Regioselective chlorination and bromination of unprotected anilines under mild conditions using copper halides in ionic liquids

Article abstract of DOI:10.3762/bjoc.8.84

By using ionic liquids as solvents, the chlorination or bromination of unprotected anilines at the para-position can be achieved in high yields with copper halides under mild conditions, without the need for potentially hazardous operations such as supplementing oxygen or gaseous HCl.

Full text of DOI:10.3762/bjoc.8.84

Regioselective chlorination and bromination of
unprotected anilines under mild conditions
using copper halides in ionic liquids
Han Wang1,2, Kun Wen2, Nurbiya Nurahmat2, Yan Shao2, He Zhang2,
Chao Wei2, Ya Li2, Yongjia Shen*1 and Zhihua Sun*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 744–748.
1School of Chemistry and Molecular Engineering, East China
University of Science and Technology, Shanghai, 200237, China and
2College of Chemistry and Chemical Engineering, Shanghai
University of Engineering Science, Shanghai, 201620, China
doi:10.3762/bjoc.8.84
Received: 19 March 2012
Accepted: 04 May 2012
Published: 16 May 2012
Email:
Yongjia Shen* - yjshen@ecust.edu.cn; Zhihua Sun* -
zhihuasun@sues.edu.cn
Associate Editor: J. A. Murphy
© 2012 Wang et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
halogenation; ionic liquids; oxydation; regioselectivity; solvent effects
Abstract
By using ionic liquids as solvents, the chlorination or bromination of unprotected anilines at the para-position can be achieved in
high yields with copper halides under mild conditions, without the need for potentially hazardous operations such as supplementing
oxygen or gaseous HCl.
Introduction
Chlorination and bromination of aromatic rings are classical and However, from an environmental-impact point of view, prob-
widely performed transformations, which are useful in many ably the best chlorination protocol was the one reported several
multistep organic-synthesis procedures. Regioselectivity with decades ago with chloride salts of several transition metals [5].
electron-rich substrates such as aniline will predominantly Copper(II) chloride (1) was found to be one of the best reagents
produce para- and ortho-substitutions. However, chlorination or for this transformation, which yielded mostly the para-chlori-
bromination of aniline derivatives is often performed with nated product with minor ortho- and dichlorinated products.
protected anilines. This increases both the cost of synthesis and The mechanism was believed to be mediated by Cu(II) oxi-
the environmental impact due to the added protection and dation of aniline, followed by addition of the chloride.
deprotection reaction steps.
Despite the good yield and regioselectivity in the original report
Direct chlorination of unprotected aniline is possible by using on the chlorination of unprotected aniline with CuCl2, its appli-
sulfonyl chloride [1], chlorine [2,3] or N-chlorosuccinimide [4]. cation to other unprotected aniline derivatives over the years
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Beilstein J. Org. Chem. 2012, 8, 744–748.
has not been widely reported. Instead, many procedures still
opted for the chlorination of protected aniline derivatives or the
introduction of nitrogen (for example, through nitration) to
chlorobenzenes. Part of the reason could be that the procedure
with CuCl2 is carried out in concentrated aqueous HCl and
requires the flow of both oxygen gas and HCl gas to achieve
high yield. The supplemented oxygen gas reoxidizes the Cu(I)
product back to Cu(II) to keep the reaction efficient, while the
HCl gas keeps the polymerization of unprotected aniline to a
minimum under these conditions. This promoted us to search
for conditions of this transformation that do not require the use
of aqueous or gaseous HCl and oxygen gas.
Scheme 1: Chlorination (or bromination) of unprotected aniline by
CuX2.
Our search for suitable conditions focused on the use of ionic
liquids as solvents. The amount of research into the application
of ionic liquids in organic synthesis has experienced a dramatic The reactions were carried out on a 10 mmol scale (~1 g) of
expansion over the past decade. This has been driven by the 2-methylaniline (2a). To compare the conversion efficiencies of
desires to both find green alternatives to conventional solvents, all trials, we used GC–MS-based analysis of crude reaction
through the physical properties of ionic liquids, and to explore samples after normal workup procedures. For a few represen-
improvements in organic transformations in terms of efficiency tative cases, isolated yields were also listed in Table 1.
and ease of operation [6-11]. In this article, we show that ionic
liquids enable the title transformation to be achieved under mild For reactions in 36% aqueous HCl, the use of oxygen and
conditions without the need for oxygen or gaseous HCl gaseous HCl was essential to obtain good yields for para-chlori-
(Scheme 1).
