Ortho-amide-directed oxidation of internal aryl alkynes mediated by cerium(IV) ammonium nitrate

Article abstract of DOI:10.1055/s-0031-1290434

A fascinating oxidation of alkynes mediated by CAN directed by the amide group to synthesize N-[2-(2-oxo-2-phenylacetyl)phenyl]benzamide derivatives under mild conditions is reported. Excellent yields were obtained with various substituents by this method. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0031-1290434

2132▌
LETTER
letter
ortho-Amide-Directed Oxidation of Internal Aryl Alkynes Mediated by
Cerium(IV) Ammonium Nitrate
ortho-Amide-Directed Oxidation of Internal Aryl Alkyne
Chieh-Fu Su,a Wan-Ping Hu,b Jaya Kishore Vandavasi,a Chao-Cheng Liao,a Chen-Ya Hung,a Jeh-Jeng Wang*a
a
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung City 807, Taiwan
E-mail: jjwang@kmu.edu.tw
b
Department of Biotechnology, Kaohsiung Medical University, Kaohsiung City 807, Taiwan
Received: 22.05.2012; Accepted after revision: 13.06.2012
ketone compounds. 1,2-Diketone formation from alkynes
Abstract: A fascinating oxidation of alkynes mediated by CAN di-
may be of interest to chemists working in the field of dy-
rected by the amide group to synthesize N-[2-(2-oxo-2-phenylace-
namic combinatorial chemistry.
tyl)phenyl]benzamide derivatives under mild conditions is
reported. Excellent yields were obtained with various substituents
by this method.
Furthermore, we also explored the 1,2-diketone formation
with a variety of oxidative reagents,15 viz., CAN, Dess–
Key Words: CAN, diketones, oxidation, internal aryl alkynes
Martin periodinane (DMP), cerium(IV) sulfate
[Ce(SO4)2], CeF4, AgSbF6, ZrCl4, InCl3, NiCl3, PCC,
AgNO3, Mn(OAc)3, KMnO4, ZnCl2, PdCl2, and potassi-
um ferricyanide K3Fe(CN)6. Herein, we describe our stud-
ies of general investigations of the 1,2-diketone formation
from alkynes.
Synthetic drugs and natural products mainly consist of ox-
ygen widely present in their compounds.1 1,2-Dicarbonyl
functionalities are mostly available in bioactive natural
products. Moreover, they are versatile building blocks for
the major pharmaceutical drugs.2 Therefore, the develop-
ment of oxygen-containing molecules and their applica-
tions are playing an important role in medicinal
chemistry.
To examine the feasibility of this idea, we investigated the
treatment of N-(2-phenylethynylphenyl)benzamide 1a
with 1.5 equivalents of various oxidizing reagents in the
presence of air and acetonitrile as solvent at 28–80 °C for
24 hours (Table 1, entries 1–10). Raise of temperature re-
sulted in the decomposition of the starting material. With
CAN and Ce(SO4)2 (Table 1, entries 1 and 2) the com-
pound 2a furnished yields 62% and 60%, respectively.
Very low yields were obtained for CeF4 and AgSbF6 (Ta-
ble 1, entries 3 and 4) as 32% and 41%, respectively. Re-
actions were not successful with ZrCl4, InCl3, NiCl3, PCC,
AgNO3, Mn(OAc)3, and K3Fe(CN)6 (Table 1, entries 5–
10 and 16). Compound 1a was studied for the possibility
of reaction with the other oxidizing reagents in dichloro-
methane as solvent at 28–40 °C for 24 hours (Table 1, en-
tries 11–14 and 17). With Mn(OAc)3, DMP, and ZnCl2
(Table 1, entries 11, 12, and 14), reactions were unsuc-
cessful. Low yields were observed with KMnO4 and CAN
as 51% and 37% respectively (Table 1, entries 13 and 17).
