4.6- Dinitrobenzofuroxan adducts

Spear, R.J., Norris, W.P. Structure and properties of the potassium hydroxide-dinitrobenzofuroxan adduct (KDNBF) and related explosive salts. Propellants Explosives Pyrotechnics 8, 85-88 (1983)... 

Thorough thermodynamic and kinetic investigation of solvolysis of 4,6-dinitrotetrazolo[l,5- ]pyridine 11 in water and methanol has been carried out <2003OBC2764>. It has been shown that in water the anionic rr-complex 12 is formed exclusively, whereas addition of methanol results in partial formation of the neutral carbinolamine-type adduct 13 at low pH (Scheme 4). All these results indicate that 11 is an even more powerful electrophile than dinitrobenzofuroxane. 

Although the tendency of benzofuroxan derivatives to interact with electron donors has been known since Drost s early report197 199 firm experimental evidence on the formation of Meisenheimer adducts was not reported until the 60s for the reaction of 4,6-dinitrobenzofuroxan with RO nucleophiles200 202 (Eq. 25). Information on benzofurazans became available even later with an NMR study by Terrier, Simonnin, and their co-workers.203... 

The powerfully electrophilic character of 4,6-dinitrobenzofuroxan is further illustrated by the fact that ketones of low acidity, such as acetone and cyclopentanone, form n-adducts by carbon attachment to the 7-position... 

Some interesting reactions of the super-electrophile 4,6-dinitrobenzofuroxan (151) with aryloxide ions have been reported 130 (151) with phenoxide ion gives the O-bonded ff-adduct (152) which has been fully characterized for the first time. On... 

There have been several studies utilizing the ability of 4,6-dinitrobenzofuroxan (DNBF) to act as a super-electrophile. It has been shown that the initial product of its reaction with phenoxide ions is the O-bonded adduct (29), although this rearranges to give die C-bonded adduct.117 The reaction of DNBF with ethyl vinyl ether yields a dihydrooxazine A -oxidc cycloadduct as a mixture of two diastereoisomers, and in the presence of excess ether a di-adduct (30) is formed.118 Rate constants have been reported for the reaction of DNBF with 3-methoxythiophene in DMSO-water mixtures. The reaction results in a er-adduct which is the product of Se-At substitution in the thiophene, and the results were used to probe the carbon basicity of the thiophene derivative.119... 

The presence or absence of the dioxolane protecting group in dienes dictates whether they participate in normal or inverse-electron-demand Diels-Alder reactions.257 The intramolecular inverse-electron-demand Diels-Alder cycloaddition of 1,2,4-triazines tethered with imidazoles produce tetrahydro-l,5-naphthyridines following the loss of N2 and CH3CN.258 The inverse-electron-demand Diels-Alder reaction of 4,6-dinitrobenzofuroxan (137) with ethyl vinyl ether yields two diastereoisomeric dihydrooxazine /V-oxide adducts (138) and (139) together with a bis(dihydrooxazine A -oxide) product (140) in die presence of excess ethyl vinyl ether (Scheme 52).259 The inverse-electron-demand Diels-Alder reaction of 2,4,6-tris(ethoxycarbonyl)-l,3,5-triazine with 5-aminopyrazoles provides a one-step synthesis of pyrazolo[3,4-djpyrimidines.260 The intermolecular inverse-electron-demand Diels-Alder reactions of trialkyl l,2,4-triazine-4,5,6-tricarboxylates with protected 2-aminoimidazole produced li/-imidazo[4,5-c]pyridines and die rearranged 3//-pyrido[3,2-[Pg.460]

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... 

The reaction of 7-chloro-4,6-dinitrobenzofurazan with 2-(4,-bromophenyl)indolizine has been shown to result in carbon-carbon bond formation by substitution of the 7-chloro substituent.58 However, in the reaction of 7-chloro-4,6-dinitrobenzofuroxan the initial substitution is followed by intramolecular oxygen transfer from the N-oxide function and by rearrangement to give the zwitterionic spiro-adduct (15). An... 

Keyes, 1965 Norris and Osmundsen, 1965 Boulton and Clifford, 1965). The NMR spectrum of the potassium salt in dimethyl sulphoxide shows bands at — 8-93 and — 6-20 p.p.m. due to ring protons and an absorption at —6-55 p.p.m. (which disappears in the salt formed in D20) attributed to the added hydroxyl group. The latter two resonances are spin-spin coupled (J = 7 Hz). This spectrum does not by itself distinguish 49 from 50. However, Brown and Keyes experiments with the 5-deuterio compound have established that the addition occurs at C3, to give 49. The high reactivity of 4,6-dinitrobenzofuroxan is shown by the observation that the adduct is almost quantitatively formed in water without added hydroxide ions. 

