Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Palladium-Catalyzed Regio- and Stereoselective Coupling−Addition of Propiolates with Arylsulfonyl Hydrazides: A Pattern for Difunctionalization of Alkynes Lixin Liu,† Kang Sun,† Lebin Su,† Jianyu Dong,*,† Lei Cheng,† Xiaodong Zhu,† Chak-Tong Au,‡ Yongbo Zhou,*,† and Shuang-Feng Yin*,† †
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China College of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China
‡
S Supporting Information *
ABSTRACT: A new pattern for difunctionalization of alkynes via a palladium-catalyzed regio- and stereoselective coupling− addition of propiolates with arylsulfonyl hydrazides is disclosed. The approach enables the synthesis of various highly functionalized (E)-vinylsulfones in satisfactory yields. Arylsulfonyl hydrazides act as both aryl and sulfonyl sources via selective cleavage of Ar(C)−S and S−N bonds, which are simultaneously incorporated onto the terminal carbon atom of an alkyne molecule.
T
Interestingly, arylsulfonyl hydrazides can simultaneously act as coupling partners and addition reagents via selective cleavage of the C−S and N−S bond, respectively. Arylsulfonyl hydrazides are usually employed as aryl or sulfonyl sources via desulfitation or denitrification,7 but there is no report on the dual roles of arylsulfonyl hydrazides being simultaneously utilized. The reaction that incorporates two different functional groups onto the terminal carbon atom of an alkyne for the synthesis of 1,1-disubstituted alkene derivatives via a coupling-addition represents a new pattern for alkyne difunctionalization. Vinylsulfone derivatives are useful synthetic intermediates8 and unique motifs in biologically active molecules.9 A plethora of methods,10−14 including sulfonamination,11 sulfonyloxidation,12 halosulfonylation,13 and sulfonylcarbonation of alkynes,14 have been developed for efficient construction of these compounds. However, the sulfonative functionalization of alkynes was limited to 1,2-difunctionalization, and the introduction of aryl and sulfonyl groups at a terminal carbon atom of alkynes has not been realized. We commenced our study with the treatment of benzenesulfonyl hydrazide (1a, 0.2 mmol) and ethyl propiolate (2a, 0.2 mmol) in the presence of Pd(OAc)2 (5 mol %), 1,3bis(diphenylphosphino)propane (dppp) (5 mol %), and Cu(OAc)2·H2O (2.0 equiv) in N,N-dimethylformamide (DMF) at 80 °C for 30 min. A difunctionalized product (E)ethyl 3-phenyl-3-(phenylsulfonyl)acrylate 3a was obtained in 44% yield (Table 1, entry 1, yield based on 0.1 mmol), in which the corresponding Sonogashira coupling product was not observed,6b probably due to the strong electron-withdrawing
ransition-metal-catalyzed transformation of alkynes into polyfunctionalized compounds is at the forefront of research activities in modern organic and organometallic chemistry.1−4 In this context, one-pot difunctionalization of alkynes is an appealing strategy that provides straightforward and flexible access to polyfunctionalized alkene derivatives.2−4 In recent decades, efficient protocols for 1,2-difunctionalization of alkynes have been developed and extensively applied in organic synthesis, material chemistry, and biochemistry,3d,5 in which two functional groups are incorporated onto the two carbon atoms of an alkyne, respectively (Scheme 1).2,3 In Scheme 1. Difunctionalization of Alkynes to Alkenes
contrast, 1,1-difunctionalization of alkynes that requires dehydrogenation and addition of two functional groups onto the terminal carbon atom of a alkyne, especially in a regio- and stereoselective manner, is challenging, and thus, successful achievements of this kind are rarely reported.4 Herein, as a result of our continuous interest in selective transformation of alkynes,6 we report the regio- and stereoselective 1,1-difunctionalization of alkynes with arylsulfonyl hydrazides for the formation of (E)-β-aryl sulfonyl acrylates. © XXXX American Chemical Society
Received: May 19, 2018
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DOI: 10.1021/acs.orglett.8b01585 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Table 1. Optimization of the Reaction Conditionsa
Scheme 2. Substrate Scopea
yield (%)b
entry
catalyst
ligand
additive
solvent
1 2 3 4 5 6 7 8 9 10c 11
Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 PdCl2 Pd(TFA)2 Pd(PPh3)2Cl2 Pd(PPh3)4 Pd(OAc)2 Pd(OAc)2
dppp dppm dppe PPh3 Phen dppp dppp dppp dppp dppp dppp
− − − − − − − − − − −
44 25 39 27 32 39 45 41 42 none 52
12 13
Pd(OAc)2 Pd(OAc)2
dppp dppp
14 15e 16f
Pd(OAc)2 Pd(OAc)2 Pd(OAc)2
dppp dppp dppp
Na2CO3 salicylic acid β-alanine β-alanine β-alanine
DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF/DMAc (3:1) DMF/DMAc DMF/DMAc DMF/DMAc DMF/DMAc DMF/DMAc (3:1)
70(64)d 65 71
trace 58
a
Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), catalyst (5.0 mol %), ligand (5.0 mol %), Cu(OAc)2·H2O (2.0 equiv), additive (15 mol %), solvent (2.0 mL), N2, 80 °C, 30 min. bGC yield using dodecane as an internal standard. cOther oxidants, such as O2, BQ, K2S2O8, PhI(OAc)2, and TBHP. dIsolated yields. e70 °C. f100 °C.
