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Tautomers
Kinds of Tautomers Selected AbstractsPseudo-Octahedral Schiff Base Nickel(II) Complexes: Does Single Oxidation Always Lead to the Nickel(III) Valence Tautomer?EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 27 2008Olaf Rotthaus Abstract With the aim of establishing correlations between the ligand structure and the oxidation site in nickel complexes from Schiff base ligands, five ligands and their nickel complexes have been synthesized. The prototypical asymmetric Schiff base ligand HL1 contains both phenol and pyridine pendant arms with a pivotal imine nitrogen atom. Ligands HL2,5 differ from HL1 by either their phenolate para substituent, the hybridization of the pivotal nitrogen atom, and/or the N-donor properties of the pyridine moiety. The five complexes [Ni(L1,5)2] are obtained by treating the corresponding ligands with 0.5 equiv. of Ni(OAc)2·4H2O in the presence of NEt3. X-ray crystal-structure diffraction studies as well as DFT calculations reveal that [Ni(L1,5)2] involves a high-spin nickel(II) ion within a pseudo-octahedral geometry. The two ligands are arranged in a meridional fashion when the pivotal nitrogen atom is an imine {as in [Ni(L1,2)2] and [Ni(L4,5)2]}, while the fac isomer is preferred in [Ni(L3)2] (amino pivotal nitrogen atom). [Ni(L1)2] is characterized by an oxidation potential at ,0.17 V vs. Fc+/Fc. The one-electron-oxidized species [Ni(L1)2]+ exhibits an EPR signal at g = 2.21 attributed to a phenoxyl radical that is antiferromagnetically coupled to a high-spin NiII ion. [Ni(L2)2] differs from [Ni(L1)2] by the phenolate para substituent (a tert -butyl instead of the methoxyl group) and exhibits an oxidation potential that is ca. 0.16 V higher. Compared to [Ni(L1)2]+ the cation [Ni(L2)2]+ exhibits a SOMO that is more localized on the metal atom. The EPR and electrochemical signatures of [Ni(L3)2]+ are similar to those of [Ni(L1)2]+, thus showing that an imino to amino substitution compensates for a methoxy to tert -butyl one. Replacement of the pyridine by a quinoline group in [Ni(L4,5)2] makes the complexes slightly harder to oxidize. The EPR signatures of the cations [Ni(L4,5)2]+ are roughly similar to those of the pyridine analogs [Ni(L1,2)2]+. The oxidation site is thus not significantly affected by changes in the N-donor properties of the terminal imino nitrogen atom.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] ChemInform Abstract: Borane Complexes of the H3PO2 P(III) Tautomer: Useful Phosphinate Equivalents.CHEMINFORM, Issue 11 2009Yamina Belabassi Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Enol Forms of 1,3-Indanedione, Their Stabilization by Strong Hydrogen Bonding, and Zwitterion-Assisted InterconversionEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2010Mark Sigalov Abstract By analyzing NMR spectroscopic data, and supported by IR, UV/Vis, Raman, dielectrometry, and DFT techniques, a comprehensive study of the 1:2 adducts of picolinaldehyde and 1,3-indanediones is presented. The parent indanedione derivative 5 exists in an equilibrium between all-keto and enol forms, the latter being stabilized by an intramolecularO,H···N hydrogen bond. Only the all-keto form was observed in the 5,6-dimethoxy compound 6, whereas solely the enol tautomer was observed with its 5,6-dichloro analogue 7. Polar solvents and low temperatures shift the equilibrium towards the enol tautomer in 5. The structure of adduct 8, formed with isonicotinaldehyde, prevents the formation of intramolecular O,H···N hydrogen bonds and thus it exists in the all-keto form in low polar solvents. However, in DMSO solutions it adopts a zwitterionic form with a strong anionic O,···H···O hydrogen bond. Thus, the enol form in indanedione adducts was unequivocally characterized in solution and the factors that determine the keto,enol tautomerism, namely electronic effects, solvent, temperature, and intramolecular hydrogen bonds, have been methodically studied by spectroscopic and quantum mechanical methods. [source] Synthesis of Pyrazoles by Treatment of 3-Benzylchromones, 3-Benzylflavones and Their 4-Thio Analogues with Hydrazine,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 12 2006Albert Lévai Abstract The synthesis of pyrazoles 13,24 has been accomplished by treatment of 3-benzylchromones 1,5, 3-benzylflavones 6,12 and their 4-thio analogues 25,29 with hydrazine hydrate in hot pyridine. A plausible reaction mechanism for the formation of pyrazoles 13,24 is discussed. A 1H NMR study in [D6]DMSO allowed the presence of both pyrazole annular tautomers to be observed, due to the presence of intramolecular hydrogen bonds in each tautomer (OH--N and NH--O). GIAO/B3LYP/6-311++G** calculations were carried out on some model pyrazoles to provide a theoretical basis for the NMR experimental observations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Solid-State