Unimolecular rearrangement of alpha-silylcarbenium ions. An ab initio study

The unimolecular rearrangements of hydrogen, methyl and phenyl groups at the Si atom in alpha-silylcarbenium ions have been investigated using an ab initio molecular orbital method. MP2/6-31 + G*//HF/6-31G* calculations predict that all three groups migrate from the Si to an adjacent C-alpha with no energy barrier. Thus, the silicenium ion is the only stable species in each potential energy surface. The conformation of the benzylsilicenium ion, (C6H5)CH2-SiH2+, indicates that the phenyl ring is significantly bent toward the silyl cationic center in order to interact with the vacant 3p(Si+) orbital. In contrast to MP2 results, Hartree-Fock calculations (both HF/3-21G* and HF/6-31G* levels) predict small energy barriers for 1,2-migrations of H and Me (1.4 kcal mol(-1) for H migration, and 1.5 kcal mol(-1) for Me migration, respectively, at the HF/6-31G* level). This difference provides convincing evidence that the incorporation of electron correlation is of particular importance in describing the potential energy surface for the rearrangement of alpha-silylcarbenium ions to silicenium ions. The results of the calculations have also been applied to the possible rearrangement mechanism of alpha-chlorosilanes to chlorosilanes, assuming that the experimental conditions are favorable toward the generation of ionic species. Various factors which may govern the migratory aptitudes of various R groups, i.e. (1) activation energies, (2) overall reaction energies and (3) the conformational preference of reactants have been investigated. The calculated activation energy obtained, namely the energy for the generation of the silicenium ion and the Cl- ion from an cu-chlorosilane, is consistent with the experimental migratory aptitude in the gas phase observed in mass spectrometers.