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

• ) photocatalyst (Scheme 1) [16]. The [Cu(I)(dap)2]Cl complex had a strong absorption under irradiation at 530 nm using a green LED. Different organic halides and alkenes were reacted, leading to the product by an ATRA reaction pathway with moderate to good yield at room temperature (Scheme 1). The authors

• copper photocatalyst initiated the formation of the azidyl radical, which abstracted the benzylic hydrogen atom from the substrate. Then, the benzylic radical reacted with the Zhdankin reagent, producing the azidated product and propagating the radical chain through the reaction of the iodane radical

• radical was oxidized to the corresponding carbocation, regenerating the photocatalyst in the ground state. The benzylic carbocation was finally trapped with MeOH, which was used as the solvent to form the trifluoromethyl methoxylated product. In the same publication, Dilman and co-workers reported the

Visible-light-induced addition of carboxymethanide to styrene from monochloroacetic acid

• Kaj M. van Vliet,
• Nicole S. van Leeuwen,
• Albert M. Brouwer and
• Bas de Bruin

Beilstein J. Org. Chem. 2020, 16, 398–408, doi:10.3762/bjoc.16.38

• rapid redox reaction between the oxidant and the reducing agent instead of converting the substrate). Excitation of the photocatalyst, on the other hand, allows continuous formation of low concentrations of both oxidized and reduced radical forms of the substrate(s), and the excited catalysts (and/or

• photosensitizer. Because the use of polar solvents did not lead to the desired reactivity, we turned to nonpolar solvents. Since [Cu(dap)2]Cl is insoluble in nonpolar solvents, we continued with the more reducing fac-[Ir(ppy)3] photocatalyst. The choice of benzene as a solvent led to a significant formation of

• organic photocatalyst, showing hyperfine interactions with two equivalent nitrogen nuclei (giso = 2.0032; ANiso = 18.6 MHz). The HRMS of this species showed the mass of the catalyst, thus confirming that the oxidized catalyst precipitates as a salt from solution when using benzene as a solvent. Based on

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

• ]. In photoredox catalysis, visible light gets absorbed by the photocatalyst (PC), which transitions into a photoexcited state (*PC) that can undergo either energy transfer or redox pathways. As can be seen in Figure 4, the redox pathway consists of reductive and oxidative quenching pathways

• reaction conditions. Similar to Scheme 2, there was no product obtained without a semiconductor photocatalyst. Therein, they utilized the efficient photoredox catalysts 16 (band gap: 2.4 eV) and 17 (band gap: 2.6–3.0 eV). However, better results were obtained with a heterogeneous semiconductor, photoredox

• seen in Figure 8, the mechanism of the reaction commences with the deprotonation of the biphenyl carboxylic acid 36, followed by the reaction of 38 with dimethyl dicarbonate (DMDC) to generate compound 39. On the other hand, the photocatalyst is excited by metal–ligand charge transfer, which produces

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

• catalysis (Scheme 61). The method tolerates a myriad of primary, secondary and tertiary carboxylic acids and provides the corresponding CF3 analogue in good to excellent yields. Details of the proposed dual copper–photoredox cycle are shown in Scheme 61. The Ir(III) photocatalyst Ir[dF(CF3)ppy]2(4,4

• Cu(II) complex 6, along with reduced Ir(II) photocatalyst 7. The resulting carboxyl radical extrudes CO2 and sequentially recombines to generate Cu(III) species 9. At this stage, SET from 7 to 9 closes the photoredox catalytic cycle and produces an alkylcopper(II) species 10. Under the addition of

• efficient method to access ortho-CF3 acetanilides and anilines (Scheme 66b). Recently, Wang and co-workers [127] reported a visible-light-induced Pd-catalyzed ortho-trifluoromethylation of acetanilides. Without the need of an external photocatalyst and additive, various N-substituted anilides and

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

• species – a photocatalyst. If the interacting mode between the sensitizer and the reactant is via charge transfer, it is named photoredox catalysis. This research field has been established over the past decade [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. In principle, it is a

A review of the total syntheses of triptolide

• Xiang Zhang,
• Zaozao Xiao and
• Hongtao Xu

Beilstein J. Org. Chem. 2019, 15, 1984–1995, doi:10.3762/bjoc.15.194

• cationic polyene cyclization, transition-metal- or photocatalyst-mediated radical polyene cyclization [72]. The key to such transformation is to install a proper initiator within the substrate such as an allylic alcohol, an acetal, an aziridine, an N-acetal, a hydroxylactam, or a 1,3-dicarbonyl moiety. van

