Abstract
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. Herein, we report the isolation of two acyclic gold and copper carbene intermediates in either Au(I)- or Cu(I)-catalysed cyclization of N-alkynyl formamidines through five-exo-dig cyclization. X-ray diffraction, 13C NMR spectra data and computational analyses provide evidence for the formation of a gold carbene intermediate with a carbocation-like electronic character. Using the intrinsic bond orbital (IBO) approach, we also evaluate the π-stabilizing effects of organic substituents at the carbene carbon atom in the gold carbene intermediate. Another rare six-membered copper carbene complex is also obtained through 6-endo-dig cyclization. These metal carbenes have proven reactive toward oxidation. The metal-promoted cyclization of N-alkynyl formamidine provides a facile approach to synthesize metal carbene species.
Highlights
Metal carbenes are often proposed as reactive intermediates in the late transition metal-catalysed transformations of alkynes
Like B, both Cu(I)-stabilized indazolylcarbene C reported by Haley group[24,25,26] and Au(I) imidazopyridinylcarbene D reported by Cao group[27] can trap molecular oxygen to generate the corresponding 3-acylindazole or 3-acylimidazo-pyridine product, respectively. Owing to their high reactivity, isolation and structural characterization of late transition metal carbene intermediates in catalytic transformation of alkynes has remained unknown; information regarding the property of the metal carbene intermediates is limited to in situ NMR study and computational analysis[28,29]
As part of our long-standing interest in the design and synthesis of N-heterocyclic carbenes (NHCs) and abnormal NHC for catalysis, we previously reported the isolation of a rare vinyl silver species through 6-endo-dig cyclization of a formamidine with a s,p-silver-activated terminal alkyne[56]
Summary
Phenylsubstituted formamidine 1 was initially examined in the amination cyclization (Fig. 2a). In the presence of a stoichiometric amount of IPrCuOTf (IPr 1⁄4 2,6-bis(diisopropyl-phenyl)imidazol2-ylidene), 1 underwent 5-exo-dig cyclization to form a Known late transition metal carbenes
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