Vilsmeier-Haack formylation of coumarin derivatives. A solvent dependent kinetic study

Article abstract of DOI:10.1002/(SICI)1097-4601(1996)28:12<865::AID-KIN1>3.0.CO;2-L

A kinetic study of the reaction of coumarin derivatives with Vilsmeier-Haack (VH) reagent (1 : 1 DMF-POCl₃) in various solvent media revealed second-order kinetics with a first-order in [Substrate] and first-order in [VH reagent]. The reaction rates altered nonlinearly with an increase in the dielectric constant of the medium and the data did not fit completely well with either Amis or Kirkwood's theories of ion-dipole and dipole-dipole type reactions. On the basis of kinetic and spectroscopic results, participation of VH-adduct and coumarin molecule in the rate limiting step, has been proposed. Kinetic and activation parameters have been evaluated and discussed in terms of isokinetic relationship and as a function of solvent compositions. Linearity of Leffler's and Exner's plots indicate a similar type of mechanism to be operative in different dielectric media at all temperatures.