Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Copper-Catalyzed Direct Oxidative C−H Functionalization of Unactivated Cycloalkanes into Cycloalkyl Benzo[b]phosphole Oxides Dumei Ma,† Jiaoting Pan,‡ Lu Yin,† Pengxiang Xu,‡ Yuxing Gao,*,‡ Yingwu Yin,† and Yufen Zhao‡ †
Department of Chemical and Biochemical Engineering, and ‡Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China S Supporting Information *
ABSTRACT: The first simple and efficient Cu-catalyzed radical addition/cyclization of various unactivated cycloalkanes with diaryl(arylethynyl)-phosphine oxides has been developed, providing a general, one-step approach to construct a new class of important benzo[b]phosphole oxides via sequential C−H functionalization along with two new C−C bond formations.
P
reactions and relatively lower yields. Furthermore, the properties of various obtained benzo[b]phospholes have already been extensively investigated, and these results have demonstrated that their photoelectric properties are highly dependent on their structure, and selecting an appropriate chemical modification of the benzo[b]phosphole core could finely tune their electronic and structural properties for desired applications.2,6 However, only a limited number of benzo[b]phosphole derivatives have been reported to date. Therefore, the development of convenient and efficient procedures for synthesizing a new class of benzo[b]phospholes from simple starting materials is still desirable and essential. Over the past several years, transition-metal-catalyzed direct functionalization of saturated C−H bonds for selective C−C and C−heteroatom bond formation has emerged as an appealing and powerful methodology in modern organic synthesis, owing to its remarkable advantages of step- and atom-economy and environmental sustainability in industrial and green chemistry, and has made significant progress.7 One of the most attractive methods for the inactive C−H functionalization is the direct activation of unreactive C(sp3)−H bonds of cheap cycloalkanes through the oxidation involving C(sp3)−H bond cleavage from environmental and economic points of view.8 On the basis of this strategy, a variety of commercially available and low-cost cycloalkanes could be transformed to a wide range of synthetically useful molecular frames. However, the cycloalkanes are difficult to activate in reactions, probably due to their nonpolarity leading to weak interactions between the bond and other species such as transition metals. Thus, looking for the further synthetic application of cycloalkanes is a continuously challenging task for organic chemists. Recently, we have reported a TBAI-catalyzed radical addition/ cyclization of diaryl(arylethynyl)phosphine oxides with simple toluene derivatives for the preparation of benzo[b]phosphole
hosphorus-containing heterocycles have gained considerable attention from synthetic chemists over the past few decades, owing to their wide applicability in organic synthesis, medicinal chemistry, and material science.1 Among them, benzo[b]phosphole derivatives, the phosphorus analogue of indole, have recently received much attention as advanced organic optoelectronic materials due to their unique electronic and photophysical properties.2 However, synthetic approaches to such promising frameworks are relatively scarce. The majority of these methods involve the intermolecular cyclization of alkynylarenes bearing ortho-phosphorus substituents with a stoichiometric amount of an organometallic strong base (Scheme 1a),3 but their general use poses severe limitations due to the Scheme 1. Synthetic Strategies towards Benzo[b]phosphole Derivatives
complicated multistep synthesis of the cyclization precursors and poor functional group compatibility. Recently, protocols based on the intermolecular oxidative annulation of secondary phosphine oxides and internal alkynes have been developed (Scheme 1b),4 but these methods used large amounts of environmentally unfriendly Mn or Cu salts, noble Ag salts, or other oxidants. Multicomponent approaches to benzo[b]phospholes have also been reported recently,5 which suffer from complicated multistep © XXXX American Chemical Society
Received: April 9, 2018
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DOI: 10.1021/acs.orglett.8b01108 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Optimization of the Reaction Conditionsa
