Hydrazone Tautomer Research Articles

A correction of the neglected tautomerism in the spectra prediction of o-methoxyaniline terminated monoazonaphthols: DFT and 15N NMR study

In a paper of Zhu and co-workers (“Study of the electronic effect and quantitative spectra predictions of o-methoxyanilineterminated monoazonaphthols: a combined experimental and DFT study” by Guoxun Zhu, Yan Lin, Wenxian Zhou, Huacan Song and Zhengquan Li, RSC Advances, 2023, 13, 33736), the authors derived a protocol for the absorption spectra prediction is sulphonated azo naphthols assuming that they exist as single azo tautomers. However, as it is well known before and as shown by the available experimental data and the calculations, performed in this note, the studied dyes exist almost solely in hydrazone tautomeric form, which unavoidably affects the conclusions and linear relations made in the original paper. A detailed benchmarking on the spectra prediction, involving 39 density functionals (33 of them hybrid and 6 – pure) indicates that there is no a functional correctly predicting the absorption spectra of the tautomers in azo naphthols.

Investigation of the Dyeing Ability of Some Reactive Triazine Azo Dyes Containing Pyrazole Fragment.

The IR, 1H NMR, and DFT studies indicated that the prepared reactive disperse dyes predominately exist as hydrazone tautomers. The electronic absorption spectra in methanol were observed and compared to those computed using B3LYP/6-311G(d,p). The dyeing efficiency of the produced dispersed reactive dyes was examined on polyester, cotton, and polyester/cotton blended fabrics. The degree of exhaustion and the quick properties of the dyed samples in terms of perspiration, washing, scorch and light fastness were assessed. It was found that reactive disperse dyes under investigation have a higher affinity for dyeing polyester textiles than cotton textiles. Moreover, the reflectance and color strength of the synthesized dyes were measured and discussed.

Synthesis, physicochemical, nonlinear optical properties of azonaphtharylmide dyes: Dyeing and UV-protection performance on wool, silk, polyester

This study focuses on the synthesis and characterization of three azonaphtharylmide dyes, APAS, APASD, and APASTR, using naphthol AS, ASD, and ASTR couplers. The research also observes the presence of a hydrazone tautomer in a DMSO solution. It was characterized through spectroscopic analysis, which revealed three characteristic carbonyl carbon peaks in the 13C NMR spectrum. The calculated low LUMOs (−3.46 eV to −3.54 eV) align closely with experimental calculations (−3.45 eV to −3.48 eV), indicating the potential of these dyes as electron acceptors. Furthermore, the alignment of frontier molecular orbital (FMO) values supports their suitability for electron acceptance. When applied to wool, silk, and polyester materials, these dyes confer UV protection properties, with wool-dyed fabrics achieving a UV Protection Factor (UPF) of 50+. Additionally, these dyes exhibit significantly improved nonlinear optical responses, with beta (β0) values ranging from 11 × 10−30 to 23 × 10−30 esu and gamma (γ0) values ranging from 275 × 10−36 to 365 × 10−36 esu, indicating their functionality as nonlinear optical materials.

Semi-empirical design and synthesis of novel 4-hydroxy-6-sulphonaphthyl azohydroxynaphthalenes with predominant hydrazone form