nation of 2-methylaniline (Table 1, entries 1–5). The reaction
needs to be carried out at 60 °C for several hours with at least
2 equiv of CuCl2. Increasing the amount of CuCl2 to 3 equiv
Results and Discussion
We first used 2-methylaniline (2a) to explore and compare did not significantly change the reaction outcome (Table 1,
chlorination conditions using CuCl2 either in water or in an entry 3). Without supplementary oxygen, no product was
ionic liquid, with or without the use of either oxygen, gaseous formed after 3 h at 60 °C. Without supplementary gaseous HCl,
HCl, or both. The results are summarized in Table 1. The choice only 10% of the desired product was formed after 3 h. Under
of 2-methylaniline will leave one position for either para- or the optimal conditions listed in Table 1 entry 2, the desired pro-
ortho-chlorination, thus simplifying the product analysis.
duct was converted in 82% and isolated in 75% yield in this
Table 1: Exploring solvents and other conditions for the chlorination of 2-methylaniline (2a, 10 mmol scale) using CuCl2.a
entry solvent
CuCl2 e O2
quiv
HCl (g)
T (°C) time
2-methyl-
aniline
remaining
4-Cl
product
4a
2-Cl
di-Cl
product product
4a’
4a’’
1
2
3
4
5
6
36% aq HCl
2
2
3
2
2
3
Yes Yes
Yes Yes
Yes Yes
No Yes
Yes No
40
60
60
60
60
40
2 d
6 h
6 h
3 h
3 h
4 h
55%
10%
8%
43%
1%
—
36% aq HCl
36% aq HCl
36% aq HCl
36% aq HCl
82% (75%)b 5.8%
1.6%
2.1%
—
82%
—
6.5%
—
100%
85%
5%
10%
1.5%
—
1-hexyl-3-methyl-
imidazolium chloride (3a)
No
No
No
No
No
No
92% (91%)b 2.5%
—
7
8
1-isobutyl-3-methyl-
imidazolium chloride (3b)
3
3
40
40
4 h
4 h
7%
89%
88%
3.2%
2.8%
—
—
1-pentyl-3-methyl-
10%
imidazolium chloride (3c)
aProduct distribution was analyzed by GC–MS.
bIsolated yields.
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Beilstein J. Org. Chem. 2012, 8, 744–748.
10 mmol trial. It is of interest to note that, even under optimal the unreacted aniline substrate [5], thus supplementary oxygen
conditions, such as those listed under entries 2 and 3 in Table 1, is necessary to convert Cu(I) back to Cu(II).
no significant amount of any product was formed within the
first 30 min or so of the reaction, as indicated by TLC analysis In ionic liquids, Cu(II) and aniline were both dissolved and thus
of the reaction mixtures. It seems that there is a kinetic delay of optimally mixed. This allows the reaction to proceed at
the chlorination reaction.
maximum speed without delay. In addition, Cu(I) produced
during the reaction did not hinder the reaction with the unre-
The best results for the chlorination of 2-methylaniline were acted aniline, thus, no supplement of oxygen was necessary
obtained with ionic liquids as solvents (Table 1, entries 6–8), during the reaction. In fact, in our experience, with 3 or 4 equiv
which were prepared as reported [12-14]. The conversions to of CuCl2, one can repeatedly perform the chlorination reaction
the desired product were detected to be around 90%, and impor- two or three times, respectively, with no loss of efficiency
tantly, neither oxygen nor gaseous HCl was needed to achieve a before regeneration of Cu(II). In ionic liquids, chloride as a
high yield of conversion. The temperature used to carry out the counter ion of the solvent is in large excess, and Cu(II) regener-
reaction was also lower than that used for the aqueous solvent. ation can be performed offline by passing either oxygen or
The best result was obtained with 1-hexyl-3-methylimida- simply air through the ionic liquid that contains the Cu(I)/Cu(II)
zolium chloride (3a) as solvent, in which a 92% conversion mixture, to restore full reaction power. This important feature
with 91% isolated yield was achieved.
makes the chlorination in ionic liquids much more attractive
than in aqueous HCl in terms of operational safety and environ-
In contrast to reactions in aqueous HCl, reactions in ionic mental impact.
