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 (Ke) and kinetic (k1) constants for the formation of the adducts. For a given X-substituent, log Ke 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 Ke values. A further outcome is that, within a single series of adducts, a linear relationship of either log Ke 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.