Kinetics, isotope effects, and mechanism of the reaction of 1-nitro-1(4-nitrophenyl)alkanes with DBU in acetonitrile

Abstract

The kinetics of the reaction of O₂NC₆H₄C(L)(R)NO₂(R = Me, Et, Prⁱ, NNPE, NNPP, or MNNPP, respectively; L = H, D) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in acetonitrile (MeCN) are reported. The nature of the product indicates that substantial dissociation into free ions occurs. The usefulness of Benesi–Hildebrand relationship for distinguishing between ion pairs and ions of the product is discussed in detail.

The reaction shows low activation enthalpy value ΔH^‡= 15.4, 17.8, and 19.9 kJ mol^–1 and large negative entropies of activation ΔS^‡=–131, –134, and –147 J mol^–1 K^–1 for NNPE, NNPP, and MNNPP respectively.

The kinetic isotope effects k_H/k_D(12.5, 12.4, and 12.3) are large, showing no variation, the more sterically hindered the substrate. The values of the isotope effects exerted on the activation parameters indicate the contribution of a tunnelling effect Q_H/Q_D= 1.35 at 25 °C.

The influence of water on the kinetics is also examined and discussed with respect to reliability of kinetic measurements of such reactions systems.