N-杂环卡宾金属配合物在酮硅氢加成反应中的新进展

N-杂环卡宾是一类新型催化剂和配体, 在有机化学中得到了极大的重视. N-杂环卡宾金属配合物的研究在近几年来得到迅速的发展,总结了酮硅氢加成反应中N-杂环卡宾金属配合物催化剂的应用新进展.     

The first N-heterocyclic carbene-based nickel catalyst for C-S coupling

We have developed the first N-heterocyclic carbene (NHC)-based transition metal catalysts for C-S coupling reactions. Ni-NHC catalysts showed good to excellent activities toward various aryl halides in C-S coupling reactions. The catalytic activities were greatly affected by the electronic and steric properties of the NHC ligands. The new catalysts were inexpensive, easy to synthesize, and environmentally friendly. They could be excellent candidates to replace Pd-organophosphanes for C-S coupling catalysis.     

Highly Active Ruthenium Catalysts for Olefin Metathesis: The Synergy of N-Heterocyclic Carbenes and Coordinatively Labile Ligands

Two N-heterocyclic carbene ligands at once may be one too many , at least if you intend to have highly active ruthenium catalysts for olefin metathesis. Density functional calculations recommend the replacement of the second carbene ligand in the successful ROMP catalysts 1 by coordinatively more labile ligands as in 2 or 3 . In both cases, the catalytic activity is greatly improved.     

Coinage metal complexes with N-heterocyclic carbene ligands as selective catalysts in diboration reaction

We describe the improved catalytic reactivity of terminal alkenes with 1,2-diboranes in the presence of Au(I) and Ag(I) complexes when N-heterocyclic carbene ligands are used. The new catalytic systems are able to diminish the undesired β-H-elimination of the alkylboryl–metal intermediates, which leads to the formation of hydroborated byproducts. The electronic properties and molecular the structure of the precursors of the catalysts could explain the modest asymmetric induction provided.     

N-杂环卡宾配体在聚合反应中的研究进展

N-杂环卡宾配体的优异催化性能引起了人们的广泛关注, 已成功应用于多种烯烃聚合反应。本文结合N-杂环卡宾的相关研究报道,首先简要介绍N-杂环卡宾,随后重点介绍N-杂环卡宾在开环易位聚合、烯烃配位聚合和原子转移自由基聚合等聚合反应的应用。在此基础上,指出了今后N-杂环卡宾配体在聚合反应研究的发展方向。     

Highly active iridium(I) complexes for catalytic hydrogen isotope exchange

Practically convenient methods have been developed for the preparation of new iridium complexes, possessing bulky N-heterocyclic carbene and phosphine ligands; these routinely handled complexes are highly active catalysts within directed hydrogen isotope exchange processes.     

Ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene ligands: synthesis, structure, and catalytic activity

A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy(3))(2)Cl(2)Ru==CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ring-opening metathesis polymerization of 1,5-cyclooctadiene, and the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60 %. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(I) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail.     

Ruthenium alkylidenes: modulation of a new class of catalysts for controlled radical polymerization of vinyl monomers

Air-stable and readily available ruthenium benzylidene complexes of the general type [RuCl2(=CHPh)(L)(L')] (L, L' = PCy3 and/or N-heterocyclic carbene) constitute a new class of catalyst precursors for atom-transfer radical polymerization (ATRP) of methyl methacrylate and styrene, and provide an unprecedented example for the involvement of ruthenium alkylidenes in radical reactions. They promote the polymerization of various monomers with good to excellent yields, and in a controlled way with methyl methacrylate and styrene. Variations of their basic structural motif provide insights into the essential parameters responsible for catalytic activity. The ligands L (PCy3 and/or N-heterocyclic carbene) turned out to play a particularly important role in determining the rate of the polymerizations. A similarly pronounced influence is exerted by the substituents on the N-heterocyclic carbene. Our results indicate that the catalysts decompose quickly under ATRP conditions, and polymerizations are mediated by both [RuCl2(=CHPh)(L)(L')] complexes and ruthenium species bereft of the benzylidene moiety, through a pathway in which both tricyclohexylphosphane and/or N-heterocyclic carbene ligands remain bound to the metal center. Polymerization of n-butyl acrylate and vinyl acetate is not controlled and most probably takes place through a redox-initiated free-radical process.     

Chelated ruthenium catalysts for Z-selective olefin metathesis

We report the development of ruthenium-based metathesis catalysts with chelating N-heterocyclic carbene (NHC) ligands that catalyze highly Z-selective olefin metathesis. A very simple and convenient procedure for the synthesis of such catalysts has been developed. Intramolecular C-H bond activation of the NHC ligand, promoted by anion ligand substitution, forms the appropriate chelate for stereocontrolled olefin metathesis.     

Anti-Markovnikov hydroaminations of styrene catalyzed by palladium(II) N-heterocyclic carbene complexes under conventional and microwave heating

Six palladium(II) complexes with benzimidazole-based N-heterocyclic carbene ligands were synthesized by transmetallation reactions between silver(I) N-heterocyclic carbene complexes and PdCl₂(PhCN)₂. The complexes were characterized by physicochemical and spectroscopic methods. The palladium complexes were tested as catalysts for intermolecular hydroamination reactions of styrene with various anilines in ionic liquids under both conventional and microwave heating. All of these complexes proved to be catalytically active in these reactions. The anti-Markovnikov addition products were selectively obtained by using 1 mol% of the palladium complex.     

