Mechanistic investigation of the transfer hydrogenation of alkynes catalysed by an MLC catalyst with multiple ancillary ligand sites

Abstract

Homogeneous organometallic catalysts based on ancillary ligands have important applications in organic synthesis. In recent years, the introduction of multiple functional sites into the ancillary ligand represents a new direction for metal–ligand cooperation (MLC) catalyst design. However, the involvement of multiple functional sites in the active species leads to complicated influence factors and uncertain reaction mechanisms. Herein, a theoretical study is presented to illuminate the mechanistic preference in transfer hydrogenation of alkynes catalyzed by an MLC catalyst with multiple functional sites in the ancillary ligand. The calculations reveal that the conventional M–L bond MLC mode possesses a high activation energy barrier (34.3 kcal mol⁻¹). In contrast, the catalyst adopts an unusual MLC mode where the NEt₂ group behaves as the non-innocent ligand and facilitates the transfer hydrogenation with a more accessible activation free energy barrier (22.6 kcal mol⁻¹). Further theoretical analysis indicates that the hemilability of the NEt₂ group increases the nucleophilicity of the Co(I) hydride complex during the insertion reaction and reduces the ring strain of the transition state in the proton transfer process. The flexible coordination modes of the catalyst avoid the unfavored steric repulsion, leading to the enhanced activation of the substrate from the cobalt center. Our study demonstrates the mechanistic diversity of the MLC catalyst with multiple ancillary ligand sites, which is anticipated to provide inspiration for future homogeneous catalyst design.