Carbene adduct of cyclopalladated ferrocenylimine-assisted synthesis of aminopyridine derivatives by the amination of chloropyridines with primary and secondary amines

Article abstract of DOI:10.1002/aoc.3026

An efficient, simple way to synthesize aminopyridine derivatives is presented, based on Buchwald-Hartwig aminations. Using 1 mol% N-heterocyclic carbene adduct of cyclopalladated ferrocenylimine in the presence of 1.5 equiv. ^tBuOK as base in dioxane at 110°C offered moderate to excellent yields in the reaction of chloropyridines with primary and secondary amines, including sterically hindered amines and alkyl amines. Copyright

Full text of DOI:10.1002/aoc.3026

Full Paper
Received: 13 April 2013
Revised: 23 May 2013
Accepted: 29 May 2013
Published online in Wiley Online Library: 30 July 2013
(wileyonlinelibrary.com) DOI 10.1002/aoc.3026
Carbene adduct of cyclopalladated
ferrocenylimine-assisted synthesis of
aminopyridine derivatives by the amination of
chloropyridines with primary and secondary
amines
Bing Mua,b, Jingya Lia,c* and Yangjie Wua*
An efficient, simple way to synthesize aminopyridine derivatives is presented, based on Buchwald–Hartwig aminations. Using
t
1 mol% N-heterocyclic carbene adduct of cyclopalladated ferrocenylimine in the presence of 1.5 equiv. BuOK as base in
dioxane at 110°C offered moderate to excellent yields in the reaction of chloropyridines with primary and secondary amines,
including sterically hindered amines and alkyl amines.
Supporting information may be found in the online version of this article.
Keywords: N-heterocyclic carbene adduct; Buchwald–Hartwig amination; chloropyridine; primary and secondary amines; aminopyridine
derivatives
3-bromo-2-aminopyridine, but the reactions proceed with catalyst
Introduction
loadings of 4 mol%. J. G. Ji et al.[30] found that 2 mol% palladium–
Aminopyridine derivatives are commonly used as ligands in
inorganic and organometallic chemistry,[1,2] fluorescent dyes,[3]
biologically active pharmaceutical ingredients and numerous
natural products.[4–6] In addition, the aminopyridine moiety is still
regarded as one of the most interesting heteroaromatic ring sys-
tems.[7] Consequently, there has been tremendous interest in de-
veloping various methods for the synthesis of aminopyridine
derivatives. Traditionally, the most common method to access
these compounds is the amination of the corresponding
(hetero)aryl halides with free amines as the nucleophile under
high pressure[8] and/or high temperature.[9] The palladium-
catalyzed amination of (hetero) aryl halides has evolved as an
available alternative to traditional nucleophilic aromatic substitu-
tion because of mild conditions necessary and tolerance to a
wide range of functional groups.[10–12] More recently, owing to
development of strong σ-donor ligands such as tertiary bulky
phosphines[13,14] and air-stable N-heterocyclic carbenes,[15,16] sig-
nificant progress in the palladium-catalyzed amination reactions
has been achieved, demonstrating low reaction temperature, less
reactive aryl chlorides, heteroaryl halides or extremely hindered aryl
halides as the coupling partner and low catalyst loadings.[17–24]
Despite all these advances, the Buchwald–Hartwig amination of
less reactive heteroaryl chlorides with amines, especially sterically
hindered arylamines and alkylamines, has been less explored.[25,26]
For instance, the amination of halo-(pyridine or pyrimidine) via
microwave irradiation at 160–180°C was reported by M. Schnürch
et al. and H. Q. Zhang et al.[27,28] F. Perez and A. Minatti[29]
developed palladium-catalyzed CÀN cross-coupling reactions of
xantphos complex in toluene at 100°C exhibited high catalytic
activity in the amination of bromopyridines. L. X. Shao et al.[31]
reported an efficient method to obtain aminopyridine derivatives,
but only 3-chloropyridine and primary amines proved to be suit-
able reaction partners. Accordingly, from academic and industrial
viewpoints, the design of catalytic systems for Buchwald–Hartwig
aminations of cheap and available chloropyridines with various
arylamines and alkylamines under conventional heating without
other external assistance with low catalyst loadings is still a promis-
ing direction for establishing a general and convenient synthetic
method for the synthesis of aminopyridine derivatives.
