Novel Ruthenium-Based Metathesis Catalysts Containing Electron- Withdrawing Ligands: Synthesis, Immobilization, and Reactivity

Fluorine-containing ruthenium-based olefin metathesis catalysts

The review summarizes literature data on the methods for the introduction of fluorine atoms and fluoralkyl groups into different ligands to construct metathesis-active ruthenium carbene complexes. It also analyzes the influence of fluorinated ligands on the catalytic activity of the complexes. The choice, structure and positions of fluorinated substituents in NHC ligands are generally dictated by the desire to increase the electrophilicity of the ruthenium atom due to the electron-withdrawing effect of fluorine atoms and fluoroalkyl groups, resulting, as a rule, in an increase in the activity of the ruthenium complex. In catalysts with unsymmetrical fluorine-containing NHC ligands, there is a possibility of additional Ru–F coordination, making the complexes much more stable and, consequently, more active. The presence of fluorine in chelating alkylidene ligands provides an increase in the catalyst initiation rate due to a weakening of the ruthenium – heteroatom bond. Besides, the introduction of polyfluoroalkyl groups into ligands solves the problem of catalyst recovery using fluorous biphasic systems for reuse.

The bibliography includes 172 references.

Integrated Batch and Continuous Flow Process for the Synthesis of Goniothalamin

An integrated batch and continuous flow process has been developed for the gram-scale synthesis of goniothalamin. The synthetic route hinges upon a telescoped continuous flow Grignard addition followed by an acylation reaction capable of delivering a racemic goniothalamin precursor (16) (20.9 g prepared over 3 h), with a productivity of 7 g·h^-1. An asymmetric Brown allylation protocol was also evaluated under continuous flow conditions. This approach employing (-)-Ipc₂B(allyl) provided an (S)-goniothalamin intermediate in 98% yield and 91.5% enantiomeric excess (ee) with a productivity of 1.8 g·h^-1. For the final step, a ring-closing metathesis reaction was explored under several conditions in both batch and flow regimes. In a batch operation, the Grubbs second-generation was shown to be effective and highly selective for the desired ring closure product over those arising from other modes of reactivity, and the reaction was complete in 1.5 h. In a flow operation, reactivity and selectivity were attenuated relative to the batch mode; however, after further optimization, the residence time could be reduced to 16 min with good selectivity and good yield of the target product. A tube-in-tube reactor was investigated for in-situ ethylene removal to favor ring-closing over cross-metathesis, in this context. These results provide further evidence of the utility of flow chemistry for organometallic processing and reaction telescoping. Using the developed integrated batch and flow methods, a total of 7.75 g of goniothalamin (1) was synthesized.

Supported Ru olefin metathesis catalysts via a thiolate tether

Ruthenium alkylidene complexes can be successfully immobilized on hybrid mesostructured silica via thiolate tethers to give heterogeneous, thiolate-coordinated olefin metathesis catalysts.

Forged and fashioned for faithfulness-ruthenium olefin metathesis catalysts bearing ammonium tags

In this article, the synthesis and applications of selected ammonium tagged Ru-alkylidene metathesis catalysts were described. Because of the straightforward synthesis, the first generation of onium-tagged catalysts have the ammonium group installed in the benzylidene ligand. Such catalysts usually give relatively pure metathesis products, and are used in polar solvents and water, or immobilised on various supports. Later, catalysts tagged in the N-heterocyclic carbene ligand (NHC) were developed to offer higher stability and even lower metal contamination levels. Due to minimal leaching, the non-dissociating ligand tagged systems were successfully immobilised on various supports, including zeolites and Metal Organic Frameworks (MOFs) and used in batch and in continuous flow conditions.

Effective immobilisation of a metathesis catalyst bearing an ammonium-tagged NHC ligand on various solid supports

An ammonium-tagged ruthenium complex, 8, was deposited on several widely available commercial solid materials such as silica gel, alumina, cotton, filter paper, iron powder or palladium on carbon. The resulting catalysts were tested in toluene or ethyl acetate, and found to afford metathesis products in high yield and with extremely low ruthenium contamination. Depending on the support used, immobilised catalyst 8 shows also additional traits, such as the possibility of being magnetically separated or the use for metathesis and subsequent reduction of the obtained double bond in one pot.

