Abstract
In this Special Issue on N-Heterocyclic Carbenes and Their Complexes in Catalysis, we report the first example of Suzuki–Miyaura cross-coupling of amides catalyzed by well-defined, air- and moisture-stable nickel/NHC (NHC = N-heterocyclic carbene) complexes. The selective amide bond N–C(O) activation is achieved by half-sandwich, cyclopentadienyl [CpNi(NHC)Cl] complexes. The following order of reactivity of NHC ligands has been found: IPr > IMes > IPaul ≈ IPr*. Both the neutral and the cationic complexes are efficient catalysts for the Suzuki–Miyaura cross-coupling of amides. Kinetic studies demonstrate that the reactions are complete in < 1 h at 80 °C. Complete selectivity for the cleavage of exocyclic N-acyl bond has been observed under the experimental conditions. Given the utility of nickel catalysis in activating unreactive bonds, we believe that well-defined and bench-stable [CpNi(NHC)Cl] complexes will find broad application in amide bond and related cross-couplings of bench-stable acyl-electrophiles.
Highlights
Nickel catalysis has recently garnered significant attention, enabling cleavage of unreactive bonds by this abundant 3D transition metal [1,2,3]
Major advances have been made in amide cross-coupling, wherein highly selective oxidative addition of the N–C(O) bond enables to exploit the traditionally unreactive amides as a novel class of acyl and aryl electrophiles [4,5,6,7,8,9,10]. This unconventional amide bond disconnection is relevant in the view of common presence of amides in natural products, pharmaceuticals, and biopolymers, where the emergence of new catalytic methods has a potentially major impact on the way chemists perceive synthetic routes
To the best of our knowledge, there are no methods for the use of well-defined, air- and moisture-stable nickel/NHC complexes as efficient precatalysts in amide bond activation
Summary
Nickel catalysis has recently garnered significant attention, enabling cleavage of unreactive bonds by this abundant 3D transition metal [1,2,3]. Major advances have been made in amide cross-coupling, wherein highly selective oxidative addition of the N–C(O) bond enables to exploit the traditionally unreactive amides as a novel class of acyl and aryl electrophiles [4,5,6,7,8,9,10] This unconventional amide bond disconnection is relevant in the view of common presence of amides in natural products, pharmaceuticals, and biopolymers, where the emergence of new catalytic methods has a potentially major impact on the way chemists perceive synthetic routes. In spite of the advances made by in situ formed Ni(0) catalysts, the lack of air-stability of Ni(cod) severely limits the potential broad applications of the powerful Ni catalysis platform in amide bond activation [15,16,17] In this Special Issue on N-Heterocyclic Carbenes and Their Complexes in Catalysis, we report the first example of Suzuki–Miyaura cross-coupling of amides catalyzed by well-defined, air- and moisture-stable nickel/NHC (NHC = N-heterocyclic carbene) complexes (Figure 1). Ni–NHC complexes in selective activation of amide N–C(O) bonds (this work)
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