Highly efficient dinitration of aromatic compounds in fluorous media using ytterbium perfluorooctanesulfonate and perfluorooctanesulfonic acid as catalysts

Article abstract of DOI:10.1080/00397910600773700

Ytterbium perfluorooctanesulfonate [Yb(OPf)₃] and perfluorooctanesulfonic acid [PfOH] catalyze the highly efficient dinitration of toluene, benzene, benzyl chloride, and chlorobenzene in fluorous media. Notably the process produces almost no waste acid, as opposed to the traditional case. The fluorous phase-containing catalyst could be easily and efficiently recovered for reuse by simple phase separation. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600773700

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Highly Efficient Dinitration of Aromatic
Compounds in Fluorous Media using
Ytterbium Perfluorooctanesulfonate and
Perfluorooctanesulfonic Acid as Catalysts
Wen‐Bin Yi a & Chun Cai a
a Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing, China
Published online: 16 Feb 2007.
To cite this article: WenBin Yi & Chun Cai (2006) Highly Efficient Dinitration of Aromatic Compounds in
Fluorous Media using Ytterbium Perfluorooctanesulfonate and Perfluorooctanesulfonic Acid as Catalysts,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
36:20, 2957-2961, DOI: 10.1080/00397910600773700
To link to this article: http://dx.doi.org/10.1080/00397910600773700
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Synthetic Communicationsw, 36: 2957–2961, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600773700
Highly Efficient Dinitration of Aromatic
Compounds in Fluorous Media using
Ytterbium Perfluorooctanesulfonate and
Perfluorooctanesulfonic Acid as Catalysts
Wen-Bin Yi and Chun Cai
Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing, China
Abstract: Ytterbium perfluorooctanesulfonate [Yb(OPf)3] and perfluorooctanesulfonic
acid [PfOH] catalyze the highly efficient dinitration of toluene, benzene, benzyl
chloride, and chlorobenzene in fluorous media. Notably the process produces almost
no waste acid, as opposed to the traditional case. The fluorous phase-containing
catalyst could be easily and efficiently recovered for reuse by simple phase separation.
Keywords: Dinitration, fluorous biphasic catalysis, fluorous solvents, perfluorooctane-
sulfonic acid, ytterbium perfluorooctanesulfonate
Nitration is one of the key processes in synthetic chemistry and is widely used
industrially.[1,2] However, nitration in a manufacturing process requires the use
of potent mixtures of concentrated or fuming nitric acid with sulfuric acid
leading to excessive acid waste streams and added expense. Although a lot
of catalysts or catalytic systems that complete the nitration by catalytic use
have been reported, subjects are limited to mononitration in most cases.
Thus, development of more efficient and powerful catalysts is strongly
demanded for dinitration.[3] Recently, it was found that ytterbium perfluoro
octanesulfonates [Yb(OPf)3] can catalyze the mononitration of aromatic
compounds in moderate yield using concentrated nitric acid in fluorous
Received in Japan March 1, 2006
Address correspondence to Chun Cai, Chemical Engineering College, Nanjing
University of Science and Technology, Nanjing 210094, China. E-mail: caichunh@
mail.njust.edu.cn
2957

