An Efficient Palladium‐NHC (NHC=N‐Heterocyclic Carbene) and Aryl Amination Pre‐Catalyst: [Pd(IPr*)(cinnamyl)Cl]

The well‐defined [Pd(IPr*)(cinnamyl)Cl] complex is reported as one of the best N‐heterocyclic carbene (NHC)‐based pre‐catalysts for the Buchwald–Hartwig amination reaction. This catalytic system displays high efficiency for the coupling of numerous (hetero)aryl chlorides, with a wide range of amines, at room temperature or at extremely low catalyst loading (as low as 0.025 mol%).     

Highly Active,Well‐Defined (Cyclopentadiene)(N‐heterocyclic carbene)palladium Chloride Complexes for Room‐Temperature Suzuki–Miyaura and Buchwald–Hartwig Cross‐Coupling Reactions of Aryl Chlorides and Deboronation Homocoupling of Arylboronic Acids

A new class of well‐defined N‐heterocyclic carbene (NHC)‐(cyclopentadiene)palladium chloride complexes such as CpPd(NHC)Cl wasw synthesized from the readily available starting NHC‐palladium(II) chloride dimers. These air‐stable, coordinatively saturated NHC‐Pd complexes bearing the cyclopentadiene (Cp) unit exhibit high catalytic activity in the room temperature Suzuki–Miyaura and Buchwald–Hartwig cross‐coupling reactions involving unactive aryl chlorides as the substrates. In addition, they are found to be extremely efficient catalysts in the deboronation homocoupling of arylboronic acids at room temperature.     

Pyridin‐, Quinolin‐ and Acridinylidene Palladium Carbene Complexes as Highly Efficient C–C Coupling Catalysts

A series of four new complexes bearing N‐heterocyclic carbene ligands (NHCs) as well as four compounds bearing N‐heterocyclic carbene ligands with remote heteroatoms (rNHCs) of the general types [(NHC)(PPh₃)₂PdCl]⁺BF₄⁻ and [(rNHC)(PPh₃)₂PdCl]⁺BF₄⁻, respectively, have been prepared in high yields. Crystal and molecular structures have been determined for four representative examples. These compounds proved to be efficient catalysts for aryl coupling reactions of the Heck and Suzuki types (reaching TONs of as high as 6,200,000). Both aryl bromides and aryl chlorides can be used as substrates. Like the well known mixed, standard (NHC)(phosphine) compounds, the new six‐numbered, one‐N‐heterocyclic carbene complexes (and in particular certain rNHC‐containing ones) also combine the advantageous stability of bis(carbene) and the high activity of bis(phosphine) complexes. Furthermore, their good catalytic performance and, especially, their easy synthesis based on cheap and commercially available starting materials, make them by far superior when compared to the mixed (NHC)(phosphine) catalysts known thus far.     

Improved Syntheses of Versatile Ruthenium Hydridocarbonyl Catalysts Containing Electron‐Rich Ancillary Ligands

Clean, high‐yield routes are established to the important catalyst chlorobis(tricyclohexylphosphine)ruthenium hydridocarbonyl [RuHCl(CO)(PCy₃)₂] 2 and its N‐heterocyclic carbene (NHC) derivatives RuHCl(CO)(NHC)(PCy₃) ( 3a : NHC=IMes; 3b , NHC=H₂IMes; IMes=1,3‐dimesitylimidazol‐2‐ylidene). These complexes are obtained by treating chlorotris(tricyclohexylphosphine)ruthenium hydridocarbonyl [RuHCl(CO)(PPh₃)₃] 1 with tricyclohexylphosphine [PCy₃], or with the appropriate NHC ligand, then PCy₃. Advantages over prior routes to these complexes lie in the high yields from a conveniently accessible precursor, the absence of by‐products that otherwise prove difficult to remove, and the short reaction times under experimentally convenient conditions.     

