Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• electron transfer (SET) steps photoinitiated using visible light as the energy source, often leading to the generation of radicals and subsequent reaction of these radicals with the ground-state substrates [3]. Energy transfer catalysis is another significant branch of photocatalysis, in which

• ][33]. When an alkylating agent containing an aryl ring 40 (e.g., phenacyl/benzyl bromides) is used, the enamine intermediate 41 forms a coloured electron-donor acceptor (EDA) complex that can absorb visible light via an intermolecular charge-transfer state (EDA route) [14][34]. Mechanistic

Photosensitized direct C–H fluorination and trifluoromethylation in organic synthesis

• Shahboz Yakubov and
• Joshua P. Barham

Beilstein J. Org. Chem. 2020, 16, 2151–2192, doi:10.3762/bjoc.16.183

• reactions. Keywords: C–H activation; energy transfer; fluorination; photocatalysis; photosensitization; visible light; Review 1 Introduction 1.1 Importance of direct C–H fluorination/trifluoromethylation and photosensitization in organic synthesis 1.1.1 Importance of fluorine atoms in organic molecules

• –H bond is matched to the HAT catalyst. For example, an electrophilic radical abstracts H atoms selectively from the most electron-rich or “hydridic” C–H bond [65]. 1.1.3 Importance of visible light and visible-light photosensitization in synthesis: Unlike many other traditional energy resources

• processes [72][73][74][75][76][77]. However, the adoption of photochemistry into the toolbox of the synthetic organic chemist has been hindered by three phenomena: i) simple organic-molecule targets typically do not absorb (or have very small extinction coefficients for) visible light, which is among the

Access to highly substituted oxazoles by the reaction of α-azidochalcone with potassium thiocyanate

• Mysore Bhyrappa Harisha,
• Pandi Dhanalakshmi,
• Rajendran Suresh,
• Raju Ranjith Kumar and
• Shanmugam Muthusubramanian

Beilstein J. Org. Chem. 2020, 16, 2108–2118, doi:10.3762/bjoc.16.178

• synthesis of 2,4,5-trisubstituted oxazole from azirine. a) I2, PPh3; b) NaH, 1H-pyrazole; c) 2-bromoacetyl bromide, NaN3; d) heating; e) t-BuOK; f) Ph-CHO, visible light; g) KSCN, K2S2O8. Scope of the α-azidochalcones. The reactions were carried out at reflux temperature, using 1 (1 mmol), 2 (3 mmol

Metal-free synthesis of phosphinoylchroman-4-ones via a radical phosphinoylation–cyclization cascade mediated by K₂S₂O₈

• Qiang Liu,
• Weibang Lu,
• Guanqun Xie and
• Xiaoxia Wang

Beilstein J. Org. Chem. 2020, 16, 1974–1982, doi:10.3762/bjoc.16.164

• appropriate radical precursors through visible-light promoted systems [15][16], transition-metal-catalyzed systems [17][18], or transition-metal-free systems [19][20], have emerged as a powerful strategy for the synthesis of diversely functionalized chroman-4-one derivatives. Organophosphorus compounds are

Synthesis and highly efficient light-induced rearrangements of diphenylmethylene(2-benzo[b]thienyl)fulgides and fulgimides

• Vladimir P. Rybalkin,
• Sofiya Yu. Zmeeva,
• Lidiya L. Popova,
• Valerii V. Tkachev,
• Andrey N. Utenyshev,
• Olga Yu. Karlutova,
• Alexander D. Dubonosov,
• Vladimir A. Bren,
• Sergey M. Aldoshin and
• Vladimir I. Minkin

Beilstein J. Org. Chem. 2020, 16, 1820–1829, doi:10.3762/bjoc.16.149

• as compared with 2-thienyl and 3-benzo[b]thienyl analogues. Exposure of the colored solutions to the visible light leads to the reverse conversion into the primary colorless hexatriene form. Here, we report the synthesis of 2-benzo[b]thienylfulgides with bulky diphenylmethylene substituents and the

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

• last decade particular attention has been focused on two modern fields, C–H bond activation, and visible-light-induced photocatalysis. Couplings through C–H bond activation involve the use of non-prefunctionalized substrates that are directly converted into more complex molecules, without the need of a

• catalytic system is extremely appealing. In that perspective, the scope of this review aims to present innovative reactions combining C–H activation and visible-light induced photocatalysis. Keywords: C–H activation; C–H functionalization; dual catalysis; photoredox catalysis; radical chemistry