Structure and Tautomerism of 2-Aminotroponimines Studied by X-ray Crystallography and Multinuclear NMR SpectroscopyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2004Rosa M. Claramunt Abstract Structural studies in the solid state by X-ray crystallography and by 13C and 15N CPMAS NMR spectroscopy carried out on a series of 2-aminotroponimine derivatives 2,5 has allowed to establish the existence of hydrogen bonding and to determine the most stable tautomer. Almost all the structures reflect the classical double-well potential function for the N,H···N hydrogen bonds. Only in the case of the compound N -(pyrrol-1-yl)-2-(pyrrol-1-ylamino)troponimine (5) the crystal structure shows two independent molecules, one with a classical hydrogen bond and another with either a single-well or a low-barrier hydrogen bond. The structure of this compound is discussed with the use of the solid-state NMR spectroscopic data. 2-Aminotropones, as intermediates to the 2-aminotroponimines, show the oxo-tautomer as the stable form. B3LYP/6-31G* calculations are used to rationalise the experimental results. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Further Studies on the Synthesis of meso -Tetraarylazuliporphyrins under Lindsey,Rothemund Reaction Conditions and Their Conversion into BenzocarbaporphyrinsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 23 2003Timothy D. Lash Abstract Azulene has been shown to react with pyrrole and a series of aromatic aldehydes in the presence of boron trifluoride etherate to give meso -tetraarylazuliporphyrins 6. Good yields of azuliporphyrins were obtained for benzaldehyde, 4-chlorobenzaldehyde, 4-bromobenzaldehyde, and 4-iodobenzaldehyde, and under dilute conditions p -tolualdehyde gave respectable yields. In each case, substantial amounts of meso -tetraarylporphyrins were also formed and a minor fraction of carbaporphyrin by-products could be detected, but otherwise no other macrocyclic products could be identified. 4-Nitrobenzaldehyde gave relatively poor yields of the corresponding azuliporphyrin, while p -anisaldehyde only gave trace amounts of product. Pentafluorobenzaldehyde gave variable results, although in this case a large number of additional by-products were identified including N -fused pentaphyrin, hexaphyrin, and higher order porphyrinoids, but no expanded azulene-containing macrocycles could be detected. Azuliporphyrins undergo reversible nucleophilic substitution on the seven-membered ring with pyrrolidine, benzenethiol, hydrazine, or benzylamine to give carbaporphyrin adducts. This property appears to facilitate an oxidative ring contraction of azuliporphyrins 6 with tert -butyl hydroperoxide in the presence of potassium hydroxide to produce mixtures of benzocarbaporphyrins 19 and 20. Tetraaryl-benzocarbaporphyrins exhibit slightly reduced diatropic ring currents compared to their meso -unsubstituted counterparts, although their UV/Vis spectra are very porphyrin-like and exhibit strong Soret bands near 450 nm. The benzocarbaporphyrins undergo reversible protonation to give monocationic and dicationic species. The latter involves C -protonation to generate an internal CH2 within the macrocyclic cavity. X-ray crystallography of tetraphenylbenzocarbaporphyrin 19a confirms that the preferred tautomer has the two NHs on either side of the indene subunit, in agreement with previous theoretical and spectroscopic studies. In addition, the presence of phenyl substituents at the 5,20-positions was found to tilt the indene moiety substantially by 27.4(1)° relative to the [18]annulene substructure. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Pyridoxal 5,-phoshate Schiff base in Citrobacter freundii tyrosinephenol-lyaseFEBS JOURNAL, Issue 6 2000Ionic, tautomeric equilibria Spectral properties of the internal Schiff base in tyrosine phenol-lyase have been investigated in the presence of an activating cation K+ and a cation-inhibitor Na+. The holoenzyme absorption spectra in the pH range 6.5,8.7 were recorded in the presence of K+. No apparent pKa value of the coenzyme chromophore was found in this pH range, indicating that the internal Schiff base does not change its ionic form on going from pH 6.5 to 8.7. To determine the ionic state and tautomeric composition of the Schiff base in tyrosine phenol-lyase, the absorption and circular dichroism spectra were analyzed using lognormal distribution curves. The predominant form of the internal Schiff base is that with protonated pyridinium and aldimine nitrogen atoms and deprotonated 3,-hydroxy group, i.e. the ketoenamine. This form is in prototropic equilibrium with its enolimine tautomer. The internal aldimine ionic form is changed upon replacement of K+ with Na+. This replacement leads to a significant decrease in the pKa value of pyridinium nitrogen of the pyridoxal- P. [source] Solid-State and Solution Structural