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The M06/6-31+G(d) method has been used to study the reactivity of the two forms of curcumin (enol and keto). The energies needed for the three thermodynamic mechanisms: HAT, SET-PT and SPLET, have been calculated, in different solvents, to determine the most probable hydrogen atom transfer mechanism. The solvent effect is evaluated using an implicit solvation model (IEF-PCM). The results show the existence of an intramolecular hydrogen bond strength, in the enol form, which prevents the dissociation of hydrogen atom. In nonpolar solvent, the value of BDFE is lower than PA and IP; this means that HAT is the most favorable mechanism of the two forms of curcumin, while the SPLET mechanism is thermodynamically preferred in polar solvent. The UV/Vis spectra have been determined by the time-dependent density functional theory (TD-DFT) to show the maximum absorption wavelength value of curcumin and the nature of the excited states.

In this study, the molecular structure, tautomerization, conformational stability, and electronic energies of 25 enols and keto forms of methyl acetoacetate (MAA) were examined theoretically using DFT and experimentally by IR, Raman, and UV spectroscopy. According to the theoretical and experimental results, three more stable tautomer’s including an enol (2EU1-I) and two ketos (DK1 and DK2) forms in the gas phase, and solutions have been obtained. Using equilibrium ratios in the gas phase and different solvents at various levels of theory, the abundance of enol was determined. Outcomes indicated that the content of cis-enol or keto forms depended on to phase. In gas phase, cis-enol form is as major form. Conventional and unconventional intramolecular hydrogen bonds for cis-enol and keto forms, respectively, were considered via AIM. Also, using NBO in terms of E(2) values, the enol and keto forms, and the hydrogen bond strength interpreted. By recording the vibrational spectra in various polar and non-polar solvents, deuterated analog, deconvolution, and calculating potential energy distribution of cis-enol for MAA, the strong IR bands were obtained at about 1755–1715 cm−1 regions show the existence of two keto forms in the sample. Also, two medium intensity bands at 1654 and 1630 cm−1 were assigned to the chelated ring of cis-enol form. Finally, the UV spectra in different solvents and frontier molecular orbitals were shown that hydrogen bond strength in the enol form of MAA is weaker than AA. Also, the content of keto forms increased in polar solvents.

The reaction mechanisms of cyclic 1,3-diketones with trialkyl orthoformates and trimethyl orthoacetate in the absence of activators were theoretically and experimentally studied. The reactions proceed via C-C-coupling of the ionized forms of the reactants giving dialkyl acetals, which further transform into vinyl ethers. The reactions of the latter with ortho esters afford aldehydes (ketones). Related 2-acetylcyclopentanone reacts with trimethyl orthoacetate under argon to form a mixture of methyl enol ethers, whereas in the presence of air oxygen dimethyl glutarate is formed.

The enolization of simple carbonyl compounds is a key reaction for many chemical and biochemical processes. Numerous theoretical and experimental studies have been done to probe aspects of the mechanism of this reaction. In this work, the effect of small water clusters, (H2O)n: n=1-9, on the enol content of acetone is investigated by using density functional theory calculations at the M06 level of theory in the gas and solution phases. The calculations indicated that the formation of hydrogen-bonded assemblies between water clusters and both tautomers of acetone affect the enolization reaction. Among them, the trimeric water cluster has the highest binding energy difference (DEb) in the solution phase and greatly shift the equilibrium in the favor of the enol form. The results also shown that under this condition, the enol content of acetone increased by decreasing the polarity of the solvent. The practical conclusion of this study is that the enol content of carbonyl compounds can be maximized only by addition a defined amount of water.

The hydration of alkyne represents the most straightforward and simplest route toward the synthesis of ketone. Herein, Bronsted acidic zeolites are explored as potential catalysts for the liquid-phase phenylacetylene hydration. The topology structure and Si/Al ratio are disclosed to be key factors controlling the catalytic activity of zeolites. Typically, H-MFI zeolite with a Si/Al molar ratio of 13 exhibits the highest catalytic activity, with turnover frequency of 6.0 h−1 at 363 K. Besides, H-MFI zeolite shows good catalytic stability and recyclability in the reaction of phenylacetylene hydration, and the substrate scope can be simply extended to other soluble aromatic alkynes. The reaction mechanism of phenylacetylene hydration is investigated by means of kinetic and spectroscopic analyses. The Markovnikov electrophilic addition of phenylacetylene by hydrated protons is established as the rate-determining step, followed by deprotonation and enol isomerization to derive acetophenone product.

The self-condensation of cyclopentanone has been studied over calcined and uncalcined TiO2–ZrO2. The catalyst properties were examined by XRD, FTIR, SEM, N2 adsorption–desorption, and pyridine FTIR. Compared with calcined TiO2–ZrO2, uncalcined TiO2–ZrO2 exhibited superior catalytic performance (94% conversion of cyclopentanone and 86% yield of dimer). This might be because uncalcined TiO2–ZrO2 has both Lewis and Bronsted acids, while calcined TiO2–ZrO2 only contains Lewis acids. Kinetics analysis indicated that C–C coupling was the rate-limiting step on the two catalysts. For uncalcined TiO2–ZrO2, the C–C coupling occurred between the two species on the catalyst surface. Through the H bond, the cyclopentanone was firmly adsorbed on the catalyst surface by Bronsted acid sites, then the enol intermediate could attack another cyclopentanone polarized by adjacent Lewis acid sites. As a consequence, the coexistence of Bronsted and Lewis acids in catalysts exhibited enhanced activity in cyclopentanone self-condensation.

The photoinduced birefringence of two 4-substituted phthalimide 2-hydroxy Schif bases, containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties has been investigated in PMMA matrix. Their optical behaviour as nanocomposite flms was revealed by combined use of DFT quantum chemical calculations (in ground and excited state) and experimental optical spectroscopy (UV–Vis and fuorescence). The results have shown that solid-state reversible switching takes place by enol/keto tautomerization and Z/E isomerization. Birefringence study was performed in the PMMA nanocomposite flms using pump lasers at λrec = 355 nm and λrec = 442 nm. Fast response time and high stability of anisotropy up to 58% for (4) and 95% for (5) after turning of the excitation laser, was observed, which makes these materials appropriate candidates for cutting-edge optical information technology materials. The possibility for multiple cycles of recording, reading and optical erasure of the photoinduced birefringence at λrec = 442 nm in 5 has been demonstrated. The obtained results have shown that the maximum value of the measured birefringence is close to the anisotropic characteristics of the most frequently used azo materials.

