Communication pubs.acs.org/JACS
Enantioselective C−H Functionalization−Addition Sequence Delivers Densely Substituted 3‑Azabicyclo[3.1.0]hexanes Julia Pedroni and Nicolai Cramer* Laboratory of Asymmetric Catalysis and Synthesis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland S Supporting Information *
electrophiles such as vinyl bromides 13 and triflates, 14 chloroacetamides,15 and carbamoyl chlorides.16 Methodologies introducing fluorine-containing substituents are sought-after in drug discovery, due to their favorable physicochemical properties 17 and metabolic stability. 18 In this respect, trifluoroacetimidoyl chlorides are highly attractive. They are stable, readily prepared, and in principle competent for Pd(0)catalyzed C−H functionalizations.19 We selected this group to illustrate a concept where the initial C−H functionalization forges a valuable bond and awakes a unique property on the intermediate for further complexity generating transformation, ideally in a cascade or one-pot manner. Herein, we disclose an enantioselective access to 3azabicyclo[3.1.0]hexenes 2 using trifluoroacetimidoyl chloride electrophiles 1 for the Pd(0)-catalyzed C−H cyclization (Scheme 1). The resulting chiral ketimine displays electrophilic
ABSTRACT: An enantioselective C−H functionalization route to perfluoroalkyl-containing 3-azabicyclo[3.1.0]hexanes is disclosed. A modular and bench-stable diazaphospholane ligand enables highly enantioselective Pd(0)-catalyzed cyclopropane C−H functionalization using trifluoroacetimidoyl chlorides as electrophilic partners. In turn, the resulting cyclic ketimine products react smoothly with a broad variety of nucleophiles in onepot processes enabling the rapid and modular construction of heavily substituted pyrrolidines.
N
itrogen-containing heterocycles are omnipresent in biologically active molecules, and more than half of FDA approved drugs contain a nitrogen heterocycle.1 Rigid molecules with a higher proportion of sp3-hybridized carbon atoms have attracted recently more attention.2 An intriguing subset of saturated N-heterocyclic scaffolds that displays diverse biological activities and great potential for pharmaceutical use are 3-azabicyclo[3.1.0]hexanes (Figure 1).3 This scaffold is
Scheme 1. Merging Enantioselective C−H Functionalization with Nucleophilic Addition for a Rapid and Modular Access to CF3-Substituted 3-Azabicyclo[3.1.0]hexanes
character which could be exploited for additions of nucleophiles, giving unprotected secondary pyrrolidines 3. The fused cyclopropane unit ensures perfect diastereocontrol in subsequent nucleophilic additions. Overall, our strategy consists of a enantio- and diastereoselective modular approach for highly substituted perfluoroalkylated 3-azabicyclo[3.1.0]hexanes. The envisioned transformation was investigated with substrate 1a (Table 1). TADDOL-derived P(III)-ligands, previously very successful for asymmetric C−H functionalizations, failed to provide good enantioselectivities for 2a (entries 1−4). However, chiral alkoxy diazaphospholidines20 were suitable for this transformation. While the smallest version L5 failed to induce any selectivity (entry 5), the increased steric bulk of L7 promoted the reaction to the desired bicyclic imine 2a in high yield and an excellent enantiomeric ratio of 98:2 (entry 7). Increasing the steric demand further (OiPr instead of
Figure 1. Selection of biologically active molecules with the 3azabicyclo[3.1.0]hexane scaffold.
