Supported ionic liquid phase catalysis on functionalized carbon nanotubes

Highly active rhodium catalysts have been prepared by immobilization of an ionic liquid film on carbon nanotubes functionalized with imidazolium-based ionic moieties.     

Recent advances in oxidation catalysis using ionic liquids as solvents

Ionic liquids are an interesting alternative to classical solvents presenting several advantages. A variety of catalysts show good or even enhanced activities when applied in ionic liquids. Oxidation catalysis represents a large segment of industrial chemistry, providing both bulk chemicals and intermediates for high value added special products and pharmaceuticals. Particularly for the latter products organometallic and inorganic catalysts are being developed that can be applied in systems consisting of or containing ionic liquids. This work provides an overview on recent developments in this field.     

Lipase catalysis in organic solvents

The conditions for esterification and transesterification catalyzed by porcine pancreatic lipase in organic media were studied. It was found that the enzyme reaction was dependent on the following factors: the pH at which the enzyme powder was prepared from its solution, the polarity of organic media, the reaction temperature, the water content in reaction system, and the substrate structures. Effects of the above factors on enzyme activity were discussed.     

离子液体中的不对称催化反应

对价格昂贵的手性催化剂进行回收和重复利用是目前不对称催化领域面临的难题之一,受到学术界和工业界的共同关注。化学家们已经尝试了许多方法,其中使用离子液体来替代常规有机溶剂使催化剂得到分离和重新使用已经引起他们极大的兴趣。本文综述了近年来在离子液体中进行不对称催化反应的研究进展,对离子液体中过渡金属和有机小分子催化的各种反应进行重点介绍,强调了离子液体不但在催化剂回收方面有独特优势,而且在许多反应中能够提高催化效率。     

Supported ionic liquid catalysis

Supported ionic liquid catalysis is a concept which combines the advantages of ionic liquids with those of heterogeneous support materials. The viability of this concept has been confirmed by several studies which have successfully confined various ionic phases to the surface of support materials and explored their potential catalytic applications. Although the majority of the evaluated supports were silica based, several studies focused on polymeric materials including membranes. The preparation of these materials was achieved by using two different immobilization approaches. The first approach involves the covalent attachment of ionic liquids to the support surface whereas the second simply deposits the ionic liquid phases containing catalytically active species on the surface of the support. Herein recent advances made in this area are described.     

Transition metal nanoparticle catalysis in green solvents

Nanoparticle (NP) catalysis in liquid phase, which is usually called “soluble” NP catalysis, is an old topic but is now well advanced due to the great progress in nano-chemistry and nano-technology in green chemistry. After a short introduction of the history, this review describes the current status of NP catalysis in solvents and then discusses the main drawbacks hindering the particles from industrial practice. Efficiency, stability, sustainability, and recyclability (ESSR) criteria were suggested to evaluate NP catalytic systems. A state-of-the-art approach to satisfy ideal ESSR criteria is to produce cohesion over the individual contributions of metal center, stabilizer and solvent (MSS), i.e., a cohesive MSS approach. Based on reported examples, the roles that the metal core, the stabilizer and the solvent play in NP catalysis are discussed in detail. For clarity, a fairly complete list of NP catalytic systems in various green solvents reported in recent decade is provided.     

Applications of functionalized ionic liquids

Recent developments of the synthesis and applications of functionalized ionic liquids (including dual-functionalized ionic liquids) have been highlighted in this review. Ionic liquids are at-tracting attention as alternative solvents in green chemistry, but as more functionalized ILs are pre-pared, a greater number of applications in increasingly diverse fields are found.     

Nitrile functionalized methimazole-based ionic liquids

The alkylation reaction of 2-mercapto-1-methylimidazole 1b with 2-chloroacetonitrile and 2-chloropropionitrile produced S-alkyl methimazole chlorides 2a and 2b which were subjected to anion metathesis with lithium bis(trifluoromethanesulfonyl)amide, LiNTf(2), to afford nitrile functionalized methimazole-based room temperature ionic liquids 3a and 3b in 94% and 89% yields, respectively. Ionic liquids 3a and 3b have reasonably wide electrochemical windows. The efficient extraction of Ag(+) from aqueous media into 3a and 3b is also reported.     

