Phase-transfer catalysts, phosphonium salts

Another important use of the susceptibility of phosphonium salts to undergo alkaline hydrolysis concerns their use as liquid-liquid phase-transfer catalysts. Phosphonium salts can be decomposed much more easily than ammonium salts, under alkaline conditions726-729, and they have to be used under much milder conditions [i.e. low temperature (< 25 °C) and moderate aqueous base concentration (< 15%)] in all cases reaction conditions should be used which prevent the extraction of OH - into the organic phase or minimize its reactivity730. 

Halex rates can also be increased by phase-transfer catalysts (PTC) with widely varying stmctures quaternary ammonium salts (51—53) 18-crown-6-ether (54) pytidinium salts (55) quaternary phosphonium salts (56) and poly(ethylene glycol)s (57). Catalytic quantities of cesium duoride also enhance Halex reactions (58). 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

Phase-tiansfei catalysis (PTC) is a technique by which leactions between substances located in diffeient phases aie biought about oi accelerated. Typically, one OI more of the reactants are organic Hquids or soHds dissolved in a nonpolar organic solvent and the coreactants are salts or alkah metal hydroxides in aqueous solution. Without a catalyst such reactions are often slow or do not occur at ah the phase-transfer catalyst, however, makes such conversions fast and efficient. Catalysts used most extensively are quaternary ammonium or phosphonium salts, and crown ethers and cryptates. Although isolated examples of PTC can be found in the early Hterature, it is only since the middle of the 1960s that the method has developed extensively. 

Benzylic quaternary phosphonium and ammonium salts are dealky-lated by mild heating and/or nucleophilic anions, particularly iodide (9) and thiolate (10), but also hydroxide (11). Most N-benzyl-pyridinium or quaternary aryl ammonium compounds are particularly susceptible (12). Decompositions of this sort have seriously limited the usefulness of solid phase-transfer catalysts derived from (chloromethyl)polystyrene (13, 14). 

The phosphonium salt 21 having a multiple hydrogen-bonding site which would interact with the substrate anion was applied to the phase transfer catalyzed asymmetric benzylation of the p-keto ester 20,[18 191 giving the benzylated P-keto ester 22 in 44% yield with 50% ee, shown in Scheme 7 Although the chemical yield and enantiomeric excess remain to be improved, the method will suggest a new approach to the design of chiral non-racemic phase transfer catalysts. 

Onium salts, crown ethers, alkali metal salts or similar chelated salts, quaternary ammonium and phosphonium are some of the salts which have been widely used as phase transfer catalysts (PTC). The choice of phase transfer catalysts depends on a number of process factors, such as reaction system, solvent, temperature, removal and recovery of catalyst, base strength etc. 

Long-chain alkyl 14B>146 and polymer-bound147 phosphonium salts have been used as phase-transfer catalysts. 

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... 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

Polymer phase-transfer catalysts (also referred to as triphase catalysts) are useful in bringing about reaction between a water-soluble reactant and a water-insoluble reactant [Akelah and Sherrington, 1983 Ford and Tomoi, 1984 Regen, 1979 Tomoi and Ford, 1988], Polymer phase transfer catalysts (usually insoluble) act as the meeting place for two immiscible reactants. For example, the reaction between sodium cyanide (aqueous phase) and 1-bromooctane (organic phase) proceeds at an accelerated rate in the presence of polymeric quaternary ammonium salts such as XXXIX [Regen, 1975, 1976]. Besides the ammonium salts, polymeric phosphonium salts, crown ethers and cryptates, polyethylene oxide), and quaternized polyethylenimine have been studied as phase-transfer catalysts [Hirao et al., 1978 Ishiwatari et al., 1980 Molinari et al., 1977 Tundo, 1978]. 

The dependence of kobsd on stirring speed for Br-I exchange reactions with polymer-supported crown ethers 34 and 35 has been determined under the same conditions as with polymer-supported phosphonium salts 1 and 4149). Reaction conditions were 90 °C, 0.02 molar equiv of 100-200 mesh catalyst, 16-17% RS, 2% CL, 20 mmol of 1-bromooctane, 200 mmol of KI, 20 ml of toluene, and 30 ml of water. Reaction rates with 34 and 35 increased with increased stirring speed up to 400 rpm, and were constant above that value. This result resembles that with polymer-supported onium ion catalysts and indicates that mass transfer as a limiting factor can be removed in experiments carried out at stirring speeds of 500-600 rpm, whatever kind of polymer-supported phase transfer catalyst is used. 

The most straightforward way to obtain polymeric phosphonium salts involves introducing the phosphonio groups on to a suitable polymeric structure, for example by reacting tertiary phosphines with a poly(chloromethylstyrene) (reaction 99). The polymeric phosphonium salts obtained in this way are mostly used as polymer-supported phase-transfer catalysts for nucleophilic substitutions reactions under triphase conditions. 

The alkylation of phthalimide has been carried out under phase-transfer conditions by Landini and Rolla,272 using soluble phosphonium salts and later by Tundo,273 using heterogenous phase transfer catalysts immobilized on silica gel. Later the preparation of N-alkylphthalimide has been carried out directly from phthalimide by Santaniello and Ponti.274... 

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. 

Apart from these well-known catalysts, much effort has been expended in the synthesis and applications of chiral phase-transfer catalysts that include various quaternary ammonium salts, metal-salen complexes, phosphonium salts, and chiral amines. However, few of these catalysts have shown promising levels of asymmetric induction in asymmetric reactions. 

Quaternary phosphonium salts are organophosphorous compounds used as Wittig olefination reagents, phase transfer catalysts, electrolytes, ionic liquids, and as surface active reagents. Their preparation involves the C-P bond formation in tertiary phosphines. We envisaged that addition of phosphines to unsaturated compounds should be preferable as compared to the conventional method using a substitution reaction of organohalogen compounds (Scheme 1). In this chapter, we describe our recent study on this subject. 

For the oxidation with potassium permanganate in water it is necessary to use a phase-transfer catalyst to obtain high yields.The phase-transfer agents are generally quaternary ictra-alkylammoniiim or phosphonium salts in concentrations of about 0.5-5 mol % relative to the alkene. Using this method several carboxylic acids, such as tetrafluoro-3-iodopropanoic acid, perfluoroheptanoic acid. perfluoroundecanoic acid, perfluorononanoic acid, and pcrfluorooctanedioic acid can be prepared. In a similar way, 3-(perfluoro-1,l-dimethyl-butyl)prop-l-ene can be oxidized. ... 

The nucleophilic curing system is most common and is used in about 80% of all applications. It is based on the cross-linker (bisphenol AF) and accelerator (phase transfer catalyst, such as phosphonium or amino-phosphonium salt). Both diaminic and bisphenol type cure systems are permitted by U.S. Food and Drug Administration (FDA) regulations governing rubber articles in contact with food. The diaminic curing system is also used in some coating and extrusion applications [42]. 

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