Olefin metathesis in supercritical carbon dioxide

Unsaturated Macrolactones from Renewable Feedstocks: Synthesis, Ring-Opening Polymerization and Application Prospects

Unsaturated macrolactones (UMs) have long attracted researchers' attention due to a combination of a reactive ester fragment and C=C bond in their structures. UMs of natural origin are comparatively few in number, and the task of developing synthetic approaches to new UMs is relevant. Recent advances in the synthesis of UMs cannot be dissociated from the progress in design of metathesis catalysts, since this catalytic approach is an atom-economy alternative to conventional organochemical methods. In the present review, we summarized and discussed the use of ring-closing metathesis, catalyzed by Ru and Group 6 metal complexes, in the synthesis of Ums and the advantages and shortcomings of the catalytic approach to UMs in comparison with organochemical methods. In a separate section, the use of UMs in the synthesis of unsaturated polyesters, the functionalization of these (co)polymers, and the prospects for practical use of the material obtained are also presented. It is essential that the actual approaches to UMs are often based on the use of renewable feedstocks, thereby meeting Green Chemistry principles.

Sustainable C-H Methylation Employing Dimethyl Carbonate

The use of CO2 and CO2-derived chemicals offers society sustainable and biocompatible chemistry for a variety of applications, ranging from materials to medicines. In this context, dimethyl carbonate (DMC) stands out owing to its low toxicity, high biodegradability, tunable reactivity, and sustainable production. Further, the ability of DMC to act as an ambient electrophile at varied temperatures and reaction conditions in order to produce methoxycarbonylated (via BAC2) and methylated products (via BAL2) is very promising. While the methylation of hetero-H (N-, O-, and S-methylation) with DMC is established and well-reviewed, the C-H methylation reaction with DMC is limited, and there is no specific literature detailing the C-methylation reaction using DMC, creating new opportunities as well as challenges in the same domain. In this context, the present perspective focuses on the new breakthroughs, recent advances, and trends in C-H methylation reactions employing DMC. A critical analysis of the mechanistic course of reactions under each category was undertaken. We believe this timely perspective will offer an in-depth analysis of existing literature with critical remarks, which will certainly inspire fellow researchers across disciplines to understand and pursue cutting-edge research in the area of C-H methylation (alkylation) using DMC and related organic carbonates.

Assessing hydrophobic deep eutectic solvents for intramolecular excimer formation

Intramolecular excimer formation by a dipyrenyl probe, 6-(1-pyrenyl)hexyl-11(1-pyrenyl)-undecanoate [1-Py(CH2)10COO(CH2)61-Py], is used to assess hydrophobic deep eutectic solvents (HDESs) for the purpose. n-Decanoic acid (DA), L(-)-menthol (Men) and thymol (Thy) have been utilized to form HDESs with different pairs of constituents in different molar ratios, namely Men : DA (2 : 1, 1 : 1, and 1 : 2), Thy : DA (2 : 1, 1 : 1, and 1 : 2), and Thy : Men (5 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 5). The maximum of the excimer-to-monomer emission intensity ratio, (IE/IM)max, is observed at 343.15-353.15 K for all DESs irrespective of the constitution, and it varies in a narrow range exhibiting no correlation with the dynamic viscosity (η) of the DES which varies between 2.05 and 3.56 mPa s. Excited-state intensity decay data reveal excimer dissociation back to the excited monomer to be negligible in all DESs at lower temperatures (T ≤ 323.15 K); the simplistic Birks scheme is followed at higher temperatures (T > 323.15 K). The rate constant for excimer formation/association, ka, ranges from (3.00 ± 0.50) × 106 s-1 to (103 ± 10) × 106 s-1, which is similar to that reported in other media. The temperature-dependence of the equilibrium constant for excimer formation follows the van't Hoff equation with recovered standard enthalpy (ΔaH*⊖) and standard entropy (ΔaS*⊖) changes, indicating the reversible intramolecular excimer formation to be exothermic and energetically-favorable but entropically unfavorable. A plot of kavs. T/η for all the DES systems investigated exhibits a fairly good linear correlation, indicating the adherence to the Stokes-Einstein formulation within the HDESs further emphasizing the homogeneous nature of the solubilizing media. The work helps to highlight the potential of HDESs for intramolecular excimer formation involving non-polar reactants.

