Quaternary ammonium salts catalytic activity

Cinchona alkaloids, of course, have occupied the central position in the design of chiral PTCs. By employing a simple chemical transformation of the tertiary amine ofthe natural cinchona alkaloids to the corresponding quaternary ammonium salts, using active halides (e.g., aryl-methyl halides), a basic series of PTCs can be readily prepared. Cinchona alkaloid-derived PTCs have proved their real value in many types of catalytic asymmetric synthesis, including a-alkylation of modified a-amino acids for the synthesis of higher-ordered a-amino acids [2], a-alkylation of... 

In our previous works[8,9] on the synthesis of various 5-membered cyclic carbonate, quaternary ammonium salts such as tetrabutylammonium halides showed excellent catalytic activities in relatively mild reaction conditions, under atmospheric pressure and below 140 U. hi this work, several kinds of quaternary ammonium salts have been used for the transesterification reactions of the ethylaie carbonate with methanol to DMC and ethylene glycol. 

In the synthesis of DMC fiom the transesterification of EC and methanol, quaternary ammonium salt catalysts showed good catalytic activity. The main byproduct was ethylene glycol. The quaternary salt with the cation of bulkier alkyl chain laigth and witii more nucleophilic anion showed better reactivity. Hi temperature and large amount of catalyst increased the conversion of EC. The EC conversion and DMC selectivity increased as the pressure of CO2 increased from 250 to 350 psig. 

Loss of catalytic activity resulting from internal displacements is not usually a serious problem below temperatures of about 100 C. However, highly active R-groups, such as benzyl, methyl and allyl, undergo internal displacement more readily, particularly in the presence of strong nucleopfiles. For instance, the presence phenolates and thiolates may lead to the formation of benzyl alcohol, ethers, or sulphides from benzyl-substituted quaternary ammonium salts. 

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. 

Herriott and Picker (1975) have studied the reaction between sodium thiophenoxide and 1-bromobutane in benzene-water catalysed by various quaternary ammonium salts and by the dicyclohexyl-18-crown-6 isomers ([20] + [21]). The catalytic activities, as judged from the second-order rate constants, span a range of 104. The best catalyst appeared to be dicyclohexyl- 18-crown-6, directly followed by tetrabutylphosphonium chloride and tetrabutylammonium iodide. 

The catalytic effect of quaternary ammonium salts in the basic liquid liquid two-phase alkylation of amines [1-3] is somewhat unexpected in view of the low acidity of most amines (pKfl>30). Aqueous sodium hydroxide is not a sufficiently strong base to deprotonate non-activated amines in aqueous solution and the hydroxide ion is not readily transferred into the organic phase to facilitate the homogeneous alkylation (see Chapter 1). Additionally, it is known that ion-pairs of quaternary ammonium cations with deprotonated amines are decomposed extremely rapidly by traces of water [4]. However, under solidrliquid two-phase conditions, the addition of a quaternary ammonium salt has been found to increase the rate of alkylation of non-activated amines by a factor of ca. 3-4 [5]. Similarly, the alkylation of aromatic amines is accelerated by the addition of the quaternary ammonium salt the reaction is accelerated even in the absence of an inorganic base, although under such conditions the amine is deactivated by the formation of the hydrohalide salt, and the rate of the reaction gradually decreases. Hence, the addition of even a weak base, such as... 

Early procedures used stoichiometric amounts of the quaternary ammonium catalyst to solubilize the preformed sodium or potassium salts of the active methylene compounds (prepared under anhydrous conditions) in the organic medium. Subsequently, liquidrliquid two-phase procedures using catalytic amounts of the quaternary ammonium salts were developed, and solidrliquid two-phase conditions have been used to improve yields. In some cases, only the solidrliquid two-phase procedures are effective. 

