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Review
. 2023 May 3;145(17):9369-9388.
doi: 10.1021/jacs.3c01000. Epub 2023 Apr 20.

Iron Photoredox Catalysis-Past, Present, and Future

Lisa H M de Groot  1 , Aleksandra Ilic  1 , Jesper Schwarz  1 , Kenneth Wärnmark  1
Affiliations
Review

Iron Photoredox Catalysis-Past, Present, and Future

Lisa H M de Groot et al. J Am Chem Soc. .

Abstract

Photoredox catalysis of organic reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramolecular electron transfer involving charge-transfer states, for example, through visible-light-induced homolysis. (3) Improving the excited-state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design. We seek to give an overview and evaluation of recent developments in this rapidly growing field and, at the same time, provide an outlook on the future of iron-based photoredox catalysis.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
General mechanism of photoredox catalysis via SET; D = (sacrificial) electron donor, A = (sacrificial) electron acceptor, PC = photoredox catalyst.
Scheme 1
Scheme 1. Steps That Commonly Occur upon Light Absorption by a TM-PC and Reductive Quenching
Q = quencher, S = substrate, kr = rate of radiative decay, knr = rate of nonradiative decay, kq = rate of quenching, ηq= quenching yield, kbc = rate of back-combination, kce = rate of cage escape, ηce = cage escape yield, kbd = rate of back donation, ks = rate of SET to the substrate.
Figure 2
Figure 2
Schematic ES energies of Ru(II)L6 and Fe(II)L6 complexes. Adapted with permission from ref (27). Copyright 2016 ACS.
Scheme 2
Scheme 2. Visible-Light-Mediated Enantioselective Alkylation of Aldehydes Using [Fe(bpy)3]Br2 (4) as PC
Scheme 3
Scheme 3. Visible-Light-Mediated Enantioselective Alkylation of Aldehydes Using [Fe(bpy)3]Br2 (4) as PC
Adapted with permission from ref (28). Copyright 2015 ACS.
Scheme 4
Scheme 4. Visible-Light-Mediated Synthesis of Carbazoles Using [Fe(phen)3](NTf2)2 (5) as PC and O2, under Flow Conditions
Scheme 5
Scheme 5. Proposed Mechanism for the Oxidative Quenching of [Fe(II)(tren(py)3)]2+ (8) by DDQ Via SET from the 5T2 State Following MLCT-MC Conversion
D = electron donor. Adapted with permission from ref (41). Copyright 2020 ACS.
Scheme 6
Scheme 6. Visible-Light-Mediated Oxidative Fragmentation of Ethers and Acetals Using [Fe(acac)3] (9) as PC
Scheme 7
Scheme 7. Proposed Mechanism for the Visible-Light-Mediated Oxidative Fragmentation of Ethers and Acetals Using [Fe(acac)3] (9) as PC
Adapted with permission from ref (44). Copyright 2022 ACS.
Figure 3
Figure 3
General mode of action for an inner-sphere visible-light-induced homolysis (VLIH) process; S = substrate.
Scheme 8
Scheme 8. Overview of Inner-Sphere CT Photoredox Reactions Employing Iron
Scheme 9
Scheme 9. Proposed Mechanistic Cycle of the Intramolecular Aromatic C–H Oxygenation Reaction
Adapted with permission from ref (57). Copyright 2020 ACS.
Scheme 10
Scheme 10. Proposed Mechanism for the Photocatalytic Aerobic Oxidation of Alcohols to Acids
Adapted with permission from ref (58). Copyright 2021 ACS.
Scheme 11
Scheme 11. Proposed Mechanism for the Photocatalytic Amination of Unstrained Cyclic Alcohols
Adapted with permission from ref (60). Copyright 2022 ACS.
Scheme 12
Scheme 12. Proposed Mechanism for the Aerobic Oxidation of Olefins
Adapted with permission from ref (63). Copyright 2020 Wiley.
Scheme 13
Scheme 13. General Catalytic Cycle for Photoredox Catalysis Using Free Chlorine Radicals Generated from a Photoreactive Iron Species
Scheme 14
Scheme 14. Overview of Inner-Sphere CT Photoredox Reactions Utilizing In Situ-Generated Chlorine Radicals
Scheme 15
Scheme 15. Overview of Reactions Utilizing In Situ-Generated Iron Catalysts Operating Via Mechanisms Other than VLIH
Figure 4
Figure 4
(left to right) The first Fe-NHC complex with an extended CT ES lifetime (9 ps) and the three iron complexes with demonstrated photoactive CT ESs to date (0.1–2 ns). Ar = p-tolyl.
Scheme 16
Scheme 16. (A) Dehalogenation Reaction Studied by Troian-Gautier and Co-Workers; R3N = Amine Used as Sacrificial Reductant. (B) Follow-up Dehalogenation/Cyclization Reaction by Troian-Gautier and Co-Workers. (C) General Mechanism of the HAT Reaction
Adapted with permission from refs (84) (Copyright 2021 ACS) and (85) (Copyright 2021 RSC).
Scheme 17
Scheme 17. (A) Radical Cationic [4 + 2] Cycloaddition Driven by Green Light Irradiation of [Fe(btz)3](PF6)3 (11). (B) Mechanism proposed by Kang and Co-Workers
Adapted with permission from ref (87). Copyright 2022 ACS.
Scheme 18
Scheme 18. (A) The ATRA Reaction Driven by Green Light Irradiation of [Fe(btz)3](PF6)3 (11). Reductive Quenching: 0.5 mol % 11, TEA (0.34 equiv) in Acetonitrile/Methanol 4:3. Oxidative Quenching: 1.5 mol % 11 in Acetonitrile. (B) The Reaction Mechanism of the Reductive Green-Light-Driven ATRA Reaction
Adapted with permission from ref (88). Copyright 2022 RSC.
Scheme 19
Scheme 19. Stoichiometric Photoinduced Aryl–Aryl Coupling Driven by Blue-Light Irradiation of [Fe(II)(phenN,N’^C)2] (13)
Adapted with permission from ref (96). Copyright 2022 ACS.
See this image and copyright information in PMC

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