Letter Cite This: Org. Lett. 2018, 20, 5510−5514
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Co(II)/Ag(I) Synergistically Catalyzed Monoinsertion Reaction of Isocyanide to Terminal Alkynes with H2O: Synthesis of Alkynamide Derivatives Rong Zhang, Zheng-Yang Gu, Shun-Yi Wang,* and Shun-Jun Ji* Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China
Org. Lett. 2018.20:5510-5514. Downloaded from pubs.acs.org by UNIV STRASBOURG on 11/10/18. For personal use only.
S Supporting Information *
ABSTRACT: A Co(II)/Ag(I) synergistically catalyzed three-component reaction of isocyanide with terminal alkyne and water to afford alkynamide derivatives is reported. The insertion of monoisocyanide into the C−H bond of terminal alkynes is an efficient, straightforward, atom-economical route to alkynamides, which are useful synthons in organic synthesis. This synergistic process achieves the cleavage of a C−H bond and the construction of new C−C and CO bonds under mild conditions through the reaction of Co(II)-activated isocyanides and a Ag(I)-complex-activated terminal alkyne. This reaction has broad substrate versatility and functional group tolerance.
A
insertion of isocyanides into the C−H bond of terminal alkynes. In 2004, Eisen’s group reported the first catalytic monoinsertion of isocyanide to terminal alkynes by actinide metallocene complexes to construct (E)-1-aza-1,3-enynes as a major product (Scheme 1, eq 1).4a Komeyama, Takaki, and co-workers developed the cross-coupling of isocyanides with terminal alkynes catalyzed by an Sm complex to give 1-aza-1,3-enynes as a mixture of E and Z isomers (Scheme 1, eq 2).4b,c In 2008, Hou’s
mide groups are widely present in natural compounds. Among them, polypeptides and proteins with important biological activities consist of multiple amide groups. They are increasingly used in drug discovery, disease diagnosis, biomedical materials, and performance materials.1 In the past few decades, methods for synthesizing alkynamides have been limited to the reaction between alkynoic acid and amine compounds2a or by the Suzuki−Miyaura coupling reaction2b to construct amide structures for synthesizing alkynamides. In 2016, Zhao’s group applied ynamides as new coupling reagents for amide and peptide synthesis,1 which are also suitable for the synthesis of alkynamides. In 2017, Charette’s group developed a new method for the construction of alkynamides utilizing diphenylsilane.2c Although this method has better substrate universality, acetylenic amides are all prepared from alkynoic acid. Some alkynoic acids are more difficult to obtain, which limits its applications. Therefore, it is more desirable to develop a new method to construct alkynamides directly from terminal alkynes. Isocyanide is an important active reactant containing stable divalent carbon atoms. It has been widely used in the construction of nitrogen-containing compounds, the development of new drugs, and the synthesis of natural products. In recent years, the oxidative coupling reaction catalyzed by transition-metal-catalyzed isocyanide has been widely developed. The insertion reaction of isocyanide can efficiently and conveniently construct C−C and C−N bonds, providing another new strategy for the synthesis of nitrogen compounds.3 Organoactinide and rare-earth metal complex catalyzed coupling of isocyanides with terminal alkynes provides a straightforward strategy to afford 1-aza-1,3-enynes via 1,1© 2018 American Chemical Society
Scheme 1. Monoinsertion Reaction of Isocyanide into the C− H Bond of Terminal Alkynes
Received: August 7, 2018 Published: August 28, 2018 5510
DOI: 10.1021/acs.orglett.8b02516 Org. Lett. 2018, 20, 5510−5514
Letter
Organic Letters
