Lanthanide Bis[(trifluoromethyl)sulfonyl]imides as reusable catalysts for mononitration of substituted benzenes in ionic liquids

Article abstract of DOI:10.1002/adsc.200900243

Lanthanide bis[(trifluoromethyl)sulfonyl]imides as a kind of effective catalyst were used in quaternary ammonium ionic liquids to afford a useful new method for the nitration of substituted benzenes with concentrated nitric acid (95%) as nitrating agent. The less expensive lanthanum bis[(trifluoromethyl) sulfonyl]imide was found to be the most effective catalyst employed in the ionic liquid tributylammonium bis[(trifluoromethyl)sulfonyl]imide. The lanthanum bis[(trifluoromethyl)sulfonyl]-imide/tributylammonium bis[(trifluoromethyl) sulfonyl]imide system demonstrated high catalytic activity for the nitration of halobenzenes and rendered good to excellent yields. This catalyst/ionic liquid system could be recovered by simple procedures and recycled for at least 5 times.

Full text of DOI:10.1002/adsc.200900243

FULL PAPERS
DOI: 10.1002/adsc.200900243
Lanthanide Bis[(trifluoromethyl)sulfonyl]imides as Reusable
Catalysts for Mononitration of Substituted Benzenes in
Ionic Liquids
a
a
a,
Shuojin Wang, Shaojie Jiang, and Jin Nie *
a
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074,
Peopleꢀs Republic of China
Fax : (+86)-27-8754-3632; phone: (+86)-27-8754-3232; e-mail: niejin@mail.hust.edu.cn
Received: April 8, 2009; Published online: July 7, 2009
Abstract: Lanthanide bis[(trifluoromethyl)sulfonyl]-
ACHTUNGTRENNUNiG mide/tributylammonium bis[(trifluoromethyl)sulfo-
ACHTUNGTRENNUNGi mides as a kind of effective catalyst were used in nyl]imide system demonstrated high catalytic activity
quaternary ammonium ionic liquids to afford a for the nitration of halobenzenes and rendered good
useful new method for the nitration of substituted to excellent yields. This catalyst/ionic liquid system
benzenes with concentrated nitric acid (95%) as ni- could be recovered by simple procedures and recy-
trating agent. The less expensive lanthanum bis[(tri- cled for at least 5 times.
fluoromethyl)sulfonyl]imide was found to be the
most effective catalyst employed in the ionic liquid
tributylammonium
bis[(trifluoromethyl)sulfonyl]- Keywords: catalyst recycling; electrophilic substitu-
ACHTUNGTRENNUNGi mide. The lanthanum bis[(trifluoromethyl)sulfonyl]- tion; ionic liquids; lanthanides; Lewis acids
Introduction
tion of aromatic compounds with 2 or 3 equiv. of
aqueous nitric acid using SO H-functional Brønsted
3
Nitration of aromatic compounds is one of the most acidic ILs as a recoverable catalyst. In their study,
important fundamental and unit reactions in the good yields were obtained for activated aromatic
chemical industry, providing very important inter- compounds, whereas the yields for halobenzenes were
[
1]
mediates for the preparation of various compounds.
Traditionally in industry and academic laboratories,
only moderate.
In our previous work, lanthanide bis[(trifluorome-
nitration of aromatic compounds requires the use of thyl)sulfonyl]imides {Ln[N AHCTUNTGRENNUNG( CF SO ) ] or Ln AHCNUTRGTENNNUG( NTf ) ]
3 2 2 3 2 3
mixtures of concentrated nitric acid with sulfuric acid have been developed for catalyzing diverse organic
leading to an excessive acid waste stream that was en- reactions in organic solvents and exhibited high cata-
[
6]
vironmentally unfriendly. Therefore, various clean ap- lytic activity. Therefore, we envisioned that the com-
proaches have been developed to improve this tradi- bination of these excellent Lewis acid catalysts and
[
1,2]
tional mixed-acid method.
environmentally benign ILs could render a clean and
As an important alternative approach, the nitration efficient catalyst/solvent system for the nitration of
of aromatic compounds in ionic liquids (ILs) has been substituted benzenes with nitric acid as nitrating
[
3–5]
documented in the past few years.
