Synthesis and characterization of novel chiral ionic liquids and investigation of their enantiomeric recognition properties

We report the synthesis and characterization of amino acid ester based chiral ionic liquids, derived from L- and D-alanine tert butyl ester chloride. The synthesis was accomplished via an anion metathesis reaction between commercially available L- and D-alanine tert butyl ester chloride using a variety of counterions such as lithium bis (trifluoromethane) sulfonimide, silver nitrate, silver lactate, and silver tetrafluoroborate. Both enantiomeric forms were obtained as confirmed by bands of opposite sign in the circular dichroism spectra. The L- and D-alanine tert butyl ester bis (trifluoromethane) sulfonimide were obtained as liquids at room temperature and intriguingly exhibited the highest thermal stability (up to 263 degrees C). In addition, the ionic liquids demonstrated enantiomeric recognition ability as evidenced by splitting of racemic Mosher's sodium salt signal using a liquid state (19)F nuclear magnetic resonance (NMR) and fluorescence spectroscopy. The L- and D-alanine tert butyl ester chloride resulted in solid salts with nitrate, lactate, and tetrafluoroborate anions. This illustrates the previously observed tunability of ionic liquid synthesis, resulting in ionic liquids of varying properties as a function of varying the anion.     

New chiral imidazolium ionic liquids from isomannide

New chiral bis and mono-imidazolium ionic liquids derived from isomannide were synthesized. The structural features of the chiral organic cations impart a special arrangement of the chiral cavity. The new chiral chloride salts of isomannide derivatives are pivotal compounds for the synthesis of different organic ionic liquids. After metathesis different anions were associated to the chiral cations providing a new class of chiral ionic liquids.     

Exploring the thermal stability of α-chymotrypsin in protic ionic liquids

Ammonium based ionic liquids (ILs) are biocompatible co-solvents that stabilize the native state of proteins. Experimentally, we have explored the stability of α-chymotrypsin (CT) in the presence of nine ILs, i.e., diethylammonium acetate (DEAA), diethylammonium hydrogen sulfate (DEAS), diethylammonium dihydrogen phosphate (DEAP), triethylammonium acetate (TEAA), triethylammonium hydrogen sulfate (TEAS), triethylammonium dihydrogen phosphate (TEAP), trimethylammonium acetate (TMAA), trimethylammonium hydrogen sulfate (TMAS), trimethylammonium dihydrogen phosphate (TMAP). Thermodynamic folding properties such as transition temperature (T_m), Gibbs free energy change of unfolding (ΔG_U), enthalpy change (ΔH) and heat capacity change (ΔC_p) of CT in ILs are obtained by fluorescence spectra analysis. Fluorescence and circular dichroism (CD) spectroscopy experiments were performed to probe CT stabilization and structural changes in the presence of ILs. Our experimental results suggest that the ILs act as stabilizers for the CT structure and the stability of CT depends on the structural arrangement of the ions of ILs. Our experimental results reveal that ILs (DEAA, DEAS and DEAP) having more hydrophobic ammonium cations [DEA⁺] are weak stabilizers for CT, while trimethyl ammonium cations [TMA⁺] ILs having small alkyl chain length such as TMAA, TMAS and TMAP are strong stabilizers and therefore more biocompatible for the native structure of CT.     

疏水离子液体中生物不对称还原制备手性醇

针对近平滑假丝酵母全细胞不对称还原2-羟基苯乙酮制备光学纯(R)-苯基乙二醇反应中底物的质量浓度、产量及质量平衡低的问题,运用多相萃取生物转化的原理,比较不同非水介质对不对称还原反应效率的影响,构建具有良好生物相容性和高质量平衡的水/疏水离子液体1-丁基-3-乙基咪唑六氟磷酸盐([BEIM]PF6)双相反应体系。考察该体系下辅助底物种类、辅助底物用量、底物质量浓度、催化剂用量、离子液体比例、p H和反应温度对生物催化反应的影响,通过正交试验设计和响应面法优化不对称还原2-羟基苯乙酮的反应条件,在最优反应条件下,产物质量浓度、产率和质量平衡得率分别达到15.35 g/L、76.8%和84.3%,产物的对映消旋值(e.e.值)大于99.9%。     

Influence of protic ionic liquids on the structure and stability of succinylated Con A

