A facile three-component route to powerful 5-aryldeazaalloxazine photocatalysts

• Ivana Weisheitelová,
• Radek Cibulka,
• Marek Sikorski and
• Tetiana Pavlovska

Beilstein J. Org. Chem. 2024, 20, 1831–1838, doi:10.3762/bjoc.20.161

• -deazaflavins [11][12][13]. Recently, it has been discovered that both 5-deazaflavins 1 and 5-deazaalloxazines 2, which have an aryl substituent in position C(5), form stable radicals that act as powerful reductive photocatalysts with a reducing power comparable to that of lithium [E*(1/1•) = −3.3 V vs SCE

• of photocatalysts by tuning their redox and photophysical properties. Thus, we successfully developed a one-pot, three-component synthetic method with those substituents in 5-aryldeazaflavins 1 on the deazaisoalloxazine core or on the phenyl ring by condensation of N-substituted anilines, aromatic

• substituents at positions 7 and 8 of the deazaalloxazine ring (see Figure 1 for numeration) showed the best reactivity as reductive photocatalysts. Such an outcome of MCR opens up the possibility of their production on a larger scale for commercial needs. However, the synthesis of the 7- and 8-methoxy

Benzylic C(sp³)–H fluorination

• Alexander P. Atkins,
• Alice C. Dean and
• Alastair J. J. Lennox

Beilstein J. Org. Chem. 2024, 20, 1527–1547, doi:10.3762/bjoc.20.137

• -fluorobenzoates as both photocatalysts or photo-auxiliaries and was demonstrated on a number of benzylic examples. Photochemical Photochemical methods have proven to be powerful tools in the generation of reactive intermediates, including benzylic radicals [64][65][66][67]. Oxidative photochemical

Generation of alkyl and acyl radicals by visible-light photoredox catalysis: direct activation of C–O bonds in organic transformations

• Mithu Roy,
• Bitan Sardar,
• Itu Mallick and
• Dipankar Srimani

Beilstein J. Org. Chem. 2024, 20, 1348–1375, doi:10.3762/bjoc.20.119

• photocatalysts, transition-metal photoredox catalysts, and metallaphotocatalysts to produce acyl and alkyl radicals driven by visible light. Keywords: acyl radical; alkyl radical; sustainable catalysis; visible light; Introduction The growing awareness of the necessity for sustainable developments has been

• . Different photocatalysts, such as transition metal complexes [23][24], organic dyes [25], and semiconductors [26], can be employed for visible-light-induced chemical processes. The choice of photocatalyst depends on the specific requirements of the catalytic process, including the type of reaction, the

• targeted absorption wavelength, and the overall efficacy. Researchers continue to explore and design photocatalysts to enhance the performance in various photocatalytic applications. Visible-light-induced photoredox catalysis has been used in a variety of chemical reactions, including C–C, C–N, C–O, and C

Mechanistic investigations of polyaza[7]helicene in photoredox and energy transfer catalysis

• Johannes Rocker,
• Till J. B. Zähringer,
• Matthias Schmitz,
• Till Opatz and
• Christoph Kerzig

Beilstein J. Org. Chem. 2024, 20, 1236–1245, doi:10.3762/bjoc.20.106

• Johannes Rocker Till J. B. Zahringer Matthias Schmitz Till Opatz Christoph Kerzig Department of Chemistry Johannes Gutenberg University, Duesbergweg 10–14, 55128 Mainz, Germany 10.3762/bjoc.20.106 Abstract Organic photocatalysts frequently possess dual singlet and triplet photoreactivity and a

• recent years was pioneered by the introduction of photocatalysts (PC) based on metals such as Ru and Ir [1][2][3][4][5][6]. However, due to the high cost and limited availability of precious metals, organic photocatalysts have become a focal point of academic and industrial research [7][8][9][10][11][12

• rule [43]. However, helicenes have not been broadly considered as potential photocatalysts or sensitizers or their applications were unsuccessful [44]. Recently, one of our groups exploited the highly reducing polyaza[7]helicene (Aza-H, see Scheme 1 for its structure) for sulfonylation/arylation three

SOMOphilic alkyne vs radical-polar crossover approaches: The full story of the azido-alkynylation of alkenes

