Abstract
As part of our continuing studies into the electron deficient nature of five-membered ring heterocycles, specifically 2-methoxy-3-X-5-nitrothiophenes and 3-X-5-nitrothiophene, we report here a kinetic study of σ-complexation reactions involving 2-bromo-3,5-dinitrothiophene 1 and various nitroalkyl anions in aqueous solution at 20°C. Theoretical calculations at the density functional theory (DFT) level were performed to confirm the proposed mechanism. From the previously reported nucleophilicity parameters N and sN for the nucleophiles 2 and the derived second-order rate constants, the electrophilicity parameter (E) at the C-4 position of thiophene 1 was quantified according to the linear free enthalpy relationship log k (20°C) = sN (E + N). Mayr's approach accurately predicted the rate constants for the reactions of this thiophene 1 with a series of para-substituted phenoxide anions in water at 20°C. A reasonable correlation between calculated global electrophilicity index ω values and experimental electrophilic reactivities was observed and discussed.
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