Transition-Metal-Free Tandem Cyclization/ N-Arylation Reaction: A Method to Access Biaryl Sultam Derivatives via a Diradical Pathway

Article abstract of DOI:10.1021/acs.orglett.0c02393

A novel procedure for the transition-metal-free tandem cyclization/N-arylation reaction sequence of an aryne with a 1,2,3,4-benzothiatriazine-1,1-dioxide is reported. This reaction goes through the intramolecular homolytic cyclization to generate an N-H b

Full text of DOI:10.1021/acs.orglett.0c02393

pubs.acs.org/OrgLett
Letter
Transition-Metal-Free Tandem Cyclization/N‑Arylation Reaction: A
Method To Access Biaryl Sultam Derivatives via a Diradical Pathway
Vijaykumar H. Thorat, Jen-Chieh Hsieh,* and Chien-Hong Cheng*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c02393
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: A novel procedure for the transition-metal-free tandem cyclization/N-arylation reaction sequence of an aryne with a
1,2,3,4-benzothiatriazine-1,1-dioxide is reported. This reaction goes through the intramolecular homolytic cyclization to generate an
N−H biaryl sultam intermediate, which enables aryne insertion to access diversely functionalized biaryl sultam derivatives with high
yields. The mechanism study indicates that homolytic cyclization is executed by a diradical species, initiated from the thermal
decomposition of 1,2,3,4-benzothiatriazine-1,1-dioxide to release a nitrogen molecule.
ulfonamide is considered to be an important building
radical annulations were reported mainly for the construction
Sblock, widely distributed in medicinally bioactive mole- of biaryl sultams.10 A denitrogenative cyclization from 1,2,3,4-
cules, natural alkaloids, and material compounds.1−4 Among
benzothiatriazine-1,1-dioxide is also an accessible pathway to
offer the sultam derivatives. However, there are only two
related works reported to date (Scheme 1). In 2010, Murakami
indicated a nickel-catalyzed enantioselectively denitrogenative
annulation reaction for the synthesis of benzosultams.11 Years
later, a visible-light promoted denitrogenative cyclization to
furnish biaryl sultams had been revealed to be catalyzed by the
ruthenium photocatalyst.12 Despite the disclosure of many
synthetic methods, a distinct strategy or a concise process to
synthesize sultams is still desirable in response to the diverse
structural requirement. In this regard, the use of an aryne as a
reaction partner to participate in a denitrogenative cyclization
would be an efficient route to reach the polyaromatic sultams.
Aryne is a highly reactive species, which can be generated in
situ from ortho-(trimethylsilyl)phenyl triflate with a suitable
fluoride source under mild conditions.13 This aryne precursor
has been widely studied in the multiple-component coupling
reactions, the pericyclic reactions, and the nucleophilic
arylations.14−16 Our previous studies concerning the aryne
reactions16 inspired us to introduce the aryne as a unit block in
a polyaromatic sultam, and this is also highly related to our
experience in the construction of heterocycles.17 Herein, we
them, the cyclic sulfonamide is also known as sultam, which
plays a vital role in pharmaceutical chemistry. Its bioactivity
profile inspires synthetic chemists to design diversely potent
drug candidates. This structural motif is a well established
platform for several crucial medicinal and bioactive properties,
such as antitumor, antibacterial, diuretic, hypoglycemic,
antithyroid, inhibitor of protease, etc. (Figure 1).3,4
Figure 1. Selected bioactive sultams.
Because of the importance, sultam derivatives, in particular,
the benzosultam and the biaryl sultam, have attracted much
attention. Many synthetic approaches have been developed for
the formation of these two structures. The facile synthesis via
the C−H functionalization catalyzed by Rh,5 Co,6 and Pd7
complexes with simple substrates has dominated in this field
over the past decade. Transition-metal catalysis associated with
the intermolecular cyclizations through a bond cleavage by the
oxidative addition8 or the intramolecular cyclizations through a
C−N bond formation9 has also demonstrated the feasibility to
form diverse sultam derivatives. Meanwhile, the photoinduced
Received: July 18, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02393
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
fluoride source, the reaction yields were significantly reduced
even with different solvents such as DME, DCE, toluene, etc.