Synthesis and Characterization of N‐Heterocyclic Carbene Substituted Phosphine and Phosphite Rhodium Complexes and their Catalytic Properties in Hydrogenation Reactions

Mixed N‐heterocyclic carbene‐substituted phosphine and phosphite complexes of rhodium were prepared, starting from [Rh(COE)₂Cl]₂ (COE=cyclooctene) by addition of free N‐heterocyclic carbenes (NHC) and PR₃. All new complexes were characterized by spectroscopy. In addition, the structures of trans‐chloro(1,3‐dicyclohexylimidazol‐2‐ylidene)‐bis(triphenylphosphite)rhodium(I) ( 5 ), chloro‐trans‐bis(1,3‐dicyclohexylimidazol‐2‐ylidene) (triphenylphosphine)rhodium(I) ( 6 ), and chloro(η⁴‐1,5‐cyclooctadiene)(1,3‐di‐[(1R,2S,5R)‐2‐isopropyl‐5‐menthylcyclohex‐1‐yl]imidazol‐2‐ylidene)rhodium(I) ( 8 ) were determined by single crystal X‐ray analyses. The hydrogenation of cyclohexene using molecular hydrogen has been optimized for some N‐heterocyclic carbene‐substituted phosphine and phosphite rhodium complexes by variation of the reaction conditions.     

Comparative Investigation of Hoveyda–Grubbs Catalysts bearing Modified N‐Heterocyclic Carbene Ligands

This paper reports the structural modification of Hoveyda–Grubbs complexes through the introduction of either an N‐alkyl‐N′‐mesityl heterocyclic carbene, an N‐alkyl‐N′‐(2,6‐diisopropylphenyl) heterocyclic carbene, or an N‐alkyl‐N′‐alkyl heterocyclic carbene. The effect of the modified N‐heterocyclic carbene (NHC) ligand was investigated in representative ring‐opening metathesis polymerization (ROMP), ring‐closing metathesis (RCM) and cross metathesis (CM) reactions. A pronounced influence on both catalyst activity and selectivity was found to be exerted by the NHC amino substituents, which emphasizes that a rigorously selected steric environment is critical in olefin metathesis catalyst design.     

Polystyrene‐Supported N‐Heterocyclic Carbene–Silver Complexes as Robust and Efficient Catalysts for the Reaction of Carbon Dioxide and Propargylic Alcohols

Three polystyrene‐supported N‐heterocyclic carbene–silver complexes [PS‐NHC‐Ag(I)] and a polystyrene‐supported N‐heterocyclic carbene–copper complex [PS‐NHC‐Cu(I)] catalyst were synthesized and characterized by elemental analysis, Fourier transform infrared spectroscopy, inductively coupled plasma‐atom emission spectrometer, thermogravimetric analysis and scanning electron micrographs. The catalytic activity of the supported catalysts was investigated for the reaction of propargylic alcohols and carbon dioxide. PS‐NHC‐Cu(I) showed no catalytic activity to the reaction, while PS‐NHC‐Ag(I) showed a considerable high activity and selectivity for the reaction, yielding the corresponding α‐alkylidene cyclic carbonates in high to excellent yields under mild conditions. Most importantly, the supported catalysts could be separated easily from the products and reused up to 15 times without loss of their high catalytic activity, showing excellent stability. The effect of various reaction parameters such as carbon dioxide pressure, temperature, time, and catalyst loading on the reaction was also investigated.     

N‐Heterocyclic Carbene‐Mediated Organocatalytic Transfer of Tin onto Aldehydes: New Access to α‐Silyloxyalkylstannanes and γ‐Silyloxyallylstannanes

A new, highly efficient and mild N‐heterocyclic carbene (NHC)‐mediated organocatalytic procedure for the transfer of tin from tributyl(trimethylsilyl)stannane (Bu₃SnSiMe₃) onto aldehydes for the preparation of α‐silyloxyalkylstannanes and γ‐silyloxyallylstannanes has been developed.     

