Lipase-catalyzed enantioselective reaction of amines with carboxylic acids under reduced pressure in non-solvent system and in ionic liquids

Brønsted Acid-Catalyzed Atroposelective Coupling of Carboxylic Acids with Amines and Alcohols via Ynamide Mediation

Carboxylic acids are readily available chemicals with broad applications in chemistry-related areas, and their coupling with amines and alcohols is a fundamental transformation in organic synthesis. However, the catalytic enantioselective coupling of carboxylic acids remains elusive, especially for the atroposelective reaction. Here we report a chiral Brønsted acid (CBA)-catalyzed atroposelective coupling of carboxylic acids with amines and alcohols using ynamides as coupling reagents. The novel enantiocontrol involving CBA-catalyzed ester addition enables the straightforward construction of axially chiral amides and planar-chiral esters with high enantioselectivities through atroposelective intermolecular amidation and intramolecular macrolactonization. Diverse medicinally relevant carboxylic acids can undergo direct late-stage modification by this method. Importantly, this reaction represents the first atroposelective coupling of carboxylic acids with amines, as well as the first chemocatalytic atroposelective coupling of carboxylic acids with alcohols. The resulting atropisomeric skeletons can be readily derivatized to chiral ligands and catalysts for asymmetric catalysis.

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Enzymatic Reactions using Ionic Liquids for Green Sustainable Chemical Process; Stabilization and Activation of Lipases

The enzymatic reaction is highly respected from an environmentally-friendly point-of-view. Optimization of the reaction media and supporting materials of enzymes must be investigated in parallel with the effort to develop new enzymes. Lipases are frequently used for organic syntheses as synthetic tools even industry because of their acceptance of having a broad range of substrates, stability, and availability. We have investigated the possibility of ILs as both a solvent and activating or stabilization agent of enzymes, in particular, lipase as a model enzyme. ILs allowed recyclable use of a lipase and significant acceleration of transesterification, and also enhanced the stability and reaction activity of a lipase by immobilization through a lyophilization process. We discuss how we enhanced the enzyme capability using the IL engineering focusing on lipase-catalyzed reactions, i. e., realization of the recyclable use of an enzyme, how ILs mediated the enhanced reaction rate, and improved the stability of the enzyme.

A Sustainable Green Enzymatic Method for Amide Bond Formation

A sustainable enzymatic strategy for the preparation of amides by using Candida antarctica lipase B as the biocatalyst and cyclopentyl methyl ether as a green and safe solvent was devised. The method is simple and efficient and it produces amides with excellent conversions and yields without the need for intensive purification steps. The scope of the reaction was extended to the preparation of 28 diverse amides using four different free carboxylic acids and seven primary and secondary amines, including cyclic amines. This enzymatic methodology has the potential to become a green and industrially reliable process for direct amide synthesis.

Coupling of Bioreaction and Separation via Novel Thermosensitive Ionic Liquids Applied in the Baker's Yeast-Catalyzed Reduction of Ethyl 2-oxo-4-phenylbutyrate

The use of baker's yeast to reduce ethyl 2-oxo-4-phenylbutyrate (EOPB) in conventional biphasic systems is hindered by low productivities due to mass transfer resistance between the biocatalyst and the substrate partitioned into two different phases. To overcome the limitation, a new reaction-separation coupling process (RSCP) was configured in this study, based on the novel thermosensitive ionic liquids (ILs) with polyoxyethylene-tail. The solubility of ILs in common solvents was investigated to configure the unique thermosensitive ionic liquids-solvent biphasic system (TIBS) in which the reduction was performed. [(CH₃)₂N(C₂H₅)(CH₂CH₂O)₂H][PF₆] (c₂) in 1,2-dimethoxyethane possesses the thermosensitive function of homogeneous at lower temperatures and phase separating at higher temperatures. The phase transformation temperature (PTT) of the mixed system of c₂/1,2-dimethoxyethane (v/v, 5:18) was about 33 °C. The bioreaction takes place in a "homogeneous" liquid phase at 30 °C. At the end of each reduction run, the system temperature is increased upon to the PTT, while c₂ is separated from 1,2-dimethoxyethane with turning the system into two phases. The enantiomeric excesses (e.e.) of ethyl (R)-2-hydroxy-4-phenylbutyrate ((R)-EHPB) increased about 25~30% and the yield of ethyl-2-hydroxy-4-phenylbutyrate (EHPB) increased 35% in TIBS, compared with the reduction in 1,2-dimethoxyethane. It is expected that the TIBS established in this study could provide many future opportunities in the biocatalysis.

