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

• transformative synthetic strategy, including in enantioselective synthesis. From the pioneering work by MacMillan [1] and Bach [2], enantioselective photocatalysis has grown into a well-established field of its own. A large proportion of photocatalysis focuses on photoredox catalysis, which involves single

• with photoredox catalysis was reported by Nicewicz and MacMillan [1] for the alpha alkylation of aldehydes 1 with various alkyl bromides bearing an electron-withdrawing substituent 2, which while seemingly trivial, was not possible with enamine catalysis alone (Scheme 1). The proposed mechanism

• /ketones 33 with glycine derivatives 34 to synthesise the corresponding products 35 in good yields and excellent enantioselectivities (35 examples, up to 99:1 er) (Scheme 4b) [30]. The previous examples of enamine/photoredox catalysis have all required two or more separate catalysts. Currently, three

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 proceed under photoredox catalysis (PRC), involving Dexter electron transfer. Such photoredox reactions begin with the excitation of the photocatalyst (PC) by visible light, followed by a single-electron transfer (SET) between the excited photocatalyst and another molecule (quencher, Scheme 2A

A complementary approach to conjugated N-acyliminium formation through photoredox-catalyzed intermolecular radical addition to allenamides and allencarbamates

• Olusesan K. Koleoso,
• Matthew Turner,
• Felix Plasser and
• Marc C. Kimber

Beilstein J. Org. Chem. 2020, 16, 1983–1990, doi:10.3762/bjoc.16.165

• radical addition, using photoredox catalysis, to allenamides and allencarbamates is reported. This transformation synthesizes N-acyl-N’-aryl-N,N’-allylaminals, and proceeds by a conjugated N-acyliminium intermediate that previously has principally been generated by electrophilic activation methods. The

• in the synthesis of N,N’-aminals [50]. Therefore, photoredox-catalysis would be employed to generate an electrophilic radical that would add to the central carbon of the allenamide 1 to give a transient radical 13, whose oxidation, facilitated by the photoredox catalyst [47][48], would provide 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

• previous functionalization, thus considerably reduce waste generation and a number of synthetic steps. In parallel, transformations involving photoredox catalysis promote radical reactions in the absence of radical initiators. They are conducted under particularly mild conditions while using the visible

• 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

• catalysis with a second catalytic system has sparked significant interest from the scientific community. Accordingly, numerous unprecedented reactions involving photoredox catalysis in synergy with other activation modes including Brønsted/Lewis acid catalysis, organocatalysis, enzymatic biocatalysis, or

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

• some interesting reactor designs that could be implemented to enhance organic synthesis. Keywords: air purification; flow chemistry; heterogeneous photoredox catalysis; organic synthesis; reactor design; water purification; Review 1 Introduction 1.1 Scope of the review This review aims to be of

• discussion of the advantages and disadvantages to guide the reader in selecting the reactor best suited to their system. Following this, in Section 4, we review the recent applications of HPCats in flow reactors for synthetic organic chemistry through photoredox catalysis (PRC, Section 4.1) and energy

• 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

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

• is a quite unexplored field, a notable exception being the seminal work published in 1984 by Cano-Yelo and Deronzier, where the authors reported one of the first applications of the Ru(bpy)32+ complex in photoredox catalysis (Scheme 1). This contribution described a photo-Pschorr cyclization

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

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

• . Under photoirradiation, organic disulfides can be easily cleaved into free thiyl radicals (RS•) and can reversibly add to unsaturated multiple bonds to catalyze a variety of functionalization reactions under mild conditions. In photoredox catalysis reactions, an excellent electron transfer ability and

• variety of reactions, renders them a class of green, economic, mild, and chemoselective radical catalyst. Apart from this, they are also excellent HAT catalysts in photoredox catalysis systems [2][3]. In various types of organic photochemistry reactions, such as cyclizations, anti-Markovnikov additions

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

• , partially due to the general interest of the community into C–H functionalization methodologies [18][19], and partially thanks to the synthetic possibilities enabled by photoredox catalysis, a field widely explored in recent years [20][21][22]. In 2018, Ngai [23][24] and Togni [25] reported simple

• residence time up to 16 times in comparison to the batch procedure, while achieving similar or higher yields. In addition, the use of inorganic bases was demonstrated to increase the reaction yield under batch conditions. Keywords: C–H functionalization; continuous-flow; organic synthesis; photoredox

• catalysis; trifluoromethoxylation; Introduction The number of fluorine-containing compounds and their influence in medicine and agrochemistry are increasing every year [1][2]. The introduction in a drug (or a bioactive compound in general) of single fluorine atoms, or multiple fluorine-containing

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

• -, nitrogen-, oxygen-, and sulfur-centered radicals, open-shell charged species, and sensitized organic compounds. Keywords: organic dyes; photocatalysis; photoredox catalysis; radicals; reactive intermediates; Review Introduction In the last decade, synthetic organic chemistry has experienced the

