Quaternary salts as catalysts

Brunelle, Chapter 5, also demonstrated that bis-quaternary salts with appropriate spacing between the quaternary nitrogens are dramatically better than mono-quaternary salts as catalysts for transfer of divalent anions, such as the di-anion of bisphenol A. Thus the ion pair formed from the di-anion and a bis-quat appears to be more easily formed and transferred than the species formed from the di-anion and two mono-quaternary cations. 

Use of solvents having higher polarity facilitates distribution of quaternary salts into organic solvents. Hence, it also allows use of smaller quaternary salts as catalysts. With di-... 

The high activity of M(L )2 + L (L = crown-ethers or ammonium quaternary salts) as catalysts of the ethylbenzene oxidation into a-phenylethylhydroperoxide is connected with the formation active primary complexes (M(II)(L )2), (L ), and homo poly nuclear hetero ligand complexes M(II) (L )y(L ) L j, ( A (Ni), B (Fe)) (formed through dioxygenase-like mechanisms). The stability of A ( B ) to the L dioxygenation seems to be due to the intramolecular and intermolecular H-bonding interactions. 

Table 6. Michael Addition of Carbonyl Compounds to Enones and Enoates with Crown Ethers or Quaternary Salts as Chiral Catalysts Crown Ethers ...

M. Horikawa, J. Bush-Petersen, E. J. Corey, Enantioselective Synthesis of P-Hydroxy-a-amino Acid Esters by Adol Coupling Using a Chiral Quaternary Ammonium Salt as Catalyst , Tetrahedron Lett. 1999, 40, 3843-3846. 

Other classic examples illustrating the use of quaternary salts as phase transfer catalysts were published by Makosza(2), and by Brandstrom(3). Subsequent development of crown ethers(4-7) and crvptands(7-8) as phase transfer catalysts gave PTC an entirely new dimension since now the inorganic reagent, as sodium cyanide in the above equation, need no longer be dissolved in water but can be used... 

The use of polyethers and quaternary salts as liquid-liquid and solid-liquid phase transfer catalysts has been well-documented in the literature. It has been shown that (1) the catalyst functions as a vehicle for transferring the anion of a metal salt from the aqueous or solid phase into the organic phase where reaction with an organic substrate ensues, (2) the rate of reaction is proportional to the concentration of the catalyst in the organic phase, and (3) small quantities of water have a significant effect on the catalytic process. This Communication specifically addresses the role of cyclic polyethers as phase transfer catalysts and the influence of water with regard to the location of the catalyst. 

With soluble quaternary ammonium salts as catalysts the reaction is thought to take place at the aqueous/organic interface because a) the solubilities of quaternary ammonium hydroxides in organic solvents are too low to account for the observed reaction rates, and b) the most active catalysts are benzyltriethylammonium and... 

Quaternary stereocenters can be obtained with high selectivity with ot-amino acid amides as chiral auxiliaries, which were first converted with P-oxo esters to give enamines such as compounds 58. According to a combinatorial strategy, various enamino esters 58 were screened in Michael additions with MVK (41a) and several metal salts as catalysts. With FeCl3, however, the maximum stereoselectivity achieved was only 77% ee (with enamine 58a derived from L-isoleucine dimethylamide). Cu(0Ac)2H20 turned out be the optimal catalyst for this transformation. With L-valine diethylamide as chiral auxiliary in compound 58b, reaction proceeds with 86% yield and 98% ee after aqueous workup [79]. Importantly, this valuable method for the construction of quaternary stereocenters [80] under ambient conditions seems to be generally applicable to a number of Michael donors [81]. In all cases, the auxiliary can be quantitatively recovered after workup. 

Rozwadowska and coworkers carried out the asymmetric alkylation of isoquino-line Reissert compounds under phase-transfer conditions using cinchonine-derived quaternary ammonium salts as catalysts. The best enantioselectivity was achieved in the benzylation and allylation of 1 -cyano-2-phenoxy carbonyl-1,2-dihydroisoquinoline (17) catalyzed by 2a (Scheme 2.14) [34]. 

