Keto Enol Research Articles

Multiple Enol-Keto Isomerization and Excited-State Unidirectional Intramolecular Proton Transfer Generate Intense, Narrowband Red OLEDs.

A novel series of excited-state intramolecular proton transfer (ESIPT) emitters, namely, DPNA, DPNA-F, and DPNA-tBu, endowed with dual intramolecular hydrogen bonds, were designed and synthesized. In the condensed phase, DPNAs exhibit unmatched absorption and emission spectral features, where the minor 0-0 absorption peak becomes a major one in the emission. Detailed spectroscopic and dynamic approaches conclude fast ground-state equilibrium among enol-enol (EE), enol-keto (EK), and keto-keto (KK) isomers. The equilibrium ratio can be fine-tuned by varying the substitutions in DPNAs. Independent of isomers and excitation wavelength, ultrafast ESIPT takes place for all DPNAs, giving solely KK tautomer emission maximized at >650 nm. The spectral temporal evolution of ESIPT was resolved by a state-of-the-art technique, namely, the transient grating photoluminescence (TGPL), where the rate of EK* → KK* is measured to be (157 fs)-1 for DPNA-tBu, while a stepwise process is resolved for EE* → EK* → KK*, with a rate of EE* → EK* of (72 fs)-1. For all DPNAs, the KK tautomer emission shows a narrowband emission with high photoluminescence quantum yields (PLQY, ∼62% for DPNA in toluene) in the red, offering advantages to fabricate deep-red organic light-emitting diodes (OLED). The resulting OLEDs give high external quantum efficiency with a spectral full width at half-maximum (FWHM) as narrow as ∼40 nm centered at 666-670 nm for DPNAs, fully satisfying the BT. 2020 standard. The unique ESIPT properties and highly intense tautomer emission with a small fwhm thus establish a benchmark for reaching red narrowband organic electroluminescence.

Molecular modelling of luciferyl adenylate deprotonation in the active site of Photinus pyralis luciferase

Firefly bioluminescence is a product of chemical reactions that involve luciferin chromophore oxidation in the active site of luciferase proteins. We perform a series of classical molecular dynamic simulations and combined quantum and molecular mechanics (QM/MM) calculations to expose the molecular mechanism of C4 carbon atom deprotonation in luciferyl adenylate molecule. QM/MM calculations confirm that ND-protonated His245 residue is a suitable proton acceptor in the wild-type Photinus pyralis luciferase. Classical molecular dynamic simulations reveal oxygen-binding cavities inside the protein including the one located close to the C4 atom of luciferin. In mutant forms that lack direct interactions with the His245 side chain, a proton wire comprising water molecules stimulates either protonation of the phosphate group of the luciferyl adenylate forming an unstable intermediate or keto–enol tautomerisation of luciferin. The comparison of the ionisation potentials of molecular systems with the ‘deprotonated’ C4 carbon atom revealed that the ionisation energy for the enol tautomeric form is close to the system with the His245 proton acceptor. Thus, the existence of the keto–enol tautomerisation channel might explain bioluminescence in case of the absence of the amino acid proton acceptor.

Synthesis of 5-(4-Formylphenyl)barbituric Acid to Access Enolizable Chromophoric Barbituric Acids

AbstractBarbituric acids mono-substituted at the 5-position show keto–enol tautomerism. In the keto form, conjugation to an aryl substituent is interrupted due to the sp³-hybridized carbon atom at the 5-position of barbituric acid. The enol form generates a conjugated π-system to the aryl substituent and acts as an electron-donating group. If the aryl substituent is electron-deficient, a push-pull system is generated that shows typical UV/Vis absorption. These types of compounds are difficult to access synthetically due to their intrinsic convertibility. The synthesis of barbituric acids with a 4-formylphenyl functionality at the 5-position is reported. This compound, 5-(4-formylphenyl)barbituric acid, could be used to introduce extended π-systems with electron-withdrawing groups in great variety by simple condensation reactions. We demonstrate this by a Horner–Wadsworth–Emmons reaction that forms the enolizable dye (E)-5-(4-(4-nitrostyryl)phenyl)barbituric acid.

