Amination of aryl halides using copper catalysis

Article abstract of DOI:10.1016/S0040-4039(01)00458-0

Bromopyridine 4 was converted into aminopyridine 5 under Cu₂O catalysis with an ethylene glycol solution of ammonia in excellent yield (90%). The amination reaction features low (0.5 mol%) catalyst loading, mild reaction temperature (80°C) and low reaction pressure (50 psi). This protocol is further studied in the amination of a variety of aryl halides.

Full text of DOI:10.1016/S0040-4039(01)00458-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3251–3254
Amination of aryl halides using copper catalysis
Fengrui Lang,* Daniel Zewge, Ioannis N. Houpis and R. P. Volante
Merck Research Laboratories, Department of Process Research, PO Box 2000, Rahway, NJ 07065, USA
Received 16 February 2001; revised 12 March 2001; accepted 13 March 2001
Abstract—Bromopyridine 4 was converted into aminopyridine 5 under Cu2O catalysis with an ethylene glycol solution of
ammonia in excellent yield (90%). The amination reaction features low (0.5 mol%) catalyst loading, mild reaction temperature
(80°C) and low reaction pressure (50 psi). This protocol is further studied in the amination of a variety of aryl halides. © 2001
Elsevier Science Ltd. All rights reserved.
Aryl amines in general, and aminopyridines in particular,
are useful pharmacophores or intermediates1 in several
classes of drug candidates. Traditionally, the most com-
mon way to access these compounds is by amination of
the corresponding aryl halides using ammonia or ammo-
nium hydroxide as the nucleophile under high tempera-
ture and pressure. More recently, significant progress in
palladium-catalyzed amination reactions has been
achieved by Buchwald2 and Hartwig3, demonstrating
mild conditions and tolerance for a wide range of
functional groups. Subsequently, this methodology has
seen useful applications4 in the preparation of pharma-
ceuticals and other interesting compounds. However, the
lone shortcoming of this palladium-catalyzed amination
technology is that it does not provide direct entry to
primary aryl amines, and additional step(s) are necessary
to unmask latent or protected primary amine surrogates.
Hence, a direct amination of aryl halides under mild
conditions will complement the existing palladium-cata-
lyzed aminations technology.
prepared by amination of the bromopyridine 4 using
benzophenone imine as a primary amine surrogate fol-
lowing the Buchwald/Hartwig protocol. However, a
more direct amination strategy was desirable.5 Herein we
report a direct, practical, and efficient conversion of 4
to the desired aminopyridine 3 using copper(I) oxide
(Cu2O) catalysis (Scheme 1).
Copper salts have been reported to catalyze the amina-
tion of halopyridines6 and earlier examples have shown
that the amination generally initiates at temperatures
between 150°–250°C with either aqueous ammonium
hydroxide or anhydrous ammonia.7,8 More recently9, the
amination of an aryl bromide was reported using liquid
ammonia as solvent and Cu/CuCl couple to induce the
transformation. Although a mild temperature (70°C) was
used and a good yield was achieved in the reaction, high
pressure and a long reaction time were associated with
this process and 75 mol% Cu loading was required. In
our studies, we were constrained by safety concerns and
scale-up issues and thus required considerably milder
reaction conditions.
In our recent efforts toward the synthesis of muscarinic
(M3) antagonist 1,5 the amine portion 3 could indeed be
O
OH
OH
+
F
N
NH2
2
O
N
F
N
Br
N
N
H
N
NH2
OH
F
F
N
RHN
1
4a: R = Ac-
4b: R = Boc-
H2N
3
Scheme 1. Synthesis of M3-antagonist 1.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00458-0

3252
F. Lang et al. / Tetrahedron Letters 42 (2001) 3251–3254
Table 1. Amination of bromopyridine 4a in different solventsa
Entry
Cu
Solvents
Conversion (%)
5a:6a
Assayed yield (%)
1
2
3
4
5
6
CuCl2
CuCl2
Cu/CuCl
Cu/CuCl
CuCl2
H2O
100
B10
100
B10
B5
0
78:22
83:17
83:17
65
–
70
–
–
–
MeOH
MeOH
EtOH
iPrOH
ACN
–
–
–
CuCl2
7
CuCl2
NMP
50
100:0
–
8
9
CuCl2
CuCl2
Cu/CuCl
Cu2O
DMF
THF
Ethylene glycol
Ethylene glycol
B2
B2
100
100
–
–
94:6
94:6
–
–
91
91
10b
11b
a General procedure for the amination reactions: In a 25 mL stainless steel vessel were mixed 1.0 g bromopyridine 5a, 10 mL solvent, 2 mL liquid
ammonia and 5.0 mg Cu catalyst. The vessel was sealed and heated to 100°C overnight. Reaction products were analyzed by HPLC.
b In the absence of the copper catalysts, under otherwise identical conditions, no conversion to the product was observed.
