Showing papers on "Schiff base published in 1995"

Cobalt-Schiff base complex catalyzed oxidation of para-substituted phenolics. Preparation of benzoquinones

Abstract: Para-substituted phenolics, serving as models for lignin (a renewable source of carbon), are oxidized to the corresponding benzoquinone with oxygen in the presence of catalytic amounts of Co-Schiff base complexes. The reaction products observed depend on the structure of the catalyst. The 5-coordinate catalysts (pyridine)[bis(salicylidene)ethylenediamine]cobalt[(pyr)Co(salen)]and[bis(salicylideneamino)ethylamine]cobalt [Co(n-Me salpr)] convert syringyl alcohol (3,5-dimethoxy-4-hydroxybenzyl alcohol) to 2,6-dimethoxybenzoquinone in high yield. In contrast, syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) is unreactive toward these catalysts. However, the 4-coordinate Co(salen) converts syringaldehyde to 2,6-dimethoxybenzoiquinone in 72% isolated yield. Phenols bearing a single methoxy group on the ring are unreactive toward any catalyst in MeOH. However, vanillyl alcohol (3-methoxy-4-hydroxybenzyl alcohol) is converted to 2-methoxybenzo-quinone with Co(N-Me salpr) and oxygen in 43% yield in CH{sub 2}Cl{sub 2} and 58% yield in CH{sub 2}Cl{sub 2} in the presence of 1% CuCl{sub 2}. The success of the oxidations appears to be related to the ease of removal of the phenolic hydrogen by the Co/O{sub 2} complex. Competitive deactivation of the catalyst occurs with substrates of lower reactivity. 84 tabs.

pKa of the protonated Schiff base and aspartic 85 in the bacteriorhodopsin binding site is controlled by a specific geometry between the two residues.

Abstract: The structure and function of the light-driven proton pump bacteriorhodopsin appear to be determined by the exact geometrical conformation of specific groups in the retinal binding site, including bound water molecules. This applies to the pKa values of the protonated Schiff base, which links the retinal chromophore to Lys216, and to Asp85. In the present work we show that the geometrical constraints imposed by the ring structures of several synthetic retinals can induce substantial changes in the pKa values of the Schiff base and of Asp85. Thus, the artificial pigments derived from 13-demethyl-11,14-epoxyretinal (2) and 13-demethyl-9,12-epoxyretinal (3) show protonated Schiff base pKa values of 8.2 +/- 0.1 and 9.1 +/- 0.1, respectively, as compared with 13.3 in the native (all-trans-retinal) pigment. We also suggest that in both systems the pKa of Asp85 increases from 3.2 in the native bR to above 9. Analogous, though smaller, effects are obtained for artificial bR pigments derived from 12,14-ethanoretinal (4), 11,13-propanoretinal (5), 11,13-ethanoretinal (6), and p-(CH3)2N-C6H4-HC = CH-C(CH3) = CH-CHO 7. The effects of geometry on the pKa values (those on Asp85 being more pronounced) are attributed to the disruption of the original, well-defined, structure in which the Schiff base and its Asp85 counterion are bridged by bound water molecules. These results are the first to show that it is possible to modify the pKa values of the Schiff base and Asp85 in appropriate artificial pigments, without inducing intrinsic pKa changes in the chromophore or introducing a mutation in the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Complexes of Trivalent Metal Ions with Potentially Heptadentate N4O3 Schiff Base and Amine Phenol Ligands of Varying Rigidity

Abstract: The synthesis and characterization of several potentially heptadentate N{sub 4}O{sub 3} Schiff bases and amine phenols, as well as a series of their mononuclear and dinuclear complexes with indium and the lanthanides are reported. Schiff bases containing imidazolidine rings were the products of the known condensation reaction of triethylenetetramine with 3 equiv of 5-substituted salicylaldehydes to form H{sub 3}api (5-H-substituent), H{sub 3}Clapi (5-Cl-substituent), or H{sub 3}Brapi (5-Br-substituent); KBH{sub 4} reduction of these Schiff bases gave the appropriate isomeric N{sub 4}O{sub 3} amine phenols H{sub 3}(1,2,4-btt) and H{sub 3}(1,1,4-btt), as well as an acetone adduct, H{sub 3}(1,2,4-ahi). The Schiff bases reacted with 1 equiv of a lanthanide (Ln{sup 3+}) nitrate to produce mononuclear nine-coordinated [Ln(H{sub 3}Xapi)-(NO{sub 3}){sub 3}] complex wherein the ligand adopts a tridentate capping coordination mode, whereas the amine phenols formed mononuclear seven-coordinate complexes with the lanthanides and indium; homodinuclear complexes [LnL]{sub 2} were also obtained with the Schiff bases. The X-ray structures of the Schiff bases H{sub 3}api and H{sub 3}Clapi, the mononuclear amine phenol complexes Yb(1,2,4-btt){center_dot}0.5CH{sub 3}OH and In(1,1,4-btt), and the homodinuclear Schiff base complex [La(Brapi)]{sub 2}{center_dot}2CHCl{sub 3} have been determined.

