Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Cobalt-Catalyzed Annulation Reactions of Alkylidenecyclopropanes: Access to Spirocyclopropanes at Room Temperature Arnab Dey, Neetipalli Thrimurtulu, and Chandra M. R. Volla* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
Org. Lett. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 05/02/19. For personal use only.
S Supporting Information *
ABSTRACT: A bidentate chelation-assisted cobalt-catalyzed C(sp2)−H activation and annulation of benzamides and alkylidenecyclopropanes (ACPs) has been realized. The unique reactivity of organocobalt species led to selective migratory insertion across the more electron-rich CC bond of the ACP followed by faster reductive elimination from the seven-membered cobaltacycle leading to spiro-dihydroisoquinoline derivatives with conservation of the cyclopropyl ring. The operationally simple reaction conditions allowed the C−H activation of both aryl and heteroaryl amides at room temperature.
T
range rapidly into a homoallyl-metal intermediate by ring opening. Addressing this colossal challenge, Ackermann and co-workers in a pioneering report described the Ru-catalyzed hydroarylation reaction of ACPs and 2-phenylpyridine to access anti-Markovnikov products with conservation of the cyclopropyl ring.11 In 2013, Cui extended this idea to Rh(III)-catalyzed C−H activation/cycloaddition of N-pivaloylbenzamide and ACPs.12 The conservation of the cyclopropyl ring was limited to aryl amides, whereas N-pivaloyl-heteroarylamides readily underwent ring opening to form azepinones by cyclopropylcarbinylbutenyl rearrangement. The β-carbon elimination and ring scission tendency of ACPs was also elegantly exploited independently by the groups of Ackermann and Li in Mn(I)catalyzed C−H activaion of imines13 and Rh(III)-catalyzed C− H activation of nitrones.14 More recently, the apparent advantages of cobalt salts prompted Kwong and co-workers to explore cobalt catalysis for C−H activation and insertion reaction with methylenecyclopropanes, and an interesting tandem C−H activation/C−C cleavage and C−H cyclization sequence emerged for generating dihydronaphthalene skeletons (Scheme 1a).15 Spiro-cyclopropane system is present in the core of various natural products and pharmaceuticals and has attracted considerable attention due to its prominent biological and medicinal activities (Figure 1).16 We therefore aimed to develop a general protocol for accessing spirocyclopropane derivatives employing commercially viable cobalt catalysis. In our previous work17 on the Co-catalyzed heterocyclization reaction with allenes, we observed that migratory insertion of arylcobalt(III) species proceeds selectively on the more
ransition metal-catalyzed C−H bond functionalization followed by annulation for the construction of complex heterocyclic scaffolds in a one-pot manner is emerging as one of the most versatile and powerful tools in organic synthesis.1 Notable advances in this direction have been made using precious second-row transition metal catalysts.2 However, more recently, significant consideration has also been directed toward the use of low-cost and environmentally benign firstrow transition metal catalysts.3 In particular, cobalt-catalyzed directed C−H bond functionalization reactions attracted a great deal of attention due to easy accessibility and unique reactivity.4 The higher nucleophilic character of organocobalt species in comparison to those of its contegers opens up a new direction for improved positional selectivities and chemoselectivities.5 Seminal contributions in this area were made by the groups of Daugulis, Kanai, Yoshikai, Ackermann, Glorius, Ellman, and others.6 The most common coupling partners employed in transition metal-catalyzed C−H bond functionalization are alkenes, alkynes, and allenes, due to the high regioselectivity of migratory insertion across these πcomponents by the organometallic intermediate formed by C−H activation. More recently, C−H bond functionalization with uncommon π-organic components, alkylidenecyclopropanes (ACPs) in particular, also has attracted considerable interest due to their high reactivity and switchable reactivity pattern.7 ACPs are highly strained molecules (ring strain of ∼38.8 kcal/mol)8 but extremely important entities due to their easy transformation into appealing organic building blocks and their ability to rapidly generate molecular complexity.9,10 The intramolecular strain present in the cyclopropyl ring and in its CC exocyclic bond help it to endure several reactions to release the strain. Carbometalation of the organometallic intermediate onto the exocyclic CC bond leads to highly reactive (cyclopropylmethyl)metal species, which can rear© XXXX American Chemical Society
Received: April 21, 2019
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DOI: 10.1021/acs.orglett.9b01392 Org. Lett. XXXX, XXX, XXX−XXX
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delighted to see the formation of the desired product, 4-(1′oxo-2′,3′-dihydro-1′H-spiro[cyclopropane-1,4′-isoquinolin]3′-yl)benzonitrile (3a), as the sole product without having any other regioisomeric product, which was further confirmed by X-ray crystallography. After rigorous optimization of various reaction parameters like cobalt salt, oxidant, base, and solvent, we found that Mn(OAc)3·2H2O and NaOPiv were crucial as an oxidant and base, respectively, in TFE solvent for achieving better yield (see the Supporting Information). With the optimized reaction conditions in hand, we started to explore the versatility of the reaction for various substituted benzamides and 4-(cyclopropylidenemethyl)benzonitrile 2a (Scheme 2). We were delighted with the applicability of the
Scheme 1. Overview of This Work
Scheme 2. Synthesis of Spirodihydroisoquinolines with Different Benzamidesa
Figure 1. Examples of bioactive molecules containing a spirocyclopropane ring.
