Catalytic claisen rearrangement by intercepting ketenimines with propargylic alcohols: A strategy to generate and transform ketenimines from radicals

Article abstract of DOI:10.1021/acs.orglett.0c02306

An efficient strategy for facilitating the cross-coupling of two radicals has been established via the coordination of a radical with a metal catalyst. This strategy provides a remarkable ability to harness the reactivity of nitrile-containing azoalkanes and enables a novel cascade reaction with nitrile-containing azoalkanes and propargylic alcohols to be established. By using this reaction, a range of acetylenic and allenic amides were obtained that provides a versatile platform for further derivatizations.

Full text of DOI:10.1021/acs.orglett.0c02306

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Letter
Catalytic Claisen Rearrangement by Intercepting Ketenimines with
Propargylic Alcohols: A Strategy to Generate and Transform
Ketenimines from Radicals
Xuyang Yan, Hongchi Liu, Shenquan Wei, and Hanmin Huang*
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ABSTRACT: An efficient strategy for facilitating the cross-
coupling of two radicals has been established via the coordination
of a radical with a metal catalyst. This strategy provides a
remarkable ability to harness the reactivity of nitrile-containing
azoalkanes and enables a novel cascade reaction with nitrile-
containing azoalkanes and propargylic alcohols to be established.
By using this reaction, a range of acetylenic and allenic amides were
obtained that provides a versatile platform for further derivatiza-
tions.
Scheme 1. Strategy for Generating Ketenimines from Free
Radicals
he development of new reactions is a fundamental goal
Tand a big challenge in synthetic organic chemistry.
Because almost all inert starting materials have to be
transferred to reactive intermediates prior to furnishing the
desired transformations, this task is associated with the
discovery of new valuable transient intermediates and the
design of efficient strategies for making and transforming them.
Ketenimines (KIs) have long been established as one of the
most versatile and transient intermediates in nucleophilic,
electrophilic addition, and cyclization reactions via both
noncatalytic and catalytic manners.1 As such, the development
of efficient strategies to generate and control the reactivity of
such transient intermediates is crucial for the success of these
reactions. In this context, a particularly straightforward strategy
to access silyl ketenimines (SKIs) is the deprotonation of
simple alkyl nitriles with strong bases (Scheme 1a).2,3
Unfortunately, the super stoichiometric quantities of strong
amide or alkyllithium bases as well as silyl reagents are
required, which severely limits the functional group tolerance
and the extension of this process to a catalytic reaction.
Azobis(isobutyronitrile) (AIBN) is possibly the most widely
used radical initiator in polymer chemistry and radical-
mediated organic synthesis. The solution thermolysis/
photolysis of AIBN has been extensively studied, and it has
been disclosed that a pair of cyanoisopropyl radicals I would be
produced by the extrusion of N2.4 Both tetramethylsuccino-
dinitrile (TMSN) and dimethyl-N-(2-cyano-2-propyl)-keteni-
mine (DKI) were produced via the radical−radical coupling
reaction, in which the DKI was produced from the cross-
coupling of the C-centered radical resonance I and N-centered
radical resonance II.4 These results indicated that AIBN might
be utilized as a latent ketenimine. However, catalytic reactions
employing AIBN as a latent ketenimine source to deliver
Received: July 11, 2020
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© XXXX American Chemical Society
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a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst
additive
solvent
yield (%)
1
2
3
4
5
6
7
8
Cu(CH3CN)4BF4
Cu(OTf)2
CuCl
AgOTf
Zn(OTf)2
DME
DME
DME
DME
DME
DME
DME
toluene
NMP
THF
i-PrOH
DME
DME
DME
DME
DME
DME
74
trace
trace
30
71
30
17
58
16
54
0
45
27
90 (82)
87
Fe(OTf)2
BF3·Et2O
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
9
10
11
12
13
14
15
16
17
CF3CO2H
PhSO3H
4-NO2C6H4B(OH)2
4-NO2C6H4B(OH)2
4-NO2C6H4B(OH)2
4-NO2C6H4B(OH)2
c
d
89
55
e
a
b
Reaction conditions: 1a (0.5 mmol), 2a (1.0 mmol), catalyst (0.025 mmol), additive (0.05 mmol), solvent (2.0 mL), 100 °C, 12 h. Yields of 3aa
c
d
were determined by GC analysis using n-tetradecane as the internal standard. Isolated yield is given in parentheses. 120 °C. 4-NO2C6H4B(OH)2
(0.1 mmol). 2a (0.75 mmol).
