Chiral Mitsunobu reactions with (1S)-(+)-ketopinic acid: kinetic resolutions of secondary alcohols

Several secondary alcohols undergo the Mitsunobu reaction with triphenylphosphine, diethyl azodicarboxylate and (1S)-(+)-ketopinic acid (0.5 equiv. each relative to alcohol) in CH2Cl2 solution at -23degreesC, to furnish the chiral secondary alcohol and its ketopinate ester (d.e. >95%,). Chromatographic separation of these and subsequent hydrolysis of the ketopinate ester (KOH EtOH/0degreesC) provides the chiral secondary alcohol in overall yields of similar to75% and e.e. of similar to80%. When the above Mitsunobu reaction is performed with 1 equiv. of all the reactants. an effective dynamic kinetic resolution of the alcohol is observed in two cases, the ketopinate esters being isolated in 63 and 75% yields and >95% d.e. (C) 2002 Elsevier Science Ltd. All rights reserved.

Mechanistic investigations on the efficient catalytic decomposition of peroxynitrite by ebselen analogues

In this study, ebselen and its analogues are shown to be catalysts for the decomposition of peroxynitrite (PN). This study suggests that the PN-scavenging ability of selenenyl amides can be enhanced by a suitable substitution at the phenyl ring in ebselen. Detailed mechanistic studies on the reactivity of ebselen and its analogues towards PN reveal that these compounds react directly with PN to generate highly unstable selenoxides that undergo a rapid hydrolysis to produce the corresponding seleninic acids. The selenoxides interact with nitrite more effectively than the corresponding seleninic acids to produce nitrate with the regeneration of the selenenyl amides. Therefore, the amount of nitrate formed in the reactions mainly depends on the stability of the selenoxides. Interestingly, substitution of an oxazoline moiety on the phenyl ring stabilizes the selenoxide, and therefore, enhances the isomerization of PN to nitrate.

Effect of molecular association and reactivity on substitution in chromium(III)-salprn complexes

A new chromium(III)-Schiff base complex, [Cr(5-chlorosalprn)(H2O)(2)]ClO4, where salprn=N,N'-propylenebis(salicylideneimine) has been prepared and characterized by electrospray ionization mass spectrometric (ESIMS) analysis and other spectroscopic techniques. Single crystal X-ray data reveal that the complex assumes a trans-diaquo structure, [Cr(C17H18Cl2N2O4)]ClO4.H2O. The effect of phenyl ring substituents on the rate of formation of [O=Cr-V Schiff base](+) has been investigated. The bimolecular rate constant for the formation of O=Cr-V species by the [Cr(Schiff base)(H2O)(2)]ClO4, where the Schiff base=salprn, (1) and 5-chlorosalprn, (2) with PhOI was compared. In the case of (2) the rate was found to be faster by an order of magnitude at pH=4 compared to (1). The introduction of a chloro-substituent on the phenyl ring not only influences the rate of redox reactivity but also the pKa values of aquo ligands of the complexes, indicating the difference in the electronic environment around the metal ion in both (1) and (2).

Electrochemically deposited crystalline thin film of polyaniline on nickel for redox reactions at positive potentials

Redox reactions which occur at positive potentials such as ferrous/ferric, hydroquinone/quinone, ferrocyanide/ferricyanide etc. in aqueous acidic electrolytes cannot be studied on non-platinum metals, for example, a Ni electrode. On the contrary, these reactions occur on polyaniline (PANI) modified Ni electrodes, as evidenced from cyclic voltammetry, amperometry and steady-state polarization experiments. Under identical experimental conditions of scan rate (v) and concentration (C), the peak current density (i(p)) values of Fe2+/Fe3+ redox reaction are greater on the PANI modified Ni than on Pt. Additionally, the peak potential separation (DeltaE(p)) of the voltammogram is lesser on the PANI modified Ni. With an increase in thickness of the PANI, DeltaE(p) increases suggesting that the redox reactions tend to depart from the reversibility. Scanning electron micrographs reveal the presence of a crystalline deposit of PANI on Ni when the thickness of PANI is about 0.08 mum. However, the PANI becomes amorphous and porous at higher thickness values. Raman spectroscopy and X-ray diffraction studies corroborate the observations made out of scanning electron microscopy. Higher catalytic activity of PANI is attributed to crystalline nature of PANI on Ni. Exchange current density and standard rate constant of Fe2+/Fe(3+)redox reaction are evaluated. (C) 2002 Published by Elsevier Science B.V.

