Stability of gamma-valerolactone under neutral,acidic, and basic conditions

Dry gamma-valerolactone (GVL) is stable for several weeks at 150 °C and its thermal decomposition only proceeds in the presence of appropriate catalysts. Since GVL does not react with water up to 60 °C for several weeks, it could be used as a green solvent at mild conditions. At higher temperatures, GVL reacts with water to form 4-hydroxyvaleric acid (4-HVA) and reaches the equilibrium in a few days at 100 °C. Aqueous solutions of acids (HCl and H₂SO₄) catalyze the ring opening of GVL even at room temperature, which leads to the establishment of an equilibrium between GVL, water, and 4-HVA. Although the 4-HVA concentration would be below 4 mol% in the presence of acids, it could be higher than the concentration of a reagent or a catalyst precursor, not to mention a catalytically active species. The latter could be especially worrisome as 4-HVA could be an excellent bi- or even a tri-dentate ligand for transition metals. Aqueous solution of bases (NaOH and NH₄OH) also catalyzes the reversible ring opening of GVL. While in the case of NaOH, the product is the sodium salt of 4-hydroxyvalerate, the reversible reaction of GVL, with NH₄OH results in the formation of 4-hydroxyvaleric amide. The reversible ring opening of (S)-GVL in the presence of HCl or NaOH has no effect on the stability of the chiral center.     

Crystal structure and characterization of pyrroloquinoline quinone disodium trihydrate

ABSTRACT: BACKGROUND: Pyrroloquinoline quinone (PQQ), a tricarboxylic acid, has attracted attention as a growthfactor, and its application to supplements and cosmetics is underway. The product used forthese purposes is a water-soluble salt of PQQ disodium. Although in the past, PQQdisodiumpentahydrates with a high water concentration were used, currently, low hydrationcrystals of PQQ disodiumpentahydrates are preferred. RESULTS: We prepared a crystal of PQQ disodium trihydrate in a solution of ethanol and water, studiedits structure, and analyzed its properties. In the prepared crystal, the sodium atom interactedwith the oxygen atom of two carboxylic acids as well as two quinones of the PQQ disodiumtrihydrate. In addition, the hydration water of the prepared crystal was less than that of theconventional PQQ disodium crystal. From the results of this study, it was found that the colorand the near-infrared (NIR) spectrum of the prepared crystal changed depending on the watercontent in the dried samples. CONCLUSIONS: The water content in the dried samples was restored to that in the trihydrate crystal by placingthe samples in a humid environment. In addition, the results of X-ray diffraction (XRD) andX-ray diffraction-differential calorimetry (XRD-DSC) analyses show that the phase of thetrihydrate crystal changed when the crystallization water was eliminated. The dried crystalhas two crystalline forms that are restored to the original trihydrate crystals in 20% relativehumidity (RH). This crystalline (PQQ disodium trihydrate) is stable under normalenvironment.     

Complex and charge transfer between TiO2 and pyrroloquinoline quinone

Pyrroloquinoline quinone (PQQ) forms a tridentate complex with coordinatively unsaturated titanium atoms on the surface of approximately 4.5 nm TiO2 particles; an association constant of K = 550 M-1 per Ti(IV)surf has been determined. Low-temperature electron paramagnetic resonance was employed in identification of localized charges and consequently produced radicals and in determination of charge-transfer processes. The photoexcitation of the PQQ-TiO2 complex results in the transfer of conduction band electrons from TiO2 to bound PQQ and the formation of the semiquinone radical. Attaching dopamine (DA) as an electron donor and PQQ as an electron acceptor on the surface of TiO2 results in spatial separation of photogenerated charges; the holes localize on dopamine and electrons on PQQ, with higher yields than for each component separately. In this triad-type assembly (PQQ-TiO2/DA) the PQQ that is bound to the particles acts as a sink for electrons allowing their almost complete scavenging even at temperature as low as 4 K.     