• oxidation of the tertiary radical and reduction of the [Au-Au]3+ ion could give the cyclization product and regenerate the dimeric gold photocatalyst. Later, the utility of this photoredox methodology was demonstrated in a concise formal synthesis of triptolide (1) via the reaction of bromobutenolide 19

Multicomponent reactions (MCRs): a useful access to the synthesis of benzo-fused γ-lactams

• Edorta Martínez de Marigorta,
• Jesús M. de Los Santos,
• Ana M. Ochoa de Retana,
• Javier Vicario and
• Francisco Palacios

Beilstein J. Org. Chem. 2019, 15, 1065–1085, doi:10.3762/bjoc.15.104

• , including a thiophene derivative. With the aid of several dedicated experiments, the researchers proposed a mechanism initiated by the formation of an arylsulfonyl radical 72, which then would add to the alkene moiety in 67 to produce a radical intermediate 73 (Scheme 22). The photocatalyst-assisted

Dirhodium(II)-catalyzed [3 + 2] cycloaddition of N-arylaminocyclopropane with alkyne derivatives

• Wentong Liu,
• Yi Kuang,
• Zhifan Wang,
• Jin Zhu and
• Yuanhua Wang

Beilstein J. Org. Chem. 2019, 15, 542–550, doi:10.3762/bjoc.15.48

• cycloaddition chemistry based on compound 1. Zheng et al. [7] first reported on the [3 + 2] cycloaddition reaction of 1 with an alkene or alkyne mediated by visible light by the aid of the photocatalyst [Ru(bpz)3](PF6)2. Our group reported the metal catalyst itself, particularly the dinuclear rhodium complex

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

• photocatalyst can be coupled to the reduction of Cu(I) to Cu(0), which can be observed precipitating from solution over the course of the reaction. Copper(II) salts have been demonstrated to be convenient terminal oxidants in a variety of synthetically useful catalytic reactions [23][24][25][26]. They are

• of carbamate 1 (Table 1), a reaction we had previously studied under stoichiometric Cu(II) conditions and found to proceed in good yield using 2.5 mol % 2,4,6-triphenylpyrylium tetrafluoroborate (TPPT, 3) as a photocatalyst and 1.2 equiv of Cu(TFA)2 as a stoichiometric oxidant. We lowered the loading

• nitrogen atom source (16–18). Finally, alkene diamination is also readily achieved using N-phenylureas as nucleophiles, although acridinium photocatalyst 6 afforded modestly higher yields in these reactions (19–21). A complete mechanistic picture of this reaction will require additional experimentation

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

• Yu Liu,
• Qiao-Lin Wang,
• Zan Chen,
• Cong-Shan Zhou,
• Bi-Quan Xiong,
• Pan-Liang Zhang,
• Chang-An Yang and
• Quan Zhou

Beilstein J. Org. Chem. 2019, 15, 256–278, doi:10.3762/bjoc.15.23

• (dtbbpy)PF4 as photocatalyst, and K2HPO4 as base in MeCN under the irradiation of 24 W blue LED light at room temperature for 12–36 h. A plausible mechanism is shown in Scheme 19. Firstly, the substrate 84a underwent oxidative quenching under the action of an iridium photoredox catalyst to afford the

N-Arylphenothiazines as strong donors for photoredox catalysis – pushing the frontiers of nucleophilic addition of alcohols to alkenes

• Fabienne Speck,
• David Rombach and
• Hans-Achim Wagenknecht

Beilstein J. Org. Chem. 2019, 15, 52–59, doi:10.3762/bjoc.15.5

• ]. The photochemical reactivity can be tuned by the absorption and excited state characteristics of the photocatalyst. In this context, organic dyes represent a perfectly suited class of photocatalysts as they can easily be modified by the introduction of functional groups that allow fine-tuning the

• transfer from the N-phenylphenothiazine (1) as photocatalyst to 13a as substrate. The resulting substrate radical anion 13a−· is instantaneously protonated to radical 13a· and back-electron transfer to the intermediate phenothiazine radical cation 1+· yields the substrate cation 13a+. The latter is

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

• degenerative transfer of xanthates to olefins is enabled by the iridium-based photocatalyst [Ir{dF(CF3)ppy}2(dtbbpy)](PF6) under blue LED light irradiation. Detailed mechanistic investigations through kinetics and photophysical studies revealed that the process operates under a radical chain mechanism, which

