RSC Advances REVIEW View Article Online View Journal | View Issue Eosin Y catalysed photoredox synthesis: a review a b Cite this: RSC Adv.,2017,7,31377 Vishal Srivastava and Praveen P. Singh * In recent years, photoredox catalysis using eosin Y has come to the fore front in organic chemistry as a powerful strategy for the activation of small molecules. In a general sense, these approaches rely on the ability of organic dyes to convert visible light into chemical energy by engaging in single-electron Received 14th May 2017 transfer with organic substrates, thereby generating reactive intermediates. In this perspective, we Accepted 13th June 2017 highlight the unique ability of photoredox catalysis to expedite the development of completely new DOI: 10.1039/c7ra05444k reaction mechanisms, with particular emphasis placed on multicatalytic strategies that enable the rsc.li/rsc-advances construction of challenging carbon–carbon and carbon–heteroatom bonds. 1. Introduction However, the transition metal based photocatalysts disadvantageously exhibit high cost, low sustainability and Visible light photoredox catalysis has recently received much potential toxicity. Recently, a superior alternative to transition Creative Commons Attribution 3.0 Unported Licence. attention in organic synthesis owing to ready availability, metal photoredox catalysts, especially metal-free organic dyes sustainability, non-toxicity and ease of handling of visible particularly eosin Y has been used as economically and 1–13 light but the general interest in the eld started much ecologically superior surrogates for Ru(II)andIr(II)complexes earlier.14 Unlike thermal reactions, photoredox processes occur in visible-light promoted organic transformations involving 18–21 under mild conditions and do not require radical initiators or SET (single electron transfer). These organic dyes have got stoichiometric chemical oxidants or reductants. much more attention with the last few years also due to easy Ruthenium and iridium polypyridyl complexes are commonly handling, eco-friendly and have great potential for applica- 22–24 employed visible light photocatalysts and their chemistry tions in visible-light-mediated organic synthesis which This article is licensed under a and application in organic synthesis has recently been fulls the basic principle of green chemistry. In this article, summarized.15–17 wediscussrecentapplicationsofeosinYasavisiblelight photocatalyst in organic synthesis. The general scheme out- Open Access Article. Published on 19 June 2017. Downloaded 9/30/2021 2:23:54 AM. aDepartment of Chemistry, United College of Engineering & Management, Naini, lining the important photocatalytic reactions with eosin Y as Allahabad-211010, U.P., India photocatalyst is given in Fig. 1. bDepartment of Chemistry, United College of Engineering & Research, Naini, Allahabad-211010, U.P., India. E-mail: [email protected] Vishal Srivastava is working as Praveen P. Singh is an Assistant Assistant Professor, Department Professor in the Department of of Chemistry, United College of Chemistry at the United College Engineering and Management of Engineering and Research (AKT University). He has (A.K.T. University), Allahabad, completed Graduation (B.Sc.), India. He obtained his Post-Graduation (M.Sc.) in B.Sc., M.Sc. in Organic Chem- Organic Chemistry and Doctoral istry from T. D. College (V. B. S Degree (D.Phil.) from Depart- Purvanchal University) Jaunpur ment of Chemistry, University of and D.Phil. from Department of Allahabad, India. His current Chemistry, University of Allaha- research work involves the bad, India in 2009. His current designing of novel photoredox research interests include the catalysed synthetic organic compounds. Photoredox catalysis and development of synthetic receptors for the recognition of biological organocatalysis represent two powerful elds of molecular acti- target structures and the application of visible light chemical vation that have found widespread application in the areas of photocatalysis towards organic synthesis. inorganic and organic chemistry. This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7,31377–31392 | 31377 View Article Online RSC Advances Review À À coefficient 3 ¼ 60 803 M 1 cm 1. Upon excitation eosin Y becomes more reducing and more oxidizing compared to in its ground state. The redox potentials of the excited state can be estimated from the standard redox potentials of the ground state, determined by cyclic voltammetry, and the triplet excited state energy. The measured ground state and the estimated excited state oxidation and reduction potentials are given in Scheme 1.28,29 In addition, the photoexcited state of eosin Y may also undergo energy transfer.30 2. Acid–base chemistry ff Fig. 1 General scheme involving application of eosin Y in photo- Amajordi erence between organic and metallic photocatalysts organocatalysis. is the pronounced acid–base chemistry of the former due to the availability of electron lone pairs at heteroatoms. For example, eosin Y and other uorescein dyes exist as an equilibrating mixture of four components:31 two neutral forms Y (e.g.,spi- rocyclic eosinYH2spiro and ring-opened eosinYH2) and upon sequential deprotonations the monoanionic eosinYHNa and dianionic eosinYNa2. The negative charge at the long-wavelength absorbing xanthene core exerts a signicant effect on the pho- 32 tophysical properties. pKa values of 2.0 and 3.8 were derived. 