Ruthenium diacetate-catalysed oxidative alkenylation of C-H bonds in air: Synthesis of alkenyl N-arylpyrazoles

Article abstract of DOI:10.1039/c1gc15875a

Ru(OAc)₂(p-cymene) catalyses the directed dehydrogenative alkenylation of N-aryl pyrazoles by styrene and alkyl acrylates in the presence of a catalytic or stoichiometric amount of Cu(OAc)₂·H ₂O in air; the acetic acid solvent plays a key role. With arene electron-donating groups, ruthenium-catalysed ortho-dialkenylation with alkyl acrylates can be obtained. A new method to generate the oxidative homocoupling of N-phenylpyrazole is provided with the Ru(OAc)₂(p-cymene) catalyst. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1gc15875a

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Ruthenium diacetate-catalysed oxidative alkenylation of C–H bonds in air:
synthesis of alkenyl N-arylpyrazoles†
Percia B. Arockiam, Cedric Fischmeister, Christian Bruneau* and Pierre H. Dixneuf*
Received 19th July 2011, Accepted 8th August 2011
DOI: 10.1039/c1gc15875a
Ru(OAc)2(p-cymene) catalyses the directed dehydrogena-
tive alkenylation of N-aryl pyrazoles by styrene and alkyl
acrylates in the presence of a catalytic or stoichiometric
amount of Cu(OAc)2·H2O in air; the acetic acid solvent
plays a key role. With arene electron-donating groups,
ruthenium-catalysed ortho-dialkenylation with alkyl acry-
lates can be obtained. A new method to generate the
oxidative homocoupling of N-phenylpyrazole is provided
with the Ru(OAc)2(p-cymene) catalyst.
ible with industry developments. Comparatively less expensive
ruthenium(II) catalysts for the oxidative alkenylation of arenes
have just started to be explored.8–13 Whereas the ruthenium(II)-
catalysed oxidative Heck reaction with boronic acids has been
described by Brown et al.,8 Milstein et al. have pioneered the
ruthenium-catalysed direct alkenylation of benzene by an alkyl
acrylate.9 Miura et al. have recently described the alkenylation
of thiophene-2-carboxylic acids with acrylates in the presence
of a [RuCl2(p-cymene)]2 catalyst,10 showing that the carboxylic
group directs the regioselective alkenylation at the neighbouring
C–H bond without decarboxylation, as occurred with palladium
catalysts. Related work on the synthesis of alkenyl arene
derivatives has involved the formal insertion of alkynes into
C–H bonds promoted by Ru(III) or Ru(II) catalysts, such as the
alkenylation of phenylpyridine with RuCl3·H2O in the presence
of benzoic peroxide.11 The annulation of alkynes with arylamides
in the presence of a [RuCl2(p-cymene)]2 catalyst to generate
a variety of isoquinolones via both aryl C–H and amide N–
H functionalisation has recently been reported by Ackermann
et al.,12a as the catalytic addition of acrylamides to alkynes
for the formation of 2-pyridones.12b Moreover, Ackermann et
al. have recently reported the dehydrogenative alkenylation
of benzoic acids with acrylates in the presence of [RuCl2(p-
cymene)]2 with 2 equiv. of Cu(OAc)2·H2O in de-aerated water,
leading to annulated lactones.13
The transition metal-catalysed Heck reaction has proved to be
one of the most useful methods for the synthesis of unsaturated
molecules and conjugated materials via cross-coupling and C–
C bond formation.1 A potentially more efficient and greener
process to reach the same family of unsaturated compounds
consists of the catalytic oxidative alkenylation of aromatic C–
H bonds, as pioneered by Fujiwara and Moritani.2 It offers an
atom-economical strategy to directly functionalise arenes with
olefins, thus avoiding the prehalogenation of the substrate, as in
the Heck reaction (eqn (1)).
(1)
In the course of our study on the regioselective functionali-
sation of aromatic C–H bonds with carboxylato-ruthenium(II)
catalysts,14,15 we have now found that the selective catalytic
ortho-alkenylation of N-arylpyrazoles, which constitute an
important class of heterocycles, can be easily performed. We
now report that Ru(OAc)2(p-cymene) catalyses the dehydro-
genative alkenylation of N-arylpyrazoles by styrene and alkyl
acrylates in acetic acid and air in the presence of a catalytic
or stoichiometric amount of Cu(OAc)2·H2O. We also report
a complementary method to perform the oxidative homocou-
pling of N-phenylpyrazole promoted by a Ru(OAc)2(p-cymene)
catalyst.
