Dual behavior of 4-aza-6-nitrobenzofuroxan.: A powerful electrophile in hydration and σ-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% H2SO4(H-0 similar to -3) for complete recovery of 2). The equilibrium constant K-H2O for hydration of 2 is estimated to be greater than or equal to 100, being comparable only with the K-H2O 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 (pK(a)(NH) = 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 bf 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 Dials-Alder trapping;of the o-dinitroso intermediate involved in the exchange of the 1-oxide and 3-oxide tautomers of 2.