Severalc obalt complexes bearing tridentate (NNN) ligands were synthesized and served as precatalysts for alkyne hydrosilylation with Ph 2 SiH 2 .F or terminal alkynes, the catalyst L2 b-CoCl 2 was selected, and resulted in the corresponding a-vinylsilanes with high (Markovnikov) regioselec-tivity and extensive functional-group tolerance. For internal diaryl alkynes, the catalyst L2 c-CoCl 2 exhibited the best activity,a nd afforded E-selective vinylsilanes through syn-addition in excellenty ield under mild conditions. Scheme1.Cobalt complexes for the hydrosilylation of alkynes. IAd = 1,3-diadamantylimidazol-2-ylidene,CH 2 TMS = (trimethylsilyl)methyl, Mes PDI = (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-mesitylethan-1-imine), acac = acetylacetone,d pephos = bis[(2-diphenylphosphino)phenyl] ether, xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, dppb = 1,4-bis(diphenylphosphino)butane, Ar PDI = (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-arylethan-1-imine), P C NN = (E)-N-(2,6-diisopropylphenyl)-1-(6-((diisopropylphosphaneyl)methyl)pyridin-2-yl)ethan-1-imine, IPO = iminopyridine-oxazoline, PyBox = pyridine-2,6-bis(oxazoline), API = amine-pyridine-imine, 4-Me bipy = 4,4'-dimethyl-2,2'-bipyridine.

Four Ru­(II)-NC complexes were tested as catalysts for semihydrogenation of internal alkynes to (E)-alkenes with ethanol, and the complex {(C5H4N)­(C6H4)}­RuCl­(CO)­(PPh3)2 (1a) showed the highest activity. The reactions proceeded well with 1 mol % catalyst loading and 0.1 equiv of t-BuONa at 110 °C for 1 h, and 32 alkenes were synthesized with excellent E:Z selectivity. This is the first ruthenium-catalyzed semihydrogenation of internal alkynes to (E)-alkenes using ethanol as the hydrogen donor.

The transfer hydrogenation of azoarenes to hydrazoarenes using ethanol as a hydrogen source by a bidentate Ru­(II)-NC complex is developed. A weak base is crucial for this semihydrogenation that can efficiently avoid hydrazoarene dehydrogenation under strongly basic conditions. Control experiments and density functional theory calculations demonstrate the mechanism via a Meerwein–Ponndorf–Verley mechanism, and the only byproduct is ethyl acetate. This study offers a new and simple approach for the synthesis of hydrazoarenes.

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