Great progress has been recently made by Dr. Zhang Jun, Dr. Cao Xiaoming and Master student Wang Jiwei on the Isolation of Key Intermediates in Metal-Catalyzed Reactions, which has been published on Nat. Commun. (“Synthesis and structures of gold and copper carbene intermediates in catalytic amination of alkynes” DOI: 10.1038/ncomms14625)
Metal carbenes are often proposed as reactive intermediates in the late transition metal-catalysed transformations of alkynes. Owing to their high reactivity, however, isolation and structural characterization of metal carbene intermediates in these transformations has remained unknown. Very recently, Zhang group designed a silver-catalyzed amidiniumation of N-alkynyl formamidine to for the first time isolate a silver intermediate through σ,π-silver activation mode. The stability of the silver intermediate is attributed to the formation of NHC after the cyclization of the N-alkynyl formamidine. (Angew. Chem. Int. Ed., 2015, 54, 14941-14946)
Zhang group has further investigated the Au(I)- or Cu(I)-catalysed cyclization of formamidines bearing a internal alkyne. A vinylgold intermediate I was first isolated through five-exo-dig cyclization, and its oxidation reaction was observed, suggesting a metal carbene intermediate (I’ or I’’) involved. X-ray diffraction, 13C NMR spectra data and computational analyses provide evidence for the formation of a gold carbene intermediate I’’ with a carbocation-like electronic character. Another rare copper carbene complex II was also obtained through the similar cyclization.
This work is supported by the National Natural Science Foundation of China. This project was successfully carried out by Associate Prof. Zhang Jun, Associate Prof. Cao Xiaoming, and Master student Wang Jiwei with the help and support from Prof. Shi Min and Associate Prof. Xu Shen.