Phase transfer catalysis quats

Quaternary ammonium compounds (quats) are prepared - by moderate heating of the amine and the alkyl halide in a suitable solvent - as the chlorides or the bromides. Subsequently conversion to the hydroxides may be carried out. Major applications of the quat chlorides are as fabric softeners and as starch cationizing agent. Several bio-active compounds (agrochemicals, pharmaceuticals) possess the quat-structure. Important applications of quat bromides are in phase transfer catalysis and in zeolite synthesis. 

Phase-transfer catalysis offers a variety of conceptual and practical advantages. Among these advantages unique to PTC are the abihty of quats to transfer the anionic forms of metal carbonyls in the organic phase, in which CO is about 20 times... 

Figure 12.1 General mechanism of phase-transfer catalysis in the presence of quat salts ... 

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. 

Alkyl cyanides have been prepared by three major variations of the phase transfer method. These include the liquid-liquid phase transfer method (including catalysis by quats and amines), the crown catalyzed solid-liquid method and the tri-phase catalytic method. Each of these methods has been discussed with reference to the mechanism in the preceding section and it remains largely to exemplify these approaches. It should be noted, however, that numerous stoichiometric syntheses of aliphatic nitriles are available, including those involving dipolar aprotic solvents [12]. 

---