Primordial aqueous alteration recorded in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu

We report primordial aqueous alteration signatures in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 spacecraft of JAXA. Newly identified low-molecular-weight hydroxy acids (HO-R-COOH) and dicarboxylic acids (HOOC-R-COOH), such as glycolic acid, lactic acid, glyceric acid, oxalic acid, and succinic acid, are predominant in samples from the two touchdown locations at Ryugu. The quantitative and qualitative profiles for the hydrophilic molecules between the two sampling locations shows similar trends within the order of ppb (parts per billion) to ppm (parts per million). A wide variety of structural isomers, including α- and β-hydroxy acids, are observed among the hydrophilic molecules. We also identify pyruvic acid and dihydroxy and tricarboxylic acids, which are biochemically important intermediates relevant to molecular evolution, such as the primordial TCA (tricarboxylic acid) cycle. Here, we find evidence that the asteroid Ryugu samples underwent substantial aqueous alteration, as revealed by the presence of malonic acid during keto–enol tautomerism in the dicarboxylic acid profile. The comprehensive data suggest the presence of a series for water-soluble organic molecules in the regolith of Ryugu and evidence of signatures in coevolutionary aqueous alteration between water and organics in this carbonaceous asteroid.

Calculating Thermodynamic Stabilities of Keto–Enol Tautomers of Aldehydes, Ketones, Esters, and Amides

Calculating Thermodynamic Stabilities of Keto–Enol Tautomers of Aldehydes, Ketones, Esters, and Amides

Decoding the commutable first hyperpolarisability of keto–enol tautomer using DFT and AI-based soft computing method

ABSTRACT Non-linear optical (NLO) parameter of a series of donor–acceptor substituted N-salicylidene-4-benzenesulfonylaniline (SBSA) derivatives with a contrast of first hyperpolarisability has been investigated using density functional-based theory. The derivative of SBSA with both the donor and acceptor substitution (NMe2-SBSA-NO2) exhibits the highest first hyperpolarisability ( ∼ 3 × 103 a.u in gas phase) value. Frequency and solvent polarity responsive behaviours of first hyperpolarisability are utilised to demonstrate the variation of NLO as switchable electrical parameter. Each of the studied compounds displays very high NLO response at frequency corresponding to the absorption maxima of their electronic state. Likewise, the increase of solvent polarity induces a substantial increase of first hyperpolarisability. The computation of NLO responsive first hyperpolarisability by varying the frequency and solvent dielectric constant within a wide domain is very tedious and time-consuming. To overcome the lacuna, we implemented machine learning tools such as artificial neural networks (ANNs), fuzzy logic (FL) and adaptive neuro-fuzzy inference system (ANFIS) to predict the computation data of the NMe2-SBSA-NO2 and moderately good results were obtained using ANFIS as soft computing tool.

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)).

5-chloro-3-(2-(2,4-dinitrophenyl) hydrazono)indolin-2-one: synthesis, characterization, biochemical and computational screening against SARS-CoV-2

