A highly active aqueous olefin metathesis catalyst bearing a quaternary ammonium group

Oligo(ethylene glycol) Length Effect of Water-Soluble Ru-Based Olefin Metathesis Catalysts on Reactivity and Removability

A study of reaction kinetics and removal efficiency of a family of ruthenium (Ru)-based olefin metathesis catalysts containing ethylene-glycol-oligomer-tethered N-heterocyclic carbene (NHC) ligands has been carried out, with a focus on variation of ethylene glycol oligomer length. The length of ethylene glycol oligomer was precisely defined by sequential addition of repeating units. Due to the dual solubility of ethylene glycol oligomer, the produced catalyst was highly soluble in both aqueous and organic solvents (dichloromethane). In aqueous solution, the polarity increase with longer ethylene glycol oligomers enhanced the reactivity in homogeneous solution. The length of ethylene glycol oligomer did not significantly affect olefin metathesis rate in organic solution. Yet the removal efficiency of catalyst strongly relies on the length of ethylene glycol oligomer. A longer ethylene glycol oligomer demonstrated better catalyst removal efficiency. The tested catalyst removal method was aqueous extraction from organic solution using its higher water solubility property compared to its lower organic solvent (dichloromethane) solubility property. The results obtained from the aqueous extraction catalyst removal method demonstrated similar and/or better removal rates compared to previously reported host-guest catalyst removal methods.

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.

Catalytic Organic Reactions in Water toward Sustainable Society

Traditional organic synthesis relies heavily on organic solvents for a multitude of tasks, including dissolving the components and facilitating chemical reactions, because many reagents and reactive species are incompatible or immiscible with water. Given that they are used in vast quantities as compared to reactants, solvents have been the focus of environmental concerns. Along with reducing the environmental impact of organic synthesis, the use of water as a reaction medium also benefits chemical processes by simplifying operations, allowing mild reaction conditions, and sometimes delivering unforeseen reactivities and selectivities. After the "watershed" in organic synthesis revealed the importance of water, the development of water-compatible catalysts has flourished, triggering a quantum leap in water-centered organic synthesis. Given that organic compounds are typically practically insoluble in water, simple extractive workup can readily separate a water-soluble homogeneous catalyst as an aqueous solution from a product that is soluble in organic solvents. In contrast, the use of heterogeneous catalysts facilitates catalyst recycling by allowing simple centrifugation and filtration methods to be used. This Review addresses advances over the past decade in catalytic reactions using water as a reaction medium.

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.

Hexacoordinate Ru-based olefin metathesis catalysts with pH-responsive N-heterocyclic carbene (NHC) and N-donor ligands for ROMP reactions in non-aqueous, aqueous and emulsion conditions

Three new ruthenium alkylidene complexes (PCy₃)Cl₂(H₂ITap)Ru=CHSPh (9), (DMAP)₂Cl₂(H₂ITap)Ru=CHPh (11) and (DMAP)₂Cl₂(H₂ITap)Ru=CHSPh (12) have been synthesized bearing the pH-responsive H₂ITap ligand (H₂ITap = 1,3-bis(2’,6’-dimethyl-4’-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene). Catalysts 11 and 12 are additionally ligated by two pH-responsive DMAP ligands. The crystal structure was solved for complex 12 by X-ray diffraction. In organic, neutral solution, the catalysts are capable of performing standard ring-opening metathesis polymerization (ROMP) and ring closing metathesis (RCM) reactions with standard substrates. The ROMP with complex 11 is accelerated in the presence of two equiv of H₃PO₄, but is reduced as soon as the acid amount increased. The metathesis of phenylthiomethylidene catalysts 9 and 12 is sluggish at room temperature, but their ROMP can be dramatically accelerated at 60 °C. Complexes 11 and 12 are soluble in aqueous acid. They display the ability to perform RCM of diallylmalonic acid (DAMA), however, their conversions are very low amounting only to few turnovers before decomposition. However, both catalysts exhibit outstanding performance in the ROMP of dicyclopentadiene (DCPD) and mixtures of DCPD with cyclooctene (COE) in acidic aqueous microemulsion. With loadings as low as 180 ppm, the catalysts afforded mostly quantitative conversions of these monomers while maintaining the size and shape of the droplets throughout the polymerization process. Furthermore, the coagulate content for all experiments stayed <2%. This represents an unprecedented efficiency in emulsion ROMP based on hydrophilic ruthenium alkylidene complexes.

