Palladium(II)/copper(I)-catalyzed sequential C[sbnd]H arylation and oxidative C[sbnd]N bond cleavage of aryl sulfonamino acids: Efficient one-pot synthesis of primary biaryl sulfonamides

Article abstract of DOI:10.1016/j.tet.2016.10.056

A versatile strategy for the one-pot synthesis of primary biaryl-based sulfonamides has been developed via a tandem process consisting of palladium-catalyzed C[sbnd]H arylation and subsequent copper-catalyzed oxidative C[sbnd]N bond cleavage of aryl sulfonamino acids. Both electron-withdrawing and electron-donating functionalities can be introduced into the ortho positions of arenes bearing a variety of substituents. The amio acid moiety not only acts as a directing group but also as an ammonia synthetic equivalent. Importantly, the directing group was smoothly removed in the presence of catalytic CuI by using air as a sole oxidant.

Full text of DOI:10.1016/j.tet.2016.10.056

Accepted Manuscript
Palladium(II)/copper(I)-catalyzed sequential C–H arylation and oxidative C–N bond
cleavage of aryl sulfonamino acids: Efficient one-pot synthesis of primary biaryl
sulfonamides
Wei Liu, Yongli Zhao, Fei Yi, Junmin Chen
PII:
S0040-4020(16)31115-2
10.1016/j.tet.2016.10.056
TET 28197
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 26 July 2016
Revised Date: 17 October 2016
Accepted Date: 22 October 2016
Please cite this article as: Liu W, Zhao Y, Yi F, Chen J, Palladium(II)/copper(I)-catalyzed sequential C–
H arylation and oxidative C–N bond cleavage of aryl sulfonamino acids: Efficient one-pot synthesis of
primary biaryl sulfonamides, Tetrahedron (2016), doi: 10.1016/j.tet.2016.10.056.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphic abstract
Palladium(II)/copper(I)-catalyzed sequential C-H arylation and oxidative C-N bond cleavage of aryl
sulfonamino acids: efficient one-pot synthesis of primary biaryl sulfonamides
Wei Liua,b, Yongli Zhaoa, Fei Yia, Junmin Chena,b٭
a Department of Chemistry, Jiangxi Normal University, Nanchang, 330022, PR China
bKey Laboratory of Functional Small Organic Molecule, Ministry of Education, Nanchang, 330022,
PR China
E-mail: jxnuchenjm@163.com
Tel./fax: +86 0791 8812 0380
1) Pd(OAc)2 (10 mol%)
NaOAc (2.0 equiv)
O
S
O
S
Me
Me
OH
O
O
I
CH3CN
100 oC, 12h
NH2
Ar/Het
N
+
Ar/Het
Ar/Het
H
O
2) CuI (10 mol%)
100 oC, 8h
Ar/Het
3a-t
1a-h
2a-i
1

ACCEPTED MANUSCRIPT
Palladium(II)/copper(I)-catalyzed sequential C-H arylation and oxidative C-N bond cleavage of
aryl sulfonamino acids: efficient one-pot synthesis of primary biaryl sulfonamides
Wei Liua,b, Yongli Zhaoa, Fei Yia, Junmin Chena,b٭
a Department of Chemistry, Jiangxi Normal University, Nanchang, 330022, PR China
bKey Laboratory of Functional Small Organic Molecule, Ministry of Education, Nanchang, 330022, PR China
E-mail: jxnuchenjm@163.com
Tel./fax: +86 0791 8812 0380
Abstract: A versatile strategy for the one-pot synthesis of primary biaryl-based sulfonamides has
been developed via a tandem process consisting of palladium-catalyzed C-H arylation and subsequent
copper-catalyzed oxidative C-N bond cleavage of aryl sulfonamino acids. Both electron-withdrawing
and electron-donating functionalities can be introduced into the ortho positions of arenes bearing a
variety of substituents. The amio acid moiety not only acts as a directing group but also as an
ammonia synthetic equivalent. Importantly, the directing group was smoothly removed in the
presence of catalytic CuI by using air as a sole oxidant.
Keywords: Palladium(II), copper(I), one-pot, C-H arylation, primary biaryl sulfonamides
1 Introduction
Sulfonamides are common motifs in many drugs and medicinal compounds and play a significant
role in their bioactivity since the development of sulfa antibiotics in the 1930s.1 Common drugs such
as glibenclamide,2 sultiame,3 and COX-II inhibitors Piroxicam,4 Ampiroxiam,5 and Celecoxib6
containing a sulfonyl moiety, which displays potential activity across a variety of biological targets
(Figure 1). Recently, biaryl-based sulfonamides represent a significant class of promising bioactive
compounds for drug discovery, for example, compound MK-996 and their analogues L-159,894 are a
series of new potent angiotensin II antagonists for treating hypertension,7 BPBTS represents a
1

