Drying with microwaves

Wang, Z.H. Chen, G. Theoretical study of fluidized-bed drying with microwave heating. Ind. Eng. Chem. Res. 2000, 39, 775-782. 

Heating and drying with microwave and dielectric energy is distinctly different from conventional... 

Microwave chemistry has been found to be a useful method for accelerating reactions or catalyzing reactions that are difficult to carry out by other methods. A modification of the Hantzsch method to directly obtain pyridines has been communicated. A dry medium using ammonium nitrate bentonitic clay system with microwave irradiation affords pyridines 96 in a single pot within 5 minutes. When the pyridine is not the major product (> 75% yield), the dealkylated pyridine 97 becomes an... 

Benzenediamine (185) and benzoin (186) gave a separable mixture of 2,3-diphenyl-1,2-dihydroquinoxaline (187, R = Ph) and 2,3-diphenylquinoxaline (188, R = Ph) (dry mixture, microwave irradiation under reflux, 4 min 21% and 67%, respectively) in contrast, similar treatment with m,m -dichloro-benzoin gave only the aromatized product, 2,3-bis(m-chlorophenyl)quinoxa-line (188, R = C6H4Cl-m) (94%). ... 

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

Pyrrole is one of the most prominent heterocycles, having been known for more than 150 years, and it is the structural skeleton of several natural products, synthetic pharmaceuticals, and electrically conducting materials. A simple access to the pyrrole ring system involves the conversion of cyclic anhydrides into five-membered imides. Mortoni and coworkers have described the conversion of 2-methylquinoline-3,4-dicarboxylic acid anhydride to a quinoline-3,4-dicarboximide library by treatment of the anhydride with a diverse set of primary amines under microwave conditions (Scheme 6.180) [341]. The authors studied a range of different conditions, including dry media protocols (see Section 4.1) whereby the starting materials were adsorbed onto an inorganic support and then irradiated with microwaves. For the transforma-... 

Differential heating is more easily achieved with microwave technology than by conventional conductive methods and as Cundy suggested in his excellent review, could well account for apparent rate enhancements obtained by others with organic reactions on dry media [40]. 

As demonstrated above with examples, the CMR and MBR offer many advantages for synthetic processes that benefit from rapid heating and cooling. These systems are less useful and may be inappropriate when the reaction requires low temperature conditions throughout, when materials or reactions that are incompatible with microwave energy (e. g. reactions involving predominantly nonpolar organics) are to be employed or for reactions with dry media. 

A simple and rapid synthesis of tetrapyrrolic macrocycle has been achieved under dry media conditions with microwave activation. Pyrrole and benzaldehyde adsorbed on silica gel afford tetraphenylporphyrin within 10 min (Scheme 8.26), whereas with conventional methods (e. g. acetic acid in the presence of pyridine) 24 h were necessary. 

A dry-media technique with microwave irradiation for synthesis of aziridines has recently appeared in the literature [45] (Scheme 8.28). 

In 1999, Hajipour reported that silica gel, without any base, could be a useful catalyst for the preparation of oximes in dry media coupled with microwave irradiation. Hydroxylamine hydrochlorides were reacted with several aliphatic and aromatic aldehydes and ketones affording the desired oximes. 

Potassium borohydride (1.0 g, 20 mmol), anliydrous lithium chloride (0.8 g, 20 mmol) were thoroughly mixed in a mortar and transferred to a flask (100 mL) connected with reflux equipment, then dry THF (10 mL) was added and the mixture was heated to reflux for 1 h. After cooling, the ester (10 mmol) was added and stirred for 0.5 h at room temperature, then the THF was removed under reduced pressure. After the mixture was irradiated by microwave for 2-8 min, the mixture was cooled to room temperature, water (20 mL) was added, extracted with ether (3 x 15 mL), dried with magnesium sulfate, and evaporated to give the crude product, which was purified by crystallization, distillation or column chromatography. 

A mixture of phtlialimide 1 (0.70 g, 4.8 mmol), alkyl halide (6.0 mmol), tetrabu-tylammonium bromide - TBAB (0.15 g, 0.45 mmol), and potassium carbonate (2.6 g, 18.8 mmol) was heated in a domestic microwave oven in an open Erlen-meyer flask for an appropriate time. After cooling down, the reaction mixture was extracted with methylene chloride (2x25 mL). Then the extracts were dried with MgS04, filtered, and the solvent was evaporated to dryness. If necessary before a recrystallization the solid material was purified by means of flash chromatography to afford desired /V-alkylphthalimide 2, yield 49-95%. 

General Procedure for Preparation of Triazones 5a-f. 264 mg (3 mmol) /V,/V-di-metliylurea, 1 g paraformaldehyde, 3 mmol primary amine 4a-f and 2 g montmorillonite K-10 were irradiated by microwave in a Teflon vessel. The reaction mixture was filtered and washed with water. The organic phase was separated and dried with Na2S04 and concentrated by vacuum distillation. Purification of the... 

A mixture of 4-methoxybenzyl alcohol le (5 mmol) and acetic anhydride (5 mmol) in a beaker covered with watch glass was irradiated by microwaves for 10 min (heating and cooling at the interval of 1 min). After completion of die reaction (TLC), the product was extracted with ether (3x15 mL). The ether layer was washed with 10% NaOH and then dried with anhydrous sodium sulfate. Removal of the solvent under reduced pressure gave 4-methoxybenzyl acetate 2e in excellent yield (95%). 

Zs-3-Phenylpropenoic acid (0.025 g, 0.16 mmol), Amberlite IRA 938-supported formate (0.2 g) and Wilkinson s catalyst (0.04 g, 0.0043 mmol) were suspended in dimethylsulfoxide (0.5 ml) in a reaction tube. The mixture was irradiated with microwaves at 100 W for 30 s. On cooling, the mixture was diluted with dichloromethane (5 ml) and filtered. The filtrate was washed with water (2x5 ml). The organic layer was dried over anhydrous magnesium sulfate, the solvent removed under reduced pressure, and the residue was purified by filtration through a plug of alumina to give 3-phenylpropanoic acid (0.024 g, 95% yield). 

Palacios, M.A., M. Gomez, C. Camara, and M.A. Lopez. 1997. Stability studies of arsenate, monomethyl-arsonate, dimethylarsinate, arsenobetaine and arsenocholine in deionized water, urine and clean-up dry residue from urine samples and determination by liquid chromatography with microwave-assisted oxidation-hydride generation atomic absorption spectrometric detection. Anal. Chim. Acta 340 209-220. 

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