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

Development of N-F fluorinating agents and their fluorinations: Historical perspective

• Teruo Umemoto,
• Yuhao Yang and
• Gerald B. Hammond

Beilstein J. Org. Chem. 2021, 17, 1752–1813, doi:10.3762/bjoc.17.123

Methodologies for the synthesis of quaternary carbon centers via hydroalkylation of unactivated olefins: twenty years of advances

• Thiago S. Silva and
• Fernando Coelho

Beilstein J. Org. Chem. 2021, 17, 1565–1590, doi:10.3762/bjoc.17.112

• Markovnikov-type addition to generate a carbon-centered radical (B) that attacks the electron-deficient olefin (Scheme 22A). The newly formed alkyl radical (C) is then reduced by a Fe(II) species to an enolate (D) in an electron transfer (ET) step; a proton abstraction then delivers the hydroalkylated product

• aldol reaction (Scheme 22B), which was confirmed by the observation of the aldol product in low yield. The ET step initially proposed by Baran has undergone insightful discussions about its nature, and a recent theoretical study postulated a proton-coupled electron transfer (PCET) pathway (Scheme 22C

• reaction rate-determining step in a radical mechanism in which the alkane participates in both the initiation and propagation steps of the radical chain (Scheme 37C). Photoinduced electron transfer Under photoinduced electron transfer (PET) conditions, olefins generate cation radical species that are

Icilio Guareschi and his amazing “1897 reaction”

• Gian Cesare Tron,
• Alberto Minassi,
• Giovanni Sorba,
• Mara Fausone and
• Giovanni Appendino

Beilstein J. Org. Chem. 2021, 17, 1335–1351, doi:10.3762/bjoc.17.93

• + 2H2 → CH2O + H2O [57]. The energy to fuel the process is associated with the vectorial chemistry of the electron transfer chain and the mitochondrial chemiosmotic generation of ATP, a suggestion by Mitchell long considered heretic in the biochemistry community, which searched for decades the highly

Synthesis of 10-O-aryl-substituted berberine derivatives by Chan–Evans–Lam coupling and investigation of their DNA-binding properties

• Peter Jonas Wickhorst,
• Mathilda Blachnik,
• Denisa Lagumdzija and
• Heiko Ihmels

Beilstein J. Org. Chem. 2021, 17, 991–1000, doi:10.3762/bjoc.17.81

• suppressed rotation about the Ar–O bond. Nevertheless, as the emission quantum yield of the derivatives 5a–e still remained low, even at high viscosity of the medium, there obviously exist additional relaxation pathways in the excited state, most likely a photo-induced electron transfer (PET) from the 10

Prins cyclization-mediated stereoselective synthesis of tetrahydropyrans and dihydropyrans: an inspection of twenty years

• Asha Budakoti,
• Pradip Kumar Mondal,
• Prachi Verma and
• Jagadish Khamrai

Beilstein J. Org. Chem. 2021, 17, 932–963, doi:10.3762/bjoc.17.77

• activation via DDQ oxidation, followed by nucleophilic attack of an unactivated olefin to obtain all-cis-trisubstituted Prins products with high stereochemical precision [111]. A single-electron transfer (SET) mechanism was proposed for the above transformation (Scheme 69). A SET from an arene or alkene to

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

• the excited state, causing an electron transfer to give radicals and to initiate subsequent reactions. Besides light as an external energy source, reactions involving the participation of EDA complexes are mild, obviating transition metal catalysts or photosensitizers in the majority of cases and are

• Electron transfer (ET) is a very common occurrence in the field of natural science, including photochemical, electrochemical, and enzymatic reactions and even major organic synthesis. From 1950 to 1952, Mulliken suggested an electron transfer hypothesis that could more precisely explain electron transfer

• electron transfer and forming a pair of radical ions trapped in the solvent cage. The pair of radical ions escapes the solvent cage by diffusion to give radical ions, which could initiate chemical reactions or reverse electron transfer (Scheme 1) [6]. The continuously increasing demand for sustainable

α,γ-Dioxygenated amides via tandem Brook rearrangement/radical oxygenation reactions and their application to syntheses of γ-lactams

• Mikhail K. Klychnikov,
• Radek Pohl,
• Ivana Císařová and
• Ullrich Jahn

Beilstein J. Org. Chem. 2021, 17, 688–704, doi:10.3762/bjoc.17.58

• reported. The sequence starts with a new tandem nucleophilic substitution/Brook rearrangement/single electron transfer-induced radical oxygenation furnishing orthogonally protected α,γ-dioxygenated N-allylamides with wide scope, mostly good yields, and partly good diastereo- and enantioselectivity for

