Photocatalyzed syntheses of phenanthrenes and their aza-analogues. A review

• Alessandra Del Tito,
• Havall Othman Abdulla,
• Davide Ravelli,
• Stefano Protti and
• Maurizio Fagnoni

Beilstein J. Org. Chem. 2020, 16, 1476–1488, doi:10.3762/bjoc.16.123

• conditions finally afforded the desired phenanthridine 8.7 in 90% yield [69]. Carbon-based radicals could be likewise generated via a C–H hydrogen-atom transfer path. As an example, ethers were used as hydrogen donors and underwent a C–H cleavage step promoted by a photogenerated tert-butoxyl radical. The so

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

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

• Yeersen Patehebieke School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China 10.3762/bjoc.16.118 Abstract Disulfides are versatile catalysts. They can be photocatalysts, hydrogen atom transfer (HAT) catalysts, cocatalysts, or initiators in photocatalytic reactions

Oxime radicals: generation, properties and application in organic synthesis

• Igor B. Krylov,
• Stanislav A. Paveliev,
• Alexander S. Budnikov and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2020, 16, 1234–1276, doi:10.3762/bjoc.16.107

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

• combination of eosin Y (OD13) with a sacrificial electron donor can trigger the reductive debromination of several α-carbonyl halides [53]. Riboflavin (OD11) [54] and thiaporphyrin [55] have been applied as well as organic photocatalysts for similar reductive dehalogenations. Hydrogen atom transfer

• Photocatalytic hydrogen atom transfer (HAT) represents a valuable strategy for accessing C(sp3) radicals. This method allows the direct cleavage of a C–H bond and the consequent generation of alkyl radicals without relying on the presence of redox-active functional groups. This results in a superior atom economy

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

• UV as the light source and a nickel catalyst [60]. The authors suggested that the product 172, a substituted benzophenone, could act as the photocatalyst and the hydrogen atom transfer agent in this reaction (Scheme 40). They optimized the reaction conditions with regard to the nickel catalyst, the

Visible-light-induced addition of carboxymethanide to styrene from monochloroacetic acid

• Kaj M. van Vliet,
• Nicole S. van Leeuwen,
• Albert M. Brouwer and
• Bas de Bruin

Beilstein J. Org. Chem. 2020, 16, 398–408, doi:10.3762/bjoc.16.38

• . Single-electron oxidation can produce carbon-centered radicals from substrates such as amines [17], alkenes [18], and carboxylates [19], or by hydrogen-atom transfer to an oxidatively formed thiolate radical [20]. Single-electron reduction produces carbon-centered radicals from, for example, aryl

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

• functionalizations. Herein, we will broadly discuss the different catalytic systems that facilitate ortho and para C–H functionalization by utilization of effective and feasible photoredox catalysts (with the aid of transition metals), hydrogen atom transfer, and aerobic oxidation. Over the last two decades, direct

• . This system is being used in C–C, C–N, C–O, C–S, and C–X bond-breaking and -forming processes [55]. Similarly, photoinduced direct hydrogen atom transfer (HAT) catalysis also plays a major role in the functionalization of intricate molecules. Photocatalysts that can undergo this process are uranyl

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

• co-workers [117] explored a simple and facile method to access δ-trifluoromethylated carboxamides and sulfonamides through a copper-catalyzed 1,5-hydrogen atom transfer (Scheme 58). Other catalysts: In 2013, Gouverneur and co-workers [118] described a photoredox-based catalytic approach to afford

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

• 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

Cobalt bis(acetylacetonate)–tert-butyl hydroperoxide–triethylsilane: a general reagent combination for the Markovnikov-selective hydrofunctionalization of alkenes by hydrogen atom transfer

• Xiaoshen Ma and
• Seth B. Herzon

Beilstein J. Org. Chem. 2018, 14, 2259–2265, doi:10.3762/bjoc.14.201

• reagent combination suggests it as a useful starting point to develop HAT reactions in complex settings. Keywords: HAT; hydrogen atom transfer; hydrofunctionalization; Introduction Many powerful methods to effect alkene hydrogenation [1][2][3][4] and Markovnikov-selective hydroheterofunctionalization (H

• –X addition, X = O [5][6][7][8][9], I [3], Br [3], Se [3], S [8][9][10], Cl [8][11], F [12][13], and N [8][14][15][16][17]) by metal-mediated hydrogen atom transfer (HAT) [18][19][20][21] are now known. Additionally, methods to achieve carbon–carbon bond formation to alkenes by HAT have been

