A novel task-specific ionic liquid for Beckmann rearrangement: A simple and effective way for product separation

Article abstract of DOI:10.1016/j.tetlet.2004.01.131

Under mild conditions and without any additional organic solvents, Beckmann rearrangement of ketoximes was performed in a novel task-specific ionic liquid consisting sulfonyl chloride. Especially for the conversion of cyclohexanone oxime to ε-caprolactam, ε-caprolactam has good solubility in water while the task-specific ionic liquid is immiscible with water, therefore, ε-caprolactam could be easily separated from the reaction system by water extraction.

Full text of DOI:10.1016/j.tetlet.2004.01.131

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2681–2683
A novel task-specific ionic liquid for Beckmann rearrangement: a
simple and effective way for product separation
Jianzhou Gui,a Youquan Deng,b Zhide Hua and Zhaolin Suna,c,
*
aCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
bCentre for Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
cCollege of Petroleum and Chemical Technology, Liaoning University of Petroleum & Chemical Technology,
Fushun 113001 Liaoning, China
Received 27 September 2003; revised 25 November 2003; accepted 19 January 2004
Abstract—Under mild conditions and without any additional organic solvents, Beckmann rearrangement of ketoximes was per-
formed in a novel task-specific ionic liquid consisting sulfonyl chloride. Especially for the conversion of cyclohexanone oxime to
e-caprolactam, e-caprolactam has good solubility in water while the task-specific ionic liquid is immiscible with water, therefore,
e-caprolactam could be easily separated from the reaction system by water extraction.
ꢀ 2004 Elsevier Ltd. All rights reserved.
The rearrangement of a ketoxime to the corresponding
amide, known as Beckmann rearrangement, is a pow-
erful method both in organic synthesis and chemical
manufacturing, particular for the preparation of e-cap-
rolactam from cyclohexanone oxime in industry. This
reaction, however, generally requires high reaction
temperature (approximately 130 ꢁC) and large amount
of strong Brønsted acid such as sulfuric acid, phospho-
rous pentachloride in ether, or hydrogen chloride in a
mixture of acetic acid and acetic anhydride in a non-
catalytic fashion. The use of excessive amount of such
chemicals, the large amount of by-products, and the
corresponding problem of corrosion make this process
environmentally questionable.1 In order to overcome
these problems, a few examples of catalytic Beckmann
rearrangement, proceed either in the vapor-phase pro-
cess or in the liquid-phase process, have been reported.2
advance in ionic liquids research provided another route
for achieving task-specific ionic liquids (TSILs) in which
a functional group is covalently tethered to the cation or
anion of the ionic liquid, especially to the two N atoms
of the imidazole ring. It was expected that these TSILs
may further enlarge the application scope of ionic liq-
uids in chemistry.3
Deng et al. have recently reported a new approach for
catalytic Beckmann rearrangement involving using ionic
liquids as reaction medium in the presence of PCl5.4 A
following work by Ren et al. showed that P2O5 or
EatonÕs reagent is also a suitable catalyst for this reac-
tion in ionic liquid.5 In both cases, cyclohexanone oxime
was converted to e-caprolactam with satisfactory con-
version rate and selectivity. This may be contributed to
the stabilization of cyclic iminium intermediate and the
enhancement of the acidity of Brønsted acids in the ionic
liquid media. One of the most serious shortcoming
concerning the catalytic Beckmann rearrangement, not
only in ionic liquids but also in the industry manufac-
turing and other catalytic system, is that there is no an
effective way to separate the e-caprolactam product
from the catalysts. This is why ammonia is used to
neutralize the sulfuric acid in industry and therefore
produce ammonium sulfate as a by-product. It is also
inevitable to produce volatile hydrogen chloride as by-
product when PCl5 and P2O5 are used as catalysts in
ionic liquids. Meanwhile, the oximes investigated as
substrates are restricted within a narrow scope and the
Room temperature ionic liquids have received recogni-
tion as novel and promising solvents for synthetic
chemistry. They have also been refereed to as Ôdesigner
solventsÕ as their chemical and physical properties could
be adjusted by a careful choice of cation/anion. Recent
Keywords: Beckmann rearrangement; Task-specific ionic liquid;
Ketoxime; Extraction.
* Corresponding author. Tel.: +86-413-6650364; fax: +86-413-6650-
866; e-mail: zlsun62@hotmail.com
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.131

