When metal-catalyzed C–H functionalization meets visible-light photocatalysis

• Lucas Guillemard and
• Joanna Wencel-Delord

Beilstein J. Org. Chem. 2020, 16, 1754–1804, doi:10.3762/bjoc.16.147

• , few examples have been reported in which the photoredox process modifies the oxidation state of a catalyst [55][56]. Subsequently, C–H activation protocols benefiting from mild photocatalytic reoxidation have spread rapidly [60][61][62]. In such a case, a photocatalyst (PC) is introduced in the

• elimination, producing a Co(I) species. Finally, the photoexcited eosin Y reoxidizes Co(I) to Co(II), while the photosensitizer is reoxidized by molecular oxygen, thus completing the overall catalytic process. In addition, a complementary strategy towards photocatalytic C–H annulation was disclosed by Rueping

• the naphthyl ring after an intramolecular electron transfer. In the meantime, the sulfonyl radical is generated by oxidation of the sulfinate substrate via the photocatalytic cycle with the Ru(bpy)3 catalyst and K2S2O8 (the direct oxidation of sulfinate by persulfate could also be envisioned

Heterogeneous photocatalysis in flow chemical reactors

• Christopher G. Thomson,
• Ai-Lan Lee and
• Filipe Vilela

Beilstein J. Org. Chem. 2020, 16, 1495–1549, doi:10.3762/bjoc.16.125

• irradiation. Herein, we review some important developments of heterogeneous photocatalytic materials and their application in flow reactors for sustainable organic synthesis. Further, the application of continuous flow heterogeneous photocatalysis in environmental remediation is briefly discussed to present

• transfer catalysis (EnT, Section 4.2). The final two subsections review the importance of HPCats in flow reactors for the photocatalytic remediation of wastewater (Section 4.3) and air pollution (Section 4.4), an increasingly significant area of research to prevent damage to the environment and human

• topic in 2019 alone (Figure 1A). The credit for the development of this field is often given to MacMillan [24], Yoon [25], and Stephenson [26], whose seminal papers in 2008 and 2009 elegantly demonstrated the photocatalytic ability of ruthenium–bipyridyl complexes to drive chemical reactions with

Photocatalyzed syntheses of phenanthrenes and their aza-analogues. A review

• Alessandra Del Tito,
• Havall Othman Abdulla,
• Davide Ravelli,
• Stefano Protti and
• Maurizio Fagnoni

Beilstein J. Org. Chem. 2020, 16, 1476–1488, doi:10.3762/bjoc.16.123

• Phenanthrenes and their aza-analogues have important applications in materials science and in medicine. Aim of this review is to collect recent reports describing their synthesis, which make use of radical cyclizations promoted by a visible light-triggered photocatalytic process. Keywords: phenanthrenes

• photocatalyst, have revolutionized the way chemists can arrive to important chemical scaffolds [24][25][26]. Indeed, the photocatalytic approach combines unparalleled mild conditions, due to the use of photons as traceless reagents that leave no residue behind [27][28], with the exploitation of rather

• construct the phenanthridine core, somophilic (radical) isocyanide addition [45][46][47] is probably the most adopted one, in view of the versatility and low cost of the starting substrates. Accordingly, several protocols for the synthesis under photocatalytic conditions of phenanthridines starting from 2

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

Beilstein J. Org. Chem. 2020, 16, 1418–1435, doi:10.3762/bjoc.16.118

• Yeersen Patehebieke School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China 10.3762/bjoc.16.118 Abstract Disulfides are versatile catalysts. They can be photocatalysts, hydrogen atom transfer (HAT) catalysts, cocatalysts, or initiators in photocatalytic reactions

• , mild, and chemoselective radical catalysts that deserve more attention. The present review highlights the recent progress in the field of disulfide-catalyzed and -cocatalyzed photocatalytic reactions for different reaction types. Keywords: cycloaddition; disulfide catalyst; isomerization; oxidation

Distinctive reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines under photoredox conditions

• Shrikant D. Tambe,
• Kwan Hong Min,
• Naeem Iqbal and
• Eun Jin Cho

Beilstein J. Org. Chem. 2020, 16, 1335–1342, doi:10.3762/bjoc.16.114

• Shrikant D. Tambe Kwan Hong Min Naeem Iqbal Eun Jin Cho Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea 10.3762/bjoc.16.114 Abstract A simple photocatalytic method was developed for the synthesis of unsymmetrical 1,2-diamines by the

