Nickel(IV) Intermediates in Aminoquinoline-Directed C(sp²)-C(sp³) Coupling.

Area of Science:

• Organometallic Chemistry
• Catalysis
• Organic Synthesis

Background:

• C-H functionalization is a vital transformation in organic synthesis.
• Aminoquinoline-directed C-H functionalization often involves proposed nickel(IV) intermediates.
• Isolation and characterization of these nickel(IV) species have been challenging.

Purpose of the Study:

• To isolate and characterize a nickel(IV) complex analogous to intermediates in C-H functionalization.
• To investigate the stabilizing effect of LX-type ligands on nickel(IV) species.
• To demonstrate the accessibility of nickel(IV) intermediates in catalysis.

Main Methods:

• Ligand design strategy utilizing a picolinate ligand.
• Oxidative addition of a diaryliodonium reagent to a nickel(II) precursor.
• Characterization of the resulting cyclometalated nickel(IV) complex.

Main Results:

• Isolation of a stable, cyclometalated nickel(IV) σ-aryl complex.
• The nickel(IV) complex is stable at room temperature.
• Reductive elimination to form a C(sp2)-C(sp3) bond occurs under mild conditions (70 °C).

Conclusions:

• Nickel(IV) intermediates in C-H functionalization catalysis are accessible.
• LX-type ligands, such as picolinate, significantly stabilize nickel(IV) species.
• This work provides direct evidence for the role of nickel(IV) in catalytic cycles.