Among various agent. In this research, the catalytic performance of
(NTf ) in ILs with quaternary ammonium cations
nitrating agents, acetyl nitrate has proved to be very LnAHCTUNGTRENNUNG
efficient for the nitration of halobenzenes . How- paired with the [Tf N] anion (Figure 1) are investi-
2
3
[
4]
À
2
ACHTUNGTRENNUNGe ver, from the environmental or industrial point of gated for the nitration of toluene and halobenzenes
view, nitric acid should be the ideal nitrating reagent with 95% HNO (Scheme 1).
3
because it is inexpensive and only generates water as
waste. Until now, only a few catalyst systems that in-
volved the use of nitric acid as nitrating agent in ionic Results and Discussion
[
5a]
liquids have been reported. Handy et al. reported
the ytterbium
tion of activated aromatic compounds with 1 equiv. of are both common cations in ILs, but the 1,3-dialkyl-
nitric acid (70%) in [bmpy][NTf ]. Quite recently, imidazolium cations were reported to be less stable
ACHTUNGTRENNUNG( III) triflate [Yb ACHTUNGTNERNUNG( OTf) ] catalyzed nitra- 1,3-Dialkylimidazolium and quaternary ammonium
3
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
2
[5b]
[5c]
[4a,4b]
Qiao et al. and Fang et al. investigated the nitra- towards the nitrating reagents
. Consequently, the
Adv. Synth. Catal. 2009, 351, 1939 – 1945
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1939

Shuojin Wang et al.
FULL PAPERS
Table 1. Kamlet–Taft parameter b for ILs at 558C.
Ionic liquid
[NH444][NTf ]
b
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
0.200
0.254
0.299
0.321
0.390
2
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG[ NTf ]
2
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
2
A
H
U
G
R
N
U
G
A
T
N
T
E
N
N
[NTf ]
2
A
H
U
G
R
N
U
G
A
T
N
T
E
N
N
[NTf ]
2
On the other hand, it has been reported that the
halochromism of azo dyes could be used in the mea-
surement of acid strength of Lewis catalysts in organic
[
9]
solvents: Unfortunately, the attempt to investigate
the Lewis acidity of Ln(NTf ) in ILs also failed. For
example, in the case of [NH ]
AHCTUNGTRENNUNG
2
3
AHCTUNGTNERNUG[ NTf ] it was difficult
44 2
4
to evaluate the difference of Lewis acidity of the
3+
3+
3+
metal cations (Yb , Sm , La ) because the protic
ionic liquid [NH ][NTf ] itself had a strong interac-
Figure 1. Cations and anion used to prepare the ILs.
AHCTUNGTRENNUNG
44
4
2
tion with the azo dye, 2,4,4’,6-tetramethoxyazoben-
[
9c]
AHCTUNGTRENNUNG
reaction to optimize the reaction conditions. Our ini-
tial effort aimed to use aqueous nitric acid (68%) as
the nitrating reagent in ILs, but the results obtained
were not satisfying. In an attempt to improve the re-
action, concentrated nitric acid (95%) was found to
be a suitable nitrating agent.
Scheme 1. Nitration of substituted benzenes.
Parallel experiments of the nitration of toluene
were carried out with the foresaid five structurally dif-
quaternary ammonium cations were selected in the ferent ionic liquids. The reactions were conducted at
present work (Figure 1). 858C for certain times (Table 2), and a stoichiometric
For comparison, five structurally different ILs, such amount of 95% HNO was employed. The products
3
as [N2227
]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ], [N4446]
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
were analyzed by gas chromatography.
2
2
2
444
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ], and [NH ]
A
H
N
T
E
N
N
[NTf ], were used in this work. As
As can be seen from Table 2, the nitration of tolu-
2
666
2
[
7]
the Kamlet–Taft three-parameter equation is a rig- ene is a somewhat self-catalyzing reaction in neutral
orous approach for the treatment of multiple interact- ILs when nitric acid was used as nitrating agent in the
ing solvent effects, we tried first to measure the three absence of any catalyst. However, the yields of nitro-
parameters to probe the fundamental properties of toluenes obtained were in the range 35.7 to 63.9%,
five structurally different ILs. But as the Reichardt manifesting that the structure of the cation of ILs has
dye was confirmed not to be applicable to the polarity a great effect on the nitration of aromatic com-
[
8]
[4a]
measurement of protic ILs, only the b and p* values pounds. The tendency for increases in the yield of
were measured by using 4-nitroaniline and N,N-dieth- nitrotoluenes was approximately coincident with the
yl-4-nitroaniline in this work, and the Kamlet–Taft pa- decrease of the b value of the ILs (Table 1). The high-
rameter b ,which provides a measure of a solventꢀs est product yield was observed in the protic quaterna-
hydrogen bond-accepting basicity (HBA), was found ry ammonium ionic liquid [NH ] ACTHUNGTRNEUNG[ NTf ] and after
4
44 2
to have a good correlation with the results of the ni- 24 h a 75.9% of yield of nitrotoluenes could be ach-
tration of toluene. As shown in Table 1, the values of ieved. We inferred that it might be attributable to the
b varied with the structural transformation of quater- higher Brønsted acidity and polarity of [NH ]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ].
