Base-catalyzed System Research Articles

Mechanistic study on the reaction of pinB-BMes2 with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent.

Transition metal-free direct and base-catalyzed 1,2-diborations of arylacetylenes using pinB-BMes2 provided a syn/anti-isomeric mixture of diborylalkenes. The kinetic analysis showed that the reaction rate and isomer ratio were affected by reaction conditions and substituents on the aryl ring. DFT calculations indicated that direct addition proceeded via the interaction of acetylene-π with the BMes2 fragment. In contrast, for the base-catalyzed diboration, the previously isolated sp2–sp3 diborane and borataallene were confirmed as stable intermediates by calculations. The whole reaction pathways can be divided into the Bpin-migration and deprotonation steps, where the borataallene should be considered as a common intermediate. It should be noted that the deprotonation step is reversible and affords the kinetically less favoured isomer under the thermodynamic conditions. As a result, the composition of isomeric products, in the base-catalyzed diboration, is attributed to the small difference of activation barriers between direct and base-catalyzed systems.

Investigation of microwave-assisted transesterification reactor of waste cooking oil

In this work, the esterification and trans-esterification reactions of waste cooking oil (WCO) with high free fatty acids (FFA) (≥1% by weight) were investigated. This investigation used a two-step batch process with kinetic based reaction mechanisms and examined potential benefits of combining the conventional Fatty Acid Methyl Ester (FAME) trans-esterification method with microwave technology. Optimization of an acid-catalyzed FAME process to minimize FFA content in the feedstock found that a feed volume ratio of 0.3 L of methanol per liter of WCO, 2% grams of sulfuric acid (H2SO4) per gram of WCO for a reaction time of 200 min at a reaction temperature of 60 °C produced a biodiesel yield of 24%. By comparison, optimization of a base-catalyzed FAME process found that a feed volume ratio of 0.3 L methanol per liter WCO oil, 1 g of potassium hydroxide (KOH) per gram of WCO for a reaction time of 60 min at a reaction temperature of 60 °C produced a biodiesel yield of 93–98%. When microwave irradiation was included, the reaction time for the acid-catalyzed system was 80 min (compared to 200 min) and 10 min for the base-catalyzed system (compared to 60 min). These preliminary results suggest microwave technology merits further investigation for industrial application to biodiesel production.

Mechanical properties of thiol-ene UV-curable thermoplastic polysilsesquioxanes

A series of thiol-ene UV-curable inorganic–organic hybrid polysilsesquioxanes of various phenyl:methacryloxypropyl and phenyl:mercaptopropyl copolymer compositions were synthesized in one-batch using a base-catalyzed system. These thermoplastic photocurable ladder-like structured materials were examined as scratch-resistant, hard-coating materials on both glass and PET substrates. The unique fully condensed structure of the hybrid polymers allowed for simplified processing and through variation of the organic functional group, films with high transparency, tunable refractive indices in the range of 1.467–1.546, pencil hardness values of 9H, and high nanoindentation modulus were obtained (7 GPa). These scratch-resistant polysilsesquioxanes may provide insight into the different organic functional group compositions which allow for optimal mechanical properties.

Synthesis and characterization of organic-inorganic hybrid block copolymers containing a fully condensed ladder-like polyphenylsilsesquioxane

A series of inorganic–organic hybrid block copolymers were synthesized via atom transfer radical polymerization using a fully condensed, ladder-like structured polyphenylsilsesquioxane end-functionalized macroinitiator. The inorganic portion, ladder-like polyphenylsilsesquioxane, was synthesized in a one-batch, base-catalyzed system, whereas organic hard and soft monomers, styrene, and n-butyl acrylate, were polymerized and copolymerized on the ends of the linear, inorganic backbone. Synthesized hybrid diblock, triblock, and random copolymers were characterized by 1H NMR, 29Si NMR, gel permeation chromatography, static light scattering, Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. Hybrid block copolymers were well-defined with low polydispersity (<1.4) and exhibited enhanced thermal properties in the form of increased glass transition and degradation onset temperatures over their organic analogues.© 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Exploring Catalyst and Solvent Effects in the Multicomponent Synthesis of Pyridine-3,5-dicarbonitriles