Theoretical insights in enzyme catalysis

In this tutorial review we show how the methods and techniques of computational chemistry have been applied to the understanding of the physical basis of the rate enhancement of chemical reactions by enzymes. This is to answer the question: Why is the activation free energy in enzyme catalysed reactions smaller than the activation free energy observed in solution? Two important points of view are presented: Transition State (TS) theories and Michaelis Complex (MC) theories. After reviewing some of the most popular computational methods employed, we analyse two particular enzymatic reactions: the conversion of chorismate to prephenate catalysed by Bacillus subtilis chorismate mutase, and a methyl transfer from S-adenosylmethionine to catecholate catalysed by catechol O-methyltransferase. The results and conclusions obtained by different authors on these two systems, supporting either TS stabilisation or substrate preorganization, are presented and compared. Finally we try to give a unified view, where a preorganized enzyme active site, prepared to stabilise the TS, also favours those reactive conformations geometrically closer to the TS.     

Terminally functionalized polyisobutylene oligomers as soluble supports in catalysis

A new phase selective hydrocarbon soluble polymer support is described.     

Green solvents from ionic liquids and deep eutectic solvents to natural deep eutectic solvents

Natural deep eutectic solvents (NADESs) are defined as mixtures of certain molar ratios of natural compounds such as sugars, organic acids, amino acids, and organic bases that are abundant in organisms. The melting points of these mixtures are considerably lower than those of their individual ingredients and far below ambient temperature. The first publications on the NADES concept in 2011 created a great expectation regarding their potential as green solvents that could replace conventional organic solvents in a wide range of applications. This was largely because many of the drawbacks of conventional synthetic ionic liquids (ILs) and deep eutectic solvents (DESs), particularly their toxicity and environmental hazards, could be solved using NADESs. Throughout the last 7 years, the interest in NADESs has increased enormously as reflected by the exponential growth of the number of related publications. The research on NADESs has rapidly expanded particularly into the evaluation of the feasibility of their application in diverse fields such as the extraction of (targeted) bioactive compounds from natural sources, as media for enzymatic or chemical reactions, preservatives of labile compounds, or as vehicles of non–water-soluble compounds for pharmaceutical purposes. Along with the exploration of these potential applications, there have been a large number of other studies related to their physicochemical features, the search for new NADESs, the research into the interactions between NADES components or with solutes, the recovery of solutes from NADES solutions, and the ways of circumventing inherent problems of NADESs such as their high viscosity and the consequent difficulties in handling them. This article contains a review of the applications of NADESs as extraction solvents, reaction media, and preservative, providing also a perspective of their future.     

Surface characterization of functionalized imidazolium-based ionic liquids

The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied by means of X-ray photoelectron spectroscopy (XPS). For [Me(EG)MIM][Tf 2N], [Et(EG) 2MIM][Tf 2N], and [Me(EG) 3MIM][Tf 2N], which vary by the number of ethylene glycol (EG) units (from 1 to 3), we have shown that the stoichiometry of the surface near region is in excellent agreement with the bulk stoichiometry, which confirms the high purity of the ionic liquid samples investigated and rules out pronounced surface orientation effects. This has been deduced from the experimental observation that the angle-resolved XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surfaces sensitive geometry, i.e., at 0 degrees and 70 degrees emission angle, respectively. The relative intensity ratios of all elements were found to be in nearly perfect agreement with the nominal values for the individual ILs. In contrast to these findings, we identified surface-active impurities in [Me(EG)MIM]I, which is the starting material for the final anion exchange step to synthesize [Me(EG)MIM][Tf 2N]. Sputtering of the surface led to a depletion of this layer, which however recovered with time. The buildup of this contamination is attributed to a surface enrichment of a minor bulk contamination that shows surface activity in the iodide melt.     

Conductive polypyrrole via enzyme catalysis

Laccase catalyzes the polymerization of pyrrole into a conducting polymer using dioxygen as the terminal oxidant. This finding is significant, because it identifies an enzymatic route, and thus an environmentally benign method, for preparing a technologically important polymer. In addition, the rate of oxidation of pyrrole increases when the redox molecule, ABTS [2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonate)], is included in a reaction medium that contains laccase. This increase in rate occurs because laccase catalyzes the oxidation of ABTS to ABTS*. In addition to laccase, the biocatalytically generated ABTS* oxidizes pyrrole to its corresponding radical cation to yield polypyrrole. Moreover, oxidation of pyrrole by ABTS* regenerates ABTS for subsequent biocatalytic turnover. Including ABTS in the reaction medium has two important consequences for the final product: (a) The reaction proceeds rapidly enough to form polymeric films instead of oligomeric precipitates, and (b) ABTS remains within the polymeric film as a redox-active dopant. The charge transport properties of the resulting polymers, both with and without ABTS as the counteranion, are compared to those of other conducting materials including polypyrrole prepared electrochemically or chemically.     

