Substituent effects on the tautomer ratios between the hydra-zone imine and diazenyl enamine forms in 3-(arylhydra-zono)methyl-2-oxo-1,2-dihydroquinoxalines. Correlation of the hammett constants σp with the tautomeric equilibrium constants KT

The 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-e and 2a-i showed tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The sub-stituent effects on the tautomer ratios of A to B in compounds 1a-e and 2a-i were studied by the nmr spec-troscopy. The electron-donating or electron-withdrawing p-substituents R¹ in compounds 2a-i represented a tendency to increase the ratios of the tautomer A or the tautomer B , respectively, exhibiting the linear correlation of the Hammett constants σ_p (-0.17 to +0.78) with the tautomer ratios of A to B or the tautomeric equilibrium constants K_T. However, the presence of the ester group R² in compounds 1a-e induced the exclusive existence of the tautomer A regardless of the nature of the p-substituents R¹. In the tautomeric thermodynamic study, the elevating temperature increased the ratios of the hydrazone imine tautomer A in compounds 2a-i . The tautomeric thermodynamic parameters ΔG°, ΔH° and ΔS° were derived from the van't Hoff plots for compounds 2a , b , h , i , wherein the entropy term dominated the free-energy difference between the A and B tautomers.     

13C NMR and structural study of solid state naphthol-1 and naphthol-2 arylazo-derivatives

Solid state ¹³C NMR spectra of a series of naphthol-1 and naphthol-2-arylazo-derivatives were studied and compared with respective results in solutions. Signals of carbon nuclei of naphthalene ring were assigned. Tautomeric forms of compounds were determined. It was shown that 4-(p-NO₂C₆H₄)-azonaphthol-1 and 1-(p-NO₂C₆H₄)-azonaphthol-2 in solid state existed exclusively in quinohydrazone form. The other two compounds—1-(C₆F₅)-azonaphthol-2 and 2-(p-CH₂C₆H₄)-azonaphthol-1 in solid state were not found in the form of individual tautomeric mixtures but in respective tautomeric equilibrium form. Thus, during transition from liquid to solid state, the tautomeric equilibrium was practically unchanged. In solid state it was found that rotation around aryl-N bond was hindered in the time scale of NMR spectroscopy.     

Ring-chain tautomerism of 2-mercaptobenzoylhydrazones of aromatic aldehydes

It has been shown by ¹H NMR spectroscopy that 2-mercaptobenzoylhydrazones of aromatic aldehydes 2-HSC₆H₄CONHN=CHC₆H₄X (X = 4-NO₂, 3-NO₂, 4-Br, H, 4-Me, 4-MeO, 4-Me₂N) exist in DMSO-d₆ solution as tautomeric mixtures of linear and cyclic benzo-1,3,4-thiadiazepine forms. The linear hydrazone form is represented by (E,Z′)-conformational isomers, differing in the disposition relative to the amide C–N bond. It was shown that the logarithm of the tautomeric equilibrium constant K _T correlates with the σ-constants of the substituents in the aromatic nucleus.     

Nouveaux types de sucres triazotés: triazènes et phénylimino-2-oxadiazoles-1,3,4. Communication préliminaire

Novel types of sugars bearing three nitrogen atoms: triazenes and 2-phenylimino-1, 3, 4-oxadiazoles . A series of aminodeoxysugars treated with p-nitrobenzenediazonium tetrafluoroborate led to the corresponding triazenes, each of which in chloroform solution existing as an equilibrium between its two tautomeric forms. The free energy of activation of the exchange of the proton between the two nitrogen atoms has been estimated by variable temperature ¹H-NMR. measurements. Each triazenylsugar gave on acetylation an unique positional isomer bearing its acyl group on the nitrogen atom directly attached to the glycosyl group. Phenylsemicarbazones of two keto-sugars were oxidized with El Khadem's reagent (I₂, HgO, MgO) to give the corresponding spiro-2-phenylimino-1, 3, 4-oxadiazoles.     