give 3aa without AgNO3 (Table 1, entry 5). A range of silver salts Ag2CO3, AgOTf, and AgF and oxidant K2S2O8 were also evaluated instead of AgNO3, and all failed to afford the desired product 3aa (Table 1, entries 6, 8−10). The target product can also be produced when Cs2CO3 is used instead of AgNO3 (Table 1, entry 7). Among the tested solvents, CH3CN provided the best yield (Table 1, entries 11−13). We further evaluated the effects of ligand. It was found that Ph3P could also afforded the desired product in 71% yield (Table 1, entry 14), and the reaction was unsuccessful using 4,7-diphenyl-1,10-phenanthroline as the ligand (Table 1, entry 15). Finally, we determined that the optimal conditions for the reaction were ethynylbenzene (1a, 1.0 equiv), 2-isocyano-2-methylpropane (2a, 1.5 equiv), H2O (5.0 equiv), CoBr2·6H2O (5 mol %), AgNO3 (1.0 equiv), and DPEphos (20 mol %) in MeCN (2.5 mL) at 80 °C for 6 h. With the optimized reaction conditions in hand, we first explored the substrate scope of alkyne derivatives. The monoinsertion of 2a to a broad range of alkyne derivatives was examined (Figure 1). In most cases, the reaction occurred
group found that half-sandwich rare-earth metal complexes could catalyze the monoinsertion of isocyanide to terminal alkynes to form (Z)-1-aza-1,3-enynes selectively (Scheme 1, eq 3).4d To the best of our knowledge, the reaction of synthesizing alkynamides by the monoinsertion of isocyanide into the terminal alkyne has not been reported previously. The search for new catalysts for the efficient and selective cross-coupling reaction of terminal alkynes and isocyanides is therefore of interest and importance. Based on the study of cobalt-catalyzed isocyanide insertion reactions,5 herein, we developed a Co(II)/ Ag(I) synergistically catalyzed three-component reaction of isocyanide with terminal alkyne and water to afford alkynamide derivatives via monoinsertion of isocyanide into the terminal alkynes (Scheme 1, eq 4). We started our studies with ethynylbenzene 1a and tert-butyl isocyanide 2a as the model substrates. In the presence of 5 mol % of CoBr2·6H2O, 1.0 equiv of AgNO3, and 20 mol % of ligand L2 (DPEphos) in anhydrous CH3CN at 80 °C under an argon atmosphere, N-(tert-butyl)-3-phenylpropiolamide 3aa was generated in 10% yield (Table 1, entry 1). To improve this result, we added 1.0 equiv of H2O to the reaction system. To our delight, the yield of 3aa was increased to 92% (Table 1, entry 2). We tested other cobalt salts such as Co(acac)2 and Co2(CO)8, but only CoBr2·6H2O was effective (Table 1, entries 2−4). Control experiments revealed that the reaction did not work to Table 1. Optimization of Reaction Conditionsa
entry
deviation from standard conditions
yield (%)b
c
anhydrous none Co(acac)2 instead of CoBr2·6H2O Co2(CO)8 instead of CoBr2·6H2O without AgNO3 K2S2O8 instead of AgNO3 Cs2CO3 instead of AgNO3 Ag2CO3 instead of AgNO3 AgOTf instead of AgNO3 AgF instead of AgNO3 DCE instead of MeCN THF instead of MeCN DMF instead of MeCN L1 instead of L2 L3 instead of L2
10 92 38e 56e ND ND 20 ND ND ND 42e 23e 19e 71 ND
1 2d 3 4 5 6 7 8 9 10 11 12 13 14d 15d
Figure 1. Substrate scope of alkyne derivatives. The reaction conditions were alkyne 1 (0.5 mmol), 2a (0.6 mmol), H2O (5.0 equiv), CoBr2· 6H2O (5.0 mol %), L2 (20 mol %), AgNO3 (0.5 mmol), MeCN (2.5 mL) at 80 °C, 6 h.
a
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), H2O (5.0 equiv), CoBr2·6H2O (5.0 mol %), AgNO3 (0.5 mmol), MeCN (2.5 mL) at 80 °C, 4 h. bIsolated yield. cReaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), Co(OAc)2 (5.0 mol %), L2 (20 mol %), AgNO3 (0.5 mmol), dry MeCN (2.5 mL) at 80 °C, 6 h. dReaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), H2O (5.0 equiv), CoBr2· 6H2O (5.0 mol %), L2 (20 mol %), AgNO3 (0.5 mmol), MeCN (2.5 mL) at 80 °C, 6 h. DMF = N,N-dimethylformamide; THF = tetrahydrofuran; DCE = 1,1,2,2-tetrachloroethane. eLC yield using biphenyl as the internal standard.