• ; electron transfer; γ-lactams; tandem reactions; Introduction Nitrogen-containing heterocycles are widely distributed in biologically active compounds [1][2][3][4]. Saturated nitrogen heterocycles such as pyrrolidines [5][6][7][8][9], piperidines, pyrrolizidines or indolizidines [10][11][12][13][14][15][16

• rearrangement/single electron transfer-induced radical oxygenation proceeds efficiently with silylacetamides 8a–g,j,k giving α,γ-dioxygenated amides 9a–p in good yields (cf. Table 2). This transformation represents a rare geminal C–C/C–O functionalization of the starting silylacetamides 8. However, neither the

CF₃-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

• Anthony J. Fernandes,
• Armen Panossian,
• Bastien Michelet,
• Agnès Martin-Mingot,
• Frédéric R. Leroux and
• Sébastien Thibaudeau

Beilstein J. Org. Chem. 2021, 17, 343–378, doi:10.3762/bjoc.17.32

• very poor stabilizing power by π-electron donation. A trend exists in the magnitude of the parameter according to the nature of the carbenium ions, which is in line with the carbenium ion stability (alkyl < allylic < benzylic). Thus, an increased π-electron transfer is present in the least-stabilized

• of 93%, the Pummerer rearrangement of sulfoxide 214 under harsh conditions turned out to be less efficient, affording 204f in only 42% yield. This reaction is thought to proceed stepwise via a first oxidative electron transfer, followed by deprotonation, a second oxidative electron transfer, and

• the electron-withdrawing CF3 group and the electron donor sulfur atom through a captodative effect. Further oxidative electron transfer produces α-(trifluoromethyl)-substituted sulfonium ion 206, leading to 204a,f after reacting with the solvent. α-(Trifluoromethyl)alkylcarbenium ions Hypothetical

Insight into functionalized-macrocycles-guided supramolecular photocatalysis

• Minzan Zuo,
• Krishnasamy Velmurugan,
• Kaiya Wang,
• Xueqi Tian and
• Xiao-Yu Hu

Beilstein J. Org. Chem. 2021, 17, 139–155, doi:10.3762/bjoc.17.15

• explained as follows: the zinc(II)–cyclen complex can absorb light and facilitate the intramolecular electron transfer from benzyl alcohol to the excited flavin, and thus the benzaldehyde and the photoreduced flavin were produced. The study indicates the significance of connecting a photosensitizer and a

• and photocatalysis [33][34][35]. Su and co-workers reported a hybrid material based on a calixarene-modified dye and TiO2 (HO-TPA–TiO2) [36]. The calixarene could combine with TiO2, providing efficient electron transfer between them (Figure 11). The TPA–TiO2 system exhibits an efficient H2 evolution

• titanium–thiacalix[4]arene-based clusters for H2 evolution, and it also provides a useful method for tuning the band gaps of the catalysts. Apart from reducing the distance between the electron donor and acceptor, calixarenes can also promote the charge separation to enhance the electron transfer [38

Synthesis of tetrafluorinated piperidines from nitrones via a visible-light-promoted annelation reaction

• Vyacheslav I. Supranovich,
• Igor A. Dmitriev and
• Alexander D. Dilman

Beilstein J. Org. Chem. 2020, 16, 3104–3108, doi:10.3762/bjoc.16.260

• substitution could be catalyzed by iodide anions accumulating in the reaction mixture. Finally, the deoxygenation of the N-oxide fragment may proceed via consecutive protonation and electron-transfer steps [28]. Conclusion In summary, a one-step method for the synthesis of tetrafluorinated piperidines starting

Metal-free nucleophilic trifluoromethylselenolation via an iodide-mediated umpolung reactivity of trifluoromethylselenotoluenesulfonate

• Kevin Grollier,
• Alexis Taponard,
• Arnaud De Zordo-Banliat,
• Emmanuel Magnier and
• Thierry Billard

Beilstein J. Org. Chem. 2020, 16, 3032–3037, doi:10.3762/bjoc.16.252

• ][46][47]. Very recently, we demonstrated that under reductive conditions, such compounds succeeded to perform nucleophilic substitutions [48]. In this reaction, the CF3Se− anion was in situ generated by reduction through a double electron transfer of 1a with TDAE (tetrakis(dimethylamino)ethylene

