
==== Front
RSC Adv
RSC Adv
RA
RSCACL
RSC Advances
2046-2069
The Royal Society of Chemistry

d4ra06178k
10.1039/d4ra06178k
Chemistry
Incorporating azaheterocycle functionality in intramolecular aerobic, copper-catalyzed aminooxygenation of alkenes†
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra06178k

https://orcid.org/0000-0001-7463-5728
DeCicco Ethan M. a
https://orcid.org/0009-0007-6496-7981
Tlapale-Lara Neively a
https://orcid.org/0000-0002-6181-7431
Paradine Shauna M. a
a Department of Chemistry, University of Rochester 120 Trustee Road Rochester NY 14627 USA sparadin@ur.rochester.edu

10 9 2024
4 9 2024
10 9 2024
14 39 2882228826
26 8 2024
3 9 2024
This journal is © The Royal Society of Chemistry
2024
The Royal Society of Chemistry
https://creativecommons.org/licenses/by-nc/3.0/ This article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given and it is not used for commercial purposes.
Despite the maturity of alkene 1,2-difunctionalization reactions involving C–N bond formation, a key limitation across aminofunctionalization methods is incompatibility with substrates bearing medicinally relevant N-heterocycles. Using a cooperative ligand-substrate catalyst activation strategy, we have developed an aerobic, copper-catalyzed alkene aminooxygenation method that exhibits broad tolerance for β,γ-unsaturated carbamates bearing aromatic azaheterocycle substitution. The synthetic potential of this methodology was demonstrated by engaging a densely-functionalized vonoprazan analogue and elaborating an amino oxygenated product to synthesize a heteroarylated analogue precursor of the FDA-approved antibiotic chloramphenicol.

A key limitation across aminofunctionalizations is incompatibility with substrates bearing medicinally relevant N-heterocycles; in contrast, this aerobic, Cu-catalyzed aminooxygenation engages a diverse range of N-heterocycle-bearing substrates.

National Institutes of Health 10.13039/100000002 R35GM150584 National Science Foundation 10.13039/100000001 MRI-2215973 pubstatusPaginated Article
==== Body
pmcIntroduction

Alkene 1,2-aminofunctionalization is a powerful tool for synthetic chemists, rapidly constructing useful difunctionalized building blocks and targeted scaffolds.1 This includes such transformations as diamination,2 aminooxygenation,3 aminohalogenation,4 carboamination,5 and less commonly, aminoborylation,6 aminocyanation,7 aminophosphorylation8 and thioamination9 (Fig. 1A). While diverse inter- and intramolecular aminofunctionalization methods are available to construct a host of highly functionalized products, there is a notable lack of functional group tolerance in most methods with respect to substrates bearing N-heterocyclic scaffolds. Although several aminofunctionalization methods have shown isolated examples of N-heterocycle-bearing substrates,6,10 a method that demonstrates broad tolerance across a range of azaheterocycles has not yet been reported in the literature. These coordinating moieties can lead to catalyst poisoning, and reactions with substrates bearing these groups are prone to undesired side reactions under the oxidizing conditions typically necessary for alkene aminofunctionalization. This means that many otherwise useful methods cannot be confidently used in medicinal chemistry endeavors due to the lack of this medicinally relevant functionality in substrate scopes. This is consequential as over 85% of biologically active molecules contain at least one heterocycle,11 and more than 75% of FDA approved therapeutics contain a nitrogen heterocycle.12 Heterocycle incorporation into drug targets is often associated with more beneficial therapeutic and pharmacokinetic properties.13 Thus, it is essential to develop methodologies that tolerate N-heterocyclic moieties to facilitate their application in the synthesis of biologically active compounds.

Fig. 1 (A) Overview of alkene aminofunctionalization strategies. (B) Our prior work: aminooxygenation of cinnamyl N-alkoxy carbamates featuring an atypical amidyl radical pathway. This work: alkene aminooxygenation of γ-heteroaryl-β,γ-unsaturated carbamates.