• is initiated through triplet-sensitization of xanthates by the long-lived triplet state of the iridium-based photocatalyst. Keywords: energy transfer; olefin; photocatalysis; radical; xanthate; Introduction A degenerative radical transfer of xanthates to olefins has been developed as a robust

• 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

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

• created as illustrated in Figure 1 to regenerate the PC. As illustrated, for an oxidative cycle, the excited photocatalyst (PC*) reacts first with an electron acceptor (also named oxidation agent, OA1 in Figure 1) which leads to PC●+ and OA1●− . Then, PC can be regenerated with an electron donor (also

• observe, this process offers the possibility to regenerate the catalyst. Consequently, the amount of PC used for the photochemical transformation is added only in catalytic amount and recovered after the reaction. That’s why the definition of photocatalyst is fulfilled. For a compound to be efficient as

• using a catalytic system instead of “traditional” PIs, can drastically reduce the final price of the system. Secondly, we noticed that there is no notably difference between the reactivities using a metal-based and a metal-free photoredox catalyst. The choice of the photocatalyst has to be done

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

• photocatalyst, Cu(tmp)(BINAP)BF4, was found to be active in a photoredox Appel-type conversion of alcohols to bromides. The catalyst was identified from a screening of 50 complexes and promoted the transformation of primary and secondary alcohols to their corresponding bromides and carboxylic acids to their

• 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

• a suitable copper-based catalyst was performed under identical reaction conditions whereby the Ru-based photocatalyst was substituted for the Cu-based complex. Control reactions performed in the absence of light or in the absence of catalyst at either 394 or 450 nm revealed no conversion to the

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

• charge transfer using Cs2CO3 as base [41]. Two recent reports showed the synthesis of C-3 sulfenylated indoles and 3-sulfenylimidazopyridine via C–H functionalization using Rose Bengal as photocatalyst [42][43]. In general, the arylation reactions use the reductive cycle of the photocatalyst and for this

• (dtbpy)]PF6 as the photocatalyst. The reaction was carried out under nitrogen under visible-light irradiation at 455 nm. The oxidation potential of this test arene is 1.02 V vs SCE, which allows oxidation by [Ir(dF(CF3)ppy)2(dtbpy)]PF6 having an estimated excited state oxidation potential of 1.21 V vs

• SCE. Other photocatalysts like Ru(bpy)3Cl2, Ru(bpz)3PF6, DDQ, acridinium dyes, Eosin Y, Eosin Y disodium salt and 4-CzIPN were evaluated, but under our reaction conditions either low substrate conversion or the degradation of the photocatalyst was observed (see Supporting Information File 1, Table S1

Applications of organocatalysed visible-light photoredox reactions for medicinal chemistry

• Michael K. Bogdos,
• Emmanuel Pinard and
• John A. Murphy

Beilstein J. Org. Chem. 2018, 14, 2035–2064, doi:10.3762/bjoc.14.179

• photophysical overview. There are several factors that affect the ability of an organic molecule to act as a photocatalyst. In a typical organocatalysed photoredox reaction, the photocatalyst transitions from a singlet ground state (S0) to a long-lived and relatively stable excited state, either a singlet

• excited state (S1) or a triplet excited state (T1), by absorption of a photon with energy hν, which then undergoes photoinduced electron transfer (PET). Following this, the photocatalyst is reduced or oxidised accordingly, such that it returns to its ground state and native oxidation state (Figure 1 and

• Figure 2). It is ideal if a photocatalyst has a local absorbance maximum (λmax) at a relatively long wavelength. Lower energy photons avoid exciting other reactants and prevent competing photochemistry from occurring, cf. ultraviolet light. However, the energy of the absorbed photon also determines the

Graphitic carbon nitride prepared from urea as a photocatalyst for visible-light carbon dioxide reduction with the aid of a mononuclear ruthenium(II) complex

• Kazuhiko Maeda,
• Daehyeon An,
• Ryo Kuriki,
• Daling Lu and
• Osamu Ishitani

Beilstein J. Org. Chem. 2018, 14, 1806–1812, doi:10.3762/bjoc.14.153

• (773–923 K) in air, and was examined as a photocatalyst for CO2 reduction. With increasing synthesis temperature, the conversion of urea into g-C3N4 was facilitated, as confirmed by X-ray diffraction, FTIR spectroscopy and elemental analysis. The as-synthesized g-C3N4 samples, further modified with Ag

