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

• , followed by the insertion of an alkyne into the C–M bond. The key step of the dual catalysis is the reductive quenching of the photoexcited catalyst by Rh(I), a low-valent metal obtained after C–N reductive elimination. Hence, the Rh(III) active catalyst is regenerated and the resulting reduced

Clickable azide-functionalized bromoarylaldehydes – synthesis and photophysical characterization

• Dominik Göbel,
• Marius Friedrich,
• Enno Lork and
• Boris J. Nachtsheim

Beilstein J. Org. Chem. 2020, 16, 1683–1692, doi:10.3762/bjoc.16.139

• ,π*) excited states, owing to the poor orbital overlap, resulting in a decreased spin-orbit coupling. Quenching processes of triplet states, induced by molecular motions, oxygen, or humidity, restrict the versatile application of such organic materials [24]. Thus, low temperatures [25][26][27] or

• defined up to 8.66 ns for iminium bromide 25, with quantum yields Φ below 1% (see Figure S32, Supporting Information File 1). The absence of any long living triplet species in all fluorene derivatives in solution as well as in the solid-state again indicates undesirable quenching events, induced by the

Facile synthesis of 7-alkyl-1,2,3,4-tetrahydro-1,8-naphthyridines as arginine mimetics using a Horner–Wadsworth–Emmons-based approach

• Rhys A. Lippa,
• John A. Murphy and
• Tim N. Barrett

Beilstein J. Org. Chem. 2020, 16, 1617–1626, doi:10.3762/bjoc.16.134

• -1,8-naphthyridine analogues. The investigation initially began using commercially available N-Boc-protected tetrahydro-1,8-naphthyridine 8 [16]. Upon deprotonation and quenching with diethyl chlorophosphate, migration of the Boc group from the nitrogen atom to the exocyclic methyl group was observed

• would afford phosphonate 12 upon quenching with diethyl chlorophosphate via formation of the dianion. This could then be used in a subsequent Horner–Wadsworth–Emmons reaction to construct the carbon skeleton of amine 6. Upon the addition of a single equivalent of diethyl chlorophosphate, phosphoramidate

• to account for the deprotonation of the more acidic phosphonate product 7 versus the starting material 13, and potential lithium sequestration by chelation between an oxygen atom of the phosphonate and the nitrogen atom of the unsaturated ring. Monitoring of the deprotonation followed by quenching

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

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

• -isocyano-1,1'-biaryls 4.1 have been reported, as summarized in Scheme 4. Along with substrate 4.1, a radical source R–X and a photocatalyst (PC), which is activated upon visible-light irradiation, are usually required. Oxidative quenching of the photoexcited PC* by R–X (path a) affords, upon loss of the

• the bromoalkanes through an oxidative quenching mechanism [48]. Phenanthridines may be also formed by the initial addition of an electrophilic radical onto isonitriles. Thus, a library of 6-alkylated phenanthridines (5.2a–d in Scheme 5) and other nitrogen-based heterocycles have been prepared from

• quenching of the excited photocatalyst, in turn triggering the formation of radicals 12.2·a–d. These smoothly underwent radical cyclization to give the corresponding methyl 5,6-dihydrophenanthridine-6-carboxylates and then the desired phenanthridine 6-carboxylates 12.3a–d in good yields. Noteworthy, the

Synthesis of new fluorescent molecules having an aggregation-induced emission property derived from 4-fluoroisoxazoles

• Kazuyuki Sato,
• Akira Kawasaki,
• Yukiko Karuo,
• Atsushi Tarui,
• Kentaro Kawai and
• Masaaki Omote

Beilstein J. Org. Chem. 2020, 16, 1411–1417, doi:10.3762/bjoc.16.117

• concentrated solutions by aggregation-caused quenching (ACQ) [8]. On the other hand, there are molecules that exhibit strong emission even in poor solvents or in the solid state. This property is referred to as aggregation-induced emission (AIE) and has attracted much attention in the field of fluorescence

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

• quenched by single-electron transfer from Cy2NMe, resulting in the generation of the highly reducing [IrII] species and the radical cation A. To validate the reductive quenching pathway, we carried out Stern−Volmer quenching experiments (Figure S1, Supporting Information File 1). The emission intensity of

