Friedel–Crafts approach to the one-pot synthesis of methoxy-substituted thioxanthylium salts

• Kenta Tanaka,
• Yuta Tanaka,
• Mami Kishimoto,
• Yujiro Hoshino and
• Kiyoshi Honda

Beilstein J. Org. Chem. 2019, 15, 2105–2112, doi:10.3762/bjoc.15.208

• green light irradiation [24][25]. In the course of this study, we found that these thioxanthylium photocatalysts efficiently oxidized styrene derivatives such as trans-anethole, and promoted radical cation Diels–Alder reactions. Based on the background mentioned above, in order to expand the utility of

Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

• Gagandeep Kour Reen,
• Ashok Kumar and
• Pratibha Sharma

Beilstein J. Org. Chem. 2019, 15, 1612–1704, doi:10.3762/bjoc.15.165

• the first step which was converted to the final product via radical cation intermediate 17 in the second step (Scheme 6). This additive-free approach offered an easy separation and reusability of heterogeneous catalyst along with the use of air as environmentally benign oxidant. The use of zinc and

A diastereoselective approach to axially chiral biaryls via electrochemically enabled cyclization cascade

• Hong Yan,
• Zhong-Yi Mao,
• Zhong-Wei Hou,
• Jinshuai Song and
• Hai-Chao Xu

Beilstein J. Org. Chem. 2019, 15, 795–800, doi:10.3762/bjoc.15.76

• reaction. A mechanism for the electrochemical synthesis was proposed based on the results from our previous work [31] and of this work (Scheme 3). The redox catalyst 1 is oxidized at the anode to give radical cation I. In the meanwhile, H2O is reduced at the cathode to afford HO− and H2. The base generated

• at the cathode deprotonates 2a to give its conjugate base II. The anionic II is oxidized by radical cation I through single electron transfer (SET) to give radical intermediate III, which undergoes a biscyclization to give V. Further oxidation of V followed by hydrolysis of the cyclic carbamate

Dirhodium(II)-catalyzed [3 + 2] cycloaddition of N-arylaminocyclopropane with alkyne derivatives

• Wentong Liu,
• Yi Kuang,
• Zhifan Wang,
• Jin Zhu and
• Yuanhua Wang

Beilstein J. Org. Chem. 2019, 15, 542–550, doi:10.3762/bjoc.15.48

• generated distonic radical cation can be further trapped by an alkene, alkyne, or triplet oxygen to initiate radical cyclization (Scheme 1) [7][8][9][10][11][12][13][14][15]. Thus, as key synthons, this class of molecules may play an important role in organic synthesis during construction of a series of

• radical cation C resulting from cyclopropane ring opening reacts with alkyne substrate 2a generating radical D. The intermediate radical D yielded E through intramolecular radical addition. After hydrogen atom transfer (HAT) from complex A, the desired product is obtained with regeneration of the N

Tandem copper and photoredox catalysis in photocatalytic alkene difunctionalization reactions

• Nicholas L. Reed,
• Madeline I. Herman,
• Vladimir P. Miltchev and
• Tehshik P. Yoon

Beilstein J. Org. Chem. 2019, 15, 351–356, doi:10.3762/bjoc.15.30

• we have proposed for photocatalytic oxyamination is outlined in Figure 1c. Photoinduced one-electron oxidation of an appropriately electron-rich styrene 1 results in the formation of a radical cation 1•+ that is susceptible to attack by various heteroatomic nucleophiles, including carbamates [21][22

N-Arylphenothiazines as strong donors for photoredox catalysis – pushing the frontiers of nucleophilic addition of alcohols to alkenes

• Fabienne Speck,
• David Rombach and
• Hans-Achim Wagenknecht

Beilstein J. Org. Chem. 2019, 15, 52–59, doi:10.3762/bjoc.15.5

• fast access to a wide variety of catalysts. Recently it was shown that the radical cation of the photoredox catalyst can play a key role in photoinduced oxidation chemistry [16]. This is rather unusual due to the usually short lifetime of radical cations in solution attributed to their low-lying

• dynamics of the radical cation of N-phenylphenothiazine was investigated by Wasielewski et al. This radical cation had a high reduction potential of about +2.1 V (vs SCE) [17] allowing the reduction of poorly oxidizing agents. The combination of both properties in one system is a remarkable feature for