commonly accessed by intramolecular C−N bond formation of a cyclopropane-tethered precursor4 or cyclopropanation of an unsaturated 5-membered N-heterocycle.5,6 Limitations on the decoration of the pyrrolidine moiety remain, along with the scarcity of enantioselective methods. Enantioselective C−H functionalizations have enabled new synthetic strategies with a rapid increase in molecular complexity,7,8 and functionalizations of cyclopropane C−H bonds made significant progress.9 Asymmetric Pd(0)-catalyzed C−H functionalizations have emerged as complementary processes for stereoselective access to chiral cyclic compounds.10 Most asymmetric Pd(0)-catalyzed C−H functionalizations rely on aryl halides11 or triflates12 as electrophilic reaction partners. Few reports showcase the potential of other © 2017 American Chemical Society
Received: July 10, 2017 Published: August 23, 2017 12398
DOI: 10.1021/jacs.7b07024 J. Am. Chem. Soc. 2017, 139, 12398−12401
Communication
Journal of the American Chemical Society Table 1. Ligand Screening and Reaction Optimizationa
entry
L*
base
yield (%)b
er
1 2 3 4 5 6 7c 8e 9 10 11f
L1 L2 L3 L4 L5 L6 L7 L8 L7 L7 L7
CsOAc CsOAc CsOAc CsOAc CsOAc CsOAc CsOAc CsOAc KOAc AdCO2Cs CsOAc
63 75 59 61 66 33 81d 22 71 72 90d [g-scale]
67:33 72.5:27.5 64:36 74:26 51:49 60:40 98:2 92:8 97:3 95.5:4.5 98.5:1.5
Scheme 2. Scope for Imines 2a
a 50 μmol of 1a, 2.5 μmol of Pd(allyl)Cp, 5.0 μmol of L, 60 μmol of base, 0.1 M in toluene at 110 °C for 1 h. bDetermined by 1H NMR with internal standard. c0.1 mmol of 1a. dIsolated yield. e40% conversion. f3.0 mmol of 1a, 75 μmol of Pd(allyl)Cp, 150 μmol of L, 4.5 mmol of CsOAc, 0.2 M in toluene.
OMe in L8) resulted in sluggish cyclization with loss of selectivity (entry 8). KOAc could be used instead of CsOAc with very little impact on the reaction outcome (entry 9). AdCO2Cs slightly decreased the enantioselectivity (entry 10). The reaction was scaled up 60-fold with 2.5 mol % Pd-catalyst with an at least similar favorable reaction outcome (entry 11). The scope of the transformation was investigated with imidoyl chlorides 1 bearing different substituents R1, R2, and RF (Scheme 2). Variations of R1 include hydrogen, heteroatomcontaining chains (2b), and benzyl (2c) and aryl groups (2d− 2j). In all cases high reactivity and enantioselectivities were obtained. Notably, for aryl-bearing substrates 1d−1j, chiral cyclic imines 2d−2j were selectively obtained using L7 (for a ligand-promoted reversal of the selectivity see Table 2). Concerning substituents R2, substrates 1 bearing valuable oxetane and azetidine rings21 delivered spirocyclic compounds 2j−2l. The cyclization proceeds as well in high enantioselectivity without substituent R2 (2n), albeit in reduced yield. The reaction proceeds with high efficiency and selectivity irrespective of length of the perfluoroalkyl chain (2o−2p). The high fidelity in the cyclopropane C−H enantiodiscrimination prompted us to explore the potential of a parallel kinetic resolution,22,11c selecting between a cyclopropane Csp3−H and an aryl Csp2−H bond to functionalize (eq 1). Phenyl-substituted
0.10 mmol of 1, 5.0 μmol of Pd(allyl)Cp, 10 μmol of L7, 0.12 mmol of CsOAc, 0.1 M in toluene at 110 °C for 1−6 h; isolated yield. b Determined by 1H NMR with internal standard. c1.5 equiv of Cs2CO3 added. dWith 10 μmol of Pd(allyl)Cp and 20 μmol of L7. a
Table 2. Ligand-Controlled Chemoselectivity Switch between Aryl Csp2−H vs Cyclopropyl Csp3−Ha
entry
1
L
ratio 2:7b
major product, yield (%)c
1 2 3 4 5 6 7 8 9 10 11 12 13 14
1d 1d 1e 1e 1f 1f 1g 1g 1h 1h 1i 1i 1j 1j
PPh3 L7 PPh3 L7 PPh3 L7 PPh3 L7 PPh3 L7 PPh3 L7 PPh3 L7
1:14 6:1 1:15 4:1 1:11 6:1 1:13 2:1 1:>99 2:1 1:11 8:1 1:10 9:1
7d, 82 2d, 73 7e, 87 2e, 77 7f, 89 2f, 88 7g,d 74 2g, 63 7h,e 87 2h, 59 7i, 73 2i, 78 7j, 74 2j, 70
a 0.1 mmol of 1, 5.0 μmol of Pd(allyl)Cp, 10 μmol of L, 0.12 mmol of CsOAc, 0.1 M in toluene at 110 °C (1 h with L7, 6 h with PPh3). b Determined by 1H NMR. cIsolated yield. do/p = 1.1:1. eo/p = 8:1.
