Phenolic Bond Research Articles

N-Heterocyclic Carbene Derived 3-Azabutadiene as a π-Base in Classic and Frustrated Lewis Pair Chemistry.

N-Heterocyclic carbene (NHC) derived 3-azabutadienes 1 and 2 have been prepared by a single-step reaction of the corresponding NHC with cyclohexyl isocyanide. Compound 1 features π-basic, delocalized nucleophilic sites over the 3-azabutadiene moiety, therefore allowing for coordinating with small Lewis acids, such as AlCl3 , GaCl3 , and Me2 SAuCl, to form diverse classic Lewis adducts 3-5. Combination of 1 with B(C6 F5 )3 or [Ph3 C][B(C6 F5 )4 ] resulted in single-electron transfer and the obtained radical cation was detected by EPR. In addition, a frustrated Lewis pair comprised of the π-basic 1 and BPh3 effects the splitting of the O-H bond of phenol and the N-H bond of imidazole to give 7 and 8, respectively. An intrinsic bond orbital (IBO) analysis of the pathway leading to 8 showcases the transformation of the delocalized π-electrons of 1 to a newly formed C-H localized σ-bond.

Depolymerization and hydrodeoxygenation of lignin to aromatic hydrocarbons with a Ru catalyst on a variety of Nb-based supports

Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hydrodeoxygenation is important. Previously, we found that NbOx species played a key role in the activation and cleavage of C–O bonds in lignin and its model compounds. In this study, commercial niobic acid (HY-340), niobium phosphate (NbPO-CBMM) and lab-made layered niobium oxide (Nb2O5-Layer) were chosen as supports to study the effect of Brönsted and Lewis acids on the activation of C–O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all the Ru/Nb-based catalysts produce high mole yields of C7–C9 hydrocarbons (82.3–99.1%). What's more, Ru/Nb2O5-Layer affords the best mole yield of C7–C9 hydrocarbons and selectivity for C7–C9 aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers and the cleavage of the C–O bond of phenols. Additionally, the electronic state and particle size of Ru are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the selectivity for aromatic hydrocarbons.

Selective activation of C[sbnd]H bond into C[dbnd]O bond of phenols in para-position via aerobic oxidation

An efficient method for the oxidation of phenols to 1,4-benzoquinones catalyzed by cuprous(I) chloride was achieved in a solution of acetonitrile and water using molecular dioxygen as an oxidant. Particularly, the inert phenols, such as phenol and mono-alkyl substituted phenols, were effectively oxidized to 1,4-benzoquinones via the selective activation of CH bond in para-position into CO bond under mild conditions. The catalyst shows high activity for unsubstituted or alkyl substituted phenols, but no effect on substituted phenols with electron-withdrawing groups. This study offers an aerobic method for the selective oxidation of aromatic phenols to 1,4-benzoquinones.

Sequential Functionalization of meta-C-H and ipso-C-O Bonds of Phenols.

The use of a template as a linchpin motif in directed remote C-H functionalization is a versatile yet relatively underexplored strategy. We have developed a template-directed approach to realizing one-pot sequential palladium-catalyzed meta-selective C-H olefination of phenols, and nickel-catalyzed ipso-C-O activation and arylation. Thus, this bifunctional template converts phenols to synthetically useful 1,3-disubstituted arenes.

Phenolic hydrogen bond donors in the formation of non-ionic deep eutectic solvents: the quest for type V DES.

Mixtures of non-ionic compounds have been reported as DES but most are just ideal mixtures. In the thymol-menthol system, an abnormal strong interaction was identified stemming from the acidity difference of the phenolic and aliphatic hydroxyl groups. This type of interaction is found to be the key to prepare non-ionic DES, that may be classified as type V.

Palladium‐Catalyzed Aerobic Oxidative Carbonylation of C–H Bonds in Phenols for the Synthesis of p‐Hydroxybenzoates

This work reports the synthesis of p‐hydroxybenzoates directly from phenols by oxidative carbonylation of phenolic C–H bonds, proceding through oxidative iodination. The developed methodology is efficient and economically attractive because phenols are cheap and easily available starting materials. This one‐pot strategy was expediently applied to the synthesis of a variety of p‐hydroxybenzoates by utilizing simple primary and secondary alcohols with different phenols under mild reaction conditions. Advantageously, the procedure has no need for co‐catalysts, co‐solvents or external ligands. The utilization of molecular oxygen as a terminal oxidant for C–H bond oxidation represents an additional benefit.

Effect of Nonplanarity on Excited-State Proton Transfer and Internal Conversion in Salicylideneaniline.

