Redox Activity of Pyridine-Oxazoline Ligands in the Stabilization of Low-Valent Organonickel Radical Complexes

Abstract

Low-valent organonickel radical complexes are common intermediates in cross-coupling reactions and metalloenzyme-mediated processes. The electronic structures of N-ligand supported nickel complexes appear to vary depending on the actor ligands and the coordination number. The reduction products of a series of divalent (pyrox)Ni complexes establish the redox activity of pyrox in stabilizing electron-rich Ni­(II)–alkyl and −aryl complexes by adopting a ligand-centered radical configuration. The reduced pyrox imparts an enhanced trans-influence. In contrast, such redox activity was not observed in a (pyrox)­Ni­(I)–bromide species. The excellent capability of pyrox in stabilizing electron-rich Ni species resonates with its proclivity in promoting the reductive activation of C­(sp3) electrophiles in cross-coupling reactions.

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

“…The electronic effect of the ligand appears to be insignificant on the activation of C­(sp 3 ) electrophiles, as evident by the similar rates between bpy and 4,4′- t Bu-bpy (Figure ). The lack of an electronic effect is attributed to the redox activity of bpy and derivatives, , which dilutes the electronic effect of spectator ligands on the nickel center. The success of 4,4′- t Bu-bpy in nickel-catalyzed cross-coupling reactions may be attributed to its excellent solubility in organic solvents or an improved reactivity of other steps in the catalytic cycle.…”

Controlling Ni redox states by dynamic ligand exchange for electroreductive Csp3–Csp2 coupling

“…On the basis of these reports, we hypothesized that XEC is restricted to electrophiles that react with comparable rates at Ni I , whereas electrophiles that are exceedingly reactive (3° alkyl halides) or unreactive (Ar–Cl/OTf) at Ni I are incompatible coupling partners (Fig. 1C) ( 20 , 25 , 26 ). This competition between activation of alkyl and aryl electrophiles is circumvented in conventional Suzuki or Negishi methodologies that preactivate aryl halides in separate synthetic sequences as organoboron or organozinc reagents, respectively ( 27 – 30 ).…”

Nickel-catalyzed reductive coupling of unactivated alkyl bromides and aliphatic aldehydes

“…This intermediate can then engage the alkyl halide coupling partner via either a radical-chain process as noted by Weix 30 , or through a sequential reductionoxidative addition pathway involving Ni(I) intermediate 1c, to produce 1d. 48,49 Regardless of the exact nature of this step, the resulting dialkyl Ni(III) intermediate 1d can then undergo reductive elimination to furnish the coupled product and a Ni(I) salt. This Ni(I) salt is either reduced by Mn to regenerate the active Ni(0) catalyst or activates another equivalent of alkyl halide to propagate the radical chain process.…”