electron-rich C1C2 bond of the allene forming a Co-σ-allyl intermediate, which undergoes faster reductive elimination leading to cyclization at the more sterically hindered side (Scheme 1b).16a To extend this mechanistic rationale further, coupled with the synthetic relevance of spirocyclopropanes, we envisioned ACPs as the suitable coupling partner in the annulation reaction with benzamides. The initial challenges associated with the envisioned project are controlling the regioselectivity of migratory insertion across methylenecyclopropanes and suppressing the ring opening tendency of (cyclopropylmethly)cobalt(III) intermediates. Herein, we report the successful establishment of a protocol to access spiroheterocycles with conservation of the cyclopropyl ring (Scheme 1c). Overall, this transformation not only represents a substantial step forward for engaging methylenecyclopropanes as privileged building blocks in C−H activation but also provides useful insight into the divergence in cobalt-catalyzed C−H bond functionalization. For the optimization studies, we chose N-quinolinyl benzamide (1a) and 4-(cyclopropylidenemethyl)benzonitrile (2a) as model substrates and they reacted at room temperature with readily available Co(acac)2 catalyst, and we were
a
Reaction conditions: 1 (0.2 mmol), 2a (0.2 mmol), Co(acac)2 (20 mol %), Mn(OAc)3·2H2O (0.4 mmol), NaOPiv (0.4 mmol), TFE (2 mL), 48 h, rt, isolated yield.
Co(II) catalyst with diversely substituted benzamides affording the corresponding annulated product with conservation of the cyclopropyl ring. Benzamides having electron-donating substituents such as methyl and 2,4-dimethyl at different positions of the aromatic ring were found to be suitable for accessing the corresponding annulation products in good to moderate yields (3b−3d). Various electron-withdrawing functional groups like F, Cl, Br, NO2, and CN were also well tolerated under our optimized reaction conditions (3e−3k). The structure and regioselectivity of the annulated products were further confirmed by single-crystal X-ray diffraction analysis of 3a and 3g. Notably, having a NO2 substitution at the ortho position of the benzamide furnished spiro-heteroB
DOI: 10.1021/acs.orglett.9b01392 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters cycle 3j in 45% yield probably due to increased steric hindrance. The reaction was found to proceed in a highly regioselective manner as meta-chloro and 2-naphthyl benzamides afforded only one regioisomer, 3h and 3l in 64% and 66% yields, respectively, with selective activation of the sterically less hindered C−H bond. In addition, the reaction was carried out with 2-thienyl amide and we were pleased to see that 2-thienyl amide reacted smoothly under the standard reaction conditions and provided the expected product 3m in 60% isolated yield. This is in sharp contrast with the Rh(III)catalyzed C−H activation/annulation reaction with ACPs, where ring opening took place in the case of heteroaryl amides leading to fused azepinones.12 The generality and scope of the reaction were further investigated with various ACPs (2). As one can clearly see in Scheme 3, a variety of ACPs are compatible regardless of their electronic properties on the phenyl ring and delivered the products (3n−3ai) in moderate to excellent yields of 50−90%. The structures of products 3aa and 3ac were unambiguously confirmed by X-ray crystallography. When the reaction was carried out using 1.0 g of 1a, a 73% yield was observed for the formation of 3n. The reaction between (cyclopropylidenemethyl)benzene 2b and N-(quinolin-8-yl)furan-2-carboxamide (1q) under the optimized reaction condition led to the annulated spiro product (3q) with conservation of the cyclopropyl ring. When the phenyl ring of the ACP having a NO2 substituent at the ortho position was tested, the corresponding products (3ad−3af) were isolated in moderate yields of 50−53% probably due to the strong electronwithdrawing nature of the nitro group and increased steric hindrance. The effectiveness of the reaction was further tested with 5-acetoxy substitution on the quinoline ring (1aj), which afforded the spiro-dihydroisoquinoline derivative (3aj) with a moderate yield of 50%. Interestingly, diphenyl phosphinamide also reacted smoothly with 1-bromo-4-(cyclopropylidenemethyl)benzene 2d under the optimized conditions and afforded the corresponding phosphaheterocycle 6 in 60% yield. The structure of the product was confirmed by X-ray crystallography. It is worth mentioning that arylsulfonamides under similar reaction condition did not participate in the reaction. This is probably because of the weaker binding of the amide nitrogen of arylsulfonamides to Co(II), due to the higher acidity of sulfonamides in comparison to those of benzamides and phosphinamides. Interestingly, while performing the control studies, we observed that when the reaction was carried out in the absence of methylenecyclopropane, C−N bond formation takes place leading to the formation of homocoupling product 8a. After minor optimization (see the Supporting Information), we synthesized various aryl and heteroaryl amide homocoupling products 8 (C−N bond formation) in moderate to good yields (Scheme 4) (8a−8f). The structure of product 8b was further confirmed by X-ray crystallography. To gain additional insight into the mechanism of the annulation reaction, detailed intermolecular competition and deuterium labeling experiments were conducted. The H/D exchange experiments led to no H/D exchange for 1a and [D]5-1a, which indicates C−H activation is irreversible in nature (Scheme 5a,b). When a 1:1 mixture of p-methylbenzamide (1c) and p-cyanobenzamide (1k) was subjected to reaction under the standard conditions, a 1:1.26 mixture of products was obtained, suggesting that electrophilic cobaltation is unlikely (Scheme 5c). The competitive experiment was
Scheme 3. Synthesis of Spirodihydroisoquinolines with Different ACPs and Benzamidesa
a
Reaction conditions: 1 (0.2 mmol), 2a (0.2 mmol), Co(acac)2 (20 mol %), Mn(OAc)3·2H2O (0.4 mmol), NaOPiv (0.4 mmol), TFE (2 mL), 48 h, rt, isolated yield. bYield in parentheses for the reaction on a 4 mmol scale (1.0 g).
also studied with p-cyano and p-methoxy substitution on the phenyl ring of the ACP, and a 1:1.05 mixture of products was obtained under the standard condition, indicating a slight preference in electronics (more electron-rich aryl) for migratory insertion (Scheme 5d). A kinetic isotope effect study in two parallel reactions with 1a and [D]5-1a gave a kinetic isotopic value (kH/kD) of 1.12 (Scheme 5e) and a product distribution value (PH/PD) of 1.1 was observed when a C
DOI: 10.1021/acs.orglett.9b01392 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Scheme 4. Co-Catalyzed Homocoupling of Benzamidesa
revealed that the C−H cobaltation step is irreversible and not rate-limiting. On the basis of our preliminary mechanistic studies and the literature,18 we proposed a plausible reaction mechanism shown in Scheme 6. The catalytic reaction starts with the Scheme 6. Proposed Reaction Mechanism
a
Reaction conditions: 1 (0.2 mmol), Co(acac)2 (20 mol %), Mn(OAc)3·2H2O (0.2 mmol), NaOPiv (0.4 mmol), TFE (2 mL), 36 h, rt, isolated yield.
