Debasish Sengupta

Pd/Cu bimetallic nanoparticles embedded in macroporous ion-exchange resins: an excellent heterogeneous catalyst for the Sonogashira reaction

Cationic and macroporous amberlite resins with formate (HCOO−) as the counter anion (ARF) have been used to prepare a new class of heterogeneous Pd/Cu bimetallic composite nanoparticles (NPs) (Pd/Cu–ARF). The physicochemical characteristics of Pd/Cu–ARF were examined with the help of FTIR spectroscopy, X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and atomic absorption spectroscopy (AAS). XRD and TEM showed the existence of composite NPs made of metallic Pd, PdO and CuO. The TEM analysis revealed fairly uniform distributions of composite NPs of average size ∼4.9 nm. The as-synthesized nanocomposite material (Pd/Cu–ARF) exhibited high catalytic activity in the Sonogashira cross-coupling reaction between aryl iodide and terminal alkynes. Heterogeneity of the catalytic activity was evidenced from different tests (hot-filtration and catalyst-poisoning experiments) and the recycling ability of the catalyst was examined for five consecutive runs without any significant loss of activity.

Graphene oxide as a carbocatalyst: the first example of a one-pot sequential dehydration–hydrothiolation of secondary aryl alcohols

An efficient and mild one-pot graphene oxide (GO)-catalyzed sequential dehydration–hydrothiolation reaction has been developed for the first time from a mixture of secondary aryl alcohols and thiols. The resulting unsymmetrical thioethers were prepared under metal-free conditions. The catalyst can be reused for five runs with appreciable conversions.

Reduced graphene oxide supported Ni nanoparticles: a high performance reusable heterogeneous catalyst for Kumada-Corriu cross-coupling reactions

Reduced graphene oxide (RGO) has emerged as an excellent 2D catalyst support. A high concentration of air stable Ni(0) nanoparticles (∼40 wt%) of average size 11 nm can be loaded on reduced graphene oxide. In this article Kumada–Corriu cross-coupling reactions have been studied extensively using RGO supported 40 wt% Ni nanoparticles (Ni/RGO-40) as a heterogeneous catalyst. The reaction conditions were optimized by performing the cross-coupling between 4-iodoanisole and phenylmagnesium chloride in the presence of a catalytic amount of Ni/RGO-40. A detailed study of the catalysis was performed by varying the haloarenes and Grignard reagents under the optimized conditions with yields of 91–72%. The products were characterized by 1H and 13C NMR spectroscopy. The catalyst was found to be significantly reusable. Finally the recovered Ni/RGO-40 was characterized by X-ray diffraction (XRD) and Raman spectroscopy and found to be unaffected. Considering the catalytic performance, reusability and economic reasons, Ni/RGO-40 can be of technological importance.

MnO2 nanowires anchored on amine functionalized graphite nanosheets: highly active and reusable catalyst for organic oxidation reactions

A facile method of synthesizing MnO2 nanowires on amine functionalized graphite nanosheet (AFGNS) has been accomplished. A probable mechanism has been proposed where the –NH2 groups on the AFGNS surface create a weakly basic environment assisting the reduction of KMnO4 to MnO2 nanoparticles followed by their anchoring and oriented growth to form MnO2 nanowires. The as prepared MnO2@AFGNS composite has been efficiently used as a selective heterogeneous catalyst for oxidation of primary and secondary benzyl alcohols to corresponding carbonyl compounds under aerobic condition in high yields in the absence of any other oxidizing or activating agent. While MnO2 is considered as the most efficient and selective catalyst for the oxidation of benzyl and allyl alcohols, its large excess use (200 mol% with respect to the substrate) has been circumvented in this work by using heterogeneous AFGNS supported MnO2 (only 7 mol% with respect to the substrate). The synergistic effect between the 1-D MnO2 nanowires and AFGNS facilitates very fast e− transfer enabling such huge enhancement of the catalytic activity of the MnO2@AFGNS composite. The composite also shows sufficient reuse capability and stability after 3 cycles of catalysis thus making it a potentially cheap and active catalyst in the field of organic catalysis.

