Current Catalysis - Volume 5, Issue 1, 2016

Background: Pyrazoles are useful intermediates in pharmaceutical, agrochemical, photographic and other fields. Pyrazole nucleus has been reported to possess a wide spectrum of biological properties such as anti-inflammatory, anti-depressant, anti-bacterial, anti-tumor, anti-microbial, anti-viral, anti-fungal and anti-filarial agent. The aims of this paper to introduce readers the polyvinylsulfonic acid (PVSA) as a new catalytic system for the synthesis of pyrazoles. The PVSA was found to be an inexpensive, stable, easy to handle and biodegradable catalyst. Methods: The catalyst is well characterized using various techniques such as IR, XRD and DSC. All products were characterised using melting point, GC-MS, 1H and 13C NMR techniques. Results: The various acidic catalysts were compared with PVSA under optimized reaction condition and it was found that PVSA giving better yield of desired product. Different reaction parameters such as solvent, temperature, time, and substrate were studied to achieve optimal catalyst performance using PVSA as a catalytic system. Conclusion: We have described PVSA as a novel Bronsted acid catalyst for the synthesis of pyrazoles. The catalyst was isolated by simple filtration process and recycled up to four cycles providing the excellent yield of the pyrazole derivatives.

Background: Nitration of aromatic and hetero aromatic compounds has received enormous attention for the past several decades because many of these nitro products are used as chemical feed stocks and important intermediates for the synthesis of several lifesaving drugs, dyes, pharmaceuticals, perfumes, plastics and explosives. The classical nitration method demands the use of acid mixture (a potent mixture of concentrated nitric and sulfuric acids), which results in large amounts of inorganic acid waste pumped into the industrial drains. Such a large acid waste is not only hazardous but also causes environmental pollution and often leads to over nitration or by-products. Over the years, numerous useful methods have been developed for environmentally safe nitration. Poly ethylene glycol (PEG) and its derivatives have been extensively used as phase-transfer catalyst (PTC) in many commercial processes. Inspired by these procedures we report herein, a clean, economically cheap and, easy to operate, and efficient protocol for the nitration of aromatic compounds, using laboratory desktop chemicals such as nanocrystalline manganese (II) carbonate and polyethylene glycols (PEG). Materials and Methods: Chemicals were purchased from Merck, Sigma-Aldrich. Nitration reactions were conducted under sonication and microwave irradiation in addition to the conventional method of organic synthesis. The progress of the reaction was monitored by Thin Layer Chromatography on commercial precoated TLC plates. Melting points were determined by using open capillary tubes with a Buchi 510 apparatus and corrected. UV spectra were recorded on a Shimadzu UV-2700 model UV-VIS Spectrophotometer. Conclusion: In conclusion, a regioselective nitration of a various aromatic compounds using green, available, inexpensive and easy to handle catalyst PEG/Mn (II) has been reported. The scope and generality of this protocol was illustrated with several aromatic compounds, short reaction times with good to excellent yields. Non-conventional methods such as sonochemical and microwave affords the nitration of aromatic compounds of both ring activated and deactivated compounds.

Background: The hetero-Diels-Alder (HDA) reaction of the carbonyl compounds with conjugated dienes, sometimes referred to as oxa-DA reaction, is an important method for the preparation of dihydropyrans, which are useful starting materials for the synthesis of several natural products including carbohydrates. However, carbonyl group as such is a less reactive dienophile and usually requires the use of a catalyst to enhance its reactivity. Methods: The catalyst, namely ((R)-1,1´-bi-2-naphthoxy)titanium dichloride (15% mol) was generated in situ. On adding aldehyde, red brown colour developed and the solution was cooled to -15ºC followed by addition of dimethylbutadiene. The reaction mixture was stirred at the same temperature for 4-5 h under nitrogen atmosphere. Results: The hetero-Diels-Alder reaction of X-C6H4CHO (X=m-NO2, o-OMe, p-CF3, m-CF3) with DMB in the presence of ((R)-1,1´-bi-2-naphthoxy)titanium dichloride (15% mol) at ~ -10 - -15°C (ice-sodium chloride bath)) gave 3,6-dihydro-2H-pyrans in 37- 74% yields with 68- 95% ee. Diels-Alder reaction of the 1:1 complex of benzaldehyde with the catalyst, ((R)-1,1´-bi-2-naphthoxy)titanium dichloride with 2,3-dimethyl-1,3- butadiene was investigated at the DFT (B3LYP) level using minimal basis set, (STO-3G). The values of the activation enthalpies (ΔH≠) for the attack of DMB on the Re and Si faces differ by 1.37 kcal mol-1 indicating ee >90% for the Si face. Conclusion: Aromatic aldehydes undergo HDA reaction with DMB in the presence of ((R)-1,1´-bi-2- naphthoxy)titanium dichloride as catalyst to give the cycloadducts in 37-74% yields with the ee ranging from 68 to 95%.

