Hydroxy-α-amino acids modified by ionic liquid moieties: recoverable organocatalysts for asymmetric aldol reactions in the presence of water

Evaluation of new L-amino acids triethanolammonium salts usability for controlling protease activity

Twenty new triethanolammonium amino acid salts (TEA AA) have been prepared from triethanolammonium hydroxide and L-amino acids. The physicochemical properties of TEA AA depended on the applied amino acid. Five of the synthesised salts, i.e. mono- and bis-salts of L-glutamic acid, L-aspartic acid, and TEA salt of l-glutamine were solids with melting points between 127.32 °C to 171.51 °C. The other TEA AA exhibited glass transition temperatures from -68.45 °C for TEA Ser to -6.27 °C for TEA Trp and were assigned as amino acid ionic liquids (AAILs). The TEA His was characterised by the highest thermal stability, with an average temperature of 5 % weight loss at 186.4 °C, whereas the lowest stability was determined for TEA Asp (107.5 °C). The developed salts were tested as reaction medium additives for proteolytic enzymes (papain, subtilisin, bromelain). Most AAILs showed an inhibitory effect on tested proteases but with different mechanisms related to the enzyme substrate specificity and structural diversity. The TEA Ser was the most effective competitive inhibitor (K_i = 0.24 10^-4 mol/L) for bromelain, while TEA Val uncompetitive inhibitor for papain (K_i = 0.25 10^-4 mol/L). The developed TEA AA salts exhibit potential as enzyme-controlling agents for use in industrial processes.

Chiral Ionic Liquids Based on l-Cysteine Derivatives for Asymmetric Aldol Reaction

Structure, and consequently properties, of ionic liquids can be easily tailored by changing cation/anion combinations and/or attaching functional groups. By grafting enantiopure moieties to the framework of ionic liquid it is possible to prepare bioinspired chiral molecules that can serve as a reaction medium, additive or even asymmetric catalyst. In this context, new chiral ionic liquids (CILs), based on biomolecules, such as aminoacids (l-Cysteine derivatives), have been synthesised and tested in asymmetric aldol condensation of aldehydes and ketones. The best results were obtained for CILs composed of S-methyl-l-cysteine cation and bis(trifluoromethane)sulfonimide anion, in the reaction of 2- or 4-nitrobenzaldehyde with acetone or cyclohexanone, giving the aldol product in moderate yields 70–76% and high ee values (up to 96%).

Chiral Imidazolium Prolinate Salts as Efficient Synzymatic Organocatalysts for the Asymmetric Aldol Reaction

Chiral imidazolium l-prolinate salts, providing a complex network of supramolecular interaction in a chiral environment, have been studied as synzymatic catalytic systems. They are demonstrated to be green and efficient chiral organocatalysts for direct asymmetric aldol reactions at room temperature. The corresponding aldol products were obtained with moderate to good enantioselectivities. The influence of the presence of chirality in both the imidazolium cation and the prolinate anion on the transfer of chirality from the organocatalyst to the aldol product has been studied. Moreover, interesting match/mismatch situations have been observed regarding configuration of chirality of the two components through the analysis of results for organocatalysts derived from both enantiomers of prolinate (R/S) and the trans/cis isomers for the chiral fragment of the cation. This is associated with differences in the corresponding reaction rates but also to the different tendencies for the formation of aggregates, as evidenced by nonlinear effects studies (NLE). Excellent activities, selectivities, and enantioselectivities could be achieved by an appropriate selection of the structural elements at the cation and anion.

Recent advances in reactions promoted by amino acids and oligopeptides

During the last 20 years, Organocatalysis has become one of the major fields of Catalysis. Herein, we provide a recent overview on reactions where the use of amino acids and peptides as the organocatalysts was employed. All aspects regarding aldol reactions, Michael reactions, epoxidation, Henry reactions and many others that are crucial for the reaction conditions and reaction mechanisms are discussed.

The Relationship between the Structure and Properties of Amino Acid Ionic Liquids

Ionic liquids based on different l-amino acids (glycine, l-valine, l-leucine, l-isoleucine, l-histidine, l-methionine, l-tyrosine, l-tryptophan, l-arginine, and l-threonine) and different cations (tetrabutylammonium (TBA), tributylmethylammonium (tBMA), didecyldimethylammonium (DDA), (2-hydroxyethyl)trimethylammonium (choline) (Chol), alkyl(C₁₂-C₁₄) dimethylbenzylammonium (benzalkonium) (BA), dodecyltrimethylammonium (DDTMA), hexadecyltrimethylammonium (HDTMA), octadecyltrimethylammonium (ODTMA) and 1-ethyl-3-methylimidazolium (EMIM)) have been synthesized and characterized by NMR and FTIR. Viscosity, specific rotation, surface activity, thermal stability (TG), and phase transformations (DSC) have been determined and compared with available data. Furthermore, benzalkonium, didecyldimethylammonium, dodecyltrimethylammonium, hexadecyltrimethylammonium, and octadecyltrimethylammonium amino acid ionic liquids have been shown to exhibit surface activity. The dissolution of cellulose in amino acid ionic liquids (AAILs) composed of various cations was also investigated. Cellulose was only dissolved in EMIM salts of amino acids. In particular, the influence of the cation type on selected physicochemical and spectroscopic properties were discussed. The article is a mini review on amino acid ionic liquids.

