Showing papers on "Schiff base published in 1976"

Visual-pigment spectra: implications of the protonation of the retinal Schiff base.

Abstract: Various models of visual-pigment spectra are critically discussed in terms of the spectral properties of protonated Schiff bases and the common structural features of most proteins. The opsin apoprotein is capable of regulating visual pigment wavelengths in ways that are difficult or impossible to reproduce in model systems. Theories based on solvent effects of the spectra of protonated Schiff bases may be misleading. Careful parameterization using known polyene spectra allows accurate calculation of the spectral properties of protonated Schiff bases. It is shown that an isolated protonated Schiff base of retinal should absorb near 600 nm and that blue-shifted spectra seen in solution arise from associated counterions or solvent molecules. We conclude that the most plausible specific model of chromophore-protein interactions is one in which the protonated Schiff base is closely associated with its counterion and where additional negatively charged or polar groups are positioned by the protein in the vicinity of the ring half of the chromophore. Pigment absorption maxima, bandwidths, and the A2-A1 pigment absorption differences arise naturally from these simple models of pigment spectra.

Asymmetric synthesis of α-amino-acid derivatives by alkylation of a chiral Schiff base

Abstract: An efficient asymmetric synthesis of D-α-amino-acid derivatives has been achieved by alkylation of a Schiff base prepared from glycine t-butyl ester and (1S,2S,5S)-2-hydroxypinan-3-one.

The structure determination of a binuclear copper(II) complex of aTetra-Schiff base macrocycle

Abstract: The structure of the complex Lcu2Cl2,6H2O (where LH2 is the tetra-Schiff base macrocycle formed as the condensation product of propane-1,3-diamine and 2-hydroxy-5-methylisophthal- aldehyde in the mole ratio 2: 2) was determined by single-crystal X-ray diffraction methods. The crystals are monoclinic, a 7.720(1), b 17.079(1), c 11.171(1) A and β 91.50(1)o, with two molecules per unit cell. It has proved difficult to resolve the ambiguity between the three possible space groups (C2, Cm, and C2/m) and the initial model has been refined in each using a full-matrix least-squares method. The space group C2/m seems the most likely and the structure is described in it; R 0.049 for the 1369 independent reflections measured using counter methods. The structure analysis has confirmed the anticipated cyclic structure of the ligand with the two copper atoms held together in a binuclear arrangement by the planar N4O2 donor set; the Cu.. .Cu distance is 3.133(1)A. Each copper atom has a distorted square-pyramidal environment with the apical position of each being occupied by a chlorine atom; the two chlorine atoms are on the opposite sides of the macrocycle. The water molecules are not coordinated, but form an extensive system of hydrogen bonding throughout the crystal with discrete binuclear molecules of the LCu2Cl2 complex.

The pyridoxal 5' -phosphate site in rabbit skeletal muscle glycogen phosphorylase b: an ultraviolet and 1H and 31P nuclear magnetic resonance spectroscopic study.

Abstract: 1 H NMR spectra of the 3-0-methylpyridoxal 5'-phosphate-n-butylamine reaction product indicated that this analogue forms a Schiff base in aprotic solvent. The uv spectral properties of 3-0-methylpyridoxal-5'-phosphate phosphorylase b correspond to those of the n-butylamine Schiff base derivative in dimethyl sulfoxide. On the basis of that and auxiliary uv and 1H NMR spectra of pyridoxal and pyridoxal 5'-phosphate and the corresponding Schiff base derivatives we have verified that pyridoxal 5' -phosphate is also bound as a Schiff base to phosphorylase and not as an aldamine. Since 3-0-methylpyridoxal-5'-phosphate phosphorylase is active, a proton shuttle between the 3-hydroxyl group and the pyridine nitrogen is excluded. This directs attention to the 5' -phosphate group of the cofactor as a candidate for a catalytic function. 31P NMR spectra of pyridoxal 5' -phosphate in phosphorylase b indicated that deprotonation of the 5' -phosphate group was unresponsive to external pH. Interaction of phosphorylase b with adenosine 5' -monophosphate, the allosteric effector required activity, and arsenate, which substitutes for phosphate as substrate, triggered a conformational change which resulted in deprotonation of the 5' -phosphate group of pyridoxal 5' at pH 7.6. It now behaved like in the pyridoxal-phosphate-epsilon-aminocaproate Schiff base in aqueous buffer, where the diionized form is dominant at this pH. Differences of line widths of the adenosine 5' -monophosphate signal point to different life times of the allosteric effector- enzyme complexes in the presence and absence of substrate (arsenate).

