Noor-ul H. Khan

Enantioselective epoxidation of non-functionalised alkenes using a urea–hydrogen peroxide oxidant and a dimeric homochiral Mn(III)-Schiff base complex catalyst

Abstract The catalytic enantioselective epoxidation of chromenes, indene and styrene using a urea–hydrogen peroxide adduct as an oxidising agent and the novel dimeric homochiral Mn(III)-Schiff base catalyst 1 has been investigated in the presence of carboxylate salts and nitrogen and oxygen coordinating co-catalysts. Conversions of more than 99% were obtained with all alkenes except styrene. Absolute chiral induction, as determined by 1 H NMR using the chiral shift reagent (+)-Eu(hfc) 3 , was obtained in the case of nitro- and cyanochromene. The catalyst could be re-used for up to five cycles with some loss of activity due to degradation of the catalyst under epoxidation condition with retention of e.e.s.

Heterogeneous Chiral Copper Complexes of Amino Alcohol for Asymmetric Nitroaldol Reaction

Chiral amino alcohols supported on mesoporous silicas were synthesized and evaluated as a new class of chiral ligands in copper-catalyzed nitroaldol reaction under heterogeneous and mild reaction conditions. The activity and enantioselectivity of the present catalytic system is immensely influenced by the presence of achiral and chiral bases as an additive. The heterogenized chiral copper(II) complex of amino alcohol was found to be an effective recyclable catalyst for the nitroaldol reaction of different aldehydes such as aromatic, aliphatic, alicyclic, and α-β unsaturated aldehydes to produce nitroaldol products with remarkably high enantioselectivity (≥99%) and yields.

Chiral Mn(III) salen complex-catalyzed enantioselective epoxidation of nonfunctionalized alkenes using urea–H2O2 adduct as oxidant

AbstractEnantioselective epoxidation of chromenes, indene, and styrene mediated by manganese salen complexes 1a–b, 2a–b (1 mol%) as catalystswith urea–H 2 O 2 adduct as an oxidant is observed to give excellent epoxide yield ( > 99%) in 0.5–4 h with enantiomeric excess (ee) in therange 56–99% except for styrene in which case 23–39% ee was obtained in 20 h. Even with a catalyst loading of 0.4 mol%, the system worksefficiently with retention of enantioselectivity, albeit with an increase in reaction time. Kinetic investigations of a representative substrate,indene, with these catalysts indicated a kinetic profile having first-order dependence with respect to the concentrations of the catalyst andoxidant and independent of-initial concentration of the substrate. Based on kinetic, catalytic and experimental evidence, the mechanism ofthe epoxidation reaction is suggested.  2003 Elsevier Inc. All rights reserved. Keywords: Enantioselective; Chiral; Manganese; Nonfunctionalized alkenes; Catalysis; Urea–H

Influence of chirality of V(V) Schiff base complexes on DNA, BSA binding and cleavage activity.

New chiral V(V) Schiff base complexes (S)-[VO(OMe)L] and (R)-[VO(OMe)L] were synthesized and characterized by microanalysis, infrared (IR), UV-Visible, Circular dichroism (CD) spectroscopy and single crystal X-ray studies. The interaction of these complexes with calf thymus (CT) DNA and bovine serum albumin (BSA) protein showed chiral expression DNA/protein binding strength. The influence of chirality was also observed in cytotoxicity assay of Hep 2 cells. (R)-[VO(OMe)L] enantiomer exhibited higher binding constant (5 ± 1 × 10(5) M(-1)) as compared to (S)-[VO(OMe)L] (8 ± 1 × 10(4) M(-1)). The fluorescence quenching, thermal melting and viscosity data suggest DNA surface and/or groove binding nature of the complexes and electrophoresis studies also showed greater activity for (R)-[VO(OMe)L] in cleaving DNA and protein as against (S)-[VO(OMe)L].

Dimeric chiral Mn(III) Schiff base complex-catalysed enantioselective epoxidation of non-functionalised alkenes

Abstract Dimeric chiral Mn(III) Schiff base complex 1 has been investigated as a catalyst for enantioselective epoxidation of chromenes, indene and styrene with an objective to explore its efficiency and recycling capability. Excellent conversions were obtained with all alkenes. More than 99% chiral induction, as determined by 1 H NMR using the chiral shift reagent Eu(hfc) 3 , was obtained in the case of electron deficient chromenes, viz. nitro and cyano chromene.

Chiral Ru(II) Schiff base complex-catalysed enantioselective epoxidation of styrene derivatives using iodosyl benzene as oxidant. II

Abstract Six-coordinated chiral Ru(II) Schiff base complexes of the type [RuLX(Y) 2 ] where L=terdentate chiral Schiff bases derived from l -tyrosine, l -phenylalanine with salicylaldehyde, 3- tertiary -butyl-, 3,5-di- tertiary -butyl-, 3,5-dichloro- and 3,5-dinitrosalicylaldehyde, X=PPh 3 and Y=H 2 O have been investigated as catalysts for enantioselective epoxidation of styrene, 4-chloro-, 4-nitro- and 4-methylstyrene in fluorobenzene in order to explore the efficiency of catalytic system by varying the substituents on the ligand moiety of the catalysts as well as on the substrates using iodosyl benzene as terminal oxidant. Much better results were obtained with catalyst 5 and 10 with 4-nitrostyrene. The enantiomeric excess of the resulting epoxide was evaluated by chiral capillary column.

