Catalytic hydrosilylation of terminal alkynes promoted by organoactinides

Organoactinide complexes of the type Cp*₂AnMe₂ (An = Th, U) have been found to be efficient catalysts for the hydrosilylation of terminal alkynes. The chemoselectivity and regiospecificity of the reactions depend strongly on the nature of the catalyst, the nature of the alkyne, the silane substituents, the ratio between the silane and alkyne, the solvent, and the reaction temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH₃ at room temperature produces the trans-vinylsilane as the major product along with the silylalkyne and the corresponding alkene. At higher temperatures (50-80°C), besides the products obtained at room temperature, the cis-vinylsilane and the double-hydrosilylated alkene, in which the two silicon moieties are connected at the same carbon atom, are obtained. The catalytic hydrosilylation of (TMS)C≡CH and PhSiH₃ with Cp*₂ThMe₂ was found to proceed only at higher temperatures, although no cis-vinylsilane or double-hydrosilylated products were observed. When the catalytic hydrosilylation reaction is carried out using a 1:2 ratio of ⁱPrC≡CH to PhSiH₃ with Cp*₂ThMe₂, the yield of the double-hydrosilylated product is increased from 6 to 26%. When the same reaction is conducted using a 2:1 ratio between ⁱPrC≡CH and PhSiH₃, the alkene was found to be the major product with the concomitant formation of the tertiary silane ⁱPrCH≡CHSi(HPh)(C≡CPrⁱ). For bulky silanes, nonselective alkyne oligomerization and trace amounts of the hydrosilylation products were produced. Mechanistic studies on the hydrosilylation of ⁱPrC≡CH and PhSiH₃ in the presence of Cp*₂ThMe₂ show that the first step in the catalytic cycle is the insertion of an alkyne into a thorium-hydride bond. A delicate balance between alkyne protonolysis and σ-bond metathesis by the silane determines the ratio among the vinylsilanes, the double-hydrosilylated product, the silylalkyne, and the alkene. The kinetic rate law is first order in organoactinide, silane, and alkyne, with ΔH‡= 6.3(3) kcal mol^-1 and ΔS‡ = -51.1(5) eu. The turnover-limiting step is the release of the hydrosilylated product from the alkenyl-actinide complex. The key organoactinide intermediates for the cis-vinylsilane and the double-hydrosilylation products are the Cp*₂An(C≡CR)(C(PhSiH₂)=CHR) (An = Th, U) complexes. These complexes have been trapped (for R = ⁱPr) and characterized by spectroscopic methods and water poisoning experiments. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes.