A comparison of the structure and bonding in the aliphatic boronic R–B(OH)<Subscript>2</Subscript> and borinic R–BH(OH) acids (R=H; NH<Subscript>2</Subscript>, OH,and F): a computational investigation

Boronic acids, R–B(OH)₂, play an important role in synthetic, biological, medicinal, and materials chemistry. This investigation compares the structure and bonding surrounding the boron atoms in the simple aliphatic boronic acids, R–B(OH)₂ (R=H; NH₂, OH, and F), and the analogous borinic acids, R–BH(OH). Geometry optimizations were performed using second-order Møller–Plesset perturbation theory (MP2) with the Dunning–Woon aug-cc-pVTZ, aug-cc-pVQZ, and aug-cc-pV5Z basis sets; single-point CCSD(FC)/aug-cc-pVTZ//MP2(FC)/aug-cc-pVTZ level calculations were used to generate a QCI density for natural bond orbital analyses of the bonding. The optimized boron–oxygen bond lengths for the X–B–O_t–H trans-branch of the endo-exo form of the boronic acids and for the X–B–O–H cis-branch of the boronic and borinic acids (X=N, O, and F, respectively) decrease as the electronegativity of X increases. The boron–oxygen bond lengths are generally longer in the endo-exo or anti forms of the boronic acids than in the corresponding borinic acids. NBO analyses suggest the boron–oxygen bond in H₂BOH is a double bond; the boron–oxygen bonding in the remaining boronic and borinic acids in this study has a significant contribution from dative pπ–pπ bonding. Values for Δ\({\text{H}}_{298}^{0}\) for the highly balanced reaction, R–B(OH)₂ + R–BH₂ → 2 R–BH(OH), suggest that the bonding surrounding the boron atom is stronger in the borinic acid than in the corresponding boronic acid.     

Synthesis of Benzo‐ and Naphthoquinonyl Boronic Acids: Exploring the Diels–Alder Reactivity

Substituted 2‐quinonyl boronic acids have been synthesised from 1,4‐dimethoxy aromatic derivatives in two steps: regiocontrolled boronation and oxidative demethylation. The study of their dienophilic behaviour evidenced that the boron substituent significantly increases the reactivity and triggers an efficient domino process in which the Diels–Alder reaction was followed by a protodeboronation or dehydroboronation, depending on the substitution on both the quinone and diene partners. The boronic acid acts as a temporary controller, opening a direct access to trans‐fused meta‐regiosomeric adducts when 3‐methyl‐substituted 2‐quinonyl boronic acids react with dienes with a substituent at C‐1. A particularly valuable synthetic result was obtained in the reaction between 3,6‐dimethyl‐2‐quinonyl boronic acid and piperylene under an oxygen atmosphere; trans‐fused 8a‐hydroxy‐2,4a,8‐trimethyl tetrahydronaphthoquinone was formed directly, in excellent yield and in a highly diastereoselective manner.     

A Cu-based metal-organic framework with two types of connecting nodes as catalyst for oxygen activation

Using a ditopic organic linker 4-(1H-pyrazol-4-yl)benzoic acid (H₂pba), FICN-6, a metal-organic framework containing both Cu₂(O₂CR)₄ and Cu₃(OH)(pyz)₃(O₂CR) secondary building units (SBUs), was synthesized. FICN-6 adopts in an unusual intercatenated structure with SBUs from two distinct networks connecting to each other. Presence of Cu₃ clusters makes FICN-6 a good heterogeneous catalyst for oxygen activation and aerobic oxidative C-C coupling of organic boronic acids.     

Two closely related {4‐[(N‐substituted amino)(diethoxyphosphoryl)methyl]phenyl}boronic acids

Organic phosphonic acids and organic phosphonic acid esters have been of much interest due to their applications in the fields of medicine, agriculture and industrial chemistry. Boronic acids can act as synthetic intermediates and building blocks and are used in sensing, protein manipulation, therapeutics, biological labelling and separation. The additional introduction of an aminophosphonic acid group into a boronic acid may give new opportunities for application. To study the structure of such multifunctional compounds, we prepared two new derivatives which can be easily converted to the corresponding phosphonic acids. In the title compounds, {4‐[(butylamino)(diethoxyphosphoryl)methyl]phenyl}boronic acid monohydrate, C₁₅H₂₇BNO₅P·H₂O, (I), and {4‐[(diethoxyphosphoryl)(4‐nitroanilino)methyl]phenyl}boronic acid, C₁₇H₂₂BN₂O₇P, (II), three different substituents are attached to a central C—H group, namely 4‐boronophenyl, diethoxyphosphoryl and amine. Compound (I) crystallizes as a monohydrate and O_B—H…N hydrogen bonds link neighbouring molecules into chains along the [001] direction. The solvent water molecule connects two such chains running in opposite directions. Compound (II) crystallizes as an ansolvate and classical hydrogen bonds result in a layer structure in the (001) plane.     