Chemical prototypes with broad-spectrum antiviral activity are important toward developing new therapies that can act on both existing and emerging viruses. Binding of the SARS-CoV-2 spike protein to the host angiotensin-converting enzyme 2 (ACE2) receptor is required for cellular entry of SARS-CoV-2. Toward identifying new chemical leads that can disrupt this interaction, including in the presence of SARS-CoV-2 adaptive mutations found in variants like omicron that can circumvent vaccine, immune, and therapeutic antibody responses, we synthesized 5-chloro-3-(2-(2,4-dinitrophenyl)hydrazono)indolin-2-one (H2L) from the condensation reaction of 5-chloroisatin and 2,4-dinitrophenylhydrazine in good yield. H2L was characterised by elemental and spectral (IR, electronic, Mass) analyses. The NMR spectrum of H2L indicated a keto–enol tautomerism, with the keto form being more abundant in solution. H2L was found to selectively interfere with binding of the SARS-CoV-2 spike receptor-binding domain (RBD) to the host angiotensin-converting enzyme 2 receptor with a 50% inhibitory concentration (IC50) of 0.26 μM, compared to an unrelated PD-1/PD-L1 ligand–receptor-binding pair with an IC50 of 2.06 μM in vitro (Selectivity index = 7.9). Molecular docking studies revealed that the synthesized ligand preferentially binds within the ACE2 receptor-binding site in a region distinct from where spike mutations in SARS-CoV-2 variants occur. Consistent with these models, H2L was able to disrupt ACE2 interactions with the RBDs from beta, delta, lambda, and omicron variants with similar activities. These studies indicate that H2L-derived compounds are potential inhibitors of multiple SARS-CoV-2 variants, including those capable of circumventing vaccine and immune responses.

Omadacycline dihydrate, C29H40N4O7·2H2O, from X-ray powder diffraction data.

The crystal structure of the title compound {systematic name: (4S,4aS,5aR,12aR)-4,7-bis-(di-methyl-amino)-9-[(2,2-di-methyl-propyl-amino)-meth-yl]-1,10,11,12a-tetra-hydroxy-3,12-dioxo-4a,5,5a,6-tetra-hydro-4H-tetra-cene-2-carb-oxamide dihydrate, C29H40N4O7·2H2O} has been solved and refined using synchrotron X-ray powder diffraction data: it crystallizes in space group R3 with a = 24.34430 (7), c = 14.55212 (4) Å, V = 7468.81 (2) Å3 and Z = 9. Most of the hydrogen bonds are intra-molecular, but two classical N-H⋯O inter-molecular hydrogen bonds (along with probable weak C-H⋯O and C-H⋯N hydrogen bonds) link the mol-ecules into a three-dimensional framework. The framework contains voids, which contain disordered water mol-ecules. Keto-enol tautomerism is apparently important in this mol-ecule, and the exact mol-ecular structure is ambiguous.

Computational insights into the binding modes, keto-enol tautomerization and stereo-electronically controlled decarboxylation of oxaloacetate in the active site of macrophomate synthase.

Oxaloacetic acid (OAA) is a β-ketocarboxylic acid, which plays an important role as an intermediate in some metabolic pathways, including the tricarboxylic acid cycle, gluconeogenesis and fatty acid biosynthesis. Animal studies have indicated that supplementing oxaloacetic acid shows an increase of lifespan and other substantial health benefits including mitochondrial DNA protection, and protection of retinal, neural and pancreatic tissues. Most of the chemical transformations of OAA in the metabolic pathways have been extensively studied; however, the understanding of decarboxylation of OAA at the atomic level is relatively lacking. Here, we carried out MD simulations and combined quantum mechanical/molecular mechanical (QM/MM) calculations as an example to systematically elucidate the binding modes, keto-enol tautomerization and decarboxylation of OAA in the active site of macrophomate synthase (MPS), which is a Mg(II)-dependent bifunctional enzyme that catalyzes both the decarboxylation of OAA and [4+2] cycloaddition of 2-pyrone with the decarboxylated intermediate of OAA (pyruvate enolate). On the basis of our calculations, it was found that the Mg2+-coordinated oxaloacetate may exist in enol forms and keto forms. The four keto forms can be transformed into each other by simply rotating the C2-C3 single bond, nevertheless, the keto-enol tautomerization strictly requires the assistance of pocket water molecules. In addition, the decarboxylation is stereo-electronically controlled, i.e., it is the relative orientation of the terminal carboxyl anion that determines the rate of decarboxylation. As such, the chemistry of oxaloacetate in the active site of MPS is complex. On one hand, the most stable binding mode (K-I) may undergo enol-keto tautomerization to isomerize to the enol form, which may further react with the second substrate; on the other hand, K-I may isomerize to another binding mode K-II to proceed decarboxylation to generate pyruvate enolate and CO2. Starting from K-I, the enol-keto tautomerization corresponds to a barrier of 16.2 kcal mol-1, whereas the decarboxylation is associated with an overall barrier of 19.7 kcal mol-1. These findings may provide useful information for understanding the chemistry of OAA and the catalysis of related enzymes, and they are basically in agreement with the available experimental kinetic data.

