Superelectrophilic dications

Superelectrophilic dications 50 have been observed by Coustard,197 who used 1H and 13C NMR spectroscopy in triflic acid at low temperature (Scheme 5.25). They... 

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. 

The protonation of the methyleneiminium cation gives rise to the superelectrophilic methyleniminium dication, 19, which is a gitonic 1,2-dicationic species (carbenium ammonium species). Such a carbodication has been observed by charge stripping mass spectrometry by Schwarz and coworkers.Olah and coworkers have shown by theoretical calculations that the carbenium ammonium species, 19a, is 27.6 kcal/mol more stable than the three-center two-electron (3c-2e) bonded structure, 19b. ... 

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 onium dications have been the subject of extensive studies and their chemistry is discussed in chapters 4-7. Other multiply charged carbocationic species are shown in Table 2. These include Hogeveen s bridging, nonclassical dication (14)26 the pagodane dication (15)27 Schleyer s l,3-dehydro-5,7-adamantane dication (16)28 the bis(fluroenyl) dication (18)29 dications (17 and 19) 19a trications (20-21)19a,3° and tetracations (22-23).31 Despite the highly electrophilic character of these carbocations, they have been characterized as persistent ions in superacids. 

Although electrophilic reactions involving dications with deactivated arenes may suggest the formation of superelectrophilic intermediates, there are a number of well-known examples of monocationic electrophiles that are capable of reacting with benzene or with deactivated aromatic compounds. For example, 2,2,2-trifluoroacetophenone condenses with benzene in triflic acid (eq 12).13 A similar activation is likely involved in the H2SO4 catalyzed reaction of chloral (or its hydrate) with chlorobenzene giving DDT (eq 13). 

It was reported that arylpinacols (48a) can undergo a superacid-catalyzed dehydrative cyclization to give the aryl-substituted phenan-threnes (eq 27).23 Superelectrophilic intermediates were proposed in the conversion. Tetraarylethylene dications have been studied by several methods and were observed directly by NMR as well as by UV-vis spectroscopy and X-ray crystallography.28 The low temperature oxidation of tetraaiylethylenes gives the dicationic species (50, eq 28). 

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). 

Carboxonium ions are indicated to be involved in a number of super-electrophilic reactions. In several cases, the direct observation of the superelectrophiles and reactive dications has been possible using low... 

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). 

Mass spectrometric techniques also enable experimentalists to react dications and trications with neutral substrates in the gas phase to explore the chemical reactions of these multiply charged species. Although only a few superelectrophiles have thus far been examined experimentally for their gas phase chemical reactivities, other electrophilic gas phase reactions have demonstrated the potential of these methods. For example, the... 

Computational methods have also been used frequently to estimate the thermodynamic stabilities of superelectrophiles. These calculations have often involved the estimation of barriers to gas phase dissociation or deprotonation, and the proton affinities of conventional electrophilic intermediates. Other useful studies have calculated the heats of reactions for isodesmic processes. An interesting example of these calculations comes from a study of the protoacetyl dication (Cf COH2"1- ).42 In calculations at the 6-31G //4-31G level of theoiy, the protoacetyl dication (83) is estimated to react with methane by hydride abstraction with a very favorable... 

As discussed previously, several types of reactive dications and superelectrophiles have been directly observed using NMR spectroscopy. These experiments have all used low temperatures (— 100°C to — 30°C) and superacidic conditions to generate the observable reactive dications and superelectrophiles. Some reactive dications and superelectrophiles are stable at low temperatures and can be directly observed by NMR, but at higher temperatures they readily cleave and decompose. The low temperatures also slow down proton exchange reactions and enable the ions to be observed as static species. 

A considerable amount of work has also been done to show that superelectrophilic sulfonium dications can be generated. For example, like the... 

In the case of the dimethylbromonium cation 46, ab initio calculations have found two stable minima for the protosolvated superelectrophile, the bromine and carbon protonated forms, 51 and 52, respectively (Figure 3).44a Dication 51 is estimated to be more stable than 52 by about 21 kcal/mol at the B3LYP/6-31G //B3LYP/6-31G level. When dimethylbromonium cation (46) is reacted in DF/SbFs in SO2 at —78°C, no deuterium incorporation is observed into the methyl groups. This observation is in accord with the theoretical calculations in that Br-protonation is expected to be the preferred process. 

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