Ab initio study of silyloxonium ions

The geometries and proton and silylenium cation (H3Si+) affinities as well as ring strain energies of several hydrogen-substituted cyclic siloxanes, cyclotrisiloxane (H2SiO)(3) (5a), cyclotetrasiloxane (H2SiO)(4) (6a), and 1,3-oxadisilacyclopentane (7a), were calculated by ab initio quantum-mechanical methods using the polarized 6-31* basis set. Protonation and silylenium cation addition to siloxanes lead to secondary and tertiary silyloxonium ions, respectively. The calculated strain energies follow the order: 7a > 5a > 6a approximate to 0. Upon protonation or silylation, the strain in the five-membered ring of 7b and 7c is significantly reduced, while in the cyclotrisiloxane silyloxonium ions 5b,c the strain is preserved. The endocyclic Si-O bonds in 7a and 5a are weakened upon protonation or addition of H3Si+ more than the exocyclic bonds and are therefore predicted to be cleaved more readily by nucleophiles, resulting in a ring opening rather than in splitting of the exocyclic SiH3 group. 7a is by ca. 10 kcal/mol more basic than the other siloxanes due to the angular strain in the five-membered ring. Its basicity is comparable to that of dialkyl ethers and alkoxysilanes. A linear correlation was found between the gas-phase proton and H3Si+ affinities. On the basis of SCRF calculations interaction with solvent (cyclohexane or CH2Cl2) has only a moderate effect on the energies of protonation and ring-opening reactions. The role of silyloxonium ions as possible active centers in the cationic ring-opening polymerization of cyclosiloxanes is discussed in light-of the calculated basicities and ring strain energies. The calculations suggest that the polymerizations of the cyclic monomers 5a, 6a, and 7a should reveal different kinetic and thermodynamic behavior. 7a is predicted to be the most reactive monomer, and its polymerization is the most favored thermodynamically.