Synthesis of C4-Substituted Indoles via a Catellani and C-N Bond Activation Strategy

Article abstract of DOI:10.1021/acs.orglett.0c02897

This paper describes the case of a cross study between the C-N bond cleavage reaction field and the Catellani-Lautens reaction system. A series of highly functionalized C4-substituted indoles were synthesized using this strategy. By screening the alkyl groups of amines, the energy barrier of C-N bond cleavage reaction was reduced and the corresponding allenization products were avoided. Finally, the density functional theory calculation shows that the inert C-N bond activation reaction is not a concerted process; on the contrary, the coupling reaction first generates indole quaternary ammonium salt, and then C-N bond cleavage occurs via an SN2 process.

Full text of DOI:10.1021/acs.orglett.0c02897

pubs.acs.org/OrgLett
Letter
Synthesis of C4-Substituted Indoles via a Catellani and C−N Bond
Activation Strategy
Bo-Sheng Zhang,*,∇ Fan Wang,∇ Ying-Hui Yang, Xue-Ya Gou, Yi-Feng Qiu, Xi-Cun Wang,
Yong-Min Liang,* Yuke Li,* and Zheng-Jun Quan*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c02897
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: This paper describes the case of a cross study
between the C−N bond cleavage reaction field and the Catellani−
Lautens reaction system. A series of highly functionalized C4-
substituted indoles were synthesized using this strategy. By screening
the alkyl groups of amines, the energy barrier of C−N bond cleavage
reaction was reduced and the corresponding allenization products
were avoided. Finally, the density functional theory calculation
shows that the inert C−N bond activation reaction is not a
concerted process; on the contrary, the coupling reaction first
generates indole quaternary ammonium salt, and then C−N bond
cleavage occurs via an SN2 process.
ndole is one of the most popular heterocyclic frameworks in
Scheme 1. Synthesis of C4-Substituted Indoles via a
Catellani−Lautens and C−N Bond Activation Strategy
I
natural products and pharmaceutical chemistry.1 Among
them, C4-substituted indoles are widely studied by pharma-
ceutical chemists, because of their unique biological activities.2
For instance, Umifenovir is a C4-dimethylaminomethane-
substituted indole with hydrophobicity. Umifenovir is widely
used to treat and prevent influenza and other respiratory
infections, and it is currently being studied as a potential
prophylactic agent and treatment for COVID-19.3 Rucaparib is
“the world’s first PARP inhibitor for the third line treatment of
ovarian cancer”.4 In addition, C4-substituted indoles are also
common in the field of natural products, such as chanoclavine.5
Traditional synthesis methods have some limitations in the
synthesis of C4-substituted indoles. Fischer indole synthesis is
one of the most practical methods for indole synthesis.6 The
corresponding indole products can be directly synthesized from
phenylhydrazine through one-step conversion. To synthesize
C4-substituted indoles, m-substituted phenylhydrazine should
be used as the substrate. The method has no regioselectivity, so
the product is a mixture of isomers. In particular, the steric
hindrance of C4-substituted indoles is greater than that of C6-
substituted indoles, which makes the formation of C4 products
more difficult. Among the transition-metal catalysts, Larock
indole synthesis is the most widely used method in the synthesis
of indole, and it has the advantages of mild reaction conditions,
high yield, and good selectivity.7 This method can effectively
synthesize C4-substituted indoles, but it needs 3-substituent-o-
haloaniline, which is relatively expensive and difficult to
synthesize, as a substrate (Scheme 1). The use of nitrenes as
amination reagents provides a new method for the synthesis of
indole. However, other types of C−H amination for indole
synthesis have not been developed.8b−g
Received: August 31, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02897
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
To synthesize indole directly from iodobenzene, C−H
amination must be realized first. One of the most remarkable
methods in the o−C-H amination of aryl halides8 is realized by
the Pd/norbornene (NBE) cocatalysis strategy.9 In recent years,
this reaction has been proven to be a widely applicable and
convenient method for the synthesis of highly functionalized
aromatics.10 However, this reaction is only suitable for C−H
amination of secondary amines, so the corresponding indole
products cannot be obtained by the reaction in series with the
Larock indole synthesis method. At present, transition-metal-
catalyzed C−N bond activation has developed rapidly and has
become a powerful tool for the synthesis of amines, amides, and
other nitrogen-containing molecules.11 Therefore, we assume
that if the C−N bond activation reaction is connected with the
termination step of the Catellani−Lautens reaction, the C4-
substituted indole derivatives can be synthesized effectively in
one step.