[3 + 2] Cycloaddition with photogenerated azomethine ylides in β-cyclodextrin

• Margareta Sohora,
• Leo Mandić and
• Nikola Basarić

Beilstein J. Org. Chem. 2020, 16, 1296–1304, doi:10.3762/bjoc.16.110

• and intercepted with AN to yield cycloadducts 7 or 11, respectively. However, the formation of cycloadducts is very inefficient, which may be ascribed to a smaller rate constant for the quenching due to steric hindrance imposed by the bulky cyclohexane or adamantine moiety. Thus, irradiation of 2 gave

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

• quenching cycle, PC* acts as a reductant donating an electron to A. This generates the oxidized form of the photocatalyst, PC•+, and a reduced acceptor, A•−. Alternatively, in a reductive quenching cycle, PC* acts as an oxidant promoting an SET oxidation of the electron donor D. This leads to the reduced

• different redox-active functional groups (Figure 2) [48]. Among them, dihydropyridines (DHPs), silicates, and tetrafluoroborate salts were recently exploited in organophotocatalytic reactions. These functionalities can act as donors in reductive quenching manifolds and release the desired C(sp3) radicals

Aryl-substituted acridanes as hosts for TADF-based OLEDs

• Naveen Masimukku,
• Dalius Gudeika,
• Oleksandr Bezvikonnyi,
• Ihor Syvorotka,
• Rasa Keruckiene,
• Dmytro Volyniuk and
• Juozas V. Grazulevicius

Beilstein J. Org. Chem. 2020, 16, 989–1000, doi:10.3762/bjoc.16.88

• 6 are relatively small, and their LE emissions are mainly ultraviolet. There was practically no positive solvatochromism observed for the dilute solutions of compounds 3, 5, and 6. Only tails related to CT can be observed in the PL spectra of these compounds. Quenching of internal molecular motion

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

• catalysts in both oxidative and reductive quenching. The use of porphyrins as a photosensitizer for singlet oxygen generation is presented in the second topic, which was subdivided into two sections: pericyclic reactions and heteroatom oxidations. The first section describes the use of singlet oxygen in

• photoredox catalysts up to now [10][22]. Remarkably the same system can be applied for both pathways, oxidative and reductive processes, beyond singlet oxygen generation [9][10][23]. Only a few photocatalysts can be applied in both photoredox processes (oxidative and reductive quenching), for example, [Ru

• this transformation involves the formation of an aryl radical by SET between the diazo compound and the porphyrin in its excited state (Scheme 3). The authors demonstrated that meso-arylated porphyrins can efficiently act by an oxidative quenching. However, issues about why an electron-poor porphyrin

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

• reaction conditions (0.5 N citronellol, 1 mL/min,1 mol % catalyst, 2 equiv oxygen, 8 bar, 20 °C, reactor volume: 2.7 mL, solvent: ethanol (B, C, D, E, and F) or dichloromethane (A, G, H, and I)). aThe conversion was determined by GC after quenching with PPh3. bTransmission (in %) of the starting solution

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

• irradiation, supporting the other two pathways. Also, mechanistic studies by Cui et al. showed that the energy levels of the reactions (1) and (2) in Scheme 5 were closely tied, so distinguishing them is not always trivial [32]. The triplet state quenching of 8 and related compounds, such as aryl ketones, has

• of an α-hydroxybenzyl radical (11) and a benzoyl radical (10). This upper statement shows the potential of benzaldehyde (8) compared to the related aryl ketones, which are known to be capable of HAT. Until recently, the quenching of the fluorescence of excited benzaldehyde (9) was unreasonably

• thought to originate from the basicity of the aldehyde. A recent publication though came to empower the statement that the HAT properties of benzaldehyde (8) are responsible for the observed quenching of the fluorescence [34]. Aldehydes as photoinitiators for photochemical polymerizations A number of

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

• , probably due to their intrinsic reduction potential. Interestingly, the bromide substrate derived from menthol was readily reduced in an excellent 74% isolated yield. The authors conducted some mechanistic studies (including cyclic voltammetry and Stern–Volmer quenching experiments, for instance), and

• the reaction mechanism and suggested the following one: First, the excited in situ-formed copper complex reduced the NHP ester, as demonstrated by the Stern–Volmer quenching experiment. The formed radical anion collapsed into the corresponding alkyl radical and the phthalimide anion. Then, the

• oxidative quenching. First, the excited [Cu(I)]* species was oxidized in the presence of iodine, furnishing a [Cu(II)] complex. Then, an oxidation of the diarylamine occurred, generating the N-centered radical cation, which undergoes an intramolecular cyclization. A final oxidation of the aryl radical

