BiCl3-catalyzed hydroamination of norbornene with aromatic amines

Article abstract of DOI:10.1002/ejoc.200700483

A BiCl₃-catalyzed hydroamination reaction of norbornene, in which a variety of electron-withdrawing groups were tolerated on amines, was presented. The current transformation possesses the advantages of being highly selective, cheap, and eco-fr

Full text of DOI:10.1002/ejoc.200700483

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.200700483
BiCl3-Catalyzed Hydroamination of Norbornene with Aromatic Amines
Hua Wei,[a] Guimin Qian,[b] Yuanzhi Xia,[b] Kai Li,[a] Yahong Li,*[a,c] and Wu Li[b]
Keywords: Hydroamination / Lewis acids / Bismuth / Catalysis
A BiCl3-catalyzed hydroamination reaction of norbornene, in
which a variety of electron-withdrawing groups were toler-
ated on amines, was presented. The current transformation
possesses the advantages of being highly selective, cheap,
and eco-friendly, and this process also represents a rare sys-
tem for main-group Lewis acid catalyzed intermolecular hy-
droamination of unactivated alkenes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
project with an aim to discover novel catalysts for olefin
hydroamination, we found that only limited attention has
been paid to main-group metals in hydroamination reac-
tions.[11] Among the few publications in this field, the recent
work of Shibasaki and co-workers has achieved remarkable
progress,[12] in which Bi(OTf)3 was found to be an efficient
catalyst for diene and vinylarene hydroaminations. We won-
dered whether commonly used main-group Lewis acids
could also serve as competent catalysts; therefore, we
studied the intermolecular hydroamination of norbornene
with aromatic amines by using several main-group metal
salts. Herein our preliminary results are reported with the
focus on the performance of BiCl3. The bismuth salt is
highly selective and air and moisture tolerable, and it is also
recoverable with additional procedures; its activity is supe-
rior to that of early transition metals. A plausible mecha-
nism for the reaction is proposed and discussed.
Introduction
The hydroamination of alkenes represents an atom eco-
nomic process for the synthesis of amine and amine deriva-
tives that are prevalent in natural products and compounds
of pharmaceutical significance.[1] Efforts to develop such
processes have been intensified in recent years. To achieve
efficient transformation, the late transition metals are
widely used as catalysts. Both intramolecular and intermo-
lecular hydroamination of alkenes have been successfully
developed by using Au,[2] Pt,[3] Rh,[4] and other noble-tran-
sition-metal complexes.[5] Recently, through the elegant
work of the Bergman and Ackermann groups, great ad-
vances have been made to use early transition metals, as
evidenced by the hydroamination of norbornene and sty-
rene.[6] Along with the transition-metal catalysts, there also
have been examples using lanthanide complexes,[7] Bronsted
acids,[8] and other catalysts,[9] which provide alternative pro-
cedures for olefin hydroamination.
Whereas transition-metal catalyzed hydroamination reac-
tions are widely pursued, it should be noted that these cata-
lysts have the drawbacks of high cost, toxicity, and de-
manding reaction conditions, as most of the transition met-
als are sensitive to moisture and air. Recent work on olefin
hydroamination with commonly used transition-metal
Lewis acids are outstanding,[6b,6c,10] although the mecha-
nisms of these reactions are still unrevealed. As an ongoing
Results and Discussion
Initial experiments were performed with norbornene (1)
and 2,5-dichloroaniline (2a), and several readily available
Lewis acids were screened as the catalyst for the reactions.
Under ambient conditions, the catalyst, 1, 2a, and toluene
were sealed in a pressure tube and heated in an oil bath at
169 °C. Subsequently, the resulting mixture was analyzed
by GC-MS, and the products were isolated by flash
chromatography. The results are summarized in Table 1.
As shown in Table 1, when the strong Lewis acid
B(C6F5)3 was used (Table 1, Entry 1), no product was de-
tected during a period of 5 h, and 80% of norbornene start-
ing material was recovered. This may be attributed to the
fact that the coordination of the amine to B(C6F5)3 some-
what hindered the hydroamination and hydroalkylations.[13]
All the chloride salts, AlCl3 (Table 1, Entry 2), FeCl3
(Table 1, Entry 3),[10] BiCl3 (Table 1, Entry 4), and SnCl4
(Table 1, Entry 8), were competent catalysts and afforded
[a] Key Laboratory of Organic Synthesis of Jiangsu Province,
College of Chemistry and Chemical Engineering, Suzhou
University,
Suzhou, 215123, China
Fax: +86-512-65880089
E-mail: liyahong@suda.edu.cn
[b] Qinghai Institute of Salt Lakes, Chinese Academy of Science,
Xining, 810008, China
[c] State Key Laboratory of Applied Organic Chemistry (Lanzhou
University),
Lanzhou 730000, China
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2007, 4471–4474
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H. Wei, G. Qian, Y. Xia, K. Li, Y. Li, W. Li
SHORT COMMUNICATION
Table 1. Hydroamination reaction between norbornene (1) and 2,5- BiCl3-catalyzed intermolecular hydroamination of norbor-
dichloroaniline (2a) catalyzed by various Lewis acids.
nene with various aniline derivatives was studied.
As shown in Table 2, BiCl3 catalyzed the reactions be-
tween norbornene (1) and a number of aniline derivatives,
and the hydroamination products were obtained as the
major products.[16] The reaction of aniline itself gave a mod-
erate yield of 3b, whereas a comparable amount of byprod-
uct 4 was isolated (Table 2, Entry 1). The electron-with-
Entry
Catalyst
Isolated yield of 3a [%]
