Rhodium-Catalyzed Reaction of Sulfoxonium Ylides and Anthranils toward Indoloindolones via a (4 + 1) Annulation

Article abstract of DOI:10.1021/acs.orglett.9b02249

A rhodium-catalyzed annulation between aroyl sulfoxonium ylides and anthranils has been developed to synthesize 10H-indolo[1,2-a]indol-10-one derivatives. This reaction started with an unpredented (4 + 1) annulation toward N-(2-formylphenyl) indolones, pr

Full text of DOI:10.1021/acs.orglett.9b02249

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium-Catalyzed Reaction of Sulfoxonium Ylides and Anthranils
toward Indoloindolones via a (4 + 1) Annulation
Xiaopeng Wu, Yan Xiao, Song Sun, Jin-Tao Yu, and Jiang Cheng*
School of Petrochemical Engineering and Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology, Changzhou
University, Changzhou 213164, P. R. China
S
* Supporting Information
ABSTRACT: A rhodium-catalyzed annulation between aroyl sulfoxonium ylides and anthranils has been developed to
synthesize 10H-indolo[1,2-a]indol-10-one derivatives. This reaction started with an unpredented (4 + 1) annulation toward N-
(2-formylphenyl) indolones, proceeding with the sequential ortho-amination of the C−H bond in aroyl sulfoxonium ylides by
anthranils and the insertion of N−H to carbene. Finally, the Aldol condensation constructed the second indole ring. This
procedure features the formation of two C−N bonds and one CC bond in one pot.
Scheme 1. Reaction of Aroyl Sulfoxonium Ylides
ecently, much progress has been made in the field of aroyl
R
sulfoxonium ylides,1 which served as novel carbene
surrogates catalyzed by transition metals in the rapid
construction of new C−C,2 C−N,3 and C−O4 bonds.5
Pioneered by Vaitla’s elegant work on the coordination of
6
7
8
9
̈
aroyl sulfoxonium ylides with iridium, Aιssa, Li, Kim, and
Ellman10 independently developed the rhodium−carbon
carbene-involved sequential C−H functionalization to access
the ortho-acylation or cyclization products,11 such as
isoquinolones, isocoumarins, and pyrimidines, etc., where
ylides served as either C1 or C2 synthons (Scheme 1, eq 1).
Alternatively, the carbonyl in aroyl sulfoxonium ylides could
enable the metal-catalyzed chelation-assisted ortho-C−H
functionalization (Scheme 1, eq 2), developed by Li12 and
Fan,13 leading to new aroyl sulfoxonium ylides, naphthols, and
naphthalenones.
Meanwhile, aroyl sulfoxonium ylides could also undergo the
(4 + 1) annulation with amidine toward 3-acylindole (Scheme
1, eq 3).14 In light of the potential coordination of the N atom
in the indole ring, we envisioned to develop the cascade
annulation of aroyl sulfoxonium ylides with ideal substrates
toward hetero polycycles, where the in situ formed potential
directing group enabled the ortho-C−H functionalization to
furnish the second cyclization. With this regard, anthranil was
selected as the proper substrate since it served as the surrogate
of 2-aminobenzaldehyde nitrene in metal-catalyzed ortho-C−H
aromatization reactions.15 Thus, the sequential insertion of
nitrene to the α-C−H in sulfoxonium ylides and Knoevenagel
condensation may achieve the (4 + 1) annulation toward 2-
Received: June 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02249
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
acylindole (path a in Scheme 1, eq 4). Afterward, the in situ
formed indolyl-assisted intramolecular ortho-amination of the
C−H bond selectively furnishes the second cyclization toward
indoloindolones. Alternatively, the ortho-amination of the C−
H bond in aroyl sulfoxonium ylides by anthranils followed by
the (4 + 1) annulation may also access indoloindolones (path
b in Scheme 1, eq 4).16
With the development of rhodium-catalyzed C−H activa-
tion17 and our recent achievements,18 herein, we report our
study on the rhodium-catalyzed reaction of aroyl sulfoxonium
ylides and anthranils toward 10H-indolo[1,2-a]indol-10-ones.
