Effects of Hydrogen Bonding to Amines on the Phenol/Phenoxyl Radical Oxidation

Electrostatically tuned phenols: a scalable organocatalyst for transfer hydrogenation and tandem reductive alkylation of N-heteroarenes

One of the fundamental aims in catalysis research is to understand what makes a certain scaffold perform better as a catalyst than another. For instance, in nature enzymes act as versatile catalysts, providing a starting point for researchers to understand how to achieve superior performance by positioning the substrate close to the catalyst using non-covalent interactions. However, translating this information to a non-biological catalyst is a challenging task. Here, we report a simple and scalable electrostatically tuned phenol (ETP) as an organocatalyst for transfer hydrogenation of N-arenes using the Hantzsch ester as a hydride source. The biomimetic catalyst (1-5 mol%) displays potential catalytic activity to prepare diverse tetrahydroquinoline derivatives with good to excellent conversion under ambient reaction conditions. Kinetic studies reveal that the ETP is 130-fold faster than the uncharged counterpart, towards completion of the reaction. Control experiments and NMR spectroscopic investigations elucidate the role of the charged environment in the catalytic transformation.

Remote Stereoelectronic Effects in Pyrrolidone- and Caprolactam-Substituted Phenols: Discrepancies in Antioxidant Properties Evaluated by Electrochemical Oxidation and H-Atom Transfer Reactivity

New antioxidants are commonly evaluated via two main approaches, i.e., the ability to donate an electron and the ability to intercept free radicals. We compared these approaches by evaluating the properties of 11 compounds containing both antioxidant moieties (mono- and polyphenols) and auxiliary pharmacophores (pyrrolidone and caprolactam). Several common antioxidants, such as butylated hydroxytoluene (BHT), 2,3,5-trimethylphenol (TMP), quercetin, and dihydroquercetin, were added for comparison. The antioxidant properties of these compounds were determined by their rates of reaction with 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and their oxidation potentials from cyclic voltammetry. Although these methods test different chemical properties, their results correlate reasonably well. However, several exceptions exist where the two methods give opposite predictions! One of them is the different behavior of mono- and polyphenols: polyphenols can react with DPPH more than an order of magnitude faster than monophenols of a similar oxidation potential. The second exception stems from the size of a "bystander" lactam ring at the benzylic position. Although the phenols with a seven-membered lactam ring are harder to oxidize, the sterically nonhindered compounds react with DPPH about 2× faster than the analogous five-membered lactams. The limitations of computational methods, especially those based on a single parameter, are also evaluated and discussed.

Molecular Mechanisms on the Selectivity Enhancement of Ascorbic Acid, Dopamine, and Uric Acid by Serine Oligomers Decoration on Graphene Oxide: A Molecular Dynamics Study

The selectivity in the simultaneous detection of ascorbic acid (AA), dopamine (DA), and uric acid (UA) has been an open problem in the biosensing field. Many surface modification methods were carried out for glassy carbon electrodes (GCE), including the use of graphene oxide and amino acids as a selective layer. In this work, molecular dynamics (MD) simulations were performed to investigate the role of serine oligomers on the selectivity of the AA, DA, and UA analytes. Our models consisted of a graphene oxide (GO) sheet under a solvent environment. Serine tetramers were added into the simulation box and were adsorbed on the GO surface. Then, the adsorption of each analyte on the mixed surface was monitored from MD trajectories. It was found that the adsorption of AA was preferred by serine oligomers due to the largest number of hydrogen-bond forming functional groups of AA, causing a 10-fold increase of hydrogen bonds by the tetraserine adsorption layer. UA was the least preferred due to its highest aromaticity. Finally, the role of hydrogen bonds on the electron transfer selectivity of biosensors was discussed with some previous studies. AA radicals received electrons from serine through hydrogen bonds that promoted oxidation reaction and caused the negative shifts and separation of the oxidation potential in experiments, as DA and UA were less affected by serine. Agreement of the in vitro and in silico results could lead to other in silico designs of selective layers to detect other types of analyte molecules.

