Rh2(OAc)4/CeCl3-catalyzed olefination of carbonylferrocenes with α-diazocarbonyl compounds: A convenient synthesis of alkenylferrocenes

Article abstract of DOI:10.1055/s-0031-1290612

A concise and efficient protocol for the synthesis of al-kenylferrocene derivatives based on the olefination of carbonylferrocenes with α-diazocarbonyl compounds using Rh₂(OAc)₄/CeCl ₃ as efficient catalysts was developed. The present method was applicable to many kinds of substituted carbonylferrocenes and α-diazocarbonyl compounds providing good to excellent yields of desired products. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290612

LETTER
943
Rh2(OAc)4/CeCl3-Catalyzed Olefination of Carbonylferrocenes with
a-Diazocarbonyl Compounds: A Convenient Synthesis of Alkenylferrocenes
Synthesis of
A
h
lkenylferroc
u
College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, P. R. of China
Fax +86(471)4992982; E-mail: bauoguol@sohu.com
Received 26 December 2011
dition reactions were successfully applied in the synthesis
Abstract: A concise and efficient protocol for the synthesis of al-
kenylferrocene derivatives based on the olefination of carbonylfer-
rocenes with a-diazocarbonyl compounds using Rh2(OAc)4/CeCl3
as efficient catalysts was developed. The present method was appli-
of ferrocenyl-based conjugated compounds.11 However,
all these approaches were encountered with high reaction
temperatures and the restricted availability of substrates.
cable to many kinds of substituted carbonylferrocenes and a-diazo- Considering the limited methods available for their prep-
carbonyl compounds providing good to excellent yields of desired
products.
aration, further development of synthetically useful meth-
odologies for alkenylferrocene derivatives is highly
Key words: carbonylferrocenes, diazo compounds, olefination,
alkenylferrocene
desirable. In this paper, we wish to report an efficient
method for the preparation of alkenylferrocene deriva-
tives based on the olefination of carbonylferrocenes with
a-diazocarbonyl compounds using Rh2(OAc)4/CeCl3 as
Ferrocene and its derivatives are a class of important efficient catalysts (Scheme 1).12
building blocks1 and widely used in many areas such as
asymmetric catalysis,2 nonlinear optics,3 material sci-
CO2Et
ence,4 and bioorganometallic chemistry.5 Among which,
synthesis of alkenylferrocenes has attracted considerable
attention due to their increasing applications in the asym-
metric catalysis2 and diversities of the double bonds in or-
ganic transformations.
CHO
+
Rh2(OAc)4, CeCl3
Ph3P, DCE
CO2Et
N2
Fe
Fe
H
1a
2a
3a
Scheme 1 Synthesis of alkenylferrocene derivative
Over the past decades, there have been some investiga-
tions on the development of synthetic methodologies of
alkenylferrocene derivatives.6 The most commonly ap-
plied synthesis of such compounds is the Wittig reaction
of ferrocenecarboxaldehyde with various phosphorous
ylides.7 However, the Wittig protocol often produces a
mixture of E/Z stereoisomers; the separation of which is
not always easy. It is noted that unlike the conventional
Wittig reaction, the Horner–Wadsworth–Emmons (HWE)
reaction unambiguously affords the E-isomers with mod-
erate to good yields.8 A different but interesting approach
is developed by Blakemore’s group, using ethyl (ben-
zothiazol-2-ylsulfonyl)acetate as olefination reagent,
which reacts with ferrocenecarboxaldehyde under mild
reaction conditions (DBU, CH2Cl2, r.t.) to give alkenyl-
ferrocene in good yield.9 Another successful method for
the synthesis of alkenylferrocenes is the dehydration of
the corresponding alcohols with suitable dehydration re-
agents.10 The major drawbacks of this method are that
they require preliminary preparation of ferrocenyl alco-
hols using moisture-sensitive lithium aluminum hydride
as reduction reagent as well as involving multistep manip-
ulations. Alternatively, transition-metal-catalyzed cross-
coupling reactions of iodoferrocene or iodovinylferrocene
with electron-deficient alkenes and the organometallic ad-
On the outset of this study, we employed ferrocenecar-
boxaldehyde (1a) and ethyl diazoacetate (EDA, 2a) as the
substrates (Table 1). After the initial experiment using
Rh2(OAc)4 as the catalyst in the presence of Ph3P in 1,2-
dichloroethane at room temperature, we obtained the ole-
fination product 3a in 50% yield with an excellent E ste-
reoselectivity (Table 1, entry 1). Furthermore, we were
delighted to find that the isolated yield of 3a enhanced to
64% when the reaction was carried out at 80 °C (Table 1,
entry 2). Encouraged by these results, we next carefully
examined the effect of various transition-metal catalysts
on this olefination reaction. For comparison, we first ex-
amined CuI, Cu(OAc)2·H2O and FeCl3 as the catalysts.
