Nickel‐Catalyzed Stereodivergent Synthesis of E‐ and Z‐Alkenes by Hydrogenation of Alkynes

A convenient protocol for stereodivergent hydrogenation of alkynes to E‐ and Z‐alkenes by using nickel catalysts was developed. Simple Ni(NO3)2⋅6 H2O as a catalyst precursor formed active nanoparticles, which were effective for the semihydrogenation of several alkynes with high selectivity for the Z‐alkene (Z/E>99:1). Upon addition of specific multidentate ligands (triphos, tetraphos), the resulting molecular catalysts were highly selective for the E‐alkene products (E/Z>99:1). Mechanistic studies revealed that the Z‐alkene‐selective catalyst was heterogeneous whereas the E‐alkene‐selective catalyst was homogeneous. In the latter case, the alkyne was first hydrogenated to a Z‐alkene, which was subsequently isomerized to the E‐alkene. This proposal was supported by density functional theory calculations. This synthetic methodology was shown to be generally applicable in >40 examples and scalable to multigram‐scale experiments. EZ selectivity between E and Z! Stereodivergent hydrogenation of alkynes to E‐ or Z‐alkenes is achieved by using nickel catalysts with or without the addition of multidentate phosphine ligands. Mechanistic studies reveal that the Z‐alkene‐selective catalyst is heterogeneous whereas the E‐alkene‐selective catalyst is homogeneous.