Enol Research Articles

Influence of pyridinic nitrogen on tautomeric shifts and charge transport in single molecule keto enol equilibria

Keto-enol tautomerism in organic molecules presents a potential for modulating the charge transport at the nanoscale. The reduction of the isomerization barrier and favoring the highly conductive enol form are the main challenges towards practical implementation of this phenomenon. Using density functional theory calculations, we have demonstrated that pyridinic nitrogen in biphenyl molecules with keto-enol tautomerism can successfully make the conductive enol form energetically more favorable. This enhanced stability originates from the hydrogen bonding between the pyridinic nitrogen and the hydroxyl group in the enol state. The presence of the pyridinic unit further enhances the conductance of the enol form while reducing that of the keto form compared to the pristine molecule. The novelty of our findings lies in the use of pyridinic nitrogen to control isomerization through hydrogen bonding, offering a new approach to tuning electronic properties for molecular devices. These findings can be valuable in the development of functional molecular devices based on the keto-enol isomerization phenomenon.

Superconjugated Anthraquinone Carbonyl-Based Covalent Organic Framework as Anode Material for High-Performance Aqueous Ammonium-Ion Batteries.

Covalent organic frameworks (COFs) have recently been emerged as promising anode candidates for aqueous ammonium ion batteries (AAIBs), but suitable COFs need to be developed and the energy storage mechanisms need to be further investigated. Here we report an anthraquinone carbonyl-containing COF as anode for AAIBs, which exhibits a high specific capacity of 141 mAh g-1 at 0.1 A g-1 and good cycling stability with 90% retention after 8000 cycles at 6 A g-1. We find that the anthraquinone carbonyls (denoted as C=O1) with superconjugated structure in the COF can form hydrogen bonds with NH4+ ions, thus serving as optimal sites and providing the major contribution to NH4+ ion storage, whereas another type of carbonyls (denoted as C=O2) in the same COF suffers from enol-keto tautomerism, which prevents the formation of hydrogen bonds and is therefore unfavourable for NH4+ ion storage. This is further verified by the theoretical calculations where the hydrogen bonding rate of C=O1 groups is much faster than that of C=O2. This work not only provides insights into understanding the mechanism of NH4+ ion storage, but also offers new ideas for the design of advanced COF-based materials for AAIBs.

1,3,5-Triformylphloroglucinol Derived β-Ketoenamine-Linked Functional Covalent Organic Frameworks with Enhanced Crystallinity and Stability-Recent Advances.

Crystallinity, stability, and complexity are significant factors to consider in the design and development of covalent organic frameworks (COFs). Among various building blocks used, 1,3,5-triformylphloroglucinol (Tp) is notable for enhancing both crystallinity and structural stability in COFs. Tp facilitates the formation of β-ketoenamine-linked COFs through keto-enol tautomerism when reacted with aromatic amines. This review article examines the stability, crystallinity, and flexibility of synthetic methodologies involving Tp-based COFs, while highlighting their recent applications. We emphasize the critical roles of non-covalent interactions and keto-enol tautomerism in achieving high levels of crystallinity and stability. Additionally, the diverse and straightforward synthesis methods available for Tp-based COFs contribute to the prevalence of 1,3,5-triformylphloroglucinol in COF development. We conclude by addressing the challenges and future prospects in this area, underscoring the significant potential of Tp-based COFs for environmental and energy-related applications due to their exceptional structural tunability and functionality.

Single GAF Domain Phytochrome Exhibits a pH-Dependent Shunt on the Millisecond Timescale.

The light-sensing activity of phytochromes is based on the reversible light-induced switching between two isomerization states of the bilin chromophore. These photo-transformations may not necessarily be only unidirectional, but could potentially branch back to the initial ground state in a thermally driven process termed shunt. Such shunts have been rarely reported, and thus our understanding of this process and its governing factors are limited. Here, we aim to close this gap by providing coherent experimental evidence of a shunt process using UV/Vis laser flash photolysis. We studied the Pfr to Pr dynamics of the single GAF domain (g1) construct of the knotless phytochrome All2699 from cyanobacterium Nostoc punctiforme. We identified a shunt that can be switched on and off by ambient buffer conditions. In combination with H/D exchange and kinetic modeling, we propose a keto-enol tautomerism to allow for the thermal isomerization of the chromophore and act as the driver of the shunt transition.