Non-coordinated and Hydrogen Bonded Phenolate Anions as One-Electron Reducing Agents

In this work, the syntheses of non-coordinated electron-rich phenolate anions via deprotonation of the corresponding alcohols with an extremely powerful perethyl tetraphosphazene base (Schwesinger base) are reported. The application of uncharged phosphazenes renders the selective preparation of anionic phenol-phenolate and phenolate hydrates possible, which allows for the investigation of hydrogen bonding in these species. Hydrogen bonding brings about decreased redox potentials relative to the corresponding non-coordinated phenolate anions. The latter show redox potentials of up to -0.72(1) V vs. SCE, which is comparable to that of zinc metal, thus qualifying their application as organic zinc mimics. We utilized phenolates as reducing agents for the generation of radical anions in addition to the corresponding phenoxyl radicals. A tetracyanoethylene radical anion salt was synthesized and fully characterized as a representative example. We also present the activation of sulfur hexafluoride (SF6 ) with phenolates in a SET reaction, in which the nature of the respective phenolate determines whether simple fluorides or pentafluorosulfanide ([SF5 ]- ) salts are formed.

Amino-based magneto-polymeric-modified mixed iron hydroxides for magnetic solid phase extraction of phenol residues in environmental samples

Mixed iron hydroxides (MIHs) modified with different amino-based polymeric materials, including aminopropyltriethoxysilane, polydopamine, diaminobenzoic acid, polyaniline, and polyphenylenediamine, were comparatively investigated as sorbents for the extraction of phenol compounds. Polyphenylenediamine-modified mixed iron hydroxides (MIH@PPDA) showed high adsorption capability for most target analytes. Its ferromagnetic behavior, with a magnetization of 17.38 emu g^-1, was sufficient for subsequent use in magnetic solid-phase extraction (MSPE). The functional groups, morphology, and magnetic properties of this magnetic nanomaterial were investigated using Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometry, X-ray diffraction, and CHN analysis. High-performance liquid chromatography with a photodiode array detector was used to quantify phenol compounds. The experimental parameters affecting the efficiency of the entire MSPE process were optimized. Good linearity in the range of 0.5-1000 µg L^-1 was obtained (depended on the compound). The detection and quantitation limits varied from 0.01 to 0.3 µg L^-1 and 0.03 to 0.9 µg L^-1, respectively. The enrichment factors for all phenol compounds were in the range of 80-285. The precision in terms of intra- and inter-day relative standard deviations were below 5.8% and 6.2%, respectively. The developed MSPE method was applied to analyze phenol compounds in diverse samples, including soil, drinking water, and fruit. Relative recoveries of 76.7-130.1% were obtained. The MIH@PPDA magneto-polymeric sorbent exhibits good stability and is reliable for a variety of phenol compounds.

Antiradical Properties of trans-2-(4-substituted-styryl)-thiophene

2-substituted thiophene compounds with electron donating and electron withdrawing p-phenyl substitution were synthesized and studied their radical scavenging properties using DPPH assay and DFT method. It is shown that p-hydroxy and p-amino phenyl substituted compound exhibit radical scavenging activity. From DFT and radical scavenging studies, a correlation between IC₅₀ with the bond dissociation enthalpy, proton affinity, ground state dipole moment and optical band gap of compound is found. Compounds 1-3 with electron withdrawing substituent (NO₂, CN, Cl) do not show any radical scavenging properties, whereas compounds 6-7 with electron donating substituent (OH, NH₂) show antiradical properties. Further, the antiradical activity is reduced drastically by replacing the -OH and -NH₂ with methoxy and -N-alkylating group respectively in 6 and 7. The compound with p-hydroxy phenyl substitution, exhibits stronger antiradical activity as compared to the p-amino phenyl substitution due to smaller O-H bond dissociation energy as compared to the N-H bond. From DPPH and DFT studies, it is suggested that the radical scavenging activity in 2-substituted thiophene is occurred through proton transfer mechanism. The other possible SET, SPLET mechanisms are also corroborated. Graphical Abstract Antiradical properties of trans-2-(4-substituted-styryl)-thiophene Anamika Gusain, Naresh Kumar, Jagdeep Kumar, Gunjan Pandey, Prasanta Kumar Hota.

Antioxidant properties of ethenyl indole: DPPH assay and TDDFT studies

Ethenyl indole exhibits antioxidant activity in a substituent dependent manner. Ethenyls bearing strong electron withdrawing substituents show weak or no antioxidant activities, whereas ethenyls with electron donating substituents exhibit antioxidant properties comparable to vitamin E.