• transfer from the N-phenylphenothiazine (1) as photocatalyst to 13a as substrate. The resulting substrate radical anion 13a−· is instantaneously protonated to radical 13a· and back-electron transfer to the intermediate phenothiazine radical cation 1+· yields the substrate cation 13a+. The latter is

Oxidative and reductive cyclization in stiff dithienylethenes

• Michael Kleinwächter,
• Ellen Teichmann,
• Lutz Grubert,
• Martin Herder and
• Stefan Hecht

Beilstein J. Org. Chem. 2018, 14, 2812–2821, doi:10.3762/bjoc.14.259

• spectra see Figure S29, Supporting Information File 1). To identify the nature of this product, SEC was performed on the photochemically generated closed isomer (Figure 2c,d). Herein, the reversible first oxidation step yields the radical cation C+• (Figure 2c, light blue), identified by its

• characteristic red-shifted absorption at 731 nm and 912 nm due to an unpaired electron. The radical cation C+• is stable even at the slow scan rates of SEC and builds up continuously as evidenced by clean isosbestic points. In a subsequent, second oxidation step, the radical cation C+• is converted to the

• red dots marking when UV–vis–NIR spectra were measured. A stable intermediate is formed upon oxidation of: a) E-sDTE66-Me and b) Z-sDTE66-Me. c) Oxidation of photogenerated C-sDTE66-Me to the stable radical cation C+•. d) Further oxidation of C+• to the dication FP2+, measured in a separate experiment

Synthesis of aryl sulfides via radical–radical cross coupling of electron-rich arenes using visible light photoredox catalysis

• Amrita Das,
• Mitasree Maity,
• Simon Malcherek,
• Burkhard König and
• Julia Rehbein

Beilstein J. Org. Chem. 2018, 14, 2520–2528, doi:10.3762/bjoc.14.228

• )piperidine was obtained in both cases. See Supporting Information File 1 for the HRMS analysis of the TEMPO adduct. These radical trapping experiments show that initially a radical cation of the arene is formed by the excited photocatalyst, which then is trapped by the radical scavenger TEMPO. S–S bond

• )]PF6 and also did not give a sulfenylated product under our photocatalytic conditions. This is rationalized by the oxidation potential of anisole of 1.76 V vs SCE, which is higher than the estimated excited state oxidation potential of the photocatalyst. To elucidate, if the 1,3,5-TMB radical cation

• investigations and literature reports, we propose a photocatalytic mechanism (Scheme 5). Upon photoexcitation, [Ir(dF(CF3)ppy)2(dtbpy)]PF6 accepts an electron from the arene and converts it into the corresponding radical cation 1. Ammonium persulfate present in the reaction mixture could oxidize the reduced

Tetrathiafulvalene – a redox-switchable building block to control motion in mechanically interlocked molecules

• Hendrik V. Schröder and
• Christoph A. Schalley

Beilstein J. Org. Chem. 2018, 14, 2163–2185, doi:10.3762/bjoc.14.190

• radical cation or dication. For over 20 years, TTFs have been key building blocks for the construction of redox-switchable mechanically interlocked molecules (MIMs) and their electrochemical operation has been thoroughly investigated. In this review, we provide an introduction into the field of TTF-based

• molecular switch. A first one-electron oxidation [23] converts neutral TTF (1) into the radical-cationic species 1●+ (Figure 1). The TTF radical cation is one of the rare organic radicals that are long-term stable and even isolable. A second oxidation step yields the dication 12+. Both redox-transitions are

• change of the electronic structure is also accompanied by conformational changes [25][26] of the TTF skeleton. Neutral TTF has a boat-shaped structure with C2v symmetry. In the radical-cation state, TTF●+ planarizes into a D2h-symmetric structure due to its partial aromatization. This property change is

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

• the sulfonamidation of other heterocycles and was not successful. The authors attribute this lack of reactivity to the limited oxidising power of the excited acridinium salts and to the relative instability of the heterocyclic radical cation, which is a key intermediate in the proposed mechanism