Simple immobilization of pyrroloquinoline quinone on few-walled carbon nanotubes

Pyrroloquinoline quinone (PQQ) was immobilized on glassy-carbon electrodes (GCE) modified with single-walled carbon nanotubes (SWCNT), few-walled carbon nanotubes (FWCNT) and carbon black (Vulcan XC72R). Modified electrodes were prepared by drop-casting. Immobilization was achieved with an extremely simple dipping procedure and without any further modification to the electrodes. Electrochemical performance of the electrodes was studied by cyclic voltammetry and spectroelectrochemistry. FWCNT adsorbed 30 times more PQQ than the other carbon materials. Compared to more complicated immobilization methods, PQQ/FWCNT/GCE showed well-defined electrochemistry in a considerably wide pH area from 2 to 12. The dipping process is affected by pH and electrostatic forces. At dipping pH 9.5, where both FWCNTs and PQQ have strong negative charge, the adsorption was halved compared to dipping pH 2, where the charges are smaller.     

A biofuel cell based on pyrroloquinoline quinone and microperoxidase-11 monolayer-functionalized electrodes

A pyrroloquinoline quinone (PQQ) monolayer-functionalized-Au-electrode and a microperoxidase-11 (MP-11)-modified Au-electrode are used as catalytic anode and cathode in a biofuel cell element, respectively. The cathodic oxidizer is H₂O₂ whereas the anodic fuel-substrate is 1,4-dihydronicotinamide adenine dinucleotide, NADH. The PQQ-monolayer electrode catalyzes the oxidation of NADH in the presence of Ca²⁺ ions. The MP-11-functionalized electrode catalyzes the reduction of H₂O₂. The biofuel cell generates an open-circuit voltage, V_oc, of ca. 320 mV and a short-circuit current density, I_sc, of ca. 30 μA·cm⁻². The maximum electrical power, W_max, extracted from the cell is 8 μW at an external load of 3 kΩ. The fill factor of the biofuel cell, f=W_max·I_sc⁻¹·V_oc⁻¹, is ca. 25%.     

Iron-catalyzed sulfonylimine synthesis under neutral conditions

A convenient FeCl₃-catalyzed synthesis of N-sulfonylimines via the condensation of aldehydes with N-sulfonylamides in mild and neutral conditions (in ethanol at room temperature) is reported. This procedure constitutes the first iron-catalyzed synthesis of N-sulfonylimines and is adapted to the condensation of both aromatic and aliphatic aldehydes.     

Cyclen-catalyzed Henry reaction under neutral conditions

A convenient cyclen-catalyzed Henry reaction of aldehydes with nitroalkanes under mild and neutral conditions is reported. This procedure constitutes the first cyclen-catalyzed synthesis of nitroalcohols and is adapted to the condensation of both aromatic and aliphatic aldehydes with nitromethane in THF at room temperature without addition of stoichiometric amount of the base. A wide range of substrates, β-nitroalcohols, were obtained in moderate to good yields (up to 98%) using this methodology.     

Synthesis of hydrophosphorylated fullerene under neutral conditions

[chemical reaction: see text]. A variety of phosphorous compounds such as secondary phosphines, phosphine oxides, phosphinates, and phosphonates undergo addition to [60]fullerene in DMSO/C6H5Cl without extraneous reagents to produce hydrophosphorylated fullerene derivatives in moderate to high yield.     

Synthesis of imidazolo analogues of the oxidation-reduction cofactor pyrroloquinoline quinone (PQQ)

Parallel syntheses of 2-hydro-, 2-methyl-, and 2-methoxycarbonylimidazo-7,9-dimethoxycarbonyl analogues of the oxidation-reduction cofactor pyrroloquinoline quinone [4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylic acid] have been developed. The properties of the imidazolo analogues in relation to the corresponding pyrrole analogues will be important in assessing the origins of catalysis and biological activity in the cofactor, which has recently been shown to be a vitamin.     

Asymmetric phase-transfer reactions under base-free neutral conditions

Although quaternary onium salt-catalyzed phase-transfer reactions are generally believed to require base additives, we discovered even without any base additives conjugate additions of 3-substituted oxindoles proceeded smoothly in the presence of lipophilic quaternary onium bromide under water-organic biphasic conditions. The mechanism of this novel base-free neutral phase-transfer reaction system was investigated, and the assumed catalytic cycle was presented together with interesting effects of water and lipophilicity of the phase-transfer catalyst. The base-free neutral phase-transfer reaction system could be applied to highly enantioselective conjugate additions, aldol reaction, sulfenylation, and chlorination under the influence of chiral bifunctional onium bromides as key catalysts.     

