Superelectrophiles concept

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. 

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. 

The concept of superelectrophilic activation was first proposed 30 years ago.20 Since these early publications from the Olah group, superelectrophilic activation has been recognized in many organic, inorganic, and biochemical reactions.22 Due to the unusual reactivities observed of superelectrophiles, they have been exploited in varied synthetic reactions and in mechanistic studies. Superelectrophiles have also been the subject of numerous theoretical investigations and some have been directly observed by physical methods (spectroscopic, gas-phase methods, etc.). The results of kinetic studies also support the role of superelectrophilic activation. Because of the importance of electrophilic chemistry in general and super-acidic catalysis in particular, there continues to be substantial interest in the chemistry of these reactive species. It is thus timely to review their chemistry. 

As noted previously in Chapter 1, the electrophilic reactivities of acetyl salts increase dramatically as the acidity of the reaction medium increases. This was one of the observations that lead Olah and co-workers to first propose the concept of superelectrophilic activation, or protosolvation of the acetyl cation, in 1975.2 This seminal paper described the chemistry of acetyl hexafluoroantimonate (CHsCO+SbFg-) and the reaction with alkanes in various solvents. In aprotic solvents such as SO2, SO2CIF, AsF3, and CH2CI2, there was no reaction. However in HF-BF3, acetyl salts react with Ao-alkanes and efficient hydride abstraction is observed.27 This was interpreted by Olah as evidence for protonation of the acetyl... 

Since the concept of superelectrophilic activation was proposed 30 years ago, there have been many varied superelectrophiles reported both in experimental and theoretical studies. Superelectrophiles can be involved in both gas and condensed phase reactions, ranging from interstellar space down to the active sites of certain enzymes. Moreover, synthetic conversions involving superelectrophiles are increasingly used in the synthesis of valuable products. Superelectrophilic activation has also been useful in the development of a number of new catalytic processes. It is our belief that superelectrophilic chemistry will continue to play an increasing role in both synthetic and mechanistic chemistry. 

Superelectrophilic Activation or Superelectrophilic Solvation. Trifluoromethanesulfonic acid (triflic acid, TfOH) has been extensively employed as a superacid Ho= —14.1) in superelectrophilic activation (or superelectrophilic solvation), both concepts advanced by Olah. Superelectrophilic activations may occur when a cationic electrophile reacts with a Bronsted or Lewis acid to give a dicationic (doubly electron-deficient) superelectrophile. However, it should be recognized that the activation may proceed through superelectrophilic solvation without necessarily forming limiting dicationic intermediates. The frequently used depiction of protosolvated species as their limiting dications is just for simplicity. ... 

To explain this process, Berkessel and Thauer proposed the mechanism outlined in Figure 11.63. In this proposal, they extended, for the first time, the idea of superelectrophilic activation, a concept that had previously been postulated only under superacidic conditions, into the realm of enzymatic reactions. Furthermore, this mechanism has a number of interesting stereoelectronic features that we will discuss below. 

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