Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles

Redox-active ligand promoted electrophile addition at cobalt

The reactivity of an electron-rich cobalt complex bearing an o-phenylenediamide ligand with electrophilic CF3+ and F+ sources is reported. These reactions lead to generation of a Co(III)-CF3 or Co(III)-F complex, promoted by redox-active ligand-to-substrate two-electron transfer. The rate of trifluoromethyl addition at cobalt correlates with the potential difference between the cobalt complex and the CF3+ source. We present initial demonstrations of radical trifluoromethylation and nucleophilic fluorination of organic substrates, setting the stage for the development of electrocatalytic pathways for these bond-forming reactions.

Evidence for a High-Valent Iron-Fluoride That Mediates Oxidative C(sp3)-H Fluorination

[Fe^II(NCCH₃)(NTB)](OTf)₂ (NTB = tris(2-benzimidazoylmethyl)amine, OTf = trifluoromethanesulfonate) was reacted with difluoro(phenyl)-λ³-iodane (PhIF₂) in the presence of a variety of saturated hydrocarbons, resulting in the oxidative fluorination of the hydrocarbons in moderate-to-good yields. Kinetic and product analysis point towards a hydrogen atom transfer oxidation prior to fluorine radical rebound to form the fluorinated product. The combined evidence supports the formation of a formally Fe^IV(F)₂ oxidant that performs hydrogen atom transfer followed by the formation of a dimeric μ-F-(Fe^III)₂ product that is a plausible fluorine atom transfer rebound reagent. This approach mimics the heme paradigm for hydrocarbon hydroxylation, opening up avenues for oxidative hydrocarbon halogenation.

Non-Native Anionic Ligand Binding and Reactivity in Engineered Variants of the Fe(II)- and α-Ketoglutarate-Dependent Oxygenase, SadA

Mononuclear non-heme Fe(II)- and α-ketoglutarate-dependent oxygenases (FeDOs) catalyze a site-selective C-H hydroxylation. Variants of these enzymes in which a conserved Asp/Glu residue in the Fe(II)-binding facial triad is replaced by Ala/Gly can, in some cases, bind various anionic ligands and catalyze non-native chlorination and bromination reactions. In this study, we explore the binding of different anions to an FeDO facial triad variant, SadX, and the effects of that binding on HO• vs X• rebound. We establish not only that chloride and bromide enable non-native halogenation reactions but also that all anions investigated, including azide, cyanate, formate, and fluoride, significantly accelerate and influence the site selectivity of SadX hydroxylation catalysis. Azide and cyanate also lead to the formation of products resulting from N3•, NCO•, and OCN• rebound. While fluoride rebound is not observed, the rate acceleration provided by this ligand leads us to calculate barriers for HO• and F• rebound from a putative Fe(III)(OH)(F) intermediate. These calculations suggest that the lack of fluorination is due to the relative barriers of the HO• and F• rebound transition states rather than an inaccessible barrier for F• rebound. Together, these results improve our understanding of the FeDO facial triad variant tolerance of different anionic ligands, their ability to promote rebound involving these ligands, and inherent rebound preferences relative to HO• that will aid efforts to develop non-native catalysis using these enzymes.