Synthesis of catalytically active polymer-bound Mn(III) salen complexes for enantioselective epoxidation of styrene derivatives

Abstract Catalytically active metal-complexing polymer-containing chiral Mn(III) salen moieties derived from (1 R , 2 R )-(−)-diphenylethylenediamine, (1 S , 2 S )-(+)-cyclohexanediamine, and ( S )-(+)-diaminopropane with α-naphthyl salicylaldehyde anchored to the polymeric matrix obtained from styrene-4-vinyl pyridine-divinyl benzene (PVPD) have been synthesised. These catalysts were used for enantioselective epoxidation of styrene and substituted styrenes, viz. 4-chloro-, 4-methyl and4-nitrostyrene using iodosyl benzene as terminal oxidant by GLC. The enantiomeric excess of the resulting epoxide was determined by GLC using chiral capillary column or by 1 H-NMR using chiral-shift reagent Eu(hfc) 3 . Each catalyst/substrate combination was examined under epoxidation condition and the results for catalysts 1–3 are presented as Hammet plots. A mechanism involving formation of an Mn-oxo complex and oxygen transfer from a reactive Mn-oxo intermediate to styrene was also proposed for the reaction. These catalysts can be recycled at least ten times without loss of its activity.

Asymmetric epoxidation of styrene by novel chiral ruthenium(II) Schiff base complexes, synthesis and characterization.

Abstract Synthesis of some novel Chiral Ru(II) Schiff base complexes of the type [RuL(PPh3)(H2O)2] 1–6 where L = Chiral Schiff bases derived from salicylaldehyde and L-amino acids namely, L-alanine, L-valine, L-Serine, L-Arginine, L-Cystein and L-aspartic acid are reported. The characterisation of the complexes has been accomplished by microanalysis. IR-,UV/visible,[1H], 13C[1H] and 31P[1H] NMR spectroscopy, conductance measurements, electrochemical studies, optical rotation and circular dichroism soectroscopy. The conformational aspects regarding the asymmetric arrangement of substituents R at aminoacid moiety of the Schiff bases around Ru(II) metal ion has been discussed. The complexes show quasireversible behaviour and the redox potential of Ru(II)/Ru(I) couple lie in the range −0.34 to −0.18 volts. In the asymmetric epoxidation of styrene by the complexes 1 – 6 and idosylbenzene, we observed that on employment of the R form of the catalyst resulted in the formation of (S) styrene oxide as dominant enantiomer by GLC and NMR. Enantiomeric excess for the resulting epoxides were determined by [1H] NMR spectroscopy using Chiral shift reagent, tris[3-(heptafluoropropyl hydroxymethylene − (+) camphoratoleuropium(III), Eu(hfc)3.

Chiral Ni(II) Schiff base complex-catalysed enantioselective epoxidation of prochiral non-functionalised alkenes

The synthesis, characterisation and single crystal X-ray structures of square planar Ni(II) chiral Schiff base complexes with N2O2-type ligands have been described. These catalysts were used for enantioselective epoxidation of non-functionalised alkenes viz. 1-hexene, 1-octene, styrene, 4-chloro-, 4-nitrostyrene and 1,2-dihydronaphthalene using NaOCl as oxidant, giving excellent conversions with long chain alkenes while ees were moderate to good. A mechanism for the Ni(II)-catalysed epoxidation with NaOCl is proposed.

Synthesis, physicochemical studies and aerobic enantioselective epoxidation of non functionalized olefins catalyzed by new Co(II) chiral salen complexes

Abstract Co(II) chiral salen complexes 1 – 3 derived from α -naphthyl salicylaldehyde with 1 S ,2 S (+) diaminocyclohexane, 1 R ,2 R (−) diaminodiphenylethane and S (+) 1,2-diaminopropane have been prepared. The characterization of the complexes was done by microanalysis, magnetic moment, IR-, UV/Vis-, CD spectral studies, optical rotation, conductance measurements and cyclic voltammetry. Epoxidation of non-functionalized prochiral olefins viz. styrene, trans 3-nonene and trans 4-octene was achieved by the combined use of an atmospheric pressure of molecular oxygen and sacrificial reductant isobutyraldehyde catalyzed by the above synthesized Co(II) chiral salen complexes with and without pyridine N -oxide as cooxidant. Good yields of the desired epoxide were obtained with the substrate trans 3-nonene and trans 4-octene by GLC. Enantiomeric excess of the epoxide were evaluated by 1 H NMR using chiral shift reagent Eu(hfc) 3 and by chiral capillary column.