• –H activations, visible-light-induced photocatalysis, electrosynthesis, enzyme catalysis, and others. Each of these techniques aims at accessing complex molecules while limiting ecological footprint. Over the last decade, the metal-catalyzed C–H activation established itself as one of the most

Pauson–Khand reaction of fluorinated compounds

• Jorge Escorihuela,
• Daniel M. Sedgwick,
• Alberto Llobat,
• Mercedes Medio-Simón,
• Pablo Barrio and
• Santos Fustero

Beilstein J. Org. Chem. 2020, 16, 1662–1682, doi:10.3762/bjoc.16.138

• endotermic process is the rate-limiting step and long reaction times are generally associated to this. However, the reaction can be accelerated in conditions that facilitate the dissociation of CO ligands such as heating, microwave irradiation [37][38], visible light, or ultrasonication [39]. Alternatively

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

• visible light through single-electron transfer processes, now referred to as visible light photoredox catalysis (PRC). A similar query on the Web of Science for the term “Photoredox” clearly shows the surge in PRC research following those reports, from 2010 onwards (Figure 1A). However, what is

• processes by an upconversion system to produce higher-frequency, ultraviolet (UV), or visible light photons that are reemitted in close proximity to, or directly from within the HPCat solid matrix [51]. This is discussed in more detail in Section 2.5. The “Achilles’ heel” of all heterogeneous catalysts is

• . Synthetic TiO2 commonly has oxygen atom vacancy defects, especially at its surface, resulting in Ti3+ ions that produce quantum states close to the conduction band minimum. These states lower the Eg, enabling visible light absorption and producing reactive centres on the catalyst surface, which has been

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

• ; phenanthridines; photocatalysis; radicals; visible light; Introduction Phenanthrenes are widely investigated compounds, due to the impressive number of diverse applications involving this scaffold, ranging from medicinal chemistry [1] to materials sciences, including their use in optoelectronics [2][3] and in

• inexpensive visible light (or sunlight, when possible) irradiation [29]. In general terms, photocatalysis smoothly gives access to reactive radical intermediates [30], mainly carbon-centered [31][32][33], or nitrogen-centered radicals [34][35]. In turn, these species have been extensively employed in radical

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

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

• of visible light, oxygen, and disulfide [15]. The diaryl-1,2-diketone products can serve as an important structural component of many natural products and bioactive molecules. This method works for a broad range of substrates with high yields (77–97%). A plausible mechanism for this reaction is that

• by the excitation with visible light, the homolytic cleavage of disulfide generates an arylthiyl radical (ArS•), which adds to diphenylacetylene to form a free-radical intermediate 28. Subsequently, this intermediate traps a molecule of singlet oxygen (1O2), and the thiyl radical is regenerated to

• give the four-membered ring intermediate 29. Finally, the rearrangement of the four-membered intermediate provides the diketone 30 as the product (Scheme 8). In 2017, Wang and co-workers reported an oxidative cleavage of aromatic alkenes at ambient temperature with visible-light irradiation, using

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

• . The developed method can be employed for the synthesis of various bulky vicinal diamines, which are potential ligands in stereoselective synthesis. Keywords: 1,2-diamine; diversity; imine; photocatalysis; visible light; Introduction The selective formation of distinct valuable compounds from the

• same starting material is a highly attractive divergent approach, though it represents significant synthetic challenges. Recent advances in visible-light photocatalysis, mediated by visible-light-absorbing photosensitizers, have allowed ready access to complex molecules in a controlled manner, where

• extensively explored under visible-light photocatalysis [10][11][12][13][14][15][16]. N-Benzylidenes can undergo facile single-electron reduction to generate α-amino radical intermediates, which can participate in diverse processes, depending upon the nature of the substrates and the reaction conditions

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

• 1,2,4-oxadiazolines 88 by oxidative cyclization of amidoximes 87 under the action of molecular oxygen and visible light in the presence of catalytic amounts of 2,4,6-tris(4-fluorophenyl)pyrilium tetrafluoroborate (T(p-F)PPT) was proposed (Scheme 32) [122]. Pyrrolidinyl oxime derivatives having both

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

• growth resides in the availability of visible light-absorbing transition metal complexes. These catalysts can harvest the energy of visible-light photons and transfer it to organic molecules, giving access to key reactive intermediates. For instance, ruthenium and iridium polypyridyl complexes played a