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

• solvents did not lead to any significant increase of the yield or selectivity (Table 1, entries 5–9). The reduction of catalyst loading resulted in a lower yield and a lower selectivity for the acid compound 3 (Table 1, entries 10–14). The products indicate an oxidative quenching of the photoredox catalyst

• the formation of the cation of the photoredox catalyst, we believe that after oxidative quenching the catalyst is not fully reduced back to the neutral complex, accumulates in solution and as a result precipitates as chloride salt (with Cl− stemming from the substrate). We assume that the same happens

• product was not formed at all. Addition of [Ag]PF6 to remove the chloride and solubilize the oxidized photocatalyst also did not lead to any product formation. [Ag]PF6 is poorly soluble in benzene and the Ag+ ion is likely to act as an oxidizing agent quenching the excited state of the photocatalyst

Architecture and synthesis of P,N-heterocyclic phosphine ligands

• Wisdom A. Munzeiwa,
• Bernard Omondi and
• Vincent O. Nyamori

Beilstein J. Org. Chem. 2020, 16, 362–383, doi:10.3762/bjoc.16.35

• to obtain cyclopropyl intermediate 45. The latter was converted into compound 46 by reaction with lithium diphenylphosphide in dry THF. Finally, the desired ligand 47 was obtained after quenching the intermediate compound 46 with a saturated solution of ammonium chloride. Ligand 47 was investigated

• magnesium-containing triazoles 66 which, upon quenching with ammonium chloride, afforded the triazoles 67. Lithiation followed by coupling with the appropriate chlorophosphines resulted in the desired 1,5-disubstitued triazolylphosphine ligands 68. The procedure could be performed in one pot by directly

• quenching the metalated triazole 66 with chlorophosphine. However, a separation of the triazole before phosphorylation makes purification of the final ligand easier [74]. The direct ortho-metalation of pyridyltriazole 69 and subsequent reaction with chlorophosphines gave the isomeric ligands 71 and 72 in

p-Pyridinyl oxime carbamates: synthesis, DNA binding, DNA photocleaving activity and theoretical photodegradation studies

• Panagiotis S. Gritzapis,
• Panayiotis C. Varras,
• Nikolaos-Panagiotis Andreou,
• Katerina R. Katsani,
• Konstantinos Dafnopoulos,
• George Psomas,
• Zisis V. Peitsinis,
• Alexandros E. Koumbis and
• Konstantina C. Fylaktakidou

Beilstein J. Org. Chem. 2020, 16, 337–350, doi:10.3762/bjoc.16.33

• conditions (λexcitation = 540 nm at room temperature). Thus, the quenching observed in an EB–DNA solution upon addition of the compounds 11 and 12 may reveal their competition to EB for the DNA-intercalation sites as monitored by fluorescence emission spectroscopy with λexcitation = 540 nm. A significant

• quenching of the EB–DNA fluorescence (up to 68.5% of the initial fluorescence for compound 12 (Figure 4) was found in the presence of the compounds. The as-observed quenching (which is in good agreement (R = 0.99) with the linear Stern–Volmer equation [72]) may be attributed to the competition of compounds

• (up to the value of r = 0.35) [11]. EB-displacement studies The ability of compounds 11 and 12 to displace EB from its DNA–EB conjugate was investigated by fluorescence emission spectroscopy. The Stern–Volmer constant (KSV, in M−1) was used to evaluate the quenching efficiency for each compound

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

• hand, S1 can also transition to the spin-forbidden T1 through intersystem crossing (ISC), which then decays by (longer) radiative processes to S0 (phosphorescence. Figure 3) [41][42][43]. The triplet excited state is oxygen-sensitive as O2 carries out quenching in solution, thereby leading to the

• ]. 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

• hydroxylation of arenes (Scheme 18) [157]. They realized that the photoredox catalyst 8 possesses a great oxidizing ability (Ered vs SCE = 2.72 V) at ambient conditions. The mechanism of the reaction was studied by fluorescence quenching and transient absorption spectroscopy. They observed that the one-electron

The reaction of arylmethyl isocyanides and arylmethylamines with xanthate esters: a facile and unexpected synthesis of carbamothioates

• Narasimhamurthy Rajeev,
• Toreshettahally R. Swaroop,
• Ahmad I. Alrawashdeh,
• Shofiur Rahman,
• Abdullah Alodhayb,
• Seegehalli M. Anil,
• Kuppalli R. Kiran,
• Chandra,
• Paris E. Georghiou,
• Kanchugarakoppal S. Rangappa and
• Maralinganadoddi P. Sadashiva

