Karol Grela

Ruthenium-Based Olefin Metathesis Catalysts Bearing N-Heterocyclic Carbene Ligands

N-Heterocyclic carbene (NHC) ligands, introduced as analogues to phosphines, are recently getting wide attention in the design of diverse homogeneous catalytic systems.1-6 During recent years, olefin metathesis has gained a position of increasing significance.7-9 The ruthenium complex (PCy3)2(Cl2)RudCHPh 1 (Cy ) cyclohexyl) developed by Grubbs et al.10 constitutes a highly efficient metathesis catalyst11 tolerating most functional groups. In spite of the generally superb application profile of 1, its limited stability and the low activity toward substituted double bonds are major drawbacks. The initial success of olefin metathesis has spurred the intense investigation of new catalysts for this transformation. Inter alia, the recent introduction of NHCs * To whom correspondence should be addressed. E-mail: klgrela@ gmail.com. † Polish Academy of Sciences. ‡ Warsaw University. Cezary Samojłowicz was born in 1983 in Sokołów Podlaski, Poland. He obtained his MSc Eng. degree in chemical technology from the Warsaw University of Technology, studying sigmatropic rearrangements of sulfur ylides under the supervision of Tadeusz Zdrojewski. Before moving to olefin metathesis, he conducted work on supramolecular chemistry with David Reinhoudt at Twente University. Since 2007, he is conducting his PhD study under the supervision of Karol Grela.

A Highly Efficient Ruthenium Catalyst for Metathesis Reactions

The development of accessible metathesis catalysts that combine high activity with excellent tolerance to a variety of functional groups has been key to the widespread application of olefin metathesis in organic synthesis. In spite of the general superb application profile of the ruthenium carbene 1a, its limited thermal stability and the low activity towards substituted double bonds are major drawbacks.[1] Specifically, the preparation of substituted olefins with electron-withdrawing functionality (such as a,b-unsaturated carbonyl compounds, nitriles, sulfones, etc.) through cross metathesis (CM) with terminal alkenes remains a difficult task. The newly introduced highly active ruthenium alkylidene complexes with sterically demanding N-heterocyclic carbene (NHC)[3] ligands have dramatically alleviated this limitation.[2] Compounds of type 1b and 1c were found to be efficient catalysts in the reactions of previously metathesisinactive substrates, including a,b-unsaturated olefins (Scheme 1).[2,4]

Aqueous Olefin Metathesis

According to popular belief, oxygen and water are the natural enemies of organometallic reactions and therefore must be excluded rigorously from the reaction vessel. This belief is founded in the case of the highly reactive nucleophilic metal alkylidene complexes that were used in early catalytic olefin metathesis. However, owing to the high stability of the ruthenium carbene complexes introduced by Grubbs, metathesis in water has become reality.

Olefin metathesis : theory and practice

This is a complete examination of the theory and methods of modern olefin metathesis, one of the most widely used chemical reactions in research and industry. • Provides basic information for non-specialists, while also explaining the latest trends and advancements in the field to experts • Discusses the various types of metathesis reactions, including CM, RCM, enyne metathesis, ROMP, and tandem processes, as well as their common applications • Outlines the tools of the trade—from the important classes of active metal complexes to optimal reaction conditions—and suggests practical solutions for common reaction problems • Includes tables with structures of commercial catalysts, and recommendations for commercial catalyst suppliers

A green catalyst for green chemistry: Synthesis and application of an olefin metathesis catalyst bearing a quaternary ammonium group

The novel catalyst 8, bearing a polar quaternary ammonium group, is very stable and can be easily prepared from commercially available reagents. Catalyst 8 can be efficiently used for olefin metathesis not only in traditional but also in aqueous media. Various ring closing-, cross- and enyne-metathesis reactions were conducted in water–methanol mixtures in air. The electron withdrawing quaternary ammonium group not only activates the catalyst chemically, but at the same time allows its efficient separation after reaction. Application of 8 leads to organic products of high purity, which exhibit very low ruthenium contamination levels (12–68 ppm) after filtering through a pad of silica gel.

A PS-DES immobilized ruthenium carbene: a robust and easily recyclable catalyst for olefin metathesis

The butyldiethylsilyl polystyrene (PS-DES) supported ruthenium carbene 7 is a robust, practical and easily recyclable catalyst for olefin metathesis of substituted olefins.

