Mechanistic Insight into the Formal [1,3]-Migration in the Thermal Claisen Rearrangement

摘要

The thermal formal [1,3]-sigmatropic shift ofnallyl aryl ethers has been studied in depth experimentally withnthe aid of the density functional theory (DFT) calculations ofnthe B3LYP function. Three mechanistic possibilities, referred tonas the radical, ionic, and concerted mechanisms, have previouslynbeen put forth to explain the thermal [1,3]-rearrangementnprocess. However, the intercrossing and radical trappingnexperiments indicate the rearrangement is an intramolecularnprocess. The computational studies reveal that the concertednC[1,3]-sigmatropic shift suffered from a higher energetic barriernto allow the rearrangement to proceed under the conditionsnused. However, a tandem O[1,3]-sigmatropic shift with anconfiguration inversion of the oxygen atom and [3,3]-nsigmatropic shift (the Claisen rearrangement) is the most likely pathway for the formal [1,3] rearrangement. Furthermore,nthe rearrangement experiments with a designed optically active substrate and O[1,3]-sigmatropic shift examples verify the newncascade rearrangement. In addition, computational and experimental studies indicate that water molecule assists the proton shiftnduring the isomerization. The combined methods provide the new insight into the mechanism of the thermal formal [1,3]-nmigration in the Claisen rearrangement and the novel O[1,3]-sigmatropic shift as well.