Showing papers on "Schiff base published in 1998"
742 citations
TL;DR: The first example of the catalytic asymmetric oxidation of tert-butyl disulfide is described and thiosulfinate ester 2 is obtained with 91% enantiomeric excess in yields of ≥92% on scales as large as 1 mol.
Abstract: The first example of the catalytic asymmetric oxidation of tert-butyl disulfide (1) is described. The product, tert-butyl tert-butanethiosulfinate (2) is obtained with 91% enantiomeric excess in yields of ≥92% on scales as large as 1 mol. The application of H2O2 as stoichiometric oxidant in the presence of 0.25 mol % of VO(acac)2 and 0.26 mol % of a chiral Schiff base ligand, 6a, is both convenient and cost-effective. Thiosulfinate ester 2 is chemically and optically stable and serves as an excellent precursor to chiral tert-butanesulfinyl compounds by the stereospecific nucleophilic displacement of tert-butyl thiolate. Addition of LiNH2 in liquid ammonia and THF provides tert-butanesulfinamide (3; 91% yield). A single recrystallization provides enantiomerically pure 3 in 71−75% overall yield from disulfide 1. Enantiomerically pure thiosulfinate ester 2 also reacts readily and stereospecifically with Grignard reagents, organolithiums, lithium amides, and lithium imine salts to provide enantiomerically pur...
316 citations
TL;DR: In this article, a series of Schiff-base-substituted ruthenium carbene complexes have been prepared by the treatment of RuCl_2(=CHPh)(PCy_3)_2 with a variety of Schiffbase ligands as the thallium salts.
207 citations
TL;DR: In this paper, the corrosion inhibition by Schiff base compounds derived from diamines and o -hydroxy, o -methoxy aromatic aldehydes was investigated by weight loss, electrochemical measurements and surface analysis.
202 citations
TL;DR: In this paper, the emission spectra of EuIII β-diketone complexes were analyzed and it was shown that the Na[Eu(L)2] complexes exhibit higher quantum yields than the corresponding Eu2L3 species, and that luminescence is negligible when the ligand triplet-state lies below the 5Do state of the euIII.
Abstract: New EuIII Schiff base compounds and polyelectrolytes luminesce with quantum yields consistent with the intramolecular energy transfer mechanism established for EuIII β-diketone complexes. The emission spectra clearly show that the two EuIII ions in Eu2L3 complexes, where L is a tetradentate Schiff base ligand, are in different environments, that the Na[Eu(L)2] complexes exhibit higher quantum yields than the corresponding Eu2L3 species, and that luminescence is negligible when the ligand triplet-state lies below the 5Do state of EuIII.
176 citations
TL;DR: In this paper, the structure of 2-hydroxy Schiff bases has been characterized by elemental analysis, IR, 1H-NMR and UV-visible techniques and the structure has been examined crystallographically.
157 citations
TL;DR: In this paper, eight chiral Schiff-base ligands were prepared from (S)-tert.-leucinol and four chiral, racemic salicylic aldehydes.
154 citations
TL;DR: The preparation, crystal structures, and variable temperature magnetic susceptibility data are presented for two azido-bridged Schiff base complexes of Mn(III) and Fe(III).
Abstract: The preparation, crystal structures, and variable temperature magnetic susceptibility data are presented for two azido-bridged Schiff base complexes of Mn(III) and Fe(III). Mn(salpn)N3 (1), where s...
142 citations
TL;DR: In this article, the reaction of chitosan with aromatic aldehydes gave Schiff base polymers, which were characterized and identified using microanalysis and IR spectroscopy.
139 citations
TL;DR: In this article, a chiral Salen Schiff base ligand was developed via a stepwise approach, which has two different salicylaldehyde units, and the steric factor can be fixed on one side, electronic or/and steric factors can be tuned from the other side with different substituents.
96 citations
TL;DR: In this article, the cobalt complex of a Schiff base diquinone ligand CoIII(Cat-N-BQ) undergoes an entropy-driven valence tautomeric equilibrium in solution yielding the CoII(Cat N BQ)2 species.
Abstract: The cobalt(III) complex of a Schiff base diquinone ligand CoIII(Cat-N-BQ)(Cat-N-SQ) undergoes an entropy-driven valence tautomeric equilibrium in solution yielding the CoII(Cat-N-BQ)2 species. This suggestion is supported by variable temperature electronic and 1H NMR spectra, as well as by solution magnetic susceptibility data. The calculated thermodynamic quantities associated to the redox equilibrium are ΔH = 42.0(0.8) kJ mol-1 and ΔS = 140 (3) J mol-1 K-1.
TL;DR: In this article, several nickel(II) complexes with tetradentate N2O2 Schiff-base ligands (H2L) have been synthesized and their oxidative chemistry studied in dimethylformamide and dimethyl sulfoxide.