Compound 1a with PdCl2 in DMSO as solvent at 140 °C
for 24 hours was not tolerated (Table 1, entry 15). Under
screened conditions, CAN showed the most prominent re-
sults compared to the other oxidizing reagents, and further
optimization reactions were therefore carried out with
CAN (Table 1, entries 18–21). When the reaction was
conducted in the presence of CH2Cl2 and MeCN in a 1:1
ratio, the yield afforded 65% (Table 1, entry 18). In a sim-
ilar way, the reactions were performed in different ratios
of CH2Cl2 and MeCN such as 1:2 and 1:3 resulting in an
improved yield of 73% and 74%, respectively (Table 1,
entries 19 and 21). The reaction with 50 mol% CAN in the
presence of O2 (1.0133 bar) and CH2Cl2 and MeCN in a
1:3 ratio at 28 °C for 24 hours furnished a yield as high as
92% (Table 1, entry 20). The role of oxygen was also stud-
ied: The reaction was carried out in the presence of nitro-
Various methods have been reported to synthesize 1,2-
diketones, such as oxidation of carbon-substituted al-
kynes,3 heteroatom-substituted alkynes,4 substitution of
oxalyl chloride,5 oxidation of α-hydroxy ketones,6 oxida-
tion of α-keto acid chloride,5 and also with other meth-
ods.7 Despite this, several drawbacks such as high
temperatures, using metal catalysts like palladium,13j, 8
gold,9 ruthenium,10 KMnO4,11 and via other reagents,12
low chemoselectivity, high toxicity, and low functional-
group tolerance have been reported; accordingly, there is
a need to develop a new pathway for the synthesis of dik-
etones. Cerium(IV) compounds mainly act as oxidants
from lanthanide reagents. They are expected to be very
strong one-electron oxidants, where the oxidation reac-
tions in organic synthesis with Ce(IV) are dominated by
radical and radical-cation chemistry.13 Oxidation of al-
kyne has been reported with cerium(IV) ammonium ni-
trate (CAN),14 but, unfortunately, the yields were very low
as well as the scope of the reaction was very narrow in
forming the other byproducts. Herein, we describe our
studies to a more general investigation of this phenome-
non, with the formation of 1,2-diketones from alkynes.
We observed the influence of the ortho-amide group
which supported the oxidation of internal aryl alkyne.
Based on our literature survey, CAN has not yet been ex-
ploited as a reagent for the synthesis of various 1,2-di-
SYNLETT 2012, 23, 2132–2136
Advanced online publication: 03.08.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290434; Art ID: ST-2012-W0442-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
ortho-Amide-Directed Oxidation of Internal Aryl Alkynes
2133
gen, and the desired compound obtained a yield of 20% and NO2) on ring B and ring C at the para positions pro-
(Table 1, entry 22).
duced high yields of 84–95%, respectively (2f, g, and 2i–
l).
With the optimized reaction conditions in hand (Table 1,
entry 20), we explored the scope and limitations of this Ring C with F at the para position also resulted in a yield
method. As shown in Figure 1, a variety of functional of 86% (2h). The reaction proceeded smoothly with CF3
groups like NO2, F, Cl, CF3, Me, and Et were tolerated un- on ring A and obtained a yield of 81% (2m). The reaction
der optimized reaction conditions. The lack of the pres- examined with CF3 on ring A and F on ring C was also
ence of substituents on the aromatic rings was tolerated well tolerated to produce the desired product with an 88%
with an excellent yield of 92% (2a).17 Ring B with Me, Cl, yield (2n). Ring A and ring B, having substituents like
F, and NO2 at the para position afforded excellent yields NO2, F, and CF3, transformed into their desired products
of 80–94%, respectively (2b–e).17 Substituents (Et, Cl, F, 2o and 2p with yields of 78% and 79%, respectively. The
O
O
O
O
O
O
O
O
O
O
O
O
NH
NH
NH
NH
Cl
F
2a
92%
2b
85%
2c
87%
2d
94%
F
F
F
O
O
O
O
O
O
O
O
O
O
NH
NH
NH
NH
O
O
O2N
O2N
Cl
2e
80%
2f
95%
2g
95%
2h
86%
O
O
O
O
O
O
O
O
O
O
NH
NH
NH
NH
O
O
F3C
O2N
F
F
Cl
2j
2i
2k
2l
84%
90%
88%
93%
O
O
O
O
F3C
F3C
F3C
F3C
O
O
O
O
O
O
NH
NH
NH
NH
O
O
O2N
F
2m
81%
2n
88%
2o
78%
2p
79%
F
F
F
O
O
O
F3C
F3C
O
O
O
O
NH
NH
NH
O
O
O2N
F3C
Cl
2q
85%
2r
88%
2s
83%
Figure 1 CAN-mediated oxidation for the formation of N-[2-(2-oxo-2-phenyl-acetyl)phenyl]benzamide derivatives
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2132–2136

2134
C.-F. Su et al.