Compared with the previous reported synthetic strategies,19
this procedure features not only the simultaneous construction
of indole and indolone rings by simple starting materials but
also the diversity and complexity of the final compounds. Being
a class of bioactive polycyclic heteroarenes combining indole
and indolone scaffolds, indolo[1,2-a]indols are ubiquitous in
the fields of dyes and external fluorescent indicators.20
We started our work by exploring the reaction of benzoyl
sulfoxonium ylide 1a (0.1 mmol) and anthranil 2a (0.11
mmol) in the presence of [Cp*RhCl2]2 (2.5 mol %) and
AgSbF6 (10 mol %) in toluene under 110 °C (Table 1).
the presence of HOAc (entry 11). However, pivalic acid
(PivOH, 60%) was not so effective (entry 12). The reaction
efficiency was not improved under elevated (73%, 120 °C) or
lower (65%, 80 °C; 68%, 100 °C) reaction temperatures (entry
11).
With the optimized reaction conditions in hand, the scope
and limitation of aroyl sulfoxonium ylides were studied, as
shown in Figure 1. As expected, some functional groups, such
Table 1. Selected Results for Screening the Optimized
a
Reaction Conditions
b
entry
catalyst
additive
solvent
yield (%)
c
1
2
3
4
5
6
7
8
9
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2
-
-
-
-
-
-
-
-
toluene
DCM
CH3CN
THF
DCE
DCE
DCE
DCE
DCE
DCE
trace
c
16
22
13
c
c
c
Figure 1. Scope of aroyl sulfoxonium ylides. Reaction conditions:
aroyl sulfoxonium ylides 1 (0.1 mmol), anthranil 2a (0.11 mmol),
[Cp*RhCl2]2 (2.5 mol %), AgSbF6 (10 mol %), HOAc (0.1 mmol),
DCE (3.0 mL), and N2 (1.0 atm), at 110 °C for 2 h, in a sealed
Schlenk tube.
35 , 55
n.r.
n.r.
48
43
n.r.
AgSbF6
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgBF4
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
-
10
11
Cs2CO3
HOAc
d
DCE
81, 65
68 , 73
as methyl (3ba−da, 73−90%), methoxy (3ea, 76%; 3fa, 70%),
halogen (3ga, 3ha, 3ka, and 3la, 65−83%), and trifluoromethyl
(3ia, 82%), were well compatible under the reaction conditions
with good yields. Unfortunately, strong electron-withdrawing
groups such as NO2 inhibited the reaction. Importantly,
various substrates possessing substituents at the para- (3ba,
82%, 3fa, 70%; 3ia, 82%; 3ka, 65%; 3la, 66%), meta- (3ca,
90%, 3ea, 76%; 3ga, 83%), and ortho- (3da, 73%, 3ha, 77%)
positions of the phenyl ring all ran smoothly under the
standard procedures. Notably, the naphthyl analogue was also
a good reaction partner, providing 3ja in 60% yield. In a 1.0
mmol scale reaction, 3aa was isolated in 45% yield.
Next, the scope of substituted anthranils was studied, as
shown in Figure 2. Generally, all substrates ran smoothly to
afford the 10H-indolo[1,2-a]indol-10-ones in 65%−88%
yields. The procedure tolerated chloro (3cb, 85%; 3cf, 76%),
fluoro (3cc, 79%; 3cg, 70%), and bromo (3ad, 74%; 3cd, 86%;
3dd, 70%) groups well, which provided handles for potentially
further functionalization. In addition to halogens, substrates
containing electron-donating groups (3ce, 88%; 3ch, 83%) are
also suitable reaction partners under the standard procedure.
Specially, anthranils with unsaturated functional groups (3ak,
e
f
12
[Cp*RhCl2]2/AgSbF6
PivOH
DCE
60
a
Reaction conditions: Benzoyl sulfoxonium ylide 1a (0.1 mmol),
anthranil 2a (0.11 mmol), [Cp*RhCl2]2 (2.5 mol %), AgSbF6 (10 mol
%), other indicated additives (0.1 mmol), solvent (3.0 mL), and N2
(1.0 atm), at 110 °C for 2 h, in a sealed Schlenk tube. Isolated yield.
