Chloro-substituted Hoveyda-Grubbs ruthenium carbene: Investigation of electronic effects

A series of applications of cross and ring-closing metathesis has been made to investigate the application profile of the chloro-substituted Hoveyda-Grubbs ruthenium carbene in order to evaluate electronic effects resulting from the introduction of a chlorine atom para to the isopropoxy moiety of its parent catalyst.     

Chiral Ru-based complexes for asymmetric olefin metathesis: enhancement of catalyst activity through steric and electronic modifications

Design, synthesis, characterization, and catalytic activity of six enantiomerically pure Ru-based metathesis catalysts are disclosed (3a-3f). The new chiral catalysts were prepared through steric and electronic alterations of the parent catalyst system (3). The present studies indicate that the effect of structural modifications of chiral complex 3 does not always correspond to those of the related achiral complexes. The present findings illustrate that modified Ru complexes (3e and 3f) deliver reactivity levels that are more than 2 orders of magnitude higher than 3. Reactivity and physical data are provided that shed light on the origin of activity differences. Some members of the new generation of chiral Ru catalysts promote asymmetric ring-opening (AROM) and ring-closing (ARCM) metatheses that cannot be effected by the first generation chiral catalyst (3).     

Hybrid silica materials derived from Hoveyda-Grubbs ruthenium carbenes. Electronic effects of the nitro group on the activity and recyclability as diene and enyne metathesis catalysts

The preparation of several organic-inorganic hybrid materials by sol-gel process derived from Hoveyda-type monomers is described. One of them presents a nitro group at the para position with respect to the alkoxy moiety. These materials were treated with Grubbs catalysts to generate the corresponding Hoveyda-Grubbs carbene ruthenium complexes covalently bonded to the silica matrix, which were tested as recyclable catalysts for diene and enyne RCM. Electronic effects of the nitro group resulted in enhanced activity of the catalyst. Whereas the recyclability decreased in RCM of dienes, the presence of this electron-withdrawing group was highly advantageous for the RCM of enynes, the reusability being greatly improved.     

Ortho- and para-substituted Hoveyda-Grubbs carbenes. An improved synthesis of highly efficient metathesis initiators

A novel highly efficient and general route to various 3- and 5-substituted 2-alkoxystyrenes, required for the preparation of Hoveyda-Grubbs catalysts, is described.     

Tuning the electronic properties of N-heterocyclic carbenes

The electron-donating properties of 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, OC(12)H(25), Me, H, Br, S(4-tolyl), SO(4-tolyl), SO2(4-tolyl)) were studied. Twelve new N-heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (DeltaE1/2) and IR (nu (CO)) can be used to measure the electron-donating properties of the carbene ligands. Modifying the 4-positions with electron-withdrawing substituents (4-R=-SO(2)Ar, DeltaE1/2=+0.92 V) results in NHC ligands with virtually the same electron-donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (DeltaE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4-R=NEt2 (DeltaE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties.     

Epimerization reaction of a substituted vinylcyclopropane catalyzed by ruthenium carbenes: mechanistic analysis

A novel ruthenium carbene-catalyzed epimerization of vinylcyclopropanes is reported. The reaction rate strongly depends on the presence of ruthenium ligands in solution. When the first-generation Grubbs catalyst is employed, a 5.3:1 equilibrium ratio of epimers is established quickly, but when a first-generation Hoveyda catalyst is employed, epimerization is observed only if an additional phosphine or nitrogen ligand is added. NMR and kinetic studies suggest that the isomerization reaction occurs through the intermediacy of a ruthenacyclopentene. The observation suggests that cyclopropylmethylidene ruthenium carbenes of synthetic utility may be accessible via ruthenacyclopentenes obtained via other routes.     

An ionic liquid-tagged second generation Hoveyda-Grubbs ruthenium carbene complex as highly reactive and recyclable catalyst for ring-closing metathesis of di-, tri- and tetrasubstituted dienes

A second generation Hoveyda-Grubbs ruthenium carbene complex bearing an ionic liquid tag was prepared and shown to be a highly reactive catalyst for the ring-closing metathesis of di-, tri- and tetrasubstituted diene and enyne substrates in minimally ionic solvent systems ([Bmim]PF₆/CH₂Cl₂, 1:9-1:1 v/v). Both the catalyst and the ionic liquid can be conveniently recycled and repeatedly reused (up to 17 cycles) with only a very slight loss of activity. The ionic liquid tag is crucial to the high level of recyclability of the catalyst since the original second generation Grubbs and Hoveyda-Grubbs catalysts rapidly lose their activity when recycled in the ionic liquid layer.     

Hoveyda-Grubbs catalyst confined in the nanocages of SBA-1: enhanced recyclability for olefin metathesis

Via a simple adsorption, the second generation Hoveyda-Grubbs catalyst was successfully immobilized on a mesoporous material SBA-1, leading to a highly recyclable solid catalyst for olefin metathesis.     