Density functional theory (DFT) has been employed to study the structure, stability and reactivity of curcumin and some of its important analogues by computing HOMO-LUMO energy gap (H-L gap), ionization potential (IP), electron affinity (EA), chemical potential (μ), chemical hardness (η), electrophilicity index (ω) and density of states (DOS). In this study, curcumin is found to be more reactive in dimethyl sulfoxide (DMSO) solvent than in the gas phase in both of its enol and keto isomeric forms. Next, reactivities of the keto-based curcumin analogues are found to follow the order of 1,5-bis(3,4-dimethoxyphenyl)penta-1,4-dien-3-one (GO-035) > 1,5-bis(3,4,5-trimethoxyphenyl)penta-1,4-dien-3-one (GO-Y016) > 1,5-bis[3,5-bis(methoxymethoxy)phenyl]penta-1,4-dien-3-one (GO-Y031) > 1,5-bis[3,5-dimethoxy-4-(methoxymethoxy)phenyl]penta-1,4-dien-3-one (GO-Y030) > dibenzalacetone (DBA) in both gas and DMSO solvent phases. However, there is enhancement in reactivities of the complexes in DMSO solvent from those in the gas phase. The reactivity order is observed on the basis of H-L gap, IP, μ, η and DOS analyses. Moreover, GO-Y030, GO-Y031 and DBA possess similar electrophilic behaviours as indicated by their comparable EA and ω values, while GO-Y016 and GO-035 behave as the strongest and the weakest electrophiles. Furthermore, the nature of the reactive sites in the curcumin analogues has been identified via local reactivity descriptors and the molecular electrostatic potential (MEP) maps, results of which are consistent with the molecular orbital analyses.

Hydroxybenzophenone (HBP) molecules have their applications in commercial sunscreens as an UV chromophore because of their photostability and lack of reactivity after several hours of irradiation. They are usually photoexcited from the ground state chelated enol form to excited state where it undergoes proton transfer to chelated keto form and undergoes dihedral twisting to the ground state twisted keto form before going back to enol form. In this research, the three proposed isomers of five substituted 2-hydroxybenzophenones were modelled with additional three conformational changes in the ground state chelated enol and the results are related to the available experimental observations in solvents acetonitrile, n-dimethylformamide and dimethylsulfoxide. A significant difference in reduction potential, electron affinity and molecular properties was observed within the four ground state conformations and also the two excited-state proton transfer geometries for each of the five derivatives. The nature of the solvents also had significant effects on the electron affinity, HOMO, LUMO and energy bandgap of the molecules. The best possible conformation from the theoretical models that better described the experimental observations is the ground state trans chelated enol (named M1d) because of the close range of its reduction potential values and their strong level of correlation to that of the experiment and also a close range of its λmax values to that of the experiment.

The remarkable properties of deuterium have led to many exciting and favourable results in enhancing material properties, for applications in the physical, medical, and biological sciences. Deuterated isotopologues of avobenzone, a sunscreen active ingredient, were synthesised to examine for any changes to the equilibrium between the diketone and enol isomers, as well as their UV photostability and photoprotective properties. Prior to UV irradiation, deuteration of the diketone methylene/enol moiety (i.e. avobenzone-d2) led to an increase in the % diketone compared to non-deuterated, determined by 1H NMR experiments in CDCl3 and C6D12. This can be rationalised from two angles; mechanistically by a deuterium kinetic isotope effect for the CH vs. CD abstraction step during tautomerisation from the diketone to the enol, and a weaker chelating hydrogen bond for the enol when deuterated allowing increased equilibration to the diketone. Avobenzone-d2 was further examined by solid state 13C NMR. The higher % diketone for avobenzone-d2 was postulated to favour increased photodegradation by a non-reversible pathway. This was investigated by UV irradiation of the avobenzone isotopologues in C6D12, both in real time in situ within the NMR by fibre optic cable as well as ex situ using sunlight. An increase in the relative amount of photoproducts for avobenzone-d2 compared to non-deuterated was observed by 1H NMR upon UV irradiation ex situ. Overall, the study demonstrates that deuteration can be applied to alter complex equilibria, and has potential to be manifested as changes to the properties and behaviour of materials.

In the present work, a detailed investigation of synergistic effects between the intramolecular hydrogen bond (IMHB) and π-electron delocalization (π-ED) of 3-hydroxy prop-2-en thial (HPT) and its halogenated derivatives was performed. For this purpose, at first, the π-ED in the enol form of the benchmark systems by various aromaticity indices such as λ, λ, HOMA, NICS, PDI, ATI, and FLUπ were evaluated. On the other hand, the strength of IMHB by various descriptors such as energetical, geometrical, spectral, topological, and molecular orbital parameters was also estimated. For better understanding the nature of the synergistic phenomenon, we examined and compared the linear relationships between the π-ED indices with the HB descriptors. Our results show that the geometrical indicators have the best linear relationships with all of the mentioned HB parameters. Also, according to their absolute linear correlation coefficients, the following order is concluded: λ > λ > HOMA > FLUπ > ATI > NICS (1) > PDI > NICS (0)Finally, the synergistic effect between the π-ED and IMHB from the position and nature point of views is discussed. These results clearly show that the synergistic effect of R1 derivatives is negative, while the corresponding effects of R2 and R3 ones are positive. Moreover, the synergetic effects also depend on the nature of substitutions especially their electronegativity values (F > Br > Cl).

Organic acids play a key role in central metabolic functions of organisms, are crucial for understanding regulatory processes and are ubiquitous inside the cell. Therefore, quantification of these compounds provides a valuable approach for studying dynamics of metabolic processes, in particular when the organism faces changing environmental conditions. However, the extraction and analysis of organic acids can be challenging and validated methods available in this field are limited. In this study, we developed a method for the extraction and quantification of organic acids from microbial samples based on solid-phase extraction on a strong anionic exchange cartridge and gas chromatographic-mass spectrometric analysis. Full method validation was conducted to determine quality parameters of the new method. Recoveries for 12 of the 15 aromatic and aliphatic acids were between 100 and 111% and detection limits between 3 and 272 ng/mL. The ranges for the regression coefficients and process standard deviations for these compound classes were 0.9874–0.9994 and 0.04–0.69 μg/mL, respectively. Limitations were encountered when targeting aliphatic acids with hydroxy, oxo or enol ester functions. Finally, we demonstrated the applicability of the method on cell extracts of the bacterium Escherichia coli and the dinoflagellate Prorocentrum minimum. Graphical abstract Electronic supplementary material The online version of this article (10.1007/s00216-020-02883-3) contains supplementary material, which is available to authorized users.