oxides via sequential C−H functionalization.9 Considering the importance of developing a new family of benzo[b]phospholes during the exploration process of advanced optoelectronic materials, it would be synthetically valuable and demanding if this protocol could be expanded to the construction of various novel cycloalkyl benzo[b]phosphole oxides. However, when we attempted to extend the substrate scope to readily available cycloalkanes, we found the yields are relatively low under the previously optimized conditions. Obviously, an alternative catalyst is necessary to make the direct C−H functionalization of cycloalkanes more efficient. We rationalized that introducing a transition-metal catalyst instead of TBAI might be beneficial to the generation and stability of a cycloalkyl radical and would enhance the yields dramatically. As part of our ongoing endeavors to exploit the new approaches to phosphorus-containing heterocycles,4e,9,10 herein, we would like to report our further significant progress in this area: the synthesis of structurally diverse cycloalkyl benzo[b]phosphole oxides directly from diaryl(arylethynyl)phosphine oxides and cycloalkanes via successive cleavage of C(sp3)−H and C(sp2)−H bonds along with the formation of two new C−C bonds by a Cu-catalyzed radical addition/cyclization using DTBP as an oxidant. To the best of our knowledge, the example of cycloalkyl benzo[b]phosphole oxide formation via a Cu-catalyzed oxidative radical addition/cyclization of unactivated cycloalkanes with diaryl(arylethynyl)phosphine oxides has yet to be reported (Scheme 1c). Initially, we began our studies by evaluating the previously optimized conditions9 for the direct C−H functionalization of cyclohexane (2a) with diphenyl(phenylethynyl)phosphine oxide (1a); however, this TBAI-DTBP catalytic system only gave the desired product 3a in a lower yield of 32% (entry 1). We first envisioned that introducing a transition-metal catalyst would improve the yield of 3a. To our delight, when CuI was used as the catalyst, the yield of 3a could be significantly increased to 84% (entry 2). Encouraged by this result, other transition-metal catalysts, including CuBr, CuCl, Cu(OAc)2, NiCl2, FeCl2, and FeCl3 were further investigated (entries 3−8), and the results showed that CuBr was the optimal catalyst for this reaction and could enhance the yield up to 90% (entry 3). Only in the presence of DTBP, the reaction still occurred, but gave an unsatisfactory yield of 49% (entry 9). The loading of CuBr was also evaluated, yet using 5 mol % and 15 mol % of CuBr resulted in a decrease of the yield (entries 10−11). To advance the process further, other oxidants such as TBHP and H2O2 were also surveyed, with the findings that TBHP only afforded a moderate yield of 50% and that H2O2 was ineffective for the reaction (entries 12−13). In addition, changing the loading of DTBP did not increase the yield (entries 14−15). Notably, no desired product was observed in the absence of DTBP (entry 16). The results revealed that DTBP plays a crucial role in achieving a high yield of 3a. Finally, the effect of temperature was detected, and we found that increasing or decreasing the temperature from 140 °C gave rise to the yield reduction (entries 17−18). With the optimized reaction conditions in hand (Table 1, entry 3), other cycloalkanes were further tested in this reaction (Scheme 2). We found that various cycloalkanes such as cyclopentane 2b, cycloheptane 2e, cyclodecane 2f, cyclododecane 2g, norbornane 2h, adamantine 2i, and norbornane 2j were all suitable reaction partners in this transformation and generated the corresponding products (3b−3j) in moderate to excellent yields. The C−H bond functionalization of adamantine 2i containing four methine C−H bonds and 12 methylene C−H
entry
catalyst
oxidant
temp (°C)
yield (%)b
1c 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
TBAI (20) CuI (10) CuBr (10) CuCl (10) Cu(OAc)2 (10) NiCl2 (10) FeCl2 (10) FeCl3 (10)
DTBP (3) DTBP (3) DTBP (3) DTBP (3) DTBP (3) DTBP (3) DTBP (3) DTBP (3) DTBP (3) DTBP (3) DTBP (3) TBHP (3) H2O2 (3) DTBP (2) DTBP (4)
140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 140 150 120
32 84 90 83 39 65 42 70 49 81 83 50 0 55 65 0 75 56
CuBr (5) CuBr (15) CuBr (10) CuBr (10) CuBr (10) CuBr (10) CuBr (10) CuBr (10) CuBr (10)
DTBP (3) DTBP (3)
a
Reaction conditions: 1a (0.3 mmol), cyclohexane 2a (2 mL), catalyst (mol %), and oxidant (equiv) at the indicated temperature for 12 h under Ar. DTBP = di-tert-butyl peroxide. TBHP = tert-butyl hydroperoxide 70% in water. H2O2 30% in water. bIsolated yields. c Using pyridine (1.0 equiv) at 140 °C for 30 h under argon.
