Direct Observation of the Molecular Structural Changes during the Claisen Rearrangement Including the Transition State

The detailed processes in the Claisen rearrangement were observed. The process was vibrationally excited in the electronic ground state by a stimulated Raman process using a 5-fs pulse. The Claisen rearrangement was found to follow a three-step pathway. At first, the C4–O bond is weakened to generate a bis-allyl-like intermediate. Next, the formation of a weak C1–C6 bond results in the generation of an aromatic-like intermediate. Finally, C4–O breaking and C1–C6 formation occur simultaneously to generate the product.