Tetrahydrofuran solvent structure

Carboanionic Intermediates.—Whereas characterization of the carbocations active in cationic polymerization has been difficult, identification and quantification of the carboanionic centres in anionic propagations have been somewhat easier, largely as a result of a considerably reduced tendency to rearrange and isomerize. Nevertheless, detailed investigations continue in particular areas. Thus Bywater and Worsfold ° have used n.m.r. spectroscopy to probe the structure of various model compounds which closely resemble those presumed to be responsible for anionic propagations. Whereas in tetrahydrofuran solvent 2,5-diphenyl-2,5-dipotassiohexane and 2-lithio-4,4-dimethyl-2-phenyl-pentane display coupling constants consistent with the carbon atoms... 

The final difference in the copolymerization of carbon monoxide with propene or styrene is the overall connectivity of the initial polymer generated under some conditions. The polymer generated from the copolymerization of carbon monoxide and propene in protic solvents consists of the fused tetrahydrofuran ketal structure shown in Figure 17.17. This polymer reopens to the polymer shown in Figure 17.13 upon addition of acid in alcohol. Several mechanisms for formation of this product have been proposed, and the origin of the ketal structure remains unresolved. Polymers formed in aprotic solvents form the acylic polymer. 

As seen in Table 3.7 and Figure 3.18, at 40°C, the removal of residual solvent takes place from film which was cast from chloroform solution tetrahydrofuran solvent is removed at 40°C and 50°C, then at 60°C the film starts to swell, while NMP solvent remains in the polymer film even at 60°C, and at 80°C the degree of swelling is so low as it can be neglected. The highest degree of swelling of polymer 2 synthesized in NMP was attained when the film was cast form chloroform solution, but it did not exceed 8.8% (Table 3.8). The structure of this polyimide 2 is similar to the stmcture of polyimide 1 (Table 3.6) which was synthesized in m-cresol. 

Hu et al. have synthesised ordered mesoporous and macroporous carbon monoliths by template method using silica monoliths as template. In this preparation, the mesophase pitch was used as the carbon precursor [54]. The silica monolith templates were filled with a 10 wt.% solution of mesophase pitch in tetrahydrofuran solvent and then carbonised at different temperatures. The physical characteristics of these various mesoporous and macro-porous carbon are presented in Table 7.8, which demonstrates these carbons have more prominent graphitic structures as the carbonisation temperature increases. SEM images of the carbon monoliths obtained at carbonisation temperature of 700°C and 2500°C are shown in Figure 7.59. 

By reaction of an a-halo ester 1 with zinc metal in an inert solvent such as diethyl ether, tetrahydrofuran or dioxane, an organozinc compound 2 is formed (a Grignard reagent-like species). Some of these organozinc compounds are quite stable even a structure elucidation by x-ray analysis is possible in certain cases ... 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

A phenomenological study was performed to determine the effect of solvent on Sn NMR spectra of these organoraetallic polymers. Samples were dissolved in chloroform, benzene, n-hexane, acetone, tetrahydrofuran, methanol, and pyridine. The Sn NMR spectra in these solvents are given in Figure 1. The appearance and location of the H Sn resonance changes drastically over the range of selected solvents. The chemical shift moves upfield in the order chloroform, benzene, n-hexane, acetone, tetrahydrofuran, pyridine, and methanol. The amount of structural information and, conversely, the broadening of the resonance increases in the same order with methanol and pyridine reversed. 

Again, weak coordination is expected for ether ligands to zinc, however, the larger number of structurally characterized examples is at least partially attributable to the frequent use of solvents such as diethyl ether or tetrahydrofuran which may provide additional ligands to the metal center. 

The spontaneous isomerization of all-trans- carotenoids at room temperature is a slow process, and its rate depends on the solvent and the pigment structure. For example, the initial solutions of P-carotene in a mixture of tetrahydrofuran (THF), methanol, and acetonitrile containing ca. 95% of all -trans- and 5% of 9-cis- plus 13-co-isomers was transformed to 90% all-fra ns- p -ca rot ene and 9% of 9-cis- plus 13-cA-carotene after 24h of spontaneous isomerization at 25°C (Pesek et al. 

The crystal structures of the 2 1 solvates formed by phenylbutazone with benzene, cyclohexane, 1,4-dioxane, tetrahydrofuran, and tetrachloromethane were found to be isostructural, while the structure of the chloroform solvate differed [57]. In all of the solvatomorphs, the solvent molecules were found to be located in channels along the (0 1 0) direction, and their inclusion served to increase the length of the unit cell along the n-axis. The solvent inclusion was also found to alter the //-angle. 

LDA (0.118 g, 1.1 mmol) was added to (SP)-f-butyl(phenyl)phosphine oxide (0.182 g, 1 mmol) in tetrahydrofuran (5 ml) under an atmosphere of nitrogen at -78°C. After 15 min, the solution was treated with a solution of benzaldehyde (0.117 g, 1.1 mol) in tetrahydrofuran (2 ml), and the resultant mixture was stirred at -78°C for 3 h. Evaporation of the solvent and flash chromatography of the residue provided the (SP)-f-butyl(phenyl)(a-hydroxybenzyl)phosphine oxide (0.22 g, 77%) with a diastereoisomeric ratio of 98 2, which exhibited spectral data in accord with the proposed structure. 

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