Palladation of diimidazolium salts at the C4 position: access to remarkably electron-rich palladium(ii) centers

Heckenroth, Marion; Kluser, Evelyne; Neels, Antonia; Albrecht, Martin

doi:10.1039/b812405a

Cited by 57 publications

(63 citation statements)

References 76 publications

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“…The latter two precursors have been used predominantly for C pyridinium -H bond activation, while Pd(OAc) 2 has been more widely applied, including C-H activation of imidazolium, pyridinium, and triazolium salts. The bond activation with Pd(OAc) 2 presumably involves the formation of an azolium palladate precursor (diimidazolium)[PdI 2 (OAc) 2 ] originating from coordination of the halide of the azolium salt to the palladium center [16]. Support for such a palladate formation has been obtained through NMR investigations and from the fact that palladation using azolium salts with noncoordinating PF 6 -or BF 4 -anions fail to undergo C-H bond activation.…”

Section: C-h Bond Activationmentioning

confidence: 95%

“…Experiments using the diimidazolium system 1 have indicated that the first Pd-C formation is irreversible (Scheme 1; Mes = mesityl) [16]. Unlike strong acids such as HCl, weaker acidic HOAc-which is the side product from acetate-mediated palladation of azolium salts-cannot cleave the Pd-C bond in intermediate C. The irreversibility of the bond forming process 7 together with the high yield of cyclopalladated dicarbene complex 2 point to an early preequilibrium that triggers the regioselectivity of C-H bond activation to the 'inner' carbon.…”

Section: C-h Bond Activationmentioning

confidence: 99%

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Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity

Poulain¹,

Iglesias²,

Albrecht

2011

COC

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Developments in palladium chemistry have been spurred predominantly by the outstanding application potential of this metal in catalysis. The quest for new ligands in order to modulate the catalytic activity and selectivity of the palladium center has been greatly stimulated by the discovery of N-heterocyclic carbenes as formally neutral, strongly donating, and covalently binding ligands. Abnormal variations of N-heterocyclic carbenes, even though known (yet not recognized) for 30 years, have received very little attention until recently. In parts this may have been due to the fact that the free abnormal carbene ligand is much less stable than the normal carbene analogues. In the last decade, significant progress has been made in abnormal carbene palladium chemistry and reliable synthetic routes as well as promising catalytic applications have been developed. As a consequence, these type of complexes have gradually transformed from laboratory curiosities to unique formally neutral ligands with exceptional donor ability. Here, the advances in abnormal carbene palladium chemistry are summarized. In an attempt to stimulate the entry of newcomers in this fascinating field of research, elementary aspects of synthesis are discussed as well as progress in characterization of the complexes. Most recent (catalytic) applications may highlight the potential of this rapidly growing area of palladium chemistry.

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Section: C-h Bond Activationmentioning

confidence: 95%

Section: C-h Bond Activationmentioning

confidence: 99%

Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity

Poulain¹,

Iglesias²,

Albrecht

2011

COC

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“…119 It has been suggested that the efficient discrimination between the C4-H and C5-H for bond activation occurs through ion pairing of the diimidazolium dication with the -ate complex that forms upon coordination of the anions from the diimidazolium salt to the metal precursor (D, Scheme 7). 120 Alternatively, multiple anion-π interactions may be operational. 121 Such weak interactions provide a thermodynamic control that is principally related to that exerted by donor atom coordination to the metal precursor as discussed above (cf.…”

Section: Methodsmentioning

confidence: 99%

Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

2009

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“…In contrast, the normal analogues 28 are stable under identical conditions, even at elevated temperatures. 36 The former reactivity has been suggested to reflect an increased nucleophilicity of the palladium(II) center in 27, because of the stronger donor ability of the C4-bound carbene, compared to the 2-imidazolylidenes. Theoretical studies by Frenking and co-workers support this conclusion.…”

Section: C-h Bond Activation Of C2-substituted Imidazolium Saltsmentioning

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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Palladation of diimidazolium salts at the C4 position: access to remarkably electron-rich palladium(ii) centers

Cited by 57 publications

References 76 publications

Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity

Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity

Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

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

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