A New Catalytic Route for the Activation of sp-Hybridized Carbon–Hydrogen Bonds

Mechanistic investigation of zinc-promoted silylation of phenylacetylene and chlorosilane: a combined experimental and computational study

The zinc-promoted silylation method is of great importance to synthesize high-performance silicon-containing arylacetylene (PSA) resins in the industry. However, it is difficult to eliminate the accompanied by-product of terminal alkenes due to the lack of mechanistic understanding of the silylation. The initiation of zinc-promoted silylation is facilitated by the interaction between zinc and phenylacetylene. Our DFT calculations indicated that the intermolecular hydrogen transfer of phenylacetylene follows an ionic pathway, which generates a phenylacetylene anion and the corresponding alkene moieties on the zinc surface. The styrene by-product is observed in this stage, with its alkene moieties desorbing as radicals into the solvent under the high reaction temperature. Three possible intermediates of surface phenylacetylene anions were proposed including PhC[triple bond, length as m-dash]C-Zn, PhC[triple bond, length as m-dash]CZnCl, and (PhC[triple bond, length as m-dash]C)2Zn. These carbanion-zinc intermediates undergo an SN2 reaction with Me3SiCl to afford the alkynylsilane on the zinc surface, which is calculated to be the rate-determining step for the zinc-promoted silylation reaction.

Highly selective dehydrogenative silylation of alkenes catalyzed by rhenium complexes

Choosy chemicals: Rhenium(I) complexes of type [ReBr(2)(L)(NO)(PR(3))(2)] (L=H(2) (1), CH(3)CN (2), ethylene (3); R=iPr (a), cyclohexyl (b)) proved to be suitable catalyst precursors for the highly selective dehydrogenative silylation of alkenes. Two types of rhenium(I) hydride species, [ReBrH(NO)(PR(3))(2)] (4) and [ReBr(eta(2)-CH(2)=CHR(1))H(NO)(PR(3))(2)] (5), were found in the [ReBr(2)(L)(NO)(PR(3))(2)]-catalyzed dehydrogenative silylation of alkenes.Rhenium(I) complexes of type [ReBr(2)(L)(NO)(PR(3))(2)] (L=H(2) (1), CH(3)CN (2), and ethylene (3); R=iPr (a) and cyclohexyl (Cy; b)) catalyze dehydrogenative silylation of alkenes in a highly selective manner to yield silyl alkenes and the corresponding alkanes. Hydrosilylation products appear only rarely depending on the type of olefinic substituent, and if they do appear then it is in very minor amounts. Mechanistic studies showed that two rhenium(I) hydride species of type [ReBrH(NO)(PR(3))(2)] (R=iPr (4 a) and Cy (4 b)) and [ReBr(eta(2)-CH(2)=CHR(1))H(NO)(PR(3))(2)] (R(1)=p-CH(3)C(6)H(4), R=iPr (5 a), Cy (5 b); R(1)=H, R=iPr (5 a'), Cy (5 b')) are involved in the initiation pathway of the catalysis. The rate-determining steps of the catalytic cycle are the phosphine dissociation from complexes of type 5 and the reductive eliminations to form the alkane components. The catalytic cycle implies that the given rhenium systems have the ability to activate C-H and Si-H bonds through the aid of a facile redox interplay of Re(I) and Re(III) species. The molecular structures of 4 b and 5 a were established by means of X-ray diffraction studies.