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.     

Chemodivergent Metathesis of Dienynes Catalyzed by Ruthenium–Indenylidene Complexes: An Experimental and Computational Study

A study on the enyne metathesis reaction leading to the formation cyclic compounds using ruthenium–indenylidene complexes is presented. Several 1,11‐dien‐6‐ynes have been subjected to ruthenium metathesis cyclization by using ruthenium–indenylidene complexes bearing various phosphine and N‐heterocyclic carbene (NHC) ligands. Interestingly, for some substrates chemodivergent metathesis occurs and is a function of the catalyst employed. This led us to investigate the competing “ene‐then‐yne” or “yne‐then‐ene” reaction pathways apparently at play in these systems using both experimental observations and DFT calculations. Experimental and computational studies were found in good agreement and permit to conclude that for phosphine‐containing catalysts, the “ene‐then‐yne” pathway is exclusively adopted. On the other hand, for catalysts bearing NHC ligands, both pathways are possible.     

The influence of N-heterocyclic carbene steric and electronic properties in Ru-catalysed cross metathesis reactions

The present study examines the influence of N-heterocyclic carbene (NHC) ligand electronic and steric parameters on the activity of ruthenium indenylidene complexes in cross metathesis. The NHC ligands tested lead to varied E/Z selectivities with the pre-catalyst bearing an IMes ligand exhibiting high activity.     

Olefin metathesis catalyst: stabilization effect of backbone substitutions of N-heterocyclic carbene

Ruthenium olefin metathesis catalysts bearing an N-phenyl-substituted N-heterocyclic carbene (NHC) ligand that are resistant to decomposition through C-H activation have been prepared and tested in ring closing metathesis (RCM), cross metathesis (CM), and ROMP reactions. The N, N'-diphenyl-substituted NHC complex proved to be one of the most efficient catalysts in RCM to form tetrasubstituted olefins.     

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.     

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo-, cross and ring-opening cross metathesis reactions. The catalysts remain active even in 2-PrOH and are applicable in ring-opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3-dimesitylimidazol-2-ylidene NHC ligand was found essential for reactive and yet robust catalysts.     

The influence of electronic modifications on rotational barriers of bis‐NHC‐complexes as observed by dynamic NMR spectroscopy

There has been much debate about the σ‐donor and π‐acceptor properties of N‐heterocyclic carbenes (NHCs). While a lot of synthetic modifications have been performed with the goal of optimizing properties of the catalyst to tune reactivity in various transformations (e.g. metathesis), direct methods to characterize σ‐donor and π‐acceptor properties are still few. We believe that dynamic NMR spectroscopy can improve understanding of this aspect. Thus, we investigated the intramolecular dynamics of metathesis precatalysts bearing two NHCs. We chose four systems with one identical NHC ligand (N,N′‐Bis(2,4,6‐trimethylphenyl)‐imidazolinylidene (SIMes) in all four cases) and NHC_ewg ligands bearing four different electron‐withdrawing groups (ewg). Both rotational barriers of the respective Ru‐NHC‐bonds change significantly when the electron density of one of the NHCs (NHC_ewg) is modified. Although it is certainly not possible to fully dissect σ‐donor and π‐acceptor portions of the bonding situations in the respective Ru‐NHC‐bond via dynamic NMR spectroscopy, our studies nevertheless show that the analysis of the rotation around the Ru‐SIMes‐bond can be used as a spectroscopic parameter complementary to cyclic voltammetry. Surprisingly, we observed that the rotation around the Ru‐NHC_ewg‐bond shows the same trend as the initiation rate of a ring‐closing metathesis of the four investigated bis‐NHC‐complexes. Copyright © 2013 John Wiley & Sons, Ltd.     

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts.     