• Julien Borrel and
• Jerome Waser

Beilstein J. Org. Chem. 2024, 20, 701–713, doi:10.3762/bjoc.20.64

• since it is known to be reduced by photocatalysts such as Cu(dap)2Cl [17]. This perfectly fits a catalytic cycle involving the reduction of Ts-ABZ (3) followed by oxidation of the carbon radical to form a carbocation and regenerate the ground state catalyst. Styrene (1a) was used as model substrate

• a non-complexed copper catalyst formed during the transformation [24][51]. When iridium-based photocatalysts were tested, no product formation or only traces were observed (Table 2, entries 2 and 3). Using Ru(bpy)3Cl2·6H2O afforded 17% of 4a, a similar yield as with Cu(dap)2Cl with a reduced

• except for 4ClDPAIPN which afforded 10% of yield of 4a (Table 2, entries 6–10). No correlation between the different redox potentials of the photocatalysts and the yield of the reaction could be established. Ru(bpy)3Cl2·6H2O was selected as the optimal catalyst since it afforded the highest yield and

Green and sustainable approaches for the Friedel–Crafts reaction between aldehydes and indoles

• Periklis X. Kolagkis,
• Eirini M. Galathri and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2024, 20, 379–426, doi:10.3762/bjoc.20.36

Mechanisms for radical reactions initiating from N-hydroxyphthalimide esters

• Carlos R. Azpilcueta-Nicolas and
• Jean-Philip Lumb

Beilstein J. Org. Chem. 2024, 20, 346–378, doi:10.3762/bjoc.20.35

• ], have been employed as suitable photocatalysts (Scheme 4B). Under visible light irradiation the photocatalyst (PC) is excited into its corresponding excited state (*PC), where it can be reduced by a suitable electron donor such as DIPEA or Hantzsch ester to generate the reduced form of the photocatalyst

• , the aminodecarboxylation reaction proved unsuccessful when employing alternative photocatalysts such as Ru(bpy)3Cl2 or eosin Y, underscoring the distinctive ability of q-OAc to activate TCNHPI esters via EDA complex formation. Photoinduced transition metal-catalyzed mechanisms The in situ formation of

• significant advantages compared to other methods. It allows for single electron reduction to be facilitated by cost-effective carbon-based cathodes, eliminating the requirement for precious metal photocatalysts or exogeneous reductants such as Zn0. In the final section of this perspective, we explore examples

Visible-light-induced radical cascade cyclization: a catalyst-free synthetic approach to trifluoromethylated heterocycles

• Chuan Yang,
• Wei Shi,
• Jian Tian,
• Lin Guo,
• Yating Zhao and
• Wujiong Xia

Beilstein J. Org. Chem. 2024, 20, 118–124, doi:10.3762/bjoc.20.12

• radicals. This method allows the efficient synthesis of various indole derivatives without the need of photocatalysts or transition-metal catalysts. Mechanism experiments indicate that the process involves a radical chain process initiated by the homolysis of Umemoto's reagent. This straightforward method

• usually occur under mild conditions, they typically require expensive metal-based photocatalysts or structurally complex organic dyes [18]. Therefore, the development of a photoinduced cascade reaction without the need of additional catalysts or additives remains highly desirable [19]. The introduction of

• to furnish trifluoromethylated dihyropyrido[1,2-a]indolones under mild conditions, without the need of photocatalysts or transition metals [28]. Results and Discussion We initialized our study by employing Ru(bpy)3Cl2·6H2O and Umemoto’s reagent to generate trifluoromethyl radicals via a photo

Optimizing reaction conditions for the light-driven hydrogen evolution in a loop photoreactor

• Pengcheng Li,
• Daniel Kowalczyk,
• Johannes Liessem,
• Mohamed M. Elnagar,
• Dariusz Mitoraj,
• Radim Beranek and
• Dirk Ziegenbalg

Beilstein J. Org. Chem. 2024, 20, 74–91, doi:10.3762/bjoc.20.9

• in a proper way, solar energy can be converted to hydrogen fuels. Hydrogen as an energy carrier has zero carbon emission and a high energy density [11][12]. While significant efforts are directed towards developing effective photocatalysts for solar water splitting [13][14][15][16], a crucial

