Aprotic organic cations

Saf, R. Mirtl, C. Hummel, K. ESI-MS Using Potassium Iodide in Aprotic Organic Solvents for the Ion Formation by Cation Attachment. Tetrahedron Lett. 1994, 35, 6653-6656. 

Ion exchange from organic solvents and mixed organic aqueous solvents offers interesting possibilities for the extraction and separation of metals because of the different nature of the solvation processes in these systems. Only cation solvation is significant in dipolar, aprotic, organic... 

Where solubility alone is the issue, simply changing solvent to permit all species to be dissolved allows the chemistry to proceed essentially as it would in aqueous solution were species soluble. Typical molecular organic solvents used in place of water include other protic solvents such as alcohols (e.g. ethanol), and aprotic solvents such as ketones (e.g. acetone), amides (e.g. dimethylformamide), nitriles (e.g. acetonitrile) and sulfoxides (e.g. dimethylsulfoxide). Recently, solvents termed ionic liquids, which are purely ionic material that are liquid at or near room temperature, have been employed for synthesis typically, they consist of a large organic cation and an inorganic anion (e.g. lV, lV,-butyl(methyl)-imidazolium nitrate) and their ionic nature supports dissolution of, particularly, ionic complexes. 

Because ion pairs are formed in low polarity aprotic solvents, the anion of organic cations also contributes to the complexation. It has been found that the stability of complexes between aromatic receptors and ammonium and iminium ions is decreased in the order picrate>trifluoroacetate>r>Br >Cr>tosylate>acetate. Thus, the lower the cation-anion attraction, the stronger is the cation-rr interaction. ... 

Saf, R., Mirtl C., and Hummel, K. Electrospray mass-spectrometry using potassium-iodide in aprotic organic solvents for the ion formation by cation attachment. Tetrahedron Lett., 35, 6653, 1994. 

The organic cation is again thought to modify the electrode reaction by creating an aprotic layer on the cathode surface where the anion radical resulting from the le reduction of acrylonitrile does not immediately protonate. 

For many years, prior to the development of current phase-transfer catalytic techniques, tetraalkylammonium borohydrides have been used in non-hydroxylic solvents [see, e.g. I, 2], Originally, the quaternary ammonium borohydrides were obtained by metathesis in water or an alcohol [3, 4], However, with greater knowledge of the phase-transfer phenomenon, an improved procedure has been developed in which the ammonium salt is transferred into, and subsequently isolated from, dichloromethane [5, 6], In principle, it should be possible to transfer the quaternary ammonium borohydride for use in any non-miscible organic solvent. It should be noted, however, that quaternary ammonium cations are susceptible to hydrogeno-lysis by sodium borohydride in dipolar aprotic solvents to yield tertiary amines [4]. 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

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