Copper-catalyzed efficient direct amidation of 2-methylquinolines with amines

Article abstract of DOI:10.1039/c5ob00915d

A novel Cu-catalyzed direct amidation of 2-methylquinolines with amines is described. This method afforded an efficient approach for the synthesis of biologically important aromatic amides from readily available coupling partners using molecular oxygen as the oxidant.

Full text of DOI:10.1039/c5ob00915d

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COMMUNICATION
Copper-Catalyzed Efficient Direct Amidation of 2-Methylquinolines
with Amines
Hao Xie,a Yunfeng Liao,a Shuqing Chen,a Ya Chen,a Guo-Jun Deng*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A novel Cu-catalyzed direct amidation of 2-methylquinolines 30 were successfully exploited for this transfromation. In recent
with amines is described. This method afforded an efficient
approach for the synthesis of biologically important aromatic
amides from readily available coupling partners using
years, much efforts has been paid on using readily available
alcohols as the coupling partners for amide synthesis. Murahashi
and Naota reported the synthesis of lactams from an
intramolecular reaction of amino alcohols.13 In 2007, the Milstein
35 group made a breakthrough for intermolecular amidation of
alcohols with amines using ruthenium pincer as the catalyst.14
This amide synthetic route is very environmentally benign since
molecular hydrogen is the only by-product. Since that, various
catalytic procedures have been developed for efficient synthesis
40 of amides and ployamides from alcohols and amines.15 In these
processes, alcohols were oxidized into the corresponding
aldehydes via dehydrogenation reaction.16 The Milstein group
also found that esters could smoothly reacted with amines with
liberation of hydrogen under neutral conditions.17 Johnston and
45 co-workers developed an efficient procedure for amide and
peptide synthesis using α-halo nitroalkanes as the coupling
partners in the presence of an electrophilic iodine source.18 In
recent years, direct amidation of C-H bond with amides or other
nitrogen sources provided another efficient route for amide
50 synthesis.19
In general, aromatic aldehydes and benzylic alcohols are
prepared from methylarenes via oxidation/reduction processes.
Therefore, direct amidation of methylarenes with amines can
potentially lead to a more efficient synthesis by eliminating the
55 need for activation one of the coupling partners. However,
efficient direct amidation of methyl group under mild conditions
is rare.20, 21 Herein, we wish to report a copper-catalyzed efficient
direct amidation of 2-methylquinolines with amines to provide
various aromatic amides containing a quinoline motif in good
60 yields.22 The methyl group at C2 position of quinolines was
activated in situ with catalytic amount of CuI catalyst using
molecular oxygen as a green oxidant.
10 molecular oxygen as the oxidant.
Amide motifs are present in many natural products,
pharmaceuticals, functional materials and biological systems.1
Amides are also of great importance as key intermediates for
the preparation of various useful organic compounds.2
15 Conventional amide bond formation utilizes carboxylic acids
and amines as coupling partners in the presence of
stoichiometric activating agents for the acid functionality.3
Recently, great efforts have been made to explore
environmentally benign processes toward amide synthesis.4
20 Among the various methods developed, replacement of the
carboxylic acids with other organic chemicals was proved to
be very powerful for clean amide synthesis.5
1) Conventional amide formation
O
coupling
reagent
O
R2
H
N
R1
N
R3
+
R2 R3
R1
X
2) Oxidative amidation of aldehyde
O
O
O
R2
H
cat.
R1
N
N
+
R2 R3
R1
H
[O]
R3
3) Dehydrogenative amide synthesis
R2
H
cat.