Synthesis, crystal structure, and Hirshfeld analysis of a novel Schiff base compound 5-hydroxy-2-{[(propan-2-yl)iminio]methyl}phenolate

5-hydroxy-2-{[(propane-2-yl)iminio]methyl}phenolate, with an empirical formula of C 10 H 13 NO 2 was synthesized by a reaction between 2, 4-Dihydroxybenzaldehyde and isopropyl amine in an aqueous ethanolic medium. The Schiff base compound was characterized by elemental analysis, infrared (IR) spectroscopy, Nuclear magnetic spectroscopy (NMR), Mass spectrometry (MS), thermogravimetry (TG), and single-crystal X-ray diffraction (XRD). The compound was crystallized in an orthorhombic crystal system with a space group of Pna21. The azomethine functional group (-HC=N-) was confirmed by the band at 1638 cm -1 in its IR spectrum. The compound is thermally stable from 40-154 o C losing only 0.2% of its weight. The thermogram (TG) indicated the absence of any water of hydration or lattice water. The compound exhibits keto-enol tautomerism with a zwitterionic structure. The non-covalent interactions (NCI) were studied by various computational methods such as Hirshfeld surface analysis, NCI plot, and scatter plot (RDG vs sign (λ 2 )ρ). The major contributors to the Hirshfeld surface (HS) contacts are O--/H--O (21.5%) and C--/H--C (25.3%). The non-covalent interactions (NCI) and the crystal parameters are, a (Å): 10.3839(9), b (Å):9.9442(8), c (Å): 9.2223(8), α=β=γ= 90 o , Z = 4.

Influence of Keto-Enol Tautomerism in Regulating CO2 Photoreduction Activity in Porous Organic Porphyrinic Photopolymers.

Photoassisted CO2 reduction employing a metal-free system is both challenging and fascinating. In our study, we present a structural engineering strategy to tune the potential energy barrier, which, in turn, affects the photoreduction ability. A series of porphyrin-based porous organic polymers (POPs) were hydrothermally synthesized and the influence of keto-enol tautomerization on the CO2 photoreduction potential has been rigorously investigated. Among the screened photocatalysts, POP-1 demonstrated the highest CO2/CO conversion efficacy, producing 518 μmol g-1 h-1 of CO selectively under light illumination for 2 h. Density Functional Theory computational investigations concretely highlighted the reaction mechanistic pathway supporting the CO2 conversion reaction. Additionally, the electron density mapping underpinned the thermodynamic energy barrier requirements for the progress of the reaction and elucidated the reason for the enhanced photocatalytic activity seen in POP-1. In situ Fourier-Transform Infrared spectroscopy was carried out for real-time investigations to understand the synergistic reaction dynamics and unlock the generation of key reaction intermediates during the CO2 reduction reaction process. Additionally, ultrafast transient absorption spectroscopy plays a vital role in understanding the surface interaction dynamics of our designed catalysts. Overall, this straightforward modulation strategy not only enhances CO2 reduction performance but also contributes toward presenting a crisp and concrete understanding of the structure-property relationship, opening up the possibilities for the development of artificial photocatalysts. The results introduce a strategy for photocatalytic CO2 reduction using an efficient, stable, and recyclable metal-free photocatalytic system.

Synthesis of novel hydrazide-substituted phthalonitrile resin: Effect of keto-enol tautomerism on structure–property relationship

Synthesis of novel hydrazide-substituted phthalonitrile resin: Effect of keto-enol tautomerism on structure–property relationship

Functional biosynthetic stereodivergence in a gene cluster via a dihydrosydnone N-oxide

Chirality plays a critical role in the biochemistry of life and often only one enantiomeric series is observed (homochirality). Only a few natural products have been obtained as racemates, e.g. the signalling molecule valdiazen produced by Burkholderia cenocepacia H111. In this study, we investigated the ham biosynthetic gene cluster and discovered that both the enantiomerically pure (R)-fragin and the racemic valdiazen result from the same pathway. This stereodivergence is based on the unusual heterocyclic intermediate dihydrosydnone N-oxide, as evident from gene knockout, stable isotope feeding experiments, and mass spectrometry experiments. Both non-enzymatic racemisation via keto-enol tautomerisation and enzyme-mediated dynamic kinetic resolution were found to be crucial to this stereodivergent pathway. This novel mechanism underpins the production of configurationally and biologically distinct metabolites from a single gene cluster. Our findings highlight the intricate design of an intertwined biosynthetic pathway and provide a deeper understanding of microbial secondary metabolism related to microbial communication.

