Photoredox Ni-Catalyzed Selective Coupling of Organic Halides and Oxalates to Esters via Alkoxycarbonyl Radical Intermediates

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022Photoredox Ni-Catalyzed Selective Coupling of Organic Halides and Oxalates to Esters via Alkoxycarbonyl Radical Intermediates Wen-Duo Li, Yi-Qian Jiang, Yan-Lin Li and Ji-Bao Xia Wen-Duo Li State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Excellence in Molecular Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000 University of Chinese Academy of Sciences, Beijing 100049 , Yi-Qian Jiang State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Excellence in Molecular Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000 , Yan-Lin Li State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Excellence in Molecular Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000 University of Chinese Academy of Sciences, Beijing 100049 and Ji-Bao Xia *Corresponding author: E-mail Address: [email protected] State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Excellence in Molecular Synthesis, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.021.202100920 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail A new approach for radical cross coupling of organic halides and oxalates toward esters has been developed via photoredox nickel dual catalysis. This method has been demonstrated for transformation of a wide range of aryl, heteroaryl, alkenyl, and alkyl bromides to various esters under mild conditions. Notably, fluoro-, chloro-, or iodo-substituents on the aryl bromides remain after the coupling reaction, which has been applied for the easy synthesis of drug molecules from simple aryl dihalides. Mechanistic studies indicate that an (alkoxycarbonyl)Ni(I) species might be generated via oxidation of Ni(0) species with an alkoxycarbonyl radical intermediate. Selective alkoxycarbonyl radical coupling over decarboxylative alkyl radical coupling is achieved here. Download figure Download PowerPoint Introduction Esters are one class of the most ubiquitous organic compound that is widespread in numerous natural products, drug molecules, and useful synthetic intermediates.1 Conventionally, esters are prepared by condensation of carboxylic acids with alcohols in the presence of activating reagents. Transition metal–catalyzed alkoxycarbonylation of organic halides with carbon monoxide (CO) and alcohols is another widely practiced and important method to incorporate ester groups into readily available simple materials, particularly with Pd catalyst.2–7 Meanwhile, notable esterification methods have been developed to couple with organic halides using other carbonyl sources, including various CO surrogates.8–16 However, usual requirements of high-pressure autoclaves for CO and its toxicity as well as harsh reaction conditions often limit the scope and utility of these methods. Thus, development of novel and mild methods toward esters remains of great significance. Radicals are a highly reactive and common intermediate in synthetic transformations. Significant advances have been achieved to enhance their applications in recent decades.17–26 Particularly, carbon-centered radicals are intriguing intermediates among various radicals and have found extensive utilization in C−C bond-forming reactions.27–32 In this respect, carbonyl radicals, such as acyl and alkoxycarbonyl radicals, are useful carbonyl intermediates for the synthesis of carbonyl compounds.33–44 Despite the remarkable achievements with widely investigated acyl radicals, applications of alkoxycarbonyl radicals in synthetic chemistry are rather limited due to its lower levels of nucleophilicity.45,46 Two general reaction modes have been disclosed involving alkoxycarbonyl radicals. One is the addition of alkoxycarbonyl radical to unsaturated multiple bonds generates alkyl radicals, which undergo the following transformations to produce valuable products (Figure 1a, top). This strategy has been applied in the alkoxycarbonylation of (hetero)arenes,47–51 olefins,52–64 and isocyanide65,66 with several