Recently, we developed
a
new carbene adduct of
cyclopalladated ferrocenylimine, and discovered it to be an effi-
cient catalyst in the amination reactions of aryl chlorides with
amines and to be handled easily because of its stability to oxygen
*
Correspondence to: Yangjie Wu and Jingya Li College of Chemistry and Molecu-
lar Engineering, Key Laboratory of Chemical Biology and Organic Chemistry of
Henan Province, Zhengzhou University, Zhengzhou 450052, People's Republic
of China. Email: wyj@zzu.edu.cn; jingya.li@tetranov.com
a College of Chemistry and Molecular Engineering, Key Laboratory of Chemical
Biology and Organic Chemistry of Henan Province, Zhengzhou University,
Zhengzhou 450052, People's Republic of China
b Department of Chemistry, Zhengzhou Normal University, Zhengzhou 450044,
People's Republic of China
c
Tetranov Biopharm LLC, Zhengzhou 450052, People's Republic of China
Appl. Organometal. Chem. 2013, 27, 537–541
Copyright © 2013 John Wiley & Sons, Ltd.

B. Mu et al.
and moisture, synthetic convenience and facile modification.[32]
Encouraged by these results, we further explored the application
Results and Discussion
In previous studies[32] we found complex 1 to be an efficient cat-
alyst in the Buchwald–Hartwig amination reactions of a range of
aryl chlorides with amines under nitrogen atmosphere with
of carbene adduct of cyclopalladated ferrocenylimine
1
(Scheme 1) in the amination reactions of chloropyridines with
various amines. Herein we report the results of a preliminary inves-
tigation into complex 1 in Buchwald–Hartwig aminations of
chloropyridines with primary and secondary amines, including
sterically hindered primary amines and alkyl amines with catalyst
loadings of 1 mol% in dioxane at 110°C. The results indicated that
aminations occurred in high yield and with high turnover numbers.
t
catalyst loadings of 1 mol% in the presence of BuOK as base in
dioxane at 110°C for 3 h. In view of the results, we decided to re-
act 2-chloropyridine with morpholine under the same conditions
as described previously.[32] We were pleased to obtain 98% of the
desired product. To determine the scope of the amination
method, a series of secondary amines were reacted with
chloropyridines to generate the corresponding aminopyridines
under the same conditions. The obtained results are summarized
in Table 1. High yields were obtained for a range of secondary
amines such as morpholine, piperidine, N-methylaniline,
dibenzylamine with chloropyridines like 2-chloropyridine,
3-chloropyridine and 4-chloropyridine hydrochloride (Table 1).
2-, 3- and 4-chloropyridines were viable substrates for
catalytic amination reactions, and 4-chloropyridine reacted
with N-methylaniline to form the desired coupling product
in the highest isolated yield (entries 13–15). For the coupling of
2-chloropyridine or 3-chloropyridine with morpholine or piperi-
dine, they could be coupled very efficiently with a reaction
time as short as 1 h, and the reaction of morpholine with
3-chloropyridine gave the coupled product in 90% yield for
Scheme 1. Structure of complex 1.
Table 1. Amination of chloropyridines with secondary aminesa
Cat. 1, tBuOK
R1
N
R2
R1
+
CI
HN
R2
Dioxane, 110 °C
N
N
Entry
Chloropyridine
Amine
Time (h)
Product
Yield (%)b
1
2
3
2-Chloropyridine
2-Chloropyridine
2-Chloro-6-(trifluoro
methyl)pyridine
2-Chloro-5-(trifluoro
methyl)pyridine
3-Chloropyridine
3-Chloropyridine
3-Chloropyridine
3-Chloropyridine
3-Chloropyridine
3-Chloropyridine
2-Chloropyridine
3-Chloropyridine
2-Chloropyridine
3-Chloropyridine
4-Chloropyridine
hydrochloride
Morpholine
Morpholine
Morpholine
3
1
1
2a
2a
2b
98
94
82
4
Morpholine
1
2c
88
5
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Piperidine
1
0.5
1
2d
2d
2d
2d
2d
2d
2e
2f
99
90
6
7
68c
57d
70e
30f
66
8
1
9
1
10
11
12
13
14
15
1
1
Piperidine
1
81
Ph–NH–CH3
Ph–NH–CH3
Ph–NH–CH3
3
2g
2h
2i
86
3
85
95g
3
16
17
2-Chloropyridine
3-Chloropyridine
Ph–NH–Ph
6
3
2j
54
80
PhCH2–NH–CH2Ph
2k
aReaction conditions: chloropyridine 1.0 mmol, secondary amine 1.2 mmol, tBuOK 1.5 mmol, dioxane 2 ml, 1 mol% catalyst 1, at 110°C.