31 Creative Commons Attribution 3.0 Unported Licence. The neutral forms of uoresceins adopt spirocyclic structures in which the xanthenoid p-system is disrupted and visible absorp- tion and photocatalytic activity are extinguished (Scheme 2).33 Unfortunately, the recent literature has not entirely appreciated the relevance of acid–base behavior in photocatalysis. The rst a Fig. 2 Intermediates involved in photo-organocatalysis. reports of eosin Y photocatalysis involved -amine oxidations, which proceeded in the presence of stoichiometric amines to ensure sufficient formation of the dibasic eosin Y.34,35 In some The possibilities of generation of following intermediates in occasions, weakly basic reactants such as sul nates can enable This article is licensed under a 36 a photocatalytic reaction may occurs by oxidative and reductive conversion to the photoactive forms of eosin Y. quenching (Fig. 2). The organic dye eosin Y has been found more synthetic 3. Early work Open Access Article. Published on 19 June 2017. Downloaded 9/30/2021 2:23:54 AM. utility in photocatalysed organic reactions due to its better yield capacity in comparison to other organic dye of uorescein The rst applications of photoredox catalysis to organic family.50 synthesis were reported almost 40 years ago, and these seminal The photochemistry of eosin Y is well investigated: upon excitation by visible light, eosin Y undergoes rapid intersystem crossing to the lowest energy triplet state, which has a life time of 24 ms.25–27 Eosin Y absorbs green light; the UV-Vis spectrum shows a characteristic peak at 539 nm with a molar extinction Scheme 1 Different forms of eosin Y and the redox potentials of eosin YinCH3CN–H2O (1 : 1) in ground and corresponding excited states. Scheme 2 Acid–base behaviour of eosin Y. 31378 | RSC Adv.,2017,7,31377–31392 This journal is © The Royal Society of Chemistry 2017 View Article Online Review RSC Advances 38 method B). A large stoichiometric excess of NaBH4 (250 equiv) was used as the sacricial reductant, but this process was greatly improved upon in 2014 with the conditions shown in Scheme 4, method A, which uses triethanolamine (TEOA) as the stoichiometric reductant.39 The reaction is chemoselective and Scheme 3 Kellogg 1978 – reductive desulfuration. tolerates the presence of other functional groups, such as carbonyls, halogen atoms, and nitriles. The nitro group is a better electron acceptor. Important factors to achieve the publications laid the foundations for the recent developments optimal reaction yield are the pH value of the reaction mixture in the eld of modern photoredox catalysis. In 1978, Kellogg in the deoxygenated ethanol–water (3 : 2, v/v) mixture and the demonstrated that the visible light induced reduction of phe- 37 amount of added TEOA. Nitro groups of substrates bearing nacyl sulfonium salts by 1,4-dihydropyridines (Scheme 3). either electron donating or electron withdrawing substituents Irradiation of a mixture of 3.1 and 3.2 in CD CN or CD COCD 3 3 3 are smoothly reduced. without any photosensitizer provided the reduced product 3.3 Accompanying photophysical studies revealed that electron in quantitative yield a er 48 h using normal room light (neon 3 transfer from TEOA to [EY]* was second orders of magnitude uorescent lamp at ca. 2 m distance) at 25 C. Addition of 1 slower than electron transfer from 3[EY]* to nitrobenzene. mol% of Na –eosin Y accelerated the reaction resulting in 2 Accordingly, the mechanism is likely to involve an oxidative complete conversion within 1 h of irradiation. The authors PETs (photoinduced electron transfer) cycle, in which electrons speculated that light induced single electron transfer (SET) are repeatedly donated from 3[EY]* to the arene intermediates. steps are responsible for the formation of the reduced product Ultimately, the production of the aniline from a single equiva- and suggested an acceleration effect upon addition of the lent of nitroarene is a 6-electron reduction which proceeds photocatalyst. However, the exact role of the photocatalyst in through nitrosobenzene 4.5