Due to their enhanced photochromic effects, hydrazone forms of tautomeric azo compounds are commercially preferred as photoreceptors and binary switches. While ortho-hydroxyazo compounds readily exhibit an intramolecular hydrazone re-arrangement, the predominance of hydrazone tautomers of para-hydroxyazo compounds is less frequently observed. The objective of this study was to design and synthesize sulphonated 4-naphthylazo naphth-1-ols with preferential hydrazone shift.PM6 calculations were used to quantify contributions of selected structural features to hydrazone stability in a set of model compounds. Synthesis of five computationally-optimized compounds (3a-e) was subsequently carried out via diazo coupling of 5‑hydroxy-7-sulphonaphthalene-2-diazonium ion with naphthol derivatives. The azo-hydrazone equilibria of 3a-e were further characterized using spectroscopic techniques and density functional theory (DFT).Computational design revealed that sulphonic acid substitution on the coupling components of the dyes and, solvation effect of the substituents were the highest contributors to hydrazone shift of the compounds. The ring position and electron withdrawing resonance effect of substituents were less contributory. Increasing substitution with sulphonic acid or another hydrophilic group (amino) also resulted in higher hydrazone stability. Mass spectral, IR and NMR data confirmed computational results with mono sulphonated 3a and poly substituted 3c-e existing predominantly as azo and hydrazone forms respectively. UV–Visible spectral analysis revealed that azo and hydrazone forms of 3a predominated in hydrogen bond acceptor and donor solvents respectively. In contrast, absorption spectra of 3b-e showed lone hydrazone bands in all solvents investigated. The DFT/M06-2X/6-31G(dp) characterization of the azo-hydrazone equilibrium of the compounds confirmed experimental results and showed that hydrazone tautomers of 3b, 3c and 3d were more stable than corresponding azo forms by 1.536, 0.289 and 1.359 kcal/mol respectively. DFT also confirmed involvement of sulphonic acid and imino groups in the transition of the dyes’ structures to the keto form.Three novel p-hydroxyazo dyes with increased hydrazone stability have been synthesized.

Synthesis, characterization, and application of novel aryldiazenyl disperse dyes on polyester fabrics

Azo dyes are widely used for dyeing polyester fabrics but require optimization of properties like color strength and fastness. Fourteen novel disperse azo dyes were synthesized from 2,3-naphthalenediol and aniline derivatives to examine their potential for polyester dyeing. The dyes were prepared via diazotization and coupling reactions and characterized using FT-IR, UV–Vis, 1H NMR, 13C NMR, and elemental analysis. Furthermore, several techniques were employed to study the azo-hydrazone tautomerism, including UV–Vis spectroscopy, NMR spectroscopy, and computational methods. DFT computations revealed hydrazone tautomers were more stable than azo tautomers. The prepared azo dyes were applied on polyester fabrics at 2% depth using a high temperature pressure technique in water utilizing DYEWELL-002 as a dispersing agent. The color shading of dyed polyester samples ranged from peach amber to apple of my eye, depending on the coupler moieties. The fastness properties, assessed using a grey scale of dyed polyester fabrics, indicated very good to excellent grades for most dyes. Additionally, measurements of color strength (K/S), dye exhaustion (%E), as well as colorimetric colors CILAB of dyed polyester fabrics values, were measured and discussed in terms of the effect of substituents. The findings provide new insights into structure-performance relationships to design optimized disperse dyes for polyester coloration. Overall, the synthesized aryldiazenyl dyes are promising candidates for dyeing polyester fabrics across a spectrum of shades with good fastness properties.

Reversal in solvatochromism, photochromism and thermochromism in a new bis-azo dye based on naphthalen-1-amine

