Superelectrophilic species

MeZrCp2Cl, by virtue of the presence of the dipolar Cl+-AI bond. Most likely, this reaction exemplifies a widely observable principle of activation of an electrophile by another electrophile to generate a superelectrophilic species that has been termed the two-is-better-than-one principle.13 1... 

These results can be interpreted in terms of protosolvation of the nitronium ion. While the monocationic nitronium ion is a sufficiently polarizible electrophile to react with strong nucleophiles such as olefins and activated arenes, it is generally not reactive enough to react with weak nucleophiles including methane. Partial or complete protonation of the nitronium oxygen then leads to the superelectrophilic species 8. The... 

Two types of interactions have been shown to be involved in superelectrophilic species. Superelectrophiles can be formed by the further interaction of a conventional cationic electrophile with Brpnsted or Lewis acids (eq 16).23 Such is the case with the further protonation (protosolvation) or Lewis acid coordination of suitable substitutents at the electron deficient site, as for example in carboxonium cations. The other involves further protonation or complexation formation of a second proximal onium ion site, which results in superelectrophilic activation (eq 17).24... 

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. 

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. 

Calculations by Perez have examined superelectrophilic species within the context of both electron deficiency and polarizability of the active sites of superelectrophiles.46 Using the model equation proposed by Parr and associates,... 

Table 2. Superelectrophilic species generated in ISOHUSbl solution.

As discussed in Chapter 2, Shudo and Ohwada found that acetyl and benzoylhexafluoroantimonate salts (CH3CO+SbF6 and CfiHsCO+SbFf,-, respectively) acylate aromatic compounds with increasing reaction rates and greater yields in progressively more acidic media.44 These data are consistent with the formation of the superelectrophilic species 137 and 141 (eq 39).65... 

To account for these conversions, two mechanistic proposals have been made either the halogenating system involves a highly reactive and solvated X+, or the reaction involves X+ transfer from a protonated form of N -halosuccinimide. In the later case, X+ transfer could occur from the neutral A-halosuccinimide, monoprotonated, or multiply protonated, superelectrophilic species. These mechanistic possibilities were further examined using DFT calculations. For A-chlorosuccinimide and its pro-tosolvated intermediates, a series of calculations were done at the B3LYP/... 

There have been several types of gitonic superelectrophiles having acyl cationic groups as part of a 1,3-dicationic system. Monocationic acyl cations (168) can be prepared as persistent species in superacidic media and salts have even been studied by X-ray crystallography.113 Much of the interest in superelectrophilic species from acyl cations has focused on the protioacyl dications (169, vide supra).61... 

Among other distonic superelectrophiles described in the literature, there are carbo-onium dications. These include carbo-carboxonium dications, carbo-ammonium dications, and related ions. Despite the separation of charge in these superelectrophiles, some have been shown to have very high electrophilic reactivities. I. ike the carbodications described previously, the discussion here is limited to those systems that have been shown to have electrophilic reactivities greater than the related monocationic onium ions, as well as structural criteria supporting their designation as a distonic superelectrophilic species. 

As mentioned, 2-oxazolines may form a ring-opened distonic superelectrophile in reactions in superacid. These carboxonium-carbenium dications are capable of reacting with benzene and moderately deactivated substrates.2 For example, the optically active oxazoline (94) reacts in CF3SO3H to generate the chiral dication (95) and this superelectrophilic species is capable of reacting with o -dichlorobenzene in fair to modest yield and diastereoselectivity (eq 35). 

As described in the previous chapter, a number of diketones have been diprotonated in superacid to generate distonic superelectrophilic species. Several types of 1,4-diketones yield the respective distonic superelectrophiles (eq 49).47... 

The activating effects of ammonium groups on carboxonium electrophiles has also been exploited in the Friedel-Crafts acylations with amides.50 For example, in comparing the superacid-catalyzed reactions of acetanilide, the monoprotonated species (198) is found to be unreac-tive towards benzene (eq 67), while the diprotonated, superelectrophilic species (199) reacts with benzene to give the acyl transfer product in reasonably good yield (eq 68). 

The above results clearly demonstrate that the reactivities of electrophilic centers can be enhanced by adjacent positive charge. We have also found several interesting exanq)les suggesting that positive charge centers can influence the regiochemistry of nucleophilic attack. There are two reported examples of superelectrophilic species undergoing nucleophilic attack at the... 

Smooth aza-Nazarov cyclization to form dihydroisoindolones is observed with TfOH where CE3COOH is totally ineffectual. Eormation of dicationic superelectrophilic species is apparently important for overcoming the energy barrier of the cyclization in such cases. ... 

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