Hydrosilylation of alkenes and ketones catalyzed by nickel(II) indenyl complexes

Abstraction of Cl – from the complexes (indenyl)Ni(PPh 3 )Cl generates cationic species that are effective precatalysts for the hydrosilylation of some olefins and ketones. For instance, the mixture of (1-Me-indenyl)Ni(PPh 3 )Cl and NaBPh 4 (or methylaluminoxane) reacts at room temperature with ca. 100 equiv. each of PhSiH 3 and styrene to produce [1-phenyl-1-ethyl](phenyl)silane, PhCH(CH 3 )(SiPhH 2 ), in 50%–80% yield. The same system can also catalyze the hydrosilylation of 1-hexene and norbornene, but the products arising from these substrates consist of mixtures of regio- and stereoisomers. On the other hand, ketone hydrosilylation is regiospecific, giving the corresponding silyl ethers in high yields. A number of experimental observations have indicated that the initially generated Ni-based cation is not the catalytically active species. Indeed, the cationic initiators may be replaced by LiAlH 4 or AlMe 3 , which generate the corresponding Ni-H or Ni-Me derivatives, respectively. Moreover, the observed regioselectivity for the addition of PhSiH 3 to styrene (i.e., predominant addition of the silyl fragment to the α-C) is opposite of what would be expected if the reaction mechanism involved carbocationic intermediates. A new mechanism is proposed in which the active species is a Ni-H species originating from the transfer of H – from PhSiH 3 to the initially generated Ni cation.