Anna Szadkowska

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.

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.

Is the Hoveyda–Grubbs Complex a Vinylogous Fischer‐Type Carbene? Aromaticity‐Controlled Activity of Ruthenium Metathesis Catalysts

Three naphthalene-based analogues (4 a-c) of the Hoveyda-Grubbs metathesis catalyst exhibited immense differences in reactivity. Systematic structural and spectroscopic studies revealed that the ruthenafurane ring present in all 2-isopropoxyarylidene chelates possesses some aromatic character, which inhibits catalyst activity. This aromatic stabilization within the chelate ring may be controlled by variation of the polycyclic core topology as was demonstrated for tetraline and phenanthrene derivatives (4 d, e). General conclusions about a new mode of ligand-structure tuning in catalytic systems are presented.

Olefin cross-metathesis with vinyl halides

The first successful example of olefin cross-metathesis with chloroalkenes is reported.

Ruthenium-amido complexes: synthesis, structure, and catalytic activity in olefin metathesis.

Olefin metathesis has become a powerful reaction for the formation of carbon–carbon bonds in organic and polymer chemistry. Availability of well-defined ruthenium-based catalysts (e.g., 1–3) tolerant of moisture, oxygen, various functional groups and normal organic or polymer processing conditions has greatly expanded the scope and applications of this process and has made it widely used in synthesis.

Enhancement of ruthenium-catalyzed olefin metathesis reactions: Searching for new catalyst or new reaction conditions?

Olefin metathesis as a catalytic process constantly gains interest among organic chemists. Over the last decade, it became an efficient tool to accomplish the synthesis of many complex molecules. The development of new well-defined catalysts and continuous examination of novel ligands led to the establishment of metathesis methodology in a group of widespread chemical transformations. Not only does the selection of the catalyst seem to be of crucial importance, but modifying the reaction conditions, such as choice of the solvent and temperature, also allows one to make olefin metathesis a practical industrial process. This contribution, based on examples from our research, is devoted to answering the question “What may have a greater impact on the performance of metathesis reaction: a sophisticated catalyst design or unique reaction conditions?” Based on the data reported in the paper, we discuss two complementary strategies concerning the tuning of the olefin metathesis process.

X‐Ray Photoelectron Spectroscopy and Reactivity Studies of a Series of Ruthenium Catalysts

X‐Ray photoelectron spectroscopy (XPS) was applied to six selected ruthenium precatalysts. The XPS data obtained were compared against reactivity and structural results. The XPS data confirmed some dependencies such as the electron‐donor properties of the substituents at the ruthenium center. Additionally, the data combined with structural and reactivity results explain the differences between the character of Grubbs and Hoveyda catalysts. It was found that changing the PCy3 ligand to OiPr (PCy3=tricyclohexylphosphane, OiPr=isopropoxy) has a major influence on relative electron‐donating properties of the N‐heterocyclic carbene ligand (NHC) and PCy3 groups, which was supported by the relative charges on the Ru center for the examined compounds. Moreover, the turnover frequency (TOF) of a selected example reaction decreased when introducing a NHC group in the case of Grubbs catalysts, but increased in the case of Hoveyda‐type catalysts. The XPS data also explained the relative activity values of some catalysts (higher reactivity of nitro‐Hoveyda than Hoveyda second‐generation catalysts). However, the binding energies do not predict TOFs. Sole examination of the XPS data does not provide a base for reaching unambiguous and binding conclusions as to the relative reactivity of all the investigated systems.

Design and Application of Latent Olefin Metathesis Catalysts Featuring S-Chelating Alkylidene Ligands

This review article is devoted to recent advances in the design and application of so-called “dormant” or “latent” ruthenium olefin metathesis catalysts bearing S-chelating alkylidene ligands. Selected ruthenium complexes containing S-donor ligands, which possess controllable initiation behaviour are presented. Applications of these complexes in olefin metathesis are described.