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

Abstract

Abstraction of Cl– from the complexes (indenyl)Ni(PPh3)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(PPh3)Cl and NaBPh4 (or methylaluminoxane) reacts at room temperature with ca. 100 equiv. each of PhSiH3 and styrene to produce [1-phenyl-1-ethyl](phenyl)silane, PhCH(CH3)(SiPhH2), 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 LiAlH4 or AlMe3, which generate the corresponding Ni-H or Ni-Me derivatives, respectively. Moreover, the observed regioselectivity for the addition of PhSiH3 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 PhSiH3 to the initially generated Ni cation. Key words: hydrosilylation, nickel indenyl complexes, cationic complexes, hydride intermediates.

Development of Nickel Hydrosilylation Catalysts

“…Therefore, it is to be noted that the reaction catalyzed by the (salicylaldiminato)Ni(II)methyl complex would be one of the rare examples of a Ni‐catalyzed hydrosilylation that follows the metathesis mechanism 22. This could be one reason for the good reaction selectivity and rather wide substrate scope of this system.…”

“…It was indicated that the stoichiometric reaction of 1a with Ph 2 SiH 2 resulted in the formation of silyl complex A based on 1 H and 29 Si NMR spectroscopies. Because a Chalk–Harrod or modified Chalk–Harrod‐type mechanism via Ni(II)/Ni(IV) intermediates is less likely,21,22b we proposed a metathesis mechanism for this system (Scheme ) 12. Complex 1 initially reacts with the hydrosilane to give silyl intermediate A , which further undergoes successive alkene coordination and insertion to give C via B .…”

Indenylmetal Catalysis in Organic Synthesis

“…(1-MeInd)Ni(PPh 3 )Cl, which had previously been studied as acatalyst for dehydropolymerzation of phenylsilane, [98] can be used as ac atalyst for the hydrosilylation of styrene with phenylsilane with 2mol %catalyst loading. [99] Te thered amino complex 141 displays longer catalytic lifetimes and as little as 1mol %o ft his complex can be used to give aq uantitative yield of the a-silylated product 142 as the sole regioisomer (Scheme 48). [100] Thet ethered ligand prevents catalyst deactivation via decomposition to an unknown number of nickel complexes.A lternatively,b ulky indenyl complexes [1,3-(TMS) 2 Ind]Ni(PPh 3 )Cl) [101] can be used instead of 141 to deliver 142 in an 82 %yield.…”

Indenylmetallkatalyse in der organischen Synthese

“…Im Vergleich zu anderen Übergangsmetallen sind die katalytischen Eigenschaften von Indenylnickel‐ und Indenylpalladiumkomplexen im Zusammenhang mit der organischen Synthese wenig erforscht. (1‐MeInd)Ni(PPh 3 )Cl, das zuvor als Katalysator für die Dehydropolymerisierung von Phenylsilan untersucht wurde, katalysiert in einer Menge von 2 Mol‐% die Hydrosilylierung von Styrol mit Phenylsilan . Der Komplex 141 mit angebundener Amin‐Donorgruppe ist ein Katalysator mit längerer Lebensdauer, und mit nur 1 Mol‐% dieses Komplexes wird in quantitativer Ausbeute das α‐silylierte Produkt 142 als einziges Regioisomer erhalten (Schema ) .…”