Quaternary ammonium salts supported

Polymer-supported amino alcohols and quaternary ammonium salts catalyze the enan-tioselective addition of dialkylzinc reagents to aldehydes (Table 31). When the quaternary ammonium salt F is used in hexane, it is in the solid state, and it catalyzes the alkylation of benzaldehyde with diethylzinc in good chemical yield and moderate enantioselectivity. On the other hand, when a mixture of dimethylformamide and hexane is used as solvent, the ammonium salt is soluble and no enantioselectivity is observed21. 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

Silica gel-based catalytic systems have been described as efficient promoters for a number of organic reactions.28 Illustrative examples include the oxidative cleavage of double bonds catalyzed by silica-supported KM11O4,29 reaction of epoxides with lithium halides to give /i-halohydrins performed on silica gel,30 selective deprotection of terf-butyldimethylsilyl ethers catalyzed by silica gel-supported phosphomolybdic acid (PMA),31 and synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts.32... 

J.-Q. Wang, D.-L. Kong, J.-Y. Chen, F. Cai, and L.-N. He, Synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts under supercritical conditions, /. Mol. Cat. A Chem., 249 (2006) 143-148. 

In contrast with the reactions involving sulphide or hydrogen sulphide anions, aryl alkyl thioethers and unsymmetrical dialkyl thioethers (Table 4.3) are obtained conveniently by the analogous nucleophilic substitution reactions between haloalkanes and aryl or alkylthiols under mildly basic conditions in the presence of a quaternary ammonium salt [9-15] or polymer-supported quaternary ammonium salt [16]. Dimethyl carbonate is a very effective agent in the formation of methyl thioethers (4.1.4.B) [17]. 

In a manner analogous to that used for the formation of 5-alkyl thioacetates using a polymer-supported quaternary ammonium salt (4.1.31), the dithiocarbamate anion can be 5-alkylated under mild conditions [3]. The corresponding arylation reaction with activated aryl systems requires more vigorous conditions ... 

In the main, the original extractive alkylation procedures of the late 1960s, which used stoichiometric amounts of the quaternary ammonium salt, have now been superseded by solid-liquid phase-transfer catalytic processes [e.g. 9-13]. Combined soliddiquid phase-transfer catalysis and microwave irradiation [e.g. 14-17], or ultrasound [13], reduces reaction times while retaining the high yields. Polymer-supported catalysts have also been used [e.g. 18] and it has been noted that not only are such reactions slower but the order in which the reagents are added is important in order to promote diffusion into the polymer. 

The use of polymer-supported quaternary ammonium salts to catalyse the Wadsworth-Emmons reaction produces acceptable yields from both aromatic and aliphatic aldehydes and ketones [16]. 

In 2000, Benaglia and coworkers reported preparation of MeO-PEG supported quaternary ammonium salt (10) and examined the catalytic efficiency in a series of phase-transfer reactions (Fig. 5.3) [69]. The reactions occurred at lower temperatures and with shorter reaction times than with comparable insoluble 2% cross-linked polystyrene-supported quaternary ammonium salts, although yields varied with respect to classical solution phase quaternary ammonium salt catalyzed reactions. It was observed that yields dropped with a shorter linker, and that PEG alone was not responsible for the extent of phase-transfer catalysis. While the catalyst was recovered in good yield by precipitation, it contained an undetermined amount of sodium hydroxide, although the presence of this byproduct was found to have no effect on the recyclability of the catalyst... 

The typical concentration profile of solute in an SLM system with quaternary ammonium salt as carrier is schematically shown in Fig. 6. To model the facilitated transport within a supported liquid membrane [58,59], the following assumptions are usually made ... 

In both MALDI-TOF MS and ion-spray MS, molecules must be positively charged to be detected. To facilitate detection of all types of support-bound compound, linkers incorporating a charged spacer (e.g. a quaternary ammonium salt), which is also released during MALDI, have proven particularly convenient [6,14,15]. 

The preparation of tertiary amines with the aid of insoluble supports has mostly been performed by 3-elimination of support-bound quaternary ammonium salts or by N-alkylation of secondary amines with support-bound alkylating agents (Figure 3.28). 

Quaternary ammonium salts were used as supporting electrolytes in this reaction by Mitsubishi168) and others ... 

Du Y, Wang J-Q, Chen J-Y et al (2006) A polyethylene glycol)-supported quaternary ammonium salt for highly efficient and environmentally friendly chemical fixation of C02 with epoxides under supercritical conditions. Tetrahedron Lett 47(8) 1271—1275... 

Another interesting feature of polymer-supported catalysts containing quaternary ammonium salts involves the development of enantioselective catalysis using salts derived from cinchonia or ephedra alkaloids.11341 The first application of such chiral supported catalysts in the Michael reaction between methyl 1-oxoindan-2-carboxylate and methyl vinyl ketone revealed a high chemical yield in condensation product (60-100 %) although the enantioselectivities were only moderate (ee <27 %). 

Hodge, P., Khoshdel, E. and Waterhouse, J. Michael reactions catalyzed by polymer-supported quaternary ammonium-salts derived from cinchona and ephedra alkaloids, J. Chem. Soc., Perkin Trans. 1, 1983, 2205-2209. 

A quaternary ammonium salt was easily synthesized on a modified MeOPEG, and this supported catalyst was shown to be an efficient and recoverable promoter of several reactions carried out under PTC conditions. Catalyst 13 showed a catalytic activity that was similar to, or even better than that of the non-supported catalysts (Benaglia et al. 2000). 

Benaglia M, Annunziata R, Cinquini M, Cozzi F, Tocco G (2000) A polyethylene glycol)-supported quaternary ammonium salt an efficient, recoverable, and recyclable phase-transfer catalyst. Qrg Lett 2 1737-1739... 

The hydroxymethyl resin-supported regenerated Michael (REM) linker 1.38 (96), has been used to support secondary amines during elaboration followed by release as tertiary amines by classical Hoffman elimination [DIEA/(diisopropyl ethylamine)-DMF at rt over 18 h] of the resin-bound quaternary ammonium salt. 

The EW of aliphatic quaternary ammonium systems based on fluoroanions has the widest EW compared with that of aromatic systems. This result can be explained by the presence of quaternary ammonium salts, which are usually used as a supporting electrolyte in the conventional electrolyte and which have wide electrochemical windows. 

Gelbard et al. have prepared a numbor of neutral and acidic supported ammonium chromates. Neutral resins woe found to be generally more effective dian acidic resins. The large differences in leactiviQr between different unbound, soluble alkylammonium chromates (Section 2.7.6.1) are not so pronounced with the polymer-supported oxidants. Some commoits about the reactivity of these supported oxidants woe made (i) quaternary ammonium salts are more reactive than the tertiary pyridinium chromates (ii) the reactivity of a conqilex chromate XCtOr was found to increase as the basicity of X decreases. Best... 

Although the electrochemical coupling to form ADN had been known since the 1940s, the breakthrough came only when Baizer found out that the use of quaternary ammonium salts such as tetraethylammonium /7-toluenesulfonate as supporting electrolytes led to mass yields of 90% of ADN [75]. Following Baizer s discovery, an intensive process development effort was undertaken at Monsanto s Technical Centre in Pensacola, Florida [76]. 

Procedural variations of the oxidation of sulfides with Phl=0 include micellar medium containing a quaternary ammonium salt, and dispersion on a solid support. While the former reaction gives sulfoxides, the latter can lead to C-S bond cleavage. 

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