ACCEPTED MANUSCRIPT
structurally novel and potent sodium channel blocker.8 Conventionally, the biaryl-based sulfonamides
are synthesized via palladium-catalyzed Suzuki and Stille cross-coupling reactions.9 However, these
tactics rely on strategically installed metal-active groups and such starting materials require additional
steps to prepare them from feedstock chemical sources.
Me
Me
N
H
N
N
N
Et
O
OH
O
O
NH
S
HN
N
O
N
N
O
S
O
S
S
H
N
O
NHCOR
S
Me
H N
2
O
O
O
O
S
MK-996 (R = -Ph)
L-159,894 (R = -CHPh )
Piroxicam
BPBTS
PF-4455242
2
Figure 1 Selected bioactive biaryl-based sulfonamide skeletons
Transition metal-catalyzed C-H activation/functionalization is a fascinating synthetic strategy for
the construction of new chemical bonds. The past decades have witnessed considerable progress in
the construction of C-C and C-hetero bonds and represent a useful method for the modification of
organic molecules in terms of atom and step economy.10 However, due to the ubiquity of C-H bonds
in organic molecules, the presence of a nearby directing group (DG) is usually required in order to
direct positioning of a metal catalyst so that specific C-H bond activation occurs. Based on this
strategy, a variety of directing groups (such as oxygen, nitrogen and sufur containing)-assisted C-H
bond activation is frequently used to realize regio- and chemoselective transformations of selected
C-H bonds.11 However, the sulfonamide-assisted C-H activation is not well explored.12 Recently, Yu
and co-workers successfully developed a kind of different Pd-catalyzed sulfonamide C-H
functionalization reactions using SO2NHC6F5 as a directing grouping.13 More recently, Li and
co-workers established a Rh(III)-catalyzed ortho C-H olefination of aryl sulfonamide directed by the
2

ACCEPTED MANUSCRIPT
SO2NHAc group.14 In spite of these significant progresses, developing a more practical traceless C-H
activation-based one-pot synthetic protocol for the synthesis for primary biaryl sulfoamides is still
desirable.
Recently, we first reported a Pd-catalyzed mono-selective C-H arylation of aryl sulfonamides in
water by using an amino acid moiety as a bidentate directing group to afford the biaryl-based
sulfonamide derivatives, and the amino acid auxiliary were removed with a 30 mol % amount of CuO
as catalyst.15 However, the C-H arylation reaction and removal of the directing group were conducted
in different reaction system. In order to improve synthetic efficiency,16 we attempt to explore a novel
traceless C-H activation-based synthetic protocol for the synthesis for the primary biaryl-based
sulfonamides derivatives. Herein, we report the preliminary result of an efficient one-pot
palladium(II)/copper(I)-catalyzed sequential C-H arylation and oxidative C-N bond cleavage of aryl
sulfonamino acids to afford primary biaryl sulfoamides.
2. Results and discussion
Initially, the N-arylsulfonyl amino acids 1a, and aryl iodide 2a was used to optimize the reaction
conditions (Table 1). We began our investigation by screening various solvents and copper sources.
Expectably, the C-H arylation reaction can smoothly take place in the presence of Pd(OAc)2/NaOAc
and using H2O as the solvent. However, only trace amount of primary biaryl sulfonamide 3a were
detected when 2.0 equiv of CuO was added (Table, entry 1). When DMSO was used as the solvent,
the yield of primary biaryl sulfonamide 2a increased to 48% (Table, entry 2). Then, various copper
salts were screened; we found that CuCl2 and CuBr2 only deliver the moderate yields (Table 1, entries
3-4). Interestingly, CuI can afford the desired product in 57% yield (Table, entry 5). Other copper
salts such as Cu(OAc)2, CuSO4 were also studied, but gave poor results (Table 1, entries 6-7). Next, a
multitude of solvents were screened in the presence of CuI, DMF provided the desired product in
3

