DABCO-promoted photocatalytic C–H functionalization of aldehydes

• Bruno Maia da Silva Santos,
• Mariana dos Santos Dupim,
• Cauê Paula de Souza,
• Thiago Messias Cardozo and
• Fernanda Gadini Finelli

Beilstein J. Org. Chem. 2021, 17, 2959–2967, doi:10.3762/bjoc.17.205

• a reactive species, often used in catalytic amounts, capable of promoting a highly selective homolytic cleavage of the C–H bond that results in a carbon-centered radical [5][6]. Nitrogenated structures are easily oxidized under mild conditions into their radical or radical cation forms [7], being

• a common inexpensive organic base with two nitrogen atoms in a bicyclic cage structure. The interaction between these two nitrogen atoms makes DABCO easier to oxidize and improves the lifetime of the radical cation species when compared to quinuclidine [7]. Investigation of DABCO as a hydrogen

• accessibility, it is still underused, and has only recently started to gain attention from the synthetic community. Murphy and co-workers reported the use of the DABCO radical cation, generated by a stoichiometric oxidant (TPTA-PF6), as a hydrogen abstractor for alpha-nitrogen C–H functionalization [21] (Figure

Visible-light-mediated copper photocatalysis for organic syntheses

• Yajing Zhang,
• Qian Wang,
• Zongsheng Yan,
• Donglai Ma and
• Yuguang Zheng

Beilstein J. Org. Chem. 2021, 17, 2520–2542, doi:10.3762/bjoc.17.169

• ] discovered the photoinduced, copper-catalyzed cyanofluoroalkylation of alkenes and fluoroalkyl iodides 12. The reaction was initiated by the reduction of CuII with tertiary amines, which formed CuICN and an amine radical cation [55]. Under irradiation by ultraviolet light, CuICN was excited and transformed

• cation I and a CuI species. This process regenerated CuII in the presence of molecular oxygen. The deprotonation of the nitrogen radical cation produces an α–amino radical II, which was further oxidized to the iminium ion III to which the copper alkynylide added forming the desired product (Scheme 17

• photophysical properties (Scheme 16). In 2019, Vlla’s group [76] explored the copper-catalyzed alkynylation of dihydroquinoxalin-2-ones 34 with terminal alkynes under irradiation. 4-Benzyl-3,4-dihydroquinoxalin-2(1H)-one 35 was subjected to an oxidation process with a CuII salt to generate a nitrogen radical

Synthesis of phenanthridines via a novel photochemically-mediated cyclization and application to the synthesis of triphaeridine

• Songeziwe Ntsimango,
• Kennedy J. Ngwira,
• Moira L. Bode and
• Charles B. de Koning

Beilstein J. Org. Chem. 2021, 17, 2340–2347, doi:10.3762/bjoc.17.152

• synthesis of trisphaeridine to afford the product in four linear steps in an overall yield of 6.5% from 1-bromo-2,4,5-trimethoxybenzene. Keywords: aromatic compounds; cyclization; iminyl radical; phenanthridines; radical cation; synthesis; UV irradiation; Introduction Phenanthridine derivatives have

• having a methoxy substituent as the leaving group for the formation of phenanthridines. Structure–reactivity relationship studies also indicated that an ortho- or para-methoxy group must be present to stabilize the incipient radical or radical cation intermediate. We propose that the reaction is

• proceeding by means of the initial generation of an iminyl radical that cyclizes onto the electron-rich aromatic ring or through the formation of a radical cation on the electron-rich benzene ring. Finally, the methodology has successfully been applied to synthesizing the natural product trisphaeridine

Towards new NIR dyes for free radical photopolymerization processes

• Haifaa Mokbel,
• Guillaume Noirbent,
• Didier Gigmes,
• Frédéric Dumur and
• Jacques Lalevée

Beilstein J. Org. Chem. 2021, 17, 2067–2076, doi:10.3762/bjoc.17.133

• as an electron-donating component with the radical cation dye•+ and therefore, the dye/borate system is able to generate additional free radicals in the reaction medium, improving, in turn, the polymerization process (Scheme 7). This behavior has already been reported in the literature and is in full

Methodologies for the synthesis of quaternary carbon centers via hydroalkylation of unactivated olefins: twenty years of advances

• Thiago S. Silva and
• Fernando Coelho

Beilstein J. Org. Chem. 2021, 17, 1565–1590, doi:10.3762/bjoc.17.112

• reaction was observed in its absence. The authors highlighted the role of the solvent hexafluoro-2-propanol (HFIP) in the stabilization of the radical cation induced by PET and its assistance in the hydrogen shift process. Miscellaneous Lewis acid catalysis in olefin hydroalkylation reactions The ability

Breaking paracyclophane: the unexpected formation of non-symmetric disubstituted nitro[2.2]metaparacyclophanes