Scheme 3. Testing the Participation of
Norbornenylpalladium Species in the C−N Bond Cleavage
generate the corresponding palladium intermediate, and then,
transfer and insertion with alkynes. Finally, the C−N bond
activation reaction occurs between alkynes and dimethylamine
to form the C4-methyl-indole target product.
Morpholino benzoate was used as an electrophilic amination
reagent. When 3-hexyne was added, the corresponding C−N
bond cleavage product was not obtained, but the allene product
was obtained (see Scheme 4). Through the screening of
a
Scheme 4. Variations on Electrophilic Amination Reagent
After a preliminary exploration, the main difficulties of this
series of reaction are as follows. First, whether arylnorborne-
nylpalladium can cleave C−N bond directly after C−H
amination must be determined. If the C−N bond cannot be
cleaved directly, then the β-carbon elimination process of
norbornene should occur smoothly to obtain the corresponding
aromatic Pd(II) intermediate. Second, the energy barrier of the
C−N bond cleavage reaction should not be too high after the
migration and insertion reaction occurs between the alkyne and
the Pd (II) intermediate; otherwise, the alkenyl palladium
intermediate will undergo β-hydrogen elimination or the
concerted metalation deprotonation (CMD) process to form
allene compounds.10h Therefore, it is a challenge to realize the
series reaction of C−N cleavage and the Catellani−Lautens
reaction (see Scheme 2).
a
Reaction conditions: substrate 1a (0.2 mmol), 2 (0.4 mmol, 2.0
equiv), 3a (0.4 mmol, 2.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (25
mol %), norbornene (0.4 mmol, 2.0 equiv), Cs2CO3 (0.8 mmol, 4.0
equiv), toluene (3.0 mL), 140 °C, 24 h. Isolated yields.bThe yield of
the compound was estimated using gas chromatography−mass
spectroscopy (GC-MS).
Scheme 2. Challenges for Overcoming the Compatibility
between C−N Bond Activation and Catellani−Lautens
Reaction
electrophilic amination reagents, surprisingly, the strategy
worked, and we achieved the desired product with an isolated
yield of 49%. Note that the C−H amination process of the
diethylamine electrophilic amination reagent (2ab) was not
smooth, and a large amount of norbornene product 5 was
obtained. After the temperature was reduced to 120 °C, the
phosphorus ligands were screened. Tri(2-furyl)phosphine
(TFP) was found to be the best ligand and could effectively
inhibit the formation of 4-membered products 5 of aryl
norbornene and dehalogenation product 6, and the target
product was obtained in a 61% yield (see Table S1, entries 1−6,
in the Supporting Information). Subsequently, we screened the
reaction temperature and norbornene derivative and found no
better reaction conditions (Table S1, entries 7−12). Note that a
small amount of direct allenization product was formed when
the N2 amide-substituted norbornene was used as a co-catalyst.
This may be due to the relatively slow migration and insertion
reaction occurring between the norbornene and palladium aryl
intermediate. According to our previous studies, electron-
donating benzoyl hydroxamine (2a′) may promote the C−H
amination reaction, but the actual effect is not ideal (Table S1,
entry 13). Finally, we selected other solvents, but the reaction
results were not as good as those obtained with toluene.
Initially, the following compounds were used to begin the
experiment: o-iodotoluene (1a), which is used as a substrate; O-
benzoyl-N,N-dimethylhydramine or morpholino benzoate,
which is used as an electrophilic amination reagent; and
palladium acetate, which is used as a catalyst. After a series of
screening conditions, there was no expected C−N cleavage
product (see Scheme 3). Therefore, we added alkynes to the
reaction system and hoped that the denorbornene process will
occur through the β-carbon elimination after C−H amination to
After the optimum reaction conditions were obtained, the
substrate scope of iodobenzene was investigated (Scheme 5).
