Stereoselective Transfer Semi-Hydrogenation of Alkynes to E-Olefins with N-Heterocyclic Silylene-Manganese Catalysts

The synthesis and structures of the first Si-II-donor supported manganese(II) complexes [L1]MnCl2, [L2]MnCl2, and [L3](2)MnCl2 are reported, bearing a pincer-type bis(NHSi)-pyridine ligand L1, bidentate bis(NHSi)-ferrocene ligand L2, and two monodentate NHSi ligands L3 (NHSi = N-heterocyclic silylene), respectively. They act as unprecedented very active and stereoselective Mn-based precatalysts (1mol% loading) in transfer semi-hydrogenations of alkynes to give the corresponding E-olefins using ammonia-borane as a convenient hydrogen source under mild reaction conditions. Complex [L1]MnCl2 shows the best catalytic performance with quantitative conversion rates and excellent E-stereoselectivities (up to 98%) for different alkyne substrates. Different types of functional groups can be tolerated, except CN, NH2, NO2, and OH groups at the phenyl group of 1-phenyl substituted alkynes.