A novel bis-azo dye, 4,4′-((1E,1′E)-(oxybis(4,1-phenylene))bis(diazene-2,1-diyl))bis(naphthalen-1-amine) (abbreviated as 4odna) was synthesized and characterized by IR and 1H, 13C and 2D COSY-NMR spectroscopies, and mass spectrometry. Optical properties were evaluated using UV–vis absorption spectroscopy. The observed solvatochromism was ascribed to the presence of different azo or/and hydrazone forms in solution. The azo forms show both positive and negative solvatochromism, with the reversal occurring for solvents with ET(30) ∼45 kcal mol−1, while the hydrazone tautomer shows negative solvatochromism. Application of the multiparametric Catalán and Kamlet-Taft linear solvation energy models allowed to evaluate the dependence of the solvatochromism exhibited by 4odna on the hydrogen bond donating (HBD) and accepting (HBA) abilities of the solvent, as well as on their dipolarity (SdP) and polarizability (SP). Upon UV (λ = 311 nm) irradiation at room temperature, the compound was found to exhibit similar photochromic behavior in the polar-protic solvents methanol and ethanol, which is distinct from that observed in the polar-aprotic solvents dichloromethane, chloroform and chlorobenzene, with the hydrazone tautomer being photoconverted into the azo tautomer in the first group of solvents and vice-versa in the second group. In acetone, UV irradiation extensively transforms the compound into species with no absorption in the visible range, leading to fast discoloration of the solution. Temperature dependence of the color of the solutions of 4odna in ethanol and chlorobenzene was also evaluated, and reversible thermochromic behavior was observed in the first solvent. In chlorobenzene, no thermochromism was observed, but a change of color of the solution was promptly induced by the UV–vis broadband source beam of the spectrometer when the absorbance spectrum of the solution was being recorded at T = 115 °C, which demonstrates that the azo forms of 4odna undergo easy phototransformation into the hydrazone forms in this solvent at high temperature.

A pH-dependent CT sensitive method for detection of trace Mn in milk by UV-spectrophotometry

In this study, a new method was developed for the extractive pre-concentration/detection of trace Mn in milk prior to analysis by UV-spectrophotometry. The method is based on the pH-controlled intra-ligand charge transfer (ICT) complexation of modifier, 2-HBT in presence of trace Mn (II), and then the extraction of the complexes, where the hydrazine and hydrazone tautomers are predominant at pHs of 2.0/ 3.0, and pH 6.0 because of CT at pH 4.0. The free Mn (II) and total Mn levels of the samples were detected at 269 nm by UV-spectrophotometry. For total Mn, the method was validated by analysis of two certified milk samples. The results were statistically in good agreement with the certified values, and the precision was lower than 6.4%. After a pre-concentration of 62.5-fold, the limits of detection were 1.45/1.92 and 1.54/2.12 μg L−1 in range of 5–165 and 5–150 µg L−1 from the solvent-based and matrix-matched calibration curves, respectively. There is not a matrix effect from comparison of the calibration curves’s slopes. The method was successfully applied to analysis of milk samples. In terms of speciation, the free Mn (II) and total Mn levels in milk were in the range of 7.65–30.4 μg L−1, and 10.2–34.7 and 10.7–35.5 μg L−1 after the microwave and ultrasound assisted extraction.

Molecular docking and ADMET studies of the interaction of 4-carboxyl-2,6-dinitrophenylazohydroxynaphthalenes with bovine serum albumin

Previous spectrophotometric investigations revealed strong binding affinities between four potential monoazo colourants (code-named AZ-01 to 04) and bovine serum albumin (BSA) which could dictate the tissue distribution and toxicity of the additives. The molecular docking interactions of the dyes with BSA were analyzed using AutoDock vina and PatchDock in order to elucidate the functional groups involved in complex stabilization. Docking conformations confirmed the ligands preferentially inserted into the hydrophobic cavities of BSA site I. Structure-BSA binding relationships revealed the binding of AZ-02 was driven by hydrogen bond donation from its free phydroxynaphthalene substituent to Ser-479 while the predominantly hydrazone form of its positional isomer, AZ-01, increased its lipophilicity and tendency for hydrophobic interactions. The relatively higher C/H ratio of AZ-03 and - 04, which contain additional C-7 substituents, was responsible for their stronger binding and the extensive involvement of their aromatic rings in ligand-site I complex stabilization via Pi-Pi T-shaped, Pi-alkyl and alkyl-alkyl interactions. Moreso, AZ-01, -03 and -04 exist predominantly as hydrazone tautomers with an overall positive charge which provided complementary modes for interaction with negatively charged aspartic and glutamic acids. The structure-BSA binding relationships of the molecules, which can be employed in synthesis of safer congeners, have been elucidated.

