Organometallic vs organic photoredox catalysts for photocuring reactions in the visible region

• Aude-Héloise Bonardi,
• Frédéric Dumur,
• Guillaume Noirbent,
• Jacques Lalevée and
• Didier Gigmes

Beilstein J. Org. Chem. 2018, 14, 3025–3046, doi:10.3762/bjoc.14.282

• with the monomer or a monomer blend. The PI is a component which absorbs light and initiates polymerization alone whereas a photoinitiating system comprises different compounds [8][9][10]. Under irradiation, PI or PIs generate active species: free radicals and/or ions and/or acid. When the active

• catalytic cycles used in organic chemistry, the development of photoredox catalysis for photopolymerization reactions has been proposed. It has emerged as a significant innovation in the field of photoinitiated polymerization. Photoredox catalysis is a new strategy to generate radicals and/or cations upon

• named reduction agent, RA2 in Figure 1) by a redox reaction. In a reductive cycle, the PC* reacts first with a reduction agent (in Figure 1, RA1) which leads to PC●− and RA1●+. Then, PC can be regenerated with an oxidation agent (OA2). Radicals and cations are generated in these cycles [16]. As we can

An overview on recent advances in the synthesis of sulfonated organic materials, sulfonated silica materials, and sulfonated carbon materials and their catalytic applications in chemical processes

• Hashem Sharghi,
• Pezhman Shiri and
• Mahdi Aberi

Beilstein J. Org. Chem. 2018, 14, 2745–2770, doi:10.3762/bjoc.14.253

• carbonyl of acetylacetone 90 to form enamine intermediate II. In the next step, hydroxyl radicals are produced through activation of molecular oxygen in the presence of SSA. Intermediate III is created via the addition of a hydroxyl radical to enamine II. This intermediate loses hydrogen radical to form

• intermediate IV. Finally, this intermediate is split by a nucleophilic attack of hydroxyl radicals to afford byproducts (including acetic acid, acetanilide, and formic acid) and desired product. The proposed mechanism was confirmed by EPR spectrum. The SSA catalyst is an inexpensive and reusable solid acid

Learning from B₁₂ enzymes: biomimetic and bioinspired catalysts for eco-friendly organic synthesis

• Keishiro Tahara,
• Ling Pan,
• Toshikazu Ono and
• Yoshio Hisaeda

Beilstein J. Org. Chem. 2018, 14, 2553–2567, doi:10.3762/bjoc.14.232

• and has an unpaired electron in the axial dz2 orbital. It acts as a high efficient “radical trap” and reacts with alkyl radicals to yield alkylcob(III)alamin (Figure 1b). Four-coordinated cob(I)alamin has a paired electron in the axial dz2 orbital, resulting in high nucleophilicity with a Pearson

• ligand. B12 is reduced to Co(I) species in the active center by reductases in sustainable processes. The partially π-conjugated system of the corrin ring is less easy to be adducted by free radicals than those of porphyrins. B12 is bound to a number of proteins and acts as a module. Different chemical

• (II) species and an alkyl radical intermediate A [54]. The prolonged lifetime of the radical intermediate A could be provided by the channel of MOF, enabling conversion to the acetyl-migrated radical B. The radicals A and B may abstract hydrogen radicals to form the reduced product and the acetyl

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

• cleavage has been reported for alkyl and aryl disulfides in an oxidative [44][45][46] and triplet sensitized mechanism [47]. It is also well known in literature that aromatic disulfides are cleaved homolytically under UV irradiation yielding the corresponding radicals [48]. A recent study from Nicewicz

Synthesis of eunicellane-type bicycles embedding a 1,3-cyclohexadiene moiety

• Alex Frichert,
• Peter G. Jones and
• Thomas Lindel

Beilstein J. Org. Chem. 2018, 14, 2461–2467, doi:10.3762/bjoc.14.222

• have also been obtained (TiCl4/Zn [12][13][14][15][16][17] or TiCl3/Zn-Cu [18][19][20]), often as mixture of diastereomers. The use of samarium diiodide to achieve the pinacol coupling was not advised, since we had observed that intially formed ketyl radicals would add to the alkyne moiety [11], even

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

• , and medicinal chemistry [56][57], we investigated the scope of this transformation (Scheme 3). By varying the alkene substitution pattern, we determined that the coupling of tertiary radicals is more efficient than secondary radicals. For example, methallyl p-methoxybenzoate (3a) and prenyl p

