Dicyclohexyl-18-crown-6-Potassium

Dicyclohexyl[18]crown-6, potassium complex equilibrium constant, 7, 742 (73M152101) Dicyclohexyl[18]crown-6, sodium complex equilibrium constant, 7, 742 (73MI52101) Dicyclopenta[6,e]pyridine... 

Cyanosilylation of ketones (4,542-543 5,720). In a total synthesis of natural camptothecin (9), Corey et al used this f-butyldimethylsilyl derivative rather than trimethylsilyl cyanide (5, 720-722) to effect cyanosilylation of a ketone (1). Hydrolysis of the resulting cyano silyl ether to the required amide was not accompanied by desilylation with reversal of cyanohydrin formation. By use of carefully controlled conditions and with dicyclohexyl-18-crown-6-potassium cyanide as catalyst, they were able to convert (1) into the a-hydroxy... 

Alkali Metal Catalysts. The polymerization of isoprene with sodium metal was reported in 1911 (49,50). In hydrocarbon solvent or bulk, the polymerization of isoprene with alkaU metals occurs heterogeneously, whereas in highly polar solvents the polymerization is homogeneous (51—53). Of the alkah metals, only lithium in bulk or hydrocarbon solvent gives over 90% cis-1,4 microstmcture. Sodium or potassium metals in / -heptane give no cis-1,4 microstmcture, and 48—58 mol % /ram-1,4, 35—42% 3,4, and 7—10% 1,2 microstmcture (46). Alkali metals in benzene or tetrahydrofuran with crown ethers form solutions that readily polymerize isoprene however, the 1,4 content of the polyisoprene is low (54). For example, the polyisoprene formed with sodium metal and dicyclohexyl-18-crown-6 (crown ether) in benzene at 10°C contains 32% 1,4-, 44% 3,4-, and 24% 1,2-isoprene units (54). 

Reductive removal of fluorme from alk I fluorides requires a potent reducing agent and so is not noimally encountered However, hydrogenolysis of an unacuvated carbon-fluorine bond in, for example, 3 (3-fluorocholestane has been efficiently accomplished in 88% yield with a solution of potassium and dicyclohexyl 18 crown-6 in toluene at 25 C [/] Similarly, sodium naphthaiene in tetrahydrofuran converts 6 tluorohexene-1 and 1-fluorohexane to hydrocarbons in 50% yield at 25 C over a 7-h penod [2]... 

In the presence of dicyclohexyl-18-crown-6 ether, potassium fluoride converts fluonnated vinylic iodides to acetylenes [2] (equation 2)... 

Dehydrochlorination of bis(tnfluoromethylthio)acetyl chloride with calcium oxide gives bis(trifluoromethylthio)ketene [5] (equation 6) Elimination of hydrogen chloride or hydrogen bromide by means of tetrabutylammonium or potassium fluoride from vinylic chlorides or bromides leads to acetylenes or allenes [6 (equation 7) Addition of dicyclohexyl-18-crown-6 ether raises the yields of potassium fluoride-promoted elimination of hydrogen bromide from (Z)-P-bromo-p-ni-trostyrene in acetonitrile from 0 to 53-71 % In dimethyl formamide, yields increase from 28-35% to 58-68%... 

Figure 3.57. Crown ethers a) hexaoxaoctadecane (18-crown-6) b) dibenzo-18-crown-6 c) complex of dicyclohexyl-18-crown-6 with potassium salt.

Using dicyclohexyl-18-crown-6 it is possible to dissolve potassium hydroxide in benzene at a concentration which exceeds 0.15 mol dm-3 (Pedersen, 1967). The free OH- has been shown to be an excellent reagent for ester hydrolysis under such conditions. The related solubilization of potassium permanganate in benzene, to yield purple benzene , enables oxidations to be performed in this solvent (Hiraoka, 1982). Thus, it is possible to oxidize a range of alkenes, alcohols, aldehydes, and alkylbenzenes under mild conditions using this solubilized reagent. For example, purple benzene will oxidize many alkenes or alcohols virtually instantaneously at room temperature to yield the corresponding carboxylic acids in near-quantitative yields (Sam Simmons, 1972). 

