Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Copper-Catalyzed Intermolecular Carboamination of Alkenes Induced by Visible Light Yang Xiong, Xiaodong Ma, and Guozhu Zhang* State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Center for Excellence in Molecular Synthesis, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P.R. China
Org. Lett. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 02/25/19. For personal use only.
S Supporting Information *
ABSTRACT: A photoinduced copper-catalyzed three-component reaction involving carbohalide, alkene and amine has been developed, leading to valuable fluoroalkyl-containing amines. A sole inexpensive CuCl is used as the photo- and coupling catalyst. A broad array of substrates are capable coupling partners. The diverse method is compatible with a broad range of functional groups and can be further applied to the late-stage functionalization of bioactive pharmaceuticals.
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on photoexcited copper(I)−hetero nucleophile (N, S, O) complexes which engage in facile SET reduction of alkyl halides to produce alkyl radicals.10a Recombination of these radicals with nucleophilic ligands lead to new C−C or hetero bond formation.10 Inspired by those elegant works, also in line with our continuing interest in radical-type reaction of carbohalides,11a we suspect a photoinduced copper-catalyzed three-component coupling reaction involving halides, simple amines, and alkenes might be viable by intercepting the alkyl radical with alkene before coupling with copper-chelated amine. Herein, we report the successful implementation of this hypothesis which allows a rapid construction of valuable amines containing a fluoroalkyl group, where an in situ formed copper complex serves as the photo- and coupling catalyst without additional transition metals and sacrificing reagents. The broad substrate scope and mild and green reaction conditions are remarkable. Given that the copper carbozolide complex has been established to be a capable photocatalyst to activate carbohalide in light,10e we chose readily available 9H-carbazole, a common framework in natural products, bioactive molecules, and chiral ligands,11 1,1,1-trifluoro-2-iodoethane, and styrene as model substrates to initiate the study. After extensive experiments, the desired product 9-(4,4,4-trifluoro-1-phenylbutyl)-9H-carbazole d1 was attained in 20% yield in CH3CN (1.5 mL) via CuCl catalysis by using KOtBu as base (Table 1, entry 1). When different bases were investigated (Table 1, entries 2−4), LiOtBu was the optimal choice for this reaction.10 However, when the reaction was performed in other solvents, no detectable amount of the desired product
itrogen-containing organic compounds are of paramount importance in pharmaceuticals and functional materials.1 Amines bearing a fluoroalkyl group, such as perfluoroalkyl, trifluoromethyl, or difluoromethyl, are vital synthetic building blocks and advanced intermediates for pharmaceutical and agricultural chemicals because of the ability of these fluorinated moieties to change the physical and biological properties of host compounds.2 Indeed, the development of novel C−N bond-forming methods for making amines containing fluoroalkyl moieties has attracted sustained interest.3 The carboamination of alkenes is one imperative means of such reactions and features simultaneous introduction of two functionalities into alkene frameworks.4 For instance, recently, several groups have reported copper-catalyzed intramolecular aminotrifluoromethylation of alkenes with nitrogen-based nucleophiles by using Togni’s reagents or trifluoromethylsulfonyl chlorides as the CF3 source, affording fluoroalkylcontaining azirines, aziridines, pyrrolidines, lactams, and indolines.5 Additionally, photocatalyzed aminotrifluoromethylation of unactivated alkenes by using Umemoto’s reagent has also been established by several other groups.6 Although continual progress has been achieved in this transformation over the past few years,7 the use of more commercially accessible and cheap fluoroalkyl precursor reagents and simple amines as the amino source are still underexplored. Thus, a new general platform with green and mild reaction conditions, enabling more effective, synthetically convenient, and diverse aminofluoroalkylation of alkenes, is highly desirable. Despite a well-explored and increasing body work regarding photoredox catalysis during the past decades,8 the potential of copper serving as a photocatalyst induced by visible light still in its infancy.9 Fu and Peters recently reported a series of studies © XXXX American Chemical Society
Received: January 21, 2019
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DOI: 10.1021/acs.orglett.9b00252 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Scope Studiesa,b
Table 1. Optimization of the Reaction Conditions
entrya
catalyst
base
solvent
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14c 15d
CuCl CuCl CuCl CuCl CuCl CuCl CuCl CuI CuBr (CH3CN)4CuPF6 CuOAc NO CuCl CuCl CuCl
KOtBu NaOtBu LiOtBu KF LiOtBu LiOtBu LiOtBu LiOtBu LiOtBu LiOtBu LiOtBu LiOtBu NO LiOtBu LiOtBu
CH3CN CH3CN CH3CN CH3CN DMF EtOAc toluene CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN
20 30 85 52 ND ND ND 70 72 60 60 ND trace ND 82e
a
0.2 mmol scale. bMeasured by 1H NMR analysis using diethyl phthalate as internal standard. cNo light. d0.4 mmol scale. eIsolated yield.
