16-Electron Nickel(0)-Olefin Complexes in Low-Temperature C(sp²)–C(sp³) Kumada Cross-Couplings

Abstract

Investigations into the mechanism of the low-temperature C­(sp2)–C­(sp3) Kumada cross-coupling catalyzed by highly reduced nickel-olefin-lithium complexes revealed that 16-electron tris­(olefin)­nickel(0) complexes are competent catalysts for this transformation. A survey of various nickel(0)-olefin complexes identified Ni­(nor)3 as an active catalyst, with performance comparable to that of the previously described Ni-olefin-lithium precatalyst. We demonstrate that Ni­(nor)3, however, is unable to undergo oxidative addition to the corresponding C­(sp2)–Br bond at low temperatures (<−40 °C), thus indicating that the canonical cross-coupling cycle is not operative under this condition. Instead, such binary nickel(0)-olefin complexes capitalize on the long-known Lewis acidity of the Ni(0) center when it is coordinated to olefins to accommodate the incoming charge from the Grignard reagent, forming a nickel(0)-alkylmagnesium complex. We demonstrate that this unique heterobimetallic complex is now primed for reactivity, thus cleaving the C­(sp2)–Br bond and ultimately delivering the C­(sp2)–C­(sp3) bond in high yields.

The Anionic Pathway in the Nickel‐Catalysed Cross‐Coupling of Aryl Ethers

“…TheT HF solvate Li 2 (THF) 4 Ph 2 NiCOD (2a)w as isolated in 65 %c rystalline yield, whereas Li 2 - Organometallic Ni 0 -ates have been reviewed by Jonas and Krüger; [28,42] however, only examples with ethylene as the ligand have been structurally authenticated by XRD studies. [19,25,26,31,36] Thel ithium nickelates (2a-c)a re extremely sensitive to air and moisture,a nd must be stored at low temperatures to prevent decomposition.…”

“…It has also been proposed that this co-complexation is an equilibrium that often lies towards the starting components, [19] and only nickelates derived from highly nucleophilic organometallic reagents in combination with ah ighly Lewis acidic Ni 0 source have been isolated to date. [19,[25][26][27][28][29][30][31]36] Nevertheless, this does not discount the possibility that transient nickelate intermediates may still form when milder nucleophiles such as organoboron reagents are used, but the requirement for electron-rich ligands and harsh reaction conditions does suggest that ad ifferent mechanism may be in operation. [15] Then ext step of the catalytic cycle is the coordination of the aryl ether substrate to the Lewis acidic metal (C or G).…”

“…[8][9][10] In 1979, Wenkert et al [11] reported that aryl ethers could serve as electrophiles in Kumada-Corriu type cross-coupling reactions with Grignard reagents under Ni catalysis.U nlike aryl esters and other activated phenol-derived electrophiles [8,10] in which the C(sp 2 )ÀOb ond can be cleaved by Ni 0 species,t he direct oxidative addition of aryl ethers is thermodynamically and kinetically unfavourable, [12][13][14][15][16] which suggests that non-conventional mechanisms are in operation, particularly for transformations occurring under mild conditions.W ang, Uchiyama, and co-workers have provided theoretical support for an alternative anionic pathway (Scheme 1) through DFT calculations,w hich identified that anionic Ni 0 -ate complexes are key intermediates that enable C À Ob ond cleavage. [14,16] Nickelate intermediates have been proposed in several Ni-promoted functionalisation reactions, [17][18][19][20] but catalytically relevant examples have rarely been isolated [21] and direct experimental evidence for their role in the anionic pathway has yet to be established.…”

“…[25][26][27][28][29][30][31] However,t he ability to observe or isolate these sensitive complexes depends on the Lewis acidity and basicity of the two species,which is influenced by the choice of ligand and substituents,and this interaction can be represented as an equilibrium that often lies towards the mono-metallic components. [19] Herein, we provide detailed spectroscopic and structural insights into the anionic pathway through the isolation and characterisation of as eries of structurally diverse lithium Scheme 1. Proposed anionicp athway in the nickel-catalysed crosscoupling of aryl ethers.…”

The Anionic Pathway in the Nickel‐Catalysed Cross‐Coupling of Aryl Ethers

“…[25][26][27][28][29][30][31] However,t he ability to observe or isolate these sensitive complexes depends on the Lewis acidity and basicity of the two species,which is influenced by the choice of ligand and substituents,and this interaction can be represented as an equilibrium that often lies towards the mono-metallic components. [19] Herein, we provide detailed spectroscopic and structural insights into the anionic pathway through the isolation and characterisation of as eries of structurally diverse lithium nickelates derived from Ni(COD) 2 and PhLi. In addition, we demonstrate dramatic solvent and donor effects that suggest that the hetero-bimetallic nickelates work cooperatively to enable the smooth activation of the aryl ether substrate.…”