Ring-Opening Metathesis Polymerization and Related Olefin Metathesis Reactions in Benzotrifluoride as an Environmentally Advantageous Medium

A tremendous number of solvents, either as liquids or vapors, contaminate the environment on a daily basis worldwide. Olefin metathesis, which has been widely used as high-yielding protocols for ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), and isomerization reactions, is typically performed in toxic and volatile solvents such as dichloromethane. In this study, the results of our systematic experiments with the Grubbs G1, G2, and Hoveyda-Grubbs HG2 catalysts proved that benzotrifluoride (BTF) can replace dichloromethane (DCM) in these reactions, providing high yields and similar or even higher reaction rates in certain cases. The ROMP of norbornene resulted not only in high yields but also in polynorbornenes with a high molecular weight at low catalyst loadings. Ring-closing metathesis (RCM) experiments proved that, with the exception of the G1 catalyst, RCM occurs with similar high efficiencies in BTF as in DCM. It was found that isomerization of (Z)-but-2-ene-1,4-diyl diacetate with the G2 and HG2 catalysts proceeds at significantly higher initial rates in BTF than in DCM, leading to rapid isomerization with high yields in a short time. Overall, BTF is a suitable solvent for olefin metathesis, such as polymer syntheses by ROMP and the ring-closing and isomerization reactions.

Supercritical Fluid Application in the Oil and Gas Industry: A Comprehensive Review

The unique properties of supercritical fluid technology have found wide application in various industry sectors. Supercritical fluids allow for the obtainment of new types of products with special characteristics, or development and design of technological processes that are cost-effective and friendly to the environment. One of the promising areas where supercritical fluids, especially carbon dioxide, can be used is the oil industry. In this regard, the present review article summarizes the results of theoretical and experimental studies of the use of supercritical fluids in the oil and gas industry for supercritical extraction in the course of oil refining, increasing oil recovery in the production of heavy oil, hydraulic fracturing, as well as processing and disposal of oil sludge and asphaltenes. At the end of the present review, the issue of the impact of supercritical fluid on the corrosion of oil and gas equipment is considered. It is found that supercritical fluid technologies are very promising for the oil industry, but supercritical fluids also have disadvantages, such as expansion or incompatibility with materials (for example, rubber).

Mechanochemical ring-opening metathesis polymerization: development, scope, and mechano-exclusive polymer synthesis

Ruthenium-alkylidene initiated ring-opening metathesis polymerization has been realized under solid-state conditions by employing a mechanochemical ball milling method. This method promotes greenness and broadens the scope to include mechano-exclusive products. The carbene- and pyridine-based Grubbs 3^rd-generation complex outperformed other catalysts and maintained similar mechanistic features of solution-phase reactions. High-speed ball milling provides sufficient mixing and energy to the solid reaction mixture, which is composed of an initiator and monomers, to minimize or eliminate the use of solvents. Therefore, the solubility and miscibility of monomers and Ru-initiators are not limiting factors in solid-state ball milling. A wide variety of solid monomers, including ionomers, fluorous monomers, and macromonomers, were successfully polymerized under ball milling conditions. Importantly, direct copolymerization of immiscible (ionic/hydrophobic) monomers exemplifies the synthesis of mechano-exclusive polymers that are difficult to make using traditional solution procedures. Finally, the addition of a small amount of a liquid additive (i.e., liquid-assisted grinding) minimized chain-degradation, enabling high-molecular-weight polymer synthesis.