was found (Table 1, entries 5−7). CuCl could give a higher yield than other copper salts (Table 1, entries 8−11). Some control experiments establish the essential role of CuCl, LiOtBu, and light (Table 1, entries 12−14). The optimized reaction conditions could afford d1 in 82% isolated yield (Table 1, entry 15). First, we surveyed a variety of alkenes, and the results are shown in Scheme 1. The styrenes containing electron-donating and -withdrawing groups could work well in this method. Meanwhile, the functional groups of halides, ester, ether, and CF3 could also be tolerated to afford products d2−d11 in satisfactory yields. For the disubstituted styrenes, the reactions proceeded efficiently to give d12 and d13. Alkenes bearing a heterocycle and ferrocene also reacted well to produce products d14 and d15 in satisfactory yields. Compared with monosubstituted styrenes, the prop-1-en-2-ylbenzene exhibited a little lower reactivity to provide product d16 in 53% yield. In addition, 1,3-dienes were used in the carboamination to give the desired products with good 1,4-selectivity (d17−d18). Notably, when unactivated aliphatic alkene was treated with the above optimized reaction conditions, the yield of product (d19) was 40%. Difluoromethylene (CF2), which is a significant bioisostere of the oxygen or a carbonyl group, could increase the dipole moment and acidity of neighboring groups, often resulting in conformational changes.12 Encouraged by the above results, the commercially accessible 1,1-difluoro-2-iodoethane was used in the difluoromethylation. The reaction proceeded successfully to afford the corresponding difluoromethylated product d20 in 53% yield (Scheme 2). The easily prepared (2-bromo-2,2-difluoroethoxy)-tert-butyldimethylsilane and ((2-bromo-2,2-difluoroethoxy)methyl)benzene were also good difluoroalkylating reagents to obtain d21 and d22 in good yields. The three-component coupling of other polyfluoroalkyl iodides including 1,1,2,2-tetrafluoro-3iodopropane and 1,1,1,2,2-pentafluoro-4-iodobutane performed well and produced the desired products d23 and d24
a
0.2 mmol scale. bIsolated yield. c0.3 mmol scale, for 72 h.
Scheme 2. Scope Studiesa,b
a
0.2 mmol scale. bIsolated yield.
in good yields. In further substrate scope surveys, some typical nonfluoro-substituted alkyl halides including secondary and tertiary alkyl halides were shown to be good substrates as well (d24−d28). Interestingly, dichloromethane could be photoexcited as well, leading to monochlorinated product in moderate yield. Reaction of 2-iodo-1,3-dimethylbenzene could also give product d30 albeit in relatively low yield. In further substrate scope studies on variation of amines, we were delighted to find substituted 9H-carbazoles (d31 and B
DOI: 10.1021/acs.orglett.9b00252 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
d42−d49. In addition, the nonfluoro-substituted alkyl halide iodocyclohexane was a capable substrate as well (d50). The 1,1-disubstituted styrene also exhibited good reactivity to give product d51 in 65% yield. In an effort to show the synthetic potential in late-stage functionalization of relatively medicinal amines, two pharmaceutical agents were selected and subjected to the carboamination reaction.1e Both architecturally complex amines were compatible with the reactions to furnish the desired products d52 and d53. To understand the mechanism of this transformation, some preliminary control experiments were conducted. With addition of TEMPO, the reactions were inhibited and no desired product d1 or d37 was observed [Scheme 4, eq (1)].
