Silsesquioxanes Synthesised in Acetonitrile

the hydrolytic condensation of cyclopentyltrichlorosilane in acetonitrile (Section 9.2.2) was repeated on a 125 ml scale (50 times up-scaling). Next, the silsesquioxane products were reacted with a titanium alkoxide, yielding a catalyst with the same epoxidation activity of the HTE lead and, therefore, confirming the applicability of HTE techniques to the synthesis of silsesquioxanes. The silsesquioxane products obtained prior to reaction with the titanium centre could be divided into two fractions one as a precipitate (A) and the other as solute in the reaction mixture (B). Fraction B was dried under reduced pressure and redissolved in tetrahydrofuran. 

Both fractions were characterised by NMR spectroscopy and mass spectrometry. Fraction A mainly consists of silsesquioxane a7b3 (Fig. 9.5) [38], while fraction B is a mixture of different silsesquioxanes, mostly incompletely condensed species, with the main species assigned to silsesquioxane structure 6i 2 (Fig. 9.5). Finally, both fractions were reacted with a titanium alkoxide and tested for catalytic activity in the epoxidation of 1-octene as a function of the reaction time and the results compared with those of HTE lead (all three catalysts are homogeneous) (Fig. 9.6). 

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A new parameter space for the synthesis of silsesquioxane precursors was defined by six different trichlorosilanes (R=cyclohexyl, cyclopentyl, phenyl, methyl, ethyl and tert-butyl) and three highly polar solvents [dimethyl sulfoxide (DMSO), water and formamide]. This parameter space was screened as a function of the activity in the epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) [26] displayed by the catalysts obtained after coordination of Ti(OBu)4 to the silsesquioxane structures. Fig. 9.4 shows the relative activities of the titanium silsesquioxanes together with those of the titanium silsesquioxanes obtained from silsesquioxanes synthesised in acetonitrile. The values are normalised to the activity of the complex obtained by reacting Ti(OBu)4 with the pure cyclopentyl silsesquioxane o7b3 [(c-C5H9)7Si7012Ti0C4H9]. 

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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