(entries 5−9; also see Supporting Information for details). The
effect of other fluoride sources such as TBAT (tetrabutyl
ammonium difluorosilicate) and TBAF (tetrabutyl ammonium
fluoride) was investigated as well, and they showed only a
small difference in the product yields (entries 10, 11). We then
tried to introduce Cs2CO3 as an additive in replacing the
fluoride source; however, we afforded 3a in only 65% yield
(entry 12).
Scheme 1. Synthesis of the Sultam Derivatives through the
Denitrogenative Cyclization Reactions
After the optimized reaction conditions were established, we
then studied reaction capacity by investigating the substrate
scope for the present transition-metal-free tandem reaction. As
indicated in Scheme 2, we first surveyed the scope of substrate
a b
,
Scheme 2. Scope of the Substrate 1
report the concise access to the biaryl sultams via a transition-
metal-free tandem cyclization/N-arylation reaction by using an
aryne with a 1,2,3,4-benzothiatriazine-1,1-dioxide.
Our study commenced with using 2-phenyl-2H-benzo[e]-
[1,2,3,4]thiatriazine-1,1-dioxide (1a) as the model substrate to
optimize the reaction conditions, and the results are shown in
Table 1 (see Supporting Information in detail; substrate 1a can
a
Table 1. Optimization of Reaction Conditions
b
entry
[F]
solvent
temp (°C) yield (%)
c
1
KF/18-crown-6
KF/18-crown-6
KF/18-crown-6
KF/18-crown-6
CsF
CsF
CsF
CsF
CsF
THF
THF
THF
CH3CN
THF
CH3CN
DME
DCE
toluene
THF
50
70
80
80
80
80
80
80
80
80
80
80
41
83
94
92
85
77
85
83
52
93
90
65
c
2
c
3
c
4
5
6
7
8
a
9
Reactions were carried out using 0.1 mmol (1.0 equiv) of 1, 0.15
mmol (1.5 equiv) of 2a with 0.3 mmol of KF and 0.2 mmol of 18-
10
11
12
TBAT
TBAF (1.0 M in THF)
Cs2CO3
b
c
THF
THF
crown-6 in 2.0 mL of THF at 80 °C for 8 h. Isolated yield. 1.0 g of
1b was used.
a
Reaction conditions: 1a (0.1 mmol, 1.0 equiv), 2a (0.15 mmol, 1.5
equiv), fluoride source [F] (0.3 mmol), solvent (2.0 mL), indicated
temperature, under N2, 8 h. Isolated yield. 0.2 mmol of 18-crown-6
was used.
1 and found that the reactions proceeded smoothly for a wide
range of substituents. Both electron-donating group (EDG)
and electron-withdrawing group (EWG) on the two aryl
moieties are well tolerated, and the reaction yields did not
demonstrate a big difference. In general, when the product
included an electron-withdrawing group on either of the two
aryl moieties, it was afforded in a slightly lower yield compared
to those with an electron-donating group on it. For the
substrates with a para-substituent on the N-aryl moiety (red
ring, 3b−3i), only the strong electron-withdrawing group CF3
(3d) and the carbonyl-containing groups (3e, 3f) resulted in
lower yields. Reactions for the substrates with a substituent on
the ortho-position of the N-aryl moiety (3j, 3k) proceeded
successfully with excellent yields. When a 3,5-disubstituted N-
aryl ring was introduced into the structures (3l−3n), the
b
c
be prepared from a 2-nitrobenzenesulfonyl chloride with an
aniline through the synthetic sequence of substitution/nitro-
reduction/diazotization). We first treated 1a (0.10 mmol) and
benzyne precursor 2a (0.15 mmol) with KF (0.30 mmol) as a
fluoride source and 18-crown-6 (0.20 mmol) as an additive in
THF (2.0 mL) at 50 °C for 8 h, which afforded the expected
product 3a in 41% isolated yield (entry 1). Further increase of
the reaction temperature significantly improved the yields of
desired product 3a (entries 2, 3). In addition, upon switching
to CH3CN as the solvent, product 3a could be maintained in a
very good yield (entry 4). When the CsF was introduced as a
B
https://dx.doi.org/10.1021/acs.orglett.0c02393
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
products remained in good yields. Substrates with a meta-
substituted N-aryl ring were also investigated, and the reactions
provided high yields of their corresponding products with exact
1:1 ratios for the product mixtures (3o/3o′, 3p/3p′).
