Tautomerism of derivatives of azines. 17. Effect of solvents on the position of the azinyl-ylidene tautomeric equilibrium of substitued azinylmethanes

The effect of solvents on the position of the azinyl-ylidene tautomeric equilibrium in series of azinylmethanes was studied. It was shown that an increase in the polarity of the solvent leads to stabilization of the ylidene tautomer; the sensitivity of the tautomeric equilibrium to the effects of the solvent depends on the form of the side fragment that undergoes tautomerization. It was concluded that stabilization of the ylidene tautomeric form by polar solvents is a general tendency in series of prototropic equilibrium of the azinyl-ylidene type as a whole.See [1] for Communication 16.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1668–1672, December, 1987.     

Tautomeric equilibria in 8-oxopurines: Implications for mutagenicity

Free-radical-induced DNA damage by ionizing radiation leads to a number of oxidized purines, of which 7H-8-oxoguanine (8OG) and 7H-8-oxoadenine (8OA) are predominant and known to cause an appreciable amount of cellular damage. A detailed quantum mechanical study at various levels of theory in both the gas phase and in an aqueous solution has been carried out in order to assess the tautomeric preferences of the bases. The calculated energies of various plausible tautomers suggest that at higher levels of ab initio theory with inclusion of electron correlation, the 8-keto-6-enolic form of 8-oxoguanine (8OG2) would predominate over the 6,8-diketo form (8OG1) in the gas phase whereas the 6-amino-8-keto form (8OA1) predominates over the other possible tautomers of 8-oxoadenine. Aqueous solvation, however, changes the gas-phase order for 8-oxoguanine, 8OG1 turning out to be the major tautomeric species in an aqueous medium. The estimated free energies of hydration by polarized continuum models are indicative that the mutagenically significant amounts of minor tautomeric forms of 8-oxoguanine and 8-oxoadenine exist in the aqueous phase and might be held responsible for inducing transversional as well as transitional mutations.     

Synthesis, spectral studies of salicylidine-pyridines: crystal and molecular structure of 2-[(1E)-2-aza-2-(5-methyl(2-pyridyl)ethenyl)]-4-bromobenzen-1-ol

Schiff bases derived from different meta-substituted salicylaldehyde and 5-methylaminopyridine have been synthesized and characterized by elemental analysis, FT-IR, NMR and UV-vis techniques. NMR assignments were made using (1)H, (13)C NMR and aided by 2D HETCOR and HMBC heteronuclear correlation techniques. The UV-vis spectra of the compounds were found useful in understanding the existence of tautomeric equilibria [phenol-imine (O-H...N) and keto-amine (O...H-N) forms] in polar and non-polar solvents. In order to rationalize the stabilization of tautomer in solid state, X-ray structure of 2-[(1E)-2-aza-2-(5-methyl(2-pyridyl)ethenyl)]-4-bromobenzen-1-ol (6) was determined. According to our crystallographic result, it has enol-imine tautomeric form.     

Hydrolytic degradation and thermooxidative stability of polyimides based on 3,5-diaminodiphenyl oxide and 2-methyl-3,5-diaminodiphenyl sulfide

For a number of new polyimides prepared from 3,5-diaminodiphenyl oxide, 2-methyl-3,5-diaminodiphenyl sulfide, and various dianhydrides of aromatic tetracarboxylic acids, the hydrolytic stability in DMF and 96% H₂SO₄ and the thermooxidative stability in the bulk have been studied. Hydrodynamic techniques have been employed to determine the molecular parameters of these polymers at various stages of degradation. It has been shown that the polymers under study form stable solutions in DMF but turn out to be unstable in 96% H₂SO₄ even at room temperature. Degradation accompanies dissolution of the polymer. The correlation between the chemical structure of polymer molecules and their hydrolytic stability in both solvents has been established. It has been demonstrated that the majority of the said polyimides are stable in the solid state at temperatures up to 400°C and marked degradation begins only above 500°C.     

