Nanoporous Gold Catalyst for Highly Selective Semihydrogenation of Alkynes: Remarkable Effect of Amine Additives

We report for the first time the highly selective semihydrogenation of alkynes using the unsupported nanoporous gold (AuNPore) as a catalyst and organosilanes with water as a hydrogen source. Under the optimized reaction conditions, the present semihydrogenation of various terminal- and internal-alkynes affords the corresponding alkenes in high chemical yields and excellent Z-selectivity without any over-reduced alkanes. The use of DMF as solvent, which generates amines in situ, or pyridine as an additive is crucial to suppress the association of hydrogen atoms on AuNPore to form H2 gas, which is unable to reduce alkynes on the unsupported gold catalysts. The AuNPore catalyst can be readily recovered and reused without any loss of catalytic activity. In addition, the SEM and TEM characterization of nanoporosity show that the AuNPore catalyst has a bicontinuous 3D structure and a high density of atomic steps and kinks on ligament surfaces, which should be one of the important origins of catalytic activity.