Tautomeric Equilibrium Research Articles

The Reaction of Fenchone and Camphor Hydrazones with 5-Acyl-4-Pyrones as a Method for the Synthesis of New Polycarbonyl Conjugates: Tautomeric Equilibrium and Antiviral Activity.

Selective synthesis of polycarbonyl conjugates of (+)-fenchone and (+)-camphor was carried out (44-91 % yields) via the ring-opening transformation of 5-acyl-4-pyrones with hydrazones of the corresponding monoterpenoids. A strong influence of the hydrazone fragment on the observed tautomeric equilibrium of the tricarbonyl system was shown. Although the major tautomer of the conjugates is the acyclic polycarbonyl form, the camphor-based conjugates undergo new type of ring-chain tautomerism, diketoenaminone-dihydropyridone equilibrium, and predominantly exist in the cyclic dihydropyridone form in DMSO-d6. The polyketones can undergo intramolecular cyclization to form N-amino-4-pyridones in high selectivity. In vitro screening for activity against the influenza virus H1 N1 and vaccinia virus was estimated for the obtained conjugates. The (+)-fenchone derivatives demonstrated the higher activity against vaccinia virus than camphor derivatives. The conjugate, which was prepared from diethyl isochelidonate and hydrazone (+)-fenchone, showed the highest activity against vaccinia virus (SI=17).

Exploring the factors influencing the ketoenamine-enolimine tautomeric equilibrium of pyridoxal 5′-phosphate in branched-chain aminotransferases

Pyridoxal 5′-phosphate (PLP), the active form of vitamin B6, is a critical coenzyme for various enzymes. It generates a Schiff base with the substrate and exhibits ketoenamine and enolimine tautomeric forms due to intramolecular proton transfer. This study aims to ascertain the predominant tautomeric form of the PLP Schiff base in MtIlvE and analyze its influencing factors. Molecular dynamics simulations indicate that the ketoenamine tautomer has higher binding free energy than the enolimine tautomer. Density functional theory calculations suggest that, despite their ability to interconvert at a relatively low energy barrier, the ketoenamine tautomer is thermodynamically more stable. Factors affecting the keto-enol tautomeric equilibrium were investigated by constructing various QM-cluster models. Our results demonstrate that both the protonation of the pyridine nitrogen and the presence of Tyr209, which stabilizes the O3 anion, shift the tautomeric equilibrium toward the ketoenamine configuration. These findings provide a theoretical basis for investigating enzyme catalytic mechanisms.

Synthesis, characterization, DFT and molecular docking study of an antiviral drug, the crystallized 6-methoxylated derivative of favipiravir, in the presence of an iron (III) Lewis acid.

Few years ago, the COVID-19 infection, caused by SARS-CoV-2 virus, presented a worldwide health pandemic that taken the life of millions of people. In this concern, Favipiravir (FAV) has attracted considerable attention as an effective antiviral drug against this disease. In this study, the favipiravir's derivative (3‑hydroxy-6-methoxypyrazine-2-carboxamide) was synthetized, in presence of iron (III), and fully characterized through various analytical techniques including single-XRay, IR, UV and 1H/13C NMR. The refinement show the exclusive existence of enol form in the solid state. In solution, UV and NMR data were used to highlight the tautomeric equilibrium of this molecule. Additionally, the density functional theory (DFT) calculations were achieved to estimate the chemical parameters, electronic affinity and molecular electrostatic potential of the compound. Mulliken atomic charges as well as the chemical reactivity descriptors were investigated. Finally, the two tautomer forms of synthetized derivative were docked on SARS-CoV-2 main protease (PDB: 6LU7) and spike receptor-binding domain (PDB: 7BZ5) to evaluate their binding affinity. The docking simulation studies was predicted that the two forms had high binding affinity the critical amino acids residues of the receptor. This affinity was attributed to the presence of several hydrogen bonds for both tautomer along with five different hydrophobic interactions with the enol form. These results conduct to conclude that the synthetized product could be an interest potential drug for that type of diseases.