ability of the ester group, making the internal alkyne very electrophilic, and resulting in the consequent addition (vide inf ra). On the basis of this finding, we tuned the reaction parameters for the optimum reaction conditions. Initially, ligands such as bis(diphenylphosphino)methane (dppm), 1,2bis(diphenylphosphino)ethane (dppe), PPh3, and Phen were examined (Table 1, entries 2−5; for details, see Supporting Information (SI)), and dppp gave the best yield (44%, Table 1, entry 1). For palladium catalysts, PdCl2, Pd(TFA)2, Pd(PPh3)2Cl2, and Pd(PPh3)4 showed good efficiency (Table 1, entries 6−9). An assessment of oxidants revealed that Cu(OAc)2·H2O was optimal for the reaction (Table 1, entry 1), whereas molecular oxygen, benzoquinone (BQ), K2S2O8, PhI(OAc)2, and tert-butyl hydroperoxide (TBHP) were ineffective (Table 1, entry 10). Other solvents such as N,Ndimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), 1methyl-2-pyrrolidinone (NMP), acetonitrile, and toluene were inferior to DMF (for details, see SI). The mixed solvents were more effective than DMF (for details, see SI), and a DMF to DMAc ratio of 3:1 gave the best yield (52%, entry 11). Excitedly, a catalytic amount of additives could further improve the reaction performance (Table 1, entries 13 and 14; for details, see SI). When 15 mol % β-alanine was added,15 a 70% yield of 3a was achieved (Table 1, entry 14), whereas the loading of Na2CO3 completely blocked the reaction (Table 1, entry 12). A slightly lower 3a yield was observed at 70 °C (Table 1, entry 15), and the rise of temperature to 100 °C did not result in significant enhancement of 3a yield (Table 1, entry 16). With the optimized conditions in hand, we investigated the scope and generality of the reaction. As shown in Scheme 2,
a Reaction conditions: 1 (0.2 mmol), 2 (0.2 mmol), Pd(OAc)2 (5.0 mol %), dppp (5.0 mol %), β-alanine (15 mol %), Cu(OAc)2·H2O (2.0 equiv), DMF/DMAc (2.0 mL, v/v = 3:1), N2, 80 °C, 30 min, isolated yields. b1.0 mmol gram scale. c1 (0.4 mmol), 2 (0.4 mmol). d 1:1 ratio of two different arylsulfonyl hydrazides.
