Oxidative Addition of Aryl Halides to a Ni(I)-Bipyridine Complex

Abstract

The oxidative addition of aryl halides to bipyridine- or phenanthroline-ligated nickel(I) is a commonly proposed step in nickel catalysis. However, there is a scarcity of complexes of this type that both are well-defined and undergo oxidative addition with aryl halides, hampering organometallic studies of this process. We report the synthesis of a well-defined Ni(I) complex, [(CO2Etbpy)NiCl]4 (1). Its solution-phase speciation is characterized by a significant population of monomer and a redox equilibrium that can be perturbed by π-acceptors and σ-donors. 1 reacts readily with aryl bromides, and mechanistic studies are consistent with a mechanism proceeding through an initial Ni(I) → Ni(III) oxidative addition. Such a process was demonstrated stoichiometrically for the first time, affording a structurally characterized Ni(III) aryl complex.

Halide Noninnocence and Direct Photoreduction of Ni(II) Enables Coupling of Aryl Chlorides in Dual Catalytic, Carbon–Heteroatom Bond-Forming Reactions

“…Oxidative addition at Ni(I) has been directly observed in recent reports with aryl iodides, with a reduced Ir complex, a radiolytic pulse, or photolysis of an organonickel(II) complex as the source of the Ni(II) to Ni(I) reduction. 15,37,42 Furthermore, oxidative addition of aryl bromides has been followed via EPR in recent work 28 and also through stoichiometric studies of a dimeric Ni(I) complex, 43 both of which support oxidative addition occurring at a monomeric Ni(I) center. Although useful, none of these studies followed Ni(I) generated directly through photoinduced electron transfer (PET) as in our approach.…”

Exogenous Ligand‐Free Nickel‐Catalyzed Carboxylate O‐Arylation: Insight into NiI/NiIII Cycles**

“…[14] Dimeric nickel complexes have been identified before as important intermediates in nickel-catalyzed cross-coupling, but this represents the first observation of a Ni I À Ni II dimer under exogenous ligand-free conditions. [14,20,21] Next, the reactivity of the in situ formed Ni I À Ni II dimer was examined. Therefore we treated the pink-colored nickel solution, obtained after reduction, with 5 equiv.…”

“…A variety of electron-poor aryl bromides were found to be effective coupling partners (11)(12)(13)(14)(15). For the carboxylic acid coupling partner, sterically hindered pivalic acid and cyclohexanecarboxylic acid (16,17) furnished the corresponding ester efficiently, as well as various aromatic carboxylic acids (19)(20)(21). Only acetic acid hampered catalysis and afforded the ester product (18) in poor yield (< 10 %).…”

“…Investigations have unveiled that specific Ni I -complexes bearing a bipyridine (bpy) based ligand [14,[18][19][20] are indeed able to activate aryl halides [14,19] and that the required Ni III state can be accessed via oxidative addition of a Ni I complex. [21,22] So far mechanistic insights through catalytic studies and stoichiometric reactions have demonstrated the validity of the elementary steps proposed in the self-sustained Ni I /Ni III catalytic cycle. However, these catalytic and stoichiometric studies do not account for the non-equimolar ratios of substrates often required for productive CÀ heteroatom bond formations.…”

Nickel-Catalyzed Radical Mechanisms: Informing Cross-Coupling for Synthesizing Non-Canonical Biomolecules

“…Our recent study delineated that a cross-electrophile coupling reaction proceeds through a sequential reduction mechanism (Scheme 1C). 1 The preference for cross-electrophile coupling over homocoupling stems from the distinct activation of C(sp 2 ) and C(sp 3 ) electrophiles by two separate nickel(I) species, nickel-halide 2 and nickel-aryl 4, via oxidative addition 20 and a radical mechanism, respectively. Our program centers around dissecting each radical-involved step: (I) radical generation from the interaction of Ni(I)-aryl 4 with C(sp 3 )-halides; (II) radical capture by Ni(II) 5; and (III) reductive elimination from nickel(III) 6.…”