Synthesis, Antimicrobial, DNA Interactions, In Vitro Cytotoxicity, and Cell Cycle Analysis Efficiency of Newly Nano‐Sized Cu (II), Ni (II), Co (II), and Zn (II) Thio‐Schiff Base Complexes of 2‐((E)‐((4,5‐Dimethyl‐2‐(((E)‐4‐Methylbenzylidene)Amino)Phenyl)imino)Methyl)‐Benzenethiol

ABSTRACTNano‐sized bivalent metal complexes, specifically [M(L)2] with L = 2‐((E)‐((4,5‐dimethyl‐2‐(((E)‐4‐methylbenzylidene)amino)phenyl)‐imino)methyl)benzenethiol] (C23H22N2S) and M = Cu (II) (C1), Co (II) (C2), Ni (II) (C3), and Zn (II) (C4), underwent synthesis and subsequent characterization. Elemental analyses, infrared, NMR, mass, electronic, magnetic susceptibility, conductivity assessments, and X‐ray diffraction studies were used to assess our bivalent metal complexes. DFT studies were used to study the tautomeric equilibrium of the tridentate thio‐Schiff base ligand (HL), via the DFT\B3LYP method in connection with a 6–311G* correlation consistent basis set. Two tautomers, which are thione and thiol forms, were studied to estimate the predominant one. The metal formed four coordinated with the tridentate N2S donor thio‐Schiff base to form octahedral geometry complexes. The SEM, TEM, XRD, AFM, and EDX of the studied complexes unveiled distinct and strong diffraction peaks, signifying their crystalline nature and providing evidence of their particle sizes being within the nano‐size. The crystal sizes calculated for all complexes were determined to be ranging from 27.73 to 76.39 nm. The interactions between metal complexes and calf thymus DNA, and their potential for mimicking insulin activity, were investigated in a controlled lab setting by measuring their ability to inhibit alpha‐amylase. The antimicrobial potential of thio‐Schiff base ligand (HL) plus complexes (C1–C4) were tested. The viscosity and UV–Vis absorption determinations were utilized to assess the calf thymus DNA (CT‐DNA) interaction with the nano‐sized metal (II) chelates. Our flow cytometry data indicate significant levels of apoptosis and cell cycle arresting in both liver and breast cancer cell lines.

Chitosan-azo dye bioplastics that are reversibly resoluble and recoverable under visible light irradiation.

Biopolymer composite materials were prepared by combining bio-sourced cationic water-soluble chitosan with bi-functional water-soluble anionic azo food dyes amaranth (AMA) or allura red (ALR) as ionic cross-linkers, mixing well in water, and then slow-drying in air. The electrostatically-assembled ionically-paired films showed good long-term stability to dissolution, with no re-solubility in water, and competitive mechanical properties as plastic materials. However, upon exposure of the bioplastics to low power light at sunlight wavelengths and intensities stirring in water, the stable materials photo-disassembled back to their water-soluble and low-toxicity (edible) constituent components, via structural photo-isomerization of the azo ionic crosslinkers. XRD, UV-vis, and IR spectroscopy confirmed that these assemblies are reversibly recoverable and so can in principle represent fully recyclable, environmentally degradable materials triggered by exposure to sunlight and water after use, with full recovery of starting components ready for re-use. A density functional theory treatment of the amaranth azo dye identified a tautomeric equilibrium favouring the hydrazone form and rationalized geometrical isomerization as a mechanism for photo-disassembly. The proof-of-principle suitability of films of these biomaterial composites as food industry packaging was assessed via measurement of mechanical, water and vapour barrier properties, and stability to solvent tests. Tensile strength of the composite materials was found to be 25-30 MPa, with elongation at break 3-5%, in a range acceptable as competitive for some applications to replace oil-based permanently insoluble non-recyclable artificial plastics, as fully recyclable, recoverable, and reusable low-toxicity green biomaterials in natural environmental conditions.

4-Phenyldiazenyl-substituted 3,6-di-tert-butylcatechol: synthesis and azo–hydrazone equilibrium

Reaction of 3-hydroxy-2,5-di-tert-butyl-1,4-benzoquinone with phenylhydrazine hydrochloride mainly leads to the corresponding hydrazone at the position 1 existing in tautomeric equilibrium with 3,6-di-tert-butyl-4-(phenyl- diazenyl)catechol. Thermodynamic parameters of the tautomerization were evaluated by NMR spectroscopy. The hydrazone structure was confirmed by XRD study.