We report a new facile synthesis procedure for the C-alkylation reaction of indole and indole derivative at 3-position with cyclic enol ethers in the presence of 2.5 mol% of I2 under mild conditions. The optimum composition of the reaction mixture and catalyst were determined for maximum yield of C-alkylated indole derivative with molar ratio of indole derivative: cyclic enol ether was fixed at 1.0 equivalence: 1.5 equivalence with the presence of 2.5 mol% of I2 in acetonitrile solvent and room temperature conditions. The results showed that under optimum conditions indole derivatives can be obtained good to excellent yields (80–92%) by the C-alkylation of indoles with cyclic enol ethers.

In this research, using nonequilibrium green’s function integrated with density functional theory, we investigate the electronic transport properties of a β-diketone (2,6-dimethyl-3,5-heptanedione) molecular wire induced by hydrogen transfer. The title molecule can be converted between two enol and keto forms. The electronic transmission factors, spatial spreading of molecular projected self-consistent Hamiltonian orbitals, on–off ratio, I–V characteristics, three different adsorption types (hollow, top, and bridge), the alteration of the electrode materials, Y, (Y = Au, Ag, and Pt), and HOMO–LUMO gaps relevant to these forms are thoroughly discussed. It can be concluded that due to the deformation of the title molecule (enol → keto), there is a noticeable change in conductivity. As a result of this deformation, the conductivity is switched from on state (high conductivity and low resistance) to off state (low conductivity and high resistance).

Chemoselective domino ring opening and selective O/S-alkylation of ethers/thioethers over enol C/O alkylation has been observed. Various 2-aryl chromanones/thiochromanones with alkyl/allyl bromides were smoothly converted into the corresponding highly functionalized ethers and thioethers in excellent yields with high selectivity. An unusual, chemoselective domino ring opening and selective ether/thioether O/S-alkylation over. An unusual, chemoselective domino ring opening and selective ether/thioether O/S alkylation over enol C/O alkylation has been observed.

Vitamin C or ascorbic acid is an indispensable micronutrient for human health found principally on citrus species such as lemon and orange fruits and vegetables. It was involved in the production of proteins such as collagen. Its biochemical mechanism is related to its antioxidant capacity; however, its function at the cellular level is still unclear. Several theoretical studies about antioxidant and redox mechanisms for ascorbic acid were suggested; however, no derivative was proposed. Thereby, an electronic study of antioxidant capacity for ascorbic acid derivatives was performed using theoretical chemistry at the DFT/ B3LYP/6-311 + + (2d,2p) level of theory. Simplified derivatives show that enol hydroxyls are more important than any other functional group. The vicinal enolic hydroxyl on β position is more important for antioxidant capacity of ascorbic than hydroxyl on α position. According to our molecular modifications, the keto-alkene compound showed the best values when compared to ascorbic acid in some molecular characteristics. No lactone derivatives have superior application potential as antioxidant when compared with ascorbic acid. Several structures are possible to be proposed and were related to spin density contributions and the increase of chemical stability. New promising structural derivatives related to ascorbic acid can be developed in the future.

The structure of nitisinone (β, β′-triketone reversible inhibitor of the enzyme 4-hydroxyphenylpyruvate dioxygenase) in various protic and aprotic solvents was studied using PMR and 13C NMR spectroscopy. Nitisinone was shown to exist in solutions at room temperature as the endo-enol tautomer while the exo-enol tautomer was not found. Prototropic ring tautomerism (migration of a proton between the 1- and 3-positions), the rate of which increases with increasing solvent polarity, occurs in the endo-enol tautomer. The mineral composition of the biological fluid does not affect the tautomeric equilibrium. The conformational rigidity of the nitisinone molecule accounts for the nonequivalence of the 1- and 3-positions in the cyclohexene fragment from the standpoint of binding of nitisinone to the Fe(II)*4-hydroxyphenylpyruvate dioxygenase complex.

The thermodynamic aspects of keto-enol tautomerism of hyperforin were investigated theoretically using density functional theory methods. At the B3LYP/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ level of theory the enol tautomer dominates the tautomeric mixture and the second enol tautomer 1OH-HB has Gibbs free energy higher by 1.2 kcal/mol, despite possessing an intramolecular hydrogen bond. The purely keto tautomer is less stable by 3.3 kcal/mol compared with the 1OH tautomer, which means that the percentage of the keto tautomer in the tautomeric mixture is only about 0.4%. This is a different picture than in the parent compound of hyperforin—the phloroglucinol, where the keto tautomer is more stable than corresponding enol 1OH tautomer by 0.6 kcal/mol. To explain this difference, several in-between model molecules reflecting gradual transformation from phloroglucinol to hyperforin were build, and all the tautomeric forms were optimized for each molecule. It turned out that the addition of an aliphatic three-carbon bridge to phloroglucinol ring is crucial for the reversal of the tautomer stability order to that for hyperforin. The probable reason is the unfavorable strain in the keto tautomer introduced by the carbon bridge, which forces a specific geometric configuration which destabilizes in consequence the keto tautomer. This picture of hyperforin tautomerism underlines the dominance of enol tautomers, which can be important when studying the antidepressant activity of hyperforin—its interactions with neurotransmitters receptors.

In the solid state, some 3-substituted 4-hydroxycoumarins β-ketoester enols form infinite translational hydrogen-bonded β-chains with varying degrees of alignment between adjacent delocalized systems. Nine related structures have been studied. At the strongest, intermolecular associations are polar, purely translation neighbors interact essentially along a 717 pm crystallographic repeat with shortened 260 pm intermolecular O·OH-bond contacts. Four distinctive features characterize these structures: (1) moderately delocalized β-ketoester enol structures, (2) translational misalignment angles between oxygen donors and acceptors less than 10°, (3) buttressing intermolecular C–H·O contacts co-planar with and near the intermolecular O–H·O interactions, and (4) fully extended ketoester enol hydrogen-bond (ap-anti-anti) geometries. For non-polar β-chains in related coumarin systems, β-ketoester enol alignments are typically poorer, involve hydrogen-bonding between glide relatives, ap-syn-(anti) geometry, and the intermolecular O·OH-bond contacts are longer. Substituted 4-hydroxycoumarins related to phenprocoumon can form well-aligned polar translational β-chains between enolones showing resonance assisted Hydrogen-bonding and a 717 pm repeat along a crystallographic axis.

