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

• carried out under the action of a PC-I/TEMPO catalytic system and the irradiation of blue LEDs (Scheme 37) [127]. A variety of electronically rich and electronically poor aryl or heteroaryl groups at the oxime group (R1) are well-tolerated (products 115a–e). Oximes with disubstituted double C=C bond also

Synthesis of esters of diaminotruxillic bis-amino acids by Pd-mediated photocycloaddition of analogs of the Kaede protein chromophore

• Esteban P. Urriolabeitia,
• Pablo Sánchez,
• Alexandra Pop,
• Cristian Silvestru,
• Eduardo Laga,
• Ana I. Jiménez and
• Carlos Cativiela

Beilstein J. Org. Chem. 2020, 16, 1111–1123, doi:10.3762/bjoc.16.98

• spectra were measured on a Thermo Scientific Evolution 600BB spectrophotometer. Cinnamoylglycine (1) was prepared by the Schotten–Baumann method [49]. Irradiation setup for batch synthesis The irradiation setup consisted of a flask (100 mL) irradiated by a printed circuit board (PCB) formed by 24 LEDs of

• 10 mm diameter each LED. The LEDs were serially connected in blocks of 6, with the minimum voltage possible (3 V) and a resistance of 60 Ω (according to Ohm's law). The output power per LED unit (blue, 465 nm) was 250 kmcd; the optical output power of the PCB of LEDs measured with a photometer

• emitting diodes (LEDs) were purchased from Topbright. X-ray crystallography Single crystals of 2c and 4a of suitable quality for X-ray diffraction measurements were grown by slow diffusion of n-pentane into CH2Cl2 solutions of the crude product at –18 °C for several weeks. One selected single crystal was

Accelerating fragment-based library generation by coupling high-performance photoreactors with benchtop analysis

• Quentin Lefebvre,
• Christophe Salomé and
• Thomas C. Fessard

Beilstein J. Org. Chem. 2020, 16, 982–988, doi:10.3762/bjoc.16.87

• conditions compared to a single vial reactor. Active water cooling protects the LEDs from overheating, while additional air cooling with bespoke fans prevents the vessels to heat over 50 °C. Each vessel is placed at equal distance of a 35 W high power blue LED (Figure 1). Reaction monitoring by TLC–MS

A method to determine the correct photocatalyst concentration for photooxidation reactions conducted in continuous flow reactors

• Clemens R. Horn and
• Sylvain Gremetz

Beilstein J. Org. Chem. 2020, 16, 871–879, doi:10.3762/bjoc.16.78

• can be delivered to the catalyst. These limitations are defined by the photoflow setup, specifically the channel height and the emission peak of the light source. This method was tested and shown to work well for three catalysts with different absorption properties through using LEDs with emission

• sources over mercury arcs and other light sources commonly used in industry and classical synthetic organic photochemistry. LEDs typically have much narrower emission spectra when compared to these light sources, all of which have broad emission spectra except for low-pressure sodium vapor lamps [15][16

• emission wavelength, the appropriate wavelength above 365 nm can be selected for every reaction, and the scale-up can be achieved by simply increasing the number of the LEDs applied [19]. The narrower spectra, while providing many benefits and likely reducing the occurrence of side reactions, also

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

• catalyst loading could be decreased to 2 mol % without negative effects (cf. entries 15 and 16 in Table 1). Further dilution gave no further improvement in the yield (see Supporting Information File 1). Next, increasing the power of the LEDs from 6 to 18 W gave 73% GC yield and a 64% isolated yield of 3

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

• 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

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

• facilitated to shift the absorption between 750–1000 nm. This enabled their use in applications with light sources emitting in the near-infrared (NIR) region selected from high power LEDs or lasers with line-shaped focus. The absorbers considered were discussed regarding their function as sensitizer for

• their use in applications based on sensitized photoinduced electron transfer. This can be NIR-sensitized photopolymerization resulting in formation of initiating radicals and conjugate acid [5][6][13][14][15][63][64]. Recently, the use of NIR-LEDs exhibiting high excitation intensity brought more light

• differ. It facilitates their use with modern light sources such LEDs and lasers with emission in the NIR between 800–1100 nm. A purposeful use requires a more detailed understanding of the photochemistry occurring in such compounds. Photophysics and photochemistry General aspects Figure 1 depicts

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

• acetonitrile under an atmosphere of N2 to form the lactone 5-phenyldihydrofuran-2(3H)-one (and HCl) or the acid 4-chloro-4-phenylbutanoic acid via an ATRA reaction (Scheme 2). The photoredox catalysts are excited by using blue LEDs with a peak excitation of 458 nm (see the experimental section for a detailed

• reacted irradiated mixtures of 3 with either the photoredox catalyst, monochloroacetic acid or both. We did not see any formation of the lactone (1) after irradiating the mixtures overnight with 458 nm LEDs. Therefore, it is unlikely that in benzene direct conversion from 3 to 1 plays any significant role

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

• neutrality; (iii) the use of household bulbs or LEDs as light sources under operationally simple reaction conditions; (iv) the high redox potential of photocatalysts that can manipulate the oxidation states of transition metal catalysts [54][55]. They have also found applications in novel solar cell

