Microwave-assisted Esterification Research Articles

Efficient Microwave-Assisted Esterification Reaction Employing Methanesulfonic Acid Supported on Alumina as Catalyst

A rapid and efficient protocol assisted by microwave irradiation for the synthesis of esters using methanesulfonic acid (CH3SO3H) supported on Al2O3 (AMA) as catalyst and free of solvent is described. The products were obtained in good yields and purity, with reduced reaction time, and the process is simple and environmentally benign.

The Synthesis of 3-Phosphabicyclo[3.1.0]- hexane 3-Oxides and 1,2-Dihydrophosphinine 1-Oxides with Lipophilic P-Alkoxy Substituents by Ring Enlargement

A series of 1-alkoxy-3-phospholene 1-oxides available from the microwave-assisted direct esterification of 1-hydroxy-3-phospholene oxide was converted to the two diastereomers of 6,6-dichloro-3-phosphabicyclo[3.1.0]hexane 3-oxides by the addition of dichlorocarbene to the double bond. Thermolysis of the 3-phospholene oxide–dichlorocarbene adducts afforded the corresponding 1,2-dihydrophosphinine 1-oxides as a ca. 3:1 mixture of two double bond isomers. Relative stability of the isomers of the intermediates and the products and their stereostructures were evaluated by B3LYP/6-31G(d,p) calculations.

Microwave assisted esterification of free fatty acid over a heterogeneous catalyst for biodiesel production

Biodiesel fuel is gaining significant attention in recent years because of its environmental benefits and the growing interest in finding new resources and alternatives for conventional fuels. Biodiesel production from waste cooking oil with high free fatty acids usually requires esterification step to produce fatty acid methyl/ethyl ester. In the present work, the heterogeneous catalyst aminophosphonic acid resin D418 has been successfully utilized in the energy-efficient microwave-assisted esterification reaction of fatty acid ethyl ester (FAEE) from free fatty acid (FFA) stearic acid with short-chain alcohol ethanol. Under the reaction conditions of 9wt% D418 and 11: 1M ratio of ethanol to stearic acid at 353K and atmospheric pressure, more than 90% conversion of the esterification was achieved in 7h by microwave heating, while it took about 12h by conventional heating. Moreover, the kinetics of this esterification reaction has been studied, and the relevant values of activation energy and pre-exponential factor were obtained.

Mercapturic acids derived from toluene in rat urine samples: identification and measurement by gas chromatography–tandem mass spectrometry

Toluene is one of the most widely used CMR chemicals in industry. Worker exposure to this compound is regulated in France, but new, more sensitive methods are required to effectively monitor this exposure. A gas chromatography-tandem mass spectrometry (GC-MS/MS) method was developed and fully validated for the simultaneous determination of urinary toluene mercapturic acids derived from side chain and ring oxidation, i.e., benzylmercapturic acid and the three isomers o-, m- and p-toluylmercapturic acids, respectively. The method involves a simple and efficient two-step preparation procedure consisting of liquid-liquid extraction of the urinary acids followed by a microwave-assisted esterification of the isolated compounds using 2-propanol. The method meets all the required validation criteria: high selectivity, intra-day and inter-day precision ranges between 1.0 % and 12.4 %, with close to 100 % recovery. Linearity has been shown over the reduced concentration range 0.03-0.5 mg/L whereas a multiplicative model (ln-ln transformation) had to be used to describe the full range of concentrations 0.03-20 mg/L. The limits of detection for the four analytes, ranging from 2.8 to 5.5 μg/L, made the method suitable for their identification and quantification in urine from rats inhaling toluene in the 2 to 200 ppm concentration range. All urine samples from exposed rats contained measurable amounts of all metabolites. This is the first time that o- and m-toluylmercapturic acids have been shown to occur. Our results confirm the hypothesis that toluene mercapturic acids derived from ring oxidation exist in three forms.