this one-pot coupling−addition approach displayed a wide scope of substrates and outstanding tolerance toward functional groups, producing various (E)-β-aryl sulfonyl acrylates in satisfactory yields with excellent regio- and stereoselectivity. The structure of the products was unambiguously confirmed by single-crystal X-ray crystallography (3q and 3af, for details, see SI). For arylsulfonyl hydrazides derivatives, functional groups such as alkyl, methoxyl, halogen, nitro, cyano, trifluoromethyl, and trifluoromethoxyl are all tolerated in this method of alkyne arylsulfonylation, and the corresponding products (3a−o) were gained in 30−72% yields. Notably, the halogenated (Cl, Br, and B
DOI: 10.1021/acs.orglett.8b01585 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters I) substrates that are reactive in palladium-catalyzed coupling reactions were found to be suitable for the 1,1-difunctionalization reaction, producing the corresponding products 3g−k in 35−62% yields. This chemoselectivity could be utilized for further functionalization to realize the synthesis of more complex targets by stepwise coupling. The reaction of sterically hindered naphthalene substrates 2p and 2aa could also proceed, giving the corresponding products 3p and 3aa in 30% and 32% yields, respectively. Different electronic effects of substituent groups existed in the steps of desulfitative cross-coupling and sulfonative addition; i.e., the electron-withdrawing (donating) groups facilitate (disfavor) the cross-coupling reaction via cleavage of the Ar(C)−S bond, but disfavor (facilitate) the subsequent addition reaction via cleavage of the S−N bond (vide inf ra). However, the attachment of strong electron-withdrawing groups, such as NO2 (3l), CN (3m), and CF3 (3n and 3z), resulted in reaction efficiency (yields: 30−40%) much inferior to that of electron-donating groups (3b−d, and 3y, yields 62− 74%). Indeed, the direct addition of benzenesulfonyl hydrazide bearing an electron-withdrawing group (CF3, Scheme 4, eq 3) to internal alkyne 4A gave a much lower yield (32%) of sulfonative addition product (3am) in comparison with the direct addition of the other benzenesulfonyl hydrazides (3a, 70% yield; 3al, 68% yield; Scheme 4, eq 3). These results demonstrate that sulfonative addition is more difficult and may be the rate-determining step of the reaction. Other propargylic carboxylates, bearing benzyl (3q), alkyl (3r−t), hydroxyl (3u), morpholine (3v), sulfide (3w), and bromine (3x), also reacted smoothly with benzenesulfonyl hydrazide 1a to produce a range of (E)-β-aryl sulfonyl acrylates in 38−66% yields. It is considered that the yields are satisfactory, albeit moderate to good. It is because the strategy is more efficient than the twostep Sonogashira-type cross-coupling and addition reactions (for details, see SI). Gratifyingly, when two different arylsulfonyl hydrazides were subjected to the reaction system, the reaction took place selectively, furnishing the desired products in 30−43% yields (3ab−ag). The arylsulfonyl hydrazides substituted with an electron-donating group afford the sulfonyl sources via S−N bond cleavage, and those substituted with an electron-withdrawing group afford the aryl groups via C−S bond cleavage. In addition to propiolates, terminal alkynes substituted with carbonyl (1-phenylprop-2-yn-1-one, 2ah) and acylamino (Nethynyl-N-methylbenzamide, 2ai) were also suitable for the reaction, producing the desired product in 37% (3ah) and 33% (3ai) yields, respectively, with concomitant generation of internal alkynes (4aa, 30% yield; 4ab, 37% yield; for details, see SI). It is worth noting that this one-pot coupling−addition reaction can be