Crown ethers phase transfer catalysis involving

The versatile solubility properties of the crown ethers and cryptands are important in two of their major applications, phase transfer catalysis and anion activation. Phase transfer catalysis involves the transport of guest species from one phase to another. The two phases in question are usually two immiscible liquids (liquid-liquid phase transport). In practice, this usually means the use of a... 

Hydroxy-bridged complexes [Pt2(,u-OH)2(PEt3)4]2+ can also be prepared. The structure consists of two square planar platinum(II) centers bridged by hydroxide ligands with an angle of 36.4° between the mean plane normals.1569 A useful method to prepare these complexes involves the use of phase-transfer catalysis with crown ethers to facilitate the reaction of KOH with platinum(II) chloro complexes.1570... 

A new and convenient synthesis of 3-(substituted)-2,4,6-triphenylverdazyls involves crown ether assisted solid-liquid phase transfer catalysis of reactions of triphenylformazans with alkyl bromides (or 1-alkylbenzotriazoles) also using barium hydroxide hydrate catalysis (Scheme 8) [94CJC1849]. 

Reviews. Gokel and Weber have reviewed the principles involved in phase-transfer catalysis and the applications to synthesis (128 references). The review includes crown ethers and cryptates as well as quaternary ammonium and phos-phonium salts. 

Although the hydrolysis of alkyl halides to alcohols has been extensively investigated, an alternative two-step sequence involving substitution with carboxylate ion is more practical for the preparation of alcohols. Activation of the carboxylate anion prepared by the reaction of the acid with a base can be achieved (i) by use of a polar aprotic solvent and (ii) by use of aprotic apolar solvents under phase transfer catalysis, polymer conditions, or with crown ethers. 

The history of supramolecular carbene chemistry is not long. Preliminary trials, during the 1970s, involved the generation of carbenes in the presence of micelles, e.g., quaternary ammonium salts, and crown ethers.72 However, carbene host ICs were not formed, nor were they sought. Instead, these hosts enable phase transfer catalysis (PTC) of Lewis acid-Lewis base reactions, e.g., the SN2 reaction (Equation (1)). 

Phase transfer catalyzed reactions in which ylides are formed from allylic and ben-zylic phosphonium ions on cross-linked polystyrenes in heterogeneous mixtures, such as aqueous NaOH and dichloromethane or solid potassium carbonate and THF, are particularly easy to perform. Ketones fail to react under phase transfer catalysis conditions. A phase transfer catalyst is not needed with soluble phosphonium ion polymers. The cations of the successful catalysts, cetyltrimethylammonium bromide and tetra-n-butylammonium iodide, are excluded from the cross-linked phosphonium ion polymers by electrostatic repulsion. Their catalytic action must involve transfer of hydroxide ion to the polymer surface rather than transport of the anionic base into the polymer. Dicyclohexyl-18-crown-6 ether was used as the catalyst for ylide formation with solid potassium carbonate in refluxing THF. Potassium carbonate is insoluble in THF. Earlier work on other solid-solid-liquid phase transfer catalyzed reactions indicated that a trace of water in the THF is necessary (40). so the active base for ylide formation is likely hydrated, even though no water is included deliberately in the reaction mixture. 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

The long known Reissert reaction involves the kinetic trapping by cyanide of an -acylquinolinium or -isoquinolinium salt in the classical process the acylating agent is benzoyl chloride. Reissert compounds are usually prepared using a dichloromethane/water two-phase medium recent improvements include utilising phase-transfer catalysts with ultrasound or crown ether catalysis. 

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