rac-1q was efficiently resolved yielding constitutional isomers 2q and 4q in high er. Substrate rac-1r with an electron-rich aryl moiety had very little effect on the outcome of the parallel
kinetic resolution. Notably, imines 2q and 2r were obtained in excellent diastereoselectivity. 12399
DOI: 10.1021/jacs.7b07024 J. Am. Chem. Soc. 2017, 139, 12398−12401
Communication
Journal of the American Chemical Society
Scheme 4. One-Pot Synthesis of Pyrrolidines 3a
The reactivity of the constructed electrophilic ketimine toward nucleophiles was investigated next (Scheme 3a). The Scheme 3. Addition of Nucleophiles to Chiral Imines 2
0.10 mmol of 1, 5.0 μmol of Pd(allyl)Cp, 10 μmol of L7, 0.15 mmol of CsOAc, 0.1 M in toluene; isolated yields 98:2 er for 2c. bNu = LiAlH4. cNu= allylMgBr. dNu = RLi. eNu = TMSCN, BF3·OEt2.
a
The chemoselectivity of the C−H bond activation (cyclopropyl vs aryl C−H) can be modulated by the phosphine ligand. Under otherwise identical conditions, selective cyclopropane functionalization is observed with L7 (see Scheme 2). However, using PPh3 reverses the selectivity and leads to aryl Csp2−H functionalization generating spirocyclic trifluoromethylated dihydroisoquinolines 7 (Table 2). No reaction occurred in the absence of palladium, excluding a Bischler−Napieralsky pathway. In conclusion, we reported modular and bench-stable diazaphospholane ligands enabling highly enantioselective Pd(0)-catalyzed cyclopropane C−H functionalization with trifluoroacetimidoyl chloride electrophiles. The cyclic CF3ketimines products are versatile intermediates and react smoothly with a broad variety of nucleophiles providing access to important building blocks. The native bis-electrophilic nature of the trifluoroacetimidoyl chlorides was directly exploited in one-pot processes enabling the rapid and modular construction of heavily substituted pyrrolidines. Additionally, we have investigated the selectivity between cyclopropane and aryl C− H bonds leading to the development of parallel kinetic resolutions. Moreover, we discovered a ligand-controlled chemoselective C−H functionalization allowing efficient access to two different scaffolds from the same starting materials.
fused cyclopropane efficiently guides additions from the opposite face leading to a diastereoselective construction of the CF3-substituted quaternary stereogenic center. For instance, completely diastereoselective reduction of 2a to pyrrolidine 3aa proceeded quantitatively under mild conditions with NaBH4. Organolithium species exemplified by 3-lithio-pyridine delivered selectively 3ab. Moreover, a Friedel−Crafts reaction with N-methyl pyrrole occurred at C3, giving 3ac. Aminophosphonate 3ad could be obtained in excellent yield by a Pudovic reaction with diethyl phosphonate. The Sc(OTf)3catalyzed reaction between 2a and Danishefsky’s diene furnished indolizidine 3ae in low yield. The absolute configuration of products 2 was determined by X-ray