Half-sandwich Ni(II) complexes [Ni(Cp)(X)(NHC)]: From an underestimated discovery to a new chapter in organonickel chemistry

Transamidation of Amides and Amidation of Esters by Selective N–C(O)/O–C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)–NHC Complexes

“…Although we have identified well-defined Pd(II)–NHC complexes for transamidation reactions of activated amides and esters [ 18 , 19 , 20 , 21 ], we have been investigating air- and moisture-stable Ni(II)–NHCs based on naturally more abundant Ni as 3d transition metal [ 26 , 27 , 28 ]. We were attracted to the well-defined, air- and moisture-stable, half-sandwich, cyclopentadienyl [CpNi(IPr)Cl] complex ( Figure 2 ) owing to its ready availability, ease of handling and the potential to prepare more reactive cyclopentadienyl Ni(II)–NHC analogues [ 29 , 30 , 31 , 32 , 33 ]. Notably, [CpNi(IPr)Cl] has emerged as a highly attractive catalyst for several classes of cross-coupling reactions [ 29 , 30 , 31 , 32 , 33 ]; however, transamidations and amidation reactions using this well-defined catalyst have been elusive.…”

“…We were attracted to the well-defined, air- and moisture-stable, half-sandwich, cyclopentadienyl [CpNi(IPr)Cl] complex ( Figure 2 ) owing to its ready availability, ease of handling and the potential to prepare more reactive cyclopentadienyl Ni(II)–NHC analogues [ 29 , 30 , 31 , 32 , 33 ]. Notably, [CpNi(IPr)Cl] has emerged as a highly attractive catalyst for several classes of cross-coupling reactions [ 29 , 30 , 31 , 32 , 33 ]; however, transamidations and amidation reactions using this well-defined catalyst have been elusive.…”

“…These reactions provide a variety of novel methods for the synthesis of ubiquitous amide bonds and have been extended to catalytic amidation reactions of activated phenolic and unactivated methyl esters by O–C(O) cleavage [ 22 , 23 , 24 , 25 ]. In continuation of our studies on activation of amide bonds and organometallic catalysis, in this Special Issue of Editorial Board members of the Organometallic Section of Molecules, we report transamidation of N -activated amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N -heterocyclic carbenes) complexes [ 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. Most importantly, we demonstrate that readily available cyclopentadienyl complex extensively developed by Chetcuti, namely [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 ], promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines ( Figure 1 ).…”

Suzuki–Miyaura Cross-Coupling of Amides Using Well-Defined, Air- and Moisture-Stable Nickel/NHC (NHC = N-Heterocyclic Carbene) Complexes

“…Reaction profile studies demonstrated that these reactions are complete in < 1 h at 80 • C. In a broader context, the present method should be evaluated in comparison with other known approaches to biaryl ketones from amides [3][4][5][6][7][8][9][10] and acyl electrophiles [15]. The use of Ni catalysis [1-3] and the beneficial performance of Ni-NHC complexes [25][26][27][28][29] may accelerate the development of new approaches to activating amide bonds. Considering the utility of nickel catalysis in activation of unreactive bonds, we anticipate that [CpNi(NHC)Cl] complexes will be of interest in activation of bench-stable acyl electrophiles.…”

“…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). We were attracted to the recent elegant advances made in the design of half-sandwich, cyclopentadienyl [CpNi(NHC)X] practical [CpNi(NHC)Cl] precatalysts [25][26][27][28][29][30][31] are capable of selective activation of amide N-C(O) bonds. The following features of our study are noteworthy: (1) The reaction represents, to the best of our knowledge, the first example of acyl-type cross-coupling achieved by half-sandwich [CpNi(NHC)X] complexes.…”