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

• 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

• paramagnetic radicals, the resulting dimer has a diamagnetic character due to radical pairing. Although the distance of ≈3.5 Å between the two 1●+ molecules in the dimer is considerably large in comparison to a C–C bond (≈1.5 Å), the interaction can be considered as a type of multi-centered two-electron bond

Hypervalent iodine compounds for anti-Markovnikov-type iodo-oxyimidation of vinylarenes

• Igor B. Krylov,
• Stanislav A. Paveliev,
• Mikhail A. Syroeshkin,
• Alexander A. Korlyukov,
• Pavel V. Dorovatovskii,
• Yan V. Zubavichus,
• Gennady I. Nikishin and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2018, 14, 2146–2155, doi:10.3762/bjoc.14.188

• . It was shown that the iodine atom in the prepared iodo-oxyimides can be substituted by various nucleophiles. Keywords: free radicals; hypervalent iodine; imide-N-oxyl radicals; iodination; N-hydroxyimides; oxidative functionalization; Introduction The presented work opens a new chapter in the

• chemistry of N-hydroxyimides in combination with hypervalent iodine compounds with formation of imide-N-oxyl radicals. These radicals were used as reagents for the addition to a terminal position of the double bond of styrenes with subsequent iodination of the resulting benzylic radical. It is important to

• note, that nitroxyl radicals are widely used in organic and biological chemistry, and in material design [1][2][3]. These radicals are applied in the development of monomolecular magnets [4][5], spintronics [2][6], magneto-LC effect studies [7][8], organic voltaic cells [9], electrodes for

Functionalization of graphene: does the organic chemistry matter?

• Artur Kasprzak,
• Agnieszka Zuchowska and
• Magdalena Poplawska

Beilstein J. Org. Chem. 2018, 14, 2018–2026, doi:10.3762/bjoc.14.177

• plausibly followed by a reaction between the generated radical species (Figure 7, step c), which is based on the addition of aryl radicals to the graphene sheet. One common approach is to conduct such a functionalization in an organic solvent and to use amyl or isoamyl nitrite to generate the diazonium salt

• formation of phenolic compounds from the aryl radicals (Figure 7, step d); this formation can be considered to take place before the desired SET process with the graphene sheet (Figure 7, step b). Importantly, it has been well documented that carbon materials are some of the best known adsorbents of

β-Hydroxy sulfides and their syntheses

• Mokgethwa B. Marakalala,
• Edwin M. Mmutlane and
• Henok H. Kinfe

Beilstein J. Org. Chem. 2018, 14, 1668–1692, doi:10.3762/bjoc.14.143

• alkyl radicals than the corresponding diaryl disulfides and benzyl radicals, respectively, are presumed to be the reasons behind the failure of the reaction to work with aliphatic alkenes [67]. 3.3. Synthesis of masked β-hydroxy sulfides via difunctionalization of alkenes Alkenes are versatile

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

• fluorinated alkyl radicals is typically realized either by single-electron reduction or by radical abstraction of iodine [41][42][43]. Furthermore, the carbon–iodine bond is prone to homolytic cleavage under UV irradiation [44]. In this regard, we were inspired by the work of Ryu describing the use of

• decreased reactivity of the sulfone double bond toward fluorinated radicals. In this case, two equivalents of the alkene have to be used to achieve good yields of products 3t,u. It should also be pointed out that the reaction tolerates aromatic bromide substituents (products 3b,c,n,o), a boryl fragment

Glycosylation reactions mediated by hypervalent iodine: application to the synthesis of nucleosides and carbohydrates

• Yuichi Yoshimura,
• Hideaki Wakamatsu,
• Yoshihiro Natori,
• Yukako Saito and
• Noriaki Minakawa

Beilstein J. Org. Chem. 2018, 14, 1595–1618, doi:10.3762/bjoc.14.137

• out by treatment with diacetoxyiodobenzene and iodine under irradiation with visible light to give acetoxy acetals 130 and 131 in good yields with high stereoselectivities. As shown in Scheme 17, the reaction was expected to proceed via the formation of anomeric alkoxyl radicals, which underwent

Phosphoramidite building blocks with protected nitroxides for the synthesis of spin-labeled DNA and RNA

• Timo Weinrich,
• Eva A. Jaumann,
• Ute M. Scheffer,
• Thomas F. Prisner and
• Michael W. Göbel

Beilstein J. Org. Chem. 2018, 14, 1563–1569, doi:10.3762/bjoc.14.133

• could be incorporated by unmodified standard procedures into four different self-complementary DNA and two RNA oligonucleotides. After photochemical removal of the protective group, elimination of formic aldehyde and spontaneous air oxidation, the nitroxide radicals were regenerated in high yield. The