Synthesis of 1,3-diketo derivatives of calix[4]arene with nonyl substituents at the lower rim as novel efficient sensibilizers of Tb<Superscript>3+</Superscript> luminescence

Novel bis- and tetra-1,3-diketo derivatives of calix[4]arene with nonyl substituents at the lower rim have been synthesized. Their conformational and tautomeric composition have been determined; spectral parameters and complex formation properties with Tb³⁺ have been studied. Substitution of two 1,3-diketo groups with bromine under going from tetra- to bis 1,3-diketone derivatives was resulted in twofold increase luminescence intensity in their terbium complexes. Lifetime of the excited state of the terbium luminescence of the corresponding complexes as well as lifetime of excited triplet level of the 1,3-diketonate ligands have been determined.     

Spectroscopic study of the keto-enol equilibrium of N-aryldiacetylthioacetamides and their reactivity

An NMR study of the equilibrium between keto-enol tautomeric forms of N-aryl-diacetylthioacetamides enables us to estimate their population ratio in solvents of increasing polarity. An X-ray analysis confirmed the structure of the thermodynamically most stable tautomer. We presume that the course of heterocyclization processes with N-aryldiacetylthioacetamides is affected by the structure of the reacting tautomeric form. The treatment of N-aryldiacetylthioacetamides with oxalyl or bromoacetyl bromides leads to thiazolidine derivatives. The reactivity of the 5-methylene group in the obtained thiazolidin-4-one derivatives was investigated. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 452–459, March 2008.     

Theoretical and experimental study of imine-enamine tautomerism of condensation products of propanal with 4-aminobenzoic acid in ethanol

The tautomerism of the reaction products of propanal with 4-aminobenzoic acid in ethanol was studied by J-modulated spin-echo (JMOD) ¹³C NMR spectroscopy and gradient-enhanced heteronuclear (ge-2D) ¹H–¹³C HSQC spectroscopy. The existence of imine and enamine tautomeric forms of the reduced compounds in solution was established. The tautomeric equilibrium of the condensation product of propanal with 4-aminobenzoic acid in ethanol was found to be shifted toward the imine form. Quantum chemical calculations by the density functional theory (DFT) method demonstrated that the 4-(N-propylidene)aminobenzoic acid molecule forms a stronger hydrogen bond with an ethanol solvent molecule compared to the enamine molecule, resulting in a higher stability of the ethanol adduct of azomethine compared to the adduct of enamine.     

Definition of the predominant tautomeric form of 3-methyl-1-phenylpyrazoline-5-thione in solution by 1 H and 1 3C NMR spectroscopy

Conjoint use of pmr allylic coupling constants and of ^{1 3}C nmr chemical shift patterns defines the predominant tautomeric form of 3-methyl-1-phenylpyrazoline-5-thione in solution in aprotic dipolar and in non-polar solvents as the thiol species, 3-methyl-1-phenylpyrazole-5-thiol. The tautomeric preference thus differs from that of the oxygen analog, and is opposite to that noted for thiol-thione tautomerism in six-membered heteroaromatic species.     

Carbon-13 magnetic resonance spectra of aldosterone, 18-hydroxydeoxycorticosterone and 18-hydroxyprogesterone

The ¹³C NMR spectra of aldosterone in various solvents show the presence of only the tautomeric forms with hemi-acetal (11–18) and hemi-ketal (18–20) bridges. Solutions of 18-hydroxy-11-deoxycorticosterone (6) and of 18-hydroxyprogesterone in CDCl₃ contain mainly the hemi-ketal (18–20) tautomer. Solutions of 6 in more polar solvents also contain—although to a lesser extent—a second form which may be a dimer or, more probably, a form representing two retamers with appreciable populations at the 21-CH₂OH group.     

Solvent effects on the electronic absorption spectra and acid strength of some substituted pyridinols.

The electronic absorption spectra of some substituted pyridinols in organic solvents of different polarities are studied. Also, the solvent effects on the intramolecular charge transfer bands are discussed using various solvent parameters. The acid-base equilibria of the compounds used are studied spectrophotometrically in various mixed aqueous solvents at 25 degrees C and 0.1 M ionic strength (NaClO4). Furthermore, the influence of the solvents on the dissociation constants and tautomeric equilibria of a pyridinol derivatives are discussed. The effect of molecular structure of the pyridinols on the pK's is also examined.     

