DOI: 10.1002/cctc.201100363 Dyes as Visible Light Photoredox Organocatalysts Davide Ravelli and Maurizio Fagnoni* [a] Dedicated to Professor Angelo Albini on the occasion of his 65th birthday Light-driven reactions are of crucial importance for the exis- tence of mankind on Earth. [1] In the aim to exploit photons for synthetic purposes, photocatalysis has recently proven to be a powerful method to perform several organic reactions includ- ing oxidations, reductions, valuable C ÀC bond formation reac- tions, and many others. [2] The photocatalyst (PC, Scheme 1) in the excited state (PC*) is able to activate one of the reagents by means of a chemical reaction, such as an atom or electron transfer process. Highly reactive species, such as radicals or radical ions are then formed and used for synthetic purposes. The photocata- lytic processes require that PC is regenerated from its deacti- vated form (PC D ) by reaction with another intermediate formed in the process (R I , blue part of Scheme 1). [2a] As a result, the photochemical reactions can be extended to classes of molecules usually unreactive under irradiation or transparent to the wavelength used. [2a] Recently, convenient photoredox catalysts [3] such as ruthe- nium(II), mainly [Ru(bpy) 3 Cl 2 ], or iridium(III) complexes have been successfully adopted in organic synthesis. Indeed, oxida- tive quenching of the excited state of an iridium complex ena- bled the photogeneration of electron poor radicals and the subsequent addition onto olefins, resulting in an atom transfer radical addition reaction. [4a] Reductive quenching is also viable, as demonstrated by Stephenson and co-workers in the aza- Henry reaction on N-aryl tetrahydroisoquinolines. [4b] Indeed, a slightly different mode of action requires that the PC D form (either M (nÀ1) + or M (n+1) + ) is the active species, able to promote a monoelectronic reduction or oxidation of an organic mole- cule with the concomitant regeneration of the starting metal complex (red part of Scheme 1). In this case, however, either a sacrificial donor (often a tertiary amine) or a sacrificial acceptor is mandatory for the reaction. [2c,d] A skilful variation of this ap- proach was proposed by the group of MacMillan, in which a photoredox and an organocatalytic cycle were merged to allow for enantioselective processes, [5a] such as the asymmetric a-alkylation [5b] and trifluoromethylation [5c] of aldehydes, avoid- ing the use of any sacrificial molecule. Here the radical species is photocatalytically generated and then trapped by a chiral enamine generated in situ by the reaction of an aldehyde with the organocatalyst, a secondary amine. Ir 3 + or Ru 2 + based cat- alysts gave access to the so-called “visible light photocataly- sis” [2c,d] because these complexes absorb efficiently in the visi- ble region, as recently reviewed. [2c] The intriguing question is whether an organic molecule could also be a photocatalyst, that is, if an excited organocatalyst could efficiently promote a chemical reaction (as a metal catalyst does) and be regenerat- ed in the process, thus acting as a photo-organocatalyst (POC). The use of organic photocatalysts is not new; the first pho- tocatalysts ever known were simple (aromatic) ketones, the most striking example being benzophenone. [2a] An ideal photo-organocatalyst, in analogy with Ru 2 + complexes, should absorb in the visible region. Accordingly, POCs have to be strongly coloured, that is, they must be dyes. Dyes, such as methylene blue or Rose Bengal (RB), are widely employed in photochemical reactions but mainly to photogenerate singlet oxygen by a photosensitised process. [6] On the other hand, the use of coloured compounds as photocatalysts is not new; 9,10- dicyanoanthracene, acridinium or pyrilium salts, among others, have been sparsely used in synthesis. [2a, 3, 7] Very recently, however, cheap organic dyes have been ap- plied as effective visible light photoredox organocatalysts (PROC). The reactions are initiated by an electron transfer reac- tion between the excited dye and the reagent; the photogen- erated radical ion of the dye can likewise promote a chemical reaction. To demonstrate the versatility and the potential of dyes, the electrochemical characteristics of the ground (E red Scheme 1. Modes of action of a photocatalyst. [a] D. Ravelli, Prof. M. Fagnoni Department of Chemistry University of Pavia Via Taramelli 12, 27100 Pavia (Italy) Fax: (+ 39) 0382987323 E-mail: fagnoni@unipv.it ChemCatChem 2012, 4, 169 – 171  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 169