Current Organocatalysis - Volume 9, Issue 2, 2022

The supported ionic liquid (SIL) membranes have demonstrated huge potential for numerous applications in current separation science and catalysis. Membrane technology allows for the separation of complex mixtures of gases, vapors, liquids and /or solids below trivial conditions. Simultaneous chemical transformations can also be achieved in membranes by using catalytically active materials comprising the membrane or embedded catalysts in the custom-built membrane reactors. In the present editorial, the remarkable contribution of liquid membranes in catalysis is highlighted. Some recent applications are presented and compared with conventional methods. In addition, SILs and their applications in catalysis, catalytic membranes, and recent advances in membrane separation processes are briefly described.

Aim and Objective: In this work, we have prepared two bis-dicationic ionic liquids with the same cationic core (Bis-imidazole) and different counter-anions using sulfuric acid and perchloric acids. After that, the efficiency and ability of these compounds as catalysts were investigated and compared with respect to the promotion of Knoevenagel condensation and synthesis of benzo[ b] pyran derivatives to see the effect of the anionic counter-part in the reaction. Materials and Methods: In a 25 mL round-bottomed flask, a mixture of aldehyde (1.0 mmol), 1,3-- cyclo dicarbonyl (2.0 mmol), and the desired amount of the above-mentioned acidic ionic liquids was heated at 90°C in the absence of solvent (Reaction A). In a 25 mL round-bottomed flask, a mixture of aldehyde (1.0 mmol), 1,3-cyclo dicarbonyl (1.0 mmol), malononitrile, (1.1 mmol) and calculated amounts of the ionic liquid in water (3.0 mL) was heated at 80°C (Reaction B) for the appropriated time. After the completion of the reaction which was monitored by TLC of (n-hexane: EtOAc; 3:1). 10 mL of water was added and the mixture was stirred for 2 minutes. Then, the products were separated by filtration and washed several times with water, after drying, the pure products were obtained. Results: Comparison of the obtained results from both the ionic liquids revealed that [H2- Bisim][HSO4]2, because of its more acidic structure, had shown a more catalytic activity in the preparation of 1,8-dioxo-octahydro-xanthene derivatives but [H2-Bisim][ClO4]2 was relatively more efficient for the synthesis of tetrahydrobenzo[b]pyran derivatives. Because the stronger acidic nature of [H2-Bisim][HSO4]2 may prevent the simple activation of malononitrile in the reaction media. Conclusion: In this study, we have introduced efficient methods for the synthesis of 1,8-dioxo-octahydro- xanthene and tetrahydrobenzo[b]pyran derivatives in the presence of catalytic amounts of [H2-Bisim][ClO4]2 and [H2-Bisim][HSO4]2. These methods have several advantages such as ease of preparation and handling of the catalysts, high reaction rates, excellent yields, eco-friendly procedures, and simple work-up.

An efficient and green method for the Friedländer quinoline synthesis has been described. The synthesis was performed under microwave irradiation using ionic liquid [Bmim]HSO4 as a catalyst. A diverse range of quinoline derivatives was obtained in high yields from 2-aminoaryl aldehydes and ketones under solvent-free conditions.

The use of proline-based organocatalysts has acquired significant attention in organic synthesis, especially in enantioselective synthesis. Proline and its derivatives are proven to be quite effective chiral organocatalysts for a variety of transformations, including the aldol reaction, which is considered as one of the important C-C bond forming reactions in organic synthesis. The use of chiral organocatalysts has several advantages over its metal-mediated analogues. Subsequently, a large number of highly efficient proline-based organocatalysts, including polymer-supported chiral analogues, have been identified for aldol reaction. The use of polymer-supported organocatalysts exhibited remarkable stability under the reaction conditions and offered the best results, particularly in terms of its recyclability and reusability. These potential benefits along with their economic and green chemistry advantages have led to the search for many polymer-supported proline catalysts. In this review, recent developments in exploring various polymer immobilized proline- based chiral organocatalysts for asymmetric aldol reactions are described.