Scheme 2. Cu-Catalyzed Radical Addition/Cyclization of 1 with Alkanesa
a Reaction conditions: 1 (0.3 mmol), alkane 2 (2 mL), CuBr (10 mol %), DTBP (3 equiv) at 140 °C for 12 h under Ar. Isomer ratio determined by 1H NMR and 31P NMR analysis; structure and yield given for the major isomer isolated.
bonds occurred selectively at the less hindered tertiary site, and no methylene C−H bond functionalization product was isolated. According to the analysis of NMR spectra,11 oxidative C−H functionalization of norbornane 2j proceeded with single-site selectivity and furnished two isomers in an excellent total yield of 90% with a ratio of exo-3j/endo-3j of 3.85:1, without any detectable amounts of the alternative C−H functionalization product at the bridgehead position due to the effect of steric hindrance. In addition, straight-chain substrates such as n-hexane B
DOI: 10.1021/acs.orglett.8b01108 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
(phenylethynyl)di-p-tolylphosphine oxide was employed, a mixture of regioisomeric products 3w were formed as a result of a radical mechanism involving the similar rearrangement of the radical aryl migration on the P atom.4a,b To demonstrate the utility of the current method, a gram-scale experiment was performed by employing 1a (10 mmol, 3.02 g) with 2a under the standard reaction conditions and provided the desired product 3a in a good yield of 61% (Scheme 4a), revealing
were also evaluated, and the different regioisomers (3k) were obtained in a good overall yield of 70% with a ratio of C1-3k/C23k/C3-3k of 1:2:2. These results clearly indicate that the reactivity of the tertiary C−H bond is higher than that of the secondary one and that the secondary C−H bond was more active than primary C−H bond, which is consistent with the stability order of the radicals. It is noteworthy that a cyclic ether dioxane was also an effective substrate, and the expected product 3l was obtained in a moderate yield of 52%. To gain more insight into the substrate scope, the direct C−H functionalization of cyclohexane 2a with a variety of diaryl(arylethynyl)phosphine oxides 1 was evaluated, and the results are summarized in Scheme 3. Gratifyingly, this method was found
Scheme 4. Application Studies
Scheme 3. Cu-Catalyzed Radical Addition/Cyclization of Diaryl(phenylethynyl)phosphine Oxides with 2aa
this reaction could be effectively scaled up with high efficiency. Furthermore, as expected, 2a could react with diyne 1x to produce a bis(benzophosphole-3-yl)benzene framework, which has been suggested to have potential applications in organic electronics (Scheme 4b).2b,c,3a To gain insight into the reaction mechanism, the kinetic isotope effect was detected through the reaction of deuterated cyclohexane with 1a, and a significant KIE was observed with the KH/ KD = 7.33 at a low conversion level (Scheme 5a), showing that the Scheme 5. Experiments for the Mechanism Study
a
Reaction conditions: 1 (0.3 mmol), cycloalkane 2a (2 mL), CuBr (10 mol %), DTBP (3 equiv) at 140 °C for 12 h under Ar. Isolated yields.
