Synthetic Applications of Eosin Y in Photoredox Catalysis

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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/chemcomm Page 1 of Journal12 Name ChemComm Dynamic Article Links ► Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx ARTICLE TYPE Synthetic Applications of Eosin Y in Photoredox Catalysis Durga Prasad Hari, and Burkhard König* Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Eosin Y, a long known dye molecule, has recently been widely applied as a photoredox catalyst in organic synthesis. Low cost and good availability make eosin Y an attractive alternative to typical inorganic transition metal photocatalysts. We summarize the key photophysical properties of the dye and the recent synthetic applications in photoredox catalysis. 24 μs.31-33 Eosin Y absorbs green light; the UV-Vis spectrum 1. Introduction shows a characteristic peak at 539 nm with a molar extinction coefficient ε = 60803 M-1cm-1. By excitation eosin Y becomes Visible light photoredox processes have recently found many more reducing and more oxidizing compared to its ground state. applications in organic synthesis,1-13 but the general interest in the 14 The redox potentials of the excited state can be estimated from field started much earlier. Unlike thermal reactions, photoredox the standard redox potentials of the ground state, determined by Manuscript processes occur under mild conditions and do not require radical cyclic voltammetry, and the triplet excited state energy. The initiators or stoichiometric chemical oxidants or reductants. measured ground state and the estimated excited state oxidation Typical irradiation sources are LEDs or household lamps, which 34, 35 and reduction potentials are given in Scheme 1. In addition, are cheaper and easier to apply than specialized UV reactors used the photo excited state of eosin Y may also undergo energy in classical photochemistry. Ruthenium and iridium polypyridyl 36 transfer. complexes are commonly employed visible light photocatalysts 1 and their chemistry and application in organic synthesis has 1EY* 14, 15 recently been summarized. COOR 3EY* Despite the excellent photophysical properties of ruthenium Br Br - EY Br Br - 1.11 V + 0.83 V Accepted and iridium polypyridyl complexes in visible light photocatalysis, ev hν 1.89 ev RO O O the compounds are expensive and potential toxic, causing + - Br Br EY + EY EY - 16 +0.78 V EY -1.06 V disadvantages on larger scale. Organic dyes have been used as +0.78 V R = H, eosin Y spirit soluble Eosin Y an attractive alternative to transition metal complexes in Eosin Y 17-20 R = Na, Na -eosin Y photoredox catalysis. They are typically less expensive and 2 = n less toxic, easy to handle and even outperform organometallic R = (nC4H9)4, TBA-eosin Y 16, 21-24 and inorganic catalysts in some cases. Particularly eosin Y Scheme 1 Different forms of eosin Y and the redox potentials of eosin Y was widely used as organo-photocatalyst in synthetic in CH3CN/H2O (1:1) in ground and corresponding excited states. transformations. The classic dye is known for a long time and 25 26 found use in cell staining, as pH indicator, as indicator in the 3. Reduction reactions analytical halide determination by Fajans27, 28 and as dye pigment, 29, 30 The first reaction demonstrating the use of eosin Y e.g. in lip sticks. In this article, we discuss recent applications photocatalysis in organic synthesis was the photoreduction of of eosin Y as visible light photocatalyst in organic synthesis. ChemComm sulfonium salts. 2. Photochemistry of Eosin Y 3.1 Reduction of phenacyl sulfonium salt The photochemistry of eosin Y is well investigated: upon In 1978, Kellogg and coworkers reported the visible light excitation by visible light, eosin Y undergoes rapid intersystem induced reduction of phenacyl sulfonium salts by 1,4 dihydro- 37 crossing to the lowest energy triplet state, which has a life time of pyridines (Scheme 2). Irradiation of a mixture of 1 and 2 in CD3CN or CD3COCD3 without any photosensitizer provided the reduced product 3 in quantitative yield after 48 h using normal Department of Chemistry and Pharmacy room light (neon fluorescent lamp at ca. 2 m distance) at 25°C. Universität Regensburg Addition of 1 mol% of Na2-eosin Y accelerated the reaction Universitätsstr. 