crystallography of hemiaminal hydrochloride 3df (Scheme 3b).23 The cyclopropane unit of 2d underwent endo-hydrogenolysis with concomitant reduction of the imine leading to piperidine 5d as the major product. In contrast, exposure of pyrrolidine 3da to similar hydrogenation conditions led to complementary exo-hydrogenolysis of the cyclopropane yielding selectively pyrrolidine 6d. The pyrrolidines 3 can be directly accessed from 1 via a convenient one-pot process, without any change in the er (Scheme 4). For instance, a C−H functionalization/reduction sequence (3ca) was performed by the addition of LiAlH4 to the reaction mixture after completion of the Pd-catalyzed C−H functionalization. Suitable nucleophiles for the one-pot process encompass a broad range of Grignard and lithium organometallics. Aminonitrile 3cl was obtained in 84% yield by the addition of TMSCN and BF3·OEt2.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07024. Synthetic procedures, characterization data for all new compounds, and HPLC traces of the chiral products (PDF) Crystallographic data (CIF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: nicolai.cramer@epfl.ch. ORCID
Nicolai Cramer: 0000-0001-5740-8494 Notes
The authors declare no competing financial interest. 12400
DOI: 10.1021/jacs.7b07024 J. Am. Chem. Soc. 2017, 139, 12398−12401
Communication
Journal of the American Chemical Society
■
2016, 55, 15387. (i) Zhang, F.-L.; Hong, K.; Li, T.-J.; Park, H.; Yu, J.Q. Science 2016, 351, 252. (j) Smalley, A. P.; Cuthbertson, J. D.; Gaunt, M. J. J. Am. Chem. Soc. 2017, 139, 1412. (k) Jain, P.; Verma, P.; Xia, G.; Yu, J.-Q. Nat. Chem. 2016, 9, 140. (11) (a) Nakanishi, M.; Katayev, D.; Besnard, C.; Kündig, E. P. Angew. Chem., Int. Ed. 2011, 50, 7438. (b) Anas, S.; Cordi, A.; Kagan, H. B. Chem. Commun. 2011, 47, 11483. (c) Katayev, D.; Nakanishi, M.; Burgi, T.; Kundig, E. P. Chem. Sci. 2012, 3, 1422. (d) Saget, T.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51, 12842. (e) Martin, N.; Pierre, C.; Davi, M.; Jazzar, R.; Baudoin, O. Chem. - Eur. J. 2012, 18, 4480. (f) Saget, T.; Cramer, N. Angew. Chem., Int. Ed. 2013, 52, 7865. (g) Deng, R.; Huang, Y.; Ma, X.; Li, G.; Zhu, R.; Wang, B.; Kang, Y.B.; Gu, Z. J. Am. Chem. Soc. 2014, 136, 4472. (h) Gao, D.-W.; Yin, Q.; Gu, Q.; You, S.-L. J. Am. Chem. Soc. 2014, 136, 4841. (i) Liu, L.; Zhang, A.-A.; Zhao, R.-J.; Li, F.; Meng, T.-J.; Ishida, N.; Murakami, M.; Zhao, W.-X. Org. Lett. 2014, 16, 5336. (j) Ma, X.; Gu, Z. RSC Adv. 2014, 4, 36241. (k) Lin, Z. Q.; Wang, W. Z.; Yan, S. B.; Duan, W. L. Angew. Chem., Int. Ed. 2015, 54, 6265. (l) Liu, L.; Zhang, A.-A.; Wang, Y.; Zhang, F.; Zuo, Z.; Zhao, W.-X.; Feng, C.-L.; Ma, W. Org. Lett. 2015, 17, 2046. (m) Holstein, P. M.; Vogler, M.; Larini, P.; Pilet, G.; Clot, E.; Baudoin, O. ACS Catal. 2015, 5, 4300. (n) Pedroni, J.; Saget, T.; Donets, P. A.; Cramer, N. Chem. Sci. 2015, 6, 5164. (o) Gao, D.W.; Zheng, C.; Gu, Q.; You, S.