• catalysts, such as low toxicity, their functioning at room temperature, and smooth irradiation requirements with low-energy lights (e.g., LEDs, fluorescent bulbs, etc.) made this area of research very interesting. Within a very short time, photoredox catalysis has emerged as an important future direction

Synthesis and characterization of bis(4-amino-2-bromo-6-methoxy)azobenzene derivatives

• David Martínez-López,
• Amirhossein Babalhavaeji,
• Diego Sampedro and
• G. Andrew Woolley

Beilstein J. Org. Chem. 2019, 15, 3000–3008, doi:10.3762/bjoc.15.296

• . Photoisomerization UV irradiation was performed by placing a 365 nm LED (897-LZ440U610; LED Engin) operating at 68 mW/cm2 above the sample tube for 1 minute. For blue light irradiation, a 440 nm LED (Luxeon III Star LED Royal Blue Lambertian; Luxeon Star LEDs) operating at 40 mW/cm2 at 700 mA was used in the

Chemical tuning of photoswitchable azobenzenes: a photopharmacological case study using nicotinic transmission

• Lorenzo Sansalone,
• Jun Zhao,
• Matthew T. Richers and
• Graham C. R. Ellis-Davies

Beilstein J. Org. Chem. 2019, 15, 2812–2821, doi:10.3762/bjoc.15.274

• ). LEDs centered at 405 and 530 nm were used for photoswitching. a) Representative responses evoked by all trans-2 and cis-2 in the green PSS. b) Representative responses evoked by cis-2 in the green PSS without and with violet light (10 mW). c) Sequential responses from the same neuron when all trans-2

A photochemical determination of luminescence efficiency of upconverting nanoparticles

• Baptiste Amouroux,
• Clément Roux,
• Jean-Claude Micheau,
• Fabienne Gauffre and
• Christophe Coudret

Beilstein J. Org. Chem. 2019, 15, 2671–2677, doi:10.3762/bjoc.15.260

• sources. Since the recent renewal of photochemistry caused by the use of LEDs and microfluidic devices, actinometry has become a convenient tool to parameterize the performances of photoreactors [23][24][25]. Actinometer choice is guided by the operating conditions and by the spectral overlap between the

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

• potential was estimated to be approximately +1.0 V (vs SCE) which is sufficient to photocatalyze typical organic reactions. The photoredox catalytic activity in the visible light range was tested by the α-alkylation of 1-octanal as benchmark reaction. Irradiations were performed with LEDs in the visible

• sustainable method for catalysis because sunlight is an essentially unlimited and thereby “green” natural light source and LEDs – conveniently used for irradiation experiments in the laboratory – are cheap and energy-saving artificial sources for irradiations. The current “working horse” for photoredox

• following paragraphs because enantioselectivity is not a matter of discussion in this work. The photoredox catalysis with NDI 1 was performed by LEDs with 387 nm maximum emission wavelength and an irradiation time of 18 h. In order to ensure solubility of all components, a solvent mixture of DMF/CH2Cl2 = 1

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

• observed after a strong absorption of the complex centered at 380 nm (Table 4). Thus, it can react through a redox cycle with similar reaction than the Ru-based complexes under irradiation by different visible light such as a halogen lamp, laser diodes (405 and 457 nm) or LEDs (405 and 455 nm) [63]. Iron

• photochemical properties are given in Table 7. We observe from Table 7 that by irradiation in the near-UV range, the excited state of C2 is reached. This phenomenon can be observed upon exposure to different light irradiation such as light emitting diodes (LEDs) from 405 to 477 nm or a household device for

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

• of 50 different catalysts was evaluated in the conversion of alcohol 1 to bromide 2 (Figure 2 and Figure 3). Several homoleptic complexes were not evaluated due to problematic oxidation or low solubility. Reactions were irradiated at either 394 nm (purple LEDs) or 450 nm (blue LEDs), depending on the

• were then transferred to continuous flow (Table 2). Initially, an experimental set-up using a previously reported reactor for purple LEDs was selected for the reaction [30][31]. Following injection of the reaction mixture with a target residence time of 60 min, only traces of the desired bromide 2 were

• 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

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

• irradiation with blue LEDs at room temperature (Scheme 7) [48]. The authors report that in general their procedure allowed for better transformation of electron-poor anilines compared to their electron-rich counterparts. Unfortunately, it is also reported that the scope of the reaction cannot be extended to

• compatibility. Hajra et al. have reported the direct C–H thiocyanation of substituted imidazo[1,2-a]pyridines, using ammonium thiocyanate, in combination with Eosin Y under irradiation by blue LEDs (Scheme 9) [52]. This is another photoredox example of C–S bond formation, in this case to a highly versatile

• reactions using traditional chemistry and organophotoredox synthesis can offer some interesting options as well. Wu et al. reported the alkylation of unfunctionalised allylic and benzylic sp3 C–H bonds by reaction with Michael acceptors, using blue LEDs and acridinium salts (Scheme 12) [55]. The main

Phosphoramidite building blocks with protected nitroxides for the synthesis of spin-labeled DNA and RNA