Studies of 4-{[C(E)]-1H -Azol-1-ylimino)methyl}pyridin-3-olsHELVETICA CHIMICA ACTA, Issue 7 2006Dionisia Sanz Abstract The new N -salicylideneheteroarenamines 1,4 were prepared by reacting the biologically relevant 3-hydroxy-4-pyridinecarboxaldehyde (5) with 1H -imidazol-1-amine (6), 1H -pyrazol-1-amine (7), 1H -1,2,4-triazol-1-amine (8), and 1H -1,3,4-triazol-1-amine (9). Solution 1H-, 13C-, and 15N-NMR were used to establish that the hydroxyimino form A is the predominant tautomer. A combination of 13C- and 15N-CPMAS-NMR with X-ray crystallographic studies confirms that the same form is present in the solid state. The stabilities and H-bond geometries of the different forms, tautomers and rotamers, are discussed by using B3LYP/6-31G** calculations. [source] Cycloaddition Reactions of 7-Benzylidenecycloocta-1,3,5-triene with Ethenetetracarbonitrile and 4-Phenyl-3H -1,2,4-triazole-3,5(4H)-dioneHELVETICA CHIMICA ACTA, Issue 7 2005Philip Clements An (E)/(Z) mixture (3,:,2) of 7-benzylidenecycloocta-1,3,5-triene (5) is obtained when 1-benzylcycloocta-1,3,5,7-tetraene (7), prepared by an improved procedure, is treated with t -BuOK in THF. Alternatively, a ca. 9,:,1 mixture (E)/(Z)- 5 can be prepared in a Wittig reaction involving benzaldehyde and cycloocta-2,4,6-trien-1-ylidenetriphenylphoshorane (9). Treatment of (E)/(Z)- 5 88,:,12 with ethenetetracarbonitrile (TCNE) gave a complex mixture of products, from which seven mono-adducts and two bis-adducts were isolated (Sect.,2.2.1). Of the mono-adducts, four are ,4+,2 adducts: two ((E)- and (Z)-isomers) are derived from valence tautomers of the two isomers of (E)/(Z)- 5, while it is tentatively suggested that the other two (again (E)- and (Z)-isomers) are formed from the intermediacy of a pentadienyl zwitterion (Sect.,2.3). The remaining three mono-adducts, two of which are epimers, are ,8+,2 adducts. It is suggested that they are derived from the intermediacy of homotropylium zwitterions (Sect.,2.3). For the two bis-adducts, it is postulated that they are derived from an initial ,2+,2 cycloaddition involving the homotropylium zwitterions followed by ,4+,2 cycloaddition to the valence tautomer of each of the ,2+,2 cycloadducts. With 4-phenyl-3H -1,2,4-triazole-3,5(4H)-dione (6), (E)/(Z)- 5 91,:,9 yielded two ,4+,2 cycloadducts ((E)- and (Z)-isomers) as well as two epimeric ,8+,2 cycloadducts (Sect.,2.2.2). The intermediacy of pentadienyl (tentative suggestion) and homotropylium zwitterions accounts for the formation of the products (Sect.,2.3). [source] Efficient Enantioselective Syntheses of Sertraline, 2-Epicatalponol and Catalponol from Tetralin-1,4-dioneADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2010Alvaro Enriquez Garcia Abstract Tetralin-1,4-dione, the stable tautomer of dihydroxynaphthalene, was reduced with catecholborane in the presence of 3,3-diphenyl-1-butyltetrahydro-3H -pyrrolo[1,2- c][1,3,2]oxazaborole as catalyst to give enantiomerically highly enriched 4-hydroxy-1-tetralone (99% ee) in an efficient one-pot procedure. The R -enantiomer provided a rapid access to sertraline while the S -enantiomer was converted into 2-epicatalponol and catalponol. A more selective enantioselective route to the antithermitic catalponol made use of the planar chiral tricarbonylchromium complex of hydroxytetralone. Its precursor chromium(tricarbonyl)[,6 -(1-4,4a,8a)-tetralin-5,8-dione] was obtained via direct complexation of 1,4-dihydroxynaphthalene using chromium(tricarbonyl)- tris(ammonia) and boron trifluoride etherate as source of the chromium(tricarbonyl) fragment. Enolate prenylation was best carried out in the presence of a tetraamine ligand. Complete inversion of the stereogenic center bearing the prenyl group of the initially obtained tetralone complex was achieved via enolate formation followed by protonation. [source] A family of thioxanthato ruthenium and osmium arylsISRAEL JOURNAL OF CHEMISTRY, Issue 3 2001Swarup Chattopadhyay The title complexes of type M(RL2)(PPh3)2(CO)(S2CSEt) (2a: M = Ru; 2b: M = Os) have been synthesized in excellent yields by reacting M(RL1)(PPh3)2(CO)X (1a: M = Ru, × = Cl; 1b: M = Os, × = Br) with potassium ethyl thioxanthate and have been characterized with the help of spectral and electrochemical data. The RL2 ligand in 2 is the imine-phenol tautomer of N-C6H4R(p)-4-methylsalicylaldimine (R = Me, MeO, Cl) coordinated at the carbanionic-C2 atom only while RL1 in 1 is the iminium-phenolato tautomer chelated via carbanionic-C2 and phenolato-O atoms. The synthetic reaction is thus attended with tautomerization of the Schiff base ligand. It is also associated with a rotation of the ligand by ,180° around the M,C bond in order to exclude steric repulsion. These features have been revealed by structure determination of 2a (R = Me). The metallated aldimine ring is found to be highly noncoplanar (dihedral angle ,40°) with the thioxanthate