Rhodium(0) Metalloradicals in Binuclear C−H Activation

Puschmann, Florian Frank; Bruin, Bas de

doi:10.1021/ja909022p

Cited by 27 publications

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“…However, although open-shell hydrido complexes are very rare, [19] the presence of a hypothetical iridium(IV) complex [Ir(H) 2 Cl(L 4 )] (R = tBu) could not be fully excluded on the basis of these results. Interestingly, the synthesis of iridium(IV) dihydrido complexes [Ir(H) 2 (Cl) 2 -(PR 3 ) 2 ] (R = iPr, cyclohexyl) had been reported earlier, but reevaluation of these results could not confirm their existence. [20,21] Therefore, single crystals of 1 were also characterized by neutron diffraction ( Supporting Information, Figure S4).…”

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“…[1] However, the importance of the latter was more recently emphasized for platinum metal complexes, for example in radical H 2 , CÀH, and CÀC activation reactions or catalytic oxidations. [2][3][4][5] Nevertheless, fully characterized metalloradical complexes of these metals remain scarce. [6] Alternatively, radical complexes with the spin density mainly located on redox non-innocent ligands have been described, such as strongly N-centered radical complexes resulting from oxidation of Rh I and Ir I dialkylamides.…”

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confidence: 99%

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Square‐Planar Iridium(II) and Iridium(III) Amido Complexes Stabilized by a PNP Pincer Ligand

et al. 2011

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Square‐Planar Iridium(II) and Iridium(III) Amido Complexes Stabilized by a PNP Pincer Ligand

et al. 2011

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“…B. bei radikalischen H 2 -, C-H-und C-C-Aktivierungen oder bei katalytischen Oxidationen, wurde bereits demonstriert, [2][3][4][5] dennoch sind voll charakterisierte Metalloradikale noch immer selten.…”

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Quadratisch‐planare Iridium(II)‐ und Iridium(III)‐Amidokomplexe mit einem PNP‐Pinzettenliganden

et al. 2011

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Koordinationsverbindungen der Edelmetalle nehmen eine zentrale Stellung in der homogenen Katalyse ein. Die Präfe-renz zur geschlossenschaligen elektronischen Struktur prä-destiniert solche Komplexe zu formalen Zwei-ElektronenRedoxreaktionen (oxidative Addition/reduktive Eliminierung).[1] Die Bedeutung von Ein-Elektronen-Reaktionen der Platinmetalle, z. B. bei radikalischen H 2 -, C-H-und C-C-Aktivierungen oder bei katalytischen Oxidationen, wurde bereits demonstriert, [2][3][4][5] dennoch sind voll charakterisierte Metalloradikale noch immer selten.[ [8]Ebenso sind Platinmetallkomplexe mit gerader Valenzelektronenzahl und elektronischer High-Spin-Konfiguration wegen der generell hçheren Ligandenfeldaufspaltung der 4d/ 5d-Metallionen als bei 3d-Metallen sehr selten. Eine Ausnahme bilden quadratisch-planare d 6 -Disilylamidokomplexe vom Typ [MX (L 5 )] (M = Ru, Os; X = F, Cl, I, Trifluormethansulfonat (OTf); L 5 = N(SiMe 2 CH 2 PtBu 2 ) 2 ), die in Analogie zu Eisen(II) eine elektronische Intermediate-SpinKonfiguration aufweisen. [9,10] [Ir(H)Cl(C 8 H 13 )(HL 1 )] (3; R = tBu) [13] wird in situ mit Benzochinon (2.5 ¾quiv.) zum türkisfarbenen 1 in Ausbeuten an isoliertem Produkt von bis zu 60 % oxidiert (Schema 2). [14] Im 31 P-NMR-Spektrum von 1 werden keine Signale beobachtet. Die drei breiten Signale im 1 H-NMR-Spektrum sind stark paramagnetisch verschoben und kçnnen den tBu-Substituenten (d = 10.5 ppm) und je einem Satz von Ligand-

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“…[17] An ormal KIE of 3.0 was observed in aRh-based CÀHactivation that was proposed to use as imilar bimetallic mechanism. [18] Overall, we conclude that the rate-determining step is intramolecular addition of the Fe À Hbond across the N = Cbond, as depicted in the lower left of Scheme 4. Theability to explain the kinetics using the mechanistic model in Scheme 4also supports its applicability to the reverse reaction:t hat is, b-hydride elimination as the step that breaks the CÀHbond of pyridine.Additional studies…”

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C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

et al. 2016

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Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105-111 kcal/mol) C-H bonds of pyridine substrates are unknown. In this work, we report the rapid room-temperature cleavage of C-H bonds in pyridine, 4-tertbutylpyridine, and 2-phenylpyridine by an iron(I) species, to give well-characterized iron(II) products. In addition, 4-dimethylaminopyridine (DMAP) undergoes room-temperature C-N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl-bridged tetrairon(II) square. These facile bond-cleaving reactions are proposed to occur through intermediates having a twoelectron reduced pyridine that bridges two iron centers. Thus, the redox non-innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions. Graphical abstractAn iron(I) complex can activate C-H and C-N bonds in pyridines, using a mechanism that takes advantage of pyridine's redox activity. KeywordsPyridine; Iron; Redox-Active; Bond Cleavage Pyridines are widespread in pharmaceuticals, natural products, and polymers, and therefore their expedient synthesis is crucial. [1] However, pyridines are relatively unreactive in the absence of prior activation. [2] Direct cleavage of C-H bonds in unfunctionalized pyridines is uncommon, but has made substantial progress recently. [3] A recent exciting development has been the C-H arylation of pyridines using catalytic mixtures generated in situ from simple iron salts and an excess of Grignard reagent. [4] This recipe presumably generates reduced Correspondence to: Patrick L. Holland. a These authors contributed to this work equally, and should be considered co-first authors.Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.XXXX HHS Public Access Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript iron species that are the catalysts, but little is known about reduced iron-pyridine compounds and what stoichiometric steps to expect under these conditions. In this communication, we report well-characterized iron products that result from pyridine C-H bond cleavage. We also show an unprecedented C-N bond cleavage of 4-dimethylaminopyridine (DMAP) and construct a reasonable mechanistic scaffold that is based on the ability of pyridines to accept charge from low-valent iron.Pyridines are well-known redox-active ligands. [5] We have shown that the addition of one or more equiv of pyridine to the iron(I) complex LFe(C 6 H 6 ) (L = β-diketiminate ligand, shown in Scheme 1) yields products in which the pyridine ligands are reduced by 1-2 electrons. [6] However, we have now discovered that addition of smaller amounts of pyridine leads to formation of a surprising new compound (1). Solutions of 1 are in equilibrium with the previously characterized pyridine-bound complexes and with LFe(C 6 H 6 ), as shown by 1 H NMR spectra of C 6 D 6 solutions of 1 with various amounts of pyridine ( Figure S5). Compound 1 can be isolated by...

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Rhodium(0) Metalloradicals in Binuclear C−H Activation

Cited by 27 publications

References 39 publications

Square‐Planar Iridium(II) and Iridium(III) Amido Complexes Stabilized by a PNP Pincer Ligand

Square‐Planar Iridium(II) and Iridium(III) Amido Complexes Stabilized by a PNP Pincer Ligand

Quadratisch‐planare Iridium(II)‐ und Iridium(III)‐Amidokomplexe mit einem PNP‐Pinzettenliganden

C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

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