Investigation of a novel Schiff base Catena-((μ6-(E)-2-((4-methoxy-2-oxidobenzylidene)ammonio)ethane-1-sulfonato potassium as a potential antibacterial agent.

Schiff bases, which have intriguing properties in many areas, have been studied extensively in recent years due to their structural properties and biological activities. In this research, a novel water-soluble Schiff base complex, Catena-((μ6-(E)-2-((4-methoxy-2-oxidobenzylidene) ammonio) ethane-1-sulfonato potassium, C10H14KNO6S (CMOAESP), was synthesized by a one-step condensation reaction of 2-hydroxy-4-methoxy benzaldehyde and taurine with the yield of 65%, 0.333g. Spectral analysis of water-soluble crude product showed equilibria keto-enol tautomerism, by a resulting Zwitterionic form of the structure. The ADMET prediction showed that the CMOAESP obeys Lipinski's rule and was in bioavability range. The molecular docking results showed an inhibition ability for 1JIJ and MBT1 proteins which play an important role in antibacterial studies. The experimental end theoretical calculations were in a good agreement that CMOASP has inhibition efficiency against Escherichia coli and has inhibition at high concentrations against Staphylococcaceae aureus. The WinGX software system SHELXS direct technique was used to solve the structure, and SHELXL full-matrix least-squares approach on F2 was used to refine the results. The Gaussian 09W program was used to carry out the DFT calculations. The gap values for CMOAESP were calculated by using the B3LYP/6- 311 + + G(d,p) method in order to predict its reactivities and behaviors in the gas phase. The frontier orbitals, LUMO and HOMO together with some global parameters as electronegativity (μ), chemical potential (χ), hardness (η), electrophilicity index (ω), and softness (S) values were predicted from DFT calculations.

Visible-Light-Induced Photocatalytic Degradation of Naproxen Using 5% Cu/TiO2, Transformation Products, and Mechanistic Studies.

The presence of drugs in wastewater effluent is of concern due to their effects on the aquatic fauna and flora and there are growing efforts for their removal from the environment. In this paper, we study the photocatalytic visible-light degradation of naproxen, an over-the-counter anti-inflammatory drug, using 5% copper-doped TiO2. The photocatalyst was characterized by XRD and BET surface area measurements. The optimal conditions for the degradation of 1 × 10-3 M of naproxen were found to be 3 h, with a catalyst loading of 50 mg/100 mL of the drug solution, and an acidic pH of 4.55. The degradation followed pseudo-first order kinetics and achieved a photodegradation efficiency of 44.8%. HPLC was used to separate the degradation products and their structures were determined using MS/MS data. A pathway for the degradation of naproxen is proposed along with degradation mechanisms. The major degradation events involve the formation of hydroxyl radicals, hydroxylation, keto-enol tautomerism, and decarboxylation.

Phenolic -OH-Induced Fluorescence and Chemoselectivity in a Triptycene-Based trans-Azo Oligomer for Sensing Applications.

A novel triptycene-based trans-azo fluorescent oligomer exhibiting phenolic groups has been designed. The presence of phenolic -OH in conjugation with the azo group rendered the oligomer fluorescence active by keto-enol tautomerism, which was evidenced by quenching the fluorescence intensity of TP1 in 1 M aq. NaOH. A green synthetic protocol was employed to synthesize this oligomer as a dark-brown solid, and it was characterized by using diverse analytical tools such as FTIR, 13C-CPMAS NMR, GPC, FESEM, EDS, TGA, and PXRD techniques. FESEM and PXRD confirmed its existence as amorphous nanoclusters. The oligomer displayed efficient chemosensing properties toward the detection of picric acid with the LOD value of 391 nM. The specific recognition of PA in the presence of other explosives and the results of real water sample analysis further indicated the high efficacy of the TP1 as a chemosensor.

Asymmetric Synthesis of β-Ketoamides by Sulfonium Rearrangement.