ACCEPTED MANUSCRIPT
59% (Table 1, entry 8). Remarkably, the use of CH3CN dramatically improved the catalytic
efficiency, and 87% yield was obtained (Table 1, entry 9). However, the yield of 2a decreased,
affording 2a in 41%, 44% and 47% yields, respectively using EtOAC, DCE and Dioxane as the
solvent (Table 1, entries 10-12). We found that protic solvent such as AcOH were inferior and only
29% yield was obtained (Table 1, entry 13). Whereas, xylene and dioxane were ineffective and only
trace amount of the product were obtained (Table 1, entries 14, 15). It was worth mentioning that the
first C-H arylation performed well in the above mentioned reactions. Finally, results of solvents and
copper salts screening indicated that CH3CN and CuI was still the best choice, respectively (Table 1,
entry 9). The amount of CuI was also screened; we found that 0.1 equiv of CuI were sufficient,
affording the desired product in 86% yield under air (Table 1, entry 18). However, a drop in yield was
observed when the amount of CuI was reduced to 0.05 equiv (Table 1, entry 19). Therefore, the
optimized conditions employed CuI (0.1 equiv) as the catalyst in CH3CN by using air as a sole
oxidant (Table 1, entry 9).
Table 1. Optimization of one-pot palladium(II)/copper(I)-catalyzed sequential C-H arylation and
oxidative C-N bond cleavage of aryl sulfonamino acidsa
1) Pd(OAc)2 (10 mol%)
NaOAc (2.0 equiv)
Solvent
O
S
O
S
Me
Me
OH
O
O
I
NH2
100 oC, 12h
N
+
H
O
2) Copper salt
100 oC, 8h
3a
1a
2a
Entry Solvent Additive Yieldb (%)
1
2
3
H2O
CuO
CuO
trace
48
DMSO
DMSO
CuCl2
35
4

ACCEPTED MANUSCRIPT
4
5
DMSO
DMSO
CuBr2
CuI
46
57
6
DMSO Cu(OAc)2
42
7
DMSO
DMF
CuSO4
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
44
8
59
9
MeCN
EtOAC
DCE
87
10
11
12
13
14
15
16c
17d
18e
19f
41
44
Dioxane
AcOH
Xylene
Hexane
MeCN
MeCN
MeCN
MeCN
47
29
trace
trace
87
87
86
62
a
Reaction conditions: step 1: 1a (0.25 mmol), 2a (0.5 mmol), Pd(OAc)2 (10 mol%), NaOAc (0.5
mmol), 100 °C, solvent (0.5 mL), 12h; step 2: copper source (2.0 equiv), 100 °C, 8h.
b Isolated yield.
c 1.0 equiv of CuI.
d 0.5 equiv of CuI.
e 0.1 equiv of CuI.
f 0.05 equiv of CuI.
5

ACCEPTED MANUSCRIPT
Having established the optimal conditions, we next examined the substrate scope of N-arylsulfonyl
amino acids 1, and aryl iodide 2 (Table 2). Firstly, the substrate scope of aryl iodides was examined.
We found that the arenes of aryl iodides 2 with both electron-donating and electron-withdrawing
groups could smoothly react to afford the desired products 3b-d in good to high yields. For example,
the substrates containing p-MeO, p-Me, and p-Br gave the corresponding primary biaryl
sulfonamides 3b-d in 82%, 80%, and 63% yields, respectively. Then, the reactions of N-arylsulfonyl
amino acids 1 containing different groups on arenes were investigated, a broad range of
N-arylsulfonyl amino acids 1 can participate in this reaction efficiently. For example, the substrates 1
bearing p-Me, and p-MeO groups in aromatic ring afforded the corresponding products in high yields
Table 2 Substrate scope of N-arylsulfonyl amino acids and aryl iodidesa,b
1) Pd(OAc)2 (10 mol%)
NaOAc (2.0 equiv)
CH3CN
O
S
O
S
Me
Me
OH
O
O
I
NH2
100 oC, 12h
N
Ar/Het
+
Ar/Het
Ar/Het
H
O
2) CuI (10 mol%)
100 oC, 8h
Ar/Het
3a-t
1a-h
2a-i
O
O
S
O
S
O
O
O
S
NH2
NH
NH
2
2
OMe
Me
3b: 82%
3c: 80%
3a: 86%
O
S
O
O
O
S
O
O
S
NH
NH
2
NH
2
2
Me
Me
Br
Me
3d: 63%
3f: 83%
3e: 81%
O
O
O
O
O
O
S
S
S
NH
NH2
NH
2
2
Me
Me
MeO
Me
F
Me
3g: 79%
3h: 74%
3i: 77%
6

