One-pot synthesis of silica gel confined functional ionic liquids: Effective catalysts for deoximation under mild conditions

Article abstract of DOI:10.1016/j.tetlet.2003.10.175

Several acid-functional ionic liquids were physically confined into the silica gel through a sol-gel process and used as effective catalysts for deoximation reactions, which proceeded under ambient temperatures without addition of any co-catalysts or oxidizing and reducing agents.

Full text of DOI:10.1016/j.tetlet.2003.10.175

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 265–268
One-pot synthesis of silica gel confined functional ionic liquids:
effective catalysts for deoximation under mild conditions
Dongmei Li, Feng Shi, Shu Guo and Youquan Deng*
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, China
Received 3 September 2003; revised 29October 2003; accepted 30 October 2003
Abstract—Several acid-functional ionic liquids were physically confined into the silica gel through a sol–gel process and used as
effective catalysts for deoximation reactions, which proceeded under ambient temperatures without addition of any co-catalysts or
oxidizing and reducing agents.
ꢀ 2003 Elsevier Ltd. All rights reserved.
Recovery of ketone and aldehyde from oximes is an
important reaction because oxime serves as an efficient
protective group for carbonyls, which are extensively
used for the purification of carbonyl compounds.1 At
the same time, oximes could be prepared from non-
carbonyl compounds, and the regeneration of carbonyl
compounds from oximes represents a potential route for
syntheses of ketones and aldehydes.2 Various methods,
including acid-catalytic hydrolysis, reductive deoxima-
tion, and oxidative deoximation, have been developed
for these processes.3 Unfortunately, stoichiometric acid,
oxidizing or reducing agents were generally needed to
neutralize, oxidize or reduce the hydroxylamine formed
during the reactions as mentioned above. In addition,
the using of large amounts of Ni, Cr, Mn, and Zn, etc.
usually causes environmental pollution seriously.
Therefore, the development of clean, highly efficient
catalytic process for deoximation reactions should be
highly desired.
recently, it was found that the ionic liquids could be
efficient dehydrating agents, in which methyl tert-butyl
ether could be produced from the dehydration between
tert-butyl alcohol and methanol, and disubstituted ureas
could be obtained from the reactions between amines
and carbon dioxide,5 and therefore, ionic liquids may
also be effective catalysts for cleavage of hydroxylamine
from the oximes especially with the ionic liquids in
which an acid group was incorporated. Herein, a series
of acid-functional ionic liquids were synthesized (Fig. 1)
for the deoximation reactions and high conversion and
selectivity were achieved. Then, immobilization of these
ionic liquids with silica gel by a sol–gel method6 was
further conducted for easy separation of catalyst and
increase of the catalytic efficiency for the deoximation
reactions.
Firstly, the cyclohexanone oxime was used as a probe
substrate to investigate the possibility of the pure func-
tional ionic liquids as the deoximation catalysts. As it is
shown in Table 1, all the pure functional ionic liquids
exhibited high catalytic activities on the deoximation of
cyclohexanone oxime. The conversions of cyclohexa-
none oxime could reach to 80–90% with ca. 100%
selectivities. The presence of the substituted alkyls on
the imidazolium cation was found to have strong impact
on the conversion and selectivity. With the increasing of
the carbon number of the side chain, the conversion of
cyclohexanone oxime decreased remarkably while the
selectivity was maintained ca. 99%.
As a new kind of catalytic media with very low vapor
pressure and high dissolvabilities towards many organic
and inorganic substances, room temperature ionic liq-
uids have been attracted growing interests and many
catalytic reactions proceeded in ionic liquids as reaction
media were reported with excellent performance.4 More
Keywords: Deoximation; Oxime; Ionic liquid; Immobilization; Cataly-
sis.
Based on the results achieved, the silica gel confined
these acid-functional ionic liquids as catalysts were then
* Corresponding author. Tel.: +86-931-8277683; fax: +86-931-82770-
88; e-mail: ydeng@ns.lzb.ac.cn
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.175