• the contrary, expanding photocatalytic systems, particularly those employing powdered photocatalysts, to a larger scale is notably more straightforward [17][18]. However, complications arise when attempting to scale up reactors solely based on geometric similarity, as alterations in mixing and

• , identifying photocatalysts capable of efficient performance across broad sunlight wavelength ranges proves to be a complex endeavor [26][27][28]. Therefore, artificial light sources are typically applied for lab testing of photocatalytic reactions. Compared to conventional light sources, light-emitting diodes

Beyond n-dopants for organic semiconductors: use of bibenzo[d]imidazoles in UV-promoted dehalogenation reactions of organic halides

• Kan Tang,
• Megan R. Brown,
• Chad Risko,
• Melissa K. Gish,
• Garry Rumbles,
• Phuc H. Pham,
• Oana R. Luca,
• Stephen Barlow and
• Seth R. Marder

Beilstein J. Org. Chem. 2023, 19, 1912–1922, doi:10.3762/bjoc.19.142

• of alkyl halides, RX, by CoCp2 (Cp = η5-cyclopentadienyl; E = −1.3 V vs FeCp2+/0), gives CoCp2+ and X−, but the organic radicals R• react with another molecule of CoCp2 to afford CoCp(η4-C5H5R) [7]. In some cases, issues of reductant air sensitivity can be circumvented by the use of photocatalysts in

• and photocatalysts used, and (c) schematic for the present approach, along with structures of 1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo[d]imidazoles, (Y-DMBI)2, and the corresponding monomer cations, specifically the first reported in the literature and those used in this work. (a) A

Recent advancements in iodide/phosphine-mediated photoredox radical reactions

• Tinglan Liu,
• Yu Zhou,
• Junhong Tang and
• Chengming Wang

Beilstein J. Org. Chem. 2023, 19, 1785–1803, doi:10.3762/bjoc.19.131

• esters 8, 9 without the requirements of phosphine or other photocatalysts (Scheme 5). Through the use of density functional theory (DFT) calculations, they elucidated the mechanism behind this process. It was revealed that the formation of a photoactive EDA complex, which subsequently generated alkyl

Quinoxaline derivatives as attractive electron-transporting materials

• Zeeshan Abid,
• Liaqat Ali,
• Sughra Gulzar,
• Faiza Wahad,
• Raja Shahid Ashraf and
• Christian B. Nielsen

Beilstein J. Org. Chem. 2023, 19, 1694–1712, doi:10.3762/bjoc.19.124

• compound with the donor polymer PM6 to create BHJ nanoparticles and employed it in the hydrogen evolution reaction. This approach substantially reduced trap density, increasing the hydrogen evolution rate by 2–3 times compared to conventional inorganic/organic hybrid photocatalysts [30]. Computational

Visible-light-induced nickel-catalyzed α-hydroxytrifluoroethylation of alkyl carboxylic acids: Access to trifluoromethyl alkyl acyloins

• Feng Chen,
• Xiu-Hua Xu,
• Zeng-Hao Chen,
• Yue Chen and
• Feng-Ling Qing

Beilstein J. Org. Chem. 2023, 19, 1372–1378, doi:10.3762/bjoc.19.98

• complex between Hantzsch ester and N-trifluoroethoxyphthalimide was subsequently engaged in a nickel-catalyzed coupling reaction with in situ-activated alkyl carboxylic acids. This convenient protocol does not require photocatalysts and metal reductants, providing a straightforward and efficient access to

• photocatalysts, providing a direct and robust access to trifluoromethyl aliphatic acyloins. Results and Discussion Initially, we commenced our exploration by choosing 4-phenylbutyric acid (1a) as the model substrate to react with N-trifluoroethoxyphthalimide (2, Table 1). On basis of the previously reported

Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp³)–H to construct C–C bonds

• Hui Yu and
• Feng Xu

Beilstein J. Org. Chem. 2023, 19, 1259–1288, doi:10.3762/bjoc.19.94

• , noble metals have been extensively studied as photocatalysts for CDC reactions [116][117][118][119][120][121] and these methods fill the gap of traditional thermocatalytic CDC reactions. In 2017, Ryu et al. developed tetrabutylammonium decatungstate (TBADT, (n-Bu4N)4[W10O32] as a photocatalyst to