LETTER
three rings while containing substituents also afforded structure of 2q was confirmed by single-crystal X-ray
high yields of 85% and 88% (2q18 and 2r). Finally, all the crystallographic analysis (Figure 2).
reactions were successful with high yields irrespective of
Control experiments were performed with different func-
their functional group. Even when the reaction was per-
tional groups on both of the rings to prove that the reaction
formed with strong or weak electron-withdrawing or elec-
was directed by the amide (Figure 3). Reactions with mol-
tron-donating group still resulted in good yields. The
ecules A and B containing fluoro and ethyl groups at the
meta position were not successful under the optimized re-
action conditions. Hydroxy, amine, and nitro groups at or-
tho positions in reactions with molecules C, D, and I were
not tolerated. We have studied the influence of amine and
amide groups in the meta and para positions; in this case
also the reaction did not progress (reactions with com-
pounds E–H). These results have shown that mainly the
ortho-amide-group effect influenced the synthesis of the
desired diketones.
A plausible mechanism was proposed by Wille,14 with
CAN to produce the diketone compounds from alkynes.
In their report the reaction was not selective to give only
the diketones, and they also observed the formation of
monoketone with the CAN. In our reaction we have ob-
served the formation of diketone product selectively in the
presence of oxygen with high yields. The role of oxygen
was unknown in this reaction. In our mechanism (Scheme
1) the CAN coordinates with the amide oxygen and nitro-
Figure 2 ORTEP diagram of 2q
F
OH
A
D
B
E
C
H2N
NH2
H2N
F
O
O
HN
O
N
H
N
H
NO2
H
I
G
Figure 3 Control experiments to prove the amide is a directing group: no reaction in all cases (A–I)
O3N
NO3
Ce
NO3
O
R
O
O3N
CAN
NO3
Ce
NH
N
N
HN
ONO2
– Ce(IV)
NO3
O
O
R
O2NO
O
O
R
R
NO2 NO2
I
1
II
III
R
R
R
HN
– NO2
– NO2
HN
HN
O2NO
O
O
O2NO
O
O
O
O
O
IVA
IVB
2
Scheme 1 Plausible mechanism for the formation of diketones
Synlett 2012, 23, 2132–2136
© Georg Thieme Verlag Stuttgart · New York

LETTER
ortho-Amide-Directed Oxidation of Internal Aryl Alkynes
2135
gen atoms to direct the nitrate ion easily to attack the al- work are in progress. Further biological activities of these
kyne and further oxidization of alkyne to form diketone.
compounds are also under investigation.
In summary, we have developed a facile and efficient
method for the synthesis of N-[2-(2-oxo-2-phenylace-
tyl)phenyl]benzamides 2 from N-(2-phenylethynylphe-
nyl)benzamide 1 under mild reaction conditions. Further
studies on the reaction mechanism and extension of the
Acknowledgment
We thank the National Science Council of the Republic of China for
financial support. This work was also supported by a grant from the
Kaohsiung Medical University Research Foundation.
Table 1 Optimization of Reaction Conditions to Synthesize 1,2-Di-
ketones 2a
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
O
B
B
References and Notes
A
(1) For selected examples, see: (a) Rozwadowska, M. D.;
Chrzanowska, M. Tetrahedron 1985, 41, 2885.