to be quite general, and diverse benzo[b]phosphole oxides can be conveniently and efficiently obtained by this simple and cheap Cu-catalyzed radical addition/cyclization. Generally, diaryl(arylethynyl)phosphine oxides bearing electron-donating groups and electron-withdrawing groups at the aryl ring were all suitable reaction partners in this transformation and afforded the corresponding oxidative addition/cyclization products (3m−3w) in moderate to excellent yields. Thus, several substituents on the aromatic ring, such as CH3, t-Bu, MeO, F, Cl, Br, CF3, CN, and CO, were all well tolerated for this method, and their electronic nature was not evident for this reaction. Notably, chloro- and bromo-substituted substrates could participate in this reaction to generate the desired products (3q and 3r) in 79% and 70% yields, respectively, thereby serving as handles for further synthetic manipulations. Additionally, 1 having strong electron-withdrawing groups such as trifluoromethyl and cyano could also be compatible with this protocol and gave the relative products 3s and 3t in good yields. Interestingly, the substrate containing a reactive carbonyl unit could also undergo the reaction to produce the desired benzo[b]phosphole oxide 3u selectively in 82% yield without needing any protection of the carbonyl group. The reaction could also tolerate a heterocyclic substrate 1v, which afforded the desired heterocyclic product 3v in 46% yield. It is noteworthy that when
C−H bond cleavage on the cyclohexane may be one of the ratelimiting steps of this procedure.12 In addition, the radical scavenger effect was also examined, and the addition of 2 equivalents of BHT as a radical scavenger completely suppressed this reaction, indicating the reaction probably proceeded via a free radical process (Scheme 5b). On the basis of these experimental results and the literature survey,4a,b,13 a reasonable radical-triggered mechanism is proposed in Scheme 6. Initially, the tert-butoxyl radical and CuII(OBu-t) were generated from DTBP under heating with the aid of the active CuI species.13 Then, the tert-butoxyl radical abstracted the hydrogen atom of cyclohexane 2a to afford alkyl radical 4, followed by the selective addition of 4 to the α-position of the PO bond in 1a that produced an alkenyl radical 5. Finally, based on the generation of two regioisomeric products, the radical 5 might undergo two pathways to offer the desired product 3a through the dehydrogenation of intermediate 6 or 9 with CuII(OBu-t) via a similar process originally proposed by Duan and Miura.4a,b In conclusion, we have developed a facile and efficient Cucatalyzed radical addition/cyclization of various unfunctionalized cycloalkanes with diaryl(arylethynyl)phosphine oxides, affording a rapid strategy for a structurally diverse array of cycloalkyl C
