Ni(0)/NHC-catalyzed amination of N-heteroaryl methyl ethers through the cleavage of carbonoxygen bonds

Article abstract of DOI:10.1016/j.tet.2012.04.005

Ni(0)/NHC-based catalyst system can promote the amination of N-heteroaryl methyl ethers via the cleavage of normally unreactive carbon-oxygen bonds. Electron-deficient N-heteroarenes including pyridine, quinoline, isoquinoline, and quinoxaline undergo amination to afford aminopyridine and related heteroarenes, which constitute an important class of compounds.

Full text of DOI:10.1016/j.tet.2012.04.005

Tetrahedron 68 (2012) 5157e5161
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Ni(0)/NHC-catalyzed amination of N-heteroaryl methyl ethers through
the cleavage of carbon‒oxygen bonds
,
b
,
b
b
b
,
,
*
Mamoru Tobisu a , Ayaka Yasutome , Ken Yamakawa , Toshiaki Shimasaki y, Naoto Chatani b
*
a Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
b Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Ni(0)/NHC-based catalyst system can promote the amination of N-heteroaryl methyl ethers via the
cleavage of normally unreactive carboneoxygen bonds. Electron-deficient N-heteroarenes including
pyridine, quinoline, isoquinoline, and quinoxaline undergo amination to afford aminopyridine and re-
lated heteroarenes, which constitute an important class of compounds.
Received 4 February 2012
Received in revised form 30 March 2012
Accepted 1 April 2012
Available online 7 April 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Nickel
Catalysis
Amination
CeO bond activation
Heteroarene
1. Introduction
catalyzed cross-couplings with the aid of BEt3 additive.7 Itami
reported that azole CeH bonds can also be used in nickel-
Catalytic cross-coupling reactions between aryl halides and
carbon- or heteroatom-based nucleophiles have now become
among the most versatile tools in the field of organic synthesis.1
Recent efforts to further increase the utility of these cross-
coupling reactions have been devoted toward the use of more
common but less reactive phenol-derived electrophiles.2 In con-
trast to the established reactivity of electronically activated aryl
sulfonates, such as triflates, tosylates, and mesylates, aryl ethers,
and carboxylates have been considered poor electrophiles in
cross-coupling reactions despite their accessibility, low cost, and
ease of handling. On the basis of pioneering works by Wenkert
and Dankwardt on nickel-catalyzed KumadaeTamaoeCorriu re-
action of aryl ethers,3 our group developed SuzukieMiyaura-type
coupling using aryl methyl ethers.4 The laboratories of Shi, Garg,
and Snieckus independently reported the use of aryl carboxylates
and carbamates in SuzukieMiyaura cross-coupling reactions.5
Negishi type reaction of aryl pivalates6a and aryl ethers6b using
organozinc reagents has also been reported. Shi’s group demon-
strated that naphthols can be employed directly in some nickel-
catalyzed cross-coupling of phenolic electrophiles.8 Quite re-
cently, MizorokieHeck type reaction using aryl pivalates has
also been developed by Watson.9 Despite the use of inert
phenol-derived electrophiles in nickel-catalyzed cross-coupling
reactions with carbon-centered nucleophiles, their application to
carboneheteroatom bond-forming cross-coupling is relatively
unexplored.10 Considering the widespread use of CeN bond-
forming cross-couplings of aryl halides (Scheme 1a) in modern
organic synthesis,11 the development of catalytic protocols that
effect the amination of inert phenolic electrophiles should provide
a useful method for the preparation of arylamines. In this context,
our group recently reported Ni(0)/IPr-catalyzed amination of aryl
methyl ethers (Scheme 1b)12 and aryl pivalates (Scheme 1c,
R¼tert-butyl).13 Our group13 and Garg et al.14 also demonstrated
that aryl carbamates are suitable electrophilic partners in nickel-
catalyzed amination (Scheme 1c, R¼NEt2). In our preliminary
studies, aryl methyl ethers proved to be much less reactive toward
amination than aryl pivalates and carbamates, and only methoxy
groups on polyaromatics, such as naphthalene were aminated in
an efficient manner. Nevertheless, the method allows direct in-
troduction of amino functionality into widely available methox-
yarenes, making it appealing to synthetic chemists. We report
herein that methoxy groups on N-heteroarenes can also partici-
pate in Ni(0)/IPr-catalyzed amination via the activation of
a CeOMe bond.
* Corresponding authors. E-mail address: tobisu@chem.eng.osaka-u.ac.jp
(M. Tobisu).
y
Current address: Department of Life and Environmental Sciences, Faculty of
Engineering, Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan.
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.04.005

