Iridium(III)-Induced Isomerization of 2-Substituted Pyridines to N-Heterocyclic Carbenes

Álvarez, Eleuterio; Conejero, Salvador; Paneque, Margarita; Petronilho, Ana; Poveda, Manuel L.; Serrano, Oracio; Carmona, Ernesto

doi:10.1021/ja0646592

Cited by 131 publications

(67 citation statements)

References 23 publications

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“…The iridium trispyrazolborate complex, IrTp(Ph) 2 (N 2 ), which contains a labile N 2 ligand activates the C6-H bond of picoline 168a at elevated temperatures and yields the carbene complex 169a, thus inducing a formal proton shift from C6 to nitrogen to afford the tautomeric carbene ligand (see Scheme 56). 117 The course of the reaction is sensitive to the applied reaction conditions, the substituents on the pyridine ring, and modifications to the iridium complex. For example, at 60°C 168a was shown to coordinate with the N atom to the metal precursor to form isomer 170a, which rearranged to 169a at 90°C.…”

Section: Scheme 49mentioning

confidence: 99%

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

et al. 2009

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theme are found via the replacement or displacement of one N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared, 7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

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Section: Scheme 49mentioning

confidence: 99%

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

et al. 2009

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“…Depending on the ring substitution pattern, both 2-and 4-pyridylidene complexes have been prepared by such a rearrangement. [30][31][32][33][34] In contrast, C-H activation of N-alkylated pyridinium precursors that could allow for C2-or C4-coordination, exclusively leads to metallation at the 4-position. [35][36][37][38][39] This observation has been attributed to the steric bulk of the N-alkyl group.…”

Section: Introductionmentioning

confidence: 99%

Pyridine-Derived N-Heterocyclic Carbenes: An Experimental and Theoretical Evaluation of the Bonding in and Reactivity of Selected Normal and Abnormal Complexes of Nickel(II) and Palladium(II)

et al. 2010

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“…The piano-stool geometry is slightly flatter in the iodide complex 2b, as indicated by the consistently wider angles between the three "legs", while most of the angles involving the Cp* centroid are more acute than in the chloride analogue 2a. In both complexes, bond length analysis suggests a pronounced localization of the double bonds in the annelated pyridine ring and formation of a 2-pyridylidene-type ground structure [65][66][67][68]. Specifically, the pyridine c and e bonds are significantly shorter than average conjugated C-C bonds and indicate partially localized C=C double bonds, while the b and d bonds are elongated (Fig.…”

Section: Resultsmentioning

confidence: 99%

Iridium, ruthenium, and palladium complexes containing a mesoionic fused imidazolylidene ligand

Petronilho

Müeller-Bunz

Albrecht

2015

Journal of Organometallic Chemistry

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Imidazo[1,2-a]pyridine consisting of a pyridine fused to an imidazolium salt at the imidazolium N1-C2 bond and hence protected from forming normal imidazole-2-ylidene complexes undergoes selective activation of the C5-H bond with Ag 2 O, i.e. at the imidazolium carbon that is proximal to the pyridine nitrogen. While the silver carbene complex is unstable, transmetallation with [IrCp*Cl 2 ] 2 , [RuCl 2 (cym)] 2 , and [PdCl(allyl)] 2 afforded stable mesoionic carbene complexes. Two iridium(III) complexes containing one fused carbene ligand and one palladium(II) complex containing two carbene ligands at the metal center were structurally characterized. The absence of substituents adjacent to the carbene carbon prevents wingtip group activation, and it imparts a reduced stability of the complexes in particular under (mildly) acidic conditions.

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Iridium(III)-Induced Isomerization of 2-Substituted Pyridines to N-Heterocyclic Carbenes

Abstract: Steric strains (F-strain or front-strain) in the acid-base interaction between the Lewis acid [TpMe2Ir(C6H5)2] and 2-substituted pyridines Lewis bases, are responsible for the stabilization of the C-adducts (2) of the respective pyridine carbene tautomers over the expected N-adducts.

Cited by 131 publications

References 23 publications

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

Pyridine-Derived N-Heterocyclic Carbenes: An Experimental and Theoretical Evaluation of the Bonding in and Reactivity of Selected Normal and Abnormal Complexes of Nickel(II) and Palladium(II)

Iridium, ruthenium, and palladium complexes containing a mesoionic fused imidazolylidene ligand

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