Asymmetric aldol reactions of chiral Ni(II)-complex of glycine with aliphatic aldehydes. Stereodivergent synthesis of syn-(2S)- and syn-(2R)-β-alkylserines

Abstract: Stereoselectivity of aldol reactions between aliphatic aldehydes and Ni(II)-complex of chiral non-racemic Schiff base of glycine with ( S )- o -[ N -( N -benzylprolyl)amino]benzophenone (BPB) in the presence of excess of MeONa, has been studied as a function of time, reaction conditions and nature of an aldehyde. Two salient features of the reaction, very high pseudokinetic syn -(2 S )-diastereoselectivity, and dependence of thermodynamic syn -(2 R )-diastereoselectivity on the steric bulk of an aldehyde side chain, were disclosed and used for efficient (more than 90% de and ee) asymmetric synthesis of both syn -(2 S ) and syn -(2 R )-3-alkyl substituted serines. Synthetic potential and reliability of this asymmetric method are demonstrated with the large scale (2–20 g) preparation of enantiomerically pure amino acids.

Identification and spectral characterization of the external aldimine of the O-acetylserine sulfhydrylase reaction.

Abstract: The O-acetylserine sulfhydrylase (OASS) reaction has been studied using a number of spectral probes including UV--visible, fluorescence, circular dichroism, and 31P NMR spectroscopy. The addition of L-cysteine, L-alanine, and glycine to OASS results in a shift in lambda max of 412 nm for the internal Schiff base to 418 nm resulting from the formation of the external Schiff base. The addition of L-serine or O-methyl-D,L-serine gives decreases of the absorbance of unliganded enzyme at 412 nm of about 50% and 20%, respectively, concomitant with an increase in the absorbance at 320 nm and a shift in the lambda max of the remaining visible absorbance to 418 nm. The spectral shifts observed in the presence of L-serine are suggestive of establishing an equilibrium between different forms of external Schiff base. The concentration dependence of the changes at 440 (L-cysteine) and 320 nm (L-serine) provides an estimate of the dissociation constant for the external aldimine. The pH dependence of the dissociation constant suggests the alpha-amine of the amino acid must be unprotonated for nucleophilic attack at C4' of PLP, and an enzyme side chain must be unprotonated to hydrogen-bond the thiol or hydroxyl side chain of the amino acid. When L-cysteine is the amino acid, the thiol side chain must be protonated to hydrogen-bond to the unprotonated enzyme side chain. The 31P NMR chemical shift is increased from 5.2 ppm for unliganded enzyme to 5.3 ppm in the presence of L-cysteine, signaling a tighter interaction at the 5'-phosphate upon formation of the external Schiff base.(ABSTRACT TRUNCATED AT 250 WORDS)

Asymmetric synthesis XXII: Asymmetric catalytic trimethylsilylcyanation of benzaldehyde by novel Ti(IV)-chiral schiff base complexes

Abstract: Benzaldehyde has been trimethylsilylcyanated with a catalyst prepared in situ from titanium tetraisopropoxide and a novel chiral Schiff base. Both (R) and (S) cyanohydrins are obtained in moderate e.e. values.