Synthesis and Tautomeric Equilibrium of Polyfluoroacyl-Containing 1,5-Benzodiazepines

The reaction of bis(polyfluoroalkyl)-containing 1,3,5-triketones with o-phenylenediamine yielded 2-polyfluoroacylmethylene-4-polyfluoroalkyl-1,3- or 1,5-dihydro-1,5-benzodiazepines. The tautomeric equilibrium of the obtained benzodiazepines in CDCl₃, CD₃CN, DMSO, and DMF solution was studied.     

Synthesis and Tautomeric Equilibrium of Polyfluoroacyl-Containing 1,5-Benzodiazepines

Summary. The reaction of bis(polyfluoroalkyl)-containing 1,3,5-triketones with o-phenylenediamine yielded 2-polyfluoroacylmethylene-4-polyfluoroalkyl-1,3- or 1,5-dihydro-1,5-benzodiazepines. The tautomeric equilibrium of the obtained benzodiazepines in CDCl₃, CD₃CN, DMSO, and DMF solution was studied.     

Substituent and solvent effects on the tautomer ratios between the hydrazone imine and diazenyl enamine forms in p- and m-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines

The p- and m-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-i and 2a-d exhibited tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in a series of mixed dimethyl sulfoxide/trifluoroacetic acid media. The substituent and solvent effects on the tautomer ratios of A to B in a series of mixed media were studied for compounds 1a-i and 2a-d by the nmr spectroscopy. The linear correlation of the Hammett σp and σm values with the tautomeric equilibrium constants K_T ([A]/[B]) was found in the dimethyl sulfoxide media of compounds 1a-i and 2b-d . On the other hand, the linear correlation of the Hammett σp and σm values with the log C'(A:B = 1:1) was also observed in a series of mixed media of compounds 1a-h and 2a-c , wherein C'(A:B = 1:1) indicated the concentrations of trifluoroacetic acid (mol/l) giving 1:1 tautomer ratios in a series of mixed media. The increase in the Hammett σp or σm values decreased the K_T values in dimethyl sulfoxide media and augmented the C'(A:B = 1:1) values in a series of mixed media. The Hammett σp or σm values controlled the electron density of the side chain nitrogen atom, which influenced the C'(A:B = 1:1) values. In the K_T value temperature dependence, the higher temperature provided the larger K_T values in dimethyl sulfoxide media regardless of the Hammett σp or σm values.     

1H-NMR study on the tautomer ratios between the hydrazone imine and diazenylenamine forms in 3-(arylhydrazono)-methyl-2-oxo-1,2-dihydroquinoxalines

The o-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-c and 2a , c were synthesized to investigate the tautomeric behavior between the hydrazone imine A and diazenylenamine B forms in a series of mixed dimethyl sulfoxide/trifluoroacetic acid media. The chemical shifts of the hydrazone NH, N₄-H, hydrazone CH, and diazenyl CH protons for o-, m-, and p-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1 and 2 synthesized so far are summarized in Tables 3 and 4, respectively, which are found to be useful for the specification of the proton signals due to the hydrazone imine form A (hydrazone NH, hydrazone CH) and diazenylenamine form B (N₄-H, diazenyl CH).     

Spectroscopic and quantum chemical investigation of enol-enol tautomerism of 2-acetyltetronic acid

Vibratonal spectra of 2-acetyltetronic acid (ATA) (3-acetyloxolane-2,4-dione) were investigated in different aggregate states.Ab initio quantum chemical calculations (3-21G basis set) of 2-formyltetronic acid as analogue of ATA and semi empirical calculation of ATA were carried out. The tautomeric forms of ATA can be arranged in the following series with respect to thermodynamic stability (in decresing order):1a,1b,2a,2b.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1043–1048, June, 1995.     

UV and MO study on the deprotonation of some 2-aryl-Δ2-1,3,4-oxadiazoline-5-thiones

Substituent effects on the deprotonation processes of a series of 2-aryl-Δ²-1,3,4-oxadiazoline-5-thione (1) derivatives have been studied experimentally as well as theoretically. The acid dissociation constants pK_a have been determined spectrophotometrically in ethanol-water solutions (7.5-92.5%) and vary between 3.76 and 5.80. Semiempirical molecular orbital (MO) calculations (AM1 and PM3) were used for the investigation of the existence of possible tautomeric thione and thiol forms of the studied compounds. Strong correlation between the pK_a values and the deprotonation enthalpies were evaluated.     

