title: Combined experimental and theoretical study on the reductive cleavage of inert C-O bonds with silanes: ruling out a classical Ni(0)/Ni(II) catalytic couple and evidence for Ni(I) intermediates.
abstract: A mechanistic and computational study on the reductive cleavage of C-OMe bonds catalyzed by Ni(COD)(2)/PCy(3) with silanes as reducing agents is reported herein. Specifically, we demonstrate that the mechanism for this transformation does not proceed via oxidative addition of the Ni(0) precatalyst into the C-OMe bond. In the absence of an external reducing agent, the in-situ-generated oxidative addition complexes rapidly undergo β-hydride elimination at room temperature, ultimately leading to either Ni(0)-carbonyl- or Ni(0)-aldehyde-bound complexes. Characterization of these complexes by X-ray crystallography unambiguously suggested a different mechanistic scenario when silanes are present in the reaction media. Isotopic-labeling experiments, kinetic isotope effects, and computational studies clearly reinforced this perception. Additionally, we also found that water has a deleterious effect by deactivating the Ni catalyst via  formation of a new Ni-bridged hydroxo species that was characterized by X-ray crystallography. The order in each component was determined by plotting the initial rates of the C-OMe bond cleavage at varying concentrations. These data together with the in-situ-monitoring experiments by (1)H NMR, EPR, IR spectroscopy, and theoretical calculations provided a mechanistic picture that involves Ni(I) as the key reaction intermediates, which are generated via comproportionation of initially formed Ni(II) species. This study strongly supports that a classical Ni(0)/Ni(II) for C-OMe bond cleavage is not operating, thus opening up new perspectives to be implemented in other related C-O bond-cleavage reactions.
authors: Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007, Tarragona, Spain.
authors: Josep
authors: J
authors: Cornella
authors: Enrique
authors: E
authors: Gómez-Bengoa
authors: Ruben
authors: R
authors: Martin
mesh_terms: D002244:Carbon / Q000737:chemistry*; D002384:Catalysis; D018360:Crystallography, X-Ray; D004987:Ethers / Q000737:chemistry; D006841:Hydrocarbons, Aromatic / Q000138:chemical synthesis / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011789:Quantum Theory; D012821:Silanes / Q000737:chemistry*
chemical_list: D004987:Ethers; D006841:Hydrocarbons, Aromatic; D009942:Organometallic Compounds; D012821:Silanes; D002244:Carbon; D009532:Nickel; D010100:Oxygen