Effect of Long Chain Fatty Acids on Organocatalytic Aqueous Direct Aldol Reactions

Structure optimization of lipopeptide assemblies for aldol reactions in an aqueous medium

Four amphiphilic peptides were synthesized, characterized, and evaluated regarding their efficiency in the catalysis of direct aldol reactions in water. The lipopeptides differ by having a double lipid chain and a guanidinium pyrrole group functionalizing one Lys side chain. All the samples are composed of the amino acids l-proline (P), l-arginine (R), or l-lysine (K) functionalized with the cationic guanidiniocarbonyl pyrrole unit (GCP), l-tryptophan (W), and l-glycine (G), covalently linked to one or two long aliphatic chains, leading to surfactant-like designs with controlled proline protonation state and different stereoselectivity. Critical aggregation concentrations (cac) were higher in the presence of the GCP group, suggesting that self-assembly depends on charge distribution along the peptide backbone. Cryogenic Transmission Electron Microscopy (Cryo-TEM) and Small Angle X-ray Scattering (SAXS) showed a rich polymorphism including spherical, cylindrical, and bilayer structures. Molecular dynamics simulations performed to assess the lipopeptide polymorphs revealed an excellent agreement with core-shell arrangements derived from SAXS data and provided an atomistic view of the changes incurred by modifying head groups and lipid chains. The resulting nanostructures behaved as excellent catalysts for aldol condensation reactions, in which superior conversions (>99%), high diastereoselectivities (ds = 94 : 6), and enantioselectivities (ee = 92%) were obtained. Our findings contribute to elucidate the effect of nanoscale organization of lipopeptide assemblies in the catalysis of aldol reactions in an aqueous environment.

Assessment of Chitosan Based Catalyst and their Mode of Action

Introduction:

The popularity of chitosan is increasing among the researchers due to its environment friendly nature, high activity and easy approachability. Chitosan based catalysts are not only the most active and selective in catalytic reaction, but their “green” accessibility also makes them promising in organic catalysis. Chitosan is commonly extracted from chitin by alkaline deacetylation and it is the second abundant biopolymer in nature after cellulose. Chitosan based catalysts are advantageous by means of non-metallic activation as it involves small organic molecules. The robustness, nontoxicity, the lack of metal leaching possibility, inertness towards moisture and oxygen, easy handling and storage are the main advantages of organocatalysts. Traditional drawbacks associated with the metal-based heterogeneous catalysts, like longer reaction times during any synthesis, metal-leaching after every reaction and structural instability of the catalyst for prolonged recycling experiments are also very negligible for chitosan based catalysts. Besides, these catalysts can contribute more in catalysis due to their reusability and these special features increase their demand as the functionalized and profitable catalysts.

Objective:

The thorough description about the preparation of organocatalysts from chitosan and their uniqueness and novel activities in various famous reactions includes as the main aim of this review. Reusable and recycle nature of chitosan based organocatalysts gain the advantages over traditional and conventional catalyst which is further discussed over here.

Methods and Discussions:

In this article only those reactions are discussed where chitosan has been used both as support in heterogeneous catalysts or used as a catalyst itself without any co-catalyst for some reactions. Owing to its high biodegradability, nontoxicity, and antimicrobial properties, chitosan is widely-used as a green and sustainable polymeric catalyst in vast number of the reactions. Most of the preparations of catalyst have been achieved by exploring the complexation properties of chitosan with metal ions in heterogeneous molecular catalysis. Organocatalysis with chitosan is primarily discussed for carbon-carbon bond-forming reactions, carbon dioxide fixation through cyclo- addition reaction, condensation reaction and fine chemical synthesis reactions. Furthermore, its application as an enantioselective catalyst is also considered here for the chiral, helical organization of the chitosan skeleton. Moreover, another advantage of this polymeric catalyst is its easy recovery and reusability for several times under solvent-free conditions which is also explored in the current article.

Conclusion:

Important organocatalyzed reactions with either native chitosan or functionalized chitosan as catalysts have attracted great attention in the recent past. Also, chitosan has been widely used as a very promising support for the immobilization of catalytic metals for many reactions. In this review, various reactions have been discussed which show the potentiality of chitosan as catalyst or catalyst support.

