Potassium sorbate as an efficient and green catalyst for Knoevenagel condensation

Potassium sorbate has been utilized as a novel, efficient and green catalyst for the Knoevenagel condensation of aromatic aldehydes with active methylene compounds to afford substituted ole-fins through the conventional stirring or under ultrasound irradiation. Improvements were observed by carrying out the reactions under ultrasound irradiation. The advantages of this procedure are mild reaction conditions, high yields, cleaner reaction profiles and operational simplicity.     

Ultrasound-enhanced reactivity of calcium in the reduction of aromatic hydrocarbons

Reductions of aromatic hydrocarbons by calcium in ethylenediamine-n-alkylamine mixture were investigated under ultrasonic conditions. Using an ultrasonic probe, with naphthalene as test molecule, it has been demonstrated that under ultrasonic action the reactions proceed faster (x10) and require a lower metal quantity (0.5) than the reactions conducted with an efficient mechanical stirrer. In addition, at ambient temperature and depending on the specific alcohol addition, selective naphthalene reduction can be performed using ultrasound. 1,2-Dihydronaphthalene (88% yield) results from the reaction in the presence of 2-propanol, and 1,4,5,8-tetrahydronaphthalene (88% yield) is obtained with tert-butanol. Investigation of the metal surface points out the characteristics of the calcium ultrasonic activation. The procedure was efficiently tested with several aromatic hydrocarbons.     

Sono-thermal oxidation of dihydropyrimidinones

Combination of ultrasound and heat has been used for the oxidation of some ethyl 3,4-dihydropyrimidin-2(1H)-one-5-carboxylates to their corresponding ethyl pyrimidin-2(1H)-one-5-carboxylates by using potassium peroxydisulfate in aqueous acetonitrile. An ultrasonic probe of 24 kHz frequency has been used for this study. Whereas the use of ultrasound increases the rate of reactions compared with reactions at reflux conditions, the nature of 4-substituent on the dihydropyrimidinone ring affects also the rate of reaction.     

An efficient and practical synthesis of bis(indolyl)methanes catalyzed by aminosulfonic acid under ultrasound

Synthesis of bis(indolyl)methanes via electrophilic substitution reactions of indoles with aromatic aldehydes and ketones catalyzed by aminosulfonic acid was carried out in 23-96% yield at 30-38 degrees C in EtOH aqueous solution under ultrasound irradiation.     

A novel approach for the quantitative kinetic study of reactions at solid/liquid interfaces in the presence of power ultrasound

A novel cell and procedure is described which permits the quantitative mechanistic study of ultrasonically enhanced reactions which occur at solid/liquid interfaces. A model of a controlled and calculable time-average rate of mass transport to and from the interface is used in order to compare experimental results with theoretical predictions based on mechanistic reaction schemes. In this way concentrations of mechanistically significant species near the interface can be related to those in bulk solution and hence the sonochemical effects of ultrasound dissected from those arising purely from mass transport. The effect of ultrasound is demonstrated for the reaction dissolution of p-chloranil in the presence of aqueous base and for the reaction of the same substrate with the aromatic amine, N,N-dimethyl-phenylenediamine, both systems which have been studied previously in the absence of ultrasound. Complementary atomic force microscopy images are also reported.     

Ultrasound-assisted solvent-free synthesis of lactic acid esters in novel SO3H-functionalized Brønsted acidic ionic liquids

Mild and efficient Fischer esterification reactions of lactic acid with a variety of straight chain aliphatic alcohols, cyclohexanol and benzyl alcohol were successfully performed using two novel Brønsted acidic ionic liquids that bear an aromatic sulfonic acid group on the imidazolium or pyridinium cation under ultrasound irradiation. These reactions carried out smoothly with good to excellent conversion rate (78–96%) and satisfactory yields (73–92%) in shorter reaction time (4–6 h) at room temperature when the amount of ionic liquids was 20 mol%. These ionic liquids could be recovered readily and recycled five times without any significant loss in their catalytic activity.     

Sonochemical effects on free phenolic acids under ultrasound treatment in a model system

Sonochemical effects on seven free phenolic acids under ultrasound treatment in a model system have been investigated. The degradation products have also been tentatively identified by FTIR and HPLC-UV-ESIMS. Five phenolic acids (protocatechuic acid, p-hydroxybenzoic acid, vanillic acid, p-coumaric acid, and ferulic acid) proved to be stable, while two others (caffeic acid and sinapic acid) were degraded under ultrasound treatment. The nature of the solvent and the temperature has been identified as important factors in determining the degradation reaction. Liquid height, ultrasonic intensity, and duty cycle of the ultrasound exposure affected only the degradation rate and did not change the nature of the degradation. The degradation rates of caffeic acid and sinapic acid decreased with increasing temperature. The degradation kinetics of these two acids under ultrasound conformed to zeroth-order reactions at ?5 to 25 °C. Both decomposition and polymerization reactions occurred when caffeic acid and sinapic acid were subjected to ultrasound. Degradation products, such as the corresponding decarboxylation products and their dimers, have been tentatively identified.     