Metal(III) compounds of potentially septadentate[N4O3] ligands derived from tris(2-aminoethyl)amine and substituted salicylaldehydes. Part I. Preparation of gallium, chromium, manganese, iron, and cobalt compounds, and the crystal and molecular structure of the iron compound of tris[2-(5-chloro-2-hydroxybenzylidene)ethyl]amine, [Fe(C27H24Cl3N4O3)]·3H2O

Abstract: The potentially septadentate and trianionic Schiff base ligands, prepared from tris(2-aminoethyl)amine and three moles of salicylaldehyde, react with GaIII, CrIII, MnIII, FeIII and CoIII species to form 1 : 1 neutral compounds. This paper reports a range of such compounds, prepared from substituted aldehydes (3-OMe, 3-NO2-, 5-Cl, 5-Br, 5-OMe, and 5-NO2) as well as the parent salicylaldehyde; details are given of their electronic spectra; some polymorphs and isomorphous series are classified from the X-ray powder diffraction patterns; and a full X-ray structural analysis of the iron(III) compound of the 5-chloro-substituted ligand is reported. Crystals of the latter were cubic with a = 22.497(1)A, space group la3 (Tn7, No. 206). Data were collected by diffractometer, and the structure solved from 1 081 independent reflections by conventional Patterson and Fourier techniques; it was refined by block-diagonal least-squares to a final R of 0.060. The molecule is essentially an [Fe(O3N2)] octahedral species [Fe–O 1.953(6), Fe–N 2.185(7)A], lying on a three-fold crystallographic axis. The latter passes through the iron and the other (apical) nitrogen N(1), which appears to be antibonding with respect to the metal. This nitrogen is 3.26(2)A, from the metal atom, and almost co-planar with its three carbon-atom substituents. C–N(1)–C anglesare 117.8 (15)°; and N(1) isonly 0.2A below the C3plane. Thewater molecules, in the crystal, form a flattened octahedral set about the crystallographic C3 axis, hydrogen-bonded to themselves and to the phenolic oxygens of the ligand.

Quadridentate Schiff-base metal complexes as chelating ligands of alkali metals. Synthesis and structure of ammonium and sodium tetra-phenylborate addition complexes with [NN′-ethylenebis(salicylideneiminato)]nickel(II)

Abstract: Transition-metal complexes of the Schiff-base NN′-erhylenebis(salicylaidimine)(H2salen) co-ordinate ammonium and alkali-metal cations to give addition complexes of different stoicheiometry. The complex [Ni(salen)] with ammonium or potassium tetraphenylborate, M′[BPh4], gives addition compounds of formula [{Ni(salen)3-,M′(BPh4)]·nL [L = solvent, e.g. tetrahydrofuran (thf), acetonitrile, or acetone]. For n= 2 and L = thf the complexes are isomorphous. Reaction with Na [BPh4] in acetonitrile yields again a compound with Ni : Na 3 : 1, from which [{Ni(salen)}2Na(NCMe)2(BPh4)]·2MeCN is obtained by very slow crystallization. The crystal structures of [{Ni(salen){3(NH4)(BPh4)]·2thf, (I), and [{Ni(salen)}2Na(NCMe)2]·2MeCN, (II), have been determined from three-dimensional X-ray data. Crystals of both compounds are triclinic, space group P, Z= 2, with cell parameters: (I), a= 18.93(1), b= 16.94(1), c= 14.84(1)A, α= 91.6(2), β= 111.4(2), γ= 116.8(2)°; (II), a= 14.64(1), b= 15.56(1), c= 14.91 A, α= 85.2(1), β= 114.2(1), γ= 98.7(1)°. The structures have been solved by Patterson and Fourier methods and refined by block-diagonal matrix least squares to R 0.11 for (I) and 0.080 for (II). The structure of (I) consists of discrete [BPh4]– anions and [{Ni(salen)}3(NH4)]+ macrocations in which the ammonium ion is surrounded by the six oxygen atoms (mean N H4+⋯ O 2.92 A) of the three Ni(salen) moieties. The co-ordination polyhedron is intermediate between a trigonal prism and an octahedron. as determined by a balance of steric and electrostatic factors. Similarly the structure of (II) has discrete [BPh4]– anions and [{Ni(salen)}2Na(NCMe)2]+ macrocations with the sodium ion on an approximately two-fold axis, co-ordinated by the four oxygen atoms of the two Ni(salen) molecules and by the nitrogen atoms of the two acetonitrile molecules occupying cis positions. The co-ordination polyhedron approximates to an octahedron with mean Na+⋯ O 2.41 and Na+⋯ N 2.49 A.

