Visible Light Photoredox Catalyst

09 March, 2016 / by SK / in Molecules

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General Characteristics

Transition metal complexes such as ruthenium(II) polypyridyl and iridium(III) phenylpyridyl are known to function as photoredox catalysts under irradiated conditions with visible light (λ=400-700 nm).

Because these systems do not rely on high energy ultraviolet light, side reactions can be expected to be minimized. The easy operation and environmental friendliness are also positive aspects from the perspective of green chemistry.

The utilization of visible light as an energy source itself is drawing an attention as an important technological paradigm.

General References

<Ru(bpy)₃²⁺>

Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. doi:10.1016/0010-8545(82)85003-0
Julis, V.; Balzani, V. Coord. Chem. Rev. 1988, 84, 85. doi:10.1016/0010-8545(88)80032-8
Broomhead, J. A.; Young, C. G. Inorg. Synth. 1990, 28, 338. doi:10.1002/9780470132593.ch86

<Ir(ppy)₂(dtb-bpy)⁺>

Bernhard, S.; Malliaras, G. G. et al. J. Am. Chem. Soc. 2004, 126, 2763. DOI: 10.1021/ja0345221

Zeitler, K. Angew. Chem. Int. Ed. 2009, 48, 9785. doi:10.1002/anie.200904056
Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527. doi:10.1038/nchem.687
Teply, F. Collect. Czech. Chem. Commun. 2011, 76, 859. doi:10.1135/cccc2011078
Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102. doi:10.1039/b913880n
Tucker, J. Q.; Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617. DOI: 10.1021/jo202538x
Allen, A. E.; MacMillan, D. W. C. Chem. Sci. 2012, 3, 633. DOI: 10.1039/c2sc00907b
Shi, L.; Xia, W. Chem. Soc. Rev. 2012, DOI: 10.1039/c2cs35203f
Ravelli, D>; Fagnoni, M. ChemCatChem 2012, 4, 169. DOI: 10.1002/cctc.201100363

Reaction Mechanism

(Extracted from Collect. Czech. Chem. Commun. 2011, 76, 859 and used with modification)

The mechanism of how Ru(bpy)₃²⁺ is thought to operate is depicted above with basic parameters (vs. SCE).

(1) Excitation from the ground state by the energy of visible light.

(2) Intersystem crossing (ISC) to the triplet state (³MLCT).

(3)(3’) In the presence of a sacrificial oxidant or reductant, quenching leads to the formation of oxidizing species (Ru³⁺) or reductive species (Ru⁺), respectively.

(4) Ru³⁺ or Ru⁺ promotes the chemical reaction and returns to the ground state.

Therefore, the ruthenium can act as either an oxidant or reductant depending on the type of substrates and reagents.

Fine-tuning of redox potential is also possible by modifying the ligand’s electronic property.

Examples

Merging of photoredox catalysis with organocatalysis.^[1]

The late-stage trifluoromethylation of Lipitor.^[2]

Experimental Tips
References

Nicewicz, D. A.; MacMillan, D. A. Science 2008, 322, 77. doi:10.1126/science.1161976
Nagib, D. A.; MacMillan, D. W. C. Nature 2011, 480, 224. doi:10.1038/nature10647
< Other Representative Report in Organic Synthesis>
Ischay, M. A.; Anzovino, M. E.; Du, J.; Yoon, T. P. J. Am. Chem. Soc. 2008, 130, 12886. doi:10.1021/ja805387f

Dai, C.; Narayanam, J. M. R.; Stephenson, C. R. J. Nat. Chem. 2011, 3, 140. doi:10.1038/nchem.949
Neumann, M.; Fuldner, S.; Konig, B.; Zeitler, K. Angew. Chem. Int. Ed. 2011, 50, 951. DOI: 10.1002/anie.201002992
McNally, A.; Prier, C. K.; MacMillan, D. W. C Science 2011, 334, 1114. DOI:10.1126/science.1213920