Trapping a Highly Reactive Nonheme Iron Intermediate That Oxygenates Strong C—H Bonds with Stereoretention

Joan Serrano-Plana^†, Williamson N. Oloo^‡, Laura Acosta-Rueda^§, Katlyn K. Meier^∥, Begoña Verdejo^⊥, Enrique García-España^⊥, Manuel G. Basallote^*§, Eckard Münck^*∥, Lawrence QueJr.^*‡, Anna Company^*†, and Miquel Costas^*†

^† Grup de Química Bioinspirada, Supramolecular i Catàlisi (QBIS-CAT), Institut de Química Computacional i Catàlisi (IQCC), Departament de Química, Universitat de Girona, Campus de Montilivi, Girona E17071, Catalonia, Spain

^‡ Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United States

^§ Universidad de Cádiz, Facultad de Ciencias, Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Apdo. 40, Puerto Real, Cádiz 11510, Spain

^∥ Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

^⊥ Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/Catedrático José Beltrán, Paterna, Valencia 2 46980, Spain

J. Am. Chem. Soc., 2015, 137 (50), pp 15833–15842

DOI: 10.1021/jacs.5b09904

http://pubs.acs.org/doi/10.1021/jacs.5b09904

http://pubs.acs.org/doi/pdf/10.1021/jacs.5b09904

An unprecedentedly reactive iron species (2) has been generated by reaction of excess peracetic acid with a mononuclear iron complex [Fe^II(CF₃SO₃)₂(PyNMe₃)] (1) at cryogenic temperatures, and characterized spectroscopically. Compound 2 is kinetically competent for breaking strong C—H bonds of alkanes (BDE ≈ 100 kcal·mol^–1) through a hydrogen-atom transfer mechanism, and the transformations proceed with stereoretention and regioselectively, responding to bond strength, as well as to steric and polar effects. Bimolecular reaction rates are at least an order of magnitude faster than those of the most reactive synthetic high-valent nonheme oxoiron species described to date. EPR studies in tandem with kinetic analysis show that the 490 nm chromophore of 2 is associated with two S = 1/2 species in rapid equilibrium. The minor component 2a (∼5% iron) has g-values at 2.20, 2.19, and 1.99 characteristic of a low-spin iron(III) center, and it is assigned as [Fe^III(OOAc)(PyNMe₃)]²⁺, also by comparison with the EPR parameters of the structurally characterized hydroxamate analogue [Fe^III(tBuCON(H)O)(PyNMe₃)]²⁺ (4). The major component 2b (∼40% iron, g-values = 2.07, 2.01, 1.95) has unusual EPR parameters, and it is proposed to be [Fe^V(O)(OAc)(PyNMe₃)]²⁺, where the O—O bond in 2a has been broken. Consistent with this assignment, 2b undergoes exchange of its acetate ligand with CD₃CO₂D and very rapidly reacts with olefins to produce the corresponding cis-1,2-hydroxoacetate product. Therefore, this work constitutes the first example where a synthetic nonheme iron species responsible for stereospecific and site selective C—H hydroxylation is spectroscopically trapped, and its catalytic reactivity against C—H bonds can be directly interrogated by kinetic methods. The accumulated evidence indicates that 2 consists mainly of an extraordinarily reactive [Fe^V(O)(OAc)(PyNMe₃)]²⁺ (2b) species capable of hydroxylating unactivated alkyl C—H bonds with stereoretention in a rapid and site-selective manner, and that exists in fast equilibrium with its [Fe^III(OOAc)(PyNMe₃)]²⁺ precursor.