Cyclopentadienyl–Silsesquioxane Titanium Catalysts: Factors Affecting Their Formation and Activity in Olefin Epoxidation with Aqueous Hydrogen Peroxide

The reaction of titanium chlorosilyl-substituted cyclopentadienyl (Cp) complexes, Ti(η⁵-C₅H₃R′SiMe₂Cl)Cl₃(R′ = H, 1b; SiMe₃, 1c), with 1 equiv. of various silsesquioxane trisilanols, R₇Si₇O₉(OH)₃(R = iBu, 2a; Ph, 2b), affords either corner-capped Cp derivatives, Ti(η⁵-C₅H₃R′SiMe₂Cl)(R₇Si₇O₁₂-κ³O₃) (R′ = H, R = iBu, 5a, Ph, 5b; R′ = SiMe₃, R = iBu, 7a, Ph, 7b), or cyclopentadienyl–silsesquioxane complexes, Ti(η⁵-C₅H₄SiMe₂OR₇Si₇O₁₁-κ²O₂)Cl (R′ = H, R = iBu, 6a, Ph, 6b; R′ = SiMe₃, R = iBu, 8a, Ph, 8b), depending on the reaction conditions. In any case, upon heating, kinetic products 5 and 7 are transformed into the corresponding thermodynamic products 6 and 8, respectively. The electron-donating ability of the Cp ring is a relevant controlling parameter: a strong π-donor character facilitates the isomerization process. In addition, the nature of the silicon substituents in the silsesquioxane framework, the type of solvent, and the reaction temperature are also factors that significantly affect this process. Cyclopentadienyl–silsesquioxane complexes 6b and 8b are efficient and selective catalysts for epoxidation with aqueous hydrogen peroxide under mild conditions. Such a catalytic efficiency is attributed to the hydrophobic environment generated about the titanium atom by the Cp ring incorporated into the cyclopentadienyl–silsesquioxane ligand.