Manoj Mondal

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.

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.

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 (1 : 1) 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.

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.

Chemoselective acyl C–O bond activation in esters for Suzuki–Miyaura coupling

Exploitation of the low binding affinity of NHC–palladium complex for the carbonyl oxygen of ester to form three-centered transition state led to the discovery of Suzuki–Miyaura coupling via unprecedented C(acyl)–O bond cleavage. The use of bulky NHC ligand resulted in direct coupling as it prevented decarbonylation of ester. This access to alternative chemoselectivity by careful selection has extended the synthetic portfolio of esters for the diversification of interesting molecular scaffolds.

Nickel-catalyzed reductive defunctionalization of esters and amides to aromatic hydrocarbons

The removal of ester and amide groups is of fundamental significance in organic syntheses. Under non-catalytic conditions, hydride sources are chiefly used for their reduction. Recently developed Ni-catalyzed one-pot reductive activation of esters and amides followed by tandem C–CO bond cleavage–decarbonylation facilitates their cleavage to aromatic hydrocarbons. Isolation and characterization of key reaction intermediates provide insight into this acyl C–O bond activation pathway.