Scheme 5. Preliminary Mechanistic Investigation
coordination of Co(II) with amide 1 to give complex A, which is oxidized by Mn(OAc)3·2H2O to form Co(III) complex B. Subsequently, ortho C−H activation takes place by concertated metalation deprotonation (CMD) to furnish the key cobaltacyclic intermediate C. Coordination and subsequent regioselective insertion of the double bond of ACP into a C−Co bond of intermediate D affords the seven-membered cobaltacycle intermediate E. Faster reductive elimination of (cyclopropylmethyl)cobalt(III) complex E before the ring opening of cyclopropane leads to annulated product 3, and the Co(II) species regenerates back from Co(I) after reoxidation with Mn(OAc)3·2H2O. In conclusion, we have successfully developed a bidentate chelation-assisted cobalt-catalyzed C−H activation of benzamides and annnulation reaction with ACPs. Due to the high ring strain in ACPs, strategies for accessing cyclopropyl ringcontaining spiro-heterocycles are still obscure. In the methodology presented here, a cobalt-catalyzed regioselective spirodihydroisoquinoline synthesis with conservation of the cyclopropyl ring has been demonstrated. Diaryl-phosphinamides also display similar reactivity furnishing phosphaheterocycles. The use of a robust and cheap Co(II) catalyst makes this room-temperature transformation most valuable and operationally simple.
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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.9b01392. Additional experimental procedures, X-ray crystallographic analysis, and spectroscopic data for synthesized compounds (PDF) Accession Codes
1:1 mixture of 1a and [D]5-1a reacted with 2a (Scheme 5f). These values imply that C−H bond cleavage might not be involved in the rate-limiting step. These combined results
CCDC 1465055, 1559197, 1559202, 1563081, 1563087, and 1895021 contain the supplementary crystallographic data for D
DOI: 10.1021/acs.orglett.9b01392 Org. Lett. XXXX, XXX, XXX−XXX
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Angew. Chem., Int. Ed. 2013, 52, 2207−2211. (f) Gao, K.; Lee, P.-S.; Fujita, T.; Yoshikai, N. J. Am. Chem. Soc. 2010, 132, 12249−12251. (g) Gao, K.; Yoshikai, N. J. Am. Chem. Soc. 2011, 133, 400−402. (h) Yu, D.-G.; Gensch, T.; de Azambuja, F.; Vásquez-Céspedes, S.; Glorius, F. J. Am. Chem. Soc. 2014, 136, 17722−17725. (i) Li, J.; Ackermann, L. Angew. Chem., Int. Ed. 2015, 54, 3635−3638. (j) Ma, W.; Ackermann, L. ACS Catal. 2015, 5, 2822−2825. (k) Hummel, J. R.; Ellman, J. A. J. Am. Chem. Soc. 2015, 137, 490−498. (l) Kalsi, D.; Sundararaju, B. Org. Lett. 2015, 17, 6118−6121. (m) Sen, M.; Kalsi, D.; Sundararaju, B. Chem. - Eur. J. 2015, 21, 15529−15533. (n) Planas, O.; Whiteoak, C. J.; Company, A.; Ribas, X. Adv. Synth. Catal. 2015, 357, 4003−4012. (o) Wu, X.; Yang, K.; Zhao, Y.; Sun, H.; Li, G.; Ge, H. Nat. Commun. 2015, 6, 6462−6471. (p) Zhang, J.; Chen, H.; Lin, C.; Liu, Z.; Wang, C.; Zhang, Y. J. Am. Chem. Soc. 2015, 137, 12990− 12996. (q) Wang, H.; Zhang, S.; Wang, Z.; He, M.; Xu, K. Org. Lett. 2016, 18, 5628−5631. (r) Yamaguchi, T.; Kommagalla, Y.; Aihara, Y.; Chatani, N. Chem. Commun. 