The C4-ester and methoxy-indoles were synthesized smoothly
from iodobenzenes, whose ortho-substitute was an electron-
B
https://dx.doi.org/10.1021/acs.orglett.0c02897
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
only one product was obtained, when ethylphenylacetone was
used as the substrate. Subsequently, an N-alkyl-N-methyl
electrophilic amination reagent was used to study the reaction,
and the results showed that the C−N bond at the methyl end
was selectively activated. Because a large amount of norbornene
compound 5 was detected by GC-MS, the key reason for the
large difference between the yields of 4ab and 4ac is the difficulty
of the reaction between the ANP intermediates and correspond-
ing electrophilic amination reagents. This problem has always
existed in the Catellani reaction, since the researchers have yet to
further solve it (see Scheme 6).
Scheme 5. Investigation of Substrate Scope for Iodobenzenes
and Alkynes
a
Scheme 6. Investigation of Substrate Scope for N-Alkyl-N-
Methyl Electrophilic Amination Reagent
After the completion of the substrate scope study, the
mechanism of C−N bond activation was studied by density
functional theory (DFT) calculations. Three possible mecha-
nisms of the C−N bond cleavage reaction are proposed (see
Figure S2 in the Supporting Information). The first mechanism
is derived from the CMD process of C−H bond activation, and
we infer that the cleavage of the C−N bond was directly realized
through the concerted metallization process promoted by
benzoic acid. The second mechanism we speculate is that the
intramolecular coupling reaction between N and alkyne is the
first step to form indole quaternary ammonium salt, and then the
target product was obtained by the SN2 process occurring
between benzoic acid and methyl. At this point, we realize that
palladium in the intermediate of indole quaternary ammonium
salt is tricoordinated. Therefore, we left a ligand to investigate
the reaction in the presumed third mechanism. After confirming
the mechanism of C−N bond cleavage, we hope to explain the
reason why norbornene cannot cleave the C−N bond.
a
Reaction conditions: substrate 1 (0.2 mmol), 2a (0.4 mmol, 2.0
equiv), 3 (0.4 mmol, 2.0 equiv), Pd(OAc)2 (10 mol %), TFP (25
mol %), norbornene (0.4 mmol, 2.0 equiv), Cs2CO3 (0.8 mmol, 4.0
equiv), toluene (3.0 mL), 120 °C, 24 h. Isolated yields. bThe amount
of substrate 1a was amplified to 2 mmol scale.
According to the above ideas and our previous mechanism
research work,10h we first constructed intermediate A via alkyne
coordination with palladium (Figure 1). Intermediate B was
obtained by the migration and insertion process of intermediate
A, and the energy barrier of this process was only 13.0 kcal/mol.
drawing or electron-donating group. Furthermore, we also tried
to synthesize a series of C4-alkyl indoles by using various types
of o-alkyl iodobenzenes in one step. Note that this reaction can
also be applied to the synthesis of indole derivatives with fused
rings, which is difficult to achieve by the previous synthesis
methods. This reaction is also suitable for the synthesis of highly
functionalized indoles. Various substituted indoles, such as C6-
or C5-ester groups, -nitro, -methoxy, -halogen (-F, -Cl), etc., can
be obtained in moderate yields. In addition, we also tried some
other asymmetric alkynes and electrophilic amination reagents,
and the yields were not ideal (see the Supporting Information).
The next step is to investigate the scope of alkynes. All types of
alkyl alkynes (4-octyne, 5-alkyne, and 6-dodecyne) can be
obtained in high yields. Electron-donating, electron-with-
drawing, and halogen (-Cl)-substituted diaryldindoles can be
successfully synthesized by using corresponding diphenylacety-
lene derivatives. In particular, dithienylacetylene can also be
successfully applied to the reaction. It is worth mentioning that
Figure 1. Mechanism of C−N bond cleavage (concerted metalliza-
tion).
C
https://dx.doi.org/10.1021/acs.orglett.0c02897
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Figure 2. Mechanism of C−N bond cleavage.
We then determined the concerted metallization transition state
TS2, which has a high energy barrier (75.4 kcal/mol).
