Catalytic efficiency, quaternary ammonium salts

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. 

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. 

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]. 

Based on the catalyst design limitation arising from their synthesis, other chiral quaternary ammonium salts were developed, such as Ca-symmetrical PTC catalysts XXVIII [53], spermidine- and spermine-based catalysts XXIX [54], bis-biphenyl quaternary ammonium salts XXX [55], and other chiral quaternary ammonium salts of types XXXI and XXXII [56]. V-spiro chiral ammonium salt XXXI and XXXII (the later, readily available from gallic acid) possessing flexible alkyl chains showed high catalytic efficiency in alkylations of glycine-imine esters even in the presence of 0.01-0.05 mol% of XXXI or XXXII (Scheme 8.6) [57]. 

Tertiary amines are also known to effect the phase transfer addition of cyanide ion to primary, allylic, and benzylic halides [9]. The reported effect of amine structure on catalytic efficiency closely parallels that reported by Hennis for ester formation in a two-phase system (see Sect. 1.7). Both the nitrogen of the amine and the carbon bearing halide of the alkyl bromide must be sterically accessible for the reaction to succeed. Thus, -hexylamine is effective in concert with -butyl bromide but the combinations of either 5-butyl bromide and -hexylamine or -butyl bromide and cyclohexylamine are not. Tertiary amines are generally more effective than secondary or primary amines. In addition, the yields of primary nitriles decrease dramatically with the size of the primary alkyl bromide from quantitative with n-butyl to only 6% with -decyl bromide when -hexylamine is used as phase transfer catalyst. On the other hand, tributylamine was equally useful as a catalyst for the quantitative conversion of either 1-bromohexane or 1-bromodecane to the corresponding nitriles [9]. In general, these observations accord with those of Hennis and coworkers indicating that this reaction is an example of in situ formation of and catalysis by quaternary ammonium salts [10]. 

Preparation.—Ethers. A new method for benzylation of hindered glucidic hydroxy groups e.g. 73) requires reaction of the alkoxide with benzyl bromide and a catalytic quantity of a quaternary ammonium salt, and it is suggested that weakened ion-pairing might account for the increased efficiency. A simple, high yield, methy-lenation of catechols with dihalomethanes, in the absence of the usual strong... 

In our works we have modeled efficient catalytic systems (ML + L (M = Ni, Fe, are crown ethers or quaternary ammonium salts) for ethylbenzene oxidation to PEH, that was based on the established (for Ni complexes) and hypothetical (for Fe complexes) mechanisms of formation of catalytically active species and their operation. 2 Selectivity (Spgjj), conversion, and yield of PEH in ethylbenzene oxidation catalyzed by these systems were substantially higher than those observed with conventional catalysts of ethylbenzene oxidation to PEH. - ... 

It is clear from the numerous accounts in literature that DMCs can efficiently catalyze the copolymerization of CO2 and epoxides. DMCs can however also be used to develop systems that selectively catalyze the CO2 cycloaddition rather than the copolymerization (Scheme 1.4) as is illustrated by the work of Dharman et al. [20]. By itself, a Zn-Co-DMC is an efficient catalyst for the copolymerization reaction. However, the addition of a quaternary ammonium salt to the reaction mixture switches the selectivity of the catalytic system toward the exclusive formation of the cyclic carbonate. The quaternary ammonium ion plays two important roles in the catalytic system it accelerates the diffusion of CO2 into the reaction mixture and it favors a backbiting mechanism. As such, it hinders the growth of the polymer chain and it enables the selective cyclic carbonate production. Although most zinc-containing catalysts for this reaction are very sensitive toward water, Wei et al. have shown that, for example, the combination of Zn-Co-DMC with CTAB (cetyltrimethylammonium bromide) could even use water-contaminated epoxides as an epoxide feed [21]. 

A comparison between the PT catalytic activity at the solid-liquid interface of Aliquat 336 (a quaternary ammonium salt) and 18-crown-6, for the reaction of alkyl halides and the solid potassium salts of several nucleophilic anions, has shown that, whereas for cyanide ion the crown compounds were more efficient, in the other cases investigated the quaternary salt was as good as, or better than, a crown ether.The extra flexibility with quaternary salts in that selection of a particular catalyst for a particular cation is not necessary is one of the reasons that makes them the PT catalysts of choice in the opinion of these authors. 

An important observation with respect to these catalytic systems is the fact that the presence of quaternary ammonium salts (R N X ) notably enhances their catalytic efficiencies. As mentioned earlier (see Section 3.2.5), in these ligandless systems, complexes such as [(Ar) (Solvent)PdXJ and [Ar(alkene)PdX2] are detected by extended X-ray absorption fine structure (EXAFS). 

Asymmetric phase-transfer catalytic addition of cyanide to C=N, C=0, and C=C bonds has been recently explored, which has been demonstrated to be an efficient method toward the synthesis of a series of substituted chiral nitriles. In this context, Maraoka and coworkers disclosed an enantioselective Strecker reaction of aldimines by using aqueous KCN [140]. In this system, the chiral quaternary ammonium salts (R)-36e bearing a tetranaphthyl backbone were found to be remarkably efficient catalysts (Scheme 12.25). Subsequently, this phase-transfer-catalyzed asymmetric Strecker reaction was further elaborated by use of a-amidosulfones as precursor of N-arylsulfonyl imines. Interestingly, the reaction could be conducted with a slight excess of potassium cyanide [141] or acetone cyanohydrin [40] as cyanide source, and good to high enantioselectivities were observed. In contrast, the asymmetric phase-transfer-catalytic cyanation of aldehydes led to the cyanation products with only moderate enantioselectivity [142]. 

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