The Effects of Microenvironment Polarity and Dendritic Branching of Aliphatic Hydrocarbon Dendrons on the Self‐Assembly of 2‐Ureido‐4‐pyrimidinones

Two series of aliphatic hydrocarbon‐based G1–G3 dendritic 2‐ureido‐4‐pyrimidinones (UPy) ( S‐Gn )₂ and ( L‐Gn )₂, differing from one another by the distance between the branching juncture to the urea end, were prepared and characterized. These hydrocarbon dendrons were also appended to a p‐aminonitrobenzene solvatochromic chromophore in order to probe their microenvironment polarity. While positive solvatochromism was observed which indicated the chromophore was solvent accessible, there was no significant difference between the microenvironment polarities on going from the G1 to the G3 dendrons. The self‐assembling behavior and tautomeric preference of the dendritic UPy derviatives were examined by ¹H NMR spectroscopy. The dimerization constants (K_dim*) of the DDAA tautomers were unchanged at 10⁷ M ^?1 in CDCl₃ at both 25 and 50 °C, which were comparable to those of UPy compounds bearing other nonpolar substitutents. Furthermore, the lower limits on the K′_dim* of the DADA tautomeric forms of the ( S‐Gn )₂ and ( L‐Gn )₂ series were determined to be 10⁶ and 10⁵ M ^?1 in CDCl₃, respectively. It was found that a closer proximity of the dendron branching juncture to the UPy unit could lead to a destabilization effect on the dimeric states. Hence, the ( L‐Gn )₂ dimers are more stable than those of ( S‐Gn )₂ in the DDAA form, but the latter are more stable than the former in the tautomeric DADA state. This study showed that both the highly nonpolar microenvironment and the proximity of the dendritic branching juncture to the UPy motif could alter the strength and profile of the hydrogen bond‐mediated self‐assembling process.     

Aluminiumorganyle mit pentakoordiniertem Aluminium-Atom: Synthese und Molekülstrukturen von [AlX2{2,6-(NEt2CH2)2C6H3}] (X = Cl,Et, H)

Aluminium Organyls with Pentacoordinate Aluminium: Syntheses and Molecular Structures of [AlX₂{2,6-(NEt₂CH₂)₂C₆H₃}] (X = Cl, Et, H) The reaction of [Li{2,6-(NEt₂CH₂)₂C₆H₃}]₂ with AlCl₃ or Et₂AlCl gives [AlX₂{2,6-(NEt₂CH₂)₂C₆H₃}] [X = Cl ( 1 ), Et ( 2 )] in good yield. 1 reacts with NaH in toluene to give [AlH₂{2,6-(NEt₂CH₂)₂C₆H₃}] ( 3 ). 1–3 were characterised spectroscopically (¹H, ¹³C, ²⁷Al n.m.r., i.r., mass spectroscopy). In solution at room temperature 1–3 exhibit dynamic behaviour. For 1 and 3 this can be frozen out below 278 K (¹H n.m.r.), indicating the presence of monomeric molecules with pentacoordinate Al at low temperature. Such species are also observed in the solid state as shown by an X-ray structure determination on 1 (monoclinic space group P2₁/n, a = 9.7325(14), b = 13.552(5), c = 28.858(7) Å, β = 99.57(2)°, V = 3753(2) ų, Z = 8, at 223(2) K) and 2 (monoclinic space group C2/c, a = 15.0045(12), b = 9.2986(8), c = 14.9955(12) Å, β =99.512(1)°, Z = 4, at 223(2) K).     