Beilstein J. Org. Chem. 2020, 16, 159–167, doi:10.3762/bjoc.16.18

• thiomethyl group via transition state TS3 forms the third intermediate Int3, consisting of a carbene and a thiolate anion. The steps leading to the observed product carbamothioate 4a occur from the final quenching hydrolysis of Int3, which occurs via several energetically favorable steps (e.g., A–C), as has

• subsequent dilute aqueous/DMF quenching conditions, we were unable to detect the hydrolysis products by HRMS. Furthermore, using the same reaction conditions, which were employed to the S-methyl dithiocarbonates 1a–f, but using S-ethyl or S-benzyl dithiocarbonates, none of the corresponding expected reaction

• products were obtained. As well, reductive quenching with aqueous NaCNBH3 failed to trap Int3 and only afforded the same reaction products. Conclusion There are several reports, which have discussed interesting reactions or reactivity of isocyanides. Among these are those which showed that the isocyanide

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

• , lithium diisopropylamide (LDA)-mediated cyclisation of 2-(methylthio)benzamides, which were obtained by directed ortho-metalation of the respective benzamides followed by quenching with dimethyl disulfide [27] (Supporting Information File 1, Scheme S1). In general, we found the LDA-mediated cyclisation

Automated glycan assembly of arabinomannan oligosaccharides from Mycobacterium tuberculosis

• Alonso Pardo-Vargas,
• Priya Bharate,
• Martina Delbianco and
• Peter H. Seeberger

Beilstein J. Org. Chem. 2019, 15, 2936–2940, doi:10.3762/bjoc.15.288

• linker as solid support [25]. A typical AGA cycle consisted of three modules. The acidic wash module prepared the resin for glycosylation by quenching any remaining base from a previous cycle. In the glycosylation module, the thioglycoside donor was coupled to the resin upon activation with NIS and TfOH

A green, economical synthesis of β-ketonitriles and trifunctionalized building blocks from esters and lactones

• Daniel P. Pienaar,
• Kamogelo R. Butsi,
• Amanda L. Rousseau and
• Dean Brady

Beilstein J. Org. Chem. 2019, 15, 2930–2935, doi:10.3762/bjoc.15.287

• also investigated this option for the first time in the synthesis of β-ketonitriles from esters, as described later (see Table 1). Finally, it is probable that the presence of protic IPA suppresses enolization as the reaction progresses, by quenching KOt-Bu generated enolates and similarly, reactive

Preparation of anthracene-based tetraperimidine hexafluorophosphate and selective recognition of chromium(III) ions

• Qing-Xiang Liu,
• Feng Yang,
• Zhi-Xiang Zhao,
• Shao-Cong Yu and
• Yue Ding

Beilstein J. Org. Chem. 2019, 15, 2847–2855, doi:10.3762/bjoc.15.278

• paramagnetic nature of chromium(III) can cause fluorescence quenching of the fluorophore via the enhancement of spin–orbit coupling [30][31][32][33][34][35]. So far, only a few successful examples of fluorescence enhancement sensors for Cr3+ have been reported [36][37][38][39][40]. Thus, developing new and

Design, synthesis and investigation of water-soluble hemi-indigo photoswitches for bioapplications

• Daria V. Berdnikova

Beilstein J. Org. Chem. 2019, 15, 2822–2829, doi:10.3762/bjoc.15.275

• extent depends on the particular sequence of RNA. Photoswitching of the RNA-bound hemi-indigo Z-2 to the E-form results in emission quenching. The ON–OFF fluorescence switching of Z-2–RNA complexes can be performed reversibly by repeated irradiation with blue (470 nm) and amber (590 nm) light. Conclusion

Photoreversible stretching of a BAPTA chelator marshalling Ca²⁺-binding in aqueous media

• Aurélien Ducrot,
• Arnaud Tron,
• Robin Bofinger,
• Ingrid Sanz Beguer,
• Jean-Luc Pozzo and
• Nathan D. McClenaghan

Beilstein J. Org. Chem. 2019, 15, 2801–2811, doi:10.3762/bjoc.15.273

• electron-transfer reaction, while ion binding blocks this quenching pathway restoring emission. Thus, ion liberation from 1 and transfer to 3 would result in a fluorescence off–on switching of the latter. To perform the ion-transfer experiment, the fluorescent dye is loaded with one equivalent of Ca2