| Abstract
| - Organoactinide complexes of the type Cp*2AnMe2 (An = Th, U) have been found to beefficient catalysts for the hydrosilylation of terminal alkynes. The chemoselectivity andregiospecificity of the reactions depend strongly on the nature of the catalyst, the nature ofthe alkyne, the silane substituents, the ratio between the silane and alkyne, the solvent,and the reaction temperature. The hydrosilylation reaction of the terminal alkynes withPhSiH3 at room temperature produces the trans-vinylsilane as the major product along withthe silylalkyne and the corresponding alkene. At higher temperatures (50−80 °C), besidesthe products obtained at room temperature, the cis-vinylsilane and the double-hydrosilylatedalkene, 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 toproceed only at higher temperatures, although no cis-vinylsilane or double-hydrosilylatedproducts were observed. When the catalytic hydrosilylation reaction is carried out using a1:2 ratio of iPrC⋮CH to PhSiH3 with Cp*2ThMe2, the yield of the double-hydrosilylatedproduct is increased from 6 to 26%. When the same reaction is conducted using a 2:1 ratiobetween iPrC⋮CH and PhSiH3, the alkene was found to be the major product with theconcomitant formation of the tertiary silane iPrCH⋮CHSi(HPh)(C⋮CPri). For bulky silanes,nonselective alkyne oligomerization and trace amounts of the hydrosilylation products wereproduced. Mechanistic studies on the hydrosilylation of iPrC⋮CH and PhSiH3 in the presenceof Cp*2ThMe2 show that the first step in the catalytic cycle is the insertion of an alkyne intoa thorium−hydride bond. A delicate balance between alkyne protonolysis and σ-bondmetathesis 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 inorganoactinide, silane, and alkyne, with ΔH⧧= 6.3(3) kcal mol-1 and ΔS⧧ = −51.1(5) eu. Theturnover-limiting step is the release of the hydrosilylated product from the alkenyl−actinidecomplex. 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 methodsand water poisoning experiments. A plausible mechanistic scenario is proposed for thehydrosilylation of terminal alkynes.
- Organoactinides of the form Cp*2AnMe2 (An = Th, U), in the presence of silanes, are active catalysts for the hydrosilylation of terminal alkynes. The stoichiometric reactions of Cp*2An(C⋮CR)2 with PhSiH3 allowed us to isolate complex A, a key organoactinide intermediate in the proposed catalytic cycle and the precursor for the double-hydrosilylated product.
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