4
44
3+
2
3
+
3+
3+
nary ammonium cations and in an order of [NH ]
Different metal cations (La , Sm , Yb , In ,
4
44
2+
À
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]<[NH ]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]<[N2227]
A
H
N
T
E
N
G
A
H
U
T
E
N
N
Cu ) paired with the [Tf N] anion were investigated
2
2
666
2
2
2
<
[N6666] ACHTUNGTRENNUNG[ NTf ]. This result implies that the protic as Lewis catalysts for the nitration of toluene. All
2
ionic liquid has a lower tendency for accepting hydro- these metal bis[(trifluoromethyl)sulfonyl]imides could
gen bonds than the aprotic ones. Consequently, the catalyze the nitration and achieve nearly the same
active proton could be less restrained in protic ILs level of equilibrium in different ILs. When using
and might exhibit a higher catalytic activity in reac- 2 mol% of catalyst, nitrotoluenes were obtained in
tion.
the yields ranging from 70.6 to 81.7% with a para/
1940
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1939 – 1945

Lanthanide Bis[(trifluoromethyl)sulfonyl]imides as Reusable Catalysts
FULL PAPERS
[a]
Table 2. Nitration of toluene in different ILs.
[b]
[b]
[b]
Entry
Ionic liquid
Time [h]
Yield [%]
Product distribution [%]
para/ortho
ortho
meta
para
1
2
3
4
5
6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[N6666]
[N4446]
[N2227]
[NH666]
[NH444]
[NH444]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
4
4
4
4
4
24
35.7
39.6
54.1
52.0
63.9
75.9
54.0
56.0
54.0
52.9
53.6
53.8
3.7
2.9
3.1
4.2
2.9
3.1
4.23
41.1
42.9
42.8
43.5
43.2
0.8
0.7
0.8
0.8
0.8
0.8
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
H
U
G
R
N
N
[NTf ]
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
H
U
G
R
N
U
G
2
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A[ NTf ]
H
U
T
E
N
N
[
[
a]
b]
Reaction conditions: 3 mmol toluene, 3 mmol 95% nitric acid, ILs 5 mmol, 858C.
By quantitative GC analysis.
[a]
Table 3. Nitration of toluene in different ILs catalyzed by various catalyst.
[b]
[b]
[b]
Entry
Ionic liquid
Catalyst
Yield [%]
Product distribution [%]
para/ortho
ortho
meta
para
1
2
3
4
5
6
7
8
9
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NH444]
A
T
N
T
E
N
N
[NTf ]
La(NTf )
A
H
U
G
R
N
U
G
81.7[
54.0
52.8
53.2
53.7
53.2
52.9
53.0
53.1
53.3
53.4
53.7
52.9
3.1
3.0
3.5
3.1
2.8
2.8
2.8
2.9
2.9
3.0
2.9
3.0
42.9
42.2
43.3
43.2
44.0
44.4
44.2
44.0
43.8
43.7
43.4
44.1
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
2
2 3
c]
89.8
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NH444]
[NH444]
[N4446]
[N4446]
[N4446]
[N4446]
[N4446]
[N2227]
[N6666]
[NH666]
A
H
U
G
R
N
U
G
Sm
Yb(NTf )
La(NTf )
Sm
Yb(NTf )
In(NTf )
Cu
A
H
U
G
R
N
N
(NTf )
78.8
75.7
72.0
76.2
76.6
71.3
70.6
78.8
76.9
76.9
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
ACHTUNGTRENNUNG
2
2 3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
A
H
U
G
R
N
N
(NTf )
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
ACHTUNGTRENNUNG
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
ACHTUNGTRENNUNG
2
2 3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
A
H
U
G
R
N
N
(NTf )
2
2
2
1
1
1
0
1
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
La
La
La
A
T
N
T
E
N
N
(NTf )
2
2
2
2
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ]
A
H
U
G
R
N
U
G
2
3
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
2
[
[
[
a]
Reaction conditions: 3 mmol toluene, 3 mmol 95% nitric acid, ILs 5 mmol, 2 mol% catalyst, 85 8C, reaction time 4 h.