Computational study on the prototropic tautomerism between simple oxo-, thio-, carbon-, aza-hydrazones, and their respective azines.

Most hydrazone compounds prefer the azine tautomeric states. However, oxygen-/sulfur-substituted compounds prefer hydrazone tautomers. In this study, density functional theory at M062X level with the basis set of 6-311 + + g(2d, 2p), with MP2/cc-pVTZ for reference, was used to investigate the different tautomeric mechanisms between hydrazone and azine forms with oxygen, sulfur, carbon, and nitrogen as negative centers. The energetic stabilities are in the order as oxygen- < sulfur- < imine- < amidino- < carbene-substituted hydrazones with respect to their azine tautomeric structures. Resonance of the molecular structures might be the geometrical basis for their energy stabilities and were estimated based on HOMED indices. Further, the increased proton affinities in the trend as hydroxyl < sulfhydryl < imine terminal groups account for their increasing azine preferences. Proton release was examined for the reversible equilibrium from azine to hydrazone by calculating the transition energy barrier of H transfer. It is favorable to form hydrazone tautomers for oxygen and sulfur containing groups, while it is less favorable for amino-substituted ones. Azine form is the most stable tautomer for methyl substituted.

Spectrophotometric studies on some new hydroxyl-azo dyes derived from 6-ethyl-4-hydroxyquinoline-2(1H)-one

PurposeThis paper aims to synthesize 6-ethyl-4-hydroxyquinolin-2(1H)-one as a new enol-type coupling component in the preparation of some 3-arylazo-4-hydroxyquinolin-2(1H)-one dyes and evaluate the solvent effects on their absorption in ultraviolet-visible spectra.Design/methodology/approach6-Ethyl-4-hydroxyquinolin-2(1H)-one was synthesized by thermal cyclocondensation reaction of N, N′-bis(4-ethylphenyl) malonamide at 130–140°C in polyphosphoric acid. This compound was then applied in the azo-coupling reaction with some aniline-based diazonium salts, so as to prepare seven new mono-heterocyclic azo dyes. The structures of the compounds were confirmed using mass spectroscopic technique. Fourier transform infra red (FT-IR) and 1H proton nuclear magnetic resonance (1H NMR) and carbon-13 nuclear magnetic resonance (13 C NMR) studies on the structure of the azo compounds revealed that they exist as two E- and Z-isomers of hydrazone tautomer both in solid and solution state. The effects of acid and base on the visible absorption spectra of the dyes were also evaluated and discussed.FindingsUltra violet-visible UV-vis absorption spectra of the dyes didn’t show significant variation by changing of solvents because of intramolecular H-bonding between proposed hydrazone forms and 2- and 4-keto functions in their structures. The spectra of the dyes were not sensitive to the addition of acid but were very sensitive to base.Originality/valueThe synthesized 3-arylazo-4-hydroxyquinolin-2(1H)-one dyes are new members in the 4-hydroxyquinolin-2(1H)-one azo dyes family, where very few details regarding the synthesis of such dyes are reported before in the literature. They are unique in terms of synthesis and spectral properties.

Unsymmetrical Structure of the Co(III) Complex with Bisheteroarylhydrazone as Hydrazone and Quinolone Tautomers Stabilized by Hydrogen Bond

Unsymmetrical Structure of the Co(III) Complex with Bisheteroarylhydrazone as Hydrazone and Quinolone Tautomers Stabilized by Hydrogen Bond

Synthesis, characterization, spectroscopic properties and computational studies of some new azo dyes derived from 5-chloro-8-hydroxy quinoline

