Mechanism of Formation of Active Catalytic Species in Nickel-Catalysed Asymmetric Hydrogenation

Initial stages of two representative Ni-catalysed asymmetric hydrogenations were investigated computationally featuring the formation of the active catalytic species [R,R-BenzP*NiH]+ from the R,R-BenzP*Ni(OAc)2 and [S,S-Ph-BPENiH]+ from the S,S-Ph-BPENi(OAc)2. Among several computed pathways the most feasible is dissociation of acetic acid from LNiH(HOAc) formed by H2 metathesis in LNi(H2)(OAc)2. The rate-limiting stage is the initial metathesis of H2 requiring high effective activation barriers of 31.0 and 28.6 kcal/mol, in accord with high pressures and elevated temperatures essential for the Ni-catalysed hydrogenations.