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M. Tobisu et al. / Tetrahedron 68 (2012) 5157e5161
(a)
(b)
(c)
As depicted in Table 2, several N-heteroaryl methyl ethers
proved to serve as suitable substrates for the nickel-catalyzed
amination reaction. In addition to a methoxy group at the 6-
position of a quinoline ring (Entry 1), that located at the 3-
position can be aminated under ’standard’ conditions (Entry 2).
Catalytic amination also proceeded with a relatively hindered 8-
methoxyquinoline substrate (Entry 3). On the other hand, 2-
methoxyquinoline did not afford the corresponding aminated
product. Nucleophilic substitution by the amine toward the
methyl group occurred instead to form 2-hydroxyquinoline in 24%
yield (Entry 4).16 On the basis of the well-known electrophilicity of
the C2 position of quinolines,17 nucleophilic attack or CeH acti-
vation of this position may lead to undesirable side reactions and
reduction in yield of the aminated product. Thus, we examined
methoxyquinoline bearing a methyl group at the 2-position. In
contrast to our expectation, however, this substrate was less re-
active toward the present amination and required higher reaction
temperature to obtain an aminated product probably because of
the electron-donating nature of the methyl group (Entry 5). It
should be noted that the methoxyquinolines used in the present
study are commercially available or facilely synthesized by
methylation of the corresponding hydroxyquinolines. Thus, the
present method allows straightforward access to a series of
aminoquinolines.
This catalytic amination is also applicable to other fused N-
heteroarenes, such as isoquinoline (Entry 6) and quinoxaline (Entry
7). Importantly, the use of a fused heteroarene is not essential, and
methoxypyridines underwent amination. A methoxy group located
at 3- and 4-position of pyridine can be substituted by an amine
under these nickel-catalyzed conditions (Entries 8 and 9). These
results are in sharp contrast to the reactivity profile observed in the
SuzukieMiyaura reaction of methoxyarenes, where substrates
without a fused ring are much less reactive.4 On the other hand, the
yield was significantly decreased when an anisole derivative
bearing a 2-pyridyl group was employed (Entry 10). These results
indicate that the use of a methoxy group directly connected to N-
heteroarene is required for an efficient reaction. A current limita-
tion of this catalytic amination is its inapplicability of azoles, such
as indole and benzothiazole (Fig. 1).
Scheme 1. Catalytic amination of aryl electrophiles.
2. Results and discussion
Our attempt to expand the scope of our Ni/IPr-catalyzed
amination reaction to heteroaromatic systems began with ex-
amining the amination of 6-methoxyquinoline (1) because 2-
methoxynaphthalene is one of the most efficient substrates in
various nickel-catalyzed CeOMe bond transformations.4,12,15 Re-
action of 1 with morpholine (2) in the presence of Ni(cod)2
(20 mol %), IPr$HCl (40 mol %), and NaOtBu as a stoichiometric
base in toluene afforded aminated product 3 in 74% isolated yield,
as expected (Entry 1 in Table 1). No aminated product was ob-
served in the absence of nickel catalyst, which clearly excludes
base-promoted amination pathways via nucleophilic aromatic
substitution or heteroaryne (Entry 2). PCy3 was ineffective in this
amination reaction even though it is an optimal ligand in most
nickel-catalyzed CeOMe bond activation reactions (Entry 3).2
Moreover, the use of SIPr, which was an optimal ligand in
nickel-catalyzed amination of aryl carbamates, proved to be less
effective (Entry 4).14 The nature of the stoichiometric base has
a significant effect on the efficiency of the catalysis. The use of
NaOEt (Entry 5), NaHMDS (Entry 6), or Cs2CO3 (Entry 7) in place
of NaOtBu did not afford 3. When amount of amine, reaction
temperature, or catalyst loading was decreased, the amination
proceeded with slightly lower yield (Entries 8e10). As a result,
we decided to employ the conditions shown in Entry 1 for the
scope and limitation studies of this nickel-catalyzed amination
reaction.
Table 1
Optimization studiesa
We also found that an array of secondary amines can be coupled
with methoxy-substituted N-heteroarenes (Table 3). Cyclic amines
including pyrrolidines (Entry 1) and piperidines (Entries 2 and 3)
proved to be suitable for this nickel-catalyzed amination reaction.
Piperazines also served as an amine component to afford an N-
arylated product (Entries 4 and 5). Because a Boc protecting group
remains intact under these condition, the other nitrogen of a pi-
perazine ring can be used for further modification. The steric bulk
around the nitrogen center has a profound impact on the efficiency
of this amination reaction, as is evident from the lower yields
obtained with acyclic amines compared with cyclic ones (Entry 6).
An aniline derivative also underwent this amination although the
yield was low (Entry 7). A current limitation of this amination
protocol regarding the scope of the amine coupling partner is that
primary amines are unsuitable.
Entry
Variation from the ‘standard’ conditions
Yield/%b
1
2
3
4
5
6
7
8
9
None
75
0
0
54
0
Trace
0
54
69
73
No Ni(cod)2 and IPrꢀHCl
PCy3 instead of IPrꢀHCl
SlPrꢀHCl, instead of IPrꢀHCl
NaOEt, instead of NaOtBu
NaHMDS, instead of NaOtBu
Cs2CO3, instead of NaOtBu
2.0 equiv of 2
90 ꢁC
The present method allows readily available and robust
methoxy-substituted N-heteroarenes in catalytic amination
reactions. In addition, sequential formation of two different
CeN bonds is also possible by combining MeO amination with
10
10 mol % Ni(cod)2, 20 mol; % IPrꢀHCl
a
Reaction conditions: 1 (0.5 mmol), 2 (2.5 mmol), Ni(cod) 2 (0.10 mmol), ligand
(0.20 mmol), base (3.0 mmol), toluene (1.5 mL) at 100 ꢁC for 12 h in a sealed tube.
b
Isolated yield of 3 based on 1.