• each of these steps, the role of A or D is assumed by a redox-active agent, either the substrate, a sacrificial electron donor/acceptor, or a reactive intermediate. This approach, usually named photoredox catalysis, has known a remarkable growth in the last decade and has given access to both neutral

• [32]. Recently, photoredox catalysis has emerged as a powerful tool to access C(sp3) radicals, and organic dyes (ODs) have been demonstrated to act as competent photocatalysts for these light-mediated reactions. The main strategies used include decarboxylations from carboxylic acid derivatives (CO2X

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

• metal-organic (MOF) and covalent-organic frameworks (COF), have significantly expanded the use of these compounds in photoredox catalysis due to the singular electronic features of these materials and chemical robustness as catalysts. The appearance of porphyrins as photoredox catalysts for C–C bond

• the same authors using 2 mol % of a Ru complex as photocatalyst (44–88%) [24]. The mechanism proposed by the authors was supported by the detection of some reaction intermediates, and suggest that TPP works in both energy transfer and photoredox catalysis (Scheme 2). Subsequently, Gryko’s group

• photoarylation of isopropenyl acetate. The corresponding α-aryl ketone was obtained in 28–53% overall yield depending on the scale. An 8 h experiment was conducted in a continuous steady-state mode, producing the same α-aryl ketone in 48% yield on a 3 gram-scale (Scheme 5B). Photoredox catalysis is not limited

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

• presented. Keywords: aldehyde; green chemistry; photochemistry; photoinitiation; sustainable chemistry; Introduction Photochemistry, and especially photoredox catalysis have altered the way that modern researchers treat radical species [1][2][3][4]. In most cases, a metal-based photocatalyst is employed

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

• (sp3) coupling; deaminative coupling; Katritzky salt; photoredox catalysis; Introduction The selective formation of new carbon–carbon bonds via direct C–H functionalization bears the potential of being a process of high efficiency [1][2][3]. Since C–H bonds are omnipresent in organic compounds it is

• ], alkynation [17][18], or allylation [19]), this method has certain drawbacks, most importantly the frequent requirement of superstoichiometric amounts of an oxidant. Hence, alternative methods were investigated and photoredox catalysis proved to be a viable option [20][21][22]. By now, several different

• methods for C–C bond formation at the α position of these amines have been studied under photoredox catalysis (Scheme 1) [23][24][25][26]. Among the functionalized THIQs, 1-benzyl-substituted analogues were shown to be able to modulate Ca/K channels, and the synthesis of these compounds is therefore of

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

• traditional reactivity of this compound. Here, we investigated the possibility of applying monochloroacetic acid as a substrate for photoredox catalysis with styrene to directly produce γ-phenyl-γ-butyrolactone. Instead of using nucleophilic substitution, we cleaved the carbon chlorine bond by single-electron

• 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

• , resulting in reactivity different from common two-electron pathways. Photoredox catalysis reactivity is very different from traditional redox reactions, and the same reactivity cannot be achieved by stoichiometric addition of both a reductant and an oxidant to a reaction mixture (as that would lead to a

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

• compounds, including bioactive natural and nonnatural products, but also due to its impact on the discovery of pharmaceutical candidates and the total synthesis of intricate natural products. On the other hand, more recently, the field of photoredox catalysis has become an indispensable and unparalleled

• in this vital area of research, to our best knowledge, this is the first review that focuses on ortho and para C–H functionalizations by photoredox catalysis to provide atom- and step-economic organic transformations. We are certain that this review will act as a promoter to highlight the application

• of photoredox catalysts for the functionalization of inert bonds in the domain of synthetic organic chemistry. Keywords: dual catalysis; light; ortho and para C–H bond functionalization; photoredox catalysis; Introduction Over a short period of time, direct C–H bond functionalizations by photoredox

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

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

• light range between 520 nm and 640 nm. The irradiation by visible light together with the use of an organic dye instead of a transition metal complex as photoredox catalyst improve the sustainability and make photoredox catalysis “greener”. Keywords: chromophore; dyes; electrochemistry; photochemistry

• ; photoredox catalysis; Introduction Photocatalysis couples the physical process of light absorption to an organic-chemical reaction by means of time, space and energetics. In order to apply visible light for photocatalysis despite its rather low energy this coupling requires to be mediated by a sensitizing

• 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

• trimethylsilyl trifluoromethanesulfonate (TMSOTf) in a diastereoselective and stepwise manner. This novel methodology provides a shorter access to the intermediate 97, which is a key intermediate for the synthesis of triptolide. Recently, photoredox catalysis has emerged as a powerful and high-yielding method

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

• ) salts proved to be unique in their ability to turn over the copper cocatalyst without deleteriously impacting the reactivity of the organoradical intermediates. Keywords: copper; diamination; oxidative functionalization; oxyamination; photoredox catalysis; radical; Introduction Over the past decade, a

• photoinduced electron transfer processes. A major theme of research that has emerged from these studies is the application of various cocatalysts to intercept the organoradical intermediates of photoredox reactions and modulate their subsequent reactivity [5][6]. The combination of photoredox catalysis with

