3-Phenylpyrazolo(4,3-c)pyridine and derivatives: structure determination

The structures of 3-phenylpyrazolo(4,3-c)pyridine (1) and 5-methyl-3-phenyl-4,5,6,7-tetrahydropyrazolo(4,3-c)pyridine (2) are determined by detailed ¹H and ¹³C NMR analysis and theoretical calculations in comparison with their N-methyl derivatives as fixed models. CNDO/2 and PPP calculations are performed on the three prototropic forms admissible for the tautomeric system 1. By comparison of the ¹³C NMR and UV spectra of 1 with those of its three fixed N-methyl derivatives and of the UV spectra calculated for the tautomers of 1 with that experimentally obtained in the range 200–450 nm, 3-phenylpyrazolo(4,3-c)pyridine (1) is established to exist entirely in the 1H tautomeric form 1A.By ¹³C NMR investigations of 5-methyl-3-phenyl-4,5,6,7-tetrahydropyrazolo(4,3-c)-pyridine (2), in comparison with its fixed 1-methyl-1H derivative 9 and other substituted pyrazoles, 2 is determined to be a mixture of the 1H- (2A) and 2H- (2B) tautomers with 2A largely predominating. The compounds investigated are described here for the first time.     

Structural tautomerism of 4-acylpyrazolone schiff bases and crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-dihydro-pyrazol-3-one

The Schiff base derivatives prepared from 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one and alkyl amines are shown to remain exclusively in the amine-one(I) tautomeric form in chloroform solutions at room temperature using a combination of ¹H, ¹³C, APT, COSY, HMQC, and HMBC NMR spectroscopic methods. The crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-dihydro-pyrazol-3-one showed that this 4-acylpyrazolone Schiff base stays in the amine-one(I) form in the solid state as well, and the solid state structure supports the fact that strong hydrogen bonding between amine hydrogen and the pyrazolone C₃ carbonyl oxygen helps to stabilize the amine-one(I) tautomer.     

Theoretical and experimental study of triphenylphosphonium Schiff base of 5-hydroxy-3-methyl-1-phenyl-4-formylpyrazole

The Schiff base (HLBr), containing a chelating unit and triphenylphosphonium moiety has been synthesized in the reaction of 4-aminobenzyl(triphenyl)phosphonium bromide with 5-hydroxy-3-methyl-1-phenyl-4-formylpyrazole. The composition and structure of HLBr have been determined by elemental analysis, IR, 1D and 2D NMR, electronic spectroscopy and mass spectrometry. Density functional theory (DFT) calculations (6-311G(d,p) level of theory) have been carried out to investigate tautomeric forms of HL⁺ and the reaction mechanism of its formation and spectral properties. The most stable form in the solid state and in DMSO solution is pyrazolone (keto-amine) tautomeric form.     

Tautomerism of 1-methyl-5-amino-2,3-dihydro-1,2,4-triazolium-1-alkylidene(or arylidene)-2-methylaminoguanidinium iodide

¹H and ¹³C NMR spectroscopy has been used to examine the structures of the products of the condensation of aldehydes and ketones with 1-methyl-1-aminoguanidinium iodide, which in some instances in solution are involved in a ring-chain tautomeric equilibrium between 1-methyl-5-amino-2,3-dihydro-1,2,4-triazolium iodide and 1-alkylidene(or arylidene)-2-methylaminoguanidinium iodide.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1071–1074, August, 1987.     

New 1H-1-alkyl-6-methyl-3-phenyl-7-phenylazo-pyrazolo[5,1-c][1,2,4]triazoles through regioselective alkylation of 1H-6-methyl-3-phenyl-7-phenylazopyrazolo[5,1-c][1,2,4]triazoles

1H-1-Alkyl-6-methyl-3-phenyl-7-phenylazopyrazolo[5,1-c][1,2,4]triazoles (2aa-ad) were obtained by regioselective alkylation of 1H-6-methyl-3-phenyl-7-phenylazopyrazolo[5,1-c][1,2,4]triazoles (2a). 1H-1-Alkyl-6-methyl-3-phenyl-7-phenylazopyrazolo[5,1-c][1,2,4]triazoles 2aa and 2ab were also prepared by coupling phenyldiazonium chloride with 1H-1-alkyl-6-methyl-3-phenyl-pyrazolo[5,1-c][1,2,4]triazoles 1aa and 1ab. The new compounds were characterized by IR, UV-VIS, ¹H-NMR, ¹³C-NMR, and ¹⁵N-NMR spectroscopy and their structures and actual tautomeric forms were established unequivocally.      

