5.5.5- trialkylsulfonium cations

Matsumoto, H., Matsuda, T., and Miyazaki, Y., Room temperature molten salts based on trialkylsulfonium cations and bis(trifluoromethylsulfonyl)imide, Chem. Lett, 12, 1430-1431,2000. 

Most ILs are based on quaternary ammonium cations such as 1,3-diaIkyHmidazoHum, N,N,N,N-tetraalkylammonium, N,N-dialkylpyrrolidinium, N-alkylpyridinium, or N-alkylpiperidinium. Other options are, for example, P,P,P,P-tetraaIkylphosphonium and S,S,S-trialkylsulfonium cations. Preferred anions can be either organic anions such as dicyanamide (DCA ), triflate... 

In some cases only the first step is required, as with the formation of ethylam-monium nitrate. In many cases the desired cation is commercially available at reasonable cost, most commonly as a halide salt, thus requiring only the anion exchange reaction. Examples of these are the symmetrical tetraalkylammonium salts and trialkylsulfonium iodide. 

The formation of a further single bond between sulfur and carbon, as in the trimethylsulfonium cation, may be pictured as involving a 3sp3 unshared pair orbital on sulfur and an empty 2sp3 orbital on carbon in a methyl cation. Thus the three a bonds and the remaining unshared pair (in a 3sp3 orbital) in a trialkylsulfonium ion are distributed approximately tetrahedrally, i.e. the ion is pyramidal, with the sulfur atom at the apex (2). 

The catalysts must supply the system with lipophilic cations in order to form, with required anions, ion pairs able to enter nonpolar media. The most typical catalysts are tetraalkyl ammonium (TAA) salts R4N+X, particularly those having at least 16 carbon atoms in the four R groups. Similar lipophilic catalysts are tetraalkylphosphonium and -arsonium or trialkylsulfonium salts, which are less available and usually less stable. They are therefore of negligible practical use. There are a few reports on the use of trialkylamines as catalysts in some two-phase reactions. Usually these amines are qua-ternized by a reactant actually these reactions are catalyzed by TAA salts. More complicated is the generation of dihalocarbenes with trialkylamines. The amines form, with the carbene, an ammonium ylide, which acts as a base in the organic phase. 

Let us discuss in more detail all these factors. First of all, it should be stressed that onium ions (e.g., trialkyloxonium, tetraalkylammonium, trialkylsulfonium) are inherently stable different onium salts may be isolated and stored in a pure state, some of them are commercially available. This is in contrast to cationic vinyl polymerization, where carbenium ion species are not inherently stable due to the possibility of proton expulsion ... 

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). 

Ionic liquids differ from classical molten salts by being liquids at room temperature, or their melting points are below 100 °C. They have an ionic structure and usually consist of an organic cation and an inorganic or organic anion. The most common cations found in ionic liquids are the tetraalkylammonium, tetraalkyl-phosphonium, N-alkylpyridinium, 1,3-dialkylimidazolium, and trialkylsulfonium moieties [11]. Some common ionic liquid cations and anions are presented in Fig. 7.1. 

Besides the mentioned earlier TAA salts typically used as PT catalysts—source of lipophilic cations—other lipophilic, stable organic cations such as trialkylsulfonium, tet-raalkylphosphonium, or arsonium, etc., can also function as PT catalysts. Another category of PT catalysts consists of neutral organic molecules able to form relatively stable... 

Many prospective photoinitiators fail to meet the basic requirements outlined above. Thus, for example, trialkylsulfonium salts are not useful photoinitiators because they do not absorb light in the ultraviolet and because as alkylating agents, they spontaneously initiate cationic polymerization even in the absence of light... 

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