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>. 

S Ar reactions have been studied mostly with dinitro and trinitro activated benzenes. Recently, the aniline reactions with the highly activated super-electrophile, 4,6-dinitrobenzofuroxan (DNBF), have attracted considerable attention due to its ambident reactivity114. In acidic solvent, where aniline is almost completely protonated, the reaction by nitrogen attack is inhibited. However, a carbon-carbon bonded adduct is slowly formed by reaction at the 4-position of the aniline. Kinetic studies in H2O-DMSO have shown that this adduct formation involves two steps a carbon-carbon bond formation ( a), followed by elimination of a proton, Kiep (vide infra). The reactions of anilines with DNBF in DMSO have shown that the N-bonded adduct (a-N) is kinetically preferred, but the C-bonded adduct (a-C), formed in equilibrium with its deprotonated form, is the thermodynamically favored product (Scheme 14). 

In the absence of a leaving group, several well-defined reactions may still occur. The adducts (235) of 4,6-dinitrobenzofuroxan with hydroxide ion have long been known, and have received further study on account of their explosive properties the barium492 and potassium493 salts are the subjects... 

The kinetics and equilibria of the reaction of 4,6-dinitrobenzofuroxan with water (forming 235),494 496and of the cyclization of 7-(hydroxyethoxy)-4-nitrobenzofuroxan with alkali, in water and in dimethyl sulfoxide (Eq. II)497 have received detailed study. Proton NMR spectroscopy has shown that methoxide ions add more quickly to the 5-position of 7-methoxy-4-nitrobenzofuroxan, but the thermodynamic product is the 7,7-dimethoxy adduct (237) (Eq. 12).498 Methoxide adducts are also formed with 4-nitro-benzofuroxan.499 The impetus for much of this work was provided by the suggestion that antitumor and antileukemic properties which have been observed in some benzofuroxans might be connected with their ability to form Meisenheimer complexes of the types illustrated in this section.300,416,500... 

The SNAr reactions of heteroarenes can be realized in a similar manner, as in the series of arenes [71, 82]. These nucleophilic reactions are analogous to electrochemical Sn transformations of arenes [22, 24, 25]. Terrier and co-workers considered an opportunity for electrochemical methoxylation of 4,6-dinitrobenzofuroxan by action of the methoxide ion via the SnAt mechanism [21]. The intermediate o -complex formed was oxidized successfully into the corresponding substitution product. Analogously, the formation of heteroaromatic amines has been suggested to occur via intermediacy of the corresponding amino adducts, as exemplified by the oxidation of the o -complex derived fi om the reaction of pyrimidine with NH2 (Scheme 26) [3, 102-104]. 

There is continued interest in the super-electrophile4,6-dinitrobenzofuroxan (DNBF). Reaction in DMSO with aniline, which may show ambident nucleophilic character, initially yields the nitrogen-bound adduct (18) and then, more slowly, the carbon-bound adduct (19) in which the DNBF has effected electrophiUc substitution of hydrogen. 

An ab initio smdy suggests that the formation of a Diels-Alder adduct [i.e. (23)] from reaction between 1-methylpyrrole and dimethyl acetylenedicarboxylate is favoured over the formation of a possible Michael-type addition product (24). Both pathways involve an initial electrophilic substitution at the 2-position of the pyrrole to form a zwitterionic intermediate. A review, in Chinese, concerns a number of electrophilic substitutions of pyridine. A kinetic study has been made of the reaction between 4,6-dinitrobenzofuroxan (25) and various anilines in acidic H2O-DMSO... 

The potassium salt of 7-hydroxy-4,6-dinitrobenzofuroxan (KDNP) has a very similar chemical structure to KDNBF. However, as opposed to KDNBF which forms Meisenheimer adducts, this compound is aromatic. The aromaticity of KDNP has been confirmed by X-ray analysis [46]. 

The rearrangement of 2-phenyl-3-butyn-2-ol to 3-phenybut-2-en-l-al was examined computationally. The carbocation PhMeC+-C=CH is considered an intermediate or, under some conditions, a transition state. The stepwise Diels-Alder reaction of 1-trimethylsiloxy-1,3-butadiene and 4,6-dinitrobenzofuroxan was studied by computational methods. A stepwise mechanism with a persistent intermediate is supported. However, this intermediate is suggested to be a heterocyclic adduct and not the zwitterionic allyl cation/furoxan anion adduct proposed in the previous studies. 

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