Current Organocatalysis - Volume 7, Issue 1, 2020

Carbon Dots (CDs) represent a new class of nanoparticles carbon-based having many application fields. This perspective summarizes the catalytic applications of CDs with potential future perspectives.

Background: Nuclear Magnetic Resonance (NMR) is one of the most employed techniques in structural elucidation of organic compounds. In addition to its use in structural characterization, it has been widely employed in the investigation of reaction mechanisms, especially those involving catalysis. Objective: In this review, we aim to provide recent examples of the interface of NMR and organocatalysis reaction mechanism. Methods: Selected examples of different approaches for mechanism elucidation will be presented, such as isotopic effect, catalyst labelling and online reaction monitoring. A discussion involving the use of solid-state NMR will also be disclosed. Conclusion: NMR consists of a non-destructive technique, extremely useful in the real-time identification of intermediates in crude reaction mixtures. With the advent of two-dimensional experiments and high field NMR spectrometers, the reports of studies involving mechanistic elucidation were greatly enhanced. In this context, nowadays NMR appears as a powerful tool for the comprehension of reaction mechanisms, including the possibility of the proposal of unknown reaction mechanisms within organocatalysis.

Background: The N-aryl piperazines are an important component of many drug products used for the treatment of malaria, depression, anxiety and Parkinson diseases. Buchwald-Hartwig amination is the latest and well-known reaction for Pd catalyzed direct synthesis of N-aryl piperazine from aryl halides. Although several Pd-ligand systems have already been discovered for this conversion, Pd nanoparticles are recently being used for this useful coupling reaction due to their recyclability and durability. Metal nanoparticles show enhanced catalytic activity compared to their bulk counterparts due to increased surface area at the edges and corners. The use of green algal extract in place of chemical ligands makes this process more environment-friendly and cost-effective. In this research, Pd nanoparticles synthesized using green alga C. Vulgaris were utilized as an alternative approach for the coupling reaction during the preparation of N-aryl piperazines. Methods: Synthesized Pd nanoparticles from C. Vulgaris were characterized by FTIR, SEM and XRD techniques. The catalytic activity of the synthesized nanoparticles was monitored for the synthesis of N-aryl piperazines by Buchwald-Hartwig reaction. The synthesized N-aryl piperazines were characterized by NMR, FTIR and mass analysis. Results: A very good catalytic activity of the synthesized Pd nanoparticles from green alga Chlorella vulgaris extract was observed. The green alga not only reduces the size of the Pd metal to nanoparticles but also acts as a green ligand for reduction of Pd(II) to Pd(0) during nanoparticle synthesis. Using this Pd nanoparticles-green ligand system, several N-aryl piperazines were synthesized in good to excellent yields. Reaction conditions for better conversion were optimized. The comparative advantage of the catalytic system with recently published works on Buchwald-Hartwig C-N coupling reaction is given. Recyclability and durability of the catalyst were explored and the results were found to be promising. A plausible mechanism of Pd nanoparticle catalyzed reaction is also proposed. Conclusion: Catalytic activity of the Pd nanoparticle synthesized from Chlorella vulagris in the synthesis of N-aryl piperazines by Buchwald-Hartwig reaction is reported first time to the best of our knowledge and understanding. The green approach of Pd catalyst to facilitate the reaction and its environmental impact is the main characteristic of the process.

Background: 1-Amidoalkyl-2-naphthols are an attractive group of organic compounds that can be converted to oxazine heterocycles and aminoalkyl naphthols. The derivatives of 1- amidoalkyl-2-naphthols have significant biological activities and act as drug candidates. Methods: 1-Amidoalkyl-2-naphthols were synthesized via the three-component condensation reaction of 2-naphthol, acetamide/benzamide and various aldehydes in the presence of 10 mol% of adipic acid as an organocatalyst under solvent-free conditions at 120°C. Results: A simple, efficient, and operative method for the synthesis of 1-amidoalkyl-2-naphtholes in the presence of adipic acid as the biodegradable catalyst is introduced. Easy operation, acceptable reaction times, eco-friendly, availability of starting materials, simple separation of products, and high yields of products are the significant results of this method. Conclusion: In this study, 1-amidoalkyl-2-naphthols were synthesized using commercially available reactants in excellent yields and relatively shorter times. In this process, microwave or ultrasonic waves were not used to provide energy for the reaction.