chelate ring due to steric repulsion originating from the relatively large size of the sulfur atom. This phenomenon, which is absent in both the precursor 1 (R = Me) and in the carboxylate analogue Ru(MeL2)(PPh3)2(CO)(O2CMe), 7, has distinctive effects on bond parameters of 2a (R = Me). Thus the two Ru,P bonds in 2a (R = Me) differ in length by as much as 0.06 Å. The thioxanthate 2 is thermodynamically more stable than the precursor 1 as well as the carboxylate 7. Accordingly, both of these are irreversibly transformed to 2a (R = Me) upon treatment with thioxanthate. [source] QM/MM calculation of solvent effects on absorption spectra of guanineJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2010Maja Parac Abstract Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited-state calculations were extracted from ground-state molecular dynamics (MD) simulations using the self-consistent-charge density functional tight binding (SCC-DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited-state calculations used time-dependent density functional theory (TDDFT) and the DFT-based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto-N7H and keto-N9H guanine, with particular focus on solvent effects in the low-energy spectrum of the keto-N9H tautomer. When compared with the vertical excitation energies of gas-phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC-DFTB-based rather than B3LYP-based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas-phase and solution spectra, typically ca. 0.1,0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent-induced structural changes and from electrostatic solute,solvent interactions, the latter being dominant. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010 [source] Aromatization and ring cyclization: A better understanding on the ring cyclization mechanism of 3-amino-6-hydrazino-1,2,4-triazin-5(2H)-one reacted with acetic acid in N,N -dimethylformamideJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2005Long-Chih Hwang In this paper we report that the title compound (3) reacts with excess N,N -dimethylformamide (DMF) containing two equivalents of acetic acid to afford 6-amino-1,2,4-triazolo[3,4- f][1,2,4]triazin-8(7H)-one (1). When 3-amino-2-benzyl-6-hydrazino-1,2,4-triazin-5(2H)-one (6), the N-2 benzylated derivative of 3, is treated under the same conditions, ring cyclization does not occur; instead, 3-amino-2-benzyl-6-(2-formyl-hydrazino)-1,2,4-triazin-5(2H)-one (7) is formed. Single-crystal X-ray analysis of a 3-ethyl derivative of compound 1 reveals the predominant tautomeric structure to be the 7H -tautomer (7H - 1). From these results, we propose a reasonable cyclization mechanism that incorporates two important points: (1) the tautomerism of the N-2 hydrogen with the C-5 oxo group aromatizes the 1,2,4-triazine ring, and (2) the DMF is proto-nated by acetic acid on the nitrogen atom, then deamination occurs where DMF is attacked by the 6-hydrazino group of 3 or 6. [source] Gas-phase tautomers of protonated 1-methylcytosine.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 11 2005Preparation, dissociation mechanisms, energetics Abstract Tautomers of 1-methylcytosine that are protonated at N-3 (1+) and C-5 (2+) have been specifically synthesized in the gas phase and characterized by tandem mass spectrometry and quantum chemical calculations. Ion 1+ is the most stable tautomer in aqueous and methanol solution and is likely to be formed by electrospray ionization of 1-methylcytosine and transferred in the gas phase. Gas-phase protonation of 1-methylcytosine produces a mixture of 1+ and the O-2-protonated tautomer (3+), which are nearly isoenergetic. Dissociative ionization of 6-ethyl-5,6-dihydro-1-methylcytosine selectively forms isomer 2+. Upon collisional activation, ions 1+ and 3+ dissociate by loss of ammonia and [C,H,N,O], whose mechanisms have been established by deuterium labeling and ab initio calculations. The main dissociations of 2+ following collisional activation are losses of CH2CNH and HNCO. The mechanisms of these dissociations have been elucidated by deuterium labeling and theoretical calculations. Copyright © 2005 John Wiley & Sons, Ltd. [source] Gas-phase theoretical prediction of the metal affinity of copper(I) ion for DNA and RNA basesJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 3 2003Nino Russo Abstract The most stable tautomeric forms of free DNA and RNA bases were considered as substrates for the interaction of Cu+ ion. Several suitable attachment sites were selected that involved mono- and bi-coordination of the cation. B3LYP/6,311 + G(2df,2p) bond energies showed that copper ion has the major affinity for guanine and cytosine bases. The proposed values of Cu+ ion affinity are 59.9, 60.0, 80.2, 88.0 and 69.0 kcal mol,1 for uracil, thymine, cytosine, guanine and adenine, respectively. The preference for the mono- or