These transition-metal complexes have been proven to be
the efficient catalysts for diazo decomposition reactions.
However, in the reaction of ferrocenecarboxaldehyde (1a)
with EDA (2a), the expected product 3a was obtained
only with trace to moderate yields using these catalysts
under the similar conditions (Table 1, entries 3–5). Inter-
estingly, when Rh2(OAc)4 and CeCl3 were used as combi-
nation catalysts in this reaction, the olefination product 3a
was isolated with higher yield (Table 1, entry 6). Next we
also screened the commercial available SmCl3 as cocata-
lyst in this reaction; the result showed that only moderate
yield of 3a was obtained (Table 1, entry 7). Finally, when
we changed the cocatalyst loading of CeCl3 from 5 mol%
to 50 mol% (Table 1, entries 8–10), it was found that us-
SYNLETT 2012, 23, 943–947
x
x.
x
x.
2
0
1
2
Advanced online publication: 15.03.2012
DOI: 10.1055/s-0031-1290612; Art ID: W80711ST
© Georg Thieme Verlag Stuttgart · New York

944
S. Chen et al.
LETTER
Table 1 Transition-Metal-Catalyzed Olefination Reaction of Fer-
rocenecarboxaldehyde (1a) with Ethyl Diazoacetate (2a)a
ing 50 mol% of CeCl3 could give the best result under the
same conditions (Table 1, entry 10).
CO2Et
With the optimized reaction conditions in hand, the scope
of this transformation was investigated by using a variety
of carbonylferrocenes as the substrates (Table 2).13 Vari-
ous substitutions on the cyclopentadiene ring could be tol-
erated, and the reaction gave moderate to good yields of
the products 3a–f with excellent E stereoselectivities. It
was found that the yield was decreased along with the in-
fluence of steric effect on cyclopentadiene rings of the
substrates (Table 2, entries 2–4). Moreover, when 3-ferro-
cenylpropenal was used as the substrate in this reaction,
only 42% yield product 3e was obtained after workup
(Table 2, entry 5). However, when 3-chloro-3-ferrocenyl-
propenal (1f) was employed as the substrate in this reac-
tion, we were delighted to find that the reaction proceeded
smoothly under the present conditions providing a higher
yield and stereoselectivity of the desired product (Table 2,
entry 6).
CHO
N2
catalyst
Fe
+
Fe
Ph3P, DCE
H
CO2Et
1a
2a
3a
Entry Catalyst (mol%)
Temp
(°C)
Time
Yield
(%)b
(h)a
48
24
24
48
48
24
24
24
24
24
1
2
Rh2(OAc)4 (0.5)
Rh2(OAc)4 (0.5)
CuI (20)
25
80
80
80
80
80
80
80
80
80
50
64
60
trace
38
83
62
74
76
87
3
4
Cu(OAc)2·H2O (20)
FeCl3 (20)
5
6
Rh2(OAc)4 (0.5), CeCl3 (20)
Rh2(OAc)4 (0.5), SmCl3 (20)
Rh2(OAc)4 (0.5), CeCl3 (5)
Rh2(OAc)4 (0.5), CeCl3 (10)
Rh2(OAc)4 (0.5), CeCl3 (50)
7
Next, we examined the scope of diazo compounds, as
shown in Table 3. The reaction tolerates a relatively small
range of substituents and functional groups on the aryldi-
azoacetates (Table 3, entries 2–7). It was found that sub-
strates with phenyl ring bearing halogen and nitro group
generally gave moderate yields (Table 3, entries 2–5),
while substrates with electron-donating substituent on the
phenyl ring, such as 3,4-(MeO)2, only trace of olefination
product was detected (Table 3, entry 6). Moreover, it was
noted that the ester moiety of the diazo compounds did not
affect the reaction (Table 3, entry 7).