A Persulfide Donor Responsive to Reactive Oxygen Species: Insights into Reactivity and Therapeutic Potential

Persulfides (RSSH) have been hypothesized as critical components in sulfur‐mediated redox cycles and as potential signaling compounds, similar to hydrogen sulfide (H₂S). Hindering the study of persulfides is a lack of persulfide‐donor compounds with selective triggers that release discrete persulfide species. Reported here is the synthesis and characterization of a ROS‐responsive (ROS=reactive oxygen species), self‐immolative persulfide donor. The donor, termed BDP‐NAC, showed selectivity towards H₂O₂ over other potential oxidative or nucleophilic triggers, resulting in the sustained release of the persulfide of N‐acetyl cysteine (NAC) over the course of 2 h, as measured by LCMS. Exposure of H9C2 cardiomyocytes to H₂O₂ revealed that BDP‐NAC mitigated the effects of a highly oxidative environment in a dose‐dependent manner over relevant controls and to a greater degree than common H₂S donors sodium sulfide (Na₂S) and GYY4137. BDP‐NAC also rescued cells more effectively than a non‐persulfide‐releasing control compound in concert with common H₂S donors and thiols.     

Biocatalytic Oxidative Cascade for the Conversion of Fatty Acids into α‐Ketoacids via Internal H2O2 Recycling

The functionalization of bio‐based chemicals is essential to allow valorization of natural carbon sources. An atom‐efficient biocatalytic oxidative cascade was developed for the conversion of saturated fatty acids to α‐ketoacids. Employment of P450 monooxygenase in the peroxygenase mode for regioselective α‐hydroxylation of fatty acids combined with enantioselective oxidation by α‐hydroxyacid oxidase(s) resulted in internal recycling of the oxidant H₂O₂, thus minimizing degradation of ketoacid product and maximizing biocatalyst lifetime. The O₂‐dependent cascade relies on catalytic amounts of H₂O₂ and releases water as sole by‐product. Octanoic acid was converted under mild conditions in aqueous buffer to 2‐oxooctanoic acid in a simultaneous one‐pot two‐step cascade in up to >99 % conversion without accumulation of hydroxyacid intermediate. Scale‐up allowed isolation of final product in 91 % yield and the cascade was applied to fatty acids of various chain lengths (C6:0 to C10:0).     

Facile and effective approach for oxidation of boronic acids

This present work illustrates facile and effective approach for oxidation of boronic acids using environmentally benign dimethyl carbonate (DMC) as a solvent with H₂O₂ as an oxidant at room temperature. In contrast to previous reaction reports, which make use of metal catalyst, hazardous reagent and oxidants that creates environmental concern. This method provides good to excellent yield of products and showed better tolerance towards various functional groups present on boronic acids. Moreover, this developed process is an alternative in terms of inexpensive, non toxic and easy reaction conditions.     

Reaction of Hydantoin with Boronic Acids

We have examined the reaction of hydantoin (=imidazolidine‐2,4‐dione) with (formylphenyl)boronic acids, where the addition of a boronic acid group is hoped to increase bioactivities. Addition of (2‐formylphenyl)boronic acid to hydantoin gave an unexpected azaborine compound, which presumably arises by initial formation of the (phenylmethylidene)hydantoin, with subsequent loss of H₂O to give the cyclized product. Reactions of (3‐formylphenyl)‐ and (4‐formylphenyl)boronic acids with hydantoin gave the corresponding [(Z)‐phenylmethylidene]hydantoins in good‐to‐excellent yields. Attempts to use (3‐formylthiophen‐2‐yl)boronic acid gave a product where the boronic acid group has been cleaved.     

