Phase transfer catalysis quaternary salts

This property of quaternary ammonium salts is used to advantage m an experi mental technique known as phase transfer catalysis Imagine that you wish to carry out the reaction... 

Quaternary ammonium salts compounds of the type R4N" X find application m a technique called phase transfer catalysis A small amount of a quaternary ammonium salt promotes the transfer of an anion from aqueous solution where it is highly solvated to an organic solvent where it is much less solvated and much more reactive... 

Phase-transfer catalysis (Section 22.5) Method for increasing the rate of a chemical reaction by transporting an ionic reactant from an aqueous phase where it is solvated and less reactive to an organic phase where it is not solvated and is more reactive. Typically, the reactant is an anion that is carried to the organic phase as its quaternary ammonium salt. 

It is important to make the distinction between the multiphasic catalysis concept and transfer-assisted organometallic reactions or phase-transfer catalysis (PTC). In this latter approach, a catalytic amount of quaternary ammonium salt [Q] [X] is present in an aqueous phase. The catalyst s lipophilic cation [Q] transports the reactant s anion [Y] to the organic phase, as an ion-pair, and the chemical reaction occurs in the organic phase of the two-phase organic/aqueous mixture [2]. 

Phase transfer catalysis (PTC) refers to the transfer of ions or organic molecules between two liquid phases (usually water/organic) or a liquid and a solid phase using a catalyst as a transport shuttle. The most common system encountered is water/organic, hence the catalyst must have an appropriate hydrophilic/lipophilic balance to enable it to have compatibility with both phases. The most useful catalysts for these systems are quaternary ammonium salts. Commonly used catalysts for solid-liquid systems are crown ethers and poly glycol ethers. Starks (Figure 4.5) developed the mode of action of PTC in the 1970s. In its most simple... 

Catalysis, enzymatic, physical organic model systems and the problem of, 11,1 Catalysis, general base and nucleophilic, of ester hydrolysis and related reactions, 5,237 Catalysis, micellar, in organic reactions kinetic and mechanistic implications, 8,271 Catalysis, phase-transfer by quaternary ammonium salts, 15,267 Catalytic antibodies, 31,249... 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25... 

C. M. Starks, Phase-Transfer Catalysis. L Heterogeneous Reactions Involving Anion Transfer by Quaternary Ammonium and Phosphonium Salts , J. Am. Chem. Soc 1971, 93,195-199. 

It was a result of demand from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures that led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic [1, 2]. Additionally, comparisons of the efficiencies of the various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols and comparable with the cryptands [e.g. 3, 4], which have fewer proven applications and require higher... 

A large number of reviews of phase-transfer catalysis in general, incorporating the use of quaternary ammonium salts, have appeal ed since the 1960s [e.g. 18, 51-66]. Many of the early articles are of important historical interest, whereas the later reviews illustrate the manner in which applications of the technique have mush-... 

More detailed discussions of the optimum choice of quaternary ammonium salt for phase-transfer catalysis are available elsewhere [1, 5-8]. 

The application of phase-transfer catalysis to the Williamson synthesis of ethers has been exploited widely and is far superior to any classical method for the synthesis of aliphatic ethers. Probably the first example of the use of a quaternary ammonium salt to promote a nucleophilic substitution reaction is the formation of a benzyl ether using a stoichiometric amount of tetraethylammonium hydroxide [1]. Starks mentions the potential value of the quaternary ammonium catalyst for Williamson synthesis of ethers [2] and its versatility in the synthesis of methyl ethers and other alkyl ethers was soon established [3-5]. The procedure has considerable advantages over the classical Williamson synthesis both in reaction time and yields and is certainly more convenient than the use of diazomethane for the preparation of methyl ethers. Under liquidrliquid two-phase conditions, tertiary and secondary alcohols react less readily than do primary alcohols, and secondary alkyl halides tend to be ineffective. However, reactions which one might expect to be sterically inhibited are successful under phase-transfer catalytic conditions [e.g. 6]. Microwave irradiation and solidrliquid phase-transfer catalytic conditions reduce reaction times considerably [7]. 

Phase-transfer catalysis succeeds where the classical method fails in the synthesis of e ,v-3,5-diacctoxycyclopcntene, which is a key compound in prostaglandin synthesis. In contrast with the formation of a mixture of the cis and mrmv-diesters and the 3,4-diacetoxy derivative, e/,v-3,5-diacetoxycyclopcntene is obtained >90% purity from the 3,5-dibromo derivative, when treated with potassium acetate in the presence of a quaternary ammonium salt [4],... 

In the main, the original extractive alkylation procedures of the late 1960s, which used stoichiometric amounts of the quaternary ammonium salt, have now been superseded by solid-liquid phase-transfer catalytic processes [e.g. 9-13]. Combined soliddiquid phase-transfer catalysis and microwave irradiation [e.g. 14-17], or ultrasound [13], reduces reaction times while retaining the high yields. Polymer-supported catalysts have also been used [e.g. 18] and it has been noted that not only are such reactions slower but the order in which the reagents are added is important in order to promote diffusion into the polymer. 

Starks, C. M. J. Am. Chem. Soc. 93 (1971) 195 Phase transfer catalysis I. Heterogeneous reactions involving anion transfer by quaternary phosphonium salts. 

The general concept of phase transfer catalysis applies to the transfer of any species from one phase to another (not just anions as illustrated above), provided a suitable catalyst can be chosen, and provided suitable phase compositions and reaction conditions are used. Most published work using PTC deals only with the transfer of anionic reactants using either quaternary ammonium or phosphonium salts, or with crown ethers in liquid-liquid or liquid-solid systems. Examples of the transfer and reaction of other chemical species have been reported(24) but clearly some of the most innovative work in this area has been done by Alper and his co-workers, as described in Chapter 2. He illustrates that gas-liquid-liquid transfers with complex catalyst systems provide methods for catalytic hydrogenations with gaseous hydrogen. 

The first examples of the application of phase-transfer catalysis (PTC) were described by Jarrousse in 1951 (1), but it was not until 1965 that Makosza developed many fundamental aspects of this technology (2,3). Starks characterized the mechanism and coined a name for it (4,5), whilst Brandstrom studied the use of stoichiometric amounts of quaternary ammonium salts in aprotic solvents, "ion-pair extraction" (6). In the meantime Pedersen and Lehn discovered crown-ethers (7-9) and cryptands (10,11), respectively. 

A. Brandstrom, "Principles of Phase Transfer Catalysis by Quaternary Ammonium Salts," in "Advances in Physical Organic Chemistry." Vol. 15, V. Gold, Ed., Academic Press, London and New York, 1977, p. 267. 

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... 

Jones, R. A. Quaternary Ammonium Salts Their Use in Phase-Transfer Catalysis, Academic Press, London, 2001. 

Alkylation of Schiff bases, derived from amino acid and non-optically active aromatic aldehydes by phase-transfer catalysis in the presence of cinchona alkaloid derived quaternary ammonium salts, gave ce values of up to 50% l42. 

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