Computational Study of the Reaction between Singlet Silylene and Propene and of Rearrangement Reactions of Methylsilacyclopropane

The reaction between singlet silylene and propene has been studied by CASPT2N/6-31G*//CASSCF/6-31G* and MP2/6-31G*//RHF/6-31G* calculations where also ring-closing and ring-opening reactions of the resulting methylsilacyclopropane have been included. The predicted reaction energy for the addition is 49.1 kcal/mol, modified to 44.7 kcal/mol when the ΔZPE correction is included. ΔZPE-corrected barriers obtained are 11.3 and 13.2 kcal/mol for the concerted ring closing and hydrogen migration in methylsilacyclopropane starting from the silylenes 4 and 5, respectively. These values are in good agreement with experimental estimates of 10.4 kcal/mol for the corresponding reaction of alkylsilylenes. The diradicals 6 and 7 resulting from bond ruptures in methylsilacyclopropane have energies that are around 30 kcal/mol above the TS's for the reactions leading to the silylenes. Carbenes, which are also conceivable as reaction products, have energies that are about 40 kcal/mol above the same TS's. Calculations using QCISD(T)/cc-pVDZ//MP2/cc-pVDZ gave a reaction energy of 36.0 kcal/mol, and ring-closing barriers of 12.9 and 14.3 kcal/mol, respectively. The energies of the diradicals 6 and 7 are around 20 kcal/mol above the TS's for the concerted ring opening and hydrogen migration using this method.