Among the many aminofunctionalization variants, alkene aminooxygenation is an excellent strategy to access 1,2-aminoalcohols and 1,2-aminocarbonyls, which are prevalent functional motifs within building blocks for the construction of bioactive compounds (Fig. 1A).14 Seminal reports detail the use of transition metal catalysts, often paired with a stoichiometric oxidant for catalyst turnover, and/or as a source of oxygen functionality.15 Recent reports have demonstrated an attractive alternative approach, using molecular oxygen to serve the dual role of oxidant and source of oxygen functionality; this approach also avoids the generation of stoichiometric byproducts and expensive reagents.3h,10d,16 Advantageously, aerobic aminooxygenation methods are typically mediated by earth-abundant metals such as Mn,17 Co,17 and Cu.18 Despite these notable advances in aerobic transition metal-catalyzed alkene aminooxygenation, typically monosubstituted or 1,1-disubstituted alkenes are used. Further, the incorporation of N-heteroaromatic substitution in substrates for these reactions is scarce.

Our lab recently developed an aerobic, copper-catalyzed aminooxygenation method for internal alkenes in the synthesis of 4-benzoyl-oxazolidin-2-ones from cinnamyl N-alkoxy carbamates (Fig. 1B).10d This reaction occurs under mild conditions, uses low catalyst loadings, and operates under an ambient atmosphere at room temperature. Our mechanistic findings were consistent with C–N bond formation proceeding via an amidyl radical cyclization pathway, as opposed to the aminometallation mechanism that is commonly observed in other transition metal-catalyzed alkene aminofunctionalizations.3c,16a,19 This mechanistic divergence was made possible through chelation of the substrate to the copper metal center, triggering a rapid reduction of Cu(ii) to Cu(i) and subsequent generation of a potent one-electron copper oxidant. We hypothesized that the preference for O-binding of the substrate and the mild reaction conditions would allow for efficient reactivity in the presence of N-heterocyclic substituents. Herein, we report the successful implementation of this strategy through the synthesis of a variety of carbonyl-linked, heteroaryl-oxazolidin-2-ones (Fig. 1B). This method is compatible with a broad range of 5- and 6-membered N-heterocycles, including those containing multiple heteroatoms.

In our initial investigations (Table 1), we applied our previously published conditions to the reaction with 3-pyridyl substituted allyl carbamate 1a, and found that aminooxygenated product 2a was formed in 54% yield (entry 1). Similar Cu sources bearing acetate counterions performed well (entries 3–4), but a slight improvement in yield was found when using CuTC (entry 4, 59%). It is worth noting that while the original conditions from our previous report worked well in this case, Cu(OAc)2 is highly sensitive to trace impurities in the starting material. Indeed, in that report we tested several azaheterocycle-containing substrates and found them all to be unreactive under those conditions.10d In contrast, CuTC is much more robust to minor impurities, giving us more consistent reactivity. Model substrate 1a reacted well even at 1 mol% loading of CuTC (entry 5), but 5 mol% loading gave better results across a wider range of substrates (Fig. 2, see below).20 Base and acid additives both hindered the reaction (entries 6–7). Further increasing catalyst loading to 20 mol% did not provide any benefit (entry 8), furnishing 2a in a slightly diminished yield (42%). Increasing the reaction temperature (entry 9) led to decomposition of the reaction components, presumably from overoxidation. Similar results were observed when increasing both temperature and catalyst loading (entry 10). Investigation into ligand structure–reactivity relationships revealed 2,9-dimethyl-1,10-phenanthroline (neocuproine) was optimal, consistent with our previous studies.10d,20 Other 1,10-phenanthroline ligands bearing alkyl substitution adjacent to the nitrogen centers (i.e. ortho) showed similar results, in addition to 2,2′-bipyridyl ligands bearing 6,6′-dialkyl substitution.20

Optimization screening. Yields correspond to quantitative NMR results using 1,3,5-trimethoxybenzene as an internal standard and are an average of three runs. Reaction conditions: 1a (0.200 mmol), Cu source (10.0–40.0 μmol), 2,9-dimethyl-1,10-phenanthroline (neocuproine) (12.0–44.0 μmol), MeCN (1.0 mL, 0.2 M), open vial, rt, 2 h. Cu(OAc)2 = copper(ii) acetate, Cu(EH)2 = copper(ii) 2-ethylhexanoate, CuTC = copper(i) thiophene-2-carboxylate, PivOH = pivalic acid

	
Entry	Changes to conditions	Yield 2a	Yield 3a	Recovered 1a	
1	None	54%	12%	0%	
2a	5 mol% Cu(OAc)2	50%	11%	0%	
3a	5 mol% Cu(EH)2	51%	10%	0%	
4a	5 mol% CuTC	59%	13%	0%	
5	1 mol% CuTC	59%	12%	0%	
6a	As in entry 4, 1 eq. K2CO3 added	0%	3%	61%	
7a	As in entry 4, 1 eq. PivOH added	0%	0%	>95%	
8b	20 mol% CuTC	2%	10%	0%	
9a	5 mol% CuTC, 70 °C	3%	8%	42%	
10b	20 mol% CuTC, 70 °C	4%	8%	41%	
a 6 mol% of neocuproine was used.

b 24 mol% of neocuproine was used.