Palladium catalysts have already been used to promote
this useful transformation, and the regioselectivity is mainly
controlled by functional directing groups linked to the arenes or
heterocycles.3 In parallel, rhodium catalysts4–7 have also allowed
the oxidative coupling of arene and heteroarene C–H bonds
with functional alkenes for a variety of directing groups, such as
amides,5 imines6 or carboxylates.7 The development of this useful
synthetic method is expected to depend on the use of stable,
efficient and inexpensive metal catalysts, allowing the use of
complementary directing functional groups in solvents compat-
UMR 6226-CNRS-Universite´ de Rennes 1, Institut Sciences Chimiques,
Catalyse et Organome´talliques, Campus de Beaulieu, 35042, Rennes
cedex, France. E-mail: pierre.dixneuf@univ-rennes1.fr
† Electronic supplementary information (ESI) available: Experi-
mental procedures, spectroscopic and analytical data. See DOI:
10.1039/c1gc15875a
Results and discussion
The alkenylation with styrene of N-phenylpyrazole (1a) was
first attempted with Ru(OAc)2(p-cymene)14b,f in the presence
of an oxidant. As the C–H bond activation of functional
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Table 1 Influence of the oxidant on the alkenylation of 1a with styrenea
(3)
Some examples of the ruthenium-catalysed dehydrogenative
homocoupling of arenes functionalized by a heterocycle have
already been reported15–19 using a sacrificial electrophile such
as methallyl acetate16 or aryl chloride,18 or an oxidant such as
FeCl3.17 The above results show that the Ru(OAc)2(p-cymene)
catalyst in AcOH with 20 mol% of Cu(OAc)2·H2O as the oxidant
provides an excellent way to conduct the homocoupling of a
functional arene.19 It is noteworthy that the alkenylation of 1a
with styrene is faster than the homocoupling of 1a, as under
almost the same conditions, the presence of 2.5 equiv. of styrene
inhibited the formation of 4a to the benefit of 3a formation
(Table 1, entry 6).
With the optimal conditions for the preparation of 3a in
hand (Table 1, entry 6), the alkenylation o◦f 1a was attempted
with a variety of functional alkenes at 100 C, but varying the
reaction time and the ratio of Cu(OAc)2·H2O (20 mol% or 1
equiv.) (Table 2). For the less-reactive electrophilic acrylates 2b–
2e, the complete conversion could not be reached after 30 h
with 20 mol% of Cu(OAc)2·H2O, leading to the high production
of 4a. In this case, 1 equiv. of Cu(OAc)2·H2O was required to
obtain full conversion with a high selectivity (>98%) for 3b–3d,
which were isolated in 65–77% yields (Table 2, entries 1–4). In
contrast, methyl methacrylate (2f) was inactive and the reaction
led to the formation of 4a as the only product, most likely due to
the double substitution on one carbon atom (Table 2, entry 5).
Alkyl-substituted styrenes 2g and 2h were less reactive than
styrene itself and lead to full conversion but with moderate
3/4 selectivity (Table 2, entries 6 and 7). It is noteworthy that
acrylamide (2i) led to an important ratio of the alkenylation
(Table 2, entry 8), whereas N,N-dimethyl-substituted acrylamide
2j inhibited the alkenylation to the benefit of derivative 4a
formation (Table 2, entry 9). Thus, each time the alkene is not
reactive the homocoupling reaction takes place (Table 2, entries
5 and 9).