alkoxycarbonyl radical precursors, including S-alkoxycarbonyl xanthates, carbazates, oxalates, alkyl formate, and so on. On the other hand, the alkoxycarbonyl radical can serve as an alkyl radical precursor via further decarboxylation (Figure 1a, bottom).67–69 Although it is a competitive process, this decarboxylative strategy has been successfully demonstrated in a variety of reactions, such as trapping of the alkyl radical with olefins70–74 and other unsaturated compounds,75–79 alkyl radical coupling with halides merged with transition metal catalysis,80–82 and alkyl radical borylation.83 Despite these achievements, the direct alkoxycarbonyl radical coupling reaction has remained scarce. Therefore, exploring novel strategies to broaden the efficient transformations of alkoxycarbonyl radicals is still rewarding. Figure 1 | Reactions involving alkoxycarbonyl radical intermediates. Download figure Download PowerPoint Recently, visible-light photoredox metal dual catalysis has emerged as a powerful and practical strategy to synthesize valuable compounds under mild conditions.84–94 Continuing our interest in photoredox nickel dual catalysis95 and catalytic carbonyl coupling reactions,96–100 we questioned whether selective coupling of alkoxycarbonyl radical and organic halides for the direct synthesis of esters could be achieved by merging photoredox with nickel catalysis. It is well-known that Ni-catalyzed carbonylation with CO is rather challenging due to the highly toxic Ni(CO)n species.101–107 In addition, we need to mention that Macmillan and co-workers80 have reported coupling of alkoxycarbonyl radicals and aryl bromides toward alkylarenes by decarboxylation of alkoxycarbonyl radicals to an alkyl radical intermediate in 2016. It would be a challenge to achieve the selective cross coupling of alkoxycarbonyl radicals and organic halides to esters. As shown in Figure 1b, the desired ester product could be obtained by reductive elimination from the key (alkoxycarbonyl)NiIII species C by two possible pathways. After formation of the alkoxycarbonyl radical under photoredox conditions, C could be formed by trapping of the alkoxycarbonyl radical with NiII species A, which is generated by oxidative addition of organic bromides with a Ni0 species (Path a). For an alternative pathway, single-electron oxidation of Ni0 species with alkoxycarbonyl radical can afford the (alkoxycarbonyl)NiI species B (Path b). Then oxidative addition of organic bromides with B could also lead to C. Herein, we report our results on the selective alkoxycarbonyl radical coupling of organic bromides and oxalates toward esters under mild visible-light photoredox nickel dual catalysis. Experimental Methods In an nitrogen-filled glovebox, a 25 mL Schlenk tube was charged with organic bromide (0.2 mmol), potassium hemioxalate (0.3 mmol), Ir[dF(CF3)ppy]2(dtbbpy)PF6 (PC I, 0.006 mol, 6.7 mg), NiBr2(PCy3)2 (0.02 mmol, 15.8 mg), dtbbpy ( L1, 0.02 mmol, 5.4 mg), and anhydrous 1,4-dioxane (1.5 mL). Then the tube was removed from the glove box and stirred under irradiation with blue LED (2 × 30 W, about 1 cm away) at room temperature for 16 h. The mixture was then concentrated in vacuo, and the crude product was purified by flash column chromatography (silica gel, Petroleum Ether (PE) /Ethyl Acetate (EA) or Dichloromethane (DCM) /Methanol (MeOH)) to afford the desired product esters. (Note: The addition of materials and solvent under nitrogen atmosphere not in the glovebox does not affect the yield of the reaction.) Results and Discussion Reaction development We initiated our studies by investigating cross coupling of 4-Br-biphenyl with oxalate 1a under photoredox nickel dual catalysis. After extensive screenings, we were delighted that the desired ester product 2 was isolated in 75% yield with commercially available NiBr2(PCy3)2 as the catalyst, 4,4′-di-tert-butyl-2,2′-bipyridine (dtbbpy, L1) as the ligand, and Ir[dF(CF3)ppy]2(dtbbpy)PF6 (PC I) as the photocatalyst in 1,4-dioxane under visible-light irradiation (Table 1, Entry 1). Decarboxylative alkyl radical cross-coupling