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Homogeneous Catalysis Part II. Ziegler Systems as Catalysts for Hydrosilylation

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Ziegler type systems, e.g., M(acac)n—AIEt3 (M = Ni, Co, or Fe) catalyse the hydrosilylation of 1,3-dienes or terminal acetylenes. With isoprene or 1,3-pentadiene the 1,4-adducts CH3 CH=C(CH3)CH2 SiX3 or CH3 CH2 CH=CHCH2- SiX3 are the major products, and Ni salts [NiCl2 or Ni(acac)2] provide the best catalysts. For 1,3-pentadiene and HSi(OR)3 a significant by-product is CH3 CH= CHCH(SiX3)CH3, the formation of which is suppressed by addition of phosphines. A chain-reaction mechanism consistent with these results involves the formation successively of a Ni0 complex, its oxidative adduct L(diene)Ni(H)- SiX3, a p-allyl complex L(p-allyl)NiSiX3, isomeric s-pentenyl derivatives, and (I)-(III) + the Ni0 complex. Terminal acetylenes RC CH yield the products in which linear dimerisation accompanies hydrosilylation. The head-to-head adduct H2 C=C(R)C(R)=CH(SiX3) preponderates over the head-to-tail isomer R(H)C= C(H)C(R)=CH(SiX3); structures of the former were established by IR and NMR studies and examination of the Diels—Alder adducts with tetracyanoethylene, including comparisons with results on RC CD. The catalytic cycle is believed to be similar to that proposed for 1,3-dienes; the linear dimerisation arising from successive insertion of RC CH into the Ni—H and Ni—C [in Ni—C(R)= CH2] bond. Competition experiments demonstrate that hydrosilylation is favoured by electronegative substituents X at Si, and that terminal acetylenes are more reactive than 1,3-dienes. Bis(p-1,5-cyclooctadiene)nickel(0) is also shown to be an effective hydrosilylation catalyst for 1,3-pentadiene.

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