O
O
reagent
O
NH
A
(b) Angelstro, M. R.; Mehdi, S.; Burkhart, J. P.; Peet, N. P.;
Bey, P. J. Med. Chem. 1990, 33, 11. (c) Maurya, R.; Singh,
R.; Deepak, M.; Handa, S. S.; Yadav, P. P.; Mishra, P. K.
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Phytochemistry 2004, 65, 1185. (e) Nicolaou, K. C.; Gray,
D. L. F.; Tae, J. J. Am. Chem. Soc. 2004, 126, 613.
solvent, 24 h
N
C
C
H
1
2
Entry
Reagenta,b
Solvent
Temp Yield
(°C)
(%)
(f) Wadkins, R. M.; Hyatt, J. L.; Wei, X.; Yoon, K. J. P.;
Wierdl, M.; Edwards, C. C.; Morton, C. L.; Obenauer, J. C.;
Damodaran, K.; Beroza, P.; Danks, M. K.; Potter, P. M.
J. Med. Chem. 2005, 48, 2906.
1
2
CAN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMSO
MeCN
CH2Cl2
r.t.
r.t.
r.t.
r.t.
80
80
80
80
80
80
40
r.t.
40
40
140
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
62
60
32
41
0
Ce(SO4)2
CeF4
3
(2) For recent examples, see: (a) Wolkenberg, S. E.; Wisnoski,
D. D.; Leister, W. H.; Wang, Y.; Zhao, Z.; Lindsley, C. W.
Org. Lett. 2004, 6, 1453. (b) Shipe, W. D.; Yang, F.; Zhao,
Z.; Wolkenberg, S. E.; Nolt, M. B.; Lindsley, C. W.
Heterocycles 2006, 70, 655. (c) Deng, X.; Mani, N. Org.
Lett. 2006, 8, 269. (d) Held, I.; Xu, S. J.; Zipse, H. Synthesis
2007, 1185. (e) Rong, F.; Chow, S.; Yan, S.; Larson, G.;
Hong, Z.; Wu, J. Bioorg. Med. Chem. Lett. 2007, 17, 1663.
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2008, 73, 3384. (g) Boyce, G. R.; Johnson, J. S. Angew.
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Hamze, A.; Bignon, J.; Brion, J.-D.; Alami, M. Tetrahedron
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Liu, J.; Chen, L.; Wan, X. Adv. Synth. Catal. 2010, 352,
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Monguchi, Y.; Sajiki, H. Adv. Synth. Catal. 2010, 352, 1630.
(4) For recent reviews on the synthesis and reaction of
ynamides, see: (a) Evano, G.; Coste, A.; Jouvin, K. Angew.
Chem. Int. Ed. 2010, 49, 2840. (b) DeKorver, K. A.; Li, H.;
Lohse, A. G.; Hayashi, R.; Lu, Z.; Zhang, Y.; Hsung, R. P.
Chem. Rev. 2010, 110, 5064. (c) For a recent example on the
synthesis of α-keto imides, see: Al-Rashid, Z. F.; Johnson,
W. L.; Hsung, R. P.; Wei, Y.; Yao, P.-Y.; Liu, R.; Zhao, K.
J. Org. Chem. 2008, 73, 8780. (d) For a related example on
the synthesis of α-keto amides, see: Zhang, C.; Jiao, N. J.
Am. Chem. Soc. 2010, 132, 28.
4
AgSbF6
ZrCl4
5
6
InCl3
0
7
NiCl3
0
8
PCC
0
9
AgNO3
Mn(OAc)3
Mn(OAc)3
DMP
0
10
11
12
13
14
15
16
17
18
19
20
21
22
0
0
0
KMnO4
ZnCl2
51
0
PdCl2
0
K3Fe(CN)6
CAN
0
37
65
73
92c
74
20d
CAN
MeCN–CH2Cl2 (1:1)
MeCN–CH2Cl2 (2:1)
MeCN–CH2Cl2 (3:1)
MeCN–CH2Cl2 (3:1)
MeCN–CH2Cl2 (3:1)
CAN
CAN (O2)
CAN
CAN (N2)
a All reactions were carried with 1.5 equiv.
b Reaction time was 24 h.
c Reaction was carried with 50 mol% of the reagent.
d Under nitrogen.