Fig. 2 Substrate scope. Yields correspond to isolated products and are averages of three runs at 0.500 mmol scale. Quantitative NMR yields in parentheses, determined using 1,3,5-trimethoxybenzene as internal standard. Reaction conditions: 1 (0.500 mmol), CuTC (25.0 μmol), neocuproine (30.0 μmol), MeCN (0.2 M), rt, 2 h.a70 °C. CuTC = copper(i) thiophene 2-carboxylate.

With optimized conditions in hand, we evaluated the scope of heteroaryl alkenes in the reaction (Fig. 2). Model product 2a was isolated in 55% yield at 0.500 mmol scale. The position of the pyridine nitrogen relative to the tether did not substantially impact the reaction, with 2-pyridyl substitution providing 2b in 39% yield, and synthetically versatile bromide substituents were well tolerated (2c, 52%). Pyrimidines were effective as substituents on phenyl rings (2d, 50%), although product yields were poor when the pyrimidine was directly attached to the alkene.20 This method is also compatible with a variety of 5-membered azaheterocycles, including thiazole (2e, 34%), isoxazole (2f, 46%), pyrazole (2g, 54%), and oxazole (2h, 49%). Substrates bearing arenes with heteroaromatic substitution are also effective in this reaction (2i–2k, 52–57%). We had previously demonstrated that protected indoles were compatible with aerobic aminooxygenation;10d under our revised conditions, the related but more challenging indazole 2l is also a good substrate (50%). Oxygen-containing heterocycles can also be employed,10d exemplified by benzodioxine 2m (41%), although elevated temperatures were required for efficient reaction of this substrate within 2 h. Notably, a precursor compound in the synthesis of the ulcer medication vonoprazan could be elaborated to generate analogue 1n, which underwent the desired aminooxygenation. Product 2n was isolated in reasonable yield (44%) showcasing this method's tolerance for densely functionalized, drug-like substrates. Some heterocycles bearing reactive C–X bonds, such as α-halopyridines, were not compatible in the reaction. The primary limitation of this method was our ability to access the substrate; substrates containing azaheterocycles such as imidazole, pyrroles, pyrazoles, halopyrimidines, and pyrrolopyridine were incompatible with the substrate synthesis and were unable to be tested in the aminooxygenation reaction.20

To showcase the synthetic utility of our aerobic aminooxygenation method, we synthesized a heteroaryl analogue precursor of the amphenicol antibiotics, which includes FDA-approved therapeutics chloramphenicol and thiamphenicol. Aminooxygenation product 2i was selected by analogy to the chloramphenicol structure (Fig. 3). Preparation of 2ivia aerobic aminooxygenation scaled readily to 5.0 mmol, proceeding in identical yield to the scope investigation scale (0.500 mmol), and the reaction was performed in an open beaker (57% yield, 826 mg). Reduction with NaBH4 fashioned alcohol 4 in 68% yield. Subsequent N–O cleavage with SmI2 generated free carbamate 5 (57%), which underwent base mediated ring opening in 52% yield (21% isolated) to generate heteroarylated amphenicol analogue 6. Subsequent N-acetylation of free amines in the presence of alcohols has been reported in previous chloramphenicol syntheses.21

Fig. 3 Synthesis of amphenicol antibiotic analogues. Reaction conditions: (A): 1i (1.37 g, 5.00 mmol, 1.0 equiv.) CuTC (47.7 mg, 0.250 mmol, 0.05 equiv.), neocuproine (62.5 mg, 0.300 mmol, 0.06 equiv.), MeCN (25 mL, 0.2 M), rt, open beaker, 2 h. (B): 2i (318 mg, 1.11 mmol, 1.0 equiv.), NaBH4 (46.0 mg, 1.1 equiv.), MeOH (0.2 M), 0 °C → rt, 1 h. (C): 4 (180 mg, 0.622 mmol, 1.0 equiv.), SmI2 (0.1 M in THF) (50 mL, 5.0 mmol, 8.0 equiv.), THF (16 mL, 0.4 M), rt, 2 h. (D): 5 (40.0 mg, 0.154 mmol, 1.0 equiv.), LiOH·H2O (19.4 mg, 0.462 mmol, 3.0 equiv.), 16 h.aQNMR yield using 1,3,5-trimethoxybenzene as internal standard.