The effects of an ionic base, tetrabutylammonium hydroxide (TBAH), and an amine base, piperidine, on the direct synthesis of pyridine-3,5-dicarbonitriles using a multicomponent reaction (MCR) from aldehydes, malononitrile, and thiols were systematically investigated. The amine base showed better results when the MCR was performed in ethanol, whereas employing the ionic base in acetonitrile resulted in similar yields but in a much shorter reaction time. A modified protocol to overcome the difficulty in the direct synthesis of pyridine-3,5-dicarbonitriles via the MCR from sterically hindered aldehydes using either base was realized by changing the reaction solvent from ethanol to acetonitrile. Mechanistically, the two catalysts were found to each promote different pathways in the final oxidation step of the penultimate product, 1,4-dihydropyridine 6. A reaction intermediate, Knoevenagel adduct 7, plays the major role in the amine base-catalyzed system, while in the presence of an ionic base, aerobic oxygen acts as the primary oxidant.

Structure of Arylene-Bridged Polysilsesquioxane Xerogels and Aerogels

Arylene-bridged polysilsesquioxanes are an interesting class of porous materials prepared by sol−gel processing of ethoxysilane monomers in which there are two or more trialkoxysilyl groups positioned about an arylene bridging group. The majority of these materials are highly porous with surface areas as high as 1880 m2/g. In an effort to understand the nature of porosity in these materials, small-angle X-ray and neutron scattering were employed to characterize phenylene-, biphenylene-, and terphenylene-bridged polysilsesquioxanes. Phenylene-bridged polysilsesquioxane xerogels and aerogels were also compared to understand the effect of drying protocol on pore structure. The effect of catalyst concentration is also reported for the base-catalyzed system. In all cases studied here, we find evidence for domains in the nanometer range with distinct fractal character. We associate these domains with porosity rather than microphase separation of organic and inorganic moieties. The nature of this porosity depend...

Dynamic Light Scattering Studies on Network Formation of Bridged Polysilsesquioxanes Catalyzed by Polyoxometalates

The sol−gel process of 1,8-bis(triethoxysilyl)octane (TES-Oct) catalyzed by phosphotungstic acid (PWA) has been investigated by time-resolved dynamic light scattering. The time−intensity correlation functions (ICF) exhibited two modes, corresponding to the cooperative diffusion of entangled chains and the translational diffusion of cross-linked clusters. Below the gelation threshold the ICFs were well described by sum of two exponential functions, allowing the successful analysis of the relaxation times and the corresponding relative intensity for the fast and slow modes. On the contrary to general observation of acid-catalyzed systems, the specific gelation process was found for rPWA = 0.100, the molar ratio of PWA to TES-Oct. Namely, the relative amplitude for the fast relaxation, Af, and the averaged slow relaxation time, 〈τslow〉, exhibit exceptionally larger values in the gelation process, suggesting the existence of the ionized larger clusters as found for the base-catalyzed system. The hydrodynamic ...

Transition-Metal Catalysis of the Michael Reaction of 1,3-Dicarbonyl Compounds and Acceptor-Activated Alkenes

Transition-metal catalysis of the Michael reaction of 1,3-dicarbonyl compounds with acceptor-activated alkenes is introduced as a valuable alternative to the classic base catalysis of this reaction. Owing to the mild, neutral reaction conditions, the chemoselectivity of these processes is superior to that offered by the base catalysis, since the latter suffers from various unwanted side- and subsequent reactions, such as aldol cyclizations and ester solvolyses. The most efficient transition-metal catalysts do not require inert or anhydrous conditions, even solvents are unnecessary in some cases, and quantitative conversions can be achieved at room temperature. Furthermore, the development of transition-metal catalysts on solid supports has allowed very simple work-up procedures. Despite the extraordinary chemoselectivity, in terms of diastereoselectivity transition-metal catalysis gives better results than base catalysis only in a very few special cases. Also, in terms of enantioselectivity, results recently achieved with basic rare-earth metal catalysts cannot be reproduced by transition metals. Nevertheless, with transition-metal catalysis, even new reactivities can be observed, which have hitherto been unknown for base-catalyzed systems. For example, Michael donor molecules have been found to react in an unprecedented vinylogous fashion.

