Monovalent Nickel-Mediated Radical Formation: A Concerted Halogen-Atom Dissociation Pathway Determined by Electroanalytical Studies

Abstract

The recent success of nickel catalysts in stereoconvergent cross-coupling and cross-electrophile coupling reactions partly stems from the ability of monovalent nickel species to activate C(sp 3 ) electrophiles and generate radical intermediates. This electroanalytical study of the commonly applied (bpy)Ni catalyst elucidates the mechanism of this critical step. Data rule out outersphere electron transfer and two-electron oxidative addition pathways. The linear free energy relationship between rates and the bond-dissociation free energies, the electronic and steric effects of the nickel complexes and the electrophiles, and DFT calculations support a variant of the halogen-atom abstraction pathway, the inner-sphere electron transfer concerted with halogen-atom dissociation. This mechanism accounts for the observed reactivity of different electrophiles in cross-coupling reactions and provides a mechanistic rationale for the chemoselectivity obtained in crosselectrophile coupling over homocoupling.

Comparing Halogen Atom Abstraction Kinetics for Mn(I), Fe(I), Co(I), and Ni(I) Complexes by Combining Electroanalytical and Statistical Modeling

“…The rationalization was described in our previous study on the Co(Pyrox)Br 2 complex (Figure 3c), wherein the curved pattern is indicative of a halogen atom abstraction mechanism. The reaction is facilitated by the stabilization of radical character on the substrates’ benzylic carbon by either electron‐rich or electron‐deficient substituents, [11b] of which the same effect was disclosed by Liu and Diao's study [9c] . As a result, the curved Hammett response for the new complexes suggests an inner‐sphere halogen atom abstraction mechanism of the benzylic halides.…”

“…Furthermore, in contrast to the conflicted conclusion drawn by the Hammett and secondary substrate effect studies, a unified pattern accounting for substrates’ kinetic behavior is afforded by the MLR analysis, where the Mn, Fe, Ni, and two Co's mechanistic models were integrated by one set of molecular descriptors ( NBO X , and Spin rad ), suggesting they react through the same general process. Alternatively, the C−X bond dissociation energy (BDE) has been proposed to determine the halogen abstraction kinetics, [9c,25] thus a study of BDE as a parameter is also conducted, where the result and discussion is given in the supporting information.…”

“…In contrast, investigation of a Co(I) complex bearing two 2,6‐bis(pyrazol‐1‐yl)pyridine (BPP) tridentate ligands [12] provided evidence of an alternative mechanism of an outer‐sphere electron transfer pathway of benzylic halides. A more recent study by Liu and Diao et al disclosed a halogen‐atom dissociation mechanism for the activation of alkyl halides by a catalysis‐relavent square planar Ni(I)(BPy) complex [9c] . The central outcome of these studies was two‐fold: 1) the electroanalytical tools are enabling due to their ability to be rapidly applied to new systems, and 2) the kinetic data and resulting statistical modelling can be used in concert to determine subtle mechanistic features not available through the study of a singular substrate or catalyst.…”

Mechanistic Understanding of Arylation vs Alkylation of Aliphatic Csp3–H Bonds by Decatungstate–Nickel Catalysis

“…On the other hand, 1e OA through the Br atom transfer transition state [6−10′] ‡ has a free-energy barrier of 19.4 kcal/mol, which is clearly favored over classic OA via [6− 5′] ‡ and thus leads to T 10-Ni II and CyCH 2 • (Figure 3b). 77 According to the overall M1−M4 reaction pathways (Figure S4), T To elucidate the distinct reactivity of ArBr versus AlkBr toward classic 2e OA to 6-Ni I , we performed fragment analysis of transition states [6−5] ‡ and [6−5′] ‡ using the distortioninteraction model and the EDA-NOCV model (Table 1 and Table S2 in SI). The distortion-interaction analysis indicates a small difference in the distortion term, ΔE dist , between [6−5] ‡ and [6−5′] ‡ , 6.6 kcal/mol, while the contribution of the stabilizing interaction term, ΔE int , is 28.3 kcal/mol stronger in [6−5] ‡ compared to [6−5′] ‡ (Table 1).…”

Reactivity of (bi-Oxazoline)organonickel Complexes and Revision of a Catalytic Mechanism

“…Therefore, we attribute the formation of benzyl radical from benzyl chloride to the cooperative reactivity of manganese and TMSCl, catalyzed by Ni­(II) species. This mechanism, involving manganese and TMSCl, complements the reactivity of Ni­(I) complexes in C­(sp 3 ) electrophile activation but is restricted to benzyl halides, evident by the inactivity of chlorocyclohexane (cf. SI).…”