Ligand-Free (Z)-Selective Transfer Semihydrogenation of Alkynes Catalyzed by in situ Generated Oxidizable Copper Nanoparticles

Herein, we present (Z)-selective transfer semihydrogenation of alkynes based on in situ generated CuNPs in the presence of hydrogen donors, such as ammonia-borane and a protic solvent. This environmentally-friendly method is characterized by operational simplicity combined with high stereo- and chemoselectivity and functional group compatibility. Auto-oxidation of CuNPs after the semihydrogenation reaction is completed results in the formation of water-soluble ammonia complex, so that the catalyst may be reused several times by simple phase-separation with no need of any special regeneration process. Formed NH₄B(OR)₄ can be easily transformed back to ammonia-borane or to boric acid. In addition, one-pot tandem sequence involving Suzuki reaction followed by semihydrogenation was presented.