Polymer‐Supported,Carbon Dioxide‐Protected N‐Heterocyclic Carbenes: Synthesis and Application in Organo‐ and Organometallic Catalysis

The synthesis of a resin‐supported, carbon dioxide‐protected N‐heterocyclic carbene (NHC) and its use in organocatalysis and organometallic catalysis are described. The resin‐bound carbon dioxide‐protected NHC‐based catalyst was prepared via ring‐opening metathesis copolymerization of 1,4,4a,5,8,8a‐hexahydro‐1,4,5,8‐exo,endo‐dimethanonaphthalene ( DMNH6 ) with 3‐(bicyclo[2.2.1]hept‐5‐en‐2‐ylmethyl)‐1‐(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐1‐ium‐2‐carboxylate ( M1 ), using the well‐defined Schrock catalyst Mo[N‐2,6‐(2‐Pr)₂‐C₆H₃](CHCMe₂Ph)(OCMe₃)₂ and was used for a series of organocatalytic reactions, i.e., for the trimerization reaction of isocyanates, as well as for the cyanosilylation of carbonyl compounds. In the latter reaction, turn‐over numbers (TON) up to 5000 were achieved. In addition, the polymer‐supported, carbon dioxide‐protected N‐heterocyclic carbene served as an excellent progenitor for various polymer‐supported metal complexes. It was loaded with a series of rhodium(I), iridium(I), and palladium(II) precursors and the resulting Rh‐, Ir‐, and Pd‐loaded resins were successfully used in the polymerization of phenylacetylene, in the hydrogen transfer reaction to benzaldehyde, as well as in Heck‐type coupling reactions. In the latter reaction, TONs up to 100,000 were achieved. M1 , as a non‐supported analogue of poly‐M1‐b‐DMNH6 , as well as the complexes PdCl₂[1,3‐bis(2‐Pr)tetrahydropyrimidin‐2‐ylidene]₂ ( Pd‐1 ) and IrBr[1‐(norborn‐5‐ene‐2‐ylmethyl)‐3‐(2‐Pr)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidine](COD) ( Ir‐1 ) were used as homogeneous analogues and their reactivity in the above‐mentioned reactions was compared with that of the supported catalytic systems. In all reactions investigated, the TONs achieved with the supported systems were very similar to the ones obtained with the unsupported, homogeneous ones, the turn‐over frequencies (TOFs), however, were lower by up to a factor of three.     

Sulfonated N‐Heterocyclic Carbenes for Pd‐Catalyzed Sonogashira and Suzuki–Miyaura Coupling in Aqueous Solvents

The reactions of the N,N′‐diarylimidazolium and N,N′‐diarylimidazolinium salts with chlorosulfonic acid result in the formation of the respective disulfonated N‐heterocyclic carbene (NHC) precursors in reasonable yields (46–77%). Water‐soluble palladium catalyst complexes, in situ obtained from the respective sulfonated imidazolinium salt, sodium tetrachloropalladate (Na₂PdCl₄) and potassium hydroxide (KOH) in water, were successfully applied in the copper‐free Sonogashira coupling reaction in isopropyl alcohol/water mixtures using 0.2 mol% catalyst loading. The preformed (disulfonatedNHC)PdCl(cinnamyl) complex was used in aqueous Suzuki–Miyaura reactions at 0.1 mol% catalyst loading. The coupling protocol reported here is very useful for Sonogashira reactions of N‐ and S‐heterocyclic aryl bromides and chlorides with aryl‐ and alkylacetylenes.     

Direct Amide Synthesis from Alcohols and Amines by Phosphine‐Free Ruthenium Catalyst Systems

Amides are synthesized directly from alcohols and amines in high yields using an in situ generated catalyst from easily available ruthenium complexes such as the (p‐cymene)ruthenium dichloride dimer, [Ru(p‐cymeme)Cl₂]₂, or the (benzene)ruthenium dichloride dimer, [Ru(benzene)Cl₂]₂, an N‐heterocyclic carbene (NHC) ligand, and a nitrogen containing L‐type ligand such as acetonitrile. The phosphine‐free catalyst systems showed improved or comparable activity compared to previous phosphine‐based catalytic systems. The in situ generated catalyst from [Ru(benzene)Cl₂]₂, an NHC ligand, and acetonitrile showed excellent activity toward reactions with cyclic secondary amines such as piperidine and morpholine.     