Subsequently, the substituent effect on the backbone aryl
ring was studied. We introduced both a strong EDG and a
strong EWG into the backbone aryl moiety and found that the
desired products (3q, 3r) were generated in good yields.
Moreover, four product structures (3i, 3k, 3n, and 3ac) have
been verified by the single-crystal X-ray diffraction analysis.
Next, we explored the structural diversity of aryne precursor
2 toward this tandem reaction (Scheme 3). The diverse aryne
To gain insight into the mechanism, we carried out a series
of control experiments to study the reaction pathway (Scheme
4). First, when 1a reacted under the standard condition in the
Scheme 4. Control Experiments
a ba
,
Scheme 3. Scope of the Substrate 2
absence of aryne precursor 2, we could isolate the product 4a
in a very good yield. This compound (4a) is also able to react
with 2a to afford the desired product 3a in an excellent yield,
which well explains that 4a is the key intermediate in this
tandem reaction. In addition, to rule out the involvement of
household light sources, we performed the reaction in the dark
and observed the same reaction yield. Second, since there was
no aryne precursor participating in the reaction, we removed
the fluoride source and found that the reaction had not clearly
been affected. Third, we further introduced different amounts
of the radical trapping reagents BHT and TEMPO into the
reactions, and the yields performed obvious changes only when
a large amount of radical scavenger was loaded. Fourth, we
conducted deuteration experiments to understand the proton
source in this tandem reaction. When the reaction proceeded
under the standard conditions in THF-d8, it was found that the
product 3a was provided in good yield without scrambling
deuterium on the N-phenyl ring. This result clearly indicates
that the proton exchange between solvent and substrates or
product does not exist. We further introduced the substrate
D5-1a to execute the tandem reaction, and the expected
product D5-3a afforded illustrates that the deuterium on the N-
phenyl ring comes from the substrate D5-1a (see Supporting
a
Reactions were carried out using 0.1 mmol (1.0 equiv) of 1a, 0.15
mmol (1.5 equiv) of 2 with 0.3 mmol of KF and 0.2 mmol of 18-
b
c
crown-6 in 2.0 mL of THF at 80 °C for 8 h. Isolated yield. CH3CN
was used as solvent.
precursors 2 are well tolerated in this reaction to furnish the
corresponding products in excellent yields for most cases, and
both electron-rich and the electron-deficient arynes do not
significantly influence the reaction yields. In addition, when a
disubstituted aryne was introduced into the reaction, the yield
was not reduced by its steric congestion (3ab−3ag). However,
the standard protocol is not applicable for reaction with the
pyridyne precursor (2h). In this reaction, the solvent THF
incorporated with two substrates to afford the product 3ah′ in
very good yield, while the product 3ah could be provided by
replacing THF with CH3CN. When the reactions proceeded
by the unsymmetrical arynes, the desired products were
obtained in comparatively lower yields (3aj/3aj′, 78%; 3ak/
3ak′, 76%), and the ratios of isomers depended on the
electronic properties of arynes.14h Notably, the single product
3ai came from ortho-methoxybenzyne, while the isomers 3ak
and 3ak′ came from meta-methoxybenzyne.
1
2
Information for H NMR, H NMR, and ESI mass spectrum
analysis in detail).
Besides, we also carried out the EPR measurement of 1b to
detect the presence of an active radical species in the solution
state (Figure 2; the recorded nitrogen-centered π-radical is in
the high field region whereas the σ-radical is a transient species,
unable to detect signals at low field regions and low
temperatures).18 The EPR spectrum consists of a triplet
1:1:1, and the hyperfine splitting with triplet intensity 1:2:1
C
https://dx.doi.org/10.1021/acs.orglett.0c02393
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
In conclusion, we have developed a concise method for the
tandem cyclization/N-arylation reaction in a transition-metal-
free process to synthesize diversely substituted biaryl sultams
starting from 1,2,3,4-benzothiatriazine-1,1-dioxides and ortho-
(trimethylsilyl)phenyl triflates. This is the first work to reveal
that 1,2,3,4-benzothiatriazine-1,1-dioxide can form a diradical
species just by thermal activation without any assistance from a
transition-metal complex or photocatalyst. The mechanistic
study indicates that the reaction proceeds through the
formation of a diradical species, which implements homolytic
cyclization and subsequent N-arylation of an aryne to generate
the desired product. Further studies to explore the application
of related reactions are currently underway.