Conclusions

By employing the catalytic strategy of substrate-promoted catalyst activation, we have developed an operationally simple, mild, and efficient approach to use N-heteroaryl alkene substrates in copper-catalyzed alkene aminooxygenation. This method displays broad compatibility across a diverse range of heteroaromatic groups that has not previously been demonstrated in alkene aminofunctionalization reactions. Further, the practicality and applicability of our strategy to the synthesis of drug-like compounds shows promise for its adoption in the rapid preparation of valuable α-amino-oxygenated motifs for diverse synthetic applications.

Data availability

The data supporting this article, including experimental procedures, additional reaction screening data, and full compound characterization, have been included as part of the ESI.†

Author contributions

E. M. D. and S. M. P. conceived of and designed the project. E. M. D. and N. T.-L. conducted all the experimental work, and all authors analyzed the data and discussed the results. E. M. D. and S. M. P. wrote the manuscript with support from all authors. S. M. P. directed the research.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-014-D4RA06178K-s001

The authors thank the National Institutes of Health (R35GM150584) for financial support. The authors acknowledge the use of JEOL NMR spectrometers acquired with support from the National Science Foundation (MRI-2215973). We thank Shannon O'Neil for checking the experimental procedure for preparation of compound 2i in Fig. 2.
==== Refs
References

