Meisenheimer-type adducts from thiophene derivatives. Part 7. Interdependence of 13C NMR, thermodynamic and kinetic data

Abstract

A ¹³C NMR study has been carried out in (CD₃)₂SO on the thiophene derivatives 5 and 7 and the related Meisenheimer adducts 6 and 8. The data obtained, and in particular the sums of the chemical-shift changes (ΣΔδ) at C-3, C-4 and C-5, accompanying the formation of the adducts, are compared with the corresponding data previously obtained for analogous substrates 1 and 3 and adducts 2 and 4. The four series of ΣΔδ values are then examined in the light of the already reported thermodynamic (K_e) and kinetic (k₁) constants for the formation of the adducts. For a given X-substituent, log K_e values increase linearly as the corresponding absolute ΣΔδ values decrease. On these grounds, the smaller π-electron-density rearrangement accompanying the formation of gem-dimethoxy adducts emerges as an important factor, contributing to the larger observed K_e values. A further outcome is that, within a single series of adducts, a linear relationship of either log K_e or log k₁ with Δδ holds only when excluding the terms with X = Ac or NO₂. Such findings are interpreted on the basis of the two-component nature of the substituent –R effects: while for the nitro and the acetyl substituents the π-electron acceptor component is prevalent, the stabilizing effect of the other substituents is due mainly to a charge–dipole interaction. The analysis of some literature data for the benzene series fits the above conclusions.