Mechanistic study on C(sp2)–F bond activation by a Ni0(silyl)-ate complex: an outer-sphere pathway via Ni0-mediated nucleophilic aromatic substitution

Abstract

In this study, we conducted a theoretical investigation to clarify the mechanism of C–F bond activation using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, with Et₃SiBpin and KO^tBu serving as reaction partners. Results suggest that a reactive nucleophile KSiEt₃ is generated in situ, which activates the C(sp³)–F bond via a bimolecular nucleophilic substitution (S_N2) with a lower energy barrier (23.6 kcal mol⁻¹) than that (28.2 kcal mol⁻¹) for activation of the C(sp²)–F bond via a nucleophilic aromatic substitution reaction (S_NAr). This is consistent with the experimental result that silylation of the C(sp³)–F bond succeeded without a Ni⁰-catalyst, while that of the C(sp²)–F bond did not. In the presence of Ni⁰(cod)₂, a more reactive ate-complex K⁺[Ni⁰(cod)₂(SiEt₃)]⁻ is generated via the strong coordination of the (SiEt₃)⁻ anion to the Ni⁰-center. The inert C(sp²)–F bond activation thus succeeded via an unusual Ni⁰-mediated S_NAr. Surprisingly, results show the activation energy (21.8 kcal mol⁻¹) for nucleophilic attack by the Ni⁰ center is significantly lower than that (36.2 kcal mol⁻¹) by the (SiEt₃)⁻ anion. Furthermore, the outer-sphere transition state is more stable than the well-known inner-sphere one because the radius of K⁺ is too large to form a multicomponent ring with synergistic stabilization. In addition to the Ni⋯Si coordination, this unprecedented transition state in the C(sp²)–F bond activation is stabilized through multiple non-covalent interactions. Additionally, AIMD simulations were employed to elucidate the dynamic effects of the Ni⁰(silyl)-ate complex initiated C(sp²)–F bond activation, highlighting the pivotal role of multiple non-covalent interactions in the reaction.