Optimization of the Lipase-Catalyzed Selective Amidation of Phenylglycinol

Ceramides and their analogs have a regulatory effect on inflammatory cytokines expression. It was found that a kind of ceramides analog synthesized from phenylglycinol could inhibit the production of cytokine TNF-α. However, two active hydrogen groups are present in the phenylglycinol molecule. It is difficult to control the process without hydroxyl group protection to dominantly produce amide in the traditional chemical synthesis. A selective catalytic the amidation route of phenylglycinol by lipases was investigated in this research. The results indicated that the commercial immobilized lipase Novozym 435 has the best regio-selectivity on the amide group. Based on the experimental results and in silico simulation, it was found that the mechanism of specific N-acyl selectivity of lipase was not only from intramolecular migration and proton shuttle mechanism, but also from the special structure of active site of enzyme. The optimal reaction yield of aromatic amide compound in a solvent-free system with lipase loading of 15 wt% (to the weight of total substrate) reached 89.41 ± 2.8% with very few of byproducts detected (0.21 ± 0.1% ester and 0.64 ± 0.2% diacetylated compound). Compare to other reported works, this work have the advantages such as low enzyme loading, solvent free, and high N-acylation selectivity. Meanwhile, this Novozym 435 lipase based synthesis method has an excellent regio-selectivity on most kinds of amino alcohol compounds. Compared to the chemical method, the enzymatic synthesis exhibited high regio-selectivity, and conversion rates. The method could be a promising alternative strategy for the synthesis of aromatic alkanolamides.

Conductivity, Viscosity, Spectroscopic Properties of Organic Sulfonic Acid solutions in Ionic Liquids

Sulfonic acids in ionic liquids (ILs) are used as catalysts, electrolytes, and solutions for metal extraction. The sulfonic acid ionization states and the solution acid/base properties are critical for these applications. Methane sulfonic acid (MSA) and camphor sulfonic acid (CSA) are dissolved in several IL solutions with and without bis(trifluoromethanesulfonyl)imine (HTFSI). The solutions demonstrated higher conductivities and lower viscosities. Through calorimetry and temperature-dependent conductivity analysis, we found that adding MSA to the IL solution may change both the ion migration activation energy and the number of “free” charge carriers. However, no significant acid ionization or proton transfer was observed in the IL solutions. Raman and IR spectroscopy with computational simulations suggest that the HTFSI forms dimers in the solutions with an N-H-N “bridged” structure, while MSA does not perturb this hydrogen ion solvation structure in the IL solutions. CSA has a lower solubility in the ILs and reduced the IL solution conductivity. However, in IL solutions containing 0.4 M or higher concentration of HTFSI, CSA addition increased the conductivity at low CSA concentrations and reduced it at high concentrations, which may indicate a synergistic effect.

Effects of Ionic Liquid Alkyl Chain Length on Denaturation of Myoglobin by Anionic, Cationic, and Zwitterionic Detergents

The unique electrochemical properties of ionic liquids (ILs) have motivated their use as solvents for organic synthesis and green energy applications. More recently, their potential in pharmaceutical chemistry has prompted investigation into their effects on biomolecules. There is evidence that some ILs can destabilize proteins via a detergent-like manner; however, the mechanism still remains unknown. Our hypothesis is that if ILs are denaturing proteins via a detergent-like mechanism, detergent-mediated protein unfolding should be enhanced in the presence of ILs. The properties of myoglobin was examined in the presence of a zwitterionic (N,N-dimethyl-N-dodecylglycine betaine (Empigen BB^®, EBB)), cationic (tetradecyltrimethylammonium bromide (TTAB)), and anionic (sodium dodecyl sulfate (SDS)) detergent as well as ILs based on alkylated imidazolium chlorides. Protein structure was measured through a combination of absorbance, fluorescence, and circular dichroism (CD) spectroscopy: absorbance and CD were used to monitor heme complexation to myoglobin, and tryptophan fluorescence quenching was used as an indicator for heme dissociation. Notably, the detergents tested did not fully denature the protein but instead resulted in loss of the heme group. At low IL concentrations, heme dissociation remained a traditional, cooperative process; at high concentrations, ILs with increased detergent-like character exhibited a more complex pattern, which is most likely attributable to micellization of the ionic liquids or direct denaturation or heme dissociation induced by the ILs. These trends were consistent across all species of detergents. 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence was further used to characterize micelle formation in aqueous solutions containing detergent and ionic liquid. The dissociation thermodynamics show that EBB- and TTAB-induced dissociation of heme is not significantly impacted by room temperature ionic liquids (RTILs), whereas SDS-induced dissociation is more dramatically impacted by all RTILs examined. Together, these results indicate a complex interaction of detergents, likely based on headgroup charge, and the active component of RTILs to influence heme dissociation and potentially protein denaturation.

Rhodium-catalyzed asymmetric hydrogenation of β-branched enamides for the synthesis of β-stereogenic amines

β-Branched simple enamides were hydrogenated to give β-stereogenic amines in quantitative yields and with excellent enantioselectivities.