2016, 52, 10129−10132. (s) Williamson, P.; Galván, A.; Gaunt, M. J. Chem. Sci. 2017, 8, 2588−2591. (t) Barsu, N.; Bolli, S. K.; Sundararaju, B. Chem. Sci. 2017, 8, 2431−2435. (u) Lan, T.; Wang, L.; Rao, Y. Org. Lett. 2017, 19, 972−975. (7) (a) Nakamura, I.; Yamamoto, Y. Adv. Synth. Catal. 2002, 344, 111−129. (b) Yu, L.; Liu, M.; Chen, F.; Xu, Q. Org. Biomol. Chem. 2015, 13, 8379−8392. (c) Wang, F.; Yu, S.; Li, X. Chem. Soc. Rev. 2016, 45, 6462−6477. (8) (a) Johnson, W. T. G.; Borden, W. T. J. Am. Chem. Soc. 1997, 119, 5930−5933. (b) Bach, R. D.; Dmitrenko, O. J. Am. Chem. Soc. 2004, 126, 4444−4452. (9) (a) For selected reviews, see: Rubin, M.; Rubina, M. M.; Gevorgyan, V. Chem. Rev. 2007, 107, 3117−3179. (b) Shao, L.-X.; Shi, M. Curr. Org. Chem. 2007, 11, 1135−1153. (c) Shi, M.; Shao, L.X.; Lu, J.-M.; Wei, Y.; Mizuno, K.; Maeda, H. Chem. Rev. 2010, 110, 5883−5913. (d) Shi, M.; Lu, J. M.; Wei, Y.; Shao, L.-X. Acc. Chem. Res. 2012, 45, 641−652. (e) Zhang, D.-H.; Tang, X.-Y.; Shi, M. Acc. Chem. Res. 2014, 47, 913−924. (f) Brandi, A.; Cicchi, S.; Cordero, F.M.; Goti, A. Chem. Rev. 2014, 114, 7317−7420. (10) Zhu, Y.-Q.; Niu, Y.-X.; Hui, L.-W.; He, J.-L.; Zhu, K. Adv. Synth. Catal. 2019, DOI: 10.1002/adsc.201900176. (11) (a) Kozhushkov, S. I.; Yufit, D. S.; Ackermann, L. Org. Lett. 2008, 10, 3409−3412. (b) Ackermann, L.; Kozhushkov, S. I.; Yufit, D. S. Chem. - Eur. J. 2012, 18, 12068−12077. (c) Schinkel, M.; Wallbaum, J.; Kozhushkov, S. I.; Marek, I.; Ackermann, L. Org. Lett. 2013, 15, 4482−4484. (12) Cui, S.; Zhang, Y.; Wu, Q. Chem. Sci. 2013, 4, 3421−3426. (13) Liang, Y.-F.; Müller, V.; Liu, W.; Münch, A.; Stalke, D.; Ackermann, L. Angew. Chem., Int. Ed. 2017, 56, 9415−9419. (14) Bai, D.; Xu, T.; Ma, C.; Zheng, X.; Liu, B.; Xie, F.; Li, X. ACS Catal. 2018, 8, 4194−4200. (15) Li, M.; Kwong, F. Y. Angew. Chem., Int. Ed. 2018, 57, 6512− 6516. (16) (a) Nagamoto, Y.; Takemoto, Y.; Takasu, K. Synlett 2012, 24, 0120−0124. (b) Smith, L. K.; Baxendale, I. R. Org. Biomol. Chem. 2015, 13, 9907−9933. (c) Talele, T. T. J. Med. Chem. 2016, 59, 8712−8756. (d) Fan, Y.-Y.; Gao, X.-H.; Yue, J.-M. Sci. China: Chem. 2016, 59, 1126−1141. (e) Ebner, C.; Carreira, E. M. Chem. Rev. 2017, 117, 11651−11679. (17) (a) Thrimurtulu, N.; Dey, A.; Maiti, D.; Volla, C. M. R. Angew. Chem., Int. Ed. 2016, 55, 12361−12365. (b) Thrimurtulu, N.; Nallagonda, R.; Volla, C. M. R. Chem. Commun. 2017, 53, 1872− 1875. (c) Nallagonda, R.; Thrimurtulu, N.; Volla, C. M. R. Adv. Synth. Catal. 2018, 360, 255−260. (d) Dey, A.; Rathi, A.; Volla, C. M. R. Asian J. Org. Chem. 2018, 7, 1362−1367. (18) Zeng, L.; Tang, S.; Wang, D.; Deng, Y.; Chen, J.-L.; Lee, J.-F; Lei, A. Org. Lett. 2017, 19, 2170−2173.
this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[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 Author
*E-mail:
[email protected]. ORCID
Arnab Dey: 0000-0001-6176-8775 Chandra M. R. Volla: 0000-0002-8497-1538 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This activity is supported by SERB, India (EMR/2015/ 002047). Financial support was received from DST under Fast Track Scheme (N.T.) and IIT-B (A.D.).
