Phase transfer catalysis solvents

The benzoic acid derivative 457 is formed by the carbonylation of iodoben-zene in aqueous DMF (1 1) without using a phosphine ligand at room temperature and 1 atm[311]. As optimum conditions for the technical synthesis of the anthranilic acid derivative 458, it has been found that A-acetyl protection, which has a chelating effect, is important[312]. Phase-transfer catalysis is combined with the Pd-catalyzed carbonylation of halides[3l3]. Carbonylation of 1,1-dibromoalkenes in the presence of a phase-transfer catalyst gives the gem-inal dicarboxylic acid 459. Use of a polar solvent is important[314]. Interestingly, addition of trimethylsilyl chloride (2 equiv.) increased yield of the lactone 460 remarkabiy[3l5]. Formate esters as a CO source and NaOR are used for the carbonylation of aryl iodides under a nitrogen atmosphere without using CO[316]. Chlorobenzene coordinated by Cr(CO)j is carbonylated with ethyl formate[3l7]. 

Phase transfer catalysis succeeds for two reasons First it provides a mechanism for introducing an anion into the medium that contains the reactive substrate More important the anion is introduced m a weakly solvated highly reactive state You ve already seen phase transfer catalysis m another form m Section 16 4 where the metal complexmg properties of crown ethers were described Crown ethers permit metal salts to dissolve m nonpolar solvents by surrounding the cation with a lipophilic cloak leav mg the anion free to react without the encumbrance of strong solvation forces... 

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... 

A method for the polymerization of polysulfones in nondipolar aprotic solvents has been developed and reported (9,10). The method reUes on phase-transfer catalysis. Polysulfone is made in chlorobenzene as solvent with (2.2.2)cryptand as catalyst (9). Less reactive crown ethers require dichlorobenzene as solvent (10). High molecular weight polyphenylsulfone can also be made by this route in dichlorobenzene however, only low molecular weight PES is achievable by this method. Cross-linked polystyrene-bound (2.2.2)cryptand is found to be effective in these polymerizations which allow simple recovery and reuse of the catalyst. 

Industrial examples of phase-transfer catalysis are numerous and growing rapidly they include polymerisa tion, substitution, condensation, and oxidation reactions. The processing advantages, besides the acceleration of the reaction, include mild reaction conditions, relatively simple process flow diagrams, and flexibiHty in the choice of solvents. 

Phase transfer catalysis processes (Starks and Liotta, 1978 Starks, 1987) for the synthesis of many organic materials use less, or sometimes no, organic solvent may use less toxic solvent may allow use of less hazardous raw materials (for example, aqueous HCl instead of anhydrous HCl) and may operate at milder conditions. Some types of reactions where phase transfer catalysis has been applied include ... 

Phase-transfer catalysis describes the action of special catalysts that assist the transfer of reactive molecules from a polar ( aqueous ) solvent to a nonpolar ( organic ) solvent. In the absence of the phase-transfer catalyst, one of the reagents is confined to one solvent, and the other reagent is confined to the other solvent, so no reaction occurs. Addition of a small amount of catalyst, however, enables one of the reagents to pass into the other solvent thereby initiating a reaction. 

Interests in the phase transfer catalysis (PTC) have grown steadily for the past several years [68-70]. The use of PTC has recently received industrial importance in cases where the alternative use of polar aprotic solvents would be prohibitively expensive [71-74]. Thus, the potential application of the phase transfer catalyzed aromatic nucleophilic displacement reactions between phenoxide or thiophenoxide and activated systems has... 

The oxidation methods described previously are heterogeneous in nature since they involve chemical reactions between substances located partly in an organic phase and partly in an aqueous phase. Such reactions are usually slow, suffer from mixing problems, and often result in inhomogeneous reaction mixtures. On the other hand, using polar, aprotic solvents to achieve homogeneous solutions increases both cost and procedural difficulties. Recently, a technique that is commonly referred to as phase-transfer catalysis has come into prominence. This technique provides a powerful alternative to the usual methods for conducting these kinds of reactions. 

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

Quaternary ammonium compounds (quats) are prepared - by moderate heating of the amine and the alkyl halide in a suitable solvent - as the chlorides or the bromides. Subsequently conversion to the hydroxides may be carried out. Major applications of the quat chlorides are as fabric softeners and as starch cationizing agent. Several bio-active compounds (agrochemicals, pharmaceuticals) possess the quat-structure. Important applications of quat bromides are in phase transfer catalysis and in zeolite synthesis. 

