Electrophiles complexes Superelectrophilic

The key of alkane transformation was assigned to the formation of CX3+-type cations that are electrophilic enough (probably due to a second complexation of A1X3), to abstract a hydride anion from linear and cycloalkanes. When these cations are generated in superacidic media, a protosolvation induces a superelectrophilic character, which was supported by Olah on the basis of high-level ab initio calculations 65 The generation of these cations was also used by various teams66,67 to initiate selective low temperature alkane activation. 

Carboxylation of aromatics with carbon dioxide with AI2CI,/AI has been studied by Olah, Prakash, and co-workers425 and shown to be a chemoselective process to give aromatic carboxylic acids in good to excellent yields (20-80°C, CO pressure = 57 atm). Two possible mechanistic pathways with the involment of organoaluminium intermediates and complexes of C02 with AICI3 were postulated. On the basis of extensive experimental studies and theoretical calculations, the authors concluded that the most feasible mechanism involves CO2 activated with superelectrophilic aluminum chloride. Complex 116 reacts with aromatics in a typical electrophilic substitution. 

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... 

The electrophile initially formed is further complexed by a Lewis acid, thus decreasing neighboring group participation into the electrophilic center, generating the superelectrophile. 

In most of the examples of superelectrophilic reactions involving Lewis acids, they are conducted using an excess of the Lewis acid. This is in accord with electrophilic solvation by the Lewis acid, i.e. activation of the electrophile requires interaction with two or more equivalents of Lewis acid. As an example, superelectrophilic nitration can be accomplished with NO2CI and at least three equivalents of AICI3 (eq 23).46 This powerful nitrating reagent involves a superelectrophilic complexed nitronium ion (33). 

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). 

A DFT study of the molecular orbitals of pyridine and a number of heteroaromatics unreactive to electrophilic substitution shows that the HOMOs of these compounds are not r-orbitals and so their low reactivity can be explained by assuming frontier orbital control of their substitution reactions.1 Consistent with this rationalization is the fact that in the case of pyridine-A-oxide and a number of other reactive substrates the HOMOs are n-orbitals. 4,6-Dinitrobenzofuroxan (1) is a superelectrophile and reacts with some supernucleophilic l,3,5-tris(A,A-dialkylamino)benzenes to form the first observed Meisenheimer-Wheland zwitterionic complexes [e.g. (2)].2... 

Positively-charged fragments such as [ML,]+, CH , and H + are all strong electrophiles ( superelectrophiles in the extreme sense (13)) towards the Lewis basic H2, but transition metals can uniquely stabilize H2 and other cr-bond coordination by back donation from d-orbitals that main group analogues cannot do. This bonding is then remarkably analogous (14) to the Dewar-Chatt- Duncanson model (15) for ji-complexes (6). 

This study reports on the reactions of ambident nucleophiles with electron-deficient nitroaromatic and heteroaromatic substrates anionic complex formation or nucleophilic substitution result. Ambident behavior is observed in the case of phenoxide ion (O versus C attack) and aniline (N versus C attack). O or N attack is generally kinetically preferred, but C attack gives rise to stable thermodynamic control. Normal electrophiles such as 1,3,5-trinitrobenzene or picryl chloride are contrasted with superelectrophiles such as 4,6-dinitrobenzofuroxan or 4,6-dinitro-2-(2,4,6-trinitrophenyl)benzotriazole 1-oxide (PiDNBT), which give rise to exceptionally stable a complexes. Further interesting information was derived from the presence in PiDNBT of two electrophilic centers (C-7 and C-l ) susceptible to attack by the ambident nucleophilic reagent. The superelectrophiles are found to exhibit lesser selectivity toward different nucleophilic centers of ambident nucleophiles compared with normal electrophiles. 

Possibly a stronger electrophile than TNB might react with aniline via its carbon center. Recent work (36-38) has shown that DNBF (16) is a much stronger electrophile than TNB. For example, DNBF reacts with neutral H20 or MeOH to give the respective a complexes 17 (equation 2), whereas TNB requires HO- or CH30 for reaction to occur. DNBF has therefore been called a superelectrophile. (In equation 2, R is H or CH3.)... 

Superelectrophiles Electrophiles that are further activated by Bronsted or Lewis superacid complexation. 

The high electrophilicities of 4,6-dinitrobenzofuroxan (DNBF) and 4,6-dinitrotetrazo-lopyridine place them in the superelectrophile category. Their reactions in acetonitrile with 2,4-dipyrrolidin-l-yl-l,3-thiazole, a strong nucleophile, allow the isolation of zwitterionic Meisenheimer-Wheland complexes, which are stable enough for X-ray... 

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