A new method for the highly stereoselective cis-semihydrogenation of internal alkynes is described based on in situ generated Ni(0) nanoparticles and molecular hydrogen. This reduction system also allows the semihydrogenation of terminal alkynes.Keywords: (Z)-alkenes; alkynes; cis-hydrogenation; nanoparticles; nickel; stereoselectivityThe semihydrogenation of the carbon-carbon triple bond is a particularly valuable reaction in synthetic organic chemistry. The difficulty to control the stereochemistry and to minimise the formation of over-reduced products make it a challenging transformation. In the last decades, different general methods have been developed for this purpose and applied depending on the stereochemistry desired for the final product, including: (a) heterogeneous catalytic hydrogenation, [1] (b) homogeneous catalytic hydrogenation, [2] and (c) non-catalytic chemical methods [3a] (e.g., reduction by diimide, [3b] dissolving metals, [3c, d] low-valent metal species, [3e] hydroalumination, [3f] or metal hydride-transition metal halide combinations [3g] ). Concerning the cis semireduction of alkynes and that of terminal alkynes, Lindlar catalyst, [4] trialkylammonium formates under palladium catalysis, [5] complex reducing agents of the type NaH-NaOR-MX n , [6] and dispersed nickel on graphite, [7] are some of the formerly utilised reducing systems, but they do not always lead to an optimum selectivity. In fact, partial isomerisation of the (Z)-alkene to the (E)-alkene, shift of the double bond, over-reduction to the alkane, and problems with reproducibility were observed, especially with the Lindlar catalyst. In the more recent literature, the higher selectivities achieved with a montmorillonite-supported complex, [8] nickel boride or palladium catalysts on borohydride exchange resin, [9] a homogeneous palladium(0) catalyst, [10] palladium on pumice, [11] or using a variant of Lindlars catalyst [12] are noteworthy. At any rate, most of the above-mentioned methods are not general but are rather applied to a specific type of substrate. Therefore, any new and efficient method for the semihydrogenation of alkynes is welcome.In the recent years we have applied the system composed of NiCl 2 · 2 H 2 O-Li-arene(cat.) to the reduction of a wide range of functional groups.[13] As regards the reduction of alkynes, a variety of substrates could be completely reduced to the corresponding alkanes, whereas the semihydrogenation reaction could be only controlled in a few cases without a general product stereochemistry.[14] On the other hand, we have recently discovered that very fine nickel(0) nanoparticles with diameters of 2.5 AE 1.5 nm can be prepared by reduction of anhydrous nickel(II) chloride with lithium powder and a catalytic amount of DTBB (4,4'-di-tert-butylbiphenyl) in THF at room temperature. [15] We report herein a new and efficient methodology for the stereoselective cis-semihydrogenation of alkynes, under the promotion of highly reactive nickel(0) nanoparticles generated from the red...