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DOI: 10.1039/C9CC07372H (Communication) Chem. Commun., 2020, 56, 960-963

Facile access to versatile aza-macrolides through iridium-catalysed cascade allyl-amination/macrolactonization†

Wei Fu a, Lianhui Wang *a, Zi Yang a, Jiang-Shan Shen b, Fei Tang a, Jiayi Zhang a and Xiuling Cui *a
aEngineering Research Centre of Molecular Medicine, Ministry of Education, Key Laboratory of Fujian Molecular Medicine, Key Laboratory of Xiamen Marine and Gene Drugs, School of Biomedical Sciences, Huaqiao University, Xiamen, 361021, P. R. China. E-mail: lianhui.wang@hqu.edu.cn; cuixl@hqu.edu.cn
bCollege of Materials Science and Engineering, Huaqiao University, Xiamen 361021, P. R. China

Received 19th September 2019 , Accepted 6th December 2019

First published on 6th December 2019

Abstract

Direct access to benzo-fused aza-macrolides was successfully achieved through iridium-catalysed intermolecular decarboxylative coupling of vinylethylene carbonates with isatoic anhydrides under relatively mild reaction conditions. Notably, this reaction proceeded through sequential allyl-amination/macrolactonization upon extrusion of CO2. Moreover, favourable fluorescence properties could be observed in the title macrocyclic products.

Macrocyclic compounds represent a huge family of molecular entities that play a crucial role in molecular recognition,1 self-assembly2 and fluorescence sensing3 by virtue of innate characteristics of recognition and binding properties. They also provide a viable and valuable area of structural space for drug discovery.4 Among them, 14-membered frameworks, especially for the macrolides, constitute one of the most commonly occurring macrocyclic scaffolds,5 and are widespread structural motifs in many natural products and bioactive molecules6 with erythromycins6b–d and resorcylic acid lactones (RALs)6e,f as the representative examples (Fig. 1). Therefore, expanding efficient strategies for the construction of such skeletons has been a long-standing goal for organic and medicinal chemists. Significant procedures have been realized through direct C–C bond constructions, e.g. transition-metal-catalysed ring-closing metathesis (RCM),7 cross-coupling reactions,8 Horner–Wadsworth–Emmons reaction and photo-induced radical macrolactonization.9 Alternative strategies via C–O bond formation, including the transformation of seco-acid,10 and carbonylative or oxidative macrolactonization,11 have also been explored. However, these reported protocols usually require tedious preparation of precursors, whereas the intermolecular synthetic processes always occur along with competitive reactions, leading to the undesired polymer as by-products. Consequently, more efficient protocols from readily available and inexpensive reactants under mild conditions are highly desirable.
image file: c9cc07372h-f1.tif
Fig. 1 The representative bioactive 14-membered lactones.

In recent years, vinylethylene carbonates (VECs) have emerged as promising dipole precursors to form zwitterionic π-allyl palladium intermediates via facile palladium-catalysed decarboxylation,12–14 which might either tautomerize to six-membered palladacyclic species or generate dienolate intermediates (Scheme 1a).15 These in situ-generated intermediates would further lead to diverse transformations, including allylic substitution14 and formal cycloadditions for the constructions of various versatile cyclic scaffolds.12,13 In spite of the significant progress that has been made, the intermolecular cycloadditions with VECs have thus far been limited to medium-sized rings. In contrast, the direct formation of macrocycles from VECs is still in its infancy. Continuing our studies on versatile heterocyclic constructions,16 we herein disclose the first iridium-catalysed decarboxylative transformations of VECs with readily available isatoic anhydrides, which directly delivered the benzo-fused 14-membered aza-macrolides via the sequential allylic amination/macrocyclisation using VECs as a 3-atom synthon (Scheme 1b). Notably, this reaction proceeded under relatively mild conditions, and generated CO2 as the sole byproduct. Moreover, the obtained macrocyclic products exhibited favourable fluorescence properties, of which appending different substituents could result in remarkable diversities of fluorescence emission intensity as well as slight red-shift in the fluorescence spectra.


image file: c9cc07372h-s1.tif
Scheme 1 Transition-metal-catalysed decarboxylative transformations of VECs.