ACCEPTED MANUSCRIPT
Me
O
S
O
S
O
O
O
O
S
NH
NH2
NH
2
2
Me
MeO
Me
MeO
Me
Br
3k: 68%
3j: 72%
3l: 79%
O
S
O
O
O
O
O
S
S
NH
NH2
NH
2
2
O2N
OMe
Cl
3n: 67%
3m: 75%
3o: 61%
O
O
S
O
O
O
O
S
O2N
S
S
S
NH2
NH2
NH
2
Me
3q: 78%
3p: 76%
3r: 77%
O
O
O
O
S
S
NH2
N
NH2
S
3s: 72%
3t: 78%
a Reaction conditions: step 1: 1 (0.25 mmol), 2 (0.5 mmol), Pd(OAc)2 (10 mol%), NaOAc (0.5 mmol),
100 °C, CH3CN (0.5 mL), 12h; step 2: CuI (0.1 equiv), 100 °C, 8h.
b Isolated yield.
(products 3e-3k). As for different substitution patterns of arenes of substrates 1, ortho- and
meta-substituted substrates all worked well to give the desired products. For example, substrate 1d
bearing an ortho-substituent such as Me proceeded well, and offered 79% isolated yield (product 3l).
Substrate 1e with a MeO group in meta-position reacted smoothly with various aryl iodides and gave
the corresponding products 3m, and 3n in 75%, and 67% yields, respectively. Substrates 1 containing
an electron-withdrawing group such as NO2 in either meta- or para-positon were less efficient in
present catalysis system. Fortunately, satisfactory yields were obtained when AgOAc (2.0 equiv) was
used in place of NaOAc (products 3o, 3p). We were delighted to find that heterocyclic substrate 1h
7

ACCEPTED MANUSCRIPT
also react with various aryl iodides and afford the products 3q, and 3r in good yields. The reaction
also preceded well with heterocyclic coupling partners, for example, 3-iodopyridine 2h and
3-iodothiophene 2i gave the desired products 3s, and 3t in good yields, respectively. Unfortunately, an
attempt to employ the ortho-substituted aryl iodides failed to yield the expected product.
O
S
O
O
O
S
CuI(10% mol)
O
NH2
H
+
O
N
H
CH3CN, 100 oC, 8h
in air
OH
4
1i
3u
Scheme 1. Preliminary mechanistic studies
To aid understanding of the reaction mechanism, the substrate 1i was carried out in the optimal
reactions; the desired product 3u and benzaldehyde 4 could be obtained in 97% yield (Scheme 1).
Cu(I)
O
S
aerobic oxidantion
Cu(II)
O
S
O
N
H
O
N
H
OH
+
Cu(I)
O
A
1i
+
-H
O
O
S
O
O
O
N
+H O
2
S
H
NH
2
+
B
4
3u
Scheme 2. Plausible reaction mechanism
On the basis of the results obtained and previous reports, a plausible reaction mechanism is proposed
as shown in Scheme 2. Initially, Cu(I) was oxidized into Cu(II) intermediate by air, and then the 1i
underwent the Cu-catalyzed oxidative decarbonylation reactions to generate the corresponding
iminium intermediate A and simultaneously release CO2. The imine intermediate B is delivered after
deprotonation. Finally, the imine B was hydrolyzed to afford the primary aryl sulfonamide 3u and
8