drawing groups enhance the chemoselectivity with the
yields of the hydroamination products from moderate-to-
excellent (Table 1, Entry 4; Table 2, Entries 2–12). Notably,
when the aniline was monosubstituted with NO2 (Table 2,
Entry 2), Cl (Table 2, Entry 7), or Br (Table 2, Entry 9)
groups in the para position, type 3 products were obtained
as the sole product, and 80% yield of 3m was obtained with
2,4-dichloroaniline (Table 2, Entry 12). However, in some
cases, hydroarylation occurred during the hydroamination
process as a minor side reaction (Table 2, Entries 4–6, 8,
10, and 11). Two hydroarylation byproducts were detected
occasionally (Table 2, Entries 5, 6, 8, and 10), yet the yields
of these byproducts were significantly lower relative to
those of the side reaction of unsubstituted aniline (Table 2,
Entry 1). These results are different from TiCl4-catalyzed
hydroamination of norbornene, in which an ortho hydro-
arylation product was the only side product.[6b] It is inter-
esting to find that electron-donating groups in the para po-
sition of the aniline inhibit the reaction, as no products
were observed with 4-methoxyaniline or 4-methylaniline as
the nitrogen sources (Table 2, Entries 13 and 14).
1
2
3
4
5
6
7
8
B(C6F5)3
AlCl3
FeCl3
BiCl3
ZnF2
ZnCl2
I2
0
86
88
92
0
trace
6
SnCl4
84
quite good yields of 3a. To our disappointment, no reaction
occurred when ZnF2 was used (Table 1, Entry 5), and only
a trace amount of product was detected in the case of ZnCl2
(Table 1, Entry 6). It is noteworthy that molecular iodine
(Table 1, Entry 7), which can be defined as a weak Lewis
acid,[14] could also promote the hydroamination reaction,
yet the yield was only 6% under the nonoptimized condi-
tions.
These initial results indicated that several main-group
Lewis acids could serve as catalysts in the intermolecular
hydroamination of norbornene with aromatic amines, and
BiCl3 was found to give the best yield. Given that BiCl3 can
be easily recovered from the heterogeneous reaction sys-
The hydroamination of secondary amine 6 with norbor-
tem,[15] it is believed that BiCl3-catalyzed reactions also nene was also investigated (Scheme 1). No hydroamination
have the advantage of being environmentally benign. To reaction was detected, and ortho-substituted hydroarylation
further test the performance of this useful catalyst, the product 7 was obtained in moderate yield.[17]
Table 2. Scope of the BiCl3-catalyzed hydroamination reaction of norbornene (1).[a]
Entry
Amine
Time [h]
Ratio of 3/4/5[b]
3
Isolated yield of 3 [%]
1
2
3
4
5
6
7
8
aniline
5
6
6
6
4
4
3
4
5
4
6
4
6
6
58:42:0
100:0:0
100:0:0
98:2:0
83:13:4
82:8:10
100:0:0
96:3:1
100:0:0
87:10:3
78:22:0
100:0:0
0:0:0
3b
3c
43
67
69
80
84
65
54
61
63
73
59
80
0
4-nitroaniline
2-nitroaniline
3-nitroaniline
2-chloroaniline
3-chloroaniline
4-chloroaniline
3,4-dichloroaniline
4-bromoaniline
2-fluoroaniline
4-fluoroaniline
2,4-dichloroaniline
4-methoxyaniline
4-methylaniline
3d
3e
3f
3g
3h
3i
3j
9
10
11
12
13
14
3k
3l
3m
none
none
0:0:0
0
[a] Reaction conditions: 1 (2 mmol), 2 (8 mmol), BiCl3 (10 mol-%), toluene (4 mL). [b] By GC–MS analysis.
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BiCl3-Catalyzed Hydroamination of Norbornene with Aromatic Amines
Conclusions
We presented here efficient Lewis acidic BiCl3-catalyzed
hydroamination reactions of norbornene (1), in which a
variety of electron-withdrawing groups were tolerated in
amines. This reaction is not only an atom-economic process
with high chemoselectivity, simple operation, and further
reusable catalyst, but also represents a rare system for main-
group metal-catalyzed intermolecular hydroamination of
unactivated alkenes. A general mechanism for the reaction,
in which the carbocation was proposed as the key interme-
diate, was provided. Further studies on the development of
the catalyst, the scope of the substrate, and the mechanism
in the field of hydroamination are undergoing in our group.
Scheme 1. BiCl3-catalyzed reaction of norbornene with a secondary
amine.
A mechanistic hypothesis, which accounts for the cata-
lytic cycle for the formation of the hydroamination products
as well as hydroarylation products, is provided in Scheme 2.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures for the hydroamination re-
action and spectroscopic data.
Acknowledgments
The authors appreciate the financial support of Hundreds of Tal-
ents Program (2005012) of CAS, Natural Science Foundation of
Jiangsu Province (BK2005030) and Suzhou University. The authors
would like to thank Prof. Aaron L. Odom for editorial comments
on a previous version of the manuscript and for helpful scientific
discussion.
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In the process, complex 8 can be firstly formed from the
Lewis acid/base interaction between BiCl3 and aniline 2, in
which the acidity of the amido H is increased as the result
of the electron donation from N to the metal. Protonation
of norbornene 1 by complex 8 will give rise to carbocationic
intermediate 10 and an anionic bismuth complex 9 as well.
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hydroarylation products, respectively, through deproton-
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SHORT COMMUNICATION
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current reaction conditions. By using 4methylbenzene-
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mization is needed to obtain desirable results.
[17] A similar result was previously reported, see ref.[6b]
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Received: May 27, 2007
Published Online: July 30, 2007
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Eur. J. Org. Chem. 2007, 4471–4474