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

• . Photoredox-active metal complexes or organic dyes are used to initiate photo-induced single-electron transfer (SET) processes upon excitation with visible-light. Such photooxidations or photoreductions yield reactive organic radicals, which can undergo unique bond forming reactions, under very mild

• alternative reductive pathway, where photoexcited [Ru(bpy)3]2+* first oxidizes the sulfur anion by single-electron transfer and is re-oxidized by dioxygen could not be excluded. Lei and co-workers reported an external oxidant-free photocatalyzed procedure for the same reaction, also applying [Ru(bpy)3](PF6)2

• from alkyl and aryl thiosulfates and aryl diazonium salts (Scheme 39) [74]. They confirmed by transient absorption spectroscopy that a single-electron transfer occurs between [Ru(bpy)3]Cl2 and the aryl diazonium salt. Additionally, electron paramagnetic resonance studies showed that K2CO3 interacts

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

• single-electron transfer (SET) between CF3SO2Cl and the association CuBr/chiral phosphoric acid. In the process, SO2 and HCl were released, but the latter was scavenged by Ag2CO3, minimising its impact on the reaction process by notably avoiding hydroamination side reactions. The trifluoromethyl radical

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

• CF3SO2Na in the presence of copper(I), reacted at the more electron-rich carbon atom of the C=C double bond to give the radical species 5 that was oxidised by copper(II) into the corresponding cationic intermediate 6 via a single electron transfer (SET). Finally, the acetyl cation was eliminated to provide

• -products. The mechanism was similar to previous examples to generate the β-CF3 alkyl radical intermediate 44, which was trapped by halogen atom transfer from the halogenating agent. The nitrogen-centered radical 45 oxidised Mes-Acr* by a single-electron-transfer process to restart the catalytic cycle

• were compatible with the reaction conditions. A series of control experiments that included the inhibition of the reaction in the presence of TEMPO, deuteration and isotope effect experiments were carried out and led the authors to propose the single-electron transfer mechanism presented in Scheme 44

Reagent-controlled regiodivergent intermolecular cyclization of 2-aminobenzothiazoles with β-ketoesters and β-ketoamides

• Irwan Iskandar Roslan,
• Kian-Hong Ng,
• Gaik-Khuan Chuah and
• Stephan Jaenicke

Beilstein J. Org. Chem. 2017, 13, 2739–2750, doi:10.3762/bjoc.13.270

• silylation [72]. In these reactions, the single electron transfer (SET) is initiated by KOt-Bu/DMF [63][67][69][71] or KOt-Bu in combination with additives such as bidentate diamine ligands [61][62][63][64][65], 18-crown-6 [70] or azobisisobutyronitrile (AIBN) [62][66]. Herein, we report the synthesis of

• with 6 (Scheme 4d). After 16 h, 93% of 3a was formed, showing unambiguously that the reaction proceeds via 6 as an intermediate. We propose that KOt-Bu assists in α-bromination of 2a to form the intermediate 6 via single electron transfer (SET). Recent mechanistic work by Murphy and co-workers showed

Dialkyl dicyanofumarates and dicyanomaleates as versatile building blocks for synthetic organic chemistry and mechanistic studies

• Grzegorz Mlostoń and
• Heinz Heimgartner

Beilstein J. Org. Chem. 2017, 13, 2235–2251, doi:10.3762/bjoc.13.221

• disulfides and diselenides is explained as a redox process through a single-electron transfer (SET) mechanism with E-1a as the oxidizing reagent, which converted into a mixture of diastereoisomeric diethyl dicyanosuccinates 96 [72]. Very likely, an analogous SET mechanism governs also the reaction of E-1

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

• -workers in the year 2000 [26][27][28][29]. It oxidizes diverse functionalities such as amines, imines, alcohols etc. [30]. Later, it was demonstrated that IBX can also be used as a reagent for oxidative dehydrogenation of benzylic carbons in various aromatic systems via single electron transfer (SET) and

• with the reaction of DDQ on the C4 H-atom of 27 and 28, respectively, to form the transition state E′ (path a). Alternatively it can also attack the H-atom at C5 to generate C′ (path b). A single electron transfer and subsequent H-abstraction on E′ or C′ lead to the formation of F′ or D′, respectively