(5) (a) Sanz, R.; Castroviejo, M. P.; Guilarte, V.; Pérez, A.;
Fãnanás, F. J. J. Org. Chem. 2007, 72, 5113. (b) Maresh,
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2132–2136

2136
C.-F. Su et al.
LETTER
J. J.; Giddings, L.-A.; Friedrich, A.; Loris, E. A.; Panjikar,
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(17) General Procedure
A mixture of the substituted N-(2-phenylethynylphenyl)-
benzamide 1 with CAN (50 mol%) in MeCN and CH2Cl2 as
1:3 ratio under oxygen 1.0133 bar pressure at 28 °C for 24 h,
monitored by TLC, extracted with CH2Cl2 and H2O layer
was washed with CH2Cl2. Combined organic layers were
washed with H2O and brine, dried over anhyd Na2SO4,
filtered, concentrated, and purified by column
chromatography (CH2Cl2–hexane, 2:3) to give
the corresponding compounds 2.
(8) (a) Mousset, C.; Giraud, A.; Provot, O.; Hamze, A.; Bignon,
J.; Liu, J. M.; Thoret, S.; Dubois, J.; Brion, J.; Alami, M.
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Petsom, A.; Muangsin, N.; Chaichit, N.; Roengsumran, S.
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I. D. J. Mol. Catal. A: Chem. 1999, 145, 111. (b) Zhu, Y.;
Kulkarni, A. P.; Wu, P.; Jenekhe, S. A. Adv. Synth. Catal.
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L.; Müllen, K. J. Am. Chem. Soc. 2000, 122, 7707.
(d) Nobuta, T.; Tada, N.; Hattori, K.; Hirashima, S.; Miura,
T.; Itoh, A. Tetrahedron Lett. 2011, 52, 875. (e) Chen, K.;
Li, H.; Chen, C.; Yang, S.; Hsieh, B.; Hsu, C.
N-[2-(2-Oxo-2-phenylacetyl)phenyl]benzamide (2a)
Yellow solid; 92% yield; mp 116–118 °C. 1H NMR (400
MHz, CDCl3): δ = 12.33 (NH), 9.10–9.07 (m, 1 H), 8.13–
8.10 (m, 2 H), 7.98–7.96 (m, 2 H), 7.71–7.51(m, 9 H), 7.14–
7.15 (m, 1 H). 13C NMR (100 MHz, CDCl3): δ = 199.3,
193.0, 166.1, 142.8, 137.3, 135.2, 134.3, 134.2, 132.7,
132.3, 130.0, 129.2, 129.1, 129.0, 127.5, 127.1, 122.8,
120.9, 118.2. HRMS (EI): m/z calcd for C21H15NO3Na:
352.0950; found: 352.0948.
4-Methyl-N-[2-(2-oxo-2-phenylacetyl)phenyl]benzamide
(2b)
Yellow solid; 85% yield; mp 128–130 °C. 1H NMR (400
MHz, CDCl3): δ = 12.30 (NH), 9.09–9.07 (m, 1 H), 8.02–
7.96 (m, 4 H), 7.72–7.62 (m, 3 H), 7.56–7.52 (m, 2 H), 7.34–
7.32 (m, 2 H), 7.13–7.09 (m, 2 H), 7.33 (d, J = 8.4 Hz, 2 H),
7.11 (t, J = 8.0 Hz, 1 H), 2.44 (s, 3 H). 13C NMR (100 MHz,
CDCl3): δ = 199.3, 193.0, 166.2, 143.0, 137.3, 135.1, 134.2,
132.8, 131.5, 130.0, 129.6, 129.2, 127.6, 122.6, 120.9,
118.2, 21.5. HRMS (EI): m/z calcd for C22H17NO3Na:
366.1106; found: 366.1105.
(18) CCDC number for compound 2q is CCDC 872227.
Macromolecules 2005, 38, 8617. (f) Giraud, A.; Provot, O.;
Peyrat, J.; Alami, M.; Brion, J. Tetrahedron 2006, 62, 7667.
Synlett 2012, 23, 2132–2136
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