Hari Chand Bajaj

The origin for highly enantioselective induction of 1-naphthol to isatin-derived N-Boc ketimines catalyzed by quinine thiourea catalyst: an experimental and computational study

An enantioselective aza-Friedel–Crafts reaction of 1-naphthol with isatin derived N-Boc ketimines by cinchona based bifunctional thiourea as organo-catalyst is reported. In general the derivatives of Betti base are formed in excellent enantioselectivities (95–99%) with high yields (<99%). The combination of experimental and computational studies have revealed the origin of the stereoselectivity of the aza-Friedel–Crafts reaction of 1-naphthol with isatin derived N-Boc ketimines by cinchona based bifunctional thiourea as organo-catalyst. The attractive electrostatic interactions in the Re face transition state and the deleterious lone-pair⋯π interactions in the Si face transition state governed the formation of Re-face as a major product in this aza Friedel–Crafts reaction. The NMR studies performed to examine the formation of the complex with 1-naphthol, isatin derived N-Boc ketimines and cinchona based bifunctional thiourea catalyst has corroborated the calculated complex geometry employed in the study.

A low temperature bottom-up approach for the synthesis of few layered graphene nanosheets via C–C bond formation using a modified Ullmann reaction

A low temperature, single-pot, bottom-up approach for the synthesis of few layered graphene sheets using the modified Ullmann reaction is reported. The synthesis protocol involved a solvothermal technique under an autogenic pressure of chloroform, which was used as the carbon source. Scanning and transmission electron microscopy revealed the formation of randomly aggregated, thin, crumpled graphene sheets with a thickness of ∼2 nm. Solid state 13C nuclear magnetic resonance and Fourier transform infrared spectroscopy showed that the prepared graphene sheets have copious surface functionality. The possible growth mechanism for the formation of graphene sheets is proposed based on an analysis of the intermediate products by gas chromatography coupled with mass spectroscopy. The growth of few layered graphene sheets proceeded through addition and cyclization reactions of different chloroalkene intermediate products formed by the addition reaction of chloroform molecules, and not by the chain polymerization of chloroform molecules.

Optical Resolution of α-Amino Acids by Reverse Osmosis using Enantioselective Polymer Membrane Containing Chiral Metal-Schiff Base Complex

The optical resolution of α-amino acids, arginine and alanine was performed by reverse osmosis at 517.10 kPa and 1034.21 kPa pressures using enantioselective composite nanofiltration membrane prepared by interfacial co-polymerizing, a mixture of Zinc metal Schiffs base complex and piperazine with trimesoyl chloride in-situ on the top of polysulfone ultrafiltration membrane. The chemical composition of the enantioselective layer was determined by ATR-FTIR and X-ray Fluorescence Spectroscopy and surface morphology was studied by scanning electron microscopy. The effect of process parameters such as the operating pressure, permeation time, and concentration of the feed on the performance of membrane was studied. The volumetric flux of aqueous solution of α-amino acids through membrane follows Hegen-Poiseuillie equation. The composite membrane permeates d-enantiomers of α-amino acids preferentially; 54% enantiomeric excess in for d-arginine was achieved. The enantioselective permeability of the membrane is found to be time dependent. The enantioselective property of the membrane has arisen due to a homo-Chiral environment created in the membrane by incorporating chiral ligand Schiff complex in thin film of poly (piperazine trimesamide) polymer on the top of the polysulfone membrane. The composite membrane also exhibits the separation of α-amino acids from their aqueous solutions.

Adsorption of carbon monoxide, methane and nitrogen on alkaline earth metal ion exchanged zeolite-X: structure, cation position and adsorption relationship†

Development of zeolite based adsorbents with high adsorption capacity and selectivity is the key requirement for efficient and economic separation processes. However, less attention has been given so far towards understanding the mechanism of adsorption on the zeolites. In the present study adsorption of carbon monoxide, methane and nitrogen on zeolite-X exchanged with magnesium, calcium, strontium and barium cations was carried out using a volumetric gas adsorption method. Calcium, strontium, and barium ion exchanged zeolite-X shows increase in carbon monoxide, methane and nitrogen adsorption capacity. Strontium exchanged zeolite-X shows carbon monoxide adsorption capacity of 28.4 molecules per unit cell and calcium exchanged zeolite-X shows methane and nitrogen adsorption capacity of 18.8 and 13.8 molecules per unit cell, respectively at 303 K and 760 mm Hg pressure, maximum among the alkaline earth metal ion exchanged zeolite-X samples. However, barium exchanged zeolite-X shows methane/nitrogen selectivity of 1.78, maximum among the studied samples. The initial heat of adsorption for carbon monoxide, methane and nitrogen increases on calcium, strontium and barium ion exchange, and decreases with increase in the size of the cations due to decrease in the electrostatic interactions. However, magnesium exchanged zeolite-X shows decrease in the heat of adsorption. The significant decrease in adsorption capacity and heats of adsorption on magnesium ion exchange is due to the migration of small size magnesium ions inside the sodalite cages (I′ and II′) and D6R (site I) from the super cage (site II, III and III′) during activation, where cations are not accessible for adsorption. The change in adsorption capacity, selectivity and heats of adsorption for alkaline earth metal ion exchanged zeolite-X is discussed in terms of the size, location, and effective charges on the extra-framework cations present in the zeolite cavity and the subsequent electrostatic interactions between the adsorbed molecules and the extra-framework cations.

