Phase-transfer catalysis usual catalysts

Asymmetric phase-transfer catalysis usually stands somewhat separate from the rest of asymmetric organocatalysis and has always been dominated by metal-free catalysts. The earliest report in asymmetric phase-transfer catalysis dates back 30 years to 1984 when Dolling and coworkers first reported the use of a quaternised Cinchona alkaloid (6) as a phase-transfer catalyst for the asymmetric alleviation of ketone 7 during an asymmetric synthesis of (- -)-Indacrinone (Scheme 1.5). Quaternised Cinchona alkaloids dominated the area of asymmetric phase-transfer catalysis for the rest of the 20th century, and were especially used as catalysts for asymmetric amino... 

A new procedure for the conversion of a catechol to a methylenedioxy function utilizes phase-transfer catalysis. The catalyst is Adogen 464, a methyl trialkylammonium chloride, and the alkylating agent is methylene bromide. Usual yields are around 80%. Another efficient method utilizes potassium fluoride and dibromomethane in... 

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... 

Phase transfer catalysts can be used to increase the solubility of reactants in the phase where the reaction takes place. Usually these catalysts are organophilic salts that pair with anionic reactants to increase their solubility in organic solvents. Phase transfer catalysis is described in more detail in Chapter 5. 

Polymer-supported multi-site phase-transfer catalysis seems to require the use of less material in order to provide activity comparable to others253 (Table 27). Quaternary phosphonium ions on polystyrene latices, the particles of which are two orders of magnitude smaller than usual, were shown to be capable of higher activity coagulation of the catalysts under reaction conditions was minimized by specific treatment904. The spacers may also contain ether linkages. 

Solid-liquid solvent-free phase-transfer catalysis (PTC) is specific for anionic reactions including base-catalysed isomerisation7. Usually, a catalyst (typically a tetraalky-lammonium salt or a cationic complexing agent) is added to an equimolar mixture of an electrophile and a nucleophile, one of which serves as both a reactant and the... 

Use of a microemulsion to overcome reagent incompatibility can be seen as an alternative to the more conventional approach of carrying out the reaction in a two-phase system with the use of a phase transfer catalyst. The latter is usually either a quaternary ammonium salt or a crown ether. There are several examples in the literature of comparisons between the microemulsion concept and phase transfer catalysis. The topic has also recently been reviewed [46]. 

Phase transfer catalysis involves typically an organic/aqueous biphasic system in the presence of a transfer agent such as a tetraalkylammonium salt which facilitates the exchange of the catalyst between the two phases, while the reactants and the products are usually retained in the organic layer. Almost all types of homogeneously catalyzed reactions can be carried out in this way.173... 

The conversion of alkenes to oxiranes using ketone catalysts in the presence of a terminal oxidant such as Oxone has proved to be an important advance in the past decade, especially for the formation of chiral oxiranes (see Section 1.03.4.3.3(ii)). The conversion of alkenes to oxiranes has been comprehensively reviewed <20020R219>. The formation of the dioxirane species usually proceeds in situ, whether it be the formation of methyl(trifluoromethyl)-dioxirane in an academic setting <1995JOC3887>, or under conditions amenable to large-scale conversion of aromatic alkenes to oxiranes (oxone, acetone, ethyl acetate, no phase-transfer catalysis) <20020PD405>. 

Phase transfer catalysis and the use of crown ethers are also of particular advantage in alkanenitrile synthesis (Table 1). Usually quaternary ammonium and phosphonium salts serve quite well as catalysts. Another modification is represented by the use of a solid catalyst, which is insoluble in the two-phase system, for instance alumina or anion-exchange resins (triphase catalysis). Crown ethers again capture the cations and generate naked cyanide ions in fairly nonpolar solvents, leading to exceptionally mild reaction conditions. 

Competitive addition of dichlorocarbene to various alkenes indicates that vinyl ethers are more reactive than I-alkenes.Hence, 2-alkoxy-l,l-dichlorocyclopropanes are usually prepared in good yield. The chloroform/base/phase-transfer catalyst method is the most often used. Substrates very sensitive to aqueous conditions, such as trimethylsilyl vinyl ethers will not, with a high degree of certainty, survive the phase-transfer catalysis conditions, thus other methods are used. ... 

Asymmetric phase-transfer catalysis had a very slow development, although the principle is quite simple. A two-phase system is involved, usually liquid/liq-uid (water organic solvent), with a water-soluble base and reactants in the organic phase. A phase-transfer catalyst such as a quaternary ammonium salt with a good balance of hydrophilicity and hydrophobicity transfers the base to the or-... 

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