Under air. At 80 °C. At 100 °C. At 120 °C.
b
c
d
e
f
However, no reaction took place at all (Table 1, entry 1). To
our delight, the annulated product 10H-indolo[1,2-a]indol-10-
one 3aa was isolated in 16% yield in dichloromethane (DCM).
Replacing DCM with THF or MeCN resulted in a similar
yield, and the reaction efficiency was further increased in 1,2-
dichloroethane (DCE) (entries 2−5). Fortunately, the yield of
3aa increased to 55% under N2 (entry 5). No reaction took
place in the absence of either [Cp*RhCl2]2 or AgSbF6 (entries
6 and 7). Additionally, other Ag(I) species, such as AgNTf2
(48%) and AgBF4 (43%), showed similar reactivity under this
transformation (entries 8 and 9). The reaction was totally
inhibited in the presence of Cs2CO3 as a base (entry 10). On
the contrary, the yield of 3aa dramatically increased to 81% in
B
DOI: 10.1021/acs.orglett.9b02249
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Letter
Scheme 2. Control Experiments
Figure 2. Substrate scope of substituted anthranils. Reaction
conditions: benzoyl sulfoxonium ylide 1a (0.1 mmol), substituted
anthranils 2 (0.11 mmol), [Cp*RhCl2]2 (2.5 mol %), AgSbF6 (10 mol
%), HOAc (0.1 mmol), DCE (3.0 mL), and N2 (1.0 atm), at 110 °C
for 2 h, in a sealed Schlenk tube.
68%; 3al, 65%) and protected hydroxyl (OBn, 3cj, 69%)
worked well, which further increased the practibility of this
procedure. Furthermore, the structure of 3dd was confirmed
by X-ray crystallography (for details, see Supporting
Information). This result strongly confirmed that the carbonyl
in the final products was derived from aroyl sulfoxonium ylides
rather than anthranils.
5).13 Thus, any attempt to test the inter- and intramolecular
KIE for the benzoyl sulfoxonium ylide failed. Replacing
CH3COOH with CD3COOD under standard conditions,
almost no D-incorporation was observed in 3cb (Scheme 2,
eq 6).
More control experiments were conducted to get some
insights into this reaction. First, the 2,6-dimethylbenzoyl
sulfoxonium ylide (1n) was subjected to the reaction with
anthranil (2a) under standard procedures. However, no 2-
benzoyl indole (4) was detected at all (Scheme 2, eq 1).
Second, intermediate 5 did not transform into 3aa under the
standard procedure (Scheme 2, eq 2). These results ruled out
the possibility of the involvement of path a depicted in eq 4 of
Scheme 1. Third, during the Ullman reaction of indoxyl (7)
with 2-iodobenzaldehyde (8), 3aa rather than the expected
coupling product 6 was detected by GC-MS (Scheme 2, eq 3).
Thus, the presumed intermediate 6 may transform into the
final products, although it was too reactivated to be detected
(path b in eq 4 of Scheme 1). Moreover, the Ullman reaction
of indoxyl acetate (9) with 2-iodobenzaldehyde (8) or 5-
bromo-2-iodobenzaldehyde (10) provided 3aa and 3ad in 42%
and 78% isolated yields, where intermediate 11 was not
detected by GC-MS at all (Scheme 2, eq 4).21 The results
suggested the in situ hydrolysis of 11 to 3-hydroxyl indole,
which underwent the intramolecular Aldol condensation
immediately toward the final products. Finally, the H/D
exchange for benzoyl sulfoxonium ylide 1a confirmed that the
cleavage of the C−H bond in 1a was reversible (Scheme 2, eq
Based on the control experiments and some relevant
literature,5,12,13 a mechanism for the procedure is proposed
in Scheme 3. The catalytic cycle starts with the depolymeriza-
tion of rhodium catalyst and C−H activition with benzoyl
sulfoxonium ylide (1a), leading to a rhodacyclic intermediate
A. Next, the insertion of anthranil (2a) produces a five-
membered cyclic “RhN” nitrene species C,15 which then
transforms into a six-membered rhodacycle D and E through
intramolecular migration.12 After releasing a molecule of
DMSO, a new “RhC” carbene species F is formed. Then,
the insertion of N−H in F to the carbene takes place to furnish
the (4 + 1) annulation toward intermediate G. The
protonation of intermediate G releases Rh(III) catalyst to re-
enter the catalytic cycle, along with intermediate 6. Finally,
intermediate 6 transforms to 3aa via an intramolecular Aldol
condensation.
