Palladium-Catalyzed Tandem Reaction of Three Aryl Iodides Involving Triple C-H Activation

Article abstract of DOI:10.1021/acs.orglett.8b00982

A novel palladium-catalyzed tandem reaction of N-(2-iodoaryl)acrylamides with two aryl iodides for the synthesis of spirooxindole has been achieved. The reaction underwent the process of triple C-H activation and four C-C bond formations based on the double trapping of transient spirocyclic palladacycles which are obtained through remote C-H activation.

Full text of DOI:10.1021/acs.orglett.8b00982

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed Tandem Reaction of Three Aryl Iodides
Involving Triple C−H Activation
Xiai Luo,†,‡ Yankun Xu,† Genhua Xiao,† Wenjuan Liu,† Cheng Qian,† Guobo Deng,† Jianxin Song,†
Yun Liang,*,† and Chunming Yang*,†
†National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, Key Laboratory
of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education, Key Laboratory of the Assembly and
Application of Organic Functional Molecules, Hunan Normal University, Changsha, Hunan 410081, China
‡Hunan University of Medicine, Huaihua 418000, China
S
* Supporting Information
ABSTRACT: A novel palladium-catalyzed tandem reaction of
N-(2-iodoaryl)acrylamides with two aryl iodides for the
synthesis of spirooxindole has been achieved. The reaction
underwent the process of triple C−H activation and four C−C
bond formations based on the double trapping of transient
spirocyclic palladacycles which are obtained through remote
C−H activation.
spiral ring skeleton is ubiquitous in organic molecules and
Afound in a wide range of natural products, pharmaceut-
icals, agrochemicals, and materials.1 For this reason, substantial
efforts have been directed toward the development of step-
economical and atom-economical strategies for the construc-
tion of spirocyclic compounds. C−H functionalization
reactions2 involving tandem processes3 have emerged as
powerful tools to reach this near-ideal goal. In the past,
tremendous progress has been made in the area of directing
group-assisted C−H bond activation.2 However, the installation
and removal of directing groups is a major drawback for
synthetic applications. Alternatively, utilizing a transient
directing group instead of the covalent directing group can
avoid this limitation.4 Pioneering and systematic research has
been carried out by Catellani and co-workers, who discovered
and developed a Pd/norbornene catalytic system that could
combine selective C−H bond activation with sequential
reactions for ortho-C−H functionalization.5 Meanwhile, many
chemists developed this reaction and applied it to activate the
ortho-C−H bond in organic synthesis.6 On the basis of this
research, a strategy of remote C−H functionalization was
developed that utilized the tethered alkene moiety of aryl
halides serving as a surrogate of norbornene to generate σ-
alkylpalladium species. The σ-alkylpalladium species can
achieve remote C−H activation via spirocyclic palladacycles
or 1−4 palladium shift.7−13 Recently, outstanding work using
mono-C−H functionalization for the synthesis of spirocyclic
and polycyclic compounds has been reported.8,15−19 However,
selective multi-C−H functionalization reactions are quite
rare.9−13 In a pioneering study, Larock synthesized a tetracyclic
compounds via a sequence of intramolecular C−H activation
which is facilitated by a net 1,4-palladium shift.9 Utilizing a
similar strategy, Zhu synthesized tetracyclic [3,4]-fused
oxindoles;10 Wang and Ji synthesized pentacyclic compounds.11
Further intermolecular reactions were achieved by Lautens: (1)
two structurally distinguishable aryl iodoides reacted together
through a carbopalladation and a sequence of C−H activations
formed three new C−C bonds;12 (2) aryl iodoides reacted with
alkyl iodoides via a sequence of C−H activations and following
C−C bond cleavage formed four new C−C bonds.13 These
methods can efficiently construct the complex organic
molecular by tandem C−H activation. Therefore, developing
multi-C−H functionalization and multi-C−C formation
between structurally distinguishable aryl iodides is important
and remains challenging.
