An [(NHC)(NHCEWG)RuCl2(CHPh)] Complex for the Efficient Formation of Sterically Hindered Olefins by Ring‐Closing Metathesis

NHC with EWGs for RCM : Ruthenium complexes with two N‐heterocyclic carbenes (NHCs), one of them substituted with electron‐withdrawing groups (EWGs), are highly efficient (pre)catalysts for the synthesis of tetrasubstituted olefins and trisubstituted olefins by ring‐closing metathesis reactions (RCM, see scheme).

     

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.     

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.     

[(NHC)(NHCewg)RuCl2(CHPh)] Complexes with Modified NHCewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

Imidazolium salts (NHC_ewg ? HCl) with electronically variable substituents in the 4,5‐position (H,H or Cl,Cl or H,NO₂ or CN,CN) and sterically variable substituents in the 1,3‐position (Me,Me or Et,Et or iPr,iPr or Me,iPr) were synthesized and converted into the respective [AgI(NHC)_ewg] complexes. The reactions of [(NHC)RuCl₂(CHPh)(py)₂] with the [AgI(NHC_ewg)] complexes provide the respective [(NHC)(NHC_ewg)RuCl₂(CHPh)] complexes in excellent yields. The catalytic activity of such complexes in ring‐closing metathesis (RCM) reactions leading to tetrasubstituted olefins was studied. To obtain quantitative substrate conversion, catalyst loadings of 0.2–0.5 mol % at 80 °C in toluene are sufficient. The complex with the best catalytic activity in such RCM reactions and the fastest initiation rate has an NHC_ewg group with 1,3‐Me,iPr and 4,5‐Cl,Cl substituents and can be synthesized in 95 % isolated yield from the ruthenium precursor. To learn which one of the two NHC ligands acts as the leaving group in olefin metathesis reactions two complexes, [(FL‐NHC)(NHC_ewg)RuCl₂(CHPh)] and [(FL‐NHC_ewg)(NHC)RuCl₂(CHPh)], with a dansyl fluorophore (FL)‐tagged electron‐rich NHC ligand (FL‐NHC) and an electron‐deficient NHC ligand (FL‐NHC_ewg) were prepared. The fluorescence of the dansyl fluorophore is quenched as long as it is in close vicinity to ruthenium, but increases strongly upon dissociation of the respective fluorophore‐tagged ligand. In this manner, it was shown for ring‐opening metathesis ploymerization (ROMP) reactions at room temperature that the NHC_ewg ligand normally acts as the leaving group, whereas the other NHC ligand remains ligated to ruthenium.     

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.     

Ring‐Closing Metathesis Reactions: Interpretation of Conversion–Time Data

Conversion–time data were recorded for various ring‐closing metathesis (RCM) reactions that lead to five‐ or six‐membered cyclic olefins by using different precatalysts of the Hoveyda type. Slowly activated precatalysts were found to produce more RCM product than rapidly activated complexes, but this comes at the price of slower product formation. A kinetic model for the analysis of the conversion–time data was derived, which is based on the conversion of the precatalyst (Pcat) into the active species (Acat), with the rate constant k_act, followed by two parallel reactions: 1) the catalytic reaction, which utilizes Acat to convert reactants into products, with the rate k_cat, and 2) the conversion of Acat into the inactive species (Dcat), with the rate k_dec. The calculations employ two experimental parameters: the concentration of the substrate (c(S)) at a given time and the rate of substrate conversion (?dc(S)/dt). This provides a direct measure of the concentration of Acat and enables the calculation of the pseudo‐first‐order rate constants k_act, k_cat, and k_dec and of k_S (for the RCM conversion of the respective substrate by Acat). Most of the RCM reactions studied with different precatalysts are characterized by fast k_cat rates and by the k_dec value being greater than the k_act value, which leads to quasistationarity for Acat. The active species formed during the activation step was shown to be the same, regardless of the nature of different Pcats. The decomposition of Acat occurs along two parallel pathways, a unimolecular (or pseudo‐first‐order) reaction and a bimolecular reaction involving two ruthenium complexes. Electron‐deficient precatalysts display higher rates of catalyst deactivation than their electron‐rich relatives. Slowly initiating Pcats act as a reservoir, by generating small stationary concentrations of Acat. Based on this, it can be understood why the use of different precatalysts results in different substrate conversions in olefin metathesis reactions.     