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REFERENCES
(1) For selected reviews, see: (a) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. Acc. Chem. Res. 2008, 41, 1013−1025. (b) Ackermann, L.; Vicente, R.; Kapdi, A. R. Angew. Chem., Int. Ed. 2009, 48, 9792− 9826. (c) Giri, R.; Shi, B.-F.; Engle, K. M.; Maugel, N.; Yu, J.-Q. Chem. Soc. Rev. 2009, 38, 3242−3272. (d) Colby, D. A.; Bergman, R. G.; Ellman, J. A. Chem. Rev. 2010, 110, 624−655. (e) Newhouse, T.; Baran, P. S. Angew. Chem., Int. Ed. 2011, 50, 3362−3374. (f) Yeung, C. S.; Dong, V. M. Chem. Rev. 2011, 111, 1215−1292. (g) Ackermann, L. Chem. Rev. 2011, 111, 1315−1345. (h) Zhang, M.; Zhang, Y.; Jie, X.; Zhao, H.; Li, G.; Su, W. Org. Chem. Front. 2014, 1, 843−895. (i) Gensch, T.; Hopkinson, M. N.; Glorius, F.; Wencel-Delord, J. Chem. Soc. Rev. 2016, 45, 2900−2936. (j) Gulías, M.; Mascareñas, J. L. Angew. Chem., Int. Ed. 2016, 55, 11000−11019. (k) Chen, Z.; Wang, B.; Zhang, J.; Yu, W.; Liu, Z.; Zhang, Y. Org. Chem. Front. 2015, 2, 1107−1295. (l) He, J.; Wasa, M.; Chan, K. S. L.; Shao, Q.; Yu, J.-Q. Chem. Rev. 2017, 117, 8754−8786. (2) (a) Colby, D. A.; Tsai, A. S.; Bergman, R. G.; Ellman, J. A. Acc. Chem. Res. 2012, 45, 814−825. (b) Song, G.; Li, X. Acc. Chem. Res. 2015, 48, 1007−1020. (c) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147−1169. (d) Arockiam, P. B.; Bruneau, C.; Dixneuf, P. H. Chem. Rev. 2012, 112, 5879−5918. (3) (a) Wendlandt, A. E.; Suess, A. M.; Stahl, S. S. Angew. Chem., Int. Ed. 2011, 50, 11062−11087. (b) Castro, L. C. M.; Chatani, N. Chem. Lett. 2015, 44, 410−421. (c) Liu, W.; Ackermann, L. ACS Catal. 2016, 6, 3743−3752. (d) Shang, R.; Ilies, L.; Nakamura, E. Chem. Rev. 2017, 117, 9086−9139. (e) Gandeepan, P.; Muller, T.; Zell, D.; Cera, G.; Warratz, S.; Ackermann, L. Chem. Rev. 2019, 119, 2192−2452. (4) (a) Gao, K.; Yoshikai, N. Acc. Chem. Res. 2014, 47, 1208−1219. (b) Wei, D.; Zhu, X.; Niu, J.-L.; Song, M.-P. ChemCatChem 2016, 8, 1242−1263. (c) Wang, S.; Chen, S.-Y.; Yu, X.-Q. Chem. Commun. 2017, 53, 3165−3180. (d) Yoshino, T.; Matsunaga, S. Adv. Synth. Catal. 2017, 359, 1245−1262. (e) Usman, M.; Ren, Z.-H.; Wang, Y.Y.; Guan, Z.-H. Synthesis 2017, 49, 1419−1443. (f) Prakash, S.; Kuppusamy, R.; Cheng, C.-H. ChemCatChem 2018, 10, 683−705. (g) Santhoshkumar, R.; Cheng, C.-H. Beilstein J. Org. Chem. 2018, 14, 2266−2288. (5) (a) Moselage, M.; Li, J.; Ackermann, L. ACS Catal. 2016, 6, 498−525. (b) Park, J.; Chang, S. Chem. - Asian J. 2018, 13, 1089− 1102. (6) (a) Grigorjeva, L.; Daugulis, O. Angew. Chem., Int. Ed. 2014, 53, 10209−10212. (b) Grigorjeva, L.; Daugulis, O. Org. Lett. 2014, 16, 4684−4687. (c) Grigorjeva, L.; Daugulis, O. Org. Lett. 2014, 16, 4688−4690. (d) Grigorjeva, L.; Daugulis, O. Org. Lett. 2015, 17, 1204−1207. (e) Yoshino, T.; Ikemoto, H.; Matsunaga, S.; Kanai, M. E
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