266
D. Li et al. / Tetrahedron Letters 45 (2004) 265–268
n=1, x=Cl, m=0, MAcImCl;
m=3, BAcImCl;
n=1, m=3, x=Br, BAcImBr;
( )
( )n
+
x=BF4, BAcImBF4;
x=PF6, BAcImPF6;
n=7, m=3, x=Br, BOcImBr;
m
N
N
CO2H
-
m=5, HAcImCl;
m=7, OAcImCl;
X
m=9, DAcImCl;
+
N
CO2H
PyAcCl
Cl-
Figure 1. Ionic liquid in which an acid group was incorporated upon the side chain of imidazolium cation.
Table 1. Results of deoximation of cyclohexanone oxime over silica gel confined ionic liquids and pure ionic liquid catalystsa
Entry
Catalysts
Conversion (%)
Selectivity (%)
TONb
1
MAcImCl/silica gel
BAcImCl/silica gel
HAcImCl/silica gel
OAcImCl/silica gel
DAcImCl/silica gel
BAcImBF4/silica gel
BAcImPF6/silica gel
BAcImBr/silica gel
94.3 (94.4)c
92.5 (94.8)
90.3 (92.1)
76.9 (93.7)
72.4 (83.7)
93.8 (93.2)
94.1 (94.2)
88.2 (94.6)
>99
>99
>99
>99
>99
>99
>99
>99
118 (30)
143 (37)
158 (40)
150 (46)
155 (45)
180 (45)
219 (55)
164 (49)
2
3
4
5
6
7
8
9BOcImBr/silica gel
26.6 (84.6) 9
65 (52)
92.2 (96.8)
90.1
>9
10
11d
PyAcCl/silica gel
BAcImBF4/silica gel
>99
>99
114 (30)
173
a Reacted for 2 h.
b Turn over number of ionic liquid ¼ mole of oxime converted per mole of ionic liquid.
c The data in brackets were results of the deoximation over pure ionic liquid catalysts.
d The immobilized catalyst was reused for the second time.
tested so as to obtain a highly efficient and easily sepa-
rable deoximation catalyst system. It can be seen from
Table 1 that exceeded 90% of conversions and ca. 100%
of selectivities were achieved over most of the immobi-
lized catalysts investigated. It is worth noting that
almost the same conversions and selectivities as the pure
ionic liquid catalysts were obtained, although the
amount of ionic liquid contained in the silica gel, which
was only ca. 25% of the amount of ionic liquid needed as
pure ionic liquid catalysts under same reaction condi-
tions, indicating that the immobilization of ionic liquids
in the silica gel would not only reduce the amount of
ionic liquid greatly but also result in an effective catalyst
for deoximation. It could be observed more clearly, if
comparing the TON of cyclohexanone oxime over the
pure and immobilized ionic liquid catalysts, that the
TONs of immobilized ionic liquids as catalysts were
almost four times as higher as that of using pure ionic
liquids as catalysts. The reusability of the immobilized
ionic liquid catalyst was also examined. It can be seen
that 90.1% of conversion and 99% of selectivity were
maintained when BAcImBF4/silica gel catalyst was
reused for the second time, which suggested that the
reusability of such catalysts was possible although only
8 mg of immobilized ionic liquid catalyst was left, this
may be mainly attributed to the catalyst losing during
separation of the catalyst from the resulted reaction
mixture, after the first reaction.
system (Table 2). It can be seen that longer reaction time
was needed for deoximation of aromatic oximes. For
example, ca. 100% of conversions and selectivities could
be achieved when p-methoxyl benzaldoxime and p-nitro
benzaldoxime were used as substrates after the deoxi-
mation reaction proceeded for 72 h. The isolated yields
of p-methoxyl benzaldehyde and p-nitro benzaldehyde
were also obtained (89% and 96%). At the same time,
poor result was given when using acetophenone oxime
as substrate, and the conversion was only 19% after the
reaction proceeded for 72 h. This may be attributed to
the steric hindrance of acetophenone oxime. 73% and
93% of conversions and 99% of selectivities were also
achieved, respectively when using acetaldehyde oxime
and butanone oxime as substrates.
An interesting result was that no reaction occurred when
using acetone oxime as reactant, and nearly the same
amounts of acetone oxime was produced when using
other oximes as substrates (Fig. 2). These experimental
results suggested that the formation of acetone oxime
may be an important step for the deoximation reactions
proceeding, and the presence of water was also indis-
pensable because the reaction could not occur without
addition of water.
In conclusion, a new method for deoximation reactions
with a series of immobilized acid-functional ionic liquids
with silica gel was developed under mild reaction con-
ditions. The excellent conversions and selectivities were
achieved with higher catalytic efficiency, easy separation
and reusability. The uses of stoichiometric reducing or
oxidizing agents were also avoided.
As one of the most effective catalysts for the deoxima-
tion reactions, BAcImBF4/silica gel was further used for
the deoximation reactions of several other oximes so as
to investigate the universality of such novel catalyst