The synthesis of enantioenriched α-substituted 1,3-dicarbonyls remains a contemporary challenge in synthesis due to their tendency to undergo racemization via keto-enol tautomerization. Herein, we report a method to access enantioenriched β-ketoamides by a chiral sulfinimine-mediated [3,3]-sigmatropic sulfonium rearrangement. The transformation displays good chirality transfer, as well as excellent chemoselectivity and functional group tolerance. Diastereoselective reduction of the ketone moiety, also achievable in one-pot fashion, affords enantioenriched β-hydroxyamides.

The Interplay of Hydration and Hydrolysis upon the Keto–Enol Tautomerism of β-Diketones

The Interplay of Hydration and Hydrolysis upon the Keto–Enol Tautomerism of β-Diketones

Catalytic pyrolysis mechanism of tetrabromobisphenol A by calcium oxide: A density functional theory study

In this paper, the mechanisms of catalytic pyrolysis of tetrabromobisphenol A (TBBPA) by calcium oxide (CaO) were studied through density functional theory methods. The results indicate that the phenolic hydroxyl group of TBBPA is the preferred site for CaO to extract protons, and the generated anion further transforms into 2,6-dibromophenol via demethylation and hydrogen transfer reactions. The reaction activity of CaO with hydrogen bromide is relatively high, with an energy barrier of 133.2 kJ/mol. CaO produces calcium ions, which combine with bromine ions to form calcium bromide to achieve the purpose of fixing bromine. In addition, the participation of calcium ions results in the OH bond being easier to crack, which further lowers the reaction energy barrier of keto-enol tautomerism reactions H2O produced during catalytic pyrolysis also has an obvious catalytic effect, and the energy barrier of the keto-enol tautomerism reaction decreased from 309.1 kJ/mol to 150.6 kJ/mol with the participation of H2O.

Oxy-pyridinium Ylides Mediated 1,4-Pyridyl/Aryl Translocation.

Molecular rearrangement via carbene transfer is a powerful tool to access molecular diversity. Herein, we describe an efficient approach to selective pyridyl/aryl relocation via a rhodium-catalyzed aminoarylation of diazo compounds, providing a promising strategy to access ortho-pyridyl N-alkylated pyridone scaffolds in a single operation. This reaction features the novel reactivity of oxy-pyridinium ylide, rhodium-associated five-membered transition state, and 1,4-pyridyl/aryl relocation. A computational study discloses the initial oxy-pyridinium ylide formation, keto-enol tautomerization, and 1,4-pyridyl migration to complete the whole rearrangement.

Palladium Single Atom-supported Covalent Organic Frameworks for Aqueous-phase Hydrogenative Hydrogenolysis of Aromatic Aldehydes via Hydrogen Heterolysis.

Developing a method for the tandem hydrogenative hydrogenolysis of bio-based furfuryl aldehydes to methylfurans is crucial for synthesizing sustainable biofuels and chemicals; however, it poses a challenge due to the easy hydrogenation of the C=C bond and difficult cleavage of the C-O bond. Herein, a palladium (Pd) single-atom-supported covalent organic framework was fabricated and showed a unique 2,5-dimethylfuran yield of up to 98.2 % when reacted with 5-methyl furfuryl aldehyde in an unprecedented water solvent at 30 °C. Furthermore, it exhibited excellent catalytic universality while converting various furfuryl-, benzyl-, and heterocyclic aldehydes at room temperature. The analysis of the catalytic mechanism confirmed that H2 was heterolytically activated on the Pd-N pair and triggered the keto-enol tautomerism of the covalent organic frameworks (COFs) host, resulting in H--Pd⋅⋅⋅O-H+ sites. These sites served as novel asymmetric hydrogenation sites for the C=O group and hydrogenolysis sites for the C-OH group through a scarce SN2 mechanism. This study demonstrated remarkable bifunctional catalysis through the H2-induced keto-enol tautomerism of COF catalysts for the atypical preparation of methyl aromatics in a water solvent at room temperature.

A density functional theory study of keto-enol tautomerism in 1,2-cyclodiones: Substituent effects on reactivity and thermodynamic stability

This study systematically investigated the influence of substituents (NH2, CH3, CF3, and Cl) on the keto-enol tautomerism of 1,2-cyclodiones, focusing on reactivity and thermodynamic stability. Calculations were performed using the M06-2X/6-311G++** level of theory, with a solvation model through CPCM/M06-2X/6-311G++**. Results indicate ring-size dependent tautomerism, with three-, five-, and six-membered rings favouring the keto-enol form, while four- and seven-membered rings favour the diketo form. The NH2 substituent uniquely stabilised the keto-enol form. Electron-donating groups generally stabilised the keto-enol form. Contrary to common expectations, a direct correlation between dipole moment and stability order was not universally observed within 1,2-cyclodione systems, with the NH2 group demonstrating nuanced effects. Similarly, dipole moment and electrostatic potential variance did not exhibit consistent correlation under the influence of different substituents. The diketo tautomer was generally found to be more reactive than the keto-enol form, especially in non-polar solvents. Surprisingly, substituent groups decreased the reactivity of the diketo form relative to the unsubstituted compound.