• . Considering that Ered*(cat/cat•−) is greater than +2 V (vs SCE) for acridinium salts, the limited oxidising power is not the most probable explanation. Instability of the heteroaromatic radical cation seems more plausible. The authors explore both aromatic and heteroaromatic pendant groups on the sulfonamide

• iminium radical cation are the keys in the mechanism and to understanding the selectivity of the reaction. In an unsymmetrical reactant, the iminium carries most of the partial positive charge on the benzylic carbon (Figure 11). The radical is not stabilised by being borne on the benzylic carbon, as the

Synthesis of 9-arylalkynyl- and 9-aryl-substituted benzo[b]quinolizinium derivatives by Palladium-mediated cross-coupling reactions

• Siva Sankar Murthy Bandaru,
• Darinka Dzubiel,
• Heiko Ihmels,
• Mohebodin Karbasiyoun,
• Mohamed M. A. Mahmoud and
• Carola Schulzke

Beilstein J. Org. Chem. 2018, 14, 1871–1884, doi:10.3762/bjoc.14.161

• -type acridinium, benzo[b]quinolizinium and naphtho[b]quinolizinium derivatives [36][58][59][60][61][62][63][64]. The CS/CT leads to an intermediate excited molecule with a charge neutral quinolizinyl radical and the radical cation of the phenanthryl unit (Scheme 3) that is well stabilized in polar

Stepwise radical cation Diels–Alder reaction via multiple pathways

• Ryo Shimizu,
• Yohei Okada and
• Kazuhiro Chiba

Beilstein J. Org. Chem. 2018, 14, 704–708, doi:10.3762/bjoc.14.59

• -8588, Japan 10.3762/bjoc.14.59 Abstract Herein we disclose the radical cation Diels–Alder reaction of aryl vinyl ethers by electrocatalysis, which is triggered by an oxidative SET process. The reaction clearly proceeds in a stepwise fashion, which is a rare mechanism in this class. We also found that

• two distinctive pathways, including “direct” and “indirect”, are possible to construct the Diels–Alder adduct. Keywords: Diels–Alder reaction; radical cation; rearrangement; single electron transfer; stepwise; Introduction Umpolung, also known as polarity inversion, is a powerful approach in

• produces a radical cation species, which offers electrophilic reactivity for subsequent transformations. Enol ether radical cations are among the simplest members of this class and thus have been widely used in synthetic organic chemistry [13][14][15]. The Diels–Alder reaction is a classic reaction, and

Investigating radical cation chain processes in the electrocatalytic Diels–Alder reaction

• Yasushi Imada,
• Yohei Okada and
• Kazuhiro Chiba

Beilstein J. Org. Chem. 2018, 14, 642–647, doi:10.3762/bjoc.14.51

• , the turnover number, also referred to as catalytic efficiency, remains unclear in most cases. Herein, we disclose our investigations of radical cation chain processes in the electrocatalytic Diels–Alder reaction, leading to a scalable synthesis. A gram-scale synthesis was achieved with high current

• efficiency of up to 8000%. The reaction monitoring profiles showed sigmoidal curves with induction periods, suggesting the involvement of intermediate(s) in the rate determining step. Keywords: chain process; Diels–Alder reaction; electrocatalytic; radical cation; single electron transfer; Introduction

• ], some of which were achieved with a catalytic amount of electricity. Such electrocatalytic cycloadditions should involve radical cation chain processes, meaning that the reaction is not only triggered by an oxidative SET at the surface of the electrode but also by an intermolecular SET process in bulk

The selective electrochemical fluorination of S-alkyl benzothioate and its derivatives

• Shunsuke Kuribayashi,
• Tomoyuki Kurioka,
• Shinsuke Inagi,
• Ho-Jung Lu,
• Biing-Jiun Uang and
• Toshio Fuchigami

Beilstein J. Org. Chem. 2018, 14, 389–396, doi:10.3762/bjoc.14.27

• organosulfur and amino compounds was generally enhanced by the presence of an α-electron-withdrawing group (EWG) such as a CF3 group. Here, the deprotonation of an anodically generated radical cation intermediate is accelerated by an EWG [7][8]. Based on these facts, we successfully achieved the first anodic

