Non‐Innocence of <i>N</i>‐Heterocyclic Carbene Ligands: Intermolecular CH Activation in Allyl Palladium NHC Complexes

Scheele, Ulrich J.; Dechert, Sebastian; Meyer, Franc

doi:10.1002/chem.200800564

Cited by 51 publications

(21 citation statements)

References 61 publications

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“…Metallation of the C2 position constitutes another, rather special method for protecting the normal position. In an unusual reaction, bimetallic imidazole-based palladium systems have been isolated from intramolecular C4/5-H bond activation [22]. The formed product features a C2-Pd and a C4/5-Pd bond and may thus be formulated either as a normal or as an abnormal carbene palladium complex.…”

Section: C-h Bond Activationmentioning

confidence: 99%

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: 99%

Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity

Poulain¹,

Iglesias²,

Albrecht

2011

COC

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“…[10] Starting from cheap 3,6-dichloropyridazine, chelate arms may be readily attached to the 3-and 6-positions of the pyridazine bridge in order to increase preorganization of the dimetallic array. [11] The simple pyridazine/NHC hybrid ligands A (Scheme 1) have previously been employed in palladium chemistry, [12] and the dinucleating type B scaffolds have been used for the synthesis of oligonuclear mercury(II) and silver(I) complexes. [13] Scheme 1.…”

Section: Introductionmentioning

confidence: 99%

New Pyridazine‐Bridged NHC/Pyrazole Ligands and Their Sequential Silver(I) Coordination

et al. 2011

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A family of new pyridazine‐bridged NHC/pyrazole ligand precursors HL1–5 were prepared and fully characterized including analysis by XRD {HL1 = 3‐[3‐(2,6‐diisopropylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3‐pyridin‐2‐yl‐pyrazol‐1‐yl)‐pyridazine, HL2 = 3‐[3‐(2,4,6‐trimethylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3‐pyridin‐2‐yl‐pyrazol‐1‐yl)‐pyridazine, HL3 = 3‐[3‐(2,4,6‐trimethylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3,5‐dimethylpyrazol‐1‐yl)‐pyridazine, HL4 = 3‐(3‐tert‐butyl‐3H‐imidazolium‐1‐yl)‐6‐(3,5‐dimethylpyrazol‐1‐yl)‐pyridazine, HL5 = 3‐[3‐(2,4,6‐trimethylphenyl)‐3H‐imidazolium‐1‐yl]‐6‐(3‐methyl‐5‐phenylpyrazol‐1‐yl)‐pyridazine, X = PF6– or BF4–}. Reaction of the ligand precursors with Ag2O yielded various silver(I) complexes whose structures have been elucidated crystallographically. In complexes [(L3)2Ag](PF6) (4) and[(L4)2Ag](PF6) (4′) the single silver(I) ion is coordinated in a linear fashion by the NHC moieties of the two ligand strands. An NMR titration with AgBF4 reveals that 4 can bind two more silver(I) ions. The complex [(L5)2Ag2](PF6)2 (5′) features an additional silver(I) centre bound to the two pyrazole rings, whereas in [(L3)2Ag2](BF4)2 (6) ligand reshuffling has occurred to give antiparallel ligand strands with {CNHCNpyrazole} coordination of each metal ion. Secondary interactions with the pyridazine N‐atom are observed in some cases. An additional third silver(I) ion can be accommodated between the central pyridazine bridges, as shown in [(L3)2Ag3](PF6)2(BF4) (7). The sequence of binding events and the identity of the species in solution have been investigated by NMR spectroscopy and ESI mass spectrometry.

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“…14) [85]. Reaction of an N-pyridazine functionalized imidazolium salt 23 with allylpalladium chloride dimer initially produces the anticipated mononuclear palladium complex 24.…”

Section: Metal-mediated Activationmentioning

confidence: 99%