Highly Regioselective Carbamoylation of Electron-Deficient Nitrogen Heteroarenes with Hydrazinecarboxamides

Article abstract of DOI:10.1021/acs.orglett.7b02312

The use of hydrazinecarboxamides as a new class of carbamoylating agents has been established through the dehydrazinative Minisci reaction of electron-deficient nitrogen heteroarenes. A wide range of electron-deficient nitrogen heteroarenes, including isoquinoline, quinoline, pyridine, phenanthridine, quinoxaline, and phthalazine, underwent copper/acid-catalyzed oxidative carbamoylation with hydrazinecarboxamide hydrochlorides to afford structurally diverse nitrogen-heteroaryl carboxamides as single regioisomers in moderate to excellent yields. The functional group tolerance was substantially demonstrated in the direct carbamoylation of quinine obviating multistep sequences involving protecting groups and prefunctionalization of the heterocycle.

Full text of DOI:10.1021/acs.orglett.7b02312

Letter
pubs.acs.org/OrgLett
Highly Regioselective Carbamoylation of Electron-Deficient Nitrogen
Heteroarenes with Hydrazinecarboxamides
Zeng-Yang He, Chao-Fan Huang, and Shi-Kai Tian*
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and
Technology of China, Hefei, Anhui 230026, China
S
* Supporting Information
ABSTRACT: The use of hydrazinecarboxamides as a new class of
carbamoylating agents has been established through the dehy-
drazinative Minisci reaction of electron-deficient nitrogen hetero-
arenes. A wide range of electron-deficient nitrogen heteroarenes,
including isoquinoline, quinoline, pyridine, phenanthridine, qui-
noxaline, and phthalazine, underwent copper/acid-catalyzed
oxidative carbamoylation with hydrazinecarboxamide hydrochlor-
ides to afford structurally diverse nitrogen−heteroaryl carbox-
amides as single regioisomers in moderate to excellent yields. The functional group tolerance was substantially demonstrated in
the direct carbamoylation of quinine obviating multistep sequences involving protecting groups and prefunctionalization of the
heterocycle.
hile the hydrazinyl group in a monosubstituted
hydrazine has long been utilized to reduce multiple
low regioselectivity and overcarbamoylation were observed for
some nitrogen heteroarenes.11 To avoid using carbamoylating
agents as solvents and improve selectivity, we expected that
alternative use of hydrazinecarboxamides would be more
practical and expand the scope of the selective carbamoylation
of electron-deficient nitrogen heteroarenes.
W
bonds in chemical synthesis,1 only recent years have witnessed
growing interest in its oxidative removal for the formation of
C−C bonds, yielding molecular nitrogen and water as
byproducts. In this regard, arylhydrazines,2 arylsulfonyl
hydrazides,3 and aroyl hydrazides4 serve as arylating agents
and carbazates5 serve as sources of ester groups to functionalize
certain C−H bonds through dehydrazination under oxidative
conditions. Encouraged by these reports, along with our
interest in exploring dehydrazination reactions,3a,4b,5a,6 we
envisioned that hydrazinecarboxamides could serve as a new
class of carbamoylating agents in the C−H bond functionaliza-
tion of electron-deficient nitrogen heteroarenes through
oxidative dehydrazination. Importantly, the proposed reaction
would provide convenient and economic access to electron-
deficient nitrogen-heteroaryl carboxamides, which not only
possess a spectrum of biological activities7 but also serve as
powerful directing groups and building blocks in chemical
synthesis.8
The model reaction of hydrazinecarboxamide hydrochloride
(1a-HCl)12 with isoquinoline (2a) was performed in the
presence of 25 mol % of AlCl3 and 3 equiv of TBHP in
dimethyl sulfoxide (Table 1, entry 1). The desired C−H bond
carbamoylation reaction took place at 70 °C to afford