e
versatile nitrile-handled compounds have rarely been docu-
mented.
in Table 1, this new cascade reaction was successful with a
broad range of metal salts including Cu(CH3CN)4BF4, AgOTf,
Zn(OTf)2, and Fe(OTf)2. Moreover, the nonmetal Lewis acid
BF3·Et2O was able to promote the reaction, albeit with lower
activity. The best yield was obtained with Cu(CH3CN)4BF4 as
the catalyst (Table 1, entry 1). Inspired by this promising
result, we next sought to improve the efficiency of this reaction
by optimizing other reaction parameters with this metal salt as
the catalyst. Screening of solvents disclosed that dimethyl ether
(DME) was the best choice for the present reaction (entries
8−11), whereas i-PrOH could not be used. Further
optimizations of this reaction by using Brønsted acid as a
cocatalyst led us to find that the yield of desired product 3aa
dramatically increased to 90% when 10 mol % of 4-
NO2C6H4B(OH)2 was utilized as the cocatalyst (Table 1,
entry 14). Furthermore, changing temperature as well as the
loading of 4-NO2C6H4B(OH)2 or 2a resulted in lower
reactivities (Table 1, entries 15−17). Control experiments in
the absence of Lewis acid led to virtually no reactivity under
the standard conditions, highlighting the importance of the
coordination effect of the metal catalyst.
With the optimized reaction conditions in hand, we next
investigated the scope of this cascade reaction (Scheme 2). We
were pleased to find that the copper-catalyzed cascade reaction
proceeded well with a broad range of propargylic alcohols,
providing access to a diverse array of highly functionalized
amide motifs. By varying the substituents at the acetylenic
position of the propargylic alcohol, we found that various
substrates bearing electron-withdrawing or electron-donating
groups on the aryl ring of propargylic alcohols were well
tolerated to give the adducts products (3aa−ia) in good yields.
The reaction could be performed on a 10 mmol scale,
providing 2.9 g of 3aa in 78% yield. In addition, the steric
hindrance of the substituents on the aryl ring did not have a
strong influence on the reactivities (3ja and 3ka). Moreover,
The underlying reason might be attributed to the fact that
the amount of the in-situ-formed DKI from AIBN is dependent
on the thermolysis conditions.4 Moreover, the DKI proves to
be unstable and decomposes at a similar rate as AIBN to give
TMSN under the thermolysis conditions.5 Thus one of the
potential problems facing the development of the reaction with
AIBN as a latent ketenimine is to identify a new strategy to
promote the formation of the DKI rather than TMSN.
Recently, our research group has demonstrated that the
metal-ligated cyanoisopropyl radical A possesses lower singly
occupied molecular orbital (SOMO) in energy via enhancing
the captodative effect of the radical.6 Such kind of radical
complex holds higher electrophilicity toward reacting with
carbon nucleophiles through single-electron-transfer (SET) in
the presence of a redox metal catalyst.6 This result together
with the consideration of the Ingold−Fischer “persistent
radical effect” (PRE)7 prompted us to envision that a metal-
coordinated radical complex A might also catch the N-center
radical II to produce the ketenimine complex B in the absence
of the aforementioned carbon nucleophiles. In this context,
once a suitable nucleophile is identified, which is reluctant to
undergo a SET reaction, a typical ketenimine-involved cascade
reaction would be established. Herein we report an
unprecedented copper- and silver-catalyzed radical coupling/
nucleophilic addition/Claisen rearrangement8 cascade reaction
of propargylic alcohols and nitrile-containing azoalkanes,9 in
which the nitrile-containing azoalkanes were functionalized as
latent ketenimines and intercepted by propargylic alcohols
enabled by the coordination of a radical with a metal catalyst
(Scheme 1c).
