Current Organocatalysis - Volume 4, Issue 3, 2017

Background: 2-Substituted-1,3-benzothiazoles, 1,3-benzimidazoles and 1,3-benzoxazoles are one of the important classes of bicycle hetero-compounds because these moieties can serve as unique and versatile scaffolds for experimental drug design. Among these herocycles, 1,3- benzothiazole derivatives have attracted considerable attention in the research area of synthetic as well as pharmaceutical chemistry because of its potent and significant pharmacological activities. These moieties act as antitumor, antiviral, anti-HIV, and microbiological agents. Some of the benzothiazole derivatives have also been used for treatment of autoimmune and inflammatory diseases, in the prevention of solid organ transplant rejection, epilepsy, amyotrophic lateral sclerosis, and analgesia. Moreover, these scaffolds were also utilized in industries as antioxidants, vulcanization accelerators, and a dopant in light emitting organic electroluminescent devices. Beside these, they also have interesting chemical properties and used as chemosensors for a range of analytes. Methods: As a consequence, a wide range of methods are available for synthesizing the benzothiazole nucleus and its derivatives. These methods enable to access benzothiazole unit with structural diversity and various substitutions in its core nucleus. Conclusion: The present review deals with the general method of condensation as well as recent advance method of cyclization which were adopted and reported in the literature for the synthesis of diverse benzothiazole nucleus.

Background: Quinolines are one of the most popular N-heteroaromatic compounds incorporated into the structures of many pharmaceuticals. The classical methods for the synthesis of quinoline derivatives include Skraup, Doebner–Von Miller, Conrad–Limbach, Combes, and Pfitzinger quinoline syntheses but the best-known method for the preparation of quinolines is Friedlander quinoline synthesis. A number of methods have been reported for the synthesis of quinolines involving a variety of catalysts. However, many of these procedures are not satisfactory because of harsh reaction conditions, poor yields, prolonged time period, and use of hazardous and often expensive catalyst. Hence, mild synthetic protocol for the synthesis of quinolines is highly desirable. Objective: Mild and efficient synthesis of quinolines in the presence of sulfonyl functionalized imidazolium ionic liquids is presented in this manuscript. Method: Various functional quinolines were synthesized by one–pot condensation reaction of 2– aminoaryl carbonyl compounds with an activated α–CH acid such as cyclic or acyclic β–dicarbonyl or aromatic carbonyl compounds in the presence of recyclable sulfonyl functionalized imidazolium ionic liquids. Results: Sulfonyl functionalized imidazolium ionic liquids exhibited remarkable activity towards synthesizing a broad range of quinolines and 1-(4-sulfonylbutyl)-3-methylimidazolium trifluoromethanesulfonate ionic liquid was found to be most efficient compared to other ionic liquids used for the synthesis of parallel library of quinolines. Conclusion: A cascade protocol for the synthesis of functional quinolines is developed by the condensation of 2-amino-5-chlorobenzophenone (or 2-aminobenzophenone) with a variety of cyclic or acyclic β-diketones or aromatic ketones in the presence of SO3H-functionalized ILs. The recyclability of ionic liquids, simple work-up procedure, short reaction time and high yields are the important attributes of the present protocol.

Background: Carbonyl compounds are one of the most important starting materials used in pharmaceutical and industries. Due to their high demand, a vast number of efficient and aerobic oxidation methods have been developed using suitable oxidizing agents or catalyst. Using cheaper starting materials and efficient methods which are more favorable are very important in terms of economic and environmental view point. The present synthetic protocol involved the oxidation of alcohols catalyzed by bimetallic molybdenum(VI) complex containing multidentate hydrazone ligand. Methods: Benzylic, allylic, propargylic, and aliphatic alcohols were oxidized using bimetallic molybdenum( VI) complex in aqueous medium using 15% hydrogen peroxide as a mild oxidizing agent. The products were confirmed using 1H and 13C NMR spectroscopy. Results: We have oxidized different varieties of alcohols including primary benzylic, allylic, propargylic and aliphatic alcohols to their corresponding aldehyde using our synthesized bimetallic complex. The process went smoothly and moreover the catalyst could be recovered with almost the same amount of product formation in the next catalytic cycles. Benzyl alcohols containing electron donating groups give high yield compared to benzyl alcohol containing electron withdrawing groups. Conclusion: The bimetallic cis-dioxido-molybdenum(VI) complex containing multidentate hydrazone ligand, catalyzed the oxidation of primary benzylic, allylic, propargylic and aliphatic alcohols. The process went smoothly and moreover the complex can be easily recovered without any decrease in percentage yield in the next cycles.