bIsolated yields based on chloropyridine.
c0.5 mol% catalyst 1 was used.
dReaction temperature: 90°C
eCs2CO3 was used as base.
fK3PO4 was used as base.
g2.5 mmol tBuOK was used.
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Synthesis of aminopyridines by the amination of chloropyridines
Table 2. Amination of 3-chloropyridine with anilinesa
Cat. 1, tBuOK
H
Cl + H2N R
N R
Dioxane, 110 °C
N
N
Entry
R
Time (h)
Product
Yield (%)b
1
Ph–
Ph–
4
8
8
8
8
8
8
8
8
8
8
2l
2l
53
94
87
77
86
77
91
85
77
87
89
2
3
4–CH3–Ph–
2m
2n
2o
2p
2q
2r
4
3–CH3–Ph–
5
4–CH3O–Ph–
3–CH3O–Ph–
2–CH3O–Ph–
4–CH3CH2O–Ph–
2,5–(CH3)2–Ph–
2,6–(CH3)2–Ph–
2,4,6–(CH3)3–Ph–
6
7
8
9
2s
2t
10
11
2u
aReaction conditions: 3-chloropyridine 1.0 mmol, aniline 1.2 mmol, tBuOK 1.5 mmol, dioxane 2 ml, 1 mol% catalyst 1, at 110°C.
bIsolated yield based on 3-chloropyridine.
Table 3. Amination of 2-chloropyridine with anilinesa
Cat. 1, tBuOK
H
Cl + H2N R
R
N
Dioxane, 110 °C
N
N
Entry
R
Time (h)
Product
Yield (%)b
1
2
3
4
5
Ph–
8
6
2v
2w
2x
2y
2y
62
4–CH3–Ph–
72c
65d
20
3–CH3–Ph–
6
2,6–(CH3)2–Ph–
2,6–(CH3)2–Ph–
3
24
25
aReaction conditions: 2-chloropyridine 1.0 mmol, aniline 1.2 mmol, tBuOK 1.5 mmol, dioxane 2 ml, 1 mol% catalyst 1, at 110°C.
bIsolated yield based on 2-chloropyridine.
c15% N-(pyridin-2-yl)-N-p-tolylpyridin-2-amine 2w′ was isolated.
d18% N-(pyridin-2-yl)-N-m-tolylpyridin-2-amine 2x′ was isolated.
Table 4. Amination of chloropyridines with alkyl aminesa
Cat. 1, tBuOK
Dioxane, 110 °C
CI
NH
R
H2N
R
+
N
N
Entry
Chloropyridine
R
Time (h)
Product
Yield (%)b
1
2
3
4
2-Chloropyridine
3-Chloropyridine
2-Chloropyridine
3-Chloropyridine
Cyclohexyl-
Cyclohexyl-
6
6
3
3
2z
3a
3b
3c
55
56
58
51
n-C12H25
–
–
n-C12H25
aReaction conditions: chloropyridine 1.0 mmol, alkyl amine 1.2 mmol, tBuOK 1.5 mmol, dioxane 2 ml, 1 mol% catalyst 1, at 110°C.
bIsolated yield based on chloropyridine.
