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NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling

While iron–NHC catalysed cross-couplings have been shown to be effective for a wide variety of reactions (e.g. aryl–aryl, aryl–alkyl, alkyl–alkyl), the nature of the in situ formed and reactive iron species in effective catalytic systems remains largely undefined. In the current study, freeze-trappe...

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Autores principales: Fleischauer, Valerie E., Muñoz III, Salvador B., Neate, Peter G. N., Brennessel, William W., Neidig, Michael L.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: Royal Society of Chemistry 2018
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5890793/
https://www.ncbi.nlm.nih.gov/pubmed/29675234
http://dx.doi.org/10.1039/c7sc04750a
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author Fleischauer, Valerie E.
Muñoz III, Salvador B.
Neate, Peter G. N.
Brennessel, William W.
Neidig, Michael L.
author_facet Fleischauer, Valerie E.
Muñoz III, Salvador B.
Neate, Peter G. N.
Brennessel, William W.
Neidig, Michael L.
author_sort Fleischauer, Valerie E.
collection PubMed
description While iron–NHC catalysed cross-couplings have been shown to be effective for a wide variety of reactions (e.g. aryl–aryl, aryl–alkyl, alkyl–alkyl), the nature of the in situ formed and reactive iron species in effective catalytic systems remains largely undefined. In the current study, freeze-trapped Mössbauer spectroscopy, and EPR studies combined with inorganic synthesis and reaction studies are utilised to define the key in situ formed and reactive iron–NHC species in the Kumada alkyl–alkyl cross-coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide and 1-iodo-3-phenylpropane. The key reactive iron species formed in situ is identified as (IMes)Fe((1,3-dioxan-2-yl)ethyl)(2), whereas the S = 1/2 iron species previously identified in this chemistry is found to be only a very minor off-cycle species (<0.5% of all iron). Reaction and kinetic studies demonstrate that (IMes)Fe((1,3-dioxan-2-yl)ethyl)(2) is highly reactive towards the electrophile resulting in two turnovers with respect to iron (k(obs) > 24 min(–1)) to generate cross-coupled product with overall selectivity analogous to catalysis. The high resistance of this catalytic system to β-hydride elimination of the alkyl nucleophile is attributed to its chelation to iron through ligation of carbon and one oxygen of the acetal moiety of the nucleophile. In fact, alternative NHC ligands such as SIPr are less effective in catalysis due to their increased steric bulk inhibiting the ability of the alkyl ligands to chelate. Overall, this study identifies a novel alkyl chelation method to achieve effective alkyl–alkyl cross-coupling with iron(ii)–NHCs, provides direct structural insight into NHC effects on catalytic performance and extends the importance of iron(ii) reactive species in iron-catalysed cross-coupling.
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spelling pubmed-58907932018-04-19 NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling Fleischauer, Valerie E. Muñoz III, Salvador B. Neate, Peter G. N. Brennessel, William W. Neidig, Michael L. Chem Sci Chemistry While iron–NHC catalysed cross-couplings have been shown to be effective for a wide variety of reactions (e.g. aryl–aryl, aryl–alkyl, alkyl–alkyl), the nature of the in situ formed and reactive iron species in effective catalytic systems remains largely undefined. In the current study, freeze-trapped Mössbauer spectroscopy, and EPR studies combined with inorganic synthesis and reaction studies are utilised to define the key in situ formed and reactive iron–NHC species in the Kumada alkyl–alkyl cross-coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide and 1-iodo-3-phenylpropane. The key reactive iron species formed in situ is identified as (IMes)Fe((1,3-dioxan-2-yl)ethyl)(2), whereas the S = 1/2 iron species previously identified in this chemistry is found to be only a very minor off-cycle species (<0.5% of all iron). Reaction and kinetic studies demonstrate that (IMes)Fe((1,3-dioxan-2-yl)ethyl)(2) is highly reactive towards the electrophile resulting in two turnovers with respect to iron (k(obs) > 24 min(–1)) to generate cross-coupled product with overall selectivity analogous to catalysis. The high resistance of this catalytic system to β-hydride elimination of the alkyl nucleophile is attributed to its chelation to iron through ligation of carbon and one oxygen of the acetal moiety of the nucleophile. In fact, alternative NHC ligands such as SIPr are less effective in catalysis due to their increased steric bulk inhibiting the ability of the alkyl ligands to chelate. Overall, this study identifies a novel alkyl chelation method to achieve effective alkyl–alkyl cross-coupling with iron(ii)–NHCs, provides direct structural insight into NHC effects on catalytic performance and extends the importance of iron(ii) reactive species in iron-catalysed cross-coupling. Royal Society of Chemistry 2018-01-08 /pmc/articles/PMC5890793/ /pubmed/29675234 http://dx.doi.org/10.1039/c7sc04750a Text en This journal is © The Royal Society of Chemistry 2018 http://creativecommons.org/licenses/by-nc/3.0/ This article is freely available. This article is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported Licence (CC BY-NC 3.0)
spellingShingle Chemistry
Fleischauer, Valerie E.
Muñoz III, Salvador B.
Neate, Peter G. N.
Brennessel, William W.
Neidig, Michael L.
NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling
title NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling
title_full NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling
title_fullStr NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling
title_full_unstemmed NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling
title_short NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling
title_sort nhc and nucleophile chelation effects on reactive iron(ii) species in alkyl–alkyl cross-coupling
topic Chemistry
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5890793/
https://www.ncbi.nlm.nih.gov/pubmed/29675234
http://dx.doi.org/10.1039/c7sc04750a
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