Abstract: Several nickel(II) complexes with tetradentate N2O2 Schiff-base ligands (H2L) have been synthesized and their oxidative chemistry studied in dimethylformamide and in dimethyl sulfoxide. Electrochemical and EPR data for the oxidised solution revealed that in these strong co-ordinated solvents almost all nickel(II) complexes were oxidised to low-spin six-co-ordinate nickel(III) complexes, formulated as [NiIIIL(solv)2]+, where solv stands for a solvent molecule. Upon addition of pyridine to fluid solutions of [NiIIIL(solv)2]+ new species were formed which on the basis of their EPR spectra can be formulated as [NiIIIL(py)2]+. Extensive electronic characterisation of the nickel(III) compounds was made by coupling of EPR and electronic spectroscopic data.
TL;DR: Several new complexes of chiral and achiral tetradentate Schiff bases with copper (II), nickel (II) and oxovanadyl (IV) ions have been synthesised and characterised by X-ray analysis as mentioned in this paper.
TL;DR: The structure of the probable major ex/em 360/430-nm fluorophore that arises from cross-linking of two protein-based lysines by one 4-hydroxy-2-alkenal is shown to be a 2-alkyl- 2-Hydroxy-1,2-dihydropyrrol-3-one iminium and supports a proposed mechanistic pathway that involves two 2e oxidations following initial Schiff base formation.
Abstract: Modification of proteins by products of lipid peroxidation results in various fluorescent adducts associated with oxidative stress pathophysiology in degenerative disease. Using n-butylamine as a model for the lysine side chain, the structure of the probable major ex/em 360/430-nm fluorophore that arises from cross-linking of two protein-based lysines by one 4-hydroxy-2-alkenal is shown to be a 2-alkyl-2-hydroxy-1,2-dihydropyrrol-3-one iminium. That this fluorophore can be independently generated in higher yield from either 4-oxo-2-alkenals or 3,4-dioxoalkanals supports a proposed mechanistic pathway that involves two 2e oxidations following initial Schiff base formation.
TL;DR: In this article, a new heterogeneous catalyst based on a chemically modified mesoporous silica gel and possessing immobilised chromium ions has been prepared and successfully applied to the aerial oxidation of alkyl aromatics at atmospheric pressure and in the absence of solvent.
TL;DR: The readily prepared di-Schiff base of pyridine-2-aldehyde and ethane-1,2-diamine 1 reacts with silver(I) tetrafluoroborate to form by self-assembly the double-stranded D2-helicate (±)-[Ag212](BF4)2; the corresponding Schiff base of (1R,2R)-cyclohexane- 1,2diamines [(R,R)-2] reacts with Silver(I)-nitrate to produce the homochiral singlestranded C
TL;DR: In this paper, a condensation of 2-aminobenzothiazole with salicylaldehyde and o-nitrobenzaldehyde was used to obtain 1:1 and 1:2 complexes.
Abstract: Schiff bases of the type salicylidene 2-aminobenzothiazole and o-nitrobenzylidene 2-aminobenzothiazole were prepared by condensation of 2-aminobenzothiazole with salicylaldehyde and o-nitrobenzaldehyde. The bases react with cobalt(II), nickel(II) and copper(II) chlorides to give 1:1 and 1:2 complexes. Electronic spectra, magnetic susceptibility measurements and infrared data are used to infer the structures. The thermal decomposition of the complexes and evaluation of the thermodynamic parameters are also reported.
TL;DR: It is found that at pH 5, where proton release does not occur, but not at higher pH, the photostationary state created by illumination with yellow light contains not only the M1 and M2 states, but also the L and the N intermediates.
TL;DR: The unexpected finding, for D85N/D96N, that when the retinal is in the stable all-trans, 15-anti and 13-cis,15-syn isomeric forms access of the Schiff base is locked, suggests that in this protein reisomerization, rather than changes in the proton conductivities of the EC and CP half-channels, provides the switch function.