8
9
10
a Reactions were carried out with 0.3 mmol of 1a, 0.45 mmol of 2a,
and 0.33 mmol of Ph3P.
b Yield of isolated product after chromatography, calculated based on
the reacted 1a.
Table 2 Rh2(OAc)4/CeCl3-Catalyzed Reaction of Various Carbonylferrocenes with Ethyl Diazoacetate (2a)a
CO2Et
R1
CHO
+
Rh2(OAc)4,
CeCl3
R1
N2
Fe
Fe
Ph3P, DCE
80 °C
H
CO2Et
R2
R2
1a–f
2a
3a–f
Entry
1
Carbonylferrocene 1
Product 3
Yield (%)b
87
E/Zc
CO2Et
CHO
Fe
>20:1
Fe
1a
3a
CO2Et
CHO
Fe
2
3
48
33
>20:1
15:1
Fe
Et
Et
1b
3b
CO2Et
Et
CHO
Et
Et
Fe
Fe
Et
1c
3c
Synlett 2012, 23, 943–947
© Thieme Stuttgart · New York

LETTER
Synthesis of Alkenylferrocenes
945
Table 2 Rh2(OAc)4/CeCl3-Catalyzed Reaction of Various Carbonylferrocenes with Ethyl Diazoacetate (2a)a (continued)
CO2Et
R1
CHO
+
Rh2(OAc)4,
CeCl3
R1
N2
Fe
Fe
Ph3P, DCE
80 °C
H
CO2Et
R2
R2
1a–f
2a
3a–f
Entry
4
Carbonylferrocene 1
Product 3
Yield (%)b
28
E/Zc
CO2Et
n-Bu
CHO
n-Bu
n-Bu
Fe
15:1
Fe
n-Bu
1d
3d
3e
CHO
Fe
5
6
42
82
>20:1
>20:1
Fe
CO2Et
1e
CHO
Cl
Cl
Fe
Fe
CO2Et
1f
3f
a Reactions were carried out with 0.3 mmol of carbonylferrocenes, 0.45 mmol of 2a, and 0.33 mmol of Ph3P.
b Yield of isolated product after chromatography, calculated based on the reacted carbonylferrocenes 1a–f.
c The ratio of E- and Z-isomers was estimated by 1H NMR of the crude product.
Table 3 Rh2(OAc)4/CeCl3-Catalyzed Reaction of Ferrocenecarbox-
aldehyde (1a) with Various Diazoacetatesa
Table 4 Rh2(OAc)4/CeCl3-Catalyzed Reaction of General Alde-
hydes with Ethyl Diazoacetate (2a)a
N2
Rh2(OAc)4,
Rh2(OAc)4, CeCl3
CHO
N2
CO2Et
+
CeCl3
RCHO
5a–c
Entry
1
R
CO2R1
Fe
Ph3P, DCE
80 °C
+
Fe
H
CO2Et
Ph3P, DCE
80 °C
R2
CO2R1
2b–h
R2
2a
6a–c
1a
4a–g
5
R
6
Yield (%)b E/Zc
Entry
2
R1
Ph
R2
4
Yield E/Zc
5a
5b
5c
Ph
6a
6b
6c
91
89
81
>20:1
(%)b
2
4-ClC6H4
Pr
>20:1
>20:1
1
2
3
4
5
6
7
2b
2c
2d
2e
2f
Me
Me
Me
Me
Me
4a
4b
4c
4d
4e
4f
46
48
53
38
42
>20:1
3
2-ClC6H4
4-ClC6H4
4-BrC6H4
4-O2NC6H4
>20:1
10:1
5:1
a Reactions were carried out with 0.3 mmol of RCHO, 0.45 mmol of
2a, and 0.33 mmol of Ph3P.
b Yield of isolated product after chromatography.
c The ratio of E and Z isomers was estimated by 1H NMR of the crude
product.
