Rare earth metal triflates catalyzed electrophilic nitration using N 2O5

Article abstract of DOI:10.1016/j.cclet.2011.09.021

A mild, efficient and eco-friendly process for the electrophilic nitration is described using N₂O₅ as a green nitrating agent in the presence of rare earth metal triflates [RE(OTf)₃] under mild conditions.

Full text of DOI:10.1016/j.cclet.2011.09.021

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 73–76
www.elsevier.com/locate/cclet
Rare earth metal triflates catalyzed electrophilic nitration using N2O5
*
Xiao Ming Ma, Bin Dong Li , Ming Lu, Chun Xu Lv
Chemical Engineering College, Nanjing University of Science and Technology, Nanjing 210094, China
Received 13 July 2011
Available online 8 November 2011
Abstract
A mild, efficient and eco-friendly process for the electrophilic nitration is described using N2O5 as a green nitrating agent in the
presence of rare earth metal triflates [RE(OTf)3] under mild conditions.
# 2011 Bin Dong Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Nitration; Rare earth metal triflates; N2O5; Catalysis
Nitration of aromatic substrates is one of the widely studied organic reactions as nitro aromatic compounds are
industrially important intermediates during the synthesis of dyes, plastics and pharmaceuticals [1]. Despite this,
industry still largely relies upon the early technology involving a mixture of nitric acid and sulfuric acid. Mixed acid
nitration systems, however, have many disadvantages like low selectivity, poly-nitration, oxidized products formation
and generation of environmentally hazardous waste [2]. The obvious disadvantages of the commercial manufacturing
process currently used has led to a substantial effort to develop viable alternatives, by using solid acid catalysts of, e.g.
+
MoO3/SiO2 [3,4], SO42À/SiO2 [5], zeolite-based solid acid catalysts [6–11]; other sources of NO2 , such as nitronium
salts in organic media [12], metal nitrates of zinc [13], N2O4 [14,15] and N2O5 [16–19]; organic nitrating agents of, e.g.
acetyl nitrates [7,10] and alkyl nitrates [14], other acids replacing sulfuric acid [20], etc. However, these are associated
with the problem of leaching of acid form the support during reaction and even calcination, require higher diluted
conditions, long reaction time and high reaction temperature, and poor selectivity towards some aromatic compounds.
Rare earth metal triflates [RE(OTf)3] are currently frequently used as potent environmentally benign Lewis acids,
and it was found that rare earth metal triflates (1–10 mol%) can catalyze the nitration of a range of simple aromatic
compounds using nitric acid, the only by-product is water and the catalyst can be readily recycled by simple
evaporation [21]. Nevertheless, these nitrations require a long reaction time (12–18 h) at the reflux temperature.
In our efforts to develop a mild, efficient and eco-friendly process for electrophilic aromatic nitration, we attempted
nitration of aromatic compounds using a highly active and green nitrating agent, dinitrogen pentoxide (N2O5), in the
presence of RE(OTf)3 under mild conditions. The results are presented in this letter.
In our initial study, nitration of toluene using N2O5 in the presence of a catalytic amounts of various RE(OTf)3 (5–
10 mol%) was examined (Scheme 1), the results are also compared with blank reaction (without catalyst), and the
orientation of this nitration has been elucidated in Table 1. As can be seen from Table 1, the nitration of toluene indeed
* Corresponding author.
E-mail address: libindong@mail.njust.edu.cn (B.D. Li).
1001-8417/$ – see front matter # 2011 Bin Dong Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.09.021