• photocatalytic synthetic methods are mainly limited to aniline-based substrates and do not encompass aliphatic amines. We planned the synthesis of 1,2-diamine compounds having an aliphatic amine moiety by the intermolecular coupling of N-benzylidines with aliphatic amines that not only act as coupling partner

• , the cyclized product with the tethered phenyl ring proposed in Scheme 1c was not generated under any of the photocatalytic conditions evaluated. With the optimized conditions in hand, the generality of the transformations was investigated using a wide variety of phenyl-substituted N-benzylideneaniline

Photocatalytic trifluoromethoxylation of arenes and heteroarenes in continuous-flow

• Alexander V. Nyuchev,
• Ting Wan,
• Borja Cendón,
• Carlo Sambiagio,
• Job J. C. Struijs,
• Michelle Ho,
• Moisés Gulías,
• Ying Wang and
• Timothy Noël

Beilstein J. Org. Chem. 2020, 16, 1305–1312, doi:10.3762/bjoc.16.111

• , Road, North Chicago, Illinois 60064, United States of America 10.3762/bjoc.16.111 Abstract The first example of photocatalytic trifluoromethoxylation of arenes and heteroarenes under continuous-flow conditions is described. Application of continuous-flow microreactor technology allowed to reduce the

Oxime radicals: generation, properties and application in organic synthesis

• Igor B. Krylov,
• Stanislav A. Paveliev,
• Alexander S. Budnikov and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2020, 16, 1234–1276, doi:10.3762/bjoc.16.107

• dithionite was omitted. High yields were reached for both monosubstituted (products 113a,d,f,h) and disubstituted C=C bonds (products 113b,c,e,g). Aliphatic oximes also undergo this transformation in high yields (products 113c,g,h). The photocatalytic oxidative cyclization of β,γ-unsaturated oximes 114 was

Photocatalysis with organic dyes: facile access to reactive intermediates for synthesis

• Stephanie G. E. Amos,
• Marion Garreau,
• Luca Buzzetti and
• Jerome Waser

Beilstein J. Org. Chem. 2020, 16, 1163–1187, doi:10.3762/bjoc.16.103

• renaissance of photocatalysis. Since the early seminal reports [1][2][3][4], inspired by pioneering works in photochemistry [5][6][7], this field has attracted increasing attention, and organic chemists have developed a wide variety of photocatalytic reactions [8][9][10][11]. One of the reasons for this rapid

• central role in the rapid expansion of photocatalytic methods [12]. These catalysts typically absorb light in the blue region and promote different activation modes, including photoinduced electron transfer (PET) and energy transfer (EnT), which respectively lead to the formation of open-shell and

• found particularly illustrative. This report is therefore in no means comprehensive, and readers searching to gain deeper insight into photocatalytic processes and/or for an exhaustive coverage of applications should refer to more complete specialized reviews and books [26][27][28][29][30]. We hope this

A simple and easy to perform synthetic route to functionalized thienyl bicyclo[3.2.1]octadienes

• Dragana Vuk,
• Irena Škorić,
• Valentina Milašinović,
• Krešimir Molčanov and
• Željko Marinić

Beilstein J. Org. Chem. 2020, 16, 1092–1099, doi:10.3762/bjoc.16.96

• compound 12. Possible outcomes of future photocatalytic oxygenation reactions of new benzobicyclo[3.2.1.]octadienes. Supporting Information Supporting Information File 42: Experimental details, copies of spectra and X-ray crystallographic data. Acknowledgements The competent help by Jerome Le Cunff in

Recent applications of porphyrins as photocatalysts in organic synthesis: batch and continuous flow approaches

• Rodrigo Costa e Silva,
• Luely Oliveira da Silva,
• Aloisio de Andrade Bartolomeu,
• Timothy John Brocksom and
• Kleber Thiago de Oliveira

Beilstein J. Org. Chem. 2020, 16, 917–955, doi:10.3762/bjoc.16.83

• photoprocesses [9][10][11]. Additionally, it is possible to realize tuning in terms of chemical properties by changing substituents, thus producing robust, soluble or heterogeneous, readily available and low-cost photocatalysts. The mechanisms of the photocatalytic activities of porphyrins are similar to other

• , we have demonstrated that both porphyrins, in the excited state, are thermodynamically able to promote the first photooxidation step (Scheme 4B), however, the turnover of TPFPP+ to TPFPP is much more favored which justifies the acceleration of the photocatalytic cycle. In this protocol, the scope of