Entry
Oxidant/mmol
Conversion (%)b (3a/4a)c
1
2
3
4
5
6
Benzoquinone (1.1)
Cu(OTf)2 (0.1 = 20 mol%)
CuBr2 (0.1)
CuCl2. 2H2O (0.1)
Cu(OAc)2 (0.1)
72 (83/17)
70 (13/87)
84 (73/27)
88 (5/95)
98 (56/44)
Cu(OAc)2·H2O (0.1 = 20 mol%) 98 (93/7)
7d
8e
9f
10g
Cu(OAc)2·H2O (0.1)
—
51 (3a, 4a + products)
72 (88/12)
—
Cu(OAc)2·H2O (1.1)
Cu(OAc)2·H2O (0.1)
75 (30/70)
a Reaction conditions: 0.5 mmol of phenylpyrazole,
5 mol% of
Ru(OAc)2(p-cymene), oxidant, 10 mL of tetradecane as an internal
standard for GC, 1.25 mmol of styrene in 1 mL of AcOH. b Conversion
determined by gas chromatography. c Ratio 3a/4a determined by
gas chromatography. d Reaction performed under argon. e Without
Cu(OAc)2·H2O. f Without Ru(OAc)2(p-cymene). g At 80 ◦C.
arenes promoted by Ru(OAc)2(p-cymene) has recently been
shown to involve an autocatalytic process, in which the formed
acetic acid plays the role of a cocatalyst,15 the use of an
acetic acid-containing solvent was studied (see ESI, Table
S1†) with various oxidants in air at 100 ◦C. The reaction
led to the formation of the desired ortho-monoalkenylated
product, 3a, and the N-phenylpyrazole dehydrogenative ho-
mocoupling derivative 4a, of which the ratio strongly de-
pended on the oxidant’s nature and conditions (eqn (2) and
Table 1).
Whereas polar organic solvents NMP and DMF or DEC and
toluene did not allow the reaction to take place, the addition of
0.1 mL of AcOH to various solvents increased the conversion,
and finally acetic acid appeared to be by far the best solvent (ESI,
Table S1†). The study showed that Cu(OAc)2·H2O, especially
when used in a catalytic amount of 20 mol% (Table 1, entry 6),
was preferable to the use of benzoquinone, Cu(OTf)2 or CuBr2
(Table 1, entries 1–3). In contrast, CuCl2·2H2O strongly favoured
oxidative homocoupling product 4a formation (Table 1, entry 4).
More importantly, a hydrated copper(II) salt was crucial to reach
a high selectivity of 3a (Table 1, entries 5 and 6). The determining
role of the Ru(OAc)2(p-cymene) catalyst was shown, as in its
absence, no catalytic alkenylation or homocoupling reaction
took place, whereas without Cu(OAc)2·H2O, it led to only a 72%
conversion (Table 1, entries 8 and 9). The absence of air and a
decrease of temperature to 80 ◦C disfavoured the formation of
3a (Table 1, entries 7 and 10).
The alkenylation of substituted N-phenylpyrazoles 1b–1f
with alkyl acrylates was investigated (eqn (5)) to evaluate the
aryl substituent influence on both the rate and on mono and
dialkenylation. The results with Cu(OAc)2·H2O (0.1 and 0.5
mmol) are displayed in Table 3.
The reaction of sterically-hindered 6-methoxyphenylpyrazole
(1b) with 2.5 equiv. of alkyl acrylates lead to E-monoalkenylated
derivatives 5a–c, isolated in 43–62% yield. These reactions
were slower than those with parent compound 1a, showing
that the electron-donating ortho-methoxy group slows down
the alkenylation; however, under these conditions, the cor-
responding homocoupling product was never observed. 4-
Methoxyphenylpyrazole (1c) under similar conditions but with 1
equiv. ofCu(OAc)2·H2O afforded the correspondingortho-mono
alkenylated pyrazoles 6a and 7a, but with a significant amount
of dialkenylated pyrazoles 6b and 7b (Table 3). Products 7a
and 7b lead to a blue fluorescence in solution when irradiated at
365 nm, showing the potential of this C–H bond alkenylation for
the production of molecular materials for optical applications.
The partial and sometimes predominant formation of homo-
coupling product 4a (Table 1, entries 2 and 4) led us to improve its
formation from 1a with Ru(OAc)2(p-cymene). Actually, in acetic
acid as the solvent in the presence of 20 mol% of Cu(OAc)2·H2O
at 100 ◦C in air without styrene, the complete transformation of
1a was observed after 5 h, and 4a was quantitatively formed and
isolated in 64% yield (eqn (3)).
