Catalysts crown ethers

Recently, the above mentioned model reaction has been extended to polycondensation reactions for synthesis of polyethers and polysulfides [7,81]. In recent reports crown ether catalysts have mostly been used in the reaction of a bifunctional nucleophile with a bifunctional electrophile, as well as in the monomer species carrying both types of functional groups [7]. Table 5 describes the syntheses of aromatic polyethers by the nucleophilic displacement polymerization using PTC. 

Especially for large-scale work, esters may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl halides or tosylates. Carboxylate anions are not very reactive nucleophiles so the best results are obtained in polar aprotic solvents45 or with crown ether catalysts.46 The reactivity order for carboxylate salts is Na+ < K+ < Rb+ < Cs+. Cesium carboxylates are especially useful in polar aprotic solvents. The enhanced reactivity of the cesium salts is due to both high solubility and minimal ion pairing with the anion 47 Acetone is a good solvent for reaction of carboxylate anions with alkyl iodides48 Cesium fluoride in DMF is another useful... 

Santaniello et al.125 have N-alkylated pyrrole and indole (71) in the presence of crown-ether catalysts. The indolyl anion also behaves as an ambident nucleophile alkylation occurs at nitrogen and at C-3.126... 

There have been reports of syntheses of cyano complexes in fused potassium cyanide, though the characterization of the products has not always been satisfactory.1 The dry reaction between potassium cyanide and potassium hexaiodoplatinate(IV), however, certainly gives the impure hexacyano-platinate(IV) and substitution of cycloocta-1,5-diene bonded to platinum(II) may be achieved by the use of solid potassium cyanide in the presence of a crown ether catalyst.16... 

To enhance the UV detection, Durst et al. (22) report the formation of phenacyl esters using crown ether catalysts. Since the time of their discovery, crown ethers have been shown to have... 

Durst, H.D., Milano, M., Kikta, E.J., Connelly, S.A., and Grushka, E., Phenacyl esters of fatty acids via crown ether catalysts for enhanced ultraviolet detection in liquid chromatography, Anal. Chem., 47, 1797, 1975. 

Lam, S. and Grushka, E., Labeling of fatty acids with 4-bromomethyl-7-methoxycoumarin via crown ether catalyst for fluorimetric detection in high-performance liquid chromatography, J. Chromatogr., 158, 207, 1978. 

Fig. 10.25. Current-potential curves for crown-ether catalysts added to the electrolyte for a p-CdTe electrode in DMF-0.1 M TEAP(tetraethylam-monium phosphate)/5% H20. (Reprinted from J. O M. Bockris and J. W. Wass, J. Electrochem. Soc. 136 2523, 1989. Reproduced by permission of the Electrochemical Society, Inc.)...

In 1977, Yamamura and Murahashi (55) described the palladium(0)-cat-alyzed cyanation of vinyl halides in the presence of a crown ether catalyst. The isolated yields of unsaturated nitriles were excellent, especially... 

The most common fluorotags (fluorescent reagents) are dansyl chloride and o-phthalaldehyde (OPA). Chromotags include p-bromophenacyl bromide (PBPB) for derivatization of carboxylic acids (K-salts) with a crown ether catalyst, ninhydiin for primary amines forming complexes that have their adsorption maxima at about 570 nm, and dansyl chloride for primary and secondary... 

Many other different efforts have been directed toward the synthesis and design of an efficient crown ether catalyst able to catalyze this reaction with better results. In general, the Michael addition of methyl phenylacetate to methyl acrylate has been employed as the test reaction in order to evaluate the performance of the new catalysts prepared. As can be seen in Scheme 5.22, variable results have been obtained for many different crown ethers employed in this reaction but, as a general rule, it can be said that all these newly developed systems have been demonstrated to participate as rather efficient promoters of the reaction with regard to catalytic activity, but enantioselec-tivities have remained in moderate or low values. In all cases, the size of the crown ether macrocycle has been chosen according to the nature of the cation... 

Benzylidene halides are not sufficiently acidic to be deprotonated in the standard liquid-liquid PTC system. On the other hand, in the presence of solid KOH or CsOH and TEBA or crown ether catalyst, 2-dichloromethyl derivatives of furan and thiophene and even benzylidene chloride form appropriate carbenes, which can be trapped by alkenes (eqs. 126 and 127). 

As workers succeed in anchoring crown ether catalysts on surfaces, it will be interesting to see the extent to which catalyzed reaction rates are further enhanced by what is known as a "reduction in dimensionality"(28). The essence of this idea is that a substrate only needs to hunt for an "active site" in two dimensions once it sticks on the catalytic surface. This is in contrast to a longer three dimensional search for the catalyst in the case of homogeneous solutions. 

The copolymerization of butadiene and styrene with n-butyl-sodium-crown ether catalyst also produces polymers at high conversion and high molecular weight. The resulting copol) er prepared from this catalyst showed no block styrene at Na/crown ether ratios of 11.3 to 113. The data also showed a higher initial rate of styrene incorporation in the polymer chains at low conversion, but leveled off at about 40 percent conversion. 

Both phase transfer and crown ether catalysts have been used to promote a eliminations from chloroform and other haloalkanes. " Various trialkylammonium hydroxides catalyze formation of dichlorocarbene in the organic phase of two phase systems consisting of CHCI3 and 50% sodium hydroxide solution. 

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