The structure of the homogeneous oxidation catalyst, Mn(II)(Br(-1))x, in supercritical water: an X-ray absorption fine-structure study

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to probe the first-shell coordination structure about Mn(II) and Br(-1) ions that exist as contact ion pairs in supercritical water. This work was performed to clarify why solutions of MnBr2 in supercritical water are known to effectively catalyze the aerobic oxidative synthesis of terephthalic acid from p-xylene as well as a number of other methylaromatic compounds. The Mn and Br K-edge spectra were collected at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne National Laboratory. The first-shell coordination structure about the Mn(II) ion changes from octahedral at ambient conditions to tetrahedral at supercritical conditions. Under supercritical conditions, the measured bond distances of Mn-OH2 and Mn-Br are 2.14 and 2.46 A, respectively. Direct contact ion pairs form with about 2 Br(-1) ions present in the first coordination shell of the Mn(II) ion. The structure of dissolved MnBr2, below 1.0 m, changes from essentially [Mn(II)(H2O)6]+2 to [Mn(II)(H2O)2(Br(-1))2] in supercritical water (scH2O). When an excess of Br(-1) ion is added, the bromide coordination number increases and the number of water molecules decreases. The results show that the initial MnBr2 catalyst in scH2O is tetrahedral with two Mn-Br contact ion pairs. The presence of the acetate anion deactivates the catalyst by formation of insoluble MnO.     

Kinetics and mechanism of p-isopropenylphenol synthesis via hydrothermal cleavage of bisphenol A

Although bishydroxyarylalkanes are known to be reactive in high-temperature (T > 200 degrees C) liquid water (HTW), no mechanistic insight has been given to explain the reactivity of methylene bridge-containing diaryls under hydrothermal conditions. We examined the kinetics and mechanism of p-isopropenylphenol (IPP) synthesis via bisphenol A (BPA) cleavage in HTW. The cleavage reaction is first order in BPA. Cleavage of BPA in HTW occurs by specific acid catalysis, by specific base catalysis, and by general water catalysis. Under neutral conditions, the dominant mechanism is general base catalysis with water serving as the proton acceptor. We generated a detailed chemical kinetics model for the decomposition reaction based on a base-catalyzed mechanism in the literature. This three-parameter model fit the experimental data for BPA disappearance and formation of IPP and phenol and accurately predicted the yield of the IPP hydrolysis product acetone. Using acid- and base-catalyzed mechanisms, we explain the reactivity in HTW reported for other diaryl groups linked by methylene bridges and propose criteria for assessing the reactivity of methylene bridges under hydrothermal conditions.     

Thick Silicate Film by an Interfacial Polymerization

A thick silica gel film, corresponding to the glass film of 10–20 micron in thickness, has been formed at the interface between two immiscible liquids, hexane and water, using E-40, a partially polymerized silicon alkoxide, as the precursor. The film formation was possible using both acid- and base-catalyzed water, but was greatly dependent on the type of catalyst. Only the trace of a film was observed for the system catalyzed with a strong electrolyte such as HCl, HNO₃ or NaOH, while a gel film, corresponding to the glass film of several to 10 micron in thickness was formed with a weak electrolyte such as ammonia, organic acid like acetic acid, citric acid, etc., of similar pH value. The direct introduction of organic base catalyst like triethylamine in hexane was much more effective than the use of ammonia water, suggesting that the polymerization of E-40 to form a gel film takes place in the organic phase, where water molecules, as well as undissociated ammonia or organic acids, can diffuse in.     

An organic-based pH oscillator

In a recent paper, we suggested that the acid- or base-catalyzed dehydration of a hydrated carbonyl compound provides a suitable foundation for an organic-based pH oscillator. Here we present the first experimental example of such an oscillator in a flow reactor, utilizing the base-catalyzed dehydration of methylene glycol as a source of positive feedback (OH- autocatalysis) coupled with the base-catalyzed hydrolysis of gluconolactone for negative feedback (H+ production). The large amplitude oscillations (between pH 7 and 10) are reproduced in a kinetic model of the reaction. Such experiments present new possibilities in the design of pH oscillators.     

Divergent kinetic control of classical versus ozonolytic lactonization: mechanism-based diastereoselection

A strategy has been developed in which mechanistically distinct lactonization reactions are used to prepare diastereomeric delta-lactones relevant to the C1-C9 fragment of (+)-peloruside A. Depending upon which of two reaction types is used, the central (C5) hydroxyl group can be directed to differentiate the C1 versus C9 termini of pseudosymmetric substrates to provide diastereomeric lactones. Thus, the 5-hydroxy-1,9-diester substrate 1 (an azelaic ester) cyclizes under classical (acid- or base-catalyzed) lactonization conditions to give a predominance of one diastereomer, whereas the 5-hydroxy-1,10-diene congener 2 provides the opposite sense of diastereoselectivity when subjected to ozonolytic lactonization (O3, MeOH, NaOH, at -78 degrees C). Thus, this under-utilized oxidative transformation is mechanistically orthogonal to the classical reaction.     

