Utpal Bora

Recent advances in manganese(III) acetate mediated organic synthesis

The preceding four decades have witnessed a major expansion in the field of free-radical assisted reactions, as they are becoming a key tool in organic, polymer and complex natural product chemistry. This progress has clearly changed the traditional idea of free-radical reactions being “extremely uncontrollable”, thereby allowing chemists to deal with free radicals in a more accurate and controlled way. Manganese(III) acetate mediated radical reactions have reached distinction in this field, as a wide range of protocols involving manganese(III) acetate under mild and efficient reaction conditions have been developed for the synthesis of biologically active and important organic molecules through oxidation, addition and cyclization reactions, which are in general difficult to achieve by the established synthetic operations. The present review focuses on manganese(III) acetate mediated oxidative free-radical reactions that have been developed and adapted during the last five years. It includes the advancements made in the field of free-radical promoted carbon–carbon and carbon–oxygen bond forming reactions, illustrating the examples of furans, hydrofurans, pyrroles, bicyclic lactones, biaryls, oxepins, indole-alkaloids, 1,2-benzanthracene and C60-fused tetrahydronaphthalene synthesis. Numerous manganese(III) acetate initiated carbon–phosphorus bond formation reactions were also devised during recent years, involving both phosphonyl and phosphinoyl radicals, and are a key part of this review. Manganese(III) acetate promotes functionalization of significant organic compounds, thereby guiding the synthesis of important and complex molecules, such as the formation of propellanes, quinolinones, 1,2-dioxalanes; oxidation of indoles; lactonization of fullerene [C60], malonyl cyclopentanes, glycols; and hydroperoxidation of heterocycles. The review will also envelop several recent milestones that have been achieved in the chemistry of natural products precising the vital role of manganese(III) acetate in the key synthetic steps.

A palladium salen complex: an efficient catalyst for the Sonogashira reaction at room temperature

A tetradentate Schiff base derived palladium complex has been developed as a catalyst for the room temperature Sonogashira reaction under copper free conditions. The main catalytic species, a Pd-complex was derived from Schiff base ligand N,N′-bis(salicylidene)-arylmethanediamine and Pd(OAc)2. Electron rich, electron deficient and sterically hindered aryl iodides underwent smooth coupling to afford good to excellent yield of diaryl alkynes in isopropanol at room temperature. The protocol is also suitable for aliphatic alkynes.

Eco-friendly Suzuki–Miyaura coupling of arylboronic acids to aromatic ketones catalyzed by the oxime-palladacycle in biosolvent 2-MeTHF

Oxime-palladacycle catalyzed Suzuki–Miyaura cross-coupling reaction of acid chlorides and arylboronic acids to yield aryl ketones was developed. The remarkable benefit of this method is the use of water immiscible (practically) 2-MeTHF as solvent, which provides easy isolation of the crude reaction mixture just by separation of 2-MeTHF–water layers, and then evaporation of 2-MeTHF. Moreover, the use of relatively equal proportion of substrates and less generation of side products make the method highly atom economic.

A highly efficient copper and ligand free protocol for the room temperature Sonogashira reaction

A mild and efficient catalytic system based on PdCl2 and Na2SO4 has been developed for the Sonogashira reaction of aryl iodides at room temperature. The system provides a simple route to obtain polyfunctional alkynes under ligand and copper free conditions. The procedure is equally efficient for aliphatic alkynes and aryl bromides.

Palladium-Schiff-base-silica framework as a robust and recyclable catalyst for Suzuki–Miyaura cross-coupling in aqueous media

A silica supported palladium catalyst, Pd@imine–SiO2 was prepared by immobilizing Pd(OAc)2 onto silica gel through coordination of imine, generated via Schiff-base condensation between 3-aminopropyltriethoxysilane (APTES) functionalized silica gel and salicylaldehyde. The prepared catalyst was characterized by FT-IR, BET surface area measurements, XRD, SEM-EDX, EDS-mapping and ICP-AES analysis. The imine-based catalyst exhibited excellent activity in the Suzuki–Miyaura cross-coupling reactions of aryl bromides with arylboronic acids in iPrOH/H2O (1u2006:u20061) at room temperature. The reaction proceeds under mild reaction conditions and the catalyst is recyclable, thus offering an environmentally benign alternative to the existing protocols.

