Showing papers by "Krishnapillai Sreekumar published in 2021"

Sustainable synthesis of 1,8-dioxooctahydroxanthenes in deep eutectic solvents (DESs)

Abstract: Deep eutectic solvents are obtained by the fusion of two or more compounds: hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs). They are formed by mixing the constituents with moderate heating (via hydrogen bond interactions). In this manuscript, the dual activities (as catalysts and reaction media) of six deep eutectic solvents were compared for the synthesis of 1,8-dioxooctahydroxanthenes at room temperature. Here DES 2, a combination of ZrOCl2·8H2O with ethylene glycol at a 1 : 2 ratio, was found to be the supreme catalyst among the six DESs and was reused for five batches. The photophysical properties of the xanthene derivatives were examined by UV-vis and fluorescence spectroscopy in THF. The synthesised molecules showed absorption maxima around 285–295 nm and emission maxima around 403–404 nm. The ground state dipole moments of all the molecules were calculated by the Debye–Guggenheim method using toluene as the non-polar solvent. All the synthesised molecules showed significant dipole moment values and are in good agreement with theoretical values.

Multi- arm dendronized polymer as a unimolecular micelle: Synthesis, characterization and application as organocatalyst in the synthesis of N-unsubstituted 1,2,3-triazoles

Abstract: The present work reports dendronized polymer based catalysis in which the dendritic cavity and surface functional groups are the reactive centers due to the formation of dendritic unimolecular micelle in aqueous medium. First generation amine functionalized tris(hydroxymethyl)propane initiated polyepichlorohydrin cored dendronized polymer (TMP-PECH-Amine-G1) was synthesized and was properly characterized with different analysis techniques. High functional group capacity, thermal stability and good water solubility are the eye-catching features of the TMP-PECH-Amine-G1 catalyst. The catalytic activity of TMP-PECH-Amine-G1 was studied by synthesizing 4-aryl-1H-1,2,3-triazoles by one pot condensation reaction of aromatic aldehydes, nitromethane and sodium azide. The synthesized catalyst offered high yield of products with low catalyst loading in green reaction media without the presence of any external additives. The catalyst was regenerated for six cycles of reaction without substantial loss in activity. This is the first reported method where homogeneous amine functionalized dendronized polymer was used as an organo catalyst for the synthesis of 4-substituted triazoles.

Amine Functionalized Dendronized Polymer as a Homogeneous Base Catalyst for the Synthesis of Polyhydroquinolines and 4-Arylidene-3-Methylisoxazol-5(4H)-Ones

Abstract: Novel isosorbide initiated polyepichlorohydrin cored PPI dendronized polymer (ISO-PECH-Amine-G1) has been prepared and its structure was determined by different spectro- analytical techniques. The catalytic activity of ISO-PECH-Amine-G1 was explored for the synthesis of polyhydroquinolines and 4-arylidene-3-methylisoxazol-5(4H)-ones. From an eco-benign perspective, the notable advantages of this protocol are as follows: (i) water as reaction medium, (ii) transition metal-free, (iii) no toxic by-products, (iv) room temperature reaction, (v) simple experimental and work-up procedure, (vi) high conversion, (vii) shorter reaction time, (viii) diverse substrate scope, (ix) no need of solvent extraction and (x) facile reusability of the catalyst. In addition, computational calculations were performed using the B3LYP/6-31G level of theory to explore various density functional theory (DFT) based descriptors such as dipole moment, hardness, chemical potential, electrophilicity index, and the nucleophilicity index of the synthesized derivatives.

Green synthesis of dihydropyrano[3,2-c]chromene derivatives using amino-terminated PAMAM dendrimer as catalyst

Abstract: d-Sorbitol-cored PAMAM dendrimer (SOR-G1) was effectively synthesized by the ring opening polymerization of epichlorohydrin. The dendrimer was characterized using different spectroscopic and analytical techniques including IR and NMR spectroscopy, TG–DTA, and GPC. Dihydropyrano[3,2-c]chromene derivatives were synthesized using SOR-G1 as a catalyst, and it was synthesized within 30 min in ethanol/water medium and excellent yield was obtained. SOR-G1 acted as a good base catalyst on the basis of amine capacity and good thermal stability. The prepared dihydropyrano[3,2-c]chromene derivatives were characterized using GCMS, LCMS, IR, 1H NMR, and 13C NMR spectra. The catalyst could be reused up to three reaction cycles without losing its catalytic activity.