To explore our hypothesis, we commenced our studies by
attempting the proposed reaction using 1a and AIBN 2a as
substrates with a variety of Lewis acids as catalysts. As outlined
B
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solvent.10 Moreover, the reaction could also be performed
on the gram scale to provide 1.3 g of 5aa in 49% yield. We then
evaluated the scope of primary propargylic alcohols that could
be utilized in this protocol (Scheme 3). We were pleased to
a
Scheme 2. Scope of the Formation of Acetylenic Amides
a
Scheme 3. Scope of the Formation of Allenic Amides
a
Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), Cu(CH3CN)4BF4
(0.025 mmol), 4-NO2C6H4B(OH)2 (0.05 mmol), DME (2.0 mL),
1
100 °C, 12 h, isolated yield. The d.r. was determined by H NMR of
the crude reaction mixture.
a
Reaction conditions: 4 (0.5 mmol), 2 (1.0 mmol), AgOTf (0.05
mmol, 10 mol %), DCM (1.0 mL), 100 °C, 12 h, isolated yield. The
1
the thiophene ring was well tolerated under the reaction
conditions (3la). In addition to the aromatic propargylic
alcohols, a series of aliphatic propargylic alcohols also gave the
desired products (3ma−ra) in good yields, including those
with trimethylsilyl attached to the carbon−carbon triple bond.
Next, we turned our attention to varying the functionality at
the propargylic position. Along with phenyl and anisyl
substituents, electron-rich heteroaromatics such as thiophene
attached to the carbinol center performed well under the
reaction conditions (3va, 80% yield). Unfortunately, substrates
with simple alkyl substituents were not tolerated in the present
reaction. Finally, we examined the scope with respect to the
nitrile-containing azoalkanes. A series of commercially available
nitrile-containing azoalkanes, such as 1,1′-azobis(cyclohexane-
1-carbonitrile) 2b and 2,2′-azobis(2-methylbutyronitrile) 2c,
reacted well with 1a to generate the desired products in 66−
95% yields (3ab−ac).
However, when the primary propargylic alcohol 4a was
subjected to the previously described reaction conditions, the
allenic amide 5aa was obtained in 30% yield under the
standard reaction conditions. After evaluating various reaction
parameters (see the Supporting Information), we found that
the best yield could be obtained by using AgOTf as the
effective catalyst with dichloromethane (DCM) as the
d.r. was determined by H NMR of the crude reaction mixture.
find that a range of primary propargylic alcohols with both
aromatics and aliphatic substituents attached to the carbon−
carbon triple bond could be utilized to react with AIBN. 3-
Phenyl-2-propyn-1-ol derivatives bearing electron-donating
groups at the para and meta positions of the aromatic ring
were tolerated, providing the corresponding products in 67 and
69% yields (5ba and 5ca), respectively. The substrate 4d with
a methyl group at the ortho position of the aromatic ring
afforded the allenic amide 5da in a rather lower yield. Electron-
withdrawing groups, such as −CF3, −COMe, and −CO2Me
(5ea−ga), were also well-tolerated under this protocol. (The
structure of 5ga was confirmed by X-ray analysis.) In addition
to phenyl-substituted propargylic alcohols, naphthyl-substi-
tuted propargylic alcohol was also compatible with this new
reaction, generating the desired product 5ha in 62% isolated
yield. The aliphatic propargylic alcohols with long chains or
functional groups converted to 5ia−ka in 16−51% yields. In
addition, the simple prop-2-yn-1-ol could also give the
corresponding allenic amide 5la in moderate yield. Further-
more, along with AIBN, other nitrile-containing azoalkanes
C
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could be utilized for this reaction, giving the desired products
in good yields (5ab and 5ac).
