Electrochemical oxidation of sigma-complex-type intermediates in aromatic nucleophilic substitutions.

A series of sigma-adducts (1H-...7H-) derived from the addition of 2-nitropropenide ion to various nitrobenzofuroxans and nitrobenzofurazans have been oxidized electrochemically. The results show that the rearomatization of the carbocyclic ring of these adducts as well as that of a few additional 4,6-dinitrobenzofuroxan adducts (8 H- a-c) is associated with much higher oxidation potentials than found for the same process in the dinitro- and trinitrobenzene series. Especially high Eo values are measured for the oxidation of the 2-nitropropenide 4,6-dinitro- and 4-nitro-6-trifluoromethylsulfonylbenzofuroxan adducts 1H- and 4H- in acetonitrile: E (1H-)= 1.15 V versus SCE; Eo(4H-)=1.33V versus SCE. These values fit well with the available evidence that the chemical oxidation of these adducts requires the use of very strong oxidizing agents to proceed efficiently. The mechanism for the oxidation process has been established. It is shown to involve transfer of two electrons and liberation of one proton per sigma-complex precursor with no evidence whatsoever for the intermediacy of radical anionic species.

Dual behavior of 4-aza-6-nitrobenzofuroxan. A powerful electrophile in hydration and sigma-complex formation and a potential dienophile or heterodiene in diels-alder type reactions

In investigating the reactivities of aza analogues of super-electrophile 4,6-dinitrobenzofuroxan (DNBF, 1), we have found that the nitro-substituted pyridofuroxan 2 gives a remarkably stable hydrate 3 in aqueous solution (as evidenced by the requirement of ca. 50% H(2)SO(4) (H(0) approximately -3) for complete recovery of 2). The equilibrium constant K(H)()2(O) for hydration of 2 is estimated to be >/= 100, being comparable only with the K(H)()2(O) values reported for hydration of highly activated neutral polyazaaromatics such as 2- and 6-hydroxypteridines or 7-azapteridine. Interestingly, the NH group of 3 undergoes ionization at rather low pH (p = 5.79), affording an anionic hydroxy sigma-adduct 4 which is thermodynamically 10(8) times more stable than the related sigma-adduct of pteridine. The experimental evidence is that 4 is slightly more stable than the hydroxy sigma-adduct of DNBF, indicating not only that 2 ranks among the most electrophilic heteroaromatics known to date but also than an aza group may in fact be as efficient as a nitro group in promoting sigma-complex formation. 2 is also found to be a versatile Diels-Alder reagent, as a result of the low aromatic character of its six-membered ring. Upon treatment of 2 with cyclopentadiene and 2,3-dimethylbutadiene, various reactivity patterns have been observed. These led to different cycloadducts arising from normal as well as inverse electron-demand condensations involving the pyridine ring as the dienophile or the heterodiene contributor. Altogether, the results reveal major differences between the reactivity of 2 and that of DNBF, with in particular a remarkable tendency of the pyridofuroxan adducts to undergo covalent hydration, resulting in the formation of stable carbinolamines. Also noteworthy is the characterization of a diadduct which results from a Diels-Alder trapping of the o-dinitroso intermediate involved in the exchange of the 1-oxide and 3-oxide tautomers of 2.