nitrogen−heteroaryl carboxamide 3a as a single regioisomer
in 51% yield. To our delight, addition of 5 mol % of copper(II)
salts to the reaction mixture dramatically enhanced the yield,
and the use of CuCO3 as the catalyst gave the highest yield:
89% (entry 7). Increasing the scale from 0.20 to 2.0 mmol gave
a slightly lower yield (85%). In sharp contrast, the yield
decreased dramatically after addition of FeSO4·7H2O, CoSO4,
or NiCl2·6H2O (entries 8−10). We then replaced AlCl3 with a
few other Lewis acids and Bronsted acids, TBHP with a few
̈
In general, electron-deficient nitrogen−heteroaryl carbox-
amides are prepared by treating the corresponding carboxylic
acids or their derivatives with amines under various conditions.9
Nevertheless, it constitutes a more promising strategy to
execute direct carbamoylation of electron-deficient nitrogen
heteroarenes, which is more atom-economic and employs more
readily accessible starting materials and reagents. In this regard,
the Minisci reaction of electron-deficient nitrogen heteroarenes
was disclosed using formamides as carbamoylating agents as
well as solvents.10 In the cases of N-alkylformamides,
unfavorable site selectivity was observed between carbamoyla-
tion and alkylation of electron-deficient nitrogen heteroar-
enes.10b−e Minisci and co-workers modified this reaction by
using oxalic acid monoamides as carbamoylating agents, but
other inexpensive oxidants, and dimethyl sulfoxide with a few
other solvents, and obtained lower yields or even did not
observe the desired carbamoylation reaction (entries 11−26).
In the presence of 5 mol % of CuCO3, 25 mol % of AlCl3,
and 3 equiv of TBHP, a range of N-substituted hydrazine-
carboxamide hydrochlorides smoothly participated in carba-
moylation with isoquinoline (2a) to afford structurally diverse
secondary isoquinoline-1-carboxamides as single regioisomers
in moderate to excellent yields (Scheme 1). When compared to
the previously reported Minisci reaction on the carbamoylation
Received: July 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02312
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 1. Scope of Hydrazinecarboxamide
a,b
Hydrochlorides
c
yield
(%)
b
entry
catalyst
none
acid
AlCl3
oxidant
solvent
1
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
MeCN
dioxane
EtOH
51
86
63
65
76
66
89
37
27
0
2
CuBr2
AlCl3
3
CuCl2·2H2O
CuSO4·5H2O
Cu(OTf)2
Cu(OAc)2
CuCO3
AlCl3
4
AlCl3
5
AlCl3
6
AlCl3
7
AlCl3
8
FeSO4·7H2O
CoSO4
AlCl3
9
AlCl3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
NiCl2·6H2O
CuCO3
AlCl3
Al(OTf)3
Al(OEt)3
TiCl4
CoCl2
ZnCl2
HCl
63
79
53
51
45
49
73
33
84
73
85
33
trace
0
CuCO3
CuCO3
CuCO3
CuCO3
CuCO3
CuCO3
TsOH·H2O
AlCl3
CuCO3
CuCO3
AlCl3
TBPB
DTBP
K2S2O8
TBHP
TBHP
TBHP
TBHP
TBHP
CuCO3
AlCl3
a
Reaction conditions: 1-HCl (0.40 mmol), 2a (0.20 mmol), CuCO3
CuCO3
AlCl3
(5 mol %), AlCl3 (25 mol %), TBHP (aqueous 70 wt %, 3 equiv),
CuCO3
AlCl3
b
DMSO (1.0 mL), 70 °C (oil bath), 12 h. Isolated yields were given.
CuCO3
AlCl3
CuCO3
AlCl3
CuCO3
AlCl3
EtOAc
DCE
31
15
functional group tolerance was further demonstrated in the
direct carbamoylation of quinine, a biologically important and
synthetically useful natural alkaloid having an oxidizable benzyl
alcohol, a monosubstituted alkene, and a nucleophilic
quinuclidine nitrogen, obviating multistep sequences involving
protecting groups and prefunctionalization of the heterocycle
(3ab). The dehydrazinative Minisci reaction was further
extended to pyridine, phenanthridine, quinoxaline, and
phthalazine with very high regioselectivity (3ac−af). Finally,
replacing hydrazinecarboxamide hydrochloride (1a-HCl) with
N-phenylhydrazinecarboxamide hydrochloride (1i-HCl) per-
mitted the facile synthesis of various secondary nitrogen−
heteroaryl carboxamides as single regioisomers (3ag−ak).