We report the synthesis of a series of ionic liquids (ILs) from various ions having different kosmotropicity including dihydrogen phosphate (H(2)PO(4)(-)), hydrogen sulfate (HSO(4)(-)) and acetate (CH(3)COO(-)) as anions and chaotropic cation such as trialkylammonium cation. To characterize the biomolecular interactions of ILs with protein, we have explored the stability of succinylated Con A (S Con A) in the presence of these aqueous ILs, which are varied combinations of kosmotropic anion with chaotropic cation such as triethylammonium dihydrogen phosphate [(CH(3)CH(2))(3)NH][H(2)PO(4)] (TEAP), trimethylammonium acetate [(CH(3))(3)NH][CH(3)COO] (TMAA), trimethylammonium dihydrogen phosphate [(CH(3))(3)NH][H(2)PO(4)] (TMAP) and trimethylammonium hydrogen sulfate [(CH(3))(3)NH][HSO(4)] (TMAS). Circular dichroism (CD) and fluorescence experiments have been used to characterize the stability of S Con A by ILs. Our data distinctly demonstrate that the long alkyl chain IL TEAP is a strong stabilizer for S Con A. Further, our experimental results reveal that TEAP is an effective refolding enhancer for S Con A from a thermally denatured protein structure.     

Application of L-proline derivatives as chiral shift reagents for enantiomeric recognition of carboxylic acids

Four proline-derived chiral receptors 5-8 were readily synthesized starting from L-proline. The enantiomeric recognition ability of chiral receptors was examined with a series of carboxylic acids by (1) H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds with each of the enantiomers of guest molecules were determined by using Job plots and a nonlinear least-squares fitting method, respectively. The Job plots indicate that the hosts form 1:1 instantaneous complexes with all guests. The receptors exhibited different chiral recognition abilities toward the enantiomers of racemic guests. Among the chiral receptors used in this study, prolinamide 6 was found to be the best chiral shift reagent and is effective for the determination of the enantiomeric excess of chiral carboxylic acids.     

An efficient catalytic dehydration of fructose and sucrose to 5-hydroxymethylfurfural with protic ionic liquids

The renewable furan-based platform chemical, 5-hydroxymethylfurfural (HMF), has been efficiently synthesized from d-fructose and sucrose in the presence of a catalytic amount of protic ionic liquids. The 1-methylimidazolium-based and N-methylmorpholinium-based ionic liquids are employed. As a result, 74.8% and 47.5% yields of HMF are obtained from d-fructose and sucrose, respectively, at 90 °C for 2 h under nitrogen atmosphere when N-methylmorpholinium methyl sulfonate ([NMM]⁺[CH₃SO₃]⁻) is used as the catalyst in an N,N-dimethylformamide-lithium bromide (DMF-LiBr) system. The acidities of ionic liquids are determined by the Hammett method, and the correlation between acidity and catalytic activity is discussed. Moreover, the effects of reaction temperature and time are investigated, and a plausible reaction mechanism for the dehydration of d-fructose is proposed.     

A rapid analytical method for monitoring the enantiomeric purity of chiral molecules synthesized in ionic liquid solvents

Ionic liquids (ILs) have been widely used as reaction solvents in asymmetric synthesis due to their interesting physical and chemical properties. However, monitoring reactant-to-product conversion and the enantiopurity of formed stereoisomers often involves a tedious extraction step before chromatographic analysis. In this study, a rapid and sensitive sampling method using headspace solid-phase microextraction (SPME) coupled to chiral gas chromatography was developed for the "on-line" analysis of chiral molecules in the IL solvent. Three different SPME sorbent coatings, namely polydimethylsiloxane, polyacrylate, and a polymeric ionic liquid-based fiber, were examined in this study. The analytical performance of the developed method was evaluated in terms of reproducibility, slope of calibration curve, linear range, calibration linearity, and the determination of detection limits. The SPME method was successfully applied in the determination of enantiomeric excess from selected mixtures of chiral molecules. A preliminary study was performed using an "on-fiber" derivatization approach revealing that the stereoisomers extracted by the SPME fiber can be efficiently derivatized using a short "on-fiber" derivatization step. The developed SPME method eliminates the need of sequestering the reaction, separating the compounds of interest from the IL solvent, and the addition of a derivatizing reagent.     

Thiazolinium and imidazolium chiral ionic liquids derived from natural amino acid derivatives

Starting from commercially available amino acid derivatives, two novel families of chiral ionic liquids having either a thiazolinium or an imidazolium cation were prepared by simple and straightforward procedures in good overall yields. The properties of these new salts can be finely tuned by careful selection of the anion and the cation.     

Novel chiral ionic liquids stationary phases for the enantiomer separation of chiral acid by high‐performance liquid chromatography

Novel chiral ionic liquid stationary phases based on chiral imidazolium were prepared. The ionic liquid chiral selector was synthesized by ring opening of cyclohexene oxide with imidazole or 5,6‐dimethylbenzimidazole, and then chemically modified by different substitute groups. Chiral stationary phases were prepared by bonding to the surface of silica sphere through thioene “click” reaction. Their enantioselective separations of chiral acids were evaluated by high‐performance liquid chromatography. The retention of acid sample was related to the counterion concentration and showed a typical ion exchange process. The chiral separation abilities of chiral stationary phases were greatly influenced by the substituent group on the chiral selector as well as the mobile phase, which indicated that, besides ion exchange, other interactions such as steric hindrance, π‐π interaction, and hydrogen bonding are important for the enantioselectivity. In this report, the influence of bulk solvent components, the effects of varying concentration, and the type of the counterion as well as the proportion of acid and basic additives were investigated in detail.     

