Facile approach to N,O,S-heteropentacycles via condensation of sterically crowded 3H-phenoxazin-3-one with ortho-substituted anilines

• Eugeny Ivakhnenko,
• Vasily Malay,
• Pavel Knyazev,
• Nikita Merezhko,
• Nadezhda Makarova,
• Oleg Demidov,
• Gennady Borodkin,
• Andrey Starikov and
• Vladimir Minkin

Beilstein J. Org. Chem. 2024, 20, 336–345, doi:10.3762/bjoc.20.34

• –H proton. Therefore, three tautomeric forms are possible for 5 (Scheme 4), one of which, the 7H-tautomer 7b, inevitably adopts a bipolar or biradical structure. According to the data from the DFT calculations performed at the B3LYP/6-311++G(d,p) approximation (Figure S6, Supporting Information File

• unpaired electrons and the singlet biradical form. In previous studies on the coupling of 3H-phenoxazin-3-one derivatives 8 and 9 with o-phenylenediamine [10][11], the preference was given to the 12Н-quinoxaline[2,3-b]phenoxazine form 7 (Scheme 5). A series of N-aryl derivatives of this form was also

Photochromic derivatives of indigo: historical overview of development, challenges and applications

• Gökhan Kaplan,
• Zeynel Seferoğlu and
• Daria V. Berdnikova

Beilstein J. Org. Chem. 2024, 20, 228–242, doi:10.3762/bjoc.20.23

• solvents and under high pressures were performed. It was found that N,N'-dibenzoylindigos underwent the thermal relaxation much more slowly than the compounds without the aromatic ring in the acyl group. The biradical mechanism (Figure 11) was found to be the preferable pathway for the thermal Z–E

• the Z-isomer with metal cations. Schematic representation of indigo-type (left) and amide-type (right) resonances in N,N'-acetylindigo (9a). Suggested intermediates for the double bond cleavage for the thermal relaxation of N,N'-diacylindigos: (A) a biradical transient species, (B) a dipolar transient

Radical chemistry in polymer science: an overview and recent advances

• Zixiao Wang,
• Feichen Cui,
• Yang Sui and
• Jiajun Yan

Beilstein J. Org. Chem. 2023, 19, 1580–1603, doi:10.3762/bjoc.19.116

• addition. The termination step occurs by either disproportionation (radical β-elimination, Equation 4) or biradical coupling (Equation 5). Chain transfer (Equations 6–8) is usually considered as a type of side effect in radical polymerization [18]. It occurs between the growing chain and a transfer agent

• carbon-centered radical to another thiol group (propagation 2), and biradical termination between either carbon-centered or thiyl radicals (termination). Polymerization by thiol–ene coupling is a step-growth polymerization, which means it can produce polymers with no theoretical upper-limited molecular

• radicals on polymer surfaces. Its photoexcitation and subsequent reaction with polymers have been studied for decades [164][165][166]. When irradiated at around 360 nm, benzophenone undergoes excitation to a triplet state with biradical behavior. It then abstracts a hydrogen atom from the polymer resulting

Germacrene B – a central intermediate in sesquiterpene biosynthesis

• Houchao Xu and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2023, 19, 186–203, doi:10.3762/bjoc.19.18

• photochemical E/Z isomerisation to 13 prior to [2 + 2] cycloaddition. Further photochemical products from 1 include 5, 15 that may be formed through a biradical mechanism, and rearranged 16 [51]. Germacrene B (1) has planar chirality (Scheme 4D), but recovery of the starting material from an incomplete

Vicinal ketoesters – key intermediates in the total synthesis of natural products

• Marc Paul Beller and
• Ulrich Koert

Beilstein J. Org. Chem. 2022, 18, 1236–1248, doi:10.3762/bjoc.18.129

• with a medium pressure mercury lamp in Pyrex® glassware triggered a 1,6-HAT leading to biradical X which combined to the racemic pyrrolizidine 68 as a 1:1 mixture of diastereomers. Three more steps gave the target compound 69 in 31% overall yield. Corynoxine Hiemstra et al. used the α-ketoester moiety

Site-selective reactions mediated by molecular containers

• Rui Wang and
• Yang Yu

Beilstein J. Org. Chem. 2022, 18, 309–324, doi:10.3762/bjoc.18.35

• irradiation, biradical species 12 was generated and immediately abstracted a hydrogen atom from the methyl group of 11. Site-selective radical coupling at the oxygen atom between 13 and 14 produced the 1,4-adduct 15. The unusual site-selectivity of this reaction was also traced from the restricted geometry