Identification and characterization of a new family of catalytically highly active imidazolin-2-ylidenes

A new class of easily accessible and stable imidazolin-2-ylidenes has been synthesized where the side chains are comprised of substituted naphthyl units. Introduction of the naphthyl groups generates C 2 -symmetric ( rac) and C s- symmetric ( meso) atropisomers, and interconversion between the isomers is studied in detail both experimentally and computationally. Complete characterization of the carbenes includes rare examples of crystallographically characterized saturated NHC structures. Steric properties of the ligands and an investigation of their stability are also presented. In catalysis, the new ligands show versatility comparable to the most widely used NHCs IMes/SIMes or IPr/SIPr. Excellent catalytic results are obtained when either the NHC salts (ring-opening alkylation of epoxides), NHC-modified palladium compounds (C-C and C-N cross-couplings), or NHC-ruthenium complexes (ring-closing metathesis, RCM) are employed. In several cases, this new ligand family provides catalytic systems of higher reactivity than that observed with previously reported NHC compounds.     

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.     

Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands

This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.     

Chiral iridium(I) bis(NHC) complexes as catalysts for asymmetric transfer hydrogenation

The common use of NHC complexes in transition‐metal mediated C–C coupling and metathesis reactions in recent decades has established N‐heterocyclic carbenes as a new class of ligand for catalysis. The field of asymmetric catalysis with complexes bearing NHC‐containing chiral ligands is dominated by mixed carbene/oxazoline or carbene/phosphane chelating ligands. In contrast, applications of complexes with chiral, chelating bis(NHC) ligands are rare. In the present work new chiral iridium(I) bis(NHC) complexes and their application in the asymmetric transfer hydrogenation of ketones are described. A series of chiral bis(azolium) salts have been prepared following a synthetic pathway, starting from L ‐valinol and the modular buildup allows the structural variation of the ligand precursors. The iridium complexes were formed via a one‐pot transmetallation procedure. The prepared complexes were applied as catalysts in the asymmetric transfer hydrogenation of various prochiral ketones, affording the corresponding chiral alcohols in high yields and moderate to good enantioselectivities of up to 68%. The enantioselectivities of the catalysts were strongly affected by the various, terminal N‐substituents of the chelating bis(NHC) ligands. The results presented in this work indicate the potential of bis‐carbenes as stereodirecting ligands for asymmetric catalysis and are offering a base for further developments. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

A readily available chiral Ag-based N-heterocyclic carbene complex for use in efficient and highly enantioselective Ru-catalyzed olefin metathesis and Cu-catalyzed allylic alkylation reactions

A new chiral bidentate N-heterocyclic carbene (NHC) ligand has been designed and synthesized. The NHC ligand bears a chiral diamine backbone and an achiral biphenol group; upon metal complexation (derived from Ag(I), Ru(II), or Cu(II)), the diamine moiety induces >98% diastereoselectivity such that the biaryl unit coordinates to the metal center to afford the desired complex as a single atropisomer. Because the ligand does not require optically pure biaryl amino alcohols, its synthesis is significantly shorter and simpler than the related first generation ligands bearing a chiral binaphthyl-based amino alcohol. The chiral NHC ligand can be used in the preparation of highly effective Ru- and Cu-based complexes (prepared and used in situ from the Ag(I) carbene) that promote enantioselective olefin metathesis and allylic alkylations with scope that is improved from previously reported protocols. In many cases, transformations promoted by the chiral NHC-based complexes proceed with higher enantioselectivity and reactivity than was observed with previously reported complexes.     

Cyclic ruthenium-alkylidene catalysts for ring-expansion metathesis polymerization

A series of cyclic Ru-alkylidene catalysts have been prepared and evaluated for their efficiency in ring-expansion metathesis polymerization (REMP). The catalyst structures feature chelating tethers extending from one N-atom of an N-heterocyclic carbene (NHC) ligand to the Ru metal center. The catalyst design is modular in nature, which provided access to Ru complexes having varying tether lengths, as well as electronically different NHC ligands. Structural impacts of the tether length were unveiled through (1)H NMR spectroscopy as well as single-crystal X-ray analyses. Catalyst activities were evaluated via polymerization of cyclooctene, and key data are provided regarding propagation rates, intramolecular chain transfer, and catalyst stabilities, three areas necessary for the efficient synthesis of cyclic poly(olefin)s via REMP. From these studies, it was determined that while increasing the tether length of the catalyst leads to enhanced rates of polymerization, shorter tethers were found to facilitate intramolecular chain transfer and release of catalyst from the polymer. Electronic modification of the NHC via backbone saturation was found to enhance polymerization rates to a greater extent than did homologation of the tether. Overall, cyclic Ru complexes bearing 5- or 6-carbon tethers and saturated NHC ligands were found to be readily synthesized, bench-stable, and highly active catalysts for REMP.     