Background: Baylis-Hillman reaction requires cheap starting materials, easy reaction protocol, and possibility to create the chiral center in the reaction product has increased the synthetic efficacy of this reaction which also suffers from high catalyst loading, low reaction rate, and poor yield. Objective: The extensive use of various functional or non-functional ionic liquids (ILs) with organocatalyst acts not only as reaction medium but also as a support to anchor the catalysts to increase the reaction rate of various organic transformations. Methods: In this manuscript, we have demonstrated the synthesis of quinuclidine-supported trimethylamine-based functionalized ionic liquid as a catalyst for the Baylis-Hillman reaction. Results: We obtained the Baylis-Hillman adducts in good, isolated yield along with low catalyst loading, short reaction time, wide substrate scope, easy product, and catalyst recycling. N- ((E,3S,4R)-5-benzylidene-tetrahydro-4-hydroxy-6-oxo-2H-pyran-3-yl) palmitamide was also successfully synthesized using CATALYST-3 promoted Baylis-Hillman reaction. Conclusion: We successfully isolated the 25 types of Baylis-Hillman adducts using three different quinuclidine-supported ammonium-based ionic liquids such as Et3AmQ][BF4] (CATALYST-1), [Et3AmQ][PF6] (CATALYST-2), and [TMAAmEQ][NTf2](CATALYST-3) as new and efficient catalysts. Generally, all the reactions demonstrated higher activity and gave good to high yield in competition with various previously reported homogenous and heterogeneous catalytic systems. Easy catalyst and product recovery followed by 6 times of catalysts recycling were the added advantages of the prosed catalytic system. Tedious and highly active N-((E,3S,4R)-5-benzylidene-tetrahydro- 4-hydroxy-6-oxo-2H-pyran-3-yl) palmitamide derivative was also synthesized using CATALYST- 3 followed by Baylis-Hillman reaction.

Aim: The objective of the work is to synthesize, characterize and biochemically analyze Azo-metal complex of Embelin with Second Group Transition Metals. Background: The genus Embelia is a well-known herb and has considerable importance in the field of pharmaceutical chemistry. The plant species has been used considerably as a traditional medicine in Ayurveda, old Chinese medicine, and Siddha for a long time. The dried berries of this plant, called “vidanga” have boundless biochemical properties, like anthelmintic, carminative, antibacterial, antibiotic, and hypoglycemic. Objective: Embelin, metal-embelin (EM) and azo-metal-embelin (EAM) complexes were synthesized, analysed for antioxidant and antimicrobial properties. Methods: The metal-embelin (EM) complexes and metal-azo-embelin (EAM) complexes were synthesized by pure mixing of embelin, azo-embelin and metals viz, Tc, Ru, Rh, Pd, Ag and Cd. The embelin and EAN complexes were analysed by various spectroscopic techniques, viz, UV-visible, FTIR, NMR, TGA, MS and CHNS analysis. Results: The results authenticate the reaction between metals, and bidentate embelin occurs via quinonic and enolic oxygen atoms as [M (Emb) 2 (H2O)] 2H2O and [M (Emb-Azo)2 (H2O)2]. The antioxidant results show that the complexation between metals and compounds decreases the antioxidant potential significantly. In contrast, the antimicrobial activity shows that cobalt and nickel embelin complexes showed more than 74% growth inhibition against the concerned microbes in comparison to embelin alone. Conclusion: As the results are taken into consideration, the scavenging property of embelin shows the reduction in power upon complexation with metals and azo-metals. Also, embelin and its associates as metal-embelin and metal-azo-embelin can be used as antioxidant and antimicrobial agents significantly.

Background: Polycarboxylic acids are of interest as simple mimics for cellulase enzyme- catalyzed depolymerization of cellulose. In this study, DFT calculations were used to investigate the effect of structure on dicarboxylic acid organo-catalyzed hydrolysis of cellulose model compound D-cellobiose to D-glucose. Methods: Binding energy of the complex formed between D-cellobiose and acid (Ebind), as well as glycosidic oxygen to dicarboxylic acid closest acidic H distance, were studied as key parameters affecting the turn over frequency of hydrolysis in water. Results: α-D-cellobiose - dicarboxylic acid catalyst down face approach showed high Ebind values for five of the six acids studied, indicating the favorability of the down face approach. Maleic, cis-1,2-cyclohexane dicarboxylic, and phthalic acids with the highest catalytic activities showed glycosidic oxygen to dicarboxylic acid acidic H distances 3.5-3.6 Å in the preferred configuration. Conclusion: The high catalytic activities of these acids may be due to the rigid structure, where acid groups are held in a fixed geometry.