An initiator- and catalyst-free hydrogel coating process for 3D printed medical-grade poly(ε-caprolactone)

• Jochen Löblein,
• Thomas Lorson,
• Miriam Komma,
• Tobias Kielholz,
• Maike Windbergs,
• Paul D. Dalton and
• Robert Luxenhofer

Beilstein J. Org. Chem. 2021, 17, 2095–2101, doi:10.3762/bjoc.17.136

• exposed PCL fibers in the PHEMA-filled MEW scaffolds. Schematic representation of the self-initiated photografting and photopolymerization (SIPGP) of 2-hydroxyethyl methacrylate (HEMA) including intersystem crossing from singlet to triplet state which is mandatory for the formation of a biradical (left

[2 + 1] Cycloaddition reactions of fullerene C₆₀ based on diazo compounds

• Yuliya N. Biglova

Beilstein J. Org. Chem. 2021, 17, 630–670, doi:10.3762/bjoc.17.55

• latter involves the closure into a [5,6]-closed fullerene, which is then regrouped into a [6,6]-closed fullerene via a biradical intermediate (Scheme 1). Nevertheless, examples of the synthesis of both individual isomeric cycloadducts and mixtures of isomeric fullerene and fulleroid structures upon

The preparation and properties of 1,1-difluorocyclopropane derivatives

• Kymbat S. Adekenova,
• Peter B. Wyatt and
• Sergazy M. Adekenov

Beilstein J. Org. Chem. 2021, 17, 245–272, doi:10.3762/bjoc.17.25

• . Such selectivity is caused by the possibility of the resonance stabilization of the biradical that is formed. If other reagents are absent, the biradical can rearrange and recombine, leading to isomerization of the starting material as was observed in the case of 1,1-difluoro-2,3-dialkylcyclopropanes

All-carbon [3 + 2] cycloaddition in natural product synthesis

• Zhuo Wang and
• Junyang Liu

Beilstein J. Org. Chem. 2020, 16, 3015–3031, doi:10.3762/bjoc.16.251

• tricyclic compound 25, which led to the synthesis of (±)-hirsutene (14) [22] (Scheme 1A). Refluxing azo compound 22 in acetonitrile generated the proposed biradical intermediate 23 through nitrogen extrusion. This intermediate underwent isomerization to 24 and intramolecular diyl trapping through a [3 + 2

Metal-free synthesis of biarenes via photoextrusion in di(tri)aryl phosphates

• Hisham Qrareya,
• Lorenzo Meazza,
• Stefano Protti and
• Maurizio Fagnoni

Beilstein J. Org. Chem. 2020, 16, 3008–3014, doi:10.3762/bjoc.16.250

• of the biradical intermediate 6 [55] preceding the loss of ROP(O)(OH)2 (paths (b), (c), Scheme 3). The long irradiation time required to achieve a complete consumption of the substrates 1 and 3 is in accordance with the low quantum yield values reported for this process [58][61]. Furthermore, a

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

• : a biradical capable of abstracting an H atom from C–H bonds. Recently, Martin exploited this feature in a nickel-catalyzed process for the alkylation of arenes (Scheme 8) [57]. In this report, the excited state of a push–pull benzophenone OD10 can abstract an H atom from the substrate

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

• reaction were described. The first one included an attack on the π-system of the olefin by the oxygen atom of the excited carbonyl compound, forming a biradical intermediate 72. Then, the biradical intermediate 72 could either cyclize to produce an oxetane 73 or dissociate to form again the carbonyl

• Hanovia 450 W mercury arc [17]. If the reaction proceeded stereospecifically, cis-2-butene would have led to the formation of the oxetanes 75a and 75b, while trans-2-butene would have led to the oxetanes 75c and 75d (Scheme 19). It was suggested that the reaction proceeded through a long-lived biradical

• biradical intermediate may lead to all four conformations. Photolysis products of benzaldehyde (8) in different solvents. a) In benzene or ethanol. b) In hex-1-yne. N-tert-Butylbenzamide formation proceeds via a benzoyl radical. Photochemical pinacol coupling. Photochemical ATRA catalyzed by 4-anisaldehyde

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

• (thio)indigo, the energy maximum in the ground state corresponds to the 90° rotation about the central double bond resulting in formation of a state with biradical-like character that is polarized along the molecule’s long axis [22]. The close structural similarity allows to expect a similar character