Abstract: The accessibility of the retinal Schiff base in bacteriorhodopsin was studied in the D85N/D96N mutant where the proton acceptor and donor are absent. Protonation and deprotonation of the Schiff base after pH jump without illumination and in the photocycle of the unprotonated Schiff base were measured in the visible and the infrared. Whether access is extracellular (EC) or cytoplasmic (CP) was decided from the effect of millimolar concentrations of azide on the rates of proton transfers. The results, together with earlier work on the wild-type protein, suggest a new hypothesis for the proton-transfer switch: (i) In the metastable 13-cis, 15-anti and all-trans, 15-syn photoproducts, but not in the stable isomeric states, access flickers between the EC and CP directions. (ii) The direction of proton transfer is decided both by this local access and by the presence of a suitable donor or acceptor group (in the wild type), or the proton conductivity in the EC and CP half-channels (in D85N/D96N). (iii) Thermal reisomerization of the retinal can occur only when the Schiff base is protonated, as is well-known. In the wild-type transport cycle, the concurrent local EC and CP access during the lifetime of the metastable 13-cis, 15-anti state enables the changing pKa's of the proton acceptor and donor to determine the direction of proton transfer. Proton transfer from the Schiff base to Asp-85 in the EC direction is followed by reprotonation by Asp-96 from the CP direction because proton release to the EC surface raises the pKa of Asp-85 and a large-scale protein conformation change lowers the pKa of Asp-96. The unexpected finding we report here for D85N/D96N, that when the retinal is in the stable all-trans, 15-anti and 13-cis, 15-syn isomeric forms access of the Schiff base is locked (in the EC and CP directions, respectively), suggests that in this protein reisomerization, rather than changes in the proton conductivities of the EC and CP half-channels, provides the switch function. With this mechanism, the various modes of transport reported for Asp-85 mutants (CP to EC direction with blue light, and EC to CP direction with blue plus green light) are understood also in terms of rules i-iii.
TL;DR: The strain of the protonated internal aldimine is interpreted to enhance the catalytic ability of the enzyme by increasing the energy level of the free enzyme plus substrate at neutral pH relative to the transition state.
Abstract: In aspartate aminotransferase, pyridoxal 5‘-phosphate (PLP) forms a Schiff base with the e-amino group of Lys258 (internal aldimine). The internal aldimine has a pKa value of 6.8. Binding of a subs...
TL;DR: In this article, a facile two-step synthesis of chiral ligands for bonding to transition metals was reported, which were easily prepared from trans-1,2-diaminocyclohexane by reaction with sulfonyl chlorides to give amino-sulfonamide compounds.
Abstract: We report a facile two step synthesis of chiral ligands for bonding to transition metals. The ligands are easily prepared from trans -1,2-diaminocyclohexane by reaction with sulfonyl chlorides to give amino-sulfonamide compounds. These intermediates are then condensed with salicylaldehyde derivatives to provide sulfonamide/Schiff base compounds which represent a new class of chiral ligands.
TL;DR: Some ruthenium(II) Schiff base complexes have been derived from condensation of salicylaldehyde with 1,2-diaminobenzene and 4-chlorobenzaldehyde with 4-aminobenzoic acid.
TL;DR: In this paper, unsaturated α-amino acids are synthesized through condensation of allyl and propargyl bromides or of 9-bromoundecene with a Ni(II) complex of the Schiff base derived from glycine and BPB.
Abstract: ω-Unsaturated α-amino acids are synthesized through condensation of allyl and propargyl bromides or of 9-bromoundecene with a Ni(II) complex of the Schiff base derived from glycine and BPB. Hydroboration with Ipc 2 BH followed by oxidation with acetaldehyde affords enantiomerically pure ω-borono-α-aminocarboxylic acids.
Patent•
19 Nov 1998TL;DR: The Schiff base derivatives of the ruthenium and osmium carbene catalysts show unexpectedly improved thermal stability while maintaining high metathesis activity, even in polar protic solvents as mentioned in this paper.
Abstract: The present invention generally relates to ruthenium and osmium carbene catalysts for use in olefin metathesis reactions. More particularly, the present invention relates to Schiff base derivatives of ruthenium and osmium carbene catalysts and methods for makig the same. The inventive catalyst are generally prepared by the treatment of unmodified catalysts with the salts of the desired Schiff base ligands, in which an anionic and a neutral electron donating ligands of the unmodified catalysts are simultaneously replaced. The Schiff base derivatives of the ruthenium and osmium carbene catalysts show unexpectedly improved thermal stability while maintaining high metathesis activity, even in polar protic solvents. Although the inventive catalysts may be used in all metathesis reactions, use of these catalysts for ring-closing metathesis ("RCM") reactions is particularly preferred.
TL;DR: Neutron diffraction from oriented purple membrane fragments at various hydration levels, coupled with H2O/2H2O exchange, was used to compare the structure and hydration of the light-adapted initial state (B-state) and the M photointermediate of bacteriorhodopsin mutant D96N to show that contrary to certain models, the structural changes in the M-state are not correlated with major hydrationChanges in the proton channel projection.
TL;DR: An asymmetric insertion reaction of α-diazoesters derived from methyl arylacetates into the silicon-hydrogen bond of silanes was achieved using a copper (I) catalyst associated with a chiral, C 2 symmetric Schiff base.
