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Radical Reactivity: Overview01:11

Radical Reactivity: Overview

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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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Radical reactions can occur either intermolecularly or intramolecularly. In an intermolecular radical reaction, a nucleophilic radical adds to an electrophilic alkene or vice versa. In such reactions, the radical and generally the alkene, which is also called the radical trap, are two different molecules. Additionally, for such intermolecular reactions to occur, the radical trap must be active, present in an excess concentration, and the radical starting material must have a weak...
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Radical Reactivity: Nucleophilic Radicals01:16

Radical Reactivity: Nucleophilic Radicals

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Radicals adjacent to electron-donating groups are called nucleophilic radicals. These radicals readily react with electrophilic alkenes. The SOMO–LUMO interactions are the driving force for the reaction, where the high-energy SOMO of the electron-rich, nucleophilic radicals interacts with the low-energy LUMO of the electron-deficient, electrophilic alkenes. Such SOMO–LUMO interactions are the basis of reactive radical traps, affecting the selectivity in radical reactions. For...
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Radical Formation: Addition00:47

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Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.
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The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
Along with electronic...
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Radical Formation: Overview01:03

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A bond can be broken either by heterolytic bond cleavage to form ions or homolytic bond cleavage to yield radicals. A fishhook arrow is used to represent the motion of a single electron in homolytic bond cleavage. There are two main sources from which radicals can be formed:
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Nickel-Mediated Radical Capture: Evidence for a Concerted Inner-Sphere Mechanism.

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Area of Science:

  • Organometallic Chemistry
  • Catalysis
  • Radical Chemistry

Background:

  • Nickel catalysis is vital for C(sp3) cross-coupling via carbon-centered radicals.
  • A stepwise mechanism involving nickel(III) intermediates is widely accepted.
  • A concerted radical capture and bond formation pathway has been largely unexplored.

Purpose of the Study:

  • To investigate the ligand effect and kinetics of nickel-mediated radical capture and reductive elimination.
  • To differentiate between stepwise and concerted mechanisms in nickel catalysis.
  • To provide design principles for nickel-catalyzed cross-coupling reactions.

Main Methods:

  • Radical clock experiments
  • Spectroscopic investigation
  • Electrochemical studies
  • Multivariate linear regression analysis of [(pybox)Ni(Ar)]BArF4 complexes

Main Results:

  • Strong correlation between radical capture rate and HOMO/LUMO energies, and charge stabilization.
  • Data support a concerted pathway, ruling out stepwise nickel(III) intermediate formation.
  • Redox-active nitrogen ligands facilitate radical capture, unlike nonredox-active phosphine ligands.

Conclusions:

  • Ligand redox activity is critical for nickel-catalyzed radical capture and C-C bond formation.
  • The concerted pathway is supported by electronic and orbital contributions.
  • Ancillary ligand effects are minimal, with bidentate and tridentate ligands showing similar rates.