bi-coordination depends on the particular tautomer for each base. Copyright © 2003 John Wiley & Sons, Ltd. [source] A theoretical density functional study of association of Zn2+ with oxazolidine and its thio derivatives in the gas phaseJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2010Zaki S. Safi Abstract We have performed density functional theory (DFT) calculations in order to study the gas-phase interaction of oxo- and thio-oxazolidine derivatives with Zn2+. The calculations were performed at B3LYP/6-311+(2df,2p) level of theory. It has been found, in all cases, that the direct association of Zn2+ with the carbonyl and thiocarbonyl groups takes place at the heteroatom attached to position 2 irrespective of its nature. This preference has been attributed to the resonance effects caused by the nearest heteroatoms (oxygen and nitrogen). The most stable complexes correspond to structures with Zn2+ bridging between the heteroatom at position 2 or 4 of the 4- or 2-enol (or the 4- or 2-enethiol) tautomer and the dehydrogenated ring nitrogen atom, N3. Zn2+ association has a clear catalytic effect on the tautomerization processes which connect the oxo,thione forms with the enol,enethiol tautomers. Hence, although the enol,enethiol tautomers of oxazolidine and its thio derivatives should not be observed in the gas phase, the corresponding Zn2+ complexes are the most stable species and should be accessible, because the tautomerization barriers are smaller than the Zn2+ binding energies. Copyright © 2010 John Wiley & Sons, Ltd. [source] Mechanism of 4-methyl-1,2,4-triazol-3-thione reaction with formaldehydeJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2008Monika Wujec Abstract We have recently described theoretically the mechanism of the reaction between 4-methyl-1,2,4-triazol-3-thiol and formaldehyde that leads to the N- substituted product, N1 -hydroxymethyl-4-methyl-1,2,4-triazol-3-thione. New experimental findings indicate that the thione tautomeric form in reaction with formaldehyde also yields this product. This observation could not be explained on the basis of previous calculations, which predicted that the thione tautomer undergoes nucleophilic substitution at the sulfur atom, leading to the S- substituted product. We present theoretical explanation of the observed reactivity. We show that under experimental conditions this reaction proceeds with the intervention of the anionic form of the triazole with the Gibbs free energy of activation of only 1.8,kcal/mol. Copyright © 2008 John Wiley & Sons, Ltd. [source] Long-range substituent and temperature effect on prototropic tautomerism in 2-(acylmethyl)quinolinesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2001Ryszard Gawinecki Abstract Tautomeric equilibria between 2-(cinnamoylmethyl)quinoline, (Z)-1,2-dihydro-2-(cinnamoylmethylene)quinoline and (Z)-4-phenyl-1-(2-quinolyl)-1,3-butadien-2-ol were studied by 1H, 13C and 15N NMR methods. The ,CHCH, fragment conjugated with phenyl and a strong electron donor p -(1-pyrrolidine) substituent were found to favour the enolimine tautomer. This undergoes fast exchange (on the NMR time-scale) with the enaminone form. The amount of the latter tautomer was found to increase at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd. [source] Synthesis and characterization of benzothiazolyl-substituted anilsMAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2010Taracad K. Venkatachalam Abstract New Schiff bases containing a hydroxynaphthyl ring and substituted benzothiazolyl groups have been synthesized. High-resolution NMR spectra confirmed that these anils exist as enol,keto tautomers in solution. The results from NMR data demonstrated that the proportion of enol tautomer exceeded 90% in these substituted anils. Some compounds exhibited thermochromism in solid state. Copyright © 2010 John Wiley & Sons, Ltd. [source] Channel-forming solvates of 6-chloro-2,5-dihydroxypyridine and its solvent-free tautomer 6-chloro-5-hydroxy-2-pyridoneACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2009Sean R. Parkin On crystallization from CHCl3, CCl4, CH2ClCH2Cl and CHCl2CHCl2, 6-chloro-5-hydroxy-2-pyridone, C5H4ClNO2, (I), undergoes a tautomeric rearrangement to 6-chloro-2,5-dihydroxypyridine, (II). The resulting crystals, viz. 6-chloro-2,5-dihydroxypyridine chloroform 0.125-solvate, C5H4ClNO2·0.125CHCl3, (IIa), 6-chloro-2,5-dihydroxypyridine carbon tetrachloride 0.125-solvate, C5H4ClNO2.