Our initial examination of the decarboxylative coupling of isatoic anhydride (1a) with vinyl-1,3-dioxolan-2-one (2a) was carried out using [Ir(cod)(OMe)]2 (2.0 mol%) and PPh3 (4.5 mol%) as the catalyst combination in 1,2-DCE at 60 °C for 12 h, which afforded the serendipitous aza-macrolide product 3a in 8% yield with 50[thin space (1/6-em)]:[thin space (1/6-em)]50 d.r. (Table 1, entry 1). The structures of syn- and anti-3a were unambiguously confirmed by X-ray single crystal diffraction (Fig. 2).17 This outcome inspired us to intensively investigate the interesting transformation. First, a variety of chelating N,N-donor ligands were screened, including bipyridines and phenanthrolines (for details see Table S1 in the ESI†), and 2,2′-bipyridine proved to be the optimal choice (Table 1, entry 2). To our disappointment, attempts to gain one preferential diastereomer of 3a failed even in the presence of chiral PyrOx and Pybox ligands (for details see Table S1 in the ESI†). Subsequently, a brief examination of the organic solvents, including DCM, MeCCl3, DME, THF, MeCN and acetone, revealed that the yield of 3a could slightly increase to 52% in DCM (Table 1, entries 3–8). To our delight, further investigations showed that the presence of co-solvent could efficiently promote the conversion and the target product 3a improved to 62% yield in DCM and acetone at 3[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) (Table 1, entry 9, and see Table S4 in the ESI†). The yield of 3a was diminished by either increasing or decreasing the reaction temperature (Table 1, entries 10 and 11), even when prolonging the reaction time to 36 h (Table 1, entry 12). In contrast to the previous reports,12–15 no conversion was observed switching to the popular palladium catalytic system, indicating the current transformations specific to iridium catalysis (Table 1, entries 13 and 14).

Table 1 Optimization of various reaction parametersa

image file: c9cc07372h-u1.tif
Entry Catalyst Solvent T/°C 3a (%)
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst (2.0 mol%), ligand (4.5 mol%) in solvent (2.0 mL) for 12 h. b Isolated yields. c PPh3 as ligand. d 36 h. e n.r. = no reaction.
1c [Ir(cod)(OMe)]2 1,2-DCE 60 8
2 [Ir(cod)(OMe)]2 1,2-DCE 60 42
3 [Ir(cod)(OMe)]2 DCM 60 52
4 [Ir(cod)(OMe)]2 MeCCl3 60 45
5 [Ir(cod)(OMe)]2 DME 60 26
6 [Ir(cod)(OMe)]2 THF 60 18
7 [Ir(cod)(OMe)]2 MeCN 60 30
8 [Ir(cod)(OMe)]2 Acetone 60 45
9 [Ir(cod)(OMe)] 2 DCM/acetone (3[thin space (1/6-em)]:[thin space (1/6-em)]1) 60 62
10 [Ir(cod)(OMe)]2 DCM/acetone (3[thin space (1/6-em)]:[thin space (1/6-em)]1) 80 51
11 [Ir(cod)(OMe)]2 DCM/acetone (3[thin space (1/6-em)]:[thin space (1/6-em)]1) 40 55
12d [Ir(cod)(OMe)]2 DCM/acetone (3[thin space (1/6-em)]:[thin space (1/6-em)]1) 60 60
13 Pd2(dba)3·CHCl3 DCM/acetone (3[thin space (1/6-em)]:[thin space (1/6-em)]1) 60 n.r.e
14 Pd(OAc)2 DCM/acetone (3[thin space (1/6-em)]:[thin space (1/6-em)]1) 60 n.r.


image file: c9cc07372h-f2.tif
Fig. 2 ORTEP plots for molecular structures of syn- (left) and anti-3a (right) with the probability at 50% level. For the crystal structure of anti-3a, two half molecules were in the asymmetric unit cell, each lying about an inversion centre.

Next, the scope and limitations of the cascade decarboxylative macrocyclisation were evaluated, as shown in Scheme 2. Isatoic anhydrides 1 bearing electron-donating and -withdrawing groups, such as –Me, –OMe, halides and –CF3, proceeded smoothly with 2a, affording the corresponding aza-macrolides 3 in moderate yields. In most cases, compounds 3 were observed with 50[thin space (1/6-em)]:[thin space (1/6-em)]50 d.r., while stereospecific selectivity was observed in 3g, 3l and 3m. Unfortunately, substituted vinylethylene carbonates 2 were not compatible with this iridium catalytic system, generating α,β-unsaturated ketones.12a,15c


image file: c9cc07372h-s2.tif
Scheme 2 Ir(I)-Catalysed cascade reaction of isatoic anhydrides 1 with vinyl-1,3-dioxolan-2-one (2a).