ACCEPTED MANUSCRIPT
benzaldehyde 4. Lastly, the Cu(I) generated in the previous step could undergo further oxidation with
air, regenerating Cu(II) to resume the catalytic cycle.
3. Conclusion
In conclusion, we have developed a straightforward route to the synthesis of primary biaryl
sulfonamides via palladium (II)/copper(I)-catalyzed sequential C-H arylation and oxidative C-N bond
cleavage of aryl sulfonamino acids. This reaction utilized N-sulfonyl amino acid as a directing group
and showed excellent regio-selectivity and high reaction activity. Furthermore, the amino acid moiety
could be sequentially removed in one pot reaction by adding CuI as the catalyst and air as a sole
oxidant. The realization of such an approach enriches the methods for the preparation of the primary
biaryl sulfonamides. Further investigations to study the mechanism of this reaction and
transition-metal-catalyzed C–H activation/functionalization of the analogous substrates are in
progress in our laboratory.
4. Experimental section
4.1. General
All reactions were performed under air, Pd(OAc)2 was purchased from Acros. AgOAc, AgOTf, and
Ag2CO3 were purchased from Alfa Aesar. Other reagents were commercially available and were used
directly without further purification unless otherwise specified. All the solvents were directly used
unless otherwise specified. 1H-NMR (400 MHz) and 13C-NMR (100 M Hz) spectra were recorded on
an AVANCE III Bruker 400 M Hz spectrometer. Chemical shifts are reported in (ppm) down field
from tetramethylsilane with reference to solvent signals [1H NMR: d-DMSO (2.50); 13C NMR:
d-DMSO (40.00)]. Signal patterns are indicated as s, singlet; d, doublet; t, triplet; q, quartet; m,
multiplet; br, broad peak, coupling constant (Hz), integration and assignment. Flash column
9

ACCEPTED MANUSCRIPT
chromatography was performed over silica gel (230-400 mesh) purchased from Qindao Puke Co.,
China. Analytical thin-layer chromatography (TLC) was performed on pre-coated, glass-backed silica
gel plates. High-resolution mass spectral (HRMS) data were recorded on Bruker APEX IV Fourier
transform ion cyclotron resonance mass spectrometer using electrospray ionization (ESI).
4.1.1 General Procedure for Palladium(II)/Copper(I)-Catalyzed Sequential C-H Arylation and
Oxidative C-N Bond Cleavage of Aryl Sulfonamino Acids
A mixture of aryl sulfonamide 1 (0.25 mmol), aryl iodide 2 (0.5 mmol), Pd(OAc)2 (0.025 mmol),
NaOAc (0.5 mmol), and CH3CN (0.5 mL) was placed in a 25 mL Schlenk tube with a rubber plug.
o
The tube was heated at 100 C for 12h. At this point, the reaction mixture was cooled to room
temperature, and the CuI (0.025 mmol) was added. The reaction mixture was reheated to 100 °C for
an additional 8h, and the product was extracted with EtOAc (3×5 mL). Combined organic layers were
dried over Na2SO4, concentrated in vacuo. The residue was purified by silica gel column
chromatography (petroleum ether/EtOAc = 5/1) to afford the desired product 3.
Acknowledgements
We are grateful for the financial support of the Science and Technology Planning Project of Jiangxi
Province, China (20142BAB203007).
Supplementary data
Compounds 3 characterization data, copies of 1H and 13C spectra.
Notes and references
1. (a) Drews, J. Science 2000, 287, 1960; (b) Scozzafava, A.; Owa, T.; Mastrolorenzo, A.; Supuran, C.
T.; Curr. Med. Chem. 2003, 10, 925; (c) Kalgutkar, A.; Jones, R.; Sawant, A. In Metabolism
Pharmacokinetics and Toxicity of Functional groups; RSC Drug Discovery; Smith, D. A. Ed.; RSC:
10

