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 Que, Anna Company, Miquel Costas

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Abstract

An unprecedentedly reactive iron species (2) has been generated by reaction of excess peracetic acid with a mononuclear iron complex [FeII(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 [FeIII(OOAc)(PyNMe3)]2+, also by comparison with the EPR parameters of the structurally characterized hydroxamate analogue [FeIII(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 [FeV(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 [FeV(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 [FeIII(OOAc)(PyNMe3)]2+ precursor.

Original languageEnglish (US)
Pages (from-to)15833-15842
Number of pages10
JournalJournal of the American Chemical Society
Volume137
Issue number50
DOIs
StatePublished - Dec 23 2015

Bibliographical note

Funding Information:
Financial support for this work was provided by the European Commission (2011-CIG-303522 to A.C. and ERC-2009-StG- 239910 to M.C.), the Spanish Ministry of Science (CTQ2012- 37420-C02-01/BQU to M.C., CTQ2012-37821-C02-02 to M.G.B., CTQ2013-43012-P to A.C., and CSD2010-00065 to M.C., M.G.B., and E.G.E) and Generalitat de Catalunya (ICREA Academia Award to M.C.). The Spanish Ministry of Science is acknowledged for a Juan de la Cierva contract to B.V. (JCI-2011-09302) and for a Ramón y Cajal contract to A.C. (RYC-2011-08683). We are thankful for the financial support from INNPLANTA project INP-2011-0059-PCT-420000- ACT1 to Dr. Xavi Ribas. We also thank Dr. Laura Gómez (Serveis Tècnics de Recerca, Universitat de Girona) for helpful advice in setting up the HR−MS experiments and helpful discussions. The work at the University of Minnesota and Carnegie Mellon University was respectively supported by grants from the U.S. Department of Energy, Office of Basic Energy Sciences (Grant DE-FG02-03ER15455 to L.Q.) and the US National Science Foundation (CHE-1305111 to E.M.).

Publisher Copyright:
© 2015 American Chemical Society.

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