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

Abstract

The rates of racemization of optically active nitropentanoic acid, and 4-deuteronitropentanoic have been compared. The rate ratio (kie) is k H/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 dimethox-yethane: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 kH2O/kD2O = 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, EaD - EaH = 5.5(±0.1) and AH/AD = 0.63(±1.03). For the acetate catalysed racemizations of the methyl ester, 25°C, k H/kD = 7.43 with EaD - E aH = 5.2 kJ mol-1 and AH/A D = 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