Bronsted superacid

The alkoxysiladioxanes described by Davis were shown to undergo selective axial addition of allyltrimethylsilane [49]. Moderate selectivity (7-13 1) was observed when the reactions were promoted by Lewis acids (TMSOTf, SnCl4), while higher selectivity was realized when a Bronsted superacid was used (Eq.23). 

In Ref. 6, the Bronsted superacidity of HCl in liquid chloroaluminate IL ([C2Cilm]Cl/AlCl3) was studied by the protonahon of arens during which the degree of protonahon was measured by using absorption spectroscopy. The arens were stable in the liquid chloroaluminate for many hours and their protonated forms (arenium ions) were stable for 1 h or more. 

Smith, G.R, Dworkin, A.S., Ragni, R.M., Zingg, S.R, Bronsted superacidity of HCl in liquid chloroaluminate, A1C13—l-ethyl-3-methyl-lH-imidazolium chloride, J. Am. Chem. Soc., Ill, 525-529,1989. 

Like the trialkyloxonium superelectrophiles, the salts of trimethyl sul-fonium (CH3)sS+, selenonium (CEL Se"1", and telluronium (CE Te4" ions have also been shown by Laali et al. to undergo superelectrophilic activation.41 These onium salts methylate toluene in FSChH-SbFj, but with the weaker Bronsted superacid CF3SO3H (triflic acid, //q —14.1), no methylation takes place (eq 12). 

Bronsted superacids Proton donor acids stronger than 100% sulfuric acid. 

Thus, the classification of Lewis superacids as those stronger than anhydrous aluminum trichloride is only arbitrary. Just as in the case of Gillespie s classification of BrOnsted superacids, it is important to recognize that acids stronger than conventional Lewis acid halides exit, with increasingly unique properties. 

Chloroaluminate melts are extremely hygroscopic and in all practically relevant applications more or less contaminated with water [131. Accordingly, chloroaluminate(lll) ionic liquids contain at least traces of 0x0- and/or hydr0x0-chloroaluminate species and molecular HCl. It has been known since the early 1980s that Bronsted superacidity results if gaseous HCl is added to acidic chloroaluminate melts [141, and quantitative studies of the acidity of HCl in [C2inimlCl—AICI3 as a function of HCl pressure and ionic liquid composition (51.0-66.4 mol... 

Having presented the chemistry of the main classes of onium salt cationic photoinitiators, this section will be used to further elaborate on their use in cationic polymerizations. Those onium salts that generate Bronsted superacids on photolysis are technically capable of initiating the cationic polymerizations of all known polymerizable electron-rich vinyl and heterocyclic monomers. Figure 4 provides an overview of the photopolymerizations of some of the more commonly used classes of monofimctional vinyl and heterocyclic monomers and is not intended to be inclusive. 

A. V. Vasilyev, Electrophilic Activation of Acetylene Compounds in Bronsted Superacids. Reactions of Vinyl Type Cations, Russ. Chem. Rev., 2013, 82,187. 

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