Multifunctional polyether grafted dendritic solid supports: Novel class of solvent like high loading hybrid supports for solid phase organic synthesis

Abstract: Three novel multifunctional polyether grafted dendritic solid supports for organic synthesis were developed via single step, grafting from and hypergrafting strategies. In order to improve both the swelling capacity and loading capacity of conventional resins simultaneously, the hybrid supports were developed by grafting either multivalent linear polymers or dendritic architectures to the conventional resin. The ring opening polymerization of strained cyclic monomers was explored for the development of multivalent polymer grafts on the solid support. The physico-chemical properties of the dendritic solid supports were studied by various characterization techniques and analytical methods. Among the hybrid supports, Het-PG was found to be a novel solvent like high loading solid support. High functional group capacity of 7 mmol g−1 which was introduced via single step approach, increased accessibility of functional sites even at high loading etc. make Het-PG a versatile solid support for organic synthesis. The applicability of dendritic solid support (Het-PG) as a high loading hybrid support for multistep solid phase organic synthesis of small peptides was investigated. The high functional group capacity and increased accessibility of functional sites in Het-PG were exploited for addressing the poor industrial scalability issue of solid phase organic synthesis. A novel approach to solid phase peptide synthesis using solvent like high loading hybrid solid support is presented.

Deep eutectic solvent assisted synthesis of dihydropyrimidinones/thiones via Biginelli reaction: theoretical investigations on their electronic and global reactivity descriptors

Abstract: Deep eutectic solvents formed from hydrated metal chlorides and hydrogen bond donors (Type 4) were prepared and their catalytic activity was compared for the synthesis of dihydropyrimidinones/thiones via Biginelli reaction at room temperature. The one-pot multicomponent reaction was performed between ethyl acetoacetate, various aldehydes and urea/thiourea using DESs. Among the five DESs, excellent conversion was observed using DES 2 (ZrOCl2·8H2O with ethylene glycol at 1 : 2 ratio). The higher activity of DES 2 might be due to the strong hydrogen bonding ability and low viscosity of DES 2, which favoured the reaction as a catalyst. Further, the catalyst was recycled and reused up to four consecutive cycles without significant loss in its activity. The efficiency of the present catalyst was compared with some previous reports. Various structural and quantum chemical parameters (like energy gap), dipole moment, ionization potential, electron affinity, chemical potential (μ), global hardness (η), softness (μ), electronegativity (χ), electrophilicity index (ω) of the pharmaceutically important Biginelli products were investigated using B3LYP functional at 6-31G* level, as implemented in Gaussian 09.

Palladium Loaded Dendronized Polymer as Efficient Polymeric Sustainable Catalyst for Heck Coupling Reaction

Abstract: The palladium incorporated amine-functionalized dendronized polymer was synthesized by the addition of palladium acetate to dendronized polymer in methanol at room temperature. Palladium species are immobilized onto the dendritic structure by their coordination with amino functional groups. The newly developed dendritic system showed high palladium content in the low generation level itself, which was found to be 4.19 mmol/g. This was fairly higher than, the other palladium-based catalysts. Energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, UV–Visible spectroscopy, and X-ray photoelectron spectroscopy were used to confirm the successful synthesis of the new catalyst. It was used as a homogeneous palladium catalyst for Heck coupling reaction between olefins and differently substituted aryl halides and the products were isolated in high yield. The products isolated were in trans configuration, which indicated the selectivity of the newly developed catalytic system. Also, this catalyst system was reused up to nine times without a significant decrease in its catalytic activity. The easy accessibility of catalytic sites, stability, resistance to metal leaching, high catalytic activity and remarkable stereoselectivity with a low amount of catalyst are all due to the dendritic support. The docking study was carried out for all the stilbene derivatives obtained by the Heck coupling reaction against DprE1 protein to study its potential antitubercular activity. All the compounds displayed superior docking score values over the range − 6.5 to − 8.2 kcal/mol, compared to the standard drug isoniazid with docking score of − 6.1 kcal/mol against DprE1.