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Photoredox Catalysis Mediated by Tungsten(0) Arylisocyanides
Javier Fajardo1, Alexandra T Barth1, Maryann Morales1
1Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States.
Journal of the American Chemical Society
|November 10, 2021
Summary
Tungsten(W) isocyanide complexes efficiently catalyze aromatic substitution reactions. These photoreductants can be regenerated, offering a sustainable approach to BHAS catalysis.
Area of Science:
- Organometallic Chemistry
- Photocatalysis
- Organic Synthesis
Background:
- Tungsten hexakis(isocyanide) complexes (W(CNAr)6) are known photoreductants.
- Base-promoted homolytic aromatic substitution (BHAS) is a valuable synthetic tool.
- Understanding photocatalyst behavior and regeneration is crucial for sustainable chemistry.
Purpose of the Study:
- To investigate the catalytic activity of W(CNAr)6 complexes in BHAS reactions.
- To explore photocatalysis using both one-photon and two-photon excitation.
- To identify pathways for catalyst deactivation and regeneration.
Main Methods:
- Photocatalysis of 1-(2-iodobenzyl)-pyrrole using W(CNAr)6 in deuterated benzene.
- Varying W(CNAr)6 loading and structure to optimize reaction efficiency.
- Stern-Volmer quenching experiments to elucidate catalytic mechanisms.
- Utilizing femtosecond-pulsed 810 nm excitation for two-photon absorption (TPA) studies.
- Characterization of reaction intermediates and products, including seven-coordinate WII-diiodo complexes.
Main Results:
- W(CNAr)6 complexes effectively catalyze BHAS reactions with moderate to high efficiencies.
- Catalyst performance correlates with excited-state reduction potentials.
- Optimal photoreductants W(CNDipp)6 and W(CNDippPhOMe3)6 achieve near-complete conversion at 10 mol% loading.
- Catalysis is initiated by substrate reductive dehalogenation.
- Photocatalysis can be driven by both one-photon (445 nm) and two-photon (810 nm) excitation.
- Reaction termination occurs via formation of seven-coordinate WII-diiodo complexes.
- W(CNDipp)6 can be regenerated through chemical reduction of its corresponding diiodo complex.
Conclusions:
- W(CNAr)6 complexes are highly effective photocatalysts for BHAS reactions.
- The catalytic efficiency is tunable via ligand modification and excited-state potentials.
- BHAS can be initiated via substrate reductive dehalogenation.
- Two-photon absorption provides an alternative excitation pathway for these catalysts.
- Catalyst deactivation is reversible, and regeneration is achievable, enhancing the practicality of this methodology.
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