(a) Wu Z. Hu M. Li J. Wu W. Jiang H. Org. Biomol. Chem. 2021 19 3036 3054 33734255
(b) Chen X. Xiao F. He W.-M. Org. Chem. Front. 2021 8 5206 5228
(a) Kumar R. Khanna Y. Kaushik P. Kamal R. Khokhar S. Chem.–Asian J. 2023 18 e202300017 36869415
(b) Tao Z.-L. Denmark S. E. Synthesis 2021 53 3951 3962
(a) Hashimoto K. Watari T. Uchida T. Tetrahedron Lett. 2023 123 154542
(b) Kirby G. Prestat G. Berhal F. J. Org. Chem. 2023 88 4720 4729 36939110
(c) Liu C. Wu J. Tan X. Zhang J. Wu W. Jiang H. ACS Catal. 2023 13 11339 11344
(d) Li X. Song H. Yu S. Mi R. Li X.-X. Angew. Chem., Int. Ed. 2023 62 e202305669
(e) Nie X. Ritter C. W. Hemming M. Ivlev S. I. Xie X. Chen S. Meggers E. Angew. Chem., Int. Ed. 2023 62 e202314398
(f) He M. Shi C. Luo M. Yang C. Guo L. Zhao Y. Xia W. J. Org. Chem. 2024 89 1967 1979 38241611
(g) Madiu R. Dellosso B. Doran E. L. Doran J. M. Pinarci A. A. TenHoeve T. M. Howard A. M. Stroud J. L. Rivera D. A. Moskovitz D. A. et al. Org. Biomol. Chem. 2024 22 2300 2306 38410027
(h) Carmo R. L. L. Galster S. L. Wdowik T. Song C. Chemler S. R. J. Am. Chem. Soc. 2023 145 13715 13729 37327484
(a) Zhan X. Gao G. Liang Y. Li F. Liu K. Fan W. Zhang S. Li M.-B. Org. Chem. Front. 2023 10 3353 3360
(b) Wang Z. Hou C. Chen P. Org. Lett. 2023 25 2685 2690 37026673
(c) Mejri E. Higashida K. Kondo Y. Nawachi A. Morimoto H. Ohshima T. Sawamura M. Shimizu Y. Org. Lett. 2023 25 4581 4585 37289073
(d) Constantinou C. T. Gkizis P. L. Lagopanagiotopoulou O. T. G. Skolia E. Nikitas N. F. Triandafillidi I. Kokotos C. G. Chem.–Eur. J. 2023 29 e202301268 37254681
(e) Rahman A. U. Zarshad N. Khan I. Faiz F. Li G. Ali A. Front. Chem. 2021 9 742399 34568286
(f) Ng W.-H. Lam Y.-P. Hu R.-B. Ng W.-L. Yeung Y.-Y. Asian J. Org. Chem. 2021 10 1131 1140
(g) Luo Y. Zhou S. Nkingwa A. A. Zeng Q. Eur. J. Org Chem. 2024 27 e202301178
(h) Schäfer M. Stünkel T. Daniliuc C. G. Gilmour R. Angew. Chem., Int. Ed. 2022 61 e202205508
(i) Wang B. Li J. Wu W. Jiang H. Chin. J. Chem. 2024 42 464 470
(a) Bertrand M. B. Neukom J. D. Wolfe J. P. J. Org. Chem. 2008 73 8851 8860 18942792
(b) White D. R. Hutt J. T. Wolfe J. P. J. Am. Chem. Soc. 2015 137 11246 11249 26313846
(c) Shi P. Wang J. Gan Z. Zhang J. Zeng R. Zhao Y. Chem. Commun. 2019 55 10523 10526
(d) Wdowik T. Galster S. L. Carmo R. L. L. Chemler S. R. ACS Catal. 2020 10 8535 8541 34306802
(e) Falk E. Makai S. Delcaillau T. Gürtler L. Morandi B. Angew. Chem., Int. Ed. 2020 59 21064 21071
Liu Z. Ni H.-Q. Zeng T. Engle K. M. J. Am. Chem. Soc. 2018 140 3223 3227 29384373
(a) Jiang H. Gao H. Liu B. Wu W. Chem. Commun. 2014 50 15348 15351
(b) Kwon Y. Wang Q. Org. Lett. 2020 22 4141 4145 32383382
Li J.-A. Zhang P.-Z. Liu K. Shoberu A. Zou J.-P. Zhang W. Org. Lett. 2017 19 4704 4706 28832163
Luo J. Zhu Z. Liu Y. Zhao X. Org. Lett. 2015 17 3620 3623 26158564
(a) Li Y. Bao J. Zhang Y. Peng X. Yu W. Wang T. Yang D. Liu Q. Zhang Q. Fu J. Chem 2022 8 1147 1163
(b) Liu Z. Wang Y. Wang Z. Zeng T. Liu P. Engle K. M. J. Am. Chem. Soc. 2017 139 11261 11270 28727452
(c) Tao Z. Gilbert B. B. Denmark S. E. J. Am. Chem. Soc. 2019 141 19161 19170 31742399
(d) McNichol C. P. DeCicco E. M. Canfield A. M. Carstairs D. P. Paradine S. M. ACS Catal. 2023 13 6568 6573
(e) Wang L. Wang C. J. Org. Chem. 2019 84 6547 6556 31090410
(f) Li Y. Liang Y. Dong J. Deng Y. Zhao C. Su Z. Guan W. Bi X. Liu Q. Fu J. J. Am. Chem. Soc. 2019 141 18475 18485 31600069