Oxidative Degradation of 2,4‐Dihydroxybenzoic Acid by the Fenton and Photo‐Fenton Processes: Kinetics,Mechanisms, and Evidence for the Substitution of H2O2 by O2

The kinetics and mechanisms of the oxidative degradation of 2,4‐dihydroxybenzoic acid (2,4‐DHBA) by the Fenton and photo‐Fenton processes were investigated in detail by a combination of HPLC, IC, and TOC analyses. The formation of 2,3,4‐trihydroxybenzoic acid (2,3,4‐THBA) at an early oxidation stage shows that hydroxylation of the aromatic ring is the first step of the process. This intermediate was able to reduce Fe^III and to contribute to the recycling of Fe^II. Complete mineralization could only be achieved under irradiation (photo‐Fenton). A detailed study of the dependence of the rate of mineralization on the concentration of H₂O₂ and dissolved O₂ was carried out. It was found that, even at a low initial concentration of H₂O₂, mineralization by the photo‐Fenton process was complete in a relatively short time, provided that the O₂ concentration was high enough, indicating that O₂ may, at least in part, substitute H₂O₂. Channeling reaction pathways toward O₂ rather than H₂O₂ consumption is of particular interest for the technical development of the photo‐Fenton process.     

Transformation of 2,4-dichlorophenol by H2O2/UV-C, Fenton and photo-Fenton processes: Oxidation products and toxicity evolution

In the present study, H₂O₂/UV-C, Fenton and photo-Fenton treatment of 2,4-dichlorophenol was compared in terms of oxidation products and acute toxicity. The oxidation products were identified by gas chromatography-mass spectroscopy, high performance liquid chromatography and ion chromatography, whereas changes in acute toxicity were evaluated by the Vibrio fischeri luminescence inhibition assay. H₂O₂/UV-C and photo-Fenton processes ensured complete 2,4-dichlorophenolremoval, detoxification and significant mineralization. Hydroquinone and formic acid were identified as the common oxidation products of the studied advanced oxidation processes investigated. 3,5-dichloro-2-hydroxybenzaldehyde, phenol, 4-chlorophenol and 2,5-dichlorohydroquinone were identified as the additional H₂O₂/UV-C oxidation products of 2,4-dichlorophenol. Acute toxicity decreased with decreasing 2,4-dichlorophenol and increasing chloride release.     

Spectrofluorimetric determination of hydrogen peroxide scavenging activity

Homovanillic acid (HVA) is widely used for the detection and imaging of oxidative enzymes—peroxidase, glucose oxidase and xanthine oxidase, but antioxidant activity has not been determined so far with the use of HVA. We have developed a simple, sensitive and in-field spectrofluorimetric method for the determination of hydrogen peroxide (H₂O₂) scavenging activity. The assay is based on the oxidation of HVA to its fluorescent biphenyl dimer in the presence of H₂O₂ and peroxidase. The presence of substances with H₂O₂ scavenging activity prevents the oxidation of HVA by removing H₂O₂. The decrease in fluorescence intensity is proportional to the antioxidative (H₂O₂ scavenging) activity. The method was evaluated using Trolox (6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid), BHA (3-t-butyl-4-hydroxyanisole) and ferulic, vanillic, caffeic, chlorogenic, protocatechuic and oxalic acids. Additionally, tea and herb infusions known for their antioxidant properties were evaluated.     

Cyclopeptid-Antibiotika aus Aspergillus-Arten. Struktur der Echinocandine C und D

Cyclopeptide antibiotics from Aspergillus species. Structure of echinocandins C and D The echinocandins B, C and D are antifungal antibiotics produced by a strain of Aspergillus rugulosus. All three metabolites are closely related representing cyclic oligopeptides composed of six amino acids and a linolic acid residue in an amide linkage. The complete structure of echinocandin B ( 1 ) has recently been established by X-ray analysis. Structural assignments to the new minor metabolites C and D have now been made by hydrolytic and oxidative cleavage reactions, formation of N-acyl-α-aminoethers as well as by chemical correlations and extensive NMR. examinations. Echinocandin C ( 2 ), C₅₂H₈₁N₇O₁₅, contains 3-hydroxyhomotyrosine in the place of 3, 4-dihydroxyhomotyrosine present in 1 . Echinocandin D ( 3 ), C₅₂H₈₁N₇O₁₃, differs in two amino acids: 3, 4-dihyroxyhomotyrosine and 4, 5-dihydroxyornithine, unusual units of 1 being replaced by 3-hydroxyhomotyrosine and ornithine.     