Reassessment of Hammett σ as an effective parameter representing intermolecular interaction energy-links between traditional and modern QSAR approaches

The Hammett σ constant has for a long time been known to be one of most important linear free-energy related parameters that correlate with biological activity. It is a conventionally used electronic parameter in studies of enzymatic quantitative structure-activity relationships (QSAR). However, it is not necessarily obvious why σ represents variations in the free-energy change associated with the complex formation between a congeneric series of ligands with their target protein. So far, several powerful molecular calculations, such as the ab initio fragment molecular orbital (FMO) one, that are directly applicable to ligand-protein complexes have emerged. In this study, we comprehensively reevaluate experimentally derived parameter σ confirming it represents intermolecular interaction energy terms, by applying molecular orbital (MO) calculations to a simple ligand-protein complex model. The current results provide a rational and quantitative basis for bridging the gap between the traditional QSAR approach and 'the modern QSAR one', which involves the molecular calculations to evaluate the overall free-energy change for complex formation.

Probing concerted proton-electron transfer in phenol-imidazoles

A series of seven substituted 4,6-di-tert-butyl-2-(4,5-diarylimidazolyl)-phenols have been prepared and characterized, along with two related benzimidazole compounds. X-ray crystal structures of all of the compounds show that the phenol and imidazole rings are close to coplanar and are connected by an intramolecular ArOHN hydrogen bond. One-electron oxidation of these compounds occurs with movement of the phenolic proton to the imidazole base by concerted proton-electron transfer (CPET) to yield fairly stable distonic radical cations. These phenol-base compounds are a valuable system in which to examine the key features of CPET. Kinetic measurements of bimolecular CPET oxidations, with E(rxn) between +0.04 and -0.33 V, give rate constants from (6.3 +/- 0.6) x 10(2) to (3.0 +/- 0.6) x 10(6) M(-1) s(-1). There is a good correlation of log(k) with DeltaG degrees , with only one of the 15 rate constants falling more than a factor of 5.2 from the correlation line. Substituents on the imidazole affect the (O-HN) hydrogen bond, as marked by variations in the (1)H NMR and calculated vibrational spectra and geometries. Crystallographic d(ON) values appear to be more strongly affected by crystal packing forces. However, there is almost no correlation of rate constants with any of these measured or computed parameters. Over this range of compounds from the same structural family, the dominant contributor to the differences in rate constant is the driving force DeltaG degrees .

Theoretical study of hydrogen-bonded complexes of chlorophenols with water or ammonia: Correlations and predictions of pKa values

A simple practical method for predicting the acidity constants (as pKa values) of chlorophenols is proposed based on density functional theory calculations of a series of hydrogen-bonded complexes of phenol and 19 different congeners of chlorophenol, with a single probe molecule, either water or ammonia. Relevant structural parameters and molecular properties of these complexes, primarily involving the acidic hydroxyl group, are examined and plotted against the known pKa values of 14 chlorophenols and phenol. Strong linear correlations are found for these compounds. Such correlations are used to determine the pKa values of five chlorophenols whose experimental acidities have large uncertainties. Similar predicted pKa values are obtained by using different structural parameters and molecular properties for the complexes with either probe molecule. The study may be extended to determine the acidity of other compounds with a single acidic functional group.

A series of unconjugated ferrocenyl phenols: prospects as anticancer agents

We recently reported that a ferrocenyl diphenol butene derivative showed a very strong cytotoxic effect on both hormone-dependent and -independent breast cancer cell lines. In order to obtain more information about the structure-activity relationship in the cytotoxicity of small ferrocene compounds, we have prepared a series of simple unconjugated ferrocenyl diphenol complexes (ortho,para; meta,para; para,para). These compounds retain a reasonable to good affinity for both estrogen receptor types, with higher values for the beta form, and superior binding for the para,para diphenol complex (RBA=28%). In vitro these complexes exhibit significant cytotoxic effects on hormone-independent prostate (PC3) and breast cancer cell lines (MDA-MB231), with IC50 values between 2.5 and 4.1 microM. This effect is more marked with PC3, the ortho,para diphenol complex proving the most effective. On the hormone-dependent MCF7 breast cancer cell line, the observed effect seems to be the result of two components, one cytotoxic (antiproliferative), the other estrogenic (proliferative). Electrochemical studies show that the cytotoxic effect of the complexes correlates with the ease of oxidation of the ferrocene group. All these complexes are much less cytotoxic than the ferrocenyl diphenol butene derivative.