We carried out a few control experiments to gain insights
into the reaction mechanism. Replacing hydrazinecarboxamide
hydrochloride (1a-HCl) with hydrazinecarboxamide (1a) in
the reaction of isoquinoline (2a) dramatically decreased the
yield (from 89% to 47%) for the formation of carboxamide 3a
(eq 1). This result suggests that HCl obviates the sequestration
of AlCl3 by the hydrazinyl group and can play an important role
in the activation of the nitrogen heteroarene probably by
combining with AlCl3 to form a stronger acid HAlCl4. As
evidenced by the unsuccessful reaction with 2-benzoylhydrazi-
necarboxamide (1a-Bz), being free of another substituent is
necessary for the dehydrazination. Addition of 3 equiv of
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) to the reaction
mixture of hydrazinecarboxamide hydrochloride (1a-HCl) and
isoquinoline (2a) completely inhibited the formation of
carboxamide 3a (eq 2). ESI-MS analysis of this mixture
permitted us to identify TEMPO−CONH2 (4a) tentatively
according to the high-resolution mass data.13 These results
CuCO3
AlCl3
a
Reaction conditions: 1a-HCl (0.40 mmol), 2a (0.20 mmol), catalyst
(5 mol %), acid (25 mol %), solvent (1.0 mL), 70 °C (oil bath), 12 h.
Aqueous 70 wt % TBHP and 30 wt % H2O2 were used. Isolated
b
c
yield.
of electron-deficient nitrogen heteroarenes,10 this method not
only obviates using carbamoylating agents as solvents and
exhibits very high site selectivity but also dramatically expands
the scope of the synthesis of the corresponding secondary
carboxamides. As demonstrated by the results shown in Scheme
1, we successfully synthesized a range of secondary
carboxamides having N-substituents such as primary, secon-
dary, and tertiary alkyl groups, functionalized alkyl groups
including an allyl group and a propargyl group, and sterically
hindered aryl groups. Nevertheless, the dehydrazinative Minisci
reaction failed to occur with N,N-disubstituted hydrazinecar-
boxamide hydrochlorides, such as N,N-dibenzylhydrazinecar-
boxamide hydrochloride, probably due to the steric repulsion
imposed by the two N-substituents.
We next surveyed the scope of the dehydrazinative Minisci
reaction with regards to electron-deficient nitrogen hetero-
arenes (Scheme 2). As demonstrated by the reaction with
hydrazinecarboxamide hydrochloride (1a-HCl), a range of
substituted isoquinolines and quinolines served as suitable
substrates, and the corresponding isoquinoline-1-carboxamides
and quinoline-2-carboxamides were obtained as single
regioisomers in moderate to good yields (3r-aa). Both
electron-donating groups and electron-withdrawing groups
were successfully introduced by employing the corresponding
isoquinolines and quinolines having such substituents. The
̇
suggest that a carbamoyl radical (CONH2) is generated during
B
DOI: 10.1021/acs.orglett.7b02312
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
Scheme 2. Scope of Nitrogen Heteroarenes
Scheme 3. Proposed Reaction Pathway
of tert-butoxyl radical to afford nitrogen−heteroaryl carbox-
amide 3.15
In summary, we have demonstrated, for the first time, the use
of hydrazinecarboxamides as a new class of carbamoylating
agents through the dehydrazinative Minisci reaction of
electron-deficient nitrogen heteroarenes. In the presence of 5
mol % of CuCO3, 25 mol % of AlCl3, and 3 equiv of TBHP, a
wide range of electron-deficient nitrogen heteroarenes,
including isoquinoline, quinoline, pyridine, phenanthridine,
quinoxaline, and phthalazine, underwent carbamoylation with
hydrazinecarboxamide hydrochlorides to afford structurally
diverse nitrogen−heteroaryl carboxamides as single re-
gioisomers in moderate to excellent yields. The functional
group tolerance was substantially demonstrated in the direct
carbamoylation of quinine, obviating multistep sequences
involving protecting groups and prefunctionalization of the
heterocycle. Preliminary mechanistic studies show that
hydrazinecarboxamides first decompose to carbamoyl radicals
and then undergo C−H bond functionalization with nitrogen
heteroarenes. This study paves the way for the use of
hydrazinecarboxamides as effective carbamoylating agents in
chemical synthesis.