0.5 h (entries 2, 5, 6, 11 and 12). 2-Chloro-6-(trifluoromethyl)
pyridine or 2-chloro-5-(trifluoromethyl)pyridine reacted with
morpholine for 1 h to give the cross-coupled product in 82%
and 88% yields, respectively (entries 3 and 4). In the case of the
coupling between sterically hindered diphenylamine and 2-
chloropyridine, the coupled product was obtained in 54% yield
Appl. Organometal. Chem. 2013, 27, 537–541
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B. Mu et al.
for 6 h (entry 16). By decreasing the catalyst loading to 0.5 mol% or
reducing the reaction temperature to 90°C, only 68% and 57% isolated
yields were obtained, respectively (entries 7 and 8). In addition, bases
play a critical role in transition metal-catalyzed organic reactions.[33] To
this end, different bases were tested. We noticed that a strong base
(tBuOK) was required to promote the reaction, and weaker bases
(Cs2CO3, K3PO4) led to lower conversion (entries 5, 9 and 10).
General Procedure for Buchwald–Hartwig Amination
In a Schlenk tube, a mixture of catalyst 1 (1 mol%), tBuOK
(1.5 mmol), chloropyridines (1 mmol) and amines (1.2 mmol) in
dry dioxane (2 ml) was evacuated and charged with nitrogen.
The reaction mixture was then placed in an oil bath and heated
at 110°C with the protection of N2 for 3 h. The reaction mixture
was allowed to cool to room temperature and was quenched
by filtering through a short silica column and then concentrated
under reduced pressure. After purification by flash chromatogra-
phy, the yield was calculated based on chloropyridines. The new
products were identified by 1H NMR, 13C NMR spectroscopy,
high-resolution mass spectra, mass spectra and melting points.
Other products herein are known compounds and were charac-
We next turned our particular attention to the aminations of a
variety of anilines with 3-chloropyridine under the same reaction
conditions. As can be seen from Table 2, all reactions took place
smoothly to give the desired products in good to excellent yields
by prolonging the reaction time to 8 h (entries 2–9). It is worth noting
that sterically hindered 2,6-dimethylaniline and 2,4,6-trimethylaniline
also worked well in these transformations to provide the target
products in 87% and 89% yields, respectively (entries 11 and 12).
Additionally, the aminations of a variety of anilines with 2-
chloropyridine are shown in Table 3. A series of substituted aniline
derivatives were coupled employing the established conditions in
order to assess the scope of the protocol. Aniline gave the cross-
coupled product in 62% yield for 8 h and no other side product
was isolated (entry 1). In the case of p-toluidine and m-toluidine used
as substrates, the mono-aminated products as the major along with
the bis-aminated products were obtained in good total yields for 6 h
(entries 2 and 3). However, sterically hindered 2,6-dimethylaniline
was found to be a poor coupling partner even by prolonging the
reaction time for 24 h (entries 4 and 5).
1
terized by comparing their H and 13C NMR, mass spectrometric
and melting point results with the literature (see supporting in-
formation for characterization data).
N-(2,5-Dimethylphenyl)pyridin-3-amine 2s
White solid; m.p. 114–116°C; 1H NMR (CDCl3): δ 8.28 (1H, d, J = 1.4
Hz, 2-Hpy), 8.11 (1H, d, J = 4.3 Hz, 6-Hpy), 7.19 (1H, d, J = 8.3 Hz, 4-
Hpy), 7.15–7.09 (2H, m, 5-Hpy and HAr), 7.01 (1H, s, HAr), 6.82
(1H, d, J =7.6 Hz, HAr), 5.49 (1H, bs, ÀNH), 2.28 (3H, s, ÀCH3),
2.21 (3H, s, ÀCH3); 13C NMR: δ 141.1 (CAr; ÀNH), 140.9
(6-Cpy), 139.7 (2-Cpy), 139.4 (CAr), 136.7 (3-Cpy), 131.0 (CAr), 126.4
(CAr), 124.1 (5-Cpy), 123.7 (4-Cpy), 122.8 (CAr), 120.5 (CAr), 21.1
(ÀCH3), 17.4 (ÀCH3). MS: 198.9 [M+H]+, 220.8 [M+Na]+. HRMS:
calcd for 199.1236 [M+H]+; found 199.1229.