Determination of protein-bound methionine oxidation in the hippocampus of adult and old rats by LC-ESI-ITMS method after microwave-assisted proteolysis

Protein-bound methionine (Met) oxidation has been associated with normal aging and a variety of age-related diseases, including Alzheimer’s disease and Parkinson’s disease. Monitoring the changes of protein-bound methionine content in the brain in response to normal aging and oxidative stress is of great interest and could be used as an indicator of oxidative stress of rats in pathological conditions. We have developed a rapid analytical method for the determination of oxidized products of protein-bound methionine in rat brain. The assay involved rapid acid proteolysis with microwave irradiation and solid-phase extraction of the free amino acids followed by LC-ESI-ITMS analysis. Detection was achieved in positive ionization with an ion trap mass spectrometer operating in multiple-reaction monitoring mode. The calibration curves of the analytes were linear (r ² > 0.99) in the range between 0.098 and 1.560 μg/mL. Intra- and inter-day relative standard deviation percentages were <9% and <8%, respectively. The assay performance was sufficient to support a rapid analytical tool for monitoring brain protein-bound methionine oxidation levels. The content of protein-bound Met and methionine sulfoxide (MetO) in the hippocampus of adult and old rats with or without H₂O₂ treatment was determined by employing the new method. The content of protein-bound MetO was significantly increased in old rats after exposure to H₂O₂. This result indicates increased sensitivity to Met oxidation in the hippocampus of old rats.     

Regenerating Spent Solutions of Vanadium‐containing Heteropoly Acids in the Presence of Additives

Vanadium‐containing heteropoly acid solutions of Keggin H_3+xPMo_12–xV_xO₄₀ and modified H_aP_zMo_yV_xO_b types (P‐Mo‐V HPAs) are promising nanosized inorganic metal‐oxygen cluster compounds with the property of reversible oxidability (V^V ↔ V^IV). The oxidation of reduced P‐Mo‐V HPAs at a temperature of 130–170 °C and an oxygen pressure of 4 atm is a convenient method for their regeneration, but results in regeneration degree of only 75–88 %. Various materials with electron transfer or oxidative properties, such as nitrogen doped carbon nanofibers (N‐CNFs), Sibunit‐4, HNO₃, and MoO₂, were investigated as additives to facilitate and accelerate the regeneration of HPA solutions. Among the studied additives HNO₃ was found to show the best efficiency, resulting in regeneration degree of higher 95 %. Rapid and efficient regeneration of spent HPA catalysts is an important criterion for achieving high productivity and sustainability of oxidative processes on their basis.     

Electron-Deficient Borinic Acid Polymers: Synthesis,Supramolecular Assembly,and Examination as Catalysts in Amide Bond Formation

The Lewis acidic character of borinic-acid-functionalized polymers suggests broad potential applications in supramolecular materials, chemo- and biosensors, as well as supported catalysts. Two highly electron-deficient borinic acid copolymers ( 3 a and 3 b ) with variable steric hindrance at the boron center were prepared by reaction of aryldibromoboranes ArBBr₂ ( 2 , Ar=2,4-Cl₂Ph, 3,5-Cl₂Ph) with a 10 % stannylated polystyrene random copolymer, followed by conversion to the desired PS-B(Ar)OH functionalities. The supramolecular assembly of these polymers through Lewis acid–Lewis base interactions and reversible covalent B−O−B bond formation was investigated. Exposure of a polymer solution of 3 a to pyridine triggered spontaneous gelation, whereas 3 b only gelled upon addition of molecular sieves to favor formation of boroxane crosslinks. The crosslinking process was readily reversed by addition of small amounts of water or wet solvent. The dynamic processes were studied in detail by variable-temperature (VT) NMR by using molecular model compounds. The polymers and their corresponding model compounds were also examined as catalysts in the amide bond formation reaction between phenylacetic acid and benzylamine. The 3,5-dichlorophenyl borinic acid derivatives proved to be the more effective catalysts. Mechanistic studies suggested that the borane Lewis acid-catalyzed coupling involves initial acid-induced protodeboronation to release the dichlorophenyl boronic acid as the active catalyst.     

The reactions of diborane with aryl-organotin compounds

A number of tetraaryltin compounds, Ar₄Sn (where Ar = phenyl, o- and p-tolyl, and p-chlorophenyl) and triphenyltin compounds, Ph₃SnX (where X = Cl, H, OH, OCOCH₃, and OCOCF₃) have been treated with diborane in tetrahydrofuran. Transmetallation occurs in which one or more aryl groups are transferred to boron. The organoboron intermediates give phenols upon oxidation and boronic and borinic acids upon hydrolysis. Pyridine complexes of organoboranes have also been isolated.     