Rate-Enhancing Role of Water in H-BEA and Sn-BEA for Keto–Enol Tautomerization of Acetone: A DFT Study

Rate-Enhancing Role of Water in H-BEA and Sn-BEA for Keto–Enol Tautomerization of Acetone: A DFT Study

Regulating Keto–Enol Tautomerism of β-Ketoenamine Covalent–Organic Frameworks for Photocatalytic Oxidative Coupling of Amines

Regulating Keto–Enol Tautomerism of β-Ketoenamine Covalent–Organic Frameworks for Photocatalytic Oxidative Coupling of Amines

Photoelectric Multilevel Memory Device based on Covalent Organic Polymer Film with Keto–Enol Tautomerism for Harsh Environments Applications

AbstractCovalent organic polymers (COPs) memristors with multilevel memory behavior in harsh environments and photoelectric regulation are crucial for high‐density storage and high‐efficiency neuromorphic computing. Here, a donor–acceptor (D–A)‐type COP film (Py‐COP‐3), which is initiated by keto–enol tautomerism, is proposed for high‐performance memristors. Satisfactorily, the indium tin oxide (ITO)/Py‐COP‐3/Ag device demonstrates multilevel memory performance, even in high temperatures, acid‐base corrosion, and various organic solvents. Moreover, the performance can be modulated by the photoelectric effect to maintain a great switching behavior. By contrast, Py‐COP‐0, with similar structure and chemical composition to Py‐COP‐3 but without keto–enol tautomerism, exhibits binary storage performance. Further studies unravel that both the formation of conductive filaments and charge transfer within D‐A Py‐COP‐3 film contribute to the resistive switching behavior of memory devices.

Difluoroboron complexes of aromatic imidazole-functionalized 2-(benzo[d]thiazol-2-yl)phenol (BTZ): Synthesis and luminescence properties

Two aromatic imidazole-functionalized ligands (BTZ-PI-OH and BTZ-IM-OH) and their difluoroboron complexes (BTZ-PI-BF2 and BTZ-IM-BF2) were successfully synthesized by linking the aromatic imidazole units into 2-(benzo[d]thiazol-2-yl)phenol (BTZ-OH) via Suzuki-Miyaura cross-coupling reaction, and characterized by NMR (1H and 13C), HR-MS and FT-IR spectra. The photophysical, electrochemical properties and thermal stability of the ligands and the difluoroboron complexes were systematically investigated by experimental and theoretical methods to reveal their relationship between the molecular structures and photoelectric properties. The ligands exhibited dual fluorescence (enol emission and keto emission) due to the characteristics of excited-state intramolecular proton-transfer (ESIPT) in solution, while they showed only one emission in solid thin films. Compared with the ligands, the difluoroboron complexes showed a significant blue-shifted and enhanced emission due to the restricted conformation change. The solution-processed doped devices using the difluoroboron complexes as the emitting materials exhibited green-blue emission with maximum external quantum efficiency (EQEmax) of 1.35% for BTZ-PI-BF2 and 3.08% for BTZ-IM-BF2, respectively.

Diethyl 2-Cyano-3-oxosuccinate

The titular compound was characterized for the first time using a full range of spectroscopic methods, including UV, IR, 1H, and 13C NMR spectra. In solution, all methods showed a keto–enol equilibrium strongly shifted to the enol form. The X-ray structures determined for all simple 2-oxosuccinates showed only the enol form packed as hydrogen-bonded dimer stacks.

Tautomerism of pyridinylbutane-1,3-diones: An NMR and DFT study.

