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

Rate 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 = NO 2 , 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 = NO 2 , 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 k H / k D (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), k H / k D (25 °C) = 7.8, with E   a D  −  E   a H  = 6.9 kJ mol −1 and A H / A D  = 0.5. For 3 (X = NO 2 , L = H or D), rates of intramolecular deprotonation are reduced 30‐fold, and an elevated kinetic isotope effect is found ( k H / k D (25 °C) = 10.7). Activation parameters ( E   a D  −  E   a H  = 17.8 kJ mol −1 and A H / A D  = 0.008) are compatible with an enhanced tunnelling contribution to reactivity in the H‐isotopomer. Copyright © 2009 John Wiley & Sons, Ltd.