Proton Exchanges between Phenols and Ammonia or Amines: A Computational Study

Density functional theory calculations at the B3LYP/6-31+G(d,p) level of theory have been performed to explore proton exchanges between phenols and ammonia or amines, which can be used to account for previous NMR experiments. For the parent phenol-NH(3) system, a transition state with a symmetric phenolate-NH(4)(+)-like structure, which lies about 35 kcal mol(-1) in energy above the hydrogen-bonded complex, has been successfully located. An intrinsic reaction coordinate (IRC) analysis indicates that the proton exchange is a concerted process, which can be roughly divided into four continuous subprocesses. A series of para-substituted phenol-NH(3) systems have been considered to investigate the substituent effect. Whereas introduction of an electron-withdrawing group on the phenol appreciably reduces the barrier, an opposite effect is observed for an electron-donating group. Moreover, it has been disclosed that there exists a good linear correlation between the activation barriers and the interaction energies between the phenols and NH(3), indicating the important role of proton transfer (or hydrogen bonding) in determining the proton exchange. Also considered are the proton exchanges between phenol and amines and those for some sterically hindered systems. The results show that the phenol tends to exchange hydrogen with the amines, preferably the secondary amines, and that the steric effect is favorable for the proton exchange, which imply that, as the IRC analysis suggested, besides the proton transfer, the flip of the ammonium-like moiety may play a significant role in the course of proton exchange. For all of these systems, we investigated the solvent effects and found that the barrier heights of proton exchange decrease remarkably as compared to those in a vacuum due to the ion pair feature of the transition state. Finally, we explored the phenol radical cation-NH(3) system; the barrierless proton transfer and remarkably low barrier (5.2 kcal mol(-1)) of proton exchange provide further evidence for the importance of proton transfer in the proton exchange.

Coupling properties of imidazole dimer radical cation assisted by embedded water molecule: Toward understanding of interaction character of hydrogen-bonded histidine residue side-chains

Geometry optimizations are performed at the DFTB3LYP6-311+G* level. Four intriguing coupling modes, totally eight stable structures are found in the potential energy surfaces of the water-assisted coupling of imidazole dimer radical cation. In these isomers, the water molecules are embedded between two imidazole moieties, and the oxygen atom is tridentate or quadridentate, respectively. The distinct redshifts of the vibrational frequencies of the O-H...N and N-H...O type H bonds indicate the strong interaction of two imidazole rings of respective isomer. Inspection of the highest occupied molecular orbital predicts the alterations of the geometry structures on oxidation and reduction. The low barrier of the fragment rotation demonstrates that the isomerization processes by experiencing the distinct transition states are easy to fulfill, especially for those with O-H...N and C-H...O H bonds. Both the energy difference of the 0 degrees-cis and 180 degrees-trans orientation and the barriers of the fragment rotation are lowered by the water assisting. The range of the zero point vibrational energy correction indicates that the influence on the complexes with N-H...O and O-H...N H bonds (0.13-0.17 kcal/mol) is more significant than those with O-H...N and C-H...O H bonds (+/-0.03 kcal/mol). The dissociation energies of these isomers indicate that the charges transfer easily through water in the dissociation process and then are distributed mainly over the imidazole ring connecting with water molecule. The isomer with proton transfer between imidazole fragments is the most stable one.

The Interaction of Monosubstituted Benzenes with the Stationary Liquid in Gas Liquid Chromatography

In gas liquid chromatography (GLC), the relative retention values log gamma was mainly expressed by van der Waals energy (the sum of the dispersion E(dis) and repulsive E(rep) energies) to the interactions between monosubstituted benzene derivatives and the nonpolar stationary liquid as squalane. The single exception was that of anilines, and it was corrected by the electrostatic energy (E(ES)) due to C-H/pi hydrogen bond. When the stationary liquid changed from the nonpolar to polar, log gamma was estimated by the inductive interaction energy (included in E(ES)) in addition to the sum of E(dis) and E(rep). In the benzene solution, the relative equilibrium values log K/K(o) introduced from the interactions between phenol and substituted benzene derivatives were estimated by E(ES). The E(ES) of COCH(3), CO(2)C(2)H(5) groups is especially originated in the excited dipole moments micro(e). The relative frequency values log nu/nu(o) derived from O-H or O-D stretching vibration of phenol or methanol-D gave the correlation to E(ES) as well as log K/K(o). That of anilines-methanol-D however had been out of a linear relation to E(ES). The cause is concluded that the aniline-methanol-D is making the proton transfer structure from the discussion about the proton affinity (PA) of the base.