2958
W.-B. Yi and C. Cai
solvents.[4] Yuan et al. used MCM-41-supported metal bis[(perfluoroalkyl)sul-
fonyl]imides [M(NPf2)n] as catalysts for nitration of aromatic compounds.[5]
A
novel fluorinated catalyst for nitration, cyclic trimeric perfluoro-o-phenylene-
mercury [(o-C6F4Hg)3], was reported by Zaraisky et al.[6] Crampton and
coworkers reported that perfluorocarbons can be used effectively as solvents
and bulking agents in the dinitration of aromatic compounds with nitric acid
or dinitrogen pentoxide to yield highly nitrated products.[7] However, this dini-
tration required at least 5 equiv nitric aicd or dinitrogen pentoxide, in addition
to rather drastic reaction conditions and tedious workup procedures required
when using dinitrogen pentoxide as a nitration reagent. Inspired by the
recent work on the Friedel–Crafts acylation of unactivated benzenes
catalyzed by Yb(OPf)3 with the coexistence of perfluorooctanesulfonic acid
[PfOH] using perfluorocarbon as solvent,[8] we have applied this catalytic
system to the dinitration of aromatic compounds, such as toluene, benzene,
benzyl chloride, and chlorobenzene. As expected high yields of the corre-
sponding dinitrobenzenes and the robustness of the catalytic system for
recycling using by simple phase-separation were obtained. We report herein
the work on this new application of the catalytic system.
The dinitration of toluene was first carried out in various fluorous using
slightly excess 95% nitric acid as the nitration reagent. Based on the
general concept of fluorous phase chemistry,[9] we also used hexane as a
cosolvent for the nitration. It was found that upon heating at 608C, the
organic phase is miscible with fluorous phase, and the nitration became a
biphasic reaction (top layer is the nitric acid; bottom layer is the fluorous
solvent with aromatic compound). Much lower yields of dinitration were
obtained in the presence of PfOH or Yb(OPf)3 (5% and 8% yields, respect-
ively). In addition, a Yb(OPf)3-PfOH catalyst loading of only 0.5 mol% was
required to produce quantitive yields of dinitrotoluene. The ratio of 2,4-
isomer to 2,6-isomer in the production is generally ca. 4:1, which is typical
for nitronium ion nitrations.[7] Other lanthanide perflate catalytic systems
were also examined for the nitration in the same reaction conditions. We
found that Sc(OPf)3-PfOH had a similar catalytic activity for the reaction
with a quantitive conversion to dinitro product over 4 h, whereas the
systems containing other lanthanide perflates, namely La(OPf)3 or
Eu(OPf)3, yielded dinitrotoluene in 46% and 59%, respectively. This is
ascribed to the higher Lewis acidity of Yb(OPf)3 and Sc(OPf)3 than those
of other lanthanide perflates. When using perfluorotoluene (C7F8) and per-
fluoromethylcyclohexane (C7F14) as fluorous solvents in Yb(OPf)3-PfOH
catalyzed cases, the nitration also proceeded smoothly to give the desired
product. However, perfluorotoluene is in fact miscible with reaction
aromatic substrates at room temperature. Thus, it is impossible to recover
the fluorous phase by phaseseparation. In addition, it was found that during
repeated nitration reactions the loss of fluorous solvent is very serious when
using perfluoromethylcyclohexane (C7F14) as a fluorous solvent because it
is very volatile (bp 768C).

Dinitration of Aromatic Compounds
2959
Thus, to seek out a practical and economical dinitration, we decided to use
relatively cheap and similarly active Yb(OPf)3-PfOH as catalysts and perfluor-
odecalin (C10F18, cis- and trans-mixture) as the fluorous solvent for the dini-
tration of other aromatic compounds. The results are summarized in Table 1.
As shown in Table 1, when 2-nitrotoluene and 4-nitrotoluene were performed
as substrates, the dinitration was completed in a shorter time. Although the
reaction became slower in nitration of benzene, the advantages of the
catalytic system were still clear: benzene gave a quantitive conversion to
the dinitrobenzene in 16 h, as opposed to the uncatalyzed case, which
proceeded to only 2% conversion. As to the isomers of product of benzene,
small proportions of 1,2- and 1,4-dinitrobenzene were formed in addition to
the major 1,3-isomer. As an extension of the present work, we have investi-
gated the dinitration of benzyl chloride and chlorobenzene under similar con-
ditions. The reaction of benzyl chloride gave 2,4- and 2,6-dinitrobenzyl
chlorides in an 86:14 ratio. In the case of chlorobenzene, the reaction
produced a 2,4-/2,6-dinitrochlorobenzene in a ratio of 97:3, which is much
higher than the mixed acid nitration case with the ratio of 90:10. The
reaction was unsuccessful for phenol and benzyl alcohol, which presumably
suffered complications due to competing oxidation of the hydroxy.
Table 1. Summary of the reactions for dinitration of aromaticsa
Isomers of
product/
composition
(%)b
Isomers of
product/
composition
(%)b
Isomers of
product/
composition
(%)b
Substrate
Time (h)
Toluene
Toluenec
4
4
2,4- (80)
2,4- (84)
2,4- (81)
2,4- (68)
2,4- (.99)
1,3- (90)
1,3- (2)
2,6- (18)
2,6- (16)
2,6- (16)
2,6- (32)
2,6- (,1)
1,4- (8)
Others (2)
Others (3)
Toluened
4
2-Nitrotoluene
4-Nitrotoluene
Benzene
Benzenee
2.5
2.5
16
16
8
Others (2)
1,4- (-)
2,6- (14)
2,6- (3)
Benzyl chloride
Chlorobenzene
2,4- (86)
2,4- (97)
18
aReaction conditions for dinitration: substrate (0.0025 mol), 2.1 equiv. 95% HNO3,
Yb(OPf)3 (0.5 mol%), PfOH (0.5 mol%), perfluorodecalin (5 mL), hexane (3 mL),
608C. Reaction conditions for mononitration: substrate (0.0025 mol), 1.1 equiv. 95%
HNO3, Yb(OPf)3 (0.5 mol%), PfOH (0.5 mol%), perfluorodecalin (5 mL), hexane
(3 mL), 608C. Convension of substrate was more than 99.5% by GC analysis.
bComposition was determined by GC using an internal standard method.
c,dPerfluorotoluene and perfluoromethylcyclohexane as fluorous solvents
respectively.
eUncatalyzed case.

