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

TLDR

Without an external reducing agent, oxidative addition complexes of Ni rapidly undergo β‑hydride elimination at room temperature, forming Ni(0)-carbonyl or Ni(0)-aldehyde complexes. The study investigates the mechanism of Ni(COD)(2)/PCy(3)-catalyzed reductive cleavage of C‑OMe bonds using silanes as reducing agents. Kinetic, isotope‑labeling, and spectroscopic studies reveal that Ni(I) intermediates generated by comproportionation of Ni(II) species drive the C‑OMe cleavage, rather than a Ni(0)/Ni(II) cycle. The results show that the reaction proceeds via Ni(I) intermediates, water deactivates the catalyst by forming a Ni‑bridged hydroxo species, and a classical Ni(0)/Ni(II) catalytic cycle is not operative.

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.