This contribution describes coordinative/insertive stereoregular homopolymerizations and copolymerizations of styrene and methyl methacrylate (MMA) mediated by a highly active single-site organotitanium catalyst. The catalyst system used to effect these polymerizations of nonpolar and polar olefinic monomers is prepared by in situ Zn reduction of the precursor derived from the reaction (Me(5)Cp)TiMe(3) + Ph(3)C(+)B(C(6)F(5))(4)(-). The resulting catalyst produces polystyrene (>95% syndiotactic, 170 000 g/mol molecular weight; s-PS) by the established coordinative/insertive pathway. The same catalyst mediates polymerization of MMA to poly(methyl methacrylate) (>65% syndiotactic, >70 000 g/mol molecular weight; s-PMMA) by a group transfer protocol-like (GTP-like) pathway (1,4 insertion mechanism). Under optimal conditions, this catalyst also mediates the copolymerization of MMA + styrene (1:19 ratio) at 50 degrees C to yield random approximately 80% coisotactic poly[styrene-co-(methyl methacrylate)] (coiso-PSMMA) which contains approximately 4% MMA. Control experiments argue that a single-site Ti catalyst is the active species for the copolymerization. The catalyst formation process is quite general, and a variety of reducing agents can be substituted for Zn and still effect copolymerization. Control experiments also indicate that known noncoordination copolymerization mechanisms (i.e., ionic or radical) cannot explain this copolymerization. We suggest a new mechanism involving sequential conjugate addition steps to explain these copolymerization results.

This contribution describes the catalytic effect observed by opening the coordination sphere at an organoactinide complex. Replacing the pentamethylcyclopentadienyl ligand in Cp* 2 -ThCl 2 (Cp* ) C 5 Me 5 ) by the bridge ligation [Me 2 SiCp′′ 2 ] 2-2[Li] + (Cp′′ ) C 5 Me 4 ) affords the synthesis of ansa-Me 2 SiCp′′ 2 ThCl 2 . The X-ray structure of this bridged complex coordinated to LiCl salt and solvent is presented, indicating the large coordinative unsaturation of the bridge organoactinides. This dichloro complex reacts with 2 equiv of BuLi, affording the corresponding dibutyl complex ansa-Me 2 SiCp′′ 2 Th(CH 2 CH 2 CH 2 CH 3 ) 2 , which was found to react extremely fast for the dimerization of terminal alkynes and also in the hydrosilylation of terminal alkynes or alkenes with PhSiH 3 . Besides the rapidity of the processes using the bridge organoactinide, as compared to Cp* 2 ThMe 2 , the chemo-and regioselectivity of the products were increased, allowing the production of only the gem-dimer, the trans-vinylsilane, and the 1-silylated alkane for the dimerization, hydrosilylation of alkyne, and hydrosilylation of alkene processes, respectively. In the latter process, the corresponding alkane is always obtained as a byproduct. The rapidity of the processes is a consequence of the opening of the coordination sphere at the metal center, whereas the chemoselectivity and regioselectivity were achieved due to the hindered equatorial plane, attributed to the disposition of the methyl groups in the bridge ligation, forcing the incoming substrates to react with a specific regiochemistry. For the dimerization of alkynes the kinetic rate law is first order in organoactinide and exhibits two domains as a function of the alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order, whereas at higher alkyne concentrations a zero order is observed. The turnover-limiting step is the carbon-carbon triple bond insertion of the terminal alkyne into the actinide acetylide bond to give the corresponding dimer. For the hydrosilylation of terminal alkyne or alkenes, the rate law for both processes follows a first order in catalyst and silane concentrations, although an inverse order is observed for either alkyne or alkene, respectively. D 2 O quenching experiments between the organoactinide complex in the presence of 1-octene under starving PhSiH 3 conditions indicate the presence of a π-alkene complex responsible for the inverse order in all three processes. Plausible mechanistic scenarios are proposed.(1) For general organolanthanide and organoactinide reviews, see: (a) Edelmann, F. T.; Gun'ko, Y. K.

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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.