N,N'-dialkyl- and N-alkyl-N-mesityl-substituted N-heterocyclic carbenes as ligands in Grubbs catalysts

Various symmetrically and asymmetrically substituted N-heterocyclic carbene (NHC) ligands bearing aliphatic nitrogen-containing side groups have been synthesised. In our attempts to isolate the corresponding second-generation Grubbs catalysts, we were unsuccessful when using the symmetrical aliphatic NHC ligands. For the asymmetrical ligands bearing an aliphatic moiety on one side and an aromatic mesityl group on the other side, substitution of a phosphine ligand was achieved. The performance of a so-formed series of Ru-based metathesis initiators has been evaluated for the ring-opening metathesis polymerisation (ROMP) of cycloocta-1,5-diene and the ring-closing metathesis (RCM) of diethyl diallylmalonate.     

Rational Exploration of N‐Heterocyclic Carbene (NHC) Palladacycle Diversity: A Highly Active and Versatile Precatalyst for Suzuki–Miyaura Coupling Reactions of Deactivated Aryl and Alkyl Substrates

As less attention has been focussed on the design of highly efficient palladium precatalysts to ensure the smooth formation of the active catalyst for metal‐mediated cross coupling reactions, we herein demonstrate that combining the bulky N‐heterocyclic carbene (NHC) 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) with cyclopalladated acetanilide as the optimal palladium precatalyst leads to superior catalytic activity compared with the state‐of‐the‐art NHC–Pd catalysts. The complex was discovered through the evaluation of a small, rationally designed library of NHC–palladacycles prepared by a novel, practical and atom‐economic method, the direct reaction of IPr?HCl with palladacycle acetate dimers.     

Fast Olefin Metathesis at Low Catalyst Loading

Reactions of the Grubbs 3rd generation complexes [RuCl₂(NHC)(Ind)(Py)] (N‐heterocyclic carbene (NHC)=1,3‐bis(2,4,6‐trimethylphenylimidazolin)‐2‐ylidene (SIMes), 1,3‐bis(2,6‐diisopropylphenylimidazolin)‐2‐ylidene (SIPr), or 1,3‐bis(2,6‐diisopropylphenylimidazol)‐2‐ylidene (IPr); Ind=3‐phenylindenylid‐1‐ene, Py=pyridine) with 2‐ethenyl‐N‐alkylaniline (alkyl=Me, Et) result in the formation of the new N‐Grubbs–Hoveyda‐type complexes 5 (NHC=SIMes, alkyl=Me), 6 (SIMes, Et), 7 (IPr, Me), 8 (SIPr, Me), and 9 (SIPr, Et) with N‐chelating benzylidene ligands in yields of 50–75 %. Compared to their respective, conventional, O‐Grubbs–Hoveyda complexes, the new complexes are characterized by fast catalyst activation, which translates into fast and efficient ring‐closing metathesis (RCM) reactivity. Catalyst loadings of 15–150 ppm (0.0015–0.015 mol %) are sufficient for the conversion of a wide range of diolefinic substrates into the respective RCM products after 15 min at 50 °C in toluene; compounds 8 and 9 are the most catalytically active complexes. The use of complex 8 in RCM reactions enables the formation of N‐protected 2,5‐dihydropyrroles with turnover numbers (TONs) of up to 58 000 and turnover frequencies (TOFs) of up to 232 000 h^?1; the use of the N‐protected 1,2,3,6‐tetrahydropyridines proceeds with TONs of up to 37 000 and TOFs of up to 147 000 h^?1; and the use of the N‐protected 2,3,6,7‐tetrahydroazepines proceeds with TONs of up to 19 000 and TOFs of up to 76 000 h^?1, with yields for these reactions ranging from 83–92 %.     

A hexafluorobenzene promoted ring-closing metathesis to form tetrasubstituted olefins

Highly efficient formation of tetrasubstituted olefins is described by ring-closing metathesis (RCM) using catalyst 2 in presence of hexafluorobenzene. This combination with hexafluorobenzene shows an unexpected promoting effect, which requires low catalysts loadings and allows the conversion of deficient olefins in high yields and very short reaction times.     