Background: Lipases are excellent biocatalysts with dual ability of hydrolysis as well as esterification with high efficiency and substrate specificity. However, in the pristine form they have inherent limitations such as lack of reusability and limited thermal and pH stability those restrict their uses in commercial applications. These limitations can be overcome by their immobilization on the suitable supports. Methods: In the present study a new poly(acrylic acid)-based functional nanogel was prepared by a simple surfactant free emulsification method and functionalized to poly(acrylic azide) form by a simple chemical protocol. Nanogels were characterized by X-Ray diffraction (XRD), Fourier Transform Infra Red (FTIR) spectroscopy, and transmission electron microscopy (TEM) to get evidence of the network formation and post reaction. The well characterized precursor nanogel and its functional derivatives were used as supports to immobilize lipase (triacyl glycerin). The activity assay of the immobilized lipase was carried out by the standard assay method. The catalytic potential of the immobilized lipase was evaluated in hydrolytic reactions using p-nitrophenol palmitate as substrate. Results: Immobilization enhanced the lipase activity and the structure of the nanogels made significant difference in enhancing their activity as the lipase immobilized on poly(Acryl azide)-cl-divinylbenzene nanogel exhibited the higher activity than the lipase immobilized on the precursor nanogel. Also, its activity was far higher than the free lipase. The immobilized lipase exhibited thermal and pH stability, which is not possible with the free lipase. Conclusion: The lipase was immobilized by the covalent linkages. The immobilized lipase exhibited good activity, storability and reusability. These results were dependent on the nanogel structure. Hence, a suitable nanogel can be tailored to obtain the maximum gains of immobilization. The reported results have good technological potential.

Background: Currently, phase transfer catalysis is an experimental technique which is environmentally benign technique have become immensely popular in promoting various reactions. Methods: The reaction between 4-nitrophenol and propargyl bromide was carried out in a solid-liquid biphasic phase transfer catalytic system using potassium carbonate as base at 40°C. Results: Trace amount of water is introduced to the reaction system to enhance the reaction rate. The higher conversion of propargyl bromide was achieved in the presence of tetra-n-butylammonium bromide (TBAB) as catalyst. From the experimental evidence there is no side products were observed during or after the reaction. Conclusion: Various parameters were conducted to increase the conversion of propargyl bromide such as, stirring speed, various PTCs, temperature and amount of TBAB loading. All the experiments were conducted to obey pseudo-first order rate law. Apparent activation energy was also determined from the Arrhenius plot.

Background: Heterogeneous catalysts are the leading areas of research interest mainly due to the facilities of recovery and reusability. Water hyacinth (Eichhornia crassipes) is a renewable aquatic plant and is considered a hazard for other useful aquatic crops. In the search of a sustainable and eco-friendly process, a novel heterogeneous catalyst has been prepared from this plant for the use in the synthesis of bis(indolyl)methanes and bis(pyrrolyl)methanes. These compounds are important building blocks for the preparation of pharmaceutically significant porphyrins and related compounds such as dipyrrins, calixpyrroles, chlorins, corroles, etc. Methods: Water hyacinth, which is available in plenty in Northeast of India at no cost, is used to prepare the heterogeneous catalyst. The catalyst has been fully characterized by AAS, FESEM, BET, TGA analysis, FT IR spectroscopy, XRF, powder XRD, EDX and TEM. The efficacy of the catalyst in the synthesis of bis(indolyl)methane and bis(pyrrolyl)methanes under microwave irradiation in solvent free condition has been examined. Results: The catalyst has good thermal stability and exhibits high catalytic activity. It consists of a mixture of carbonates, oxides and chlorides of varieties of elements such as C, Si, Mg, P, S, K, Ca, Na, etc. The catalyst has proved highly effective in as many as 20 sets of reactions leading to the formation of bis(indolyl)methanes and bis(pyrrolyl)methanes. The catalyst can be reused. Conclusion: The study provides a novel application of water hyacinth (Eichhornia crassipes) which is hitherto considered an aquatic hazard for other crops. It brings into light a novel heterogeneous solid base catalyst which is cost effective, environmentally safe and recyclable, and can be used in catalytic amount in solvent free organic transformations.

Background: The aim of this paper is to introduce about the synthetic strategy of new polyoxometalate compounds using single step reaction procedure and their oxidative catalytic activity. In this context herein, we report green oxidation process for the oxidation of alcohols to carbonyl compounds, using yttrium containing dimeric K11[Y(PW11O39)2]·35H2O (1) and tetrameric K16Na6[(PY2W10O38)4 (W3O8)(OH)4(H2O)2]·56H2O (2) phosphotungstates compounds in the presence of H2O2 oxidant and reused up to five catalytic cycles. Methods: The conventional aqueous solution method was used to synthesize a single crystalline solid material with good yields. The different parameters such as pH, temperature, time and nature of ligand were taken under consideration for the single step reaction procedure under mild conditions. The catalytic alcohol oxidation reactions were performed using very less catalyst loading and H2O2 co-oxidant in eco-friendly water as a solvent. Results: The solid state characterization of compounds 1 and 2 was performed using single crystal X-ray diffraction, FT-IR spectroscopy, thermogravimetric analysis and solution stability was also checked by 31P NMR spectroscopy. The ICP-AES analysis was performed for the chemical composition of the compounds. A variety of primary and secondary alcohols were oxidized to corresponding carbonyl compounds with excellent yields in presence of very less catalyst loading and 2:1 H2O2/substrate ratio in water. Conclusion: In conclusion, we have isolated two yttrium containing Keggin type dimeric and tetrameric phosphotungstate complexes by the single step reaction of K14[P2W19O69(H2O)]·24H2O POM ligand with Y(NO3)3·6H2O salt. The isolated compounds were further characterized with FT-IR spectroscopy, single crystal X-ray diffraction, 31P NMR, thermogravimetric and elemental analysis. The catalytic alcohol oxidation studies show good catalytic conversions, and finally FT-IR spectra of recovered catalysts after fifth cycle shows both catalysts retains their identity after fifth catalytic cycle.