>20:1
–
d
2g
2h
3,4-(MeO)2C6H3 Me
Ph allyl
–
Besides, it has been noted that this method is also applica-
ble to the general aldehydes RCHO (R = aromatic and al-
iphatic group), as shown in Table 4. It was found that
benzaldehyde, p-chlorobenzaldehyde, and butyraldehyde
were all suitable for this transformation, and the corre-
4g
48
>20:1
a Reactions were carried out with 0.3 mmol of 1a, 0.45 mmol of 2b–
h, and 0.33 mmol of Ph3P.
b Yield of isolated product after chromatography, calculated based on
the reacted ferrocenecarboxaldehyde (1a).
c The ratio of E- and Z-isomers was estimated by 1H NMR of the crude
product.
O
N2
Rh2(OAc)4, CeCl3
d Trace amount of product was detected by 1H NMR of the reaction
mixture.
R
+
Fe
no reactions
Ph3P, DCE
80 °C
H
CO2Et
R = Me, Et, Ph
Scheme 2 Reactions of ferrocenyl ketone derivatives
© Thieme Stuttgart · New York
Synlett 2012, 23, 943–947

946
S. Chen et al.
LETTER
CO2Et
CeCl3
N2
H
CO2Et
PPh3
Rh2(OAc)4
FcCHO
Fe
EtO2C
H
2a
B
3a
Ph3P=O
H
CO2Et
Fc = ferrocenyl
LnRh
N2
Ph3P
A
Scheme 3 Possible reaction pathway
2003, 36, 659. (e) Thomas, J. C. Chem. Rev. 2003, 103,
3101. (f) Arrayas, R. G.; Adrido, J.; Carretero, J. C. Angew.
Chem. Int. Ed. 2006, 45, 7674. (g) Moyano, A.; Rios, R.
Synlett 2009, 1863. (h) Sondenecker, A.; Cvengros, J.;
Aardoom, R.; Togni, A. Eur. J. Org. Chem. 2011, 78.
sponding olefins were obtained in 91%, 89%, and 81%
yields with excellent E stereoselectivity, respectively
(Table 4, entries 1–3).
Finally, we tried to apply this novel method to the reaction
of ferrocenyl ketone derivatives with EDA (Scheme 2).
However, we were disappointed to find that no reactions
occurred when acetylferrocene, propionylferrocene, and
benzoylferrocene were used as the substrates under the
above reaction condition. We attributed this result to the
poor reactivity of ferrocenyl ketone derivatives.
A possible reaction pathway12c to account for the forma-
tion of alkenylferrocenes is shown in Scheme 3. The reac-
tion is initiated by diazo decomposition of EDA (2a)
catalyzed by Rh2(OAc)4 to afford the metal carbene inter-
mediate A, which is then converted to phosphorous ylide
intermediate B in the presence of Ph3P. Next, ferrocene-
carboxaldehyde reacts with phosphorous ylide intermedi-
ate B to generate the product 3a. The action of CeCl3 may
be act as Lewis acid to coordinate with the carbonyl group
of carbonylferrocenes and increase the reactivity of this
reactant.
(3) (a) Barlow, S.; Marder, S. R. In Functional Organic
Materials; Müller, T. J. J.; Bunz, U. H. F., Eds.; Wiley-
VCH: Weinheim, 2007, 393–437. (b) Kato, S.-i.; Beels,
M. T. R.; La Porta, P.; Schweizer, W. B.; Boudon, C.;
Gisselbrecht, J.-P.; Biaggio, I.; Diederich, F. Angew. Chem.
Int. Ed. 2010, 49, 6207.
(4) (a) Hudson, S. A.; Maitlis, P. M. Chem. Rev. 1993, 93, 861.
(b) Ferrocenes: Homogenous Catalysis. Organic Synthesis.
Materials Science; Togni, A.; Hayshi, T., Eds.; VCH:
Weinheim, 1995. (c) Long, N. J. Metallocenes, 1st. ed.;
Blackwell Science: London, 1997. (d) Sutclie, O. B.; Bryce,
M. R. Tetrahedron: Asymmetry 2003, 14, 2297. (e) Kadkin,
O. N.; Han, H.; Galyametdinov, Y. G. J. Organomet. Chem.
2007, 692, 5571. (f) Ochi, Y.; Suzuki, M.; Imaoka, T.;
Murata, M.; Nishihara, H.; Einaga, Y.; Yamamoto, K. J. Am.
Chem. Soc. 2010, 132, 5061.
