Continuous-flow methylations, dimethyl

Current single-mode continuous-flow microwave reactors allow the processing of comparatively small volumes. Much larger volumes can be processed in continuous-flow reactors that are housed inside a multimode microwave system. In a 2001 publication, Shieh and coworkers described the methylation of phenols, indoles, and benzimidazoles with dimethyl carbonate under continuous-flow microwave conditions using a Milestone ETHOS-CFR reactor (see Fig. 3.11) [104]. In a typical procedure, a solution containing the substrate, dimethyl carbonate, 1,8-diazabicy-clo[5.4.0]undec-7-ene (DBU) base, tetrabutylammonium iodide (TBAI), and a solvent was circulated by a pump through the microwave reactor, which was preheated to 160 °C and 20 bar by microwave irradiation (Scheme 4.31). Under these condi-... 

A continuous-flow reactor with a fixed catalyst bed was employed at pressurized conditions. Gaseous dimethyl ether was supplied to the reactor at its vapor pressure with carbon monoxide while liquid reactants such as methyl acetate, methyl iodide, and water were fed with microfeeders. Methyl acetate used in this experiment was dehydrated by Molecular Sieve 5A before use. A part of the reaction mixture was sampled with a heated syringe and was analyzed by gas chromatography. 

Shieh et al." reported the use of a continuous flow reactor for the esterification of a 50 to 100 g scale of carboxylic acids to methyl esters with dimethyl carbonate and 1,8-Diazabicyclo[5.4.0]undec-... 

Recently, a continuous flow process was developed utilizing dimethyl carbonate as a less toxic methylating reagent (eq 10). Sulfolane was selected as a solvent due to its stability at 240 °C, producing NMI efficiently in good yield (84%). 

Dimethyl carbonate can be used for methylation of nitrogen in amine compounds at a temperature of 180°C under continuous-flow gas-liquid phase-transfer catalysis conditions (which involve transfer of organic ionic reactant species between water and an organic phase) as shown below for the methylation of aniline ... 

In the Chemische Werke Witten process, which was further developed by Dynamit Nobel and Hercules, p-xylene, air and the catalyst are fed continuously into the oxidation reactor, to which recirculated p-methylbenzoic acid methyl ester is also added. Oxidation is effected at a temperature of 140 to 170 °C and a pressure of 4 to 7 bar. The heat of reaction is removed by the vaporization of water and excess p-xylene. The further reaction with methanol is carried out at 200 to 250 °C under slightly raised pressure (20 bar) in the esterification reactor, to keep the reaction mixture in the liquid-phase. The esterification products flow to the crude ester column, where p-methylbenzoic add methyl ester is separated from the crude dimethyl terephthalate. p-Methylbenzoic acid methyl ester is recycled to the oxidation reactor, where oxidation of the second methyl group occurs. The crude dimethyl terephthalate is purified to fiber grade quality by distillation and crystallization from methanol, and subsequent redistillation in a column with around 30 trays. The yield of dimethyl terephthalate (m.p. 141 °C) is generally about 87 mol%. 

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