N‐Heterocyclic Carbene,High Oxidation State Molybdenum Alkylidene Complexes: Functional‐Group‐Tolerant Cationic Metathesis Catalysts

We synthesized the first N‐heterocyclic carbene (NHC) complexes of Schrock’s molybdenum imido alkylidene bis(triflate) complexes. Unlike existing bis(triflate) complexes, the novel 16‐electron complexes represent metathesis active, functional‐group‐tolerant catalysts. Single‐crystal X‐ray structures of two representatives of this novel class of Schrock catalysts are presented and reactivity is discussed in view of their structural peculiarities. In the presence of monomer (substrate), these catalysts form cationic species and can be employed in ring‐closing metathesis (RCM), ring‐opening metathesis polymerization (ROMP), as well as in the cyclopolymerization of α,ω‐diynes. Monomers containing functional groups, which are not tolerated by the existing variations of Schrock’s catalyst, e.g., sec‐amine, hydroxy, and carboxylic acid moieties, can be used. These catalysts therefore hold great promise in both organic and polymer chemistry, where they allow for the use of protic monomers.     

Silica-Supported Cationic Tungsten Imido Alkylidene Stabilized by an N-Heterocyclic Carbene Ligand Boosts Activity and Selectivity in the Metathesis of α-Olefins

A well-defined silica-supported cationic W imido alkylidene was prepared through surface organometallic chemistry. This catalyst shows preferential activity towards α- over internal olefins, which is atypical for W-based catalysts, but consistent with the strong σ-donating ability of the NHC ancillary ligand. Complementing the studies on tungsten-based d⁰ metathesis catalysts, the silica-supported cationic W imido alkylidene displays the highest activity among W imido catalysts for α-olefins and shows improved selectivity for this class of olefins compared to Mo-based catalysts.     

Novel metathesis catalysts based on ruthenium 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidenes: synthesis, structure, immobilization, and catalytic activity

The synthesis of novel ruthenium-based metathesis catalysts containing the saturated 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene ligand, that is, [RuCl2(NHC)[=CH-2-(2-PrO)-5-NO(2)-C6H3]] (1) and [Ru(CF3COO)2(NHC)[=CH-2-(2-PrO)-5-NO2-C6H3]] (2) (NHC=1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) is described. Both catalysts are highly active in ring-closing metathesis (RCM) and ring-opening cross-metathesis (ROCM). Compound 1 shows moderate activity in enyne metathesis. Compound 2 is not applicable to enyne metathesis since it shows high activity in the cyclopolymerization of diethyl dipropargylmalonate (DEDPM). Poly(DEDPM) prepared by the action of 2 consists of 95% five-membered rings, that is, poly(cyclopent-1-enevinylene)s, and 5 % of six-membered rings, that is, poly(cyclohex-1-ene-3-methylidene)s. The polymerization proceeds in a nonliving manner and results in polyenes with broad polydispersities (1.9< or =PDI< or =2.3). Supported analogues of 2 were prepared by immobilization on hydroxymethyl-Merrifield resin and a monolithic support derived from ring-opening-metathesis polymerization (ROMP). Catalyst loadings of 1 and 2.5%, respectively, were obtained. Both supported versions of 2 showed excellent reactivity. With 0.24-2% of the supported catalysts, yields in RCM and ROCM were in the range of 76-100%. Leaching of ruthenium was low and resulted in Ru contaminations of the products of less than 0.000014% (0.14 ppm).     

Hyperbranched macromolecules bridged salicylaldimine cobalt complexes: synthesis,characterization and ethylene oligomerization studies