Cyclic voltammetric and e.s.r. studies of a tetraaza 14-membered macrocyclic copper(II) complex derived from 3-salicylideneacetylacetone ando-phenylenediamine: stabilization and activation of unusual oxidation states

Abstract: A 14-membered macrocyclic Schiff base derived from 3-salicylideneacetylacetone ando-phenylenediamine acts as a tetradentate and strongly conjugated ligand to form a cationic solid complex with CuCl2. U.v.-vis. and e.s.r. spectral data reveal a strong ligand to metal π-interaction in the square planar complex. C.v. data reveal that the title ligand is able to stabilize the copper(III) oxidation state more effectively than comparable saturated or partially unsaturated macrocyclic ligands and confers a weaker tendency for reduction of copper(II) to copper(I) and copper(0). While the inclusion of a PPh3 ligand suppresses the Cu0 → CuI → CuII oxidation, imidazole and pyridine strongly enhance the CuII → CuIII oxidation of the complex.

Non-planar co-ordination of the Schiff-base dianion N,N′-2,2-dimethyltrimethylenebis[salicylideneiminate(2–)] to vanadium

Abstract: The Schiff-base dianion N,N′-2,2-dimethyltrimethylenebis[salicylideneiminate(2–)](salnptn) coordinates to vanadium(IV) to form [VO(salnptn)] whose crystal structure has been determined. The compound is polymeric in the solid state with ⋯ VO → VO → VO ⋯ chains, the salnptn donor atoms being coplanar and the salnptn framework umbrella-shaped. In the derivative [VO(OMe)(salnptn)], the crystal structure of which has also been determined, the OMe and the vanadyl oxygen are co-ordinated cis to each other, and the salnptn now occupies three equatorial and one axial co-ordination positions. Even N,N′-ethylenebis(salicylideneiminate) seems to behave analogously, though it is normally planar.

Asymmetric synthesis XXVII: Asymmetric catalytic trimethylsilylcyanation of aldehydes by novel Ti-chiral Schiff base complexes

Abstract: Enantioselective catalytic trimethylsilylcyanations of aldehydes with 48% to 92% e.e. have been studied using the novel Ti-chiral Schiff base complexes. We have found that the catalyst led to high enantoselectivity when the molar ratio of the Schiff base 1 to Ti(O-i-Pr) 4 was 2:1.

Catalytic and physico-chemical properties of new Schiff base complexes in zeolites

Abstract: Adsorption of tetradentate Schiff base ligands on MnIINaY zeolites yields catalysts for selective oxidation of hydrocarbons with PhIO or tert-butylhydroperoxide (tBHP). The interaction between Mn and the ligands is proved by IR, diffuse reflectance and EPR spectroscopy. Olefin epoxidation with these catalysts suffers from formation of undesirable products when tBHP is used, but reasonable epoxide selectivities are obtained with PhIO. Very active catalysts for alkane oxidation with tBHP are produced by adsorption of the pyridine derived pyren and pyrpn ligands on MnIINaY (pyren = bis(2-pyridinecarboxaldehyde)ethylenediimine; pyrpn = bis(2-pyridinecarboxaldehyde)propylenediimine). With these catalysts, very high selectivities are obtained for formation of monoketones from alkanes. A relation is proposed between the appearance of low-field features in the Mn(II) EPR spectra and a high activity in hydrocarbon oxidation.

Coordination Chemistry of Schiff Base Complexes of Uranium

Abstract: A. ABBREVIATIONS 2 B. INTRODUCTION 3 C. URANIUM (VI) COMPLEXES 4 (i) Complexes of bidentate Schiff bases 4 (ii) Complexes of tridentate Schiff bases 20 (iii) Complexes of tetradentate Schiff bases 47 (iv) Complexes of pentadentate Schiff bases 62 (v) Complexes of hexadentate Schiff bases 75 D. URANIUM (V) COMPLEXES 76 E. URANIUM (IV) COMPLEXES 78 (i) Complexes of mono and bidentate Schiff bases 78 (ii) Complexes of tridentate Schiff bases 79 (iii) Complexes of tetradentate Schiff bases 80 F. MACROCYCLIC COMPLEXES 82 G. PEROXO COMPLEXES 88 H. CONCLUDING REMARKS 89 I. ACKNOWLEDGEMENTS 91 J. REFERENCES 91

α-Aminophosphonate derivatives as nucleophiles in diastereoselective and enantioselective palladium catalysed allylic substitution reactions

Abstract: The palladium catalysed reaction between allyl acetates and Schiff base derivatives of α-aminophosphonates is reported. For suitable substrates, the observed diastereoselectivities (up to 87:13) and enantioselectivities (>96% ee) are high.

