Protosolvated

Volpin called the CH3COCI lAlCk complex an aprotic superacid. The results indicate that the acetyl cation is activated by further 0-complexation with a second molecule of AIX3 (the equivalent of protonation or protosolvation). 

Various sulfonium and carbosulfonium ions show remarkably enhanced reactivity upon superelectrophilic activation, similar to their oxygen analogs so do selenonium and telluronium ions. The alkylating ability of their trialkyl salts, for example, is greatly increased by protosolvation. 

In these (and other) solid superacid catalyst systems, bi- or multi-dentate interactions are thns possible, forming highly reactive intermediates. This amounts to the solid-state equivalent of protosolvation resulting in superelectrophilic activation. 

Olah, G. A. Germain, A. Lin, H. C. Forsyth, D. Electrophilic reactions at Single Bonds. XVIII.1 Indication of Protosolvated de facto Substituting Agents in the Reactions of Alkanes with Acetylium and Nitronium Ions in Superacidic Media. J. Am. Chem. Soc. 1975, 97, 2928-2929. 

Olah, G. A., Laali, K. K., and Sandford, G. (1992a). Comparison of the nitration of poly-fluoronitrobenzenes by nitronium salts in superacidic and aptotic media Activation of the nitronium ion by protosolvation. Proc. Natl. Acad. Sci. U.S.A. 89, 6670-6672. 

The key of alkane transformation was assigned to the formation of CX3+-type cations that are electrophilic enough (probably due to a second complexation of A1X3), to abstract a hydride anion from linear and cycloalkanes. When these cations are generated in superacidic media, a protosolvation induces a superelectrophilic character, which was supported by Olah on the basis of high-level ab initio calculations 65 The generation of these cations was also used by various teams66,67 to initiate selective low temperature alkane activation. 

Since the inception ofthe superelectrophilic concept in the 1970s1 and 1980s 2 first formulated as protosolvation of cationic intermediates, superelectrophiles as highly reactive dicationic and tricationic intermediates have been successfully observed and characterized.3-5 Consequently, selected examples of superelectrophiles are also covered in this chapter where appropriate, whereas various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are observed, are treated in Chapter 5. 

Trialkyloxonium ions have been extensively used to carry out O-alkylation of lactams, amides, sulfoxides, oxo-sulfonium ylides, iV-alkylation of iV-heterocycles, 5-alkylation of thioethers, thioacetals, thioamides, and thiolactams.1,90 Although trialkyloxonium ions do not alkylate benzene or toluene directly, alkylations readily occur in combination with highly ionizing superacids such as HS03F-SbF5. This may indicate protosolvation of a nonbonded electron pair on oxygen in the superacid medium. 

The success of carbocation chemistry lies in the stabilization of carbocations in a medium of low nucleophilicity. Superelectrophiles, in turn, are reactive intermediates generated by further protonation (protosolvation). This second protonation increases electron deficiency, induces destabilization, and, consequently, results in a profound increase in reactivity. In particular, charge-charge repulsive interactions6 play a crucial role in the enhanced reactivity of dicationic and tricationic superelectrophilic intermediates. As various examples of acidity dependence studies show, without an appropriate acidity level, transformations may occur at much lower rate or even do not take place at all. In addition to numerous examples of superacid catalyzed reactions, various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are de facto observed in the condensed state, are also included in this chapter. 

The reaction of the parent compound did not take place in trifluoroacetic acid (TFA), but did proceed on the addition of triflic acid to give the alkylated product in low yield (Table 5.24). In neat triflic acid the reaction is faster and the product is formed in much higher yield. Similar changes were observed with the other 1,3-diphenylpropanones. Substituents on the benzoyl group also induce significant differences in reactivities (compare the results for R = Me and CF3 in Table 5.24). These observations suggest that the protosolvated dicationic species 79 is involved in the rate-determining step of cyclization (Scheme 5.35). 

Addition of small amounts of bromine to the reaction mixture before adding the propane/CO mixture resulted in both an increase in conversion and increasing selectivity of the isopropylcarboxonium ion independently of the propane/CO ratio.413,414 This was accounted for by the involvement of the bromocarbonyl cation BrCO+, which, upon protosolvation, acts as a superelectrophile exhibiting high... 

Triflic acid effectively promotes the phenylamination of aromatics with phenylazide in a fast, convenient, high-yield process598 (Table 5.34). The high ortho/para selectivity with only a small amount of meta product and high substrate selectivity (kT/kB = 11) indicate the involvement of a substantially electron-deficient species, the phenylami-nodiazonium ion intermediate with the possible protosolvation by triflic acid. 

Extending the concept of superacids to varied superelectrophiles has emerged as a productive new field in recent years (G. A. Olah and D. A. Klumpp, Superelectrophiles and Their Chemistry, Wiley-Interscience, Hoboken, NJ, 2008). Highly reactive and activated protosolvated or multiply charged superelectrophilic intermediates are involved in varied chemical reactions, many of them of substantial practical significance. 

Scheme 1. Protosolvation of the acetyl cation and its reaction with isobutane.

These results can be interpreted in terms of protosolvation of the nitronium ion. While the monocationic nitronium ion is a sufficiently polarizible electrophile to react with strong nucleophiles such as olefins and activated arenes, it is generally not reactive enough to react with weak nucleophiles including methane. Partial or complete protonation of the nitronium oxygen then leads to the superelectrophilic species 8. The... 

Two types of interactions have been shown to be involved in superelectrophilic species. Superelectrophiles can be formed by the further interaction of a conventional cationic electrophile with Brpnsted or Lewis acids (eq 16).23 Such is the case with the further protonation (protosolvation) or Lewis acid coordination of suitable substitutents at the electron deficient site, as for example in carboxonium cations. The other involves further protonation or complexation formation of a second proximal onium ion site, which results in superelectrophilic activation (eq 17).24... 

The reaction yield increases with acid strength, further suggesting the intermediacy of the protosolvated, superelectrophilic formylating species (9). 

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