TL;DR: Alkoxo-rich Schiff-bases of potentially tri-, tetra-and penta-dentate binding capacity, and their sodium tetrahydroborate-reduced derivatives, have been synthesized and characterized using several analytical and spectral techniques including multinuclear NMR spectroscopy and single-crystal X-ray diffraction studies as discussed by the authors.
Abstract: Alkoxo-rich Schiff-bases of potentially tri-, tetra- and penta-dentate binding capacity, and their sodium tetrahydroborate-reduced derivatives, have been synthesized. Their oxo-vanadium(V) and -molybdenum(VI) complexes were synthesized and characterized using several analytical and spectral techniques including multinuclear NMR spectroscopy and single-crystal X-ray diffraction studies. Eight structurally different types of complexes possessing distorted square-pyramidal, trigonal-bipyramidal and octahedral geometries have been obtained. While VVO exhibits dimeric structures with 2-HOC6H4CHNC(CH2OH)3 and 2-HOC6H4CH2NHC(CH2OH)3 and related ligands through the formation of a symmetric V2O2 core as a result of bridging of one of the CH2O– groups, MoVIO gives only mononuclear complexes even when some unbound CH2OH groups are available and the metal center is co-ordinatively unsaturated. In all the complexes the nitrogen atom from a HCN or H2CNH group of the ligand occupies a near trans position to the MO bond. While the Schiff-base ligands act in a tri- and tetra-dentate manner in the vanadium(V) complexes, they are only tridentate in the molybdenum(VI) complexes. Proton NMR spectra in the region of bound CH2 provides a signature that helps to differentiate dinuclear from mononuclear complexes. Carbon-13 NMR co-ordination induced shifts of the bound CH2 group fit well with the charge on the oxometal species and the terminal or bridging nature of the ligand. The reactivity of the vanadium(V) complexes towards bromination of the dye xylene cyanole was studied. Transmetallation reactions of several preformed metal complexes of 2-HOC6H4CHNC(CH2OH)3 with VO3+ were demonstrated as was selective extraction of VO3+ from a mixture of [VO(acac)2] and [MoO2(acac)2] using this Schiff base. The unusual selectivity and that of related derivatives for VO3+ is supported by binding constants and the solubility of the final products, and was established through a.c. conductivity measurements. The cis-MoO22+ complexes with alkoxo binding showed an average Mo–Oalk distance of 1.926 A, a value that is close to that observed in the molybdenum(VI) enzyme dmso reductase (1.92 A). Several correlations have been drawn based on the data.
TL;DR: Two Schiff base−oxovanadium(IV) complexes were dehydrogenated upon heating at 210 °C for 3−15 h in the solid state in the air to form [VO(Xsalton)], which have a CC double bond in the five-membered chelate ring moiety.
Abstract: Schiff base−oxovanadium(IV) complexes, [VO(Xsal-meso-stien)] (X = 3-EtO, 5-MeO, 5-Br) were dehydrogenated upon heating at 210 °C for 3−15 h in the solid state in the air to form [VO(Xsalton)], which have a CC double bond in the five-membered chelate ring moiety. The structures of [VO(3-EtOsal-meso-stien)] and [VO(3-EtOsalton)] were determined by X-ray structure analyses. The benzylic carbon atoms of the N−N chelate ring of [VO(Xsal-meso-stien)] will be oxidized by oxygen in the air to form a double bond.
TL;DR: In this paper, the Schiff base-based tetradentate (O2N2, S2N 2, and P 2N2) complexes were prepared and their properties were investigated, and the photosubstitution reaction of the axial ligands under room light was observed.
TL;DR: In this article, the Mannich condensation was used to synthesize dicompartmental ligands bearing picolyl pendant arms on the amine nitrogen donors, including the protonated acyclic ligand salt and the mononuclear macrocyclic complex NiH2(L2b)2+ with a Schiff base.
Abstract: Examples of interesting ligands previously requiring lengthy synthetic procedures have been prepared by one-step routes, opening the way to more extensive studies of their complexes and to possible applications. New dicompartmental ligands bearing picolyl pendant arms on the amine nitrogen donors have been synthesized, via the Mannich condensation, from 5-substituted salicylaldehydes, formaldehyde, and N,N‘-bis(2-pyridylmethyl)-1,2-diaminoethane. The protonated acyclic ligand salt, two mononuclear complexes of a macrocyclic ligand with a second compartment featuring a Schiff base, and one of the decomposition products resulting from a retro-Mannich reaction have been structurally characterized. The ligand salt (L1b) has an extended conformation with the ethylenediamine fragment displaying the trans configuration, very different from that of the corresponding closed-site macrocyclic complexes NiH2(L2b)2+ and ZnH2(L2b)2+. The mononuclear macrocyclic complex NiH2(L2b)2+ has a much smaller ligand twist than t...