Thermodynamic analysis of enzyme catalysed reactions: new insights into the Michaelis-Menten equation

A simple thermodynamic analysis of the well-known Michaelis-Menten equation (MME) of enzyme catalysis is proposed that employs the chemical potential mu to follow the Gibbs free energy changes attending the formation of the enzyme-substrate complex and its turnover to the product. The main conclusion from the above analysis is that low values of the Michaelis constant KM and high values of the turnover number k(cat) are advantageous: this supports a simple algebraic analysis of the MME, although at variance with current thinking. Available data apparently support the above findings. It is argued that transition state stabilisation - rather than substrate distortion or proximity - is the key to enzyme catalysis.

Ab initio, DFT and transition state theory calculations on 1,2-HF, HCI and CIF elimination reactions from CH2F-CH2Cl

This paper reports ab intio, DFT and transition state theory (TST) calculations on HF, HCI and CIF elimination reactions from CH2Cl-CH2F molecule. Both the ground state and the transition state for HX elimination reactions have been optimized at HF, MP2 and DFT calculations with 6-31G*, 6-31G** and 6-311++G** basis sets. In addition, CCSD(T) single point calculations were carried out with MP2/6-311++G** optimized geometry for more accurate determination of the energies of the minima and transition state, compared to the other methods employed here. Classical barriers are converted to Arrhenius activation energy by TST calculations for comparisons with experimental results. The pre-exponential factors, A, calculated at all levels of theory are significantly larger than the experimental values. For activation energy, E-a DFT gives good results for HF elimination, within 4-8 W mol(-1) from experimental values. None of the methods employed, including CCSD(T), give comparable results for HCI elimination reactions. However, rate constants calculated by CCSD(T) method are in very good agreement with experiment for HCI elimination and they are in reasonable agreement for HF elimination reactions. Due to the strong correlation between A and E., the rate constants could be fit to a lower A and E-a (as given by experimental fitting, corresponding to a tight TS) or to larger A and E-a (as given by high level ab initio calculations, corresponding to a loose TS). The barrier for CIF elimination is determined to be 607 U mol(-1) at HF level and it is unlikely to be important for CH2FCH2Cl. Results for other CH2X-CH2Y (X,Y = F/Cl) are included for comparison.

NO Reduction Over Noble Metal Ionic Catalysts

In last 40 years, catalysis for NO (x) removal from exhaust gas has received much attention to achieve pollution free environment. CeO(2) has been found to play a major role in the area of exhaust catalysis due to its unique redox properties. In last several years, we have been exploring an entirely new approach of dispersing noble metal ions in CeO(2) and TiO(2) for redox catalysis. We have extensively studied Ce(1-x) M (x) O(2-delta) (M = Pd, Pt, Rh), Ce(1-x-y) A (x) M (y) O(2-delta) (A = Ti, Zr, Sn, Fe; M = Pd, Pt) and Ti(1-x) M (x) O(2-delta) (M = Pd, Pt, Rh, Ru) catalysts for exhaust catalysis especially NO reduction and CO oxidation, structure-property relation and mechanism of catalytic reactions. In these catalysts, lower valent noble metal ion substitution in CeO(2) and TiO(2) creates noble metal ionic sites and oxide ion vacancy. NO gets molecularly adsorbed on noble metal ion site and dissociatively adsorbed on oxide ion vacancy site. Dissociative chemisorption of NO on oxide ion vacancy leads to preferential conversion of NO to N(2) instead of N(2)O over these catalysts. It has been demonstrated that these new generation noble metal ionic catalysts (NMIC) are much more catalytically active than conventional nano crystalline noble metal catalysts especially for NO reduction.

Cyclometalated complexes derived from calix[4] arene bisphosphites and their catalytic applications in cross-coupling reactions

Palladium and platinum dichloride complexes of a series of symmetrically and unsymmetrically substituted 25,26;27,28-dibridged p-tert-butyl-calix[4]arene bisphosphites in which two proximal phenolic oxygen atoms of p-tert-butyl-or p-H-calix[4]arene are connected to a P(OR) ( R = substituted phenyl) moiety have been synthesized. The palladium dichloride complexes of calix[4]arene bisphosphites bearing sterically bulky aryl substituents undergo cyclometalation by C-C or C-H bond scission. An example of cycloplatinated complex is also reported. The complexes have been characterized by NMR spectroscopic and single crystal X-ray diffraction studies. During crystallization of the palladium dichloride complex of a symmetrically substituted calix[4]arene bisphosphite in dichloromethane, insertion of oxygen occurs into the Pd-P bond to give a P,O-coordinated palladium dichloride complex. The calix[4]arene framework in these bisphosphites and their metal complexes adopt distorted cone conformation; the cone conformation is more flattened in the metal complexes than in the free calix[4]arene bisphosphites. Some of these cyclometalated complexes proved to be active catalysts for Heck and Suzuki C-C cross-coupling reactions but, on an average, the yields are only modest. (C) 2011 Elsevier B.V. All rights reserved.