a
Reaction conditions: 1a-HCl or 1i-HCl (0.40 mmol), 2 (0.20 mmol),
CuCO3 (5 mol %), AlCl3 (25 mol %), TBHP (aqueous 70 wt %, 3
equiv), DMSO (1.0 mL), 70 °C (oil bath), 12 h. Isolated yields were
given.
b
the reaction through decomposition of hydrazinecarboxamide
hydrochloride (1a-HCl).
On the basis of our experimental results and previous
relevant studies, we propose the following reaction pathway for
the dehydrazinative Minisci reaction (Scheme 3). Copper-
catalyzed oxidation of hydrazinecarboxamide hydrochloride 1-
HCl with TBHP gives diazene radical 6 and tert-butoxyl
radical.5b,14 Extrusion of molecular nitrogen from diazene
radical 6 gives carbamoyl radical 7,5b,c which participates in
nucleophilic addition to the activated CN bond of salt
8,10c−h generated from nitrogen heteroarene 2, HCl, and AlCl3.
The resulting radical 9 undergoes aromatization in the presence
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.7b02312.
C
DOI: 10.1021/acs.orglett.7b02312
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
2014, 50, 11661. (c) Gao, Y.; Lu, W.; Liu, P.; Sun, P. J. Org. Chem.
2016, 81, 2482.
Experimental procedures, characterization data, and H
NMR and 13C NMR spectra (PDF)
(6) (a) Yang, F.-L.; Tian, S.-K. Angew. Chem., Int. Ed. 2013, 52, 4929.
(b) Yang, F.-L.; Wang, F.-X.; Wang, T.-T.; Wang, Y.-J.; Tian, S.-K.
Chem. Commun. 2014, 50, 2111. (c) Wang, T.-T.; Wang, F.-X.; Yang,
F.-L.; Tian, S.-K. Chem. Commun. 2014, 50, 3802. (d) Wang, T.-T.;
Yang, F.-L.; Tian, S.-K. Adv. Synth. Catal. 2015, 357, 928. (e) Wang, F.-
X.; Tian, S.-K. J. Org. Chem. 2015, 80, 12697. (f) Yang, F.-L.; Gui, Y.;
Yu, B.-K.; Jin, Y.-X.; Tian, S.-K. Adv. Synth. Catal. 2016, 358, 3368.
(g) Guo, J.-Y.; Wu, R.-X.; Jin, J.-K.; Tian, S.-K. Org. Lett. 2016, 18,
3850.
(7) (a) Seitz, L. E.; Suling, W. J.; Reynolds, R. C. J. Med. Chem. 2002,
45, 5604. (b) Thaimattam, R.; Daga, P.; Rajjak, S. A.; Banerjee, R.;
Iqbal, J. Bioorg. Med. Chem. 2004, 12, 6415. (c) St. Laurent, D. R.;
Serrano-Wu, M. H.; Belema, M.; Ding, M.; Fang, H.; Gao, M.;
Goodrich, J. T.; Krause, R. G.; Lemm, J. A.; Liu, M.; Lopez, O. D.;
Nguyen, V. N.; Nower, P. T.; O’Boyle, D. R., II; Pearce, B. C.;
Romine, J. L.; Valera, L.; Sun, J.-H.; Wang, Y.-K.; Yang, F.; Yang, X.;
Meanwell, N. A.; Snyder, L. B. J. Med. Chem. 2014, 57, 1976.
(8) For reviews, see: (a) Rouquet, G.; Chatani, N. Angew. Chem., Int.
