Kinetic isotope effect in the reaction of 4-nitrophenylnitro-methane with the cyclic amidine base DBU [1,5-diazabicyclo(5,4,0)undec-5-ene]

Abstract

Kinetic deuterium isotope effects have been determined for the reaction of the cyclic amidine base DBU [1,5-diazabicyclo(5,4,0)undec-5-ene] with the carbon acid 4-nitrophenylnitromethane in toluene solution from –5 to +25°C. The isotopic rate ratio for the forward reaction (k^H_f/k^D_f) at 25°C is 13.3 ± 1.2; this is large enough to suggest an appreciable tunnelling correction. The activation enthalpies are strikingly low, both for proton and deuteron transfer: ΔH^‡H_f= 2.0 ± 0.2 kcal mol^–1(8.4 ± 0.8 kJ mol^–1) and ΔH^‡D_f= 3.6 ± 0.3 kcal mol^–1(15.1 ± 1.3 kJ mol^–1). The interpretation of these results is discussed.