product 3 and hydrodebromination product 4 were detected as byproducts. Lower yield of 2 was obtained using other NilI precatalysts, such as NiCl2(PCy3)2, NiBr2(DME), and NiBr2 (Table 1, Entries 2–4). Moderate yield of 2 was also obtained Ni(COD)2, indicating that Ni0 might be an active on-cycle catalyst (Table 1, Entry 5). When other pyridine ligands ( L2– 4) were used instead of L1, the yield of desired product 2 significantly decreased due to the low conversion of 4-Br-biphenyl or increased hydrodebromination product, demostrating that the ligand has a significant effect on the reaction (Table 1, Entries 6–8). Further examination of the photocatalyst showed that it also played an important role in the reaction. The trace amount of product 2 was detected with other commonly used photocatalyst, including fac-Ir(ppy)3, Ir(ppy)2(dtbbpy)PF6, and 4CzIPN (see Table S3 in Supporting Information). In addition, the yield of hydrodebromination product 4 was significantly increased when replacing 1,4-dioxane with THF or DME as the solvent (Table 1, Entries 9–10). Finally, control experiments highlight the importance of the ligand, nickel catalyst, photocatalyst, and light irradiation (Table 1, Entries 11–14). No desired product 2 was obtained without nickel catalyst, photocatalyst, or light. Table 1 | Optimization of the Reaction Conditionsa Entry Variations from standard conditions Yield (%)b 4-Br-biphenyl 2 3 4 1 None Trace 80 (75)c 9 9 2 NiCl2(PCy3)2 instead of NiBr2(PCy3)2 Trace 72 7 10 3 NiBr2(DME) instead of NiBr2(PCy3)2 Trace 60 5 15 4 NiBr2 instead of NiBr2(PCy3)2 11 39 8 14 5 Ni(COD)2 instead of NiBr2(PCy3)2 Trace 54 5 20 6 L2 instead of L1 10 55 7 16 7 L3 instead of L1 32 23 3 25 8 L4 instead of L1 53 27 3 17 9 THF instead of 1,4-dioxane Trace 15 5 34 10 DME instead of 1,4-dioxane Trace 44 7 18 11 Without L1 43 31 Trace 17 12 Without Ni catalyst 78 0 0 19 13 Without photocatalyst 90 0 0 5 14 Without blue LEDs at 60 °C 90 0 0 5 aStandard conditions: 4-Br-biphenyl (0.2 mmol), 1a (1.5 equiv), NiBr2(PCy3)2 (10 mol%), L1 (10 mol%), Ir[dF(CF3)ppy]2(dtbbpy)PF6 (PC I, 3 mol%), 1,4-dioxane (1.5 mL), 2 × 30 W blue LEDs, rt, 16 h. bDetermined by gas chromatography analysis with naphthalene as an internal standard. cIsolated yield in the parenthesis. Scope of the reaction With standard conditions in hand, we first investigated the substrate scope of potassium alkyl or aryl hemioxalates 1 (Table 2). The photoredox Ni-catalyzed alkoxycarbonyl radical cross coupling occurred smoothly affording the desired products in good yields with methyl, ethyl, and n-butyl hemioxalates ( 5– 7, 60–72% yield). Moderate yields of 8 and 9 were obtained with potassium isopropyl and cyclohexyl hemioxalates. However, with potassium tert-butyl hemioxalate as the coupling partner, low yields of 10 were obtained, along with 70% yield of the hydrodebromination product of biphenyl as the byproduct. Finally, the desired ester 11 was obtained in good yield when potassium phenyl hemioxalate was used. Table 2 | Scope of Potassium Hemioxalates 1a aAll reactions were carried out with 4-Br-biphenyl (0.2 mmol) and 1 (0.3 mmol) under the standard conditions, isolated yield is provided. Next, we examined the generality of this radical cross-coupling reaction with organic halides (Table 3). A variety of aromatic bromides, heteroaryl bromides, and alkenyl bromides are all successful coupling components in this reaction. The mild reaction conditions are compatible with a number of functional groups, including cyanide ( 12), trifluoromethyl ( 13), aldehyde ( 14), trimethylsilyl ( 15), methyl ( 16), methoxy ( 17), and ester ( 18) groups, providing the corresponding products in moderate to good yields (43–84%). Coupling with polycyclic aromatic bromides, such as 2-bromonaphthalene, 9-bromophenanthrene, 1-bromopyrene, and 2-bromotriphenylene, also occurred smoothly furnishing the ester 20– 23 in 65–73% yields. Diester 24 was obtained when coupling of 2,2′-dibromo-1,1′-biphenyl and hemioxalate 1a. Remarkably, selective cross coupling of aryl bromides has been achieved when aryl dihalides were