Ed. 2013, 52, 11726. (b) Daugulis, O.; Roane, J.; Tran, L. D. Acc.
Chem. Res. 2015, 48, 1053.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tiansk@ustc.edu.cn.
ORCID
Shi-Kai Tian: 0000-0002-2938-7013
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (21472178 and
21232007), the National Key Basic Research Program of
China (2014CB931800), and the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB20000000).
(9) For reviews, see: (a) Benz, J. In Comprehensive Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon, Press: Oxford, U.K., 1991;
Vol. 6, p 381. (b) El-Faham, A.; Albericio, F. Chem. Rev. 2011, 111,
6557.
REFERENCES
■
(1) For reviews, see: (a) Pasto, D. J.; Taylor, R. T. Org. React. 1991,
40, 91. (b) Pasto, D. J. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 8, p
471.
(2) (a) Crank, G.; Gately, G. E.; Makin, M. I. H. Aust. J. Chem. 1984,
37, 2499. (b) Demir, A. S.; Reis, O.; Ozgul-Karaaslan, E. J. Chem. Soc.,
(10) (a) Minisci, F.; Gardini, G. P. Tetrahedron Lett. 1970, 11, 15.
(b) Gardini, G. P.; Minisci, F.; Palla, G.; Arnone, A.; Galli, R.
Tetrahedron Lett. 1971, 12, 59. (c) Citterio, A.; Gentile, A.; Minisci, F.;
Serravalle, M.; Ventura, S. J. Chem. Soc., Chem. Commun. 1983, 916.
(d) Citterio, A.; Gentile, A.; Minisci, F.; Serravalle, M.; Ventura, S. J.
Org. Chem. 1984, 49, 3364. (e) Minisci, F.; Citterio, A.; Vismara, E.;
Giordano, C. Tetrahedron 1985, 41, 4157. (f) Minisci, F.; Recupero, F.;
Punta, C.; Gambarotti, C.; Antonietti, F.; Fontana, F.; Pedulli, G. F.
Chem. Commun. 2002, 2496. (g) Caronna, T.; Gambarotti, C.;
Palmisano, L.; Punta, C.; Recupero, F. Chem. Commun. 2003, 2350.
(h) Han, W.; Jin, F.; Zhao, Q.; Du, H.; Yao, L. Synlett 2016, 27, 1854.
(11) Coppa, F.; Fontana, F.; Lazzarini, E.; Minisci, F. Heterocycles
1993, 36, 2687.
̈
̈
̈
̈
̆
Perkin Trans. 1 2001, 3042. (c) Demir, A. S.; Reis, O.; Emrullahoglu,
M. Tetrahedron 2002, 58, 8055. (d) Zhu, M.-K.; Zhao, J.-F.; Loh, T.-P.
Org. Lett. 2011, 13, 6308. (e) Jasch, H.; Scheumann, J.; Heinrich, M. R.
J. Org. Chem. 2012, 77, 10699. (f) Chen, Y.; Guo, S.; Li, K.; Qu, J.;
Yuan, H.; Hua, Q.; Chen, B. Adv. Synth. Catal. 2013, 355, 711. (g) Bai,
Y.; Kim, L. M. H.; Liao, H.; Liu, X.-W. J. Org. Chem. 2013, 78, 8821.
(h) Hofmann, J.; Jasch, H.; Heinrich, M. R. J. Org. Chem. 2014, 79,
2314. (i) Patil, P.; Nimonkar, A.; Akamanchi, K. G. J. Org. Chem. 2014,
79, 2331. (j) Jiang, T.; Chen, S.-Y.; Zhuang, H.; Zeng, R.-S.; Zou, J.-P.
Tetrahedron Lett. 2014, 55, 4549. (k) Jiang, T.; Chen, S.-Y.; Zhang, G.-
Y.; Zeng, R.-S.; Zou, J.-P. Org. Biomol. Chem. 2014, 12, 6922. (l) Li, Y.;
Liu, W.; Kuang, C. Chem. Commun. 2014, 50, 7124. (m) Li, M.; Ye, Y.