Ruthenium Olefin Metathesis Catalysts Featuring N-Heterocyclic Carbene Ligands Tagged with Isonicotinic and 4-(Dimethylamino)benzoic Acid Rests: Evaluation of a Modular Synthetic Strategy

A modular and flexible strategy towards the synthesis of N-heterocyclic carbene (NHC) ligands bearing Brønsted base tags has been proposed and then adopted in the preparation of two tagged NHC ligands bearing rests of isonicotinic and 4-(dimethylamino)benzoic acids. Such tagged NHC ligands represent an attractive starting point for the synthesis of olefin metathesis ruthenium catalysts tagged in non-dissociating ligands. The influence of the Brønsted basic tags on the activity of such obtained olefin metathesis catalysts has been studied.     

Chiral bidentate bis(n-heterocyclic carbene)-based palladium complexes bearing carboxylate ligands: highly effective catalysts for the enantioselective conjugate addition of arylboronic acids to cyclic enones

Axially chiral cis-chelated bidentate bis(N-heterocyclic carbene)-palladium(II) complexes with two weakly coordinating carboxylate ligands are effective catalysts for the asymmetric conjugate addition of arylboronic acids to cyclic enones, producing the corresponding adducts in moderate-to-high yields and with good-to-high enantioselectivities, in most cases under mild conditions.     

Comparative investigation of ruthenium-based metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands

Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.     

Rate acceleration in olefin metathesis through a fluorine-ruthenium interaction

The synthesis, structure, and performance of new ruthenium-based olefin metathesis catalysts, featuring fluorinated NHC ligands are presented. The introduction of halogen atoms into the N-heterocyclic carbene ligand profoundly alters the catalytic activity and can afford a more efficient catalyst. Structural investigations suggest that a fluorine-ruthenium interaction is responsible for this increased activity.     

The influence of N-substitution on the reductive elimination behaviour of hydrocarbyl-palladium-carbene complexes--a DFT study

The reductive elimination of 2-hydrocarbyl-imidazolium salts from hydrocarbyl-palladium complexes bearing N-heterocyclic carbene (NHC) ligands represents an important deactivation route for catalysts of this type. We have explored the influence that carbene N-substituents have on both the activation energy and the overall thermodynamics of the reductive elimination reaction using density functional theory (DFT). Given the proximity of the N-substituent to the three-centred transition structure, steric bulk has little influence on the activation barrier and it is electronic factors that dominate the barriers' magnitude. Increased electron donation from the departing NHC ligand acts to stabilise the associated complex against reductive elimination, with stability following the trend: Cl < H < Ph < Me < Cy < iPr < neopentyl < tBu. The intimate involvement of the carbene p pi-orbital in determining the barrier to reductive elimination means N-substituents that are capable of removing pi-density (e.g. phenyl) act to promote a more facile reductive elimination.     

Improved ruthenium catalysts for Z-selective olefin metathesis

Several new C-H-activated ruthenium catalysts for Z-selective olefin metathesis have been synthesized. Both the carboxylate ligand and the aryl group of the N-heterocyclic carbene have been altered and the resulting catalysts evaluated using a range of metathesis reactions. Substitution of bidentate with monodentate X-type ligands led to a severe attenuation of metathesis activity and selectivity, while minor differences were observed between bidentate ligands within the same family (e.g., carboxylates). The use of nitrato-type ligands in place of carboxylates afforded a significant improvement in metathesis activity and selectivity. With these catalysts, turnover numbers approaching 1000 were possible for a variety of cross-metathesis reactions, including the synthesis of industrially relevant products.     

Access to macrocyclic endocyclic and exocyclic allylic alcohols by nickel-catalyzed reductive cyclization of ynals

Ni-catalyzed reductive macrocyclizations of ynals are reported. Disubstituted alkynes afford either endocyclic or exocyclic allylic alcohols depending on the ligand. Phosphine ligands favor the formation of endocyclic olefins, whereas N-heterocyclic carbene ligands favor the formation of exocyclic olefins. Terminal alkynes provide 1,2-disubstituted olefins with N-heterocyclic carbene ligands.     

Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.     

Enantioselective synthesis of [7]helicene: dramatic effects of olefin additives and aromatic solvents in asymmetric olefin metathesis

The asymmetric synthesis of [7]helicene was accomplished in good ee (80%) by kinetic resolution by means of asymmetric olefin metathesis. Three key factors contributed to the success of the kinetic resolution: the use of new Ru-based olefin metathesis catalysts bearing C1-symmetric N-heterocyclic carbene ligands, simple olefins as additives to control the nature of the propagating alkylidene and hexafluorobenzene as a solvent.     

Electronic nature of N-heterocyclic carbene ligands: effect on the Suzuki reaction

Suzuki reactions of aryl chlorides and arylboronic acids with a range of electronically different N-heterocyclic carbene ligands derived from N,N-diadamantylbenzimidazolium salts are reported. Results indicate that an electron-rich NHC ligand enhances the rate of oxidative addition. However, reductive elimination is unchanged by the electronic nature of the supporting ligand and is primarily affected by the steric environment.