• ] to afford the corresponding products 4g and 4h in 96% and 89% yields, respectively. Carreira and co-workers reported the hydroazidation of alkenes using cobalt–salen complexes as hydrogen atom transfer agents and para-toluenesulfonyl azide as an azide source [16][48][55]. After careful optimization

Visible light-mediated difluoroalkylation of electron-deficient alkenes

• Vyacheslav I. Supranovich,
• Vitalij V. Levin,
• Marina I. Struchkova,
• Jinbo Hu and
• Alexander D. Dilman

Beilstein J. Org. Chem. 2018, 14, 1637–1641, doi:10.3762/bjoc.14.139

• with blue light is described. The reaction involves radical addition of 1,1-difluorinated radicals at the double bond followed by hydrogen atom transfer from sodium cyanoborohydride. Keywords: difluoroalkylation; organofluorine compounds; radical addition; visible light; Introduction Applications of

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

• radical. Yadav and co-workers presented a metal-free radical thiol–ene approach, using benzophenone as photoredox catalyst (Scheme 13) [43]. No sacrificial oxidant is required for this reaction as benzophenone is regenerated by hydrogen atom transfer to the anti-Markovnikov radical intermediate. Aliphatic

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

• concert for the hydrogen atom transfer with complete suppression of the oxidised trifluoromethylated byproducts. The method was regioselective for mono-, di-, and trisubstituted aliphatic alkenes and styrenyl alkenes with a broad substrate scope (Scheme 19). As an exception, 1,2-disubstituted alkenes and

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

• -butoxide anion to CBrCl3, forming the tert-butoxy radical [94]. This radical attacks the α-hydrogen of 2a via hydrogen atom transfer (HAT), to form intermediate A with a radical at the α-carbon. A then undergoes α-bromination to form the intermediate 6 [95]. Attack at the α-carbon of 6 by 2

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

• nitrile carbon to form the nitrogen centered radical 77. β-Fragmentation leads to NHC-boryl nitrile 74 and a carbon centered radical. A hydrogen atom transfer reaction between the electrophilic α-cyano radical and the nucleophile NHC-borane achieves the chain propagation (Scheme 24). The isolation of the

Solvent-free, visible-light photocatalytic alcohol oxidations applying an organic photocatalyst

• Martin Obst and
• Burkhard König

Beilstein J. Org. Chem. 2016, 12, 2358–2363, doi:10.3762/bjoc.12.229

• solvent like hydrogen-atom transfer or the formation of byproducts could be excluded. For liquid substrates, some examples for photocatalytic, solvent-free conversions are reported, such as the oxidation of benzyl alcohol to benzaldehyde [4] and the oxidation of benzenes to phenols [5]. Another field of

Conjugate addition–enantioselective protonation reactions

• James P. Phelan and
• Jonathan A. Ellman

Beilstein J. Org. Chem. 2016, 12, 1203–1228, doi:10.3762/bjoc.12.116

• enantioselective hydrogen atom transfer is an approach that provides products analogous to those accessed via conjugate addition–enantioselective protonation sequences. Due to the similar challenges when delivering both protons and hydrogen atoms enantioselectively, examples of conjugate addition–enantioselective

• hydrogen atom transfer are also included in this review. Review α,β-Unsaturated esters Lewis acids α,β-Unsaturated esters have been the most extensively studied class of electrophiles for conjugate addition–enantioselective protonation sequences. Esters are useful functional group handles for additional

• yield, with the best enantioselectivities being observed for α-arylacrylates (94:6 to 96:4 er). For the synthesis of enantioenriched α-amino esters 7a, β-amino esters 7b and 2-hydroxymethyl esters 7c, the Sibi group has utilized a conjugate addition–enantioselective hydrogen atom transfer reaction

Pyridine-promoted dediazoniation of aryldiazonium tetrafluoroborates: Application to the synthesis of SF₅-substituted phenylboronic esters and iodobenzenes

• George Iakobson,
• Junyi Du,
• Alexandra M. Z. Slawin and
• Petr Beier

Beilstein J. Org. Chem. 2015, 11, 1494–1502, doi:10.3762/bjoc.11.162

• configuration) and 6i in 16:31:28:25 GC–MS ratio. The presence of these products can be explained only by the formation of the substituted SF5-phenyl radical which undergoes borylation to 2i, hydrogen atom transfer followed by borylation to 2i’, intramolecular cyclization to 5i or hydrogen abstraction to 6i

• -trimethylpyridine) gave similar yields. Unlike borylations, the iodination reactions were not sensitive to ortho substitutions. In the case of 3i, compound 10i was the sole product; no product of hydrogen atom transfer or cyclization was observed, demonstrating that the reaction with I2 is much faster than with