Comparative Study of Enantioseparation of Racemic Tryptophan by Ultrafiltration Using BSA-Immobilized and BSA-Interpenetrating Network Polysulfone Membranes

Effectiveness of Bovine Serum Albumin (BSA) as chiral recognizing protein in enantiomers separation by ultrafiltration technique was studied by immobilizing BSA on the membrane and incorporating BSA as semi-interpenetrating network in the membrane matrix. Separation of racemic tryptophan solution was performed in closed loop cross flow ultrafiltration using BSA immobilized polysulfone membrane and polysulfone membrane having BSA semi-IPN network. The volumetric flux (Jv), the solute flux (Js), the separation factor (α), and the enantiomeric excess (%ee) of two types of membranes at different trans-membrane pressures and permeation times were determined. BSA semi-IPN membrane exhibits higher volumetric as well as solute fluxes compared to BSA immobilized membrane. Separation factor (α) to the order of 1.89 was achieved with BSA immobilized membrane after 8 h of ultrafiltration and in the same duration BSA-IPN membrane exhibited separation factor (α) to the order of 1.62. BSA immobilized membrane exhibits higher enantiomeric excess (30.8%) compared to BSA semi-IPN membrane (23.8%) after 8 hrs. BSA molecules available on membrane as immobilized or as semi-IPN under go complexion with tryptophan enantiomers differently. BSA immobilized membrane performed better separation and enantiomeric purity; however, the solute flux of the membrane decreases.

Optical Resolution of Racemic Mixtures of Amino Acids through Nanofiltration Membrane Process

Optical resolution of racemic mixtures of arginine and alanine was performed by chiral selective nanofiltration membrane. The chiral selective layer of the membrane was prepared by interfacial polymerization of metaphenylenediamine, trimesoyl chloride, and S (-)-2-acetoxypropionyl chloride (S-2-actoxpcl) in situ on the top of polysulfone ultrafiltration membrane. S-2-actoxpcl consists of a chiral carbon atom that has an induced chiral environment in the membrane. The membranes were characterized by FTIR, scanning electron microscopy, and atomic force microscopy to establish a structure-performance relationship. The optical resolution was performed on the membrane testing module and the effect of process parameters was determined. The results indicated that the incorporation of S-2-actoxpcl made membrane chiral selective hence membranes performed optical resolution. The resolution capacity increased by increasing S-2-actoxpcl in polymerizing solution up to 0.03% but any increase beyond 0.03% reduces the resolution capacity. More than 92% enantiomeric excess of D enantiomer was observed in the permeate of the membrane which was prepared from 0.07% trimesoyl chloride and 0.03% S-2-actoxpcl. The membrane prepared without the chain terminator exhibited less volumetric flux but more solute rejection compared to those prepared with the chain terminator. The flux of the membrane increases as the amount of the chain terminator in the reaction increases.

Solvent-Free Microwave Synthesis of Aryloxypropanolamines by Ring Opening of Aryloxy Epoxides

The ring opening reaction of aryloxyepoxides with isopropylamine under solvent-free microwave irradiation produced therapeutically useful 𝛽 -blockers-aryloxypropanolamines in excellent yield (up to 98%) in 10 minutes which is considerably less than the time taken in classical heating ( ∼ 1 6 hours).

Asymmetric Cyanation Reactions

The asymmetric cyanation across C C, C O, and C N bonds ranks among the most important and well-studied reactions in asymmetric catalysis to achieve useful chiral building blocks for pharmaceuticals, agrochemicals, and specialty materials. The asymmetric cyanation reaction demands careful selection of a chiral catalyst and a cyanide source. A number of efficient and successful synthetic strategies have been developed that include asymmetric cyanation-utilizing enzyme, organocatalyst, and metal complexes as catalysts. As for the concern of the source of cyanide, inorganic cyanides, for example, NaCN and KCN; and organic cyanides, for example, trimethylsilyl cyanide, alkyl cyanoformates, acetone cyanohydrin, acetyl cyanide, alkyl cyanophosphorylates, etc. have been employed, depending upon the targeted substrates. Since the subject is vast, this chapter discusses in detail chiral-metal-complex-based catalytic asymmetric synthesis of cyanohydrins using different cyanide sources. An attempt is made to discuss the reaction mechanism involved in enantioselective cyanation reactions.