Enhanced Acid/Base Catalysis in High Temperature Liquid Water

High temperature liquid water (HTLW), as an environmentally benign medium, has drawn increasing attention for organic chemical reactions and biomass conversion1-3. HTLW has a strong tendency to ionize and can act as an acid and/or base catalyst. In addition, HTLW can dissolve organic compounds to some extent allowing for a homogenous reaction within an aqueous phase. Extensive researches4-10 have been pursued on acid/base-catalysis reactions in HTLW without addition of acid or base. Howe…     

Vinylidene-quinone methides, photochemical generation and β-silicon effect on reactivity

Irradiation of 2-alkynylphenols resulted in the generation of vinylidene-quinone methides (QMs), which were detected by laser flash photolysis in organic solvents and aqueous acetonitrile. QMs' spectroscopic properties and electrophilicity were both significantly affected by β-silicon effect. The hydration of the alkynyl moiety (22 and 900 M(-1) s(-1)for QM-1 and QM-2, in aqueous acetonitrile) was an acid- and base-catalyzed process. The addition of amines was fast (9.2 × 10(3) M(-1) s(-1) < k(2) < 1.3 × 10(8) M(-1) s(-1)), yielding ketimines, with primary amines.     

Control of Chemical Reactions by Using Molecules that Buffer Non-aqueous Solutions

Control of chemical reactions is necessary to obtain designer chemical transformation products and for preventing decomposition and isomerization reactions of compounds of interest. For the control of chemical events in aqueous solutions, the use of aqueous buffers is a common practice. However, no molecules that buffer non-aqueous solutions were commonly used. Herein, we demonstrate that 1,3-cyclohexanedione derivatives have buffering functions in non-aqueous solutions. It was also shown that these molecules can be utilized to alter and control chemical reactions. 1,3-Cyclohexanedione derivatives inhibited both acid- and base-catalyzed isomerizations and decompositions in organic solvents. The reaction products obtained in the presence of the buffering molecule 2-methyl-1,3-cyclohexanedione differed from those obtained in the absence of the buffering molecule. The use of buffering molecules that work in organic solvents provides a strategy to control chemical reactions and expands the range of compounds that can be synthesized.     

Electrophilic and Nucleophilic Addition of Alkanethiols to 2-Vinyloxyethylmethacrylate

Under acid- or base-catalyzed conditions, thiols add regio- and chemos-electively either to vinyloxy or methacrylate group of vinyloxyalkyl-methacrylates to give polyfunctional methacrylates or polyfunctional vinyl ethers, respectively.     

Microwave promoted synthesis of cycl[3.2.2]azines in water via a new three-component reaction

A microwave-promoted and base-catalyzed synthesis of cycl[3.2.2]azines is accomplished in water via a three-component reaction (3-CR) of 2-picoline, α-bromoacetophenone and alkyne. The extension of the methodology to the synthesis of steroidal and carbocyclic cycl[3.2.2]azine is also reported.     

无机盐水合物对有机溶剂中酶促寡肽合成的影响

必需水的存在是酶在有机溶剂中保持活性的关键。本文研究了自由水初始水含量对有机溶剂中α－胰凝乳蛋白酶(α-chymotrypsin)和嗜热杆菌蛋白酶粗品(thermoase)催化合成寡肽的影响。用无机盐水合物Na2SO4·10H2O和Na2CO3·10H2O代替自由水重复了上述反应。实验表明,足量的无盐水合物能在有机溶剂中可逆地失去自射所带的结晶水,因而能代替自由水来提代酶所需的"必需水",并有效地控制有机溶剂中酶促反应体系的水活度(αw)值,从而有利于肽键的形成。     

An activated sulfonylating agent that undergoes general base-catalyzed hydrolysis by amines in preference to aminolysis