(5) (a) Molina, P.; Pastor, A.; Vilaplana, M. J.; Velasco, M. D.;
Ramirez de Arellano, M. C. Organometallics 1997, 16,
5836. (b) Molina, P.; Tarraga, A.; Lopez, J. L.; Martinez, J.
C. J. Organomet. Chem. 1999, 584, 147. (c) Bollo, S.;
Vergara, L. J.; Bonta, M.; Chauviere, G.; Perie, J.; Squella,
J. A. Electroanal. Chem. 2001, 511, 46. (d) Van Staveren,
D. R.; Metzler-Nolte, N. Chem. Rev. 2004, 104, 5931.
(e) Fouda, M. F. R.; Abd-Elzaher, M. M.; Abdelsamaia, R.
A.; Labib, A. A. Appl. Organomet. Chem. 2007, 21, 613.
(f) Buriez, O.; Heldt, J. M.; Labbe, E.; Vessieres, A.; Jaouen,
G.; Amatore, C. Chem. Eur. J. 2008, 14, 8195. (g) Metzler-
Nolte, N.; Salmain, M. In Ferrocenes: Ligands, Materials
and Biomolecules; Stepnicka, P., Ed.; Wiley: New York,
2008, 499. (h) Patra, M.; Gasser, G.; Wenzel, M.; Merz, K.;
Bandow, J. E.; Metzler-Nolte, N. Organometallics 2010, 29,
4312.
(6) Debroy, P.; Roy, S. Coord. Chem. Rev. 2007, 251, 203.
(7) (a) Rodriguez, J.-G.; Gayo, M.; Fonseca, I. J. Organomet.
Chem. 1997, 534, 35. (b) Thomas, K. R. J.; Lin, J. T.; Wen,
Y. S. J. Organomet. Chem. 1999, 575, 301. (c) Mata, J. A.;
Falomir, E.; Llusar, R.; Peris, E. J. Organomet. Chem. 2000,
616, 80. (d) Mata, J. A.; Uriel, S.; Llusar, R.; Peris, E.
Organometallics 2000, 19, 3797.
In conclusion, we have developed a concise and efficient
method for the preparation of alkenylferrocene deriva-
tives based on the olefination of carbonylferrocenes with
a-diazocarbonyl compounds using Rh2(OAc)4/CeCl3 as
efficient catalysts. This catalytic methodology is highly
attractive and provides a valuable choice for the organic
synthesis.
Acknowledgment
The project is generously supported by the National Natural Science
Foundation of China (No. 20902044), Natural Science Foundation
of Inner Mongolia of China (Nos. 2009BS0204 and NJ09007), and
‘Chunhui’ Programme from the Ministry of Education of China
(Z2009-1-01003).
References and Notes
(8) (a) Hradsky, A.; Bildstein, B.; Schuler, N.; Schottenberger,
H.; Jaitner, P.; Ongania, K.-H.; Wurst, K.; Launay, J.-P.
Organometallics 1997, 16, 392. (b) Chesney, A.; Bryce, M.
R.; Batsanov, A. S.; Howard, J. A. K.; Goldenberg, L. M.
Chem. Commun. 1998, 677. (c) Stepnicka, P.; Lamac, M.;
Cisarova, I. J. Organomet. Chem. 2008, 693, 446.
(d) Granzhan, A.; Teulade-Fichou, M.-P. Tetrahedron 2009,
65, 1349.
(1) (a) Togni, A.; Hayashi, T. Ferrocenes; VCH: New York,
1995. (b) Atkinson, R. C. J.; Gibson, V. C.; Long, N. J.
Chem. Soc. Rev. 2004, 33, 213.
(2) (a) Green, M. L. H.; Marder, S. R.; Thompson, M. E.;
Bandy, J. A.; Bloor, D.; Kolinsky, P. V.; Jones, R. J. Nature
(London) 1987, 330, 360. (b) Togni, A. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1475. (c) Richards, C. J.; Locke, A. J.
Tetrahedron: Asymmetry 1998, 9, 2377. (d) Dai, L. X.; Tu,
T.; You, S. L.; Deng, W. P.; Hou, X. L. Acc. Chem. Res.
(9) Blakemore, P. R.; Ho, D. K. H.; Nap, W. M. Org. Biomol.