Figure 2. EPR spectrum of 1b in 2Me-THF at 80 °C.
ASSOCIATED CONTENT
■
sı
* Supporting Information
was attributed to two ortho-hydrogens of N-phenyl ring; the
observed coupling values are aN = 9.86 G and aO‑H = 3.74
G.18c,d The g-factor for the crossover point of the peak was
found to be 2.0059. A simulation by the Winepr simfonia
software was also performed, which exhibits a highly similar
profile compared to the spectrum. The observed g-value
(2.0059) is in close agreement with a nitrogen-centered radical
in the organic reaction.19 This result assisted by radical
trapping experiments implies that a radical species generated
by the substrate 1 proceeds by a rapid intramolecular reaction
rather than the intermolecular reaction.
Additionally, the single-crystal X-ray diffraction analysis of
compound 1m (see Supporting Information and CCDC data
for details) exhibits that the N−S−C bond angle is 96.7°,
which is highly extruded and would cause the thermal
decomposition to release molecular nitrogen and form a
diradical species. Based on the above results, a plausible
mechanism for this transition-metal-free tandem cyclization/
N-arylation reaction sequence is shown in Scheme 5. The
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02393.
Experimental procedures, characterization, spectral data,
X-ray structure, NMR spectra, and X-ray data (PDF)
Accession Codes
CCDC 2004958−2004963 contain the supplementary crys-
tallographic 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
Jen-Chieh Hsieh − Department of Chemistry, Tamkang
University, New Taipei City 25137, Taiwan (R.O.C.);
orcid.org/0000-0002-1975-7842; Email: jchsieh@
mail.tku.edu.tw
Scheme 5. Proposed Reaction Pathway
Chien-Hong Cheng − Department of Chemistry, National
Tsing Hua University, Hsinchu 30013, Taiwan (R.O.C.);
orcid.org/0000-0003-3838-6845; Email: chcheng@
mx.nthu.edu.tw
Author
Vijaykumar H. Thorat − Department of Chemistry, Tamkang
University, New Taipei City 25137, Taiwan (R.O.C.);
Department of Chemistry, National Tsing Hua University,
Hsinchu 30013, Taiwan (R.O.C.)
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02393
Notes
The authors declare no competing financial interest.
reaction is initiated from the thermal decomposition of 1a by
releasing a nitrogen molecule to form the diradical species A,
which rearranges its nitrogen radical to the N-aryl ring and
furnishes the diradical species B. The close proximity of these
two aryl radicals facilitates intramolecular homolytic cyclization
to offer the intermediate C. Subsequent aromatization
generates the compound 4a (see the X-ray data). Finally,
aryne insertion into 4a occurs and provides the desired
product 3a.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of Republic
of China (MOST 108-2113-M-032-001) for financial support
of this research and the postdoctoral fellowship for Dr.
Vijaykumar H. Thorat (MOST 108-2113-M-032-506). We
also thank Dr. Hsin-Ru Wu of the Instrumentation Center of
National Tsing Hua University for HRMS measurements.
D
https://dx.doi.org/10.1021/acs.orglett.0c02393
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
(d) Jayanth, T. T.; Cheng, C.-H. Angew. Chem., Int. Ed. 2007, 46,
5921. (e) Hsieh, J.-C.; Cheng, C.-H. Chem. Commun. 2005, 2459.
(17) (a) Hsieh, J.-C.; Su, H.-L. Synthesis 2020, 52, 819. (b) Yeh, L.-
H.; Wang, H.-K.; Pallikonda, G.; Ciou, Y.-L.; Hsieh, J.-C. Org. Lett.
2019, 21, 1730. (c) Chen, M.-H.; Hsieh, J.-C.; Lee, Y.-H.; Cheng, C.-
H. ACS Catal. 2018, 8, 9364. (d) Jhang, Y.-Y.; Fan-Chiang, T.-T.;
Huang, J.-M.; Hsieh, J.-C. Org. Lett. 2016, 18, 1154. (e) Chen, W.-L.;
Chen, C.-Y.; Chen, Y.-F.; Hsieh, J.-C. Org. Lett. 2015, 17, 1613.