N-2-Halophenylacrylamides,14−18 as important synthons for
construction of spirooxindole, were widely used. In the
presence of Pd(0) catalysts, they readily transformed into
spirocyclic palladacycles through a Heck-type reaction and
following remote C−H activation which was trapped by
alkynes,15 arynes,16 CH2Br2,17 disilanes,18 and diazo19 com-
pounds to spirocyclic oxindoles. Inspired by these previous
studies, we envision that the iodobenzene can insert into the
spirocyclic palladacycles for synthesis of spirocyclic oxindoles
(Scheme 1). Herein, we wish to detail our results.
Scheme 1. Intermolecular Multi-C−H Functionalization
Received: April 1, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00982
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We began our investigation using the easily accessible N-(2-
iodophenyl-N-methyl-2-phenylacrylamide and iodobenzene as
model substrates. Under the reaction conditions we developed
previously in the related studies20 [Pd(OAc)2 as catayst, PPh3
as ligand, K2CO3 as base, DMSO as solvent, at 120 °C, under
nitrogen atmosphere], the unexpected phenyl-substituted
spirocyclic oxindole 3aa was isolated in 35% yield, while no
expected 1-methyl-10′H-spiro[indoline-3,9′-phenanthren]-2-
one (3aa′) was observed. We reasoned that the reaction
underwent remote C−H bond activation twice via formation of
twice spirocyclic palladacycles. Encouraged by this interesting
result, screening of various reaction conditions, including Pd
catalysts, P-ligands, bases, solvents, and additives, revealed the
optimal reaction conditions (see the Supporting Information
(SI) for details), and the expected arylated spirooxindole 3aa
was obtained in 65% yield (Scheme 2). It is noteworthy that the
reaction could also be performed in 61% yield at a scale of 1
mmol of N-(2-iodophenyl)-N-methyl-2-phenylacrylamide 1a.
respectively. Regrettably, the N-free and N-tosyl-substituted
acrylamides were unfavorable substrates for this cyclization
reaction. Subsequently, our study focused on the substitution
effect on the 2-iodophenyl of N-(2-iodophenyl)-N-benzyl-2-
phenylacrylamide. It was observed that substrates containing
electron-donating groups (including methyl and methoxyl) and
electron-withdrawing groups (including chloro, fluoro, and
trifluoromethyl) could be smoothly transformed into the
corresponding products in good yields. Unfortunately, the
substrate bearing a cyano group could not be tolerated well,
and the corresponding products 3ma were formed in 38% yield.
Furthermore, the compatibility of functionalities on the phenyl
groups linked to the double bonds was checked. The substrates
containing the electron-donating group such as methoxyl and
the electron-withdrawing group such as fluoro were well
tolerated, and the corresponding products were given in 79%
(3na) and 63% (3oa) yields, respectively. When a fluorine
atom was introduced to the ortho-position of benzene ring, the
reaction proceeded with high efficiency, and the monoarylated
product 3pa was afforded in 87% yield.
Scheme 2. Optimization of Reaction Conditions
We next explored the generality of substituted iodobenzenes
(Scheme 4). For para-substituted iodobenzenes, we found that
a
Scheme 4. Variation of the Iodobenzenes 2
With the optimized conditions in hand, a series of substituted
N-(2-iodophenyl)-2-phenylacrylamides 1 were first examined to
study the scope of tandem process, and the results are
summarized in Scheme 3. When electron-donating groups such
as ethyl (1b), benzyl (1c), 4-fluorobenzyl (1d), and 4-
methylbenzyl (1e) N-substituted acrylamides were subjected
to the reaction conditions, the desired products 3ba, 3ca, 3da,
and 3ea were obtained in 61%, 81%, 71%, and 65% yields,
a
Scheme 3. Variation of the Acrylamides 1
a
Reaction conditions: 1a or 1c (0.2 mmol), 2 (0.8 mmol), Pd(OAc)2
(10 mol %), PPh3 (20 mol %), Cs2CO3 (1.0 mmol), TBAI (0.1
mmol), DMSO (2 mL) at 120 °C under nitrogen atmosphere for 12 h.