Tautomerism of aza cycles: I. Structure of 3(5)-butylsulfanyl-5(3)-methyl(phenyl)-1H-1,2,4-triazole tautomers in crystal. Preference of the 3-RA-5-RD-1H-tautomer of 3(5)-mono-and 3,5-disubstituted 1,2,4-triazoles

The X-ray diffraction study of potentially tautomeric 3(5)-butylsulfanyl-5(3)-methyl-1H-1,2,4-triazole and its 5(3)-phenyl-substituted analog showed that these compounds in crystal have the structure of 3-butylsulfanyl-5-methyl(phenyl)-1H-tautomers. Analysis of our experimental and published data indicated that 3(5)-monosubstituted 1,2,4-triazoles and 3,5-disubstituted derivatives having nonequivalent substituents in crystal as exist as a tautomer in which the electron-acceptor substituent (R_A) occupies the 3 position, while the electron-donor substituent (R_D) resides in the 5 position, i.e., as 3-R_A-5-R_D-1H-1,2,4-triazoles. Symmetric 3,5-disubstituted 1,2,4-triazoles could give rise to tautomeric equilibrium between the 1H-and 2H-structures even in crystal.     

The Tautomerism of 1,3,4-Thiadiazol-2 (3H)-ones. A 15N-NMR-Study

5-Substituted 1,3,4-thiadiazol-2(3H)-ones were shown to exist almost exclusively in the oxo tautomeric form with the aid of proton-coupled ¹⁵N-NMR spectra using the corresponding 3-methyl-1,3,4-thiadiazol-2(3H)-ones and 5-substituted-2-methoxy-1,3,4-thiadiazoles as reference compounds.     

Bildung von 2-(1-Methyl-1, 4, 5, 6-tetrahydropyridyl-3)-2-oxazolin-4-on aus 1-Methyl-1, 4, 5, 6-tetrahydronicotinsäureamid und Bromessigsäureäthylester und seine Überführung in O-(1-Methylnipecotinoyl)glykolsäureamid via das dazu tautomere Cyclol

The main product of the reaction between 1-methyl-1, 4, 5, 6-tetrahydronicotin-amide and ethyl bromoacetate is shown to be the 2-(1-methyl-1, 4, 5, 6-tetrahydropyridyl-3)-2-oxazoline-4-one which on partial hydrogenation followed by reaction with alkali affords O-(1-methylnipecotinoyl)glycolic amide, presumably via the tautomeric cyclol.     

Synthesis and structure of 1-[(3-hydroxybenzo[<Emphasis Type="Italic">b</Emphasis>]thiophen-2-yl)methylidene]-3-oxo-5-phenyl-1-pyrazolidinium-2-ide

Potentially tautomeric azomethine imine, 1-[(3-hydroxybenzo[b]thiophen-2-yl)methylidene]-3-oxo-5-phenyl-1-pyrazolidinium-2-ide has been prepared. According to X-ray diffraction, ¹H, ¹³C, and ¹⁵N NMR and electronic spectroscopy, the compound exists as 3-hydroxybenzothiophene structure containing intermolecular hydrogen bond between the hydroxy group and the carbonyl oxygen of the pyrazolidone ring in crystal state. Quantum chemical calculations predict the possibility of OH and NH tautomeric forms.     

Synthesis and prototropic tautomerism of 1-n-alkyl-3-methyl-4-nitroso-5-pyrazolones

Nitrosation at C-4 of 1-n-alkyl-3-methyl-5-pyrazolones is achieved in about 50% yield with sodium nitrite in hydrochloric acid medium. A tautomeric equilibrium in solution with a proton moving from the OH at C-5 to N-2 in the nitrosated pyrazolones is proposed.     