(18) (a) Abe, M. Chem. Rev. 2013, 113, 7011. (b) Rajca, A. Chem.
Rev. 1994, 94, 871. (c) Miura, Y.; Ohnishi, T.; Kinoshita, M.;
Kinoshita, T. Bull. Chem. Soc. Jpn. 1990, 63, 57. (d) Miura, Y.;
Ohnishi, T. J. Org. Chem. 1988, 53, 3012.
(19) (a) Kaneko, T.; Aso, M.; Koga, N.; Suemune, H. Org. Lett.
2005, 7, 303. (b) DiLabio, G. A.; Scanlan, E. M.; Walton, J. C. Org.
Lett. 2005, 7, 155. (c) Miura, Y.; Ohana, T. J. Org. Chem. 1988, 53,
5770. (d) Koenig, T.; Hoobler, J. A.; Mabey, W. R. J. Am. Chem. Soc.
1972, 94, 2514.
REFERENCES
■
(1) Drews, J. Science 2000, 287, 1960. (b) Rabasseda, X.; Hopkins, S.
Drugs Today 1994, 30, 557. (c) Wroblewski, T.; Graul, A.; Castaner, J.
Drugs Future 1998, 23, 365. (d) Casini, A.; Scozzafava, A.;
Mastrolorenzo, A.; Supuran, C. Curr. Cancer Drug Targets 2002, 2, 55.
(2) (a) Zia-ur-Rehman, M.; Choudary, J. A.; Ahmad, S.; Siddiqui, H.
L. Chem. Pharm. Bull. 2006, 54, 1175. (b) Shah, S. S. A.; Rivera, G.;
Ashfaq, M. Mini-Rev. Med. Chem. 2013, 13, 70.
(3) (a) Wells, G. J.; Tao, M.; Josef, K. A.; Bihovsky, R. J. Med. Chem.
2001, 44, 3488. (b) Francotte, P.; de Tullio, P.; Goffin, E.; Dintilhac,
G.; Graindorge, E.; Fraikin, P.; Lestage, P.; Danober, L.; Thomas, J.-
Y.; Caignard, D.-H.; Pirotte, B. J. Med. Chem. 2007, 50, 3153.
(c) Cherney, R. J.; Mo, R.; Meyer, D. T.; Hardman, K. D.; Liu, R.-Q.;
Covington, M. B.; Qian, M.; Wasserman, Z. R.; Christ, D. D.;
Trzaskos, J. M.; Newton, R. C.; Decicco, C. J. Med. Chem. 2004, 47,
2981. (d) Debnath, S.; Mondal, S. Eur. J. Org. Chem. 2018, 2018, 933.
(4) (a) Bhatti, H. A.; Khatoon, M.; al-Rashida, M.; Bano, H.; Iqbal,
N.; Zaib un, N.; Yousuf, S.; Khan, K. M.; Hameed, A.; Iqbal, J. Bioorg.
Chem. 2017, 71, 10. (b) Chen, X.; Zhang, S.; Yang, Y.; Hussain, S.;
He, M.; Gui, D.; Ma, B.; Jing, C.; Qiao, Z.; Zhu, C.; Yu, Q. Bioorg.
Med. Chem. 2011, 19, 7262. (c) Brooke, E. W.; Davies, S. G.;
Mulvaney, A. W.; Okada, M.; Pompeo, F.; Sim, E.; Vickers, R. J.;
Westwood, I. M. Bioorg. Med. Chem. Lett. 2003, 13, 2527. (d) Virk, T.
S.; Ilawe, N. V.; Zhang, G.; Yu, C. P.; Wong, B. M.; Chan, J. M. W.
ACS Omega 2016, 1, 1336.
(5) Pham, M. V.; Ye, B.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51,
10610.
(6) (a) Kalsi, D.; Sundararaju, B. Org. Lett. 2015, 17, 6118.
(b) Thrimurtulu, N.; Nallagonda, R.; Volla, C. M. R. Chem. Commun.