• isoquinoline 82 from diverse 1,2,3,4-tetrahydroquinolines 81 in moderate to good yield (Scheme 29). Ferrous chloride (FeCl2) acted as a single electron transfer agent in the presence of DMSO to facilitate the reaction. Metal-catalyzed oxidant induced dehydrogenation Our next discussion involved the oxidative

Kinetic analysis of mechanoradical formation during the mechanolysis of dextran and glycogen

• Naoki Doi,
• Yasushi Sasai,
• Yukinori Yamauchi,
• Tetsuo Adachi,
• Masayuki Kuzuya and
• Shin-ichi Kondo

Beilstein J. Org. Chem. 2017, 13, 1174–1183, doi:10.3762/bjoc.13.116

• microcrystalline cellulose powder through mechanocation polymerization with isobutyl vinyl ether in vacuum at 77 K [24]. The aforementioned mechanoanion was confirmed through tetracyanoethylene (TCNE) radical anion formation. The latter radical is produced by a single-electron transfer from the mechanoanion to

The reductive decyanation reaction: an overview and recent developments

• Jean-Marc R. Mattalia

Beilstein J. Org. Chem. 2017, 13, 267–284, doi:10.3762/bjoc.13.30

• reduction of radical probes (no rearranged products formed from 25d,l,m) and deuterium-labelling experiments (no deuterium incorporation using THF-d8 and quenching with D2O) discard the possibility of a single-electron transfer pathway. Other reductions suggest a hydride addition with formation of an iminyl

• intermediate 64 via a β-hydride elimination from 63. A single-electron transfer (SET) to the nitrile oxidizes the complex at the metal center into 65 and generates an aryl radical. The electron can also be located in the ligand (non-innocent ligand, not represented in Scheme 21). Then, elimination of MgXCN and

• NHC-boryl nitrile 74, EPR spectroscopy observations [122], and polar effects fit with this proposition. Neutral organic electron donors Powerful single-electron transfer reagents have been described. Kang et al. reported the decyanation of both malononitriles and α-cyanoesters using samarium(II

Direct arylation catalysis with chloro[8-(dimesitylboryl)quinoline-κN]copper(I)

• Sem Raj Tamang and
• James D. Hoefelmeyer

Beilstein J. Org. Chem. 2016, 12, 2757–2762, doi:10.3762/bjoc.12.272

• reactions and, more specifically, direct arylation have been the subject of intense interest. Mechanistic models appear to diverge along those favoring oxidative addition/reductive elimination via Cu(I)/Cu(III) versus proposals favoring a single electron transfer (SET) pathway [55][56]. In the base-promoted

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

• , entries 7 and 8). A tentative reaction mechanism has been proposed in Scheme 3, in order to rationalize this arylation reaction. Upon visible light irradiation, [Ir(ppy)2(dtbbpy)]PF6 I was converted into an excited state II, Ir(III)* [11][33][34][35][36][37]. The THIQ undergoes a single electron transfer

Cp₂TiCl/D₂O/Mn, a formidable reagent for the deuteration of organic compounds

• Antonio Rosales and
• Ignacio Rodríguez-García

Beilstein J. Org. Chem. 2016, 12, 1585–1589, doi:10.3762/bjoc.12.154

• complexes catalyse α-deuteration of amines and alcohols [13] and palladium complexes catalyse the ortho-selective deuteration of arenes [14]. Also, SmI2/D2O-mediated the chemoselective synthesis of α,α-dideuterio alcohols directly from carboxylic acid under single-electron-transfer conditions [15]. However

• water. This complex is a single electron transfer system (SET) that has an unpaired d-electron and a vacant site, allowing heteroatoms with free valence electrons to coordinate and undergo electron transfer through an inner-sphere mechanism to generate carbon radicals or intermediate titanaoxiranes

• -ordination of water to Cp2TiCl might weakens the strength of the O–H bond. In this way a single electron transfer from titanium to oxygen might facilitate the HAT from the titanocene aqua-complex to the free radicals. Theoretical calculations supported that the coordination of water to Cp2TiIIICl weakens the

