Superelectrophiles spectroscopic studies

In several recent studies, nitro-substituted olefins have been shown to exhibit high electrophilic reactivities in superacid-promoted reactions.29 NMR studies have been used to identify some of the superelectrophilic intermediates in these reactions. For example, it was found that nitroethy-lene reacts with benzene in the presence of 10 equivalents of CF3SO3H to give deoxybenzoin oxime in 96% yield (eq 29). Since the reaction does not occur with only one equivalent of TfOH, it was proposed that the N,N-dihydroxy-iminium-methylium dication (51) is generated. In spectroscopic studies, l-nitro-2-methyl-l-propene (52) was dissolved in CF3SO3H, and at —5°C the stable dication (53) could be directly observed by and 13 C NMR spectroscopy (eq 30). 

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. 

Similarly, as spectroscopic techniques have been used to study many long-lived, electrophilic species such as carbocations, acyl and carbox-onium ions, and various onium ions, they have also been used in a number of reports directed to characterization of superelectrophiles. Both condensed and gas phase techniques have been used to study superelectrophilic systems. In the condensed phase, however, superelectrophiles are... 

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. 

Related classes of gitonic superelectrophiles are the previously mentioned protoacetyl dications and activated acyl cationic electrophiles. The acyl cations themselves have been extensively studied by theoretical and experimental methods,22 as they are intermediates in many Friedel-Crafts reactions. Several types of acyl cations have been directly observed by spectroscopic methods and even were characterized by X-ray crystal structure analysis. Acyl cations are relative weak electrophiles as they are effectively stabilized by resonance. They are capable of reacting with aromatics such as benzene and activated arenes, but do not generally react with weaker nucleophiles such as deactivated arenes or saturated alkanes. 

Our book is about the emerging field of Superelectrophiles and Their Reactions. It deals first with the differentiation of usual electrophiles from superelectrophiles, which show substantially increased reactivity. Ways to increase electrophilic strength, the classification into gitionic, vicinal, and distonic superelectrophiles, as well as the differentiation of superelec-trophilic solvation from involvement of de facto dicationic doubly electron deficient intermediates are discussed. Methods of study including substituent and solvent effects as well as the role of electrophilic solvation in chemical reactions as studied by kinetic investigations, spectroscopic and gas-phase studies, and theoretical calculations are subsequently reviewed. Subsequently, studied superelectrophilic systems and their reactions are discussed with specific emphasis on involved gitionic, vicinal, and distonic superelectrophiles. A brief consideration of the significance of superelectrophilic chemistry and its future outlook concludes this book. 

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