Chiral mono and diamide derivatives of calix[4]arene for enantiomeric recognition of chiral amines

Novel chiral mono and diamide derivatives of calix[4]arene have been prepared from the aminolysis reaction of 5,11,17,23-tetra-tert-butyl-25,27-diethoxycarbonyl-methoxy-26,28-dihydroxycalix[4]arene 1 and 25,27-diethoxycarbonyl-methoxy-26,28-dihydroxycalix[4]arene 2 with chiral (S)-(-)-1-phenylethylamine (PEA) and (1S,2S)-(+)-2-amino-1-(4-nitrophenyl)-1,3-propanediol, respectively. Spectrophotometric titrations have been performed in CHCl(3) at 20-30 degrees C in order to obtain the binding constants (K) and the thermodynamic quantities (DeltaH and DeltaS) for the stoichiometric 1:1 inclusion complexation of various chiral amines with these new host compounds. Preliminary experiments were undertaken to confirm the complexation properties of receptors 9 and 13 with PEA by (1)H NMR in CDCl(3) at room temperature. The molecular recognition abilities and enantioselectivities for guests (R and S)-alpha-PEA and (R and S)-cyclohexylethylamine (CHEA) are discussed from a thermodynamic point of view.     

DABCO-based chiral ionic liquids as recoverable and reusable organocatalyst for asymmetric Diels–Alder reaction

New DABCO-based chiral ionic liquids were synthesized and evaluated in asymmetric Diels–Alder reaction of cyclopentadiene with α,β-unsaturated aldehydes or 4-phenyl-3-buten-2-one. Chiral ionic liquid of modified MacMillan catalyst having a DABCO cation and hexafluorophosphate anion acts as organocatalyst (5 mol%) for the Diels–Alder reaction of crotonaldehyde and cyclopentadiene producing 98% of the product and 87% ee (endo) in CH₃CN/H₂O (95/5) at 25°C in 2 h. The scope and limitations of the catalysis were also studied by using cyclopentadiene and α,β-unsaturated aldehydes, and the Diels–Alder products were obtained in 18%–92% yields with 68%–93% ee. The catalyst was recycled and reused up to 6 cycles with a slight drop in ee and conversion of the product.     

Enantioseparation of flurbiprofen enantiomers using chiral ionic liquids by liquid‐liquid extraction

Flurbiprofen is a kind of nonsteroidal anti‐inflammatory drug, which has been widely used in clinic for treatment of rheumatoid arthritis and osteoarthritis. It has been reported that S‐flurbiprofen shows good performance on clinic anti‐inflammatory treatment, while R‐enantiomer almost has no pharmacological activities. It has important practical values to obtain optically pure S‐flurbiprofen. In this work, chiral ionic liquids, which have good structural designability and chiral recognize ability, were selected as the extraction selector by the assistance of quantum chemistry calculations. The distribution behaviors of flurbiprofen enantiomers were investigated in the extraction system, which was composed of organic solvent and aqueous phase containing chiral ionic liquid. The results show that maximum enantioselectivity up to 1.20 was attained at pH 2.0, 25°C using 1,2‐dichloroethane as organic solvent, 1‐butyl‐3‐methylimidazole L‐tryptophan ([Bmim][L‐trp]) as chiral selector. The racemic flurbiprofen initial concentration was 0.2 mmol L^?1, and [Bmim][L‐trp] concentration was 0.02 mol L^?1. Furthermore, the recycle of chiral ionic liquids has been achieved by reverse extraction process of the aqueous phase with chiral selector, which is significant for industrial application of chiral ionic liquids and scale‐up of the extraction process.     

Probability rule for chiral recognition

Molecular Chirality is of central interest in biological studies because enantiomeric compounds, while indistinguishable by most inanimate systems, show profoundly different properties in biochemical environments. Enantioselective separation methods, based on the differential recognition of two optical isomers by a chiral selector, have been amply documented. Also, great effort has been directed towards a theoretical understanding of the fundamental mechanisms underlying the chiral recognition process. Here we report a comprehensive data examination of enantio separation measurements for over 72000 chiral selector-select and pairs from the chiral selection compendium CHIRBASE. The distribution of alpha = k'(D)/k'(L) values was found to follow a power law, equivalent to an exponential decay for chiral differential free energies. This observation is experimentally relevant in terms of the number of different individual or combinatorial selectors that need to be screened in order to observe alpha values higher than a preset minimum. A string model for enantiorecognition (SMED) formalism is proposed to account for this observation on the basis of an extended Ogston three-point interaction model. Partially overlapping molecular interaction domains are analyzed in terms of a string complementarity model for ligand-receptor complementarity. The results suggest that chiral selection statistics may be interpreted in terms of more general concepts related to biomolecular recognition.     

