Aswini K. Dash

Dehydrocoupling reactions of amines with silanes catalyzed by [(Et2N)3U][BPh4]

Abstract Dehydrocoupling reactions of primary amines RNH2 with PhSiH3 were catalyzed by [(Et2N)3U][BPh4] to give the corresponding aminosilanes PhSiH3−n(NHR)n (n=1–3), the relative yields of the products were found to be dependent on the experimental conditions and on the nature of R. For a primary silane (PhSiH3), the reactivity of RNH2 follows the order primary>secondary>tertiary. Similar dehydrocoupling reactions using secondary amines with secondary silanes were found to be less reactive. Homodehydrocoupling of the silane was found not to be a competing reaction at room temperature. The hydride [(RNH)2UH][BPh4], which is plausibly formed in the reaction of [(RNH)3U][BPh4] with PhSiH3 is a likely intermediate in the catalytic cycle.

Diverse catalytic activity of the cationic actinide complex [(Et2N)3U][BPh4] in the dimerization and hydrosilylation of terminal alkynes. Characterization of the first f-element alkyne π-complex [(Et2N)2U(CCtBu)(η2-HCCtBu)][BPh4]

Abstract The cationic actinide complex [(Et 2 N) 3 U][BPh 4 ] is an active catalytic precursor for the selective dimerization of terminal alkynes. The regioselectivity is mainly towards the geminal dimer but for bulky alkyne substituents, the unexpected cis -dimer is also obtained. Mechanistic studies show that the first step in the catalytic cycle is the formation of the acetylide complex [(Et 2 N) 2 UC  CR][BPh 4 ] with the concomitant reversible elimination of Et 2 NH, followed by the formation of the alkyne π-complex [(Et 2 N) 2 UC  CR(RC  CH)][BPh 4 ]. This latter complex (R= t Bu) has been characterized spectroscopically. The kinetic rate law is first order in organoactinide and exhibits a two domain behavior as a function of alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order whereas at high alkyne concentrations, a zero order is observed. The turnover-limiting step is the C  C bond insertion of the terminal alkyne into the actinideacetylide bond to give the corresponding alkenyl complex with Δ H ‡ =15.6(3) kcal mol −1 and Δ S ‡ =−11.4(6) eu. The following step, protonolysis of the uraniumcarbon bond of the alkenyl intermediate by the terminal alkyne, is much faster but can be retarded by using CH 3 C  CD, allowing the formation of trimers. The unexpected cis -isomer is presumably obtained by the isomerization of the trans -alkenyl intermediate via an envelope mechanism. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes. The cationic complex [(Et 2 N) 3 U][BPh 4 ] has been found to be also an efficient catalyst for the hydrosilylation of terminal alkynes. The chemoselectivity and regiospecificity of the reaction depend strongly on the nature of the alkyne, the solvent and the reaction temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH 3 at room temperature produced a myriad of products among which the cis - and trans -vinylsilanes, the alkene and the silylalkyne are the major components. At higher temperatures, besides the products obtained at room temperature, the double hydrosilylated alkene, in which the two silicon moieties are connected at the same carbon atom, is obtained. The catalytic hydrosilylation of (TMS)C  CH and PhSiH 3 with [(Et 2 N) 3 U][BPh 4 ] was found to proceed only at higher temperatures. Mechanistically, the key intermediate seems to be the uranium–hydride complex [(Et 2 N) 2 UH][BPh 4 ], as evidenced by the lack of the dehydrogenative coupling of silanes. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes taking into account the formation of all products.

Oligomerization and hydroamination of terminal alkynes promoted by the cationic organoactinide compound [(Et2N)3U][BPh4].

The three ancillary amido moieties in the cationic complex [(Et2N)3U][BPh4] are highly reactive and are easily replaced when the complex is treated with primary amines. The reaction of [(Et2N)3U][BPh4] with excess tBuNH2 allows the formation of the cationic complex [(tBuNH2)3(tBuNH)3U][BPh4]. X-ray diffraction studies on the complex indicate that three amido and three amine ligands are arranged around the cationic metal center in a slightly distorted octahedral mer geometry. The cationic complex reacts with primary alkynes in the presence of external primary amines to primarily afford the unexpected cis dimer and, in some cases, the hydroamination products are obtained concomitantly. The formation of the cis dimer is the result of an envelope isomerization through a metal-cyclopropyl cationic complex. In the reaction of the bulkier alkyne tBuC identical to CH with the cationic uranium complex in the presence of various primary amines, the cis dimer, one trimer, and one tetramer are obtained regioselectively, as confirmed by deuterium labeling experiments. The trimer and the tetramer correspond to consecutive insertions of an alkyne molecule into the vinylic CH bond trans to the bulky tert-butyl group. The reaction of (TMS) C identical to CH with the uranium catalyst in the presence of EtNH2 followed a different course and produced the gem dimer along with the hydroamination imine as the major product. However, when other bulkier amines were used (iPrNH2 or tBuNH2) both hydroamination isomeric imines Z and E were obtained. During the catalytic reaction, the E (kinetic) isomer is transformed into the most stable Z (thermodynamic) isomer. The unique reactivity of the alkyne (TMS) C identical to CH with the secondary amine Et2NH is remarkable because it afforded the trans dimer and the corresponding hydroamination enamine. The latter probably results from the insertion of the alkyne into a secondary metal-amide bond, followed by protonolysis.

Organoactinides—novel catalysts for demanding chemical transformations

Abstract The catalytic effect obtained by opening the coordination sphere of the organoactinide complex is presented. 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 , which reacts with two equiv of BuLi affording the corresponding dibutyl complex ansa -Me 2 SiCp 2 ″Th″Bu 2 . This latter complex was found to be an active catalyst for the dimerization of terminal alkynes, and in the hydrosilylation of terminal alkynes with PhSiH 3 . In both processes a large chemoselectivity and regioselectivity are 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.