Chemistry of naphthazarin derivatives. 12. Synthesis and investigation of prototropic tautomerism of 3-(alk-1-enyl)-2-hydroxynaphthazarins

Substituted 3-(alk-1-enyl)-2-hydroxynaphthazarins were synthesized. According to data from IR spectroscopy, these compounds exist in organic aprotic solvents as mixtures of tautomeric 1,4-naphthoquinonoid forms. The compositions of tautomeric mixtures were quantitatively determined. The effects of the structure of the alkenyl substituent and the polarity of the solvent on the tautomeric equilibrium were qualitatively estimated.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2519–2525, November, 2004.     

Synthesis,characterization and luminescence of the tetranuclear gold cluster: [Au4{7,8-(PPh2)2-7,8-C2B9H10}2(PPh3)2]

A tetranuclear gold cluster has been synthesized by the reaction of [Au(PPh₃)NO₃] with the closo carborane diphosphine 1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀ in THF, and characterized by elemental analysis, FT-IR, ¹H and ¹³C?NMR spectroscopy and X-ray structure determination. The cluster crystallizes in the triclinic Pī, a?=?15.118(8)?Å, b?=?16.057(9)?Å, c?=?24.284(13)?Å, α?=?80.822(9)°, β?=?79.624(8)°, γ?=?81.938(8)°, Z?=?2, R ₁?=?0.0626, wR ₂?=?0.1894. A single crystal structure determination showed that four gold atoms form a tetrahedral framework. Among these four gold atoms, two were chelated by two nido carborane diphosphine [7,8-(PPh₂)₂-7,8-C₂B₉H₁₀]^? anions coming from the degradation of the initial closo ligand 1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀, while the other two were ligated to two PPh₃ groups. The luminescence of this cluster was also investigated in dichloromethane solution at room temperature.     

Ferroelectric phase transitions in Pb2xSn2(1-x)P2Se6 system

Heat capacities of the Pb_2xSn_2(1-x)P₂Se₆ crystals (x=0, 0.098, 0.251, 0.402 and 1.0) were measured using an adiabatic calorimeter at temperatures between 10 and 350 K. In the crystal of x=0, two heat capacity anomalies corresponding to the ferroelectric commensurate - intermediate incommensurate(C-IC) phase transition temperature T _i, and the incommensurate - paraelectric (IC-N) phase transition temperature T _c, were observed at 193.24±0.10 and 220.07±0.15 K, respectively. The phase transition temperatures decreased with an increase in Pb²⁺ concentration. The anomaly at Ti disappeared at x=0.251 in the mixed systems of the Pb_2xSn_2(1-x)P₂Se₆. In the crystal of Pb₂P₂Se₆ (x=1.0), no phase transition was observed. The normal heat capacities for the mixed crystals were determined by least squares fitting of the Debye and Einstein functions to the experimental data. The anomalous heat capacities gave the phase transition entropies of 8.5 and 1.5 J mol^-1 K^-1 for x=0. The large transition entropies are consistent with an order-disorder mechanism in the ferroelectric-paraelectric phase transitions in x=0. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synthesis,characterization, and biological properties of Ni(II), Co(II), and Cu(II) complexes of Schiff bases derived from 4-aminobenzylamine

New Schiff bases, N,N′-bis(salicylidene)-4-aminobenzylamine (H₂L¹), N,N′-bis(3-methoxysalicylidene)-4-aminobenzylamine (H₂L²), and N,N′-bis(4-hydroxysalicylidene)-4-aminobenzylamine (H₂L³), with their nickel(II), cobalt(II), and copper(II) complexes have been synthesized and characterized by elemental analyses, electronic absorption, FT-IR, magnetic susceptibility, and conductance measurements. For the ligands, ¹H and ¹³C NMR and mass spectra were obtained. The tetradentate ligands coordinate to the metal ions through the phenolic oxygen and azomethine nitrogens. The keto-enol tautomeric forms of the Schiff bases H₂L¹, H₂L², and H₂L³ have been investigated in polar and apolar solvents. All compounds were non-electrolytes in DMSO (～10^?3 M) according to the conductance measurements. Antimicrobial activities of the Schiff bases and their complexes have been tested against Acinobacter baumannii, Pseudomonas aeruginosa, Micrococcus luteus, Bacillus megaterium, Corynebacterium xerosis, Staphylococcus aureus, Escherichia coli, Candida albicans, Rhodotorula rubra, and Kluyveromyces marxianus by the disc diffusion method; biological activity increases on complexation.     

