MECHANISMS OF FRONT-SIDE SUBSTITUTIONS - THE TRANSITION-STATES FOR THE S(N)I DECOMPOSITION OF METHYL AND ETHYL CHLOROSULFITE IN THE GAS-PHASE AND IN SOLUTION

We examined the front (S(N)i)- and back (S(N)2)-side substitutions of methyl and ethyl chlorosulfite in isolated and in simulated solution conditions using ab initio MO methods. All methyl chlorosulfite structures were optimized up to the MP2(fc)/6-31+G* level, but the changes are small with increased sophistication of the computational method. Consequently, we optimized ethyl chlorosulfite structures at HF/6-31G* and computed single point energies at MP2(fc)/G-31+G*//HF/6-31G*. Solution effects were simulated by means of single point energies on the HF/6-31G* optimized geometries using the self consistent reaction field (SCRF) model. Dielectric constants epsilon of hexane, carbon tetrachloride, sulfur dioxide, methanol, and water were employed. While the front-side substitution of methyl chlorosulfite is favored in the gas phase and in nonpolar solvents, back-side attack is preferred in polar solvents due to a more favorable stabilization of the back-side transition state. The S(N)i mode is preferred for ethyl chlorosulfite, in the gas phase and in all solvents; the cationic center is stabilized more by hyperconjugation than by solvent interaction.