FTIR spectroscopic studies of the CH3S + NO2 reaction under atmospheric conditions

Product studies using FTIR absorption spectrometry have been performed in a 420 xxx reaction chamber on the 254 nm photolysis of mixtures containing CH₃SSCH₃ and NO₂ at ppm concentrations in 760 Torr of O₂/N₂ diluent. The results indicate that the CH₃S radicals formed by photolysis of CH₃SSCH₃ react primarily with NO₂, forming CH₃SO and NO. In the presence of O₂ an unstable intermediate whose IR absorption spectrum resembles a peroxynitrate compound is observed. The intermediate has been tentatively assigned to methyl sulfinyl peroxynitrate, CH₃S(O)OONO₂. Other products include SO₂, CH₃SNO, CH₃SNO₂, CH₃NO₃, CH₃SO₃H, and HCHO.     

Voltammetric and spectroscopic studies on binding of antitumor Morin, Morin-Cu complex and Morin-beta-cyclodextrin with DNA

A systematic comparative study of the binding of antitumor Morin and its complexes with DNA has been investigated in the Britton-Robison (BR) buffer solutions using voltammetric and spectroscopic methods. The results show that Morin molecule, acting as an intercalator, is inserted into the cavity of the beta-cyclodextrin (beta-CD) as well as into the base stacking domain of the DNA double helix. The interaction of Morin-Cu complex or the inclusion complex of Morin-beta-CD with ds-DNA causes hypochromism in the absorption spectra, along with pronounced changes in the electrochemical behavior of the Morin complexes. An isobestic point and a new spectrum band appeared indicating the formation of the new system of Morin-Cu-DNA at lambda(m)=391 nm and Morin-beta-CD-DNA at lambda(m)=375 nm. The intercalation of Morin-Cu and Morin-beta-CD complexes with DNA produces an electrochemically inactive supramolecular complex. The binding constants were calculated from the increase of the solubility, the strong hypochromism, and the decrease in peak current of Morin and its complexes upon the addition of the host molecules. Calculation of the thermodynamic parameters of the interaction of the inclusion complex of Morin-beta-CD with DNA, including Gibbs free energy change, Helmholz free energy and entropy change shows that the complexation is a spontaneous process of association.     

Comparative studies of the behavior of neutral and ionic ferrocene derivatives under different conditions of electrospray ionization

We have studied the behavior of ferrocene CpFeCp (FcH), ferrocenium triiodide [FcH]⁺I₃⁻, dimethylaminomethylferrocene FcCH₂NMe₂ and its trimethylammonium salt [FcCH₂NMe₃]⁺I⁻ under the conventional conditions of electrospray ionization (ESI), when the substance solution is subjected to spraying, and in two versions of desorption electrospray ionization (DESI), when the sprayed solvent bombards the surface of solid or liquid samples. In addition to these techniques, the behavior of neutral compounds under conditions of electrospray ionization of vapors of the studied compounds in a gas phase (ESI_V) has been investigated. It has been shown using the examples of ferrocene and its dimethylaminomethyl derivative that the detection limits for these compounds occurring in a gas phase are comparable within an order of magnitude with their detection limits under the ESI and DESI conditions of solid and liquid samples. The high effectiveness of ionization of analyte vapors makes it possible to use the ESI method not only in combination with liquid (conventional ESI technology) and thin layer chromatography (DESI), but also with gas liquid chromatography (ESI_V). Thus, the electrospray ionization becomes a universal method allowing studies of a compound under the natural conditions in any state of aggregation, that is, solid, liquid, and gas. With the help of statistical methods for designing experiments (complete factorial experiment), quantitative evaluation of the influence of experimental parameters on the ion-formation processes under different ESI conditions has been carried out, which makes it possible to purposefully select the optimal conditions to record the ESI mass spectra with a minimum number of experiments. Moreover, analysis of the dependences of the mass spectra on the experimental parameters can serve as an instrument for studying the details of the ion-formation mechanisms depending upon different ways of ionization.     

