Great progress has been recently made by Associate Prof. Jun Zhang on silver-catalyzed amination of alkynes, which has been published on Angew. Chem. Int. Ed. (“Silver-Catalyzed Amidiniumation of Alkynes: Isolation of a Silver Intermediate, Synthesis of Enamine Amido Carbene Precursors, and an Unprecedented Umpolung of Propiolamide” DOI: 10.1002/anie. 201508221.) 

    In metal-catalyzed organic transformation, isolation and characterization of key organometallic intermediates is very important to well understand the reaction mechanism. In the past ten years, silver-catalyzed reactions of alkynes have attracted considerable interest because of their high selectivity and mild reaction conditions. Because the Ag-C bonds in silver intermediates are vulnerable to attack by electrophiles, they are highly reactive and rarely isolated. Partially because of the lack of the isolable organosilver intermediate, the reaction mechanism for silver-catalyzed transformations is much less investigated. In this work, we isolated a first silver intermediate supported by an abnormal carbene in silver-catalyzed amidiniumation of alkynes through σ,π-silver activation mode and other organogold intermediates were also prepared. The characterization and isolation of the silver and gold intermediates supports our proposed mechanisms.

    Additionally, we first revealed that, upon activation by AgOTf, N-propiolic formamidines undergoes hydroamidiniumation in a 5-exo-dig fashion, and subsequent hydrolysis offers the five-membered formamidinium salt, which is highly reactive and represents a rare example of the umpolung of propiolamide.
    The work, collaborated with Prof. Min Shi, was mainly finished by Shichang Lv, and is financially supported by the National Natural Science Foundation of China.