Gitonic 1,3-Superelectrophiles

The destabilization and connected superelectrophilic character of some 1,3-carbodications can be seen in the unsuccessful attempts to prepare 

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

In a similar respect, ionization of 2,4-dichloro-2,4-dimethylpentane (6) does not give the 1,3-carbodication (7, eq 3).3 Despite the superacidic conditions, deprotonation occurs to give the allylic cation (8). Even substitution by phenyl groups is not enough to stabilize the 1,3-dication. For example 1,1,3,3-tetraphenyl-l,3-propanediol (9) also undergoes the deprotonaton or disproportionation reactions (eq 4).3 

In an attempt to prepare an adamanta-l,3-diyl dication 12, only the monocationic, donor-acceptor complex (11) could be observed experimentally in the superacid-promoted reaction of the difluoride (10, eq 5).4 

Although the fully formed dicationic structure (12) is not formed, the donor-acceptor complex 11 may have partial superelectrophilic character by interaction with SbFs. The adamanta-l,3-diyl dication 12 has been found to be the global minimum structure on the CioHi42+ potential energy surface.5 Theoretical studies at the B3LYP/6-31G level have shown 12 to be 0.4kcal/mol more stable than the isomeric 1,4-dication 

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There have been many studies related to tetraaryl-1,2-ethylene dications, formally gitonic superelectrophiles, which show extensive charge delocalization into the aryl rings (Scheme 1). Low temperature 13C NMR studies have shown that the para carbons are deshielded, from < 13C... 

Vicinal 1,2-carbodicationic systems are some of the most important and thoroughly studied superelectrophiles. They include inter alia 1,2-ethylene dications, related carbon-nitrogen superelectrophiles (diprotonated imines and nitriles), protosolvated carboxonium ions, and superelectrophilic tri-halomethyl cations. It is understood that many of these systems involve extensive charge delocalization, and in a sense may not be considered formal 1,2-dicationic systems. For example, diprotonated 2,3-butanedione (34) may be represented formally as a 1,2-ethylene dication (35a, a gitonic superelectrophile), but even in monocationic carboxonium ions, there is a significant amount of double-bond character retained in the carbon-oxygen bond (eq 5).22... 

Charge-charge repulsive effects increase the importance of the resonance form (35b) having dione-type structure (a 1,4-dication and representing a distonic superelectrophile). Despite the importance of the charge separated structure 35b, the system is included here with other 1,2-ethylene dications and gitonic superelectrophiles. 

Several lines of evidence in the studies of Ohwada and Shudo support the involvement of these gitonic superelectrophiles. An analagous dicationic species (47) has been directly observed by 1H and 13C NMR at low temperature in both CFsSOsH-SbFs and CF3SO3H solutions. With dication 47, the NMR spectra show an equivalence of the aromatic rings suggesting a perpendicular relationship between the two cationic centers. The dications having the 4-methylphenyl substituents (61-62) have likewise been directly observed by NMR at low temperature, and upon warming the solutions, the expected cyclization products are formed (eqs 11-12). 

Further experimental evidence for gitonic superelectrophiles comes from the catalytic activation of the monocationic species.32a When the a-chloro ester (68) is reacted with AgBE or Ag(C>2CCF3), the monocationic intermediate (69) is cleanly formed. This is evident from the observed trapping product (57, eq 14). 

An unusual class of gitonic superelectrophiles (having vicinal -dicationic structure) are the protosolvated alkylcarbenium ions (of carbenium-car-bonium dication nature). Several studies have shown that alkylcarbenium... 

Since the reaction only occurs in highly acidic media, it is suggested that the gitonic superelectrophile (94) is formed and leads to product.44... 

Kinetic evidence suggests the involvement of gitonic superelectrophiles also in superacid-promoted Houben-Hoesch reactions.44 As discussed previously, 4-phenylbutyronitrile (6) is found to cyclize at an appreciable rate only in solutions more acidic than Hq —10 (eq 28). 

In superacidic reactions, diprotonated imines form gitonic superelectrophiles.48 As described in Chapter 2, kinetic experiments have shown that diprotonated intermediates are involved in these conversions. Other experiments showed that the reaction provides higher yields in stronger acid systems (eq 32),... 

Another example of a carbon-nitrogen gitonic superelectrophile is the protonated guanidinium ion.49 The guanidinium ion (110) can be proto-nated in FSOsFFSbFj to give the gitonic superelectrophile (111 eq 33),... 

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... 

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. 

There are several reports of activation of acyl cations by superacids, suggesting the involvement of gitonic superelectrophiles.61 As discussed in Chapter 2, hydride transfer from isobutane to the acetyl cation has been reported when the reaction is carried out in excess HF-BF3. At the same... 

Another related class of gitonic superelectrophiles are the superelectrophilic halocarbonyl dications. The halocarbonyl cations (XCO+, X = F, Cl, Br, I) have been prepared under long-lived stable ion conditions and characterized by 13C NMR spectroscopy.68 Sommer and co-workers studied the bromine-assisted carbonylation of propane in superacids, and... 

No systematic study of the reactivities of these electrophiles has been done, but they are known to be easily reduced to their radical cations. The A.A-bipyridiniums and related systems have not yet been studied in detail, but some have been prepared, such the 1-pyridinopyridinium dication 200.95 Little is known about the electrophilic reactivities of these compounds, but 200 has been shown to react with water at a ring carbon (eq 55). An unusual gitonic superelectrophile has been proposed involving... 

Diazenium dications may also be considered gitonic superelectrophiles, but only a limited number of them have been prepared and studied.97 In principle, diazenium dications could be prepared by double alkylation or protonation of various azo-compounds. To date, only monalkylation has been achieved with strong alkylating agents (eq 57)97a However, Olah et al. showed that diazenium dications can be produced by double protonation of azobenzenes in FSQ3H-SbF5 at low temperature.97b It has also been demonstrated that the diazenium dications are prepared from electrochemical and chemical oxidations (eqs 58-59).97c,d... 

Another class of gitonic superelectrophiles (based on the 1,3-carbodica-tion structure) are the Wheland intermediates or sigma complexes derived from electrophilic aromatic substitution of carbocationic systems (eq 8). 

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