This contribution reports an efficient approach for preparing polycarbonate block terpolymers by immortal stepwise copolymerization of CO2 with different epoxides in the presence of enol chain transfer, mediated by robust cobalt catalyst systems consisting of the fluorine substituted salenCo(III)NO3 or biphenol-linker bimetallic Co(III) complex in conjunction with an ionic cocatalyst, PPNX (PPN = bis(triphenylphosphine)iminium, X = NO3− or 2,4-dinitrophenoxide). Various polycarbonate block terpolymers were obtained in perfectly unimodal distribution of their molecular weights with narrow polydispersity. They all possessed only one broad glass transition temperature, which could be adjusted by altering the length of different polycarbonate segments.

The tandem Cope rearrangement and [2 + 2] cycloaddition of cis-1,2-diethynyl-1,2-dihydroxycyclopropane and its mono-hetero analogues have been investigated at the B3LYP/6-31+G* level. The presence of the hydroxy group lowers the activation enthalpies for the Cope rearrangement, whereas activation enthalpies for the [2 + 2] cycloaddition are raised as compared to those for their non-hydroxy derivatives. The NBO analysis indicates that in the transition structure involved in the Cope rearrangement, lone pairs of the oxygen atoms of the hydroxy groups are transferred into the σ* C–C bond undergoing migration, as a result of which it is weakened. On the other hand, the lone pairs of the oxygen atoms interact with the π* C=C orbitals of the bis-allenic systems in the intermediate thereby stabilizing it and, thus, suppressing its driving ability for the [2 + 2] cycloaddition. In the products so formed, 6π electrons are delocalized conferring stability on them, which is further augmented by extended conjugation with the hydroxy groups. Due to high stability of these products, activation barrier for the change of enol into ketone is very high.

Catalytic asymmetric electrosynthesis combines the unique features of an electrochemical addition or removal of electrons with the catalytic asymmetric synthesis of enantioenriched molecules. However, identifying suitable catalysts that are compatible with electrochemical conditions and provide a high stereocontrol is a formidable challenge. Here we introduce a versatile electricity-driven chiral Lewis acid catalysis for the oxidative cross-coupling of 2-acyl imidazoles with silyl enol ethers. Powered by an electric current, this work provides a sustainable avenue to synthetically useful non-racemic 1,4-dicarbonyls, which include products that bear all-carbon quaternary stereocentres. A chiral-at-metal rhodium catalyst activates a substrate towards anodic oxidation by raising the highest occupied molecular orbital on enolate formation, which enables mild redox conditions, high chemo- and enantioselectivities (up to >99% enantiomeric excess) and a broad substrate scope. This work demonstrates the potential of combining asymmetric Lewis acid catalysis with electrochemistry and we anticipate that it will spur the further development of catalytic asymmetric electrosynthesis. The use of electrochemistry in asymmetric catalysis can prove challenging, not least due to the difficulty of achieving chemo- and stereoselectivity in combination with very reactive electrochemically generated intermediates. Here, catalytic asymmetric electrosynthesis is reported for the synthesis of 1,4-dicarbonyl compounds with high enantiomeric excess, including compounds with all-carbon quaternary stereocentres. The chiral-at-metal catalyst activates the substrate towards anodic oxidation in addition to controlling the enantioselectivity of the process.

In this paper, we aim to determine whether the N7-methylation can influence the base pairing properties of guanine by promoting the formation of guanine enol-tautomers. The keto- to -enol-tautomerization of N7-methylguanine (N7mG) and its base pairing patterns with all the canonical DNA bases have been investigated at the M06-2X/6-311+G(d,p) level of density functional theory. The barrier free energy calculations reveal that N7-methylation does not promote the keto- to enol- tautomerization of guanine. The Watson-Crick-like enol-N7mG:T1 or enol-N7mG:T2 base pair similar to what is observed experimentally is found to be energetically more stable than the keto-N7mG:T base pairs. However, the keto-N7mG:C1 which is structurally similar to the canonical G:C base pair is the most stable base pair among all the base pairs studied here. Thus, our calculations predict that N7mG would pair preferably with cytosine during DNA replication but there is also a probability that it can cause mutation through mispairing with thymine, in agreement with experimental observations.

The keto-enol tautomerisation in homophthalic anhydride (HA) is investigated in the ground (S0) and excited (S1) electronic states. The keto form with a dicarbonyl structure is found to be the most stable form in S0 and enol form with a monocarbonyl structure in S1 indicating an excited state intramolecular proton transfer (ESIPT) process. The computed results show consistency with the change in basis sets and methods of calculations. Apart from the two tautomers, transition states are also identified. The barrier to interconversion is found to reduce substantially in S1. Internal reaction coordinate (IRC) calculations confirm the pathway of interconversion between the two forms in S0 and S1. The observed FT-IR spectra corroborate well with our computed spectra. The appearance of two strong lines around 1800 cm−1 confirms the lowest energy structure to be the keto tautomer with a dicarbonyl form in S0. Our computations corroborate well with the crystal structure data for an analogous molecule. Electron distribution in HOMO and LUMO indicate the excitation process as π → π* in nature. The qualitative chemical concepts like hardness and electrophilicity are calculated to estimate the stability of the tautomers. The energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.

We report thermodynamic, geometric, and electronic parameters for the interaction between neutral ligands and the [Hg(H2O)]2+ dication, using the B3LYP/6–311 + G(d,p) approach. Gibbs free energies for the interaction were employed to rank the affinity order of the several neutral ligands. To identify the parameters that characterize the affinity between the two fragments, the metal-ligand interaction was analyzed according to the EDA, NBO, and QTAIM decomposition schemes. The phosphine oxide showed the highest affinity for the Hg(H2O)2+ dication, mainly due to the P=O bond polarization. Ligands containing the sulfur atom, characterized by a high covalent component for the metal-ligand interaction, are the following in the interaction order. According to the Gibbs free energy for substitution of one water molecule in the [Hg(H2O)2]2+ complex, the sequence for the affinity order is: phosphine oxide > thioketone > thioesther > lactam > amide > amine > carboxylic acid > thiophene > ketone > esther > thiol > thiocyanate > ammonia > disulfide > aldehyde > ether > haloydrin > alcohol > enol > azide.