Therefore, the reaction mechanism was excluded.
elimination of the β-carbon of norbornene, the palladium
intermediate reacted with alkynes by migration and insertion,
which was followed by the C−N bond activation of the alkyl
amine. By screening the alkyl groups of amines, the energy
barrier of the C−N bond cleavage reaction was reduced and the
corresponding allenization products were avoided. Finally, three
possible C−N bond cleavage mechanisms were proposed. The
density functional theory (DFT) calculation shows that the inert
C−N bond activation reaction is not a concerted process; on the
contrary, the coupling reaction first generates indole quaternary
ammonium salt, and then C−N bond cleavage occurs.
The cleavage of the C−N bond by a concerted metallization
process is excluded, and we consider that the electron-deficient
effect of indole quaternary ammonium salt may weaken the
strength of the C−N bond (Figure 2). Intermediate A reacted
with alkyne to obtain intermediate D. The intermediate E of
indole quaternary ammonium salt then was obtained by the
reduction elimination reaction of D. Intermediate E can be
directly attacked by the oxygen of benzoic acid, and the target
product G-B is obtained by an SN2 reaction. This is the rate-
determining step of the reaction, and the energy barrier is 27.9
kcal/mol (see Figure 3).12
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02897.
Experimental details, characterization data and NMR
spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Zheng-Jun Quan − College of Chemistry and Chemical
Engineering, Northwest Normal University, Lanzhou, Gansu
730070, China; orcid.org/0000-0002-4393-3594;
Email: quanzhengjun@hotmail.com
Bo-Sheng Zhang − College of Chemistry and Chemical
Engineering, Northwest Normal University, Lanzhou, Gansu
730070, China; orcid.org/0000-0003-1148-0065;
Email: zhangbsh@foxmail.com
Yuke Li − Department of Chemistry and Centre for Scientific
Modeling and Computation, Chinese University of Hong Kong,
Shatin, Hong Kong, China; Email: yukeli@link.cuhk.edu.hk
Yong-Min Liang − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China;
orcid.org/0000-0001-8280-8211; Email: liangym@
lzu.edu.cn
Figure 3. Mechanism of C−N bond cleavage (SN2 process).
The palladium in intermediate E is tricoordinated, so we
consider whether the energy barrier of the C−N activation
reaction can be reduced by removing a triphosphate. However,
the high energy intermediate F-A can be obtained by the
removal of TFP. Subsequently, the C−N bond cleavage product
G-A was obtained by metallization. The energy barrier of this
process is very high, which is 46.9 kcal/mol. In conclusion, the
mechanism of C−N bond cleavage should be a SN2 process.
Finally, we use the above SN2 method to calculate the C−N
bond cleavage reaction of norbornene, and the results showed
that the process could not happen (see Figure S3 in the
Supporting Information).
In conclusion, we have developed the first case of a cross study
between the C−N bond cleavage reaction field and Catellani−
Lautens reaction system. A series of highly functionalized C4-
substituted indoles were synthesized by this strategy. The
experimental results showed that norbornene could not cleave
the C−N bond directly with an alkyl amine. Therefore, after the
Authors
Fan Wang − College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, Gansu 730070, China
Ying-Hui Yang − College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, Gansu 730070, China
Xue-Ya Gou − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Yi-Feng Qiu − College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, Gansu 730070, China
D
https://dx.doi.org/10.1021/acs.orglett.0c02897
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
(9) (a) Catellani, M.; Frignani, F.; Rangoni, A. Angew. Chem., Int. Ed.
Engl. 1997, 36, 119−122. (b) Catellani, M.; Motti, E.; Baratta, S. Org.
Lett. 2001, 3, 3611−3614. (c) Cheng, H.-G.; Chen, S.; Chen, R.; Zhou,
Q. Angew. Chem., Int. Ed. 2019, 58, 5832−5844. (d) Della Ca’, N.;
Fontana, M.; Motti, E.; Catellani, M. Acc. Chem. Res. 2016, 49, 1389−
1400. (e) Lautens, M.; Piguel, S. Angew. Chem., Int. Ed. 2000, 39, 1045−
1046. (f) Wang, J.; Dong, G. Chem. Rev. 2019, 119, 7478. (g) Ye, J.;
Lautens, M. Nat. Chem. 2015, 7, 863−870. (h) Ye, J.; Lautens, M. Nat.