Proton transfer and esterification reactions in EMIMOAc-based acidic ionic liquids

Proton transfer and esterification reactions are observed when formic acid and ionic liquid EMIMOAc are mixed.

Amides in Nature and Biocatalysis

Amides are widespread in biologically active compounds with a broad range of applications in biotechnology, agriculture and medicine. Therefore, as alternative to chemical synthesis the biocatalytic amide synthesis is a very interesting field of research. As usual, Nature can serve as guide in the quest for novel biocatalysts. Several mechanisms for carboxylate activation involving mainly acyl-adenylate, acyl-phosphate or acyl-enzyme intermediates have been discovered, but also completely different pathways to amides are found. In addition to ribosomes, selected enzymes of almost all main enzyme classes are able to synthesize amides. In this review we give an overview about amide synthesis in Nature, as well as biotechnological applications of these enzymes. Moreover, several examples of biocatalytic amide synthesis are given.

Catalytic amide formation from non-activated carboxylic acids and amines

The amide functionality is found in a wide variety of biological and synthetic structures such as proteins, polymers, pesticides and pharmaceuticals. Due to the fact that synthetic amides are still mainly produced by the aid of coupling reagents with poor atom-economy, the direct catalytic formation of amides from carboxylic acids and amines has become a field of emerging importance. A general, efficient and selective catalytic method for this transformation would meet well with the increasing demands for green chemistry procedures. This review covers catalytic and synthetically relevant methods for direct condensation of carboxylic acids and amines. A comprehensive overview of homogeneous and heterogeneous catalytic methods is presented, covering biocatalysts, Lewis acid catalysts based on boron and metals as well an assortment of other types of catalysts.

"Nonsolvent" applications of ionic liquids in biotransformations and organocatalysis

The application of room-temperature ionic liquids (RTILs) as (co)solvents and/or reagents is well documented. However, RTILS also have "nonsolvent" applications in biotransformations and organocatalysis. Examples are the anchoring of substrates to RTILs; ionic-liquid-coated enzymes (ILCE) and enzyme-IL colyophilization; the construction of biocatalytic ternary reaction systems; the combination of enzymes, RTILs, membranes, and (bio)electrochemistry; and ionic-liquid-supported organocatalysts. These strategies provide more robust, more efficient, and more enantioselective bio- and organocatalysts with many practical applications. As shown herein, RTILs offer a wide range of promising alternatives to conventional chemistry.

Kinetics of reactions catalyzed by enzymes in solutions of surfactants

The effect of surfactants, both in water-in-oil microemulsions (hydrated reverse micelles) and aqueous solutions upon enzymatic processes is reviewed, with special emphasis on the effect of the surfactant upon the kinetic parameters of the process. Differences and similarities between processes taking place in aqueous and organic solvents are highlighted, and the main models currently employed to interpret the results are briefly discussed.

Solvent-free enzymatic synthesis of fatty alkanolamides

An environmentally benign and volume efficient process for enzymatic production of alkanolamides is described. Immobilized Candida antarctica lipase B, Novozym435, was used to catalyze the condensation of lauric acid with monoethanolamine. The reaction temperature of 90 degrees C was required to keep the reactants in a liquid state. Stepwise addition of the amine minimized problems caused by the formation of a highly viscous amine/fatty acid ion-pair. The enzyme was both very active and stable under the reaction conditions, with about half of the activity remaining after 2 weeks. The maximum amide yield obtained when using equimolar amounts of the reactants was 75%, which could be increased to 95% upon water removal. Special precautions to avoid co-distillation of the amine were required. Two different strategies to avoid the amine loss are presented.

Highly Selective Enzymatic Kinetic Resolution of Primary Amines at 80 °C: A Comparative Study of Carboxylic Acids and Their Ethyl Esters as Acyl Donors

Optimization of the kinetic resolution of 2-amino-4-phenyl-butane was achieved at 80 degrees C using CAL-B-catalyzed aminolysis of carboxylic acids and their ethyl esters. The reactions carried out with long chain esters and the corresponding acids as acyl donors proceeded with remarkably high enantioselectivity. The use of carboxylic acids as acylating agents led to a marked acceleration of the reaction rate compared to their ester counterparts. Lauric acid led to enantiomeric excesses superior to 99.5% for both the remaining amine and the corresponding lauramide at 50% conversion (reached in 3 h). These optimized conditions were applied to the resolution of a series of aliphatic and benzylic amines.

