Synthesis and structures of gold and copper carbene intermediates in catalytic amination of alkynes

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.

Reaction of chloroauric acid with histidine in microdroplets yields a catalytic Au–(His)2complex

“…Under the proposed MS condition, intermediates and some product were detected. We just found the intermediate product at m/z of 222.1242 for the rst step of amine reaction, which resulted from the addition reaction of phenylacetylene (1) and indoline (2) (Fig. S16 †).…”

“…In the enamine bulk reaction, the Au-(His) 2 was collected by the offline homemade-ESSI and then reacted with phenylacetylene (1) and indoline (2) under stirring for 780 min (Fig. S21 †).…”

“…Metal-based N-heterocyclic carbenes (NHCs) have found remarkable use as pre-catalysts and active intermediates in many catalytic reactions. [1][2][3][4] The most important challenge is to capture effectively pre-catalysts and active intermediates and achieve maximum catalytic efficiency. [5][6][7] This problem has been reviewed by Hahn, 8 Kuhn, 9 Braunstein, 10 and Tamm 11 from 2017 to 2019.…”

Non-heteroatom-substituted alkylidene complexes of groups 10 and 11

“…Interestingly, this approach also permitted the isolation of a rare example of cationic Au(I) carbonyl complex (36), the first of the classical type with a (CO) of 2113 cm 1 (Scheme 38). Comments already made on the copper(I) carbene published by Wang in 2017, apply essentially to its isostructural gold congener (34b, Scheme 34), 66 which should therefore be considered as an atypical heteroatom-stabilized alkylidene.…”

“…This discussion is postponed to the chapter dedicated to gold, as a final summary. Synthesis of the copper alkylidene 33.The last example of this family of complexes appeared very recently in a contribution by Wang and collaborators aimed to isolate possible intermediates generated in the course of catalytic amination of alkynes 66. Thus, copper and gold carbenes were obtained in a novel template synthesis in which the metal promoted the cyclization of a sterically demanding N-alkynyl formamidine and its transformation into an alkylidene ligand (Scheme 35).…”