The three possible 1-(n-pyridinyl)butane-1,3-diones (nPM) have been synthesized. Structures, tautomerism, and conformations are investigated by means of DFT calculations. 1 H and 13 C NMR spectra are assigned, and deuterium isotope effects on 13 C chemical shifts have been measured. Analysis of the isotope effects leads to the equilibrium constants of the keto-enol tautomers. Some interesting differences are seen between the three compounds and the phenyl analogs. The isotope effects can also rank the hydrogen bonds of the compounds, with the one with nitrogen in the three positions of the pyridine ring as the weakest. Structures, conformers, energies, and NMR nuclear shieldings are calculated using DFT calculations at the B3LYP/6-311++G(d,p) level.

3,3'-(Phenyl-methyl-ene)bis-(1-ethyl-3,4-di-hydro-1H-2,1-benzo-thia-zine-2,2,4-trione): single-crystal X-ray diffraction study, quantum-chemical calculations and Hirshfeld surface analysis.

The title compound, C27H26N2O6S2, possesses potential anti-microbial, analgesic, and anti-inflammatory activity. This compound has three tautomeric forms, which relative energies were estimated with quantum-chemical calculations. All these tautomers (dienol form 7A, keto-enol form 7B, and diketo form 7C) were optimized by the M06-2X/cc-pVTZ method in a vacuum, using the PCM model with chloro-form and DMSO as solvent. The diketo form of the title compound proved to be the most energetically favourable as compared to the keto-enol or dienol forms. The diketo form can exist as three possible stereoisomers with the same configuration of one stereogenic center and different configurations of the stereogenic centers at two other atoms: ( R , R , R ), (S , R , S ) and ( R , R , S ). The ( R , R , S ) stereoisomer was found in the crystal phase. It was revealed that the thia-zine rings of equivalent benzo-thia-zine fragments have different conformations, (a sofa or a half-chair). The two bicyclic fragments connected through the phenyl-methyl-ene group are oriented almost orthogonal to each other, subtending a dihedral angle of 82.16(7)°.

Quantum-Chemical Study of Keto–Enol Equilibrium and Global Electrophilicity of Hydroxymaleimide Derivatives

Quantum-Chemical Study of Keto–Enol Equilibrium and Global Electrophilicity of Hydroxymaleimide Derivatives

Phenothiazine-based donor–acceptor covalent–organic frameworks with keto–enol irreversible tautomerism as a promising third-order nonlinear optics material

Keto–enol tautomerization endowed TFP-TZ with more D–A interaction strengths and an electron-withdrawing CO group in the keto form facilitated charge transfer.

Investigation of first hyper-polarisability molecular switches between enol–keto equilibrium of phenyl benzodifurantrione: a DFT-based computational study

The contrast of the second-order nonlinear optical response between the enol (anti and syn) and keto tautomeric equilibrium of 7-Phenyl-7H-benzo[1,2-b;4,5-b’]difuran-2,3,6-trione (PBDT) with donor/acceptor substitution at the para position of phenyl group have been computed involving electron correlation, frequency dispersion, and solvent polarity effects. The Hyper Rayleigh Scattering (HRS) first-hyperpolarisability contrast between the enol and keto forms for PBDT and Nitro-substituted PBDT is 3.5 and 2.8, respectively. However, in NMe2-substituted PBDT, a large contrast (12 times) of HRS first hyper-polarisability between enol (anti and syn) and keto form is observed. Present study reveals that electron donor group (–NMe2) in PBDT plays a pivotal role for obtaining very high contrast in the first hyper-polarisability (β HRS and β vec) values between the enol and keto forms compare to the electron acceptor (–NO2) group substitution at the same position. The solvent effect also plays a significant role in further improving the first hyper-polarisability value and its switching behaviour between the enol and keto forms of NMe2-PBDT. NLO switching contrast between the enol and keto forms of donor/acceptor substituted PBDT is also explained using the electronic features including UV-Vis absorption spectra, HOMO–LUMO electron densities, optical energy gaps, charge transfer contribution etc.