A general synthetic route to mixed NHC-phosphane palladium(0) complexes (NHC=N-heterocyclic carbene)

Mixed NHC-phosphane palladium(0) complexes [(NHC)Pd(PR(3))] (NHC: N-heterocyclic carbene) are synthesized directly from commercially available reagents, with the possibility to tune the nature of both the NHC and the phosphane. Reaction of [(NHC)Pd(allyl)Cl] (palladium source) and PR(3), in the presence of a base afforded, in isopropanol, [(NHC)Pd(PR(3))] in good yields. We found that the nature of the solvent played a key role in the efficient reduction of the Pd(II) precursor to Pd(0). Supported by experimental evidence we propose that the reduction step is driven by the isopropoxide anion formed in situ from isopropanol and a base. Detection of acetone in the reaction mixture confirms that the isopropoxide anion acts as the reducing agent. Moreover, different bases proved efficient for the reaction. The structures of the complexes were unambiguously confirmed by X-ray analysis. Exposure of these complexes to air does not lead to decomposition, but to the oxo-complex [(NHC)Pd(PR(3))(O(2))], which is stable both in the solid state and in solution.     

Dimerization of a mixed‐carbene PdII dibromide complex by elemental iodine

A monomeric Pd^II complex bearing a mixed carbocyclic/N‐heterocyclic carbene ligand and two bromides was reacted with an excess of elemental iodine, which resulted in the surprising removal of one bromide ligand and dimerization of the mixed‐carbene complex to form di‐μ‐bromido‐bis{[1‐(cyclohepta‐2,4,6‐trien‐2‐yl‐1‐ylidene‐κC ¹)‐3‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]palladium(II)} bis(pentaiodide) dichloromethane monosolvate, [Pd₂Br₂(C₂₂H₂₄N₂)₂](I₅)₂·CH₂Cl₂. The dimeric complex features a slightly distorted square‐planar core of two Pd^II centres bridged by two bromide ligands, which lie in the same plane as the seven‐ and five‐membered rings of the bidentate carbene ligand. The counter‐ions in the single crystal were found to be pentaiodide monoanions featuring their typical V‐shape, whereas for the bulk material, a mixture of Br/I interhalides is proposed.     

Synthesis and reactions of N-heterocyclic carbene boranes

Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently. NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes. They do not exhibit "borane chemistry", but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts. They have significant potential for use in organic synthesis and in polymer chemistry. They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry. Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions. This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes.     

Bicyclic N-heterocyclic carbene (NHC) ligand precursors and their palladium complexes

Eight bicyclic amidinium precursors (3), prepared from R,S-tmcp (R,S-tmcp: (1R,3S)-diamino-1,2,2-trimethylcyclopentane) were described. Only five of the precursors (3a–e) could be converted to palladium complexes, (PdX₂(6,7-NHC)PEPPSI) (4) by treatment with PdCl₂, K₂CO₃, and pyridine (additional KBr was used for (PdBr₂(6,7-NHC)PEPPSI)). The salts and complexes were fully characterized by spectroscopic methods and X-ray crystallography.     

Hybrid ligands with N-heterocyclic carbene and chiral phosphaferrocene components

N-Heterocyclic carbenes (NHCs) possessing one or two 3,4-dimethylphosphaferrocenyl substituents and either methylene or ethylene alkyl bridges have been prepared. These carbenes turned out to be remarkably stable and were characterized by NMR methods and partly by mass spectrometry. Their molybdenum and ruthenium complexes were examined in order to determine the electronic properties and the coordination behaviour of these chiral PC- and PCP-chelate ligands, which combine a NHC unit as a strong sigma-donor with pi-accepting phosphaferrocene moieties. Crystal structures of one ligand precursor and of three complexes have been determined.     

Catalytic activities in direct arylation of novel palladium N‐heterocyclic carbene complexes

Eight novel palladium N‐heterocyclic carbene (Pd‐NHC) complexes were synthesized by the reaction of chloro 1,3‐dialkylbenzimidazolin‐2‐ylidene silver(I) complexes with bis(benzonitrile)palladium(II) chloride in dichloromethane. These eight Pd‐NHC complexes are as follows: bis[1‐phenyl‐3‐(2,4,6‐trimethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐phenyl‐3‐(2,3,5,6‐tetramethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐phenyl‐3‐(2,3,4,5,6‐pentamethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐phenyl‐3‐(3,4,5‐trimethoxybenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐(2‐diethylaminoethyl)‐3‐(3‐methylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐(2‐diethylaminoethyl)‐3‐(2,3,5,6‐tetramethylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II), bis[1‐(2‐morpholinoethyl)‐3‐naphthalenomethylbenzimidazol‐2‐ylidene]dichloropalladium(II) and bis[1‐(2‐morpholinoethyl)‐3‐(2‐methylbenzyl)benzimidazol‐2‐ylidene]dichloropalladium(II). Also, these synthesized complexes were fully characterized using Fourier transform infrared, ¹H NMR and ¹³C NMR spectroscopic methods and elemental analysis techniques. These synthesized novel Pd‐NHC complexes were tested as catalysts in the direct arylation of 2‐n‐butylthiophene, 2‐n‐butylfuran and 2‐isopropylthiazole with various aryl bromides at 130°C for 1 h. The complexes showed very good catalytic activities in these reactions. Copyright © 2014 John Wiley & Sons, Ltd.     

N-heterocyclic carbene and phosphine ruthenium indenylidene precatalysts: a comparative study in olefin metathesis

Kinetic studies on ring-closing metathesis of unhindered and hindered substrates using phosphine and N-heterocyclic carbene (NHC)-containing ruthenium-indenylidene complexes (first and second generation precatalysts, respectively) have been carried out. These studies reveal an appealing difference, between the phosphine and NHC-containing catalysts, associated with a distinctive rate-determining step in the reaction mechanism. These catalysts have been compared with the benzylidene generation catalysts and their respective representative substrates. Finally, the reaction scope of the two most interesting precatalysts, complexes that contain tricyclohexylphosphine and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (SIMes), has been investigated for the ring-closing and enyne metathesis for a large range of olefins. Owing to their high thermal stability, the SIMes-based indenylidene complexes were more efficient than their benzylidene analogues in the ring-closing metathesis of tetrasubstituted dienes. Importantly, none of the indenylidene precatalysts were found to be the most efficient for all of the substrates, indeed, a complementary complex-to-substrate activity relationship was observed.     

(N‐heterocyclic carbene)PdCl(N‐heterocyclic carboxylate) complexes: Synthesis and catalytic activities towards arylation of benzoxazoles with aryl halides

A series of N‐heterocyclic carboxylate‐stabilized N‐heterocyclic carbene palladium complexes have been synthesized and fully characterized. The solid‐state structures indicate that each of the palladium centers is coordinated by an N‐heterocyclic carbene, a chloride and a bidentate N,O‐donor N‐heterocyclic carboxylate ligand. The catalytic performance of the complexes was screened and the results revealed that the complexes exhibit moderate to high catalytic activities for the direct C─H bond arylation of benzoxazoles with aryl bromides.     

pi-Face donor properties of N-heterocyclic carbenes in Grubbs II complexes

The electron-donating properties of eighteen N-heterocyclic carbenes (N,N'-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and the respective dihydro ligands) with 4,4'-R substituted aryl rings (4,4'-R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.