Organocatalysts Derived from Unnatural α-Amino Acids: Scope and Applications

The organocatalytic properties of unnatural α-amino acids are reviewed. Post-translational derivatives of natural α-amino acids include 4-hydroxy-l-proline and 4-amino-l-proline scaffolds, and also proline homologues. The activity of synthetic unnatural α-amino acid-based organocatalysts, such as β-alkyl alanines, alanine-based phosphines, and tert-leucine derivatives, are reviewed herein. The organocatalytic properties of unnatural monocyclic, bicyclic, and tricyclic proline derivatives are also reviewed. Several families of these organocatalysts permit the efficient and stereoselective synthesis of complex natural products. Most of the reviewed organocatalysts accelerate the reported reactions through covalent interactions that raise the HOMO (enamine intermediates) or lower the LUMO (iminium intermediates).

Polymer ionic liquid network: a highly effective reusable catalyst for one-pot synthesis of heterocyclic compounds

Significant efforts have been devoted to developing immobilized chiral catalysts with high activity, selectivity, and stability. In this present study, a new heterogeneous proline catalyst system was prepared based on strong noncovalent interactions between polymer ionic liquid (PIL) and l-proline. First, pyridine PILs, which can complex with l-proline monomers through noncovalent interactions, were synthesized using reversible addition–fragmentation chain transfer (RAFT) polymerization. The polymer network-supported chiral catalysts were obtained following further free radical polymerization. Different structures were formed in response to different ratios of PIL and chiral monomer, as well as different PIL anions, in the reactions. The new formed layer structures and synergic effects of PIL resulted in heterogeneous catalysts with high catalytic activity and enantioselectivity, thus endowing them with better catalytic performance for the one-pot synthesis of heterocyclic compounds compared to homogeneous catalytic systems. These catalytic systems were able to be reused and recycled five times with no discernible loss in catalytic activity and enantioselectivity. l-Proline was efficiently loaded onto the polymer network simply based on supramolecular interactions, providing a novel method of synthesizing high performance supported catalysts for organic reactions.

Significant efforts have been devoted to developing immobilized chiral catalysts with high activity, selectivity, and stability.

Synthesis of novel chiral imidazolium stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two novel chiral stationary phases (CSPs) were prepared by bonding chiral imidazoliums on the surface of silica gel. The chiral imidazoles were derivatized from chiral amines, 1-phenylethylamine and 1-(1-naphthyl)ethylamine. The obtained CSPs were characterized by Fourier Transform Infrared (FT-IR) spectroscopy and elemental analysis (EA), demonstrating the bonding densities of CSP 1 and CSP 2 were 0.43 mmol g^-1 and 0.40 mmol g^-1, respectively. These two CSPs could be used to availably separate 8 pharmaceuticals, 7 mandelic acid/its derivatives, 2 1-phenylethylamine derivatives, 1 1,1'-bi-2-naphthol, and 1 camphorsulfonic acid in high-performance liquid chromatography (HPLC). It is found that CSP 1 could effectively enantioseparate most chiral analytes, especially the acidic components, while CSP 2 could enantiorecognize all chiral analytes, although a number of components did not achieve baseline separation. Additionally, the effects of mobile phase composition, mobile phase pH and salt content, chiral selector structures, and analyte structures on the enantiorecognitions of the two CSPs were investigated. It is found that high acetonitrile content in mobile phases was conducive to enantiorecognition. Mobile phase pH and salt content could alter the retention behaviors of different enantiomers of the same chiral compound, resulting in better enantioresolution. Moreover, both chiral selector structures and substituted groups of analytes played a significant role in the separation of chiral solutes.

Temperature-responsive hairy particle-supported proline for direct asymmetric aldol reaction in water

In this paper, three kinds of hairy particles with different brush structures were prepared and evaluated as chiral catalysts in the direct asymmetric aldol reaction.

A Snapshot of Ionic-Liquid-Tagged Proline-Based Organocatalysts

This highlight focuses on the developments in ionic-liquid (IL)-tagged proline-based organocatalysts. An overview of catalyst structure and application to asymmetric transformations is provided, and a representative synthesis of an IL-tagged organocatalyst is also discussed.

Ionic-liquid-supported 1,5,7-triazabicyclo[4.4.0]dec-5-ene — An efficient and recyclable organocatalyst for Michael addition to α,β-unsaturated ketones

A novel ionic-liquid-supported 1,5,7-triazabicyclo[4.4.0]dec-5-ene (IL–TBD) was synthesized and investigated for its ability to act as an active organocatalyst in the Michael addition of active methylene compounds and thiophenols to chalcones under solvent-free conditions. The IL–TBD afforded Michael addition products in excellent yields (82%–94%) at room temperature, and it was simply recycled and reused at least five times without significant loss of catalytic activity.

Chiral ionic liquids: a compendium of syntheses and applications (2005-2012)

In recent years, the field of chiral ionic liquids (CILs) has undergone exponential growth. As the technology has advanced, new ways of synthesizing stable and structurally diverse ionic liquids have been established. This has led to heretofore unknown applications of CILs as well as in improving efficiency of previously identified applications. In this review article we have compiled a comprehensive database containing structures and physical properties of notable CILs that have been synthesized during the last 6 years. Their applications in the fields of asymmetric organic synthesis, spectroscopy, and chromatography are also illustrated. This is an expansion of our previous review, which covered the literature before 2005.

Highly diastereo- and enantioselective direct Barbas-List aldol reactions promoted by novel benzamidoethyl and benzamidopropyl prolinamides in water

Four novel benzamido-functionalized prolinamides have been prepared and tested as organocatalysts for enantioselective aldol reaction of aldehydes and cyclic ketones in water. In particular, prolinamide derived from achiral ethylene diamine was the best catalyst leading to anti aldols in excellent diastereomeric (up to 98/2) and enantiomeric (up to 99/1) ratios, thereby showing that lateral amide functionalities might be a key issue for facilitating "in water" chemistry. These catalysts are cheaper and easier to prepare than those previously described.