Structural characterization and keto-enol tautomerization of 4-substituted pyrazolone derivatives with DFT approach

The synthesis of two pyrazolone derivative compounds, PYR-I(4-Acetyl-1-(4-chlorophenyl)-3-isopropyl-1H-pyrazol-5(4H)-one) and PYR-II1-(4-Chlorophenyl))-3-isopropyl-5-oxo-4,5-5-dihydro-1H-pyrazole-4-carbaldehyde, their characterization by FT-IR, NMR, UV–Vis and GC-MS techniques, and the evaluation of the keto-enol tautomerization process of the structures along with the DFT approach and spectral data were reported in this paper. Spectral findings indicated that PYR-I was stable at the keto state. The IR spectrum recorded in solid form showed that the PYR-II structure was stable in the enol state, while the NMR spectrum in the solution medium showed that it was stable in the keto state. DFT-based analyses were realized with the B3LYP hybrid functional and the 6–311++G(d,p) basis set. The modelled keto, transition and enol state molecular geometries of structures were optimized in the gas phase and different solvent media and the total energy and dipole moment values were investigated at the specified theoretical level. The possible keto-enol tautomerism mechanism of the structures was evaluated through some thermodynamic parameters such as the difference in free Gibbs energy (ΔG), enthalpy (ΔH), entropy (ΔS), and predictive tautomeric equilibrium constants (Keq), acidity constants (pKa) and percentages of tautomers at 298.15 K and 1 atm pressure. The results of these analyses based on the DFT approach indicated that the keto-enol tautomer equilibrium heavily favours the keto form for PYR-I and the enol form for PYR-II in all cases. Moreover, natural bond orbital (NBO) analysis was performed for the tautomers, and the chemical reactivity profiles of the most stable tautomers were examined with the values of frontier molecular orbital energy and some reactivity descriptors.

Tautomerism of herbicides metribuzin, diclosulam and clethodim: evaluation of different parameters for structure assignation

Pesticides are widely used in agriculture, and they can cause serious damage to human health and to the environment. In this work, three herbicides are studied regarding possible tautomeric equilibria: clethodim, diclosulam and metribuzin. Tautomerism is a key reaction that is directly related to action, disposition and toxicity of herbicides, and here lies the importance of this study. The specific aim of this work is to compare classical parameters (MAE and R2) with DP4+ method as a tool for obtaining relevant information about tautomeric equilibria in the three compounds, contrasting experimental NMR data with theoretical calculations carried out using DFT methods.1H and 13C NMR spectra of the three herbicides were recorded in DMSO‑d6. Additionally, HSQC and COSY spectra were obtained for clethodim. Theoretical calculations were carried out in order to obtain optimized geometries in vacuum (using B3LYP method at 6–31G(d,p) level of theory); and chemical shifts of nuclei corresponding to all possible tautomers of each herbicide were obtained by means of B3LYP-GIAO method using 6–311+G(2d,p) basis set.In the three cases, classical comparison parameters MAE and R2 fail in the assignation of tautomers since they gave doubtful or even inconsistent results. In the case of metribuzin, MAE values obtained from 1H and 13C point towards different structures. R2 and MAE values obtained from 1H and 13C are very close when comparing the two tautomers, and the assignation is not certain. The case of clethodim is more confusing, since the R2 and MAE values suggest that all signals correspond to the same structure. DP4+ method, on the other hand, gave more reliable results (within the set of proposed structures). It is important to remark that DP4+ method has been used before for tautomer assignation in only one case (to the best of our knowledge), and it has also proven successful in this context of use.We concluded that metribuzin and diclosulam exist in solution in one major tautomeric form (aminotriazinone and sulfonamide, respectively), while two tautomers of similar energy (E- and Z-ketoenolimine complexed with DMSO) are proposed for clethodim.

Indole-3-pyruvate: Analysis and Control of Tautomerism and Reactivity.