Isolated yield are given.
both electron-donating substituents (Me, OMe, N(Me)2) and
electron-withdrawing substituents (F, Cl, CF3, Ph) were well
tolerated, and moderate yields of corresponding spirooxindoles
were obtained (52%−67%). The structure of spirooxindole
product 3ac was unambiguously confirmed by single-crystal X-
ray analysis (CCDC: 1825599). However, methyl 4-iodoben-
zoate was not a suitable arylating reagent, and only 36% yield of
3ci was given. Happily, the iodobenzene containing two methyl
groups in the meta-position afforded the desired product 3cj in
65% yield when the reaction time extended to 24 h. When the
reaction of 1-iodo-2-methylbenzene or 1-chloro-2-iodobenzene
a
Reaction conditions: 1 (0.2 mmol), 2a (0.8 mmol), Pd(OAc)2 (10
mol %), PPh3 (20 mol %), Cs2CO3 (1.0 mmol), TBAI (0.1 mmol),
DMSO (2 mL) at 120 °C under nitrogen atmosphere for 12 h.
Isolated yield are given.
B
DOI: 10.1021/acs.orglett.8b00982
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
was performed under standard reaction conditions, no desired
product was isolated. This result indicated that the steric effect
affected the reaction dramatically.
Scheme 6. Possible Reaction Mechanism
To investigate the reaction intermediates, several control
experiments were performed (Scheme 5). First, the spiropalla-
Scheme 5. Control Experiments
dacycle 4 was independently synthesized, which could
efficiently react with iodobenzene to give desired product 3aa
in 42% yield. This result indicated that the reaction was
initiated by the spiropalladcycle 4, which was generated by an
intramolecular Heck-type reaction followed by remote C−H
activation. Then the possible intermediates 5 and 6 were
synthesized as well. The reaction of 4-methyliodobenzene with
5 was investigated first, and a pair of isomers of 7 and 7′ was
obtained in a 1:2 ratio. This result showed that the final C−H
activation could take place from the phenyl or 4-methylphenyl.
Finally, it was found that the intermediate 5 could not react
with iodobenzene. These findings suggested that the reaction
underwent a phenylation followed by the other phenylation.
On the basis of the present experimental results, as well as
previous reported mechanisms,7−20 the hypothetical catalytic
cycle is shown in Scheme 6. The reaction starts with oxidative
addition of Pd(0) to the C−I bond of N-(2-iodophenyl)-N-
methyl-2-phenylacrylamide 1a followed by a tandem intra-
molecular Heck reaction and C−H activation to form the key
intermediate of five-membered palladacycle E.15−18 In general,
two reaction pathways for the arylation of the palladacycle are
considered. Path A: E undergoes oxidative addition with
iodobenzene to give Pd(IV) complex H, and subsequent
reductive elimination generates alkylpalladium species G. Path
B: A transmetalation-type exchange may occur between the
palladacycle E and arylpalladium species N, and following aryl−
aryl reductive elimination it gives the intermediate G, which
takes place without a formal change of the oxidation state of
palladium. The arylated intermediate G forms a new spirocyclic
palladacycle I via a second C−H activation.13 Likewise, two
reaction pathways [A′, Pd(IV), versus B′, Pd(II)−Pd(II)] for
the cyclization cannot be ruled out, and the seven-membered
palladacycle K can be achieved through the third C−H
activation.12 Finally, intermediate K undergoes reductive
elimination, which regenerates the Pd(0) catalyst and affords
the product of 3aa.
In conclusion, a novel, efficient method for the synthesis of
spirooxindoles has been developed. In this reaction, three C−H
bond activations and four C−C bond formations are involved.
In the presence of palladium catalyst, N-(2-halophenyl)-2-
phenylacrylamides transform into spirooxindole via a sequence
of formation of spirocyclic palladacycles that is generated by an
intramolecular remote C−H activation. We anticipate that this
approach will show its application in a series of directing-group-
free remote C−H functionalization reactions.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00982.
Experimental procedures, full characterization of prod-
ucts, and NMR spectra (PDF)
Accession Codes
CCDC 1825599 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yliang@hunnu.edu.cn.
C
DOI: 10.1021/acs.orglett.8b00982
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
P.-X.; Wang, X.-C.; Wang, P.; Zhu, R.-Y.; Yu, J.-Q. J. Am. Chem. Soc.
2015, 137, 11574. (d) Dong, Z.; Wang, J.; Dong, G. J. Am. Chem. Soc.