Aza-macrolide syn-3i was successfully transformed to the corresponding alkynylated product 4 in quantitative yield through Pd(II)-catalysed Sonogashira coupling reaction, which highlights the synthetic applications of this method (Scheme 3).


image file: c9cc07372h-s3.tif
Scheme 3 Late-stage functionalization of syn-3k.

To gain insight into the reaction mechanism, two control experiments were performed. Treating N-methyl isatoic anhydride 5 with 2a under the standard conditions failed to generate the macrocyclic product, and the starting materials were almost quantitatively recovered (Scheme 4a). Meanwhile, the nucleophilic reaction of 3,4-dihydro-2(H)-quinoline 6 with 2a under the standard catalytic conditions was conducted, successfully delivering the branched allylic alcohol 7 in 32% yield (Scheme 4b). These results implied that a successful C–N bond construction through allylic amination was essential to the subsequent macrocyclisation in this transformation.


image file: c9cc07372h-s4.tif
Scheme 4 Mechanistic investigations.

On the basis of the experimental results and previous reports,14,18 a plausible reaction mechanism was proposed as shown in Scheme 5. The reaction was initiated by the Ir(I)-catalysed decarboxylation of 2a to generate zwitterionic π-allyl iridium intermediate A, which could further tautomerize to a highly electrophilic six-membered iridacyclic species B. Subsequently, the cyclic intermediate B was trapped by the nucleophilic amino group of 1a, giving the branched allylic alcohol intermediate C, as well as the active Ir(I) catalyst for the next catalytic cycle.19 Finally, intermediate C underwent the intermolecular ester exchange with a second molecular C to deliver the aza-macrolide product 3a.


image file: c9cc07372h-s5.tif
Scheme 5 Proposed mechanism.

Finally, we were pleased to find that the selected aza-macrolides 3 could exhibit favourable fluorescence emission among 397–430 nm (Fig. 3, and for details see the ESI†). Specifically, compounds 3 with both electron-withdrawing and electron-donating groups on the phenyl rings would result in red-shift in the fluorescence spectra (see Fig. S1 in the ESI†). In general, anti-3 exhibited superior fluorescence emission than syn-3 (see Fig. S2 in the ESI†). Among them, anti-3m exhibited the strongest fluorescence emission. Meanwhile, the fluorescence emission of syn-3k with an iodine substituent was almost quenched probably due to the heavy atom effect of iodine. In addition, the highest quantum yield of compound anti-3m could reach 36.19%. All these findings would probably make such aza-macrocycles promising fluorescent materials and biosensors. Moreover, some experiments were carried out to sense some metal ions, such as Pd2+, Ir3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+ to aza-macrolide anti-3m. However, no significant binding to metal ions was observed and thus they did not arouse a fluorescence change.


image file: c9cc07372h-f3.tif
Fig. 3 Fluorescence spectra of aza-macrolides 3 in DCM.

In summary, we have discovered the first iridium-catalysed intermolecular decarboxylative transformation of VECs with isatoic anhydrides, generating the benzo-fused aza-macrolides via the cascade allylic amination/macrocyclisation process. This protocol features high step economy with CO2 being the sole byproduct. Moreover, the macrocyclic products exhibit favourable fluorescence properties, and could find applications in fluorescence materials and biosensors.

This research was supported by NSF of China (No. 21602064 and 21572072), the Science and Technology Project of Quanzhou City (2018C073R), Huaqiao University (ZQN-PY317), Outstanding Youth Scientific Research Cultivation Project of Colleges and Universities of Fujian Province, and Subsidized Project for Postgraduates’ Innovative Fund in Scientific Research of Huaqiao University.

Conflicts of interest

There are no conflicts to declare.

