The primary kinetic hydrogen isotope effect in the deprotonation of a nitroalkane by an intramolecular carboxylate group

Abstract

AbstractThe rates of racemization of optically active nitropentanoic acid, and 4‐deuteronitropentanoic have been compared. The rate ratio (kie) is kH/kD = 5.68(±0.17) at 31°C, in good agreement with that determined by Lewis et al. for base‐catalysed deprotonations using iodine‐trapping methods. In a more detailed study, optically active 4‐nitro‐4‐phenylbutanoic acid (NPBA) has also been prepared and rates of racemization measured in dimethoxyethane:water. With less than a full equivalent of triethylamine, rates are proportional to [Et3N:]/[NPBA]. For 1 < [Et3N:]/[NPBA] < 2, rates are independent of the ratio, consistent with racemization being dominated by deprotonation of the nitroalkane by the intramolecular carboxylate group. The solvent isotope effect is = 0.73(±0.04) and rates of exchange with D2O are equal to rates of racemization. Comparison with rates of racemization by acetate of the methyl ester yielded an effective molarity (EM = 13.7) for the intramolecular carboxylate. The kie for racemizations of NPBA and 4‐deutero‐NPBA is kH/kD = 5.78 at 25°C, and for 20 < T < 50°C, E − E = 5.5(±0.1) and AH/AD = 0.63(±1.03). For the acetate catalysed racemizations of the methyl ester, 25°C, kH/kD = 7.43 with E − E = 5.2 kJ mol−1 and AH/AD = 1.08. In neither case is there any indication of a major tunnelling contribution on the isotopic rate ratio. A hitherto unrecognised mode of decomposition of nitronic acids, involving direct reaction with dissolved oxygen, has been identified. Copyright © 2008 John Wiley & Sons, Ltd.