liquids did not seem to have a delay in product formation, as
product formation could be detected by TLC analysis within Further expansion of the chlorination reaction to other aniline
minutes of the start of the reaction. In combination with the analogous was conducted in 1-hexyl-3-methylimidazolium
different requirements for oxygen and gaseous HCl, one may chloride (3a), as summarized in Table 2. All reactions were
attribute the optimal results in ionic liquids to the efficient attempted on larger scales (100 mmol, about 10 g) than the
dissolution and mixing of reactants. In aqueous solvent, CuCl2 initial testing. As shown in Table 2, for all 2- or 3-mono-substi-
is fully solvated by water, but dissolution of organic aniline is tuted anilines excellent results were obtained in terms of
sluggish even with the help of HCl. The reaction between the isolated yields of the desired 4-Cl products, which ranged from
highly solvated Cu(II) and the less optimally solvated aniline is 85–96% (Table 2, entries 1–9, see Supporting Information
therefore hindered by suboptimal mixing. The produced Cu(I) File 1). The substitutions ranged from electron-donating
in the aqueous solvent still hinders the further transformation of substituents, such as the methoxy group, to electron-with-
Table 2: para-Chlorination of aniline analogues (100 mmol scale) using CuCl2 in 1-hexyl-3-methylimidazolium chloride (3a).a
entry
substrate
time
yield of 4-Cl productb
isomer detected in crude
mixturec
1
2-methylaniline (2a)
2-methoxyaniline (2b)
2-fluoroaniline (2c)
4 h
3 h
4 h
6 h
16 h
4 h
3 h
6 h
8 h
4 h
4 h
4 h
4a, 91%
4b, 93%
4c, 88%
4d, 90%
4e, 85%
4f, 95%
4g, 96%
4h, 92%
4i, 94%
4j, —
2.5%
2.7%
2.0%
2.2%
1.8%
1.7%
2.2%
1.9%
1.6%
—
2
3
4
2-trifluoromethylaniline (2d)
2-nitroaniline (2e)
5
6
3-methylaniline (2f)
3-methoxyaniline (2g)
3-fluoroaniline (2h)
7
8
9
3-trifluoromethylaniline (2i)
4-nitroaniline (2j)
10
11
12
4-methoxyaniline (2k)
2-aminopyridine (2l)
4k, —
—
4l, —
—
aAll reactions were carried out with 3 equiv of CuCl2 at 40 °C.
bIsolated yields.
cBased on GC–MS analysis of crude reaction mixtures.
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Beilstein J. Org. Chem. 2012, 8, 744–748.
drawing ones, such as the nitro group. It is not surprising that a
short reaction time (3–4 h) was sufficient for the anilines with
electron-donating substituents, but a longer time (up to 16 h)
was necessary for those with electron-withdrawing substituents.
When 4-substituted anilines were used (entries 10 and 11), no
chlorination products were detected or isolated, indicating that
the most stable resonance of the aniline cation generated by
Cu(II) oxidation had the positive charge at the C4-position.
2-Aminopyridine did not give any product under similar reac-
tion conditions.
Supporting Information
Supporting Information File 1
Experimental section and characterization data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-84-S1.pdf]
Supporting Information File 2
NMR spectra of all compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-84-S2.pdf]
With the success of chlorination in ionic liquids, we also
attempted to extend the reaction to other halides. While
attempts to use fluoride and iodide failed, bromination was Acknowledgements
successfully achieved in high yields and regioselectivity, in an We acknowledge the Innovation Team Building Projects of
ionic liquid with bromide as a counter ion (Table 3). In general, Pharmaceutical Engineering in Shanghai (11K118B, and
the reaction time for bromination was shorter than that for the XKZZ1205), the 2011 Young Teachers Domestic Visiting
corresponding chlorination reaction, and was achieved at lower Scholars Program in Shanghai Universities, the Special Scien-
temperature (room temperature for bromination). In all cases, as tific Foundation for Outstanding Young Teachers in Shanghai
shown in Table 3, high regioselectivity of para-bromination was Higher Education Institutions (gjd10003) and the Students
again achieved, similar to the chlorination results.
Innovation Program in Shanghai University of Engineering
Science (cx1104008, cs1104009, cx1104010, and cs1104033)
for financial support.
Conclusion
In summary, we showed that using an ionic liquid as a solvent,
the direct chlorination or bromination of unprotected aniline
derivatives using CuCl2 or CuBr2 can be achieved in high yield
and high regioselectivity (para-substitution in most cases) under
mild conditions without the need for supplementary oxygen or
HCl gas. Thus, our new protocol offers a safer operational
choice and reduced environmental impact when compared to
direct chlorination in aqueous HCl, or halogenation of
protected/masked anilines, which requires additional deprotec-
tion processes.
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