By quantitative GC analysis.
Result of 24 h.
b]
c]
ortho ratio of 0.8. The para/ortho ratios of the prod- acid and the reaction temperatures were varied from
ucts were almost the same in the different ILs, regard- 608C to 1008C. The results are presented in Figure 2.
less of whether the reactions were carried out with or It was observed that yield of nitrotoluenes jumped
without catalysts (Table 2) and the ratios were consis- rapidly from 60.4 to 81.7% while increasing the reac-
tent with obtained those using HNO /Ac O as nitrat- tion temperature from 608C to 858C. However, when
3
2
[4]
ing agent in ILs. The results of Table 3 demonstrate the temperature exceeded 858C the yield gradually
that La(NTf ) was the most effective catalyst for ac- decreased. From both the results and the red-brown
celerating the nitration of toluene, especially in nitrogen dioxide that observed in the experiment, we
NH ][NTf ] with 1 equiv. of 95% nitric acid. Fur- concluded that the nitric acid probably decomposed
ACHTUNGTRENNUNG
2
3
[
ACHTUNGTRENNUNG
44
4
2
ther extending the reaction time to 24 h only led to a at the higher temperature.
limited increase of yield in the La(NTf ) /[NH ] The results of the nitration of toluene using nitric
(NTf ) in
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
3
2
444
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[NTf ] system (entry 2 in Table 3) and this suggests acid with different dosages of catalyst La
A
H
N
T
E
N
N
2
2 3
that the nitration reactions approximately reach the [NH ] ACHTUNGTRNENUG[ NTf ] are given in Table 4. We found that in-
4
44 2
equilibrium after 4 h under the experimental condi- creasing the catalyst loading brought about a marginal
À
tions. On the other hand, as [Tf N] is a weakly coor- increase of the yield. Even when using only 1 mol%
2
dinating organic anion, the catalytic activity order of catalyst with 1 equiv. of 95% HNO the reaction equi-
3
À
the Lewis catalysts with the [Tf N] anion normally is librium was almost attained after 4 h. However, con-
Yb >Sm >La in organic solvents, but in ILs sidering that halobenzes were much more deactivated
Table 3) this trend is not followed and further inves- than toluene, 5 mol% of catalyst was used for further
2
3+
3+
3+
(
tigation is underway.
study.
The effect of the molar ratio of nitric acid to tolu-
The temperature effect on the nitration of toluene
was also studied. The reaction was carried out in the ene on the nitration reaction at 858C over 5 mol%
presence of 2 mol% catalyst, using 1 equiv. of nitric catalyst was investigated and the result are shown in
Adv. Synth. Catal. 2009, 351, 1939 – 1945
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1941

Shuojin Wang et al.
FULL PAPERS
luenes was gradually enhanced and an excellent yield
(
96.7%) of mononitrotoluenes was obtained by using
only 1.3 equiv. of HNO (entry 4 in Table 5).
3
The nitration of halobenzenes was further studied
and the results are shown in Table 6. We can see that
the La ACTHUNGTRNENNU(G NTf ) / ACHUTGNRENNUG[ NH ] ACHUTNGRENUN[G NTf ] system demonstrated
2 3 444 2
very good catalytic activity for deactivated benzenes
at 5 mol% catalyst loading with 1.3 equiv. of 95%
HNO . Extending the reaction time from 8 to 24 h,
3
greatly increased the yield of nitrohalobenzenes.
La ACHTUNGTRNENUG( NTf ) could catalyze the nitration of haloben-
2 3
zenes affording good to excellent yield (78.4–94.3%)
after 24 h. Furthermore, it can be seen that the para-
selectivity of the nitration of halobenzenes gradually
decreased from fluorobenzene to iodobenzene along
with the decrease of electron affinity of the halogen
atoms. This indicated that a correlation exists between
the degree of para-selectivity and the ability of elec-
tron-withdrawing halogen atoms to induce an electro-
static attraction between substrate and [NH ]
Figure 2. The effect of temperature on the nitration of tolu-
ene. Toluene and HNO (95%)=3 mmol; reaction time 4 h;
3
444
2
mol% La ACHTUNGTRENNUNG( NTf ) as catalyst; yields were determined by
2 3
[5b]
AHCTUNGTNERN[GU NTf ]. For the nitration of fluorobenzene a 94.3%
2
quantitative GC analysis.