Background: The biopolymer-containing hybrid composite materials of silica have drawn attention owing to their promising properties and biocompatibility with living matter. Silica component in such hybrids is responsible for the properties like temperature and mechanical resistance, porosity, while the biopolymer offers extra functionality and framework to the hybrid matrices. This work is devoted to selective C-H bond oxidation and heterogeneous catalysis, which are important from the viewpoint of green and sustainable chemistry. Methods: A series of different metal acetylacetonates covalently anchored onto amine functionalized silica/starch composite [ASS-M(acac)n] were developed via surface functionalization with 3-aminopropyl- silica/starch composite, followed by schiff condensation of the surface -NH2 with M(acac)n [Co(acac)2, Cu(acac)2, Pd(acac)2, Ru(acac)3, Mn(acac)3, Co(acac)3]. Results: The catalytic activity of the synthesized catalysts was examined for the C-H bond oxidation of arylalkanes. Co(acac)2 supported onto amine functionalized silica/starch [ASS-Co(acac)2] was found to be efficient catalyst for the C-H bond oxidation of arylalkanes with t-BuOOH in water. Various alkyl benzenes including toluenes, ethylbenzene and alkylhydrocarbons were oxidized efficiently to their corresponding products. The catalyst was characterized by various techniques such as FT-IR, SEM, TEM, TGA and AAS analysis. Conclusion: A series of different organic/inorganic composite catalysts have been synthesized by covalently immobilizing metal acetylacetonate complexes over amine functionalized silica/starch composites. Easy preparation/heterogeneous nature of catalyst, higher yields, aqueous medium and versatility towards various substrates make this method a facile tool for desired oxidations.

Background: Although, Wittig reaction is mainly reported with aldehydes and phorphorus ylides, unfortunately, aldehyde oxidation mainly drops the efficiency of the reaction trailed by the decomposition or polymerization. There are several one-pot methods that have been testified to achieve an efficient Wittig reaction, but these protocols mainly hurt due to the requirement of toxic, oxidizing mediators and the inevitability of the process activation step, for example, the use of toxic Martin's reagent to oxidize alcohol during the Wittig olefination reaction. Methods: The exclusive physiochemical properties of imidazolium ionic liquids like low viscosity, negligible vapor pressure and high thermochemical stability make them a perfect candidate for biphasic nanocatalysis. Herein, we wish to report 1-n-butyl-3- methylimidazolium hexafluorophosphate (bmim.PF6) for the synthesis and stabilization of Ru (0) and Rh (0) nanoparticles (NPs). Further, we used these metal NPs for Wittig reaction as an effective catalytic system in liquid-liquid biphasic condition. Results: We created stable transition nanoparticles of [M0]n type in a dry ionic liquid medium under hydrogen atmosphere. The synthetic protocol of transition nanoparticles is very simple; they were prepared in dry bmim.PF6 reaction medium. The dry ionic liquid was used as a reaction medium because the presence of water may cause ionic liquid decay and may create transition metal phosphates along with hydrogen fluoride. The physiochemical properties of all the synthesized nanoparticles were analyzed by advanced analytical methods. These analyses confirmed that the particle size of well dispersed Ru (0) and Rh (0) nanoparticles ranged from 2.1 to 5.5nm. The Ru (0) and Rh (0) nanoparticles were used as catalysts under mild reaction conditions (without using any oxidizing agents) to obtain Wittig olefination reaction product in good yield and acceptable distereoselectivity in contrast to conventional solvent systems. In addition, easy product isolation and 8 times reuse of Ru (0) nanoparticles catalyst are the main results of this study. Conclusion: In summary, we synthesized and stabilized the Ru (0) and Rh (0) NPs using bmim.PF6 ionic liquid. We obtained narrow size distributed, agglomeration free and stable NPs of Ru (0) and Rh (0) with practical size ranging from 2.1 to 5.5 nm. Both the NPs were found active to catalyze Wittig olefination reaction in ionic liquid with respect to the conventional solvent system. The Ru (0) nanoparticle was recorded as an outstanding catalyst for the Wittig type elimination reaction. It was found active for 19 different types of alcohol and ylide. In most of the results, we obtained the Wittig reaction yield more than 65% with satisfactory diastereoselectivity. Additionally, no aqueous work-up is compulsory throughout the product isolation step. Simply diethyl ether extraction method (5 x 2mL) was applied to recover the reaction product without loss of catalytic systems. Ionic liquid immobilized Ru (0) NPs were recycled up to 8 runs without any important loss of product yield.