Chem. 2015, 7, 863. (i) Zhao, K.; Ding, L.; Gu, Z. Synlett 2019, 30,
129−140.
(10) (a) Chen, Z.-Y.; Ye, C.-Q.; Zhu, H.; Zeng, X.-P.; Yuan, J.-J. Chem.
- Eur. J. 2014, 20, 4237−4241. (b) Dong, Z.; Lu, G.; Wang, J.; Liu, P.;
Dong, G. J. Am. Chem. Soc. 2018, 140, 8551−8562. (c) Pan, S.; Ma, X.;
Zhong, D.; Chen, W.; Liu, M.; Wu, H. Adv. Synth. Catal. 2015, 357,
3052−3056. (d) Shi, H.; Babinski, D. J.; Ritter, T. J. Am. Chem. Soc.
2015, 137, 3775−3778. (e) Sun, F.; Gu, Z. Org. Lett. 2015, 17, 2222−
2225. (f) Ye, C.; Zhu, H.; Chen, Z. J. Org. Chem. 2014, 79, 8900−8905.
(g) Zhang, B.-S.; Li, Y.; An, Y.; Zhang, Z.; Liu, C.; Wang, X.-G.; Liang,
Y.-M. ACS Catal. 2018, 8, 11827−11833. (h) Zhang, B.-S.; Li, Y.; Gou,
X.-Y.; Zhang, Z.; An, Y.; Wang, X.-G.; Liang, Y.-M. Chem. Commun.
2020, 56, 9202−9205. (i) Zhang, B.-S.; Li, Y.; Zhang, Z.; An, Y.; Wen,
Y.-H.; Gou, X.-Y.; Quan, S.-Q.; Wang, X.-G.; Liang, Y.-M. J. Am. Chem.
Soc. 2019, 141, 9731−9738. (j) Zhou, P.-X.; Ye, Y.-Y.; Ma, J.-W.;
Zheng, L.; Tang, Q.; Qiu, Y.-F.; Song, B.; Qiu, Z.-H.; Xu, P.-F.; Liang,
Y.-M. J. Org. Chem. 2014, 79, 6627−6633.
Xi-Cun Wang − College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, Gansu 730070, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02897
Author Contributions
∇These authors contributed equally.
Funding
Financial support was received from the National Natural
Science Foundation of China (No. NSF22061038,
NSF22067018), Long Yuan Youth Innovative Project of
Gansu Province (2019−39,) and the Northwest Normal
University Young Scholars Research Capacity Improvement
Program (No. NWNU-LKQN-2020-04).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Computations reported in this paper were performed on the
computer clusters at the Centre for Scientific Modeling and
Computation, CUHK. We would like to thank ACS ChemWorx
Authoring Services for providing linguistic assistance during the
preparation of this manuscript.
(11) (a) Hao, W.; Wei, J.; Geng, W.; Zhang, W.-X.; Xi, Z. Angew.
Chem., Int. Ed. 2014, 53, 14533−14537. (b) Hie, L.; Fine Nathel, N. F.;
Shah, T. K.; Baker, E. L.; Hong, X.; Yang, Y.-F.; Liu, P.; Houk, K. N.;
Garg, N. K. Nature 2015, 524, 79−83. (c) Shi, R.; Lu, L.; Zhang, H.;
Chen, B.; Sha, Y.; Liu, C.; Lei, A. Angew. Chem., Int. Ed. 2013, 52,
10582−10585. (d) Ueno, S.; Chatani, N.; Kakiuchi, F. J. Am. Chem. Soc.
2007, 129, 6098−6099. (e) Wang, Q.; Su, Y.; Li, L.; Huang, H. Chem.
Soc. Rev. 2016, 45, 1257−1272. (f) Yu, H.; Zhang, G.; Huang, H. Angew.
Chem., Int. Ed. 2015, 54, 10912−10916.
REFERENCES
■
(1) (a) Taber, D. F.; Tirunahari, P. K. Tetrahedron 2011, 67, 7195−
7210. (b) Taylor, R. D.; MacCoss, M.; Lawson, A. D. G. J. Med. Chem.
2014, 57, 5845−5859. (c) Van Order, R. B.; Lindwall, H. G. Chem. Rev.