In conclusion, we have developed a Rh(III)-catalyzed ortho-
C−H annulation between α-aroyl sulfoxonium ylides and
anthranils toward a series of 10H-indolo[1,2-a]indol-10-ones
in moderate to good yields, proceeding with a (4 + 1)
annulation involving sequential ortho-amination of the C−H
bond in aroyl sulfoxonium ylides and insertion of N−H to
carbene. Our procedure features the rapid construction of two
C
DOI: 10.1021/acs.orglett.9b02249
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Tentative Mechanism
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(nos. 21572025, 21672028, and 21602019), “Innovation &
Entrepreneurship Talents” Introduction Plan of Jiangsu
Province, Natural Science Foundation of Jiangsu Province
(BK20171193), Jiangsu Key Laboratory of Advanced Catalytic
Materials & Technology (BM2012110), and Advanced
Catalysis and Green Manufacturing Collaborative Innovation
Center for financial support. S. Sun thanks the Young Natural
Science Foundation of the Jiangsu Province (BK20150263) for
financial support.
REFERENCES
■
(1) Reviews on sulfur ylides: (a) Li, A.-H.; Dai, L.-X.; Aggarwal, V.
K. Chem. Rev. 1997, 97, 2341. (b) McGarrigle, E. M.; Myers, E. L.;
Illa, O.; Shaw, M. A.; Riches, S. L.; Aggarwal, V. K. Chem. Rev. 2007,
107, 5841. (c) Sun, X.-L.; Tang, Y. Acc. Chem. Res. 2008, 41, 937.
(d) Lu, L.-Q.; Chen, J.-R.; Xiao, W.-J. Acc. Chem. Res. 2012, 45, 1278.
(e) Zhu, C.; Ding, Y.; Ye, L.-W. Org. Biomol. Chem. 2015, 13, 2530.
(f) Lu, L.-Q.; Li, T.-R.; Wang, Q.; Xiao, W.-J. Chem. Soc. Rev. 2017,
46, 4135.
(2) (a) Neuhaus, J. D.; Bauer, A.; Pinto, A.; Maulide, N. Angew.
Chem., Int. Ed. 2018, 57, 16215. (b) Zhou, C.; Fang, F.; Cheng, Y.; Li,
Y.; Liu, H.; Zhou, Y. Adv. Synth. Catal. 2018, 360, 2546. (c) Li, C.; Li,
M.; Zhong, W.; Jin, Y.; Li, J.; Wu, W.; Jiang, H. Org. Lett. 2019, 21,
872.
(3) (a) Lai, R.; Wu, X.; Lv, S.; Zhang, C.; He, M.; Chen, Y.; Wang,
Q.; Hai, L.; Wu, Y. Chem. Commun. 2019, 55, 4039. (b) Mangion, I.
K.; Nwamba, I. K.; Shevlin, M.; Huffman, M. A. Org. Lett. 2009, 11,
3566. (c) Mangion, I. K.; Weisel, M. Tetrahedron Lett. 2010, 51, 5490.
(d) Molinaro, C.; Bulger, P. G.; Lee, E. E.; Kosjek, B.; Lau, S.;
Gauvreau, D.; Howard, M. E.; Wallace, D. J.; O’Shea, P. D. J. Org.
Chem. 2012, 77, 2299. (e) Mangion, I. K.; Ruck, R. T.; Rivera, N.;
Huffman, M. A.; Shevlin, M. Org. Lett. 2011, 13, 5480. (f) Phelps, A.
M.; Chan, V. S.; Napolitano, J. G.; Krabbe, S. W.; Schomaker, J. M.;
Shekhar, S. J. Org. Chem. 2016, 81, 4158.
