Synthesis and Applications of Carbohydrate-Based Organocatalysts

Organocatalysis is a very useful tool for the asymmetric synthesis of biologically or pharmacologically active compounds because it avoids the use of noxious metals, which are difficult to eliminate from the target products. Moreover, in many cases, the organocatalysed reactions can be performed in benign solvents and do not require anhydrous conditions. It is well-known that most of the above-mentioned reactions are promoted by a simple aminoacid, l-proline, or, to a lesser extent, by the more complex cinchona alkaloids. However, during the past three decades, other enantiopure natural compounds, the carbohydrates, have been employed as organocatalysts. In the present exhaustive review, the detailed preparation of all the sugar-based organocatalysts as well as their catalytic properties are described.

Water-soluble triazole chitin derivative and its based nanoparticles: Synthesis, characterization, catalytic and antibacterial properties

In this work, we treated chitin with 2-(azidomethyl)oxirane and successfully involved the resultant azido chitin derivatives in the ultrasound-assisted Cu(I)-catalyzed azido-alkyne click (CuAAC) reaction with propargylic ester of N,N,N-trimethyl glycine. Thus, we obtained novel water-soluble triazole chitin derivatives. The triazole chitin derivatives and their nanoparticles are characterized by a high in vitro antibacterial activity, which is the same or even higher than that of commercial antibiotics ampicillin and gentamicin. The obtained derivatives are non-toxic. Moreover, the obtained water-soluble polymers are highly efficient green catalysts for the aldol reaction in green solvent water. The catalysts can be easily extracted from the reaction mixture by its precipitation with green solvent ethanol followed by centrifugation and they can be reused at least 10 times.

Chitosan as a chiral ligand and organocatalyst: preparation conditions–property–catalytic performance relationships

Properties of chitosan prepared by alkaline deacetylation of chitin under various conditions were correlated with their performance as ligands or organocatalysts in asymmetric catalytic reactions.

Valorization of Waste: Sustainable Organocatalysts from Renewable Resources

One of the greatest challenges facing our society is to reconcile our need to develop efficient and sophisticated chemical processes with the limited resources of our planet and its restricted ability to adsorb pollution. Organocatalysis has allowed many issues to be addressed in the development of sophisticated, but less polluting, processes. However, minimizing waste also means an efficient utilization of raw and renewable materials. Waste biomass represents an alternative to conventional petroleum-based chemical manufacturing and is a highly attractive renewable resource for the production of chemicals and high-value-added organocatalysts. Recent achievements in the use of renewable biomass feedstocks for the synthesis of organocatalysts are presented. Their application in synthetic methodologies, including multicomponent reactions, which are performed under solvent-free conditions or in eco-friendly reaction media, as well as recycling and reusing the organocatalysts, is illustrated. A few pioneering examples that demonstrate the potential of these promoters in asymmetric synthesis have also been documented. In particular, this review covers examples on the use of hetero- and homogeneous organocatalysts derived from 1) waste biopolymers, such as chitosan, alginic acid, and cellulose; ii) renewable platform molecules, such as levoglucosenone, isosorbide, mannose, d-glucosamine, and lecithin; 3) terpenes and rosin, such as pinane, isosteviol, and abietic acid; and iv) natural proteins (gelatin, bovine tendons, silk fibroin proteins).

Organocatalytic Asymmetric Aldol Reaction of Arylglyoxals and Hydroxyacetone: Enantioselective Synthesis of 2,3-Dihydroxy-1,4-diones

A highly efficient quinine-derived primary-amine-catalyzed asymmetric aldol addition of hydroxyacetone to arylglyoxals is described. Structurally diverse anti-2,3-dihydroxy-1,4-diones were generated in high yields, with good diastereoselectivities and enantioselectivities.

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.

Kinetic evaluation of chitosan-derived catalysts for the aldol reaction in water

The reaction rate and stability of chitosan as heterogeneous amine catalyst is quantified in a batch and continuous-flow aldol reaction.

Polysaccharide-Based Aerogel Bead Production via Jet Cutting Method

The aim of this work is to develop a method to produce spherical biopolymer-based aerogel particles, which is capable for scale-up in the future. Therefore, the jet cutting method is suggested. Amidated pectin, sodium alginate, and chitosan are used as a precursor (a 1⁻3 wt. % solution) for particle production via jet cutting. Gelation is realized via two methods: the internal setting method (using calcium carbonate particles as cross-linkers and citric and acidic acid for pH adjustment) and the diffusion method (in calcium chloride solutions). Gel particles are subjected to solvent exchange to ethanol and consequent supercritical drying with CO₂. Spherical aerogel particles with narrow particle size distributions in the range of 400 to 1500 µm and a specific surface area of around 500 m²/g are produced. Overall, it can be concluded that the jet cutting method is suitable for aerogel particle production, although the shape of the particles is not perfectly spherical in all cases. However, parameter adjustment might lead to even better shaped particles in further work. Moreover, the biopolymer-based aerogel particles synthesized in this study are tested as humidity absorbers in drying units for home appliances, particularly for dishwashers. It has been shown that for several cycles of absorption and desorption of humidity, aerogel particles are stable with an absorption capacity of around 20 wt. %.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Progress in aminosugar derived asymmetric organocatalysis

This review sheds light on the effect of the structural modification of sugar amines on their catalytic efficiency and the stereoselectivity of the reaction.