R1
N
R3
N
+
R2 R3
R1
OH
- H2
4) Amidation of methyl group (This work)
O
H
CuI
N
HetAr
C
R1 R2
HetAr CH3
+
N R2
PivOH, O2
First, we investigated the amidation of 2-methylquinoline
R1
o
(1a) and aniline (2a) in 1,2-dichlorobenzene at 120 C under
65 an oxygen atmosphere (Table 1). When the reaction was
carried out in the absence of catalyst, no N-phenylquinoline-2-
carboxamide (3a) was formed as determined by GC-MS and
1H NMR methods (entry 1). We found that such reactions
proceeded when catalytic amount of iodine or CuI was
70 employed as the catalyst (entries 2 and 4). Halonium ion
source such as N-iodo succinimide (NIS) was proved to be
Scheme 1 Different pathways for the amide bond formation.
25
The transition-metal-catalyzed direct amidation of aldehydes
with amines has been proved to be an attractive alternative to the
traditional amide synthesis. Transition metals such as Cu,6 Ru,7
Pd,8 Rh,9 Au10 and lanthanide series11 or even organic carbene12
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[journal], [year], [vol], 00–00 | 1

Organic & Biomolecular Chemistry
Page 2 of 4
ineffective for this kind of transformation (entry 3). Among
ring of anilines slightly improved the reaction yield. For example,
the desired product 3d was isolated in 89% yield when 4-
the various copper salts investigated, CuI showed the best
efficiency (entries 4-6). The choice of additives was very
crucial for this reaction. Basic additive such as K2CO3
significantly decreased the reaction yield (entry 7). To our
delight, the reaction yield could be improved to 53% and 60%
when 1.5 equiv of acetic acid and pivalic acid (PivOH) were
used, respectively (entries 8 and 10). The amount of acid
affected the reaction yield profoundly. When the reaction was
10 carried out in pure pivalic acid, the reaction yield could be
slightly promoted to 70% (entry 11). Decreasing or increasing
the reaction temperature both decreased the reaction yield
(entries 14 and 15). The reaction yield could be further
improved to 80% by extending the reaction time to 48 h (entry
15 16). Much lower yield was observed when the model reaction
was performed in air (entry 17).
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(trifluoromethyl)aniline (2d) was used (entry 4). Functional
DOI: 10.1039/C5OB00915D
30 groups such as cyano, fluoro and chloro were compatible under
the optimized reaction conditions (entries 5, 7-8). When 4-
bromoaniline was used as the coupling partner, the desired
product was achieved in 72% yield (entry 9). The position of the
substituents on the phenyl ring of anilines affected the reaction
35 yield slightly (entries 10-14). Heterocyclic aniline 2-amino
pyridine (2o) was also suitable amidation partner to give the
hetero amide (3o) in 81% yield (entry 15). When aromatic
secondary amine N-methylaniline (2p) was used, unexpected
5
40 Table 2 Reaction of various amines with 2-methylquinoline (1a)a
R1
H
N
CuI (10 mol%)
+
N
R1 R2
Table 1 Optimization of the reaction conditionsa
O2, PivOH, 120 o
C
N
R2
N
O
1a
2
3
catalyst
H
Entry
Amines
Product
Yieldb [%]
Ph NH2
+
N
O2
N
Ph
H
N
R
N
O
1a
2a
3a
N
NH2
R
O
Yieldb [%]
Entry
Catalyst
Additive
Solvent
2a
2b
2c
2d
2e
2f
3a
1
2
3
4
5
6
76
57
49
89
86
86
R = H
1
2
3
4
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
0
6
3b
3c
3d
3e
3f
R = 4-CH3
R = 4-OCH3
R = 4-CF3
R = 4-CN
R = 4-OCF3
R = 4-F
I2
NIS
CuI
CuBr
CuCl2
CuI
CuI
trace
11
5
6
7
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
7
trace
trace
53
2g
2h
2i
3g
3h
3i
7
85
79
72
69
88
84
56
71
K2CO3
AcOH
8
R = 4-Cl
9
R = 4-Br
8
9
2j
3j
10
11
12
13
14
R = 3-Me
R = 3-CF3
R = 3-Cl
CuI
CuI
CuI
CuI
CuI
TsOH
1,2-dichlorobenzene
1,2-dichlorobenzene
PivOH
15
60
2k
2l
3k
3l
0
PivOH
1
11
12
70
2m
2n
3m
3n
R = 2-CH3
R = 2-Cl
AcOH
30
3
H3PO4
trace
1
14c
15
2o
3o
81
CuI
PivOH
51
N
NH2
15d
16e
17f
CuI
PivOH
48
Ph
2p
2q
3a
3p
16
N
57
62
42
CuI
PivOH
80
H
CuI
PivOH
19
NH
17c
18c
O
a Conditions:
(0.2 mmol),
1a
2a
(0.4 mmol), catalyst (0.02 mmol),
additive (0.3 mmol), solvent (0.4 mL) under oxygen, 120 oC, 24 h. b
NH
2r
3q
c
o
d
110 C 130 oC. e 48 h. f Under air.
.
.
1a
GC yield based on
a Conditions: 1a (0.5 mmol), 2 (1.0 mmol), CuI (0.05 mmol), PivOH (0.8 mL),
120 oC, 48 h, under oxygen. b Isolated yield based on
c CuI (0.05 mmol),
1a
.
PivOH (0.2 mL), 1,2-dichlorobenzene (0.6 mL).
To demonstrate the general applicability of the CuI/PivOH
20 system, various amines were tested in amidation reaction with 2-
methylquinoline (1a). In general, aromatic amines with
substituents at the para position were able to smoothly react with
1a to afford the corresponding aromatic amide products in good
yields (entries 2-9). Lower yields were obtained when electron-
25 donating groups were presented at the phenyl ring (entries 2 and
3). Introduction of electron-withdrawing groups to the phenyl
product 3a was obtained by losing the methyl substituent
(entry 16). It should be noted that cyclic secondary amines
such as morpholine (2q) and piperidine (2r) were also able to
45 couple with 1a to give the desired products in moderate yields
(entries 17 and 18).
2
| Journal Name, [year], [vol], 00–00
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Organic & Biomolecular Chemistry
To further explore the scope of the reaction, various 2-
Single-electron transfer from Cu to O2 generates peroxycopper
methylquinolines were employed to react with 2a under the
optimized conditions (Table 3). To our surprise, no product
was obtained when a methoxy group was located at C4
position of the pyridine ring (entry 1). However, moderate to
good yields were achieved when the substituents were
presented at the phenyl ring of quinolines (entries 2-8).
Functional groups such as trifluoromethoxy, trifluoromethyl
and fluoro were well tolerated under the optimized reaction
intermediate C. Reaction of the oxygen radical with the benzyl
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group yields a peroxycopper intermediateDDO.I:E10li.m10i3n9a/tCio5nOBo0f0C9u15n-D
45 OH releases aldehyde E24, which can undergo spontaneous
imidization with amine to produce intermediate F. Hydrolysis
and oxidation of F provides the final product H.25 The hydrolysis
reaction could give a reasonable explanation why part of the
oxygen atom in the product comes from water.
5
10 conditions (entries 3, 4 and 6). However, much lower yields
were obtained when bromo and chloro substituents were
presented (entries 5 and 7). Interestingly, the desired product
3y was obtained in 88% when the methoxy group was situated
at C8 position (entry 8). Unfortunately, 2-methylpyridine and
15 toluene were not effective coupling partners under the current
reaction conditions.