applied to modify highly functionalized molecules, further demonstrating its applicational potential in drug discovery and synthesis of natural products. For example, propiolates bearing natural groups such as L-menthol and cholesterol could well react with benzenesulfonyl hydrazide 1a to produce the corresponding products 3aj and 3ak in 49% and 46% yields under the optimized conditions, respectively (Scheme 3). Finally, a number of control experiments were performed to gain mechanistic insight into this reaction. Only ethyl 3phenylpropiolate 4A reacted with 1a to give the desulfitative coupling product (Scheme 4, eq 3), whereas the reaction of 1a with the other probable intermediates (4B, 4C, or 4D) did not
Scheme 3. Reaction Scope of Late-Stage Modification
Scheme 4. Control Experiments
take place (Scheme 4, eq 4). The results demonstrate that the reaction proceeds via the pattern of Sonogashira-type desulfitative coupling and subsequent sulfonative addition (for more details, see SI). Based on the results of the above studies, and those reported in literature,6b,7c a plausible mechanism is illustrated in Scheme 5. The reaction of Pd(OAc)2 with arylsulfonyl hydrazide gives Scheme 5. Possible Reaction Mechanism
ArPdIIOAc intermediate III, which is formed via successive dehydrogenation of arylsulfonyl hydrazide, together with the liberation of N2 and SO2. Transmetalation of III with copper acetylides IV gives palladium arylacetylides V, which undergoes reductive elimination to generate the cross-coupling product VI, simultaneously releasing Pd0. Intermediate II undergoes migratory insertion into the internal of alkynes VI, followed by protonolysis of the alkenyl16 C−Pd bond of VII to afford product 3 and the PdII catalyst. In summary, we have developed a novel strategy for one-pot 1,1-difunctionalization of terminal alkynes via a palladiumcatalyzed coupling−addition reaction of propiolates with readily available arylsulfonyl hydrazides, providing a broad range of highly functionalized (E)-vinylsulfones in satisfactory yield. Arylsulfonyl hydrazides act as both arylation and sulfonylation reagents via selective Ar(C)−S and S−N bond C
DOI: 10.1021/acs.orglett.8b01585 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
Chem. Soc. 2017, 139, 15724. (e) Iwamoto, T.; Nishikori, T.; Nakagawa, N.; Takaya, H.; Nakamura, M. Angew. Chem., Int. Ed. 2017, 56, 13298. (4) 1,1-Diboronation of alkynes to alkenes via B−B bond cleavage is known, see: (a) Morinaga, A.; Nagao, K.; Ohmiya, H.; Sawamura, M. Angew. Chem., Int. Ed. 2015, 54, 15859. (b) Krautwald, S.; Bezdek, M. J.; Chirik, P. J. J. Am. Chem. Soc. 2017, 139, 3868. (5) For selected examples, see: (a) McKinley, N. F.; O’Shea, D. F. J. Org. Chem. 2006, 71, 9552. (b) Wang, F.; Zhu, N.; Chen, P.; Ye, J.; Liu, G. Angew. Chem., Int. Ed. 2015, 54, 9356. (c) Deng, J.-R.; Chan, W.-C.; Chun-Him Lai, N.; Yang, B.; Tsang, C.-S.; Chi-Bun Ko, B.; LaiFung Chan, S.; Wong, M.-K. Chem. Sci. 2017, 8, 7537. (d) Wan, D.; Li, X.; Jiang, R.; Feng, B.; Lan, J.; Wang, R.; You, J. Org. Lett. 2016, 18, 2876. (e) Kuram, M. R.; Bhanuchandra, M.; Sahoo, A. K. Angew. Chem., Int. Ed. 2013, 52, 4607. (6) Recent examples: (a) Su, L.; Dong, J.; Liu, L.; Sun, M.; Qiu, R.; Zhou, Y.; Yin, S.-F. J. Am. Chem. Soc. 2016, 138, 12348. (b) Qian, L. W.; Sun, M.; Dong, J.; Xu, Q.; Zhou, Y.; Yin, S. F. J. Org. Chem. 2017, 82, 6764. (c) Liu, L.; Ji, X.; Dong, J.; Zhou, Y.; Yin, S. F. Org. Lett. 2016, 18, 3138. (d) Cao, H.; Chen, T.; Zhou, Y.; Han, D.; Yin, S.-F.; Han, L.-B. Adv. Synth. Catal. 2014, 356, 765. (7) For selected examples on arylsulfonyl hydrazides employed as aryl or sulfonyl sources, see: (a) Senadi, G. C.; Guo, B.-C.; Hu, W.