Study of Superelectrophiles

Superelectrophiles and Their Chemistry, by George A. Olah and Douglas A. Klumpp Copyright 2008 John Wiley Sons, Inc. 

One of the defining features of superelectrophiles is the often-observed high level of reactivity towards nucleophiles of low strength.1 This experimental observation is frequently used as an indication for the involvement of a superelectrophiles. To illustrate, the following examples show how the electrophile s reactivity can be characterized to indicate superelectrophilic chemistry. 

Even the trityl cation has been successfully nitrated using superacidic activation of nitronium salts (eq 3).4 

Friedel-Crafts type reactions of strongly deactivated arenes have been the subject of several recent studies indicating involvement of superelectrophilic intermediates. Numerous electrophilic aromatic substitution reactions only work with activated or electron-rich arenes, such as phenols, alkylated arenes, or aryl ethers.5 Since these reactions involve weak electrophiles, aromatic compounds such as benzene, chlorobenzene, or nitrobenzene, either do not react, or give only low yields of products. For example, electrophilic alkylthioalkylation generally works well only with phenolic substrates.6 This can be understood by considering the resonance stabilization of the involved thioalkylcarbenium ion and the delocalization of the electrophilic center (eq 4). With the use of excess Fewis acid, however, the electrophilic reactivity of the alkylthiocarbenium ion can be 

Similarly, hydroxyalkylation with aldehydes or ketones is best accomplished with activated aromatic compounds, such as phenols.7 However, reaction even with chlorobenzene has been carried out with either para-banic acid or isatin using triflic acid (eqs 6-7).8,9 

---

Nuclear magnetic resonance (NMR) spectroscopy has been used to directly observe varied persistent superelectrophilic species. Although H and 13C NMR have been the most often used techniques, there have also been applications of 15N, 170, and 19F NMR in their structural characterization. Coupled with theoretical computational methods capable of estimating NMR chemical shifts, these studies have been very useful in the study of superelectrophiles. 

An impressive application of infrared and Raman spectroscopy was demonstrated in studies of superelectrophilic diprotonated thiourea, [H3NGSH jNfE 2+ 2AsFf,. The Raman spectrum (taken at — 110°C) corresponded reasonably well with calculated vibrational bands predicted by density functional theoiy.38 Coupled with computational methods for predicting vibrational frequencies, it is expected that vibrational spectroscopic techniques will be useful for the observations of these and other superelectrophiles. 

Quantum mechanical calculations are an essential part of chemistry, and these methods have been extremely useful in studies of superelectrophilic chemistry. For example, computations have been used in some studies to show that the formation of dicationic superelectrophiles lowers... 

Lower temperatures were also an important aspect of other studies of superelectrophilic chemistry. For example, Olah and co-workers studied the role of superelectrophiles in the acid-catalyzed cleavage of esters.34 One of the key experiments was carried out under highly acidic conditions and at —40°C to prevent nucleophilic attack of monocationic intermediates (eq 44). 

---