Naphthalonitriles featuring efficient emission in solution and in the solid state

• Sidharth Thulaseedharan Nair Sailaja,
• Iván Maisuls,
• Jutta Kösters,
• Alexander Hepp,
• Andreas Faust,
• Jens Voskuhl and
• Cristian A. Strassert

Beilstein J. Org. Chem. 2020, 16, 2960–2970, doi:10.3762/bjoc.16.246

• , internal conversion, intermolecular electron transfer, as well as excimer or exciplex formation and isomerization. These phenomena significantly limit the usability of luminogens for the abovementioned purposes. Several attempts were already made to prevent or restrict these non-radiative pathways by

Controlled decomposition of SF₆ by electrochemical reduction

• Sébastien Bouvet,
• Bruce Pégot,
• Stéphane Sengmany,
• Erwan Le Gall,
• Eric Léonel,
• Anne-Marie Goncalves and
• Emmanuel Magnier

Beilstein J. Org. Chem. 2020, 16, 2948–2953, doi:10.3762/bjoc.16.244

• : Pt ultramicroelectrode (∅ 20 µm, S1 = 3.14 × 10−6 cm2) and Pt macroelectrode (∅ 0.76 mm, S2 = 4.5 × 10−3 cm2). Using Pt ultramicroelectrode, the stationary current density (j1) is deduced from Equation 2: Assuming a rapid electron transfer, the current (j2) decreases exponentially with time (t

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• electron transfer (SET) steps photoinitiated using visible light as the energy source, often leading to the generation of radicals and subsequent reaction of these radicals with the ground-state substrates [3]. Energy transfer catalysis is another significant branch of photocatalysis, in which

• -position of the iminium ion generating an unstable iminyl radical 52•+ that is quickly quenched by the nearby carbazole to form a more stable carbazole centred radical 53•+. Rapid tautomerisation to imine 54•+ precludes the undesired back electron transfer. Single electron reduction of 54•+ by PC•– and

• -covalent catalysis with photoredox catalysis was reported by Rono and Knowles in 2013 (Scheme 15) [57]. They showed that using a chiral phosphoric acid (CPA), a photoredox catalyst and Hantzsch ester (HEH) as a HAT reagent, a concerted proton-coupled electron transfer (PCET) process is promoted to form

Formation of an exceptionally stable ketene during phototransformations of bicyclo[2.2.2]oct-5-en-2-ones having mixed chromophores

• Asitanga Ghosh

Beilstein J. Org. Chem. 2020, 16, 2297–2303, doi:10.3762/bjoc.16.190

• (characteristic of β,γ-enone moiety) and the type-B path (characteristic of an α,β-enone moiety). However, with the help of the photoinduced electron transfer (PET) reaction of 1a,c,d,g–h and photoreaction of the partially olefinated product of 1c,d,h, we have recognized that in a mixed α,β- and β,γ-enone system

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

• chemoselectivity. Overall, chemo- and regioselective C(sp3)–H fluorinations continue to challenge chemists. Most direct C(sp3)–H fluorinations are reported to proceed under radical pathways involving hydrogen atom transfer (HAT), although proton-coupled electron transfer (PCET) has also been reported [44][49][50

• reactions proceed under photoredox catalysis (PRC), involving Dexter electron transfer. Such photoredox reactions begin with the excitation of the photocatalyst (PC) by visible light, followed by a single-electron transfer (SET) between the excited photocatalyst and another molecule (quencher, Scheme 2A

The biomimetic synthesis of balsaminone A and ellagic acid via oxidative dimerization

• Sharna-kay Daley and
• Nadale Downer-Riley

Beilstein J. Org. Chem. 2020, 16, 2026–2031, doi:10.3762/bjoc.16.169

• ], and chromium trioxide (CrO3), which, based on its Cr(VI) oxidation state, should be able to facilitate single-electron transfer in the presence of electron-rich arenes. The dimerization of 1,2,4-trimethoxynaphthalene (17) in the presence of the metal oxidants CAN, V2O5, and CrO3, afforded binaphthyl

A complementary approach to conjugated N-acyliminium formation through photoredox-catalyzed intermolecular radical addition to allenamides and allencarbamates

