Preparation of imidazolines from aziridines and nitriles via TfOH promoted Ritter process

Article abstract of DOI:10.1016/j.cclet.2014.01.020

An efficient preparation of imidazolines from nitriles and aziridines in the presence of TfOH via Ritter reaction is described. It indicates that different kinds of nitriles can undergo the process. Among the nitriles, pivalonitrile is proven to be better than acetonitrile. The reaction is performed at room temperature and the yields are excellent.

Full text of DOI:10.1016/j.cclet.2014.01.020

Chinese Chemical Letters 25 (2014) 583–585
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Chinese Chemical Letters
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Original article
Preparation of imidazolines from aziridines and nitriles via TfOH
promoted Ritter process
b
a
a
a,
Rui Li a, , Hui Jiang , Wan-Yi Liu , Pei-Ming Gu , Xue-Qiang Li
*
*
a Key Laboratory of Energy Sources & Engineering, State Key Laboratory Cultivation Base of Natural Gas Conversion, Department of Chemistry, Ningxia
University, Yinchuan 750021, China
b Department of Energy and Chemical Technology, Ningxia Polytechnic & TV University, Yinchuan 750021, China
A R T I C L E I N F O
A B S T R A C T
Article history:
An efficient preparation of imidazolines from nitriles and aziridines in the presence of TfOH via Ritter
reaction is described. It indicates that different kinds of nitriles can undergo the process. Among the
nitriles, pivalonitrile is proven to be better than acetonitrile. The reaction is performed at room
temperature and the yields are excellent.
Received 1 September 2013
Received in revised form 20 December 2013
Accepted 25 December 2013
Available online 14 January 2014
ß 2014 Rui Li and Xue-Qiang Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All
rights reserved.
Keywords:
Ritter reaction
Nitrile
TfOH
Aziridine
Imidazoline
1. Introduction
The Lewis acids employed in the process are (frequently) moisture
sensitive, and moreover, some Lewis acids result in undesired
The preparation of imidazolines analogs has attracted consid-
erable interest [1] due to their potential biological activities [2] and
applications in organocatalysis [3]. The Ritter reaction provides an
important route to generate amides via stabilized carbocations.
Originally, the carbocations employed are generated from alkenes
[4] and alcohols [5] in the presence of Brønsted acids. Recently,
epoxides [6] and aziridines [7], as available carbonium ion sources,
have been employed and proven successful in the Ritter process.
Utilizing the Ritter reaction, the products from epoxides and
aziridines are dihydrooxazoles and imidazolines, respectively,
followed with ring closure (Scheme 1). Although several groups
have reported that the combination of nitriles and aziridines with
the promotion of a Lewis acid can give imidazolines [7], however,
the deficiencies of the reported procedures, including expensive
reagents [7e,7f], high temperature [7a,7d,7e] and unsatisfied yields
[7a,7b,7d–7g], tended to limit their applications. The reported
yields are generally from 60% to 80% and some use lanthanide
triflates [7e] as the promoter for this conversion. To our surprise,
with aziridines and epoxides only Lewis acids have been reported
to be successful, and the most widely referenced one is BF3ÁOEt2.
products, since the halide anion from the promoter can act as a
nucleophile to attack the aziridine ring, or the intermediate. For
example in reference [7f], InCl3, ErCl3 and YbCl3 do not give any
Ritter products, but deliver chlorinated compounds as the major
product, while Et3OBF4 results in some fluorinated product [7b]
accompanied with the imidazoline. Furthermore, the imidazolines
could be easily hydrolyzed to diamine derivatives with the
promotion of HCl in EtOH in very high yield [8]. Considering the
important application of diamines, the efficient preparation of
imidazolines is of potential and high demand. Herein, we describe
a practical preparation of imidazolines in excellent yields from
aziridines and nitriles at room temperature.
2. Experimental
As mentioned above, we focused our attention on the use of
Brønsted acids for the Ritter transformation. Aziridine 1a and
acetonitrile 2a were selected for the model reaction because of
their frequent appearance in other reported experiments. Initially,
we applied 1 equiv. of Brønsted acid as the promoter (Table 1,
entries 1–6), while BF3ÁOEt2 was used as comparison. Among the
Brønsted acids, H2SO4, aqueous HCl, TFA, HClO4 and trifluoro-
methanesulfonic acid (TfOH) were screened, but TfOH was
demonstrated the most efficient and gave the corresponding
*
Corresponding authors.
E-mail addresses: ruili@nxu.edu.cn (R. Li), lixq@nxu.edu.cn (X.-Q. Li).
1001-8417/$ – see front matter ß 2014 Rui Li and Xue-Qiang Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2014.01.020

