Heterolytic versus Homolytic: Theoretical Insight into the Ni⁰-Catalyzed Ph–F Bond Activation

Abstract

The Ni0-catalyzed borylation of fluorobenzene (PhF) was theoretically investigated. Density functional theory (DFT) calculations disclosed that the Ph–F bond activation occurred heterolytically via an unprecedented nucleophilic aromatic substitution reaction (SNAr) assisted by an sp2–sp3 diboron complex [B2nep2·(OPh)]‑Na+, which forms a Ni0-ate complex as an active species. The diboron-ate complex stabilizes the transition state of the Ph–F bond activation through three interactions, a Ni···O coordination, a Na+···F cationic dipole interaction, and a charge transfer arising from NaOPh. On the other hand, the Ph–F bond activation catalyzed by Ni0(dcpe) and Ni0(PCy3)2 complexes has also been studied to allow a comparison between the monophosphine and bisphosphine ligands. Results suggest that Ni0(PCy3)2 is less effective than Ni0(dcpe) for the concerted oxidative addition of the Ph–F bond because the Ni dπ orbital of Ni0(PCy3)2 is at a lower energy level than that of Ni0(dcpe) in the equilibrium geometry. The characteristic molecular orbital features of Ni0-catalyzed Ph–F bond activation via both the nucleophilic aromatic substitution reaction (heterolytic) and the concerted oxidative addition (homolytic) were theoretically disclosed.

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

“…Interestingly, despite C being somewhat more stable (2.7 kcal mol −1 ) than E , the Ni 0 (silyl)-ate complex is considered a plausible active species, because the pre-catalyst Ni 0 (cod) 2 is inactive for the inert C(sp 2 )–F bond activation. 20,36 As shown in Scheme S1,† in intermediate E , two cod molecules coordinate to the Ni 0 -center in an η 2 manner, which is more stable than a single cod molecule coordinated in an η 4 manner.…”

“…In the presence of Ni 0 (cod) 2 , the coordination of the silyl anion to the Ni 0 -center could not be ruled out because the coordination energy between Ni and the silyl anion is −52.5 kcal mol −1 , forming a Ni 0 (silyl)-ate complex, K + [Ni 0 (cod) 2 (SiEt 3 )] − , E. Interestingly, despite C being somewhat more stable (2.7 kcal mol −1 ) than E, the Ni 0 (silyl)-ate complex is considered a plausible active species, because the pre-catalyst Ni 0 (cod) 2 is inactive for the inert C(sp 2 )-F bond activation. 20,36 As shown in Scheme S1, † in intermediate E, two cod molecules coordinate to the Ni 0center in an η 2 manner, which is more stable than a single cod molecule coordinated in an η 4 manner.…”

“…The relatively higher energy of TS I/L can be attributed to a smaller charge transfer (CT) stabilization, as the d π orbital of the Ni 0 -ate complex is at a lower energy level, unfavorable for CT to the σ* orbital of the distorted C(sp 2 )-F bond. 20,36 On the other hand, the energy of TS I/L is relatively high due to the absence of the K⋯F interaction (K⋯F distance is 4.5 Å). Similarly, TS I/M , with a larger K⋯F distance (5.3 Å), as shown in Table 1, struggles to overcome the substantial distortion energy resulting from the dissociation of the Ni-Si coordination bond.…”

“…The Ni 0mediated S N Ar pathway for activating the C(sp 2 )-F bond is not unreasonable, because Ni exhibits strong nucleophilicity in ate-complexes. 20,21 Regarding the competition of these catalytic cycles, another open question is addressed: which is the most favorable one for the inert C(sp 2 )-F bond activation among the four pathways?…”

Stoichiometric and Catalytic Lithium Nickelate-Mediated C–F Bond Alkynylation of Fluoroarenes

“…While the C–F activation of polyfluorinated arenes is well established for many transition-metal and low-valent main-group complexes, the selective C–F functionalization of monofluorinated arenes is challenging due to the high bond dissociation energy (BDE) of the C(sp 2 )–F bond (125.6 kcal mol –1 for Ph–F vs 114 kcal mol –1 for C 6 F 6 and 65.1 kcal mol –1 for Ph–I) . The C–F activation of monofluorinated arenes provides opportunities for orthogonal cross-coupling strategies or the late-stage functionalization of substituted arenes, , and numerous stoichiometric, catalytic, and mechanistic studies have been reported to date using nickel complexes. − …”