The cross-metathesis of methyl oleate with cis-2-butene-1,4-diyl diacetate and the influence of protecting groups

BACKGROUND

α,ω-Difunctional substrates are useful intermediates for polymer synthesis. An attractive, sustainable and selective (but as yet unused) method in the chemical industry is the oleochemical cross-metathesis with preferably symmetric functionalised substrates. The current study explores the cross-metathesis of methyl oleate (1) with cis-2-butene-1,4-diyl diacetate (2) starting from renewable resources and quite inexpensive base chemicals.

RESULTS

This cross-metathesis reaction was carried out with several phosphine and N-heterocyclic carbene ruthenium catalysts. The reaction conditions were optimised for high conversions in combination with high cross-metathesis selectivity. The influence of protecting groups present in the substrates on the necessary catalyst loading was also investigated.

CONCLUSIONS

The value-added methyl 11-acetoxyundec-9-enoate (3) and undec-2-enyl acetate (4) are accessed with nearly quantitative oleochemical conversions and high cross-metathesis selectivity under mild reaction conditions. These two cross-metathesis products can be potentially used as functional monomers for diverse sustainable polymers.

The allylic chalcogen effect in olefin metathesis

Olefin metathesis has emerged as a powerful tool in organic synthesis. The activating effect of an allylic hydroxy group in metathesis has been known for more than 10 years, and many organic chemists have taken advantage of this positive influence for efficient synthesis of natural products. Recently, the discovery of the rate enhancement by allyl sulfides in aqueous cross-metathesis has allowed the first examples of such a reaction on proteins. This led to a new benchmark in substrate complexity for cross-metathesis and expanded the potential of olefin metathesis for other applications in chemical biology. The enhanced reactivity of allyl sulfide, along with earlier reports of a similar effect by allylic hydroxy groups, suggests that allyl chalcogens generally play an important role in modulating the rate of olefin metathesis. In this review, we discuss the effect of allylic chalcogens in olefin metathesis and highlight its most recent applications in synthetic chemistry and protein modifications.

Chemical modification of proteins at cysteine: opportunities in chemistry and biology

Chemical modification of proteins is a rapidly expanding area in chemical biology. Selective installation of biochemical probes has led to a better understanding of natural protein modification and macromolecular function. In other cases such chemical alterations have changed the protein function entirely. Additionally, tethering therapeutic cargo to proteins has proven invaluable in campaigns against disease. For controlled, selective access to such modified proteins, a unique chemical handle is required. Cysteine, with its unique reactivity, has long been used for such modifications. Cysteine has enjoyed widespread use in selective protein modification, yet new applications and even new reactions continue to emerge. This Focus Review highlights the enduring utility of cysteine in protein modification with special focus on recent innovations in chemistry and biology associated with such modifications.

PQS: a new platform for micellar catalysis. RCM reactions in water, with catalyst recycling

The first "designer" surfactant containing a covalently bound ruthenium catalyst is reported. This species dissolves freely in water, forming nanomicelles in which ring-closing metathesis reactions on water-insoluble dienic substrates can occur in pure water at room temperature. It can also be recycled continuously without reaction workup.

Bioorthogonal chemistry: fishing for selectivity in a sea of functionality

The study of biomolecules in their native environments is a challenging task because of the vast complexity of cellular systems. Technologies developed in the last few years for the selective modification of biological species in living systems have yielded new insights into cellular processes. Key to these new techniques are bioorthogonal chemical reactions, whose components must react rapidly and selectively with each other under physiological conditions in the presence of the plethora of functionality necessary to sustain life. Herein we describe the bioorthogonal chemical reactions developed to date and how they can be used to study biomolecules.

Sugars and proteins: New strategies in synthetic biology

The development of novel methodology for bond-forming processes that are compatible with biomolecules allows the assembly, alteration, or modification of proteins. Such synthetic proteins allow precise insight and investigation of function in a manner that has the potential for almost unlimited diversity.