Ruthenium‐Catalyzed E‐Selective Partial Hydrogenation of Alkynes under Transfer‐Hydrogenation Conditions using Paraformaldehyde as Hydrogen Source.

E‐alkenes were synthesized with up to 100 % E/Z selectivity via ruthenium‐catalyzed partial hydrogenation of different aliphatic and aromatic alkynes under transfer‐hydrogenation conditions. Paraformaldehyde as a safe, cheap and easily available solid hydrogen carrier was used for the first time as hydrogen source in the presence of water for transfer‐hydrogenation of alkynes. Optimization reactions showed the best results for the commercially available binuclear [Ru(p‐cymene)Cl2]2 complex as pre‐catalyst in combination with 2,2‐bis(diphenylphosphino)‐1,1‐binaphthyl (BINAP) as ligand (1 : 1 ratio per Ru monomer to ligand). Mechanistic investigations showed that the origin of E‐selectivity in this reaction is the fast Z to E isomerization of the formed alkenes. Mild reaction conditions plus the use of cheap, easily available and safe materials as well as simple setup and inexpensive catalyst turn this protocol into a feasible and promising stereo complementary procedure to the well‐known Z‐selective Lindlar reduction in late‐stage syntheses. This procedure can also be used for the production of deuterated alkenes simply using d2‐paraformaldehyde and D2O mixtures. [ABSTRACT FROM AUTHOR]