• products were obtained with good TON (up to 880) and high selectivity (91–99.5%), even though the aryl C–F bonds present a high bond dissociation energy (BDE) (Scheme 13). Using a similar photocatalytic system, 2-methyl-2,3-dihydrobenzofuran was produced by an intramolecular hydro-functionalization of

Aldehydes as powerful initiators for photochemical transformations

• Maria A. Theodoropoulou,
• Nikolaos F. Nikitas and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2020, 16, 833–857, doi:10.3762/bjoc.16.76

• highlighting organocatalytically-mediated reactions [7][8][9][10] with potential in the field of photoorganocatalysis. The power of metal-based photocatalysts is indisputable and can be pinpointed through the ease that they can catalyze difficult photocatalytic reactions, such as the decarboxylation of

• supported the energy transfer pathway. Thus, the proposed triplet sensitization mechanism of the photocatalytic ATRA reaction is depicted in Scheme 25. In 2016, Ji and co-workers developed a new photoredox cross-dehydrogenative coupling (CDC) method for the α-heteroarylation of amides (α to nitrogen, e.g

• solvent, the base, and the irradiation wavelength. Next, they investigated the reaction potential, with most aryl bromides and benzaldehydes tested being compatible to this transformation, affording the products in moderate to excellent yield. The proposed photocatalytic cycle starts with the excitation

Photocatalytic deaminative benzylation and alkylation of tetrahydroisoquinolines with N-alkylpyrydinium salts

• David Schönbauer,
• Carlo Sambiagio,
• Timothy Noël and
• Michael Schnürch

Beilstein J. Org. Chem. 2020, 16, 809–817, doi:10.3762/bjoc.16.74

• . Further investigations in this direction are ongoing in our labaratories. Kinetic profile for the benzylation of 1 to 3. Examples of photocatalytic C–C bond formation by nucleophilic trapping of a reactive THIQ intermediate. Benzylation of N-phenyl-THIQ. Benzylation of substituted N-arylTHIQs. Removal of

Recent advances in photocatalyzed reactions using well-defined copper(I) complexes

• Mingbing Zhong,
• Xavier Pannecoucke,
• Philippe Jubault and
• Thomas Poisson

Beilstein J. Org. Chem. 2020, 16, 451–481, doi:10.3762/bjoc.16.42

• authors precluded a possible photocatalytic process (path II). And indeed, even though from a comparison of the redox potential of [Cu(I)(dap)2]Cl and [Ru(bpy)3]Cl2, the latter should have been capable to promote the photocatalytic reaction, the ruthenium complex was unsuccessful. Hence, to explain the

• ) [25]. This copper-catalyzed photocatalytic reduction generated an aryl radical that was trapped with various allylating reagents. First, the phenyl radical generated from the corresponding diphenyliodonium salt was added to various allyl sulfones substituted in the 2-position. The products were

• the use of Ir or Ru complexes in this transformation led to the degradation of the reagents. As this process did not involve a photocatalytic pathway, it will not be discussed here. To explain the reaction outcome, based on literature data, the authors hypothesized a catalytic cycle involving [Cu(I

Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0

• Bernd Strehmel,
• Christian Schmitz,
• Ceren Kütahya,
• Yulian Pang,
• Anke Drewitz and
• Heinz Mustroph

Beilstein J. Org. Chem. 2020, 16, 415–444, doi:10.3762/bjoc.16.40

• cation radical Sens+• forms Sens back in the cycle. The system exhibited photocatalytic behavior with no big changes of absorption for 90 min under aerobic and anaerobic conditions. However, no polymerization occurred under air showing the inhibition of polymerization by oxygen [81]. This described

Recent developments in photoredox-catalyzed remote ortho and para C–H bond functionalizations

• Rafia Siddiqui and
• Rashid Ali

Beilstein J. Org. Chem. 2020, 16, 248–280, doi:10.3762/bjoc.16.26

• observed that hydroxylation of the fluoro-, chloro-, and bromobenzene derivatives provided low yields. The photocatalytic mechanism for this reaction was inspected by time-resolved transient absorption spectroscopy to detect the triplet–triplet photoredox catalyst spectrum via nanosecond laser flash

• reported on photocatalytic brominations using a stronger oxidizing photocatalyst, viz, sodium anthraquinone-2-sulfonate (SAS, 7a, 2.3 V vs SCE) [161][162]. In their studies, they did not only observe excellent regioselectivities but also great functional group tolerance under mild reaction conditions. For