• Timo Weinrich,
• Eva A. Jaumann,
• Ute M. Scheffer,
• Thomas F. Prisner and
• Michael W. Göbel

Beilstein J. Org. Chem. 2018, 14, 1563–1569, doi:10.3762/bjoc.14.133

• TEMPO-labeled oligonucleotides and of phosphoramidites 5–8. Structures of palindromic oligonucleotides prepared from amidites 5 (22a, 23a), 7 (24a, 25a), and 8 (26a, 27a). Hemiacetals 22b–27b were obtained after photochemical deprotection (custom built apparatus with three LEDs Nichia NCCU033, 365 nm

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

• N2) which triggers an intramolecular cyclization reaction via a nitrogen-centred radical intermediate to form sulfonylated phenanthridines. The reaction is photocatalyzed by [Ru(bpy)3]Cl2 under irradiation with blue LEDs and proceeds smoothly for a variety of substituted vinyl azides and sulfonyl

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

• delivered comparable and even higher yields of the products in longer reaction times but with lower catalyst loading (1 mol %). Both unactivated terminal alkenes and electron-deficient alkenes (Michael acceptors) were successfully hydrotrifluoromethylated under irradiation with 36 W blue LEDs in the

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

• yielded the same negative result proving that ambient light did not mediate the photoredox catalytic borylation reaction under mechanochemical conditions (Table 1, entry 3). Furthermore, neat grinding of a catalyst-free mixture of 1a and 2 under blue LEDs (light-emitting diodes) light did not afford the

• operating under solventless conditions (Table 1, entry 4). Then, a premilled mixture of 1a, B2pin2, and eosin Y was subjected to irradiation with blue LEDs for 2 h in the absence of milling (Table 1, entry 5). After the irradiation was halted, the reaction mixture was immediately analyzed by 1H NMR

• ) in the mixture. Indeed, milling the product 3a under the standard milling conditions using the LEDs led to its melt. Similarly, milling a mixture of 1a, 2 and 3a for 1 h under light irradiation reached a eutectic melt phase. The need for simultaneous light and mechanical milling was also confirmed

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

• atmosphere was not essential, we therefore decided to test our synthetic methodology with an in house-made flow apparatus. The reaction coil was formed with 1 m of FEP tubing (internal diameter 0.8 mm) wrapped around a glass cylinder. Irradiation was obtained with strips of blue LEDs attached inside a hollow

Energy down converting organic fluorophore functionalized mesoporous silica hybrids for monolith-coated light emitting diodes

• Markus Börgardts and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2017, 13, 768–778, doi:10.3762/bjoc.13.76

• concept, we prepared four examples of white light emitting LEDs with organic phosphors. Therefore, UV-emitting LEDs were early immersed into the reaction mixture upon monolith formation. After aging, according to the described method, the UV emitting diodes were coated with the white light emitting hybrid

• materials as shown in Figure 9. Two different LED designs were investigated, a conventional diode setup as well as a much smaller surface mounted device (SMD). These coated LEDs were spectroscopically analyzed displaying correlated color temperatures of 10700 K and 41100 K for the conventional LED setup

• (2nd order of excitation beam not shown). Coating of the a) conventional diode setup and b) surface-mounted device (SMD) (left: prior to the addition of the reaction mixture, right: after the addition), c) coated conventional diode setup, d) coated surface-mounted device. Pictures of the coated LEDs in

Synthesis and optical properties of new 5'-aryl-substituted 2,5-bis(3-decyl-2,2'-bithiophen-5-yl)-1,3,4-oxadiazoles

• Anastasia S. Kostyuchenko,
• Tatyana Yu. Zheleznova,
• Anton J. Stasyuk,
• Aleksandra Kurowska,
• Wojciech Domagala,
• Adam Pron and
• Alexander S. Fisyuk

Beilstein J. Org. Chem. 2017, 13, 313–322, doi:10.3762/bjoc.13.34

• oxadiazole central ring symmetrically functionalized with bithiophene end-capped with various aryl groups. Our preliminary studies show that these compounds can be used as electroluminophores in LEDs, which show superior performance as compared to the devices fabricated from 1 or 2 [18]. Results and

Solvent-free, visible-light photocatalytic alcohol oxidations applying an organic photocatalyst

• Martin Obst and
• Burkhard König

Beilstein J. Org. Chem. 2016, 12, 2358–2363, doi:10.3762/bjoc.12.229

• a stirrer, and an LED frame with 5 LEDs (λ = 455 nm). Figure 1 shows a schematic representation and photographs. Before reaction, the solid substrate and photocatalyst are gently homogenized with a spatula, followed by grinding with pestle and mortar. The reaction mixture is then filled into the

• test tube. Subsequently, the rotating glass rod is pressed into the test tube, which leads to vertical migration of the reaction mixture and the formation of a film between the inner wall of the test tube and the glass rod. The reaction is carried out by rotation and irradiation with the LEDs from the

• was observed in all cases, indicating melting of the substrate or the product, which maintains mobility. The oxidation of benzilic acid and benzhydrol was also performed using an LED setup containing only 4 LEDs instead of 5 LEDs like in the frame rod mill. Here, the obtained yields were significantly