Background: Eusiderin A is a neolignan derivate, which makes up the majority of the secondary metabolite of Eusideroxylon zwageri. It has been reported as a potent biopesticide and antifungal agent. Previous studies on the oxidation of terminal methylene of the allylic chain in Eusiderin A have been able to produce primary alcohol, pinacol, and an aldehyde which demonstrated strong activity against plant pathogenic fungi, therefore activity against dermal fungi needs to be studied. Objective: The current study aims to improve the hydrophilicity of Eusiderin A via oxidation of the allylic chain in order to derive a potent antifungal property. Methods: Transformation of Eusiderin A has been achieved by using the Wacker Oxidation Method in combination with the α-Hydroxylation-Ketone Method to produce 7,3’-epoxy-8,4’-oxyneolignane-1’- carboxylic acid. The structure of the 7,3’-epoxy-8,4’-oxyneolignane-1’-carboxylic acid was identified from spectroscopy data. The in vitro antifungal activity study was performed using the paper disc diffusion method against Trichophyton mentagrophytes. Results: New molecule of natural Eusiderin A through the oxidation of the allylic chain to increase the hydrophilicity of Eusiderin A has been designed. Based on the observed UV, IR, 1H and 13C-NMR, and MS spectra, it can be stated that the 7,3’-epoxy-8,4’-oxyneolignane-1’-carboxylic acid has been formed. At a concentration of 50 ppm, this compound showed antifungal activity against Trichophyton mentagrophytes. Conclusion: It can be concluded that the 7,3’-epoxy-8,4’-oxyneolignane-1’-carboxylic acid is a potent antifungal agent as it is able to inhibit the Trichophyton mentagrophytes colonies growth.

Background: Much attention has been focused on heterogeneous catalysts. Reactions with these recoverable and reusable catalysts are clean, selective with high efficiency. Among the heterogeneous solid acid catalyst in organic synthesis, Carbon-Based Solid Acids (CBSAs), which are important solid acid with many practical and research applications have been extensively studied. In this work, green Pistachio peel, a biomass waste, was converted into a novel carbon-based solid acid catalyst (Pis-SO3H). Objective: The aim of this work is to synthesize highly sulfonated carbon as an efficient, recyclable, nontoxic solid acid catalyst by simultaneous sulfonation, dehydration and carbonization of green Pistachio peel as biomass and investigate the catalytic activity of Pis-SO3H in acetalization, thioacetalization, acylation of aldehydes and synthesis of 3,3'-Arylmethylene-bis(4-hydroxycoumarin) derivatives. Methods: Pis-SO3H was synthesized by an integrated fast one-step hydrothermal carbonization and sulfonation process in the presence of sulfuric acid. The characterization of the physicochemical properties of Pis-SO3H was achieved by XRD, FT-IR, FE-SEM, and elemental analysis. Results: The result of acid-base titration showed that the total acidity of the catalyst was 7.75 mmol H+g−1. This new heterogeneous catalyst has been efficiently used for the chemoselective thioacetalization, acetalization and acylation of aldehyde and the synthesis of biscoumarins under solvent-free conditions. All the reactions work easily in high yields. The antimicrobial activity of some of the biscoumarins was evaluated in screening by disk diffusion assay for the zone of inhibition. Conclusion: The catalytic activity of the Pis-SO3H was investigated during acetalization, thioacetalization, acylation and synthesis of biscoumarins. The results of protection of carbonyl groups and synthesis of biscoumarins in the present work offer effective alternatives for environmentally friendly utilization of abundant biomass waste.