N
OR'
N
Br
N
NH2
Cu
N
N
N
+
RHN
NH3/R'OH
RHN
RHN
(1)
6a: R = Ac-
6b: R = Boc-
5a: R = Ac-
5b: R = Boc-
4a: R = Ac-
4b: R = Boc-
Initially, we were pleased to find that the amination of
4a (Eq. (1)) proceeded smoothly in aqueous ammonium
hydroxide under CuCl2 catalysis to afford 5a in 65%
yield. The major by-product was pyridone 6a from
competing hydroxide attack on the aryl halide. To
optimize the amination reaction, a variety of solvent
systems as well as several forms of copper catalyst were
investigated (at 100°C in a closed vessel) and the results
are summarized in Table 1.
reaction proceeded to completion affording aminopy-
ridine 5a in 91% yield and the product ratio of 5a:6a
was 16:1. Both Cu2O and Cu/CuCl promote the trans-
formation. Most importantly, the solubility of ammo-
nia in ethylene glycol even at high temperature, is
sufficient to allow the reaction to proceed at an appre-
ciable rate with moderate pressure build up. Cu2O is
generally preferred over Cu/CuCl as the catalyst of
choice due to the practical simplicity of operation and
cleanliness of the reaction profile. The reaction in ethyl-
ene glycol was further studied in order to look at and
hence optimize the correlation between the N versus O
selectivity and the reaction pressure. The results are
summarized in Table 2. A saturated solution of ammo-
nia in ethylene glycol was prepared at 10°C and was
determined to be 8 M by titration. At this concentra-
tion, the pressure generated during the reaction at
100°C was ca 100 psi. A small decrease in the ammonia
concentration only modestly reduced the selectivity and
yield of the reaction while the pressure in the reaction
vessel was reduced substantially. Even with the ammo-
nia at 2 M concentration, the reaction proceeded
smoothly at low pressure, affording the desired amina-
tion product 5a in respectable yield. Thus, the transfor-
mation has been achieved in good yield and selectivity
under pressures which can easily be implemented in the
laboratory or at pilot scale. It is also worth noting that
The experimental data shows that the amination reac-
tion worked best in aqueous or alcoholic solvents
within a reaction temperature range of 70–100°C under
concentration and catalyst loadings described in Table
1. In alcoholic solvents, a significant amount of alkoxy
pyridine 6a was also formed in the reaction. In an
attempt to avoid this by-product, several polar, aprotic
solvents, including THF, acetonitrile, 1-methyl-2-pyrro-
lidinoe (NMP) and DMF (entries 6–9) were studied. In
THF, acetonitrile and DMF, very low conversion to
products was observed while moderate (50%) conver-
sion was seen in NMP. The latter reaction (NMP)
proceeded cleanly; however, the pressure generated dur-
ing the reaction was too high and it could not be scaled
up. The low conversion and high reaction pressure are
attributed to the poor solubility of ammonia in these
solvent systems. The use of MeOH (entries 2 and 3)
instead of water as solvent improved the selectivity
slightly and the combination of Cu/CuCl was demon-
strated to be more effective than CuCl2 as the catalyst.
Interestingly, in EtOH and iPrOH (entries 4, and 5),
the amination reaction only proceeded with low conver-
sion and the pressure inside the reaction vessel during
the reaction was much higher than with water or
MeOH.
