Phase transfer ammonium salts

A great improvement in this approach was obtained thanks to chiral bifunctional phase transfer ammonium salts having a binaphthyl scaffold with 3,3 -bis-(diarylhydroxy-methyl) substituents. These catalysts have demonstrated a superior level of enantiodiscrimination in the fluorination of cyclic 3-keto esters (Scheme 44.28). ... 

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... 

One prevalent strategy for this involves tire use of a phase transfer agent, such as tetraoctyl ammonium bromide, to bring gold and silver salts into an organic phase [12, and. Reduction of tire metal 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 as we have seen are useful m synthetic organic chem istry as phase transfer catalysts In another more direct application quaternary ammo mum hydroxides are used as substrates m an elimination reaction to form alkenes... 

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... 

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). 

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. 

Reactions of the Side Chain. Benzyl chloride is hydrolyzed slowly by boiling water and more rapidly at elevated temperature and pressure in the presence of alkaHes (11). Reaction with aqueous sodium cyanide, preferably in the presence of a quaternary ammonium chloride, produces phenylacetonitrile [140-29-4] in high yield (12). The presence of a lower molecular-weight alcohol gives faster rates and higher yields. In the presence of suitable catalysts benzyl chloride reacts with carbon monoxide to produce phenylacetic acid [103-82-2] (13—15). With different catalyst systems in the presence of calcium hydroxide, double carbonylation to phenylpymvic acid [156-06-9] occurs (16). Benzyl esters are formed by heating benzyl chloride with the sodium salts of acids benzyl ethers by reaction with sodium alkoxides. The ease of ether formation is improved by the use of phase-transfer catalysts (17) (see Catalysis, phase-thansfer). 

Quaternaiy ammonium salts, as we have seen, aie useful in synthetic organic chemistry as phase-transfer catalysts. In another, more direct application, quaternaiy ammonium hydroxides aie used as substrates in an elimination reaction to fonn alkenes. 

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. 

Phase transfer catalysts were used for nucleophilic displacement reactions of activated leaving groups by hydroxyfurazanyl anions. For example, tetrachloro-pyrazine was found to react with hydroxyfurazans in benzene/Na2C03/tetraalkyl-ammonium salts giving products of mono- or disubstitution (Scheme 173) (94MI1). The course of the reaction depends on the ratio of the reactants and the nature of the ammonium salt. 

Ionic liquid synthesis in a commercial context is in many respects quite different from academic ionic liquid preparation. While, in the commercial scenario, labor-intensive steps add significantly to the price of the product (which, next to quality, is another important criterion for the customer), they can easily be justified in academia to obtain a purer material. In a commercial environment, the desire for absolute quality of the product and the need for a reasonable price have to be reconciled. This is not new, of course. If one looks into the very similar business of phase-transfer catalysts or other ionic modifiers (such as commercially available ammonium salts), one rarely finds absolutely pure materials. Sometimes the active ionic compound is only present in about 85 % purity. However, and this is a crucial point, the product is well specified, the nature of the impurities is known, and the quality of the material is absolutely reproducible from batch to batch. 

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]. 

Arai and co-workers have used chiral ammonium salts 89 and 90 (Scheme 1.25) derived from cinchona alkaloids as phase-transfer catalysts for asymmetric Dar-zens reactions (Table 1.12). They obtained moderate enantioselectivities for the addition of cyclic 92 (Entries 4—6) [43] and acyclic 91 (Entries 1-3) chloroketones [44] to a range of alkyl and aromatic aldehydes [45] and also obtained moderate selectivities on treatment of chlorosulfone 93 with aromatic aldehydes (Entries 7-9) [46, 47]. Treatment of chlorosulfone 93 with ketones resulted in low enantioselectivities. 

Another catalytic system which has been successfully applied to the autoxidation of substituted toluenes involves the combination of Co/Br" with a quaternary ammonium salt as a phase transfer catalyst (ref. 20). For example, cobalt(II) chloride in combination with certain tetraalkylammonium bromides or tetraalkylphosphonium bromides afforded benzoic acid in 92 % yield from toluene at 135-160 °C and 15 bar (Fig. 19). It should be noted that this system does not require the use of acetic acid as solvent. The function of the phase transfer catalyst is presumably to solubilize the cobalt in the ArCH3 solvent via the formation of Q + [CoBr]. ... 

In this method, a catalyst is used to carry the nucleophile from the aqueous into the organic phase. As an example, simply heating and stirring a two-phase mixture of 1-chlorooctane for several days with aqueous NaCN gives essentially no yield of 1-cyanooctane. But if a small amount of an appropriate quaternary ammonium salt is added, the product is quantitatively formed in about 2 h." There are two principal types of phase-transfer catalyst. Though the action of the two types is somewhat different, the effects are the same. Both get the anion into the organic phase and allow it to be relatively free to react with the substrate. 

A method for the depolymerisation of PETP fibres using quarternary ammonium salt phase transfer catalysts in saponification processes at atmospheric pressure and temperatures as low as room temperature is reported. Terephthalic acid was produced in yields as high as 93%. Also reported are similar processes for the depolymerisation of nylon 66 and nylon 46 fibres. Nylon 46 oligomers produced were repolymerised using solid-state polymerisation to produce high molecular weight nylon 46. Nylon 66 was depolymerised to produce oligomers and adipic acid in reasonable yields. 11 refs. USA... 

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... 

In liquid-solid processes reaction takes place between a liquid reactant and an insoluble or sparingly soluble solid which must be finely divided to speed up the process. Another measure to accelerate the process is to use an aqueous solution of a phase-transfer agent (typically a quaternary ammonium salt). The solid can also be a catalyst for reactions between liquid components, e.g. in acylations, carried out both conventionally in the presence of metal chlorides (mostly AICI3) or catalysed by zeolites and Grignard reactions. 

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). 

---