Theoretical studies on the gas‐phase smiles rearrangement

Gas‐phase Smiles rearrangements and SNi processes of ions C6H5X(CH2)nY− (X, Y = 0 or S, and n = 2–4) have been studied by the AM1 MO method. The Smiles rearrangement to an ortho position of the phenyl ring is diffcult owing to involvement of a high‐energy barrier process involving a 1,2‐hydrogen shift. The reactivity of the Smiles rearrangement to the ipso position increases in the order X  O and Y  S < X  S and Y  O < X  Y  0 as a result of steric and electronic effects in the transition state (TS). The reactivity order with respect to the side‐chain length, n = 3 < 4 < 2, is mainly determined by ring strain in the TS. For the SNi process, in which Y attacks the β‐carbon (β to phenyl), the enthalpy (δH≠) factor becomes more favourable but the entropy (δS≠) factor becomes unfavourable with an increase in the side‐chain length, in agreement with a general trend for cyclization processes. The product ratio in the Smiles rearrangement and the SNi process is largely controlled by the thermodynamic stabilities of the products, as observed in gas‐phase experiments.