Fig. 4.85 Quaternary ammonium salts as catalyst for BV-oxidation with H202.

This reaction can be conducted under solid-hquid phase transfer catalytic (PTC) conditions using sohd potassium carbonate as base and a liquid lipophihc quaternary ammonium salt as catalyst. Numerous carbodiimides with a side chain bearing a tertiary amino group, such as-(CH2) NMe2, are obtained in this manner. Sheehan and cowoikers used a modification of this procedure to synthesize several water soluble carbodiimides. ... 

The Darzens reaction (tandem aldol-intramolecular cyclization sequence reaction) is a powerful complementary approach to epoxidation (see Chapter 5) that can be used for the synthesis of a,P-epoxy carbonyl and a,p-epoxysulfonyl compounds (Scheme 8.32). Currently, all catalytic asymmetric variants of the Darzens reactions are based on chiral phase-transfer catalysis using quaternary ammonium salts as catalysts. 

A Darzens-type condensation has been employed in the synthesis of (46) by the reaction of 9-chlorofluorene anion (45) with PhCHO.A two-phase system was used to prepare (f)-(47 R = H, Cl, or NO2) almost exclusively, via a Darzens reaction of the substituted benzaldehyde with PhCOCH2Br in the presence of a quaternary ammonium salt as catalyst. ... 

Because the triphase reaction involves not merely diffusion of a single phase into the solid support, the organic reaction take places in the organic phase, and the ion-exchange reaction occurs in the aqueous phase. The catalyst support is usually lipophilic. The organic phase and aqueous phase fill the catalyst pores to form the continuous phase and the disperse phase, respectively. The interaction between quaternary salts as well as... 

Calo and coworkers reported the use of Pd nanoparticles in ionic liquids (quaternary ammonium salts) as catalysts for the Suzuki coupling reaction of aryl bromides and chlorides with phenylboronic acid. The catalytic system was found... 

Dialkylvinylidenecyciopropanes (cf 5,460). Various crown ethers, particularly tlicyclolicxyl-l8-crown-6, are somewhat more effective than quaternary am-immiiim salts as catalysts for the synthesis of these substances from 3-chloro-3-inclhyl-l-biitync and alkenes in the presenee of aqueous KOH. ... 

This reaction has been modified to occur in solid-liquid two-phase systems in thepresence of crown-ether or quaternary salt as the catalyst. °... 

B. Zaidman, Y. Sasson, and R. Neumann, General economic evaluation of the use of quaternary ammonium salts as catalysts in industrial appheations, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 390-393. 

The use of ot,p-unsaturated aldehydes as Michael acceptors always represents a challenging situation because of the tendency of enals to undergo 1,2- rather than the desired 1,4- addition reaction. Moreover, working under phase-transfer catalysis conditions incorporates an additional element of difficulty, because of the propensity of enolizable enals to undergo self-condensation side reactions. For this reason, there are only a few examples reporting enantioselective Michael reactions with ot,p-unsaturated aldehydes as Michael acceptors under PTC conditions, both coming from the Maruoka research team and also both making use of chiral tV-spiro quaternary ammonium salts as catalysts. 

Continuing with the use of cinchona alkaloid-based quaternary ammonium salts as catalysts, phenyl vinyl sulfones have also been employed as Michael acceptors in the reaction with glycine imines using cinchonidinium salt 103a as catalyst both in solution or in a solid-supported version (Scheme 5.33), furnishing similar results to those provided by the corresponding vinyl ketones and acrylates shown in Schemes 5.8 and 5.23. ... 

