Quantum mechanical calculations have been performed on the migration step of the Baeyer-Villiger (BV) rearrangements of some acetophenones, p-RC6H4COCH3 (R = CN, Cl, H, CH3, CH3O) with m-chloroperbenzoic acid. The energy barriers, charge distributions, and frontier molecular orbitals determined for the aryl migration step explain the effects of substituents on the reactivity of these ketones. A plot of the log of relative oxidation rates of the ketones versus their corresponding calculated energy barriers of the migration stage showed a downward deviation for the p-OCH3 derivative. This result is consistent with a change in the rate-determining step, from the aryl migration to the carbonyl addition, in the case of p-methoxyacetophenone, according to the suggestion that the rate-determining step of the BV oxidation can change with variations in the substituent group.
The AM1 method has been used for investigating the effects of substituents on retro-Diels-Alder reactions, and the results have been compared with experimental data. Transition structures for butadiene reacting with ethylene and acrolein are quite close to those obtained with ab initio methods. The calculated asynchonicity of the retro-Diels-Alder reaction of substituted bicyclo[2.2.2]octa-2,5-dienes and ethanoanthracenes depends on substituents, with electron-donating groups making the T S more asymmetrically. The calculated activation energies for these reactions are too high compared to experimental data, but trends in relative activation energies for different substituents are reproduced reasonably well, although there are exceptions. Calculated activation entropies for unsubstituted systems are in good agreement with ab initio values and experimental data, but the variation of activation entropy with substituents is not reproduced.
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