Studies of nitriles. XI. Preparation and chemistry of Schiff bases of ADAN, 2-amino-3,3-dichloroacrylonitrile. A highly effective conversion into 2-substituted-4(5)-chloroimidazole-5(4)-carbaldehydes.

Abstract: Schiff bases of 2-amino-3, 3-dichloroacrylonitrile (ADAN) have been prepared in excellent yields by condensation of ADAN with various aldehydes under mild conditions. These Schiff bases (2, 3, 4) are highly reactive polyfunctional compounds and useful synthetic intermediates : the chlorine atoms of Schiff base 2e react with various nucleophiles such as alkoxides, mercaptides, and primary and secondary amines giving the corresponding 3, 3-dialkoxy-, 3, 3-bis (substituted mercapto)-, and 3, 3-bis (substituted amino)-derivatives (7, 8, and 10), respectively. Reduction of Schiff base 2e with zinc powder under mild conditions resulted in formation of the monochloro derivative 5. With anhydrous hydrogen chloride in ether, these Schiff bases underwent smooth cyclization to new 4 (5)-chloro-5(4)-dichloromethylimidazoles 11, which in turn were converted into 4 (5)-chloro-imidazole-5 (4)-carbaldehydes 12 in excellent yields upon hydrolysis.

Stereochemistry and Reactivity of Metal–Schiff-base Complexes. I. Preparation and Thermodynamic Stereoselectivity of Mixed Ligand Cobalt(III) Complexes Containing N,N′-Ethylenebis(salicylideneamine) Dianion (sal2en) or N,N′-Ethylenebis(7-methylsalicylideneamine) Dianion (7,7′-Me-sal2en) and L-Amino Acid Anion

Abstract: Two series of new mixed ligand cobalt (III) complexes with the general formulas of [Co(sal2en)(L-aa)] and [Co(7,7′-Me-sal2en)(L-aa)] (where L-aa=anion of L-ala, L-val, L-leu, L-isoleu, L-met, L-thr, L-phe, L-tyr, or L-trp) have been prepared and characterized. All are labile for both isomerization and substitution reactions and each exists in an equilibrium mixture of ΛL- and ΔL-cis-β1(fac)-isomers (ΛL>ΔL)in methanol. The isomer’s ratios, ΛL/ΔL were estimated from their PMR spectra, and the following facts were found: The increasing order of the ratios is L-ala∼L-met∼L-leu

Coenzyme Models. VIII. Reduction of the Schiff Base of Pyridoxal Analogs by 1,4-Dihydronicotinamide

Abstract: Reduction of Schiff bases by 1-benzyl-1,4-dihydronicotinamide (BzlNH) was performed in refluxing methanol. Protonated imines such as N-(p-nitrobenzylidene)benzylamme hydrochloride (1H+) and N-(4-nitrosalicylidene)benzylamine hydrochloride (1(OH)H+) were reduced to the corresponding aminomethyl derivatives. The NMR product analysis for the reaction of BzlNH and 1H+ in refluxing ethanol-d established that the solvent deuteron was not incorporated into the aminomethylene product. 1(OH), unprotonated species, was partially reducible, whereas 1 was totally unreactive. The facile reduction of 1 occurred in the presence of several amine hydrochlorides (NEt3·HCl, imidazole hydrochloride, N,N,N′,N′-tetramethylethylenediamine dihydrochloride). These results show that the ortho hydroxyl group of 1(OH) facilitates imine reduction not only by polarization of the Schiff base via hydrogen bonding but also by proton transfer. The relative reactivity of the Schiff base to the BzlNH reduction is: 1H+, 1+NEt3·HCl≥1(OH)H+>1(...