It is well-known in biochemistry that structure confers function, meaning that chemical structural elucidation is critical to truly understanding the function of a given metabolite. Indole-3-pyruvate (IPyA) exists in an equilibrium between the keto and enol tautomeric forms. IPyA is suggested to play a role in immune function; however, determining whether the tautomeric forms function differently can only be studied if an analytical method is capable of distinguishing between the two forms. Herein, we describe the use of UHPLC-HRMS to gain insight into the physical variables that govern IPyA tautomer equilibrium, reactivity, and detection limit. We use hydrogen-deuterium exchange (HDX) to identify enol and keto peaks, and we show that tautomers exhibit a valley of fronting followed by a tailing peak shape (though separation is still attainable) and identical MS/MS spectra. We observed drastically different ratios of keto and enol forms in different solvents, which is an important consideration for in vitro studies. IPyA was found to be highly unstable with accelerated reactivity in peroxides. Through in vitro reactivity studies, IPyA produced a myriad of known and unknown metabolites via nonenzymatic processes, many of which were mapped in vivo via the analysis of human plasma. Finally, we show that vitamin C (ascorbic acid) can slow this reactivity and enable sensitive detection in whole blood.

Ultrasensitive detection of trace Hg(Ⅱ) in acidic conditions using DMABR loaded on sepiolite: Function, application and mechanism studies

Fast and real-time detection of trace Hg(Ⅱ) by fluorescent probes under acidic conditions is urgently required due to the high toxicity and accessibility to creatures and human being. However, fluorescent probes for Hg(Ⅱ) detection in environmental samples are rarely reported due to the protonation potential of acidic mercury sources. In this study, the SD probe was developed by 5-(p-dimethylaminobenzylidene) rhodanine (DMABR) loaded on sepiolite by hydrothermal treatment, and showed excellent Hg(Ⅱ) detection performances for mercury sources at pH 4–10 due to buffering ability of the hyperconjugated lactam rings. Sepiolite functioned as the support skeleton to decrease intermolecular transition, and thus increased the sensitivity. At pH 4, the SD probe showed high selectivity and sensitivity for Hg(Ⅱ) among various species, with low LOD and binding constant of 4.78 × 10−9 M and 1.34 × 106 M−1, respectively. Through DFT calculations, MAS 1H NMR and 2D-COS analysis, the detection mechanism was demonstrated as SN1 substitution of the spontaneous leaving H on amino groups in the transient state during tautomeric equilibrium, rather than the expected high-affinity sulphydryl. Additionally, the SD probe exhibited promising potential in quantifying water-soluble and bioavailable Hg(Ⅱ) in acidic polluted soil and water samples. Moreover, real-time detection was realized by paper-based strips.

Effect of cationic Gemini surfactants ethonium and decamethoxin on the spectral properties, solubility and tautomerism of the curcumin

AbstractThe properties of natural polyphenol curcumin in water solutions of cationic Gemini surfactants decamethoxin and ethonium, which have similar positively charged nitrogen hydrophilic head groups but different spacer sizes and lengths of hydrophobic tails have been investigated in a wide range of concentrations by using UV–Vis spectroscopy and quantum‐chemical calculations. It was found that curcumin dissolves in the organized micellar media of these surfactants in the enol tautomeric form and its solubility increases 400 times compared to water. The binding constants of curcumin with decamethoxin and ethonium were determined by the solubility method. Significant differences in the influence of premicellar concentrations of cationic Gemini surfactants on the solubility and tautomeric transformations of curcumin were revealed. In contrast to decamethoxin, ethonium at a concentration below the CMC promotes a significant increase in the solubility of curcumin and a shift in the tautomeric equilibrium towards the formation of its keto form.