In the present study, solvent effects on the molecular stability, intramolecular hydrogen bond (IMHB), and π-electron delocalization (π-ED) in some of the simple resonance-assisted hydrogen bond (RAHB) systems with different donors and acceptors are investigated. In this regard, all of the H-bonded structures (enol, thiol, and selenol) in the gas phase and presence of water/DMSO solvents with polarizable continuum (PCM)/self-consistent isodensity PCM methods at the M06-2X/6-311G++(d,p) level of theory are optimized. Relative energies clearly indicate that the enol conformers are more stable than the thiol and selenol ones in the gas phase while in the presence of polar solvents, the thiol structures are more stable than the other members. In the RAHB systems, the IMHB, π-ED, and tautomerization process are important factors in determining the stability of the H-bonded conformers. A survey of tautomeric equilibriums clearly shows that the tautomerization/activation energy of thio ⇋ thiol equilibrium is greater/smaller than the other equilibriums. These results reveal that the thiol tautomer is more preferred than the enol and selenol ones, from kinetic and thermodynamic points of view. Estimation of the IMHB by different descriptors emphasizes the presence of a stronger IMHB in the enol conformers. Moreover, the π-ED of enol tautomers are greater than those of the thiol and selenol ones. The results of IMHB and π-ED descriptors are compatible/incompatible with the stability order of the gas phase/solvents. Consequently, one can conclude that in the gas phase, the IMHB and π-ED are superior factors to determine the stability of tautomers. But in the presence of polar solvents, the tautomerization process is a dominant factor.

Due to their structural diversity and variety of biological activities, marine natural products have been the subject of extensive study. These compounds, especially phospholipid polycyclic aromatic hydrocarbons, have a wide range of pharmacological applications, including embedded DNA and central nervous system, anti-tumor, anti-virus, anti-parasite, anti-bacterial, and antithrombotic effects. Unfortunately, the insufficient drug sources have limited the development of these compounds. In this study, we isolated salinpostin compounds from a fermentation solution of marine-derived Salinospora sp., which has a common bicyclic enol-phosphotriester core framework, as well as potent and selective antimalarial activities against P. falciparum with EC50 = 50 nmol L−1. The chemical synthesis of these compounds in greater quantities is necessary for their use in bioactivity studies. Thus we explored a short route with high yields and mild reaction conditions, which can generate combinatorial libraries for drug discovery and lead optimization. We developed a new total synthesis method for six cyclic enol-phosphotriester salinipotin compounds and their diastereomers. For the total synthesis of cyclipostin P, we prepared cyclic enol-phosphotriester salinipostin compounds in 10 steps from a readily accessible starting material, 1,3-dihydroxyacetone, and obtained an overall yield of 1.29%. We fully characterized these compounds by proton nuclear magnetic resonance (1H-NMR), carbon-13 NMR (13C-NMR), and high-resolution mass spectrometry (HRMS) analyses, and found they coincide absolutely with the same compounds reported previously.

2-Cinnamoylimidazolium ions 4 have been synthesized by treatment of 2-cinnamoylimidazoles 8 with methyl triflate. They were characterised by NMR and mass spectroscopy, in one case (4f) also by X-ray analysis. The kinetics of their reactions [and also those of cinnamoyl fluoride (1)] with stabilised carbanions 9a–e and silyl ketene acetal 9f (reference nucleophiles) were measured photometrically. The correlation log k(20 °C) = sN (E + N) was used to calculate the electrophilicity parameters E of the cinnamoyl azolium ions 4 from the resulting second-order rate constants k and the previously reported N and sN parameters of the reference nucleophiles 9. All 2-cinnamoylimidazolium ions 4 were found to be 2–4 orders of magnitude more electrophilic than cinnamoyl fluoride (1) showing that the direct attack of nucleophiles at 1 can be avoided if sufficient concentrations of 4 are produced in the NHC-catalysed reactions of 1 with nucleophiles. From the range of electrophilicity(–12

In the present study, the conformational analysis, intramolecular hydrogen bond, π-electron delocalization, and tautomeric equilibriums in 2-selenoformyl-3-thioxo-propionaldehyde (STP) were investigated. First, conformational analysis of STP was performed, and our results have shown that the chelated thiol structures are more stable than the chelated enol and selenol ones; HB Thiol < HB Enol < HB Selenol. To justify this order, the affective factors on this stability such as tautomerism, hydrogen bonding, and π-electron delocalization were comprehensively investigated. Survey of tautomeric equilibriums in STP indicates that destabilization energy of keto⇋enol equilibrium (about 50 kJ/mol) is more than the seleno⇋selenol (about 22 kJ/mol) and then thio⇋thiol (about 20 kJ/mol) cases. Description of different hydrogen bonds by energetic, geometrical, topological, and molecular orbitals emphasizes on the presence of a stronger intramolecular hydrogen bond (IMHB) in the enol forms. Furthermore, evaluation of π-electron delocalization (π-ED) represents approximate superiority of enol conformers. The achieved results by HB and π-ED analyzes are in contrast with the mentioned stability order. Ultimately, investigation of above factors indicates that the tautomerism phenomenon plays a dominant role in determining of the conformational preferences.

A summary of the most recent applications of malonyl peroxides as reagents in organic synthesis is discussed. The microreview covers the latest selected examples on the usage of malonyl peroxides: dioxygenation and oxyamination of alkenes; oxidative C–O coupling with arenes, enol ethers, 1,3-dicarbonyl and N-heterocyclic compounds; sulfoxide synthesis.

A hydrazone Schiff base ligand was synthesized by the condensation of 3-formyl-4-hydroxycoumarin and oxalyldihydrazide in the molar ratio 2:1. The Schiff base ligand acts as a mono-, bi-, tri- or even tetradentate ligand with metal cations in the molar ratios 1:1 or 2:1 (M:L) to yield either mono- or binuclear complexes as keto or enol isomers, where M = Co(II), Ni(II), Cu(II), VO(IV), and Fe(III). The ligand and its metal complexes were characterized by elemental analyses, IR, 1H NMR, mass, and UV–Vis spectroscopy. Furthermore, the magnetic moments were calculated from the measured electric conductivities of the complexes. According to the received data, the dihydrazone ligand contains one or two units of ONO domains and can bind to the metal ions via the azomethine nitrogen, the carbonyl oxygen atoms, and/or the phenolic oxygen atoms. Electronic spectra and the magnetic moments of all complexes show that the complexes’ geometries are either octahedral, tetrahedral, square planar, or square pyramidal. Cyclic voltammograms of the mononuclear Co(II) and Ni(II) complexes show quasi-reversible peaks. Tests against two pathogenic bacteria as Gram-positive and Gram-negative bacteria for both, the Schiff base ligand and its metal complexes were carried out. In addition, also one kind of fungi was tested. The synthesized complexes demonstrate mild antibacterial and antifungal activities against these organisms. Graphical abstract Electronic supplementary material The online version of this article (doi:10.1007/s00706-017-2075-9) contains supplementary material, which is available to authorized users.