DOI: 10.1021/acs.orglett.8b01108 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Chem. - Asian J. 2014, 9, 1212. (f) Bu, F.; Wang, E.; Peng, Q.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z. Chem. - Eur. J. 2015, 21, 4440. (g) Joly, D.; Bouit, P.-A.; Hissler, M. J. Mater. Chem. C 2016, 4, 3686. (h) Hibner-Kulicka, P.; Joule, J. A.; Skalik, J.; Bałczewski, P. RSC Adv. 2017, 7, 9194. (2) Selected publications: (a) Matano, Y.; Imahori, H. Org. Biomol. Chem. 2009, 7, 1258. (b) Tsuji, H.; Sato, K.; Sato, Y.; Nakamura, E. J. Mater. Chem. 2009, 19, 3364. (c) Tsuji, H.; Sato, K.; Sato, Y.; Nakamura, E. Chem. - Asian J. 2010, 5, 1294. (d) Matano, Y.; Saito, A.; Fukushima, T.; Tokudome, Y.; Suzuki, F.; Sakamaki, D.; Kaji, H.; Ito, A.; Tanaka, K.; Imahori, H. Angew. Chem., Int. Ed. 2011, 50, 8016. (e) Ren, Y.; Baumgartner, T. Dalton Trans. 2012, 41, 7792. (f) Yamaguchi, E.; Wang, C.; Fukazawa, A.; Taki, M.; Sato, Y.; Sasaki, T.; Ueda, M.; Sasaki, N.; Higashiyama, T.; Yamaguchi, S. Angew. Chem., Int. Ed. 2015, 54, 4539. (g) Duffy, M. P.; Delaunay, W.; Bouit, P.-A.; Hissler, M. Chem. Soc. Rev. 2016, 45, 5296. (h) Matsumura, M.; Yamada, M.; Muranaka, A.; Kanai, M.; Kakusawa, N.; Hashizume, D.; Uchiyama, M.; Yasuike, S. Beilstein J. Org. Chem. 2017, 13, 2304. (3) (a) Tsuji, H.; Sato, K.; Ilies, L.; Itoh, Y.; Sato, Y.; Nakamura, E. Org. Lett. 2008, 10, 2263. (b) Sanji, T.; Shiraishi, K.; Kashiwabara, T.; Tanaka, M. Org. Lett. 2008, 10, 2689. (c) Fukazawa, A.; Hara, M.; Okamoto, T.; Son, E.-C.; Xu, C.; Tamao, K.; Yamaguchi, S. Org. Lett. 2008, 10, 913. (d) Fukazawa, A.; Yamada, H.; Yamaguchi, S. Angew. Chem. 2008, 120, 5664. (e) Fukazawa, A.; Ichihashi, Y.; Kosaka, Y.; Yamaguchi, S. Chem. Asian J. 2009, 4, 1729. (f) Xu, Y.; Wang, Z.; Gan, Z.; Xi, Q.; Duan, Z.; Mathey, F. Org. Lett. 2015, 17, 1732. (4) (a) Chen, Y.-R.; Duan, W.-L. J. Am. Chem. Soc. 2013, 135, 16754. (b) Unoh, Y.; Hirano, K.; Satoh, T.; Miura, M. Angew. Chem., Int. Ed. 2013, 52, 12975. (c) Zhang, P.; Gao, Y.; Zhang, L.; Li, Z.; Liu, Y.; Tang, G.; Zhao, Y. Adv. Synth. Catal. 2016, 358, 138. (d) Quint, V.; MorletSavary, F.; Lohier, J.-F.; Lalevée, J.; Gaumont, A.-C.; Lakhdar, S. J. Am. Chem. Soc. 2016, 138, 7436. (e) Ma, D.; Chen, W.; Hu, G.; Zhang, Y.; Gao, Y.; Yin, Y.; Zhao, Y. Green Chem. 2016, 18, 3522. (5) (a) Wu, B.; Santra, M.; Yoshikai, N. Angew. Chem., Int. Ed. 2014, 53, 7543. (b) Wu, B.; Chopra, R.; Yoshikai, N. Org. Lett. 2015, 17, 5666. (6) (a) Yavari, K.; Moussa, S.; Ben Hassine, B.; Retailleau, P.; Voituriez, A.; Marinetti, A. Angew. Chem. 2012, 124, 6852. (b) Hayashi, Y.; Matano, Y.; Suda, K.; Kimura, Y.; Nakao, Y.; Imahori, H. Chem. - Eur. J. 2012, 18, 15972. (c) Matano, Y.; Hayashi, Y.; Suda, K.; Kimura, Y.; Imahori, H. Org. Lett. 2013, 15, 4458. (d) Yavari, K.; Aillard, P.; Zhang, Y.; Nuter, F.; Retailleau, P.; Voituriez, A.; Marinetti, A. Angew. Chem. 2014, 126, 880. (e) Matano, Y.; Motegi, Y.; Kawatsu, S.; Kimura, Y. J. Org. Chem. 2015, 80, 5944. (f) Shameem, M. A.; Orthaber, A. Chem. - Eur. J. 2016, 22, 10718. (g) Zhuang, Z.; Bu, F.; Luo, W.; Peng, H.; Chen, S.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z. J. Mater. Chem. C 2017, 5, 1836. (7) Selected recent reviews: (a) Li, C.