1942, 30, 69−96.
(2) (a) Galliford, C. V.; Scheidt, K. A. Angew. Chem., Int. Ed. 2007, 46,
8748−8758. (b) Kochanowska-Karamyan, A. J.; Hamann, M. T. Chem.
Rev. 2010, 110, 4489−4497. (c) Sharma, V.; Kumar, P.; Pathak, D. J.
Heterocycl. Chem. 2010, 47, 491−502. (d) Trost, B. M.; Brennan, M. K.
Synthesis 2009, 2009, 3003−3025. (e) Zhang, M.-Z.; Chen, Q.; Yang,
G.-F. Eur. J. Med. Chem. 2015, 89, 421−441.
(12) The intermediate E and F-B are the same compound. In DFT
calculation, F-B is the relevant minimum of TS-F-B in the intrinsic
reaction coordinate (IRC) calculations.
̌
́
́
(3) (a) Haviernik, J.; Stefanik, M.; Fojtíkova, M.; Kali, S.; Tordo, N.;
́
Rudolf, I.; Hubalek, Z.; Eyer, L.; Ruzek, D. Viruses 2018, 10, 184.
(b) Herod, M. R.; Adeyemi, O. O.; Ward, J.; Bentley, K.; Harris, M.;
Stonehouse, N. J.; Polyak, S. J. J. Gen. Virol. 2019, 100, 1293−1302.
(c) Lu, H. BioSci. Trends 2020, 14, 69−71.
(4) (a) Jenner, Z. B.; Sood, A. K.; Coleman, R. L. Future Oncol. 2016,
12, 1439−56. (b) Nile, D. L.; Rae, C.; Hyndman, I. J.; Gaze, M. N.;
Mairs, R. J. BMC Cancer 2016, 16, 621.
(5) Kardos, N.; Genet, J.-P. Tetrahedron: Asymmetry 1994, 5, 1525−
1533.
(6) (a) Fischer, E.; Jourdan, F. Ber. Dtsch. Chem. Ges. 1883, 16, 2241−
2245. (b) Inman, M.; Moody, C. J. Chem. Sci. 2013, 4, 29−41.
(c) Robinson, B. Chem. Rev. 1969, 69, 227−250. (d) Wagaw, S.; Yang,
B. H.; Buchwald, S. L. J. Am. Chem. Soc. 1998, 120, 6621−6622.
(e) Wagaw, S.; Yang, B. H.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121,
10251−10263.
(7) (a) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. Rev. 2004, 104,
3079−3160. (b) Larock, R. C.; Yum, E. K. J. Am. Chem. Soc. 1991, 113,
6689−6690. (c) Larock, R. C.; Yum, E. K.; Refvik, M. D. J. Org. Chem.
1998, 63, 7652−7662.
(8) (a) Dong, Z.; Dong, G. J. Am. Chem. Soc. 2013, 135, 18350−
18353. (b) Zheng, H.; Zhu, Y.; Shi, Y. Angew. Chem., Int. Ed. 2014, 53,
11280−11284. (c) Zhou, B.; Wu, Z.; Ma, D.; Ji, X.; Zhang, Y. Org. Lett.
2018, 20, 6440−6443. (d) Jafarpour, F.; Ghasemi, M.; Mohaghegh, F.;
Asgari, S.; Habibi, A. Org. Lett. 2019, 21, 10143−10148. (e) Cheng, C.;
Zuo, X.; Tu, D.; Wan, B.; Zhang, Y. Org. Lett. 2020, 22, 4985−4989.
(f) Fan, L.; Hao, J.; Yu, J.; Ma, X.; Liu, J.; Luan, X. J. Am. Chem. Soc.
2020, 142, 6698−6707. (g) Zhang, L.; Chen, J.; Zhong, T.; Zheng, X.;
Zhou, J.; Jiang, X.; Yu, C. J. Org. Chem. 2020, 85, 10823−10834. (h) Qi,
X.; Wang, J.; Dong, Z.; Dong, G.; Liu, P. Chem. 2020, 6, 2810.
E
https://dx.doi.org/10.1021/acs.orglett.0c02897
Org. Lett. XXXX, XXX, XXX−XXX