ACCEPTED MANUSCRIPT
Cambridge, U.K., 2010; series No. 1, Chapter.
2. Wells, G. J.; Tao, M.; Josef, K. A.; Bihovsky, R. J. Med. Chem. 2001, 44, 3488.
3. Brooke, E. W.; Davies, S. G.; Mulvaney, W. A.; Okada, M.; Pompeo, F.; Sim, E.; Vickers, R. J.;
Westwood, I. M. Bioorg. Med. Chem. Lett. 2003, 13, 2527.
4. Bihovsky, R.; Tao, M.; Mallamo, J. P.; Wells, G. J. Bioorg. Med. Chem. Lett. 2004, 14, 1035.
5. Zia-ur-Rehman, M.; Choudary, J. A.; Ahmad, S.; Siddiqui, H. L. Chem. Pharm. Bull. 2006, 54,
1175.
6. Hendricks, R. T.; Spencer, S. R.; Blake, J. F.; Fell, J. B.; Fischer, J. P.; Stengel, P. J.; Leveque, V. J.
P.; Lepogam, S.; Rajyaguru, S.; Najera, I.; Josey, J. A.; Swallow, S. Bioorg. Med. Chem. Lett. 2009,
19, 410.
7. Chakravarty,P. K.; Naylor, E. M; Chen, A.; Chang, R. S. L.; Chen, T.-B.; Faust, K. A.; Lotti, V. J.;
Kivlighn, S. D.; Gable, R. A. J. Med. Chem. 1994, 37, 4068.
8. Eijkelkamp, N.; Linley, J. E.; Baker, M. D.; Minett, M. S.; Cregg, R.; Werdehausen, R.; Rugierol,
Wood, F. J. N. Brain 2012, 135, 2585.
9. (a) Blanchet, J.; Macklin, T.; Ang, P.; Metallinos, C.; Snieckus, V. J. Org. Chem. 2007, 72, 3199; (b)
Murai, N.; Miyano, M.; Yonaga, M.; Tanaka, K. Org. Lett., 2012, 14, 2818; (c) Zhao, Z.; Snieckus,
V. Org. Lett., 2005, 7, 2523; (d) Jørgensen, K. B.; Rantanen, T.; Dorfler, T.; Snieckus, V. J. Org.
Chem. 2015, 80, 9410.
10. For selected reviews of C-H activation: (a) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res.
2001, 34, 633; (b) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826; (c) Lersch, M.; Tilset,
M. Chem. Rev. 2005, 105, 2471; (d) Alberico, D.; Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107,
174; (e) Seregin, I. Y.; Gevorgyan, V. Chem. Soc. Rev. 2007, 36, 1173; (f) Lewis, J. C.; Bergman,
R. G.; Ellman, J. A. Acc. Chem. Res. 2008, 41, 1013; (g) Daugulis, O.; Do, H. Q.; Shabashov, D.
11

ACCEPTED MANUSCRIPT
Acc. Chem. Res. 2009, 42, 1074; (h) Giri, R.; Shi, B.-F.; Engle, K. M.; Maugel, N.; Yu, J.-Q.
Chem. Soc. Rev. 2009, 38, 3242; (i) Fagnou, K. Top. Curr. Chem. 2009, 292, 35; (j) Lyons, T. W.;
Sanford, M. S.; Chem. Rev. 2010, 110, 1147; (k) Yeung, C. S.; Dong, V. M. Chem. Rev. 2011, 111,
1215; (l) Sun, C.-L.; Li, B.-J.; Shi, Z.-J. Chem. Rev. 2011, 111, 1293; (m) Cho, S. H.; Kim, J. Y.;
Kwak, J.; Chang, S. Chem. Soc. Rev. 2011, 40, 5068; (n) Guo, X.; Gu, D.; Wu, Z.; Zhang, W.
Chem. Rev. 2015, 115, 1622.
11. Chen, Z.; Wang, B.; Zhang, J.; Yu, W.; Liu, Z.; Zhang, Y. Org. Chem. Front. 2015, 2, 1107
(references herein).
12. (a) Pham, M. V.; Ye, B.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51, 10610; (b) Wang, D.; Wang,
F.; Song, G.; Li, X. Angew. Chem., Int. Ed. 2012, 51, 12348; (c) Miura, M.; Tsuda, T.; Satoh, T.;
Pivsa-Art, S.; Nomura, M. J. Org. Chem. 1998, 63, 5211.
13. Dai, H.; Stepan, A. F.; Plummer, M. S.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 7222.
14 Xie, W.; Yang, J.; Wang, B.; Li, B. J. Org. Chem. 2014, 79, 8278.
15. Liu, W.; Wang, D.; Zhao, Y.; Yi, F. Chen, J. Adv. Synth. Catal. 2016, 358, 1968.
16. (a) Wang, J.; Wang, M.; Chen,K.; Zha, S.; Song, C.; Zhu, J. Org Lett. 2016, 1178; (b) Preshlock,
S. M.; Plattner, D. L.; Maligres, P. E.; Krska, S. W.; Maleczka Jr, R. E.; Smith III, M. R. Angew.
Chem., Int. Ed. 2013, 52, 12915; (c) Dong, J.; Wu, Z.; Liu, Z.; Liu, P.; Sun, P. Org. Lett. 2015, 17,
2852.
12