Superelectrophilic Systems

The electrophilicities of the superelectrophiles may also be compared using rate constants for their reactions with indoles, pyrroles, and enamines allowing them to be positioned on Mayr s elec-trophilicity scale [60, 61 ]. Also of interest is the ability of superelectrophiles to take part in Diels-Alder-type cycloaddition reactions to produce adducts with dienes, such as butadiene, and with ethyl vinyl ether [184]. 

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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... 

Carbon-Carbon Vicinal-Dications Ethylene dications are a major group of carbon-centered superelectrophilic systems. The parent ethylene dication (CH2CH22+, 1) has been studied experimentally in the gas-phase, as well as in several theoretical studies.1 Two electron... 

As described in Chapter 2, a unique gitonic superelectrophile is considered to be involved in an enzyme system that converts CO2 to methane. Berkessel and Thauer have studied this metal-free hydrogenase enzyme from methanogenic archaea and a mechanism is proposed involving activation through a vicinal-superelectrophilic system (eq 34).50... 

Transfer of Cl+ to the arene provides some relief of the Coulombic repulsion in the multiply charged, superelectrophilic system. Under the reaction conditions, it is not yet known to what extent the N-halosuccinimides are protonated in BF3-H2O, but this acid-catalyst has an estimated acidity around IIq —12. 

Several examples of superelectrophiles have already been described in which delocalization of charge may lead to structures that could be considered gitonic or distonic superelectrophiles, depending on which resonance form is being considered predominant. While the bis-oxonium structure (3a) is formally a distonic superelectrophile, the bis-carbenium structure (3c) is considered a gitonic superelectrophile (eq 2). These types of systems have been previously discussed and therefore will not be included in this chapter. In a similar respect, there are examples of equilibration between gitonic (4) and distonic (5) superelectrophilic systems (eq 3).2... 

Other adamantane-based dications have also been prepared. Although systems such as diadamanta-4,9-diyl dication (35) and TT-bisadamanta-3,3 -diyl dication (36) have been prepared (both 1,6-dications), experimental data suggests that these systems are more related to the monopositive adamantyl cations rather than distonic superelectrophilic systems.14 However, the adamanta-l,3-dimethyldiyl dication (38) has been prepared in superacid media (eq 16).15... 

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. 

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. 

Nucleophilic reactions take place in the homocyclic ring, SwAr or AEc when it is activated by electron-withdrawing substituents. It has been described that halides can be displaced by a great number of nucleophiles via a normal and cine substitution [54,55]. Nitro containing Bfx has represented a class of neutral lO-TT-electron-defident system which exhibit an extremely high electrophilic character in many covalent nucleophihc addition and substitution processes. 4,6-Dinitrobenzofuroxan and others 4-nitro-6-substitutedbenzofuroxans (Scheme 2) have been defined as superelectrophiles and used as convenient probes to assess to the C-basicity of... 

This may suggest that fully conjugated charge centers are an important aspect in directing nucleophilic attack to the terminal carbon. Thus, the superacid promoted reactions of the olefinic pyrazines (and related systems) may be viewed as the superelectrophilic version of Michael addition. 

In summary, we have shown that stable cationic charge centers can significantly enhance the reactivities of adjacent electrophilic centers. Most of the studied systems involve reactive dicationic electrophiles. A number of the reactive dications have been directly observed by low temperature NMR. Along with their clear structural similarities to superelectrophiles, these dicationic systems are likewise capable of reacting with very weak nucleophiles. Utilization of these reactive intermediates has led to the development of several new synthetic methodologies, while studies of their reactivities have revealed interesting structure-activity relationships. Based on the results from our work and that of others, it seems likely that similar modes of activation will be discovered in biochemical systems (perhaps in biocatalytic roles) in the years to come. 

G. Prakash, G. K. S. Efficient Chemoselective Carboxylation of Aromatics to Arylcarboxylic Acids with Superelectrophilically Activated Carbon Dioxide-Al2CI6/Al System. J. Am. Chem. Soc. 2002, 124, 11379-11391. (d) Klumpp, D. A. Rendy, R. McElrea, A. Superacid Catalyzed Ring-opening Reactions Involving 2-Oxazolines and the Role of Superelectrophilic Intermediates. Tetrahedron Lett. 2004, 45, 7959-7961. 

More recent studies with superacidic systems (TfOH, TfOH—SbF5), used also in the Gattermann reaction, indicated that strong acids significantly increase reactivities of benzene with benzonitrile.33 104 It is concluded that the superelectrophilic 14 dication formed as a results of protonation of 13 is the reactive species in the Houben-Hoesch reaction. 

Superelectrophilic activation has also been proposed to be involved, based upon the reactivity of carbocations with molecular hydrogen (a a-donor).16 This chemistry is probably even involved in an enzymatic system that converts CO2 to methane. It was found that A. A -menthyl tetrahy-dromethanopterin (11) undergoes an enzyme-catalyzed reaction with H2 by hydride transfer to the pro-R position and releases a proton to give the reduced product 12 (eq 15). Despite the low nucleophilicity of H2, cations like the tert-butyl cation (13) are sufficiently electrophilic to react with H2 via 2 electron-3 center bond interaction (eq 16). However, due to stabilization (and thus delocalization) by adjacent nitrogen atoms, cations like the guanidinium ion system (14) do not react with H2 (eq 17). 

These examples illustrate how electrophilic systems can exhibit enhanced reaction rates and yields with increasing strength of the acidic reaction media. Both qualitative and quantitative kinetic studies strongly suggest the involvement of superelectrophilic species in reactions. 

Unlike superelectrophiles generated in the condensed phase, the gas phase species are not stabilized by solvation or association to counter ions. The stabilities of multiply charged small ions are dependent on their tendencies to dissociate into fragments in the gas phase.39 For diatomic systems, there are two possible routes of dissociation to atomic fragments the cleavage to a pair of monocations (eq 41) or cleavage to a dication and neutral species (eq 42). 

Shudo and Ohwada have developed and used acid systems composed of varying ratios of CF3SO3H and CF3CO2H in order to obtain solutions having acidities between Hq — 7.7 and Hq — 13.7.27 These acid systems have been used in kinetic studies related to superelectrophiles... 

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