employed as the substrates. Cross coupling of fluoro-, chloro-, or iodo-substituted aryl bromides produced the corresponding products 25– 29 in moderate to good yields with retention of F, Cl, and I on the aryl ring, providing additional reaction cite for further synthetic elaborations of the ester products.108 Table 3 | Scope of Organic Bromidesa aStandard conditions: organic bromide (0.2 mmol), 1a or 1c (1.5 equiv), NiBr2(PCy3)2 (10 mol%), L1 (10 mol%), Ir[dF(CF3)ppy]2(dtbbpy)PF6 (PC I, 3 mol%),1,4-dioxane (1.5 mL), 2 × 30 W blue LEDs, 16 h, if otherwise noted. Isolated yield is provided. bWith 3 equivalent of 1a. In addition, we found that this photoredox Ni-catalyzed radical coupling reaction tolerates a variety of heteroaryl groups, including pyridine ( 30– 32), quinoline ( 33 and 34), isoquinoline ( 35), benzothiophene ( 36), indole ( 37), 7-azaindole ( 38), and indazole ( 39). Selective coupling of the corresponding heteroaryl bromides and hemioxalates led to the desired esters 30– 39 in moderate to good yields. Direct addition of alkoxycarbonyl radical to heteroarenes has been reported.47–51 Although alkoxycarbonyl radical is the reaction intermediate, no heteroaryl alkoxycarbonylation product was observed in this reaction. Notably, coupling of alkenyl bromides also delivered the desired esters 40 and 41 in moderate yields. Then, this radical cross-coupling reaction was applied for the synthesis of drug molecules Probenecid n-propyl ester ( 42, 81% yield) and Tamibarotene n-propyl ester ( 43, 72% yield) from the corresponding aryl bromides. Also, late-stage functionalization of natural product (Testosterone) or drug molecular (Desloratadine) containing aryl bromides also occurred smoothly, affording products 44 and 45 in good yields. Furthermore, this visible-light photoredox nickel dual catalytic system was tested in the alkoxycarbonyl radical coupling with challenging alkyl bromide. Encouragingly, using secondary alkyl bromide as a coupling component, the desired ester product 46 was obtained in a promising yield with tripyridine L4 as the ligand (Figure 2). The major side reaction observed was the hydrodebromination of alkyl bromide. Figure 2 | Alkoxycarbonyl radical coupling with alkyl bromide. Download figure Download PowerPoint Synthetic applications To further verify the applicability of this method, a scale-up reaction, hydrolysis of n-propyl ester, and facile synthesis of drug molecules have been carried out. First, a 1 mmol scale reaction was performed under photoredox nickel dual catalysis affording the corresponding n-propyl ester product in 78% isolated yield under the standard conditions (Figure 3a). Then, hydrolysis of n-propyl ester was easily realized by treatment with aqueous solution of potassium hydroxide affording gout suppressant drug Probenecid 47 in 91% yield. Next, with simple 1-bromo-2-chlorobenzene as the coupling component, Mefenamic n-propyl ester ( 48) was obtained in 53% yield via two steps by photoredox Ni-catalyzed alkoxycarbonyl radical cross coupling of aryl bromide with hemioxalate 1a and the following Pd-catalyzed amination of aryl chloride (Figure 3b). Similarly, via sequential photoredox Ni-catalyzed alkoxycarbonyl radical cross coupling of aryl bromide and Pd-catalyzed Suzuki–Miyaura coupling of aryl fluoride, Adapalene n-propyl ester ( 49) was obtained in moderate yield from 2-bromo-6-fluoronaphthalene (Figure 3c). Figure 3 | Synthetic applications. (1) Standard conditions, 78% yield for (a), 70% yield for (b), and 73% yield for (c). (2) KOH (aq), DMSO, rt, 91% yield. (3) 2,3-Dimethylaniline, Pd2(dba)3, Cy-John-Phos, K3PO4, DME, 100 °C, 75% yield. (4) (3-Adamantan-1-yl)-4-methoxyphenyl)boronic acid neopentylglycol ester, Ni(COD)2, PCy3, CsF, toluene, 100 °C, 55% yield. Download figure Download PowerPoint Mechanistic studies To provide some insights into the reaction mechanism, a series of control experiments have been performed. First, this photoredox Ni-catalyzed alkoxycarbonyl radical cross coupling was inhibited in the presence of radical scavenger TEMPO (2,2,6,6-tetramethylpiperidinooxy) (Figure 