Regioisomeric fluorine-containing 6,7-dihydro-1,2-benzisoxazol-4(5Н)-ones and 6,7-dihydro-2,1-benzisoxazol-4(5Н)-ones were prepared by reactions of 2-(fluorobenzoyl)cyclohexane-1,3-diones or their methyl enol ethers with hydroxylamine.

Aryldiketo acids (ADKs) exhibit the variety of biological activities, mainly due to large affinity toward divalent metal ions. Metal complexation ability of ADKs, as well as interactions with proteins, depend on tautomeric form present in solution. The main aim of this study was to fully explore the tautomeric preferences of 4-phenyl-2,4-dioxobutanoic acid (4PDA), as ADKs representative, in aqueous media at different pH values. 1D and 2D NMR spectroscopy in combination with quantum chemical calculations was applied in order to better understand the tautomeric preferences of 4PDA. The data in highly acidic media are especially interesting since there are no such findings in the literature due to low solubility of ADKs in molecular form. At low pH values, where 4PDA is unionized, the most abundant tautomeric form is enol with keto group closer to phenyl ring. At higher pH values, mixture of two 4PDA ionic forms coexists in solution. Their ratio calculated according to NMR data fits the values predicted using two experimentally determined pK (a) values. Based on the complexity of H-1 NMR spectrum of monoanionic 4PDA form, coexistence of two stable rotamers was assumed. In an alkaline media, 4PDA is mostly present in dianionic form. As pi-electrons of dianion are delocalized over an entire keto-enol moiety, spectral distinction between tautomers was not possible. Quantum chemical calculations were used to predict relative stability of tautomers. The predictions were in good accordance with experimental results only in case when explicit water molecule was included in calculations. This is the peer-reviewed version of the article: Cvijetić, I.N., Pešić, M.P., Todorov, M.D. et al. Struct Chem (2018) 29: 423. [https://doi.org/10.1007/s11224-017-1039-3] [http://cer.ihtm.bg.ac.rs/handle/123456789/2693]

Englerin A, a guaiane sesquiterpene isolated from Phyllanthus engleri, showed highly potent and selective growth inhibitory activities against renal cancer cell lines. We synthesized the key tricyclic intermediate from commercially available 2,2-dimethyl-1,3-dioxan-5-one via regio- and diastereoselective [4+3] cycloaddition between the formyl enol silyl ether and the disubstituted furan, in 4.8% total yield over 10 steps.

The reduction of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) with an excess of europium metal in 1,2-dimethoxyethane (dme) produces a divalent europium complex with the dpp-bian dianion, [(dpp-bian)Eu(dme)2] (1). The reactions of 1 with phenyl-acetylene and camphor proceed via protonation of the diimine ligand to form the monomeric amido-amino complexes of divalent europium — [H(dpp-bian)Eu(C≡CPh)(dme)2] (2) and [H(dpp-bian)Eu(camphor)(dme)2] (3), respectively. Compounds 2 and 3 were characterized by IR spectroscopy and elemental analysis. Their molecular structures were determined by X-ray diffraction. Compounds 2 and 3 were shown to be monomeric seven-coordinate europium(ii) complexes with terminal phenylethynyl and enol ligands, respectively. According to the IR spectroscopic data, the terminal ligands in complexes 2 and 3 undergo tautomerization involving backward proton transfer from the amido-amino ligand to the substrate. The magnetic moment of compound 2 (8.03 μB) remains constant in the temperature range of 4—300 К and confirms the presence of divalent europium.

The geometric and energetic preference of the possible isomers of four tautomeric triazine derivatives has been investigated by density functional theory and topological calculations and their thermodynamic stability was assessed. For the monoxime series the amino keto tautomer is favored by 2.6 kcal/mol (10.9 kJ/mol) as compared to the enol form, while the lowest energy imino isomer is 4.1 kcal/mol (17.1 kJ/mol) higher in energy. However, for the dioxime tautomer series the imino keto tautomer is 4.2 kcal/mol (17.6 kJ/mol) lower in energy as compared to the most stable amino keto tautomer. The computed excitation energies for the first electron transitions agree with the experimental observations.

The α-H acidity is an important chemical property of ketones that has attracted much research interest. Theoretical prediction of pKa for ketone α-H is significant. In this work, we theoretically studied the nuclear shielding of various α-Hs in a set of ketones and that of the corresponding enolic hydroxyl Hs in tautomeric enol forms. It has been demonstrated through linear regression analyses that the pKa values of these ketones correlate with both sets of the calculated nuclear shielding values. The correlation coefficient R2 of the linear correlation relationship is 0.90. The present work has provided a new approach to computationally evaluating the acidity of α-Hs in ketones, enabling us to semi-empirically predict the ketone α-H acidity from the calculated nuclear shielding values.

4′-methoxy-3-hydroxyflavone (5-HB), 2-(5-carboxyphenyl)-2-hydroxyphenyl) benzothiazole (6-HB) and o-LHBDI (7-HB), which have five-, six- and seven-membered intramolecular H-bonding ring between proton donor and proton acceptor, respectively, were chosen to investigate excited state intramolecular proton transfer (ESIPT) process in the gas phase by using density functional theory and time-dependent density functional theory methods. The intramolecular H-bond is strengthened in the excited state on account of the structural parameters and IR vibrational frequencies of the related group. The enhanced intramolecular H-bond is favorable of ESIPT process to convert enol form into keto form. 7-HB has a high chemical activity and low kinetic stability by analyzing the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. The calculated absorption and fluorescence spectra are in agreement with the experimental values. The potential energy curves (PECs) of 5-HB, 6-HB and 7-HB in the S0 and S1 states are scanned by altering O1–H2 distance in increment of 0.05 A. Our PECs results indicate that ESIPT happens easily in the S1 state with a very small barrier. The rate of ESIPT process follows this order: 6-HB~7-HB > 5-HB.