Salicylideneaniline (SA) is a prototype for excited-state intramolecular proton transfer (ESIPT) reactions in nonplanar molecules. It is generally understood that the dominant photochemical pathway in this molecule is ESIPT followed by nonradiative decay due to twisting about its phenolic bond. However, the presence of a secondary internal conversion pathway resulting from frustrated proton transfer remains a matter of contention. We perform a detailed nonadiabatic dynamics simulation of SA and definitively identify the existence of both reaction pathways, thereby showing the presence of a secondary photochemical pathway and providing insight into the nature of ESIPT dynamics in molecules with nonplanar ground-state geometries.

Thermo-responsive self-healable hydrogels with extremely mild base degradability and bio-compatibility

To face the increasing demands of self-healing hydrogels with biocompatibility and mild condition degradability, one brand new type of P(NIPAM-FPA-DMA) copolymers were prepared in this study. The copolymers were cross-linked by dihydrazide compound to form acylhydrazone containing self-healable hydrogels without additional catalysis. Herein, the chemical structure, as well as microscopic morphologies, mechanical and self-healing performances of the hydrogels were investigated. The hydrogels showed reversible thermo-responsiveness based on PNIPAM backbone and the phase transition temperature was regulated to body temperature by copolymerization of DMA. Moreover, the hydrogels could be degraded by extremely mild base of NaHCO3 based on phenolic ester bond and showed reversible gel-sol-gel transition under a variety of triggers. The in vitro toxicity experiment showed the self-healable hydrogels have good bio-compatibility. The doxorubicin-loaded hydrogels were prepared and showed controlled release profile. Altogether, these results identified the potential applications of the thermo-responsive self-healable hydrogels in bioscience and biotechnology with bio-safety.

Synthesis of steroid analogs of tubuloclustin, their cytotoxicity and effect on microtubules of A549 carcinoma cells

Synthesis of analogs of tubuloclustin (N-(7-adamant-2-yloxy-7-oxoheptanoyl)-N-deacetylcolchicine (1)) with the colchicine fragment replaced with 2-methoxyestradiol scaffold attached via phenolic hydroxy group was described. Esters 3a–c exhibit moderate cytotoxicity (EC50 = 5–6 μmol L–1) and exert a weak effect on the microtubule network in A549 human lung carcinoma cells similar to the clustering effect of tubuloclustin and its derivatives. Conjugates 6a–c and 7a–c with the phenolic ester bond are low stable and compounds 7a–c are inactive to the microtubules of A549 cells, while compounds 6a–c cause an unusual effect of curling of the microtubules.

Synergetic Effect and Product Characteristics of Coal and Salix psammophila Co-pyrolysis

The co-pyrolysis of Salix psammophila and coal was conducted in a pyrolysis reactor. The interaction between coal and S. psammophila at different ratios was investigated. It was found that the promotion and inhibition effects of the co-pyrolysis experiment were more obvious at 20 wt.% and 40 wt.% Salix psammophila, respectively, indicating a certain synergistic effect. The results of the N2 adsorption-desorption experiment showed that the addition of S. psammophila increased the surface area and decreased the pore size of the co-pyrolysis char. The Fourier transform infrared (FT-IR) spectra indicated that the pyrolysis tar contained mainly aromatic rings and fatty compounds. The addition of S. psammophila was beneficial to the decomposition of phenolic hydroxyls, ether bonds, and oxygen-containing heterocyclic rings in the coal tar. However, the addition of 50 wt.% S. psammophila blocked the pores of the coal and thus obstructed its pyrolysis, while the porosity of the co-pyrolysis char became more abundant. Richer porosity of the char implies great potential to be widely used in the sewage treatment industry.

A new perfluoromethyl aminoenone derivative and the role of the hydrogen bonding in the intra- and intermolecular interactions

The structure of a new perfluoromethyl derivative, (Z)-4,4,4-trifluoro-1-(2-hydroxyphenyl)-3-(methylamino)-2-buten-1-one (1), was investigated in both crystalline state and solution since the prototropy enable the existence of three tautomeric species in equilibrium. The preference in the configurational E/Z arrangement is strongly dependent on the intramolecular hydrogen bond interactions. The vibrational and electronic spectra of 1 were discussed and assigned by using DFT calculations. The crystal, solved by single-crystal X-ray diffraction, shows the prevalence of the tautomer aminoenone over the imino-enol and imino-ketone forms. Therefore, an almost planar molecular conformation is observed due to the formation of two intramolecular amino and phenolic hydrogen bonds to the same carbonyl oxygen acceptor atom that stabilize the Z configuration. The molecular packing is held by the OH⋯O(keto) and NH⋯O hydrogen bonds, F⋯F, CH⋯π and π-stacking interactions that contribute to the crystal stabilization. The intra and intermolecular contacts were also characterized by Natural Bond Orbital (NBO) analysis and Atoms in Molecules (AIM) approach. Hirshfeld surfaces and their associated fingerprint plots were generated for 1 and for a related structure, to visualize different intermolecular interactions and their relative contributions to the total surface.