4a). The alkoxycarbonyl-TEMPO trapping adduct 50 was obtained in 34% yield, indicating that an alkoxycarbonyl radical intermediate was generated in the reaction. Next, using NiBr2(dtbbpy) as the catalyst, the reaction was tested with or without PCy3 as the ligand (Figure 4b). The product 2 was obtained in 42% yield and hydrodebromination product 4 was obtained in 22% yield in the absence of catalytic amount of PCy3. However, in the presence of PCy3, product 2 was obtained in 80% yield together with 9% yield of biphenyl 4. These results demonstrate that the PCy3 may play an important role promoting the alkoxycarbonyl radical coupling reaction and suppressing the undesired hydrodebromination reaction of arylbromide.109 To gain more details into the reaction mechanism, the PCy3 coordinated aryl−NiII−Br complex Ni-I was first synthesized. Using Ni-I as the catalyst, the ester product 2 was obtained in 15% yield in the absence of catalytic amount of dtbbpy ( L1) but 81% yield in the presence of L1 (Figure 4c). These results demonstrate that the complex Ni-I is a precatalyst and highlights the importance of the dtbbpy ( L1) ligand. We then synthesized the dtbbpy-coordinated aryl−NiII−Br complex Ni-II. Coupling of 4-bromobenzonitrile and hemioxalate 1a afforded 12 in 74% yield with Ni-II as the catalyst without L1 as the ligand (Figure 4d). Furthermore, a stoichiometric experiment with the complex Ni-II in the presence of catalytic or stoichiometric amount of photocatalyst I failed to yield the coupling product 2 (Figure 4e). These results indicate that dtbbpy-coordinated nickel complex is an active catalyst, but Ni-II is not the intermediate in this alkoxycarbonyl radical cross-coupling reaction. Figure 4 | Control experiments. Download figure Download PowerPoint On the basis of the control experimental results as well as previous reprots,110–113 the plausible catalytic cycle is proposed in Figure 5. First, the photoexcited *IrIII( I) is reduced by hemioxalate via single-electron transfer (SET) to produce a highly reducing IrII species and the alkoxycarbonyl radical intermediate,71 which is captured by Ln–Ni0 species affording the key (alkoxycarbonyl)NiI species B (Path b). Subsequent oxidative addition of organic bromide with NiI species B generates the (alkoxycarbonyl)NiIII intermediate C. The high-valent NiIII species C undergoes facile reductive elimination to furnish the ester product and NiI complex D. Reduction of NiI complex D by highly reducing IrII species regenerates Ni0 species and IrIII to close the two catalytic cycles. The alternative possible pathway proceeds via initially oxidative addition of organic bromide with Ni0 species to give NiII species A, which was trapped by alkoxycarbonyl radical to provide the same (alkoxycarbonyl)NiIII intermediate C (Path a). However, the result with the stoichiometric coupling experiment using well-defined Ni-II complex may not support this pathway (Figure 4e). Figure 5 | Plausible reaction pathways. Download figure Download PowerPoint Conclusion In summary, we have developed a novel and practical method for selective cross coupling of organic halides and hemioxalates. The direct synthesis of esters has been achieved via alkoxycarbonyl radical coupling under visible-light photoredox nickel dual catalysis. This new strategy is demonstrated for the transformation of a range of aryl, heteroaryl, vinyl, and alkyl bromides to various esters under mild conditions. Remarkably, selective coupling of aryl bromides has been achieved leaving fluorine, chlorine, and iodine on the aromatic rings after the coupling reaction. Mechanistic studies indicate a successive SET pathway and the generation of the alkoxycarbonyl radical intermediate in this synergistic catalytic process. Further application of this strategy for the conversion of oxalates is ongoing in our laboratory. Supporting Information Supporting Information is available. Experimental procedures, characterizations, and analytical data of new compounds; and spectra of NMR for new compounds (PDF). Conflicts of Interest The authors declare no competing interest. Acknowledgments We gratefully acknowledge the financial support from the NSFC of China (21772208 and 21633013), Natural Science Foundation of Jiangsu Province (BK20201183), and the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDJSSW-SLH051). References 1. Otera J.Esterification: Methods, Reactions, and Applications; Wiley-VCH: Weinheim, 2003. Google Scholar 2. Brennführer A.; Neumann H.; Beller M.Palladium-Catalyzed Carbonylation Reactions of Aryl Halides and Related Compounds.Angew. Chem. Int. Ed.2009, 48, 4114–4133. Google Scholar 3. Wu X.