• hydroxylamines spontaneously reacted with air to form the nitroxide radicals in high yield [43]. Although the rigid spin labels introduced by Sigurdsson [31][32][33], Hopkins [27][29] and Engels [20][21] are generally considered advantageous for EPR studies, TEMPO-modified RNAs according to structures 1 and 3

• heating to 90 °C. This step also completes the conversion of hemiacetals into nitroxide radicals 22c–27c in all cases. It should be noted, however, that palindromic duplexes may coexist in equilibrium with monomeric hairpin structures (see below). HPLC measurements gave no evidence for the formation of

Hypervalent organoiodine compounds: from reagents to valuable building blocks in synthesis

• Gwendal Grelier,
• Benjamin Darses and
• Philippe Dauban

Beilstein J. Org. Chem. 2018, 14, 1508–1528, doi:10.3762/bjoc.14.128

• (Scheme 4b). The intermediate Cu–Bpin, then, could undergo an oxidative addition into the CF3–I bond to give 8 that, after a CF3 radical transfer, would afford the radicals 9 and 10. Radical recombination followed by reductive elimination would finally lead to the E-product and regenerate the Cu–Bpin

An unusual thionyl chloride-promoted C−C bond formation to obtain 4,4'-bipyrazolones

• Gernot A. Eller,
• Gytė Vilkauskaitė,
• Algirdas Šačkus,
• Vytas Martynaitis,
• Ashenafi Damtew Mamuye,
• Vittorio Pace and
• Wolfgang Holzer

Beilstein J. Org. Chem. 2018, 14, 1287–1292, doi:10.3762/bjoc.14.110

• well documented in the literature and proceeds under different reaction conditions such as, for instance, by air oxidation [22], under O2 atmosphere using an O2 balloon [23], by organic peroxides [24], phenoxy radicals [25], by treatment with phenylhydrazine at high temperatures [26][27], by

Investigations of alkynylbenziodoxole derivatives for radical alkynylations in photoredox catalysis

• Yue Pan,
• Kunfang Jia,
• Yali Chen and
• Yiyun Chen

Beilstein J. Org. Chem. 2018, 14, 1215–1221, doi:10.3762/bjoc.14.103

• (Scheme 4). Tertiary alcohols 6 were reported to be activated by cyclic iodine(III) reagents under photoredox conditions to generate alkoxyl radicals, and subsequently underwent β-fragmentation and alkynylation to yield 7 after eliminating the arylketone [25]. With tertiary alcohol 6a as the alkyl radical

• ’-alkyne 3a gave a slightly lower 63% yield of 9 [21]. β-Ketone alcohols 10 were reported to be activated by cyclic iodine(III) reagents under photoredox conditions to generate alkoxyl radicals, and subsequently underwent β-fragmentation and alkynylation to yield ynone 9 [26]. The unsubstituted BI-alkyne

One hundred years of benzotropone chemistry

• Arif Dastan,
• Haydar Kilic and
• Nurullah Saracoglu

Beilstein J. Org. Chem. 2018, 14, 1120–1180, doi:10.3762/bjoc.14.98

• Brønsted acids (Scheme 36) [142]. Bauld and Brown reported the ready generation of the first detectable dianion radicals as outlined in Scheme 36 [150]. Benzo[7]annulenide (benzotropenide) dianion radical 220 was generated in two steps from 12 via benzotropyl methyl ether 219. Holzmann’s group described

Selective carboxylation of reactive benzylic C–H bonds by a hypervalent iodine(III)/inorganic bromide oxidation system

• Toshifumi Dohi,
• Shohei Ueda,
• Kosuke Iwasaki,
• Yusuke Tsunoda,
• Koji Morimoto and
• Yasuyuki Kita

Beilstein J. Org. Chem. 2018, 14, 1087–1094, doi:10.3762/bjoc.14.94

• ; C–H activation; iodine; oxygenation; radicals; Introduction The oxidative activation of a C(sp3)–H bond in organic molecules to directly install various functional groups and new carbon–carbon networks is a topic of interest for researchers engaged in modern synthetic chemistry [1][2][3][4][5][6][7

• hypervalent iodine reagent were reported, both of which include the formation of benzyl radicals during the key initial reaction step. Togo and co-workers developed a reaction system consisting of stoichiometric amounts of PIDA with catalytic amounts of molecular iodine and p-toluenesulfonamide for the