Study of ciprofloxacin removal by biochar obtained from used tea leaves

In this study, used tea leaves (UTLs) were pyrolyzed to obtain used tea-leaf biochar (UTC), and then the UTC was used as an adsorbent to remove ciprofloxacin (CIP) from aqueous solutions. Batch experiments were conducted to investigate the CIP adsorption performance and mechanism. The results showed that the CIP-adsorbing ability first increased and then declined as the UTC pyrolysis temperature increased. The UTC obtained at 450°C presented excellent CIP-absorbing ability at pH6 and 40°C. The maximum monolayer adsorption capacity was 238.10mg/g based on the Langmuir isotherm model. The pseudo-second-order kinetic equation agreed well with the CIP adsorption process, which was controlled by both external boundary layer diffusion and intra-particle diffusion. The characterization analysis revealed that the OH groups, CC bonds of aromatic rings, CH groups in aromatic rings and phenolic CO bonds play vital roles in the CIP adsorption process, and that the NC, NO, OCO and COH groups of UTC were consumed in large quantities. π–π interactions, hydrogen bonding and electrostatic attraction are inferred as the main adsorption mechanisms. The present work provides not only a feasible and promising approach for UTLs utilization but also a potential adsorbent material for removing high concentrations of CIP from aqueous solutions.

Origins of Protons in C-H Bond Insertion Products of Phenols: Proton-Self-Sufficient Function via Water Molecules.

Water molecules can serve as proton shuttles for proton transfer in the C-H bond insertion reactions catalyzed by transition metal complexes. Recently, the control experiments performed for C-H bond insertion of phenol and anisol by gold carbenes show that large discrepancy exists in the yields of hydrogenated and deuterated products. Thus, we conducted a detailed theoretical analysis on the function of water molecules in the C-H bond insertion reactions. The comparison of calculated results and control experiments indicates that the solution water molecules play a crucial role of proton shuttle in C-H bond insertion. In particular, it was found that the hydroxyl groups in phenols were capable of donating protons via water shuttles for the production of C-H products, which had a substantial influence on the yields of inserted products. The hydroxyl groups instead of C-H bonds in phenols function like "proton reservoirs" in the C-H bond insertion, which we call the "proton self-sufficient" (PSS) function of phenol. The PSS function of phenol indicates that the substrates with and without proton reservoirs will lead to different C-H bond insertion products.

English

Polypharmacological activities of the biflavonoid fraction of Garcinia kola seed justify its development as a nutraceutical, however, quality assurance of active nutraceutical ingredient (ANI) requires conformance to appropriate standards of composition and quality. It was hypothesized that variation in extraction protocols, as previously reported for the biflavonoid fraction, would lead to variation in extract composition and potency. Computational antioxidant capacity simulation (CAOCS) assay of G. kola extracts obtained by different extraction protocols tested the hypothesis, through incremental addition (250 and 100 µL) of standard antioxidant (AOX) extract solutions in a photometric titration. Preferred model fitting was then statistically selected between mono- and bi-exponential decay. Best - fit reaction constant (kptt) was integrated into a metric for ranking antioxidant capacity (AOC) of the extracts. The AOC metric is a molecular descriptor for kinetics of phenolic bond cleavage. Three AOX extracts, namely, ethyl acetate seed extract, kolaviron and acetone seed extract were found to vary in composition, and produced optimal AOC values of 1500/g, 1150/g, and 1050/g respectively. Our findings demonstrated that the composition and potency of biflavonoid fraction of G. kola seed are critically dependent on solvent extraction protocol, and hence, consistent with the hypothesis. CAOCS assay is a suitable analytical tool for ensuring batch–to–batch sameness of ANI prepared from G. kola seed.   Key words: Garcinia kola seed extracts, biflavonoids, active nutraceutical ingredient, antioxidant capacity, quality assurance.

Carbene X[sbnd]H bond insertions catalyzed by copper(I) macrocyclic pyridine-containing ligand (PcL) complexes

A catalytic system comprising copper(I) and macrocyclic pyridine-containing ligands (Pc-L) proves capable of promoting carbene SiH bond insertions using diazo compounds as the carbene source. This catalytic system showed broad scope and a remarkable robustness as indicated by high TON numbers (up to 30000). Moreover, the use of enynones as carbene sources proved also feasible in hydrosilane insertion using this catalytic system. Finally, the insertion in OH and NH bonds of phenols and anilines, respectively, has been also demonstrated.