Microwave-Assisted Esterification of Phosphinic Acids

The usual synthesis of phosphinates involving the reaction of phosphinic chlorides with alcohols or phenols in the presence of a tertiary amine in an apolar solvent cannot be regarded as “green” and requires expensive P-reagents. We have found that in certain cases phosphinic acids can be esterified by simple alcohols under microwave (MW) conditions. Hence, cyclic phosphinic acids, such as 1-hydroxy-3- and 2-phospholene 1-oxide derivatives (A), 1-hydroxyphospholane 1-oxides (B) and 1-hydroxy-1,2,3,4,5,6- hexahydrophosphinine 1-oxide (C) could be converted to the corresponding esters by reaction with alcohols. Using a 15-fold excess of the alcohol at around 200 ° C, the preparative yields of the phosphinates amounted to 45-60%. The novel reaction is the consequence of the beneficial specific MW effect that was proved by comparative thermal experiments. Phosphinates may also be prepared by the alkylating esterification of phosphinic acids with alkyl halides. Under solventless and MW conditions in the presence of a phase transfer (PT) catalyst and K2CO3, cyclic phosphinic acids A, B and C mentioned above were converted to the corresponding phosphinates in 85-95% yields. It was found that the simultaneous use of the MW and PT catalytic techniques is beneficial when alkyl halides of normal or decreased reactivity are applied. Both methods described can be regarded as environmentally friendly and offer advantages over the traditional esterifications of phosphinic acids. Keywords: Microwave, green chemistry, esterification, phosphinic acids, phosphinates, TEBAC, Cyclic Phosphinates

Microwave-Assisted Esterification of Phosphinic Acids by Alcohols, Phenols, and Alkyl Halogenides

Cyclic phosphinic acids undergo direct esterification with alcohols at ca. 200°C under microwave conditions.

Novel Synthesis of Phosphinates by the Microwave-Assisted Esterification of Phosphinic Acids

1-Hydroxy-3-phospholene oxides (1 and 3) and phenyl-H-phosphinic acid (6) are converted to the corresponding phosphinic esters (2, 4, and 7, respectively) by reaction with simple alcohols on microwave irradiation. Under traditional heating conditions, the esterification does not take place, as in the cases of 1 and 3, or is highly incomplete, as in the case of 6. Steric hindrance in diphenylphosphinic acid prevents efficient microwave-assisted esterification.

Microwave-Assisted Esterification of Diverse Carboxylic Acids and Chiral Amino Acids

A facile and efficient synthetic method of esters from their corresponding carboxylic acids and amino acids is described. The esterification reaction of carboxylic acids and amino acids could be greatly accelerated under microwave irradiation because the reactions described in this article took place in only 5 min with almost quantitative yields, and distinct acidity of catalytic acids was well tolerated. Unlike the racemation problem in microwave-assisted N-acylation reactions, the esters of chiral amino acids could be achieved with retention of configuration under this condition.

Facile synthesis of mesoporous titanium dioxide nanocomposites with controllable phase compositions by microwave-assisted esterification

Mesoporous titanium dioxide nanocomposites with controllable phase compositions and high surface areas were synthesized through convenient, fast, and one-step microwave-assisted esterification method. The introduction of microwave in the synthetic reactions not only accelerates the esterification reaction but also promotes the fast crystallization. By changing reaction temperature, microwave irradiation time, the amount of staring materials and the composition of solvents, pure anatase, pure rutile or mixed phase titanium dioxide nanocomposites were obtained in minutes. Meanwhile, the size of crystallite can be controlled by the reaction temperature. The as-synthesized materials display significative photocatalytic activities without any further disposal, among which the material with 3.6% rutile shows the best effect in degradation of methylene blue under UV-light irradiation. Moreover, the nitrogen adsorption–desorption results illustrate the obtained samples are mesophases. It is also interesting that the pure rutile has specific surface area as high as 210 m 2 g −1 calculated by BET equation. The method can control the hydrolysis of TiCl 4 well and dramatically shorten the preparation time of titanium dioxide.

