Attenuation of Ni(0) Decomposition: Mechanistic Insights into AgF-Assisted Nickel-Mediated Silylation

Abstract

In nickel-mediated Kumada cross-coupling reactions, low valent active nickel complexes are often generated in situ and the ligands usually govern the reactivity or stability of these complexes. However, the decomposition of active nickel complexes is inevitable if the subsequent reaction is sluggish. While we recently developed AgF-assisted nickel catalysis to cross-couple methyl ethers and silylmagnesium reagents, the intriguing catalytic role of AgF and the actual active nickel species remains elusive. Recently, both Ni(0) and Ni(I) intermediate complexes are identified as active species in Kumada cross-coupling reactions. Control experiments in combination with 31 P nuclear magnetic resonance (NMR) suggest that AgF attenuates the decomposition of in situ generated Ni(0) species. The plausible Ni(0) and Ni(I) intermediate complexes were synthesized, and experimental findings are consistent with the actual catalytic cycle being Ni(0)/Ni(II) rather than Ni(I)/Ni(III).

Mechanisms of the Nickel-Catalysed Hydrogenolysis and Cross-Coupling of Aryl Ethers

“…Anionic nickelates derived from silyl anions have more recently been implicated in the enantiospecific silylation of benzyl methyl ethers. 56 57 31 P NMR spectroscopy studies support that an Ni(0)-ate complex of the constitution [L n Ni–SiMe 3 ] – [MgI(solv)] + is formed on addition of Me 3 SiMgI(TMEDA) to NiBr 2 (diglyme)/PCy 3 , or by direct treatment of the Ni(0) precursor, Ni(PCy 3 ) 2 , with Me 3 SiMgI(TMEDA). This same signal is also observed in the reaction mixture, and cyclic voltammetry and UV-Vis spectroscopy studies suggest that this Ni(0)-ate is the predominant resting state.…”

Utilizing Ni-Catalyzed Cross-Coupling as a Reversed Strategy for Homogeneous Deprotection of Benzyl Ethers in Batch and Flow

“…Rasappan and co-workers proposed catalyst decomposition to metallic nickel as a problematic pathway for efficient catalysis in a nickel-mediated Kumada coupling of ethers with silylmagnesium halides. 12,13 The nucleation of reduced metal species can proceed through slow multimetallic nanocluster formation followed by autocatalytic surface growth. 14 The rate of multimetallic nucleation events can be decreased by decreasing the concentration of reduced homogeneous metal species.…”

“…The observation of black particles suggested that catalyst decomposition to form nickel black may have inhibited product formation. Rasappan and co-workers proposed catalyst decomposition to metallic nickel as a problematic pathway for efficient catalysis in a nickel-mediated Kumada coupling of ethers with silylmagnesium halides. , The nucleation of reduced metal species can proceed through slow multimetallic nanocluster formation followed by autocatalytic surface growth . The rate of multimetallic nucleation events can be decreased by decreasing the concentration of reduced homogeneous metal species. , Decreasing the catalyst loading to 10 mol % and running the reaction at 120 °C resulted in a 1a / 2a area ratio of 9/91 (Table , entry 5).…”

Unravelling the Mechanism of Nickel Mediated Cross‐Electrophile‐ Electrophile Coupling Between Katritzky Salts and Acid Chlorides

“…Thus, a systematic study is required on distinguishing the radical chain reaction from sequential reduction, identify the active nickel species and the actual reductant. As part of our ongoing study on nickel‐mediated cross‐coupling reactions [15,17] and mechanistic investigations, [17b] we present here our experimental and spectroscopy‐based mechanistic studies, including NMR, UV, and CV. Our finding reveals the active Ni(I) species, a radical chain mechanism, a synergistic reduction of pyridinium salts by Ni(I) and Mn, the rate‐limiting reductive alkylation and the resting state Ni(II) species.…”