Hydrogenation of Alkenes Catalyzed by Mn(I) Alkyl Complexes Bearing NHC Phosphine Ligands

An efficient additive free hydrogenation of alkenes with molecular hydrogen is described. The pre-catalyst is a well-defined bench stable Mn(I) alkyl complex bearing a NHC phosphine ligand. These reactions are environ-mentally benign and atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl in-termediate which undergoes rapid hydrogenolysis to form the active 16e Mn(I) hydride catalyst [Mn(PC-iPr)(CO)2(H)]. A range of mono- and disubstituted alkenes were efficiently converted into alkanes in good to ex-cellent yields. The hydrogenation requires a reaction temperature of 60 oC. In all cases, a catalyst loading of 1 mol % and a hydrogen pressure of 50 bar was applied. A mechanism based on DFT calculations is presented which is supported by experimental studies.