NeoPHOX – a structurally tunable ligand system for asymmetric catalysis

• Jaroslav Padevět,
• Marcus G. Schrems,
• Robin Scheil and
• Andreas Pfaltz

Beilstein J. Org. Chem. 2016, 12, 1185–1195, doi:10.3762/bjoc.12.114

• reaction indeed took place. While for iodides evidence for a radical single electron transfer process as the major pathway was found, the corresponding bromides and chlorides seemed to prefer an SN2 mechanism. Rossi et al. [21] showed that substitution products can be obtained in good yields from neopentyl

Synthesis of 2-oxindoles via 'transition-metal-free' intramolecular dehydrogenative coupling (IDC) of sp² C–H and sp³ C–H bonds

• Nivesh Kumar,
• Santanu Ghosh,
• Subhajit Bhunia and
• Alakesh Bisai

Beilstein J. Org. Chem. 2016, 12, 1153–1169, doi:10.3762/bjoc.12.111

• bond [27][28][29][30], single electron transfer (SET) to a α-halo anilides followed by halide elimination [31][32], and the formation of an aryl radical followed by a 1,5-hydrogen atom translocation [33][34]. Out of these strategies, the initial two require specifically functionalized precursors such

• skeleton such as calycanthine (22c) (Scheme 5) [78]. In all the cases, IDC was feasible with substrates having substituents at the carbon atom α- to the amides. This gave a clue for a radical-mediated process where a single electron transfer (SET) mechanism might be operating. A tentative mechanism has

• –39% yield along with 43–52% of recovered starting material (Scheme 8) and no trace of C-iodide 24 was observed. These results suggest that, NIS and ICl also acts as oxidants and helping in a single electron transfer (SET) in the oxidative coupling reaction [87][88]. It is also well evident in the

Recent developments in copper-catalyzed radical alkylations of electron-rich π-systems

• Kirk W. Shimkin and
• Donald A. Watson

Beilstein J. Org. Chem. 2015, 11, 2278–2288, doi:10.3762/bjoc.11.248

• catalyst. The resultant stabilized alkyl radical then undergoes coupling with a nitronate anion, forging the C–C bond. Single electron transfer from the resultant radical anion to the Cu(II) halide results in the observed product while simultaneously reducing the metal center to regenerate the catalyst. In

Photoinduced 1,2,3,4-tetrahydropyridine ring conversions

• Baiba Turovska,
• Henning Lund,
• Viesturs Lūsis,
• Anna Lielpētere,
• Edvards Liepiņš,
• Sergejs Beljakovs,
• Inguna Goba and
• Jānis Stradiņš

Beilstein J. Org. Chem. 2015, 11, 2166–2170, doi:10.3762/bjoc.11.234

• the formation of the singlet state of 1*, is followed by single-electron transfer from 1* to 3O2 generating 1+• and O2− • in solution. Such reactions between strong nucleophiles and strong electrophiles, especially the annihilation reactions between ion radicals, have not been studied extensively

C–H bond halogenation catalyzed or mediated by copper: an overview

• Wenyan Hao and
• Yunyun Liu

Beilstein J. Org. Chem. 2015, 11, 2132–2144, doi:10.3762/bjoc.11.230

• roles of both reaction medium and halogen source. Notably, attempts in the chlorination of the alkene C–H bond under identical atmosphere were not successful. In the reaction process, a single electron transfer (SET) from the aryl ring to the coordinated Cu(II) complex 3 to the Cu(I) species 4 was the

• and 2-phenyl-1,3,4-oxadiazole were smoothly iodinated to provide iodoheteroarenes 18 (Scheme 10) [45]. As typical electron-enriched arenes, phenols and analogous arenes tend to undergo a single-electron transfer process [46], the property of these arenes also resulted in sound attention to their C–H

Metal-free one-pot synthesis of 2-substituted and 2,3-disubstituted morpholines from aziridines

• Hongnan Sun,
• Binbin Huang,
• Run Lin,
• Chao Yang and
• Wujiong Xia

Beilstein J. Org. Chem. 2015, 11, 524–529, doi:10.3762/bjoc.11.59

• mechanism was proposed as shown in Scheme 4. Initially, aziridine 1a might participate in single-electron transfer (SET) with the persulfate anion to render the radical cation A [32][34]. Concerted ring opening and nucleophilic addition leads to amino radical intermediate B, which is converted to the