Synthesis of siloxy-modified second generation Hoveyda-Grubbs catalysts and their catalytic activity

Efficient syntheses of the first ruthenium alkylidene complexes bearing siloxide ligands are described. Second generation Hoveyda-Grubbs catalyst is shown to undergo efficient functionalization with a number of potassium silanolates to give disiloxy derivatives. The complexes obtained are found catalytically active in selected metathesis transformations.     

A highly controllable latent ruthenium Schiff base olefin metathesis catalyst: Catalyst activation and mechanistic studies

A ruthenium Schiff base catalyst ( 5 ), bearing an N‐heterocyclic carbene ligand, was found to be a latent catalyst for the ring‐opening metathesis polymerization of cis,cis‐1,5‐cyclooctadiene and dicyclopentadiene and is activated efficiently on addition of hydrochloric acid. A benchmark study was performed using the Grubbs first ( 1 ), second ( 2 ), and third ( 3 ) generation catalyst. Results further illustrate that the catalyst can be stored in dicyclopentadiene at a monomer/catalyst ratio of 15,000/1 without any significant polymerization during at least 12 months. After activation of the catalyst with a Brønsted acid, no undesired loss of performance can be detected. This concept of activation can be easily adapted to a Reaction Injection Molding process, and reaction control is obtained via the in situ generation of the cocatalyst as the outcome of the reaction of alcohols with Lewis acids. Insight into the activation mechanism was gained through an in‐depth nuclear magnetic resonance study, and a plausible mechanism is proposed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 302–310, 2010     

Regioselective C-H activation of lanthanide-bound N-heterocyclic carbenes

The reaction of Ln(L)N'2 (Ln = Nd, Ce; L = t-BuNCH2CH2[C{NCHCHNt-Bu}], N' = N(SiMe3)2) with trimethylsilyl iodide regiospecifically functionalises the carbene backbone at the C4-carbene ring position to afford the silylated complex Ln(L')N'I; Ln(L')N'2 is isolated after attempted reduction (L' = t-BuNCH2CH2[C{NC(SiMe3)CHNt-Bu}]) which allows a comparison of the structurally characterised complexes Nd(L)N'2, [Nd(L')N'I]2, and Nd(L')N'2.     

N-heterocyclic carbenes with inorganic backbones: electronic structures and ligand properties

The electronic structures of known N-heterocyclic carbenes (NHCs) with boron, nitrogen, and phosphorus backbones are examined using quantum chemical methods and compared to the experimental results and to the computational data obtained for a classical carbon analogue, imidazol-2-ylidene. The sigma-donor and pi-acceptor abilities of the studied NHCs in selected transition-metal complexes are evaluated using a variety of approaches such as energy and charge decomposition analysis, as well as calculated acidity constants and carbonyl stretching frequencies. The study shows that the introduction of selected heteroatoms into the NHC backbone generally leads to stronger metal-carbene bonds and therefore improves the ligand properties of these systems. The backdonation of pi-electron density from the metal to the ligand is found to be strong in complexes of the studied NHCs with group 11 metals, where it constitutes up to nearly 35% of the total orbital interaction energy. The ligand properties of the aluminum analogues of some of the reported NHCs with boron backbones are also assessed.     

改性ZrO2负载钌氨合成催化剂的制备及其构效关系研究

由ZrO(NO₃)₂水解得到的ZrO(OH)₂水凝胶经碱液回流老化、焙烧后制备了改性ZrO₂载体材料,直接浸渍K₂RuO₄溶液,经还原后用于催化氨合成反应。并运用X射线衍射(XRD)、CO₂ 程序升温脱附(CO₂-TPD)、X射线荧光光谱 (XRF)、N₂物理吸附、H₂程序升温还原技术(H₂-TPR)和CO化学吸附对其进行了表征,重点考察了催化剂性能与载体性能间的构效关系。结果表明,KOH和NH₄OH溶液回流均可提高载体的比表面积,但是KOH回流制备的载体同时还具有较强的碱性,因此,负载钌以后表现出最佳活性。在425 ℃、5 MPa、空速为10 000 h^-1条件下,出口氨浓度为5.96%,分别较催化剂K-Ru/ZrO₂-NH₄OH、K-Ru/ZrO₂-CP和Ru/ZrO₂-NH₄OH提高了11%、143%和103%。与活性组分分散度相比,载体碱性强度对活性的促进作用更为明显。     

Cross and ring-closing metathesis of 1,3-diynes: metallotropic [1,3]-shift of ruthenium carbenes

The regio- and stereoselective enyne CM and RCM reactions involving 1,3-diynes have been developed. The ring-closing metathesis of enediynes induces a facile metallotropic [1,3]-shift of alkynyl ruthenium carbene species, thereby providing a unique entry into the synthesis of fully conjugated 1,5-diene-3-ynes.