Table 2. Study of amination reaction of 4a under different
pressures
Entry Conc. of NH3 5a:6a Pressure (psi) Assay yield (%)
1
2
3
4
8 M
6 M
4 M
2 M
94:6
92:8
91:9
89:11
100
50
30
91
89
88
86
Ethylene glycol (entries 10, and 11) was found to be the
best solvent to date for the amination reaction; the
20

F. Lang et al. / Tetrahedron Letters 42 (2001) 3251–3254
Table 3. Applications of amination protocola
3253
Entry
Substrate
Product
Yield (%)
Selectivity
1
2
3
4
5
6
7
8
2-Bromopyridine
2-Aminopyridine
65
70
75
99
0
85
85
81
62
65
72
74
94
84
80:20
86:14
92:8
100:0
N/A
100:0
95:5
97:3
91:9
91:9
2-Bromo-6-picoline
2-Bromo-6-methoxypyridine
2-Bromo-5-nitropyridine
2-Chloropyridine
2-Chloro-5-nitropyridine
3-Bromopyridine
2-Amino-6-picoline
2-Amino-6-methoxypyridine
2-Amino-5-nitropyridine
2-Aminopyridine
2-Amino-5-nitropyridine
3-Aminopyridine
4-Bromopyridine
4-Aminopyridine
9b
10
11
12
13
14
2,5-Dibromopyridine
4%-Bromoacetophenone
1-Bromo-4-trifluoromethylbenzene
Iodobenzene
2-Bromothiazole
3-Bromoquinoline
2-Amino-5-bromopyridine
4%-Aminoacetophenone
4-Trifluromethylaniline
Aniline
2-Aminothiazole
3-Aminoquinoline
75:25
86:14
100:0
91:9
a Reaction conditions: in a stainless steel vessel were mixed 1.0 g aryl bromide, 5.0 mg Cu2O, and 10 mL 8 M NH3 solution in ethylene glycol.
The vessel was sealed and the reaction mixture heated at 80°C for 16 h
b The amination of 2,5-dibromopyridine was done at 60°C for 40 h to give mono-amination product. Under the normal conditions, the major
product is 2,5-diaminopyridine.
the catalyst (Cu2O) loading can be as low as 0.5 mol%,
which makes the process very environmentally friendly.
bond, resulting in improved selectivity. The good selec-
tivity for the amination of 2-bromo-6-methoxypyridine
(entry 3) also relies significantly on complexation, as
opposed to an electronic factor. With careful control of
reaction conditions, selective mono-amination is
achieved (entry 9) and the amination takes place prefer-
entially at the 2-position rather than the 5-position of
the pyridine ring. Aryl chlorides generally are inert
toward the amination conditions (entry 5). However,
with proper activation they can be converted to the
desired product under the same reaction conditions
(entry 6) in good yield. For bromobenzene derivatives
with electron-withdrawing substituents (entries 10 and
11), the reaction proceeds smoothly while the ketone
functionality is compatible with the amination condi-
tions. Iodobenzene (entry 12) was converted success-
fully to aniline in 74% yield. Several other heterocyclic
bromides can also be converted to the corresponding
amines with good N versus O ratios and useful yields
(entries 13 and 14).
A typical experimental procedure is as follows: A 1 liter
pressure tested autoclave was charged with Cu2O (0.25
g), then 50 g of bromide 4b as a slurry in ethylene
glycol (450 mL). The mixture was cooled to 0°C and
liquid ammonia (75 g) was added over 30 min. The
temperature was kept below 10°C during the addition.
The autoclave was then sealed and the reaction mixture
was heated to 80°C for 16 h. The pressure in the
autoclave during the reaction is about 50 psi. The
reaction mixture was cooled to 10°C and drained into a
3 liter flask for workup. The reaction mixture was
adjusted to pH 10.5 with 2 M H2SO4 and extracted
with 900 mL EtOAc. The extract was solvent switched
to iPrOH (total volume 900 mL), then a solution of
PTSA monohydrate (53.92 g) in 600 mL iPrOH was
added over 2 h at 15°C. The slurry was stirred for 4 h
and the solid collected: 74.7 g (85%) product (99.9%
purity as determined by HPLC analysis) as its bis-
TsOH salt. It is noteworthy that the Boc protecting
group in 4b survived the amination conditions.
In summary, we have found general, practical amina-
tion reaction conditions that use Cu catalysts in a
solution of ammonia in ethylene glycol to convert aryl
halides into aryl amines in good yields and selectivities.
The success of this amination reaction protocol
prompted us to explore the scope of the amination
conditions. The results are summarized in Table 3.
Acknowledgements
As shown in Table 3, a series of 2-bromopyridine
substrates (entries 1–4, and 9) were investigated. With
an electron withdrawing substituent, the amination
reaction proceeded with better yield and selectivity.
