Carbon-centered radical capture at nickel(II) complexes: Spectroscopic evidence, rates, and selectivity

Area of Science:

• Organometallic Chemistry
• Catalysis
• Radical Chemistry

Background:

• Carbon-centered radical capture by nickel(II) is vital in cross-coupling and metallaphotoredox catalysis.
• This fundamental catalytic step has lacked detailed experimental characterization.
• Understanding radical capture mechanisms is key to advancing synthetic methodologies.

Purpose of the Study:

• To experimentally investigate the mechanism, kinetics, and stereoselectivity of carbon-centered radical capture by nickel(II) complexes.
• To elucidate structure-activity relationships of ligands in nickel-catalyzed radical reactions.
• To provide experimental benchmarks for radical capture rates and activation energies.

Main Methods:

• Spectroscopic analysis (e.g., EPR, UV-Vis) to detect nickel(III) intermediates.
• Kinetic studies to determine reaction rates and activation energies for radical capture.
• Stoichiometric reactions using well-defined chiral nickel complexes to assess stereoselectivity.

Main Results:

• Evidence for nickel(III) intermediate formation during radical capture was observed.
• Distinct rate-determining steps were identified for C(sp3)-C(sp3) versus C(sp2)-C(sp3) bond formation.
• Radical capture rates were quantified as 10^7 M^-1s^-1 for primary and 10^6 M^-1s^-1 for secondary radicals.
• Activation energies were found to be higher than predicted by prior computational studies.
• Chiral nickel complexes demonstrated that radical trapping can induce diastereoselectivity and enantioselectivity, strongly influenced by ligand structure.

Conclusions:

• The experimental characterization provides critical insights into the mechanism of nickel-catalyzed radical reactions.
• Ligand design plays a crucial role in controlling both reactivity and stereoselectivity in radical capture.
• This work establishes experimental benchmarks for radical capture kinetics and highlights the potential for asymmetric catalysis.