One-pot three-component synthesis of 3-hydroxy-5,5-dimethyl-2-[phenyl(phenylthio)methyl]cyclohex-2-enone derivatives under ultrasound

3-Hydroxy-5,5-dimethyl-2-[phenyl(phenylthio)methyl]cyclohex-2-enone is synthesized via one-pot three-component reactions of aromatic aldehyde, substituted thiophenol and 5,5-dimethyl-1,3-cyclohexanedione catalyzed by p-dodecylbenzene sulfonic acid (DBSA) under ultrasound. Under ultrasound irradiation the yields are much higher (sometimes substantially, by almost double) and the reaction time decreases substantially, the reaction conditions are milder. This method provides several advantages such as environment friendliness, high yields and simple work-up procedure and the protocol provides a novel alternative for the synthesis of thioether.     

Reactions of Wheland complexes: base catalysis in re‐aromatization reaction of σ complexes obtained from 1,3,5‐tris(N,N‐dialkylamino)benzene and arenediazonium salts

The usual idea on the two‐steps mechanism of aromatic electrophilic substitution reactions is that the first step (the attack of the electrophilic reagent on the activated substrate) is rate limiting, while the driving force of the reaction is the fast proton departure to recover the resonance energy of the aromatic substrate. The now examined systems allow the formation of stable σ cationic complexes (Wheland intermediates) which may be investigated by simple procedures. Data here reported represent a clear and simple instance of a measurement of the rate of the proton abstraction from a Wheland intermediate and they indicate that this proton abstraction occurs by base catalysis in a rate determining step. Probably, this feature is more frequent than that usually conceived in the mechanism of electrophilic aromatic substitution reactions, because these reactions are often carried out in reaction mixtures containing large amounts of proton acceptor species which might mask the possible base catalysis. Copyright © 2007 John Wiley & Sons, Ltd.     

Sonochemical and photochemical oxidation of organic matter

Recent developments in sonochemistry have led us to study its use to treat water and wastewater. The effects of ultrasound wave in hydrophilic chemical oxidations are mainly due to hydroxyl radical production during the cavitation-induced water decomposition. Currently, the sonochemical destruction of aromatic compounds in water solution is obtained with low rates. The aim of this work is to evaluate the efficiency of the sonochemical effect in conjunction with a photochemical irradiation. Taking phenol as an example, the combined action of sonochemistry and photochemistry has been considered in a ‘sonuv’ reactor. An important enhancement of the degradation rate of phenol has been observed. It may be the result of three different oxidative processes: direct photochemical action, high frequency sonochemistry and reaction with ozone (produced by UV irradiation of air). The process has been successfully tested to lower the chemical oxygen demand of a municipal wastewater.     

A study on fluorescence quenching of LD-425 by aromatic amines in 1,4-dioxane–acetonitrile mixtures

The fluorescence quenching of 4-methyl-7-(4-morpholiny)-2H-pyrano[2,3-b]pyridin-2-one (LD-425) by aromatic amines aniline (AN), dimethyl aniline (DMAN) and diethyl aniline (DEAN) in solvent mixtures of 1,4-dioxane and acetonitrile has been studied at room temperature by steady-state and time-resolved methods. The positive deviation from linearity has been observed in the Stern–Volmer (S–V) plots. Various quenching rate parameters have been determined using the extended S–V equation and are found to be dependent on solvent polarity. The quenching ability of amines increases with increase in their ionization energies. Further, with the use of the sphere of action static quenching model and finite sink approximation model, it is concluded that the bimolecular quenching reactions are due to the presence of both dynamic and static quenching processes.     

Oxidative cleavage of hydrobenzoin by ACC/silica gel under ultrasound irradiation

A mild and efficient method has been developed using ultrasound irradiation for oxidative cleavage of hydrobenzoin with ACC/silica gel in dichloromethane. A high yield of 72-99% has been obtained for the oxidation of a series of hydrobenzoins into aromatic aldehydes at room temperature.     

Catalytic activity of copper (II) oxide prepared via ultrasound assisted Fenton-like reaction

Copper (II) oxide nanoparticles were synthesized in an ultrasound assisted Fenton-like aqueous reaction between copper (II) cations and hydrogen peroxide. The reactions were initiated with the degradation of hydrogen peroxide by ultrasound induced cavitations at 0 °C or 5 °C and subsequent generation of the OH radical. The radical was converted into hydroxide anion in Fenton-like reactions and copper hydroxides were readily converted to oxides without the need of post annealing or aging of the samples. The products were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) surface area analysis. Catalytic activity of the nanoparticles for the hydrogen peroxide assisted degradation of polycyclic aromatic hydrocarbons in the dark was tested by UV–visible spectroscopy with methylene blue as the model compound. The rate of the reaction was first order, however the rate constants changed after the initial hour. Initial rate constants as high as 0.030 min⁻¹ were associated with the high values of surface area, i.e. 70 m²/g. Annealing of the products at 150 °C under vacuum resulted in the decrease of the catalytic activity, underlying the significance of the cavitation induced surface defects in the catalytic process.     