C−N bonds and one CC bond in one pot, which not only
provides a facile route to access indole derivatives with
diversity and complexity but also broadens the application of
aroyl sulfoxonium ylides in transition-metal-catalyzed C−H
functionalization.
(4) Jiang, H.; Zhang, H.; Xiong, W.; Qi, C.; Wu, W.; Wang, L.;
Cheng, R. Org. Lett. 2019, 21, 1125.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02249.
■
S
(5) Reviews on sulfoxonium ylides in transition-metal-catalyzed
reactions: (a) Bayer, A.; Vaitla, J. Synthesis 2019, 51, 612. (b) Wu, X.;
Sun, S.; Yu, J.-T.; Cheng, J. Synlett 2019, 30, 21. (c) Vaitla, J.; Bayer,
A.; Hopmann, K. H. Synlett 2019, 30, 1377. (d) Vaitla, J.; Bayer, A.
Synthesis 2019, 51, 612. (e) Neuhaus, J. D.; Oost, R.; Merad, J.;
Maulide, N. Top. Curr. Chem. 2018, 376, 15.
(6) Vaitla, J.; Bayer, A.; Hopmann, K. H. Angew. Chem., Int. Ed.
2017, 56, 4277.
(7) Barday, M.; Janot, C.; Halcovitch, N. R.; Muir, J.; Aissa, C.
Angew. Chem., Int. Ed. 2017, 56, 13117.
(8) (a) Xu, Y.; Zheng, G.; Yang, X.; Li, X. Chem. Commun. 2018, 54,
670. (b) Zheng, G.; Tian, M.; Xu, Y.; Chen, X.; Li, X. Org. Chem.
Front. 2018, 5, 998. (c) Xu, Y.; Zhou, X.; Zheng, G.; Li, X. Org. Lett.
2017, 19, 5256. (d) Hu, P.; Zhang, Y.; Xu, Y.; Yang, S.; Liu, B.; Li, X.
Org. Lett. 2018, 20, 2160.
(9) Oh, H.; Han, S.; Pandey, A. K.; Han, S. H.; Mishra, N. K.; Kim,
S.; Chun, R.; Kim, H. S.; Park, J.; Kim, I. S. J. Org. Chem. 2018, 83,
4070.
(10) (a) Halskov, K. S.; Witten, M. R.; Hoang, G. L.; Mercado, B.
Q.; Ellman, J. A. Org. Lett. 2018, 20, 2464. (b) Hoang, G. L.; Streit, A.
D.; Ellman, J. A. J. Org. Chem. 2018, 83, 15347.
(11) (a) Liu, C. F.; Liu, M.; Dong, L. J. Org. Chem. 2019, 84, 409.
(b) Shi, X.; Wang, R.; Zeng, X.; Zhang, Y.; Hu, H.; Xie, C.; Wang, M.
Adv. Synth. Catal. 2018, 360, 4049.
Experimental procedures along with copies of spectra
(PDF)
Accession Codes
CCDC 1935878 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jiangcheng@cczu.edu.cn.
ORCID
Song Sun: 0000-0002-9974-6456
Jin-Tao Yu: 0000-0002-0264-9407
Jiang Cheng: 0000-0003-2580-1616
Notes
(12) Xu, Y.; Yang, X.; Zhou, X.; Kong, L.; Li, X. Org. Lett. 2017, 19,
4307.
(13) Chen, X.; Wang, M.; Zhang, X.; Fan, X. Org. Lett. 2019, 21,
2541.
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.9b02249
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(14) Lai, R.; Wu, X.; Lv, S.; Zhang, C.; He, M.; Chen, Y.; Wang, Q.;
Hai, L.; Wu, Y. Chem. Commun. 2019, 55, 4039.
(15) (a) Yu, S.; Tang, G.; Li, Y.; Zhou, X.; Lan, Y.; Li, X. Angew.
