Transfer Hydrogenation of Vinyl Arenes and Aryl Acetylenes with Ammonia Borane Catalyzed by Schiff Base Cobalt(II) Complexes.

A series of bench-stable Co(II) complexes containing hydrazone Schiff base ligands were evaluated in terms of their activity and selectivity in carbon-carbon multiple bond transfer hydrogenation. These cobalt complexes, especially a Co(II) precatalyst bearing pyridine-2-yl-N(Me)N=C-(1-methyl)imidazole-2-yl ligand, activated by LiHBEt3, were successfully used in the transfer hydrogenation of substituted styrenes and phenylacetylenes with ammonia borane as a hydrogen source. Key advantages of the reported catalytic system include mild reaction conditions, high selectivity and tolerance to functional groups of substrates.

Markovnikov-selective double hydrosilylation of challenging terminal aryl alkynes under cobalt and iron catalysis.

Geminal bis(silanes) are unique compounds with interesting properties. The most straightforward way to access them is double hydrosilylation of alkynes, which was established only recently. Previous articles about transition metal-catalysed double hydrosilylation show that terminal aryl alkynes are a challenge. We report on cobalt(II) and iron(III) complexes with the easy-to-synthesise N,N,N-tridentate hydrazone ligand being active precatalysts in Markovnikov-selective double hydrosilylation of terminal aryl alkynes. The influence of the hydrazone ligand structure and the potential role of the sodium triethylborohydride activator were studied. Sets of geminal bis(silanes) with two identical or different silyl groups were synthesised, showing the applicability of the reported method.

Schiff Base Cobalt(II) Complex-Catalyzed Highly Markovnikov-Selective Hydrosilylation of Alkynes.

A bench-stable cobalt(II) complex, with 3N-donor socket-type benzimidazole-imine-2H-imidazole ligand is reported as a precatalyst for regioselective hydrosilylation of terminal alkynes. Both aromatic and aliphatic alkynes could be effectively hydrosilylated with primary, secondary, and tertiary silane to give α-vinylsilanes in high yields with excellent Markovnikov selectivity and extensive functional-group tolerance. Catalyst loading varies within 0.5-0.05 mol %, which is one of the most efficient reported so far in the literature on cobalt-catalyzed alkyne hydrosilylation.

Sodium triethylborohydride as a catalyst for the dehydrogenative silylation of terminal alkynes with hydrosilanes.

The first example of sodium triethylborohydride-catalyzed C(sp)-H bond silylation is reported. The reaction of aromatic and aliphatic alkynes with aromatic hydrosilanes and hydrosiloxanes proceeded in a highly selective manner to afford dehydrocoupling products. Competitive hydrosilylation of the terminal alkyne did not occur as a side-reaction. In view of the above it is remarkable that NaHBEt3 is commonly used as a reducing agent generating active transition-metal catalysts in situ in other hydrosilylation reactions.