2017, 53, 1872. (c) Lan, T.; Wang, L.; Rao, Y. Org. Lett. 2017, 19,
972. (d) Planas, O.; Whiteoak, C. J.; Company, A.; Ribas, X. Adv.
Synth. Catal. 2015, 357, 4003.
(7) (a) Laha, J. K.; Jethava, K. P.; Dayal, N. J. Org. Chem. 2014, 79,
8010. (b) Bheeter, C. B.; Bera, J. K.; Doucet, H. Adv. Synth. Catal.
2012, 354, 3533.
(8) (a) Debnath, S.; Mondal, S. J. Org. Chem. 2015, 80, 3940.
(b) Mi, P.; Liao, P.; Tu, T.; Bi, X. Chem. - Eur. J. 2015, 21, 5332.
(9) (a) Pertschi, R.; Weibel, J.-M.; Pale, P.; Blanc, A. Org. Lett. 2019,
21, 5616. (b) Rao, B. M.; Yadav, J. S.; Sridhar, B.; Reddy, B. V. S. Org.
Biomol. Chem. 2018, 16, 5163.
̃
(10) (a) Martínez, C.; Bosnidou, A. E.; Allmendinger, S.; Muniz, K.
Chem. - Eur. J. 2016, 22, 9929. (b) Guerra, W. D.; Rossi, R. A.; Pierini,
A. B.; Barolo, S. M. J. Org. Chem. 2016, 81, 4965.
(11) Miura, T.; Yamauchi, M.; Kosaka, A.; Murakami, M. Angew.
Chem., Int. Ed. 2010, 49, 4955.
(12) Han, Y.-Y.; Wang, H.; Yu, S. Org. Chem. Front. 2016, 3, 953.
(13) Himeshima, Y.; Sonoda, T.; Kobayashi, H. Chem. Lett. 1983,
12, 1211.
(14) For selected reviews after 2010: (a) Roy, T.; Biju, A. T. Chem.
Commun. 2018, 54, 2580. (b) Idiris, F. I. M.; Jones, C. R. Org. Biomol.
Chem. 2017, 15, 9044. (c) Dubrovskiy, A. V.; Markina, N. A.; Larock,
R. C. Org. Biomol. Chem. 2013, 11, 191. (d) Wu, C.; Shi, F. Asian J.
Org. Chem. 2013, 2, 116. (e) Gampe, C. M.; Carreira, E. M. Angew.
Chem., Int. Ed. 2012, 51, 3766. (f) Bhunia, A.; Yetra, S. R.; Biju, A. T.
Chem. Soc. Rev. 2012, 41, 3140. (g) Tadross, P. M.; Stoltz, B. M.
Chem. Rev. 2012, 112, 3550. (h) Sanz, R. Org. Prep. Proced. Int. 2008,
40, 215.
(15) For selected papers in recent four years: (a) Kwon, J.; Kim, B.
M. Org. Lett. 2019, 21, 428. (b) Huang, X.; Zhao, W.; Chen, D.-L.;
Zhan, Y.; Zeng, T.; Jin, H.; Peng, B. Chem. Commun. 2019, 55, 2070.
(c) Yang, X.; Tsui, G. C. Chem. Sci. 2018, 9, 8871. (d) Zhou, L.; Li,
H.; Zhang, W.; Wang, L. Chem. Commun. 2018, 54, 4822. (e) Yao, T.;
He, D. Org. Lett. 2017, 19, 842. (f) Cheng, B.; Bao, B.; Chen, Y.;
Wang, N.; Li, Y.; Wang, R.; Zhai, H. Org. Chem. Front. 2017, 4, 1636.
(16) (a) Thorat, V. H.; Upadhyay, N. S.; Murakami, M.; Cheng, C.-
H. Adv. Synth. Catal. 2018, 360, 284. (b) Lee, Y.-H.; Chen, Y.-C.;
Hsieh, J.-C. Eur. J. Org. Chem. 2012, 2012, 247. (c) Jeganmohan, M.;
Bhuvaneswari, S.; Cheng, C.-H. Angew. Chem., Int. Ed. 2009, 48, 391.
E
https://dx.doi.org/10.1021/acs.orglett.0c02393
Org. Lett. XXXX, XXX, XXX−XXX