DETERMINATION OF REACTIVITY BY MO THEORY .71. THEORETICAL-STUDIES ON THE GAS-PHASE SMILES REARRANGEMENT

Gas-phase Smiles rearrangements and S(N)i processes of ions C6H5X(CH2)(n)Y- (X, Y = O 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 difficult 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 = O 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 S(N)i process, in which Y attacks the beta-carbon (beta to phenyl), the enthalpy (DELTA-H double-ended dagger) factor becomes more favourable but the entropy (DELTA-S double-ended dagger) 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 S(N)i process is largely controlled by the thermodynamic stabilities of the products, as observed in gas-phase experiments.