Rethinking Basic ConceptsHydrogenation of Alkenes Catalyzed by Bench-Stable Alkyl Mn(I) Complexes

An
efficient additive-free manganese-catalyzed hydrogenation of
alkenes to alkanes with molecular hydrogen is described. This reaction
is atom economic, implementing an inexpensive, earth-abundant nonprecious
metal catalyst. The most efficient precatalyst is the bench-stable
alkyl bisphosphine Mn­(I) complex fac-[Mn­(dippe)­(CO)3(CH2CH2CH3)]. The catalytic
process is initiated by migratory insertion of a CO ligand into the
Mn-alkyl bond to yield an acyl intermediate which
undergoes rapid hydrogenolysis to form the active 16e Mn­(I) hydride
catalyst [Mn­(dippe)­(CO)2(H)]. A range of mono- and disubstituted
alkenes were efficiently converted into alkanes in good to excellent
yields. The hydrogenation of 1-alkenes and 1,1-disubstituted alkenes
proceeds at 25 °C, while 1,2-disubstituted alkenes require a
reaction temperature of 60 °C. In all cases, a catalyst loading
of 2 mol % and a hydrogen pressure of 50 bar were applied. A mechanism
based on DFT calculations is presented, which is supported by preliminary
experimental studies.