·0.125CCl4, (IIb), 6-chloro-2,5-dihydroxypyridine 1,2-dichloroethane solvate, C5H4ClNO2·C2H4Cl2, (IIc), and 6-chloro-2,5-dihydroxypyridine 1,1,2,2-tetrachloroethane solvate, C5H4ClNO2·C2H2Cl4, (IId), have I41/a symmetry, and incorporate extensively disordered solvent in channels that run the length of the c axis. Upon gentle heating to 378,K in vacuo, these crystals sublime to form solvent-free crystals with P21/n symmetry that are exclusively the pyridone tautomer, (I). In these sublimed pyridone crystals, inversion-related molecules form R22(8) dimers via pairs of N,H...O hydrogen bonds. The dimers are linked by O,H...O hydrogen bonds into R46(28) motifs, which join to form pleated sheets that stack along the a axis. In the channel-containing pyridine solvate crystals, viz. (IIa),(IId), two independent host molecules form an R22(8) dimer via a pair of O,H...N hydrogen bonds. One molecule is further linked by O,H...O hydrogen bonds to two 41 screw-related equivalents to form a helical motif parallel to the c axis. The other independent molecule is O,H...O hydrogen bonded to two related equivalents to form tetrameric R44(28) rings. The dimers are ,,, stacked with inversion-related dimers, which in turn stack the R44(28) rings along c to form continuous solvent-accessible channels. CHCl3, CCl4, CH2ClCH2Cl and CHCl2CHCl2 solvent molecules are able to occupy these channels but are disordered by virtue of the site symmetry within the channels. [source] An unusual syn conformation of 5-formyluracil stabilized by supramolecular interactionsACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2007Gustavo Portalone The asymmetric unit of the amino,oxo tautomer of 5-formyluracil (systematic name: 2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carbaldehyde), C5H4N2O3, comprises one planar amino,oxo tautomer, as every atom in the structure lies on a crystallographic mirror plane. At variance with all the previously reported small-molecule crystal structures containing the 5-formyluracil residue, the formyl substituent in the title compound exhibits an unusual syn conformation. The molecules are linked into planar sheets parallel to the bc plane by a combination of six N,H...O and C,H...O hydrogen bonds. Four of the hydrogen bonds are utilized to stabilize the formyl group in the syn conformation. [source] Tris(1,9-diethyladeninium) triiodide diiodideACTA CRYSTALLOGRAPHICA SECTION C, Issue 9 2006Elizabeth F. Day X-ray analysis of the title compound reveals three crystallographically distinct cations of 1,9-diethyladeninium, two iodide anions and one triiodide anion in the asymmetric unit, giving six residues and the formula 3C9H14N5+·I3,·2I,. Standard purine nomenclature is used to identify the atoms of each adenine moiety. Hydrogen bonding is observed between atoms N6 and N7 of a pair of cations [N,N = 2.885,(4)/2.902,(3) and 2.854,(3)/2.854,(3),Å], with additional hydrogen bonding to I, anions via the other N6 H atom [N,I = 3.708,(3), 3.738,(3) and 3.638,(3),Å]. The triiodide anion is not involved in hydrogen bonding. The bond lengths and angles of the 1,9-diethyladeninium cations are compared with literature values and confirm the formation of the imine tautomer. [source] An oximino tautomer of 1- n -decyl-4-hydroxyimino-3-methyl-1H -pyrazol-5(4H)-oneACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2006Ricardo Baggio The title compound, C14H25N3O2, consists of a five-membered heterocyclic ring to which a pendant decyl group is attached. The oximino tautomeric character of the molecule is clearly defined by the distribution of well defined double bonds in the heterocycle region (one C=O and two C=N). The most conspicuous packing interaction is the strong intermolecular hydrogen bond linking the oximino OH group and the carbonyl O atom to define broad planar hydrophilic strips running along the unique b axis. The alkyl chains adopt a fully extended conformation and lie almost at right angles to these one-dimensional structures, defining their hydrophobic counterpart. [source] Binding of quercetin with human serum albumin: A critical spectroscopic studyBIOPOLYMERS, Issue 6 2003Bidisa Sengupta Abstract Flavonols are plant pigments that are ubiquitous in nature. Quercetin (3,3,,4,,5,7-pentahydroxyflavone) and other related plant flavonols have come into recent prominence because of their usefulness as anticancer, antitumor, anti-AIDS, and other important therapeutic activities of significant potency and low systemic toxicity. Quercetin is intrinsically weakly fluorescent in aqueous solution, showing an emission maximum at ,538 nm. Upon binding to human serum albumin (HSA), quercetin undergoes dramatic enhancement in its fluorescence emission intensity, along with the appearance of dual emission behavior, consisting of normal and excited-state proton transfer (ESPT) fluorescence. In addition, the occurrence of a third emitting species has been noted for the first time. This is attributed to a electronic ground-state complex formed in the protein environment. High values of the fluorescence anisotropy (r) are obtained in the presence of HSA for the ESPT tautomer (r = 0.18), as well as the complex species (r = 0.37) of quercetin, indicating that the precursor ground-state molecules for both these emitting species of quercetin molecules are located in the motionally