Bi-Oxazoline (biOx) has emerged as an effective ligand framework for promoting nickel-catalyzed cross-coupling, cross-electrophile coupling, and photoredox-nickel dual catalytic reactions. This report fills the knowledge gap of the organometallic reactivity of (biOx)Ni complexes, including catalyst reduction, oxidative electrophile activation, radical capture, and reductive elimination. The biOx ligand displays no redox activity in (biOx)Ni(I) complexes, in contrast to other chelating imine and oxazoline ligands. The lack of ligand redox activity results in more negative reduction potentials of (biOx)Ni(II) complexes and accounts for the inability of zinc and manganese to reduce (biOx)Ni(II) species. On the basis of these results, we revise the formerly proposed "sequential reduction" mechanism of a (biOx)Ni-catalyzed cross-electrophile coupling reaction by excluding catalyst reduction steps.
Nickel-mediated
carbon–carbon bond formation is an essential
step in the catalytic cycle of cross-coupling reactions. Bi- and bis-oxazoline
(biOx and box) ligands are commonly applied to nickel catalyzed reactions
as they confer ideal redox properties for substrate activation and
new bond formation. The synthesis of square-planar biOx and (box)Ni(II)-dineosilyl
complexes allowed for the direct comparison of their redox properties.
CV studies reveal that (biOx)Ni(II) complexes can be oxidized more
easily than (box)Ni(II) complexes and that biOx is better at stabilizing
Ni(I) states than box. Several factors promote reductive elimination
from d8-organonickel complexes, including the coordination
of L- and X-type ligands, the inner-sphere transfer of one or two
electrons to alkyl halides, and the outer-sphere electron transfer
to oxidants. Analysis of the product distribution utilizing an alkyl
iodide as the oxidant unveiled the mechanism for this reductive elimination
process by distinguishing the one-electron halogen atom abstraction
pathway from the competing two-electron oxidative addition pathway.
Moreover, comparing the rates and the product distributions between
the biOx and box ligands reveals the effect of steric bulk on these
associative pathways.
An efficient gold-catalyzed cyclization/arylidene group transfer cascade reaction of N-propioloyl hydrazones has been developed. This method provides a novel approach for the synthesis of various functionalized 4-arylidenepyrazolones.
A highly efficient AgOTf catalyzed [3,3] sigmatropic rearrangement/1,3-H shift/6π aza-electrocyclization cascade reaction of N-propargylic hydrazones has been developed. This method provides a new mild synthetic route to various polysubstituted 1,6-dihydropyridazines including the 3-CF3-substituted ones with high selectivity.
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