Analysis of the water extract of waste papaya bark ash and its implications as an in situ base in the ligand-free recyclable Suzuki–Miyaura coupling reaction

The conversion of waste papaya-bark to ash–water extract via low-temperature combustion, and its utilisation as an efficient and environmentally friendly in situ basic medium for the Suzuki–Miyaura cross-coupling reaction at room temperature are reported. The papaya-bark ash was characterized by EDX, ion-exchange chromatography and flame photometry to reveal a broad range of active metal oxides. The chemical analysis reports of ash showed the presence of oxides of K, Ca, Na, Li and Mg, which possibly in the presence of water produce the corresponding hydroxides in situ, responsible for the basicity. Application of the ash–water extract as a base was highly effective for ligand-free Pd(OAc)2 catalyzed Suzuki–Miyaura cross-coupling reaction. The reaction proceeds smoothly without any promoter/ligand to give excellent yields. Moreover, after completion of the reaction, the catalytic system could be easily recovered by simple fractional separation, and recyclable at least five times, with the loss of some catalytic activity from the 3rd cycle onward.

Starch assisted palladium(0) nanoparticles as in situ generated catalysts for room temperature Suzuki–Miyaura reactions in water

A simple and efficient methodology for the in situ generation of Pd-nanoparticles in the presence of starch and NaOH at room temperature has been developed. It has been observed that starch played a dual role in controlling the reduction rate of metal ions and the aggregation process of metal atoms in solution. The in situ generated Pd-nanoparticles show excellent catalytic activity in the Suzuki–Miyaura cross coupling reaction of electronically diversified arylbromides and arylboronic acids in pure water at room temperature with low catalyst loading.

Rice husk derived nanosilica supported Cu(II) complex: an efficient heterogeneous catalyst for oxidation of alcohols using TBHP

We have developed a heterogenized catalyst of Cu(II) complex immobilized onto nanosilica obtained from rice husk (RH), through coordination of imine, generated via Schiff-base condensation between 3-aminopropyltriethoxysilane (APTES) functionalized nanosilica and acetamide. The synthesized catalysts were characterized by FT-IR, XRD, BET surface area measurements, ESR, SEM-EDX and ICP-AES. The synthesized catalyst showed excellent activity towards controlled oxidation of different primary and secondary alcohols in the presence of tert-butyl hydrogen peroxide (TBHP). Furthermore, the catalysts could be recovered by simple filtrations and can be reused upto many consecutive cycles without loss in activity.

Biosynthesis of poly(ethylene glycol)-supported palladium nanoparticles using Colocasia esculenta leaf extract and their catalytic activity for Suzuki–Miyaura cross-coupling reactions

A simple and green protocol for the synthesis of poly(ethylene glycol) stabilized palladium nanoparticles under ambient conditions from the aqueous extracts of Colocasia esculenta leaves has been reported. The nanoparticles are characterized using UV-visible spectroscopy, FTIR spectroscopy, XRD and SEM analysis. The prepared Pd NPs showed excellent catalytic activity towards Suzuki–Miyaura cross coupling reactions for a wide variety of aryl halides and phenyl boronic acid substrates. The catalytic system was found to be recyclable and could be reused in subsequent catalytic runs without significant loss of activity.

O -Arylation with nitroarenes: metal-catalyzed and metal-free methodologies

In this article, we focus on the introduction of nitroarene as an alternative electrophile for Caryl–O cross-coupling chemistry. In polar aprotic solvents and in the presence of a base at elevated temperature nitroarene undergoes cross-coupling with arylboronic acids or phenols under metal or metal-free reaction conditions. Compared to the conventional aryl halides, nitroarene provides highly attractive and environmentally friendly options for the synthesis of diaryl ether derivatives.