Dicyclohexyl-24-crown

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). 

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... 

Two of the most widely used crown ethers have been dibenzo-18-crown-6 and dicyclo-hexano-18-crown-6.(In older literature, the latter is often referred to as dicyclohexyl-18-crown-6 .) A major reason for this is that Pedersen reported complete details of the preparation of both compounds in Organic Syntheses in 1972. As a result, both compounds were readily prepared and available. 

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%... 

Oxygen chelates such as those of edta and polyphosphates are of importance in analytical chemistry and in removing Ca ions from hard water. There is no unique. sequence of stabilities since these depend sensitively on a variety of factors where geometrical considerations are not important the smaller ions tend to form the stronger complexes but in polydentate macrocycles steric factors can be crucial. Thus dicyclohexyl-18-crown-6 (p. 96) forms much stronger complexes with Sr and Ba than with Ca (or the alkali metals) as shown in Fig. 5.6. Structural data are also available and an example of a solvated 8-coordinate Ca complex [(benzo-l5-crown-5)-Ca(NCS)2-MeOH] is shown in Fig. 5.7. The coordination polyhedron is not regular Ca lies above the mean plane of the 5 ether oxygens... 

Formation constants K for complexes of dicyclohexyl-18-crown-6 ether with various cations. Note that, although the radii of Ca ", Na" and Hg + are very similar, the ratio of the formation constants is 1 6.3 225. Again, K" and Ba " have similar radii but the ratio of K is 1 35 in the reverse direction (note log scale). 

Fig. 10. Representation of the mechanism of redox driven K + transport using an electron and a cation carrier. (59-Ni°) and (59-Ni ) are the oxidized and reduced form of the electron carrier, the nickel bis-dithiolene complex 59 [] and [K+] are dicyclohexyl-18-crown-6 and its K+ complex. (Cited from Ref. 59>)...

Vinyl sulfones such as 262 are smoothly converted to a,) -unsaturated nitriles such as 263 on treatment with KCN in the presence of dicyclohexyl-18-crown-6 in refluxing t-butyl alcohol (equation 155)148. The reaction conditions are compatible with base-labile functionalities such as a methoxycarbonyl group (equation 156)148. This method can be used in the preparation of the sesquiterpene aldehyde nuciferal from allyl phenyl sulfones. 

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.

The crown ether here was named by its decoverer Pedersen51 dicyclohexyl-18-crown-6(18 = number of atoms in the heterocyclic ring, 6 = number of oxygen atoms in the ring) its membrane shows an appreciably higher K+ selectivity with respect to the other alkali metal ions. There is still much research being carried out on the synthesis and practical use of crown ethers. 

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). 

Hydroxyl groups protected as acetonides or as silyl, tetrahydropyranyl, benzyl, or methoxymethyl ethers are stable to these conditions. Yields with KMn04 are higher than those obtained with KMn04 and dicyclohexyl- 18-crown-6, Bu4NMn04, or NaMn04H20. ... 

DIAZACYCLOOCTADECANE, 54, 88 MACROCYCLIC POLYETHERS DIBEN-ZO—18-CROWN-6-POLYETHER AND DICYCLOHEXYL- 18-CROWN-6-POLYETHER, 52, 66 Malonaldehyde bis(diethyl acetal), 52, 139... 

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-... 

Fio. 1 Relation between log K and the ratio of cation diameter to cavity size for (cfa-syn-c r)-dicyclohexyl-18-crown-6 [20] in water at 2S°C. Ionic diameters taken from Pedersen (1967a), and log K values from Izatt et al. (1976a) and Frensforf (197 la)... 

The behaviour of the dibenzo-18-crown-6 derivative is similar, yet the highest attainable rate is only one eighth of that observed with dicyclohexyl-18-crown-6, which points to an important difference in reactivity between crown ether-separated ion pairs. Compared to tetrabutylammonium phenoxide, the dicyclohexyl-18-crown-6/K+ phenoxide was 2.6 times less reactive. The addition of 0.05 M dicyclohexyl-18-crown-6 to dioxan resulted in the alkylation rate constant becoming the same as that observed in pure tetraglyme. 

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 effect of crown ethers on the alkylation of sodium diethyl n-butylmalonate by 1-bromobutane has been studied by Zaug et al. (1972). The absence of a common-ion rate depression in dimethylformamide (DMF) pointed to an ion pair being the kinetically active species. The addition of dicyclohexyl-18-crown-6 (a mixture of [20] and [21]) accelerates the alkylation in both benzene and tetrahydrofuran (THF) (Table 24). The rates reach a plateau, indicating that at a crown-ether concentration of 0.5 M the ion pair is fully converted to the crown ether-separated ion pair which is slightly less reactive than the uncomplexed ion pair in DMF. The rate constant in pure dimethoxyethane (DME) is equal to that observed in THF or benzene... 

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