d32). Indole and indazole derivatives, as common subunits in bioactive compounds,10e also reacted smoothly to give the corresponding products (d33−d36) in good yield (Scheme 3). Scheme 3. Scope Studiesa,b
Scheme 4. Preliminary Mechanistic Study
To further elucidate the mechanism, a radical-clock experiment of 3-(allyloxy)-prop-1-ene was probed to give cyclized product e2 in dr = 15:1 (measured by 19F NMR) (Scheme 4, eq 2).13b−d These preliminary studies are in line with a stepwise radical process.13 When other nucleophiles (1.5 equiv) such as methanol, naphthol, 3,4-dihydroquinolin-2(1H)-one, and Nmethyl-cyclohexanamine were added, the yield of d1 was mostly unaffected, and the additives were almost recovered. Those results suggest the involvement of cationic intermediates is less likely. On the basis of literature precedents10 and our own observations, an outline of a plausible reaction mechanism is illustrated in Figure 1. First, LnCuCl undergoes ligand a
Isolated yield. b0.2 mmol scale, according to conditions A. c0.2 mmol scale, according to conditions B.
However, our initial attempt to apply the standard conditions to a further reduced π-system such as simple indoline was unsuccessful, possibly due to the decreased capability of the resulting Cu−Nu complex in photoexcitation or the next SET to electrophile for alkyl radical generation. When we adopted the reaction conditions from Fu’s elegant example using racBINOL as the photocatalyst for coupling of primary aliphatic amine with carbohalide,13a the desired product d37 did form in trivial amounts. By further optimizing the reaction conditions, we found that using 20 mol % rac-BINOL as a ligand and CH3CN/DMA (1.5 mL/0.3 mL) as solvent could provide the product d37 in 75% yield. The modified reaction conditions were successfully applied to a broad array of simple amines. Heteroindoline and tetrahydroquinoline were proven to be capable substrates (d38 and d39). Reactions of arylamines proceeded efficiently (d40 and d41); primary alkylamines and benzenesulfonamide also worked well to give the products
Figure 1. Proposed reaction mechanism.
exchange with nucleophile to provide LnCu(I)Nu. Then, as previously suggested, irradiation of the copper(I)−nucleophile complex could lead to an excited-state adduct B. The nucleophilic site within a π system (carbazole, indole, and imidazole) might be the key role for the viability of initial photoexcitation of the copper(I)−amido complex. When nucleophiles lack this feature (simple amines), copper(I)− binaphtholate complex A2 serves as the primary photoreductant.13a,14 The excited-state complex B would then C
DOI: 10.1021/acs.orglett.9b00252 Org. Lett. XXXX, XXX, XXX−XXX