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

• authors propose that the photo-excited state of the organic dye Eosin Y is reductively quenched by the aryl thiol to form the Eosin Y radical anion and the respective aryl thiyl radical cation. Neutral Eosin Y is regenerated through oxidation of the radical anion by dioxygen. The resulting superoxide

• radical anion then deprotonates the thiyl radical cation. Subsequent addition to the alkene yields the anti-Markovnikov radical intermediate. Radical addition to dioxygen leads finally to the β-ketosulfide, which subsequently is oxidized by the in situ generated hydrogen peroxide radical to the respective

• conduction band of TiO2 leads to electron holes in the valence band, which can be reductively quenched by the thiol to form the respective thiyl radical cation. After deprotonation, the thiyl radical undergoes thiol–ene coupling. The scope of the reaction includes the coupling of primary alkyl and aryl

CF₃SO₂X (X = Na, Cl) as reagents for trifluoromethylation, trifluoromethylsulfenyl-, -sulfinyl- and -sulfonylation and chlorination. Part 2: Use of CF₃SO₂Cl

• Hélène Chachignon,
• Hélène Guyon and
• Dominique Cahard

Beilstein J. Org. Chem. 2017, 13, 2800–2818, doi:10.3762/bjoc.13.273

• then reacted with the β-nitroalkene to furnish radical intermediate 38, which was reduced by the Eosin Y radical cation to yield the expected product after elimination of NO2. This proposed mechanism can rationalise several limitations of the reaction, such as its incompatibility with aliphatic β

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

• simple alkenes, sodium triflinate and diazonium salts. The CF3 radical was produced from CF3SO2Na by oxidation with H2O2 in the presence of silver nitrate. Then, CF3• was added to the terminal position of the alkene to give radical 26 that was trapped by the arenediazonium salt to form the radical cation

• radical from CF3SO2Na with extrusion of SO2. Then, CF3• underwent a radical addition to the alkene to form the radical 29, which was trapped by the aryldiazonium salt to give the radical cation 30. Finally, 30 was reduced by [Ru(bpy)3]2+* to end up with the product 31 (Scheme 13). The

Vinylphosphonium and 2-aminovinylphosphonium salts – preparation and applications in organic synthesis

• Anna Kuźnik,
• Roman Mazurkiewicz and
• Beata Fryczkowska

Beilstein J. Org. Chem. 2017, 13, 2710–2738, doi:10.3762/bjoc.13.269

• cycloalkene is higher than that for triphenylphosphine, the suggested mechanism of formation of the final 1-cycloalkenetriphenylphosphonium salts 19 is analogous to the reactions of the radical cation of triphenylphosphine with other nucleophiles (Scheme 13) [19]. 1.5. Triphenylphosphine addition to a triple

Oxidative dehydrogenation of C–C and C–N bonds: A convenient approach to access diverse (dihydro)heteroaromatic compounds

• Santanu Hati,
• Ulrike Holzgrabe and
• Subhabrata Sen

Beilstein J. Org. Chem. 2017, 13, 1670–1692, doi:10.3762/bjoc.13.162

• SET mechanism to generate a radical cation B, followed by fragmentation to afford 12 (Scheme 3b). Either of these processes provided the imine moiety, along with o-iodosobenzoic acid (IBA, Scheme 3). In general, IBX-mediated oxidative dehydrogenation is conducted at high temperatures (>50 °C

Transition-metal-catalyzed synthesis of phenols and aryl thiols

• Yajun Liu,
• Shasha Liu and
• Yan Xiao

Beilstein J. Org. Chem. 2017, 13, 589–611, doi:10.3762/bjoc.13.58

• stoichiometric amount of Cu(OAc)2 and H2O in acetonitrile at 130 °C, and the resulting acetate gave phenols in moderate yields through a simple hydrolysis (Scheme 25). A mechanistic investigation revealed that the reaction proceeded via a radical-cation pathway. Notably, their protocol could be also applied in

Identification, synthesis and mass spectrometry of a macrolide from the African reed frog Hyperolius cinnamomeoventris

• Markus Menke,
• Pardha Saradhi Peram,
• Iris Starnberger,
• Walter Hödl,
• Gregory F.M. Jongsma,
• David C. Blackburn,
• Mark-Oliver Rödel,
• Miguel Vences and
• Stefan Schulz