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J. Gui et al. / Tetrahedron Letters 45 (2004) 2681–2683
Table 1. Beckmann rearrangement of cyclohexanone oxime in TISC
results are always unsatisfactory except for cyclohexa-
none oxime. Therefore, further investigation is necessary
to elucidate the merits of using ionic liquids for catalytic
Beckmann rearrangement.
Entry TISC
(g)
TISC/substrate
(mol/mol)
T
t
Con.
(%)
Sel.
(%)
(ꢁC) (h)
1
2
3
4
2.0
2.0
2.0
2.0
1:1
1:1
1:5
1:1
60
80
3
2
2
5
82.3 76.5
99.2 98.3
98.0 98.1
Based on the designable properties of TSILs, a novel
TSIL consisting sulfonyl chloride (TISC, Scheme 1) was
synthesized.6 This TISC has been demonstrated to be
both stable in hot water (80 ꢁC)7 and highly active for
Beckmann rearrangement. The synthesis of compound
A could be found in previous literature.8
80
100
ꢀ100
97.8
The best reaction temperature was ca. 80 ꢁC for
achieving a maximum selectivity for e-caprolactam.
Reduce the molar ratio of TISC/cyclohexanone oxime to
0.2 resulted no significant decrease in either conversion
or selectivity (Table 1, entries 2 and 3). This indicated
that TISC is an effective catalyst for Beckmann rear-
rangement.
In a typical experiment, cyclohexanone oxime and TISC
were charged into a 25 mL round-bottom flask with the
molar ratio of TISC/cyclohexanone oxime ranged from
1 to 0.2. Then the reactions were allowed to proceed for
1–5 h at the desired temperature. The whole reaction
was monitored by a MAT-90 GC/MS. The concentra-
tion of the reactant and the product was directly given
by the system of GC/MS chemstation according to the
area of each chromatograph peak.
In order to investigate the scope and limitation of such
TISC catalyst for the Beckmann rearrangement, other
ketoximes substrates were also tested. The results are
summarized in Table 2. In each case, oximes were
smoothly transformed to the corresponding amine or
cyanide after 3–6 h reaction with moderate to high
yields. The results are much better than those have been
previously reported in ionic liquid systems,4;5 suggesting
that TISC would be a more suitable catalytic system for
Beckmann rearrangement.
The results of rearrangement of cyclohexanone oxime to
e-caprolactam were summarized in Table 1. We can see
that TISC is of very high activity for the Beckmann
rearrangement. Cyclohexanone oxime was smoothly
transformed to e-caprolactamin in high yield (Table 1,
entry 2) at ca. 80 ꢁC. Increasing the reaction temperature
(e.g., to 100 ꢁC) and reaction time did not offer signifi-
cant advantages (Table 1, entry 4), although the con-
version of cyclohexanone oxime would slightly increase
with higher temperature (Table 1, entries 1, 2 and 4).
Once the reaction is complete, more attention is paid to
isolate the product and recycle the catalyst. Separation
of e-caprolactam from the reaction system is always a
serious problem in chemical industry so an ionic liquid/
aqueous system separation of e-caprolactam would offer
a very attractive option. Even though previous works
had demonstrated the feasibility of catalytic conversion
of cyclohexanone oxime to e-caprolactam in ionic liq-
uids, there is yet no effective way to separate the product
from ionic liquids. e-Caprolactam has good solubility in
water while TISC is immiscible with water. Therefore,
(CH2)3
(CH2)3
NH4PF6
Et
Et
+
+
-
-
.Cl
SO2Cl
N
N
SO2Cl
N
N
+NH4Cl
. PF6
Water, 293K
compound A
TISC
Scheme 1. A novel task-specific ionic liquid consisting sulfonyl chlo-
ride.
Table 2. Catalytic Beckmann rearrangement of oxime in TISCa
Entry
Substrate
t (h)
Product
Yield (%)
72
1
6
N
NO
2
NO
2
N
OH
OH
N
N
2
6
80
OH
OH
O
N
3
4
5
5
5
3
86
78
90
OH
NH
Ph
O
O
NH
NH
N
Ph
OH
N
OH
a Yields are based on GC/MS, reaction temperature 80 ꢁC.