An efficient heterogeneous catalyst (CuO@ARF) for on-water C-S coupling reaction: an application to the synthesis of phenothiazine structural scaffold

Background Aryl sulfides have significant importance from biological and pharmaceutical aspects. Transition metal-catalyzed carbon-sulfur cross-coupling reaction represents an important tool for the synthesis of sulfides. Among various transition metals, copper salts or oxides have found vast applicability. Results A simple procedure for the preparation of poly-ionic amberlite resins embedded with copper oxide nanoparticles (CuO NPs) (denoted as CuO@ARF) has been developed, characterized, and employed for the first time as a heterogeneous ligand-free catalyst for ‘on-water’ C-S cross-coupling reaction. The NPs of CuO with an average size (approximately 2.6 nm), as determined from high resolution transmission electron microscopy (HRTEM) images, are found to be a potentially active, chemoselective, and recyclable catalyst for the preparation of symmetrical and unsymmetrical aryl sulfides. Recycling of the catalyst was performed successfully for five consecutive runs, and apparently no leaching was observed in a hot filtration test. Excellent chemoselectivity between iodo- and bromo-arene has been exploited in step-wise C-S and C-N couplings to synthesize bioactive heterocyclic scaffold phenothiazine. Conclusions An efficient method is established for the C-S cross-coupling reaction using heterogeneous catalyst CuO@ARF under ligand-free on-water condition. The catalyst is highly chemoselective among different aryl halides, which has been demonstrated in the synthesis heterocyclic scaffold phenothiazine. Furthermore, it is recyclable for five consecutive runs examined.Graphical abstract On-water C-S coupling using new heterogeneous nano-catalyst (CuO@ARF) Electronic supplementary material The online version of this article (doi:10.1186/s13588-014-0017-7) contains supplementary material, which is available to authorized users.

Microwave-assisted Formation of Organic Disulfides of Biochemical Significance

Organic compounds possessing the -S-S- bond, often called disulfide or disulfane, are extremely important and useful in various fields. They often exhibit unique and diverse chemistry in synthetic, biochemical, pharmacological areas as well as are vastly used as vulcanizing agents for rubbers and elastomers. In peptide and protein chemistry, the disulfide bridges impart stability in folding and their synthesis is a pivotal transformation in modern medicinal chemistry research. On the other hand, microwave (MW) provides heat energy source, an alternative to conventional heating, and is used for diverse chemical reactions. This review has presented the progress towards the synthesis of small organic disulfanes and acyclic/cyclic peptides bearing the -S-S- bond specifically under microwave irradiation, which often facilitated the formation of disulfides over conventional heating both in terms of reaction time and efficiency.

In Quest of “Stereoselective Switch” for On-Water Hydrothiolation of Terminal Alkynes Using Different Additives and Green Synthesis of Vicinal Dithioethers

On-water hydrothiolation reaction between terminal alkyne and thiol has been probed in the presence of various additives. Aromatic alkynes yield corresponding 1-alkenyl sulfides, whereas aliphatic alkynes undergo double-addition yielding vicinal disulfides in good to excellent yields. Formation of 1-alkenyl sulfides proceeds with a high degree of regioselectivity (via anti-Markovnikov addition), and switching the stereoselectivity between E/Z isomers has been noticeably realized in the presence of different additives/promoters.

Ni nanoparticles on RGO as reusable heterogeneous catalyst: effect of Ni particle size and intermediate composite structures in C-S cross-coupling reaction

The present work demonstrates the C–S cross-coupling reaction between aryl halides and thiols using nickel nanoparticles (Ni NPs) supported on reduced graphene oxide (Ni/RGO) as a heterogeneous catalyst. It is observed that the uniformly dispersed Ni NPs supported on RGO could exhibit excellent catalytic activity in C–S cross-coupling reactions and the catalytic application is generalized with diverse coupling partners. Although the electron-rich planar RGO surface helps in stabilizing the agglomeration-free Ni NPs, the catalytic process is found to occur involving Ni(II) species and the recovered catalyst containing both Ni(0)/Ni(II) species is equally efficient in recycle runs. A correlation of loading of Ni species, size of NPs and the intermediate Ni-related heterostructures formed during the catalytic process has been established for the first time, and found to be best in the C–S cross-coupling reaction for Ni(0) and Ni(II) NPs of the average sizes 11–12 nm and 4 nm, respectively.