Catalytic efficiency, quaternary ammonium

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... 

Increasing the hydrophobicity of quaternary ammonium salts increases the apparent extraction constants for the ion pair and therefore leads to a higher catalytic activity (Brandstrom, 1977). The same phenomenon has been observed by Cinquini and Tundo (1976) for crown ether catalysis (Table 35). The catalytic activity of 18-crown-6 [3] and alkyl-substituted derivatives [117]—[ 119] in the reaction of n-CgH17Br with aqueous KI follows the order [117], [118] > [119] s> [3]. The alkyl-substituted [2.2]-cryptand derivatives are also much more efficient than the parent compound [86]. Increasing the hydrophobicity of [2.2.2]-cryptand (Cinquini et al., 1975) and even of polypode ligands (Fornasier et al., 1976) leads to higher catalytic activity. The tetradecyl-substituted compounds show the reactivity sequence [2.2.2]-cryptand at 18-crown-6 > [2.2]-cryptand on the reactivity scale that can be distilled from Table 35. 

The Hoffman rearrangement of amides by quaternary ammonium hypochlorites is not particularly efficient under phase-transfer catalytic conditions and only low yields of nitrile, aldehydes, or ketones, which result from oxidation of the amines, are... 

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... 

Over the years it has been shown that complexes prepared from organophosphorous ligands in combination with peroxotungstic acid or its quaternary ammonium salts exhibit efficient catalytic properties. In order to make an efficient recycling of the catalyst possible and get tungsten-free products and effluents, some of these catalysts were immobilized onto polystyrene, poly benzimidazole and polymethacrylate copolymers modified by the introduction of the phosphorous(V)-containing ligands. 

Quaternary ammonium salts of heterocyclic compounds have been used in liquid-liquid phase-transfer syntheses. When these compounds are achiral, they show a behavior very similar to that of other quaternary ammonium salts. For example, 2-dialkylamino-l-alkylpyridinium tetrafluoroborates have been used by Tanaka and Mukayama282 in the alkylation of active methylene compounds PhCH2CN, PhCH(Et)CN, and PhCH(Me)COPh. However, comparative studies of the efficiency of the catalysts show that alkylpyridinium bromides283 or N-alkyl-Af-benzyl-piperidinium chloride284 have a smaller catalytic activity compared to tetraalkylammonium halides. McIntosh285 has described the preparation of azapropellane salts 186 as potential chiral phase transfer catalysts. 

Theory would predict that PTC should be useful in increasing the alkylation efficiency of hydrophobic electrophiles with cellulose ether alkoxides. However, there is very little previous work reported in using PTC in the preparation of cellulose ethers. Daly and coworkers10 reported that quaternary ammonium salts were useful in catalyzing the heterogeneous benzylation of cellulose, but when we applied this technique to the DPGE alkylation of nascent HEC in aqueous /-butyl alcohol, the presence of catalytic amounts of tetramethylammonium chloride or tetrabutylammonium bromide actually afforded lower alkylation efficiencies. 

Material 1 was also treated with 3-bromopropyltrichlorosilane to yield a propyl tether with a bromo-head group (4). Substitution with either trimethylamine or triethylamine to form the corresponding quaternary ammonium species, followed by ion exchange with potassium perruthenate afforded the catalytic species (5) and (6) respectively. The black solid 5 was found to be an equally efficient catalyst for the oxidation reactions and 6 was found to be a more highly active recoverable and reusable... 

A very efficient catalytic system has been developed by He and coworkers using, under solvent-free conditions, quaternary ammonium bromide-functionalized polyethylene glycol (0.25 mol% 373 K, 8MPa, 5-20min) [68d]. The catalyst worked well for a variety of l-alkyl-2-aryl-substituted aziridines, and could be easily recovered by centrifugation and reused, without any significant loss of catalytic activity and selectivity. 

Being inspired by Maruoka s results with the C2-symmetric binaphthyl-derived quaternary ammonium salt [21], Lygo and colleagues designed a quaternary ammonium salt 23, comprising conformationally flexible biphenyl units and commercially available chiral secondary amines [22], A library of 40 quaternary ammonium salt was synthesized and evaluated for their catalytic efficiency in the asymmetric alkylation of... 

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). 

At very negative potentials neither the tetraalkylammonium ions nor the metallic electrode are inert they combine to form reduced TAA-metals [7]. Tetraalkylammonium (TAA) metals are composed of quaternary ammonium ions, electrons, and a post-transistion metal such as Hg, Pb, Sn, Sb, Bi [5-18] or Pt [19] most of them have the composition R4N" MeJ [13] or R4N" Mc4 [20] and have been described as Zintl ion salts or Zintl phases [21,22]. They have been shown to be useful intermediates in the electrochemical reduction of certain substrates that are reducible with difficulty. On reduction of the quaternary ammonium salt, the initial layer of the metal compound is controlled by a two-dimensional nucleation, whereas the bulk phase is initiated by a three-dimensional nucleation and a growth controlled by the diffusion of R4N from the solution. In some cases (A-methylquinuclidinium (MQ" ) mercury) the catalytic efficiency of the initial layer is greater than that of the bulk phase [18], whereas in other cases (A, A-dimethylpyrrolidinium (DMP" ) lead) the opposite is found [16]. 

Evaluation of phase-transfer catalysts. Herriott and Picker have examined the catalytic effects of 21 quaternary ammonium and phosphonium salts on the Sn2 reaction of sodium thiophenoxide with 1-bromooctane in a two-phase system of aqueous sodium hydroxide and an organic solvent. They report the following conclusions The catalytic efficiency increases with the length of the... 

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