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X.M. Ma et al. / Chinese Chemical Letters 23 (2012) 73–76
Me
Me
RE(OTf)3
CH2Cl2
RE=Y, La, Pr, Nd, Sm, Eu, Gd,Tb, Dy, Ho, Er, Tm, Yb, Lu
+
N2O5
NO2
Scheme 1. Nitration of toluene using N2O5 catalyzed by RE(OTf)3.
R
R
5 mol% Nd(OTf)3
+
N2O5
NO2
CH2Cl2
R=H, F, Cl, Br, Et, iPr, tBu,
Scheme 2. Nitration of various aromatics using N2O5 catalyzed by Nd(OTf)3.
+
R
R
R
R
N2O5
NO2
NO2
+
+
RE(OTf)3
NO2
NO2
Scheme 3. A possible mechanism for electrophilic nitration.
carried out under mild conditions (20 8C) for 1 h to give the mono-nitrotoluene in good yield and the catalyst loading is
of a catalytic amounts (5–10 mol%). A variety of RE(OTf)3 in general gave the similar orientation, and Nd(OTf)3
(5 mol%) shows the highest activity for catalytic nitration (entry 5). Therefore, Nd(OTf)3 was selected as a general
Lewis acid catalyst for nitration.
Table 1
Nitration of toluene using N2O5 in the presence of rare earth metal triflates.a
Entry
Catalyst
Yield (%)c
Product isomers (%)d
Ortho
o/pd
Meta
Para
1
2
None
64
80
93
95
100
94
94
96
92
91
78
87
87
82
89
56.4
56.9
55.4
55.8
57.6
54.9
56.6
55.4
55.5
55.2
57.1
55.5
56.2
57.1
55.8
3.0
2.3
2.6
2.7
2.1
2.6
2.3
2.7
2.8
2.8
2.3
2.7
2.6
2.1
2.7
40.6
40.8
42.0
41.5
40.3
42.5
41.1
41.9
41.7
42.0
40.6
41.8
41.2
40.8
41.5
1.38
1.39
1.32
1.34
1.42
1.29
1.38
1.32
1.33
1.31
1.40
1.32
1.36
1.39
1.34
Y(OTf)3
La(OTf)3
Pr(OTf)3
Nd(OTf)3
Sm(OTf)3
Eu(OTf)3
Gd(OTf)3
Tb(OTf)3
Dy(OTf)3
Ho(OTf)3
Er(OTf)3
Tm(OTf)3
Yb(OTf)3
Lu(OTf)3
3
4b
5b
6
7b
8
9
10
11
12
13
14
15
a
Reaction conditions: toluene (5.0 mmol), N2O5/CH2Cl2 (5.0 mmol/5.0 mL), RE(OTf)3 (10 mol%), 1 h, 20 8C.
The catalyst amount is 5 mol%.
b
c
d
Yields are GC yields.
Determined by original GC data.

X.M. Ma et al. / Chinese Chemical Letters 23 (2012) 73–76
75
Table 2
Nitration of various aromatics using N2O5 catalyzed by Nd(OTf)3.a
Entry
R
Yield (%)b
Product isomers (%)c
Ortho
o/pc
Meta
Para
1
2
3
4
5
6
7
F
97
92
32.4
44.0
45.2
–
–
67.6
56.0
54.8
–
0.48
0.78
0.82
–
Cl
Br
H
–
84
–
100
100
98
–
Et
iPr
tBu
43.9
24.8
13.1
3.0
4.1
6.2
52.7
70.2
80.4
0.83
0.35
0.16
100
a
Reaction conditions: substrate (5.0 mmol), N2O5/CH2Cl2 (5.0 mmol/5.0 mL), Nd(OTf)3 (5 mol%), 1 h, 20 8C. All products are known and
identified by comparison using GC with authentic samples.
b
Yields are GC yields.
c
Determined by GC data.
By means of the optimized reaction conditions, extended investigation on a variety of other aromatic compounds
was studied (Scheme 2). The results were summarized in Table 2. High yields were obtained in most of the cases with
5 mol% catalyst loading under mild conditions. For the nitration of halogen benzene mono-nitro products were gained
in moderate yield (Table 2, entries 1–3), and nitration of alkyl benzenes gave totally conversion and para-selectivity.
Dinitration was not observed in all reactions.
A probable mechanism for electrophilic aromatic nitration (Scheme 3): Numerous papers have reported that high
nitration conversion can be obtained with strong acidic catalyst, and RE(OTf)3 is well-known as an efficient and strong
+
Lewis acid catalyst, which can promote the formation of electrophilic nitronium ions (NO2 ) from N2O5, then
aromatics are rapidly nitrated with this strong electrophile. Thus, this reaction may be related to a normal electrophilic
aromatic substitution mechanism involving the nitronium ion via N2O5 over Lewis catalyst.
In conclusion, we have developed a mild, efficient and eco-friendly process for the nitration of simple aromatic
compounds. This method is conducted under mild conditions and is suitable for scale-up, and the combination of
RE(OTf)3 and N2O5 seems to be a new direction for environmental chemistry in nitration.
1. Experimental
All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification;
N2O5 was prepared with Ref. [17]. All products are known and identified by comparison using GC with authentic
samples.
1.1. General procedure
To a cooled (0 8C) vigorously stirred mixture of the RE(OTf) (5 mol%) and toluene (5 mmol) was added a solution
of N2O5 (5 mmol) and CH2Cl2 (5 mL). The reaction was carried out at 20 8C for 1 h, then the catalyst was filtered off
and washed with some CH2Cl2, which was added to the filtrate. The CH2Cl2 solution was washed with a few portions
of saturated NaHCO3 solution, then several times with water, dried with MgSO4, and examined by GC.
Acknowledgment
We are grateful for the financial support from the National Nature Science Foundation of China Academy of
Engineering Physics (No. 10976014) and Nature Science Foundation of Jiangsu Province (No. BK2011697).
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