Bis(trimethylsilyl)ethyne and 1-phenyl-2-trimethylsilylethyne did not undergo hydrogenation, consistent with the observations regarding the hydrozincation of these substrates using 2 (vide supra). In all cases, Z:E stereoselectivity and alkene:alkane chemoselectivity were very high. The hydrogenation of dimethoxybut-2-ene leads to a useful building block for the potential synthesis of ring systems such as pyridoxine (relevant to vitamin B6). In addition to internal alkynes, the hydrogenation of terminal alkynes (hex-1-yne, phenylacetylene) was also investigated in this system. Low conversions and poor chemoselectivity was achieved for these substrates (Figure , Table ). Terminal alkynes can potentially deactivate the catalyst. For example, when 1 was combined with hex-1-yne, facile protonolysis of the anilide ligand of 1 led to the formation of the corresponding catalytically inactive zinc acetylide species. Origin of Selectivity: The origin of the remarkable selectivity achieved by 1 in semi-hydrogenation catalysis is likely determined by the reactivity of the intermediate 2 towards unsaturated substrates. The hydrozincation of alkynes with 2 occurs by a stereospecific reaction to form the syn-isomer this leads exclusively to the Z-alkene on protonation, resulting in high Z:E ratios. The high alkene:alkane ratios can be understood by considering the relative rates of hydrozincation of alkynes and alkenes with 2.