Condensation of 2-alkoxypropenals with N,N- and N,O-1,2-binucleophiles. A route to 2-(1′-alkoxyvinyl)imidazo-lidines and -oxazolidines

Condensation of 2-ethoxypropenal with diaminoethylene in different solvents (CHCl₃, MeCN, H₂O, DMSO) at room temperature gives an equilibrium mixture (1:1–1.5) of tautomeric 2-(1′-ethoxyvinyl)-1,3-imidazolidine and 2-aminoethylimine of 2-ethoxypropenal as well as 1,2-bis(2′-ethoxypropenyl-ideneamino)ethylene. The latter is readily prepared in quantitative yield using a twofold excess of the aldehyde. ¹H NMR was used to demonstrate the effect of heating on the dynamics of the ring-chain tautomeric equilibrium. Reaction of the 2-alkoxypropenals with N-methyl- and N,N′-diphenyl-1,2-diaminoethylenes and with N-phenylaminoethanol gives only the corresponding substituted imidazolidines in 43–95% yield. Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov in his 70^th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1809–1815, December, 2008.     

Theoretical investigation of solvent effects on tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol

The density functional theory has been used to study the tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol(DDNP) in the gas phase and in 14 solvents at the B3LYP/6‐31G* level. The solvent effects on the tautomeric equilibria were investigated by the self‐consistent reaction field theory (SCRF) based on conductor polarized continuum model (CPCM) in apolar and polar solvents and by the hybrid continuum‐discrete model in protic solvent, respectively. Solvent effects on the computed molecular properties, such as molecular geometries, dipole moments, E_LUMO, E_HOMO, total energies for DDNP tautomers and transition state, tautomerization energies and solvation energies have been found to be evident. The tautomeric equilibrium of DDNP is solvent‐dependent to a certain extent. The tautomer I (cyclic azoxy form) is preferred in the gas phase, while in nonpolar solvents tautomer I and II (quinold form) exist in comparable amounts, and in highly polar solvents, the tautomeric equilibrium is shifted in favor of the more polar tautomer II . © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

NMR spectra of Δ3- and Δ4-pyrrolin-2-one

The spectra of Δ³- and Δ⁴-pyrrolin-2-one were analysed and the sign of all the coupling constants determined by tickling and triple resonance experiments. A positive allylic interaction (J_xz in 2 ) is reported and four-bond couplings are discussed in particular. Deuterium exchange affords evidence for the tautomeric equilibrium between 1 and 2 .     

Nickel(II) complexes of a thiosemicarbazone prepared from 2-Acetylpyridine

Summary The ligand 3-azabicyclo[3.2.2]nonane-3-thiocarboxylic acid 2-[1-(2-pyridinyl)ethylidene]hydrazide (HL), which is observed in an unusual tautomeric form in the solid state, and its selenium analogue (HLSe) have been used to prepare a series of nickel(II) complexes. Compounds of the general formula [NiLX] (X=Cl, Br, NCS, N₃, NO₂ or NCSe) as well as [Ni(LSe)Cl] have been found to be diamagnetic, planar complexes. A single crystal study of [NiL(NCS)] shows the deprotonated ligand bound in a tridentate mannervia its pyridyl nitrogen, imine nitrogen and the thione sulphur atom with the nitrogen atom of the thiocyanato-ligand occupying the fourth coordination position. The solids prepared from the nickel(II) salts having tetrafluoroborate, nitrate and iodide ions approximate to octahedral symmetry and have neutral HL ligands coordinated in a bidentate fashionvia the pyridine and imine nitrogens with the remaining coordination sites being occupied by the anions or water molecules. The [NiL₂] solid is also octahedral with the two deprotonated ligands bonding as tridentate groupsvia the same atoms as in the [NiLX] complexes.