Experimental and computational electrochemistry of quinazolinespirohexadienone molecular switches – differential electrochromic vs photochromic behavior

• Eric W. Webb,
• Jonathan P. Moerdyk,
• Kyndra B. Sluiter,
• Benjamin J. Pollock,
• Amy L. Speelman,
• Eugene J. Lynch,
• William F. Polik and
• Jason G. Gillmore

Beilstein J. Org. Chem. 2019, 15, 2473–2485, doi:10.3762/bjoc.15.240

• to rupture upon reduction to SW•−. It is common to think of thermal ring-opening as looking more like that of the biradical (T0 or S1) than of the radical anion (D0), so we might have presumed the structure of any thermal or solvatochromic LW isomer that occurred to have been that of pLW (4). However

Excited state dynamics for visible-light sensitization of a photochromic benzil-subsituted phenoxyl-imidazolyl radical complex

• Yoichi Kobayashi,
• Yukie Mamiya,
• Katsuya Mutoh,
• Hikaru Sotome,
• Masafumi Koga,
• Hiroshi Miyasaka and
• Jiro Abe

Beilstein J. Org. Chem. 2019, 15, 2369–2379, doi:10.3762/bjoc.15.229

• acts as a singlet photosensitizer for PIC by the Dexter-type energy transfer. Visible-light sensitized photochromic reactions of PIC are important for expanding the versatility of potential applications to life sciences and materials science. Keywords: biradical; energy transfer; photochromism

• -type photochromic compounds which reversibly generate an imidazolyl radical and a phenoxyl radical (biradical form) in a molecule upon UV light irradiation [24]. The great advantage of PIC is the tunability of the thermal back reaction from tens of nanoseconds to tens of seconds by simple and rational

• A. PIC generates the biradical species upon UV-light irradiation and shows the broad transient absorption spectrum over the visible- to near infrared-light regions. The half-life of the thermal back reaction of the biradical in benzene is 250 ns (the lifetime is 360 ns) at 298 K. To investigate the

Synthesis of 1-indanones with a broad range of biological activity

• Marika Turek,
• Dorota Szczęsna,
• Marek Koprowski and
• Piotr Bałczewski

Beilstein J. Org. Chem. 2017, 13, 451–494, doi:10.3762/bjoc.13.48

• in 98% yield for both THP and MOM ethers (Scheme 62). Irradiation of aromatic γ,δ-epoxy ketones 226 with a medium-pressure UV mercury lamp (450 W) led to the formation of 1-indanones 227 via a photochemical epoxy rearrangement and 1,5-biradical cyclization tandem reaction (Scheme 63) [92]. The best

Dimerization reactions of aryl selenophen-2-yl-substituted thiocarbonyl S-methanides as diradical processes: a computational study

• Michael L. McKee,
• Grzegorz Mlostoń,
• Katarzyna Urbaniak and
• Heinz Heimgartner

Beilstein J. Org. Chem. 2017, 13, 410–416, doi:10.3762/bjoc.13.44

• radical dimerization of the reactive thiocarbonyl S-methanide 8. Computed enthalpies (free energies in parentheses) at 298 K at the SMD(tetrahydrofuran)/M06-2X/6-311+G(2df,p)//M06-2X/6-31G(d) level of theory. The species 12E–I are all biradical intermediates in the reaction mechanism and should be able to

Protonated paramagnetic redox forms of di-o-quinone bridged with p-phenylene-extended TTF: A EPR spectroscopy study

• Nikolay O. Chalkov,
• Vladimir K. Cherkasov,
• Gleb A. Abakumov,
• Andrey G. Starikov and
• Viacheslav A. Kuropatov

Beilstein J. Org. Chem. 2016, 12, 2450–2456, doi:10.3762/bjoc.12.238

• of the spin-density distribution in the protonated paramagnetic redox forms of di-o-quinone 1. Open-shell DFT calculations performed for (1)H2 at the B3LYP/6-311++G(d,p) level of theory revealed a singlet biradical as a ground state. The value of antiferromagnetic coupling was estimated as 1092 cm−1

A flow reactor setup for photochemistry of biphasic gas/liquid reactions

• Josef Schachtner,
• Patrick Bayer and
• Axel Jacobi von Wangelin

Beilstein J. Org. Chem. 2016, 12, 1798–1811, doi:10.3762/bjoc.12.170

• gas in the liquid phase which is pressure-dependent according to Henry’s law [69]. Reaction parameters of a model photooxygenation The often poor selectivities of reactions with molecular oxygen (being a triplet biradical in its ground state) [70] have prompted applications of microflow reactors to