Reaction‐Based and Single Fluorescent Emitter Decorated Ratiometric Nanoprobe to Detect Hydrogen Peroxide

A novel reaction‐based cross‐linked polymeric nanoprobe with a self‐calibrating ratiometric fluorescence readout to selectively detect H₂O₂ is reported. The polymeric nanoprobe is fabricated by using hydrophobic H₂O₂‐reactive boronic ester groups, crosslinker units, and environmentally sensitive 3‐hydroxyflavone fluorophores through a miniemulsion polymerization. On treatment with H₂O₂, the boronic esters in the polymer are cleaved to form hydrophilic alcohols and subsequently lead to a hydrophobic–hydrophilic transition. Covalently linked 3‐hydroxyflavones manifest the change in polarity as a ratiometric transition from green to blue, accompanied by a 500‐fold increase in volume. Furthermore, this nanoprobe has been used for ratiometric sensing of glucose by monitoring the H₂O₂ generated during the oxidation of glucose by glucose oxidase, and thus successfully distinguished between normal and pathological levels of glucose.     

2‐(6‐Bromopyridin‐3‐yl)‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane and (6‐bromopyridin‐3‐yl)boronic acid,new bifunctional building blocks for combinatorial chemistry

The first comparative study between two new heterocyclic boron derivatives, viz. a (6‐bromo­pyridin‐3‐yl)­boronic ester, C₁₁H₁₅BBrNO₂, and (6‐bromo­pyridin‐3‐yl)­boronic acid, C₅H₅BBrNO₂, shows a small but not significant difference in their C—B bond lengths, which cannot explain the experimentally observed difference in their stabilities. The crystal packing of the boronic ester consists principally of van der Waals interactions, while the boronic acid mol­ecules interact in their crystal through hydrogen bonds.     

General and highly efficient fluorinated-N-heterocyclic carbene–based catalysts for the palladium-catalyzed Suzuki–Miyaura reaction

A general and highly efficient trifluoromethylated-N-heterocyclic carbene (NHC)-based catalyst for the palladium-catalyzed Suzuki–Miyaura reaction was reported. In the presence of the catalyst, reactions of non-activated aryl chlorides and triflates with aryl boronic acids occurred at room temperature with good to excellent yields (63–98%). In addition, catalysts generated from a combination of Pd(OAc)₂/imidazolium salt 6a is not only effective for the coupling of heteroaryl boronic acid with aryl halides and heteroaryl halides, but also efficient for coupling of other heteroaryl halides and heteroaryl boronic acids. Finally, the catalyst is highly effective for Suzuki–Miyaura reaction of aryl bromides and chlorides with 0.01–0.1 mol % loading if the temperature was raised at refluxed THF/H₂O.     

Catalytic cross-coupling of aniline by pyrite and dissolved oxygen under alkaline conditions

Pyrite catalyzes oxidation of various organic contaminants by dissolved oxygen (DO) under acidic conditions; however, the catalytic mechanism under alkaline conditions is still not clear. In this study, we observe increased oxidation rates of aniline with increasing pHs (7.0–11.0). Electron paramagnetic resonance (EPR) analysis and quenching experiments rule out contributions of •OH, O₂^•−, ¹O₂ and Fe (IV) to aniline oxidation and suggest that the Fe (III)–OOH peroxo and/or H₂O₂ are the primary oxidative species in the oxidation of aniline at pH 11.0. In addition, 200 mg L⁻¹ H₂O₂ does not apparently increase the oxidation rate of aniline, which also rules out the predominant contribution of the produced H₂O₂ to aniline oxidation. We therefore suggest that the Fe (III)–OOH peroxo is indeed the primary oxidative species in the pyrite–DO system under alkaline conditions. Analyses of solid total organic carbon (TOC), gas chromatography–mass spectrometry and Fourier-transform infrared spectroscopy further reveal that more than 83.3% aniline has been polymerized to polyaniline, instead of being mineralized into CO₂ and H₂O, indicating that H-abstraction from aniline by the Fe (III)–OOH peroxo is an important step in the oxidation of aniline under alkaline conditions. This study provides new insight into the oxidative species in the pyrite–DO system, and opens a new door for organic degradations under alkaline conditions.