D. Li et al. / Tetrahedron Letters 45 (2004) 265–268
Table 2. Deoximation of different oxime substrates over BAcImBF4/silica gel catalyst
267
Entry
Oximes
Products
t (h)
Conversion (%)
Selectivity (%) TON
Yields (%)
OH
H
O
N
1
72
>99
>99
142
89
MeO
H
MeO
OH
O
N
2
39742
––19>9
OH
N
O
3
4
5
6
72
72
24
48
73
>99
93
>99
>99
>99
––
268
129
276
––
––
96
––
––
H
H
OH
O
N
O2N
H
O2N
H
OH
O
N
OH
O
N
––
After ageing at 60 ꢁC for 12 h, the mixture was then
dried in vacuum at 100 ꢁC for 3 h.
OH
OH
R2
ionic liquid/
silica gel
O
N
N
+
R1
R2
acetone/water
R1
General procedure for deoximation reaction: For each
reaction, oxime (0.2 g), silica gel confined functional
ionic liquids or pure functional ionic liquid catalysts
(0.01 g), acetone (2 mL) and water (6 mL) were succes-
sively added into a 25 mL round bottom, and then the
reaction proceeded at room temperature with vigorous
stirring for 2–72 h. Qualitative analyses were conducted
with a HP 6890/5793 GC–MS with a 30 m · 0.25 mm ·
0.33 lm capillary column and with chemstation con-
taining a NIST Mass Spectral Database. Quantitative
analyses were conducted with a HP 1790 GC equipped
with a FID detector, 30 m · 0.25 mm · 0.33 lm capillary
column. Conversion and selectivity were calculated
according to the chromatograph peak areas given by the
chemstation. After reaction, the catalyst was obtained
by filtration, washed with acetone twice and then reused
for the second time.
Figure 2. A possible route for the deoximation of oximes over silica gel
confined ionic liquids.
Preparation of the ionic liquids and ionic liquids confined
in silica gel: Following ionic liquids containing acid
groups were synthesized and used here: MAcImCl
(M ¼ methyl, Ac ¼ acetoxy), BAcImBr (B ¼ n-butyl),
BAcImCl, BAcImBF4, HAcImCl (H ¼ n-hexyl), BAc-
ImPF6, OAcImCl (O ¼ octyl), BOcImBr (Oc ¼ octylic
acid), DAcImCl (D ¼ decyl) and PyAcCl (Py ¼ pyrid-
inium).
Ionic liquids containing acid groups, in which the anion
were Clꢀ or Brꢀ, were synthesized by reacting of alkyl
imidazole with the corresponding halide substituted
carboxylic acid in toluene under reflux for about 24 h
As to the isolated yields of corresponding aldehydes,
only deoximation of p-methoxyl benzaldoxime and
p-nitro benzaldoxime were conducted due to the great
difference between the boiling point of acetone oxime
and corresponding aldehydes. The separation was con-
ducted as follows: after reaction, the resulted liquid
mixture containing solid catalyst was filtrated to
separate the catalyst, and a solution of acetone, water,
acetone oxime, and desired aldehyde was left.
Then this solution was distilled at 120 ꢁC with ca.
1–5 mmHg vacuum to remove acetone, water, and ace-
tone oxime. The desired product, that is aldehyde, was
given and weighted to obtain the isolated yield.
ꢀ
andꢀthe acid functional ionic liquids containing BF4 or
PF6 were further obtained by double decomposition of
these halide containing ionic liquids with corresponding
salts.
The silica gel confined ionic liquid was obtained as fol-
lowing procedure: After 10 mL of TEOS (tetraethyl
silicate) was heated to 60 ꢁC in a conical flask with
stirring, the mixture of ionic liquid (1 g)+ethanol (5 mL)
was transferred into the conical flask quickly. Then,
2 mL of HCl (35 wt%) and 3.5 mL of H2O were added
and the resulted mixture became gradually coagulated.