• emerging material as an organic semiconductor photocatalyst active for various kinds of reactions such as water splitting, CO2 reduction, and degradation of harmful organic compounds, because of its non-toxic, stable, and earth-abundant nature [2][3][4][5][6][7]. Our group has developed photocatalytic CO2

• -responsive photocatalyst mostly for H2 evolution from aqueous triethanolamine (TEOA) solution [2][3][5]. The present work also compares the activities for CO2 reduction with those for H2 evolution in order to obtain a better understanding on photocatalytic activities of g-C3N4 for different kinds of

Visible light-mediated difluoroalkylation of electron-deficient alkenes

• Vyacheslav I. Supranovich,
• Vitalij V. Levin,
• Marina I. Struchkova,
• Jinbo Hu and
• Alexander D. Dilman

Beilstein J. Org. Chem. 2018, 14, 1637–1641, doi:10.3762/bjoc.14.139

• addition step cannot be oxidized by photocatalysts. Herein we report a convenient method for performing hydroperfluoroalkylation of electron-deficient alkenes employing iodides 1 mediated by visible light. The reaction proceeds without the use of a photosensitizer or a photocatalyst. Generation of

• hydrofluoroalkylation process using fac-Ir(ppy)3 as a photocatalyst in combination with a suitable donor of hydrogen atom. With triethylamine, no expected product was observed. At the same time, with Hantzsch ester (3 equiv), which can serve as a single-electron reductant and as a source of hydrogen [16], 54% of

• product 3a was formed. However, difficulties in removing the pyridine byproduct formed from Hantzsch ester and the use of a precious metal photocatalyst make this protocol less practical compared to that with sodium cyanoborohydride. Under the optimized conditions a series of gem-difluorinated iodides 1

Photocatalytic formation of carbon–sulfur bonds

• Alexander Wimmer and
• Burkhard König

Beilstein J. Org. Chem. 2018, 14, 54–83, doi:10.3762/bjoc.14.4

• be increased. To overcome this limitation, anilines were used as redox mediators, which are first oxidized by the photocatalyst and subsequently activate the aliphatic thiol via direct hydrogen abstraction or sequential electron- and proton-transfer steps. With this concept they were now able to

• photoredox-catalyzed radical thiol–ene reaction for polymer postfunctionalization and step-growth addition polymerization (Scheme 4a) [33]. In contrast to Yoon’s conditions, they used [Ru(bpy)3]Cl2 as photocatalyst and N-methyl-2-pyrrolidone as solvent and were able to efficiently couple polybutadiene and

• concept [32] (Scheme 4b) [34]. They first introduced alkene moieties to the chemically inert lignin structure by esterification of the hydroxy groups of lignin with 4-pentenoic acid. Subsequent radical thiol–ene reaction with aliphatic thiols, using [Ru(bpy]3Cl2 as photocatalyst and p-toluidine as redox

CF₃SO₂X (X = Na, Cl) as reagents for trifluoromethylation, trifluoromethylsulfenyl-, -sulfinyl- and -sulfonylation and chlorination. Part 2: Use of CF₃SO₂Cl

• Hélène Chachignon,
• Hélène Guyon and
• Dominique Cahard

Beilstein J. Org. Chem. 2017, 13, 2800–2818, doi:10.3762/bjoc.13.273

• -sulfonyl amides failed to react. Interestingly, Zhang and co-workers demonstrated that this reaction could be performed as well using bismuth oxybromide (BiOBr) nanosheets instead of a ruthenium complex as the photocatalyst (Scheme 5) [12]. The reaction unfortunately suffered from the same limitations

• -nitroalkenes with trifluoromethanesulfonyl chloride [39]. They found out that in the presence of the photocatalyst Eosin Y, under visible-light irradiation, such substrates could be selectively converted into (E)-1-trifluoromethylalkenes in moderate to good yields (Scheme 30). A plausible mechanism for this

CF₃SO₂X (X = Na, Cl) as reagents for trifluoromethylation, trifluoromethylsulfenyl-, -sulfinyl- and -sulfonylation. Part 1: Use of CF₃SO₂Na