Carboxylation of Terminal Alkynes with Carbon Dioxide Catalyzed by Poly(N‐Heterocyclic Carbene)‐Supported Silver Nanoparticles

An N‐heterocyclic carbene (NHC) polymer supported silver nanoparticle catalyst system was developed. The novel nano‐composite catalyst demonstrated very high activity and excellent stability and reusability in the carboxylation of terminal alkynes with carbon dioxide at ambient conditions. The unique N‐heterocyclic carbene polymer and silver nanoparticle composite structure provided a synergistic effect on activation of terminal alkynes and carbon dioxide that contributed to the high catalytic activity. The poly‐NHC‐silver catalyst exhibited excellent substrate generality and tolerance to various functionalities. In addition, the catalyst is stable to air and moisture and can be easily recovered and reused.     

Palladium N‐Heterocyclic Carbene Catalysts for the Ultrasound‐Promoted Suzuki–Miyaura Reaction in Glycerol

Novel palladium N‐heterocyclic‐carbene (NHC)‐based complexes with 3,4,5‐trimethoxybenzyl, alkyl and sulfonate N‐substituents were obtained and fully characterized. The new complexes were used as pre‐catalysts in the Suzuki–Miyaura coupling of various aryl halides/boron sources in glycerol under pulsed‐ultrasound (P‐US) activation. High yields were obtained under mild reaction conditions, without formation of undesired by‐products. The pure final cross‐coupling products were easily recovered without column chromatography and the catalytic/solvent system could be recycled. TEM (transmission electron microscopy) and XPS (X‐ray photoelectron spectroscopy) were used to characterize the nanoparticles and to investigate the fate of the catalysts.     

Luminescent palladium(0) complexes bearing N-heterocyclic carbene and phosphine ligands

Palladium(0) complexes bearing a monodentate phosphine ligand and an N-heterocyclic carbene ligand have been prepared. In these complexes, photophysical properties of the complexes, [Pd(IPr)(PPh₃)] and [Pd(IPr)(P(o-tol)₃)] have been studied (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene). The emissive excited states have been tentatively assigned to ³MLCT. In addition to the results of the luminescent complexes, the synthesis of the related complexes, [Pd(O₂)(IPr)(P(m-tol)₃)] and [PdCl(CH₂Cl)(IPr)(P(MeOPh)₃)] (P(MeOPh)_3 = tris(pmethoxyphenyl)phosphine) have been studied and the structures were characterized by X-ray.     

Enantioselective N‐Heterocyclic Carbene‐Catalyzed Annulations of 2‐Bromoenals with 1,3‐Dicarbonyl Compounds and Enamines via Chiral α,β‐Unsaturated Acylazoliums

The N‐heterocyclic carbene (NHC)‐catalyzed generation of chiral α,β‐unsaturated acylazoliums from 2‐bromoenals followed by their interception with 1,3‐dicarbonyl compounds or enamines, the formal [3+3] annulation reaction, is reported. The reaction results in the enantioselective synthesis of synthetically and medicinally important dihydropyranones and dihydropyridinones, and tolerates a wide range of functional groups. It is noteworthy that the reaction takes place under mild reaction conditions utilizing relatively low catalyst loadings. In addition, based on DFT calculations, a mechanistic scenario involving the attack of the nucleophile from below the plane of the α,β‐unsaturated acylazoliums, and the mode of enantioinduction is presented.     

Development of Structurally Diverse N‐Heterocyclic Carbene Ligands via Palladium‐Copper‐Catalyzed Decarboxylative Arylation of Pyrazolo[1,5‐a]pyridine‐3‐carboxylic Acid

A series of fused non‐classical normal N‐heterocyclic carbenes, Pyrpy‐NHC precursors derived from pyrazolo[1,5‐a]pyridines, has been prepared using palladium‐copper‐catalyzed decarboxylative arylation of pyrazolo[1,5‐a]pyridine‐3‐carboxylic acid. Air‐stable palladium and rhodium complexes of these ligands have been synthesized via mild transmetallation of Ag‐Pyrpy‐NHC. The structural properties of Rh(Pyrpy‐NHC)(COD)Cl complexes were determined via X‐ray analysis. The measurement of the CO stretching frequencies of dicarbonyl Rh‐Pyrpy‐NHC complexes revealed that the electron donating strength of Pyrpy‐NHC could be tuned by varying the substituents of the aryl group. A catalytic study of the Pd‐Pyrpy‐NHC complexes revealed promising activity in the Suzuki–Miyaura reaction under ambient atmospheric conditions.

