Crown ethers tetrahydrofuran

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). 

Armstrong and Jin [15] reported the separation of several hydrophobic isomers (including (l-ferrocenylethyl)thiophenol, 1 -benzylnornicotine, mephenytoin and disopyramide) by cyclodextrins as chiral selectors. A wide variety of crown ethers have been synthesized for application in enantioselective liquid membrane separation, such as binaphthyl-, biphenanthryl-, helicene-, tetrahydrofuran and cyclohex-anediol-based crown ethers [16-20]. Brice and Pirkle [7] give a comprehensive overview of the characteristics and performance of the various crown ethers used as chiral selectors in liquid membrane separation. 

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... 

Crown ethers have also been found to lower the rate of reduction by metal hydrides. Wiegers and Smith (1978) reported that the rate of reduction of camphor by LiAlH4 in tetrahydrofuran was depressed by a factor of 6 on addition of one equivalent of crown ether [201]. They also concluded that, although the free cation shows a catalytic effect in metal hydride reduction, it is not indispensable. Dibenzo-18-crown-6 [11] was also found to lower the rate of... 

In these equations, Cr refers to a crown ether ligand while S denotes a solvent molecule. The notation Sn following Fl, Na+ signifies that In solvents such as dloxane, ethyl ether, tetra-hydropyran (THP) or tetrahydrofuran (THF) the tight ion pairs, especially when small alkali Ions are Involved, contain externally complexed solvent molecules. These must be removed before... 

Acceleration of the vinylcyclobutane to cyclohexene rearrangement of vinylcyclobutanols by conversion to the potassium alkoxide was first reported by Wilson and Mao.50 Typically, the [1,3] shift occurs at a satisfactory rate in tetrahydrofuran between room and reflux temperature. Sodium51 or lithium alkoxides52 also rearrange in this temperature range. The reaction rate can be further increased by addition of crown ethers or hexamethylphosphoric triamide.50,53... 

A variety of strong reducing agents, including solutions of alkali metals in liquid ammonia, sodium solubilized by crown ethers or cryptands in tetrahydrofuran (THF), and alkali metal naphthalenides in THF, have been found to reduce M2(CO)10 and/or [M(CO)5] (M = Mn and Re) to the respective [M(CO)4]3- however, [Re(CO)5] has often been observed to be... 

Cyclic ethers with larger rings than epoxides include tetrahydrofuran (THF), tetrahydropyran (THP), and dioxane. Large-ring cyclic polyethers, called crown ethers, can selectively bind metal ions, depending on the ring size. 

Know the meaning of cyclic ether, tetrahydrofuran, furan, tetrahydropyran, dioxane, crown ethers. 

In a recent paper, Soumillion and co-workers [49] were able to identify CIP and SSIPin the P-naphtholate anion/alkali cation/tetrahydrofuran system. They found out that with lithium, a CIP is formed whereas with sodium/crown ether, a SSIP results. Using uncomplexed sodium or potassium counterion, mixtures of CIPs and SSIP s were detected. All their conclusions are based on spectral shifts in the transient absorption and emission spectra which were gained using laser flash spectroscopy. 

The crown ether-catalyzed generation of the Co( CO)4 ion in ether or hydrocarbon solvents has been described (32). Treatment of the anion with 2,3-bis(bromomethyl)naphthalene in tetrahydrofuran gives what is formulated as a bis-7r-allyl complex, while ketones were isolated using monocyclic dibromides as substrates. 

Table 5-21 shows that the addition of even small proportions of EPD solvents affeets the reaetion rate markedly. The rate acceleration thus obtained is produced by a specific solvation of sodium ion, which tends to dissociate the high-molecular mass ion-pair aggregate of the sodio-malonic ester that exists in benzene solution (degree of aggregation n is equal to 40... 50 in benzene). This indicates that the kinetically active species is a lower aggregate of the free carbanion. Further evidence for a specific cation solvation is derived from the six-fold rate difference observed in tetrahydrofuran (fir = 7.6) and 1,2-dimethoxyethane (fir = 7.2), despite the fact that these two solvents possess nearly equal relative permittivities. The latter solvent is able to solvate sodium ions in the manner shown in Eq. (5-127). Especially noteworthy is the high reactivity exhibited on the addition of dicyclohexyl[18]crown-6. In benzene solution containing only 0.036 mol/L of this crown ether, the alkylation rate is already equal to that observed in neat 1,2-dimethoxyethane [351]. 

The ion-pair dissociation of ambident alkali enolates, which results in increasing 0/C alkylation ratios, can be promoted not only by dissociating solvents but also by specific cation solvation. In the latter case, EPD solvents cf. DMF and DMSO in Table 5-22b) or macro(poly)cyclic ligands such as coronands ( crown ethers ) or cryptands are used [376, 377, 660]. For example, the alkylation of sodium y9-naphtholate with (bromomethyl)benzene or iodomethane in the presence of benzo[18]crown-6 gives high O/C alkylation ratios when tetrahydrofuran or benzene are the solvents [660]. In dissociating solvents such as A,A-dimethylformamide or acetonitrile, however, so far no... 