• reported a visible light-promoted photochemical reductive decarboxylation/alkylation of carboxylic acid analogs (Scheme 4) [45]. In this protocol, the carboxylic acids are converted into the corresponding RAE 4.1 by a condensation with N-hydroxyphthalimide. The organic dye eosin Y (OD13, E(PC/PC−) ≈ −1.1 V

• successfully employed for the generation of C(sp3) radicals via HAT [58]. Interestingly, Wu and co-workers demonstrated that eosin Y (OD13) can also act as a direct HAT catalyst under visible-light irradiation [59]. Organic photoredox catalysis can also drive indirect HAT processes. In these reactions, the

Fluorinated phenylalanines: synthesis and pharmaceutical applications

• Laila F. Awad and
• Mohammed Salah Ayoup

Beilstein J. Org. Chem. 2020, 16, 1022–1050, doi:10.3762/bjoc.16.91

• LED light source (365 nm) and Selectfluor in MeCN [72]. Alternatively, a visible light (14 Watt CFL) mediated benzylic fluorination of a series of N- and C-terminally protected phenylalanines 147 using Selectfluor and dibenzosuberenone in acetonitrile, afforded the β-fluorophenylalanine derivatives

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

• overcome these issues, and leads to a drastic reduction of reaction time, lower photocatalyst loadings, minimization of the formation of byproducts [2] and uses visible light, which is considered a clean reagent [4]. Overall, visible light combined with organic photocatalysts such as porphyrinoids, make

• platform for dual catalysis due to their ability to promote both metallocatalysis and photocatalysis in a one-pot system [36][37][38][39]. Martin and co-workers carried out the C–O bond cleavage of alcohols using a cobalt porphyrin under visible light irradiation and a carbon monoxide atmosphere (Scheme 11

• highly regioselective photoinduced hydro-defluorination of perfluoroarenes with RhIII(TSPP) [38]. The oxidative addition of the perfluoroarene to the metal complex furnished the active rhodium aryl complex intermediate, which led to the product after visible light irradiation. The hydro-defluorination

Cation-induced ring-opening and oxidation reaction of photoreluctant spirooxazine–quinolizinium conjugates

• Phil M. Pithan,
• Sören Steup and
• Heiko Ihmels

Beilstein J. Org. Chem. 2020, 16, 904–916, doi:10.3762/bjoc.16.82

• irradiation with visible light, as is exemplarily shown for the spiroindolinonaphthoxazine 1a (Scheme 1) [15]. The merocyanine form 1aMC essentially possesses a planar structure, and because of the extended π system, the absorption maximum is significantly red-shifted in comparison to the closed form (e.g

• extended π system or an intramolecular charge transfer (ICT). This property may allow to use visible light with longer wavelength to induce the ring-opening reaction, which would be favorable, e.g., for biological applications. The stilbene etheno bridge, however, may also lead to an intricate photochromic

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

• , electrophilic alkyl radicals were used in several transformations, such as electrophilic cross couplings under nickel catalysis, either with boronic acids [35] or different (aryl)halides [36][37][38]. Furthermore, visible light-promoted uncatalyzed electron transfer via the formation of electron donor–acceptor

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

• ; reduction; Introduction For a decade, the realm of catalysis has known a significant renewal with the rise of photocatalysis [1]. The fact that a reaction can be carried out, and, more specifically, catalyzed in the presence of visible light and a photosensitive catalyst (organic or organometallic

• organic molecules absorb light in the UV region (UV-A, UV-B, or UV-C), selectivity was a key point to address in photochemistry and photocatalysis using visible light tackled this issue. Moreover, the use of inexpensive and energy-efficient visible light-emitting diodes (LEDs) was an impressive

• , photocatalysis appeared as an interesting alternative to catalyze such transformations, and copper-based catalysts provided interesting reactivities. In 2012, the Reiser group reported a visible light-driven coupling reaction of olefin derivatives with bromo- and iodoalkanes using the Sauvage catalyst as a 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

• proceeds. Several UV and visible light sensitive compounds in conjunction with or without Cu(II) complexes were used to initiate and control the photo-ATRP process (Scheme 9) [130][131][132][133][134][135][136][137]. In the absence of these photoactive materials, the direct irradiation of Cu(II) was