     

Carbenes Made Easy: Formation of Unsymmetrically Substituted N‐Heterocyclic Carbene Complexes of Palladium(II), Platinum(II) and Gold(I) from Coordinated Isonitriles and their Catalytic Activity

From palladium(II) or platinum(II) bis(isonitrile) complexes and from gold(I) isonitrile complexes, both easily available from simple precursors, the corresponding mono‐N‐heterocyclic carbene (NHC) complexes could be obtained selectively in good yields under very mild conditions. The reagents are simple β‐chloroammonium salts in the presence of a weak base. Unsymmetric NHC complexes are accessible. Thus over only two steps from simple metal precursors a broad variety of NHC complexes is available, the method is ideal to quickly assemble catalyst libraries. The palladium complexes are active pre‐catalysts in Suzuki cross‐coupling even with the additional isonitrile ligand on palladium.     

Synthesis of Methylene‐Bridge Polyarenes through Palladium‐Catalyzed Activation of Benzylic Carbon‐Hydrogen Bond

In the presence of palladium(II) acetate [Pd(OAc)₂] and an N‐heterocyclic carbene (NHC) ligand, fluorene derivatives can be generated in good to excellent yields from 2‐halo‐2′‐methylbiaryls through the benzylic C H bond activation (14 examples; 81–97% yields). The scope and limitations of this protocol have been examined. A wide range of functional groups, such as alkyl, alkoxy, ester, nitrile, and others, is able to tolerate the reaction conditions herein. The cyclization of an isotope‐labelled biphenyl gave the corresponding product with a primary kinetic isotope effect (k_H/k_D=4.8:1), which indicates that the rate‐determining step of this reaction is the activation of the benzylic C H bond. Moreover, indenofluorenes were also accessed in excellent results from terphenyls (3 examples; 91–92% yields). The cascade reaction of 2,6‐dichloro‐2′‐methylbiphenyl with diphenylacetylene produced 8,9‐diphenyl‐4H‐cyclopenta[def]phenanthrene in 60% yield through the activation of an aryl and a benzylic C H bond.     

N‐Heterocyclic Carbene‐Mediated Organocatalytic Transfer of Tin onto Aldehydes: An Easy Access to syn‐Diols and Mechanistic Studies

An update of our preliminary communication concerning an efficient organocatalytic procedure for the transfer of tin onto aldehydes is presented. This update combines (i) a full study of the preparation of γ‐silyloxyallylstannanes from β‐substituted enals, (ii) a “one‐pot” sequence (in inter‐ and intramolecular version) including N‐heterocyclic carbene (NHC)‐catalyzed silylstannation reaction/Lewis acid‐promoted allylstannation reaction to furnish the corresponding syn‐diols, and (iii) mechanistic studies on the organocatalytic 1,2‐addition.     

Construction of Contiguous Tetrasubstituted Carbon Stereocenters by Intramolecular Crossed Benzoin Reactions Catalyzed by N‐Heterocyclic Carbene (NHC) Organocatalyst

Bicyclic compounds with two contiguous tetrasubstituted carbon stereocenters at bridgehead positions were synthesized by N‐heterocyclic carbene (NHC)‐catalyzed intramolecular crossed benzoin reactions of symmetrical compounds. This desymmetrization strategy was applied to asymmetric synthesis with chiral NHC organocatalysts. Transition‐state models were proposed to explain the enantioselectivity. A tricyclic compound with three contiguous tetrasubstituted carbon stereocenters was synthesized by a stepwise strategy. The molecular structure and absolute configuration of the (+)‐enantiomer of this tricyclic compound were determined by X‐ray crystallographic analysis.