Arjun Kumbhar

Ligand-free Pd catalyzed cross-coupling reactions in an aqueous hydrotropic medium

A simple, efficient and ligand-free protocol for the Suzuki–Miyaura reaction and base-free Heck–Matsuda reactions under mild reaction conditions have been developed over palladium supported on activated carbon (Pd/C) in an aqueous hydrotropic solution. The catalyst Pd/C was fully characterized by TG-DTA, SEM, EDS, XRD, XPS, BET and ICP-AES analyses. This green methodology represents a cost-effective and operationally convenient method for the synthesis of a variety of biaryls, prochiral ketones, and acrylates under the conditions that are tolerant for a broad range of functional groups with good to excellent yields. The developed Pd/C–aqueous hydrotrope combined catalytic system is well suited for the 3R approach (reducible, robust, and recyclable) for different cross-coupling reactions without an appreciable loss of its activity.

Ultrasound promoted efficient and green synthesis of β-amino carbonyl compounds in aqueous hydrotropic medium.

Ultrasound promoted synthesis of β-amino carbonyl compounds in aqueous hydrotropic medium at ambient temperature is reported. The remarkable features of the new procedure are shorter reaction time, excellent yields in aqueous medium, cleaner reaction profile and simple experimental and work-up procedure.

A Merrifield resin supported Pd–NHC complex with a spacer(Pd–NHC@SP–PS) for the Sonogashira coupling reaction under copper- and solvent-free conditions

Synthetic applications of a polymer supported air-stable palladium NHC complex with a spacer (catalyst 6, Pd–NHC@SP–PS) and without a spacer (catalyst 7, Pd–NHC@PS) have been studied for the Sonogashira cross-coupling reaction. The catalysts were prepared by immobilizing pre-formed IL on the polymer and reacting the resultant modified polymer with Pd(OAc)2 in THF at 60 °C. Both the catalysts were characterized using IR, SEM, EDS, ICP, XPS and TGA/DTA. Catalyst 6 has been found to be more active than catalyst 7, due to the greater accessibility of active catalytic sites, for a variety of aryl bromides and terminal alkynes in solvent and copper free Sonogashira cross-coupling reactions under aerobic conditions. The effect of the spacer, nature of base, reaction temperature, the catalyst loading as well as reusability was also investigated.

Silica Tethered Pd–DABCO Complex: An Efficient and Reusable Catalyst for Suzuki–Miyaura Reaction

A palladium-based catalyst supported on DABCO-functionalized silica was successfully prepared by a facile procedure. The prepared heterogeneous catalyst showed a high activity for the Suzuki–Miyaura reaction of aryl bromides, affording excellent yield in all the cases investigated. Interestingly, the catalyst could be quantitatively recovered from the reaction mixture and recycled for five times without any significant loss in activity. Furthermore, this protocol could be extended to the palladium catalyzed synthesis of annulated pyrazines.Graphical Abstract

Hydrotrope Induced Catalysis in Water: A Clean and Green Approach for the Synthesis of Medicinally Relevant Bis(indolyl)methanes and 2-Aryl Benzimidazoles

Abstract An efficient synthesis of medicinally relevant bis(indolyl)methanes and 2-aryl benzimidazoles has been achieved in an aqueous hydrotropic solution at ambient temperature. Substantial enhancements in the rates of the reactions along with easy recovery of the products were noted. Additionally, the aqueous hydrotropic solution could be reused for several cycles with modest change in yields of products. GRAPHICAL ABSTRACT

Palladium nanoparticles supported on a titanium dioxide cellulose composite (PdNPs@TiO2–Cell) for ligand-free carbon–carbon cross coupling reactions

Well-dispersed non-spherical PdNPs with a diameter of 39–45 nm supported on a TiO2–cellulose composite (PdNPs@TiO2–Cell) can be synthesized by a simple and clean route. The catalyst was well characterized by XRD, FE-SEM, EDS, and TEM techniques. The PdNPs have good dispersity on the TiO2–Cell support. This results in excellent catalytic activities for the synthesis of biphenyls, acrylates, acetylenes and prochiral ketones using low Pd loading (1 mol%) at comparatively low temperature. The effects of the nature and amount of bases, nature of solvents, amount of catalyst and the reaction temperature on the activity of PdNPs@TiO2–Cell were thoroughly investigated. The catalyst showed at least four times reusability without decrease in catalytic activity.

Application of novel multi-cationic ionic liquids in microwave assisted 2-amino-4H-chromene synthesis

Novel multi-cationic ionic liquids containing a mesitylene backbone with acetate and methane sulphonate anions have been synthesized. These ionic liquids were used for the synthesis of 2-amino-4H-chromenes under microwave heating. The effects of nature and amount of ionic liquids on the yield and reaction time were thoroughly investigated. The ionic liquids showed a considerable level of reusability without a significant decrease in catalytic activity. We have successfully combined the advantages of microwave technology with ionic liquids to facilitate the rapid construction of chromene skeletons from readily obtainable and inexpensive materials via a multicomponent strategy.

Hydrotrope induced synthesis of 1,8-dioxo-octahydroxanthenes in aqueous medium

Abstract Hydrotrope induced synthesis of 1,8-dioxo-octahydroxanthenes from aromatic aldehydes and dimedone/1,3-cyclohexadione in aqueous medium is reported. The remarkable features of the new procedure are high conversions, shorter reaction times, cleaner reaction profiles, and simple experimental and work-up procedures.

Hydrotrope: green and rapid approach for the catalyst-free synthesis of pyrazole derivatives

An efficient synthesis of pyrazole derivatives by condensation of 1,3-diketone and hydrazines/hydrazides has been achieved in aqueous hydrotropic solution under catalyst-free conditions within a very short time. The present protocol is beneficial as it includes mild reaction conditions, shorter reaction times, use of universal solvent water, which avoids volatile organic solvents, high yields of products, and being environmentally friendly.Graphical Abstract

Synthesis of quinoxalines and pyrido[2,3-b]pyrazines by Suzuki–Miyaura cross-coupling reaction

Bromo-substituted quinoxalines and pyrido[2,3-b]pyrazines were synthesized by condensation reactions of aromatic 1,2-diamino and 1,2-diketone compounds. These compounds were used as common intermediates for postcondensation modification by Suzuki–Miyaura coupling reaction to form aryl-substituted quinoxalines and pyrido[2,3-b]pyrazines. High-efficiency with improved product yield of aryl-substituted quinoxalines and pyrido[2,3-b]pyrazine derivatives was achieved by conducting a coupling reaction in presence of Pd(dppf)Cl2·CH2Cl2 in tetrahydrofuran (THF) using K2CO3 as base at reflux temperature.Graphical abstract