Phase-transfer catalyst, role

The scope of reactions involving hydrogen peroxide and PTC is large, and some idea of the versatility can be found from Table 4.2. A relatively new combined oxidation/phase transfer catalyst for alkene epoxidation is based on MeRe03 in conjunction with 4-substituted pyridines (e.g. 4-methoxy pyridine), the resulting complex accomplishing both catalytic roles. 

Catalysis at interfaces between two immiscible liquid media is a rather wide topic extensively studied in various fields such as organic synthesis, bioenergetics, and environmental chemistry. One of the most common catalytic processes discussed in the literature involves the transfer of a reactant from one phase to another assisted by ionic species referred to as phase-transfer catalyst (PTC). It is generally assumed that the reaction process proceeds via formation of an ion-pair complex between the reactant and the catalyst, allowing the former to transfer to the adjacent phase in order to carry out a reaction homogeneously [179]. However, detailed comparisons between interfacial processes taking place at externally biased and open-circuit junctions have produced new insights into the role of PTC [86,180]. 

Similarly to classical PTC reaction conditions, under solid-liquid PTC conditions with use of microwaves the role of catalyst is very important. On several occasions it has been found that in the absence of a catalyst the reaction proceeds very slowly or not at all. The need to use a phase-transfer catalyst implies also the application of at least one liquid component (i.e. the electrophilic reagent or solvent). It has been shown [9] that ion-pair exchange between the catalyst and nucleophilic anions proceeds efficiently only in the presence of a liquid phase. During investigation of the formation of tetrabutylammonium benzoate from potassium benzoate and tetrabu-tylammonium bromide, and the thermal effects related to it under the action of microwave irradiation, it was shown that potassium benzoate did not absorb micro-waves significantly (Fig. 5.1, curves a and b). Even in the presence of tetrabutylammonium bromide (TBAB) the temperature increase for solid potassium benzoate... 

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

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. 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

Phase transfer catalysts (PTCs) are known to play an important role in the permanganate ion oxidation of alkenes46-53 and it was found that they are also important for the imine oxidation41. In the absence of PTCs only a slow reaction was observed. The influence of different PTCs on the reaction course of the permanganate ion oxidation of la has been studied and the results are listed in Table 241. It has been found that long-chain tetraalkylammonium salts are the best PTCs for that oxidation reaction41. 

Benzylic ethers (Ph CH2 OR), allylic ethers (R-CH=CH-CH2 OR) and vinylic ethers [R CH=CH(OR)] together with the most commonly encountered tetrahydropyranyl ethers [THP-ethers, (5)] and /J-methoxyethoxymethyl ethers [MEM-ethers, R0CH2 0(CH2)2 0CH3] play an important role in the protection of a hydroxyl group (p. 550). Macrocyclic ethers (the crown ethers) are important phase transfer catalysts [e.g. 18-Crown-6 (6)]. 

In 1971, Starks introduced the term phase-transfer catalysis to explain the critical role of tetraalkylammonium or phosphonium salts (Q 1 X ) in the reactions between two substances located in different immiscible phases [1], For instance, the displacement reaction of 1-chlorooctane with aqueous sodium cyanide is accelerated many thousand-fold by the addition of hexadecyltributylphosphonium bromide 1 as a phase-transfer catalyst (Scheme 1.1). The key element of this tremendous reactivity enhancement is the generation of quaternary phosphonium cyanide, which renders the cyanide anion organic soluble and sufficiently nucleophilic. 

Upon facing the difficulty of stereochemical control in peptide alkylation events, Maruoka and coworkers envisaged that the chiral phase-transfer catalyst should play a crucial role in achieving an efficient chirality transfer, and consequently examined the alkylation of the dipeptide, Gly-L-Phe derivative 57 (Scheme 5.28) [31]. When a mixture of 57 and tetrabutylammonium bromide (TBAB, 2 mol%) in toluene was treated with a 50% KOH aqueous solution and benzyl bromide at 0°C for 4h, the corresponding benzylation product 58 was obtained in 85% yield with the diastereo-meric ratio (DL-58 LL-58) of 54 46 (8% de). In contrast, the reaction with chiral quaternary ammonium bromide (S,S)-lc under similar conditions gave rise to 58 with 55% de. The preferential formation of LL-58 in lower de in the reaction with (R,R)-lc indicated that (R,R)-lc is a mismatched catalyst for this diastereofacial differentiation of 57. Changing the 3,3 -aromatic substituent (Ar) of the catalyst 1 dramatically increased the stereoselectivity, and almost complete diastereocontrol was realized with (S,S)-lg. 