efficiently to afford the desired alkynamide derivatives in moderate to excellent yields. The ortho-substituted substrates 1b−d resulted in corresponding alkynamide derivatives 3ba−da in 81, 86, and 69% yields, respectively. These results suggest that increased steric congestion impaired the reactivity weakly. The reactions of meta-substituted substrates 1e−g furnished the corresponding alkynamide derivatives 3ea−ga in 65, 73, and 71% yields, respectively. Introducing different substituents to 5511
DOI: 10.1021/acs.orglett.8b02516 Org. Lett. 2018, 20, 5510−5514
Letter
Organic Letters the para position of the ethynyl group has a slight effect on the yields. It should be noted that the reactions of 4methylphenylacetylene 1h and 4-methoxyphenylacetylene 1i with 2a reacted well to give alkynamides 3ha and 3ia in 68 and 86% yields, respectively. Reactions of methyl 4-ethynylbenzoate 1n and 4-nitrophenylacetylene 1o with 2a afforded 3oa and 3ma in 47 and 63% yields, respectively. These results indicate that electron effects of the aromatic ring have also impaired the reactivity weakly and decreased the yields of alkynamide derivatives slightly. To our delight, C−F and C−Cl bonds are also compatible for this reaction. This monoinsertion of the 2a reaction was also applied to heterocycloalkynes 1p−r. 3Ethynylthiophene 1p and 2-ethynylthiophene 1q with 2a proceeded to afford 3pa and 3qa in 67 and 23% yields, respectively. The reaction of 3-ethynylpyridine 1r with 2a resulted in 3ra in 63% yield. Ethynylcyclopropane 1s reacted with 2a smoothly to give 3sa in 82% yield, which is difficult to be prepared, as noted in the literature. Reactions of ethynylcyclopentane 1t and ethynylcyclohexane 1u with 2a could also afford the desired products 3ta and 3ua in 51 and 48% yields, respectively. We next investigated the scope of this monoinsertion of isocyanide with respect to the isocyanides (Figure 2). When
Scheme 2. Alkyne Bifunctionalization of Alkynamide
reaction of 1a and 2a in the presence of H2O18 under the optimized conditions. It was found that O18-labeled N-(tertbutyl)-3- alkynamide 3aa′ is detected by mass spectrometry. This result indicates that the oxygen in the amide group came from water (Scheme 3). Scheme 3. Reaction of 1a and 2a in the Presence of H2O18
It is well-known that phenylacetylene reacts with silver salt to afford (phenylethynyl)silver.7−9 We wondered whether (phenylethynyl)silver is the reaction intermediate or not. We tried the reaction of (phenylethynyl)silver with 2a under similar reaction conditions, but no desired product was obtained (Scheme 4, eq 7). This result means that (phenylethynyl)silver Scheme 4. Control Experiments
Figure 2. Scope of isocyanide. The reaction conditions were ethynylbenzene 1a (0.5 mmol), isocyanide 2 (0.6 mmol), H2O (5.0 equiv), CoBr2·6H2O (5.0 mol %), L2 (20 mol %), AgNO3 (0.5 mmol), MeCN (2.5 mL) at 80 °C, 6 h.
aromatic isocyanides were subjected to reactions of 2b−g with 1a under the standard conditions, the desired products were observed in 22−42% yields. Unfortunately, other alkyl isocyanides such as n-butyl isocyanide 2h and 2-isocyano2,4,4-trimethylpentane 2i only generated the corresponding products 3ah and 3ai in low yields. With alkynamide 3aa in hand, two methods for alkyne functionalization were examined to explore the synthetic utility of these compounds. Reaction of 3aa with norbornene 4 catalyzed by PdCl2 and CuCl2·2H2O through bimetallic systemmediated cross-coupling afforded the desired product 5 in 42% yield (Scheme 2, eq 5).6a In addition, reaction of 3aa with PhSO2Na in the presence of 1.1 equiv of I2 successfully led to the stereoselective (E)-iodosulfonylation product 6 (Scheme 2, eq 6).6b To gain a mechanistic understanding of this Co(II)/Ag(I) synergistically catalyzed monoisocyanide insertion reaction, we conducted several control experiments. We performed the