Eosin Y-catalyzed visible-light-mediated aerobic oxidative cyclization of N,N-dimethylanilines with maleimides

• Zhongwei Liang,
• Song Xu,
• Wenyan Tian and
• Ronghua Zhang

Beilstein J. Org. Chem. 2015, 11, 425–430, doi:10.3762/bjoc.11.48

• undergo an oxidative or reductive quenching cycle [48][49][50]. In this mechanism, a single electron transfer (SET) from 1 to 3EY* generates the amine radical cation 4, and at the same time, 3EY* is reduced to the EY•−. In the presence of oxygen, the photoredox catalytic cycle of EY is finished via a SET

Cross-dehydrogenative coupling for the intermolecular C–O bond formation

• Igor B. Krylov,
• Vera A. Vil’ and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2015, 11, 92–146, doi:10.3762/bjoc.11.13

Come-back of phenanthridine and phenanthridinium derivatives in the 21st century

• Lidija-Marija Tumir,
• Marijana Radić Stojković and
• Ivo Piantanida

Beilstein J. Org. Chem. 2014, 10, 2930–2954, doi:10.3762/bjoc.10.312

• phenanthridines in excellent yields (>92%). The radical inhibitor 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) was applied to stop the transformation by a single electron transfer process. Intriguing combination of irradiation techniques (combined microwave-assisted and photochemical) offered a new route toward

Cyclization–endoperoxidation cascade reactions of dienes mediated by a pyrylium photoredox catalyst

• Nathan J. Gesmundo and
• David A. Nicewicz

Beilstein J. Org. Chem. 2014, 10, 1272–1281, doi:10.3762/bjoc.10.128

• intermediates are trapped by O2 and furnish the shown bicyclic products after reduction. Diversification strategy for endoperoxide synthesis by single electron transfer. E*red vs SCE [20]. Proposed mechanism for endoperoxide synthesis from tethered dienes. Competing formal [3,3] pathway. Reaction Optimization

Direct C–H trifluoromethylation of di- and trisubstituted alkenes by photoredox catalysis

• Ren Tomita,
• Yusuke Yasu,
• Takashi Koike and
• Munetaka Akita

Beilstein J. Org. Chem. 2014, 10, 1099–1106, doi:10.3762/bjoc.10.108

• visible-light-induced single-electron transfer (SET) processes [24][25][26][27][28][29][30][31][32]. On the other hand, we have intensively developed trifluoromethylations of olefins by the Ru and Ir photoredox catalysis using easy-handling and shelf-stable electrophilic trifluoromethylating reagents [33

On the mechanism of photocatalytic reactions with eosin Y

• Michal Majek,
• Fabiana Filace and
• Axel Jacobi von Wangelin

Beilstein J. Org. Chem. 2014, 10, 981–989, doi:10.3762/bjoc.10.97

• Information File 1). Much effort has been directed at the oxidative quenching of eosin Y* (T1) with suitable electrophiles in order to generate aryl radicals by a light-driven single-electron transfer (SET) process (i.e., one-electron reduction of Ar–X, see Scheme 1). Due to their easy reducibility

Visible-light photoredox catalysis enabled bromination of phenols and alkenes

• Yating Zhao,
• Zhe Li,
• Chao Yang,
• Run Lin and
• Wujiong Xia

Beilstein J. Org. Chem. 2014, 10, 622–627, doi:10.3762/bjoc.10.53

• . Recently, an intriguing and promising strategy for the application of photoredox catalysts to initiate single electron transfer processes have been developed [16][17][18][19][20][21][22]. Since the pioneering work from the groups of MacMillan [23][24][25], Stephenson [26][27][28], Yoon [29][30][31] and

Direct and indirect single electron transfer (SET)-photochemical approaches for the preparation of novel phthalimide and naphthalimide-based lariat-type crown ethers

• Dae Won Cho,
• Patrick S. Mariano and
• Ung Chan Yoon

Beilstein J. Org. Chem. 2014, 10, 514–527, doi:10.3762/bjoc.10.47

• imides; Review Since the 1970s, studies by a number of organic photochemists have demonstrated that a large number of unique single electron transfer (SET) mechanistic pathways are followed in photochemical reactions [1][2][3][4][5][6][7][8][9][10]. Owing to its potentially large energetic driving force