Convenient Transfer Semihydrogenation Methodology for Alkynes Using a Pd^II-NHC Precatalyst

A convenient and easy-to-use protocol for the <i>Z</i>-selective transfer semihydrogenation of alkynes was developed, using ammonium formate as the hydrogen source and the easily prepared and commercially available, highly stable complex PdCl­(η³-C₃H₅)­(IMes) (<b>1</b>) as the (pre)­catalyst. Combined with triphenyl posphine as an additional ligand, this system provides a robust catalytic synthetic method that shows little to no over-reduction or isomerization after full substrate conversion. The system allows the direct use of solvents and reagents, as received from the supplier without drying or purification, thus providing a practical method for semihydrogenation of a broad range of alkynes. The mechanism behind these high and enhanced selectivities was determined through a set of kinetic experiments