10 mol% CuI
PivOH
3a
1a
+
2a
+
a
( )
O2, 120 o
C
N
N
Ph
4a
3a
3a
4a
4a
Conditions: 12 h;
Conditions: 24 h;
34%,
52%,
5%
2%
standard
conditions
b
( )
3a
4a
1a
86%
Table 3 Reaction of 2-methylquinolines with aniline (2a)a
10 mol% CuI
PivOH
18O2, 120 o
16O-3a
18O-3a
(c)
(d)
+
2a
2a
+
R3
H
N
CuI (10 mol%)
O2, 120 oC
C
R3
Ph NH2
98%
2%
+
N
Ph
N
O
10 mol% CuI
PivOH
1
2a
3
16O-3a
18O-3a
1a
4a
+
+
Yieldb [%]
H218O (5 equiv)
O2, 120 o
Entry
2-Methylquinolines
Product
42%
58%
C
R3 = 4-MeO
R3 = 6-Me
R3 = 6-OCF3
R3 = 6-CF3
R3= 6-Br
R3 = 7-F
R3 = 7-Cl
R3 = 8-MeO
1
2
3
4
5
6
7
8
1b
3r
trace
78
1c
1d
1e
1f
3s
3t
10 mol% CuI
PivOH
16O-3a
18O-3a
86
+
e
( )
H218O (5 equiv)
O2, 120 oC, 48 h
3u
3v
3w
3x
3y
88
41%
59%
44
50
1g
1h
1i
81
Scheme 2 Control experiments under various conditions.
47
88
a Conditions: 1 (0.5 mmol), 2a (1.0 mmol), CuI (0.05 mmol), PivOH (0.8
mL), 120 oC, 48 h, under oxygen. b Isolated yield based on 1.
PivOH
[Cun]
- H+
O2
[Cun+1
O
]
1a
[Cun]
N
N
N
H
O
A
C
B
20
To gather more information, some control experiments were
set up under various conditions. When the reaction was
stopped in 12 h, the desired product 3a was obtained in 34%
together with 5% of 4a. If prolonged the reaction time to 24 h,
the yield of 3a could be improved to 52% with a reduced
- [Cun-OH]
O
[Cun]
H
N
O
N
HNR1R2
H
O
- H2O
E
D
25 amount of 4a (Scheme 2a). Treatment of 4a under the
standard reaction conditions afforded the corresponding
amidation product 3a in 86% yield (Scheme 2b). When the
reaction of 1a and 2a was conducted under 18O2 atmosphere,
the major product is the normal 16O-3a (Scheme 2c). When 5
30 equiv. of H218O was used, both 16O-3a (42%) and 18O-3a
(58%) were detected (Scheme 2d). The 16O comes from water
existed in pivalic acid solvent as well as water generated from
the reaction. When the reaction of 4a was performed in the
presence of H218O, similar result was observed (Scheme 2e).
35 This means the oxygen atom in the product mainly comes
from water (generated during the reaction or existed in the
reaction mixture).
[Cu]/O2
H2O
H2O
- H+
NR1R2
OH
NR1R2
NR1R2
N
N
N
O
F
G
H
Scheme 3 Plausible reaction pathway.
55
In summary, we have developed a novel direct amidation of
2-methylquinolines with amines in the presence of catalytic
amount of CuI. Aromatic amides were formed in good yields
using molecular oxygen as the green oxidant. Functional
60 groups such as cyano, halogen, CF3 and OCF3 were well
tolerated under the optimized conditions. This method
afforded a novel approach for the synthesis of biologically
important aromatic amides from readily available coupling
partners using cheap copper catalyst. A detailed reaction
65 mechanism and further application of this reaction are
Based on these observations, a plausible reaction pathway for
the direct amidation of 2-methylquinolines is proposed in Scheme
40 3. Isomerization of 1a generates an enamine intermediate A23
which further reacts with copper catalyst to afford intermediate B.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

Organic & Biomolecular Chemistry
Page 4 of 4
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underway in our laboratory.
This work was supported by the National Natural Science
Foundation of China (21172185, 21372187), the Research
Fund for the Doctoral Program of Higher Education of China,
Ministry of Education of China (20124301110005) and the
Hunan Provincial Innovative Foundation for Postgraduate
(CX2014B258).
Wang and Z. Y. Wang, Angew. Chem. Int. Ed., 2011, 50, 8917; (f)
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Application of Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan 411105, China;Fax: (+86)-731-
58292251; e-mail: gjdeng@xtu.edu.cn.
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† Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
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