Tartrate-derived iminophosphorane catalyzed asymmetric hydroxymethylation of 3-substituted oxindoles with paraformaldehyde

Chiral iminophosphoranes organocatalyzed the enantioselective synthesis of 3-hydroxymethyl-2-oxindoles in 81–98% yields and up to 94% ee under mild conditions. Of note is that readily available and easily usable paraformaldehyde was employed as a hydroxymethylation C1 unit in the reaction.

Binaphthyl-based chiral bifunctional organocatalysts for water mediated asymmetric List–Lerner–Barbas aldol reactions

Binaphthyl-based organocatalysts were synthesized and successfully applied to the asymmetric List–Lerner–Barbas aldol reaction in water medium. These organocatalysts were found to be effective catalysts for the reactions of ketones with different aldehydes to give aldol products with higher yield and ee's.

Cellulose-supported chiral rhodium nanoparticles as sustainable heterogeneous catalysts for asymmetric carbon-carbon bond-forming reactions

Cellulose-supported chiral Rh nanoparticle (NP) catalysts have been developed. The Rh NPs, which were well dispersed on cellulose, catalyzed the asymmetric 1,4-addition of arylboronic acids to enones and enoates, one of the representative asymmetric carbon-carbon bond-forming reactions, in the presence of chiral diene ligands, providing the corresponding adducts in high yields with outstanding enantioselectivities without metal leaching. The solid-state NMR analysis of the chiral NP system directly suggested interactions between the Rh NPs and the chiral ligand on cellulose. This is the first example of using polysaccharide-supported chiral metal nanoparticles for asymmetric carbon-carbon bond-forming reactions.

Alginic acid aerogel: a heterogeneous Brønsted acid promoter for the direct Mannich reaction

Alginic acid, a biopolymer from brown algae, promotes a Brønsted acid catalyzed Mannich reaction in its aerogel formulation.

Chitosan as a sustainable organocatalyst: a concise overview

Increased demand for more sustainable materials and chemical processes has tremendously advanced the use of polysaccharides, which are natural biopolymers, in domains such as adsorption, catalysis, and as an alternative chemical feedstock. Among these biopolymers, the use of chitosan, which is obtained by deacetylation of natural chitin, is on the increase due to the presence of amino groups on the polymer backbone that makes it a natural cationic polymer. The ability of chitosan-based materials to form open-network, macroporous, high-surface-area hydrogels with accessible basic surface sites has enabled their use not only as macrochelating ligands for active metal catalysts and as a support to disperse nanosized particles, but also as a direct organocatalyst. This review provides a concise overview of the use of native and modified chitosan, possessing different textural properties and chemical properties, as organocatalysts. Organocatalysis with chitosan is primarily focused on carbon-carbon bond-forming reactions, multicomponent heterocycle formation reactions, biodiesel production, and carbon dioxide fixation through [3+2] cycloaddition. Furthermore, the chiral, helical organization of the chitosan skeleton lends itself to use in enantioselective catalysis. Chitosan derivatives generally display reactivity similar to homogeneous bases, ionic liquids, and organic and inorganic salts. However, the introduction of cooperative acid-base interactions at active sites substantially enhances reactivity. These functional biopolymers can also be easily recovered and reused several times under solvent-free conditions. These accomplishments highlight the important role that natural biopolymers play in furthering more sustainable chemistry.

Facile preparation of monolithic κ-carrageenan aerogels

To the best of our knowledge, it is the first study reporting the synthesis of monolithic κ-carrageenan aerogels with meso- and macroporous structures, being unique in physical and chemical properties. We demonstrate a novel method to synthesize κ-carrageenan aerogels in which potassium thiocyanate was used as the source of specific ions. Aerogels were characterized by envelope density analysis, scanning electron microscopy, nitrogen adsorption-desorption analysis, X-ray powder diffractometry and IR spectroscopy. By varying the concentration of κ-carrageenan between 0.5 and 3 wt%, the envelope density can be linearly increased from 40 to 160 kg m⁻³. The sulphate functional groups in the wet gel and the specific ions are the key factors controlling the volume shrinkage of aerogels which average about 66%. The aerogels exhibit a fibrillar structure similar to cellulose aerogels. The fibril thickness was observed to be 10-15 nm and the specific surface area was about 230 m² g⁻¹. The existing meso- and macroporous structures were confirmed by nitrogen adsorption-desorption isotherm analysis and scanning electron microscopy. The aerogels were completely pure, free of specific ions and confirmed to be amorphous by powder X-ray diffraction. Hence, these porous materials can provide a matrix with a chelating function which can be used as a host in many applications.

Always look on the "light" side of life: sustainable carbon aerogels

The production of carbon aerogels based on the conversion of inexpensive and abundant precursors using environmentally friendly processes is a highly attractive subject in materials chemistry today. This article reviews the latest developments regarding the rapidly developing field of carbonaceous aerogels prepared from biomass and biomass-derived precursors, highlighting exciting and innovative approaches to green, sustainable nanomaterial synthesis. A review of the state-of-the-art technologies will be provided with a specific focus on two complimentary synthetic approaches developed upon the principles of green chemistry. These carbonaceous aerogel synthesis strategies, namely the Starbon and carbogel approaches, can be regarded as "top-down" and "bottom-up" strategies, respectively. The structural properties can be easily tailored by controlling synthetic parameters such as the precursor selection and concentration, the drying technique employed and post-synthesis temperature annealing. In addition to these parameters, the behavior of these sustainable carbon aerogel platforms in a variety of environmental and energy-related applications will also be discussed, including water remediation and fuel cell chemistry (i.e., the oxygen reduction reaction). This Review reveals the fascinating variety of highly porous, versatile, nanostructured, and functional carbon-based aerogels accessible through the highlighted sustainable synthetic platforms.