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

• them to propose that Selectfluor® is not only a fluorine source but that its radical cation participates as a HAT agent. When they substituted AQN (T1 = 61.9 kcal⋅mol−1) for 9-fluorenone (T1 = 50.3 kcal⋅mol−1) or alizarin red S (T1 = 34.0 kcal⋅mol−1), insignificant amounts of the fluorinated amyl

• ) compared to singlet Selectfluor® (N–F = 1.37 Å, Figure 9). After the immediate dissociation of triplet Selectfluor®, the formed Selectfluor® N-radical cation undergoes HAT with the substrate to afford an alkyl radical. The authors deemed a complex between AQN and fluorine (AQN + F) more plausible than the

• formation of fluorine radicals. The generated alkyl radical could abstract fluorine atoms either from i) the AQN–F complex to regenerate AQN or ii) Selectfluor® to regenerate the Selectfluor® radical cation and thereby propagate a chain reaction. Following shortly after Tan’s report, Chen and co-workers

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

• single electron-transfer process (Figure 36). In the first place, a bidentate chelated species is formed by the coordination of 1-naphthylamine derivatives with copper salt. The subsequent oxidation of this intermediate with potassium persulfate produces a Cu(III) species, furnishing a radical cation on

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

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

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

• nonpolar solvent, PhSSPh accelerates the [4 + 2] cycloaddition of the radical cation 19, but the electron-relay catalyst promotes the [2 + 2] cycloaddition. The radical cation 19 can undergo two different types of cyclizations, subject to the relative reactivity of its radical and cation center. The α

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

• ] to the imine 1a, where the radical cation A donates a proton to 1a to form the α-amino radical intermediates B and C, which undergo cross-coupling to give the desired unsymmetrical vicinal diamine 2a. On the other hand, in CH3OH, 1a preferentially abstracts a proton from CH3OH rather than from 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

• , exploiting the reactivity of alkene radical cations generated using organic dyes [96][97]. Their seminal work reported the oxidation of the alkenols 22.1 by the Fukuzumi dye (OD2, Scheme 22) [98]. The so-formed radical cation undergoes an intramolecular nucleophilic 5/6/7-exo-trig-cyclization to give the

• cyclic ethers 22.3. Mes-Acr-Me+ (OD2) is a strong enough oxidant (E(PC+*/PC) ≈ 2.1 V), allowing the oxidation of unactivated alkenes (1.2 ≤ Eox ≤ 1.9 V). In this transformation, the cocatalyst 22.2 acts as an H atom shuttle. This alkene radical cation-based strategy has been extended to various

• oxidation of arenes under relatively strong oxidative conditions (Eox > +1.0 V). Following up their work on alkene oxidations, Nicewicz and co-workers have developed several strategies for arene functionalizations through arene radical cation intermediates. Their work relies on the careful tuning of the Mes

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

• (50), generating a benzaldehyde radical cation (56) and the radical of the alkyl group C6F13, 58, which was proposed to initiate the radical polymerization of a methacrylate monomer 59. N,N-Dimethylaniline (51) acted as the reducing agent, regenerating the ground state of the benzaldehyde catalysts 52

Recent advances in Cu-catalyzed C(sp³)–Si and C(sp³)–B bond formation

• Balaram S. Takale,
• Ruchita R. Thakore,
• Elham Etemadi-Davan and
• Bruce H. Lipshutz

Beilstein J. Org. Chem. 2020, 16, 691–737, doi:10.3762/bjoc.16.67

• leading to an electron transfer to the iodine atom, thereby liberating iodide, an alkyl radical, and a radical cation of the Cu complex. Recombination of the latter radicals leads to the formation of the desired silane along with the regeneration of the active Cu species (Scheme 8). This strategy was also

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

• -a]pyrrolo[3,4-c]quinoline derivatives were obtained for the first time. This transformation starts with the oxidation of the excited photocatalyst with O2. The aniline is then oxidized into an N-centered radical cation, which further gives the α-amino radical. The latter reacts with the maleimide to

• oxidized copper complex oxidized the glycine ester, regenerating the catalyst, furnishing the N-centered radical cation. Then, the latter underwent a 1,2-hydride shift in the presence of the base (or the phthalimide anion) to form the α-amino radical that recombined with the alkyl radical formed in 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

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

• catalyst for the oxidation of the arenes. The reaction is initiated by the oxidation of 100 through the excited photocatalyst to generate the arene radical cation 102. Here, P(OEt)3 acts as a nucleophile, capturing the radical cation of 102 and generating 103. Concomitant to the reduction of the Co(III