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Table 2 Alkenylation of phenylpyrazole 1a with various alkenesa
Cu(OAc)2·H2O 0.1 mmol
Conv. (%)
Cu(OAc)2·H2O 0.5 mmol
Conv. (%)
No.
1
2
T/h
(3/4) (Y%)b
(3/4) (Y%)b
30
78
85/15 (48)
100
100
100
98/2
3b (72)
2
3
24
30
78
70
63/37 (44)
41/59 (22)
99/1
3c (77)
99/1
3d (65)c
4
5
30
36
86
76/24 (31)
0/100
100
99/1
3e (66)
100
6
7
8
36
36
36
100
100
72
58/42
37/63
88/12
100
95
56/44
3g (25)
39/61
100
3i (71)
9
36
80
0/100
82
0/100
a Reaction conditions as in Table 1, except for the reaction time and oxidant amount in mmol. b Ratio determined by GC and (isolated yield (%)).
c Additional 15% of divinylated product was also obtained.
By way of contrast, the reaction of pyrazoles containing
an electron-withdrawing group, such as 4-chloro- and 4-
Conclusion
The above results show the first examples of the oxidative
alkenylation of N-phenylpyrazoles performed by the cooperative
action of a ruthenium(II) catalyst and acetate ligands, and in the
presence of Cu(OAc)2·H2O in air. The profitable action of the
solvent acetic acid is especially shown, suggesting its action in
C–H bond cleavage as an autocatalytic process. Alkenylation
with electrophilic acrylates requires a stoichiometric amount of
oxidant, whereas that with styrene is easier and is performed with
a catalytic amount of Cu(OAc)2·H2O in acetic acid. Conditions
have been found for the dehydrogenative homocoupling of N-
phenylpyrazole. The exploration of this direct Ru(O2CR)2Ln-
catalysed alkenylation by alkenes of functional arenes and of
the relevant mechanism are under investigation.
cyano-phenylpyrazole 1d and 1e, lead to the corresponding
monoalkenylated products 8 and 9, but in moderate yield.
The mechanism of formation of alkenyl functional arenes
directed by the pyrazole function with a ruthenium(II) catalyst
in an acetic acid medium is still not clear. However, it may
involve these plausible successive steps: (i) ortho-C–H bond
deprotonation by the assistance of the acetate/acetic acid
in an autocatalytic process to generate the metallacycle,14–15
(ii) insertion of the alkene into the C–Ru(II) bond9 of the
resulting metallacycle, (iii) b-elimination with formation of a
Ru(H)(OAc)Ln species and (iv) regeneration of an Ru(OAc)2Ln
catalyst by the action of the Cu2+ salt, oxygen and the acetic
acid.
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Table 3 Alkenylation of substituted aryl pyrazoles with various
4 (a) S. Rakshit, C. Grohmann, T. Besset and F. Glorius, J. Am. Chem.
Soc., 2011, 133, 2350; (b) M. P. Huestis, L. Chan, D. R. Stuart and
K. Fagnou, Angew. Chem., Int. Ed., 2011, 50, 1338; (c) A.-S. Tsai, M.
Brasse, R. G. Bergman and J. A. Ellman, Org. Lett., 2011, 13, 540;
(d) T. K. Hyster and T. Rovis, J. Am. Chem. Soc., 2010, 132, 10565;
(e) S. Rakshit, F. W. Patureau and F. Glorius, J. Am. Chem. Soc.,
2010, 132, 9585; (f) T.-J. Gong, B. Xiao, Z.-J. Liu, J. Wan, J. Xu, D.-F.
Luo, Y. Fu and L. Liu, Org. Lett., 2011, 13, 3235; (g) S. Mochida, K.
Hirano, T. Satoh and M. Miura, J. Org. Chem., 2011, 76, 3024; (h) F.
Wang, G. Song and X. Li, Org. Lett., 2010, 5426; (i) N. Umeda, K.
Hirano, T. Satoh and M. Miura, J. Org. Chem., 2009, 74, 7094; (j) K.
Ueura, T. Satoh and M. Miura, J. Org. Chem., 2007, 72, 5362; (k) N.