-L. Organometallics 2015, 34, 4618. (p) Xu, G.; Li, M.; Wang, S.; Tang, W. Org. Chem. Front. 2015, 2, 1342. (q) Yang, L.; Melot, R.; Neuburger, M.; Baudoin, O. Chem. Sci. 2017, 8, 1344. (12) (a) Shintani, R.; Otomo, H.; Ota, K.; Hayashi, T. J. Am. Chem. Soc. 2012, 134, 7305. (b) Saget, T.; Lemouzy, S. J.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51, 2238. (13) (a) Gao, D.-W.; Gu, Y.; Wang, S.-B.; Gu, Q.; You, S.-L. Organometallics 2016, 35, 3227. (b) Holstein, P. M.; Dailler, D.; Vantourout, J.; Shaya, J.; Millet, A.; Baudoin, O. Angew. Chem. 2016, 128, 2855. (c) Ladd, C. L.; Charette, A. B. Org. Lett. 2016, 18, 6046. (14) Albicker, M. R.; Cramer, N. Angew. Chem., Int. Ed. 2009, 48, 9139. (15) (a) Pedroni, J.; Boghi, M.; Saget, T.; Cramer, N. Angew. Chem., Int. Ed. 2014, 53, 9064. (b) Pedroni, J.; Cramer, N. Angew. Chem., Int. Ed. 2015, 54, 11826. (16) Dailler, D.; Rocaboy, R.; Baudoin, O. Angew. Chem., Int. Ed. 2017, 56, 7218. (17) (a) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37, 320. (b) Gillis, E. P.; Eastman, K. J.; Hill, M. D.; Donnelly, D. J.; Meanwell, N. A. J. Med. Chem. 2015, 58, 8315. (18) Böhm, H.-J.; Banner, D.; Bendels, S.; Kansy, M.; Kuhn, B.; Müller, K.; Obst-Sander, U.; Stahl, M. ChemBioChem 2004, 5, 637. (19) (a) Zhu, M.; Fu, W.; Zou, G.; Xu, C.; Wang, Z. J. Fluorine Chem. 2014, 163, 23. (b) Pedroni, J.; Cramer, N. Org. Lett. 2016, 18, 1932. (20) Donets, P. A.; Cramer, N. J. Am. Chem. Soc. 2013, 135, 11772. (21) Carreira, E. M.; Fessard, T. C. Chem. Rev. 2014, 114, 8257. (22) Dehli, J. R.; Gotor, V. Chem. Soc. Rev. 2002, 31, 365. (23) CCDC 1560353 contains the crystallographic data for 3df hydrochloride.
ACKNOWLEDGMENTS This work is supported by the Swiss National Science Foundation (No. 157741). We thank Dr. R. Scopelliti and Dr. F. Fadaei Tirani for X-ray analysis of 3df·HCl.
■
REFERENCES
(1) Vitaku, E.; Smith, D. T.; Njardarson, J. T. J. Med. Chem. 2014, 57, 10257. (2) (a) Lovering, F.; Bikker, J.; Humblet, C. J. Med. Chem. 2009, 52, 6752. (b) Talele, T. T. J. Med. Chem. 2016, 59, 8712. (3) (a) Gomi, S.; Ikeda, D.; Nakamura, H.; Naganawa, H.; Yamashita, F.; Hotta, K.; Kondo, S.; Okami, Y.; Umezawa, H.; Iitaka, Y. J. Antibiot. 1984, 37, 1491. (b) Takahashi, I.; Takahashi, K.; Ichimura, M.; Morimoto, M.; Asano, K.; Kawamoto, I.; Tomita, F.; Nakano, H. J. Antibiot. 1988, 41, 1915. (c) Zhang, M.; Jovic, F.; Vickers, T.; Dyck, B.; Tamiya, J.; Grey, J.; Tran, J. A.; Fleck, B. A.; Pick, R.; Foster, A. C.; Chen, C. Bioorg. Med. Chem. Lett. 2008, 18, 3682. (d) Bymaster, F. P.; Golembiowska, K.; Kowalska, M.; Choi, Y. K.; Tarazi, F. I. Synapse 2012, 66, 522. (e) Warnock, K. T.; Yang, A. R. S. T.; Yi, H. S.; June, H. L., Jr; Kelly, T.; Basile, A. S.; Skolnick, P.; June, H. L. Pharmacol., Biochem. Behav. 