Temperature-induced reversal of proton tautomerism: role of hydrogen bonding and aggregation in 7-hydroxyquinolines

UV-vis absorption spectra of 7-hydroxyquinolines in saturated hydrocarbon solvents were measured at various temperatures between 293 and 77 K. The tautomeric equilibrium was found to reverse when the temperature was lowered. At 293 K, the enol form was exclusively present. As the temperature was lowered, the enol form decreased substantially, and the keto form became predominant. A close examination of the spectral changes suggests that the reversal of the tautomeric equilibrium at lower temperatures proceeds in two steps: aggregation of the enol forms by intermolecular hydrogen bonding and further aggregation of the hydrogen-bonded aggregates.     

Polarographic reduction and tautomeric structure of 2-amino-5-arylazothiazoles

The polarographic behaviour of a series of 2-amino-4-phenyl-5-arylazothiazoles has been investigated at a dropping mercury electrode. Each of the compounds studied exhibits one wave which was shown to correspond to the reductive cleavage of the azo linkage by a 4e irreversible step. On the basis of the polarographic data, it was concluded that the compounds exist only in one tautomeric form, namely the aminoazo structure 1 . The results of E_1/2 — σ_X correlations and HMO calculations of bonding energies of the various possible tautomeric forms 1–3 indicate that the aminoazo form 1 is the most stable structure of the compounds examined.     

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     

Tautomerism of acridin-9-amines substituted at the exocyclic nitrogen atom: spectroscopic investigations and theoretical studies

In the ground electronic state, the first two (1 and 2) of the compounds investigated--9-(methoxyamino)acridine (1), 9-hydrazinoacridine (2), N-(2-chloroethyl)acridin-9-amine (3) and N-(5-methylpyridin-2-yl)acridin-9-amine (4)--exist principally in the imino tautomeric form, while the other two (3 and 4) can coexist as amino and imino tautomers in solvents of various polarities and abilities to enter into specific interactions. These features of the molecules are reflected in the experimental absorption spectra and the predicted thermodynamic and spectral data. The predicted thermodynamic characteristics suggest that in the S1 state, the imino tautomers of 1, 2 and 4 and the amino tautomer of 3 are more stable than their tautomeric counterparts. However, the predicted rates of intersystem crossing suggest that the imino tautomers of 1-3 and the amino tautomer of 4 lose excitation energy very rapidly, so that only their counterpart tautomers in fact emit radiation. This explains why 1 and 2 do not fluoresce and why the amino form of 3 and the imino form of 4 are the emitters. 3 and 4 thus represent acridin-9-amines whose imino forms are preferred in the ground state, but whose respective amino and imino forms are preferred in the excited state. Because 3 and 4 are capable of tautomeric transformations in the ground and excited states, and also enter into specific interactions with solvents, these compounds could be potent spectral indicators or probes of environmental properties.     

Solvent and Structural Effects on the UV–Vis Absorption Spectra of Some 4,6-Disubstituted-3-Cyano-2-Pyridones

A series of 4,6-disubstituted-3-cyano-2-pyridones was synthesized and their UV?CVis absorption spectra were recorded in the region 200?C600?nm in the set of selected solvents. The effects of solvent dipolarity/polarizability and solvent?Csolute hydrogen-bonding interactions on the spectral shifts were analyzed by means of the linear solvation energy relationship concept of Kamlet and Taft. The influence of solvents as well as substituents on the 2-pyridone/2-hydroxypyridine tautomeric equilibration was evaluated. The absorption band maximum of the 2-hydroxypyridine form is found to appear at a shorter wavelength than that of the 2-pyridone form in all investigated solvents. The replacement of the methyl and phenyl groups at position 6 of the pyridone ring, by a hydroxy group, significantly changes the solvatochromic behavior of the investigated pyridones.     

