenedione and acetic acid. The O₂ -dependent aliphatic C-C bond cleavage of α-hydroxy ketones containing no α-C-H bond bears similarity to the lyase activity of the heme enzyme, cytochrome P450 17A1 (CYP17A1).