• subsequently decomposes by facile homolytic cleavage of the I(III)–Br bond, generating the iodanyl and bromo radicals [51][52]. It appears that these radicals can then selectively abstract the benzylic hydrogen atom of organic substrates, even in the presence of a wide variety of functional groups, such as

Cross-coupling of dissimilar ketone enolates via enolonium species to afford non-symmetrical 1,4-diketones

• Keshaba N. Parida,
• Gulab K. Pathe,
• Shimon Maksymenko and
• Alex M. Szpilman

Beilstein J. Org. Chem. 2018, 14, 992–997, doi:10.3762/bjoc.14.84

• involve the coupling of amide enolates with ketone enolates. Baran reported that stoichiometric Cu(2-ethylhexanoate)2 or Fe(acac)3 (2 equiv) are able to selectively oxidize imides, including Evan’s-type chiral imides, to the corresponding radicals. The formed radical then reacts selectively with a ketone

On the design principles of peptide–drug conjugates for targeted drug delivery to the malignant tumor site

• Eirinaios I. Vrettos,
• Gábor Mező and
• Andreas G. Tzakos

Beilstein J. Org. Chem. 2018, 14, 930–954, doi:10.3762/bjoc.14.80

• tested in female BDF mice bearing estrogen independent MXT mouse mammary cancers, presenting stronger tumor inhibitory effects than their respective cytotoxic radicals up to 93%, while equimolar quantities of their respective radicals were more toxic [124]. Moreover, PDC AN-207 was significantly more

An air-stable bisboron complex: a practical bidentate Lewis acid catalyst

• Longcheng Hong,
• Sebastian Ahles,
• Andreas H. Heindl,
• Gastelle Tiétcha,
• Andrey Petrov,
• Zhenpin Lu,
• Christian Logemann and
• Hermann A. Wegner

Beilstein J. Org. Chem. 2018, 14, 618–625, doi:10.3762/bjoc.14.48

• attributed to the free vacant p-orbital of the boron atom leading to further transformations, such as decomposition via radicals (O2), reactions with nucleophiles (H2O) as well as the formations of adducts. From this perspective, a suitable Lewis base may form a Lewis complex and subsequently occupy the p

Diels–Alder cycloadditions of N-arylpyrroles via aryne intermediates using diaryliodonium salts

• Huangguan Chen,
• Jianwei Han and
• Limin Wang

Beilstein J. Org. Chem. 2018, 14, 354–363, doi:10.3762/bjoc.14.23

• coupling for pyrroles using a hypervalent iodine reagent and a stabilizer for pyrrolyliodonium intermediates (Scheme 1c) [9]. The reactions readily provided a variety of desired coupling products in good yields. In general, the mechanism of these arylations was postulated by generating aryl radicals with

Recent developments in the asymmetric Reformatsky-type reaction

• Hélène Pellissier

Beilstein J. Org. Chem. 2018, 14, 325–344, doi:10.3762/bjoc.14.21

• , the addition of ethyl iodoacetate (47) was accelerated in the presence of ZnMe2 and oxygen through a cycle in which ZnMe2 acted as a source of methyl radicals which reacted with ethyl iodoacetate giving ethyl acetate radical. The latter added to complex M to give complex N. Then complex N assisted the

Progress in copper-catalyzed trifluoromethylation

• Guan-bao Li,
• Chao Zhang,
• Chun Song and
• Yu-dao Ma

Beilstein J. Org. Chem. 2018, 14, 155–181, doi:10.3762/bjoc.14.11

• ’ reagent, 1h, Scheme 22), which can generate trifluoromethyl radicals at room temperature in the presence of ambient air and moisture when combined with TBHP. Electron-neutral and -rich boronic acids proceeded smoothly to give the corresponding products in excellent yields. An addition of NaHCO3 was

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

• diazonium salts and dimethyl disulfide (Scheme 37) [72]. They applied Eosin Y for the photoreduction of aryl diazonium salts, in order to generate the respective aryl radicals. Radical addition to the nucleophilic disulfide forms a stabilized trivalent sulfur-centred radical. Oxidation of this intermediate

• selenosulfonates Formation of sulfone derivatives Already in 1994, the group of Barton envisioned that the photoreduction of selenosulfonates by [Ru(bpy)3]Cl2 as photoredox catalyst could lead to reactive sulfonyl radicals (Scheme 50) [87]. They were the first to report on a visible-light induced sulfonylation of