• , the reaction is initiate with the excitation of the photocatalyst, which further oxidizes the aniline derivative 106 to generate the arene radical cation 108. Then, the intermediate 109 is formed by deprotonation, which, upon reaction with a nitrate radical, gives the desired product 107. Aryl C–H

• plausible reaction mechanism involves the excitation of the photocatalyst by blue light, oxidizing 151 to 153, a radical cation. The nucleophilic attack by TMSCN gives cyclohexadienyl radical 154, which is oxidized by molecular oxygen to give the desired product 152 (Figure 25). Conclusion C–H bond

Regioselectivity of glycosylation reactions of galactose acceptors: an experimental and theoretical study

• Enrique A. Del Vigo,
• Carlos A. Stortz and
• Carla Marino

Beilstein J. Org. Chem. 2019, 15, 2982–2989, doi:10.3762/bjoc.15.294

• between the ground-state molecule and the radical cation (fa) [41], a direct calculation of the frontier molecular orbitals (fb) [42] was carried out. For simplicity, analogs of acceptors 1α/β and 2α/β, where benzoyl and benzyl groups were replaced by acetyl and methyl moieties, respectively, were used

Synthesis and optoelectronic properties of benzoquinone-based donor–acceptor compounds

• Daniel R. Sutherland,
• Nidhi Sharma,
• Georgina M. Rosair,
• Ifor D. W. Samuel,
• Ai-Lan Lee and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2019, 15, 2914–2921, doi:10.3762/bjoc.15.285

• derivatives (3 and 4) showed irreversible oxidation waves, which is a function of the electrochemically unstable carbazole-based radical cation that can subsequently undergo dimerization [34]. The oxidation waves shifted cathodically upon increasing the donor strength from carbazole (3 and 4) to diphenylamine

A review of asymmetric synthetic organic electrochemistry and electrocatalysis: concepts, applications, recent developments and future directions

• Munmun Ghosh,
• Valmik S. Shinde and
• Magnus Rueping

Beilstein J. Org. Chem. 2019, 15, 2710–2746, doi:10.3762/bjoc.15.264

• resulted in the corresponding coupling products 108 in moderate yields and good enantioselectivities. After detailed electrochemical analysis, the authors proposed that the reaction proceeds through the intermediacy of radical cation 111, generated via anodic oxidation of enamine 110 (Scheme 37). The same

• 115 with moderate enantioselectivity (Scheme 38). As shown in Scheme 39, the mechanism involved initial formation of radical cation 117 via anodic oxidation of enamine 116 (obtained from the condensation of 114 and 105'), which then coupled with xanthene radical 119 (Scheme 39). Finally, hydrolysis of

Arylisoquinoline-derived organoboron dyes with a triaryl skeleton show dual fluorescence

• Vânia F. Pais,
• Tristan Neumann,
• Ignacio Vayá,
• M. Consuelo Jiménez,
• Abel Ros and
• Uwe Pischel

Beilstein J. Org. Chem. 2019, 15, 2612–2622, doi:10.3762/bjoc.15.254

• dye 18 is insensitive to oxygen and was tentatively attributed to the formation of a pyrene-based radical cation, resulting from photoionization [46]. Interaction with fluoride anions The presence of the boronic acid ester moiety does not only contribute to significant changes in the fluorescence

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

• substrates [11][12][13][14][15], or vinylcarbazole or alkoxystyrene derivatives for radical cation cylcloaddition and polymerization reactions [16][17][18][19][20]). We thus proposed the replacement of the naphthalene in 1a with a more electron-deficient quinoline ring. Due to the saturated spirocyclic

Recent advances in transition-metal-catalyzed incorporation of fluorine-containing groups

• Xiaowei Li,
• Xiaolin Shi,
• Xiangqian Li and
• Dayong Shi

Beilstein J. Org. Chem. 2019, 15, 2213–2270, doi:10.3762/bjoc.15.218

• has been proposed. First, a bis(terpyridyl)Pd(II) complex B is oxidized by Selectfluor with turnover-limiting to obtain Pd(III) C and a Selectfluor radical cation. Then, a transfer of a F· radical from the Selectfluor radical cation to an aryl trifluoroborate occurs, forming the C−F bond and producing

1,2,3-Triazolium macrocycles in supramolecular chemistry

• Mastaneh Safarnejad Shad,
• Pulikkal Veettil Santhini and
• Wim Dehaen

Beilstein J. Org. Chem. 2019, 15, 2142–2155, doi:10.3762/bjoc.15.211

• ·TCNQ]2+ complex by pre-reduction to a [TCNQ]−· anion radical in situ, and encapsulation in the host, photochemical excitation of the porphyrin unit in the reactor, photoinduced electron transfer from the excited porphyrin unit to a neutral TCNQ, giving rise to a porphyrin radical cation and a second

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