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 R,R-BenzP*Ni(OAc)₂ and [S,S-Ph-BPENiH]⁺ from S,S-Ph-BPENi(OAc)₂. Among several computed pathways the most feasible is dissociation of acetic acid from (ligand)NiH(HOAc) formed by H₂ metathesis in the (ligand)Ni(H₂)(OAc)₂. The rate-limiting step is the initial metathesis of H₂ requiring high effective activation barriers of 28.3 and 26.6 kcal mol^–1, in accord with high pressures and elevated temperatures essential for the Ni-catalysed hydrogenations.