is not the reaction intermediate. The ligand DPEphos can greatly increase the reaction yield, which indicated that DPEphos can coordinate Co(II) or Ag(I). It was found that the Ag(I)−DPEphos complex was detected after the reaction (Scheme 4, eq 8). AgNO3 reacted with DPEphos quickly to give Ag(I)−DPEphos in 90% yield, which was further confirmed by X-ray diffraction and has been reported10 (Scheme 4, eq 9). When Ag(I)−DPEphos was subjected to the reaction of 1a and 2a in the presence of Co(II), 3aa could be observed in 63% yield 5512
DOI: 10.1021/acs.orglett.8b02516 Org. Lett. 2018, 20, 5510−5514
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Organic Letters Accession Codes
(Scheme 4, eq 10). This result indicates that Ag(I) in situ reacted with DPEphos to afford the Ag(I)−DPEphos complex, which could activate phenylacetylene and promote the reaction smoothly. We also tried the reaction of 1a and 2a in the absence of Co(II) or Ag(I). It was found that no desired product was observed in both cases. Therefore, we proposed that Co(II)/ Ag(I) synergistically catalyzed these monoisocyanide insertion reactions (Scheme 4, eqs 11 and 12). Based on the above results and related literature reports,11,12 we proposed a plausible reaction mechanism for this reaction (Scheme 5). First, the ligand exchange between cobalt
CCDC 1856138−1856139 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
Scheme 5. Proposed Mechanism
ORCID
Shun-Yi Wang: 0000-0002-8985-8753 Shun-Jun Ji: 0000-0002-4299-3528 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the National Natural Science Foundation of China (21772137, 21542015, and 21672157), PAPD, the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201708), the Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (No. 16KJA150002), Soochow University, and State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials for financial support. We thank Xu Zhang in this group for reproducing the results of 3aa, 3oa, 3fa, 3sa, and 3ab.
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(1) Hu, L.; Xu, S.-L.; Zhao, Z.-G.; Yang, Y.; Peng, Z.-Y.; Yang, M.; Wang, C.-L.; Zhao, J.-F. Ynamides as Racemization-Free Coupling Reagents for Amide and Peptide Synthesis. J. Am. Chem. Soc. 2016, 138, 13135. (2) (a) Qian, D.-Y.; Zhang, J.-L. Catalytic oxidation/C−H functionalization of N-arylpropiolamides by means of gold carbenoids: concise route to 3-acyloxindoles. Chem. Commun. 2012, 48, 7082. (b) Tang, J.-S.; Xie, Y.-X.; Wang, Z.-Q.; Li, J.-H. Efficient PalladiumCatalyzed Suzuki-Miyaura Cross-Coupling of Iodoethynes with Arylboronic Acids under Aerobic Conditions. Synthesis 2011, 2011, 2789. (c) Sayes, M.; Charette, A. B. Diphenylsilane as a coupling reagent for amide bond formation. Green Chem. 2017, 19, 5060. (d) Jiang, H.-F.; Liu, B.-F.; Li, Y.-B.; Wang, A.; Huang, H.