2960
W.-B. Yi and C. Cai
When the reaction was finished, the reaction mixture was cooled to room
temperature. The fluorous phase with Yb(OPf)3 and PfOH catalysts can
separate from the organic layer and return to the bottom layer. Use of
fluorous phase, recycled without purification, was equally effective. The dini-
trations of toluene under the conditions mentioned previously were run for
three consecutive cycles, furnishing the corresponding product with 99, 97,
and 96% yields. Based on the 19F NMR and UV-vis spectroscopic data and
GC-MS, no loss of catalyst or fluorous solvent to the organic and water
phases can be detected. However, we found that the loss of PfOH leaking to
the water phase become obvious when the volume ratio of fluorous phase to
water phase decreased greatly. The results of 19F NMR and UV-vis showed
that the loss of PfOH was increased from 0.1% to 8.9% with the ratio
mentioned previously decreasing from 10:1 to 1:1. Thus, controlling the
high volume ratio of fluorous phase to water phase was important strategy
for the use of this novel catalytic system in nitration.
Finally, the assumed catalytic cycle is shown in Scheme 1. A key highly
efficient nitration agent is proposed as nitroperflate 1,[10] which reacts even
with nitro aromatic compounds to afford the corresponding dinitrobenzenes,
accompanied by regeneration of PfOH. Yb(OPf)3 is assumed to catalyze the
nitration involving 1 because of its complexation with 1, which easily
makes NOþ2 release from the ion pair 1. In addition, Yb(OPf)3-promoted
generation of 1 is proposed because of its high oxophilicity in the catalytic
cycle.
In conclusion, the Yb(OPf)3-PfOH-PFC system can be considered the
most attractive alternative to existing heterogeneous catalytic systems for
dinitration of toluene, benzene, benzyl chloride, and chlorobenzene, and this
report may open the door to solve the long-term dinitration of aromatic
compounds problem, such as treatment of excessive sulfuric acid and waste
water.
Scheme 1. Assumed catalytic cycle for the dinitration.

Dinitration of Aromatic Compounds
EXPERIMENTAL
2961
Typical Procedure
A mixture of Yb(OPf)3 (21 mg, 0.012 mmol), PfOH (6 mg, 0.012 mmol),
toluene (0.26 ml, 2.5 mmol), hexane (3 ml), perfluorodecalin (C10F18, cis-
and trans-mixture, 5 ml) and 95% HNO3 (0.22 ml) was stirred at 608C for
4 h. Then, the fluorous layer on the bottom was separated for the next
nitration. The reaction mixture (organic phase) was treated with the usual
workup. Conversion and composition were determined by GC or GC-MS
using an internal standard method.
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