-P.; Wang, J.-J. Chem. Commun. 2016, 52, 11410. (b) Miao, H.; Wang, F.; Zhou, S.; Zhang, G.; Li, Y. Org. Biomol. Chem. 2015, 13, 4647. (c) Yang, F.-L.; Ma, X.-T.; Tian, S.-K. Chem. - Eur. J. 2012, 18, 1582. (d) Hao, W.-J.; Du, Y.; Wang, D.; Jiang, B.; Gao, Q.; Tu, S.-J.; Li, G. Org. Lett. 2016, 18, 1884. (e) Wang, Y.; Ma, L.; Ma, M.; Zheng, H.; Shao, Y.; Wan, X. Org. Lett. 2016, 18, 5082. (8) Selected examples: (a) Yang, X.; Cheng, F.; Kou, Y.-D.; Pang, S.; Shen, Y.-C.; Huang, Y.-Y.; Shibata, N. Angew. Chem., Int. Ed. 2017, 56, 1510. (b) Noshi, M. N.; El-awa, A.; Torres, E.; Fuchs, P. L. J. Am. Chem. Soc. 2007, 129, 11242. (c) López-Pérez, A.; Robles-Machín, R.; Adrio, J.; Carretero, J. C. Angew. Chem., Int. Ed. 2007, 46, 9261. (d) Guo, H.-M.; Zhou, Q.-Q.; Jiang, X.; Shi, D.-Q.; Xiao, W.-J. Adv. Synth. Catal. 2017, 359, 4141. (e) Forristal, I. J. Sulfur Chem. 2005, 26, 163. (f) Song, R.-J.; Liu, Y.; Liu, Y.-Y.; Li, J.-H. J. Org. Chem. 2011, 76, 1001. (9) Selected examples: (a) Chauhan, P.; Hadad, C.; López, A. H.; Silvestrini, S.; La Parola, V.; Frison, E.; Maggini, M.; Prato, M.; Carofiglio, T. Chem. Commun. 2014, 50, 9493. (b) Dunny, E.; Doherty, W.; Evans, P.; Malthouse, J. P. G.; Nolan, D.; Knox, A. J. S. J. Med. Chem. 2013, 56, 6638. (c) van der Westhuyzen, R.; Strauss, E. J. Am. Chem. Soc. 2010, 132, 12853. (d) Uttamchandani, M.; Liu, K.; Panicker, R. C.; Yao, S. Q. Chem. Commun. 2007, 1518. (e) Frankel, B. A.; Bentley, M.; Kruger, R. G.; McCafferty, D. G. J. Am. Chem. Soc. 2004, 126, 3404. (10) For selected reviews, see: (a) Pan, X.-Q.; Zou, J.-P.; Yi, W.-B.; Zhang, W. Tetrahedron 2015, 71, 7481. (b) Fang, Y.; Luo, Z.; Xu, X. RSC Adv. 2016, 6, 59661. (c) Meadows, D. C.; Gervay-Hague, J. Med. Res. Rev. 2006, 26, 793. (11) (a) Zhou, K.; Xia, H.; Wu, J. Org. Chem. Front. 2017, 4, 1121. (b) Chen, F.; Meng, Q.; Han, S.-Q.; Han, B. Org. Lett. 2016, 18, 3330. (c) Ning, Y.; Ji, Q.; Liao, P.; Anderson, E. A.; Bi, X. Angew. Chem., Int. Ed. 2017, 56, 13805. (12) Recent examples on sulfonyloxidation of alkynes: (a) Lu, Q.; Zhang, J.; Zhao, G.; Qi, Y.; Wang, H.; Lei, A. J. Am. Chem. Soc. 2013, 135, 11481. (b) Senadi, G. C.; Guo, B.-C.; Hu, W.-P.; Wang, J.-J. Chem. Commun. 2016, 52, 11410. (c) Taniguchi, N. Tetrahedron 2014, 70, 1984. (d) Handa, S.; Fennewald, J. C.; Lipshutz, B. H. Angew. Chem., Int. Ed. 2014, 53, 3432. (13) For selected examples on halosulfonylation of alkynes, see: (a) Gao, Y.; Wu, W.; Huang, Y.; Huang, K.; Jiang, H. Org. Chem. Front. 2014, 1, 361. (b) Zeng, X.; Ilies, L.; Nakamura, E. Org. Lett. 2012, 14, 954. (c) Sun, Y.; Abdukader, A.; Lu, D.; Zhang, H.; Liu, C. Green Chem. 2017, 19, 1255. (d) Li, X.; Shi, X.; Fang, M.; Xu, X. J. Org. Chem. 2013, 78, 9499. (e) Zeng, K.; Chen, L.; Chen, Y.; Liu, Y.; Zhou, Y.; Au, C.-T.; Yin, S.-F. Adv. Synth. Catal. 2017, 359, 841.
cleavage. This 1,1-difunctionalization of alkynes not only complements the methods of coupling−addition of alkynes but also substantially expands the scope of sulfonylation reactions. Displaying stereoselectivity, functional group diversity, and step economy, the method has high applicational potential in organic and biochemistry.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01585. Experimental procedures, full spectroscopic data, and copies of 1H, 13C and 19F spectroscopies (PDF) Accession Codes
CCDC 1821769 and 1821777 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]. *E-mail:
[email protected]. ORCID
Yongbo Zhou: 0000-0002-3540-8618 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by the National NSF of China (Grant Nos. 21573065, 21706058, 21725602, and 21573064) and the NSF of Hunan Province (Grant No. 2016JJ1007) is appreciated.