Aminosulfonylation of Rhodium Carbene via Ylide Formation and 1,4-Sulfonyl Rearrangement.

We report here the use of pyridin-2-yl benzenesulfonates as sulfonylation reagents in a difunctionalization reaction based on oxy-pyridinium ylide chemistry, providing an effective protocol for the installation of both a sulfonyl group and a pyridone moiety into one molecule. Density functional theory (DFT) calculations disclose that the reaction process might proceed through sequential metal-bound ylide formation, keto-enol tautomerism, and the migratory rearrangement of the sulfonyl group.

Unveiling the Photoactivation Mechanism of BLUF Photoreceptor Protein through Hybrid Quantum Mechanics/Molecular Mechanics Free-Energy Calculation.

OaPAC is a photoactivated enzyme that forms a homodimer. The two blue-light using flavin (BLUF) photoreceptor domains are connected to the catalytic domains with long coiled-coil C-terminal helices. Upon photoreception, reorganization of the hydrogen bonding network between Tyr6, Gln48, and the chromophore in the BLUF domain and keto-enol tautomerization of Gln48 are thought to occur. However, the quantitative energetics of the photoisomerization reaction and how the BLUF domain's structural change propagates toward the catalytic domain are still unknown. We evaluate the free-energy differences among the dark-state and two different light-state structures by the free-energy perturbation calculations combined with QM/MM free-energy optimizations. Furthermore, we performed long-time MD simulations for the free-energetically optimized dark- and light-state structures to clarify the differences in protein dynamics upon photoisomerization. The free-energy difference between the two optimized light-state structures was estimated at ∼4.7 kcal/mol. The free-energetically optimized light-state structure indicates that the chemically unstable enol tautomer of Gln48 in the light state is stabilized by forming a strong hydrogen bonding network with the chromophore and Tyr6. In addition, the components of free-energy difference between the dark- and light-state structures show that the energy upon photoreception is stored in the environment rather than the internal photoreceived region, suggesting a mechanism to keep the photoactivated signaling state with the chemically unstable enol tautomer of Gln48. In the light state, a fluctuation of Trp90 near the C-terminal helix becomes large, which causes subsequent structural changes in the BLUF core and the C-terminal helix. We also identified residue pairs with significant differences concerning residue-wise contact maps between the dark and light states.

Programmable Plasma-Microdroplet Cascade Reactions for Multicomponent Systems.

The concept of programmable cascade reactions in charged microdroplets is introduced using carbon-carbon (C-C) bond formation via uncatalyzed Michael addition in a three-tier study culminating in programmable Hantzsch multicomponent, multistep reactions. In situ generated reactive oxygen species (ROS) from nonthermal plasma discharge are fused with charged water microdroplets (devoid of ROS) in real time for accelerated chemical reactions. This plasma-microdroplet fusion platform utilizing a coaxial spray configuration enabled product selection while avoiding unwanted side reactions. Hydrogen abstraction via ROS facilitated the formation of enolate anions without strong base use. Reaction enhancement factors >103 were calculated for plasma-microdroplet fusion versus microdroplet-only reactions. The platform programmability was showcased through (i) uncatalyzed 1,4-Michael addition of α,β-unsaturated carbonyls, (ii) novel C-C bond formation via the use of pro-electrophilic amine and alcohol substrates─activated through collisions in the microdroplet environment to serve as Michael acceptors, and (iii) selective Hantzsch cascade reaction with cross-coupling products, avoiding side reactions including N-alkylation and self-coupling product formation. Milligram quantity product collection is achieved, showcasing plasma-microdroplet fusion as an effective tool for preparative-scale synthesis. Thus, the controlled generation of ROS via plasma discharge during charged water microdroplet evolution establishes a green synthetic method for uncatalyzed C-C bond formation.