Homogeneous catalysis : II. ziegler systems as catalysts for hydrosilylation

Abstract

Ziegler type systems, e.g., M(acac) n —AlEt 3 (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 CH 3CHC(CH 3)CH 2SiX 3 or CH 3 CH 2 CH 2 CHCHCH 2SiX 3 are the major products, and Ni salts [NiCl 2 or Ni(acac) 2] provide the best catalysts. For 1,3-pentadiene and HSi(OR) 3 a significant by-product is CH 3 CH CHCH(SiX 3)CH 3, the formation of which is suppressed by addition of phosphines. A chain-reaction mechanism consistent with these results involves the formation successively of a Ni o complex, its oxidative adduct L(diene)Ni(H)SiX 3, a π-allyl complex L(π-allyl)NiSiX 3, isomeric σ-pentenyl derivatives, and (I)-(III) + the Ni o complex. Terminal acetylenes RC yield the products in which linear dimerisation accompanies hydrosilylation. The head-to-head adduct H 2 CC(R)C(R)(SiX 3 preponderates over the head-to-tail isomer R(H)C C(H)C(R)CH(SiX 3); 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 dimerisaiton arising from successive insertion of RCCH into the Ni and NiC [in NiC(R) Ch 2] 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(π-1,5-cyclooctadiene)nickel(O) is also shown to be an effective hydrosilylation catalyst for 1,3-pentadiene.