Phase transfer catalysis reagent

Although this explanation may not be very convincing, we do not have any alternative. Presence of the intermediate 90 is necessary for this transformation. Treatment of aldehyde 85 with the same mild phase transfer catalysis reagent, but without the presence of 86, was completely ineffective the aldehyde remained unchanged. 

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. 

Phase-transfer catalysis is another modern synthetic method that is currently receiving much attention. This method tends to have several advantages over traditional methods, such as higher yields, the requirement of milder reaction conditions, simplicity and the use of relatively inexpensive reagents. 

Another two-phase system using phase-transfer catalysis for the oxidation of diaryl-iV-arylsulphonyl sulphilimines to sulphoximines has also been described188. In this reaction the oxidizing reagent is sodium hypochlorite and yields are in excess of 90% in most cases (equation 70). This reaction presumably occurs by initial attack by the nucleophilic hypochlorite ion on the sulphur atom followed by chloride ion elimination. 

The development of phase transfer catalysis, of supercritical fluids, of ionic liquids and of course, new reagents, should also have considerable potential in the labeling area. Furthermore there is the possibility of combining these approaches with energy-enhanced conditions - in this way marked improvements can be expected. 

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

For those applications involving the activation of an inorganic anion (that is, generation of a naked anion), the cryptands, rather than the crowns, tend to be the reagents of choice. Such reagents are thus also ideal for applications involving phase-transfer catalysis of the type discussed previously. 

A number of other cryptand-bound polymers have been synthesized using similar procedures to those discussed previously for immobilization of crown molecules. Apart from their use in phase transfer catalysis, such polymers have been studied extensively as chromatography reagents for the separation of a range of metal-ion types (Blasius Janzen, 1982) in a number of instances quite useful separations have been achieved. 

The phase transfer catalysis not only promotes the reactions between the reagents which are mutually insoluble in immiscible phases, but also offers a number of process advantages such as, increase in rate of reactions, increase in product specificity, lowering of energy requirement, use of inexpensive solvents and catalysts, extraction of cations or even neutral molecules from one phase to another etc. 

Addition of phase transfer reagents, i.e. phase transfer catalysis. 

Water is a unique solvent because of its high polarity and ability to form a network of H-bonds. It is immiscible with many organic solvents and is therefore a suitable solvent for use in biphasic reactions in which catalysts are made preferentially soluble in the aqueous phase. Phase transfer catalysis allows the use of aqueous reagents with substrates that have low solubility in water. That water is abundant and totally non-toxic make it the perfect clean solvent, provided that solubility issues can be overcome, and it is in use as a solvent on an industrial scale for polymerization, hydroformylation, and a range of organic chemistry involving PTC. These applications are discussed further in Chapters 7-11. 

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. 

Figure 11.5. Representative example of the mechanistic pathway of phase transfer catalysis (PTC). (Z, Z — functional group M = metal Q = chiral catalyst R = alkyl or aryl reagent X = halogen).

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