Primary kinetic hydrogen isotope effects in deprotonations of a nitroalkane by intramolecular phenolate groups

Abstract

AbstractRate constants and kinetic isotope effects have been determined for the formation of nitronate anions from the ethers 1‐(2‐methoxyphenyl)‐2‐nitropropane, 7(X = H, L = H and D) and 1‐(2‐methoxy‐5‐nitrophenyl)‐2‐nitropropane, 7(X = NO2, L = H and D), and from the corresponding phenols, 1‐(2‐hydroxyphenyl)‐2‐nitropropane, 3(X = H, L = H and D), and 1‐(2‐hydroxy‐5‐nitrophenyl)‐2‐nitropropane, 3(X = NO2, L = H and D), in aqueous basic medium. For the ethers 7, rates of deprotonation by hydroxide are comparable with those found for deprotonations of 2‐nitropropane, with kH/kD (25 °C) = 7.7 and 7.8, respectively. In both the cases, the isotope effects are conventionally temperature dependent. For the corresponding phenols 3, conditions have been established under which the deprotonations of the nitroalkane are dominated by intramolecular deprotonation by the kinetically first‐formed phenolate anion, with an estimated effective molarity EM ∼ 250. For 3 (X = H, L = H or D), kH/kD (25 °C) = 7.8, with E − E = 6.9 kJ mol−1 and AH/AD = 0.5. For 3(X = NO2, L = H or D), rates of intramolecular deprotonation are reduced 30‐fold, and an elevated kinetic isotope effect is found (kH/kD (25 °C) = 10.7). Activation parameters (E − E = 17.8 kJ mol−1 and AH/AD = 0.008) are compatible with an enhanced tunnelling contribution to reactivity in the H‐isotopomer. Copyright © 2009 John Wiley & Sons, Ltd.