of yield could be achieved and the reaction was
highly para-selective. Notably, the La(NTf ) /[NH ]
AHCTUNGTREG[NNUN NTf ] system was very effective even for the nitration
A
H
U
G
R
N
N
ACHTUNGTRENUNG
3
2
444
2
Table 4. Nitration of toluene catalyzed by La
A
H
U
G
R
N
N
(NTf ) in
2
3
[
4b]
[
a]
of iodobenzene, an 87.6% of yield of nitroiodoben-
zenes could be attained by using 5 mol% of catalyst
and 1.3 equiv. of 95% HNO3.
[
NH444] ACHTUNGTRENNUNG[ NTf ] system with different dosages of catalyst.
2
Entry HNO :toluene
Amount of catalyst
[mol%]
Yield
[b]
3
molar ratio
[%]
The reusability is one of the important properties
of catalytic systems. The recycling performance of
La ACHTUNGRTENNNU(G NTf ) / ACHTUNTGRNENUG[ NH ] CAHUTNGTRENNUN[G NTf ] system was investigated by
1
2
3
4
5
1:1
1:1
1:1
1:1
1:1
0
1
2
3
5
63.9
80.6
81.7
82.4
83.3
2 3 444 2
the nitration of toluene under identical experimental
conditions. The recovered catalyst with IL was then
directly used for the next catalytic reaction, and no
further purification was done. As revealed in Table 7,
[
a]
the La ACHNUTGTRENNUNG( NTf ) / ACHUTGNRENTNUG[ NH ] AHCUTNERGNUNG[ NTf ] catalyst system could be
2 3 444 2
Reaction conditions: 3 mmol toluene, 3 mmol 95% nitric
acid, ILs 5 mmol, 858C.
By quantitative GC analysis.
recovered in high yields during each cycle and reused
for at least five times, still providing very good yield
and the similar selectivity as at the beginning. So this
[
b]
means that the ionic liquid [NH ] CAHTUNGTRNEUN[G NTf ] might be an
4
44 2
Table 5. Clearly, the molar ratio of the nitric acid to effective solvent for lanthanide-mediated organic re-
[
10]
toluene has a conspicuous positive effect on the nitra- actions
tion of toluene in [NH ][NTf ]. With the increase of It is well known that aromatic nitration is a classical
.
AHCTUNGTRENNUNG
44
4
2
molar ratio of 95% nitric acid, the yield of nitroto- electrophilic substitution involving the attack by a ni-
+
2
tronium ion (NO ), a nitrate species generally pro-
duced with a proton. Waller et al. assumed a mode
Table 5. Nitration of toluene catalyzed by La
NH444 [NTf ] system with different dosages of nitric acid.
ACHTUNGTERNNUNG
(NTf ) in producing nitronium ion in the presence of lanthanide
2
3
[a]
[
]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
triflate: hydrated trivalent lanthanide can bind nitric
acid to its center and then promote the release of an
active proton to produce the requisite active nitroni-
um ion by protonation of nitric acid. Similarly, here
we present a mechanism for the formation of nitroni-
2
[
b]
Entry
HNO :toluene molar ratio
Yield [%]
3
[
2b]
1
2
3
4
1:1
83.3
84.9
91.3
96.7
1.1:1
1.2:1
1.3:1
um ion in the presence of Ln
ammonium ILs (Scheme 2).
ACHTUNGTNERNUN(G NTf ) in quaternary
2 3
The differences of catalytic activity among metal
bis[(trifluoromethyl)sulfonyl]imides may mainly
depend on the differences of solvation of metal cat-
[
a]
Reaction conditions: 3 mmol toluene, IL 5 mmol, 858C,
mol% La(NTf ) as catalyst, reaction time 4 h.
By quantitative GC analysis.
5
ACHTUNGTRENNUNG
2
3
[
b]
[10b]
ions in ILs.
It is obvious that the protic ionic liquid
1942
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1939 – 1945

Lanthanide Bis[(trifluoromethyl)sulfonyl]imides as Reusable Catalysts
FULL PAPERS
[a]
Table 6. Nitration of halobenzenes catalyzed by La ACHTNUGTRENNUN(G NTf ) in [NH ] AHCTUNGTNRNE[UG NTf ] system.