Chem., Int. Ed. 2016, 55, 8696. (b) Tang, C.; Zou, M.; Liu, J.; Wen,
X.; Sun, X.; Zhang, Y.; Jiao, N. Chem. - Eur. J. 2016, 22, 11165. (c) Li,
L.; Wang, H.; Yu, S.; Yang, X.; Li, X. Org. Lett. 2016, 18, 3662.
(d) Zou, M.; Liu, J.; Tang, C.; Jiao, N. Org. Lett. 2016, 18, 3030.
(e) Li, H.; Jie, J.; Wu, S.; Yang, X.; Xu, H. Org. Chem. Front. 2017, 4,
250.
(16) (a) Shi, L.; Wang, B. Org. Lett. 2016, 18, 2820. (b) Kim, S.;
Han, S. H.; Mishra, N. K.; Chun, R.; Jung, Y. H.; Kim, H. S.; Park, J.
S.; Kim, I. S. Org. Lett. 2018, 20, 4010. (c) Jin, H.; Tian, B.; Song, X.;
Xie, J.; Rudolph, M.; Rominger, F.; Hashmi, A. S. Angew. Chem., Int.
Ed. 2016, 55, 12688. (d) Yu, S.; Li, Y.; Zhou, X.; Wang, H.; Kong, L.;
Li, X. Org. Lett. 2016, 18, 2812. (e) Wang, F.; Xu, P.; Wang, S. Y.; Ji,
S. J. Org. Lett. 2018, 20, 2204.
(17) Selected reviews for Rh-catalyzed C−H activation/annulation:
(a) Satoh, T.; Miura, M. Chem. - Eur. J. 2010, 16, 11212. (b) Colby,
D. A.; Tsai, A. S.; Bergman, P. G.; Ellman, J. A. Acc. Chem. Res. 2012,
45, 814. (c) Song, G.; Li, X. Acc. Chem. Res. 2015, 48, 1007.
(d) Gensch, T.; Hopkinson, M. N.; Glorius, F.; Wencel-Delord, J.
Chem. Soc. Rev. 2016, 45, 2900. (e) Yang, Y.; Li, K.; Cheng, Y.; Wan,
D.; Li, M.; You, J. Chem. Commun. 2016, 52, 2872.
(18) (a) Xiao, Y.; Xiong, H.; Sun, S.; Yu, J.; Cheng, J. Org. Biomol.
Chem. 2018, 16, 8715. (b) Wu, X.; Xiong, H.; Sun, S.; Cheng, J. Org.
Lett. 2018, 20, 1396. (c) Zhu, J.; Sun, S.; Cheng, J. Tetrahedron Lett.
2018, 59, 2284. (d) Wu, X.; Sun, S.; Xu, S.; Cheng, J. Adv. Synth.
Catal. 2018, 360, 1111. (e) Xiong, H.; Xu, S.; Sun, S.; Cheng, J. Org.
Chem. Front. 2018, 5, 2880. (f) Xiao, Y.; Wu, X.; Wang, H.; Sun, S.;
Yu, J.-T.; Cheng, J. Org. Lett. 2019, 21, 2565.
(19) (a) Wang, X.; Li, Z.; Cao, S.; Rao, H. Adv. Synth. Catal. 2016,
358, 2059. (b) Giacometti, R.; Ramtohul, Y. Synlett 2009, 2010, 2010.
(c) Kulkarni, A. M.; Srinivas, K.; Deshpande, M. V.; Ramana, C. V.
Org. Chem. Front. 2016, 3, 43. (d) Hazra, S.; Mondal, B.; De, R. N.;
Roy, B. RSC Adv. 2015, 5, 22480. (e) Rogness, D. C.; Larock, R. C.
Tetrahedron Lett. 2009, 50, 4003. (f) Liu, L.; Qiang, J.; Bai, S.; Li, Y.;
Miao, C.; Li, J. Appl. Organomet. Chem. 2017, 31, 3817.
(20) (a) Spitz, U.; Wick, L.; Bayer, A. R.; Weymuth, C.; Schabert, G.
US 8940909, 2015. (b) Spitz, U.; Wick, L.; Weymuth, C.; Bayer, A.;
Schabert, G. EP2256103A1, 2010.
(21) For details, see the Supporting Information.
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