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2015, 115, 650. (d) Gu, J.; Min, Q.; Yu, L.; Zhang, X. Angew. Chem., Int. Ed. 2016, 55, 12270. (e) Yu, L.; Xu, Q.; Tang, X.; Shi, M. ACS Catal. 2016, 6, 526. (f) Jarrige, L.; Carboni, A.; Dagousset, G.; Levitre, G.; Magnier, E.; Masson, G. Org. Lett. 2016, 18, 2906. (g) Jiang, H.; Studer, A. Angew. Chem., Int. Ed. 2018, 57, 10707. (4) For selected papers, see: (a) Mai, D. N.; Wolfe, J. P. J. Am. Chem. Soc. 2010, 132, 12157. (b) Casavant, B. J.; Hosseini, A. S.; Chemler, S. R. Adv. Synth. Catal. 2014, 356, 2697. (c) Um, C.; Chemler, S. R. Org. Lett. 2016, 18, 2515. (d) Piou, T.; Rovis, T. Nature 2015, 527, 86. (e) Cheng, J.; Qi, X.; Li, M.; Chen, P.; Liu, G. J. Am. Chem. Soc. 2015, 137, 2480. (f) Yin, G.; Mu, X.; Liu, G. Acc. Chem. Res. 2016, 49, 2413. (g) Liu, Z.; Wang, Y.; Wang, Z.; Zeng, T.; Liu, P.; Engle, K. M. J. Am. Chem. Soc. 2017, 139, 11261. (h) Liu, Y.; Yang, X.; Song, R.; Luo, S.; Li, J. Nat. Commun. 2017, 8, 14720. (i) Qian, B.; Chen, S.; Wang, T.; Zhang, X.; Bao, H. J. Am. Chem. Soc. 2017, 139, 13076. (j) Bunescu, A.; Ha, T. M.; Wang, Q.; Zhu, J. Angew. Chem., Int. Ed. 2017, 56, 10555. (k) Bao, X.; Yokoe, T.; Ha, T. M.; Wang, Q.; Zhu, J. Nat. Commun. 2018, 9, 3725. (l) Gockel, S. N.; Buchanan, T. L.; Hull, K. L. J. Am. Chem. Soc. 2018, 140, 58. (5) (a) Egami, H.; Kawamura, S.; Miyazaki, A.; Sodeoka, M. Angew. Chem., Int. Ed. 2013, 52, 7841. (b) Lin, J.; Xiong, Y.; Ma, C.; Zhao, L.; Tan, B.; Liu, X. Chem. - Eur. J. 2014, 20, 1332. (c) Wang, F.; Qi, X.; Liang, Z.; Chen, P.; Liu, G. Angew. Chem., Int. Ed. 2014, 53, 1881. (d) Kawamura, S.; Egami, H.; Sodeoka, M. J. Am. Chem. Soc. 2015, 137, 4865. (e) Shen, K.; Wang, Q. Org. Chem. Front. 2016, 3, 222. (f) Lin, J.; Dong, X.; Li, T.; Jiang, N.; Tan, B.; Liu, X. J. Am. Chem. Soc. 2016, 138, 9357. (g) Li, X.; Lin, J.; Liu, X. Synthesis 2017, 49, 4213. (h) Lin, J.; Wang, F.; Dong, X.; He, W.; Yuan, Y.; Chen, S.; Liu, X. Nat. Commun. 2017, 8, 14841. (6) (a) Yasu, Y.; Koike, T.; Akita, M. Org. Lett. 2013, 15, 2136. (b) Dagousset, G.; Carboni, A.; Magnier, E.; Masson, G. Org. Lett. 2014, 16, 4340. (c) Wei, Q.; Chen, J.; Hu, X.; Yang, X.; Lu, B.; Xiao, W. Org. Lett. 2015, 17, 4464. (d) Noto, N.; Koike, T.; Akita, M. Chem. Sci. 2017, 8, 6375. (7) For selected papers, see: (a) Kim, E.; Choi, S.; Kim, H.; Cho, E. J. Chem. - Eur. J. 2013, 19, 6209. (b) Zhang, Z.; Tang, X.; Thomoson, C. S.; Dolbier, W. R. Org. Lett. 2015, 17, 3528. (c) Yu, L.; Wei, Y.; Shi, M. Chem. Commun. 2016, 52, 13163. (d) Kawamura, S.; Dosei, K.; Valverde, E.; Ushida, K.; Sodeoka, M. J. Org. Chem. 2017, 82, 12539. (e) Tian, Y.; Chen, S.; Gu, Q.; Lin, J.; Liu, X. Tetrahedron Lett. 2018, 59, 203. (f) Koike, T.; Akita, M. Chem. 2018, 4, 409. (8) For selected recent selected papers on visible-light photocatalysis, see: (a) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527. (b) Xuan, J.; Xiao, W. Angew. Chem., Int. Ed. 2012, 51, 6828. (c) Nguyen, J. D.; D'Amato, E. M.; Narayanam, J. M.; Stephenson, C. R. J. Nat. Chem. 2012, 4, 854. (d) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322. (e) Hari, D. P.; König, B. Angew. Chem., Int. Ed. 2013, 52, 4734. (f) Brimioulle, R.; Lenhart, D.; Maturi, M. M.; Bach, T. Angew. Chem., Int. Ed. 2015, 54, 3872. (g) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. J. Org. Chem. 2016, 81, 6898. (h) Kärkäs, M. D.; Porco, J. A., Jr.; Stephenson, C. R. J. Chem. Rev. 