Reactions of Nitroheteroarenes with Carbanions: Bridging Aromatic, Heteroaromatic, and Vinylic Electrophilicity

Chemistry, 2008, 14, 6108-18 published on 30.05.2008
Chem. Eur. J.

The relative rate constants for the vicarious nucleophilic substitution (VNS) of the anion of chloromethyl phenyl sulfone with a variety of nitroheteroarenes, for example, nitropyridines, nitropyrroles, nitroimidazoles, 2-nitrothiophene, and -nitropyrazole, have been determined by competition experiments. It was shown that nitropyridines are approximately four orders of magnitude more reactive than nitrobenzene. Among the fivemembered heterocycles 2-nitrothiophene is the most active followed by nitroimidazoles and 4-nitropyrazole. Nitropyrroles are the least electrophilic nitroheteroarenes with reactivities comparable to nitrobenzene. Quantum chemically calculated methyl anion affinities (B3LYP/6–311G(d,p)//B3LYP/ 6–31G(d)) of the nitroarenes correlated only moderately with the partial relative rate constants. The correlation of these activities with the LUMO energies of nitroarenes is even worse. By measuring the second- order rate constants of the addition of 1_ to nitroarenes and to diethyl arylidenemalonates 10, it was possible to link the electrophilic reactivities of nitroheteroarenes with the comprehensive electrophilicity scale based on the linear- free-energyrelationship log k(20 °C) = s(N + E).

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