The pivotal role of symmetry in the ruthenium-catalyzed ring-closing metathesis of olefins

The synthesis of Ru-based precatalysts with N-heterocyclic carbene (NHC) ligands bearing syn- and anti-methyl groups on the NHC backbone and aryl N-substituents with differing steric bulk was carried out. The catalytic behavior of the monophospine Ru precatalysts (7a, 7b, 8a, and 8b) was compared to the corresponding family of phosphine-free catalysts (9a, 9b, 10a and 10b) in the ring-closing metathesis (RCM) of olefins. These catalysts showed high efficiency in RCM reactions and the syn-isomers 7a and 9a, in particular, proved to be among the most active catalysts in the formation of tetrasubstituted olefins through RCM. DFT studies on the entire RCM catalytic cycle of hindered olefins were performed to rationalize the different behaviors of catalysts with syn- and anti-methyl groups on the NHC backbone. Theoretical results not only disclosed how NHC symmetry influences the overall activity of the catalyst, but also gave relevant and more general indications on the crucial steps of the RCM of olefins.     

Selective arylation of aldehydes with di-rhodium(II)/NHC catalysts

Here is described the preparation of four new rhodium(II) complexes bearing axial NHC ligands. The presence of electron-withdrawing bridging ligands resulted in an enhanced reactivity in the arylation of aldehydes with boronic acids when compared with the tetraacetate counterparts. Complex 15 (Rh₂tfa₄(IPr)₂) proved to be the most active catalyst for this transformation allowing the selective conversion of aromatic, aliphatic and vinyl aldehydes into the respective alcohols in excellent yields. It was demonstrated that the good group tolerance could be further extended to aromatic and conjugated ketones. DFT calculations carried out on this system showed the complementarily of the bridging ligands and axial ligand in these dinuclear complexes. It was also disclosed that Rh(II)/NHC catalytic system can promote the racemization of 1-phenyl ethanol.     

以N-杂环卡宾为配体的金属络合物催化有机合成的反应

综述了近几年来以N-杂环卡宾为配体的金属络合物催化有机合成的反应。     

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.     

Force field parametrization and molecular dynamics simulation of flexible POSS-linked (NHC; phosphine) Ru catalytic complexes

In recent years, N-heterocyclic carbene (NHC) or phospine groups have been put forward as candidate catalysts ligands for olefin metathesis reactions to be performed using multistep methods. Some of these proposed ligands contain polyhedral oligomeric silsesquioxane (POSS) structures linked to NHC rings by means of alkyl chains. Some important properties for the prediction of catalytic activity, such as the theoretically defined buried volume, are related to the conformational characteristics of these complex ligands that can be studied through molecular dynamics simulations. However, the chemical structure of resulting catalytic complexes usually contains atoms or groups that are not included in the common forcefields used in simulations. In this work we focus on complexes formed by a catalytic metal center (Ru) with both phospine and POSS-linked NHC groups. The central part of the complexes contain atoms and groups that have bonds, bond angles, and torsional angles whose parameters have not been previously evaluated and included in existing force fields. We have performed basic ab initio quantum mechanical calculations based on the density functional theory to obtain energies for this central section. The force field parameters for bonds, bond angles, and torsional angles are then calculated from an analysis of energies calculated for the equilibrium and different locally deformed structures. Nonbonded interactions are also conveniently evaluated. From subsequent molecular dynamics simulations, we have obtained results that illustrate the conformational characteristics most closely connected with the catalytic activity.