Kinetic Isotope Effects in the Reactions of 4-Nitrophenylnitromethane with Alkylamidine Bases in Toluene

Abstract

Kinetic isotope effects have been determined for the proton-transfer and deuteron-transfer reactions of 4-nitrophenylnitromethane (4-NPNM; NO2C6H4CH2NO2, NO2C6H4CD2NO2) with two alkylamidine bases [HN = C(NEt2)R; R = n-butyl, n-nonyl] in toluene solution. The observed values of the isotopic rate ratio kHf/kDf at 25°C are 11.7 ± 1.0 and 8.0 ± 0.3, respectively. The former value indicates an appreciable tunnel correction; this is confirmed by a value of (ΔH[graphic omitted]D-ΔH[graphic omitted]H)= 2.5 ± 0.8 kcal mol–1(10 ± 3 kJ mol–1). The isotopic differences are not precise enough to allow calculations of the dimensions of the energy barrier.