Quaternary ammonium and phosphonium

The reaction of higher alkyl chlorides with tin metal at 235°C is not practical because of the thermal decomposition which occurs before the products can be removed from the reaction zone. The reaction temperature necessary for the formation of dimethyl tin dichloride can be lowered considerably by the use of certain catalysts. Quaternary ammonium and phosphonium iodides allow the reaction to proceed in good yield at 150—160°C (109). An improvement in the process involves the use of amine—stannic chloride complexes or mixtures of stannic chloride and a quaternary ammonium or phosphonium compound (110). Use of these catalysts is claimed to yield dimethyl tin dichloride containing less than 0.1 wt % trimethyl tin chloride. Catalyzed direct reactions under pressure are used commercially to manufacture dimethyl tin dichloride. 

C. M. Starks, Phase-Transfer Catalysis. L Heterogeneous Reactions Involving Anion Transfer by Quaternary Ammonium and Phosphonium Salts , J. Am. Chem. Soc 1971, 93,195-199. 

Onium salts, crown ethers, alkali metal salts or similar chelated salts, quaternary ammonium and phosphonium are some of the salts which have been widely used as phase transfer catalysts (PTC). The choice of phase transfer catalysts depends on a number of process factors, such as reaction system, solvent, temperature, removal and recovery of catalyst, base strength etc. 

Quaternary ammonium and phosphonium halides were used as the phase transfer catalysts. For effective coupling, high-shear mixing and high concentrations of polymer and base were used. 

Catalyst-philic liquid phases can be used to promote the catalytic activity of heterogeneous catalysts, and to facilitate product-catalyst separation. A variety of different constituents of such catalyst-philic phases can be used, the most attractive being quaternary ammonium and phosphonium salts, PEGs, as well as water and other kinds of low-temperature molten salts. In each system, the catalyst-philic phase is characterized as being separate from the remainder of the reaction mixture, and the catalyst should reside within this phase. 

In addition, procedures have been described in which quaternary ammonium and phosphonium permanganates are preformed and isolated as semistable solids that can then be used as general oxidants in a wide variety of solvents (32,33). Precautions should be taken to avoid violent thermal decompositions when the latter procedure is used (34-36). 

Quaternary ammonium and phosphonium permanganates exist as intimate ion pairs in nonpolar solvents such as methylene chloride and toluene (1). However, in more polar solvents, such as acetone, nmr studies indicate that they are better described as being solvent separated ion pairs (37). In water, these salts separate completely and exist as individual ions. 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

Other Onium Salt Catalysts. We have examined several quaternary ammonium and phosphonium salts in this process. And while they vary in their effectiveness, most seem to have at least some activity. In the case of symmetrical onium salts, substituents having four or more carbon atoms are preferred (Table III). This likely originates from a more favorable partitioning into the organic phase as the size of the substitutents increase. Herriott has reported that there is a... 

Triphase Catalysis by Quaternary Ammonium and Phosphonium Ions. 57... 

Crown ether, cryptand, and poly(ethylene glycol) catalysts are more stable in base than the quaternary ammonium and phosphonium ions. Only the polyethylene glycols) are likely to meet industrial requirements for low cost, although a number of more efficient, lower cost crown ether syntheses have appeared recently, such as those of sila-crowns 64 bound to silica1B9). 

Starks, C.M. (1971) Phase-transfer catalysis. I. Heterogeneous reactions involving anion transfer by quaternary ammonium and phosphonium salts. J. Am. Chem. Soc., 93, 195. 

Transition metal oxidants such as permanganate, ruthenium tetroxide and diromium(VI) oxide are convenient and efficient reagents for routine cleavage reactions. The use of phase transfer catalysts (quaternary ammonium and phosphonium ions, primarily) has made it possible to solubilize transition metal oxides such as permanganate and chromatt in nonaqueous solvents, and to therdry increase the scope of these reactions substantially. ... 

In aqueous media the highest yields of alkanediols are obtained using potassium permanganate in very basic solutions". Alternatively, in the presence of phase transfer catalysts such as quaternary ammonium and phosphonium salts100, dimethyl polyethylene glycols101, or crown ethers 102 (Table 10), solvation of the permanganate ion in apolar organic media is also possible ... 

The effect of temperature on ion transfer across the water-nitrobenzene interface was studied for a series of six quaternary ammonium and phosphonium cations and two anions using cyclic voltammetry and equilibrium impedance measurements [115]. Standard entropies (A S ) and enthalpies (A iT ) of ion transfer have been evaluated from the experimentally accessible reversible half-wave potential ( "572 and standard Gibbs energy of transfer (A G ),... 

Phase transfer catalysis and the use of crown ethers are also of particular advantage in alkanenitrile synthesis (Table 1). Usually quaternary ammonium and phosphonium salts serve quite well as catalysts. Another modification is represented by the use of a solid catalyst, which is insoluble in the two-phase system, for instance alumina or anion-exchange resins (triphase catalysis). Crown ethers again capture the cations and generate naked cyanide ions in fairly nonpolar solvents, leading to exceptionally mild reaction conditions. 

Oxetane undergoes ring-opening polymerization under the action of MAD in conjunction with onium salts, including quaternary ammonium and phosphonium halides, giving a narrow MWD polyether (Scheme 6.173) [221]. Use of MesAl in place of MAD resulted in no polymerization. The aluminum ate complex seemed to be an initiator, which underwent a trigger reaction involving halide transfer to the aluminum-oxetane complex. 

These anionic ring opening polymerizations are usually carried out either in bulk or in solution. A host of catalyst types are active. For synthetic references using specific catalysts, the reader is referred to several excellent sources (4,7,31,32). Representative catalysts include hydroxides, alcoholates, phenolates, silanolates, siloxanolates, mercaptides of the alkali metals, organolithium and potassium compounds, and quaternary ammonium and phosphonium bases and their silanolates and siloxanolates. Some physical characteristics of linear oligomers are given in Table 5 (10). 

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