Krishnan Damodaran

On the mechanism of hydroesterification of styrene using an in situ-formed cationic palladium complex

Abstract The mechanism of hydroesterification of styrene using in situ-formed Pd(OTs) 2 (PPh 3 ) 2 from Pd(OAc) 2 , PPh 3 and TsOH in methanol has been investigated by isolation and characterisation of catalytically active intermediates. From reaction mixtures, Pd–hydridocarbonyl and Pd–acyl complexes were isolated and characterised, based on which a Pd hydride mechanism has been proposed. Formation of palladium hydride species has also been confirmed by 31 P-NMR experiments.

Investigating Structural Changes in the Lipid Bilayer upon Insertion of the Transmembrane Domain of the Membrane-Bound Protein Phospholamban Utilizing 31P and 2H Solid-State NMR Spectroscopy

Phospholamban (PLB) is a 52-amino acid integral membrane protein that regulates the flow of Ca(2+) ions in cardiac muscle cells. In the present study, the transmembrane domain of PLB (24-52) was incorporated into phospholipid bilayers prepared from 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC). Solid-state (31)P and (2)H NMR experiments were carried out to study the behavior of POPC bilayers in the presence of the hydrophobic peptide PLB at temperatures ranging from 30 degrees C to 60 degrees C. The PLB peptide concentration varied from 0 mol % to 6 mol % with respect to POPC. Solid-state (31)P NMR spectroscopy is a valuable technique to study the different phases formed by phospholipid membranes. (31)P NMR results suggest that the transmembrane protein phospholamban is incorporated successfully into the bilayer and the effects are observed in the lipid lamellar phase. Simulations of the (31)P NMR spectra were carried out to reveal the formation of different vesicle sizes upon PLB insertion. The bilayer vesicles fragmented into smaller sizes by increasing the concentration of PLB with respect to POPC. Finally, molecular order parameters (S(CD)) were calculated by performing (2)H solid-state NMR studies on deuterated POPC (sn-1 chain) phospholipid bilayers when the PLB peptide was inserted into the membrane.

Noble metal-free bifunctional oxygen evolution and oxygen reduction acidic media electro-catalysts

Identification of low cost, highly active, durable completely noble metal-free electro-catalyst for oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells, oxygen evolution reaction (OER) in PEM based water electrolysis and metal air batteries remains one of the major unfulfilled scientific and technological challenges of PEM based acid mediated electro-catalysts. In contrast, several non-noble metals based electro-catalysts have been identified for alkaline and neutral medium water electrolysis and fuel cells. Herein we report for the very first time, F doped Cu1.5Mn1.5O4, identified by exploiting theoretical first principles calculations for ORR and OER in PEM based systems. The identified novel noble metal-free electro-catalyst showed similar onset potential (1.43 V for OER and 1 V for ORR vs RHE) to that of IrO2 and Pt/C, respectively. The system also displayed excellent electrochemical activity comparable to IrO2 for OER and Pt/C for ORR, respectively, along with remarkable long term stability for 6000 cycles in acidic media validating theory, while also displaying superior methanol tolerance and yielding recommended power densities in full cell configurations.

Synthesis and Polymerization of Renewable 1,3‐Cyclohexadiene Using Metathesis, Isomerization, and Cascade Reactions with Late‐metal Catalysts

Synthesis and subsequent polymerization of renewable 1,3-cyclohexadiene (1,3-CHD) from plant oils is reported via metathesis and isomerization reactions. The metathesis reaction required no plant oil purification, minimal catalyst loading, no organic solvents, and simple product recovery by distillation. After treating soybean oil with a ruthenium metathesis catalyst, the resulting 1,4-cyclohexadiene (1,4-CHD) was isomerized with RuHCl(CO)(PPh3)3. The isomerization reaction was conducted for 1 h in neat 1,4-CHD with [1,4-CHD]/[RuHCl(CO)(PPh3)3] ratios as high as 5000. The isomerization and subsequent polymerization of the renewable 1,3-CHD was examined as a two-step sequence and as a one-step cascade reaction. The polymerization was catalyzed with nickel(II)acetylacetonate/methaluminoxane in neat monomer, hydrogenated d-limonene, and toluene. The resulting polymers were characterized by FTIR, DSC, and TGA.

Diglycerol‐Based Polyesters: Melt Polymerization with Hydrophobic Anhydrides

The melt polymerization of diglycerol with bicyclic anhydride monomers derived from a naturally occurring monoterpene provides an avenue for polyesters with a high degree of sustainability. The hydrophobic anhydrides are synthesized at ambient temperature via a solvent-free Diels-Alder reaction of α-phellandrene with maleic anhydride. Subsequent melt polymerizations with tetra-functional diglycerol are effective under a range of [diglycerol]/[anhydride] ratios. The hydrophobicity of α-phellandrene directly impacts the swelling behavior of the resulting polyesters. The low E factors (<2), large amount of bio-based content (>75%), ambient temperature monomer synthesis, and polymer degradability represent key factors in the design of these sustainable polyesters.

Dictyostatin flexibility bridges conformations in solution and in the β-tubulin taxane binding site.

Dictyostatin (DCT, 1) is a complex, flexible polyketide macrolide that demonstrates potent microtubule-polymerization activity. Both a solution structure (2a) and a possible binding mode for DCT (Conf-1) have been proposed by earlier NMR experiments. In the present study, the conformational landscape of DCT in DMSO-d(6) and methanol-d(4) was explored using extensive force-field-based conformational searches combined with geometric parameters derived from solution NMR data. The results portray a diversity of conformations for dictyostatin that illustrates the molecules flexibility and excludes the previously suggested dominant solution conformation 2a. One conformation present in DMSO-d(6) with a 7% population (Conf-2, 0.6 kcal/mol above the global minimum at 298°) also satisfies the TR-NOESY NMR parameters of Canales et al. that characterize the taxane binding-site interaction between DCT and assembled microtubules in water. Application of several docking methods (Glide, Autodock, and RosettaLigand) has identified a low-energy binding model of the DCT/β-tubulin complex (Pose-2/Conf-2) that is gratifyingly compatible with the emerging DCT structure-activity data.