Purpose The purpose of this study, 3-aminopyridine, 8-aminoquinoline and some new synthesized 2-aminobenzothiazoles were diazotized with nitrosyl sulfuric acid and subsequently coupled with 5-chloro-8-hydroxy quinoline to synthesize the corresponding heteroarylazo dyes 6–13. Design/methodology/approach The structures of dyes were characterized by mass, Fourier transform infra red, 1H proton nuclear magnetic resonance and ultra violet-visible spectroscopic techniques. Absorption spectra of the dyes were measured in acetic acid, ethanol, chloroform, acetonitrile, dimethyl formamide and dimethyl sulfoxide and correlated with the nature of the solvents and substituents. The effects of varying pH on the absorption wavelengths of the azo dyes were also studied. In addition, the acidity constants (pKa) of the dyes were determined using the spectrophotometric method in an ethanol-water mixture (80:20, v/v) at 20–23°C. Besides, density functional theory (DFT) calculations were carried out to compare the energies of proposed azo and hydrazone tautomers of the dyes. Findings The results showed that the withdrawing chloro groups on the diazo moiety have significant influence (red shift) on the electron absorption spectra of these dyes. In addition, introducing electron withdrawing chloro groups into the benzothiazoles moiety increased the acidic character of dyes. Originality/value The synthesized 7-hetroarylazo-5-chloro-8-hydroxy quinoline dyes are new members in the 8-hydroxyquinoline azo dyes family, where very few details regarding the synthesis of such dyes are reported before in the literature. They are unique in terms of synthesis, spectral properties and DFT calculations.

Azo–hydrazone tautomerism in organometallic complexes triggered by a -Re(CO)3(L) core: A spectroscopic and theoretical study

The spectroscopic properties and tautomeric behavior of five novel Rhenium (I) tricarbonyl complexes bearing an azo ligand are presented. The organic ligand is stable in solution as the hydrazone tautomer. It remains as hydrazone in different medium conditions (solvent, concentration, pH, etc.) without the formation of detectable amounts of the azo tautomer. However, the complexation of this ligand to the strong electron-withdrawing fragment -Re(CO)3X (X = Cl−, Br−) causes tautomerism to appear in the organic moiety. Two well-defined regions in the electronic spectra for both tautomers were observed, allowing the azo/enol-keto/hydrazone equilibrium to be followed. TD-DFT calculations indicate that for the keto/hydrazone form, the main absorption band is attributed to an IL transition. In contrast, the azo/enol species shows a major contribution of the 1MLCT (dπ(Re) → dπ*(NN)) transition. The tautomeric equilibrium is easily shifted using solvents with different dielectric constant and hydrogen bond donor/acceptor (HBD/HBA) abilities. Also, the tautomerization process is deeply influenced by the electronic properties of the axial ligand. Therefore, this behavior represents a different strategy for the design of novel materials with optical properties. Furthermore, tautomerism is affected by the concentration of complexes and the presence of water in solvents. The acid-base behavior of these compounds in a 50% v/v ethanol-buffer system showed that the azo/enol tautomer is stabilized in acidic media. At the same time, an increase of pH promotes tautomerization toward the keto/hydrazone, followed by forming the anionic form at pH > 8.

Preferential Solvation of 4-Carboxyl-2, 6-Dinitrophenylazohydroxynaphthalenes in Mixed Hydroxylic Solvents

