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 /k D ¼ 5.68(W0.17) 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 [Et 3 N:]/[NPBA]. For 1 < [Et 3 N:]/[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 k H 2 O =k D 2 O ¼ 0.73(W0.04) and rates of exchange with D 2 O are equal to rates of racemization. Comparison with rates of racemization by acetate of the methyl ester yielded an effective molarity (EM ¼ 13.7) for the intramolecular carboxylate. The kie for racemizations of NPBA and 4-deutero-NPBA is k H /k D ¼ 5.78 at 25-C, and for 20 < T < 50-C, E D a S E H a ¼ 5.5(W0.1) and A H /A D ¼ 0.63(W1.03). For the acetate catalysed racemizations of the methyl ester, 25-C, k H /k D ¼ 7.43 with E D a S E H a ¼ 5.2 kJ mol S1 and A H /A D ¼ 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.

Solvent isotope and viscosity effects on the steady‐state kinetics of the flavoprotein nitroalkane oxidase

“…The possibility that the solvent isotope effect on the k cat /K M value arises from an effect on the free enzyme or substrate rather than the transition state must also be considered. Critically, the nonenzymatic deprotonation of nitroalkanes by a carboxylate shows an inverse solvent isotope effect of ~0.7 (32, 33). This suggests that the origin of this isotope effect is common to both the enzymatic and nonenzymatic reactions.…”

A pictorial representation of zero‐point energy and tunnelling contributions to primary hydrogen isotope effects

“…The measurements have raised the question of whether enzymes might facilitate tunnelling. With some notable exceptions45–47 it seems fair to say that this question has been addressed more actively by computational or theoretical chemists26, 33, 48 than by organic or inorganic chemists undertaking measurements of isotope effects for non‐enzymatic reactions. It would be a useful upshot of the symposium therefore if more experimentalists were encouraged to take an interest in the field.…”

Nitroalkane oxidase: Structure and mechanism

“…Solvent isotope effects have also been used to examine the mechanisms of the enzymatic and non-enzymatic reactions. Both the k cat /K m value for NAO with nitroethane as substrate [35] and the rate constant for nonenzymatic formation of nitroethane anion show solvent isotope effects of ~0.7 [36, 37]. This was ascribed to an effect of the solvent on the structure of nitroethane in solution rather than on the transition state for proton abstraction [35].…”