Chem. 2005, 3, 1365.
Synlett 2012, 23, 943–947
© Thieme Stuttgart · New York

LETTER
Synthesis of Alkenylferrocenes
947
(10) Jary, W. G.; Mahler, A.-K.; Purkathofer, T.; Baumgartner, J.
J. Organomet. Chem. 2001, 629, 208.
in anhyd DCE (2 mL). The progress of the reaction was
monitored by TLC. After completion of the reaction, solvent
was removed by evaporation, and the residue was purified
by column chromatography over silica gel to give the
olefination products 3a–f and 4a–g.
(11) (a) Kasahara, A.; Izumi, T.; Maemura, M. Bull. Chem. Soc.
Jpn. 1977, 50, 1021. (b) Naskar, D.; Das, S. K.; Giribabu,
L.; Maiya, B. G.; Roy, S. Organometallics 2000, 19, 1464.
(c) Kundu, A.; Prabhakar, S.; Vairamani, M.; Roy, S.
Organometallics 1997, 16, 4796. (d) Jong, S.-J.; Fang, J.-M.
J. Org. Chem. 2001, 66, 3533. (e) Sinha, P.; Roy, S. Chem.
Commun. 2001, 1798. (f) Debroy, P.; Roy, S. J. Organomet.
Chem. 2003, 675, 105.
(12) Forcomprehensive reviewsabouttransition-metal-catalyzed
olefinations of aldehydes or ketones with diazo compounds,
see: (a) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern
Catalytic Methods for Organic Synthesis with Diazo
Compounds; Wiley-Interscience: New York, 1998.
(b) Herrmann, W. A. Olefins from Aldehydes in Applied
Homogeneous Catalysis with Organometallic Compounds,
2nd ed., Vol. 3; Wiley-VCH: Weinheim, 2002, 1078–1086.
(c) Zhang, Z.; Wang, J. Tetrahedron 2008, 64, 6577.
(13) General Procedure
Representative Spectroscopic Data
Compound 3f: purple solid; mp 76–77 °C. IR (KBr): 3103,
3037, 1713, 1614, 1583, 1291, 1168, 1038, 1002 cm–1. 1H
NMR (500 MHz, CDCl3): d = 1.40 (t, J = 7.0 Hz, 3 H), 4.22
(s, 5 H), 4.40 (q, J = 7.0 Hz, 2 H), 4.51 (t, J = 1.5 Hz, 2 H),
4.73 (t, J = 1.5 Hz, 2 H), 6.77 (d, J = 9.5 Hz, 1 H), 7.88 (s, 1
H), 8.81 (d, J = 9.5 Hz, 1 H). 13C NMR (125 MHz, CDCl3):
d = 14.2, 62.0, 68.4, 68.6, 70.5, 71.6, 81.5, 116.8, 149.4,
151.3, 163.3, 164.7. Anal. Calcd for C17H17ClFeO2: C,
59.25; H, 4.97. Found: C, 58.90; H, 5.17.
Compound 4e: purple solid; mp 127–129 °C. IR (KBr):
3105, 1733, 1617, 1521, 1347, 1221, 1039 cm–1. 1H NMR
(500 MHz, CDCl3): d = 4.01 (s, 3 H), 4.24 (s, 5 H), 4.55 (s,
2 H), 4.74 (s, 2 H), 7.96 (d, J = 5.0 Hz, 2 H), 8.28 (d, J = 5.0
Hz, 2 H), 8.59 (s, 1 H). 13C NMR (125 MHz, CDCl3): d =
52.2, 69.6, 69.7, 72.1, 72.4, 124.0, 128.0, 137.9, 149.0,
157.0, 165.6, 166.2. Anal. Calcd for C20H17FeNO4: C, 61.40;
H, 4.38; N, 3.58. Found: C, 60.11; H, 4.70; N, 3.79.
To a solution of carbonylferrocenes 1a–f (0.3 mmol), Ph3P
(0.33 mmol), CeCl3 (0.15 mmol), and Rh2 (OAc)4 (0.5
mmol%) in anhyd DCE (3 mL) at 80 °C was added dropwise
a solution of individual diazo compounds 2a–h (0.45 mmol)
© Thieme Stuttgart · New York
Synlett 2012, 23, 943–947

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