Intercalative pyrimido[4',5':4,5]thieno(2,3-b)quinolines induce apoptosis in leukemic cells: a comparative study of methoxy and morpholino substitution

DNA intercalating molecules are promising anticancer agents. Polycyclic aromatic molecules such as ellipticine intercalate into double-stranded DNA and affect major physiological functions. In the present study, we have characterized two molecules with the same chemical backbone but different side chains, namely 8-methoxy pyrimido[4',5':4,5]thieno (2,3-b)quinoline-4(3H)-one (MPTQ) and 4-morpholino pyrimido[4',5':4,5]thieno(2,3-b)quinoline (morpho-PTQ) at the 8th and 4th position, respectively. Although both MPTQ and morpho-PTQ show similar biophysical properties with high DNA affinity, here we show that they differ in their biological activities. We find that MPTQ is many fold more potent than morpho-PTQ and is cytotoxic against different leukemic cell lines. IC(50) value of methoxy PTQ was estimated between 2-15 A mu M among the leukemic cells studied, while it was more than 200 A mu M when morpho-PTQ was used. Cell cycle analysis shows an increase in sub-G1 phase, without any particular cell cycle arrest. Annexin V staining in conjunction with comet assay and DNA fragmentation suggest that MPTQ induces cytotoxicity by activating apoptosis. Thus the observed low IC(50) value of MPTQ makes it a promising cancer chemotherapeutic agent.

Ce(0.67)Fe(0.33)O(2-delta) and Ce(0.65)Fe(0.33)Pt(0.02)O(2-delta): New water gas shift (WGS) catalysts

Ce(0.65)Fe(0.33)Pt(0.02)O(2-delta) and Ce(0.67)Fe(0.33)O(2-delta) have been synthesized by a new low temperature sonochemical method using diethylenetriamine as a complexing agent. Due to the substitution of Fe and Pt ions in CeO(2), lattice oxygen is activated in Ce(0.67)Fe(0.33)O(2-delta) and Ce(0.65)Fe(0.33)Pt(0.02)O(2-delta). Hydrogen uptake studies show strong reduction peaks at 125 C in Ce(0.65)Fe(0.33)Pt(0.02)O(2-delta) against a hydrogen uptake peak at 420 degrees C in Ce(0.67)Fe(0.33)O(2-delta). Fe substituted ceria, Ce(0.67)Fe(0.33)O(2-delta) itself acts as a catalyst for CO oxidation and water gas shift (WGS) reactions at moderate temperatures. The rate of CO conversion in WGS with Pt free Ce(0.65)Fe(0.33)O(2-delta) is 2.8 mu mol g(-1) s(-1) at 450 C and with Pt substituted Ce(0.65)Fe(0.33)Pt(0.02)O(2-delta) is 4.05 mu mol g(-1) s(-1) at 275 degrees C. Due to the synergistic interaction of the Pt ion with Ce and Fe ions in Ce(0.65)Fe(0.33)Pt(0.02)O(2-delta), the catalyst showed much higher activity for CO oxidation and WGS reactions compared to Ce(0.67)Fe(0.33)O(2-delta). A reverse WGS reaction does not occur over Ce(0.65)Fe(0.33)Pt(0.02)O(2-delta). The catalyst also does not deactivate even when operated for a long time. Nearly 100% conversion of CO to CO(2) with 100% H(2) selectivity is observed in WGS reactions even up to 550 degrees C. (C) 2011 Elsevier B.V. All rights reserved.

Metal mould reactions during casting of titanium in zircon sand moulds

Metal-mold reaction during Ti casting in zircon sand molds has been studied using scanning electron microscope, energy and wave length dispersive analysis of X-rays, X-ray diffraction, microhardness measurements, and chemical analysis. Experimental results suggest that oxides from the mold are not fully leached out by liquid Ti, but oxygen is preferentially transferred to liquid Ti, leaving behind metallic constituents in the mold as lower oxides or intermetallics of Ti. The electron microprobe analysis has revealed the depth profile of contaminants from the mold into the cast Ti metal. The elements Si, Zr and O were found to have diffused to a considerable distance within the Ti metals. A possible mechanism has now been evolved in regard to the reactions that occur during casting of Ti in zircon sand molds.