19F spin–lattice relaxation of perfluoropolyethers: Dependence on temperature and magnetic field strength (7.0–14.1 T)

Fluorine ((19)F) MRI of perfluorocarbon-labeled cells has become a powerful technique to track the migration and accumulation of cells in living organisms. It is common to label cells for (19)F MRI with nanoemulsions of perfluoropolyethers that contain a large number of chemically equivalent fluorine atoms. Understanding the mechanisms of (19)F nuclear relaxation, and in particular the spin-lattice relaxation of these molecules, is critical to improving experimental sensitivity. To date, the temperature and magnetic field strength dependence of spin-lattice relaxation rate constant (R1) for perfluoropolyethers has not been described in detail. In this study, we evaluated the R1 of linear perfluoropolyether (PFPE) and cyclic perfluoro-15-crown-5 ether (PCE) at three magnetic field strengths (7.0, 9.4, and 14.1T) and at temperatures ranging from 256-323K. Our results show that R1 of perfluoropolyethers is dominated by dipole-dipole interactions and chemical shift anisotropy. R1 increased with magnetic field strength for both PCE and PFPE. In the temperature range studied, PCE was in the fast motion regime (ωτc<1) at all field strengths, but for PFPE, R1 passed through a maximum, from which the rotational correlation time was estimated. The importance of these measurements for the rational design of new (19)F MRI agents and methods is discussed.

Streamlining the conversion of biomass to polyesters: bicyclic monomers with continuous flow

A three-step transformation of 1,4-cyclohexadiene (1,4-CHD) using continuous flow produced an aliphatic bicyclic monomer for polyester synthesis. The monomer synthesis involved catalytic alkene isomerization of 1,4-CHD to 1,3-CHD using a heterogeneous Na2O/Na/Al2O3 catalyst, a Diels Alder reaction with maleic anhydride, and hydrogenation of the bicyclic monomer. A partially continuous strategy was compared with a fully continuous method. The continuous flow process streamlined the transformation of waste by-product biomass by minimizing workup procedures and reducing the synthesis time from ∼1 day for batch processes to ∼2.5 h. The monomer synthesis was easily scalable and allowed recycling of the catalysts for alkene isomerization and hydrogenation. The resulting bicyclic monomers were polymerized with glycerol and 1,4-butanediol (BDO) to obtain renewable polyesters with high thermal stability and tunable glass transition temperatures.

Nuclear Spin Relaxation and Molecular Interactions of a Novel Triazolium-Based Ionic Liquid

Nuclear spin relaxation, small-angle X-ray scattering (SAXS), and electrospray ionization mass spectrometry (ESI-MS) techniques are used to determine supramolecular arrangement of 3-methyl-1-octyl-4-phenyl-1H-triazol-1,2,3-ium bis(trifluoromethanesulfonyl)imide [OMPhTz][Tf2N], an example of a triazolium-based ionic liquid. The results obtained showed first-order thermodynamic dependence for nuclear spin relaxation of the anion. First-order relaxation dependence is interpreted as through-bond dipolar relaxation. Greater than first-order dependence was found in the aliphatic protons, aromatic carbons (including nearest neighbors), and carbons at the end of the aliphatic tail. Greater than first order thermodynamic dependence of spin relaxation rates is interpreted as relaxation resulting from at least one mechanism additional to through-bond dipolar relaxation. In rigid portions of the cation, an additional spin relaxation mechanism is attributed to anisotropic effects, while greater than first order thermodynamic dependence of the octyl side chains spin relaxation rates is attributed to cation-cation interactions. Little interaction between the anion and the cation was observed by spin relaxation studies or by ESI-MS. No extended supramolecular structure was observed in this study, which was further supported by MS and SAXS. nuclear Overhauser enhancement (NOE) factors are used in conjunction with spin-lattice relaxation time (T1) measurements to calculate rotational correlation times for C-H bonds (the time it takes for the vector represented by the bond between the two atoms to rotate by one radian). The rotational correlation times are used to represent segmental reorientation dynamics of the cation. A combination of techniques is used to determine the segmental interactions and dynamics of this example of a triazolium-based ionic liquid.

Probing the effect of electron donation on CO2 absorbing 1,2,3-triazolide ionic liquids

Development of the next generation materials for effective separation of gases is required to address various issues in energy and environmental applications. Ionic liquids (ILs) are among the most promising material types. To overcome the many hurdles in making a new class of materials technologically applicable, it is necessary to identify, access, and scale up a range of representative substances. In this work, CO2 reactive triazolide ILs were synthesized and characterized with the aim of developing a deeper understanding of how structural changes affect the overall properties of these substances. It was found that substituents on the anion play a crucial role in dictating the physical properties for CO2 capture. Depending upon the anion substituent, CO2 capacities between 0.07 and 0.4 mol CO2 per mol IL were observed. It was found that less sterically-hindered anions and anions containing electron donating groups were more reactive towards CO2. Detailed spectroscopic, CO2 absorption, rheological, and simulation studies were carried out to understand the nature and influence of these substituents. The effect of water content was also evaluated, and it was found that water had an unexpected impact on the properties of these materials, resulting in an increased viscosity, but little change in the CO2 reactivity.