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

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

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

© Journal of the American Chemical Society, 2015, vol. 137, núm. 50, p. 15833-15842

American Chemical Society (ACS)

Author: Serrano Plana, Joan
Oloo, Williamson N.
Acosta-Rueda, Laura
Meier, Katlyn K.
Verdejo, Begoña
García-España, Enrique
Basallote, Manuel G.
Münck, Eckard
Que, Lawrence
Company Casadevall, Anna
Costas Salgueiro, Miquel
Date: 2015
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
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
Format: application/pdf
ISSN: 0002-7863 (versió paper)
1520-5126 (versió electrònica)
Document access: http://hdl.handle.net/10256/12256
Language: eng
Publisher: American Chemical Society (ACS)
Collection: MICINN/PN 2011/INP-2011-0059-PCT-420000
MINECO/PN 2013-2015/CTQ2012-37420-C02-01
MICINN/PN 2010-2016/CSD2010-00065
MINECO/PE 2014-2016/CTQ2013-43012-P
MICINN/PN 2012-2016/RYC-2011-08683
Reproducció digital del document publicat a: http://dx.doi.org/10.1021/jacs.5b09904
Articles publicats (D-Q)
info:eu-repo/grantAgreement/EC/FP7/239910
info:eu-repo/grantAgreement/EC/FP7/303522
Is part of: © Journal of the American Chemical Society, 2015, vol. 137, núm. 50, p. 15833-15842
Rights: Tots els drets reservats
Subject: Reaccions químiques
Chemical reactions
Ferro -- Reactivitat
Iron -- Reactivity
Title: Trapping a Highly Reactive Nonheme Iron Intermediate That Oxygenates Strong C-H Bonds with Stereoretention
Type: info:eu-repo/semantics/article
Repository: DUGiDocs

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