Notes and references

  1. (a) S. Zhang and L. Zhao, Acc. Chem. Res., 2018, 51, 2535 CrossRef CAS PubMed ; (b) M.-X. Wang, Acc. Chem. Res., 2012, 45, 182 CrossRef CAS PubMed .
  2. (a) R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev., 2011, 111, 6810 CrossRef CAS PubMed ; (b) S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100, 853 CrossRef CAS PubMed .
  3. (a) X. Li, Z. Li and Y.-W. Yang, Adv. Mater., 2018, 30, 1800177 CrossRef PubMed ; (b) S. J. Malthus, S. A. Cameron and S. Brooker, Inorg. Chem., 2018, 57, 2480 CrossRef CAS PubMed .
  4. (a) H. Bai, J. Wang, Z. Li and G. Tang, Int. J. Mol. Sci., 2019, 20, 2097 CrossRef ; (b) E. M. Driggers, S. P. Hale, J. Lee and N. K. Terrett, Nat. Rev. Drug Discovery, 2008, 7, 608 CrossRef CAS PubMed .
  5. A. T. Frank, N. S. Farina, N. Sawwan, O. R. Wauchope, M. Qi, E. M. Brzostowska, W. Chan, F. W. Grasso, P. Haberfield and A. Greer, Mol. Diversity, 2007, 11, 115 CrossRef CAS .
  6. (a) M. D. Delost, D. T. Smith, B. J. Anderson and J. T. Njardarson, J. Med. Chem., 2018, 61, 10996 CrossRef CAS ; (b) A. Parenty, X. Moreau, G. Niel and J.-M. Campagne, Chem. Rev., 2013, 113, PR1 CrossRef CAS PubMed ; (c) X. Yu and D. Sun, Molecules, 2013, 18, 6230 CrossRef CAS PubMed ; (d) T. C. Henninger, Expert Opin. Ther. Pat., 2003, 13, 787 CrossRef CAS ; (e) N. Jana and S. Nanda, New J. Chem., 2018, 42, 17803 RSC ; (f) N. Winssinger and S. Barluenga, Chem. Commun., 2007, 22 RSC .
  7. (a) A. Fürstner, Chem. Commun., 2011, 47, 6505 RSC ; (b) S. E. Denmark and J. M. Muhuhi, J. Am. Chem. Soc., 2010, 132, 11768 CrossRef CAS PubMed ; (c) A. Gradillas and J. Pérez-Castells, Angew. Chem., Int. Ed., 2006, 45, 6086 CrossRef CAS PubMed ; (d) A. Deiters and S. F. Martin, Chem. Rev., 2004, 104, 2199 CrossRef CAS PubMed .
  8. (a) G. A. Molander and F. Dehmel, J. Am. Chem. Soc., 2004, 126, 10313 CrossRef CAS PubMed ; (b) P. J. Mohr and R. L. Halcomb, J. Am. Chem. Soc., 2003, 125, 1712 CrossRef CAS PubMed .
  9. Horner-Wadsworth-Emmons macrolactonization, see: (a) C. C. Sanchez and G. E. Keck, Org. Lett., 2010, 12, 1460 CrossRef PubMed ; (b) K. Ando, K. Narumiya, H. Takada and T. Teruya, Org. Lett., 2005, 7, 3053 CrossRef PubMed  . photo-induced radical macrolactonization, see: ; (c) T. Iwasaki, Y. Tajimi, K. Kameda, C. Kingwell, W. Wcislo, K. Osaka, M. Yamawaki, T. Morita and Y. Yoshimi, J. Org. Chem., 2019, 84, 8019 CrossRef CAS PubMed ; (d) Y. Yoshimi, M. Masuda, T. Mizunashi, K. Nishikawa, K. Maeda, N. Koshida, T. Itou, T. Morita and M. Hatanaka, Org. Lett., 2009, 11, 4652 CrossRef CAS PubMed .
  10. (a) S. Ganss and B. Breit, Angew. Chem., Int. Ed., 2016, 55, 9738 CrossRef CAS PubMed ; (b) E. M. Stang and M. C. White, Angew. Chem., Int. Ed., 2011, 50, 2094 CrossRef CAS PubMed ; (c) E. M. Stang and M. C. White, Nat. Chem., 2009, 1, 547 CrossRef CAS PubMed ; (d) K. J. Fraunhoffer, N. Prabagaran, L. E. Sirois and M. C. White, J. Am. Chem. Soc., 2006, 128, 9032 CrossRef CAS PubMed .
