Polar aprotic organic solvents

The higher solubility of several quaternary ammonium salts of glyphosate in polar aprotic organic solvents such as acetonitrile was discovered (2), which permitted their reaction in solution with various alkyl halides. For example, GLY(n-Bu4N)2H reacted with either o-xylylene dichloride or 1,5-dibromopentane to produce the interesting quaternary glyphosate derivatives 81 and 82, whose structures have been confirmed by x-ray analysis (2). 

An ionic liquid was successfully used as a replacement for polar aprotic organic solvents in the chemoselective O-alkylation of naphthols (Freemantle, 1998). 

Solubility (1) and (2) sol most organic solvents (3) sol polar aprotic organic solvents and H2O. 

The Baylis-Hillman reaction takes place in several polar aprotic organic solvents with different kinetic data [30], In acidic aqueous media, the reaction yield can be up to 74%, depending on the substrate and tertiary amine used (Equation 4.16) [31]. 

HMF synthesis has classically involved either water or polar aprotic organic solvents such as DMSO as the reaction medium, since the sugar feedstocks are soluble in these media. Of all the technical issues that have confronted the industrialization of HMF production, first and foremost has been the high solubility of HMF in these solvents, which complicates product isolation. This problem has been mitigated to some extent by the recent development of biphasic reaction systems that involve continuous extraction of HMF into lower boiling solvents [5]. Salting out strategies can also increase the efficiency of this approach. 

Hydrophilic interaction chromatography (HILIC) is a separation process involving polar stationary phases with mobile phases made of highly polar aprotic organic solvents (>80 %) and a small amount of aqueous solvents. It is particularly suitable for the separation of very polar analytes. HILIC coupled to mass spectrometry may offer a significant increase in sensitivity. The isocratic separation of three calystegines (A5, B4, and Cl) on an Acquity HILIC (50 x 2.1 mm, 1.7 pm) column with MeOH-10 mM aqueous ammonium acetate buffer at pH 5 (95 5) and a flow rate of 400 pL min was possible in less than 1.5 min [29]. 

A more traditional procedure, which involves considerably more process steps, is to isolate the polymer and re-dissolve it in an organic solvent followed by casting by solvent evaporation and finally impregnation with phosphoric acid [2]. A few highly polar aprotic organic solvents can dissolve many of the reported polybenzimidazole derivatives. These include M,M-dimethyla-cetamide (DM Ac), M,M-dimethylformamide... 

Figure 4. Conformation of paclitaxel based on the X-ray structure of docetaxel and proposed for non-polar aprotic organic solvents (structure A). Conformation based on X-ray structure of paclitaxel and proposed for aqueous solvents (structure B).

A similar reaction was not observed for 4-pentene-l-ol. It should be noted that a Pt(III) dimer complex is released after the reaction in Eq. (18), which is in contrast to the release of olefin and a Pt(II) dimer complex in aqueous solution by reductive elimination (Eq. (16)). The difference of such reactivity depending on the alkyl and the solvent would be caused by the difference of the electron density of the a-carbon atom and the dipole structure along the Pt-Pt bond in the solvents of different polarities. In aprotic organic solvent, the electron distribution along the Pt-Pt bond would be less polar, i.e., close to... 

The characterization of the semiquinone radical anion species of PQQ in aprotic solvents was undertaken to provide information about the electrochemistry of coenzyme PQQ and to give valuable insight into the redox function of this coenzyme in living systems <1998JA7271>. The trimethyl ester of PQQ and its 1-methylated derivative were examined in aprotic organic solvents by cyclic voltammetry, electron spin resonance (ESR), and thin-layer UV-Vis techniques. The polar solvent CH3CN was found to effectively solvate the radical anion species at the quinone moiety, where the spin is more localized, whereas the spin is delocalized into the whole molecule in the nonpolar solvent CH2CI2. 

A typical solvent or co-solvent system is selected based on the ability to solvate the analyte(s) relative to the undesired matrix components and the ease with which the solvent can be eliminated after extraction. Co-solvent blends are useful because the polarity (or other properties) can be tailored to that of the analyte(s). Traditional aprotic organic solvents are useful because they can be removed quickly and at low temperatures. Because of the dramatic increase in extraction efficiencies, solvents that have only moderate extraction properties at room temperature and atmospheric pressure can perform quite well under ASE conditions. Because organic-aqueous co-solvents can be used, it is often possible to prepare a solvent that can chemically neutralize the analyte molecule, thereby further facilitating the extraction. Dilute organic acids or bases can be employed for this purpose. Strong mineral acids are generally undesirable because they attack and destroy the stainless steel bombs or other instrument system components. 

Unlike sodium azide, which - owing to its ionic structure - is only soluble in highly polar solvents and is converted in such solvents or in a two-phase system by phase transfer catalysis, TMSA is a covalently bonded azide that is stable and miscible with many aprotic organic solvents. This is why it can be used in azide syntheses for which water-sensitive organic substances are to be used. 

The oxidation methods described previously are heterogeneous in nature since they involve chemical reactions between substances located partly in an organic phase and partly in an aqueous phase. Such reactions are usually slow, suffer from mixing problems, and often result in inhomogeneous reaction mixtures. On the other hand, using polar, aprotic solvents to achieve homogeneous solutions increases both cost and procedural difficulties. Recently, a technique that is commonly referred to as phase-transfer catalysis has come into prominence. This technique provides a powerful alternative to the usual methods for conducting these kinds of reactions. 

A difficulty that occasionally arises when carrying out nucleophilic substitution reactions is that the reactants do not mix. For a reaction to take place the reacting molecules must collide. In nucleophilic substitutions the substrate is usually insoluble in water and other polar solvents, while the nucleophile is often an anion, which is soluble in water but not in the substrate or other organic solvents. Consequently, when the two reactants are brought together, their concentrations in the same phase are too low for convenient reaction rates. One way to overcome this difficulty is to use a solvent that will dissolve both species. As we saw on page 450, a dipolar aprotic solvent may serve this purpose. Another way, which is used very often, is phase-transfer catalysis ... 

For aprotic polar organic solvents only few widely applicable alignment media are known. The lyotropic mesophase of poly(y-benzyl-L-glutamate) (PBLG) with DMF [37] has the disadvantage of a relatively strong minimum alignment. Bicellar... 

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