Effect of steam explosion treatment on chemical composition and characteristic of organosolv fescue lignin

Tall fescue, a grass currently grown as forage, presents a good potential as a feedstock in the development of biorefining. In this study, combination of steam explosion and an acetic/formic acid “organosolv” pretreatments are applied on raw fescue to optimize lignin extraction yields and obtain specific products. In a first step, fescue is pretreated by steam explosion under severity factors from 1.4 to 4.2. These treatments allow hemicelluloses solubilization and open the lignocellulosic structure. Steam exploded solid residues are then treated with an acetic acid (50%)/formic acid (30%)/water (20%) mixture. Extracted lignins are precipitated and characterized by size exclusion chromatography (SEC), 31P NMR and HSQC NMR. Results show that acetic and formic acids extracted lignins present G, H and S units with ferulate and p-coumarate. Lignin typical linkages identified are of β-O-4 and spirodienone substructures. Combination of steam explosion and acetic and formic acid pretreatments increases extraction yields from 30% to 100%. Moreover, an increase of the steam explosion treatment intensity induces chemical modifications in lignin structure such β-O-4 and spirodienone substructure degradations, increase of free COOH and phenolic OH bonds, decrease of aliphatic OH ferulate and p-coumarate bonds and changes in G/H/S units proportions. Steam explosion intensity affects also lignin molecular weights.

N-Acetyl-cysteine Increases Chemical Stability of Hydroquinone in Pharmaceutical Formulations: a Theoretical and Experimental Approach

In this study, the chemistry stability of hydroquinone (HQ) was evaluated according to its effects in redox properties and compared to kojic acid (KA). The HQ oxidation was more inhibited by N-acetylcysteine (NAC) than ascorbic acid (AA). These results were elucidated using theoretical methods at the DFT/B3LYP level of theory. All electronic parameters were related between antioxidant performance and highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), HOMO-LUMO value gap (GAP), ionization potential (IP), and phenol or enol bond dissociation energy (BDEOH) values. However, the interactions between HQ and NAC cannot be related by changing of these electronic parameters. Therefore the high calculated values for electron transfer can be associated to NAC due to polarizability or chelation properties of sulfur moiety.

Self-healable polymer gels with multi-responsiveness of gel–sol–gel transition and degradability

P(NIPAM-co-FPA) contains an aldehyde group and a phenolic ester moiety is synthesized. The aldehyde group can form reversible covalent bonds with hydrazide to endow the polymer gels with self-healing properties. The self-healable polymer gel can be degraded in Na2CO3 solution based on cleavage of phenolic ester bond.

The 15N NMR chemical shift in the characterization of weak halogen bonding in solution.

We have studied the applicability of 15N NMR spectroscopy in the characterization of the very weak halogen bonds of nonfluorinated halogen bond donors with a nitrogenous Lewis base in solution. The ability of the technique to detect the relative strength of iodine-, bromine- and chlorine-centered halogen bonds, as well as solvent and substituent effects was evaluated. Whereas computations on the DFT level indicate that 15N NMR chemical shifts reflect the diamagnetic deshielding associated with the formation of a weak halogen bond, the experimentally observed chemical shift differences were on the edge of detectability due to the low molar fraction of halogen-bonded complexes in solution. The formation of the analogous yet stronger hydrogen bond of phenols have induced approximately ten times larger chemical shift changes, and could be detected and correlated to the electronic properties of substituents of the hydrogen bond donors. Overall, 15N NMR is shown to be a suitable tool for the characterization of comparably strong secondary interactions in solution, but not sufficiently accurate for the detection of the formation of thermodynamically labile, weak halogen bonded complexes.

Ethanolysis of Kraft lignin to platform chemicals on a MoC1-x/Cu-MgAlOz catalyst

The complete conversion of Kraft lignin is examined over a MoC1-x/Cu-MgAlOz catalyst in pure ethanol to give value-added chemicals with small molecular weight, including C6 alcohols, C8C10 esters, benzyl alcohols and arenes, without the formation of char or tar. MoC1-x/Cu-MgAlOz exhibited much higher activity than the previously reported MoC1-x/AC, CuMgAlOy and Cu-MgAlOz catalysts and achieved the highest yield of aromatic compounds, 575mg/g lignin, at 330°C. The complete cleavage of aryl-O bonds in phenols is observed at temperatures over 300°C. Furthermore, the MoC1-x/Cu-MgAlOz catalyst is reusable with a 22.4% loss in the yield of aromatic compounds after 5 cyclic runs.