Advances in the use of metal-free tetrapyrrolic macrocycles as catalysts

• Mandeep K. Chahal

Beilstein J. Org. Chem. 2024, 20, 3085–3112, doi:10.3762/bjoc.20.257

• porphyrin radical anion. Ultimately, protonation of intermediate E led to the final product. Formation of intermediates, such as enamine A and cation radical B, was confirmed using techniques like ESIMS, 1H NMR, and EPR, Stern–Volmer quenching experiments, respectively. All these mechanistic studies

Interaction of a pyrene derivative with cationic [60]fullerene in phospholipid membranes and its effects on photodynamic actions

• Hayato Takagi,
• Çetin Çelik,
• Ryosuke Fukuda,
• Qi Guo,
• Tomohiro Higashino,
• Hiroshi Imahori,
• Yoko Yamakoshi and
• Tatsuya Murakami

Beilstein J. Org. Chem. 2024, 20, 2732–2738, doi:10.3762/bjoc.20.231

• catC60 and PyBA from the membranes. ESR measurements for photoinduced 1O2 and O2•– generation ESR spectra were recorded on a Bruker EMX, Continuous Wave X-Band EPR spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany). Suprasil® ESR tubes with a diameter of 4 mm, length of 250 mm and a wall thickness

• of 0.8 mm were used (SP Wilmad-LabGlass, New Jersey, US). 4-Oxo-TEMP was purchased from ABCR (Karlsruhe, Germany) and purified by sublimation prior to use. The 50 µL Blaubrand® intraMark capillaries were used in the EPR measurements (Brand GMBH, Wertheim, Germany). DEPMPO was bought from Enzo Life

Hydrogen-bond activation enables aziridination of unactivated olefins with simple iminoiodinanes

• Phong Thai,
• Lauv Patel,
• Diyasha Manna and
• David C. Powers

Beilstein J. Org. Chem. 2024, 20, 2305–2312, doi:10.3762/bjoc.20.197

• of 25 µL HFIP in MeCN showed no electrochemical events between −2.0–0 V. The CV of 2c in the absence of HFIP showed a reductive current (ip = −0.80 mA) at peak potential (Epr) of –1.72 V vs Fc+/Fc. Upon addition of 5.0 µL of HFIP (1.2 equiv with respect to 2c), the current increased to −1.22 mA

• , signaling the binding of HFIP to 2c enhanced the electron transfer kinetics between the hypervalent iodine reagent and the electrode [45]. Further additions of HFIP further increased the current response and shifted the peak potential, with 10 µL and 15 µL of HFIP showing responses with Epr at −1.55 V and

• −1.52 V, respectively. The titration showed a saturation point at 25 µL of HFIP (6.0 equiv with respect to 2c), at which the CV of 2c showed an Epr = –1.47 V and –1.52 mA current. Overall, the addition of HFIP results in a 250 mV shift in the reduction of 2c. The increased facility of reduction is

Switchable molecular tweezers: design and applications

• Pablo Msellem,
• Maksym Dekthiarenko,
• Nihal Hadj Seyd and
• Guillaume Vives

Beilstein J. Org. Chem. 2024, 20, 504–539, doi:10.3762/bjoc.20.45

• . Using the same architecture, the group explored the control of magnetic properties by the mechanical motion of the tweezers using Cu(II)–salphen complexes [43]. The magnetic properties of tweezers 13 were studied by EPR spectroscopy and SQUID magnetometry. In the open state, the large intramolecular

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

• ). Importantly, electron paramagnetic resonance (EPR) spectroscopy at 25 °C supported the formation of species 107. However, NMR measurements at −40 °C showed the accumulation of BiIII complex 131, which can be prepared separately in a stoichiometric experiment. As such, two different pathways may lead to the C

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

Polymer and small molecule mechanochemistry: closer than ever

• José G. Hernández

Beilstein J. Org. Chem. 2022, 18, 1225–1235, doi:10.3762/bjoc.18.128

• -known mechanophore that generates diarylacetonitrile radicals under force. Hence, when TASN derivative 8, bearing diarylurea moieties, was ball milled, the corresponding radical 9 was detected by electron paramagnetic resonance (EPR) spectroscopy. Similar treatment proved that 6 was 28 times less prone

Structural basis for endoperoxide-forming oxygenases

• Takahiro Mori and
• Ikuro Abe

Beilstein J. Org. Chem. 2022, 18, 707–721, doi:10.3762/bjoc.18.71

• [41][42][43]. Crystal structures of COXs The structural basis for the di-peroxide formation reaction by COXs has been substantially elucidated by electron paramagnetic resonance (EPR), kinetic analysis, X-ray crystallography, and mutagenesis experiments [24][44][45]. The structural analyses of