M. Tobisu et al. / Tetrahedron 68 (2012) 5157e5161
5159
Table 2
Table 3
Ni(0)/NHC-catalyzed amination of N-heteroarenes using 2a
Ni(0)/NHC-catalyzed amination of 1a
Entry
1
Amine
Product
Yield/%b
Entry
1
N-Heteroarene
Product
Yield/%b
75
78
78
2
3
2
67
48
61
3c
4
24
65
4
65
52
5c
5c
6
78
71
6d
26e
7c,d
7d
24e (49)f
8
9
78
61
a
Reaction conditions: 1 (0.5 mmol), amine (2.5 mmol), Ni(cod)2 (0.10 mmol),
IPr$HCl (0.20 mmol), NaOtBu (3.0 mmol), toluene (1.5 mL) at 100 ꢁC for 12 h in
a sealed tube.
b
Isolated yield after column chromatography.
Run for 48 h.
At 120 ꢁC.
Isolated yield after HPLC.
NMR yield.
c
d
e
f
10c,d
44
conventional halide amination. For example, commercially avail-
able 3-bromo-5-methoxypyridine (4) underwent copper-catalyzed
reaction with indole,18 and a new CeN bond was formed at the
bromide site (Scheme 2). Subsequent treatment of thus obtained 5
with 2 and Ni(cod)2/IPr catalyst led to the formation of a second
CeN bond via the cleavage of the CeOMe bond. CeOMe amination
could potentially be combined with a number of other trans-
formations because of the inherent stability of methoxy groups
under typical conditions for palladium-catalyzed reactions as well
as strongly basic conditions, such as BuLi. In other words, the
present method allows late-stage introduction of an amino group,
because methoxy groups are normally unreactive and survives over
synthetic sequences. Common oxygen-based electrophiles, such as
OTf, OMs, and OTs, would be too reactive to be employed in ami-
nation at a late-stage. Scheme 2.
a
Reaction conditions: N-heteroarene (0.5 mmol),
2
(2.5 mmol), Ni(cod)2
(0.10 mmol), IPr$HCl (0.20 mmol), NaOtBu (3.0 mmol), toluene (1.5 mL) at 100 ꢁC for
12 h in a sealed tube.
b
Isolated yield after column chromatography.
c
At 120 ꢁC.
d
Run for 48 h.
OMe
OMe
N
S
N
H
Fig. 1. Heteroarenes unsuitable for nickel-catalyzed amination.