  11. (a) W.-W. Zhang, T.-T. Gao, L.-J. Xu and B.-J. Li, Org. Lett., 2018, 20, 6534 CrossRef CAS PubMed ; (b) Y. Bai, X. Shen, Y. Li and M. Dai, J. Am. Chem. Soc., 2016, 138, 10838 CrossRef CAS PubMed ; (c) Y. Bai, D. C. Davis and M. Dai, Angew. Chem., Int. Ed., 2014, 53, 6519 CrossRef CAS ; (d) K. Lee, H. Kim and J. Hong, Angew. Chem., Int. Ed., 2012, 51, 5735 CrossRef CAS PubMed .
  12. [3+2] annulation, see: (a) K. Liu, I. Khan, J. Cheng, Y. J. Hsueh and Y. J. Zhang, ACS Catal., 2018, 8, 11600 CrossRef CAS ; (b) I. Khan, C. Zhao and Y. J. Zhang, Chem. Commun., 2018, 54, 4708 RSC ; (c) A. Khan, J. Xing, J. Zhao, Y. Kan, W. Zhang and Y. J. Zhang, Chem. – Eur. J., 2015, 21, 120 CrossRef CAS PubMed ; (d) L. Yang, A. Khan, R. Zheng, L. Y. Jin and Y. J. Zhang, Org. Lett., 2015, 17, 6230 CrossRef CAS PubMed ; (e) A. Khan, L. Yang, J. Xu, L. Y. Jin and Y. J. Zhang, Angew. Chem., Int. Ed., 2014, 53, 11257 CrossRef CAS PubMed ; (f) A. Khan, R. Zheng, Y. Kan, J. Ye, J. Xing and Y. J. Zhang, Angew. Chem., Int. Ed., 2014, 53, 6439 CrossRef CAS PubMed  ; other examples on annulation using VECs as 3-atom synthons, see: ; (g) Y. Xia, Q.-F. Bao, Y. Li, L.-J. Wang, B.-S. Zhang, H.-C. Liu and Y.-M. Liang, Chem. Commun., 2019, 55, 4675 RSC ; (h) Y. Xu, L. Chen, Y. Yang, Z. Zhang and W. Yang, Org. Lett., 2019, 21, 6674 CrossRef CAS PubMed .
  13. [5+n] annulation, see: (a) B. Niu, X.-Y. Wu, Y. Wei and M. Shi, Org. Lett., 2019, 21, 4859 CrossRef CAS PubMed ; (b) Y. Wei, S. Liu, M.-M. Li, Y. Li, Y. Lan, L.-Q. Lu and W.-J. Xiao, J. Am. Chem. Soc., 2019, 141, 133 CrossRef CAS PubMed ; (c) H.-W. Zhao, J. Du, J.-M. Guo, N.-N. Feng, L.-R. Wang, W.-Q. Ding and X.-Q. Song, Chem. Commun., 2018, 54, 9178 RSC ; (d) C. Yuan, Y. Wu, D. Wang, Z. Zhang, C. Wang, L. Zhou, C. Zhang, B. Song and H. Guo, Adv. Synth. Catal., 2018, 360, 652 CrossRef CAS ; (e) Y. Yang and W. Yang, Chem. Commun., 2018, 54, 12182 RSC ; (f) S. Singha, T. Patra, C. G. Daniliuc and F. Glorius, J. Am. Chem. Soc., 2018, 140, 3551 CrossRef CAS PubMed ; (g) P. Das, S. Gondo, P. Nagender, H. Uno, E. Tokunaga and N. Shibata, Chem. Sci., 2018, 9, 3276 RSC ; (h) Z.-Q. Rong, L.-C. Yang, S. Liu, Z. Yu, Y.-N. Wang, Z. Y. Tan, R.-Z. Huang, Y. Lan and Y. Zhao, J. Am. Chem. Soc., 2017, 139, 15304 CrossRef CAS PubMed ; (i) L. C. Yang, Z.-Q. Rong, Y.-N. Wang, Z. Y. Tan, M. Wang and Y. Zhao, Angew. Chem., Int. Ed., 2017, 56, 2927 CrossRef CAS PubMed  ; other examples on annulation using VECs as 5-atom synthons, see: ; (j) R. Zeng, J.-L. Li, X. Zhang, Y.-Q. Liu, Z.-Q. Jia, H.-J. Leng, Q.-W. Huang, Y. Liu and Q.-Z. Li, ACS Catal., 2019, 9, 8256 CrossRef CAS ; (k) X. Gao, M. Xia, C. Yuan, L. Zhou, W. Sun, C. Li, B. Wu, D. Zhu, C. Zhang, B. Zheng, D. Wang and H. Guo, ACS Catal., 2019, 9, 1645 CrossRef CAS .