2 3 444 2
[
b]
[b]
[b]
Entry
Arene
C H F
Time [h]
Yield [%]
Product distribution [%]
para/ortho
ortho
meta
para
1
2
3
4
5
6
7
8
9
8
24
8
24
8
24
8
81.4
10.9
10.4
26.5
25.8
31.1
31.7
38.8
37.9
53.3
–
–
–
–
–
–
–
–
89.1
89.6
73.5
74.2
68.9
68.3
61.7
62.1
44.1
8.2
8.6
2.8
2.9
2.2
2.2
1.6
1.6
0.8
6
5
[c]
94.3 (91.0)
67.9
C H Cl
6
5
82.5
66.2
78.4
C H Br
6
5
[
[
c]
c]
C H I
77.4 (76.7)
87.6
96.7 (91.0)
6
5
24
4
C H CH
3
2.0
6
5
[
[
[
a]
b]
c]
Reaction conditions: 3 mmol halobenzene, 3.9 mmol 95% nitric acid, 5 mol% catalyst, IL 5 mmol, 858C.
By quantitative GC analysis
Isolated yields in parenthesis.
Conclusions
Table 7. Recycled La ACHTGNURTENUNNG( NTf ) / ACTHNUGTENRNUG[ NH444] ACHUTNGTRENNUG[ NTf ] for nitration of
2 3 2
toluene.
[a]
In conclusion, we have developed a novel Lewis acid
catalyst/solvent system that afforded a useful new
method for the nitration of aromatic compounds with
[b]
[b]
Entry Yield [%]
Product distribution [%]
Recovery[
ratio [%]
c]
ortho
meta
para
9
5% nitric acid as nitrating agent. The less expensive
La(NTf ) was found to be the most effective catalyst
employed in [NH ] AHCTUNGTRNENUG[ NTf ] for the nitration of halo-
44 2
1
2
3
4
5
96.7
96.8
95.0
94.6
93.1
53.3
53.4
53.5
53.8
53.5
2.0
2.9
2.8
2.9
3.0
44.1
43.7
43.7
43.3
43.5
–
AHCTUNGTRENNUNG
0.99
0.99
0.99
0.98
2 3
4
benzenes and good to excellent yields could be ob-
tained. The catalyst/solvent system could be recov-
ered by simple procedures and recycled at least 5
times.
[
a]
Reaction conditions: 3 mmol toluene, 3.9 mmol nitric acid
95%), 5 mol% catalyst, IL 5 mmol, 858C, 4 h.
By quantitative GC analysis.
(
[
[
b]
c]
The weight ratio of recovered and intial IL.
Experimental Section
General
All chemicals (AR grade) were commercially available and
1
19
pretreated before used. H NMR and F NMR spectra were
recorded on a Bruker AV400 spectrometer. UV- vis spectra
were obtained using a Shimadzu UV-2550 spectrometer.
The compositions of the nitration products were analyzed
by using a GC 9790 gas chromatograph (FULI Company)
equipped with a 30QC3/AC225 capillary column (SGE
Company) and a flame ionization detector using tempera-
ture programmed from 60 to 2208C.
Preparation of Metal Bis[(trifluoromethyl)sulfonyl]-
imides
The syntheses of metal bis[(trifluoromethyl)sulfonyl]imides
[6a,d]
followed procedures reported by Nie et al.
Lanthanide
+
2
Scheme 2. Proposed mechanism for the formation of NO .
bis[(trifluoromethyl)sulfonyl]imides were prepared almost
quantitatively by the reactions of HNTf with stoichiometric
2
amounts of corresponding metal oxide in aqueous solution.
Colorless solids were obtained after filtration, evaporation
of the aqueous solution and drying under vacuum at 1008C
overnight.
will favor the liberation of a proton to form the elec-
trophilic nitronium ion. At the same time, this mecha-
nism mode can also illustrate the unchangeable distri-
+
bution of nitrated products, since free NO2 is not re-
gioselective while attacking aromatic rings.
Adv. Synth. Catal. 2009, 351, 1939 – 1945
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1943

Shuojin Wang et al.
FULL PAPERS
+
Preparations of Tetraalkylammonium (R N ) Based
ILs
residue was purified with silica gel (200–300 mesh) column
chromatography (eluent: petroleum ether/EtOAc=10/1).
All products were identified by comparing their melting
4
+
The used tetraalkylammonium (R N ) Based ILs were syn-
thesized by two-step reactions according to the literature
Figure 1).