5160
M. Tobisu et al. / Tetrahedron 68 (2012) 5157e5161
purchased from Strem Chemicals and used as received. PCy3,
Cs2CO3, 3-methoxypyridine, 4-phenylpiperidine, and 1-Boc-piper-
azine were purchased from Aldrich and used as received. NaOtBu,
IPr$HCl, SIPr$HCl, 6-methoxy-2-methylquinoline, 5-methoxy-2-
methylbenzothiazole, and 2-(4-methoxyphenyl)pyridine were
purchased from TCI and used as received. N-Butylmethylamine
were purchased from Wako Chemicals and used after distillation.
6-Methoxyquinoline, pyrrolidine, and piperidine were purchased
from Aldrich and used after distillation. Morpholine and 1-
phenylpiperazine were purchased from TCI and used after distil-
lation.
3-Methoxyquinoline,19
8-methoxyquinoline,20
6-
methoxyisoquinoline,21 and 6-methoxyquinoxaline22 were pre-
pared according to the literature procedures.
4.3. Representative procedure for Ni-catalyzed amination of
72%
N-heteroaryl methyl ethers (Entry 1 in Table 2)
Ni(cod)2 (27.5 mg, 0.1 mmol), 1,3-bis(2,6-diisopropylphenyl)
imidazolium chloride (85.0 mg, 0.2 mmol), sodium tert-butoxide
(288.3 mg, 3.0 mmol), 6-methoxyquinoline (1, 79.6 mg, 0.5 mmol),
morpholine (2, 217.8 mg, 2.5 mmol), and toluene (1.5 mL) were
added to a 10 mL sample vial with a Teflon-sealed screwcap in
a glovebox filled with nitrogen. After the cap was closed, the vial
was stirred at 100 ꢁC for 12 h. After cooling to room temperature,
the crude mixture was filtered through a Celite pad. The filtrate was
concentrated in vacuo. The residue was purified by flash column
chromatography on silica gel (hexane/EtOAc¼1:1) to give 4-(qui-
nolin-6-yl)morpholine (3, 58.9 mg, 74%) as a white solid.
Scheme 2. Sequential CeN bond formations via CeBr/CeOMe bond activation.
3. Conclusion
In summary, we show that a nickel(0)/IPr catalyst promotes
amination of methoxy-substituted N-heteroarenes via the cleavage
of normally unreactive CeO bonds. Electron-deficient N-hetero-
arenes, such as pyridine and quinoline proved to be suitable sub-
strates for this amination reaction. Given the widespread use of such
heteroarenes in pharmaceuticals, organic materials, and ligands for
transition metals, the present protocol would offer several new
synthetic strategies for elaborating these scaffolds, including the use
of readily available heteroarene partners and late-stage amination.
Further exploration of catalytic transformations through the cleav-
age of a CeOMe bond is currently underway in our laboratory.
4.4. Spectroscopic data for a representative amination
product
4-(Quinolin-6-yl)morpholine (3). Rf 0.36 (EtOAc). White solid.
1H NMR (CDCl3, 399.78 MHz): 3.29 (t, J¼4.8 Hz, 4H), 3.92 (t,
J¼4.8 Hz, 4H), 7.03 (d, J¼2.8 Hz, 1H), 7.32 (q, J¼4.0 Hz, 1H), 7.49 (dd,
J¼2.8, 9.2 Hz, 1H), 7.99 (d, J¼4.8 Hz, 1H), 8.01 (d, J¼2.4 Hz, 1H), 8.73
(dd, J¼1.2, 4.0 Hz, 1H). 13C NMR (CDCl3, 100.53 MHz): 49.4, 66.8,
108.9, 121.4, 122.0, 129.3, 130.1, 134.7, 144.0, 147.8, 149.2. HRMS
calcd for C13H14N2O: 214.1106, found: 214.1107.
4. Experimental section
4.1. General
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas "Molecular Activation Directed toward
Straightforward Synthesis" from MEXT, Japan. We also thank the
Instrumental Analysis Center, Faculty of Engineering, Osaka Uni-
versity, for assistance with HRMS.
1H and 13C NMR spectra were recorded on a JEOL ECS-400 spec-
trometer (JEOL, Tokyo, Japan) in CDCl3 with tetramethylsilane as an
internal standard. Data are reported as follows: chemical shift inparts
per million
(d), multiplicity (s¼singlet, d¼doublet, t¼triplet,
q¼quartet, and m¼multiplet), coupling constant (Hz), and in-
tegration. Infrared spectra were obtained on a Horiba FT-700 spec-
trometer (Horiba, Kyoto, Japan); absorptions are reported in
reciprocal centimeters with the following relative intensities: s
(strong), m (medium), or w (weak). Mass spectra were obtained on
a Shimadzu GCeMS-QP 5000 or GCeMS-QP 2010 spectrometer
(Shimadzu, Kyoto, Japan) with ionization voltage of 70 eV. High res-
olution mass spectra (HRMS) were obtained on a JEOL JMS-DX303.
Column chromatography was performed with SiO2 (Silycycle Silica
Flash F60 (230e400 mesh)). Some aminated products indicated be-
low were purified with LC-9210NEXT HPLC. All amination reactions
were carried out in 10 mL sample vials with Teflon-sealed screwcaps.
Chemicals were manipulated in a glovebox filled with nitrogen.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2012.04.005.
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