Schiff Base Complexes of Dioxouranium(VI), II Dioxouranium(VI) Complexes of the Schiff Bases of o-Hydroxy Acetophenone, 2,4-Pentandione and Salicylaldehyde

Abstract: Dioxouranium(VI) derivatives of the type UO2(NO3)2(SBH2)2 have been synthesised by the reactions of dioxouranium(VI) nitrate hexahydrate with the nitrogen donor ligands (SBH2) in 1: 2 molar ratio. The Schiff bases used in these reactions are of the general type, o-HOC6H4C(CH3): NX1OH (where X1 = -CH2CH2-, -CH2CH(CH3)- and -CH2CH2CH2-), CH3C(OH)CHC(CH3): NX2OH and o-OHC6H4CH: NX2OH (where X2 = -CH2CH2-, -CH2CH(CH3)-, -CH2CH2CH2- and o-C6H4). The resulting new derivatives have been characterized on the basis of elemental analysis, magnetic susceptibility, conductance measurements and infrared spectra

Selective Activation of Methylene or Methine Groups of Tripeptide-Schiff Bases Coordinated around Nickel(II)

Abstract: Nickel(II) chelates of the Schiff bases derived from salicylaldehyde and tripeptides such as glycylglycylglycine, glycyl-DL-α-alanylglycine, glycylglycyl-DL-α-alanine, and DL-α-alanylglycylglycine have newly been prepared. The reactivity of methylene or methine groups in the tripeptide moieties of those Schiff base chelates has been studied by PMR spectra in D2O. It has been found that the methylene or methine protons of the N-terminal amino acid residues are most effectively activated by the coordination around nickel(II) as compared with those of the C-terminal and central amino acid residues.

Schiff base complexes of silicon(IV)

Abstract: Reactions of silicon tetraacetate with different types ofSchiff bases have been investigated in anhydrous benzene. Monofunctional bidentate, C6H5CH∶NXOH and HORCH∶NC6H5 [whereX=CH2CH2, CH2CH(CH3) or o-C6H4 and R=o-C6H4 or 2,1-C10H6], bifunctional tridentate, o-HOC6H4CH∶NYOH [whereY=CH2CH2 or CH2CH(CH3)] and bifunctional tetradentateSchiff bases, o-HOC6H4C(CH3)∶N(CH2)nN∶C(CH3)C6H4OH-o (wheren=2 or 3) have been shown to yield derivatives of the type, Si(OAc)4−mLm, Si(OAc)4−2nLn′ and Si(OAc)2L″ (wherem=1,2 or 3;n=1 or 2 and HL, H2L′ and H2L″ represent the molecules of monofunctional bidentate, bifunctional tridentate and bifunctional tetradentateSchiff bases resp.) and have been found to be monomeric in boiling benzene. Tentative structures based on IR and in a few cases PMR spectra have been indicated for the resulting derivatives.

Schiff-base complexes of tetracarbonyliron from enneacarbonyldi-iron

Abstract: The reaction between [Fe2(CO)9] and Schiff bases of the type PhCHCH–CHN–C6H4X (I; X = H, 4-Cl, 4-Br, 3-Br, 4-F, 4-I, 4-Me, and 4-OMe) and R1R2R3R4C6HCHN–C6H4OMe-4 (II; R1,R2,R3,R4= H, Me, or OMe) has been re-examined. The first stage of the reaction leads to the formation of [Fe(CO)4L] complexes (L = Schiff base)[III; L =(I)] and [IV; L =(II)]. I.r. and n.m.r. evidence suggests that the Fe is bonded to the nitrogen lone-pair electrons of the Schiff base. With ligands (I) the second stage of the reaction gives formation of [Fe(η-PhCHCH–CHN–C6H4X)(CO)3] complexes (V). Ligands (II) having unsubstituted benzene ring ortho positions give complexes of the type [Fe2(CO)6L].

Synthesis of the copper(ii) complex of bis(salicylideneamino)maleonitrile: [(2,3-dicyano-cis-2-butene-2,3-diiminomethyl)di-o-phenolato]copper(ii)

Abstract: The copper(II) complex of the Schiff base derived from a diamino-maleonitrile and two salicylaldehyde molecules was synthesized as deep wine-red crystals. The complex molecule was supported to have a planar structure by the magnetic susceptibility and the infrared and Raman spectra.