Yu Lin Hu

An Efficient Procedure for Chloromethylation of Aromatic Hydrocarbons Catalyzed by PEG1000‐Dicationic Ionic Liquids in Aqueous Media

Chloromethyl-substituted aromatic compounds are very important intermediates which have been widely applied in synthesis of a variety of fine or special chemicals, such as pharmaceuticals, agrochemicals, dyes, flavors, fragrances, polymers, and additives. The oldest method for the synthesis of this class of compounds involves the chloromethylation of aromatic hydrocarbons with hydrochloric acid and the use of trioxane or paraformaldehyde in the absence of any catalyst; however, the reaction rate is very slow. Other classic methods for chloromethylation employ various Lewis acids, such as zinc chloride, stannic chloride, aluminum chloride, and boron trifluoride as the catalysts. Zinc chloride has been found to be an effective catalyst in a hydrochloric acid solution. 7–9] The use of Lewis acids is very important to improve the reactive activity. However, these catalysts, in general, are invariably associated with one or more disadvantages, such as tedious workup procedures, corrosiveness, high susceptibility to water, difficulty of catalyst recovery, environmental hazards, and waste control. Therefore, it is important to develop environmentally conscious catalysts that are active under mild conditions and can be easily recovered after the reaction for reuse. To attain these goals in the chloromethylation of aromatic hydrocarbons, our research project group proposed surfactant micelles, particularly cetyltrimethylammonium bromide (CTAB), as catalyst to synthesize chloromethylated hydrocarbons. Kishida et al. recently reported the use of rare earth metal triflates in the chloromethylation of aromatic hydrocarbons. 15, 16] However, the use of surfactant micelles suffers from difficulties encountered in their handling and separation after the reaction and also can not be reused for new reactions. Rare earth metal triflates are very expensive, which impeded their use for practical chemical processes. Ionic liquids (ILs) are a special class of molten salts composed of organic cations and inorganic or organic anions. Ionic liquids, with their unique properties, including low volatility, high polarity, good thermal stability over a wide temperature range, and selective dissolving capacity by a proper choice of cations or anions, have attracted increasing interest in the organic transformations as reaction medium and catalyst (ligand). Up to now, examples of their application as catalysts in the chloromethylation of aromatic hydrocarbons were seldom reported. 27] However, these ILs are inevitably associated with one or more disadvantages, such as low recovery ratio, high cost or difficulty of synthesis. In view of both the advantages and disadvantages of homogeneous and heterogeneous catalysts, and to improve catalyst recovery, multiphase systems, such as phase-transfer catalysis, thermoregulated phasetransfer catalysis, and liquid–liquid biphasic catalysis, have been studied. Some novel temperature-dependent ionic liquid biphasic catalytic systems have been reported recently, and found that they showed some advantages, such as high conversions and selectivity, stability at high temperatures, and reusability in the reaction, which provide a novel route for the separating and recycling of the catalysts. We now report an efficient and convenient procedure for the chloromethylation of aromatic hydrocarbons catalyzed by PEG1000-based dicationic ionic liquid (PEG1000-DIL) in aqueous media (Scheme 1). We found that the PEG1000-DIL/methylcyclohexane temperaturedependent biphasic system is active for chloromethylation and that it is an environmentally conscious catalytic system. The chloromethylation of ethylbenzene was initially carried out in an oil–water biphasic system in the presence and absence of PEG1000-DIL at 75 8C. In the absence of PEG1000-DIL,

Efficient dichloromethylation of some aromatic hydrocarbons catalyzed by a new ionic liquid [C12minPEG800]Br under homogeneous catalysis in aqueous media

A series of new imidazolium-type ionic liquids based on polyethylene glycol have been prepared. The new recyclable temperature-dependant phase-separation system comprised of [C12minPEG800]Br and methylcyclohexane was also developed and successfully applied to the dichloromethylation of some aromatic hydrocarbons to prepare dichloromethyl-substituted hydrocarbons in excellent yields. The ionic liquid could be excellent recycled without any apparent loss of catalytic activity and little loss of weight even after 8 times recycling.

AN INEXPENSIVE AND CONVENIENT PROCEDURE FOR CHLOROMETHYLATION OF AROMATIC HYDROCARBONS BY PHASE TRANSFER CATALYSIS IN AQUEOUS MEDIA

Reaction of aromatic hydrocarbons catalyzed by a novel catalytic system consisting of zinc chloride, acetic acid, sulfuric acid and PEG-800 in aqueous media under PTC conditions results in chloromethylation in good to excellent yield.