A DFT method was used to enlighten the reaction mechanism between dimethylacetylendicarboxylate (DMAD) and acetylacetone (ACAC) in the presence of triphenylarsine (TPA) as an efficient catalyst. Different paths of mechanism and transition states with possible intermediates were proposed and evaluated thermodynamically and kinetically. The results showed that the reaction starts with a nucleophilic attack by the TPA on the DMAD, and then it was followed by other four steps. For all of the intermediates, Z conformer was more favorable than E conformer, and also the trans configuration of the product was more favorable than of cis configuration. Enol form of ACAC was more preferred than keto form in step4, but keto form was favorable for the other steps. For this reason, the reaction mechanism couldn’t proceed through the enol-form, and the mechanism regarding keto-form of ACAC was the right mechanism. There were two possibilities for the proton-transferring step4 which lead to different products. Trans di (methoxycarbonyl)-3,3-diacetylcyclopropane product 3, which was the result of 1, 3 non-linear proton-transfer arrangement and (Z)-Alkene dimethyl 2-(pentane-2,4-dione) fumarate (product 4) which was produced by 1, 2 non-linear proton-transferring arrangement. Thermodynamic parameters exhibited that Z-Alkene is more stable than cyclopropane product, but cyclopropane product was more preferred kinetically (Ea trans cyclopropane = 44.75 kcal mol−1, Ea Alkene = 66.75 kcal mol−1).

Generation of alkenes through decarboxyolefination of alkane carboxylates has significant synthetic value in view of the easy availability of a variety of carboxylic acids and the synthetic versatility of alkenes. Herein we report that palladium catalysts under irradiation with blue LEDs (440 nm) catalyze decarboxylative desaturation of a variety of aliphatic carboxylates to generate aliphatic alkenes, styrenes, enol ethers, enamides, and peptide enamides under mild conditions. The selection of a dual phosphine ligand system is the key enabler for the successful development of this reaction. The Pd-catalyzed decarboxylative desaturation is utilized to achieve a three-step divergent synthesis of Chondriamide A and Chondriamide C in overall 68% yield from simple starting materials. Mechanistic studies suggest that, distinct from palladium catalysis under thermal condition, irradiation-induced palladium catalysis involves irradiation-induced single-electron transfer and dynamic ligand-dissociation/association process to allow two phosphine ligand to work synergistically., Production of alkenes through decarboxyolefination of carboxylates is a synthetically useful process. Here, the authors report a palladium/dual ligand-enabled decarboxylative desaturation reaction under mild irradiation conditions and apply it to the synthesis of Chondriamide A and C.

Structural modification of curcumin represents a strategy to improve its stability, water solubility, pharmaceutical properties and bioactivity. In this context, numerous structural analogues of curcumin, including curcuminoids, have been developed. In this paper, the precise density functional theory computations were used for investigating the electronic and geometrical structure of curcumin and some of its derivatives. The chemical activity of the considered molecules was investigated with the help of the global softness and hardness concepts. Among the studied molecules, bisdemethoxycurcumin had the most chemical activity and hexahydrocurcumin had the most stable structure. Among two isomers of the curcumin, the enol isomer was found to be active.

The preparation and the photophysical behaviour of two benzoxazinone derivatives isomers 2-(1-hydroxynaphthalen-2-yl)-4H-benzo[e][1, 3]oxazin-4-one(1) and 2-(3-hydroxynaphthalen-2-yl)-4H-benzo[e][1, 3]oxazin-4-one(2) designed for displaying were reported. The effect of substituent position and solvent effect on the excited state intramolecular proton transfer (ESIPT) dynamics and the spectroscopic properties were investigated using a combined theoretical (i.e., time-dependent density function theory (DFT)) and experimental (i.e., steady-state absorption and emission spectra and time-resolved fluorescence spectra) study. The results showed that compound 1 would facilitate ESIPT process and favored the keto tautomer emission, while compound 2 suppressed the ESIPT process and favored the enol emission.

The DNA-dependent protein kinase and phosphoinositide 3-kinase family is one of the most frequently activated enzymes in a wide range of human cancers; consequently, inhibition of DNA-dependent protein kinase and phosphoinositide 3-kinase represents an approach for cancer therapy. In this work, we have designed and synthesized a series of novel 7- or 8-(N-substituted)-2-morpholino quinazolines 3a–f, 5a–e, 7a–e, and 9 from 7- or 8-amino-2-morpholino quinazolin-4-ones (2a, 4a), the 3-methyl analogues (2b, 4b) and the 4-alkyloxy analogues (6a–b, 8). The compounds were subsequently assayed for DNA-dependent protein kinase and phosphoinositide 3-kinase activity. Most compounds were less active than expected in spite of the strong structural resemblance to the previously studied 7- or 8-(O-substituted)-2-morpholino-1,3-benzoxazine inhibitors. Loss of DNA-dependent protein kinase activity for the quinazolin-4-ones (3a–d and 5a–d) has been attributed to tautomerization to the aromatic enol (4-OH) tautomers. Aromatization of the heterocyclic ring could alter the conformation, and thus binding position, resulting in reduced compound-receptor hydrogen bonding of the morpholine oxygen and 4-carbonyl oxygen. The hetero-aromatic compounds 7a–e and 9 also did not show any DNA-dependent protein kinase activity at 10 µM, which supports the above hypothesis. Compound 7c (R=CH2(pyridine-4-yl)) displayed selective phosphoinositide 3-kinase delta activity with 80 % inhibition at 10 µM. Similarly, compounds 5a (8-N-substituted, R=CH2Ph) and 3a (7-N-substituted, R=CH2Ph) showed selective phosphoinositide 3-kinase beta activity with 69 and 61 % inhibition, respectively. Antiplatelet inhibition assays showed that compound 7e with the 4-O-benzyloxy group and 8-CH2(pyridine-3-yl) substituents was found to be the most active (IC50 35 µM).