• prepared a library of monobrominated compounds using this simple yet effective strategy. A plausible mechanism is shown in Figure 21. Chlorination of arenes with Mes-Acr-MeClO4 (2): Ohkubo et al. observed that only under aerobic photocatalytic conditions, C–H chlorination of trimethoxybenzene (TMB) occurs

Recent advances in transition-metal-catalyzed incorporation of fluorine-containing groups

• Xiaowei Li,
• Xiaolin Shi,
• Xiangqian Li and
• Dayong Shi

Beilstein J. Org. Chem. 2019, 15, 2213–2270, doi:10.3762/bjoc.15.218

α-Photooxygenation of chiral aldehydes with singlet oxygen

• Dominika J. Walaszek,
• Magdalena Jawiczuk,
• Jakub Durka,
• Olga Drapała and
• Dorota Gryko

Beilstein J. Org. Chem. 2019, 15, 2076–2084, doi:10.3762/bjoc.15.205

• small molecule size, there are few examples of its use not only in diastereoselective synthesis but also in enantioselective reactions [9][10]. Inspired by Cόrdova’s work [11][12][13], we explored the idea of merging enamine catalysis with photocatalytic oxygenation with singlet oxygen for α

Naphthalene diimides with improved solubility for visible light photoredox catalysis

• Barbara Reiß and
• Hans-Achim Wagenknecht

Beilstein J. Org. Chem. 2019, 15, 2043–2051, doi:10.3762/bjoc.15.201

• solutions of cNDIs 2–6 in DMF. Photocatalytic α-alkylation of octanal (12): 500 mM 12, 250 mM 13, 50 mM (20 mol %) organocatalyst 15, 500 mM 2,6-lutidine, NDI 1 or cNDI 2–6 as photoredox catalyst in 1.3 mL solvent, stirring, irradiation by LED, see Table 2. Optical and electrochemical properties of NDI 1

Syntheses and chemical properties of β-nicotinamide riboside and its analogues and derivatives

• Mikhail V. Makarov and
• Marie E. Migaud

Beilstein J. Org. Chem. 2019, 15, 401–430, doi:10.3762/bjoc.15.36

• pyridinium ring (see Figure 7). A comprehensive review on the chemistry of homogeneous and heterogeneous catalytic, electrochemical, photocatalytic, and immobilized catalysts-based regeneration of NAD(P)H was published by Wang et al. [84]. 3.4. Modifications on the 5′-position of unprotected and partially

Tandem copper and photoredox catalysis in photocatalytic alkene difunctionalization reactions

• Nicholas L. Reed,
• Madeline I. Herman,
• Vladimir P. Miltchev and
• Tehshik P. Yoon

Beilstein J. Org. Chem. 2019, 15, 351–356, doi:10.3762/bjoc.15.30

• we have proposed for photocatalytic oxyamination is outlined in Figure 1c. Photoinduced one-electron oxidation of an appropriately electron-rich styrene 1 results in the formation of a radical cation 1•+ that is susceptible to attack by various heteroatomic nucleophiles, including carbamates [21][22

• with ground-state dioxygen to afford unstable hydroperoxy radicals that can also decompose unproductively [32][33]. Indeed, in our previous study of photocatalytic alkene difunctionalization, we found that dioxygen and similar commonly used terminal oxidants resulted in unproductive decomposition of

• secondary terminal oxidant, and that Ag(I) salts appear to be uniquely effective in this capacity. This work thus provides a platform for the development of enantioselective photocatalytic alkene difunctionalization reactions that can use a chiral Cu(II) complex as a substoichiometric catalyst rather than

Degenerative xanthate transfer to olefins under visible-light photocatalysis

• Atsushi Kaga,
• Xiangyang Wu,
• Joel Yi Jie Lim,
• Hirohito Hayashi,
• Yunpeng Lu,
• Edwin K. L. Yeow and
• Shunsuke Chiba

Beilstein J. Org. Chem. 2018, 14, 3047–3058, doi:10.3762/bjoc.14.283

• report a photocatalytic degenerative radical transfer of xanthates to olefins using an iridium-based photocatalyst under blue LED irradiation (Scheme 1C). A series of mechanistic investigations identified that the process involves a triplet-sensitization of the xanthates by the long-lived triplet state

• the light source used (469 nm), xanthate 1a absorbs a negligible amount of light (Figure 3) and the majority of triplet 1a formed is due to energy transfer from excited catalyst 8*. Having optimized the reaction conditions on the photocatalytic degenerative transfer of xanthates, we next explored the