2015, 137, 5887. (e) Wang, X.-C.; Gong, W.; Fang, L.-Z.; Zhu, R.-Y.;
*E-mail: chunmingyang@126.com.
ORCID
Guobo Deng: 0000-0002-5470-5706
Jianxin Song: 0000-0002-1756-5898
Yun Liang: 0000-0002-2550-9220
́
Li, S.; Engle, K. M.; Yu, J.-Q. Nature 2015, 519, 334. (f) Cardenas, D.
J.; Martín-Matute, B.; Echavarren, A. M. J. Am. Chem. Soc. 2006, 128,
5033. (g) Wu, T.; Mu, X.; Liu, G. Angew. Chem., Int. Ed. 2011, 50,
12578. (h) Mu, X.; Wu, T.; Wang, H.-L.; Guo, Y.-L.; Liu, G. J. Am.
Chem. Soc. 2012, 134, 878.
Notes
(7) (a) Mehta, V. P.; García-Lop
1149. (b) Franzoni, I.; Yoon, H.; Garcíıa-Lop
Bahamonde, A. I.; Lautens, M. Chem. Sci. 2018, 9, 1496.
́
ez, J.-A. ChemCatChem 2017, 9,
ez, J.-A.; Poblador-
The authors declare no competing financial interest.
́
ACKNOWLEDGMENTS
■
(8) (a) Wang, M.; Zhang, X.; Zhuang, Y.-X.; Xu, Y.-H.; Loh, T.-P. J.
Am. Chem. Soc. 2015, 137, 1341. (b) Ruck, R. T.; Huffman, M. A.;
Kim, M. M.; Shevlin, M.; Kandur, W. V.; Davies, I. W. Angew. Chem.,
Int. Ed. 2008, 47, 4711. (c) Zheng, H.; Zhu, Y.; Shi, Y. Angew. Chem.,
Int. Ed. 2014, 53, 11280. (d) Piou, T.; Neuville, L.; Zhu, J. Org. Lett.
2012, 14, 3760. (e) Hu, Y.; Yu, C.; Ren, D.; Hu, Q.; Zhang, C.; Cheng,
D. Angew. Chem., Int. Ed. 2009, 48, 5448. (f) Kim, K. H.; Lee, H. S.;
Kim, S. H.; Kim, S. H.; Kim, J. N. Chem. - Eur. J. 2010, 16, 2375.
(g) Grigg, R.; Fretwell, P.; Meerholtz, C.; Sridharan, V. Tetrahedron
1994, 50, 359. (h) Lu, Z.; Hu, C.; Guo, J.; Li, J.; Cui, Y.; Jia, Y. Org.
Lett. 2010, 12, 480. (i) Yao, T.; He, D. Org. Lett. 2017, 19, 842.
(9) Huang, Q.; Fazio, A.; Dai, G.; Campo, M. A.; Larock, R. C. J. Am.
Chem. Soc. 2004, 126, 7460.
(10) (a) Piou, T.; Bunescu, A.; Wang, Q.; Neuville, L.; Zhu, J. Angew.
Chem., Int. Ed. 2013, 52, 12385. (b) Bunescu, A.; Piou, T.; Wang, Q.;
Zhu, J. Org. Lett. 2015, 17, 334.
(11) Gu, Z.-Y.; Liu, C.-G.; Wang, S.-Y.; Ji, S.-J. Org. Lett. 2016, 18,
2379.
(12) Sickert, M.; Weinstabl, H.; Peters, B.; Hou, X.; Lautens, M.
Angew. Chem., Int. Ed. 2014, 53, 5147.
(13) Ye, J.; Shi, Z.; Sperger, T.; Yasukawa, Y.; Kingston, C.;
Schoenebeck, F.; Lautens, M. Nat. Chem. 2017, 9, 361.
(14) For representative papers, see: (a) Vachhani, D. D.; Butani, H.
H.; Sharma, N.; Bhoya, U. C.; Shah, A. K.; Van der Eycken, E. V.
Chem. Commun. 2015, 51, 14862. (b) Kong, W.; Wang, Q.; Zhu, J.
