Butyl Silsesquioxanes Synthesised in

The hydrolytic condensation of tert-butyltrichlorosilane ISu SiC l, in water was performed in a 25-fold up-scaling of the HTE synthesis. Bu SiCh is a solid at room temperature since the HTE workstation employed can only handle liquids, the tri-chlorosilane was dissolved in a minimum of acetonitrile. The same procedure was used for the up-scaled synthesis. After 18 h of reaction at 50 °C, the reaction mixture contained a white precipitate, which was isolated by filtration (fraction A). 

Besides the intrinsic value of the identification of a new, selective and high yield method to synthesise silsesquioxane Bu2Si20(0H)4, this experiment proved that tert-butyl silsesquioxane a2bA is a suitable precursor for titanium catalysts. Thus, silsesquioxane structures different from the known precursor silsesquioxane alb i [R7Si709(0H)3] [25, 26, 28] can effectively coordinate titanium centres to yield almost equally active epoxidation catalysts. 

Finally, the concept was broadened by supporting these titanium silsesquiox-anes on silica remarkably, the heterogeneous silica-supported catalysts displayed an epoxidation activity per mole of titanium (94% TBHP conversion after 3 h) similar to that of the homogeneous titanium-silsesquioxane a2b4 complexes, although with a lower selectivity towards 1,2-epoxyoctane (92%). These heterogeneous catalysts did not leach active species and proved to be recyclable [45]. 

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The highest catalytic activities were found for the titanium complexes obtained from tert-butyl silsesquioxanes synthesised in DM SO and water, with respectively 84% and 74% of the activity of the reference catalyst (c-C5H9)7Si7012Ti0C4H9. [With the experimental conditions employed, (c-C5H9)7Si7012Ti0C4H9 gives complete and selective conversion of TBHP towards the epoxide therefore, the relative activities of the reported catalysts correspond to their TBHP conversions towards 1,2-epoxyoctane.] These two catalysts exhibited almost the same activity as the previous best HTE catalyst (87%) obtained from cyclopentyl silsesquioxanes synthesised in acetonitrile [39, 44, 46] and are the first reported examples of tert-butyl silsesquioxanes as precursors for very active titanium catalysts. Relevant catalytic activities were also obtained with cyclohexyl silsesquioxanes synthesised in DM SO (67%) and with phenyl silsesquioxanes synthesised in H20 (61%). 

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