1
4
points and H NMR spectra with those of standard samples.
[11]
The catalyst/ionic liquid layer was directly reused for the
next run if necessary after it was heated to 1008C for 4 h
under vacuum (0.1 Torr).
(
+
Preparations of Trialkylammonium (R NH ) Based
3
ILs
The trialkylammonium based ILs were synthesized through
neutralization of trialkylamine with stoichiometric amounts Acknowledgements
of HNTf in distilled water. The structures of ILs were char-
acterized by H NMR and F NMR (Figure 1). All of the
ILs were dried under vacuum in 908C overnight before use.
2
1
19
The authors would like to thank Dr. Xiaoyong Li for his val-
uable help to this work.
Tributylammonium
bis[(trifluoromethyl)sulfonyl]imide
1
[
NH444
] ACHTUNGTRENNUNG[ NTf ]: H NMR (CDCl , TMS): d=6.86 (broad
2
3
peak, 1H, -NH), 3.09 (m, 6H, -N-CH -), 1.67 (m, 6H, -CH -
2
2
CH CH ), 1.39 (m, 6H, -CH -CH ), 0.99 (m, 9H, -CH ); References
2
3
2
3
3
1
9
F NMR (CDCl , CCl F): d=À78.89 (s).
3
3
Trihexylammonium
bis[(trifluoromethyl)sulfonyl]imide
[1] a) G. A. Olah, R. Malhotra, S. C. Narang, Nitration,
Methods and Mechanisms, Wiley-VCH, New York,
1989; b) K. Schofield, Aromatic Nitration, Cambridge
University Press, Cambridge, 1980; c) R. Taylor, Elec-
trophilic Aromatic Substitution, John Wiley & Sons,
Chichester, 1990.
1
[
NH666
]
A[ NTf ]: H NMR (CDCl , TMS): d=6.60 (s, 1H,
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
3
-
NH), 3.03 (m, 6H, -N-CH -), 1.66 (s, 6H, -CH -C H CH ),
2 2 3 6 3
1
.33 (s,18H, -C H -CH ), 0.90 (t, J=5.8 Hz, 9H, -CH );
3
6
3
3
19
F NMR (CDCl , CCl F): d=À78.85 (s).
3
3
[
2] a) F. J. Waller, A. G. M. Barrett, D. C. Braddock, D.
Ramparsad, Chem. Commun. 1997, 6, 613–614; b) F. J.
Waller, A. G. M. Barrett, D. C. Braddock, D. Rampar-
sad, Tetrahedron Lett. 1998, 39, 641–1642; c) F. J.
Waller, A. G. M. Barrett, D. C. Braddock, R. M.
McKinnell, D. Ramparsad, J. Chem. Soc. Perkin Trans.
1 1999, 867–872; d) S. Kobayashi, M. Sugiura, H. Kita-
gawa, W. L. W. Lam, Chem. Rev. 2002, 102, 2227–2302;
e) M. Shi, S. C. Cui, Chem. Commun. 2002, 9, 994–995.
Measurement of Kamlet–Taft Parameter b of ILs
4
-Nitroaniline and N,N-diethyl-4-nitroaniline were dissolved
À5
À4
respectively in ILs in a concentration range 10 to 10
mol/L and the ILs were dried under vacuum at 808C for
2 h before use. The sample was loaded into a dry cuvette in
1
a glove box and the cuvette was then capped and sealed.
The UV-vis absorbance was measured and fitted with a
[12]
Gaussian profile according to literature procedures. The
temperature was controlled with an SYC temperature con-
troller (Æ0.18C).
[3] a) T. Welton, Chem. Rev. 1999, 99, 2071–2083; b) P.
Wasserscheid, T. Welton, Ionic Liquids in Synthesis
Wiley-VCH, Weiheim, 2002, pp 191–192; c) K. K.
Laali, V. J. Gettwert, J. Org. Chem. 2001, 66, 35–40;
d) R. Rajagopal, K. V. Srinivasan, Synth. Commun.
2003, 33, 961–966; e) G. B. Cheng, X. L. Duan, X. F.
Qi, C. Lu, Catal. Commun. 2008, 10, 201–204.
[4] a) N. L. Lancaster, V. Llopis-Mestre, Chem. Commun.
