Solvent effects cryptate formation

Phase transfer processes rely on the catalytic effect of quaternary onium or crown type compounds to solubilize in organic solutions otherwise insoluble anionic nucleophiles and bases. The solubility of the ion pairs depends on lipophilic solvation of the ammonium or phosphonium cations or crown ether complexes and the associated anions (except for small amounts of water) are relatively less solvated. Because the anions are remote from the cationic charge and are relatively solvation free they are quite reactive. Their increased reactivity and solubility in nonpolar media allows numerous reactions to be conducted in organic solvents at or near room temperature. Both liquid-liquid and solid-liquid phase transfer processes are known the former ordinarily utilize quaternary ion catalysts whereas the latter have ordinarily utilized crowns or cryptates. Crowns and cryptates can be used in liquid-liquid processes, but fewer successful examples of quaternary ion catalysis of solid-liquid processes are available. In most of the cases where amines are reported to catalyze phase transfer reactions, in situ quat formation has either been demonstrated or can be presumed. 

Under mild hydrothermal conditions, around 200°C in the presence of a liquid phase, an increase in pressure of up to 10 bar does not seem to have a marked effect on the nature of the products obtained. On the other hand, thermal effects, which act by decreasing kinetic barriers, mostly influence the physicochemical properties of the solvent, favoring electrostatic interaction and the formation of hydrogen bonds. These interactions, that play a major role in the molecular recognitibn that characterizes the formation of cryptates [46,47], are probably the cause of the very surprising stnjctures observed under those conditions (see Chapter 4, Section 4.3.1). However, the state of current knowledge of these reactions requires some caution in the inteipretalion of the mechanisms involved under such conditions. 

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