Aim: Here, we have reported easy one-pot access to a series of oxazolines using a modified Castro-Stephens coupling protocol. Background: 2-oxazolines have been shown to have significant biological activity and wide-ranging applications in organic chemistry. These properties make oxazolines as heterocyclic compounds of immense importance. Objective: The objective of this study is to synthesize oxazoline derivatives via an economical and one-pot protocol. Methods: 2-oxazoline has been synthesized through Cu-powder mediated Castro-Stephens coupling and intramolecular cyclization route. The mechanism involves a rearrangement in which one of the oxygen from the N-acylamino alcohol group is liberated as water and then transferred to alkyne functionality to form 2-oxazoline derivatives. The oxazolines were characterized by NMR, mass, and XRD studies. Results: The protocol is economically viable and uses readily available Cu-powder along with DMF for cross-coupling and cyclization steps. Conclusion: We have reported a one-pot protocol to prepare 2-oxazolines using a Castro-Stephens coupling and intramolecular cyclization.

Background: The present work describes an eco-friendly and sustainable approach for the Knoevenagel condensation of an aromatic aldehyde with ethyl cyanoacetate, and salicylaldehyde with Meldrum acid for the synthesis of ethyl benzylidenecyanoacetate and 3-carboxy coumarin (2-oxo-2H-1-benzopyran) derivatives, respectively. The reaction was performed under green catalytic media-Water Extract of Watermelon Fruit Peel Ash (WEWFPA), which is an eco-friendly protocol derived from the agro-waste feedstock. Various protocols have been reported for the synthesis of Knoevenagel condensation reaction using a hazardous catalyst or/and solvents found toxic to the environment, requiring longer reaction time, giving poor yield, and requiring purification of the final product. The method at hand provides several added advantages like: being a completely green method, economic, inexpensive catalyst, and the final product isolated is in pure form with good yield. Objective: The objective of the study was to develop a green methodology for the synthesis of ethyl benzylidenecyanoacetate and 3-carboxy coumarin derivatives. Results: The agro-waste based catalyst developed in the present study avoids the use of external inorganic/ organic bases and additives. Knoevenagel condensation of ethyl benzylidenecyanoacetate and 3-carboxy coumarin derivatives is carried out under room temperature using microwave irradiation, which is a solvent-free synthesis, requiring less time and giving better yield. Methods: We have demonstrated that WEWFPA can be employed as a green homogenous agrowaste for the synthesis of ethyl benzylidenecyanoacetate and 3-carboxy coumarin derivatives under rt stirring and microwave irradiation in a very economical way. The developed method is found to be simple and robust, non-hazardous and solvent-free to obtain the target product. Conclusion: In conclusion, we have established an efficient, simple, agro-waste based catalytic approach for the synthesis of ethylbenzylidenecyanoacetate and 3-carboxy coumarin derivatives employing WEWFPA as an efficient catalyst under rt stirring and microwave synthesis. The method is a green, economical and eco-friendly approach for the synthesis of Knoevenagel condensation products. The advantages of the present approach are that the reaction is a solvent-free synthesis, requiring no external metal catalyst, chemical base free, short reaction time and excellent yield of product. The catalyst is agro-waste derived, which is abundant in nature, thus making the present approach a greener one.

Objective: Microwave technology, together with enzymatic catalysis, is a nature-friendly chemical synthesis method with low wastage of solvent and a good yield of the products. Methods: Enzymes from various microorganisms can be used in the biochemical processes of a wide range of compounds assisted by microwave irradiation. Results: In this work, the microwave-induced reaction of α-keto β-lactams by Baker's yeast in organic solvent was conducted to afford optically active cis and trans-α-hydroxy-β-lactams for the first time. Conclusion: These hydroxy compounds are the precursors of numerous natural products of medicinal significance.