-F.; Neumann H.; Beller M.Synthesis of Heterocycles via Palladium-Catalyzed Carbonylations.Chem. Rev.2013, 113, 1–35. Google Scholar 4. Wu X.-F.; Fang X.; Wu L.; Jackstell R.; Neumann H.; Beller M.Transition-Metal-Catalyzed Carbonylation Reactions of Olefins and Alkynes: A Personal Account.Acc. Chem. Res.2014, 47, 1041–1053. Google Scholar 5. Peng J.-B.; Geng H.-Q.; Wu X.-F.The Chemistry of CO: Carbonylation.Chem2019, 5, 526–552. Google Scholar 6. Kwiatkowski M. R.; Alexanian E. J.Transition-Metal (Pd, Ni, Mn)-Catalyzed C–C Bond Constructions Involving Unactivated Alkyl Halides and Fundamental Synthetic Building Blocks.Acc. Chem. Res.2019, 52, 1134–1144. Google Scholar 7. Singh J.; Sharma S.; Sharma A.Photocatalytic Carbonylation Strategies: A Recent Trend in Organic Synthesis.J. Org. Chem.2021, 86, 24–48. Google Scholar 8. Morimoto T.; Kakiuchi K.Evolution of Carbonylation Catalysis: No Need for Carbon Monoxide.Angew. Chem. Int. Ed.2004, 43, 5580–5588. Google Scholar 9. Wu L.; Liu Q.; Jackstell R.; Beller M.Carbonylations of Alkenes with CO Surrogates.Angew. Chem. Int. Ed.2014, 53, 6310–6320. Google Scholar 10. Konishi H.; Manabe K.Formic Acid Derivatives as Practical Carbon Monoxide Surrogates for Metal-Catalyzed Carbonylation Reactions.Synlett2014, 25, 1971–1986. Google Scholar 11. Gautam P.; Bhanage B. M.Recent Advances in the Transition Metal Catalyzed Carbonylation of Alkynes, Arenes and Aryl Halides Using CO Surrogates.Catal. Sci. Technol.2015, 5, 4663–4702. Google Scholar 12. Kalck P.; Urrutigoity M.Recent Improvements in the Alkoxycarbonylation Reaction Catalyzed by Transition Metal Complexes.Inorg. Chim. Acta2015, 431, 110–121. Google Scholar 13. Konishi H.; Manabe K.Recent Progress on Catalytic Heck Carbonylations Using Carbon Monoxide Surrogates.Tetrahedron Lett.2019, 60, 151147–151153. Google Scholar 14. Chen Z.; Wang L.-C.; Wu X.-F.Carbonylative Synthesis of Heterocycles Involving Diverse CO Surrogates.Chem. Commun.2020, 56, 6016–6030. Google Scholar 15. Boehm P.; Roediger S.; A.; of Aryl In of Carbon Monoxide.Angew. Chem. Int. Google Scholar R.; Li J.-B.; Liu of Esters via Decarboxylative Coupling of Potassium with Aryl and Chem. Google Scholar P.; M. in Organic Wiley-VCH: Weinheim, Google Scholar of Radicals in and Google Scholar T.; Intermediates with Chem. Google Scholar A.; D. of Radical A Radical Chemistry Chem. Int. Google Scholar in A of Radical Google Scholar J.; M. S.; D. A.; C. in Natural 47, Google Scholar Radical in Organic Chem. Int. Google Scholar Wu X.; Chem. 53, Google Scholar Chen C–C Bond in Organic Google Scholar Li Li Q.; Wang of with for the Synthesis of Radical Google Scholar M. H.; J.; D. in Organic Org. Google Scholar D. R.; to The of Catalytic Radical in Synthetic Methods 7, Google Scholar S.; S.; Reactions by to Google Scholar S.; of Alkyl the of to the Google Scholar P.; Chem. Int. Google Scholar Liu Synthesis of Heterocycles by Google Scholar S.; Radicals for Google Scholar A.; Z.; Radical Chemistry via Google Scholar L.; Chen X.; Advances on the Reactions of Org. Google Scholar C. H.; H.; as in Radical and Related Chem. Google Scholar of Google Scholar E. T.; of Radicals via of Google Scholar of with Chem. Int. Google Scholar Wang X.; X.; Wu Synthesis of from by and Acid Chem. Int. Google Scholar S.; A.; E. Radical of Unactivated to and Google Scholar A.; Synthesis of by Radical to Chem. Int. Google Scholar R.; Liu Synthesis Direct from and Commun.2020, Google Scholar L.;

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