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

• up does not exert any influence on the antioxidant efficiency and reactivity of the two dimers. The presence of the keto–enol moiety is not of significance for the hydrogen-atom-transfer (HAT) reactions and the classical chain-breaking antioxidant activity. The presence of two longer unsaturated keto

Metal-free aerobic oxidations mediated by N-hydroxyphthalimide. A concise review

• Lucio Melone and
• Carlo Punta

Beilstein J. Org. Chem. 2013, 9, 1296–1310, doi:10.3762/bjoc.9.146

• laccase-ABTS follows an electron transfer (ET) mechanism, NHPI, VLA, HBT, and NHAmediators promote a hydrogen atom transfer (HAT) route through the formation of the corresponding N-oxyl radicals as NHDs-Medox species (Scheme 11). The same research group also emphasized the specialization of mediators

• -electron-transfer interaction of antraquinone (AQ) with NHPI in zeolite HY, followed by hydrogen-atom transfer, successfully led to the formation of the PINO radical, which in turn was responsible for the propagation of the radical chain in the selective oxidation of ethylbenzene to the corresponding

Mitomycins syntheses: a recent update

• Jean-Christophe Andrez

Beilstein J. Org. Chem. 2009, 5, No. 33, doi:10.3762/bjoc.5.33

• ) [114]. 5.4. Parson. Radical cyclization The development of novel cascade (or domino) radical reactions is an active area of current research, and one approach to the mitomycin ring system focused on the application of 1-6-hydrogen atom transfer to create a pyrrolidinone radical, which could then

Radical cascades using enantioenriched 7-azabenzonorbornenes and their applications in synthesis

• David M. Hodgson and
• Leonard H. Winning

Beilstein J. Org. Chem. 2008, 4, No. 38, doi:10.3762/bjoc.4.38

• electrophile trapping with the sulfone corresponds approximately to the rate of hydrogen atom transfer from (TMS)3SiH. Attempted reaction with methyl vinyl ketone, N,N-dimethylacrylamide and methyl propiolate gave rearranged-reduced azacycle 8 in modest yields, as well as small quantities of hydrosilylated

Part 2. Mechanistic aspects of the reduction of S-alkyl- thionocarbonates in the presence of triethylborane and air

• Florent Allais,
• Jean Boivin and
• Van Tai Nguyen

Beilstein J. Org. Chem. 2007, 3, No. 46, doi:10.1186/1860-5397-3-46

• deuterium transfer is observed from either compounds 3, 4, or 5 when xanthate 1a was subjected to reduction (Method A) in the presence of one equivalent of O-ethyl-S-ethyl dithiocarbonate, using benzene as solvent to suppress any hydrogen atom transfer from the solvent. Along the same lines, we prepared

Part 1. Reduction of S-alkyl- thionocarbonates and related compounds in the presence of trialkylboranes/air

• Jean Boivin and
• Van Tai Nguyen

Beilstein J. Org. Chem. 2007, 3, No. 45, doi:10.1186/1860-5397-3-45

• following observations: – the attack of a carbon radical on the sulfur atom of a thiocarbonyl function is known to be a fast process; – the hydrogen atom transfer in the system Et3B/air is a relatively slow process as demonstrated above. In the case of S-alkylxanthates a, the production of radical R

• transfer is slow, the radical may easily react with a or e [equation (2)], thus establishing a degenerate process that does not consume the starting material a, preserves the radical character and allows slow hydrogen atom transfer to occur. For O-alkylxanthates, the situation is different. The reaction of

• Et• with f gives rise to h and R• [equation (3)]. If the hydrogen atom transfer is slow, the latter reacts with f to afford the rearranged S-alkyl S-methyldithiocarbonate j. As R• radical cannot react with the carbonyl function of j, the starting material is progressively consumed by the unwanted

Do α-acyloxy and α-alkoxycarbonyloxy radicals fragment to form acyl and alkoxycarbonyl radicals?

• Dennis P. Curran and
• Tiffany R. Turner

Beilstein J. Org. Chem. 2006, 2, No. 10, doi:10.1186/1860-5397-2-10

• atom transfer (Figure 2). The resulting radical 5 rebounds back to the enol ether in a 1,5-cyclization to provide 6. In the crucial self-terminating step, radical 6 is suggested to fragment to product 2 and radical X•. Related steps are involved in the formation of ketone 3 (not shown), except that the

• . Representative reagents, reactions conditions and product yields for this very general transformation are shown in Figure 1. The suggested mechanism for formation of 2 involves addition of an oxygen-centered radical (XO•) to 1 to generate vinyl radical 4, followed by rapid radical translocation by 1,5-hydrogen