The allylation reactions of aromatic aldehydes and ketones with tin dichloride in water

The allylation reactions of aromatic aldehydes and ketones were carried out in 31–86% yield using SnCl₂–H₂O system under ultrasound irradiation at r.t. for 5 h. The reactions in the same system gave homoallyl alcohols in 21–84% yield with stirring at r.t. for 24 h. Compared with traditional stirring methods, ultrasonic irradiation is more convenient and efficient.     

Enthalpies of formation and isomerization of aromatic hydrocarbons and ethers by G3(MP2)//B3LYP calculations

A computational method for estimation of the gas‐phase enthalpies of formation of aromatic hydrocarbons and ethers has been developed. The method is based on high‐level G3(MP2)//B3LYP calculations, atomization reactions, and structure‐dependent correction terms. By this method, enthalpies of formation Δ_fH_m°(g, 298.15 K) of 86 aromatic compounds were evaluated. The calculated enthalpies of formation raise questions of the reliability of several experimental enthalpies of formation reported in the literature. As an application of the computational enthalpies of formation, reaction enthalpies for several types of isomerization reactions of aromatic compounds were calculated. In cases in which experimental reaction enthalpies were available for comparison, the agreement between the computational and experimental data proved to be excellent. Copyright © 2008 John Wiley & Sons, Ltd.     

Ultrasound promoted reaction of Rhodamine B with sodium hypochlorite using sonochemical and dental ultrasonic instruments

The sonochemical acceleration of bleaching of Rhodamine B by sodium hypochlorite has been studied using ultrasound intensities in the range 0-7 W cm(-2). Using a 20 kHz ultrasonic horn, it was shown that ultrasound could significantly shorten the treatment time and/or the concentration of hypochlorite required for the reaction. A number of intermediate species formed during the reaction have been identified. It was demonstrated that the same sonochemical reactions occur during the use of dental ultrasound instruments of the type used in endodontics where hypochlorite solutions act as disinfectants. Results showed pseudo-first order degradation kinetics for the degradation of Rhodamine B for both types of source. Both the distribution of cavitation and the resulting bleaching reactions were dependent on the design of the tips. The bleaching reaction can therefore be used to characterise the behaviour of dental instruments and aid in the optimisation of their performance.     

Cavitational chemistry: a mild and efficient multi-component synthesis of amidoalkyl-2-naphthols using reusable silica chloride as catalyst under sonic conditions

An efficient and direct procedure for the synthesis of amidoalkyl-2-naphthol derivatives has been described. The process employs a three-component cyclocondensation reaction in one-pot using β-naphthol, aromatic aldehyde and acetamide or benzamide in the presence of silica chloride accelerated by ultrasound giving the product in excellent yield in very short duration.     

Ethoxylation of p-chloronitrobenzene using phase-transfer catalysts by ultrasound irradiation: a kinetic study

Ethoxy-4-nitrobenzene was synthesized by the reaction of 4-chloronitrobenzene with potassium ethoxide in a homogeneous system using benzyltriethylammonium chloride (QCl) as a phase-transfer catalyst at 50 degrees C under ultrasound irradiation conditions. The use of phase-transfer catalysts and ultrasound has been compared and demonstrated in this nucleophilic substitution reactions. The kinetics of the reaction depends on the effect of amount of catalyst, quaternary ammonium salts, agitation speed, amount of potassium hydroxide, amount of ethanol, temperature and the frequency of the ultrasound waves on the conversion of the reaction.     

An efficient one-pot synthesis of 2,3-epoxyl-1,3-diaryl-1-propanone directly from acetophenones and aromatic aldehydes under ultrasound irradiation

A convenient and efficient one-pot synthesis of 2,3-epoxyl-1,3-diaryl-1-propanone directly from acetophenones and aromatic aldehydes under ultrasound irradiation at room temperature has been described. In comparison to two-step methods, the present procedure has the advantages of mild conditions, shorter reaction time, without isolation of any intermediate, saves energy and no requirement of toxic solvent.     

Towards a fully optimised organic LED device: Analysis of surface synthesis using coupling reactions by ToF-SIMS

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and polarisation-modulation reflection-absorption infrared spectroscopy (PM-RAIRS) have been used to monitor the surface synthesis of self-assembled aromatic π-conjugated molecular wires on gold substrates as a step towards a novel structure for organic electroluminescent devices (OLEDs). The wires have been synthesised using a series of Schiff's base coupling reactions in solution on a self-assembled monolayer of an aromatic thiolate anchor. ToF-SIMS and PM-RAIRS measurements have demonstrated that: (i) the anchor molecules self-assemble at the gold surface, (ii) the anchor molecules selectively react through imino coupling reactions with additional wire units with high efficiency and (iii) the wire-like structure is predominantly orientated normal to the surface.