The Catalytic Effect in Opening an Organoactinide Metal Coordination Sphere:  Regioselective Dimerization of Terminal Alkynes and Hydrosilylation of Alkynes and Alkenes with PhSiH₃ Promoted by Me₂SiCp‘ ‘₂ThⁿBu₂

Abstract

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.

Recent advances in organothorium and organouranium catalysis

“…The derived DH { and DS { parameter values from thermal Eyring analysis were measured to be 10.07(5) kcal mol 21 and 222.06(5) cal K 21 mol 21 , respectively. 90 Mechanistically, in hydrosilylation reactions of organo-felement complexes two Chalk-Harrod mechanisms were proposed as plausible routes, except for inclusion of s-bond metathesis, instead of the classical oxidative addition-reductive elimination processes. The two mechanisms differ in the reactive intermediates: the hydride (M-H) route and the silane (M-SiR 3 ) route.…”

Catalytic insertion of E–H bonds (E = C, N, P, S) into heterocumulenes by amido–actinide complexes

“…Previously, the effects of opening the coordination sphere of an actinide catalyst was achieved in cyclopentadienyl–actinide systems by the use of an ansa -bridged tetramethylcyclopentadienyl ligand, resulting in markedly increased activity and selectivity. 56 In the catalytic study of the amido–actinide complexes ( 1–3 ), the coordinative unsaturation is considerably increased as compared to the analogous actinide(imidazolin-2-iminato) complex, similarly providing superior reactivity over the previously reported system. These findings open the scope of possible catalytic transformations toward new, challenging reactions.…”

Catalytic Organic Transformations Mediated by Actinide Complexes

“…In addition the thorium hydride complex B undergoes a rapid insertion of an alkyne into the Th-H bond, forming the thorium alkenyl-acetylide complex C. A subsequent reaction of C with PhSiH3, which is the rate determining step of the catalytic cycle, regenerates the catalytically active species A with formation of the trans vinylsilane D. However, the thorium alkenyl-acetylide complex C can also react with an additional equivalent of alkyne, generating the alkene product E and the thorium bis An improvement in the reaction rate, as well as in the chemo-and regio-selectivity of the reaction was achieved by using the ansa-bridged thorium complex 4 as catalyst for the hydrosilylation of terminal alkynes and alkenes with PhSiH3 [61,62]. The increased reaction rate was attributed to the more open coordination sphere in the ansa-bridged complex 4, as compared the Cp*2ThMe2 (6).…”

“…Intermediate A reacts with an incoming alkene to furnish the thorium-alkyl compound B. The thorium-alkyl species B can now react via three different pathways: the first option includes a reaction with another equivalent of alkene to give the π-alkene complex G. In addition, complex B can undergo a metathesis reaction with a Si-H moiety, to give the 1-silylalkane product F and complex A, or a protonolysis reaction with a Si-H moiety to give intermediate D and the respective alkane C. Since these two steps have similar energies of activation, both can take place, furnishing an equimolar amount of 1-silylalkane F and alkane D (Scheme 9) [61,62].…”