Activated sulfonyl derivatives, similar to acyl ones, usually undergo aminolysis with amines in water as nucleophilic attack by the amine is preferred to hydrolysis. However, despite being active sulfonyl derivatives, four-membered heterocyclic sulfonamides, beta-sultams, do not undergo aminolysis in aqueous solution but preferentially react to give hydrolysis products only. The rate of the reaction of beta-sultams in buffered solutions of simple primary amines shows a first-order dependence on amine concentrations attributed to general base-catalyzed hydrolysis by the amine. Even N-benzyl-4,4-dimethyl-3-oxo-beta-sultam, which is both a beta-sultam and a beta-lactam, undergoes hydrolysis at the sulfonyl center rather than aminolysis at either the sulfonyl or acyl center. The solvent kinetic isotope effects (SKIE, k(H(2)O)/k(D(2)O)) for the amine-catalyzed hydrolyses are 1.4 and 1.9 for the hydrolysis of N-benzoyl-beta-sultam and N-benzyl-4,4-dimethyl-3-oxo-beta-sultam, respectively, compatible with a general base-catalyzed mechanism. The amine-catalyzed hydrolysis gives a Bronsted beta value of +0.9 for both N-benzoyl beta-sultam and N-benzyl-4,4-dimethyl-3-oxo-beta-sultam, indicating that the general base amine is almost fully protonated in the transition state. A general base-catalyzed mechanism for hydrolysis rather than nucleophilic attack was also deduced for the reaction of N-benzyl-4,4-dimethyl-3-oxo-beta-sultam with carboxylate anions based on a SKIE of 1.7-1.9 and rate constants which fit the Bronsted plot for amines. In contrast to acyl transfer reactions, those for sulfonyl transfer appear to show an inverse reactivity-selectivity relationshipthe most active compounds being the most selective. The lack of reactivity of beta-sultams toward amine nucleophiles appears to be related to the mechanism of ring opening of beta-sultams with a decreased reactivity toward amines relative to hydroxide ion, probably related to the expulsion of the relatively poor leaving group amide anion.     

Silicone nanocapsules templated inside the membranes of catanionic vesicles

A simple and effective way to synthesize hollow silicone resin particles of controlled diameter is presented. The synthesis utilizes catanionic vesicles as templates for the polycondensation/polymerization processes of 1,3,5,7-tetramethylcyclotetrasiloxane (D4H) within their membranes. Two different surfactant systems were used to form the vesicular templates: mixtures of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylbenzenesulfonate (SDBS) in the cationic (the DTAB/SDBS system) or anionic (the SDBS/DTAB system) rich region of the phase diagram. The templates obtained from these surfactant mixtures form spontaneously unilamellar vesicles in aqueous solution. The vesicular templates swell upon addition of D4H, thus increasing their size. The silicone resin was obtained in acid- or base-catalyzed polycondensation and ring-opening polymerization processes of D4H. In the case of the DTAB/SDBS system the formation of a densely cross-linked silicone material with SiO3/2 units allowed the nanocapsules to retain the vesicular shape after removal of the template, whereas in the SDBS/DTAB system, the polymer produces capsules which are too smooth to support surfactant lysis. The morphology of the silicone nanocapsules was analyzed using transmission electron microscopy (TEM) and, in some cases, atomic force microscopy (AFM). TEM and AFM reveal discrete hollow particles with a small amount of linked or aggregated hollow silica shells.     

New strategies for synthesis of partially fluorinated heterocycles as precursors for trifluoromethyl-substituted polymers

The building-block strategy is applied to the synthesis of new compounds with two five-membered heteroaromatic units. The heteroaromatic rings contain a fluorine atom which is activated towards nucleophilic displacement reactions by an adjacent trifluoromethyl group. The difunctional compounds can be converted into poly(arylene ether)s by base-catalyzed polycondensation with bisphenols or bisthiophenols. Trifluoromethylated poly(arylether oxazole)s and poly(arylthioether oxazole)s are obtained by this route. All representatives of these two new classes of polymers are soluble in common organic solvents. Reaction conditions for the polycondensation as well as thermal stability, glass transition temperatures, and molar masses of the polymers are described.     

Control of carboxylic acid and ester groups on chromium (VI) binding to functionalized silica/water interfaces studied by second harmonic generation

Resonantly enhanced surface second harmonic generation (SHG) measurements carried out at pH 7 and room temperature were performed to study how surface-bound carboxylic acid and methyl ester functional groups control the interaction of chromate ions with fused silica/water interfaces. These functional groups were chosen because of their high abundance in humic and fulvic acids and related biopolymers commonly found in soils. They were anchored to the silica surface using organosilane chemistry to avoid competing complexation processes in the aqueous solution as well as competitive adsorption of the organic compounds and chromate. The SHG experiments were carried out at room temperature and pH 7 while using environmentally representative chromate concentrations ranging from 1 x10(-6) to 2 x 10(-4) M. Chromate is found to bind to the acid- and ester-functionalized silica/water interfaces in a reversible fashion. In contrast to the plain silica/water interface, chromate binding studies performed on the functionalized silica/water interfaces show S-shaped adsorption isotherms that can be modeled using the Frumkin-Fowler-Guggenheim (FFG) model. This model predicts a coverage-dependent binding constant of K(ads) x exp(gtheta). Values for g are found to be 3.2(2), 2.1(2), and 1.3(2) for the carboxylic acid-, the ester-, and the nonfunctionalized silica/water interfaces, respectively, and are consistent with stabilizing lateral adsorbate-adsorbate interactions among the Cr(VI) species adsorbed to the functionalized surfaces. The FFG model allows for the parametrization of the solid-liquid partition coefficient and chromate retardation factors in silica-rich soil particles whose surfaces contain organic adlayers rich in carboxylic acid and methyl ester groups. The straightforward model presented here predicts that chromate retardation increases by up to 200% when carboxylic acid functional groups are present at the silica/water interface. Increases up to 50% are predicted for methyl ester-containing organic adlayers, and the retardation factor remains effectively near unity for the plain silica/water interface (no siloxanes present).     

