An extremely highly electrophilic heteroaromatic structure: 4,6-dinitrotetrazolo[1,5-a]pyridine.

A detailed thermodynamic and kinetic investigation of the reactions of 4,6-dinitrotetrazolo[1 ,5-a]pyridine (DNTP) with water and methanol has been made in the corresponding solvents. In aqueous solution, covalent addition of water to DNTP occurs with the exclusive formation of an anionic hydroxy sigma-complex C-4a which is half-formed in a 0.4 M HCl solution (pKa(sigma) = 0.4). This corresponds to a 3 pK-units jump in thermodynamic stability from the most stable hydroxy sigma-complex known so far, i.e. the hydroxy adduct C-3a of 4,6-dinitrobenzofuroxan (DNBF). DNTP forms similarly a very stable methoxy sigma-complex C-4b in methanol (pKa(sigma) = 2.64). Interestingly, the addition of methanol to DNTP also results in the partial formation of a neutral carbinolamine-type adduct (C-5b) at low pH. Rate and equilibrium constants pertaining to most of the reaction pathways involved in the interactions have been determined. In particular, the following rate constants k1(H2O) and k1(MeOH) for formation of C-4a and C-4b have been measured: k1(H2O) = 1.93 s(-1); k1(MeOH) = 3.50 s(-1), to be compared with k1(H20) = 0.035 s(-1); k1(MeOH) = 0.030 s(-1) for sigma-complexation of DNBF under similar experimental conditions. Altogether, the results obtained reveal that DNTP is a considerably more powerful electrophile than DNBF, the common reference as to whether an electron-deficient aromatic or heteroaromatic substrate may be accorded superelectrophilic properties in addition or substitution processes.