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Review
. 2017 Apr 7;7(4):2563-2575.
doi: 10.1021/acscatal.7b00094. Epub 2017 Mar 14.

Photoredox-Mediated Routes to Radicals: The Value of Catalytic Radical Generation in Synthetic Methods Development

Jennifer K Matsui  1 , Simon B Lang  1 , Drew R Heitz  1 , Gary A Molander  1
Affiliations
Review

Photoredox-Mediated Routes to Radicals: The Value of Catalytic Radical Generation in Synthetic Methods Development

Jennifer K Matsui et al. ACS Catal. .

Abstract

Photoredox catalysis has experienced a revitalized interest from the synthesis community during the past decade. For example, photoredox/Ni dual catalysis protocols have been developed to overcome several inherent limitations of palladium-catalyzed cross-couplings by invoking a single-electron transmetalation pathway. This Perspective highlights advances made by our laboratory since the inception of the photoredox/Ni cross-coupling of benzyltrifluoroborates with aryl bromides. In addition to broadening the scope of trifluoroborate coupling partners, research using readily oxidized hypervalent silicates as radical precursors that demonstrate functional group compatibility is highlighted. The pursuit of electrophilic coupling partners beyond (hetero)aryl bromides has also led to the incorporation of several new classes of C(sp2)-hybridized substrates into light-mediated cross-coupling. Advances to expand the radical toolbox by utilizing feedstock chemicals (e.g., aldehydes) to access radicals that were previously inaccessible from trifluoroborates and silicates are also emphasized. Additionally, several organic photocatalysts have been investigated as replacements for their expensive iridium- and ruthenium-based counterparts. Lastly, the net C-H functionalization of the radical partner in an effort to improve atom economy is presented. An underlying theme in all of these studies is the value of generating radicals in a catalytic manner, rather than stoichiometrically.

Keywords: catalysis; cross-coupling; dual catalysis; nickel; photoredox; radicals.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Oxidative and Reductive Pathways of Photocatalysts
Scheme 2
Scheme 2. Photoredox/Nickel Dual Catalysis Cycle
Scheme 3
Scheme 3. Transition-Metal-Based and Organic Photocatalysts
Scheme 4
Scheme 4. Building a Library of Silicates
Scheme 5
Scheme 5. Coupling of Alkyl β-Trifluoroborato Ketones
Scheme 6
Scheme 6. Coupling of Secondary α-Alkoxy Alkyltrifluoroborates
Scheme 7
Scheme 7. Gram-Scale Preparation of Trifluoroboratochromanone
Scheme 8
Scheme 8. Arylation of Trifluoroboratochromanones
Scheme 9
Scheme 9. Mechanism-Based Selective Coupling
Scheme 10
Scheme 10. Three-Step Iterative Cross-Couplings
Scheme 11
Scheme 11. Coupling Boronate-Containing Aryl Bromides
Scheme 12
Scheme 12. One-Pot Iterative Cross-Couplings
Scheme 13
Scheme 13. Silicate Scope with Aryl Triflates
Scheme 14
Scheme 14. Probing Chemoselectivity and Sulfonate Scope
Scheme 15
Scheme 15. Acylation of Trifluoroborates
Scheme 16
Scheme 16. Comparing Radical Sources
Scheme 17
Scheme 17. SET and C–C Bond Cleavage of DHPs
Scheme 18
Scheme 18. Proposed Mechanism for DHP Coupling
Scheme 19
Scheme 19. Aryl Bromide and DHP Scope
Scheme 20
Scheme 20. Tricatalytic Mechanistic Proposal
Scheme 21
Scheme 21. C(sp3)–H Partner Scope
Scheme 22
Scheme 22. Aryl Bromide Scope in Ni/Ir Cross-Coupling
Scheme 23
Scheme 23. KIE and Energy-Transfer Experiments
Scheme 24
Scheme 24. Jablonski Diagram of High-Energy Nickel Complex
Scheme 25
Scheme 25. Current Mechanistic Proposal
Scheme 26
Scheme 26. Alkylation of Diverse Imines
Scheme 27
Scheme 27. Alkenylation and Allylation of Pyrrolidinyltrifluoroborate
Scheme 28
Scheme 28. Mild Cyanation of Alkyltrifluoroborates
See this image and copyright information in PMC

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