The role of cesium fluoride in aryl propargyl ether Claisen rearrangement and its mechanistic elucidation: a theoretical study

Abstract

The role of cesium fluoride (CsF) in aryl propargyl ether Claisen rearrangement and its mechanistic pathway have been investigated in gas and solvent phase using the density functional theory implemented in Gaussian 09. Our results indicate that the [3,3]-sigmatropic rearrangement is the rate-limiting step with Delta G (aEuro) value of 37.1 kcal/mol in solvent phase. Furthermore, the results show that the enolization of alpha-allenylketone intermediate (Int1-CsF) has a higher free energy barrier, which implies that the formation of benzopyran is not favored in the presence of CsF. However, the abstraction of the alpha-hydrogen atom in Int1-CsF with CsF shows a very low free energy barrier and is the most favored pathway for aryl propargyl ether Claisen rearrangement in the presence of CsF to form benzofuran. In the case of substituted aryl propargyl ethers, a methoxy group on the benzene ring lowers the activation barrier. The HOMO-LUMO, conformational and NBO analysis indicate that increasing methyl substitution on the propargyl residue enhances the rearrangement reaction.