| Abstract
| - 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-deficientaromatic and heteroaromatic substrates (1−10), resulting in the formation of stable anionicσ-adducts, have been investigated in acetonitrile at 20 °C. It is shown that the second-order rateconstants 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 electrophilicityparameters E of 1−10 and therefore the ranking of these neutral electron-deficient compounds onthe 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 arange from −13 to −5, going from 1,3,5-trinitrobenzene 1, the least reactive molecule, to4,6-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 valueof 2 is essentially the same as that for 4-nitrobenzenediazonium cation (E = −5.1), approachingthat of the tropylium cation family (E ∼ −3 to −6) as well as a number of metal-coordinatedcarbenium ions. Such a ranking holds promise for expanding the range of coupling reactions whichcan be envisioned with such strongly electron-deficient neutral heteroaromatics as nitrobenzofuroxans and related compounds. Arguments are also given which exclude the possibility for thereactions studied to proceed via an electron-transfer mechanism.
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