Interestingly, while piperidine derivative 4 is similar
electronically to 2-bromopyridine and 2-bromo-6-picol-
ine (entries 1 and 2), the amination of 4 gave much
higher yield and selectivity. Our hypothesis is that the
bidentate complexation of 4 with Cu facilitates the
delivery and insertion of Cu to the carbonꢀbromine
We thank Mr. A. L. Houck and Mr. C. Bazaral for
their support during high pressure laboratories and Dr.
T. Wang, Ms. J. T. Wu and Dr. X. Jia for their
expertise in metal analysis. We also want to thank Dr.
J. Z. Song and Dr. D. Tschaen for helpful suggestions
and we acknowledge the continuing modification and
development work by Drs. T. Wada and T. Iida of
Banyu Pharmaceutical Company, Japan.

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F. Lang et al. / Tetrahedron Letters 42 (2001) 3251–3254
4. (a) Hong, Y.; Tanoury, G. J.; Wilkinson, H. S.; Bakale, R.
References
P.; Wald, S. A.; Senanayake, C. H. Tetrahedron Lett. 1997,
38, 5607–5610; (b) Hong, Y.; Senanayake, C. H.; Xiang,
T.; Vandenbossche, C. P.; Tanoury, G. J.; Bakale, R. P.;
Wald, S. A. Tetrahedron Lett. 1998, 39, 3121–3124; (c) Bei,
X.; Guram, A. S.; Turner, H. W.; Weinberg, W. H.
Tetrahedron Lett. 1999, 40, 1237–1240.
1. Lechat, P.; Tesleff, S.; Bownan, W. C. Aminopyridines and
Similarly Acting Drugs; Pergamon Press: Oxford, 1982.
2. (a) Wagaw, S.; Buchwald, S. L. J. Org. Chem. 1996, 61,
7240–7241; (b) Wolfe, J. P.; Tomori, H.; Sadighi, J. P.;
Yin, J.; Buchwald, S. L. J. Org. Chem. 2000, 65, 1158–
1174; (c) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am.
Chem. Soc. 1996, 118, 7215–7216; (d) Old, D. W.; Wolfe,
J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1998, 120,
9722–9723; (e) Marcoux, J.-F.; Wagaw, S.; Buchwald, S.
L. J. Org. Chem. 1997, 62, 1568–1569; (f) Wagaw, S.;
Yang, B. H.; Buchwald, S. L. J. Am. Chem. Soc. 1998,
120, 6621–6622.
3. (a) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaugh-
nessy, K. H.; Alcazar-Roman, L. M. J. Org. Chem. 1999,
64, 5575–5580; (b) Stauffer, S. R.; Lee, S.; Stambuli, J. P.;
Hauck, S. I.; Hartwig, J. F. Org. Lett. 2000, 2, 1423–1426;
(c) Louie, J.; Driver, M. S.; Hamann, B. C.; Hartwig, J. F.
J. Org. Chem. 1997, 62, 1268–1273; (d) Alcazar-Roman, L.
M.; Hartwig, J. F.; Rheingold, A. L.; Liable-Sands, L. M.;
Guzei, I. A. J. Am. Chem. Soc. 2000, 122, 4618–4630.
5. (a) Manuscript submitted for publication in J. Am. Chem.
Soc. describing the synthesis of M3-antagonist 1; (b) Syn-
thesis of muscarinic receptor antagonist: Grover, P. T.;
Bhongle, N. N.; Wald, S. A.; Senanayake, C. H. J. Org.
Chem. 2000, 65, 6283–6287; Senanayake, C. H.; Fang, Q.
K.; Grover, P.; Bakale, R. P.; Vandenbossche, C. P.;
Wald, S. A. Tetrahedron Lett. 1999, 40, 819–822.
6. Lindley, J. Tetrahedron 1984, 40, 1433–1456.
7. (a) Wibaut, J. P.; Nicolai, J. R. Rec. Trac. Chim. 1939,
709; (b) Zwart, C.; Wibaut, J. P. Rec. Trac. Chim. 1955,
1062.
8. (a) Fischer, O. Ber. 1899, 32, 1297–1307; (b) Maier-Bode,
H. Ber. 1936, 69, 1534.
9. Vedejs, E.; Trapencieris, P.; Suna, E. J. Org. Chem. 1999,
64, 6724–6729.
.
.