Pseudo-Interphase of Liposome Promotes 1,3-Dipolar Cycloaddition Reaction of Benzonitrile Oxide and N-Ethylmaleimide in Aqueous Solution

The hydrophobic interior of a liposome membrane was used as a platform for the organic synthesis of hydrophobic compounds in water. The 1,3-dipolar cycloaddition of benzonitrile oxide (BNO) and N-ethylmaleimide (EMI) in liposome suspensions was carried out, and an increase in the reaction rate constant was observed depending on the liposome characteristics. While the reaction rate constant in 1,4-dioxane was 1.5 times higher than that in water, the reaction rate constant in an aqueous solution of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) liposome was 3 times higher than in water. The amount of substrate, BNO, accumulated in the DOTAP liposome was higher than that in 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), indicating that BNO prefers to be distributed in the liposome membrane in the liquid-disordered phase. The membrane polarity, GP340, as monitored by Laurdan, varied with the presence of BNO, while EMI slightly affected the membrane properties of the liposomes. These results suggest that the pseudo-interphase afforded by the liposome membrane can promote the 1,3-dipolar cycloaddition between BNO and EMI in water.

Oleic acid: a benign Brønsted acidic catalyst for densely substituted indole derivative synthesis

Oleic acid was identified to catalyse bis(indolyl)methane and densely functionalized 4H-chromene, spirooxindole and spiro[indoline-3,4′-pyrano[2,3-c]pyrazole] derivative synthesis in good to excellent yields.

Aerobic photooxidative direct asymmetric aldol reactions of benzyl alcohols using water as the solvent

We report an aerobic photooxidative direct asymmetric aldol reaction using water as the solvent.

The smallest organocatalyst in highly enantioselective direct aldol reaction in wet solvent-free conditions

The catalytic efficacy of the smallest organocatalyst, l-proline hydrazide, prepared from a cheaply available natural amino acid, such as l-proline, was studied for the direct asymmetric aldol reaction of various ketones with aromatic aldehydes at room temperature in the presence of several acid additives.

Chiral picolylamines for Michael and aldol reactions: probing substrate boundaries

Here we report on inroads concerning increased substrate breadth via the picolylamine organocatalyst template, a vicinal chiral diamine based on a pyridine-primary amine motif. The addition of cyclohexanone to β-nitrostyrene has many catalyst solutions, but cyclopentanone and isobutyraldehyde additions continue to be challenging. PicAm-3 (10 mol%) readily allows the Michael addition of cyclopentanone or isobutyraldehyde (5.0 equiv.) to β-nitrostyrene derivatives. By contrast, PicAm-1 (7.0 mol%) is optimal for catalyzing the aldol reaction of cyclohexanone or cycloheptanone (3.3 equiv.) with aromatic aldehydes. Eighteen products are reported and for each reaction type new products are reported (4b-d, 9c). Very good yields and stereoselectivities are generally noted. The reactions, which require an acid additive, proceed via a transient chiral enamine and a mechanistic case is put forth for a bifunctional catalysis model.

Organocatalysis in water at room temperature with in-flask catalyst recycling

A new designer surfactant is described containing a covalently bound organocatalyst, proline. This species is water-soluble and, via spontaneous nanomicelle formation, catalyzes aldol reactions on water-soluble or -insoluble substrates in water as the only medium. Recycling the catalyst is trivial, as the amphiphile/catalyst remains in the aqueous phase in the flask.

Low-loading asymmetric organocatalysis

Asymmetric organocatalysis is now recognized as the third pillar of asymmetric synthesis. Recent years have witnessed increasing interest towards the use of highly active and stereoselective organocatalysts. This critical review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Brønsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, phase transfer and N-heterocyclic carbene catalyses (194 references).

Investigating ionic effects applied to water based organocatalysed aldol reactions

Saturated aqueous solutions of various common salts were examined for their effect on aqueous aldol reactions catalysted by a highly active C₂-symmetric diprolinamide organocatalyst developed in our laboratory. With respect to the aldol reaction between cyclohexanone and 4-nitrobenzaldehyde, deionised water was always a superior medium to salt solutions though some correlation to increasing anion size and depression in enantiomeric excess could be observed. Additionally, the complete inhibition of catalyst activity observed when employing tap water could be alleviated by the inclusion of ethylenediaminetetraacetate (EDTA) into the aqueous media prior to reaction initiation. Extension of these reaction conditions demonstrated that these ionic effects vary on a case-to-case basis depending on the ketone/aldehyde combination.