Background: The applications of a group of 4-carboxyl-2,6-dintrophenylazohydroxynaphathalenes, AZ-01 to 04, as colourants, chemosensors or synthetic intermediates have been limited by their solubility.Aim: To investigate the effect of solvent mixture composition on the solubility, solution thermodynamics and position of equilibrium processes of the dyes.Method: The UV-visible spectral patterns of the dyes in binary mixtures including Methanol:Water, Ethanol:Water, Methanol:Ethanol, Methanol:Propan-1-ol, Methanol:Propan-2-ol, Propan-1-ol:Water and Propan-2-ol:Water were acquired. The type and quantitative estimation of solute-solvent interactions at play were determined by fitting spectral patterns to solvent parameters using multilinear regression.Results: Preferential solvation was detected by the non-ideality of the plots of E12 as against the mole fractions of co-solvent in all binary mixtures. In pure solvents, the spectral shifts of AZ-01, 03 and 04, which exist predominantly in the hydrazone form, were affected by polarity of solvent milieu while solvent basicity and acidity, in that order, were the significant parameters for AZ-02. In aqueous alcoholic mixtures, solvent polarity was contributory, although to different degrees, to the observed spectral data of the four dyes. However, solvent acidity and basicity were the primary determinants of spectral shifts observed with AZ-04 and AZ-03 respectively. Spectra-structure relationships identified the formation of the charged hydrazone tautomer which requires stabilisation by polar solvent milieu as responsible for the observed trend. In addition, interactions between new aggregated solvent-solvent species and the propionic acid substituent present in AZ-03 contributed to its spectral shifts.Conclusion: The solvatochromic properties of the phenylazonaphthalene series in binary mixtures have been successfully studied.

Spectroscopic characterization and binding interaction of heavy metal onto the surface receptor of the azobenzene: DFT and experimental approach

The azobenzene 1-arylazo-2-naphthol has been synthesized and characterized by elemental analysis, 1H NMR, IR and UV–Vis spectroscopies using both experimental and theoretical methods. The MEP, NPA and FMOs have also been performed at the DFT/B3LYP-D3 level with different solvents in order to investigate the solvent's effect. The energetic behavior of the compound has been examined in the gas phase and in solvent media using the integral equation formalism polarizable continuum model (IEF-PCM). Solvents had sligth significant effect on NPA values whereas both molecules hardness and stability decrease along with increasing of the solvent's polarity. Vibrational and absorption analysis showed no significant azo±hydrazone tautomerism. Theorically, the title molecule calculated in Acetonitrile solvent is the most reactive. The observed red shifts are explained by the large extension of the π conjugated system and the blue shifts confirm the appearance of several tautomeric forms of the studied azo-benzene which lead to a marked improvement in photochemical stability. The cation binding properties of azo benzene as absorption sensor for Cu2+, Mg2+, Ni2+ and Zn2+ cations were reported. The highest adsorption energy in both gas and liquid phase corresponds to the complexes Azoic/Ni2+ then in Azoic/Cu2+ and somewhat in Azoic/Zn2+complex. The lowest adsorption energy is given to Azoic/Mg2+ complex. After cations chemisorption on azoic molecule a bathochromic shift is observed in the optical spectra, particularly for Azoic/Ni2+ complex, which is the proof of interaction between azoic compound and mentioned cations. The electronic proprieties and the vibrational spectra can therefore be used to detect the presence of different cations and the azobenzene shows appropriate properties to constitute good cations (Ni2+ and Cu2+) sensors.

Study of the azo‐hydrazone tautomerism of Acid Orange 20 by spectroscopic techniques: UV–Visible, Raman, and surface‐enhanced Raman scattering

AbstractAcid Organge 20 is a synthetic monoazo dye, which molecular structure consists of a naphtol unit and a sulfonated benezene ring linked together by an azo chromophore group. As a phenylazonaphthol with the OH group in para position respect to the azo group, it undergoes an azo‐hydrazone tautomerism. In this work, the study of the preponderance of one tautomer over the other in different pH conditions was carried out by means of UV–Vis, Raman, and surface‐enhanced Raman scattering (SERS) spectroscopies. Raman spectra of powder and aqueous solutions of the dye were obtained at different excitation lines at the natural pH of the AO20 solutions. The best results were obtained at 633 nm, and the observed Raman bands suggested the presence of the hydrazone tautomer. The molecular structure and the theoretical Raman spectra of the two tautomers were calculated by density functional theory (DFT) methods. The obtained results were used for the assignment of the Acid Orange 20 vibrational modes. The SERS analysis of the dye solutions were carried out employing Ag nanostars, pH below and above 4.7, and excitation at 442, 532, and 633 nm. The hydrazone tautomer was prevalent in most conditions. The azo tautomer was only detected at pH ~ 5 under excitation with the 532 nm wavelength due to resonant Raman conditions. Finally, the effect of pH on the morphology of metal nanoparticles employed as SERS substrates was evaluated by TEM.