Finally, alkyl amines which are usually difficult substrates in the
Buchwald–Hartwig amination were also tested. The results are
summarized in Table 4. Cyclohexylamine or dodecylamine
reacted with 2-chloropyridine or 3-chloropyridine to give the cor-
responding mono-aminated products in moderate yields, but a
longer reaction time did not give improved yields (entries 1–4).
N-(Pyridin-2-yl)-N-m-tolylpyridin-2-amine 2x′
White solid; m.p. 102–103°C; 1H NMR (CDCl3): δ 8.33–8.31 (2H, m,
2-Hpy), 7.56–7.52 (2H, m, 4-Hpy), 7.26 (1H, m, HAr), 7.04–6.97 (5H,
m, HAr and Hpy ), 6.92–6.89 (2H, m, HAr), 2.31 (3H, s, CH3); 13C
NMR: δ 158.2 (2-Cpy), 148.5 (6-Cpy), 144.9 (CAr), 139.6 (CAr), 137.4
(4-Cpy), 129.5 (CAr), 128.0 (3-Cpy), 126.6 (CAr), 124.4 (CAr), 118.0
(CAr), 116.9 (5-Cpy), 21.5 (ÀCH3). HRMS: calcd for 262.1345
[M+H]+; found 262.1336.
Conclusion
In summary, we have found that N-heterocyclic carbene adduct
of cyclopalladated ferrocenylimine was an efficient catalyst in
Buchwald–Hartwig aminations of chloropyridines with primary
and secondary amines, including sterically hindered amines and
alkyl amines. Typically, using 1 mol% catalyst in the presence of
N-Dodecylpyridin-3-amine 3c
White solid; m.p. 60°C; 1H NMR (CDCl3): δ 8.02 (1H, d, J = 2.5
Hz, 2-Hpy), 7.94 (1H, d, J = 4.2 Hz, 6-Hpy), 7.09–7.06 (1H, m,
5-Hpy), 6.87–6.84 (1H, m, 4-Hpy), 3.72 (1H, bs, ÀNH), 3.11 (2H,
t, J = 7.1 Hz, ÀCH2; ÀNH), 1.66–1.59 (2H, m, Halkyl), 1.41–1.26
(18H, m, Halkyl), 0.88 (3H, t, J = 6.8 Hz, ÀCH3); 13C NMR: δ
144.4 (3-Cpy), 138.3 (6-Cpy), 135.9 (2-Cpy), 123.7 (5-Cpy), 118.3
(4-Cpy), 43.6 (CH2; ÀNH), 31.9 (Calkyl), 29.66 (Calkyl), 29.64 (Calkyl),
29.60 (Calkyl), 29.4 (Calkyl), 29.3 (Calkyl), 27.1 (Calkyl), 22.7 (Calkyl),
14.1 (ÀCH3). MS: 263.1 [M+H]+, 285.1 [M+Na]+. HRMS: calcd
for 263.2488 [M+H]+; found 263.2476.
t
1.5 equiv. BuOK as base in dioxane at 110°C provided the cou-
pling products in moderate to excellent yields. Currently, further
efforts to extend the applications involving this type of palladium
complex in other palladium-catalyzed coupling reactions are un-
derway in our laboratory.
Experimental
General
Melting points were measured on a WC-1 microscopic apparatus and
are uncorrected. Mass spectra were measured on a LC-MSD-Trap-XCT
instrument. High-resolution mass spectra were measured on a Waters
Q-T Micro spectrometer. 1H NMR and 13C NMR spectra were recorded
on a Bruker DPX-400 spectrometer in CDCl3 with tetramethylsilane as
internal standard.
The carbene adduct of cyclopalladated ferrocenylimine 1 was
prepared according to published procedures.[32] Liquid amines
were distilled before used. Chloropyridines were obtained from
commercial sources and used without purification. All the
solvents were purified by standard methods.
Supporting Information
Supporting information may be found in the online version of
this article.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (No. 21172200) and the Natural Science Foundation of
Henan Province and Henan Education Department, China
(13B150315), for their financial support on this research.
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Appl. Organometal. Chem. 2013, 27, 537–541

Synthesis of aminopyridines by the amination of chloropyridines
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