J. Gui et al. / Tetrahedron Letters 45 (2004) 2681–2683
2683
3. (a) Wasserscheid, P.; Welton, T. Ionic Liquid in Synthesis;
Wiley-VCH & CoKGaA: Germay, 2002; (b) Wierzbicki,
A.; Davis, J. H. Proceedings of the Symposium on Advances
in Solvent Selection and Substitution for Extraction, 5–9
March 2000. AIChE: New York, 2000.
4. Peng, J.; Deng, Y. Tetrahedron Lett. 2001, 42, 403–405.
5. Ren, R. X.; Zueva, L.; Qu, W. Tetrahedron Lett. 2001, 42,
8441–8443.
e-caprolactam could be easily separated from the reac-
tion mixture by water extraction. We believe it is the first
example of an easy separation of e-caprolactam in ionic
liquids. The reaction mixture was extracted thrice with
water (5 mL · 3) after the reaction (Table 1, entry 3) to
give e-caprolactam at a total yield of 83%. This product
contains a very small amount of TISC due to the TISC
having a 0.86 g/100 mL in water.9 It is no doubt that a
further refine of the primary product is needed. The
remaining TISC was vacuumized for 2 h at 100 ꢁC, in
hoping that it could be used for another cycle. Repeat-
ing the Beckmann rearrangement reaction of cyclo-
hexanone oxime with the recycled TISC under the same
reaction conditions gives, however, a remarkable drop
of conversion (34%). In order to study the change of
TISC during the process, the pH value of the extractive
water (the first 5 mL) was examined. The pH value of 4.7
indicated that some TISC was transformed into sulfonic
acid in the reaction. The mechanism study is now
undergoing.
6. A typical procedure: 9.5 g (34.8 mmol) compound A and
5.67 g (34.8 mmol) NH4PF6 (commercial grade) were dis-
solved in 30 mL water. The mixture was stirred for 24 h at
room temperature. The upper aqueous phase was decanted
and the ionic liquid was washed several times with 10 mL
water, up to the point where the washing water was
chloride-free (tested with AgNO3). The residual water was
removed under vacuum for 6–8 h at 100 ꢁC and 10.0 g
(26.1 mmol) of the dark brown ionic liquid was obtained,
which corresponds to 75% of the theoretical yield. NMR
spectrum of the ionic liquid recorded on a Bruker AM-400:
1H NMR (400 MHz, acetone-d6); d ¼ 8:94 (s, 1H), 7.53 (s,
1H), 7.45 (s, 1H), 4.23 (t, J ¼ 7, 2H), 4.12 (q, J ¼ 7, 2H),
2.82 (t, J ¼ 7:8, 2H), 2.10–2.21 (m, 2H), 1.46 (t, J ¼ 7, 3H).
Furthermore, the thermal decomposition point of the ionic
liquid was determined by TGA (Perkin–Elmer TGA Pyris1
instrument, 10 ꢁC minꢁ1 heating rate under nitrogen) to be
235 ꢁC.
In conclusion, TISC has demonstrated to be a suitable
catalyst for catalytic Beckmann rearrangement. Owing
to the good solubility of e-caprolactam in water and the
immiscibility of TISC with water, e-caprolactam could
be extracted by water. This offers an attractive route for
e-caprolactam manufacturing. To our knowledge, this
should be the first report concerning the Beckmann
rearrangement of cyclohexanone oxime in TSILs under
mild conditions and without any additional organic
solvents and an easy separation of product.
€
7. (a) Wasserscheid, P.; Van Hal, R.; Bosmann, A. Green
Chem. 2002, 4, 400–404; (b) To confirm the hydrolysis
stability of TISC, 5 g of TISC and 5 g of water were heated
to 80 ꢁC. Sample was taken at 15 min intervals and the pH
value of the solution was checked with a pH indicator from
Shanghai. No decrease of pH value was determined during
6 h, indicating that TISC did not hydrolyze.
8. Yoshizawa, M.; Hirao, M.; Ito-Akita, K.; Ohno, H. J.
Mater. Chem. 2001, 11, 1057–1062.
9. (a) Chun, S.; Dzyuba, S. V.; Bartsch, R. A. Anal. Chem.
2001, 73, 3737–3741; (b) Solubility in water: In a 10 mL
polypropylene centrifuge tube, 1.00 g of the TISC and
2.5 mL of deionized water were shaken on a vortex mixer
for 30 min and centrifuged for 20 min. A 10 lL aliquot of
the aqueous phase was removed with a microsyringe and
diluted to 2.5 mL with deionized water. The absorbance of
this solution at 211 nm was measured by UV–vis spectro-
photometer (Perkin–Elmer Lamda 900) and compared with
that obtained from dissolving a weighed amount (1–9 mg)
of the TISC in 2.5 mL of deionized water.
References and notes
1. Izumi, Y.; Sato, S.; Urabe, K. Chem. Lett. 1983, 1649.
2. (a) Ichihashi, H.; Kitamura, M. Catalysis Today 2002, 73,
23–28; (b) Chandrasekhar, S.; Gopalaiah, K. Tetrahedron
Lett. 2002, 43, 2455–2457; (c) Chandrasekhar, S.; Gopal-
aiah, K. Tetrahedron Lett. 2003, 44, 577–755.