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The primary kinetic hydrogen isotope effect in the deprotonation of a nitroalkane by an intramolecular carboxylate group
Author(s) -
Backstrom Nicholas,
Burton Neil A.,
Turega Simon,
Watt C. Ian F.
Publication year - 2008
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.1330
Subject(s) - chemistry , racemization , deprotonation , kinetic isotope effect , medicinal chemistry , intramolecular force , carboxylate , triethylamine , reaction rate constant , stereochemistry , organic chemistry , kinetics , deuterium , ion , physics , quantum mechanics
The rates of racemization of optically active nitropentanoic acid, and 4‐deuteronitropentanoic have been compared. The rate ratio (kie) is k H / k D = 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 [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^{ {\rm \bf H}_{\bf 2} {\rm \bf O}} /k^{{\rm \bf D}_2 {\rm \bf O}} $ = 0.73(±0.04) 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 = 13.7) for the intramolecular carboxylate. The kie for racemizations of NPBA and 4‐deutero‐NPBA is k H /k D = 5.78 at 25°C, and for 20 < T < 50°C, E a D − E a H = 5.5(±0.1) and A H / A D = 0.63(±1.03). For the acetate catalysed racemizations of the methyl ester, 25°C, k H /k D = 7.43 with E a D − E a H = 5.2 kJ mol −1 and A H / 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.