Structure and singly occupied molecular orbital analysis of anionic tautomers of guanine

Recently, we reported the discovery of adiabatically bound anions of guanine that might be involved in the processes of DNA damage by low-energy electrons and in charge transfer through DNA. These anions correspond to some tautomers that have been ignored thus far. They were identified using a hybrid quantum mechanical-combinatorial approach in which an energy-based screening was performed on the library of 499 tautomers with their relative energies calculated with quantum chemistry methods. In the current study, we analyze the adiabatically bound anions of guanine in two aspects: (1) the geometries and excess electron distributions are analyzed and compared with anions of the most stable neutrals to identify the sources of stability; (2) the chemical space of guanine tautomers is explored to verify if these new tautomers are contained in a particular subspace of the tautomeric space. The first task involves the development of novel approaches-the quantum chemical data like electron density, orbital, and information on its bonding/antibonding character are coded into holograms and analyzed using chemoinformatics techniques. The second task is completed using substructure analysis and clustering techniques performed on molecules represented by 2D fingerprints. The major conclusion is that the high stability of adiabatically bound anions originates from the bonding character of the pi orbital occupied by the excess electron. This compensates for the antibonding character that usually causes significant buckling of the ring. Also, the excess electron is more homogenously distributed over both rings than in the case of anions of the most stable neutral species. In terms of 2D substructure, the most stable anionic tautomers generally have additional hydrogen atoms at C8 and/or C2 and they do not have hydrogen atoms attached to C4, C5, and C6. They also form an "island of stability" in the tautomeric space of guanine.     

Solvent‐Catalyzed Ring–Chain–Ring Tautomerization in Axially Chiral Compounds

The mechanism of ring–chain–ring tautomerization and the prominent effect of the solvent environment have been computationally investigated in an effort to explain the enantiomeric interconversion observed in 2‐oxazolidinone derivatives, heterocyclic analogues of biphenyl atropisomers, which were isolated as single stable enantiomers and have the potential to be used as axially chiral catalysts. This study has shed light on the identity of the intermediate species involved in the ring–chain–ring tautomerization process as well as the catalytic effect of polar protic solvents. These mechanistic details will prove very useful in predicting and understanding ring–chain tautomeric equilibria in similar heterocyclic systems and will further enable experimentalists to devise appropriate experimental conditions in which axially chiral catalysts remain stable as single enantiomers.     

2,6-二巯基吡啶互变异构平衡体系溶剂效应的理论研究

在气相及甲苯、氯仿、乙腈和水等溶剂中对2,6-二巯基吡啶及其硫酮式互变异构体进行了HF/6-31G^**水平上的优化,其中溶液中的计算采用Onsager自洽反应场(SCRF)模型.探讨了溶剂对体系几何结构和能量的影响.结果表明:溶剂的存在与极性的增加有利于平衡体系中硫酮式异构体的存在.     

Synthesis and properties of 5-aroylamino-2H-1,2,4-thiadiazol-3-ones

5 -Aroylamino-2H-1,2,4-thiadiazol-3-ones 3 were obtained from the corresponding 1-aroyl-2-thiobiurets 2 by oxidative cyclization with bromine. 5 -Aroylamino-2H-1,2,4-thiadiazol-3-ones 3 can exist in two tautomeric forms a lactam form and a lactim form. On the basis of the ¹³C nmr spectra and additional experimental information, it has been established that the stable form, in which these compounds exist, is the lactam form.     

The mechanism of the alkylation of isoxazolidin-3-one

In solvents of different polarities, the lactam-lactim tautomeric equilibrium of isoxazoli-din-3-one is strongly displaced in the direction of the lactam. The lactim form cannot be detected. It has been concluded that there is no connection between the dual chemical behavior of isoxazolidin-3-one and its potassium salt in the alkylation reaction with the equilibrium isomerization of these compounds. It has been shown that the mechanism of alkylation is connected with the structure of the lactam anion. Evidence is given in favor of a mesomeric structure of the anion with the main charge on the nitrogen at which substitution chiefly takes place.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 898–901, July, 1973.