New application of solid base to regioselective ring openings of functionalized epoxides and oxetanes with cyanotrimethylsilane

Solid bases such as magnesium oxide, calcium oxide, and hydroxyapatite were successfully applied to the reactions of cyanotrimethylsilane with epoxides and oxetanes bearing alkyl substituents and diverse functional groups. The solid base-catalyzed systems were found to induce the ring openings of epoxides and oxetanes much more efficiently and regioselectively, as compared with the conventional, homogeneous acid-promoted systems. The high regioselection is ascribed to a nucleophilic ring opening of a cyanide ion through the coordination of cyanotrimethylsilane to base sites on solid bases.

Effects of reaction parameters on the preparation of thick silica gel films by an interfacial polymerization. 1. Effects of pH and type of catalyst

The polymerization of different silicon precursors at the interface of two immiscible liquids (“organic” and “inorganic” phase) led to the formation of thick (up to 40 μm) and crack-free silica gel films. The pH of the “inorganic” phase and the type of catalyst strongly governed the rate of film formation along with the gel film properties. The preparation of thick and crack-free gel films within 24 hours of reaction time was possible for both acid-catalyzed and base-catalyzed systems in the vicinity of pH=2 and 11, respectively. The use of acetic acid as an catalyst led to gel films with a weight eight times higher than those prepared using HCl and HNO3 catalysts. The weight of formic acid-catalyzed gel films was four times higher, and the weight of citric acid-catalyzed gel films was twice as high as the mineral acid-catalyzed gel films, respectively.

Base-Catalyzed Silica Gels: Structure and Chemistry

ABSTRACTThe chemical and physical properties of two base-catalyzed silica gels systems were characterized. The sol-gel transition was studied by titration, rheometry, and NMR. Pyrolysis behavior was monitored by TGA and mass spectroscopy. Many characteristics were strikingly similar for the two base-catalyzed systems. However ammonium hydroxide additions resulted in heterogeneities at lower concentrations than ammonium carbonate. Ammonium carbonate enhanced oxidation on pyrolysis.

Precursor Chemistry and the Structure of Silica Aerogels

ABSTRACTSmall-angle X-ray scattering is used to characterize the structure of aerogels prepared by two-stage polymerization processes. Second-stage catalysis controls the resistance to collapse during drying with the base-catalyzed system being most resistant. Base catalysis in the second stage leads to compaction of the polymer network on short length scales. This short-scale rigidity makes the networks sufficiently robust to withstand the surface tension forces present during solvent extraction and re-exposure to the atmosphere. Aging in solution also improves aerogel quality. In this case, a dissolution-repolymerization process leads to short length scale circuits and improved rigidity.

A theoretical study of the reactive dimerization of HCN

The reaction of two molecules of HCN to produce isomeric forms of (HCN) 2 has been studied theoretically through the use of CNDO/2 calculations together with a method of energy partitioning. The present work predicts that the most stable product of such a reaction is iminoacetonitrile, and that, of the mechanisms considered in the present work, the energetically most favourable, under isolated conditions, involves the dissociation of HCN to free radicals, followed by the reaction of the CN radical with an undissociated HCN molecule and the subsequent addition of hydrogen followed by rearrangement to the imine. However, the energetics are sufficiently similar that an ionic mechanism involving CN − might be predicted for a condensed, base-catalyzed system.