• Olusesan K. Koleoso,
• Matthew Turner,
• Felix Plasser and
• Marc C. Kimber

Beilstein J. Org. Chem. 2020, 16, 1983–1990, doi:10.3762/bjoc.16.165

• each reaction shown in Scheme 4. A tentative mechanism for this transformation is described in Scheme 5a. Excitation of the Ir(III) complex 17 provides *Ir(III) that subsequently undergoes reductive quenching by Et3N, delivering Ir(II) [48]. Single electron transfer from Ir(II) to 18 then generates an

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

• electron transfer to complete the catalytic cycle and regenerate the active catalytic species. The classical Fujiwara–Moritani reaction promoting the addition of arenes to olefins illustrates a general mechanism for such traditional C–H functionalization (Figure 4, left) [70][71][72]. The insertion of a

• , together with a metal hydride or a low-valent metal complex. Hence, in order to reoxidize the metal catalyst, excess of an external oxidant, such as Cu(II) or Ag(I) salts, was frequently used. On the other hand, photoredox catalysis has been mainly employed for electron-transfer reactions and, remarkably

• of the photocatalyst in the absence of oxygen, suggesting that a direct electron transfer from the photosensitizer allowed the reoxidation of the active catalyst. However, the participation of molecular oxygen cannot be excluded. Rueping further demonstrated the capacity of the dual catalytic systems

Photoredox-catalyzed silyldifluoromethylation of silyl enol ethers

• Vyacheslav I. Supranovich,
• Vitalij V. Levin and
• Alexander D. Dilman

Beilstein J. Org. Chem. 2020, 16, 1550–1553, doi:10.3762/bjoc.16.126

• strongly reducing catalysts may be associated with the ability of gold to interact with the bromine atom of silane 1 followed by inner-sphere electron transfer [27]. The radical then attacks silyl enol ether 2, and the subsequent silyloxy-substituted radical is oxidized by the photocatalyst to generate the

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

• visible light through single-electron transfer processes, now referred to as visible light photoredox catalysis (PRC). A similar query on the Web of Science for the term “Photoredox” clearly shows the surge in PRC research following those reports, from 2010 onwards (Figure 1A). However, what is

• within the proximity required for an electron transfer or energy transfer process to occur. Substrate reduction and oxidation by an excited electron and hole, respectively, returns the semiconductor to its initial state and activates the substrate to further reactivity at the surface or in the bulk

• spectrum through direct VB/adsorbate electron transfer transitions [93][94]. HPCats modified with coordinating transition metal complexes also usually display significant changes to their absorption spectrum through the introduction of metal-to-ligand, ligand-to-metal, ligand-to-ligand, and metal-to-metal

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

Beilstein J. Org. Chem. 2020, 16, 1418–1435, doi:10.3762/bjoc.16.118

• . Under photoirradiation, organic disulfides can be easily cleaved into free thiyl radicals (RS•) and can reversibly add to unsaturated multiple bonds to catalyze a variety of functionalization reactions under mild conditions. In photoredox catalysis reactions, an excellent electron transfer ability and

• . Huang and co-workers proposed a polar radical crossover cycloaddition mechanism for this Diels–Alder cycloaddition (Scheme 6). The electron transfer from the electron-rich styrene 14 to the activated acridinium photocatalyst 15 oxidizes the styrene 14 to form the styrene radical 16 and the acridine

• alcohol by the thiyl radical produces thiophenol and an allylic radical. Next, the single-electron transfer (SET) from the allylic radical to another thiyl radical generates the allylic cation. Subsequently, the proton abstraction from the hydroxy group by the SET-generated thiolate gives the final

Recent synthesis of thietanes

• Jiaxi Xu

Beilstein J. Org. Chem. 2020, 16, 1357–1410, doi:10.3762/bjoc.16.116

• -Diphenylsilacyclopentadiene (324) underwent a photo-induced [2 + 2] cycloaddition with CS2 to afford two regioisomeric fused thietane-2-thiones 325 and 326. The electron transfer from the singlet-excited state of silacyclopentadiene to CS2 was shown to play an important role in the cycloaddition [90] (Scheme 61). 3.1.2

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

• -coupled single-electron transfer in the presence of an Ir photocatalyst. On the other hand, symmetrical 1,2-diamines were selectively produced from the same starting materials by the judicious choice of the reaction conditions, showcasing the distinct reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines

• 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

• the excited Ir complex significantly decreased in proportion to the concentration of Cy2NMe, while it was much less affected by the concentration of 1a, confirming the proposed working mode. The formation of 2a might be attributed to the proton-coupled electron transfer [62][63][64][65][66] from [IrII