Immobilized Grubbs catalysts on mesoporous silica materials: insight into support characteristics and their impact on catalytic activity and product selectivity

The ideal support characteristics for immobilization of the Hoveyda–Grubbs 2 catalyst were defined in the metathesis of cyclooctene and the reaction mechanism to cyclic oligomers was unraveled.

Palladium-catalyzed β-(hetero)arylation of α,β-unsaturated valerolactams

A method for the palladium-catalyzed arylation of α,β-unsaturated valerolactams is reported.

Biopolymer-supported ionic-liquid-phase ruthenium catalysts for olefin metathesis

Original ruthenium supported ionic liquid phase (SILP) catalysts based on alginates as supports were developed for olefin metathesis reactions. The marine biopolymer, which fulfills most of the requisite properties for a support such as widespread abundance, insolubility in the majority of organic solvents, a high affinity for ionic liquids, high chemical stability, biodegradability, low cost, and easy processing, was impregnated by [bmim][PF6 ] containing an ionically tagged ruthenium catalyst. These biosourced catalysts show promising performances in ring-closing metathesis (RCM) and cross-metathesis (CM) reactions, with a high level of recyclability and reusability combined with a good reactivity.

The influence of anionic ligands on stereoisomerism of Ru carbenes and their importance to efficiency and selectivity of catalytic olefin metathesis reactions

Investigations detailed herein provide insight regarding the mechanism of stereochemical inversion of stereogenic-at-Ru carbene complexes through a nonolefin metathesis-based polytopal rearrangement pathway. Computational analyses (DFT) reveal that there are two key factors that generate sufficient energy barriers that are responsible for the possibility of isolation and characterization of high-energy, but kinetically stable, intermediates: (1) donor-donor interactions that involve the anionic ligands and the strongly electron donating carbene groups and (2) dipolar effects arising from the syn relationship between the anionic groups (iodide and phenoxide). We demonstrate that a Brønsted acid lowers barriers to facilitate isomerization, and that the positive influence of a proton source is the result of its ability to diminish the repulsive electronic interactions originating from the anionic ligands. The implications of the present studies regarding a more sophisticated knowledge of the role of anionic units on the efficiency of Ru-catalyzed olefin metathesis reactions are discussed. The electronic basis for the increased facility with which allylic alcohols participate in olefin metathesis processes will be presented as well. Finally, we illustrate how a better understanding of the role of anionic ligands has served as the basis for successful design of Ru-based Z-selective catalysts for alkene metathesis.

A thermally robust ruthenium phosphonium alkylidene catalyst — the effect of more bulky N-heterocyclic carbene ligands on catalyst performance in olefin metathesis reactions

Three new ruthenium phosphonium alkylidene complexes incorporating N-heterocyclic carbene ligands with bulky N-aryl groups (2,6-diethyl, L = 1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene (H2IDEP) and 2,6-diisopropyl, L = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (H2ID-i-PP)) were synthesized and characterized. The H2ID-i-PP supported complex was found to exhibit excellent thermal stabilities relative to the parent N-mesityl (N-Mes) complexes as well as the H2IDEP supported complexes. All three phosphonium alkylidenes were evaluated in comparison to the N-Mes derivative and Grubbs second generation catalyst using standard olefin metathesis reactions and conditions. The complex containing the bulky H2ID-i-PP ligand was found to have excellent activity and longevity in comparison to the other catalysts. Although initiation rates were slow for this sterically bulky precatalyst, its superior activity led to the best overall efficiency in test reactions.

Linker-free, silica-bound olefin-metathesis catalysts: applications in heterogeneous catalysis

A set of heterogenized olefin-metathesis catalysts, which consisted of Ru complexes with the H(2)ITap ligand (1,3-bis(2',6'-dimethyl-4'dimethyl aminophenyl)-4,5-dihydroimidazol-2-ylidene) that had been adsorbed onto a silica support, has been prepared. These complexes showed strong binding to the solid support without the need for tethering groups on the complex or functionalized silica. The catalysts were tested in the ring-opening-ring-closing-metathesis (RO-RCM) of cyclooctene (COE) and the self-metathesis of methyl oleate under continuous-flow conditions. The best complexes showed a TON>4000, which surpasses the previously reported materials that were either based on the Grubbs-Hoveyda II complex on silica or on the classical heterogeneous Re(2)O(7)/B(2)O(3) catalyst.