Chercher de l'eau: The switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate

The E/Z switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate was studied by NMR, UV–Vis, and ultrafast spectroscopy and modeled by advanced quantum-chemical calculations. Three possible mechanisms were considered theoretically: out-of-plane rotation, in-plane inversion and proton transfer. Neither of them correctly describes the experimental data by using implicit solvation. Taking into account the existence of water in the used solvents, which influence the measured rate constants, an explicit solvation was attempted in the quantum-chemical calculations. According to a simplified model, the water molecules form a wire, being able to transfer the proton from Z to E form of the switch. This leads to substantially lower transition states and corresponds to the experimentally observed rate constants. This information shines new light on the mechanism of isomerization in the rotary switches and on the understanding of the mechanism of hydrazone tautomerism as a whole.

Quantitative structure-activity relationship in the photodegradation of azo dyes

The photolysis characteristics of azo dyes are critically important in environmental pollution control, dye-sensitized solar cells, and dyeing-related industries. However, there is still lack of quantitative relationship between the structures of azo dyes and their photolysis characteristics. To address this issue, the photolysis of 22 azo dyes were conducted side by side at three pH (4.0, 6.0, 9.0). The obtained pseudo-first order photodegradation rate constants (k1) were processed with meta-analysis. Statistically, the hydrazone tautomer had a smaller excitation energy and was easier to undergo photolysis than the azo tautomer. The ortho-substituted sulfonate groups had an obvious protective effect on the photostability of azo dyes. The softness (s), the most positive and negative partial charge on a carbon atom (qC+, qC−) were found to be crucial descriptors in the establishment of QSAR models for the photostability of azo dyes. The QSAR model at pH 9.0 was robust for predicting the photostability of azo dyes under UV irradiation. N2-purging experiments and quantum chemical computation verified that the cleavage of azo bond was not a result of direct photolysis but was caused by the attack of photoinduced reactive oxygen species. The results here are helpful for the design of more stable azo dyes or the selection of suitable approaches for the treatment of dye-contaminated water bodies.

Azo Coupling of Enamino Amides of the 3,3-Dimethyl-1,2,3,4-tetrahydroisoquinoline Series with Arenediazonium Salts

The azo coupling of arenediazonium chlorides with (Z)-2-(3,3-dimethyl-3,4-dihydroisoquinolin-1(2H)-ylidene)-N-(4-methylphenyl)acetamide afforded (Z)-2-(2-arylhydrazinylidene)-2-(3,3-dimethyl-3,4-dihydroisoquinolin-1-yl)-N-(4-methylphenyl)acetamides whose structure was confirmed by 1H NMR and X-ray diffraction data. The azo coupling products containing no p-tolyl substituent on the amide nitrogen atom exist in solution as mixtures of azo and hydrazone tautomers. Electronic absorption spectra of the synthesized compounds were studied.

Synthesis of 3-(Piperidin-4-yl)-6,7-dihydro-5H-pyrrolo-[2,1-c][1,2,4]triazole and Theoretical Study of the Hydrazone-Hydrazine Tautomerism of the Intermediate Hydrazonation Product

3-(Piperidin-4-yl)-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazole was synthesized through a four-step process including etherification, hydrazonation, cyclization, and reduction with an overall yield of 39%. The final product was characterized by 1H NMR and ESI-MS/MS. The molecular structures of benzyl (Z)-4-(2-(pyrrolidin-2-ylidene)hydrazine-1-carbonyl)piperidine-1-carboxylate and related compounds were analyzed using DFT calculations at the B3LYP/6-311+G(d,p) level of theory. The results indicated a higher stability of the hydrazone tautomers.