To explore the synthetic potential of the present reaction,
several transformations of the obtained amide 3aa and 5aa
were conducted (Scheme 4). The treatment of 3aa with
deuterated 4-nitrophenylboronic acid was introduced into the
reaction system as the cocatalyst, no deuterated product was
observed. In contrast, when the deuterated propargylic alcohol
D-1a was subjected to the standard reaction conditions, the
deuterated product was obtained in 80% yield, in which 100%
D was observed at the propargylic position (Scheme 5c,d).
These results indicated that the 1,3-hydrogen transfer occurred
after the rearrangement process. When the cyclopropane-
containing 1w was subjected to the standard reaction
conditions, the homologous product 3wa was isolated in
50% yield (Scheme 5e). This result is consistent with a radical
not being involved in the subsequent rearrangement process.
The dimethyl-N-(2-cyano-2-propyl)-ketenimine (DKI) could
react with 1a under the standard reaction conditions to afford
the desired product 3aa in 81% yield, whereas no reaction took
place in the absence of copper catalysts (Scheme 5f). These
results suggested the plausible intermediacy of ketenimine in
the catalytic cycle.
Scheme 4. Derivatization of Products
On the basis of the present results and previous reports,8 a
possible reaction pathway of this novel reaction was proposed
(Figure 1). First, the thermal decomposition of AIBN could
H2SO4 and CH3COOH furnished the dihydropyridin-2(3H)-
one 6 in 82% yield. The nitrile functionality could also be
easily transformed into the corresponding ester in the presence
of H2SO4 and MeOH. Furthermore, the iodine-induced
electrophilic cyclization by the reaction of 3aa or its ester 7
with I2 in the presence of NaHCO3 afforded the corresponding
pyrrolidinone derivatives 8 and 9 in 85−88% yields.11 Under
the same reaction conditions, the allenic amide 5aa could be
also transformed into the corresponding iodine-containing
dihydro-2H-pyrrol-2-one 10 and 12 in excellent yields.
To get some insights into the mechanism of the present
reaction, several control experiments were carried out. No
desired reaction took place when 2,2,6,6-tetramethylpiperidi-
nooxy (TEMPO) was introduced into the reaction system,
suggesting the involvement of a radical intermediate in this
transformation (Scheme 5a). Then, when 18O-1a was treated
with AIBN under the standard reaction conditions, the 18O-
labeled product was observed (Scheme 5b), which demon-
strated that the oxygen atom of carbonyl in the corresponding
amide originated from the propargylic alcohol. Next, when the
Figure 1. Plausible reaction mechanism.
generate isobutyronitrile radical I, which was then coordinated
with the metal to form radical−metal complex A. The metal-
ligated radical would be more electrophilic by enhancing the
captodative effect to catch the N-centered radical II to form
ketenimine complex B. Nucleophilic addition of the prop-
argylic alcohol to ketenimine complex B led to the formation
of intermediate C, which then underwent Claisen rearrange-
ment to afford allene intermediate D and regenerated the
catalyst. Finally, isomerization of intermediate D afforded the
desired product because the attached aromatic group (Ar) may
stabilize the transition state for the 1,3-hydogen transfer.
In summary, we have developed an unprecedented copper-
and silver-catalyzed radical coupling/nucleophilic addition/
Claisen rearrangement cascade reaction between propargylic
alcohols and nitrile-containing azoalkanes. The method
delivers a series of highly functionalized unsaturated amides
that provides a versatile platform for further derivatizations.
The coordination effect of the metal not only plays a key role
in promoting the Claisen rearrangement but also facilitates the
selective radical−radical cross-coupling to enable the azoal-
kanes as latent ketenimines. Further studies on the transition-
Scheme 5. Mechanistic Studies
D
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2015, 54, 241. (d) Zheng, J.; Lin, L.; Dai, L.; Tang, Q.; Liu, X.; Feng,
X. Angew. Chem., Int. Ed. 2017, 56, 13107. (e) Nishimoto, Y.;
Nishimura, T.; Yasuda, M. Chem. - Eur. J. 2015, 21, 18301.