• . Catalytic residue in the cyclooxygenase reaction The formation of a tyrosyl radical during the catalytic cycle was proved by EPR and kinetic analyses [59][60][61]. Moreover, chemical and molecular biology analyses and a mutagenesis experiment identified the position of the tyrosyl radical. Treatment of the

• substituted with Ala or Phe generates an N-1 dealkylated product, which is non-enzymatically created from C21 hydroxylated products, as a major product. Based on these structural analyses, as well as an EPR analysis of the enzyme reaction of FtmOx1 and a transient ultraviolet–visible (UV–vis) absorption

Adjusting the length of supramolecular polymer bottlebrushes by top-down approaches

• Tobias Klein,
• Franka V. Gruschwitz,
• Maren T. Kuchenbrod,
• Ivo Nischang,
• Stephanie Hoeppener and
• Johannes C. Brendel

Beilstein J. Org. Chem. 2021, 17, 2621–2628, doi:10.3762/bjoc.17.175

• known size window for a potential enhanced permeability and retention effect (EPR) [22][23]. As an alternative for the assembly pathway control, we opted to apply easy-to-use top-down approaches to tune the length distribution in a straightforward fashion over the above-mentioned length range of

Synthesis of phenanthridines via a novel photochemically-mediated cyclization and application to the synthesis of triphaeridine

• Songeziwe Ntsimango,
• Kennedy J. Ngwira,
• Moira L. Bode and
• Charles B. de Koning

Beilstein J. Org. Chem. 2021, 17, 2340–2347, doi:10.3762/bjoc.17.152

• electron donor iminyl radical. In fact, iminyl radicals have been identified and studied by EPR spectroscopy in a number of related processes involving oxime derivatives [14]. As an example utilizing substrate 14d, intermediates such as the iminyl radical 17a and the ring closed intermediate 17b (Figure 3

Chemical syntheses and salient features of azulene-containing homo- and copolymers

• Vijayendra S. Shetti

Beilstein J. Org. Chem. 2021, 17, 2164–2185, doi:10.3762/bjoc.17.139

• -polyazulene 5 (417 nm) compared to azulene (1, 341 nm) supporting the presence of extended conjugation prevailing in the polymer. The absorption and EPR spectral patterns of 1,3-polyazulene 5 recorded in TFA and H2SO4 were contrasting to each other inferring the varied degree of protonation caused by these

• protonated 17 (1.32 eV) was also reduced compared to its neutral form (1.49 eV). These polymers, unlike 1,3-polyazulene 5, displayed reversible acid–base chemistry. With the help of absorption spectroscopy and EPR analysis, the authors could establish the fact that effective stabilization of 6 π-electron

• reversible acid–base response, which was evident from its absorption and EPR studies. The optical HOMO–LUMO gap for 45 was smaller in the protonated form (1.50 eV) compared to its neutral form (1.62 eV). In 2014, Wang, He, and co-workers [33][34] synthesized the conjugated polymers containing poly(thienyl

Constrained thermoresponsive polymers – new insights into fundamentals and applications

• Patricia Flemming,
• Alexander S. Münch,
• Andreas Fery and
• Petra Uhlmann

Beilstein J. Org. Chem. 2021, 17, 2123–2163, doi:10.3762/bjoc.17.138

On the application of 3d metals for C–H activation toward bioactive compounds: The key step for the synthesis of silver bullets

• Renato L. Carvalho,
• Amanda S. de Miranda,
• Mateus P. Nunes,
• Roberto S. Gomes,
• Guilherme A. M. Jardim and
• Eufrânio N. da Silva Júnior

Beilstein J. Org. Chem. 2021, 17, 1849–1938, doi:10.3762/bjoc.17.126

• behaves as a terminal oxidant to form α-aminoalkyl radicals, whereas the formation of an Fe-peroxo species in the catalytic cycle was confirmed using a combination of EPR and ESI mass spectrometry experiments (Scheme 31D). One-pot processes for the synthesis of benzo[b]furans from aryl- or alkylketones

Synthetic reactions driven by electron-donor–acceptor (EDA) complexes

• Zhonglie Yang,
• Yutong Liu,
• Kun Cao,
• Xiaobin Zhang,
• Hezhong Jiang and
• Jiahong Li

Beilstein J. Org. Chem. 2021, 17, 771–799, doi:10.3762/bjoc.17.67

• depends on the association of eosin Y and oxidant 4 to a donor–acceptor EY–4 ground-system complex (high reactivity). Due to the ability of aryl groups to stabilize the formed alkenyl radical, this protocol could control regioselectivity efficiently with unsymmetrical alkynes. In addition, EPR

• including EPR, NMR, and DFT have been carried out to prove that the reaction mechanism is consistent with inference (Scheme 58). The construction of C–O bonds and C–H bonds Although there have been few cases of constructing C–O bonds and C–H bonds via EDA-complex pathways in recent years, we also summarized

