Catalytic Hydrosilylation of Terminal Alkynes Promoted by Organoactinides

Abstract

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 i PrC⋮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 i PrC⋮CH and PhSiH3, the alkene was found to be the major product with the concomitant formation of the tertiary silane i PrCH⋮CHSi(HPh)(C⋮CPr i ). For bulky silanes, nonselective alkyne oligomerization and trace amounts of the hydrosilylation products were produced. Mechanistic studies on the hydrosilylation of i PrC⋮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 = i Pr) and characterized by spectroscopic methods and water poisoning experiments. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes.

Catalytic Organic Transformations Mediated by Actinide Complexes

“…Several actinide complexes have been explored as catalysts in the hydrosilylation of terminal acetylenes and olefins with PhSiH3, displaying different chemo-and regio-selectivities in dependence of the actinide center, the ancillary ligands, the substrates used and the reaction conditions. The organoactinide compounds Cp*2AnMe2 (An = U (5); Th (6)) were shown to mediate the hydrosilylation of terminal alkynes with PhSiH3, leading to a different product distribution in dependence of the reaction temperature [60]. When the reaction was carried out at room temperature, the trans vinylsilane D (Scheme 8) was obtained as the major product, as well as silylalkynes and the respective alkene as byproducts.…”

Th(IV) complexes with cis-ethylenebis(diphenylphosphine oxide): X-ray structures and NMR solution studies

“…[3] There has also been a wide variety of reports directed toward the use of Th(IV) complexes as catalysts for chemical transformations. [4] A selection of reactions mediated by Th(IV) organometallic complexes include: C-H and C-O bond activation, [5,6] hydrosilylation of alkenes and alkynes, [7] hydroamination, [7][8][9][10] aldehyde dimerization, [11] and phosphonoester hydrolysis. [12] To this end, investigations into the synthesis and structure of high-denticity Th(IV) complexes will contribute to each of the diverse set of chemical areas described above.…”

Efficient and selective catalysis for hydrogenation and hydrosilation of alkenes and alkynes with PNP complexes of scandium and yttrium

“…Selective semihydrosilation catalysts are rare, 10 although the vinyl silanes are valuable materials in industry and in organic synthesis as masked ketone functionalities, and as coupling partners in cross-coupling reactions. 11 While there are some examples of hydrosilations of alkenes and alkynes with yttrium, 2c,3b,12 lanthanum, 12d and thorium, 13 there are no examples of semihydrosilation with scandium and, as such, no corresponding comparison of first- and second-row congeners (Sc and Y) possessing the same ligand set.…”

“…Examples of catalysts for this reaction, based on thorium and uranium, require 24–48 h (sometimes with heating) and have low product selectivities of 33–62%. 13 In contrast, 1-Sc achieves quantitative conversion with 95% selectivity in 20 min at 23 °C (Table 3, entry 4); 1-Y exhibited no catalytic activity under the same conditions (Table 3, entry 9).…”