Ferrocene derivatives. Tetrahydrofuran solutions of alkylcyclopentadienes, l e(ll) chloride, potassium hydroxide, and 18-crown-6 as phase transfer catalyst afford substituted ferrocenes in 45-65% yields. Less than 12% of ferrocene is olrtained from the reaction of cyclopentadiene in the absence of the crown ether. 

The correlation of these mechanisms with solvent polarity (Lewis basicity) is strongly supported by a study of the solvent effect (% E) on the EjZ product ratio of equation (95) at 25 °C dioxane (100), THF (100), THF +AlCl, (100), 2,5-dimethyl-tetrahydrofuran (55), EtjO (60), Et20 +AICI3 (60), O-Pr) (25) . The addition of a crown ether raised the yield in /-Pr20 to 70% E, presumably by facilitating dissociation and the ionic route a drop in reaction temperature to —25 °C lowered the yield in EtjO to 45% E, presumably by facilitating association and the aggregate route . ... 

When the dichlorocarbene adduct 231 of 9-methoxyphenanthrene is reacted with 2 equivalents of (-BuOK in tetrahydrofuran in the presence of a crown ether, the phenanthro[9,10-b]furan 232 is obtained in 80% yield. The intermediate methoxy-substituted chlorocarbene is derived from cyclo-propene. The key step consists in an intramolecular carbene insertion into the C—H bond of the methoxy group (84C79). 

Likewise, the strucmre of subtilisin (pH 3.0) suspended in varying ratios of acetonitrile and water demonstrated a-helical content similar to that in the lyophilized powder (Griebenow and Klibanov, 1996). Furthermore, the rate of transesteriflcation reactions of subtilisin (pH 7.8) suspended in DMSO/acetonitrile, formamide/acetonitrile or formamide/dioxane were increased approximately 100-fold over aqueous conditions (Almarsson and Klibanov, 1996). Similar results were obtained for subtilisin (pH 7.8) in a tetrahydrofuran/1-propanol mixture (Affleck et al., 1992). These results can be attributed to the increased structural rigidity of the active conformation of the protein in the solid, and the denaturing characteristics of the solvent at the solvent-particulate interface. Preservation of this molecular memory or molecular imprint of the protein can also be used to stabilize structure and activity (Mishra et al., 1996 Rich and Dordick, 1997 Santos et al., 2001). Subtilisin was lyophilized from crown ethers, resulting in more native like structure, by FTIR, and increased enzyme activity in THF, acetonitrile and dioxane (Santos et al.,2001). 

The second method can be used to synthesize Vogtle-type podands [9,10] in two high-yielding steps from polyethylene glycol ditosylates. The ditosylate derivatives, which are also precursors of cyclic crown ethers, azacrown ethers and lariat ethers, can be prepared with pyridine as the base [11], However, they are synthesized more effectively in higher yields using aqueous sodium hydroxide and tetrahydrofuran (Figure 1.4) [12], These conditions also remove the necessity to... 

The -elimination of hydrogen chloride from arylsulfonyl(chloro)organosulfanylmethanes carried out using 50% aqueous potassium hydroxide, 18-crown-6 as a catalyst in diethyl ether/ tetrahydrofuran mixture, in the presence of an alkene (arylethenes, noncyclic and cyclic enol ethers, enediol ethers) affords l-arylsulfonyl-1-organosulfanylcyclopropanes 1 the yields of these products are usually good27 (see also Houben-Weyl, Vol. E19b, pp 1731, 1736-1738). Trapping experiments with 2-methylpropene, cyclohexene, ( )-1,2-diphenylethene and phenan-threne failed. 

The highest enantiomeric excess in the palladium(0)-assisted enantiosclcctive alkylation of racemic 3-cycloalkenyl acetate has so far been observed with the chiral ligands 2, 3a and 3b60-61 (Table 17). The addition of tetrahexylammonium bromide dramatically increases enantioselec-tivity in the case of ligand 2, as does changing the solvent from tetrahydrofuran to dichloro-methane. which is believed to enhance the formation of dimeric ionic salts of the nucleophile. In contrast, additives such as tetraalkylammonium salts or crown ethers diminish the enantiomeric excess in reactions catalyzed by the phosphinoaryl oxazoline ligands 3a and 3b. bearing a chiral phosphorus on the aryl moiety. 

Recently QU1NAP was used as the chiral ligand to alkylate the allyl acetate 18. The enantioselectivity is enhanced by the addition of the crown ether 15-crown-5, but the enantiomeric excess is still moderate (up to 47 % ee)S9. The diphenyl phosphinoaryloxazoline ligand 20 exhibits best enantioselectivities for these substrates and demonstrates its broad versatility for the enantiose-lective alkylation of allyl acetates. Almost complete chirality transfer is achieved77 (for a detailed discussion see the section entitled meso-Tt-Allylpalladium Complexes). Dimethyl-formamide and tetrahydrofuran serve equally well as solvents, but with the latter solubility problems may arise. Higher temperature enhances reactivity at the expense ot selectivity (at 80 °C 65% ee). 

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