Umeda, K. Hirano, T. Satoh and M. Miura, J. Org. Chem., 2009, 74,
7094; (l) For a review, see: T. Satoh and M. Miura, Chem.–Eur. J.,
2010, 16, 11212.
acrylatesa,b
5 (a) F. W. Patureau, T. Besset and F. Glorius, Angew. Chem., Int. Ed.,
2011, 50, 1064; (b) D. R. Stuart, M. B. Laperle, K. M. N. Burgess and
K. Fagnou, J. Am. Chem. Soc., 2008, 130, 16474; (c) N. Guimond,
C. Gouliaras and K. Fagnou, J. Am. Chem. Soc., 2010, 132, 6908.
6 (a) N. Guimond and K. Fagnou, J. Am. Chem. Soc., 2009, 131,
12050; (b) T. Fukutani, N. Umeda, K. Hirano, T. Satoh and M.
Miura, Chem. Commun., 2009, 5141; (c) A. S. Tsai, M. Brasse, R. G.
Bergman and J. A. Ellman, Org. Lett., 2011, 13, 540.
7 (a) M. Shimizu, K. Hirano, T. Satoh and M. Miura, J. Org. Chem.,
2009, 74, 3478; (b) S. Mochida, K. Hirano, T. Satoh and M. Miura,
Org. Lett., 2010, 12, 5776.
8 (a) E. J. Farrington, C. F. J. Barnard, E. Rowsell and J. M. Brown,
Adv. Synth. Catal., 2005, 347, 185; (b) E. J. Farrington, J. M. Brown,
C. F. J. Barnard and E. Rowsell, Angew. Chem., Int. Ed., 2002, 41,
169.
9 H. Weissman, X. Song and D. Milstein, J. Am. Chem. Soc., 2001, 123,
337. The reaction is performed with RuCl3·H2O or [RuCl2(CO)3]2
catalysts under atmospheres of both CO and O2 at 180 ◦C for 48 h.
10 T. Ueyama, S. Mochida, T. Fukutani, K. Hirano, T. Satoh and M.
Miura, Org. Lett., 2011, 13, 706. The reaction is performed with 2
equiv. of Cu(OAc)2·H2O under an inert atmosphere in DMF.
11 K. Cheng, B. Yao, J. Zhao and Y. Zhang, Org. Lett., 2008, 10, 5309.
12 (a) L. Ackermann, A. V. Lygin and N. Hofmann, Angew. Chem.,
Int. Ed., 2011, 50, 6379; (b) L. Ackermann, A. V. Lygin and N.
Hofmann, Org. Lett., 2011, 13, 3278. These reactions require 2
equiv. of Cu(OAc)2·H2O in t-AmOH as the solvent under an inert
atmosphere.
a Reaction conditions as in Table 1, except for time and the amount
of oxidant. b Isolated yield. c Cu(OAc)2·H2O 0.1 mmol (20 mol%).
d Cu(OAc)2·H2O 0.5 mmol (1 equiv.).
Acknowledgements
The authors are grateful to the CNRS, the French Ministry for
Research, the Institut Universitaire de France (P. H. D.), and
the ANR program 09-Blanc-0101-01 for support and for a PhD
grant to P. B. A.
13 L. Ackermann and J. Pospech, Org. Lett., 2011, 13, 4153.
14 (a) F. Pozgan and P. H. Dixneuf, Adv. Synth. Catal., 2009, 351,
1737; (b) P. Arockiam, V. Poirier, C. Fischmeister, C. Bruneau and
P. H. Dixneuf, Green Chem., 2009, 11, 1871; (c) P. B. Arockiam,
C. Fischmeister, C. Bruneau and P. H. Dixneuf, Angew. Chem.,
¨
Int. Ed., 2010, 49, 6629; (d) I. Ozdemir, S. Demir, B. C¸ etinkaya,
C. Gourlaouen, F. Maseras, C. Bruneau and P. H. Dixneuf, J.
Am. Chem. Soc., 2008, 130, 1156; (e) W. Li, P. B. Arockiam, C.
Fischmeister, C. Bruneau and P. H. Dixneuf, Green Chem., 2011,
13, DOI: 10.1039/c1gc15642j; (f) The heating of [RuCl2(p-cymene)]2
with 2 equiv. of KOAc per ruthenium atom leads to the Ru(OAc)2(p-
cymene) complex.14b Details to be published.
