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 usi ng an ab initio molecular orbital method. MP2/6-31 + G//HF/6-31G* cal culations predict that all three groups migrate from the Si to an adja cent C-alpha with no energy barrier. Thus, the silicenium ion is the o nly stable species in each potential energy surface. The conformation of the benzylsilicenium ion, (C6H5)CH2-SiH2+, indicates that the pheny l ring is significantly bent toward the silyl cationic center in order to interact with the vacant 3p(Si+) orbital. In contrast to MP2 resul ts, Hartree-Fock calculations (both HF/3-21G and HF/6-31G* levels) pr edict small energy barriers for 1,2-migrations of H and Me (1.4 kcal m ol(-1) for H migration, and 1.5 kcal mol(-1) for Me migration, respect ively, at the HF/6-31G level). This difference provides convincing ev idence that the incorporation of electron correlation is of particular importance in describing the potential energy surface for the rearran gement 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 s pecies. Various factors which may govern the migratory aptitudes of va rious R groups, i.e. (1) activation energies, (2) overall reaction ene rgies and (3) the conformational preference of reactants have been inv estigated. The calculated activation energy obtained, namely the energ y for the generation of the silicenium ion and the Cl- ion from an cu- chlorosilane, is consistent with the experimental migratory aptitude i n the gas phase observed in mass spectrometers.