Mechanochemical ball-milling ring-opening metathesis polymerization minimized solvent use and produced previously inaccessible polymers in solution.

Amine-Directed Mizoroki-Heck Arylation of Free Allylamines

The transition metal-catalyzed Mizoroki−Heck reaction is a powerful method to synthesize C–C bonds, allowing access to several important pharmaceuticals. Traditionally free amines have not been compatible with these approaches due...

Palladium-Catalyzed Regioselective Arylation of Unprotected Allylamines

Palladium-catalyzed organometallic transformations of free amines are often unsuccessful due to side reactions, such as oxidation, that can occur. However, the ability to furnish the free amine products from these reactions is important for improving the utility and sustainability of these processes, especially for accessing their potential as medicinal and agrochemical agents. Notably, the 3,3-diarylallylamine motif is prevalent in a variety of biologically relevant structures, yet there are few catalytic approaches to their synthesis, and none involving the free amine. Herein, we describe a simple protocol for the arylation of cinnamylamines and the diarylation of terminal allylamines to generate a diverse group of 3,3-diarylallylamine products using a PdII precatalyst. Key features of the method are the ability to access relatively mild conditions that facilitate a broad substrate scope as well as direct diarylation of terminal allylamine substrates. In addition, several complex and therapeutically relevant molecules are included to demonstrate the utility of the transformation.

4-Methyltetrahydropyran as a Convenient Alternative Solvent for Olefin Metathesis Reaction: Model Studies and Medicinal Chemistry Applications

A number of metathesis reactions were successfully conducted in 4-methyltetrahydropyran, including both standard model dienes, as well as more complex substrates, such as analogues of biologically active compounds and active pharmaceutical ingredients. To place this solvent in a context of pharmaceutical R + D, larger-scale syntheses of SUAM 1221, a prolyl endopeptidase inhibitor with potential application in Alzheimer disease treatment, and a derivative of sildenafil, an analogue of the popular Viagra drug, were executed. In the latter case, despite all the setup being made in air, the metathesis reaction at a 33 g scale proceeded very well with relatively low catalyst loading and without need of aqueous workup or column chromatography.

Synthesis, characterization, biological determination and catalytic evaluation of ruthenium(ii) complexes bearing benzimidazole-based NHC ligands in transfer hydrogenation catalysis

A new series of ruthenium (ii) N-heterocyclic carbene complexes has been synthesized via transmetalation. The obtained complexes were applied to transfer hydrogenation of ketone derivatives.

Ethyl Lactate: A Green Solvent for Olefin Metathesis

Compatibility of selected, commercially available ruthenium olefin metathesis catalysts with ethyl lactate as solvent was evaluated using a range of substrates and conditions. In addition, the preparation of a metathesis catalyst in simplified manner by using the advantages of ethyl lactate was accomplished. The application of ethyl lactate facilitates product isolation (also allowing for lower ruthenium contamination in crude metathesis products) and improves the overall green angle of olefin metathesis.

Ru(ii)–N-heterocyclic carbene complexes: synthesis, characterization, transfer hydrogenation reactions and biological determination