Beilstein J. Org. Chem. 2016, 12, 2731–2738, doi:10.3762/bjoc.12.269

• , leading to the radical cation 18 along pathway a. An additional bond cleavage of a C–O single bond releases a neutral molecule, e.g., methylcyclohexane, giving rise to ion m/z 126 (20). High-resolution mass spectral data support the hypothesis because the ions m/z 126 as well as m/z 182 (25) formed from 2

Copper-catalyzed asymmetric sp³ C–H arylation of tetrahydroisoquinoline mediated by a visible light photoredox catalyst

• Pierre Querard,
• Inna Perepichka,
• Eli Zysman-Colman and
• Chao-Jun Li

Beilstein J. Org. Chem. 2016, 12, 2636–2643, doi:10.3762/bjoc.12.260

• (SET), reducing the iridium complex to Ir(II) III and oxidizing the the nitrogen of THIQ IV to its radical cation V, which then undergoes a hydride abstraction to form the iminium salt form VI, of the THIQ. The pre-formed chiral PhCu–PyBox complex [38], coordinates to the iminium cation VI, followed by

The EIMS fragmentation mechanisms of the sesquiterpenes corvol ethers A and B, epi-cubebol and isodauc-8-en-11-ol

• Patrick Rabe and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2016, 12, 1380–1394, doi:10.3762/bjoc.12.132

• fragment ion m/z = 43 from C3 and C15 (Scheme 4E). As shown by HRMS analysis, this fragment ion contains oxygen (measured: 43.0183, calculated for [C2H3O]+: 43.0179). The radical cation 3a+· can undergo two sequential or consecutive α-fragmentations to K3+·. A subsequent hydrogen rearrangement to L3+· and

• ). Electron impact ionisation at the oxygen lone pairs of 4 results in the radical cation 4a+· that can undergo one of two possible α-cleavages with loss of C12 or C13 to yield A4+. The alternative ionisation of 4 with loss of an electron from the olefinic double bond leads to 4b+· that may react by hydrogen

• radical cation K4+·. Another α-cleavage of the hydroxyisopropyl group results in L4+. Finally, the PMA59 demonstrates formation of this fragment ion from the hydroxyisopropyl group which is possible by a single α-cleavage from 4a+· to M4+ (Scheme 5F). Conclusion Isotopic labelling experiments continue to

Synthesis and fluorosolvatochromism of 3-arylnaphtho[1,2-b]quinolizinium derivatives

• Phil M. Pithan,
• David Decker,
• Manlio Sutero Sardo,
• Giampietro Viola and
• Heiko Ihmels

Beilstein J. Org. Chem. 2016, 12, 854–862, doi:10.3762/bjoc.12.84

• a charge shift (CS) to generate an intermediate excited species, that is a combination of charge neutral radical, i.e., a quinolizinyl radical, and the radical cation of the aromatic substituent (Scheme 3). The proposed photoinduced charge shift in derivatives 6b–e is supported by observations that

• relaxation, the counter anion migrates to the radical cation unit at the aryl substituent. At the same time, this mechanism implies that directly after the "back CS" the bromide anion is still located in the vicinity of the aryl substituent and therefore no longer compensated by a nearby cationic charge

Determination of formation constants and structural characterization of cyclodextrin inclusion complexes with two phenolic isomers: carvacrol and thymol

• Miriana Kfoury,
• David Landy,
• Steven Ruellan,
• Lizette Auezova,
• Hélène Greige-Gerges and
• Sophie Fourmentin

Beilstein J. Org. Chem. 2016, 12, 29–42, doi:10.3762/bjoc.12.5

• most energetically favorable conformation of inclusion complexes. Finally, the effect of encapsulation on the antioxidant properties of 1 and 2 was evaluated using the ABTS radical cation assay. Results and Discussion UV–visible competitive studies Stoichiometries and Kf values of inclusion complexes

• ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS•+) scavenging method was used to determine the radical scavenging potency of free and encapsulated 1 and 2. This method relies on the capacity of an antioxidant to scavenge and reduce ABTS•+ into its colorless reduced