constrained sites of HSA. The steady-state emission data suggest that quercetin binds to two distinct sites in HSA from which the emissions from the normal tautomer and complex species take place. The preliminary results of studies on emission decay kinetics are also reported herein. Studies by far-UV circular dichroism spectroscopy reveal that binding of quercetin induces no significant perturbation in the secondary structure of HSA. © 2003 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy), 2003 [source] Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects?CHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2010Characterization, Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear FeIII Triplesalen Complexes, Synthesis Abstract The chiral triplesalen ligand H6chand provides three chiral salen ligand compartments in a meta -phenylene arrangement by a phloroglucinol backbone. The two diastereomeric versions H6chandRR and H6chandrac have been used to synthesize the enantiomerically pure chiral complex [(FeCl)3(chandRR)] (3RR) and the racemic complex [(FeCl)3(chandrac)] (3rac). The molecular structure of the free ligand H6chandrac exhibits at the terminal donor sides the O-protonated phenol,imine tautomer and at the central donor sides the N-protonated keto,enamine tautomer. The trinuclear complexes are comprised of five-coordinate square-pyramidal FeIII ions with a chloride at the axial positions. The crystal structure of 3rac exhibits collinear chiral channels of ,11,Å in diameter making up 33.6,% of the volume of the crystals, whereas the crystal structure of 3RR exhibits voids of 560,Å3. Mössbauer spectroscopy demonstrates the presence of FeIII high-spin ions. UV/Vis spectroscopy is in accordance with a large delocalized system in the central backbone evidenced by strong low-energy shifts of the imine ,,,* transitions relative to that of the terminal units. Magnetic measurements reveal weak intramolecular exchange interactions but strong magnetic anisotropies of the FeIII ions. Complexes 3rac and 3RR are good catalysts for the sulfoxidation of sulfides providing very good yields and high selectivities with 3RR being enantioselective. A comparison of 3RR and [FeCl(salen,)] provides higher yields and selectivities but lower enantiomeric excess values (ee values) for 3RR relative to [FeCl(salen,)]. The low ee values of 3RR appeared to be connected to a strong ligand folding in 3RR, opening access to the catalytically active high-valent Fe,O species. The higher selectivity is assigned to a cooperative stabilization of the catalytically active high-valent Fe,O species through the phloroglucinol backbone in the trinuclear complexes. [source] Head-to-Head Right-Handed Cross-Links of the Antitumor-Active Bis(,- N,N,-di- p -tolylformamidinato)dirhodium(II,II) Unit with the Dinucleotides d(GpA) and d(ApG)CHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2008Helen Abstract Reactions of cis -[Rh2(DTolF)2(NCCH3)6](BF4)2 with the dinucleotides d(GpA) and d(ApG) proceed to form [Rh2(DTolF)2{d(GpA)}] and [Rh2(DTolF)2{d(ApG)}], respectively, with bridging purine bases spanning the Rh,Rh unit in the equatorial positions. Both dirhodium adducts exhibit head-to-head (HH) arrangement of the bases, as indicated by the presence of H8/H8 NOE cross-peaks in the 2D ROESY NMR spectra. The guanine bases bind to the dirhodium core at positions N7 and O6, a conclusion that is supported by the absence of N7 protonation at low pH,values and the notable increase in the acidity of the guanine N1H sites (pKa,7.4 in 4:1 CD3CN/D2O), inferred from the pH-dependence titrations of the guanine H8 proton resonances. In both dirhodium adducts, the adenine bases coordinate to the metal atoms through N6 and N7, which induces stabilization of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H adenine sites (pKa,7.0,7.1 in 4:1 CD3CN/D2O), as compared to the imino form of free adenosine. The presence of the adenine bases in the rare imino form is further corroborated by the observation of DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of [Rh2(DTolF)2{d(GpA)}] and [Rh2(DTolF)2{d(ApG)}] in CD3CN at ,38,°C. The 2D NMR spectroscopic data and the molecular modeling results suggest the presence of right-handed variants, HH1R, in solution for both adducts (HH1R refers to the relative base canting and the direction of propagation of the phosphodiester backbone with respect to the 5, base). Complete characterization of [Rh2(DTolF)2{d(GpA)}] and [Rh2(DTolF)2{d(ApG)}] by 2D,NMR spectroscopy and molecular modeling supports anti- orientation of the sugar residues for both adducts about the glycosyl bonds as well as N- and S-type conformations for the 5,- and 3,-deoxyribose residues, respectively. [source] Towards a Tunable Tautomeric Switch in Azobenzene Biomimetics: Implications for the Binding Affinity of 2-(4,-Hydroxyphenylazo)benzoic Acid to StreptavidinCHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2008Joan-Antoni Farrera Dr. Abstract The tautomeric equilibria of 2-(4,-hydroxyphenylazo)benzoic acid (HABA) and 2-(3,,5,-dimethyl-4,-hydroxyphenylazo)benzoic acid (3,,5,-dimethyl-HABA) have been studied by a combination of spectroscopic and computational methods. For neutral HABA in solvents of different polarity (toluene, chloroform, DMSO, DMF, butanol, and ethanol) the azo tautomer (AT) is largely predominant. For monoanionic HABA, the hydrazone tautomer (HT) is the only detected species in apolar solvents such as toluene and chloroform, while the AT is the only detected species in water and a mixture of both tautomers is detected in ethanol. Comparison of the results obtained for HABA and its 3,,5,-dimethylated derivative shows that dimethylation of the hydroxybenzene ring shifts the tautomeric preferences towards the hydrazone species. These findings have been used to examine the differences in binding affinity to streptavidin, as the lower affinity of HABA can be explained in terms of the larger energetic cost associated with the tautomeric shift to the bioactive hydrazone species. Overall, these results suggest that a balanced choice of chemical substituents, embedding environment, and pH can be valuable for exploitation of the azo,hydrazone tautomerism of HABA biomimetics in biotechnological applications. [source] The Tautomeric Forms of Cyameluric Acid Derivatives,CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2007Nadia Abstract The tautomerism of cyameluric acid C6N7O3H3 (1,a), cyamelurates and other heptazine derivatives has recently been studied by several theoretical investigations. In this experimental study we prepared stannyl and silyl derivatives of cyameluric acid (1,a): C6N7O3[Sn(C4H9)3]3 (3,a), C6N7O3[Sn(C2H5)3]3 (3,b), and C6N7O3[Si(CH3)3]3 (4). In order to investigate the structure of 1,a the mono- and dipotassium cyamelurate hydrates K(C6N7O3H2),2,H2O (5) and K2(C6N7O3H),1,H2O (6) were synthesized by UV/Vis-controlled titration of a potassium cyamelurate solution with aqueous hydrochloric acid. Compounds 3,6 were characterized by FTIR and solid-state NMR spectroscopy as well as simultaneous thermal analysis (TGA, DTA). The single crystal X-ray structures of the salts 5 and 6 show that the hydrogen atoms in both anions are localized on the peripheral nitrogen atoms. This indicates,in combination with the solid-state NMR studies,that the most stable tautomer of solid 1,a is the triketo form with C3h symmetry. However, derivatives of both the hydroxyl and the amido tautomers may be formed depending on the substituent atoms: The spectroscopic data and single crystal structures of compounds C6N7O3[Si(CH3)3]3 (4) and the solvate C6N7O3[Sn(C2H5)3]3,C2H4Cl2 (3,b,) show that the former is derived from the symmetric trihydroxy form of 1,a, while 3,b, crystallizes as a chain-like polymer, which contains the tin atoms as multifunctional building blocks, that is, bridging pentacoordinated Et3SnO2 and Et3SnON units as well as non-bridging four-coordinated Et3SnN units. The cyameluric nucleus is part of the polymeric chains of C6N7O3[Sn(C2H5)3]3,C2H4Cl2 (3,b,), by the action of both tautomeric forms of cyameluric acid, the amide and the ester form. [source] Reactivity of 1-Hydro-5-carbaphosphatrane Based on Tautomerization between Pentavalent Phosphorane and Trivalent Cyclic PhosphoniteCHEMISTRY - A EUROPEAN JOURNAL, Issue 14 2006Junji Kobayashi Dr. Abstract The reaction behavior of 1-hydro-5-carbaphosphatrane (1,a) was examined. Treatment of 1,a with oxidants such as 3-chloroperoxybenzoic acid (mCPBA) and tBuOCl gave cyclic phosphonate 2 and 1-chloro-5-carbaphosphatrane (4), respectively, via cyclic phosphonite 3, a tautomer of 1,a. Compound 4 was readily hydrolyzed to afford 2. Compound 1,a was also sulfurized via 3 by elemental sulfur to afford cyclic thioxophosphonate 5, which was also obtained by reactions in the presence of bases. Treatment of 1,a with bases also proceeded through 3 to give an equilibrium mixture of the corresponding phenoxide anion 10 and the phosphoranide anion 9, which was quenched with MeI to afford a mixture of 11 and 1-methyl-5-carbaphosphatrane (1,b). Such reactivities are typical for neutral phosphoranes. Theoretical investigations of these reactivities were also performed. [source] Tamed Tigers: Stabilization of Reactive CarbenesCHEMPHYSCHEM, Issue 13 2008Martin Albrecht Prof. Taming wild carbenes: Following previous achievements in characterizing free carbenes,long considerered as curiosities and intermediates too reactive to be isolated,Schreiner and coworkers succeeded in stabilizing hydroxymethylene (see picture), the simplest of all oxycarbenes, by matrix isolation. This carbene gradually rearranges into its formaldehyde tautomer by a barrierless tunnelling process. [source] | |||||||||||||||||||||||||||||||||||||