Annulation of a 1,3-dithiole ring to a sterically hindered o-quinone core. Novel ditopic redox-active ligands

• Sergey V. Norkov,
• Anton V. Cherkasov,
• Andrey S. Shavyrin,
• Maxim V. Arsenyev,
• Viacheslav A. Kuropatov and
• Vladimir K. Cherkasov

Beilstein J. Org. Chem. 2021, 17, 273–282, doi:10.3762/bjoc.17.26

• reduction with sodium dithionite in a water/methanol mixture: at these conditions o-quinone 6a converts into catechol 10. It has been shown also that the thiocarbonyl group in 6a could be easily converted into a carbonyl group by action of mercury(II) acetate in solution (Scheme 3). EPR spectroscopy studies

• semiquinonate species were studied in solution by EPR spectroscopy (Figures S8–S25 in Supporting Information File 1); the parameters of their isotropic EPR spectra are tabulated in Table S4 in Supporting Information File 1. The measured values of the splitting constants were fitted using the computer simulation

• of the experimental EPR spectra. It is noteworthy that semiquinone derivatives of 6b and 6d show resolved hyperfine splittings due to the coupling of unpaired electrons with the magnetic nuclei of 14N (quintet 1:2:3:2:1) and protons of methyl groups (septet 1:6:15:20:15:6:1), respectively. o

Changed reactivity of secondary hydroxy groups in C8-modified adenosine – lessons learned from silylation

• Jennifer Frommer and
• Sabine Müller

Beilstein J. Org. Chem. 2020, 16, 2854–2861, doi:10.3762/bjoc.16.234

• . Accordingly, the field has developed to a stage that allows custom-design of RNA probes and tools for specific application. For example, investigations of RNA structures by NMR, EPR, or fluorescence spectroscopy require labeling of the RNA molecules with specific reporter groups [2][4][7][8][9][10]. Likewise

Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes

• Henrik Hupatz,
• Marius Gaedke,
• Hendrik V. Schröder,
• Julia Beerhues,
• Arto Valkonen,
• Fabian Klautzsch,
• Sebastian Müller,
• Felix Witte,
• Kari Rissanen,
• Biprajit Sarkar and
• Christoph A. Schalley

Beilstein J. Org. Chem. 2020, 16, 2576–2588, doi:10.3762/bjoc.16.209

• spectra. These bands are indicative of the radical anion NDI•− (Figure 4a–c, red curves) [59]. The radical character of NDIC8 was additionally confirmed by spectroelectrochemical EPR measurements, which showed an isotropic signal with a g-value of 2.004 (Figure S17 in Supporting Information File 1). Upon

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

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

• used in the development of organic magnetic materials [1], organic batteries [2][3][4], in the preparation of polymers by living polymerization [5][6], in the studies of biomolecules and living systems by EPR [7] and NMR [8] techniques. Stable N-oxyl radicals occupy a central place in organic chemistry

• intramolecular reactions of oxidative cyclization. Examples of intermolecular reactions of oxime radicals, a brief description of their structure, stability, and spectral properties are also given. The chemistry of iminoxyl radicals (including their generation, structure, EPR spectroscopy, and reactions) was

• reviewed in 1978 before the discovery of their substantial synthetic potential [34]. A book chapter was dedicated to the chemistry of stable di-tert-alkyl iminoxyl radicals [35]. Kinetic [36] and EPR data [37] of iminoxyl radicals were previously compiled in tabular form. Review General information about

Copper catalysis with redox-active ligands

• Agnideep Das,
• Yufeng Ren,
• Cheriehan Hessin and
• Marine Desage-El Murr

Beilstein J. Org. Chem. 2020, 16, 858–870, doi:10.3762/bjoc.16.77

• another family of galactose oxidase mimics based on copper(II) complex 4 bearing a non-innocent iminophenol-iminopyridine hybrid ligand 3 that performed two-electron oxidations of primary alcohols to aldehydes (Scheme 2) [15]. Catalyst 4 was fully characterized by combined EPR, cyclic voltammetry and

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

• , benzaldehyde (8) has also been studied for the ability to carry out HAT processes. In 1975, Obi and co-worker studied the photochemistry of excited benzaldehyde (9) with the use of electron paramagnetic resonance (EPR) [20]. They detected that the generated radicals from this process were the α-hydroxybenzyl

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

• form in case of fac-[Ir(ppy)3]), which was proven by mass spectrometry and EPR spectroscopy for one of the catalysts (N,N-5,10-di(2-naphthalene)-5,10-dihydrophenazine) used in this work. The generation of HCl resulting from lactone formation could be an additional problem for organometallic photoredox