A Highly Active Aqueous Olefin Metathesis Catalyst Bearing a Quaternary Ammonium Group

A polar olefin metathesis catalyst that bears a quaternary ammonium group was prepared from commercially available reagents. The electron-withdrawing quaternary ammonium group not only activates the Ru catalyst electronically but at the same time makes the catalyst more hydrophilic. The catalyst can therefore be efficiently used both in traditional media, such as dichloromethane and toluene, as well as in technical-grade alcohols, alcohol-water mixtures and in neat water. Various metathesis reactions, including ring-closing, cross- and enyne metathesis, were conducted in these solvents in the presence of air. In addition, the Ru catalyst can act as an inisurf (initiator and surfactant) molecule, promoting metathesis under heterogeneous aqueous conditions.

Mechanistic Insights into the cis–trans Isomerization of Ruthenium Complexes Relevant to Catalysis of Olefin Metathesis

The mechanism of the trans to cis isomerization in Ru complexes with a chelating alkylidene group has been investigated by using a combined theoretical and experimental approach. Static DFT calculations suggest that a concerted single-step mechanism is slightly favored over a multistep mechanism, which would require dissociation of one of the ligands from the Ru center. This hypothesis is supported by analysis of the experimental kinetics of isomerization, as followed by (1)H NMR spectroscopy. DFT molecular dynamics simulations revealed that the variation of geometrical parameters around the Ru center in the concerted mechanism is highly uncorrelated; the mechanism actually begins with the transformation of the square-pyramidal trans isomer, with the Ru==CHR bond in the apical position, into a transition state that resembles a metastable square pyramidal complex with a Cl atom in the apical position. This high-energy structure collapses into the cis isomer. Then, the influence of the N-heterocyclic carbene ligand, the halogen, and the chelating alkylidene group on the relative stability of the cis and trans isomers, as well as on the energy barrier separating them, was investigated with static calculations. Finally, we investigated the interconversion between cis and trans isomers of the species involved in the catalytic cycle of olefin metathesis; we characterized an unprecedented square-pyramidal metallacycle with the N-heterocyclic carbene ligand in the apical position. Our analysis, which is relevant to the exchange of equatorial ligands in other square pyramidal complexes, presents evidence for a remarkable flexibility well beyond the simple cis-trans isomerization of these Ru complexes.

The Doping Effect of Fluorinated Aromatic Solvents on the Rate of Ruthenium‐Catalysed Olefin Metathesis

A study concerning the effect of using a fluorinated aromatic solvent as the medium for olefin metathesis reactions catalysed by ruthenium complexes bearing N-heterocyclic carbene ligands is presented. The use of fluorinated aromatic hydrocarbons (FAH) as solvents for olefin metathesis reactions catalysed by standard commercially available ruthenium pre-catalysts allows substantially higher yields of the desired products to be obtained, especially in the case of demanding polyfunctional molecules, including natural and biologically active compounds. Interactions between the FAH and the second-generation ruthenium catalysts, which apparently improve the efficiency of the olefin metathesis transformation, have been studied by X-ray structure analysis and computations, as well as by carrying out a number of metathesis experiments. The optimisation of reaction conditions by using an FAH can be regarded as a complementary approach for the design of new improved ruthenium catalysts. Fluorinated aromatic solvents are an attractive alternative medium for promoting challenging olefin metathesis reactions.

Studies on Electronic Effects in O‐, N‐ and S‐Chelated Ruthenium Olefin‐Metathesis Catalysts

A short overview on the structural design of the Hoveyda-Grubbs-type ruthenium initiators chelated through oxygen, nitrogen or sulfur atoms is presented. Our aim was to compare and contrast O-, N- and S-chelated ruthenium complexes to better understand the impact of electron-withdrawing and -donating substituents on the geometry and activity of the ruthenium complexes and to gain further insight into the trans-cis isomerisation process of the S-chelated complexes. To evaluate the different effects of chelating heteroatoms and to probe electronic effects on sulfur- and nitrogen-chelated latent catalysts, we synthesised a series of novel complexes. These catalysts were compared against two well-known oxygen-chelated initiators and a sulfoxide-chelated complex. The structures of the new complexes have been determined by single-crystal X-ray diffraction and analysed to search for correlations between the structural features and activity. The replacement of the oxygen-chelating atom by a sulfur or nitrogen atom resulted in catalysts that were inert at room temperature for typical ring-closing metathesis (RCM) and cross-metathesis reactions and showed catalytic activity only at higher temperatures. Furthermore, one nitrogen-chelated initiator demonstrated thermo-switchable behaviour in RCM reactions, similar to its sulfur-chelated counterparts.