Autoxidation of salvinorin A under basic conditions

[reaction: see text] Treatment of salvinorin A (1a) with KOH in MeOH gave the enedione 3, for which the dienone structure 7 was recently proposed. Also isolated, after methylation, were the secotriesters 4a-c. A mechanism for this unusual series of autoxidations is proposed. Surprisingly, 4a showed weak affinity at the kappa-opioid receptor. Divinatorins A-C (2a-c) showed no affinity at opioid receptors. Attempted reduction of 3 to a novel salvinorin diol (9d) was unsuccessful, but careful deacetylation of salvinorin C (9a) provided a viable route to this compound. A general method for identifying salvinorin 8-epimers by TLC is also presented.     

Alkylation of H-phosphinate esters under basic conditions

An efficient and general procedure was developed for the direct alkylation of H-phosphinate esters with LHMDS at low temperature. The simplicity of the reaction allows the use of various H-phosphinate esters and takes place with a wide range of electrophiles. The approach can be employed to access some GABA analogues or precursors to GABA analogues. The isolated yields are moderate to good. This is the first report of an alkylation with a secondary iodide or a primary chloride.     

Electrochemical behavior of pyrroloquinoline quinone immobilized on silica gel modified with zirconium oxide

Pyrroloquinoline quinone (PQQ) was immobilized on the silica gel surface modified with zirconium oxide, designated as Si:Zr, by the carboxylic groups of the PQQ molecule and the zirconium oxide on the silica surface. The electrochemistry of PQQ immobilized on the Si:Zr matrix, incorporated in a carbon paste electrode, was evaluated using the cyclic voltammetry technique. The Si:Zr:PQQ-modified electrode showed a redox couple at E(m)=(E(pa1)+E(pc))/2=-0.150 V vs SCE at pH 7, close to that observed in aqueous solution, and another oxidation peak, E(pa2)=-0.100 V vs SCE. Studies in different pH solutions in the range of 3-7 showed that the first oxidation peak, E(pa1), is highly dependent on the solution pH shifting from to -0.175 to 0.100 V vs SCE, while E(pa2) remains practically constant at 0.100 V as the pH decreases from 7 to 3. The immobilized PQQ electrode presented the property to electrocatalyze the NADH at 150 mV vs SCE. The effect of addition of Ca(2+) ions on the electrode electroactivity for the NADH oxidation was also verified. Different from that observed for the PQQ immobilized on other electrode materials, the Ca(2+) ions did not influence the electrocatalytical response; however, the electrode stability was considerably improved in the presence of Ca(2+) ions, indicating that the matrix surface has a great influence on the electrochemical behavior of PQQ.     

Cyclopentene-regioselective palladium-catalyzed cycloisomerization under neutral and bis-cationic reaction conditions

A series of 1,6-dienes is cyclized to cyclopentene derivatives under neutral conditions with palladium chloride in ethanol or with in situ generated LPd²⁺ in acetonitrile.     

Palladium-catalyzed alpha-arylation of esters and amides under more neutral conditions

Two procedures for the alpha-arylation of carbonyl compounds under conditions that are more neutral than those of reactions of aryl halides with alkali metal enolates are reported. The first procedure rests upon the development of catalysts bearing the hindered pentaphenylferrocenyl di-tert-butylphosphine (Q-phos) and the highly reactive dimeric Pd(I) complex {P(t-Bu)3]PdBr}2. By this procedure, zinc enolates prepared from alpha-bromo esters and amides react with aryl halides to form alpha-aryl esters and amides in high yields under mild conditions with 1-2 mol % catalyst and with remarkable functional group tolerance. By the second procedure, silyl ketene and silyl ketimine acetals react with aryl bromides in the presence of substoichiometric zinc fluoride, 1 mol % Pd(dba)2, and 2 mol % P(t-Bu)3 in DMF solvent at 80 degrees C. Reactions of zinc tert-butyl acetate and propionate enolates and trimethylsilyl ketene acetals of tert-butyl propionate and methyl isobutyrate with aryl bromides bearing electron-donating and potentially reactive, base-sensitive electron-withdrawing groups and with pyridyl bromides are reported. In addition, the diastereoselective coupling of phenyl bromide with an imide enolate bearing the Evans auxiliary is reported, and this study shows that racemization of base-sensitive stereocenters does not occur during the coupling process under these more neutral conditions.