Superelectrophiles 4,6-dinitrobenzofuroxan

Aminothiazole and its 4-methyl derivative react with the superelectrophilic 4,6-dinitrobenzofuroxan at C(5) to form, for example, (1) in spite of them exhibiting higher nitrogen basicity than aniline.7 In the case of 4,5-dimethyl-2-aminothiazole, however, attack did occur at nitrogen. 

Potentiometric measurements indicate that in water-DMSO the 3-aminothiophenes 197 undergo protonation exclusively at the nitrogen with dilute acid. But on treatment with the superelectrophile 4,6-dinitrobenzofuroxan (DNBF) they react as carbon nucleophiles giving rise directly to the corresponding C-adducts. The 3-aminothio-phenes are thus shown to possess strong enaminic character <1998CJC937>. 

Nucleophilic reactions take place in the homocyclic ring, SwAr or AEc when it is activated by electron-withdrawing substituents. It has been described that halides can be displaced by a great number of nucleophiles via a normal and cine substitution [54,55]. Nitro containing Bfx has represented a class of neutral lO-TT-electron-defident system which exhibit an extremely high electrophilic character in many covalent nucleophihc addition and substitution processes. 4,6-Dinitrobenzofuroxan and others 4-nitro-6-substitutedbenzofuroxans (Scheme 2) have been defined as superelectrophiles and used as convenient probes to assess to the C-basicity of... 

Dinitrobenzofuroxan (DNBF) is known as a superelectrophile due to its high reactivity both as an electrophile and in its pericyclic addition reactions. NMR studies show that reaction with 2-aminothiazole and its 4-methyl derivative yield anionic carbon-bonded adducts such as (11) by reaction at the 5-position, whereas the 4,5-dimethyl derivative reacts via the exocyclic amino group. Kinetic studies of the first two compounds, both in acetonitrile and in 70 30 (v/v) water-DMSO, have been used to assess their carbon nucleophilicities and place them on the Mayr nucleophilicity scale.55 In a related study, the nucleophilic reactivity, in acetonitrile, of a series of indoles with both DNBF and with benzhydryl cations have been compared and used to determine nucleophilicity parameters for the indoles.56... 

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... 

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... 

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. 

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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