2012, 103, 111. (4) Krow, G. R.; Cannon, K. C. Org. Prep. Proced. Int. 2000, 32, 103. (5) Micheli, F.; Cavanni, P.; Arban, R.; Benedetti, R.; Bertani, B.; Bettati, M.; Bettelini, L.; Bonanomi, G.; Braggio, S.; Checchia, A.; Davalli, S.; Di Fabio, R.; Fazzolari, E.; Fontana, S.; Marchioro, C.; Minick, D.; Negri, M.; Oliosi, B.; Read, K. D.; Sartori, I.; Tedesco, G.; Tarsi, L.; Terreni, S.; Visentini, F.; Zocchi, A.; Zonzini, L. J. Med. Chem. 2010, 53, 2534. (6) (a) Debrouwer, W.; Heugebaert, T. S. A.; Van Hecke, K.; Stevens, C. V. J. Org. Chem. 2013, 78, 8232. (b) Fritz, S. P.; Matlock, J. V.; McGarrigle, E. M.; Aggarwal, V. K. Chem. - Eur. J. 2013, 19, 10827. (c) Toh, K. K.; Biswas, A.; Wang, Y.-F.; Tan, Y. Y.; Chiba, S. J. Am. Chem. Soc. 2014, 136, 6011. (d) Harris, M. R.; Li, Q.; Lian, Y.; Xiao, J.; Londregan, A. T. Org. Lett. 2017, 19, 2450. (7) (a) Giri, R.; Shi, B.-F.; Engle, K. M.; Maugel, N.; Yu, J.-Q. Chem. Soc. Rev. 2009, 38, 3242. (b) Wencel-Delord, J.; Colobert, F. Chem. Eur. J. 2013, 19, 14010. (c) Zheng, C.; You, S.-L. RSC Adv. 2014, 4, 6173. (d) Asymmetric Functionalization of C−H Bonds; You, S.-L., Ed.; RSC: Cambridge, 2015. (e) Newton, C. G.; Wang, S.-G.; Oliveira, C. C.; Cramer, N. Chem. Rev. 2017, 117, 8908. (8) Selected reviews: (a) Ye, J.; Lautens, M. Nat. Chem. 2015, 7, 863. (b) Segawa, Y.; Maekawa, T.; Itami, K. Angew. Chem., Int. Ed. 2015, 54, 66. (c) Guo, X.-X.; Gu, D.-W.; Wu, Z.; Zhang, W. Chem. Rev. 2015, 115, 1622. (d) Daugulis, O.; Roane, J.; Tran, L. D. Acc. Chem. Res. 2015, 48, 1053. (e) Cheng, C.; Hartwig, J. F. Chem. Rev. 2015, 115, 8946. (f) Moselage, M.; Li, J.; Ackermann, L. ACS Catal. 2016, 6, 498. (g) Labinger, J. A. Chem. Rev. 2017, 117, 8483. (h) Hummel, J. R.; Boerth, J. A.; Ellman, J. A. Chem. Rev. 2017, 117, 9163. (i) He, J.; Wasa, M.; Chan, K. S. L.; Shao, Q.; Yu, J.-Q. Chem. Rev. 2017, 117, 8754. (j) Kim, D.-S.; Park, W.-J.; Jun, C.-H. Chem. Rev. 2017, 117, 8977. (k) Yang, Y.; Lan, J.; You, J. Chem. Rev. 2017, 117, 8787. (l) Park, Y.; Kim, Y.; Chang, S. Chem. Rev. 2017, 117, 9247. (m) Dong, Z.; Ren, Z.; Thompson, S. J.; Xu, Y.; Dong, G. Chem. Rev. 2017, 117, 9333. (9) Sustac Roman, D.; Charette, A. B. Top. Organomet. Chem. 2015, 56, 91. (10) For selected leading examples of complementary Pd(II)catalyzed C−H functionalizations, see: (a) Shi, B.-F.; Maugel, N.; Zhang, Y.-H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2008, 47, 4882. (b) Shi, B.-F.; Zhang, Y.-H.; Lam, J. K.; Wang, D.-H.; Yu, J.-Q. J. Am. Chem. Soc. 2010, 132, 460. (c) Wasa, M.; Engle, K. M.; Lin, D. W.; Yoo, E. J.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 19598. (d) Gao, D.-W.; Shi, Y.C.; Gu, Q.; Zhao, Z.-L.; You, S.-L. J. Am. Chem. Soc. 2013, 135, 86. (e) Chu, L.; Wang, X.-C.; Moore, C. E.; Rheingold, A. L.; Yu, J.-Q. J. Am. Chem. Soc. 2013, 135, 16344. (f) Du, Z.-J.; Guan, J.; Wu, G.-J.; Xu, P.; Gao, L.-X.; Han, F.-S. J. Am. Chem. Soc. 2015, 137, 632. (g) Chan, K. S. L.; Fu, H.-Y.; Yu, J.-Q. J. Am. Chem. Soc. 2015, 137, 2042. (h) Wang, H.; Tong, H.-R.; He, G.; Chen, G. Angew. Chem., Int. Ed. 12401
DOI: 10.1021/jacs.7b07024 J. Am. Chem. Soc. 2017, 139, 12398−12401
