Superelectrophilic reagents

Akhrem and co-workers417 have successfully applied aprotic organic superacids in the carbonylation of a series of alkanes. Butane was transformed into isomeric carboxylic acids depending on the superelectrophilic reagent and isolated as the... 

Novel aprotic superelectrophilic reagents (RCOX 2AIX3, where R = Alk> l, Aryl X = Br, Cl> have been described by VoTpin and co-workers. These reagents are highly active toward alkanes under mild conditions (see reviews [56] and recent publications [57]). 

Scheme IL8, Cracking and some other transformations of alkanes induced and catalyzed by superelectrophilic reagents RCOX 2AlXj.

Scheme II. 9. punctionalization of alkanes under the action of superelectrophilic reagents RCOX 2AlXj.

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). 

While many superelectrophilic reactions are accomplished at ambient or somewhat elevated temperatures, a significant number of conversions are found to work better at lower temperatures. As some of these reactions employ gaseous reagents such as HF, BF3, and low molecular weight alkanes, the lower temperatures may be necessary to help to keep the reagents in the condensed phase. In other reactions, the lower temperatures are used to control the excessive reactivities of the superelectrophiles... 

Phosphonium groups are well known for their ability to stabilize adjacent anionic sites (i.e., Wittig reagents), but the results with the dicationic species indicate that phosphonium groups can also destabilize adjacent cationic groups producing their superelectrophilic reactivities. 

There is evidence for formation of a zwitterionic Meisenheimer-Wheland complex between superelectrophilic and supernucleophilic reagents. Thus, NMR spectra in CD2CI2 show the formation of (12) from 4,6-dinitrobenzofuroxan and l,3,5-lris(/V,/V-dialkylamino)benzenes. At temperatures above — 30 °C the spectra show the presence of a dynamic process interconverting the three equivalent ring positions of the nucleophile.51... 

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. 

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