• presence of 1a in degassed DMSO recorded at different delay times, respectively (excitation wavelength = 355 nm). UV–vis absorption spectrum of 1a (1 mM solution in DMSO). Degenerative radical transfer of xanthates to olefins. Photocatalytic RAFT polymerization of xanthate 4. Determination of quantum yield

Organometallic vs organic photoredox catalysts for photocuring reactions in the visible region

• Aude-Héloise Bonardi,
• Frédéric Dumur,
• Guillaume Noirbent,
• Jacques Lalevée and
• Didier Gigmes

Beilstein J. Org. Chem. 2018, 14, 3025–3046, doi:10.3762/bjoc.14.282

• , formations of interpenetrated polymer networks (IPN) are also mentioned. For the three systems proposed above, formation of aryl radicals is observed. These radicals are able to initiate the free radical polymerization of (meth)acrylates [1]. In the photocatalytic cycle (Figure 5C), EDB(−H)• radicals are

• , such as EDB presented in photoredox catalytic cycle (Figure 5C), are also well mentioned as efficient co-initiators for free-radical-promoted cationic polymerizations [1][28][29]. In Part 2, a photoredox catalyst useable in such a photocatalytic system will be presented. To be involved properly into

• complex as described in [68], i.e., nitro-functionalization and sulfino-functionalization decreased the photocatalytic activity of the complex. Moreover, this functionalization affects the oxidative quenching rate and the stability of the complex. Thus, as for other complexes described above, the choice

Photocatalyic Appel reaction enabled by copper-based complexes in continuous flow

• Clémentine Minozzi,
• Jean-Christophe Grenier-Petel,
• Shawn Parisien-Collette and
• Shawn K. Collins

Beilstein J. Org. Chem. 2018, 14, 2730–2736, doi:10.3762/bjoc.14.251

• transfer (PCET) reactions [22][23][24][25][26]. Herein, the evaluation of Cu(I)-complexes for photocatalytic Appel reactions and demonstration in continuous flow is described. Results and Discussion The first step in identifying a heteroleptic diamine/bisphosphine Cu(I)-based photocatalyst for the

• heteroleptic copper(I)-based complexes for photocatalysis. Evaluation of the library of copper-based complexes in photocatalytic alcohol→bromide conversion. Reactions irradiated with 394 nm light (pink) or 450 nm (blue). Front entries without an indicated phosphine ligand pertain to homoleptic Cu(diamine)2BF4

• complexes and are colored in lighter blue. Entries without a color indicate reactions which could not be performed due to solubility or overoxidation of the complex. Experimental set-up for the photocatalytic conversion of alcohols to bromides. PFA tubing is wrapped around purple LEDs (394 nm) and fans are

Learning from B₁₂ enzymes: biomimetic and bioinspired catalysts for eco-friendly organic synthesis

• Keishiro Tahara,
• Ling Pan,
• Toshikazu Ono and
• Yoshio Hisaeda

Beilstein J. Org. Chem. 2018, 14, 2553–2567, doi:10.3762/bjoc.14.232

• with the report by Yoon et al. in which light irradiation to Ru(bpy)32+ resulted in rapid decomposition during the photocatalytic reaction [113]. It was remarkable that a significantly high turnover number based on 1 (10,880) was obtained in the prolonged reaction with Irdfppy. Quenching experiments

Synthesis of aryl sulfides via radical–radical cross coupling of electron-rich arenes using visible light photoredox catalysis

• Amrita Das,
• Mitasree Maity,
• Simon Malcherek,
• Burkhard König and
• Julia Rehbein

Beilstein J. Org. Chem. 2018, 14, 2520–2528, doi:10.3762/bjoc.14.228

• , electron poor arenes are required. In this article, we report the development of a mild and efficient oxidative photocatalytic method of thiolation of electron-rich di- and trimethoxybenzene arenes with aryl disulfides and (NH4)2S2O8 as terminal oxidant (Scheme 2). Results and Discussion 1,2,4

• -Trimethoxybenzene and diphenyl disulfide were employed as the model substrates to test our proposal and to optimize the reaction conditions. Our developed photocatalytic method allows the activation of electron-rich alkoxyarenes for the direct C–H sulfenylation reaction using visible light and [Ir(dF(CF3)ppy)2

• excess disulfide (e.g., 5 equivalents) resulted in the formation of thiophenol as a major side product along with other oxidized sulfur species. The amount of disulfide was varied from 0.5 equivalents to five equivalents; 1.7 equivalents of disulfide gave the best result. The photocatalytic reaction was