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

Abstract

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 σ-complex C-4a which is half-formed in a 0.4 M HCl solution (pK^σ_a = 0.4). This corresponds to a 3 pK-units jump in thermodynamic stability from the most stable hydroxy σ-complex known so far, i.e. the hydroxy adduct C-3a of 4,6-dinitrobenzofuroxan (DNBF). DNTP forms similarly a very stable methoxy σ-complex C-4b in methanol (pK^σ_a = 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 k^H₂O₁ and k^MeOH₁ for formation of C-4a and C-4b have been measured: k^H₂O₁ = 1.93 s⁻¹; k^MeOH₁ = 3.50 s⁻¹, to be compared with k^H₂O₁ = 0.035 s⁻¹; k^MeOH₁ = 0.030 s⁻¹ for σ-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.