Potassium acetate complex with dicyclohexyl-18-crown-6-polyether, 52, 71 Potassium amide, 52, 75 Potassium azide, 50, 10 Potassium tert-butoxide, 52,... 

Potassium hydroxide complex with dicyclohexyl-18-crown-... 

The small effect on reaction rates of the addition of crown ethers to the lower alcohols was also observed in the reaction of potassium acetate with 1-bromobutane in ethanol (Hirao et al., 1978a,b). The displacement of fluorine in either o-nitro- or p-nitro-fluorobenzene by a methoxy group, by reaction with potassium methoxide in methanol was hardly influenced by the presence of dicyclohexyl-18-crown-6 (Del Cima et al., 1973). Mariani et al. (1978), too,... 

The kinetics of the esterification of potassium p-nitrobenzoate by benzyl bromide in dichloromethane-water catalysed by dicyclohexyl-18-crown-6 ([20] + [21]) has been studied by Wong (1978). At low catalyst concentrations (e.g. 5.0 x 10 3 M) he found that 22% of the product was formed by the SN1 and 78% by the SN2 mechanism. Higher catalyst concentrations increased the salt concentration in the organic phase, and the contribution of the SN1 mechanism becomes negligibly small. 

The earliest example of the increased reactivity of the potassium hydroxide complex of dicyclohexyl- 18-crown-6 ((20] + [21]) was presented by Pedersen (1967b), who found that the sterically hindered esters of 2,4,6-trimethyl-benzoic acid [136], which are inert towards KOH in protic solvents, are readily... 

Saponification of esters of 2,4,6-trimethylbenzoic acid with potassium hydroxide complex of dicyclohexyl-1 S-crown-fi0,4... 

Nucleophilic substitution reactions of halide anions in aprotic solvents are often accompanied by elimination reactions. For instance, reactions of secondary alkyl halides with potassium fluoride solubilized in acetonitrile with the aid of 18-crown-6 [3] give olefins as the main reaction product (Liotta and Harris, 1974). Similarly, the dicyclohexyl-18-crown-6 complex of potassium iodide acted exclusively as a base in its reaction with 2-bromo-octane in DMF (Sam and Simmons, 1974). The strongly basic character of weakly solvated fluoride has been exploited in peptide synthesis (Klausner and Chorev, 1977 Chorev and Klausner, 1976). It was shown that potassium fluoride solubilized... 

Reactions of potassium superoxide solubilized in apolar solvents with crown ethers (see Oxidation reactions, p. 356) are also frequently accompanied by elimination reactions. Thus, in DMSO solution, secondary alkyl bromides only yield olefins when treated with the K02 complex of dicyclohexyl-18-crown-6 (Johnson et al., 1978). Scully and Davis (1978) have studied the elimination of HC1 from N-chloramines with 18-crown-6-solubilized K02, KOH, and KOAc in ether solution (27). High yields of aldimines were obtained with K02,... 

Roitman and Cram (1971) have shown that the kinetic basicity of potassium alkoxide increases on complexation with dicyclohexyl-18-crown-6 ([20] + [21]). Both the rate of isotopic exchange and the rate of racemization of (—)-4-biphenylylmethoxydeuteriomethane [163] by t-BuOK in t-BuOH were found to increase by factors between 30 and 17 000. Isotopic exchange was... 

Effect of dicyclohexyl- 18-crown-6 (120) + [21]) on the potassium alkoxide (KOR) promoted elimination of HBr from 2-bromobutane in ROH solution at 50°O ... 