Design of ionic liquids for lipase purification

Aqueous two-phase systems (ATPS) are considered as efficient downstream processing techniques in the production and purification of enzymes, since they can be considered harmless to biomolecules due to their high water content and due to the possibility of maintaining a neutral pH value in the medium. A recent type of alternative ATPS is based on hydrophilic ionic liquids (ILs) and salting-out inducing salts. The aim of this work was to study the lipase (Candida antarctica lipase B - CaLB) partitioning in several ATPS composed of ionic liquids (ILs) and inorganic salts, and to identify the best IL for the enzyme purification. For that purpose a wide range of IL cations and anions, and some of their combinations were studied. For each system the enzyme partitioning between the two phases was measured and the purification factors and enzyme recoveries were determined. The results indicate that the lipase maximum purification and recovery were obtained for cations with a C(8) side alkyl chain, the [N(CN)(2)] anion and ILs belonging to the pyridinium family. However, the highest purification parameters were observed for 1-methyl-3-octylimidazolium chloride [C(8)mim]Cl, suggesting that the IL extraction capability does not result from a cumulative character of the individual characteristics of ILs. The results indicate that the IL based ATPS have an improved performance in the lipase purification and recovery.     

Enzyme catalysis in ionic liquids

Ionic liquids offer new possibilities for the application of solvent engineering to biocatalytic reactions. Although in many cases ionic liquids have simply been used to replace organic solvents, they have often led to improved process performance. Unlike conventional organic solvents, ionic liquids possess no vapor pressure, are able to dissolve many compounds, and can be used to form two-phase systems with many solvents. To date, reactions involving lipases have benefited most from the use of ionic liquids, but the use of ionic liquids with other enzymes and in whole-cell processes has also been described. In some cases, remarkable results with respect to yield, (enantio)selectivity or enzyme stability were observed.     

Tyrosinase activity in ionic liquids

Activity of mushroom tyrosinase was studied in three ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF₆]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]) and 1-butyl-3-methylimidazolium methylsulfate ([BMIm][MeSO₄]), and was compared to that in chloroform. Kinetic parameters of the enzyme were determined and the results indicate that the enzyme in ionic liquids basically follows the same catalytic mechanism as in water, and that the ionic liquids may affect the enzyme activity by direct interacting with the enzyme and thus hindering the E–S binding due to their high hydrophilicity and polarity.     

Biocatalytic transformations in ionic liquids

Room temperature ionic liquids are non-volatile, thermally stable and highly polar; they are also moderately hydrophilic solvents. Here, we discuss their use as reaction media for biocatalysis. Enzymes of widely diverging types are catalytically active in ionic liquids or aqueous biphasic ionic liquid systems. Lipases, in particular, maintain their activity in anhydrous ionic liquid media; the (enantio)selectivity and operational stability are often better than in traditional media. The unconventional solvent properties of ionic liquids have been exploited in biocatalyst recycling and product recovery schemes that are not feasible with traditional solvent systems.     

Design of phosphonium ionic liquids for lipase-catalyzed transesterification

Ionic liquids are now recognized as solvents for use in lipase-catalyzed reactions; however, there still remains a serious drawback in that the rate of reaction in an ionic liquid is slower than that in a conventional organic solvent. To overcome this problem, attempts have been made to evolve phosphonium ionic liquids appropriate for lipase-catalyzed reaction; several types of phosphonium salts have been prepared and their capability evaluated for use as solvent for the lipase-catalyzed reaction. Very rapid lipase PS-catalyzed transesterification of secondary alcohols was obtained when 2-methoxyethyl(tri-n-butyl)phosphonium bis(trifluoromethanesulfonyl)imide ([MEBu₃P][NTf₂]) was used as solvent, affording the first example of a reaction rate superior to that in diisopropyl ether.     

Simple protocols for NMR analysis of the enantiomeric purity of chiral diols

A three-component chiral derivatization protocol for determining the enantiopurity of chiral diols by (1)H NMR spectroscopic analysis is described here. The present approach involves the derivatization of 1,2- 1,3- and 1,4-diols with 2-formylphenylboronic acid and enantiopure alpha-methylbenzylamine. This method affords a mixture of diastereoisomeric iminoboronate esters whose ratio can be determined by integration of well-resolved diastereotopic resonances in their (1)H NMR spectra, thus enabling the determination of the enantiopurity of the parent diol. The protocol as described takes less than 90 min to complete.