Notes and references
1 (a) A. B. Dounay and L. E. Overman, Chem. Rev., 2003, 103, 2945;
(b) I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100,
3009; (c) A. de Meijere and F. E. Meyer, Angew. Chem., Int. Ed.
Engl., 1994, 34, 2379; (d) R. F. Heck, Org. React., 2005; (e) R. F.
Heck, Acc. Chem. Res., 1979, 12, 146.
2 (a) Y. Fujiwara, I. Moritani and M. Matsuda, Tetrahedron, 1968, 24,
4819; (b) H. Zhang, E. M. Ferreira and B. M. Stoltz, Angew. Chem.,
Int. Ed., 2004, 43, 6144 and references therein; (c) C. Jia, T. Kitamura
and Y. Fujiwara, Acc. Chem. Res., 2001, 34, 633; (d) E. M. Beccalli,
G. Broggini, M. Martinelli and S. Sottocornola, Chem. Rev., 2007,
107, 5318.
3 (a) M. Yu, Z. Liang, Y. Wang and Y. Zhang, J. Org. Chem., 2011,
76, 4987; (b) K. J. Stowers, K. C. Fortner and M. S. Sanford, J. Am.
Chem. Soc., 2011, 133, 6541; (c) D.-H. Wang, K. M. Engle, B.-F. Shi
and J.-Q. Yu, Science, 2010, 327, 315; (d) Y. Lu, D.-H Wang, K. M.
Engle and J.-Q. Yu, J. Am. Chem. Soc., 2010, 132, 5916; (e) M. Wasa,
K. M. Engle and J.-Q. Yu, J. Am. Chem. Soc., 2010, 132, 3680; (f) K.
M. Engle, D.-H. Wang and J.-Q. Yu, Angew. Chem., Int. Ed., 2010,
49, 6169; (g) B. F. Shi, Y.-H. Zhang, J. K. Lam, D.-H. Wang and J.-Q.
Yu, J. Am. Chem. Soc., 2010, 132, 460; (h) G. Deng, L. Zhao and
C.-J. Li, Angew. Chem., Int. Ed., 2008, 47, 6278; (i) G. Cai, Y. Fu, Y.
Li, X. Wan and Z. Shi, J. Am. Chem. Soc., 2007, 129, 7666; (j) A.
Maehara, H. Tsurugi, T. Satoh and M. Miura, Org. Lett., 2008, 10,
1159.
15 E. Ferrer-Flegeau, C. Bruneau, P. H. Dixneuf and A. Jutand, J. Am.
Chem. Soc., 2011, 133, 10161.
16 S. Oi, H. Sato, S. Sugawara and Y. Inoue, Org. Lett., 2008, 10, 1823.
17 X. Guo, G. Deng and C.-J. Li, Adv. Synth. Catal., 2009, 351, 2071.
18 L. Ackermann, P. Nova`k, R. Vicente, V. Pirovano and H. K.
Potukuchi, Synthesis, 2010, 2245.
19 Dehydrogenative homocoupling of functional arenes has already
been performed with several ruthenium(II) catalysts: (a) at 120 ◦C
for 20 h in xylene with a [RuCl2(COD)]n/PPh3 catalyst and K2CO3,
especially for the homocoupling of aryl-oxazolines by S. Oi et al.16;
(b) at 110 ◦C for 16 h in chlorobenzene with [RuCl2(p-cymene)]2
(2.5 mol%) and FeCl3 (80 mol%) in the absence of a base for the
homocoupling of 2-arylpyridines by C.-J. Li et al.17; (c) at 120 ◦C
for 20 h in toluene with a [RuCl2(p-cymene)]2 catalyst (2.5 mol%)
and MesCO2H (30 mol%), with K2CO3 and an arylchloride (2-
chlorotrifluoromethylbenzene), especially for the homocoupling of a
variety of 1,2,3-triazol-4-yl arenes by L. Ackermann et al.18; (d) Thus,
the present method is performed under milder conditions at 100 ◦
C
for 5 h in acetic acid in air with a Ru(OAc)2(p-cymene) catalyst in the
presence of 20 mol% of Cu(OAc)2·H2O for N-phenylpyrazole.
3078 | Green Chem., 2011, 13, 3075–3078
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