• Suraj Patel,
• Tyson N. Dais,
• Paul G. Plieger and
• Gareth J. Rowlands

Beilstein J. Org. Chem. 2021, 17, 1518–1526, doi:10.3762/bjoc.17.109

• , protonation might not be the key step, and the highly oxidizing nature of nitration conditions that can lead to the formation of a cationic intermediate via a radical cation might control this reaction [68]. A possible mechanism for the formation of 5 and 6 starts with protonation of 1 give the Wheland

Synthetic reactions driven by electron-donor–acceptor (EDA) complexes

• Zhonglie Yang,
• Yutong Liu,
• Kun Cao,
• Xiaobin Zhang,
• Hezhong Jiang and
• Jiahong Li

Beilstein J. Org. Chem. 2021, 17, 771–799, doi:10.3762/bjoc.17.67

• additive to give corresponding thiophene radical 76 and aniline radical cation under irradiation with light. Then, 76 reacted with 73, giving rise to corresponding radical 77. Finally, product 74 was given via hydrogen atom transfer (Scheme 26). In contrast to (hetero)aryl halides with indispensable

• occurs, giving radical 146 and radical cation 147, respectively. Finally, radical 146 undergoes decarboxylation to afford an aryl radical and then combines with radical cation 147, yielding product 144 (Scheme 50). It should be noted that only when NHPI is firstly activated can it turn into an electron

CF₃-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

• Anthony J. Fernandes,
• Armen Panossian,
• Bastien Michelet,
• Agnès Martin-Mingot,
• Frédéric R. Leroux and
• Sébastien Thibaudeau

Beilstein J. Org. Chem. 2021, 17, 343–378, doi:10.3762/bjoc.17.32

• nonfluorinated analogues (Eox (PhNMe2) = +0.71 V (SCE)), the radical cation 180 is formed under the reaction conditions, and deprotonation at the methylene unit near the CF3 group is highly favored because of the higher acidity, accounting for the observed high regioselectivity. In addition, the transient

• methoxylation or acetoxylation, respectively (Scheme 53). The driving force in this reaction is assumed to be the deprotonation of radical cation 215, a highly destabilized species due to the presence of the strongly electron-withdrawing CF3 substituent, which leads to radical 216, synergistically stabilized by

Metal-free synthesis of biarenes via photoextrusion in di(tri)aryl phosphates

• Hisham Qrareya,
• Lorenzo Meazza,
• Stefano Protti and
• Maurizio Fagnoni

Beilstein J. Org. Chem. 2020, 16, 3008–3014, doi:10.3762/bjoc.16.250

• –20-fold amount. Furthermore, the aryl radical/cation addition onto the aromatic reactant may lead to a mixture of regioisomers when using non-symmetrical Ar–H. A possible solution is having recourse to an intramolecular free radical ipso substitution reaction where an XSO2 tether is placed between

On the mass spectrometric fragmentations of the bacterial sesterterpenes sestermobaraenes A–C

• Anwei Hou and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2020, 16, 2807–2819, doi:10.3762/bjoc.16.231

• radical cation 1•+ is obtained from which the methyl group C23 can directly be lost by an α-cleavage leading to fragment a1+ (Scheme 1A). However, the radical centred at the bridgehead carbon C11 is orthogonal to, or in other words, not in conjugation with the radical cation at C12–13. Therefore, an

3-Acetoxy-fatty acid isoprenyl esters from androconia of the ithomiine butterfly Ithomia salapia

• Florian Mann,
• Daiane Szczerbowski,
• Lisa de Silva,
• Melanie McClure,
• Marianne Elias and
• Stefan Schulz

Beilstein J. Org. Chem. 2020, 16, 2776–2787, doi:10.3762/bjoc.16.228

• explained by the different stabilization of the respective ions (Figure 3). The abundance of m/z 68 is higher in isoprenyl esters due to the more stable allyl radical cation (Figure 3A). In contrast, prenyl ester fragmentation produces a stabilized allyl cation m/z 69 (Figure 3B), while isoprenyl esters

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• ]•+, with the latter oxidising 22 to give radical cation 22•+ and turn over the photocatalytic cycle. The radical cation 22•+ is then proposed to participate in a two-step electron and proton exchange process with [CoII] to give [H–CoIII] and iminium ion 24, likely via a [CoI] intermediate. [H–CoIII] can

• investigations [21] showed that after excitation of the EDA complex, the electrophilic radical 40• that is formed enters the same chain propagation cycle as in Scheme 1, whereas the radical cation 41•+ is proposed to be unstable and decomposes. The third approach, also developed by Melchiorre et al., was based

• subsequently oxidise 60 to give a nucleophilic radical R• that enters a similar RCA cycle as for Scheme 7. However, in the absence of an external photocatalyst, radical cation intermediate 62•+ is reduced by another molecule of 60, thus propagating a radical chain mechanism that leads to the formation of

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