Quantum-Chemical Study of the Structure and Reactivity of Pyrazol-5-ones and Their Thio and Seleno Analogs: VI. Tautomeric and Acid-Base Properties of 3-Methyl-1-phenyl-4,5-dihydropyrazol-5-one, 3-Methyl- 1-phenyl-4,5-dihydropyrazole-5-thione,and 3-Methyl-1-phenyl-4,5-dihydropyrazole-5-selenone

The relative stabilities of tautomeric forms of 3-methyl-1-phenylpyrazol-5-one and its 5-thioxo and 5-selenoxo analogs, as well as their acid-base properties in the gas phase, were estimated in terms of nonempirical calculations and density functional theory. According to the results of both calculation methods, the CH tautomer of 3-methyl-1-phenylpyrazol-5-one is the most stable. The stabilities of the XH and CH forms of its heteroanalogs (X = S, Se) are comparable; the relative stability of the SeH (SH) tautomers increases when thermal corrections, zero-point energy, and electron correlation effects are taken into account. The two methods indicate increase in the gas-phase acidity of the title compounds on variation of the heteroatom in the series O 相似文献   

Imino-enamine tautomerism and dynamic prototropy in 1-imino-3-amino-1H-indens

The tautomeric structures and dynamic prototropic behavior of the products 1 and 2 obtained in the condensation reaction of 1,3-indandione and 2-pyridyl-1,3-indandione with p-toluidine, respectively, were investigated by ¹H NMR spectroscopy and X-ray analysis. In the solid state, compound 1 is in an imino-enamine tautomeric form, whereas in solution it coexists with an imino-imino tautomeric form. Dynamic 1,5-prototropic interconversion of the imino-enamine form was revealed to be very fast at room temperature by temperature-dependent ¹H NMR spectra. For 2, the imino-enamine form is the only species present in solution. The hydrogen of the enamine NH is hydrogen-bonded intramolecularly with the nitrogen in the pyridine ring. When the temperature is raised, the NH proton enters into dynamic 1,5-migration, which is accompanied by internal rotation around the pivot bond, which changes the hydrogen-bonding sites. For the condensation product 3 of 2-(2-quinolyl)-1,3-indandione with p-butylaniline, dynamic behavior similar to that found in 2 was observed also in ¹³C NMR spectra.     

原位生成的Ph3C+均相有机催化剂催化4,4’-芳亚甲基-二（3-甲基-1-苯基-1H-吡唑-5-醇）高效合成（英文）简

Trityl chloride (Ph₃CCl) efficiently catalyzes the condensation of 3-methyl-1-phenyl-1H-pyrazol- 5(4H)-one and aromatic aldehydes under mild and solvent-free conditions, affording 4,4''- (arylmethylene)-bis(3-methyl-1-phenyl-1H- pyrazol-5-ol)s in high to excellent yields and in short reaction time. The presence of the requisite organocatalytic trityl carbocation (Ph₃C⁺) species was confirmed by analysis of infrared, ¹H NMR, and ultra violet spectral data. A plausible mechanism was proposed for the reaction based on the observations and literature precedent.     

A theoretical study on tautomeric structures of 4′-nitroazobenzene-2,4-diol and 4-methyl-4′-nitroazobenzene-2,6-diol compounds

The goal of this study is to determine the most stable tautomeric forms, and their ground state conformers of 4′-nitroazobenzene-2,4-diol and 4-methyl-4′-nitroazobenzene-2,6-diol compounds. The calculations have shown that the most stable tautomeric forms of the compounds are hydrazo form for 4′-nitroazobenzene-2,4-diol and azo form for 4-methyl-4′-nitroazobenzene-2,6-diol. Besides, the vibrational frequencies, ¹H and ¹³C NMR shifts, frontier molecular orbital’s energies for the tautomeric forms of the compounds calculated by using density functional theory-B3LYP method with 6-311G(d) basis set were interpreted. All the assignments of the theoretical frequencies were identified by potential energy distribution (PED) analysis. Generally, theoretical spectral results were seen to be in a good agreement with the corresponding experimental data.     

Reaction of 3-oxo-N-phenylbutanethioamide with 5-amino-1H-1,2,4-triazoles

Reactions of 3-oxo-N-phenylbutanethioamide with 3-substituted 5-amino-1H-1,2,4-triazoles in acetic acid led to the formation of 5-methyl-7,8-dihydro[1,2,4]triazolo[1,5-a]pyrimidine-7-thiones, 7-methyl-5,8-dihydrol[1,2,4]triazolo[1,5-a]pyrimidine-5-thiones and 7-methyl-5-phenylamino[1,2,4]triazolo[1,5-a]pyrimidines. The structure of the products was proved by the ¹H and ¹³C NMR spectra, X-ray diffraction data, and chemical transformations.     

