Ranking the reactivity of superelectrophilic heteroaromatics on the electrophilicity scale

The kinetics of the reactions of a series of reference carbon nucleophiles, consisting of N-methylpyrrole A, indole B, N-methylindole C, and enamines D-G, with 10 electron-deficient aromatic and heteroaromatic substrates (1-10), resulting in the formation of stable anionic σ-adducts, have been investigated in acetonitrile at 20 °C. It is shown that the second-order rate constants k₁ pertaining to the carbon-carbon coupling step of these processes fit nicely the three-parameter equation log k (20 °C) = s(N + E), allowing the determination of the electrophilicity parameters E of 1-10 and therefore the ranking of these neutral electron-deficient compounds on the comprehensive electrophilicity scale defined for cationic electrophiles by Mayr et al. (Mayr, H.; Kempf, B.; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66). The E values of 1-10 are found to cover a range from -13 to -5, going from 1,3,5-trinitrobenzene 1, the least reactive molecule, to 4,6-dinitrotetrazolo[1, 5-a]pyridine 8, 4-nitro-6-trifluoromethanesulfonylbenzofuroxan 3, and 4,6-dinitrobenzofuroxan 2, the three most reactive heterocycles. Of major interest is that the E value of 2 is essentially the same as that for 4-nitrobenzenediazonium cation (E = -5.1), approaching that of the tropylium cation family (E ∼ -3 to -6) as well as a number of metal-coordinated carbenium ions. Such a ranking holds promise for expanding the range of coupling reactions which can be envisioned with such strongly electron-deficient neutral heteroaromatics as nitrobenzofuroxans and related compounds. Arguments are also given which exclude the possibility for the reactions studied to proceed via an electron-transfer mechanism.