Acid catalysed ortho-Claisen rearrangement of allyl aryl ethers in trifluoroacetic acid

Abstract

The rearrangement of allyl aryl ethers in trifluoroacetic acid (TFA) has been examined by kinetic and product studies. The addition of a co-solvent dramatically decreases the rate of rearrangement at 60°. In CDCl₃–TFA mixtures, the first-order rate constant was found to increase exponentially with the fraction of TFA present. The presence of a small amount of water (optimum 5%) accelerates while greater concentrations decrease the rate. Electron-donating substituents para to the ether group, and to the ortho-position at which new bond formation takes place affect the rate to nearly equal extents and a linear hammett plot was obtained by plotting (σ_p+σ_m)/2 versus log k. A large salt effect with an order of magnitude increase in k in the presence of LiClO₄(0·05M) and a deuterium solvent kinetic isotope effect of 1·4 were observed. It was concluded that the mechanism of the rearrangement involves a highly polar transition state with the degree of proton transfer depending upon the conditions, presence of salt, H₂O concentration, etc. A complication which can lower yields if care is not exercised, is the addition of TFA to the side chain double bond of the rearrangement product.