Arai et al. also reported another BINOL-derived two-center phase-transfer catalyst 31 for an asymmetric Michael reaction (Scheme 6.11) [8b]. Based on the fact that BINOL and its derivatives are versatile chiral catalysts, and that bis-ammonium salts are expected to accelerate the reaction due to the two reaction sites - thus preventing an undesired reaction pathway - catalyst 31 was designed and synthesized from the di-MOM ether of (S)-BINOL in six steps. After optimization of the reaction conditions, the use of 1 mol% of catalyst 31a promoted the asymmetric Michael reaction of glycine Schiff base 8 to various Michael acceptors, with up to 75% ee. When catalyst 31b or 31c was used as a catalyst, a lower chemical yield and selectivity were obtained, indicating the importance of the interaction between tt-electrons of the aromatic rings in the catalyst and substrate. In addition, the amine moiety in catalyst 31 had an important role in enantioselectivity (34d and 34e lower yield and selectivity), while catalyst 31a gave the best results. 

Consequently, Dehmlow and coworkers modified the cinchona alkaloid structure to elucidate the role of each ofthe structural motifs of cinchona alkaloid-derived chiral phase-transfer catalysts in asymmetric reactions. Thus, the quinoline nucleus of cinchona alkaloid was replaced with various simple or sterically bulky substituents, and the resulting catalysts were screened in asymmetric reactions (Scheme 7.2). The initial results using catalysts 8-11 in the asymmetric borohydride reduction of pivalophenone, the hydroxylation of 2-ethyl-l-tetralone and the alkylation of SchifF s base each exhibited lower enantiomeric excesses than the corresponding cinchona alkaloid-derived chiral phase-transfer catalysts [14]. 

The same group reported the striking observation that oxygen transfer from ox-one to substrate olefins can also be catalyzed by secondary amines alone [49]. Pyrrolidines proved particularly efficient in this process, which was originally believed to involve the amine radical cation. Subsequent work [50, 51] identified the proto-nated amine as the active species and assigned a dual role to it. It is most probable that the ammonium cation acts both as a phase-transfer catalyst and forms a com-... 

Solid-liquid phase-transfer catalysts. Diphenylphosphinic hydrazide (1) is not alkylated efficiently under usual phase-transfer conditions, but is alkylated by use of solid Na0H-K2C03 with benzene as solvent. The reaction is strongly accelerated by tetra-n-butylammonium hydrogen sulfate. The role of K2CO3 is not clear. The products are hydrolyzed by 15% HCl to pure monoalkylhydrazines. 

Utilization of a mediator and a phase-transfer catalyst may enhance the reaction rate considerably in a bimolecular reaction the rate may be enhanced by preconcentration of a mediator by adsorption to the electrode in a microemulsion [281,282]. When a desired bimolecular reaction involves ions (including radical ions), the nature of the surfactant (cationic, anionic, neutral) may play a role in the product distribution [283]. 

Alkylations. Highly enantioselective alkylation of t-butyl 4,4-bis (p-dimethyl-aminophenyl)-3-butenoate and t-butyl A -diphenylmethyleneglycine in the presence of a quatemized cinchona alkaloid results. The salt plays a dual role in asymmetric induction and as a phase-transfer catalyst. The products from the former reaction can be cleaved at the double bond to furnish chiral malonaldehydic esters which have many obvious synthetic applications. A combination of PTC, LiCl, and an organic base (e.g., DBU) favors the enantioselective alkylation of a chiral A-acylimidazolidinone in which the acyl side chain is derived from glycine. ... 

The type of phase-transfer catalyst plays a key role in the phase-transfer catalytic synthesis of l-bromo-1-chlorocyclopropanes, which are formed in good yields and with high selectivity if the reaction of dibromochloromethane with an alkene is performed using a crown ether (dibenzo-18-crown-6, " 3,5-di-fer/-butylbenzo-15-crown-5, " " 3,3, 5,5 -tetra-tert-butyldiben-zo-lS-crown-b ) or tetramethylammonium chloride.For the specific effect of the tetra-methylammonium chloride on the dichlorocyclopropanation of unconjugated dienes, see Section I.2.I.4.2.I.2., and some electrophilic alkenes, see Section I.2.I.4.2.I.8.2. The reason why these catalysts exhibit peculiar properties is not clear,other crown ethers behave like typical phase-transfer catalysts (Table 25). " ... 

Monflier et al. reported very high conversion (up to 100%) and regioselec-tivity (<95%) in the hydroformylation of various water-insoluble terminal olefins such as 1-decene with Rh/tppts catalyst system in water in the presence of per(2,6-di-0-methyl)- -cyclodextrin (or Me-p-CD) [Eq. 7] [65, 66]. These high activities and selectivities were attributed to the formation of an alkene/cyclodextrin inclusion complex and to the solubility of the cyclodextrin in both the aqueous and organic layers the cyclodextrin probably plays the role of an inverse phase transfer catalyst. 

The aim of this study was to understand the role of different phase transfer catalysts, the PTC concentration, different organic solvents and ligand to Pd ratio on the activity of the Pd complex catalyst (Pd(PPh3)2Cl2) and the selectivity to phenyl acetic acid in the carbonylation of benzylchloride. 

Role of different types of phase transfer catalysts... 

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