Numerical onium

A general reaction of this type is shown. Numerous examples are known for both base-catalyzed and onium-type cyclizations. They all belong to the category substrate-induced diastereose-lection . 

Anionic polymerization of anhydrous formaldehyde has been disclosed in numerous patents, including the use of amines, phosphines, onium compounds, etc., as initiators. Mechanistically the propagation of the anionic polymerization of formaldehyde proceeds via the polymeric alkoxide ion,... 

A review by Lund [15] listed numerous alkylation reactions by means of electrolyses of different A type compounds in aprotic organic solvents, in the presence of organic halides. Other paper [9-13] described further possibilities for such mixed electrolyses and highlighted applications for alkyl halides or onium salts (given in Table 8). 

This so-called liquid/liquid phase-transfer catalysis is what most chemists associate with the term the extraction of a vital anionic species from the aqueous phase to the organic one, either by an onium salt or with the help of a crown ether or cryptand. Its application to metal organic catalysis will be described below. First it should be stressed, however, that the concept of PTC is much wider in scope and comprises numerous variants ... 

The stability of the solid polyhalide depends on numerous factors, among them the size of the cation, the size and the nature of the polyhalide ion, and the chemical resistance of the compound to atmospheric moisture. The most stable salts are formed when the ionic sizes of the cation and the anion are similar. In the alkali metal series the stability of the polyhalides decreases in the order Cs > Rb > NH4 > K > Na, which corresponds to the order of decrease in cationic size. Not enough work has, as yet, been done on the substituted onium poly halides to allow any convincing generalizations. 

The remarkable inclusion capacity of organic onium salts has been shown for numerous representatives of this family of host compounds Thus it is obvious that optically pure onium salts, preferrably to be taken from the chiral pool, are (very) promising host resolving agents. 

Over the past ten years the development of onium salt and other cationic photoinitiators has moved from the realm of speculative investigation to the point today at which they are being employed in numerous commercial applications. Much work still needs to be done in this fields particularly to improve our understanding of the relationship between the structure and the photosensitivity of these photoinitiators. As the field advances, one can expect still other new classes of onium salt photoinitiators to be developed as well as continued improvements to be made in the efficiency of the present systems. An understanding of the mechanism of photosensitization should lead to discovery of more efficient photosensitizers and a further broadening of their spectral response in photoinitiated cationic polymerization. 

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. 

As described in this chapter, numerous recent efforts to elaborate chiral onium salts have greatly expanded their use as asymmetric catalysts in organic synthesis. However, the full potential of chiral onium salts is yet to be realized in terms of general applicability. Continuous efforts should be made toward the design and development of conceptually new catalysts, and to understanding the relationship between catalyst structure and stereocontrolling ability. 

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