584
R. Li et al. / Chinese Chemical Letters 25 (2014) 583–585
3
Table 1
R
Screening of acids for reaction of 1a and acetonirile.a
2
R
2
R
3
R
R CN, LA
N
1
N
N
Ts
Ts
1
acids
R
N
N
N
+
R
CN
R
r.t.
Ar
Entry
Ar
Scheme 1. Representative Ritter process of nitrile and aziridine.
Nitriles
Acids (equiv.)
Time (h)
Yield (%)b
1
2
CH3CN (2a)
CH3CN (2a)
CH3CN (2a)
CH3CN (2a)
CH3CN (2a)
CH3CN (2a)
t-BuCN (2b)
t-BuCN (2b)
t-BuCN (2b)
t-BuCN (2b)
CH3CN (2a)
CH3CN (2a)
HCl (1.0)
24
24
24
24
24
24
24
24
24
6
20
38
48c
42
60
72
84
92
95
96
89
95
H2SO4 (1.0)
HClO4 (1.0)
TFA (1.0)
imidazoline 3a in 72% yield, as the control subject, and in contrast,
BF3ÁOEt2 delivered 3a in 60% yield.
3
4
5
BF3ÁOEt2 (1.0)
TfOH (1.0)
TfOH (1.0)
TfOH (1.2)
TfOH (1.5)
TfOH (2.0)
TfOH (2.0)
TfOH (2.5)
After TfOH was identified as the promoter, we switched our
attention to the nitriles, since acetonitrile was widely used in the
literature. Apart from it, aliphatic nitriles, including propiononi-
trile and isobutyronitrile, were expanded, and gave the corre-
sponding imidazolines in similar yields to that of acetonitrile
6
7
8
9
10
11
12
6
[7b,7g]. There are a variable number of protons at the position
a to
0.7
the nitrile group, and we reasoned that these protons could
probably affect the process. To the best of our knowledge,
pivalonitrile 2b was not reported for this transformation. We
conducted the experiment with pivalonitrile 2b and aziridine 1a in
the presence of 1 equiv. of TfOH for 24 h and obtained the
imidazoline 3b in 84% yield (Table 1, entry 7). This indicated that
a
Combination of 1 equiv. of aziridine 1a with the corresponding equiv. of TfOH in
nitrile for the required duration.
b
Isolated yield after column chromatography.
c
Accompanied with some unknown compound.
Table 2
TfOH promoted Ritter reaction of alkylaziridines and nitriles.a
Entry
1
Aziridines 1
Nitriles 2
Products 3
Ph
Time (h)
6
Yieldb (%)
80c (90d)
CN
CN
Ts
Ts
N
N
Ts
N
N
Ph
1a
2c
Ph
3c
2
12
80c (91d)
Ph
1a
2d
2b
Ts
N
N
Ph
3d
3
12
90
Ts
CN
N
Ts
C6H13
N
N
1e
C6H13
O
3e
3f
4
6
86
Ts
N
CN
CN
NH
NH
NHTs
NHTs
2b
2b
1f
5
6
85
Ts
N
O
3g
1g
a
2.0 equiv. of TfOH used as the promoter.
Isolated yield after column chromatography.
Average yield based on three runs.
The best yield of three runs.
b
c
d

R. Li et al. / Chinese Chemical Letters 25 (2014) 583–585
585
nitrile 2b would be better than acetonitrile 2a (Table 1, entry 7 vs.
entry 6). In order to achieve a higher yield, we increased TfOH from
1 equiv. to 1.2 equiv. and 1.5 equiv., the yield of 3b was improved
to 92% and 95%, while the reaction time remained at 24 h (Table 1,
entry 8 and entry 9). Then 2.0 equiv. of TfOH was attempted, 3b
was obtained in 96% yield, and the reaction time was shortened to
6 h (Table 1, entry 10). The acetonitrile 2a was also reacted with
aziridine 1a under the above conditions, and 3a was generated in
89% yield (Table 1, entry 11). The reaction conditions were
optimized as follows: Treatment of 1 equiv. of the aziridine with
2.0 equiv. of TfOH in nitrile at room temperature for the required
time. Furthermore, the imidazoline 3a could be obtained in 95%
yield in 0.7 h by slightly increasing the amount of the promoter to
2.5 equiv. (Table 1, entry 12), which was the best yield with the
same substrates in the reports, and the common yield was only
around 70% [7].
imidazoline could be obtained in 80%–96% yield. Furthermore,
even the acetonitrile could deliver the imidazoline in excellent
yield by slightly increasing the amount of TfOH. The reaction was
performed at room temperature and no expensive promoters were
used, and the yields were much better than those of the reported
procedures.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Nos. 21262024, 21062014), the Key Project of
Chinese Ministry of Education (No. 211193), the Scientific
Research Foundation for Returned Scholars (Ministry of Education
of China), the Natural Science Foundation of Ningxia (No. NZ1165),
the Key Project of Department of Education in Ningxia (2010-
Preparation of Capsaicin), the 100 Talents Program of Ningxia,
and the ‘‘211’’ Project in Ningxia University.
A represent procedure for preparation of imidazoline 3: To a
stirred solution of 1a (272 mg, 1.0 mmol) in pivalonitrile 2b (2 mL)
was added TfOH (300 mg, 2.0 mmol). After stirring at room
temperature for an additional 6 h, the mixture was concentrated
and the residue was chromatographed to afford imidazoline 3b as a
yellow oil (342 mg, 96% yield from 1a): 1H NMR (400 MHz, CDCl3):
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2014.01.020.
d
1.56 (s, 9 H), 2.42 (s, 3 H), 3.59 (dd, 1H, J = 8.2 Hz and 10.8 Hz),
4.13 (dd, 1H, J = 9.6 Hz and 10.8 Hz), 4.78 (t, 1H, J = 9.2 Hz), 7.06–
7.08 (m, 2 H), 7.24–7.29 (m, 5 H), 7.74 (d, 2H, J = 8.0 Hz); 13C NMR
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In conclusion, we have developed an efficient procedure for
the preparation of imidazolines from aziridines and nitriles. The
procedure employed the Brønsted acid TfOH as the promoter,
and it indicated that pivalonitrile was better than acetonitrile. The
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