A series of ruthenium(ii) complexes with N-heterocyclic carbene ligands were successfully synthesized by transmetalation reactions between silver(i) N-heterocyclic carbene complexes and [RuCl₂(p-cymene)]₂ in dichloromethane under Ar conditions. All new compounds were characterized by spectroscopic and analytical methods. These ruthenium(ii)–NHC complexes were found to be efficient precatalysts for the transfer hydrogenation of ketones by using 2-propanol as the hydrogen source in the presence of KOH as a co-catalyst. The antibacterial activity of ruthenium N-heterocyclic carbene complexes 3a–f was measured by disc diffusion method against Gram positive and Gram-negative bacteria. Compounds 3d exhibited potential antibacterial activity against five bacterial species among the six used as indicator cells. The product 3e inhibits the growth of all the six tested microorganisms. Moreover, the antioxidant activity determination of these complexes 3a–f, using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azinobis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS) as reagent, showed that compounds 3b and 3d possess DPPH and ABTS antiradical activities. From a concentration of 1 mg ml⁻¹, these two complexes presented a similar scavenging activity to that of the two used controls gallic acid (GA) and butylated hydroxytoluene (BHT). From a concentration of 10 mg ml⁻¹, the percentage inhibition of complexes 3b and 3d was respectively 70% and 90%. In addition, these two Ru–NHC complexes exhibited antifungal activity against Candida albicans. Investigation of the anti-acetylcholinesterase activity of the studied complexes showed that compounds 3a, 3b, 3d and 3e exhibited good activity at 100 μg ml⁻¹ and product 3d is the most active. In a cytotoxicity study the complexes 3 were evaluated against two human cancer cell lines MDA-MB-231 and MCF-7. Both 3d and 3e complexes were found to be active against the tested cell lines showing comparable activity with examples in the literature.

A series of ruthenium(ii) complexes with N-heterocyclic carbene ligands were successfully synthesized by transmetalation reactions between silver(i) N-heterocyclic carbene complexes and [RuCl₂(p-cymene)]₂ in dichloromethane under Ar conditions.

'Green' synthesis of metals and their oxide nanoparticles: applications for environmental remediation

In materials science, “green” synthesis has gained extensive attention as a reliable, sustainable, and eco-friendly protocol for synthesizing a wide range of materials/nanomaterials including metal/metal oxides nanomaterials, hybrid materials, and bioinspired materials. As such, green synthesis is regarded as an important tool to reduce the destructive effects associated with the traditional methods of synthesis for nanoparticles commonly utilized in laboratory and industry. In this review, we summarized the fundamental processes and mechanisms of “green” synthesis approaches, especially for metal and metal oxide [e.g., gold (Au), silver (Ag), copper oxide (CuO), and zinc oxide (ZnO)] nanoparticles using natural extracts. Importantly, we explored the role of biological components, essential phytochemicals (e.g., flavonoids, alkaloids, terpenoids, amides, and aldehydes) as reducing agents and solvent systems. The stability/toxicity of nanoparticles and the associated surface engineering techniques for achieving biocompatibility are also discussed. Finally, we covered applications of such synthesized products to environmental remediation in terms of antimicrobial activity, catalytic activity, removal of pollutants dyes, and heavy metal ion sensing.

Microwave-ultrasound simultaneous irradiation: a hybrid technology applied to ring closing metathesis

Glycerol micelles were suitable nanoreactors for ring-closing metathesis reaction activated by a hybrid microwave-ultrasound reactor. Nine different Ru-catalysts were investigated.

Selective free radical reactions using supercritical carbon dioxide

We report herein a means to modify the reactivity of alkenes, and particularly to modify their selectivity toward reactions with nonpolar reactants (e.g., nonpolar free radicals) in supercritical carbon dioxide near the critical point. Rate constants for free radical addition of the light hydrogen isotope muonium to ethylene, vinylidene fluoride, and vinylidene chloride in supercritical carbon dioxide are compared over a range of pressures and temperatures. Near carbon dioxide's critical point, the addition to ethylene exhibits critical speeding up, while the halogenated analogues display critical slowing. This suggests that supercritical carbon dioxide as a solvent may be used to tune alkene chemistry in near-critical conditions.

The effects of CO2 pressure and pH on the Suzuki coupling of basic nitrogen containing substrates

The Suzuki coupling reaction of basic nitrogen containing substrates (2-bromo- and 2-chloro-4-aminopyridine, and 2-bromo and 2-chloropyridine) with phenylboronic acid using Pd(TPP)₂Cl₂/K₃PO₄ in acetonitrile–water biphasic solvent systems under a CO₂ or a N₂ atmosphere is discussed.