2003, 22, 2812–2813; b) K. Smith, S. F. Liu, A. G. El-
Hiti, Ind. Eng. Chem. Res. 2005, 44, 8611–8615;
c) B. D. Powell, G. L. Powell C. P. Reeves, Lett. Org.
Chem. 2005, 550–553; d) S. J. Wang, Z. Y. Sun, J. Nie,
Chin. J. Chem. 2008, 26, 2256–2260.
Measurements of the Lewis Acidity
Typically, 1.0 mL of IL, 1.0 mL of nitromethane solutions of
À1
the Ln
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(NTf ) (0.15 mmolmL ) and 0.1 mL of nitromethane
2
3
[9b]
solutions of the 2,4,4’,6-tetramethoxyazobenzene
(0.3
À1
mmolmL ) were mixed. The resultant ILs were dried under
vacuum at 808C for 12 h before use. A sample was loaded
into a dry cuvette in a glove box and the cuvette was then
capped and sealed. The UV-vis determination was carried
out in the range of 350–700 nm.
[
5] a) S. T. Handy, C. R. Egrie, ACS Symp. Ser. 2002, 818,
1
3
34–146; b) K. Qiao, C. Yokoyama. Chem. Lett. 2004,
3, 808–809; c) D. Fang, Q. R. Shi, J. Cheng, K. Gong,
Catalytic Reactions
Typically, 5 mmol of IL with catalyst were dried under
vacuum for 1 h before the reaction. 0.32 mL of toluene
Z. L. Liu, Appl. Catal. A: Gen. 2008, 345, 158–163.
[6] a) J. Nie, H. Kobayashi, T. Sonoda, Catal. Today 1997,
36, 81–84; b) J. Nie, J. Xu, G. Y. Zhou, J. Chem. Res.
Syn. 1999, 7, 446–447; c) Y. B. Yuan, J. Nie, Z. B.
Zhang, S. J. Wang, Appl. Catal. A: Gen. 2005, 295, 170–
176; d) Y. B. Yuan, J. Nie, S. J. Wang, Z. B. Zhang,
Chin. J. Org. Chem. 2005, 25, 394–398.
[7] a) M. J. Kamlet, R. W. Taft, J. Am. Chem. Soc. 1976,
98, 377–383; b) R. W. Taft, M. J. Kamlet, J. Am. Chem.
Soc. 1976, 98, 2886–2894; c) M. J. Kamlet, J. L.
Abboud, M. H. Abraham, R. W. Taft, J. Org. Chem.
1983, 48, 2877–2887.
(
3.0 mmol) were added into the IL, followed by dropwise
addition of 0.13 mL of 95% nitric acid (3.0 mmol). The reac-
tion mixture was stirred at 858C for several hours. After
stopped the reaction, the product was extracted with a mix-
ture of hexane and ether. The combined organic extracts
was washed with saturated NaHCO3 solution, dried over
MgSO , and analyzed by gas chromatography using the in-
4
ternal standard method (n-decane, 1.54 mmol).
In order to determine the isolated yields of nitration prod-
ucts, we removed the solvent under reduced pressure; the
1944
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1939 – 1945

Lanthanide Bis[(trifluoromethyl)sulfonyl]imides as Reusable Catalysts
FULL PAPERS
[
[
8] W. Ogihara, T. Aoyama, H. Ohno, Chem. Lett. 2004,
3, 1414–1415.
[10] a) K. Mikami, Y. Mikami, H. Matsuzawa, Y. Matsumo-
to, J. Nishikido, F. Yamamoto, N. Nakajima, Tetrahe-
dron 2002, 58, 4015–4021; b) K. Binnemans, Chem.
Rev. 2007, 107, 2592–2614.
[11] J. Z. Sun, D. R. MacFarlane, Ionics 1997, 3, 356–362.
[12] J. M. Lee, S. Ruckes, J. M. Prausnitz, J. Phys. Chem. B
3
9] a) C. Reichardt, Chem. Soc. Rev. 1992, 21, 147–153;
b) H. Iwamoto, H. Kobayashi, P. Murer, T. Sonoda, H.
Zollinger, Bull. Chem. Soc. Jpn. 1993, 66, 2590–2602;
c) H. Kobayashi, K. Fujiki, A. Nagira, J. Nie, A.
Sonoda, T. Sonoda, Rep. Inst. Adv. Mater. Study 1993,
2
008, 112, 1473–1476.
7
, 103–113.
Adv. Synth. Catal. 2009, 351, 1939 – 1945
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1945