The principle of extraction method used to separate PTC and product is based on solubility of quaternary ammonium salt in alkaline aqueous solution. " For example, tetrabutylammonium bromide is soluble to the extent of 27% in dilute (1% NaOH) aqueous solutions, but when the solution is made more concentrated (15% NaOH), the solubility of Bu4N Br decreases to 0.07%. When the products are obtained in PTC system, they can be usually separated from PTC by distillation method. PTC catalyst in the distillation residue may sometimes be reusable. With quaternary ammonium salts as catalysts, temperatures above 100-120 C usually result in partial or total decomposition of the quaternary salts to trialkylamines and other products. Mieczynska et al. and Monflier et al. investigated the hydrogenation and hydroformylation under phase transfer catalytic conditions. They found that the yield of aldehydes obtained in hydroformylation of 1-hexene strongly depends on solvent 24% in toluene, 53-86% in toluene-water-ethanol mixture and 77-94% in water-ethanol solution. The mixture of water-ethanol as a solvent was also found to be the best for hydrogenation of 1-hexene (96% of hexane). Conversion of Ph2PCH(CH3)(COOH) phosphine into sodium salt Ph2PCH(CH3)(COONa) yields aldehyde in toluene, 92% in toluene-water and 94% in toluene-water-ethanol mixture. 

Chemical stability is a factor which should be carefully considered. In the case of quaternary salts, as such or bonded to a polymer, the catalyst can be destroyed both by thermal effects and chemical reactions in the presence of active nucleophiles This gives rise to some doubts regarding the possibility of the general employment of such catalysts, especially in the industrial field. The situation is completely different in the case of more sophisticated and relatively more expensive catalysts, i.e. crown ethers and cryptands. The latter systems are much more stable than quaternary salts. In principle, polymer bonding allows a broader use, due to the possibility of recycling... 

The above reaction was carried out in the presence of quaternary amine salts as catalysts. 

Poly(monothiocarbonate)s (70) (211-214), (71) (215), (72) (216), and (73) (217) containing oxythiocarbonyloxy linkages (69) are prepared from thiophosgene (carbon chlorosulfide, thiocarbonylchloride, carbonothioic dichloride) [463-71-8] and corresponding diphenols imder phase-transfer conditions with quaternary ammonium and phosphonium salts as catalysts. The influence of the structure of the bisphenols and the nature of the catalyst on yields and jinh have been studied. Under optimum conditions, polymers in 90-97% yields and jinh up to 1.8 (70), 0.09 (71), 0.24-0.63 (72), and 0.24-0.36 dL/g (73) (0.5 g/dL, CHCI3, at 25°C) were obtained. 

Various chlorinating agents, such as thionyl chloride [965], phosphorus trichloride [966], PPhs/tetrachloromethane [967], PPhs/hexachloroacetone [968], phosgene [969-973] or, more economically, phosgene with HCl gas in the presence of an amine or a quaternary ammonium salt as catalyst [974-977], have been described. 

Typical conditions for these processes are simply to stir an aqueous solution of the metal salt of Y with the organic substrate alone, or in a solvent such as chloroform or benzene, in the presence of 5—10 mole % of a quaternary ammonium salt as catalyst. A recent example is to be found in a synthesis of alkyl azides from alkyl iodides (or other halides) using commercially available Aliquat 336 [mainly (1)] as catalyst, and a reaction temperature of 100 °C. The conversion of alkyl methanesulphonates to alkyl halides has been used to synthesize optically active secondary fluorides, chlorides and bromides via an 5 n2 inversion mechanism (the iodides racemize before isolation). Ammonium salt (1), or phosphonium salt (2), are used to catalyse these mesylate displacements. ... 

Three different highly enantioselective addition reactions of sulfones to in situ generated N-Boc and N-Cbz imines have been reported recently, all rmder PTC using structurally similar chiral ammonium salts as catalysts (Scheme 29.31). In 2007, Shibata, Toru, and coworkers developed a PTC reaction of imines, generated in situ from a-amido sulfones, with l-fluorobis(phenyIsulfonyl)methane (FBSM) [67]. A quinidine derived quaternary ammonium chloride (52) is used as catalyst, and the Mannich-type reaction proceeds smoothly to afford the 1-fluorobis(phenylsulfonyl)methylated amines in high yields and excellent enantioselectivities. Desulfonation of adducts by treatment with Mg in EtOH yields the monofluoromethylated adducts 55 with nearly complete retention of the enantiomeric excess [Scheme 29.31 (1)]. 

Chiral Quaternary Ammonium Salts as Catalysts for the Strecker Reaction... 

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