(f) Yoshimura, F.; Saito, H.; Abe, T.; Tanino, K. Synlett 2017, 28,
1816.
(4) (a) Hammond, G. S.; Sen, J. N.; Boozer, C. E. J. Am. Chem. Soc.
1955, 77, 3244. (b) Hammond, G. S.; Lucas, G. B. J. Am. Chem. Soc.
1955, 77, 3249. (c) Erben, M. T.; Bywater, S. J. Am. Chem. Soc. 1955,
77, 3710. (d) Erben, M. T.; Bywater, S. J. Am. Chem. Soc. 1955, 77,
3712. (e) Van Hook, J. P.; Tobolsky, A. V. J. Am. Chem. Soc. 1958, 80,
779. (f) Smith, P.; Rosenberg, A. M. J. Am. Chem. Soc. 1959, 81, 2037.
(g) Hammond, G. S.; Trapp, O. D.; Keys, R. T.; Neff, D. L. J. Am.
Chem. Soc. 1959, 81, 4878. (h) Smith, P.; Carbone, S. J. Am. Chem.
Soc. 1959, 81, 6174. (i) Jaffe, A. B.; Skinner, K. J.; Mcbride, J. M. J.
Am. Chem. Soc. 1972, 94, 8510.
metal-catalyzed radical cascade reactions with this new strategy
are in progress and will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02306.
Experimental procedures and characterization data
(PDF)
Accession Codes
CCDC 2011261 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.
̌
(5) (a) Talat-Erben, M.; Isfendiyaroglu, A. N. Can. J. Chem. 1959,
37, 1165. (b) Hammond, G. S.; Wu, C.-H. S.; Trapp, O. D.;
Warkentin, J.; Keys, R. T. J. Am. Chem. Soc. 1960, 82, 5394.
(6) (a) Xie, Y.; Guo, S.; Wu, L.; Xia, C.; Huang, H. Angew. Chem.,
Int. Ed. 2015, 54, 5900. (b) Gao, B.; Xie, Y.; Shen, Z.; Yang, L.;
Huang, H. Org. Lett. 2015, 17, 4968. (c) Gao, B.; Xie, Y.; Yang, L.;
Huang, H. Org. Biomol. Chem. 2016, 14, 2399. (d) Zhang, X.; Huang,
H. Org. Lett. 2018, 20, 4998.
AUTHOR INFORMATION
Corresponding Author
■
(7) (a) Fischer, H. J. Am. Chem. Soc. 1986, 108, 3925. (b) Fischer,
H. Chem. Rev. 2001, 101, 3581. (c) Studer, A. Chem. - Eur. J. 2001, 7,
1159. (d) Focsaneanu, K.-S.; Scaiano, J. C. Helv. Chim. Acta 2006, 89,
2473. (e) Leifert, D.; Studer, A. Angew. Chem., Int. Ed. 2020, 59, 74.
(8) For reviews on the catalysis of Claisen rearrangement, see:
(a) Lutz, R. P. Chem. Rev. 1984, 84, 205. (b) Ito, H.; Taguchi, T.
Chem. Soc. Rev. 1999, 28, 43. (c) Hiersemann, M.; Abraham, L. Eur. J.
Org. Chem. 2002, 2002, 1461. (d) Zhu, Y.; Sun, L.; Lu, P.; Wang, Y.
ACS Catal. 2014, 4, 1911.
Hanmin Huang − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei 230026,
P. R. China; Center for Excellence in Molecular Synthesis of
CAS, Hefei 230026, P. R. China; orcid.org/0000-0002-
0108-6542; Email: hanmin@ustc.edu.cn
Authors
(9) (a) Wang, R.; Bao, W. RSC Adv. 2015, 5, 57469. (b) Usman, M.;
Ren, Z.-H.; Wang, Y.-Y.; Guan, Z.-H. RSC Adv. 2016, 6, 107542.