(g) Zhao J. Huang H.-G. Li W. Liu W.-B. Org. Lett. 2021 23 5102 5106 34156853
(h) Ariyarathna J. P. Alom N.-E. Roberts L. P. Kaur N. Wu F. Li W. J. Org. Chem. 2022 87 2947 2958 35142512
(i) Pan Z. Wang S. Brethorst J. T. Douglas C. J. J. Am. Chem. Soc. 2018 140 3331 3338 29465996
(j) Lai S.-Q. Wei B.-Y. Wang J.-W. Yu W. Han B. Angew. Chem., Int. Ed. 2021 60 21997 22003
(k) Li Y. Dong Y. Wang X. Li G. Xue H. Xin W. Zhang Q. Guan W. Fu J. ACS Catal. 2023 13 2410 2421
(l) Zheng Y. Wang Z.-J. Ye Z.-P. Tang K. Xie Z.-Z. Xiao J.-A. Xiang H.-Y. Chen K. Chen X.-Q. Yang H. Angew. Chem., Int. Ed. 2022 61 e202212292
(m) Holst D. E. Dorval C. Winter C. K. Guzei I. A. Wickens Z. K. J. Am. Chem. Soc. 2023 145 8299 8307
(n) Xue W. Zhu Z. Chen S. You B. Tang C. J. Am. Chem. Soc. 2023 145 4142 4149
(o) Holst D. E. Wang D. J. Kim M. J. Guzei I. A. Wickens Z. K. Nature 2021 596 74 79 34157720
(p) Zeng H. Li H. Li C. Jiang H. Zhu C. Org. Chem. Front. 2022 9 1383 1388
(q) Fang X. Yu P. Morandi B. Science 2016 351 832 836 26912891
(r) Feng G. Ku C. K. Zhao J. Wang Q. J. Am. Chem. Soc. 2022 144 20463 20471 36278925
(s) Legnani L. Prina-Cerai G. Delcaillau T. Willems S. Morandi B. Science 2018 362 434 439 30361368
(t) Tan G. Das M. Kleinmans R. Katzenburg F. Daniliuc C. Glorius F. Nat. Catal. 2022 5 1120 1130
(u) Dai J.-J. Yin X. Li L. Rivera M. E. Wang Y.-C. Dai M. Nat. Commun. 2023 14 1774 36997504
Heravi M. M. Zadsirjan V. RSC Adv. 2020 10 44247 44311 35557843
Vitaku E. Smith D. T. Njardarson J. T. J. Med. Chem. 2014 57 10257 10274 25255204
Ritchie T. J. Macdonald S. J. Young R. J. Pickett S. D. Drug Discov. Today 2011 16 164 171 21129497
(a) Hemric B. N. Org. Biomol. Chem. 2021 19 46 81 33174579
(b) Allen L. A. T. Raclea R.-C. Natho P. Parsons P. J. Org. Biomol. Chem. 2021 19 498 513 33325975
(c) Karjalainen O. K. Koskinen A. M. P. Org. Biomol. Chem. 2012 10 4311 4326 22535485
(d) Bergmeier S. C. Tetrahedron 2000 56 2561 2576
(a) Rao W.-H. Jiang L.-L. Chen F.-Y. Zhang M. Guo Y.-Y. Chen J.-J. Cui Y. Zou G.-D. Tang L. Tetrahedron Lett. 2020 61 151540
(b) Fuller P. H. Kim J.-W. Chemler S. R. J. Am. Chem. Soc. 2008 130 17638 17639 19049311
(c) Paderes M. C. Chemler S. R. Org. Lett. 2009 11 1915 1918 19331361
(d) Karyakarte S. D. Smith T. P. Chemler S. R. J. Org. Chem. 2012 77 7755 7760 22870912
(a) Wen K. Wu Z. Huang B. Ling Z. Gridnev I. D. Zhang W. Org. Lett. 2018 20 1608 1612 29481092
(b) Kou X. Li Y. Wu L. Zhang X. Yang G. Zhang W. Org. Lett. 2015 17 5566 5569 26523852
(c) Shen H.-C. Wu Y.-F. Zhang Y. Fan L.-F. Han Z.-Y. Gong L.-Z. Angew. Chem., Int. Ed. 2018 57 2372 2376
Balkenhohl M. Kölbl S. Georgiev T. Carreira E. M. JACS Au 2021 1 919 924 34337605
(a) Liu Z. Wang P. Ou H. Yan Z. Chen S. Tan X. Yu D. Zhao X. Mu T. RSC Adv. 2020 10 7698 7707 35492186
(b) Li J. Wei J. Zhu B. Wang T. Jiao N. Chem. Sci. 2019 10 9099 9103 31827752
(c) Wdowik T. Chemler S. R. J. Am. Chem. Soc. 2017 139 9515 9518 28678493
(a) Wu F. Stewart S. Ariyarathna J. P. Li W. ACS Catal. 2018 8 1921 1925
(b) Paderes M. C. Belding L. Fanovic B. Dudding T. Keister J. B. Chemler S. R. Chem.–Eur. J. 2012 18 1711 1726 22237868
(c) Paderes M. C. Keister J. B. Chemler S. R. J. Org. Chem. 2013 78 506 515 23244027
For additional details, see (ESI).†

Liu J. Li Y. Ke M. Liu M. Zhan P. Xiao Y.-C. Chen F. J. Org. Chem. 2020 85 15360 15367 33169603