Several conclusions can be reached from these data. The first is that on a per mole basis, the quaternary ammonium salt is the most favorable catalyst for this reaction. Among the other compounds, the catalytic activity of 1.5 mole-% of crown, PEG, or PEG-MME are similar. If equal weights of PEG-400 and PEG-3400 are used, quite different reaction rates are observed. This is because each polymer chain is capable of transporting one cation across the phase boundary at a time. The ratio of molecular weights is 8.5, so there are 8.5 more catalysts available in the PEG-400 catalyzed reaction than in the one involving the higher molecular weight compound. The actual ratio of rates for these two processes is 12.5, or nearly the expected value. 

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. 

Cyclopropanation, Horner-Wadsworth Emmons Reaction, and Darzens Condensation Although induction in the cyclopropanation of alkenes was reported early, this work was disputed [49]. Other reports of cyclopropanations have yielded, at best, low asymmetric inductions [llh,50]. The first example of a catalytic asymmetric Horner-Wadsworth Emmons reaction, which is promoted by a chiral quaternary ammonium salt, was reported recently by the Shioiri group (Scheme 10.10) [51]. The reaction of the prochiral ketone 74 gives optically active a,P-unsaturated ester 76 with 57% ee. 

The oxidation of chalcogen compounds by hypervalent iodine reagents is a known procedure. The oxidation of sulfides only leads to the formation of mixtures of sulfoxides and sulfones under drastic conditions. Usually only sulfoxides are formed and can be obtained in excellent yields [46-48]. Recent investigations showed that sulfide oxidation can be catalyzed by quaternary ammonium salts in micellar systems. Iodosobenzene 5 is catalytically activated by cetyltrimethylammonium bromide (CTAB) and the sulfoxides 24 can be obtained in high yields under very mild conditions, Scheme 6 [49]. Other micelle forming surfactants have also been employed, but CTAB showed the best results in this reaction. It is also possible to perform such oxidations to sulfoxides with (terf-butylperoxy)iodanes of type 13 [50]. 

Shi and coworkers have reported that the rate and product distribution of Baylis-Hillman reactions of aldehydes with a,/ -unsaturated ketones can be drastically affected by the reaction temperature and by the presence of Lewis bases [18]. When the reaction was carried out at -78 °C using catalytic amounts of quaternary ammonium salts as Lewis bases, in the presence of titanium] IV) chloride, chlorinated syw-aldol adducts were obtained as the major products. Quaternary ammonium bromides and iodides showed higher catalytic activity than... 

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). 

Quaternary ammonium salts of alkaloids have been used for the synthesis of optically active oxiranes from electron-poor olefins under phase-transfer conditions. The enantiomer yield is inversely proportional to the dielectric constant of the solvent,Asymmetric epoxidation in the presence of catalytic amounts of poly-(S)-amino-acids in a triphase system has been described with optical yields up to 96% ... 

The catalytic activity of quarternary ammonium salt usually depends on the corresponding catalyst cation and counter anion[2]. For a series of tetraalkylammonium chlorides, the activity increased in the order of TP AC < TBAC < TOAC. Bulky quaternary salts, having longer distance between cation and anion, are generally known to exhibit higher activity in activating anions[3]. This explains why they are more effective in nucleophilic attack of the anion to oxirane ring of GVE. Table 1 also shows that the rate constant with different halide anions of the quaternary ammonium salts decreases in the order of Cf > Bf > T. This is consistent with the nucleophilicity of the halide anions. 

Carbon dioxide can effectively be added to the epoxide ring of GVE to produce the corresponding cyclic carbonate, OVE. Quaternary ammonium salt catalysts showed good catalytic activity even at atmospheric pressure of carbon dioxide. Since the blends of poly(OVE-co-AN) and SAN showed good miscibility, catalytic fixation of carbon dioxide to polymer blends via cyclic carbonate could be one of choice for the reduction and utilization of the greenhouse gcis. 

Modification of Catalytic Activity of Complexes of Acetylacetonates Fe(II,III) with Quaternary Ammonium Salts in the Ethylbenzene Oxidation with Molecular 02 in the Presence of Small Amounts of H20... 

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