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Primary kinetic hydrogen isotope effects in deprotonations of a nitroalkane by intramolecular phenolate groups
Author(s) -
Backstrom Nicholas,
Burton Neil A.,
Watt C. Ian F.
Publication year - 2010
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1631
Subject(s) - chemistry , deprotonation , kinetic isotope effect , intramolecular force , medicinal chemistry , reactivity (psychology) , reaction rate constant , aqueous solution , nitronate , protonation , stereochemistry , organic chemistry , ion , kinetics , nitro , deuterium , alkyl , medicine , physics , alternative medicine , quantum mechanics , pathology
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.