Angew. Chem., Int. Ed. 2016, 55, 9714. (c) Liu, X.; Ma, X.; Huang, Y.;
Gu, Z. Org. Lett. 2013, 15, 4814. (d) Tong, S.; Limouni, A.; Wang, Q.;
Wang, M.-X.; Zhu, J. Angew. Chem., Int. Ed. 2017, 56, 14192. (e) Zhou,
M.-B.; Huang, X.-C.; Liu, Y.-Y.; Song, R.-J.; Li, J.-H. Chem. - Eur. J.
2014, 20, 1843. (f) Wang, D.-C.; Wang, H.-X.; Hao, E.-J.; Jiang, X.-H.;
Xie, M.-S.; Qu, G.-R.; Guo, H.-M. Adv. Synth. Catal. 2016, 358, 494.
(g) Wei, W.-T.; Zhou, M.-B.; Fan, J.-H.; Liu, W.; Song, R.-J.; Liu, Y.;
Hu, M.; Xie, P.; Li, J.-H. Angew. Chem., Int. Ed. 2013, 52, 3638.
(h) Kong, W.; Wang, Q.; Zhu, J. Angew. Chem., Int. Ed. 2016, 55, 9714.
(i) Kong, W.; Wang, Q.; Zhu, J. J. Am. Chem. Soc. 2015, 137, 16028.
This work was supported by the National Natural Science
Foundation of China (21572051, 21602057), Opening Fund of
Key Laboratory of Chemical Biology and Traditional Chinese
Medicine Research (Ministry of Education of China), and
Hunan Normal University (KLCBTCMR201707,
KLCBTCMR201708).
REFERENCES
■
(1) For representative reviews, see: (a) Galliford, C. V.; Scheidt, K. A.
Angew. Chem., Int. Ed. 2007, 46, 8748. (b) Singh, G. S.; Desta, Z. Y.
Chem. Rev. 2012, 112, 6104. (c) Klajn, R. Chem. Soc. Rev. 2014, 43,
148. (d) Rios, R. Chem. Soc. Rev. 2012, 41, 1060. (e) Yu, B.; Yu, D.-Q.;
Liu, H.-M. Eur. J. Med. Chem. 2015, 97, 673.
(2) For representative reviews, see: (a) Chen, X.; Engle, K. M.;
Wang, D.-H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009, 48, 5094.
(b) Daugulis, O.; Do, H.-Q.; Shabashov, D. Acc. Chem. Res. 2009, 42,
1074. (c) Colby, D. A.; Bergman, R. G.; Ellman, J. A. Chem. Rev. 2010,
110, 624. (d) Engle, K. M.; Mei, T.-S.; Wasa, M.; Yu, J.-Q. Acc. Chem.
Res. 2012, 45, 788. (e) Neufeldt, S. R.; Sanford, M. S. Acc. Chem. Res.
2012, 45, 936. (f) Huang, Z.; Lim, H. N.; Mo, F.; Young, M. C.; Dong,
G. Chem. Soc. Rev. 2015, 44, 7764. (g) Gensch, T.; Hopkinson, M. N.;
Glorius, F.; Wencel-Delord, J. Chem. Soc. Rev. 2016, 45, 2900.
(h) Huang, H.; Ji, X.; Wu, W.; Jiang, H. Chem. Soc. Rev. 2015, 44,
1155. (i) He, J.; Wasa, M.; Chan, K. L.; Shao, Q.; Yu, J.-Q. Chem. Rev.
2017, 117, 8754.
(3) For representative reviews, see: (a) Tietze, L. F. Chem. Rev. 1996,
96, 115. (b) Volla, C. M. R.; Atodiresei, I.; Rueping, M. Chem. Rev.
2014, 114, 2390. (c) Platon, M.; Amardeil, R.; Djakovitch, L.; Hierso,
J.-C. Chem. Soc. Rev. 2012, 41, 3929. (d) Sears, J. E.; Boger, D. L. Acc.
Chem. Res. 2016, 49, 241. (e) Chauhan, P.; Mahajan, S.; Enders, D.
Acc. Chem. Res. 2017, 50, 2809.
(4) (a) Jun, C.-H.; Lee, H.; Hong, J.-B. J. Org. Chem. 1997, 62, 1200.