A series of novel 1.0 generation (1.0G) hyperbranched macromolecules bridged salicylaldimine cobalt complexes were synthesized in high yields. The compounds were characterized by fourier transform infrared (FT-IR) spectroscopy, ultraviolet (UV) visible spectroscopy, electrospray ionization mass spectrometry (ESI–MS), elemental analysis and thermal gravimetric analysis (TGA), as well as were investigated as precatalysts for the oligomerization of ethylene. Upon activation with methylaluminoxane (MAO) and diethylaluminumchloride (DEAC), the cobalt precatalysts showed moderate catalytic activities in the range of 10⁵ g/(mol Co h) in ethylene reactivity with the high selectivity for the butenes and high carbon number olefins products. The correlation between cobalt complexes and their catalytic activities and product distribution were investigated in detail under various reaction parameters. The research results showed that the catalytic activities of precatalysts increased with the increase of ethylene pressure and Al/Co molar ratio; however, the catalytic activities firstly increased and then decreased with the increase of reaction temperature. The highest activity of 2.54 × 10⁵ g/(mol Co h) and 50.18% selectivity of high number carbon olefins was obtained under the reaction temperature of 25 °C, ethylene pressure of 0.5 MPa, and Al/Co molar ratio of 1500. In addition, the nature of solvent and co-catalyst, as well as the structure of precatalysts, significantly affected both the activity and the product distribution of the resultant catalysts.     

Ring-Closing Metathesis of Ene-Ynamide: Application to the Synthesis of Medium-Sized Cyclic Dienamide

Summary: Ring-closing metathesis (RCM) of ene-ynamide, which could be applied to the synthesis of various heterocycles containing 7- and 8-membered rings, was investigated. Ene-ynamides are easily synthesized by the known method. When a toluene solution of ene-ynamide was stirred in the presence of a catalytic amount of second-generation ruthenium carbene complex 1 under an ethylene atmosphere, RCM proceeded smoothly to provide a heterocyclic compound having a diene moiety in good to high yield. A substituent of the ynamide moiety affected the yield of the cyclized product.     

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.     

Symmetric and dissymmetric N‐heterocyclic carbene rhodium(I) complexes: a comparative study of their catalytic activities in transfer hydrogenation reaction

Six new [RhBr(NHC)(cod)] (NHC = N‐heterocyclic carbene; cod = 1,5‐cyclooctadiene) type rhodium complexes ( 4–6 ) have been prepared by the reaction of [Rh(μ‐OMe)(cod)]₂ with a series of corresponding imidazoli(in)ium bromides ( 1–3 ) bearing mesityl (Mes) or 2,4,6‐trimethylbenzyl (CH₂Mes) substituents at N¹ and N³ positions. They have been fully characterized by ¹ H, ¹³ C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO)₂] (NHC = imidazol‐2‐ylidene) ( 7b–9b ) were also synthesized to compare σ‐donor/π‐acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4–6 as catalysts. The symmetrically CH₂Mes‐substituted rhodium complex bearing a saturated NHC ligand ( 5a ) showed the highest catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

胺基膦钌卡宾化合物的合成及催化烯烃复分解反应

合成并表征了一类含新型胺基膦配体的Grubbs二代型钌卡宾烯烃复分解催化剂[RuCl₂(H₂IMes)·(R¹HNPR²₂)(=CHPh)], 采用核磁共振波谱和单晶X射线衍射确定了催化剂的结构. 在室温条件下, 以N,N-二烯丙基-对甲苯磺酰胺的关环复分解反应(RCM)为模型, 考察了不同胺基膦配体对钌卡宾催化反应速率的影响. 结果表明, G2?1表现出最佳的催化活性. 通过底物研究发现, G2?1催化剂(摩尔分数, 1%)对双端烯及多端烯的RCM反应具有较好的活性和官能团适应性, 产物收率均>95%; G2?1催化剂同样适用于同(异)端烯底物的交叉复分解反应(CM), 其催化苯乙烯与3-苯氧基丙烯的CM反应时产物收率高达92%.     

New latent metathesis catalysts equipped with exchangeable boronic ester groups on the NHC

Abstract

Latent metathesis catalysts equipped with boronate esters of diols as exchangeable end-groups on their NHC ligands and an S-chelated ruthenium-benzylidene core were synthesized. The stable S-chelated ruthenium complexes underwent hydrolysis under mild acidic conditions, allowing easy exchange of terminal units by several 1,2- and 1,3-diols, without degrading the central ruthenium benzylidene. Using this strategy, we also prepared metathesis catalysts equipped with diallyl substrates at the termini that showed concentration dependency on RCM reactions. Notably, the larger dendritic catalysts were more efficient at the more dilute condition.     

Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in Olefin Metathesis Reactions

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis‐diiodo/sulfur‐chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis‐cyclooctene by the RCM reaction of 1,9‐undecadiene. Moreover, cis‐1,4‐polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5‐cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.