Microwave-Assisted Esterification of N-Acetyl-L-Phenylalanine Using Modified Mukaiyama’s Reagents: A New Approach Involving Ionic Liquids

Inspired by the concept of ionic liquids (ILs), this study modified the original Mukaiyama's reagent, 2-chloro-1-methylpyridinium iodide (m.p. 200-dec), from ionic solid into liquids by changing its anion. The esterification of N-acetyl-L-phenylalanine was investigated as a model reaction. The microwave irradiation was more effective in esterifying N-acetyl-L-phenylalanine than the conventional reflux method. The original Mukaiyama's reagent was modified into ILs through manipulating its anion. However, only non-nucleophilic anions (such as EtSO(4) (-) and Tf(2)N(-)) were favorable since nucleophilic ones (such as CF(3)COO(-) and CH(3)COO(-)) could exchange with chlorine resulting in non-reactive coupling reagents. Two modified Mukaiyama's compounds (i.e. hydrophilic [2-ClMePy][EtSO(4)] and hydrophobic [2-ClMePy][Tf(2)N]) have been identified as the best ILtype coupling reagents. The esterification reaction was greatly enhanced by using 1- methylimidazole as the base instead of conventional toxic tertiary amines, and by using excess amount of alcohols as solvents instead of dichloromethane. Overall, the method reported is effective and 'greener'.

Consecutive microwave irradiation induced substrate inhibition on the enzymatic esterification

The effect of consecutive microwave irradiation on the enzymatic esterification of pentanol with caprylic acid was investigated. Lipase from Mucor miehei was used as the catalyst under consecutive microwave irradiation and conventional heating, respectively. Microwave irradiation was found to increase the enzyme affinity to pentanol, which has not been reported in the literature. The conventional heating reaction conducted at an initial concentration of caprylic acid lower than 0.25 mol/L was found to fit the Ping-Pong Bi-Bi mechanism better than the microwave-assisted reaction. The microwave-assisted esterification departed from the Ping-Pong Bi-Bi mechanism independent of the initial caprylic acid concentration when the caprylic acid concentration was 0.25 mol/L and above. In addition, a pentanol inhibition under microwave irradiation was observed if the pentanol concentration was higher than 0.75 mol/L and caprylic acid concentration was over 0.25 mol/L. The pentanol inhibition caused by the microwave irradiation did not happen in the conventional heating reaction. This phenomenon has not previously been reported in the literature. The experimental results are correlated to the polarity of the substrates. These findings will benefit the understanding of nonthermal effect of microwave irradiation.

Application of microwave method to the solid phase synthesis of pseudopeptides containing ester bond

A new procedure was developed for reducing the reaction time and improving the yield of esterification reaction in solid phase synthesis of pseudopeptides containing an ester bond by utilizing microwave irradiation. We selected a pseudodipeptide (Fmoc-LysΨ[COO]Leu-NH 2) and optimized the microwave-assisted esterification reaction in solid phase synthesis using Fmoc chemistry. For this, microwave-assisted esterification reactions with different reaction time, temperature, and solvents were performed using 1,3-diisopropylcarbodiimide (DIC) as the coupling reagent. We synthesized several pseudodipeptides containing an ester bond by using the optimized microwave irradiation method. The purity and yield of the pseudodipeptides synthesized in this way were better than those obtained without microwave irradiation. Furthermore, we applied this methodology for synthesizing pseudopeptides (6- and 12-mer) corresponding to the α helical peptide. The microwave-assisted esterification reaction afforded the target pseudopeptides with high yield (∼80%) and purity within 12 min, whereas the reaction without microwave irradiation afforded the target compound with poor yield (∼45%) and low purity.

Microwave-assisted Synthesis of Starch Maleate by Dry Method

Starch maleate was prepared by microwave-assisted reactions. Microwave-assisted esterification in a dry medium for 5 min gives a reaction efficiency of 98%. Influences of reaction time, reagent molar ratio and water content of starch on the degree of substitution (DS) of the product and reaction efficiency were investigated. The structure of starch maleate was characterized by FTIR and NMR spectroscopy. Changes in the starch granules before and after modification were studied by scanning electron microscopy (SEM). Compared with the conventional heating synthesis method, microwave-assisted synthesis in a dry medium is rapid and energy saving with high reaction efficiency.

A Rapid Microwave-Assisted Esterification Utilizing the Mukaiyama Supported Reagent

Microwave heating was applied to a modified Mukaiyama reagent supported on a PS-DVB resin to develop a rapid and efficient protocol for the solution-phase synthesis of esters and lactones starting from alcohol and carboxylic acids. The products were obtained in high purity (NMR analysis) after a simple resin filtration.