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

• glycine and diethyl malonate, are based on nucleophilic substitution of the chlorine [6][7][8][9][10]. In an attempt to find new synthetic applications of monochloroacetic acid we turned our attention to photoredox catalysis. We were inspired by the rebirth of visible light photoredox catalysis, induced

• by the work of Yoon, Stephenson and MacMillan [11]. By selective excitation of an organic or organometallic dye by visible light a species is formed that can act as a single-electron oxidant and single-electron reductant. In this way reactive radical intermediates can be formed catalytically in situ

• , Kokotos and co-workers published a visible light photoredox catalyzed lactone formation from iodoacetic acid and alkenes [29] (Scheme 1). In addition, it has been shown that monochloroacetic acid can be reduced by hydrated electrons [30][31]. The group of Goez developed a strategy for the photoredox

Room-temperature Pd/Ag direct arylation enabled by a radical pathway

• Amy L. Mayhugh and
• Christine K. Luscombe

Beilstein J. Org. Chem. 2020, 16, 384–390, doi:10.3762/bjoc.16.36

• radical; visible light; Introduction π-Conjugated polymers are of significant interest as they have the potential to combine the mechanical flexibility and affordability of synthetic polymers with the optical and electronic properties of semiconductors. One of the limitations to this field’s continued

• the polymerization conditions, no polymerization occurred (Table 1). Additionally, the reaction was completely inhibited when run in the dark. These results indicate that visible light may be responsible for radical generation in the polymerization. To further investigate the possibility of a radical

• mechanism has been recently supported by 13C and 2H KIE experiments for the arylation of benzo[b]thiophene, although at C3 [13]. In contrast to these pathways, the radical trap and dark experiments reported above indicate a hybrid Pd(I) radical species induced by visible light is involved in the catalytic

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

• issue to the scientific community. In this context, visible light-induced photoredox catalysis, which is thought to be an abundant, inexpensive, renewable, and nonpolluting chemical transformation, has attracted increasing attention during the past years due to the extraordinary competence and

• = C, B, N, O, S) bonds for architecturally simple, yet challenging molecules, which are otherwise highly difficult or impossible to be formed by other methods. These practices count on the competence of metal complexes and organic dyes to convert visible light into chemical energy via SET events

• ]. 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

Potent hemithioindigo-based antimitotics photocontrol the microtubule cytoskeleton in cellulo

• Alexander Sailer,
• Franziska Ermer,
• Yvonne Kraus,
• Rebekkah Bingham,
• Ferdinand H. Lutter,
• Julia Ahlfeld and
• Oliver Thorn-Seshold

Beilstein J. Org. Chem. 2020, 16, 125–134, doi:10.3762/bjoc.16.14

• hemithioindigo-based indanone-like tubulin inhibitors (HITubs) and optimised their cellular potency as antimitotic photopharmaceuticals. These HITubs feature reliable and robust visible-light photoswitching and high fatigue resistance. The use of the hemithioindigo scaffold also permitted us to employ a para

• , all-visible-light photoswitching. Results and Discussion Design strategy for HTIs The HTI-based colchicinoid HOTubs (e.g., HOTub-31) that we previously explored had the HTI photoswitch embedded inside a methoxylation pattern, such that one isomer obeyed the structure–activity relationship (SAR) of

Light-controllable dithienylethene-modified cyclic peptides: photoswitching the in vivo toxicity in zebrafish embryos

• Sergii Afonin,
• Oleg Babii,
• Aline Reuter,
• Volker Middel,
• Masanari Takamiya,
• Uwe Strähle,
• Igor V. Komarov and
• Anne S. Ulrich

Beilstein J. Org. Chem. 2020, 16, 39–49, doi:10.3762/bjoc.16.6

• -closed isomer, while irradiation with visible light (>400 nm) converts the ring-closed isomer back to the ring-open photoform (Figure 1B), thereby affecting the structure and flexibility of the immediate molecular surrounding. We have constructed a number of DAE-containing peptides by incorporating a DAE

Starazo triple switches – synthesis of unsymmetrical 1,3,5-tris(arylazo)benzenes

• Andreas H. Heindl and
• Hermann A. Wegner

Beilstein J. Org. Chem. 2020, 16, 22–31, doi:10.3762/bjoc.16.4

• isomerization can be reversed either by irradiation with visible light (ca. 450 nm) or thermally, upon heating. The thermal back-isomerization rate can be controlled by various factors: Functional group substitutions on the phenyl rings determine the thermal half-lives of (Z)-ABs [1]. For example, o-fluoro