Calcium peroxide diperoxohydrate as a storable chemical generator of singlet oxygen for organic synthesis

Calcium peroxide diperoxohydrate (CaO(2).2H(2)O(2)) is an environmentally friendly generator of singlet oxygen ((1)O(2), (1)Delta(g)) that can be used in organic synthesis as an alternative to the regular photochemical method. This compound produces (1)O(2) in various solvents and can be easily recovered by filtration for further regeneration. Both monitoring of (1)O(2) luminescence at 1270 nm and specific trapping have shown that CaO(2).2H(2)O(2) can be stored for several days at -80 degrees C and that the yield of (1)O(2) is equal to 25%. Oxidation of typical organic substrates in methanol or THF through [4 + 2] or [2 + 2] cycloaddition and ene reaction have been carried out on a preparative scale with total conversion and selectivity.     

Facilitated Intramolecular Conjugate Addition of Amides of 3-(3',6'-Dioxo-2',4'-dimethyl-1',4'-cyclohexadienyl)-3,3-dimethylpropionic Acid. 2. Kinetics of Degradation

The chemical stability studies of amides of 3-(3',6'-dioxo-2',4'-dimethyl-1',4'-cyclohexadienyl)-3,3-dimethylpropionic acid (Qa) [Qop(a-j)] were conducted in order to determine the utility of this redox-sensitive system as a potential prodrug promoiety or redox-sensitive protecting group in organic synthesis. This study showed that quinone propionic amides of aniline derivatives [Qop(a-d)] underwent rapid degradation in mildly acidic conditions (pH 4.5-6.0) to yield degradation products resulting from the intramolecular 1,2- or 1,4- conjugate addition of the amide nitrogen to the quinone ring. This conjugate addition was found to be specific base-catalyzed and independent of the para substituent on the aromatic ring of the amine. The predominant route of degradation yielded a five-membered ring spirolactam. By altering the nature of the amine component of the amide, these degradation reactions were prevented. Amides of Qa other than those of the aniline type [Qop(e-j)] were found to be substantially more stable and were thus proposed as the more suitable candidates for this potential redox-sensitive prodrug system and redox-sensitive protecting group for amines and alcohols in organic synthesis.     

Extraction of soluble fiber from distillers' grains

The feasibility of using coproducts from dry grind corn ethanol production as a substrate for the production of soluble fiber was examined. Acid- and base-catalyzed hydrolysis experiments were performed using sulfuric acid and sodium hydroxide to partially hydrolyze hemicellulose content of whole stillage, a precursor to distillers' grains, to soluble fiber. The influences of temperature, reaction time, and hydrolyzing agent concentration on the formation of soluble fiber were studied. Soluble fiber was recovered by precipitation in a 95% ethanol solution. Results indicate that appreciable quantities of soluble fiber may be extracted using either acid- or base-catalyzed reactions. The highest yield of soluble fibers was 13.2 g per 100 g-db of treated whole stillage using one weight percent sodium hydroxide at 80oC for 1 h. HPLC analysis was used to quantify the amount of monomeric sugars which were formed during the hydrolysis procedures.     

On the synthesis and reactivity of the Z-2,4-dinitrophenylhydrazone of 5-amino-3-benzoyl-1,2,4-oxadiazole

The synthesis of the title compound (4b) has been completed: its rearrangement (in dioxane/water; 1:1, v/v) into N-(2,4-dinitrophenyl)-5-phenyl-2H-1,2,3-triazol-4-ylurea (7) has been quantitatively studied in a wide reactivity (at 293 K, k(A) 10(-8) -4 s(-1)) and pS+ (4.5-14.1) range and compared with that of the Z-2,4-dinitrophenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole (10), of the 3-(p-nitro)phenylureine of 5-phenyl-1,2,4-oxadiazole (13), and of N-(5-phenyl-1,2,4-oxadiazol-3-yl)-N'-p-nitrophenylformamidine (14). The results (reactivity, occurrence of specific or general base-catalysis, evidence for or absence of rate-limiting constants) have been well interpreted considering the structure of the side-chains involved and the stability of the final rings obtained in the rearrangements.