(c) Irudayanathan, F. M.; Lee, S. Org. Lett. 2017, 19, 2318.
(d) Zheng, C.; Lu, F.; Lu, H.; Xin, J.; Deng, Y.; Yang, D.; Wang,
S.; Huang, Z.; Gao, M.; Lei, A. Chem. Commun. 2018, 54, 5574.
(e) Li, Y.; Chang, Y.; Li, Y.; Cao, C.; Yang, J.; Wang, B.; Liang, D.
Adv. Synth. Catal. 2018, 360, 2488. (f) Kang, Q.-Q.; Liu, Y.; Huang,
X.-J.; Li, Q.; Wei, W.-T. Eur. J. Org. Chem. 2019, 2019, 7673. (g) Yu,
J.; Sheng, H.-X.; Wang, S.-W.; Xu, Z.-H.; Tang, S.; Chen, S.-L. Chem.
Commun. 2019, 55, 4578. (h) Cao, Z.; Nie, Z.; Yang, T.; Su, M.; Li,
H.; Luo, W.; Liu, Q.; Guo, C.-C. J. Org. Chem. 2020, 85, 3287.
(10) For selected reviews on silver-catalyzed reactions, see:
(a) Naodovic, M.; Yamamoto, H. Chem. Rev. 2008, 108, 3132.
(b) Weibel, J.-M.; Blanc, A.; Pale, P. Chem. Rev. 2008, 108, 3149.
(c) Pellissier, H. Chem. Rev. 2016, 116, 14868. (d) Zheng, Q.-Z.; Jiao,
N. Chem. Soc. Rev. 2016, 45, 4590. (e) Sekine, K.; Yamada, T. Chem.
Soc. Rev. 2016, 45, 4524. (f) Li, J.-H.; Song, R.-J.; Xiao, Y.-T.; Wu, Y.-
C.; Yang, Y.-Z. ChemCatChem 2020, DOI: 10.1002/cctc.202000900.
(11) Yao, T.; Larock, R. C. J. Org. Chem. 2005, 70, 1432.
Xuyang Yan − Hefei National Laboratory for Physical Sciences
at the Microscale and Department of Chemistry, University of
Science and Technology of China, Hefei 230026, P. R. China
Hongchi Liu − Hefei National Laboratory for Physical Sciences
at the Microscale and Department of Chemistry, University of
Science and Technology of China, Hefei 230026, P. R. China
Shenquan Wei − Hefei National Laboratory for Physical
Sciences at the Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei 230026,
P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02306
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Natural Science
Foundation of China (21925111, 21672199, 21790333, and
21702197).
REFERENCES
■
(1) For selected reviews on ketenimines, see: (a) Lu, P.; Wang, Y. G.
Synlett 2010, 2010, 165. (b) Lu, P.; Wang, Y. G. Chem. Soc. Rev. 2012,
41, 5687. (c) Alajarin, M.; Marin-Luna, M.; Vidal, A. Eur. J. Org.
Chem. 2012, 2012, 5637. (d) Allen, A. D.; Tidwell, T. T. Chem. Rev.
2013, 113, 7287. (e) Dodd, R. H.; Cariou, K. Chem. - Eur. J. 2018, 24,
2297.
(2) Denmark, S. E.; Wilson, T. W. Angew. Chem., Int. Ed. 2012, 51,
9980.
(3) For selected examples, see: (a) Denmark, S. E.; Wilson, T. W.
Nat. Chem. 2010, 2, 937. (b) Zhao, J.; Liu, X.; Luo, W.; Xie, M.; Lin,
L.; Feng, X. Angew. Chem., Int. Ed. 2013, 52, 3473. (c) Zhao, J.; Fang,
B.; Luo, W.; Hao, X.; Liu, X.; Lin, L.; Feng, X. Angew. Chem., Int. Ed.
E
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