Effects of additional ring-fusion site on dual reactivity based dynamic covalent chemistry

Tautomers are widespread in organic chemistry, and their unique reactivity allows diverse transformations. In this work, the incorporation of nearby amide ring-fusion site offered a versatile platform for controlling dual reactivity based dynamic covalent chemistry (DCC) of aldehyde ring-chain tautomers. A suite of 2-formylbenzosulfonamide derived tautomers were prepared, and the intramolecular ring-chain equilibrium and intermolecular dynamic covalent reactions along the underlying mechanistic foundation were investigated. By regulating the substituent electronic effect and solvent effect, ring-chain tautomerization equilibrium was readily regulated in solution. The adjacent amide NH participates in intramolecular hydrogen bonding, as revealed by crystal analysis. Furthermore, the addition of acid and the change of solvent allowed turning on/off the intramolecular cyclization from the amide group to create fused ring scaffolds. Finally, the reversible covalent reactions with amine nucleophile reagents maintained the intermolecular reactivity successfully. The molecular and pathway diversity reported herein sets the scene for future design and applications.

A new novel series of vanillin-based hexaimine-Schiff bases: Synthesis, characterization and investigation of chromic and fluorescence properties

Here, new symmetrical diimines D1 and D2 were prepared by the condensation reaction of o-vanillin with 2-nitro-p-phenylenediamine or 4-nitro-o-phenylenediamine, respectively. Diimines were reduced selectively to their amino derivatives, and reacted with terephthaldehyde to obtain of a novel symmetrical hexaimines (H1 and H2). Their structures were identified on the basis of microanalyses, IR, 1H/13C NMR, HRMS and UV–vis spectroscopy. The enol–keto tautomeric equilibrium in these Schiff bases were elucidated by chromically and fluorescence spectra with respect to the influence of solvent, acid–base and light. They acted non-emissive in the solid state (λexc = 365 nm). Diimines displayed a weak green, bluish-green/blue emission in basic DMSO and DMF solution (v/v, 1:1), but they were non-fluorescent in other media. In contrast to this, hexaimines emitted a strong greenish-blue fluorescence in pure, bluish-green/greenish-blue fluorescence in acidic, and purplish-blue/blue fluorescence in basic aqueous DMSO and DMF media.Mass spectrum of Schiff bases exhibited the ion peaks as following positions: (D1): m/z (RI%) = 447.1329 (100 %), 424.1517 (60 %), 322.1454 (20 %), 288.0988 (30 %), 102.1277 (55 %) (Fig. S7). (D2): m/z (RI%) = 422.1361 (20 %), 288.0996 (100 %) (Fig. 6). (H1): m/z (RI%) = 883.4259 (10 %), 528.2244 (100 %), 510.2038 (20 %), 392.1605 (10 %), 102.1276 (10 %) (Fig. S8). (H2): m/z (RI%) = 882.3320 (10 %) [M + 2H]+, 434.1278 (40 %), 392.1710 (50 %), 374.1588 (100 %), 322.1523 (10 %), 256.1142 (15 %), 102.1297 (15 %) (Fig. S9). Mass spectral pattern of D1 displayed [M + 3H]+ ion peak at m/z 424.1517. The base peak was observed at m/z 447.1329, which may be due to [M + 3H+Na]+ ion. And the following fragment ion appeared at m/z 288.0988, which may be due to [C14H14N3O4]+ ion (4-((2‑hydroxy-3-methoxybenzylidene)amino)-3-nitrobenzenaminium)). D2 showed [M + H]+ molecular ion at m/z 422.1361. The similar fragmentation was observed at m/z 288.0996. This base peak may be due to fragment [C14H14N3O4]+ ion (2-((2‑hydroxy-3-methoxybenzylidene)amino)-4-nitrobenzenaminium)). H1 exhibited [M + 3H]+ molecular ion at m/z 883.4259. The base peak was observed at m/z 528.2244, which may be due to [C35H24N6]+ ion (3,3′-(((2-((4-(((2-amino-5-(methyleneamino)phenyl)imino)methyl)benzylidene)amino)-1,4-phenylene) bis(azanylylidene))bis(methanylylidene))bis(benzene-1,2-diylium)). In the case of H2, [M + 2H]+ ion peak was seen at m/z 882.3320. The following fragmentation was observed at m/z 434.1278, which may be belonged to fragment [C30H18N4]+ ion (3,3′-(((4-((4-((vinylimino)methyl)benzylidene)amino)1,2phenylene)bis(azanylylidene))bis(methanylylidene))bis(benzene-1,2-diylium)). The base peak was recorded at m/z 374.1588, which may be due to fragment [C26H208N3]+ ion (3,3′-(((4-((2-vinylbut-2-en-1-ylidene)amino)-1,2-phenylene)bis(azanylylidene))bis(methanylylidene))bis(benzene-1-ylium)).

