R.I. Kureshy

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.

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.

Enantioselective catalytic epoxidation of styrenes by iodosylbenzene using chiral ruthenium(II) Schiff base complexes

Abstract Chiral ruthenium (II) Schiff base complexes 1–3 derived from l -histidine with salicylaldehyde, 5-chloro and 5-methoxy salicylaldehyde were prepared and used for catalytic enantioselective epoxidation of non-functionalised olefins, viz., styrene, 3-methyl-, 3-methoxy-, 3-chloro- and 3-nitrostyrene, with iodosylbenzene as an oxidant, giving the highest ee (80%) for nitro-styrene with catalyst 3. Each catalyst/substrate combination was examined under epoxidation conditions and the results for catalysts 1–3 are presented as Hammet plots. The existence of a possible intermediate and the mechanism of chiral induction are discussed. The stacking/charge transfer interaction between the substrate and triphenyl phosphine of chiral Ru(II) catalyst at oxo-transfer stage seem to function favouring S( − ) styrene oxide as dominant enantiomer.

Enantioselective catalytic epoxidation of nonfunctionalized prochiral olefins by dissymmetric chiral Schiff base complexes of Mn(III) and Ru(III) metal ions II

A series of new dissymmetric chiral Schiff base complexes has been obtained by a systematic condensation of (1S,2S)(+)-diaminocyclohexane and 3-acetyl-4-hydroxy-6-methyl-2-pyrone with salicylaldehyde, 5-chloro-, 5-methoxy-and 5-nitrosalicylaldehyde and by subsequent metallation with manganese and ruthenium. The characterization of the complexes 1–8 was accomplished by physico chemical studies viz. microanalysis, IR-, UVVIS-, and CD spectral studies, optical rotation, molar conductance measurements and cyclic voltammetry. Enantioselective epoxidation of non functionalised olefins, viz. cis-stilbene, trans-3-nonene and trans-4-octene with iodosyl benzene as oxidant was demonstrated in the presence of catalytic amounts of chiral Mn(III) and Ru(III) dissymmetric Schiff base complexes. Good optical yields of epoxides were obtained for the catalyst 4 with the substrates trans-3-nonene and cis-stilbene.

Enantiomer self-disproportionation of chiral compounds on achiral ordered mesoporous silica M41S and regular silica gel as a stationary phase

Chromatographic behavior of nonracemic mixtures, viz., mandelic acid and stilbene oxide as analytes has been studied in detailed by enantiomer self-disproportionation on achiral ordered mesoporous material M41S and regular silica gel as stationary phases. Enantiomer self-disproportionation gave enhanced separation of analytes. The extent and magnitude of enantiomer self-disproportionation is dependent on the optical purity of the starting non-racemic molecules, presence of intermolecular hydrogen bonding/pi-pi interactions and the nature of eluents used. The present study and previous literature data suggest that percentage ee of a nonracemic mixture needs to be determined before any chromatographic purification is taken up as enantiomer self-disproportionation phenomenon could occur during purification. The data show that enantiomer self-disproportionation of nonracemic mixtures can be harnessed for its enantioenrichment on inexpensive achiral stationary phases.

Asymmetric catalytic epoxidation of styrene by dissymmetric Mn(III) and Ru(III) chiral Schiff base complexes synthesis and physicochemical studies

Some dissymmetric Mn(III) and Ru(III) chiral Schiff base complexes derived from 1R,2R(−)1,2-diaminocyclohexane with 3-acetyl-4-hydroxy-6-methyl-2-pyrone and salicylaldehyde, 5-chloro-5-methoxy-and 5-nitrosalicylaldehyde have been synthesized. The characterization of the complexes was accomplished by microanalysis, IR, UV-Vis, CD spectroscopy, conductance measurements, magnetic susceptibility, optical rotation and electrochemical studies. The asymmetric epoxidations catalyzed by the complexes were examined with styrene using the terminal oxidant, iodosylbenzene to assess the stereoselectivity in the epoxidation of styrene by changing substituents on the catalyst. In all cases the R form of the catalyst resulted in S(−) form of the product as a dominant enantiomer. Optical yield of the resulting epoxide was determined by GLC using chiral capillary column/1H NMR using Eu(hfc)3 as a chiral shift reagent.

Chiral Mn(III) Schiff base complex catalyzed aerobic enantioselective epoxidation of prochiral non-functionalized olefins

Abstract Dissymmetric Mn(III) chiral Schiff base complexes derived from 1R,2R(−)-1,2 diaminocyclohexane with 3-acetyl 4-hydroxy 6-methyl 2-pyrone and 2-hydroxy benzaldehyde, 5-chloro-, 5-methoxy-, 5-nitro-, 3-tertiary butyl- and 3,5 ditertiary butyl 2-hydroxy benzaldehyde have been prepared. Microanalysis, IR, UV/Vis spectroscopy, conductance measurement, optical rotation, CD spectroscopy and cyclic voltammetry accomplished the characterization of the complexes. These complexes were used for aerobic enantioselective epoxidation of 1-hexene, 1-octene, and 1,2-dihydronaphthalene using molecular oxygen in the presence of 2-methylpropanal as sacrificial reductant. Reaction progress was monitored on gas chromatography (GC) and the enantiomeric excess of the resulting epoxide was evaluated by chiral capillary column on GC and 1H NMR using Eu (hfc)3 as chiral shift reagent. Good to excellent conversions and epoxide selectivity for all the substrates were obtained with all the catalysts while enantiomeric excess was relatively better in the case of 1,2-dihydronaphthalene. The extent of enantioselectivity with respect to the substrates and catalysts is shown by 3-D view presentation.

Chiral Ru(III) metal complex-catalyzed aerobic enantioselective epoxidation of styrene derivatives with co-oxidation of aldehyde

Ru(III) chiral Schiff base complexes 1–3 derived from dehydroacetic acid with 1S,2S-(+) diaminocyclohexane, 1R,2R-(−)1,2 diphenyl ethylenediamine 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. The enantioselective epoxidation of styrene and substituted styrenes viz., 4-chloro-, 4-nitro- and 4-methyl styrene was achieved by the combined use of molecular oxygen and sacrificial reductant isobutyraldehyde catalyzed by the above synthesized Ru(III) chiral Schiff base complexes. Good yields of the desired epoxides were obtained with styrene and 4-chlorostyrene by GLC. Enantiomeric excess of the epoxide was evaluated by 1H-NMR using chiral shift reagent Eu(hfc)3 and by chiral capillary column. The extent of enantioselectivity is shown on Hammet plots.