• was used proved to be soluble in DMSO, allowing us to characterize the material. After we discovered that the redissolved precipitate was NMR-silent, a room temperature EPR spectrum was recorded (Figure 3). The multiplicity in this spectrum is in agreement with formation of the 1e-oxidized form of the

• ferrocene with 0.1 M [TBA][PF6] in MeCN. The potential is referenced to the Fc/Fc+ redox couple (0 V). The excited state reduction potentials of two different reducing photoredox catalysts (in red and green) are shown in the figure. EPR spectra measured (black) and simulated (red) based on the structure of

Unexpected one-pot formation of the 1H-6a,8a-epiminotricyclopenta[a,c,e][8]annulene system from cyclopentanone, ammonia and dimethyl fumarate. Synthesis of highly strained polycyclic nitroxide and EPR study

• Sergey A. Dobrynin,
• Igor A. Kirilyuk,
• Yuri V. Gatilov,
• Andrey A. Kuzhelev,
• Olesya A. Krumkacheva,
• Matvey V. Fedin,
• Michael K. Bowman and
• Elena G. Bagryanskaya

Beilstein J. Org. Chem. 2019, 15, 2664–2670, doi:10.3762/bjoc.15.259

• a sterically shielded polycyclic nitroxide. The EPR spectra and spin relaxation behavior of the nitroxide were studied in solution. The spin relaxation seems well suited for the use as a biological spin label and are comparable with those of cyclic nitroxides with two spirocyclic moieties adjacent

• to the N–O· group. Keywords: domino reactions; EPR; nitroxide; spin relaxation; Introduction Domino reactions have attracted much attention as an approach for the synthesis of complex molecules in a few steps [1]. The utility of multicomponent reactions involving amines, activated olefins and

• molecules along the a-axis. EPR measurements Figure 4 demonstrates the X-band CW EPR spectra of nitroxide 6 in water/glycerol solution at 180 K and at room temperature with simulations (red) using the parameters listed in the caption. The electron spin relaxation of nitroxides with different bulky

Synthesis of 1-azaspiro[4.4]nonan-1-oxyls via intramolecular 1,3-dipolar cycloaddition

• Yulia V. Khoroshunova,
• Denis A. Morozov,
• Andrey I. Taratayko,
• Polina D. Gladkikh,
• Yuri I. Glazachev and
• Igor A. Kirilyuk

Beilstein J. Org. Chem. 2019, 15, 2036–2042, doi:10.3762/bjoc.15.200

• the range of 54–75%. Of note, the use of a similar three-step procedure for the synthesis of 1 from 19 increased the yield of this nitroxide from 20% [6] to 60% (Scheme 8). The EPR spectra of nitroxides 12a–c and 1 acquired in a deoxygenated buffer revealed a significant difference in line widths (see

• Table 1 and Supporting Information File 1, Figures S7–S10), with the broadest lines expectedly being shown by 12b. Introduction of spirocyclohexane moieties to α-carbons of pyrrolidine nitroxides was found to cause strong broadening of lines in the EPR spectra, presumably owing to unresolved hfc on the

• hydrogens at positions 2 and 6 of the cyclohexane ring [22]. It has been reported that EPR spectra of pyrrolidine or imidazolidine nitroxides with pair(s) of geminal ethyl groups at α-carbon atoms may feature large doublet hyperfine splittings [23][24][25]. For imidazolidine nitroxides, these splittings

Photochemical generation of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical from caged nitroxides by near-infrared two-photon irradiation and its cytocidal effect on lung cancer cells

• Ayato Yamada,
• Manabu Abe,
• Yoshinobu Nishimura,
• Shoji Ishizaka,
• Masashi Namba,
• Taku Nakashima,
• Kiyofumi Shimoji and
• Noboru Hattori

Beilstein J. Org. Chem. 2019, 15, 863–873, doi:10.3762/bjoc.15.84

• addition to their ease of handling, nitroxides are highly sensitive to electron paramagnetic resonance (EPR) spectroscopy and redox reactions. Therefore, nitroxides have been developed and utilized in diverse and crucial applications, not only in chemistry, but also in biology, physiology, and energy

• stable in benzene below 320 K (47 °C), as confirmed by electron paramagnetic resonance (EPR) spectroscopic analysis. Significant thermal decomposition of 2a and 2b was observed at ≈340 K (67 °C), as indicated by the typical EPR signals (see Supporting Information File 1, Figure S1). The photophysical

• 365 nm light (6.02 × 1015 photons s−1) under atmospheric conditions (Figure 1). Clean release of the TEMPO radical was confirmed by measuring the electron paramagnetic resonance (EPR) signals of the typical nitroxide, AN = 15.5 G (g = 2.00232, Figure 1 and Figure 2c). The first-order rate constant for