Efficient enantioselective hydrolysis of d,l-phenylglycine methyl ester catalyzed by immobilized Candida antarctica lipase B in ionic liquid containing systems

Immobilized Candida antarctica lipase B (Novozym 435)-catalyzed enantioselective hydrolysis of D,L-phenylglycine methyl ester to enatiopure D-phenylglycine was successfully conducted in the systems with ionic liquids (ILs). Novozym 435 exhibited excellent activity and enantioselectivity in the system containing the IL BMIMxBF(4) compared to several typical organic solvents tested. It has been found that the cations and, particularly, the anions of ILs have a significant effect on the reaction, and the IL BMIMxBF(4), which shows to be the most suitable for the reaction, gave the highest initial rate and enantioselectivity among various ILs examined. The reaction became much less active and enantioselective in the systems with BMIMxHSO(4). Also, it was noticed that the enzymatic hydrolysis was strongly dependent on BMIMxBF(4) content in the co-solvent systems and the favorable content of the IL was 20% (v/v). Of the assayed four co-solvents and phosphate buffer, the lowest apparent K(m) and activation energy, and the highest V(max) of the reaction were achieved using 20% (v/v) BMIMxBF(4) co-solvent with phosphate buffer. Additionally, various influential variables were investigated. The optimum pH, substrate concentration, reaction temperature and shaking rate were 8.0, 80mM, 25-30 degrees Celsius and 150rpm, respectively, under which the initial rate, the residual substrate e.e. and the enantioselectivity were 2.46mM/min, 93.8% (at substrate conversion of 53.0%) and 38, respectively. When the hydrolysis was performed under reduced pressure, the initial rate (2.64mM/min) and the enantioselectivity (E=43) were boosted.

Efficient kinetic resolution of (R,S)-1-trimethylsilylethanol via lipase-mediated enantioselective acylation in ionic liquids

Efficient enantioselective acylation of (R,S)-1-trimethylsilylethanol {(R,S)-1-TMSE} with vinyl acetate catalyzed by immobilized lipase from Candida antarctica B (i.e., Novozym 435) was successfully conducted in ionic liquids (ILs). A remarkable enhancement in the initial rate and the enantioselectivity of the acylation was observed by using ILs as the reaction media when compared to the organic solvents tested. Also, the activity, enantioselectivity, and thermostability of Novozym 435 increased with increasing hydrophobicity of ILs. Of the six ILs examined, the IL C4MIm.PF6 gave the fastest initial rate and the highest enantioselectivity, and was consequently chosen as the favorable medium for the reaction. The optimal molar ratio of vinyl acetate to (R,S)-1-TMSE, water activity, and reaction temperature range were 4:1, 0.75, and 40 -50 degrees C, respectively, under which the initial rate and the enantioselectivity (E value) were 27.6 mM/h and 149, respectively. After a reaction time of 6 h, the ee of the remaining (S)-1-TMSE reached 97.1% at the substrate conversion of 50.7%. Additionally, Novozym 435 was effectively recycled and reused in C4MIm.PF6 for five consecutive runs without substantial lose in activity and enantioselectivity. The preparative scale kinetic resolution of (R,S)-1-TMSE in C4MIm.PF6 is shown to be very promising and useful for the industrial production of enantiopure (S)-1-TMSE.

Thermal stability and activity regain of horseradish peroxidase in aqueous mixtures of imidazolium-based ionic liquids

Thermal deactivation kinetics of horseradish peroxidase (HRP) were studied from 45 to 90 degrees C in phosphate buffer and 5-25% (v,w/v) 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] and 1-butyl-3-methylimidazolium chloride [BMIM][Cl]. HRP activity at 25 degrees C was not affected by the presence of ionic liquids up to 20% (v,w/v). Increasing the ionic liquids concentration up to 25% (v,w/v) changed the biphasic character of deactivation kinetics to an apparent single first-order step. The presence of 5-10% (v/v) [BMIM][BF4] significantly improved HRP thermal stability with lower activation energies for the deactivation second phase (83-87 kJ mol(-1)). After deactivation, enhanced activity regain of the enzyme, up to 70-80% of the initial activity, was found in 25% (v/v) [BMIM][BF4] and 10% (w/v) [BMIM][Cl] and correlated to prevalence of the deactivation first phase.

Ionic liquids as an alternative to formalin in histopathological diagnosis

Asymmetry of cations and the type of anions play a key role in the properties of ionic liquids (ILs) as fixatives for tissue preservation. 1-Methyl-3-octyloxymethylimidazolium tetrafluoroborate has proven to be a very good fixative, with similar effects as formalin. Our study shows that it is applicable for both histological and immunohistochemical purposes. After treatment with 1-methyl-3-octyloxymethylimidazolium tetrafluoroborate, tissue sections are more intensely stained. With respect to expression patterns and staining intensity, immunohistochemical staining is comparable in tissues fixed in formalin and the selected ILs. The present study demonstrates the properties of 1-methyl-3-octyloxymethylimidazolium tetrafluoroborate for tissue preservation in histopathological procedures, eliminating the requirement of formalin.