By saturating the co-ordination sites of the cation with a crown ether, the relative contribution of the syn-pathway decreases, as was shown by Zavada et ai (1972) for t-BuOK-promoted eliminations from 5-decyl tosylate. Furthermore, trans/cis ratios in both anti- and syn-eliminations were affected by the presence of dicyclohexyl- 18-crown-6 and the nature of the leaving group (Zavada et ai, 1976). Fiandanese et ai (1973) observed that elimination from fluorosulphonylethanes [177] promoted by potassium phenoxide in dioxan... 

Crown ethers have been used successfully as phase-transfer catalysts for liquid-liquid and liquid-solid oxidation reactions. Sam and Simmons (1972) observed that potassium permanganate can be solubilized in benzene by dicyclohexyl-18-crown-6 to yield concentrations as high as 0.06 M. From... 

Valentine and Curtis (1975) extended the synthetic utility of potassium peroxide by reporting the successful solubilization of K02 in dry dimethyl sulfoxide using dicyclohexyl- 18-crown-6 ([20] + [21]). Corey et al. (1975) used 18-crown-6 to solubilize KOz in dimethylformamide, dimethoxyethane and diethyl ether, whilst Johnson and Nidy (1975) reported its solubilization in benzene. A wide variety of chemical transformations have been realized with K02 complexes of crown ethers. With alkyl halides the main reaction products are peroxides, alcohols and olefins (Johnson and Nidy, 1975). Peroxides are... 

Figure 1. Dependence of butyl acrylate percent conversion to polymer on the stability constants for potassium ion complexation in methanol of the various crown ethers. Line calculated by regression analysis o, experimental values 1, 18-crown-6 2, dicyclohexyl-18-crown-6 3, 21-crown-7 4, dibenzo-18-crown-6 5, 15-crown-5 6, cyclohexyl-1 5-crown-5 8, 1,10-diaza-18-crown-6. Reproduced from Ref. 2. Copyright 1981, American Chemical Society.

This residue is a mixture of stereoisomerio dicyclohexyl-18-crown-6 polyethers which may be contaminated with some unchanged dibenzo-18-crown-6 polyether and with alcohols arising from hydrogenolysis of the polyether ring. The submitter reports that this residue is sufficiently pure for many purposes such as the preparation of complexes with potassium hydroxide which are soluble in aromatic hydrocarbons. 

The checkers prepared a crystalline complex of potassium acetate with isomer B of dicyclohexyl-18-crown-6 polyether by the following procedure. To a stirred solution of 15.0 g. (0.0404 mole) of dicyclohexyl-18-crown-6 polyether (mixture of isomers) in 50 ml. of methanol was added a solution of 5.88 g. (0.0600 mole) of anhydrous potassium acetate (dried at 100° under reduced pressure) in 35 ml. of methanol. The resulting solution was concentrated under reduced pressure with a rotary evaporator, and the residual white solid was extracted with 35 ml. of boiling methylene chloride. The resulting mixture was filtered and the filtrate was cooled in a dry ice-acetone bath and slowly diluted with petroleum ether (b.p. 30-60°, approximately 200 ml. was required) to initiate crystallization. The... 

The physical properties of many macrocyclic polyethers and their salt complexes have been already described. - Dibenzo-18-crown-6 polyether is useful for the preparation of sharpmelting salt complexes. Dicyclohexyl-18-crown-6 polyether has the convenient property of solubilizing sodium and potassium salts in aprotic solvents, as exemplified by the formation of a toluene solution of the potassium hydroxide complex (Note 13). Crystals of potassium permanganate, potassium Lbutoxide, and potassium palladium(II) tetrachloride (PdClj + KCl) can be made to dissolve in liquid aromatic hydrocarbons merely by adding dicyclohexyl-18-crown-6 polyether. The solubilizing power of the saturated macrocyclic polyethers permits ionic reactions to occur in aprotic media. It is expected that this [)ropcrty will find practical use in catalysis, enhancement of... 

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