-W. Synthesis of Amides via Palladium-Catalyzed Amidation of Aryl Halides. Org. Lett. 2011, 13, 1028. (3) For recent reviews, see: (a) Qiu, G.; Ding, Q. P.; Wu, J. Recent advances in isocyanide insertion chemistry. Chem. Soc. Rev. 2013, 42, 5257. (b) Gulevich, A. V.; Zhdanko, A. G.; Orru, R. V. A.; Nenajdenko, V. G. Isocyanoacetate Derivatives: Synthesis, Reactivity, and Application. Chem. Rev. 2010, 110, 5235. (c) Vlaar, T.; Ruijter, E.; Maes, B. U. W.; Orru, R.V. A. Palladium-Catalyzed Migratory Insertion of Isocyanides: An Emerging Platform in Cross-Coupling Chemistry. Angew. Chem., Int. Ed. 2013, 52, 7084. For select literature, see: (d) Vlaar, T.; Cioc, R. C.; Mampuys, P.; Maes, B. U. W.; Orru, R. V. A.; Ruijter, E. Sustainable Synthesis of Diverse Privileged Heterocycles by Palladium-Catalyzed Aerobic Oxidative Isocyanide Insertion. Angew. Chem., Int. Ed. 2012, 51, 13058. (e) Estévez, V.; Van Baelen, G.; Lentferink, B. H.; Vlaar, T.; Janssen, E.; Maes, B. U. W.; Orru, R. V. A.; Ruijter, E. Synthesis of Pyridopyrimidines by Palladium-Catalyzed Isocyanide Insertion. ACS Catal. 2014, 4, 40. (f) Liao, J.-Y.; Shao, P.-L.; Zhao, Y. Catalytic Divergent Synthesis of 3H or 1H Pyrroles by [3 + 2] Cyclization of Allenoates with Activated Isocyanides. J. Am. Chem. Soc. 2015, 137, 628. (g) Kobiki, Y.; Kawaguchi, S.; Ogawa, A. Palladium-
hexahydrate and acetonitrile occurs to form CoBr 2 · (H2O)4(MeCN)2 A.13 Isocyanide reacts with A to give CoBr2· (H2O)4(CNR2)2 B via further ligand exchange with heat. At the same time, AgNO3 in situ reacts with DPEphos to give complex C, which activates the alkyne to afford silver complex D. The interaction between Co complex B with D results in intermediate E. The intramolecular oxidation of Ag(I) leads to intermediate Co(III) complex F and Ag(0). Homolysis of F furnishes radical G and Co(II). Co(II) reacts with isocyanide to regenerate Co complex B. The oxidation of G gives H. The reaction of H with H2O following subsequent tautomerization affords alkynamide 3. In conclusion, we have disclosed a Co(II)/Ag(I) synergistically catalyzed three-component cascade reaction of isocyanide, alkyne, and water. This reaction provides a straightforward protocol for the synthesis of alkynamides via the insertion of monoisocyanide into the C−H bond of terminal alkynes under mild conditions. This synergistic process achieves the cleavage of a C−H bond and the construction of new C−C and CO bonds through the reaction of Co(II)-activated isocyanides and a Ag(I)-complex-activated terminal alkyne.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02516. Detailed experimental procedures and characterization data (PDF) 5513
DOI: 10.1021/acs.orglett.8b02516 Org. Lett. 2018, 20, 5510−5514
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Organic Letters Catalyzed Synthesis of α-Diimines from Triarylbismuthines and Isocyanides. Org. Lett. 2015, 17, 3490. (h) Zhang, Z.; Li, Z.-Y.; Fu, B.; Zhang, Z.-H. Palladium-catalyzed cross-coupling reaction of azides with isocyanides. Chem. Commun. 2015, 51, 16312. (i) He, Y.; Wang, Y.-C.; Hu, K.; Xu, X.-L.; Wang, H.-S.; Pan, Y.-M. Palladium-Catalyzed Synthesis of 5-Iminopyrrolones through Isocyanide Double Insertion Reaction with Terminal Alkynes and Water. J. Org. Chem. 2016, 81, 11813. (j) Hu, W. G.; Zheng, J.; Li, J.-X.; Liu, B.-F.; Wu, W.-Q.; Liu, H.Y.; Jiang, H.-F. Assembly of Polysubstituted Maleimides via PalladiumCatalyzed Cyclization Reaction of Alkynes with Isocyanides. J. Org. Chem. 2016, 81, 12451. (4) (a) Barnea, E.; Andrea, T.; Kapon, M.; Berthet, J. C.; Ephritikhine, M.; Eisen, M. S. Catalytic Coupling of Terminal Alkynes with Isonitriles Promoted by Organoactinide Complexes. J. Am. Chem. Soc. 2004, 126, 10860. (b) Komeyama, K.; Sasayama, D.; Kawabata, T.; Takehira, K.; Takaki, K. Direct mono-insertion of isocyanides into terminal alkynes catalyzed by rare-earth silylamides. Chem. Commun. 