Regioselective quinazoline C2 modifications through the azide-tetrazole tautomeric equilibrium.

2-Chloro-4-sulfonylquinazolines undergo functional group swap when treated with an azide nucleophile: 1) the azide replaces the sulfonyl group at the C4 position; 2) the intrinsic azide-tetrazole tautomeric equilibrium directs the nucleofugal sulfinate from the first step to replace chloride at the C2 position. This transformation is effective with quinazolines bearing electron-rich substituents. Therefore, the title transformations are demonstrated on the 6,7-dimethoxyquinazoline core, which is present in pharmaceutically active substances. The methodology application is showcased by transforming the obtained 4-azido-6,7-dimethoxy-2-sulfonylquinazolines into the α1-adrenoceptor blockers terazosin and prazosin by further C2-selective SNAr reaction and azide reduction.

Structural analysis of Kynurenic acid and derivatives by NMR spectroscopy and DFT calculations

The 4-quinolinone derivatives present a keto-enol tautomerism, which can be studied using different analytical techniques, including NMR. In this work, the spectroscopic study in solution of numerous derivatives with different substitution patterns in positions 1–4 of the quinolone rings, and their correlation with simulated spectra by density functional theory-gauge independent atomic orbitals method (DFT-GIAO) approximation is presented. Polyfunctionalized derivatives where the tautomeric equilibrium is blocked by N- or O-substitution allow us to observe the different behavior in chemical shifts by NMR. The study of some derivatives in the solid state by ss-NMR and X-ray Crystallography is also included.

A non-extraction sequential injection method for determination of loratadine using formation of its ion-association complex with bromocresol purple in acetonitrile

The formation of an ion-association complex (IA) between sulfonephthalein dye and basic nitrogen-containing compound in an organic solvent medium has been for the first time used to develop an automated SIA method. In highly polar aprotic solvents, the tautomeric equilibrium for such dyes is strongly shifted towards the colorless lactonic form. The addition of a basic nitrogen-containing substance leads to the formation of IA with a highly colored quinonoid form, which is accompanied by an increase in the absorbance of the dye band at approximately 400 nm. Protonation of pyridine nitrogen in loratadine, structure and binding places of IA were shown using quantum-chemical calculations. The very simple, direct and non-extraction spectrophotometric SIA method with high throughput of 43 h−1 was developed based on the formation of IA between loratadine and bromocresol purple in the medium of acetonitrile used both as solvent and carrier. The calibration graph was linear in the concentration range from 1.0 to 20 mg L−1 with correlation coefficient of 0.9992. The developed method was successfully applied to the analysis of pharmaceutical formulations.

O-versus S-Metal Coordination of the Thiocarboxylate Group: An NMR Study of the Two Tautomeric Forms of the Ga(III)-Photoxenobactin E Complex.

Photoxenobactin E (1) is a natural product with an unusual thiocarboxylic acid terminus recently isolated from an entomopathogenic bacterium. The biosynthetic gene cluster associated with photoxenobactin E, and other reported derivatives, is very similar to that of piscibactin, the siderophore responsible for the iron uptake among bacteria of the Vibrionaceae family, including potential human pathogens. Here, the reisolation of 1 from the fish pathogen Vibrio anguillarum RV22 cultured under iron deprivation, its ability to chelate Ga(III), and the full NMR spectroscopic characterization of the Ga(III)-photoxenobactin E complex are presented. Our results show that Ga(III)-photoxenobactin E in solution exists in a thiol-thione tautomeric equilibrium, where Ga(III) is coordinated through the sulfur (thiol form) or oxygen (thione form) atoms of the thiocarboxylate group. This report represents the first NMR study of the chemical exchange between the thiol and thione forms associated with thiocarboxylate-Ga(III) coordination, including the kinetics of the interconversion process associated with this tautomeric exchange. These findings show significant implications for ligand design as they illustrate the potential of the thiocarboxylate group as a versatile donor for hard metal ions such as Ga(III).

