Using hydrogen gas directly in reduction reactions can be problematic due to the need for high-pressure equipment. Catalytic transfer hydrogenations with an H₂ equivalent can avoid this issue. Alcohols, for example, can be used to replace H₂ in this type of reaction. However, this approach has so far not been used for copper-catalyzed homogeneous transfer hydrogenations. Usually, these processes use hydrosilanes as reducing agents, which generates stoichiometric amounts of silicon-based waste products.

Johannes F. Teichert and colleagues, Technical University of Berlin, Germany, have developed a protocol for the copper-catalyzed transfer semihydrogenation of alkynes (pictured) that uses simple, low-cost alcohols such as isopropanol as hydrogen sources. The team used a complex of the type  [NHC–Cu–Cl] as the catalyst (NHC = N-heterocyclic carbene), NaO^tBu as a base, and isopropanol as the solvent and H₂ equivalent, simultaneously.

The reaction was performed at 140 °C and converted different disubstituted alkynes to the desired alkenes in moderate to excellent yields and with good Z/E-selectivity. The team found no overreduction to the corresponding alkanes. This stereo- and chemoselectivity is attributed to the NHC ligand: When only CuCl is used as the catalyst, selectivities drop significantly and alkanes are formed.

• Using alcohols as simple H₂-equivalents for copper-catalysed transfer semihydrogenations of alkynes,
  Trinadh Kaicharla, Birte M. Zimmermann, Martin Oestreich, Johannes F. Teichert,
  Chem. Commun. 2019.
  https://doi.org/10.1039/c9cc06637c