• Hélène Guyon,
• Hélène Chachignon and
• Dominique Cahard

Beilstein J. Org. Chem. 2017, 13, 2764–2799, doi:10.3762/bjoc.13.272

• the presence of the organic photocatalyst N-methyl-9-mesitylacridinium (17), CF3SO2Na was converted into CF3• upon visible-light irradiation. The CF3• radical reacted with the vinyl azide to give the iminyl radical 18 that was reduced by Mes-Acr• (Mes-Acr: 9-mesityl-10-methylacridinium) into the

• stoichiometric amounts of oxidant and further transformation of the azotrifluoromethyl products allowed a Fisher indole synthesis. From a mechanistic point of view, the excited photocatalyst was oxidised by the aryldiazonium salt to produce [Ru(bpy)3]3+ (bpy: 2,2’-bipyridine) as the oxidant to generate the CF3

• authors also realised the same chemical transformation under visible light irradiation at 450 nm by means of the iridium photocatalyst Ir[dF(CF3)ppy]2(dtbbpy)PF6 ([4,4’-bis(tert-butyl)-2,2’-bipyridine]bis[3,5-difluoro-2-[5-(trifluoromethyl)-2-pyridinyl]phenyl]iridium(III) hexafluorophosphate), which

Mechanochemical borylation of aryldiazonium salts; merging light and ball milling

• José G. Hernández

Beilstein J. Org. Chem. 2017, 13, 1463–1469, doi:10.3762/bjoc.13.144

• . Simultaneous neat grinding/irradiation of the reactants and the photocatalyst led to the formation of boronates in a molten state. On the other hand, the catalyst-free liquid-assisted grinding/irradiation reaction also led to product formation, featuring a direct photolysis pathway facilitated by substrate

• spectroscopy revealed just the presence of both reactants, both in the presence or absence of the organic photocatalyst eosin Y (5.0 mol %). Ruling out a sole mechanochemical activation pathway (Table 1, entries 1 and 2). Repeating the reaction in the presence of the PC in the transparent PPMA milling jar

• reaction mixture reached 6:94 (entry 12 in Table 1). Alternatively, longer reaction times allowed reducing the amount of the organic photocatalyst to 1.0 mol % and 0.5 mol % (entry 13 in Table 1; for more details, see Table S1 in Supporting Information File 1). Then, using the green LEDs an experiment in

Photocatalyzed synthesis of isochromanones and isobenzofuranones under batch and flow conditions

• Manuel Anselmo,
• Lisa Moni,
• Hossny Ismail,
• Davide Comoretto,
• Renata Riva and
• Andrea Basso

Beilstein J. Org. Chem. 2017, 13, 1456–1462, doi:10.3762/bjoc.13.143

• loading of photocatalyst (0.5% vs 2%) and excess of alkene (2 equiv vs 3 equiv) employed; moreover, the use of an inert atmosphere did not lead to significant improvements. No differences were observed when the 440 nm laser was replaced by blue LEDs (maximum wavelength 455 nm). Diazonium salts were

Chemoselective synthesis of diaryl disulfides via a visible light-mediated coupling of arenediazonium tetrafluoroborates and CS₂

• Jing Leng,
• Shi-Meng Wang and
• Hua-Li Qin

Beilstein J. Org. Chem. 2017, 13, 903–909, doi:10.3762/bjoc.13.91

• tetrafluoroborates; carbon disulfide; chemoselectivity; diaryl disulfides; photocatalyst; Findings The development of methods for the functionalization of peptides and proteins under mild conditions is a current frontier in the fields of chemistry, biology and drug discovery [1][2][3][4]. Most of the

• reactions [15][22]. Based on the above research results, we envisioned that a radical pathway may facilitate the formation of diaryl disulfides. Therefore the photocatalyst Ru(bpy)3(PF6)2 (bpy = 2,2’-bipyridine) [23] and a 20 W blue-light LED were chosen as catalyst and the source of visible light

Stabilization of nanosized titanium dioxide by cyclodextrin polymers and its photocatalytic effect on the degradation of wastewater pollutants

• Tamás Zoltán Agócs,
• István Puskás,
• Erzsébet Varga,
• Mónika Molnár and
• Éva Fenyvesi

Beilstein J. Org. Chem. 2016, 12, 2873–2882, doi:10.3762/bjoc.12.286

• nanosized titanium dioxide (nanoTiO2) applied in photodegradation-based procedures can be stabilized with cyclodextrins in order to obtain a new, more efficient photocatalyst for the purification of waters polluted by xenobiotics applying UV irradiation. During our work, on the one hand, we studied the