Enabling artificial photosynthesis systems with molecular recycling: A review of photo- and electrochemical methods for regenerating organic sacrificial electron donors

• Grace A. Lowe

Beilstein J. Org. Chem. 2023, 19, 1198–1215, doi:10.3762/bjoc.19.88

• , inorganic Z-schemes have used cobalt complexes and polyoxometalates to shuttle electrons between water oxidation and carbon dioxide reduction photocatalysts [2][4]. However, the photocatalysts of these systems are usually first developed separately with sacrificial electron donors. Other methods for

• bringing together the 2 halves of artificial photosynthesis include photoelectrochemical cells and artificial leaves, single molecule/particle photocatalysts, photovoltaic-powered electrochemical cells, and biophotoelectrochemical cells [8][9]. Some of these systems, like natural photosynthesis, are

• sulfides, or a separate deprotonation event can also result in similar behavior. Gimeno et al. highlighted this when they designed novel benzimidazole (BIH) donors for the photoreduction with [Cu(dipp)2]2+ photocatalysts [29][30]. They contrasted their work to a previous study by Cunningham and McMillin

Photoredox catalysis harvesting multiple photon or electrochemical energies

• Mattia Lepori,
• Simon Schmid and
• Joshua P. Barham

Beilstein J. Org. Chem. 2023, 19, 1055–1145, doi:10.3762/bjoc.19.81

• PEC, a main theme of this Review. Protocols for sensitization-initiated electron transfer (SenI-ET) relying on a dual catalytic system of transition-metal based photocatalysts and pyrenes to generate highly reductive species are also excluded as such reported transformations are now equally achievable

• limited to electron-poor arenes like diazonium/iodonium salts or aryl iodides with electron-withdrawing substituents as aryl radical precursors, due to the limited accessible reducing power of photocatalysts that relied on a monophotonic excitation event. However, the vast majority of inexpensive

• chlorides including 4-bromoanisole and 4-chloroanisole (2e), 1-bromo-2,4- dimethoxybenzene (2f) and 4-ethylchlorobenzene (2h) occurred in good to excellent yields (58–99%) (Figure 10B). Based on their exciting success with isophthalonitrile derived compounds as electron-primed photocatalysts in e-PRC (vide

Transition-metal-catalyzed domino reactions of strained bicyclic alkenes

• Austin Pounder,
• Eric Neufeld,
• Peter Myler and
• William Tam

Beilstein J. Org. Chem. 2023, 19, 487–540, doi:10.3762/bjoc.19.38

•  7) instead of the more commonly, and expensive, metal-based photocatalysts. While broadly successful, tertiary radicals failed to deliver any desired product. Of note, the reaction was amenable to a broad scope of derivatized heterobicyclic alkenes with mono- and disubstituted bridgeheads having

NaI/PPh₃-catalyzed visible-light-mediated decarboxylative radical cascade cyclization of N-arylacrylamides for the efficient synthesis of quaternary oxindoles

• Dan Liu,
• Yue Zhao and
• Frederic W. Patureau

Beilstein J. Org. Chem. 2023, 19, 57–65, doi:10.3762/bjoc.19.5

• disclosed a visible light-mediated radical tandem cyclization of N-arylacrylamides with N-(acyloxy)phthalimides to access 3,3-dialkylated oxindoles in the presence of [Ru(bpy)3Cl2]·6H2O [46]. However, these seminal methods remain limited by the need of noble-metal-based photocatalysts, excess additives and

Mechanochemical bottom-up synthesis of phosphorus-linked, heptazine-based carbon nitrides using sodium phosphide

• Blaine G. Fiss,
• Georgia Douglas,
• Michael Ferguson,
• Jorge Becerra,
• Jesus Valdez,
• Trong-On Do,
• Tomislav Friščić and
• Audrey Moores

Beilstein J. Org. Chem. 2022, 18, 1203–1209, doi:10.3762/bjoc.18.125

• minimize the bandgap of these metal-free photocatalysts, as well as improve their overall stability. Traditional routes to incorporate phosphorus have relied on high-temperature [7] or microwave [8] syntheses, and often proceed through the introduction of a phosphorus atom within the heptazine ring, which