2005, 634. (c) Komeyama, K.; Sasayama, D.; Kawabata, T.; Takehira, K.; Takaki, K. Rare-Earth Silylamide-Catalyzed Monocoupling Reaction of Isocyanides with Terminal Alkynes. J. Org. Chem. 2005, 70, 10679. (d) Zhang, W.-X.; Nishiura, M.; Hou, Z.-M. Synthesis of (Z)-1-Aza-1,3enynes by the Cross-Coupling of Terminal Alkynes with Isocyanides Catalyzed by Rare-Earth Metal Complexes. Angew. Chem., Int. Ed. 2008, 47, 9700. (5) (a) Zhu, T.-H.; Xu, P.; Cao, J.-J.; Wei, T.-Q.; Wang, S.-Y.; Ji, S.-J. Cobalt(II)-Catalyzed Isocyanide Insertion Reaction with Amines under Ultrasonic Conditions: A Divergent Synthesis of Ureas, Thioureas and Azaheterocycles. Adv. Synth. Catal. 2014, 356, 509. (b) Zhu, T.-H.; Wang, S.-Y.; Tao, Y.-Q.; Wei, T.-Q.; Ji, S.-J. Co(acac)2/ O2-Mediated Oxidative Isocyanide Insertion with 2-Aryl Anilines: Efficient Synthesis of 6-Amino Phenanthridine Derivatives. Org. Lett. 2014, 16, 1260. (c) Xu, P.; Zhu, T.-H.; Wei, T.-Q.; Wang, S.-Y.; Ji, S.-J. Co(acac)2/O2-catalyzed oxidative isocyanide insertion with 2-vinylanilines: efficient synthesis of 2-aminoquinolines. RSC Adv. 2016, 6, 32467. (d) Xu, P.; Wang, F.; Wei, T.-Q.; Yin, L.; Wang, S.-Y.; Ji, S.-J. Palladium-Catalyzed Incorporation of Two C1 Building Blocks: The Reaction of Atmospheric CO2 and Isocyanides with 2-Iodoanilines Leading to the Synthesis of Quinazoline-2,4(1H,3H)-diones. Org. Lett. 2017, 19, 4484. (e) Gu, Z.-Y.; Liu, Y.; Wang, F.; Bao, X.-G.; Wang, S.Y.; Ji, S.-J. Cobalt(II)-Catalyzed Synthesis of Sulfonyl Guanidines via Nitrene Radical Coupling with Isonitriles: A Combined Experimental and Computational Study. ACS Catal. 2017, 7, 3893. (f) Jiang, T.; Gu, Z.-Y.; Yin, L.; Wang, S.-Y.; Ji, S.-J. Cobalt(II)-Catalyzed Isocyanide Insertion Reaction with Sulfonyl Azides in Alcohols: Synthesis of Sulfonyl Isoureas. J. Org. Chem. 2017, 82, 7913. (g) Gu, Z.-Y.; Ji, S.-J. Recent Advances in Cobalt Catalyzed Isocyanide Coupling Reactions. Huaxue Xuebao 2018, 76, 347. (6) (a) Huang, L.-B.; Wang, Q.; Wu, W.-Q.; Jiang, H.-F. Palladium/ Copper Bimetallic System-Mediated Cross-Coupling of Alkynes and Alkenes: Two Strategies to Suppress β-H Elimination on AlkylPalladium Center. Adv. Synth. Catal. 2014, 356, 1949. (b) Kumar, R.; Dwivedi, V.; Sridhar Reddy, M. Metal-Free Iodosulfonylation of Internal Alkynes: Stereodefined Access to Tetrasubstituted Olefins. Adv. Synth. Catal. 2017, 359, 2847. (7) (a) Li, M.-L.; Kwong, F. Y. Cobalt-Catalyzed Tandem C−H Activation/C−C Cleavage/C−H Cyclization of Aromatic Amides with Alkylidenecyclopropanes. Angew. Chem., Int. Ed. 2018, 57, 6512. (b) Zhang, L. B.; Hao, X. Q.; Zhang, S. K.; Liu, Z.-J.; Zheng, X.-X.; Gong, J.-F.; Niu, J.-L.; Song, M.-P. Cobalt-Catalyzed C(sp2)-H Alkoxylation of Aromatic and Olefinic Carboxamides. Angew. Chem., Int. Ed. 2015, 54, 272. (c) Zhao, D.-B.; Kim, J.-H.; Stegemann, L.; Strassert, C. A.; Glorius, F. Cobalt(III)-Catalyzed Directed C-H Coupling with Diazo Compounds: Straightforward Access towards Extended π-Systems. Angew. Chem., Int. Ed. 2015, 54, 4508. (8) (a) Gao, M.; He, C.; Chen, H.-Y.; Bai, R.-P.; Cheng, B.; Lei, A. Synthesis of Pyrroles by Click Reaction: Silver-Catalyzed Cycloaddition of Terminal Alkynes with Isocyanides. Angew. Chem., Int. Ed. 2013, 52, 6958. (b) Liu, J.-Q.; Fang, Z.-X.; Zhang, Q.; Liu, Q.; Bi, X.-H. Silver-Catalyzed Isocyanide-Alkyne Cycloaddition: A General and
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