VILSMEIER-HAACK FORMYLATION OF COUMARIN DERIVATIVES - A SOLVENT DEPENDENT KINETIC-STUDY

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-orde r in [VH reagent]. The reaction rates altered nonlinearly with an incr ease 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 spectro scopic results, participation of VH-adduct and coumarin molecule in th e rate limiting step, has been proposed. Kinetic and activation parame ters have been evaluated and discussed in terms of isokinetic relation ship and as a function of solvent compositions. Linearity of Leffler's and Exner's plots indicate a similar type of mechanism to be operativ e in different dielectric media al all temperatures. (C) 1996 John Wil ey & Sons, Inc.