Based on the consistent stereochemistry of the zinc alkenyl products 3a-e (Figure ), it is clear the reaction proceeds via a syn-addition of Zn-H to the alkyne substrate. The regiochemistry of hydrozincation is also notable. The formation of 3e occurs with complete selectivity to place the hydrogen atom at the terminal position of the 1,3-butadiyne moiety. Similarly, addition to terminal alkenes proceeds selectively to form anti-Markovnikov products. Both these results are predicted based on established trends in hydrometallation chemistry and the expected stabilization of partial charges in the insertion transition state. The hydrozincation step also appeared susceptible to steric effects as internal alkenes react only very slowly with 2. For example, (E)-1,2-diphenylethylene and (Z)-1,2-diphenylethylene give only partial conversion (<15 %) to the corresponding hydrozincated products after at least 15 h at 100 °C. 1,3-bis(trimethylsilyl)acetylene did not react with 2 under these conditions.