Synthesis,structure, and tautomerism of formazans that contain 1-aryl-2-benzimidazolyl residues

1,5-Bis(1-aryl-2-benzimidazolyl)-3-methyl- and 1-(p-tolyl)-3-methyl-5-(1-aryl-2-benzimidazolyl)formazans were synthesized. It was shown by IR spectroscopy (from the change in the spectral characteristics of the NH, groups) that the formazans exist in the imino form in solutions in CCl₄, whereas a tautomeric equilibrium between the amino and imino forms of the investigated compounds is observed in solutions in CHCl₃. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 818–821, June, 1980.     

A molecular balance to measure the strength of N-H?π hydrogen bonds based on the tautomeric equilibria of C-benzylphenyl substituted NH-pyrazoles

A theoretical (B3LYP/6-31G^∗∗) study of 30 pyrazoles, most of them existing in two tautomeric forms, has been carried out. 3(5)-(2-Benzylphenyl)-5(3)-methyl-1H-pyrazole (11) and 3(5)-(2-benzylbenzylphenyl)-5(3)-phenyl-1H-pyrazole (20) were synthesized from 2-benzoylacetophenone, and their annular tautomeric equilibrium determined. The substituent effects were statistically analyzed and discussed with the help of Hammett substituent constants. In the case of the 5-(2-benzylphenyl) groups, the strength of the N-H?π hydrogen bond depends on the electronic effect of the substituent at position 3.     

Benzoid-quinoid tautomerism of Schiff bases and their structural analogs: LV. Crown-containing N-phenylimines derived from ortho-hydroxycarbaldehydes of the coumarin series

Schiff bases were synthesized from 3-hydroxy-6-oxo-6H-benzo[c]chromene-4-carbaldehyde, 5-hydroxy-4,7-dimethyl-2-oxo-2H-chromene-6-carbaldehyde, 6,7-dihydroxy-4-methyl-2-oxo-2H-chromene-8-carbaldehyde, 5,7-dihydroxy-4-methyl-2-oxo-2H-chromene-6,8-dicarbaldehyde, and 5-hydroxy-4,7-dimethyl-2-oxo-2H-chromene-6,8-dicarbaldehyde and (¹⁵N)aniline or aminobenzo-15-crown-5 (2,3,5,6,8,9,11,12-octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecin-15-amine), and tautomeric equilibrium between the hydroxy enimino and keto enamino forms of the 4- and 8-iminomethyl derivatives in solution was revealed by 1H NMR and electronic spectroscopy. Addition of alkaline earth cations to their solutions in acetonitrile displaced the tautomeric equilibrium toward the hydroxy enimino structure due to complex formation with the crown ether fragment.     

Structure of 1-aryl-3-alkyl-5-(2-benzothiazolyl)formazans

The isomeric and tautomeric structures of 1-aryl-3-alkyl-5-(2-benzothiazolyl)formazans in solutions were investigated by means of NMR, IR, and electronic spectroscopy. It is shown that the nature of the solvent and the alkyl substituent in the 3 position affects the ratio of the Z and E isomers relative to the C=N bond of the azohydrazone chain. An increase in the length of the alkyl substituent in the 3 position leads to preponderance of the Z isomer in solution, while branching stabilizes the Z configuration with an N²...HN⁵ intramolecular hydrogen bond (IMHB), regardless of the solvent. The nature of the solvent has a significant effect on the ratio of the amino and imino tautomeric forms of the E isomers. The benzothiazolylhydrazone form predominates in CHCl₃, while the tautomer with a benzothiazolidene fragment is the major form in DMSO.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 834–839, June, 1993.     

Acid-catalyzed rearrangement of 1-benzyl-2-methyl-3-piperidone to 1-benzyl-2-acetylpyrrolidine

We report that 1-benzyl-2-methyl-3-piperidone, conveniently prepared from 3-hydroxy-2-methylpyridine, undergoes rearrangement to 1-benzyl-2-acetylpyrrolidine in aqueous 6 N HCl at reflux. Studies showing that the 2,2-dimethyl analog is inert under the same conditions support a mechanism of reversible tautomeric equilibria via ring-opened intermediates, one of which was independently synthesized and shown to be a kinetically competent intermediate to product.