A Theoretical Study of the Mechanism for Allylic Ether Isomerization

The reaction mechanism of allylic ether isomerization has been investigated by MP2 and DFT method (different functionals) with 6-31++G(d,p) basis set. The calculated results show that M06-2X method that designed to treat dispersion and hydrogen-bonded systems do better than traditional functional-B3LYP for the calculated energetic and structural properties of allylic ether isomerization. The optimal structures of allylic ether and transition states were located and the reaction Gibbs free energy barriers were predicted at the MP2 and M06-2X level. Furthermore, the possible reaction pathways and mechanisms were proposed to explain the origin of regioselectivity observed in experiment. The calculation results show that the isomerization reaction will not readily occur in the absence of catalysis by Au. The computed potential energy barrier is quite high, and things get better when alcohol molecules are introduced, resulting in the decrease of calculated activation free energy from 67.1 to 48.6 kcal/mol. However, the Au(I)-catalyzed addition of another molecule of alcohol to an allylic ether can occur readily. A protonated diether intermediate was stabilized by a hydrogen bond and the activation energies of allylic ether isomerization were dramatically decreased, only 7.5 kcal/mol. By contrast the isomerization effect under with and without alcohol, gold catalysis, the results indicate that the allylic ether isomerization involve cationic gold coordination and proton shift reaction process, which form the intermediate that allows the interconversion of the products. This reaction mechanism can successfully explain the observed regioselectivity for the thermodynamic product. Meanwhile, the results also show that the isomerization was completely inhibited with the excess alcohol due to competing gold coordination between alcohol and ether. The discovery of gold catalysts in allylic ether isomerization not only contributes to the development of catalysts from the usual transition metals to noble metals, but also shows the potential catalytic activities by switching the reaction conditions.