Isotopic probes for ruthenium-catalyzed olefin metathesis

¹³C-labelled Grubbs catalysts, RuCl₂(L)(PCy₃)(¹³CHR) (R = H, Ph), pinpoint the fate of the methylidene (benzylidene) moiety during metathesis and deactivation.

Synthesis of tetrasubstituted alkenes via metathesis

Fully substituted olefin generation via metathesis is presented. Catalyst development, optimization of reaction conditions and substrate screening are included. In addition, asymmetric alkene metathesis, the cross metathesis reaction for this transformation and its application in natural products will be discussed.

Highly fluorous complexes of nickel, palladium and platinum: solubility and catalysis in high pressure CO2

A variety of Group 10 metal complexes [MXY(dfppp)], M = Ni, X, Y = Cl, Br, M = Pd, Pt, X, Y = Cl or CH(3), containing the recently reported highly fluorous diphosphine ligand, dfppp, 1,3-bis[di(fluoroponytail)phosphino]propane, {(p-F(13)C(6)C(6)H(4))(2)P}(2)(CH(2))(3) have been synthesised. They have been characterised by NMR, mass spectrometry and microanalysis, with two platinum complexes, [PtCl(2)(dfppp)] and [PtClMe(dfppp)], structurally characterised by single crystal X-ray diffraction studies. The highly fluorous nature of the ligands affords the complexes good supercritical CO(2) solubility as measured by supercritical fluid extraction (SFE), and has allowed for the copolymerisation of CO and ethylene using [PdClMe(dfppp)] as the catalyst precursor and CO(2) as the solvent. Additionally, PtCl(2) complexes of the new ligands dfppb, {(p-F(13)C(6)C(6)H(4))(2)P}(2)(CH(2))(4), and dfpop, {(p-F(13)C(6)C(6)H(4)O)(2)P}(2)(CH(2))(3), have also been prepared and characterised.

Organic aqueous tunable solvents (OATS): a vehicle for coupling reactions and separations

In laboratory-based chemical synthesis, the choice of the solvent and the means of product purification are rarely determined by cost or environmental impact considerations. When a reaction is scaled up for industrial applications, however, these choices are critical: the separation of product from the solvent, starting materials, and byproduct usually constitutes 60-80% of the overall cost of a process. In response, researchers have developed solvents and solvent-handling methods to optimize both the reaction and the subsequent separation steps on the manufacturing scale. These include "switchable" solvents, which are designed so that their physical properties can be changed abruptly, as well as "tunable" solvents, wherein the solvent's properties change continuously through the application of an external stimulus. In this Account, we describe the organic aqueous tunable solvent (OATS) system, examining two instructive and successful areas of application of OATS as well as its clear potential for further refinement. OATS systems address the limitations of biphasic processes by optimizing reactions and separations simultaneously. The reaction is performed homogeneously in a miscible aqueous-organic solvent mixture, such as water-tetrahydrofuran (THF). The efficient product separation is conducted heterogeneously by the simple addition of modest pressures of CO(2) (50-60 bar) to the system. Under these conditions, the water-THF phase splits into two relatively immiscible phases: the organic THF phase contains the hydrophobic product, and the aqueous phase contains the hydrophilic catalyst. We take advantage of the unique properties of OATS to develop environmentally benign and cost-competitive processes relevant in industrial applications. Specifically, we describe the use of OATS for optimizing the reaction, separation, design, and recycling of (i) Rh-catalyzed hydroformylation of olefins such as 1-octene and (ii) enzyme-catalyzed hydrolysis of 2-phenylacetate. We discuss the advantages of these OATS systems over more traditional processes. We also consider future directions that can be taken with these proven systems as well as related innovations that have recently been reported, including the use of poly(ethylene glycol) (PEG) as a tunable adjunct in the solvent and the substitution of propane for CO(2) as the external stimulus. OATS systems in fact represent the ultimate goal for a sustainable process, because in an idealized setup there is only reactant coming in and product going out; in principle, there is no waste stream.