Mononuclear cis-dioxovanadium(V) anionic complexes [VO2L]–{H2L =[1 + 1] Schiff base derived from salicylaldehyde (or substituted derivatives) and 2-amino-2-methylpropan-l-ol}: synthesis, structure, spectroscopy, electrochemistry and reactivity studies

Abstract: The compounds [H3NCMe2CH2OH][VO2L]{H2L =[1 + 1] Schiff base derived from salicyladehyde (or substituted derivatives) and 2-amino-2-methylpropan-l-ol} have been synthesized in good yields by reaction of the Schiff base (formed in situ) and a further equivalent of the amino alcohol with [VO(acac)2](Hacac = acetylacetone). The structures of three of the complexes have been solved by single-crystal X-ray studies. All the compounds were also characterized by UV/VIS, FTIR and NMR spectroscopy and by cyclic voltammetry. The three structurally characterized mononuclear complexes contain the cis-dioxovanadium(V) moiety and exhibit distorted square-pyramidal geometry at vanadium which is displaced from the equatorial plane by ≈ 0.5 A. The crystal and molecular structures of these compounds showed extensive hydrogen bonding between the anionic portion of the complexes and the counter-cations of the amino alcohol which resulted in some unusual metric features of interest to the binding of vanadium in biological systems. Two of the compounds showed the longest VO distances reported so far for mononuclear cis-dioxovanadium(V) complexes. Three of the complexes undergo photoreduction in the solid state which is attributed to their intermolecular interactions in the crystal.

Acid-base chemical mechanism of O-acetylserine sulfhydrylases-A and -B from pH studies.

Abstract: The pH dependence of kinetic parameters using natural and alternative reactants was determined in order to obtain information on the chemical mechanisms of the A and B isozymes of O-acetylserine sulfhydrylase (OASS) from Salmonella typhimurium. A general mechanism is proposed for OASS in which OAS binds with its alpha-amine unprotonated to carry out a nucleophilic attack on C4' of the protonated Schiff base and with the acetyl carbonyl hydrogen-bonded to a protonated enzyme group (or a water molecule), which aids in the beta-elimination of acetate. The enzyme lysine that was in Schiff base linkage with the active site pyridoxal 5'-phosphate deprotonates the alpha-carbon in the beta-elimination reaction, and a proton is likely released with the acetate product. Sulfide likely binds as HS- to undergo nucleophilic attack on the alpha-aminoacrylate intermediate, followed by protonation of the alpha-carbon by the enzyme lysine. In OASS-A, HS- is hydrogen-bonded to the enzyme group that assists in the beta-elimination of acetate, but this is not the case for OASS-B. The pH independent equilibrium constant for the first half-reaction of OASS-A is 1.6 x 10(-3), while the second half-reaction is practically irreversible.

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.

The complex extracellular domain regulates the deprotonation and reprotonation of the retinal Schiff base during the bacteriorhodopsin photocycle.

Abstract: During the L-->M reaction of the bacteriorhodopsin photocycle the proton of the retinal Schiff base is transferred to the anionic D85. This step, together with the subsequent reprotonation of the Schiff base from D96 in the M-->N reaction, results in the translocation of a proton across the membrane. The first of these critical proton transfers occurs in an extended hydrogen-bonded complex containing two negatively charged residues (D85 and D212), two positively charged groups (the Schiff base and R82), and coordinated water. We simplified this region by replacing D212 and R82 with neutral residues, leaving only the proton donor and acceptor as charged groups. The D212N/R82Q mutant shows essentially normal proton transport, but in the photocycle neither of this protein nor of the D212N/R82Q/D96N triple mutant does a deprotonated Schiff base (the M intermediate) accumulate. Instead, the photocycle contains only the K, L, and N intermediates. Infrared difference spectra of D212N/R82Q and D212N/R82Q/D96N demonstrate that although D96 becomes deprotonated in N, D85 remains unprotonated. On the other hand, M is produced at pH > 8, where according to independent evidence the L M equilibrium should shift toward M. Likewise, M is restored in the photocycle when the retinal is replaced with the 14-fluoro analogue that lowers the pKa of the protonated Schiff base, and now D85 becomes protonated as in the wild type.(ABSTRACT TRUNCATED AT 250 WORDS)