Heterogeneous metallaphotoredox catalysis in a continuous-flow packed-bed reactor

• Wei-Hsin Hsu,
• Susanne Reischauer,
• Peter H. Seeberger,
• Bartholomäus Pieber and
• Dario Cambié

Beilstein J. Org. Chem. 2022, 18, 1123–1130, doi:10.3762/bjoc.18.115

• visible-light photocatalysts that enable new reaction pathways that were previously difficult or impossible to realize [2]. Technical advancements, such as the rise of light-emitting diodes (LEDs) and new reactor technologies were similarly important incentives to popularize light-mediated organic

• sufficiently (photo-)stable, a higher turnover number can be achieved [16]. Issues related to the low surface-to-volume ratio that prevents efficient irradiation of heterogeneous photocatalysts in packed beds can be addressed by adding, for example, glass beads [21]. These considerations have justified the

• development of several strategies to immobilize transition-metal photocatalysts [22]. In the case of flow-metallaphotoredox catalysis packed-bed reactors were not applied to date. This is likely because these reactions are mainly carried out using homogeneous catalysis. Several studies have shown that the

Synthesis of α-(perfluoroalkylsulfonyl)propiophenones: a new set of reagents for the light-mediated perfluoroalkylation of aromatics

• Durbis J. Castillo-Pazos,
• Juan D. Lasso and
• Chao-Jun Li

Beilstein J. Org. Chem. 2022, 18, 788–795, doi:10.3762/bjoc.18.79

• halogenation reactions or oxidation. For these reasons, it would be ideal to develop an efficient methodology that allows for the generation of perfluoroalkyl radicals in a mild, redox- and pH-neutral manner, without the assistance of external photocatalysts, heavy metal catalysts, or further additives. Thus

Heteroleptic metallosupramolecular aggregates/complexation for supramolecular catalysis

• Prodip Howlader and
• Michael Schmittel

Beilstein J. Org. Chem. 2022, 18, 597–630, doi:10.3762/bjoc.18.62

• + 26), respectively, were successfully employed as visible-light photocatalysts for a cross-coupling cyclization of N,N-dimethylaniline (27) and N-alkyl/aryl maleimides 28 (Figure 6). Notably, the systems showed much higher catalytic activity compared to similar reactions with the dye or cages alone

Menadione: a platform and a target to valuable compounds synthesis

• Acácio S. de Souza,
• Ruan Carlos B. Ribeiro,
• Dora C. S. Costa,
• Fernanda P. Pauli,
• David R. Pinho,
• Matheus G. de Moraes,
• Fernando de C. da Silva,
• Luana da S. M. Forezi and
• Vitor F. Ferreira

Beilstein J. Org. Chem. 2022, 18, 381–419, doi:10.3762/bjoc.18.43

Synthesis of piperidine and pyrrolidine derivatives by electroreductive cyclization of imine with terminal dihaloalkanes in a flow microreactor

• Yuki Naito,
• Naoki Shida and
• Mahito Atobe

Beilstein J. Org. Chem. 2022, 18, 350–359, doi:10.3762/bjoc.18.39

• which realizes for the construction of medium-sized saturated nitrogen heterocycles [21]. Although the process enables the rapid construction of saturated nitrogen heterocycles from acyclic precursors, it requires homogeneous precious transition metal complexes as photocatalysts. On the other hand

DABCO-promoted photocatalytic C–H functionalization of aldehydes

• Bruno Maia da Silva Santos,
• Mariana dos Santos Dupim,
• Cauê Paula de Souza,
• Thiago Messias Cardozo and
• Fernanda Gadini Finelli

Beilstein J. Org. Chem. 2021, 17, 2959–2967, doi:10.3762/bjoc.17.205

• , increased accessibility and with structural features that may lead to different reactivities and/or selectivities that are still to be explored. The different electrochemical behaviour of DABCO may also lead to alternative photocatalysts for its activation, such as organic dyes that may not be suitable for

• catalysts and photocatalysts [9][10][11][12][21][22][23]. Previous reports of DABCO as hydrogen abstractor in HAT reactions and this work. Free energy profile for the HAT step reactions between isovaleraldehyde with (top) DABCO and (bottom) quinuclidine radical cations. The red lines are for the gas phase