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REFERENCES
(1) For selected books and reviews on transition metal-catalyzed transformations of alkynes, see: (a) Trost, B. M.; Li, C.-J. Modern alkyne chemistry; Wiley-VCH: Weinheim, 2014. (b) Diederich, F.; Stang, P. J.; Tykwinski, R. R. Acetylene Chemistry; Wiley-VCH: Weinheim, 2005. (c) Chinchilla, R.; Nájera, C. Chem. Rev. 2014, 114, 1783. (d) Koike, T.; Akita, M. Acc. Chem. Res. 2016, 49, 1937. (e) Negishi, E.-i.; Huang, Z.; Wang, G.; Mohan, S.; Wang, C.; Hattori, H. Acc. Chem. Res. 2008, 41, 1474. (f) Flynn, A. B.; Ogilvie, W. W. Chem. Rev. 2007, 107, 4698. (g) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2004, 104, 3079. (h) Boyarskiy, V. P.; Ryabukhin, D. S.; Bokach, N. A.; Vasilyev, A. V. Chem. Rev. 2016, 116, 5894. (2) For selected examples on difunctionalization of alkyne with one compound, see: (a) Suero, M. G.; Bayle, E. D.; Collins, B. S. L.; Gaunt, M. J. J. Am. Chem. Soc. 2013, 135, 5332. (b) Rabet, P. T. G.; Boyd, S.; Greaney, M. F. Angew. Chem., Int. Ed. 2017, 56, 4183. (c) Nakao, Y.; Oda, S.; Hiyama, T. J. Am. Chem. Soc. 2004, 126, 13904. (d) Wang, X.; Studer, A. J. Am. Chem. Soc. 2016, 138, 2977. (e) Tomita, R.; Koike, T.; Akita, M. Angew. Chem., Int. Ed. 2015, 54, 12923. (3) For selected examples on difunctionalization of alkyne with two compounds, see: (a) Li, Z.; García-Domínguez, A.; Nevado, C. J. Am. Chem. Soc. 2015, 137, 11610. (b) Barrado, A. G.; Zieliński, A.; Goddard, R.; Alcarazo, M. Angew. Chem., Int. Ed. 2017, 56, 13401. (c) Li, Z.; García-Domínguez, A.; Nevado, C. Angew. Chem., Int. Ed. 2016, 55, 6938. (d) Tan, G.; Zhu, L.; Liao, X.; Lan, Y.; You, J. J. Am. D
DOI: 10.1021/acs.orglett.8b01585 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters (14) Recent examples on sulfonylcarbonation of alkynes: (a) GarcíaDominguez, A.; Müller, S.; Nevado, C. Angew. Chem., Int. Ed. 2017, 56, 9949. (b) Xiang, Y.; Li, Y.; Kuang, Y.; Wu, J. Chem. - Eur. J. 2017, 23, 1032. (c) Xiang, Y.; Li, Y.; Kuang, Y.; Wu, J. Adv. Synth. Catal. 2017, 359, 2605. (d) Wen, J.; Wei, W.; Xue, S.; Yang, D.; Lou, Y.; Gao, C.; Wang, H. J. Org. Chem. 2015, 80, 4966. (e) Wei, W.; Cui, H.; Yang, D.; Yue, H.; He, C.; Zhang, Y.; Wang, H. Green Chem. 2017, 19, 5608. (15) For the catalytic ability of Pd(OAc)2 could be enhanced by acids, see; Liu, C.; Ding, L.; Guo, G.; Liu, W.; Yang, F. L. Org. Biomol. Chem. 2016, 14, 2824. (16) Zhu, G.; Chen, D.; Wang, Y.; Zheng, R. Chem. Commun. 2012, 48, 5796 and references therein.
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DOI: 10.1021/acs.orglett.8b01585 Org. Lett. XXXX, XXX, XXX−XXX