(b) Mo, F.; Dong, G. Science 2014, 345, 68. (c) Bedford, R. B.; Coles,
S. J.; Hursthouse, M. B.; Limmert, M. E. Angew. Chem., Int. Ed. 2003,
42, 112. (d) Lightburn, T. E.; Dombrowski, M. T.; Tan, K. L. J. Am.
(15) Yoon, H.; Rolz, M.; Landau, F.; Lautens, M. Angew. Chem., Int.
̈
Ed. 2017, 56, 10920.
Chem. Soc. 2008, 130, 9210. (e) Grunanger, C. U.; Breit, B. Angew.
̈
́ ́ ́
(16) (a) Perez-Gomez, M.; García-Lopez, J.-A. Angew. Chem., Int. Ed.
2016, 55, 14389. (b) Yoon, H.; Lossouarn, A.; Landau, F.; Lautens, M.
Org. Lett. 2016, 18, 6324.
(17) Shao, C.; Wu, Z.; Ji, X.; Zhou, B.; Zhang, Y. Chem. Commun.
2017, 53, 10429.
(18) (a) Lu, A.; Ji, X.; Zhou, B.; Wu, Z.; Zhang, Y. Angew. Chem., Int.
Ed. 2018, 57, 3233. (b) Xiao, G.; Chen, L.; Deng, G.; Liu, J.; Liang, Y.
Tetrahedron Lett. 2018, 59, 1836.
Chem., Int. Ed. 2008, 47, 7346. (f) Luo, J.; Preciado, S.; Larrosa, I. J.
Am. Chem. Soc. 2014, 136, 4109. (g) Mo, F.; Dong, G. Science 2014,
345, 68. (h) Xu, Y.; Young, M. C.; Wang, C.; Magness, D. M.; Dong,
G. Angew. Chem., Int. Ed. 2016, 55, 9084. (i) Zhang, F.-L.; Hong, K.;
Li, T.-J.; Park, H.; Yu, J.-Q. Science 2016, 351, 252. (j) Saidi, O.;
Marafie, J.; Ledger, A. E. W.; Liu, P. M.; Mahon, M. F.; Kociok-Kohn,
̈
G.; Whittlesey, M. K.; Frost, C. G. J. Am. Chem. Soc. 2011, 133, 19298.
(k) Paterson, A. J.; St John-Campbell, S.; Mahon, M. F.; Press, N. J.;
Frost, C. G. Chem. Commun. 2015, 51, 12807. (l) Hofmann, N.;
Ackermann, L. J. Am. Chem. Soc. 2013, 135, 5877. (m) Warratz, J.; Li,
S.; Zell, D.; De Sarkar, S.; Ishikawa, E. E.; Ackermann, L. J. Am. Chem.
Soc. 2015, 137, 13894.
(19) Per
́ ́ ́
ez-Gomez, M.; Hernandez-Ponte, S.; Bautista, D.; García-
Lopez, J.-A. Chem. Commun. 2017, 53, 2842.
́
(20) Wu, L.; Deng, G.; Liang, Y. Org. Biomol. Chem. 2017, 15, 6808.
(5) (a) Catellani, M.; Frignani, F.; Rangoni, A. Angew. Chem., Int. Ed.
Engl. 1997, 36, 119. (b) Faccini, F.; Motti, E.; Catellani, M. J. Am.
Chem. Soc. 2004, 126, 78. (c) Della Ca’, N.; Fontana, M.; Motti, E.;
Catellani, M. Acc. Chem. Res. 2016, 49, 1389. (d) Zhang, H.; Chen, P.;
Liu, G. Angew. Chem., Int. Ed. 2014, 53, 10174. (e) Zhang, H.; Wang,
H.-Y.; Luo, Y.; Chen, C.; Cao, Y.; Chen, P.; Guo, Y.-L.; Lan, Y.; Liu, G.
ACS Catal. 2018, 8, 2173.
(6) (a) Ye, J.; Lautens, M. Nat. Chem. 2015, 7, 863. (b) Martins, A.;
Mariampillai, B.; Lautens, M. Top. Curr. Chem. 2009, 292, 1. (c) Shen,
D
DOI: 10.1021/acs.orglett.8b00982
Org. Lett. XXXX, XXX, XXX−XXX