Electrophilic catalysis in nucleophilic substitution reactions in ionic liquids

2-Chlorotropone was obtained from 2-tosyloxytropone in 88% yield in the recyclable ionic medium BMIMBF₄/LiCl. That Li⁺ acts as a Lewis acid was proven by the lack of reactivity of 2-tosyloxytropone, under the above conditions, on replacing LiCl with NaCl or BMIMCl, or using BMIMCl alone, or a BMIMBF₄/MeCN/KCl mixture. 2-Bromo- and 2-iodotropone were obtained along similar lines from LiBr or LiI, respectively, whereas LiF proved unreactive.     

Synthesis of amine functionalized oxazolines with applications in asymmetric catalysis

This paper describes the synthesis of three classes of amine functionalized oxazolines that have been successfully used in asymmetric catalysis in our laboratory. Failed synthetic routes and significantly improved procedures are discussed including the synthesis of ligands for Nozaki-Hiyama-Kishi (NHK) carbonyl allylation reactions that do not require chromatography for purification.     

Preorganization and protein dynamics in enzyme catalysis

Recently, an alternative has been offered to the concept of transition state (TS) stabilization as an explanation for rate enhancements in enzyme-catalyzed reactions. Instead, most of the rate increase has been ascribed to preorganization of the enzyme active site to bind substrates in a geometry close to that of the TS, which then transit the activation barrier impelled by motions along the reaction coordinate. The question as to how an enzyme achieves such preorganization and concomitant TS stabilization as well as potential coupled motions along the reaction coordinate leads directly to the role of protein dynamic motion. Dihydrofolate reductase (DHFR) is a paradigm in which the role of dynamics in catalysis continues to be unraveled by a wealth of kinetic, structural, and computational studies. DHFR has flexible loop regions adjacent to the active site whose motions modulate passage through the kinetically preferred pathway. The participation of residues distant from the DHFR active site in enhancing the rate of hydride transfer, however, is unanticipated and may signify the importance of long range protein motions. The general significance of protein dynamics in understanding other biological processes is briefly discussed.     

Entrapping enzyme in a functionalized nanoporous support

The enzyme organophosphorus hydrolase (OPH) was spontaneously entrapped in carboxylethyl- or aminopropyl-functionalized mesoporous silica with rigid, uniform open-pore geometry (30 nm). This approach yielded larger amounts of protein loading and much higher specific activity of the enzyme when compared to the unfunctionalized mesoporous silica and normal porous silica with the same pore size. When OPH was incubated with the functionalized mesoporous silica, protein molecules were sequestered in or excluded from the porous material, depending on electrostatic interaction with the charged functional groups. OPH entrapped in the organically functionalized nanopores showed an exceptional high immobilization efficiency of more than 200% and enhanced stability far exceeding that of the free enzyme in solution. The combination of high protein loading, high immobilization efficiency and stability is attributed to the large and uniform pore structure, and to the optimum environment introduced by the functional groups.     

Rapid proton diffusion in hydroxyl functionalized imidazolium ionic liquids

There is considerable interest in using ionic liquids(ILs) as protic electrolytes. However, the reported proton transfer rate in ILs is quite slow. In this study, we report functionalizing imidazolium ILs with alcohol hydroxyls, aiming at constructing hydrogen bonding networks in the electrolyte, can stimulate fast proton hopping transfer. For demonstration, the diffusion of proton and Cl. in 1-(3-hydroxypropyl)-3-methylimidazolium tetrafluoroboride(C_3OHmimBF_4) were studied using cyclic voltammetry and potentiostatic method at 30 °C. The diffusion coefficient of proton is about one order of magnitude higher than that of Cl. in the same electrolyte, and about 5 times that of proton in the non-hydydroxyl 1-(butyl)-3-methylimidazolium tetrafluoroboride(BmimBF_4) when normalized to the diffusion coefficients of Cl. in respective ILs. In the meantime, 1H NMR spectra revealed a strong hydrogen bonding interaction between proton and C_3OHmimBF_4 which is absent between proton and BmimBF_4, thus the significantly higher diffusion coefficient of proton in C_3OHmimBF_4 may suggest the formation of effective hydrogen bonding networks, enabling rapid proton hopping via the Grotthuss mechanism.