Rearrangements of organic peroxides and related processes

• Ivan A. Yaremenko,
• Vera A. Vil’,
• Dmitry V. Demchuk and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2016, 12, 1647–1748, doi:10.3762/bjoc.12.162

Biradical vs singlet oxygen photogeneration in suprofen–cholesterol systems

• Fabrizio Palumbo,
• Francisco Bosca,
• Isabel M. Morera,
• Inmaculada Andreu and
• Miguel A. Miranda

Beilstein J. Org. Chem. 2016, 12, 1196–1202, doi:10.3762/bjoc.12.115

• biradical species. Indeed, the coupling rate constant (kd2) should be similar to that of KP-α-Ch and therefore much higher than the hydrogen abstraction rate constant (kH), which is by definition lower than kiq. Consequently, biradicals are not expected to accumulate since their consumption is much faster

• than their formation. It is interesting to note that the reverse is true that for the KP-α-Ch analogs, where kd2 is lower than kH (Table 1). Therefore, biradical accumulation is indeed observed in this case, because the coupling products are generated much more slowly. Singlet oxygen generation To

• yield (ΦΔ) of 0.52 for 1 and 2 and 0.56 for 3 (Table 1 and Figure 7B). Biradical vs singlet oxygen Although it was not possible to detect the biradical species by means of LFP in SP-α-Ch systems, its generation (5%) has been chemically proven by obtaining the coupling products 4 and 5. Moreover, the

Antioxidant potential of curcumin-related compounds studied by chemiluminescence kinetics, chain-breaking efficiencies, scavenging activity (ORAC) and DFT calculations

• Adriana K. Slavova-Kazakova,
• Silvia E. Angelova,
• Timur L. Veprintsev,
• Petko Denev,
• Davide Fabbri,
• Maria Antonietta Dettori,
• Maria Kratchanova,
• Vladimir V. Naumov,
• Aleksei V. Trofimov,
• Rostislav F. Vasil’ev,
• Giovanna Delogu and
• Vessela D. Kancheva

Beilstein J. Org. Chem. 2015, 11, 1398–1411, doi:10.3762/bjoc.11.151

• biphenyls in accordance with the ability to form radical (r) and biradical (br) species. The BDE of biradicals is equal or very close to the BDE value of the corresponding monomers, whereas the BDE of radicals is smaller than the BDE of biradicals with differences in the range of 0.45–0.84 kcal·mol−1. These

• the subsequent formation of the biradical increases the angle more noticeably. The values of the dihedral angles of 7 differ significantly between the radical and the biradical species, probably due to the higher conformational flexibility of the structure in virtue of the presence of two chains with

• and monomer 5/dimer 9 are very close in water. The changes are most pronounced for the couple 3/7, and 3 became the monomer with lowest BDE value and 7 the dimer with the lowest BDE value. The biradical of dimer 7 possesses a BDE value lower than that of the radical formed from this compound. The

Selected synthetic strategies to cyclophanes

• Sambasivarao Kotha,
• Mukesh E. Shirbhate and
• Gopalkrushna T. Waghule

Beilstein J. Org. Chem. 2015, 11, 1274–1331, doi:10.3762/bjoc.11.142

Photorelease of phosphates: Mild methods for protecting phosphate derivatives

• Sanjeewa N. Senadheera,
• Abraham L. Yousef and
• Richard S. Givens

Beilstein J. Org. Chem. 2014, 10, 2038–2054, doi:10.3762/bjoc.10.212

• the developing anionic charge on the departing nucleofuge. The resulting biradical 31, which must be a triplet by Wigner’s spin rule [47] is formed from the chromophore while on the excited state PES. Intersystem crossing (isc) to a diradicaloid ground state intermediate 32 (vide infra) is followed by

• the ‘Favorskii’ intermediate 33 [38][48][50]. The putative key intermediates for 2,6-HNA and 1,5-HNA photo-Favorskii rearrangements (Scheme 9) illustrate the stage at which disruption in the π-network takes place. As in the case for the 1,4-HNA rearrangement, neither triplet biradical 337 nor 340

• experience a change in connectivity within the aromatic nucleus. The formation of the triplet biradical is irreversible so that once generated, it must proceed on to a final product. Since no rearrangement products are formed for either the 2,6-HNA or 1,5-HNA, it is tempting to conclude that the decay to a