Excited state intramolecular proton transfer (ESIPT) time-constants of 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in high n-alcohols--1-butanol, 1-hexanol and 1-decanol--were measured to be 90 ps, 130 ps and 190 ps, respectively, which are unusually slow. At the same time, the solvation time-constants of the DMA3HF enol in the same set of solvents were measured as 100 ps, 150 ps and >300 ps, respectively. Thus, both the ESIPT and enol solvation time-constants in high n-alcohols increase monotonically with the alkyl chain-length of the solvent, although the increase is not strictly proportional. It appears that the H-bonding capacity of the solvent is the single major factor influencing both processes, causing them to become closely correlated. Solvation causes a drastic change in the solvent molecular configuration around the excited enol, E*, inducing the breakage of DMA3HF···solvent inter-molecular H-bonding, which in turn promotes ESIPT. Following previously reported theoretical work on ESIPT, a qualitative description of the S1 potential energy surface can be formulated, where the involvement of solvent relaxation with the ESIPT process is explained.

A series of B3LYP/6-311+G(d,p) calculations of the affinity of monodentate ligands for [Cd(H2O)3]2+ are performed. Three types of ligands containing O (phosphine oxide, lactam, amide, carboxylic acid, ester, ketone, aldehyde, ether, halohydrin, enol, furan), N (thiocyanate, amine, ammonia, azide), and S (thioester, thioketone, thiol, thiophene, disulfide) interacting atoms are investigated. The results show that phosphine oxide has the largest affinity for the cadmium cation due to the polarization of the P=O bond. As the P atom has a large atomic radius, the O atom can polarize the electronic cloud enhancing its amount of electronic charge and favoring the interaction with Cd2+. The affinity order found is phosphine oxide > thioester > lactam > amide > carboxylic acid > ester > thioketone > ketone > thiocyanate > amine > ammonia > aldehyde > ether > thiol > thiophene > enol > halohydrin > disulfide > azide > furan ligands. These results were also corroborated by the functional M06-2X. The electronic effects (resonance and induction) of neighboring groups of the interacting atom modulate the strength of metal-ligand binding. For almost all the O-donor ligands the electrostatic component has the same magnitude as the covalent term, while for the N- and S-donor ligands the covalent term is predominant. The polarization term accounts for twice the exchange term as part of the covalent component. The dispersion term varies less than 2 kcal mol−1 for the complexes analyzed. The Pauli repulsion term is correlated with the metal ligand distance, increasing in the compounds with decreased metal-ligand bond length. The charge between the interacting atoms is also strongly correlated with both the interacting strength and the electrostatic interaction component. The natural bond orbital analysis highlights correlations of the bond order, and S and P contributions of the interacting metal-ligand orbital with the coordination strength.

The addition of carboxylic acids onto 1-hexyne was catalyzed by [RuCl2 (p-cymene) (PTA)] complex to give enol esters. Accordingly, the Markovnikov addition-type product was selectively obtained. The microwave-assisted reactions at 160 °C confirmed the influence of the temperature, where quantitative to excellent yields were obtained in 30 min. The catalyst loading test proved that [RuCl2 (p-cymene) (PTA)] complex can be efficiently used with a 1/1500 ratio to carboxylic acid providing the TON value of 1500.

The development of general catalytic methods for the regio- and stereoselective construction of chiral N-heterocycles in a diversity-oriented fashion remains a formidable challenge in organic synthesis. N-heterocyclic carbene (NHC) catalysis has been shown to produce a variety of outcomes, but control of the reactivity has rarely been demonstrated. Here we report a switchable catalytic activation of enals with aromatic azomethine imines that provides high selectivity using NHC organocatalysts. The original selectivity corresponds to the acidity of the base used in the reaction. The catalytically generated chiral homoenolate or enol intermediate undergoes enantioselective annulation with electrophiles such as N-iminoquinolinium ylides, N-iminoisoquinolinium ylides and β-N-iminocarboline ylides. The good-to-high overall yields, high regioselectivities and excellent enantioselectivities observed are controlled by the catalyst and reaction conditions.

This study describes a simple, efficient, and environmentally benign procedure for synthesizing a novel series of 13 N-(pyrimidinyl)-N-(1H-benzo[d]imidazolyl)amines, from the cyclocondensation reaction of (benzo[d]imidazolyl)guanidine with 4-alkoxy-4-alkyl(aryl/heteroaryl)-1,1,1-trifluoroalk-3-en-2-ones or 2,2,2-trifluoro-1-(2-methoxycyclohexen-1-en-1-yl)ethanone. The (benzo[d]imidazolyl)guanidine precursor was earlier obtained from the reactions of cyanoguanidine with o-phenylenediamine. The trifluoroacetylated enol ethers were synthesized from the trifluoroacetylation reaction of enol ethers or acetals with trifluoroacetic anhydride. The main reactions were performed in pure refluxing water as the solvent (1–24 h), without catalysts, and the corresponding new N-(pyrimidinyl)-N-(1H-benzo[d]imidazolyl)amines were obtained at good to excellent yields (60–90 %). Subsequently, N-alkylation reactions were performed to obtain tertiary amines and to increase the solubility of the amines.

The excited-state intramolecular proton transfer (ESIPT) reaction of 3-hydroxyflavone (3-HF) in methylcyclohexane solvent has been investigated by using the DFT and TD-DFT methods. The geometric structure, IR vibrational spectra, frontier molecular orbitals, natural bond orbital, and potential energy curves in the ground state (S-0) and first excited state (S-1) are analyzed to reveal the mechanism of proton transfer. The results demonstrate that there are enol- and keto- two isomers for 3-HF in the S-1, which is accorded with the experimental double fluorescence bands. The 3-HF-enol can be isomerized into 3-HF-keto via ESIPT. The mechanism of proton transfer is attributed to the strengthening of hydrogen bond originated from intramolecular charge transfer. The potential energy curves in the S-0 and S-1 states also illuminate the tautomerism mechanism between 3-HF-enol and 3-HF-keto, and the ground-state 3-HF-keto might not exist long and is isomerized mostly into the 3-HF-enol due to its high energy or instability. This is the reason that only one absorption peak is observed for 3-HF in experiment.

The Schiff base compound (E)-1-(4-(4-(diethylamino)-2-hydroxybenzylideneamino)phenyl)ethanone has been synthesized and characterized by IR, UV–Vis, and 1H and 13C NMR techniques. These properties of title compound were also investigated from calculative point of view. UV–Vis spectra of the title compound were recorded in different organic solvents to investigate the dependence of tautomerism on solvent types. Calculated results reveal that its enol form is more stable than its keto form. A detailed analysis of the nature of the hydrogen bonding, using topological parameters, evaluated at bond critical points.