Ambient Reductive Hydrogenation of Aryl Alkynes with NaBH4 Catalyzed by Gold Nanowires

The traditional reductive hydrogenation of alkynes usually requires the use of suitable catalysts with high activity, sometimes under harsh reaction conditions like high temperature and high pressure.To improve the reaction efficiency, various catalysts have been developed, such as metallic, nonmetallic, metal-organic frameworks and porous materials, etc. Especially, the efficient catalysts that can catalyze the hydrogenation of alkynes under mild reaction conditions have attracted increasing attention.In recent years, with the rapid development of nanotechnology, more and more nanomaterials have been successfully applied in conventional organic reactions as catalysts, achieving exciting results.Gold nanomaterials are one of the typical metal nanomaterials.Their synthesis and application have always been a hot topic.Gold nanowires (AuNWs) are a unique class of one-dimensional nanostructures with large specific surface areas and multiple active sites, thus exhibiting excellent catalytic efficiency.In this work, AuNWs are applied to the hydrogenation of aryl alkynes with sodium borohydride (NaBH4) at ambient conditions.The results indicate that C≡C bonds on alkynes are reduced to C＝C and C-C smoothly.Here, AuNWs with different length are synthesized by using 1,3,5-triethynylbenzene as the ligand, which effectively reduces the adverse effects of competitive adsorption of ligand and substrate in subsequent catalytic reactions.The reductive hydrogenation was firstly investigated with 4-ethynylbiphenyl (17.8 mg, 0.1 mmol) as the model substrate.The results indicated that the reduction products, including olefins and alkanes, were successfully obtained with AuNWs (0.76 mol%) as the catalyst and NaBH4 (2 equiv.)as the reducing agent in 1.5 mL of ethyl acetate solvent.By high performance liquid chromatography (HPLC) analysis, the overall yield of products is determined to be 92% after 4 h of reaction, and the molar ratio of olefins to alkanes is 72∶28.Furthermore, it is noted that the proportion of alkanes increases with the reaction time and surpasses olefins after 12.5 h of reaction.It means that the selectivity of the hydrogenation of alkynes can be adjusted to a certain extent by reaction time.The reaction is applicable to various aryl alkyne substrates, including mono-alkynes, poly-alkynes, heteroaryl alkynes and internal alkynes.In conclusion, AuNWs exhibit high catalytic activity in the reductive hydrogenation of aryl alkynes, thus achieving the transformation under mild reaction conditions.It can be foreseen that AuNWs will have more excellent performances in organic catalysis due to their unique properties.Keywords ambient condition