Protonation and methylation reactions of 2-pyridyl-palladium(II) and -platinum(II) complexes

Crociani, B.; Bianca, F. Di; Giovenco, Amalia; Scrivanti, Alberto

doi:10.1016/s0022-328x(00)98785-0

Cited by 59 publications

(22 citation statements)

References 30 publications

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“…In the early 1980s, the groups of Isobe 88,89 and Crociani [90][91][92][93][94][95] successfully prepared nickel, palladium, and platinum py- These early studies focused on the effect of the metal fragment on the reactivity of the pyridine, rather than on the carbene-type bonding of the ligand. Titration experiments showed that the Brønsted basicity of pyridine in the palladium complex 123a is ∼4 orders of magnitude higher than that of unsubstituted pyridine.…”

Section: Nitrogen Functionalization Of Metallated Pyridyl Ligandsmentioning

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: Nitrogen Functionalization Of Metallated Pyridyl Ligandsmentioning

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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“…Different monodentate phosphines were used in the reaction, leading to complexes [35] 48a and [36] 20c (Scheme 16). Larger heterocycles react similarly under CH activation and complexes [37], 48b with a six-membered NH,NR-substituted NHC ligand, as well as complex [38], 49 bearing a benzodiazepine-derived mesoionic NHC ligand, have been prepared.…”

mentioning

confidence: 99%

Complexes Bearing Protic N-Heterocyclic Carbenes: Synthesis and Applications

Jahnke

Hahn

2014

Chemistry Letters

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Next to complexes bearing N,N′-dialkylazolium-derived N-heterocyclic carbenes (NR,NR-NHCs), the coordination chemistry of protic NHC ligands (NH,NR-NHCs and NH,NH-NHCs) has recently attracted attention. The protic NHC ligands offer multiple opportunities for postsynthetic modifications. This review summarizes methods for the generation of complexes with protic NHC ligands, with a special focus on the recently developed oxidative addition of 2-halogenoazoles to low-valent transition metals. Selected applications for complexes with protic NHC ligands, like the template-controlled preparation of NHC-containing macrocycles and the utilization of the NH function of protic carbene ligands as a molecular recognition unit, will also be discussed.

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“…The 13 C NMR chemical shift of the palladium-bound carbon (d C 176.2), however, compares well with related heterocyclic carbenes 6,7 and suggests considerable carbene contribution to the bonding.…”

Section: ð2þmentioning

confidence: 81%

Catalytically active palladium pyridylidene complexes: pyridinium ionic liquids as N-heterocyclic carbene precursors

Albrecht

Stœckli-Evans

2005

Chem. Commun.

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Pyridinium salts similar to those used as ionic liquid solvents readily undergo palladation in the presence of a base, thus giving palladium-pyridylidene N-heterocyclic carbene complexes that are active catalysts for Suzuki-type cross-coupling reactions.Solvents are generally used to maximize the homogeneous distribution of reaction partners and to control and balance temperature effects throughout a reaction. In addition, they may accelerate reactions by stabilizing key transition states along the reaction coordinate through non-covalent interactions. However, they are considered to be chemically inert during reactions, that is, they are not subject to breaking and making of (covalent) bonds.Recently, ionic liquids have been promoted as a novel class of solvents, 1 since they combine a variety of advantages such as environmental friendliness (virtually no vapor pressure, easiness of recovery and recycling), physical robustness within a large temperature range, and high solubilization potential, e.g., for reactions catalyzed by metal nanoparticles. 2 Interestingly, imidazolium ionic liquids have also been used as precursors of N-heterocyclic carbene ligands in organometallic chemistry. 3 It has been shown that metallation of imidazolium salts can occur under mild conditions and in the absence of a base. 4 These findings suggest that ionic liquids may act as reagents and not only as solvents in transition metal catalyzed reactions.In contrast to imidazolium salts, pyridinium ionic liquids have been assumed to be resistant towards metallation. 5 Their resistance can be rationalized by a comparatively low stabilization of the corresponding carbene by one nitrogen only. However, pyridylidene-type transition metal complexes have been proposed as products from N-protonation and -alkylation of platinum group pyridyl complexes. 6 In addition, similar complexes have recently been shown to display remarkably strong trans effects. 7 Here, we provide evidence for the formation of pyridylidene complexes using pyridinium ionic liquids as ligand precursors. Moreover these pyridylidene complexes actively catalyze Suzuki cross-coupling reactions.Our approach towards pyridylidene complexes starts from nicotine as a versatile starting material. First, coordination of the pyrrolidine nitrogen should support the M-C carbene bond through chelate formation. Second, enantiopure nicotine is readily available and offers a cheap and convenient access to chiral complexes that may be useful for asymmetric catalysis. Thus, the pyridinium salt 1 has been prepared from nicotine by selective alkylation of the imine nitrogen with iPrI. At room temperature, 1 is a highly viscous ionic liquid that dissolves readily in alcohols, though only sparingly in THF and H 2 O. Palladation of 1 occurs smoothly at room temperature in the presence of [Pd(OAc) 2 ] and a base such as KOtBu (eqn. 1). ð1ÞContrary to our expectation, metallation takes place at the 4-rather than at the most acidic 2-pyridinium position, thus affording the palladium complex 2 as an ...

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Protonation and methylation reactions of 2-pyridyl-palladium(II) and -platinum(II) complexes

Cited by 59 publications

References 30 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

Complexes Bearing Protic N-Heterocyclic Carbenes: Synthesis and Applications

Catalytically active palladium pyridylidene complexes: pyridinium ionic liquids as N-heterocyclic carbene precursors

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