2016, 116, 9683. (i) Ravelli, D.; Protti, S.; Fagnoni, M. Chem. Rev. 2016, 116, 9850. (j) Liu, Q.; Wu, L. Natl. Sci. Rev. 2017, 4, 359. (k) Zhou, Q.; Zou, Y.; Lu, L.; Xiao, W. Angew. Chem., Int. Ed. 2019, 58, 1586. (9) For selected papers, see: (a) Paria, S.; Reiser, O. ChemCatChem 2014, 6, 2477. (b) Fumagalli, G.; Rabet, P. T. G.; Boyd, S.; Greaney, M. F. Angew. Chem., Int. Ed. 2015, 54, 11481. (c) Bagal, D. B.; Kachkovskyi, G.; Knorn, M.; Rawner, T.; Bhanage, B. M.; Reiser, O. Angew. Chem., Int. Ed. 2015, 54, 6999. (d) Knorn, M.; Rawner, T.; Czerwieniec, R.; Reiser, O. ACS Catal. 2015, 5, 5186. (e) Reiser, O. Acc. Chem. Res. 2016, 49, 1990. (f) Sagadevan, A.; Charpe, V. P.; Ragupathi, A.; Hwang, K. C. J. Am. Chem. Soc. 2017, 139, 2896. (g) Li, Y.; Zhou, K.; Wen, Z.; Cao, S.; Shen, X.; Lei, M.; Gong, L. J. Am. Chem. Soc. 2018, 140, 15850. (h) Marzo, L.; Pagire, S. K.; Reiser, O.; König, B. Angew. Chem., Int. Ed. 2018, 57, 10034. (10) For selected papers, see: (a) Creutz, S. E.; Lotito, K. J.; Fu, G. C.; Peters, J. C. Science 2012, 338, 647. (b) Uyeda, C.; Tan, Y.; Fu, G. C.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 9548. (c) Bissember, A.
engage in electron transfer with the halide to afford a radical and LnCu(II)Nu (C).10e,g Next, the radical adds to the olefin, generating an internal radical. With regard to copper(II)− binaphtholate complex C2, ligand exchange with the amine leads to a copper(II)−amido complex C3. Lastly, bond formation between the nucleophile and the radical could occur through two possible pathways. In path a, rejoining of the internal radical and LnCu(II)NR2R3 could generate the Cu(III) complexes D1 and D2, which undergo reductive elimination to afford the products and regenerate the Cu(I) catalyst.10e On the other hand, a SET process (path b) could also give the same products.10a,e,14 In summary, we have reported a highly effective and synthetically convenient method of Cu-catalyzed threecomponent carboamination reaction induced by visible light. The reaction couples are readily available alkenes, a wide variety of halides, and a broad range of amines. Owing to the commercial availability of chemical substances and comparatively simple and mild conditions, this new general platform can further be applied to diverse functionalization of complex amines and positively expected to be used in synthetic and medicinal chemistry.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00252. Research details, experimental procedures, full characterization of products, and NMR spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Guozhu Zhang: 0000-0002-2222-6305 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank NSFC (21772218, 21821002), XDB20000000, the “Thousand Plan” Youth program, State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, and the Chinese Academy of Sciences for financial support.
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REFERENCES
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DOI: 10.1021/acs.orglett.9b00252 Org. Lett. XXXX, XXX, XXX−XXX
