Silsesquioxanes : alternative approaches and methods of synthesis.

Maesano, Maria Grazia (2005). Silsesquioxanes : alternative approaches and methods of synthesis. PhD thesis The Open University.



The synthesis of silsesquioxanes from chlorosilanes has been investigated in this thesis using the “non aqueous” hydrolysis approach.
DMSO (dimethylsulfoxide) has been used as a source of oxygen for the “non aqueous” hydrolysis involving di- and trichlorosilanes. With dichlorosilanes the reaction led to D rings and in particular to a D3 ring according to the following scheme:

O Cl Me2 SiCL, + H3 C-S-CH 3 (Me2 SiO)n + HjC-S^CH, Cl'

Under the conditions of the reaction, D3 is converted into larger and more stable rings such as D4 and D5. Other oxygen donor compounds have been studied and, as for DMSO, the result was the formation of D3 which was subsequently converted into larger rings. The mechanism has also been investigated and we found that D ring formation occurs via the formation of a,co-dichlorosiloxanes which are converted to D rings via an intramolecular cyclisation. DMSO has also been used to react with trichlorosilanes leading to cages. As for the conventional hydrolysis we expected the formation of T8 cages but to our surprise we obtained T6. Several trichlorosilanes are commercially available and so we performed the “non aqueous” hydrolysis using trichlorosilanes with a range of substituents. This provided a route to different T6s from the trichlorosilane which have previously been unobtainable. We also synthesised trichloroilanes via hydrosilylation in order to obtain trichlorosilanes carrying other functionalities which could be used to synthesise a new set of T6s which again could not be prepared by ordinary hydrolysis. We performed the “non aqueous” hydrolysis of trichlorosilanes using the other oxygen donor compounds but the results were not as satisfactory as with DMSO. Our last aim was the synthesis of cages (in particular T8 cages) carrying different arms on he silicon atoms. The synthesis of cyclic cis-polyols was the first step to achieve this aim. We used a range of approaches to the synthesis of the polyols but the most satisfactory was Shchegolikhina’s method. With this method we were able to isolate a number of new product such as the cis cyclopentyltriol and the cis cyclohexyltetrol.

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