Synthesis of hybrid azobenzene-chalcone dyes and studies on their photophysical properties: Molecular recognition of carbonate and phosphate ions using a novel turn-off fluorescent probe

The work presented here describes the synthesis and comparative physiochemical studies of hybrid compounds C-1 to 5 of azobenzene A-1 and chalcone B-1, which exhibited the characteristic of both. Further, electron-withdrawing and donating substituents analogues have also been synthesized. The hybrid compound is unique in nature because it contains the phenolic -OH, -N = N-, as well as α,β-unsaturated enone functional groups. The photophysical properties of hybrid C-1 behaved more or less like the azobenzene moieties and showed the same pattern of UV–vis absorption as with the change in the polarity of the solvent and the change in the pH of the medium. Thus, in the non-polar solvent or in the water medium, the phenol form dominates with its yellow colour and in polar solvents shows an intense red colour of keto form. They show a dramatic quenching of fluorescence emission in the polar solvent. Furthermore, after screening, C-1 was found to show selective sensing of the aqueous CO32− ion as well as minimally with PO43- and show a characteristic colour change in an organic solvent such as CH3CN. A few ions also changed colour in DMF, but that was due to the only shifting of the tautomerization equilibrium from keto to enol form. The UV–vis as well as fluorescent spectral studies, determination of the detection limits, and binding constants illustrated significant sensing of CO32− ions. A systematic study was performed to understand the binding interactions of the chemosensor by using Job's plot and time-dependent 1H NMR studies that supported the involvement of phenoloc -OH and β carbon of α,β-unsaturated system. The characteristic pattern of UV–vis and interaction mode was confirmed by DFT studies. The applicability of chemosensor C-1 was demonstrated by the development of a paper strip model to check the presence of CO32− ions in different kinds of water in society.

Highly acidic N-triflylphosphoramides as chiral Brønsted acid catalysts: the effect of weak hydrogen bonds and multiple acceptors on complex structures and aggregation.

N-Triflylphosphoramides (NTPAs) represent an important catalyst class in asymmetric catalysis due to their multiple hydrogen bond acceptor sites and acidity, which is increased by several orders of magnitude compared to conventional chiral phosphoric acids (CPAs). Thus, NTPAs allow for several challenging transformations, which are not accessible with CPAs. However, detailed evidence on their hydrogen bonding situation, complex structures and aggregation is still lacking. Therefore, this study covers the hydrogen bonding behavior and structural features of binary NTPA/imine complexes compared to their CPA counterparts. Deviating from the single-well potential hydrogen bonds commonly observed in CPA/imine complexes, the NTPA/imine complexes exhibit a tautomeric equilibrium between two proton positions. Low-temperature NMR at 180 K supported by computer simulations indicates a OHN hydrogen bond between the phosphoramide oxygen and the imine, instead of the mostly proposed NHN H-bond. Furthermore, this study finds no evidence for the existence of dimeric NTPA/NTPA/imine complexes as previously suggested for CPA systems, both synthetically and through NMR studies.

Theoretical investigation of tautomerism of 2- and 4-pyridones: origin, substituent and solvent effects.

Computational investigation at the BHandHLYP/6-311+G(d,p) level of theory of the gas-phase tautomerism of 2- and 4-pyridones confirmed the slight prevalence of lactim in the case of the former, but its dominance in the case of the latter, as shown previously. Examination of aromaticity by using HOMA, EDDB, NBOdel, NICS and AICD led to the conclusion that tautomerization of 4-pyridone results in greater aromaticity gain. It is also driven by the Pauli repulsion relief, which was revealed by the tautomerization energy decomposition analysis. By contrast, in the case of 2-pyridone, lactim is favoured by orbital and electrostatic interactions and disfavoured by the Pauli repulsion. Aromaticity gain in this case is smaller. The position of the tautomeric equilibrium can be modulated by substituent inductive effects (Cl and F), inductive and resonance effects (NH2 and NO2), hydrogen bonding (NO2), and medium polarity, the increase of which increases lactam population.