-J. Acc. Chem. Res. 2009, 42, 335. (b) Wencel-Delord, J.; Dröge, T.; Liu, F.; Glorius, F. Chem. Soc. Rev. 2011, 40, 4740. (c) Roizen, J. L.; Harvey, M. E.; Du Bois, J. Acc. Chem. Res. 2012, 45, 911. (d) Jeffrey, J. L.; Sarpong, R. Chem. Sci. 2013, 4, 4092. (e) Noisier, A. F. M.; Brimble, M. A. Chem. Rev. 2014, 114, 8775. (f) Huang, H.; Ji, X.; Wu, W.; Jiang, H. Chem. Soc. Rev. 2015, 44, 1155. (g) Gulías, M.; Mascareñas, J. L. Angew. Chem., Int. Ed. 2016, 55, 11000. (h) Fabry, D. C.; Rueping, M. Acc. Chem. Res. 2016, 49, 1969. (i) Lee, P.S.; Xu, W.; Yoshikai, N. Adv. Synth. Catal. 2017, 359, 4340. (j) Parasram, M.; Gevorgyan, V. Acc. Chem. Res. 2017, 50, 2038. (k) Chu, X.-Q.; Ge, D.; Shen, Z.-L.; Loh, T.-P. ACS Catal. 2018, 8, 258. (l) Mihai, M. T.; Genov, G. R.; Phipps, R. J. Chem. Soc. Rev. 2018, 47, 149. (8) For selected recent publications, see: (a) Wiese, S.; Badiei, Y. M.; Gephart, R. T.; Mossin, S.; Varonka, M. S.; Melzer, M. M.; Meyer, K.; Cundari, T. R.; Warren, T. H. Angew. Chem., Int. Ed. 2010, 49, 8850. (b) Guo, X.; Li, C.-J. Org. Lett. 2011, 13, 4977. (c) Michaudel, Q.; Thevenet, D.; Baran, P. S. J. Am. Chem. Soc. 2012, 134, 2547. (d) Zhao, J.; Fang, H.; Han, J.; Pan, Y.; Li, G. Adv. Synth. Catal. 2014, 356, 2719. (e) Du, B.; Jin, B.; Sun, P. Org. Lett. 2014, 16, 3032. (f) Zhang, H.; Pan, C.; Jin, N.; Gu, Z.; Hu, H.; Zhu, C. Chem. Commun. 2015, 51, 1320. (g) Cheng, Z.-F.; Feng, Y.-S.; Rong, C.; Xu, T.; Wang, P.-F.; Xu, J.; Dai, J.J.; Xu, H.-J. Green Chem. 2016, 18, 4185. (h) Baral, E. R.; Kim, S. H.; Lee, Y. R. Asian J. Org. Chem. 2016, 5, 1134. (i) Tortoreto, C.; Rackl, D.; Davies, H. M. L. Org. Lett. 2017, 19, 770. (j) Li, Z.; Fan, F.; Yang, J.; Liu,
Scheme 6. Proposed Mechanism
benzo[b]phospholes via sequential C(sp3)−H/C(sp2)−H functionalization. Notably, the formation of new C(sp3)−C(sp2) bonds selectively occurred at the electron-rich position of alkynes. Importantly, the use of commercially available, cheap and highly stable CuBr as a catalyst represents an added advantage of the method. Meanwhile, various novel cycloalkyl benzo[b]phosphole products can be conveniently obtained in a simple one-pot manner. In addition, the operational simplicity, remarkable functional group tolerance, and high bond-forming/annulation efficiency associated with this approach suggest its great potential for broad application in the construction of valuable benzo[b]phosphole oxide frameworks in material science. Further mechanistic investigations and research applications are underway in our laboratory.
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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.8b01108. General experimental procedures; characterization details and copies of 1H, 13C, and 31P NMR spectra of compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yuxing Gao: 0000-0002-6420-0656 Yufen Zhao: 0000-0002-8513-1354 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Chinese National Natural Science Foundation (21202135, 21642010, 41576081) and the Fundamental Research Funds for the Central Universities (20720180083, 20720160034).
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REFERENCES
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DOI: 10.1021/acs.orglett.8b01108 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.8b01108 Org. Lett. XXXX, XXX, XXX−XXX