31, 93040 Regensburg resulting in complete conversion within 1 h of irradiation. The Fax: (+499419431717) E-mail: [email protected] authors speculated that light induced single electron transfer (SET) steps are responsible for the formation of the reduced This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1 ChemComm Page 2 of 12 product and suggested an acceleration effect upon addition of the of eosin Y, which is reduced by TEOA to close the catalytic cycle photocatalyst. However, the exact role of the photocatalyst in the and produce the radical cation of TEOA. The reaction of the reaction mechanism remains undisclosed. radical anion 6 with the TEOA cation radical in the presence of water gives glycolaldehyde, diethanolamine and the further O O EtO C CO Et Na -eosin Y reduced intermediates, which are again reduced in a similar 2 2 2 (1 mol%) S BF4 25°C, rt, 1 h fashion to finally yield aniline. S N Ph CD3COCD3 or CD CN 1 2 3 3 100% visible NO2 Scheme 2 Visible light mediated reduction of phenacyl sulfonium salt. light EY* 3.2. Reduction of nitrobenzene R EY 4 Tung and coworkers utilized eosin Y as photocatalyst and NH triethanolamine (TEOA) as sacrificial reducing agent for the NO2 2 TEOA TEOA efficient photocatalytic reduction of nitrobenzene under green EY R R light irradiation (Scheme 3).38 The reaction is chemoselective and tolerates the presence of other functional groups, such as 6 5 TEOA carbonyls, halogen atoms, and nitriles. The nitro group is the better electron acceptor. Important factors to achieve the optimal Scheme 4 A plausible mechanism for the reduction of nitrobenzene to aniline via visible light photocatalysis. reaction yield are the pH value of the reaction mixture in deoxygenated ethanol-water (3:2, v/v) mixture and the amount of 3.3. Desulfonylation added TEOA. Nitro groups of substrates bearing either electron The use of sulfones as auxiliary groups is an efficient synthetic donating or electron withdrawing substituents are smoothly strategy to generate a wide range of important products. reduced. Commonly the sulfone group is removed using metal containing Manuscript NO NH 2 Eosin Y (1 mol%) 2 reducing agents, such as Bu3SnH, Al (Hg), or Sm/HgCl2. 6 equiv of TEOA Recently an environmental friendly desulfonylation reaction was R R EtOH/H2O reported by Wu and coworkers using eosin Y bis-tetrabutyl- visible light, rt 4 5 11 ammonium salt (TBA-eosin Y) as photocatalyst and diiso- examples 39 98-100% (conversion) propylethylamine (iPr2EtN) as a reducing agent (Scheme 5). NH NH NH2 2 2 O O R2 SO Ar R2 2 TBA-eosin Y (1 mol%) R R R3 R3 R1 iPr CH CN R1 Cl 2NEt, 3 7 3 W 450 nm LED 8 99% (c) 99% (c) 100% (c) Accepted 21 examples 99% (s) 98% (s) 95% (s) 55-99% yield NH NH2 2 NH2 O O O OMe O O CN O 99% 87% 90% 99% (c) 99% (c) 100% (c) 100% (s) 93% (s) 96% (s) O O c = conversion, s = selectivity N Scheme 3 Photoreduction of substituted nitrobenzenes to anilines. 92% 83% Based on quenching experiments and a flash photolysis study, the authors proposed a tentative mechanism for the photo- Scheme 5 Desulfonylation using TBA-eosin Y as a photocatalyst. catalytic reduction of nitrobenzene as shown in Scheme 4. A SET ChemComm from eosin Y* to nitrobenzene generates 6 and the radical cation Burkhard König was born in Wiesbaden, Germany. He obtained his PhD in 1991 from the Durga Prasad Hari was born in Jarajapupeta, university of Hamburg under the supervision of Nellimarla, Vizianagaram, A.P, India. He received Prof. de Meijere. He pursued postdoctoral his Bachelor degree from Silver Jubilee Degree studies with Prof. M. A. Bennett, Research College, Sri Krishnadevaraya University, School of Chemistry, Australian National Ananthpur, and then obtained his Master degree University, Canberra, and Prof. B. M. Trost, from IIT Madras. He is currently working under Stanford University. In 1996 he recieved his the supervision of Prof. Burkhard König for his Habilitation at the University of Braunschweig. PhD at the University of Regensburg. His research He became full professor of organic chemistry at interests focus on the applications of visible light the University of Regensburg from 1999. His chemical photocatalysis towards organic synthesis. current research interests include the development of synthetic receptors for the recognition of biological target structures and the application of visible light chemical photocatalysis towards organic synthesis. 2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year] Page 3 of 12 ChemComm eosin Y (2 mol%) Irradiation of a mixture of 7, TBA-eosin Y and diisopropyl- Nu nm LED N N 530 Ph ethylamine under inert atmosphere using a 3 W blue LED in Ph rt, air Nu CH3CN furnishes the desired product 8 in good yields.

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