• as a terminal oxidant. Moreover, much of the utility of photoredox catalysis has been predicated on its ability to generate radical intermediates under mild and operationally convenient conditions. The ability to intercept transient photogenerated organoradical intermediates and divert them towards

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

• photoredox catalysis oxidative radical ring-opening and cyclization of cyclopropyl olefins 83 with bromides 84 for the synthesis of partially saturated naphthalenes 85 in moderate to excellent yields (Scheme 19) [97]. It was the first example for alkylation, ring-opening and cyclization cascade reaction of

• the cyclopropyl olefins under photoredox catalysis. The alkylation reagents could be extended to other bromides, such as monofluoro-substituted bromides, trifluoro-substituted bromides, bromoacetonitrile and bromomalonate. This alkylation/ring-opening/cyclization was carried out by using Ir(ppy)2

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

• broaden the substrate scope for the nucleophilic addition of methanol to styrenes through photoredox catalysis. These N-phenylphenothiazines differ by their electron-donating and electron-withdrawing substituents at the phenyl group, covering both, σ and π-type groups, in order to modulate their

• absorbance and electrochemical characteristics. Among the synthesized compounds, alkylaminylated N-phenylphenothiazines were identified to be highly suitable for photoredox catalysis. The dialkylamino substituents of these N-phenylphenothiazines shift the estimated excited state reduction potential up to

• after 20 h of irradiation. Keywords: addition; phenothiazine; photochemistry; photoredox catalysis; redox potential; Introduction Visible-light photoredox catalysis has become a precious tool in modern synthetic organic chemistry and experiences a continuously growing interest in industrial

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

• from adhesives, coatings, packaging materials, inks, paints, optics, 3D printing, microelectronics or textiles. From a synthetic viewpoint, photoredox catalysis, originally developed for organic chemistry, has recently been applied to the polymer synthesis, constituting a major breakthrough in polymer

• networks. Keywords: photoinitiator; photopolymerization; photoredox catalysis; photoredox catalyst; Introduction Photopolymerization reactions are now widely used both in industry and in academic laboratories. These processes usually lead to the transformation of a liquid resin in a 3D solid polymer upon

• catalytic cycles used in organic chemistry, the development of photoredox catalysis for photopolymerization reactions has been proposed. It has emerged as a significant innovation in the field of photoinitiated polymerization. Photoredox catalysis is a new strategy to generate radicals and/or cations upon

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

Microfluidic light-driven synthesis of tetracyclic molecular architectures

• Javier Mateos,
• Nicholas Meneghini,
• Marcella Bonchio,
• Nadia Marino,
• Tommaso Carofiglio,
• Xavier Companyó and
• Luca Dell’Amico

Beilstein J. Org. Chem. 2018, 14, 2418–2424, doi:10.3762/bjoc.14.219

• molecules. Keywords: [4 + 2] photoenol; cycloaddition; flow chemistry; microfluidic photoreactor; photoredox catalysis; synthetic photochemistry; Introduction In recent years synthetic photochemistry has become highly sophisticated [1]. The opportunity of using renewable energy sources to transform and

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

• mechanistic considerations are highlighted in the text when appropriate. Keywords: C–H functionalisation; heterocycles; late-stage functionalisation; medicinal chemistry; organic dyes; organic photocatalysts; peptide chemistry; photoredox catalysis; Review 1 Introduction 1.1 Main advantages of

• organocatalysed photoredox chemistry Photoredox catalysis is an emerging field in organic synthesis and has been the subject of many reviews in recent years [1][2][3][4][5][6][7][8][9]. Some cover the manipulation or installation of various functional groups [10][11][12][13][14][15][16][17], the synthesis of

• ) components than traditional reactions. Organocatalysed photoredox catalysis combines the advantages of both these fields. Thus, it is not only a new field filled with exciting discoveries, but also is sustainable and beneficial in the long term. 1.2 General characteristics of photocatalysts 1.2.1 Brief

Investigations of alkynylbenziodoxole derivatives for radical alkynylations in photoredox catalysis

• Yue Pan,
• Kunfang Jia,
• Yali Chen and
• Yiyun Chen

Beilstein J. Org. Chem. 2018, 14, 1215–1221, doi:10.3762/bjoc.14.103

• radical; alkynylbenziodoxoles; photoredox catalysis; radical alkynylation; Introduction The introduction of the alkynyl group to organic molecules is an important synthetic transformation in organic synthesis [1][2][3][4]. Recently, cyclic iodine(III) reagents (CIR)-substituted alkynes

• group first used alkynylbenziodoxoles for decarboxylative radical alkynylation under silver salt and persulfate conditions [19]. In 2014, the Chen group discovered that alkynylbenziodoxoles (BI-alkyne) readily participated in photoredox catalysis as the radical alkynylation reagent [20], after which

• various applications in photoredox catalysis were reported [21][22][23][24][25][26][27]. Currently, the use of BI-alkyne for radical alkynylation is limited to unsubstituted alkynylbenziodoxoles. While effective, its reactivity with some radical precursors was compromised [19][20][21][22][23][24][25][26