Relative reactivities and activation parameters for the Vilsmeier–Haack formylation of five-membered heteroaromatic compounds containing the Group VI elements

Abstract

Rate constants and activation parameters are determined for the formylation by the HCONMe₂–COCl₂ complex in CHCl₃, of furan, thiophen, selenophen, and tellurophen. ΔS^‡ Values are identical within experimental error, and the reaction is enthalpy controlled. The activation entropies are discussed in terms of the position of the transition states along the reaction co-ordinate, whereas ΔH^‡ values are correlated with ‘empirical resonance energies’ and ΔE_loc. The results indicate that the transition states for all four compounds lie in a similar position along the reaction co-ordinate and that the differences in ground state energy play a fundamental role in determining the relative reactivities of the α-positions.