Ligand-modulated phosphine-free Ni(ii) complexes for Z-selective semi-transfer hydrogenation of alkynes and dual transfer hydrogenation of α,β-unsaturated ketones using ammonia borane

Abstract

Stereoselective semi-transfer hydrogenation of alkynes is highly challenging due to issues with cis/trans-isomerization and over-reduction. Similarly, achieving simultaneous hydrogenation of both CO and CC bonds in α,β-unsaturated ketones poses substantial challenges. A series of phosphine-free Ni(II), Mn(II), and Co(II)-based complexes bearing benzimidazole-based tri- and tetradentate ligands were synthesized. Among them, the nickel complexes exhibited superior reactivity in the Z-selective semi-transfer hydrogenation of alkynes and the dual transfer hydrogenation of chalcones using ammonia borane (AB). Selective and efficient transfer hydrogenation protocols were developed based on strategic ligand and catalyst design. Cat. 4, having a bulkier ligand, facilitated the Z-selective semi-transfer hydrogenation of alkynes, while Cat. 1, with a relatively less bulky ligand, enabled the dual transfer hydrogenation of α,β-unsaturated ketones to saturated alcohols. Employing these mild, additive-free protocols, a broad range of synthetically valuable Z-alkenes and functionalized alcohols were smoothly obtained. Several control experiments, along with DFT calculations, provided critical insights into the catalytic cycle.