Catalysts, general organic

It is generally assumed that the electron donor can have one or several of the following functions to increase the stability of the catalytically active species to increase the catalytic activity of the catalyst to allow a control over the selectivity of the catalytic reaction or to increase the solubility of the catalyst in organic media. The main effect of Lewis or Br0nsted acids is to increase the catalyst activity, but their influence on selectivity control is not considered to be of great significance (see, however, Sections IV,D,2 and IV,F). The increase in activity of the catalyst (see below) on the addition of Lewis or Brdnsted acids is frequently accompanied by a decrease in stability of the system. 

Because of the differential partitioning of hydroxide and phenoxide anions into organic solvents by quaternary ammonium cations, the catalysts generally have little effect on the Reimer-Tiemann reaction of phenols with dihalocarbenes [15]. Cetyltrimethylammonium bromide has been used in the two-phase dichloromethyl-ation of polysubstituted phenols (Scheme 7.21, Table 7.10) under Makosza s conditions [16,17] ring expansion of the reaction products provides an effective route to tropones. The rate of the reaction is enhanced by ultrasonic radiation [16]. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

The use of a Lewis acid (e.g., friethylfluoroborate, zinc chloride, stannous chloride, titanium chloride, iron(m)chloride) and other reagents (e.g., iodine, trimethylsilane, trifluoiomethane-sulfonylsilane) have also been recommended. Exhaustive lists of catalysts and conditions can be found in reviews devoted to carbohydrates [5-7], or to general organic chemistry [27,28], However, one can add the new catalyst, which has been introduced for the smooth formation of p-methoxybenzylidene acetals and p-methaxy-phenylmethyl methyl ether [29], namely 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), and has been applied very recently [30] to the synthesis of isopropylidene mixed acetals. 

The best catalysts for this reaction are inorganic acids (H2SO, HC1), organic acids (p-toluenesulfonic, methanesulfonic) and metal compounds such as tin and titanium derivatives - e.g. tetraisopropyl titanate. To achieve good yields of products, not only is a catalyst generally necessary but also the means to drive the equilibrium to the right as written in the reactions above. 

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