GROUP-4 METALLOCENE OLEFIN HYDROSILYLATION CATALYSTS

The hydrosilylation of olefins such as styrene, 1-hexene, and 2-pentene with diphenylsilane can be carried out with catalysts generated from zirconocene dichloride and 2 equiv of butyllithium. Complete regioselectivity is observed as only terminal organosilicon products are produced (approximately 90%). In the case of styrene, three major products are formed: phenethyldiphenylsilane, trans-1-phenyl-2-(diphenylsilyl)ethene, and ethylbenzene. The product distribution was found to be dependent on reagent concentrations: reactions run with excess diphenylsilane favored diphenylphenethylsilane; excess styrene favored trans-1-(diphenylsilyl)-2-phenylethene. Reactivity as a function of silane, olefin, and catalyst was examined. Secondary silanes are generally superior to primary and tertiary silanes for hydrosilylation. Silane coupling reactions compete with hydrosilylation with less sterically hindered silanes to produce oligomeric products. Sterically hindered silanes, such as triphenylsilane, react only at elevated temperature and yield vinylsilane as the major product. Extensive H/D exchange is observed in the hydrosilylation of styrene with Ph2SiD2. Stoichiometric reactions indicate that dibutylzirconocene reacts to form a zirconocene-butene complex at room temperature. The possibility of an ''olefin first'' mechanism is discussed.