ChemCatChem 2015, 7, 4137. (n) Ravi, M.; Chauhan, P.; Kant, R.;
Shukla, S. K.; Yadav, P. P. J. Org. Chem. 2015, 80, 5369. (o) Zhao, Y.;
Song, Q. Chem. Commun. 2015, 51, 13272. (p) Liu, J.-B.; Chen, F.-J.;
Liu, N.; Hu, J. RSC Adv. 2015, 5, 45843. (q) Chauhan, P.; Ravi, M.;
Singh, S.; Prajapati, P.; Yadav, P. P. RSC Adv. 2016, 6, 109. (r) Yuan,
J.-W.; Li, W.-J.; Yang, L.-R.; Mao, P.; Xiao, Y.-M. Z. Naturforsch., B: J.
Chem. Sci. 2016, 71, 1115. (s) Taniguchi, T.; Imoto, M.; Takeda, M.;
Matsumoto, F.; Nakai, T.; Mihara, M.; Mizuno, T.; Nomoto, A.;
Ogawa, A. Tetrahedron 2016, 72, 4132. (t) Yuan, J.-W.; Li, W.-J.; Xiao,
Y.-M. Tetrahedron 2017, 73, 179. (u) Taniguchi, T.; Imoto, M.;
Takeda, M.; Nakai, T.; Mihara, M.; Mizuno, T.; Nomoto, A.; Ogawa,
A. Synthesis 2017, 49, 1623. (v) Qian, L.-W.; Sun, M.; Dong, J.; Xu, Q.;
Zhou, Y.; Yin, S.-F. J. Org. Chem. 2017, 82, 6764.
(12) A hydrazinecarboxamide hydrochloride was employed instead of
a hydrazinecarboxamide because the former is much more stable than
the latter.
(13) For details, see the Supporting Information.
(14) Jiang, X.-Y.; Qing, F.-L. Angew. Chem., Int. Ed. 2013, 52, 14177.
(15) Chen, J.; Wan, M.; Hua, J.; Sun, Y.; Lv, Z.; Li, W.; Liu, L. Org.
Biomol. Chem. 2015, 13, 11561.
(3) (a) Yang, F.-L.; Ma, X.-T.; Tian, S.-K. Chem. - Eur. J. 2012, 18,
1582. (b) Liu, B.; Li, J.; Song, F.; You, J. Chem. - Eur. J. 2012, 18,
10830. (c) Yuen, O. Y.; So, C. M.; Wong, W. T.; Kwong, F. Y. Synlett
2012, 23, 2714. (d) Yu, X.; Li, X.; Wan, B. Org. Biomol. Chem. 2012,
10, 7479. (e) Chen, W.; Chen, H.; Xiao, F.; Deng, G.-J. Org. Biomol.
Chem. 2013, 11, 4295. (f) Liu, C.; Ding, L.; Guo, G.; Liu, W.; Yang, F.-
L. Org. Biomol. Chem. 2016, 14, 2824. (g) Wang, C.; Jia, H.; Li, Z.;
Zhang, H.; Zhao, B. RSC Adv. 2016, 6, 21814. (h) Yuen, O. Y.; So, C.
M.; Kwong, F. Y. RSC Adv. 2016, 6, 27584.
(4) (a) Li, C.; Li, P.; Yang, J.; Wang, L. Chem. Commun. 2012, 48,
4214. (b) Zhang, Y.-G.; Liu, X.-L.; He, Z.-Y.; Li, X.-M.; Kang, H.-J.;
Tian, S.-K. Chem. - Eur. J. 2014, 20, 2765. (c) Liu, C.; Yang, F. Chin. J.
Chem. 2016, 34, 1213.
(5) (a) Su, Y.-H.; Wu, Z.; Tian, S.-K. Chem. Commun. 2013, 49, 6528.
(b) Ding, R.; Zhang, Q.-C.; Xu, Y.-H.; Loh, T.-P. Chem. Commun.
D
DOI: 10.1021/acs.orglett.7b02312
Org. Lett. XXXX, XXX, XXX−XXX