  14. Allylic substitutions with VECs, see: (a) A. Khan, S. Khan, I. Khan, C. Zhao, Y. Mao, Y. Chen and Y. J. Zhang, J. Am. Chem. Soc., 2017, 139, 10733 CrossRef CAS PubMed ; (b) J. Xie, W. Guo, A. Cai, E. C. Escudero-Adán and A. W. Kleij, Org. Lett., 2017, 19, 6388 CrossRef CAS PubMed ; (c) N. Miralles, J. E. Gómez, A. W. Kleij and E. Fernández, Org. Lett., 2017, 19, 6096 CrossRef CAS PubMed ; (d) J. E. Gómez, W. Guo and A. W. Kleij, Org. Lett., 2016, 18, 6042 CrossRef PubMed ; (e) W. Guo, L. Martínez-Rodríguez, E. Martin, E. C. Escudero-Adán and A. W. Kleij, Angew. Chem., Int. Ed., 2016, 55, 11037 CrossRef CAS ; (f) A. Cai, W. Guo, L. Martínez-Rodríguez and A. W. Kleij, J. Am. Chem. Soc., 2016, 138, 14194 CrossRef CAS PubMed ; (g) W. Guo, L. Martínez-Rodríguez, R. Kuniyil, E. Martin, E. C. Escudero-Adán, F. Maseras and A. W. Kleij, J. Am. Chem. Soc., 2016, 138, 11970 CrossRef CAS PubMed .
  15. Umpolung reactivities of VECs, see: (a) H. Wang, S. Qiu, S. Wang and H. Zha, ACS Catal., 2018, 8, 11960 CrossRef CAS ; (b) L. C. Yang, Z. Y. Tan, Z.-Q. Rong, R. Liu, Y.-N. Wang and Y. Zhao, Angew. Chem., Int. Ed., 2018, 57, 7860 CrossRef CAS PubMed ; (c) W. Guo, R. Kuniyil, J. E. Góme, F. Maseras and A. W. Kleij, J. Am. Chem. Soc., 2018, 140, 3981 CrossRef CAS PubMed .
  16. (a) Z. Yang, Z. Song, L. Jie, L. Wang and X. Cui, Chem. Commun., 2019, 55, 6094 RSC ; (b) B. Wu, Z. Yang, H. Zhang, L. Wang and X. Cui, Chem. Commun., 2019, 55, 4190 RSC ; (c) S. Huang, H. Li, X. Sun, L. Xu, L. Wang and X. Cui, Org. Lett., 2019, 21, 5570 CrossRef CAS PubMed ; (d) Z. Shen, C. Pi, X. Cui and Y. Wu, Chin. Chem. Lett., 2019, 30, 1374 CrossRef CAS ; (e) S. Du, C. Pi, T. Wan, Y. Wu and X. Cui, Adv. Synth. Catal., 2019, 361, 1766 CrossRef CAS ; (f) T. Yuan, C. Pi, C. You, X. Cui, S. Du, T. Wan and Y. Wu, Chem. Commun., 2019, 55, 163 RSC ; (g) L. Jie, L. Wang, D. Xiong, Z. Yang, D. Zhao and X. Cui, J. Org. Chem., 2018, 83, 10974 CrossRef CAS PubMed ; (h) L. Wang, Y. Yu, M. Yang, C. Kuai, D. Cai, J. Yu and X. Cui, Adv. Synth. Catal., 2017, 359, 3818 CrossRef CAS  ; for a recent review, see: ; (i) Z. Shi, L. Wang and X. Cui, Chin. J. Org. Chem., 2019, 39, 1596 CrossRef .
  17. CCDC 1942738 (syn-3a) and 1942739 (anti-3a) contain the supplementary crystallographic data for this paper†.
  18. (a) K. Pal, A. Hoque and C. M. R. Volla, Chem. – Eur. J., 2018, 24, 2558 CrossRef CAS PubMed ; (b) L. M. Stanley and J. F. Hartwig, Angew. Chem., Int. Ed., 2009, 48, 7841 CrossRef CAS PubMed ; (c) B. M. Trost, R. C. Bunt, R. C. Lemoine and T. L. Calkins, J. Am. Chem. Soc., 2000, 122, 5968 CrossRef CAS .
  19. Q. Cheng, H.-F. Tu, C. Zheng, J.-P. Qu, G. Helmchen and S.-L. You, Chem. Rev., 2019, 119, 1855 CrossRef CAS PubMed .

Footnote

† Electronic supplementary information (ESI) available. CCDC 1942738–1942742. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9cc07372h
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