A difficulty that occasionally arises when carrying out nucleophilic substitution reactions is that the reactants do not mix. For a reaction to take place the reacting molecules must collide. In nucleophilic substitutions the substrate is usually insoluble in water and other polar solvents, while the nucleophile is often an anion, which is soluble in water but not in the substrate or other organic solvents. Consequently, when the two reactants are brought together, their concentrations in the same phase are too low for convenient reaction rates. One way to overcome this difficulty is to use a solvent that will dissolve both species. As we saw on page 450, a dipolar aprotic solvent may serve this purpose. Another way, which is used very often, is phase-transfer catalysis ... 

Although phase-transfer catalysis has been most often used for nucleophilic substitutions, it is not confined to these reactions. Any reaction that needs an insoluble anion dissolved in an organic solvent can be accelerated by an appropriate phase transfer catalyst. We shall see some examples in later chapters. In fact, in principle, the method is not even limited to anions, and a small amount of work has been done in transferring cations, radicals, and molecules. The reverse type of phase-transfer catalysis has also been reported transport into the aqueous phase of a reactant that is soluble in organic solvents. ... 

Transesterification has been carried out with phase-transfer catalysis, without an added solvent. In another procedure, RCOOR are converted to RCOOR" by treatment of the ester and an alcohol R OH with n-BuLi, which converts the R"OH to R"OLi. ... 

This reaction is similar to 13-1 and, like that one, generally requires activated substrates. With unactivated substrates, side reactions predominate, though aryl methyl ethers have been prepared from unactivated chlorides by treatment with MeO in HMPA. This reaction gives better yields than 13-1 and is used more often. A good solvent is liquid ammonia. The compound NaOMe reacted with o- and p-fluoronitrobenzenes 10 times faster in NH3 at — 70°C than in MeOH. Phase-transfer catalysis has also been used. The reaction of 4-iodotoluene and 3,4-dimethylphenol, in the presence of a copper catalyst and cesium carbonate, gave the diaryl ether (Ar—O—Ar ). Alcohols were coupled with aryl halides in the presence of palladium catalysts to give the Ar—O—R ether. Nickel catalysts have also been used. ... 

Phase transfer catalysis (PTC) has been utilized in organic synthesis to perform reactions in organic solvents when some of reactants are present in the aqueous phase (e.g., the substitution reaction involving alkylchlorides RCl),... 

Higher selectivity, easier processing, use of inexpensive solvents, use of cheaper chemicals, and ease of heat removal have been realized through phase-transfer catalysis (PTC). It appears that no catalytic method has made such an impact as PTC on the manufacture of fine chemicals (Sharma, 1996). Many times we benefit by deliberately converting a single-phase reaction to a two-phase reaction. Consider catalysis by. sodium methoxide in a dry organic. solvent. This can invariably be made cheaper and safer by using a two-pha.se. system with a PT catalyst. 

These reactions proceed more rapidly in polar aprotic solvents. In DMSO, for example, primary alkyl chlorides are converted to nitriles in 1 h or less at temperatures of 120°-140°C.36 Phase transfer catalysis by hexadecyltributylphosphonium bromide permits conversion of 1-chlorooctane to octyl cyanide in 95% yield in 2 h at 105° C.37... 

Tosylate is displaced by weak oxyanions with little elimination in aprotic solvents, providing alternative routes to polymer-bound esters and aryl ethers. Alkoxides, unfortunately, give significant functional yields of (vinyl)polystyrene under the same conditions. Phosphines and sulfides can also be prepared from the appropriate anions (57), the latter lipophilic enough for phase-transfer catalysis free from poisonning by released tosylate. 

Crown Ethers Nucleophilic Substitution Reactions in Relatively Nonpolar Aprotic Solvents by Phase-Transfer Catalysis... 

In addition to solvent-free processing, phase-transfer catalytic conditions (PTC) have also been widely employed as a processing technique in MAOS [15]. In phase-transfer catalysis, the reactants are situated in two separate phases, for example liquid-... 

Solvent-free Solid-Liquid Phase-transfer Catalysis (PTC)... 

Loupy and Soufiaoui described a comparative study of the reactivity of diphenylnitri-limine 200 with several dipolarophiles under microwave irradiation in the absence of solvent using a solid mineral support or phase-transfer catalysis (PTC) conditions (Scheme 9.62) [30b]. The results showed that the best yields of adducts were achieved upon impregnating KF-alumina with a mixture of the hydrazynoyl chloride 199 and the dipolarophile followed by irradiation of the mixture in a focused oven. Reaction of this mixture under solid-liquid PTC conditions with KF-Aliquat under microwaves afforded lower yields of cycloadducts, perhaps owing to the partial decomposition of Aliquat at the reaction temperature (140 °C). In all cases, worse yields were obtained by classical heating under comparable reaction conditions (time and temperature). 

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