268
D. Li et al. / Tetrahedron Letters 45 (2004) 265–268
S. B.; Kim, K.; Kim, Y. H. Tetrahedron Lett. 1987, 28, 645;
Acknowledgements
(d) Meloney, J. R.; Lyle, R. E.; Saavedra, J. E.; Lyle, G. G.
Synthesis 1978, 212; (e) Aizpurua, J. M.; Juaristi, M.;
Lecea, B.; Palomo, C. Tetrahedron 1987, 41, 2903; (f)
Joseph, R.; Sudalai, A.; Ravindranathan, T. Tetrahedron
1994, 35, 5493; (g) Curini, M.; Rosati, O.; Pisani, E. Synlett
1996, 333; (h) Bandgar, B. P.; Kunde, L. B.; Thote, J. L.
Synth. Commun. 1997, 27, 1149; (i) Olah, G. A.; Liao, Q.;
Lee, C. S.; Surya Prakash, G. K. Synlett 1993, 427; (j)
Drabowicz, J. Synthesis 1980, 125.
This work was financially supported by the Natural
Science Foundation of China (no. 20233040 and no.
20225309).
References and Notes
4. (a) Welton, T. Chem. Rev. 1999, 99, 2071; (b) Smith, G. P.;
Dworkin, A. S.; Pagni, R. M.; Zingg, P. J. Am. Chem. Soc.
1989, 111, 525; (c) Boon, J. A.; Levisky, J. A.; Pflug, J. L.;
Wikes, J. S. J. Org. Chem. 1986, 51, 480; (d) Adams, C. J.;
Earle, M. J.; Roberts, G.; Seddon, K. R. Chem. Commun.
1998, 2097; (e) Green, L.; Hemeon, I.; Singer, R. D.
Tetrahedron Lett. 2000, 41, 134; (f) Peng, J.; Deng, Y.
Tetrahedron Lett. 2001, 42, 403.
5. (a) Shi, F.; Xiong, H.; Gu, Y.; Guo, S.; Deng, Y. Chem.
Commun. 2003, 1054; (b) Shi, F.; Deng, Y.; SiMa, T.; Peng,
J.; Gu, Y.; Qiao, B. Angew. Chem., Int. Ed. 2003, 42,
3257.
1. (a) Bandgar, B. P.; Lalita, B. K.; Thote, J. L. Synth.
Commun. 1997, 27, 1149; (b) Donaruma, L. G.; Heldt, W.
Z. Org. React. 1960, 11, 1; (c) Bosch, A. L.; Cruz, P.; Diez-
Barra, E.; Loupy, I.; Langa, F. Synlett 1995, 1259.
2. (a) Barry, R. H.; Hartung, W. H. J. Org. Chem. 1947, 12,
460; (b) Touster, O. Org. React. 1953, 7, 327; (c) Hartung,
W. H.; Cressly, F. Org. Synth. Coll. 1943, 11, 363; (d)
Barton, D. H. R.; Beaton, J. M.; Geller, L. E.; Pechet, M.
M. J. Am. Chem. Soc. 1961, 83, 4076; (e) Barton, D. H. R.;
Beaton, J. M. J. Am. Chem. Soc. 1961, 83, 4083.
3. (a) Curran, D. P.; Brill, J. F.; Rakiewicz, D. M. J. Org.
Chem. 1983, 49, 1654; (b) Rao, C. G.; Rashakrishna, A. S.;
Singh, B. B.; Bhatnagar, S. P. Synthesis 1983, 808; (c) Shim,
6. Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.; Afeworki,
M. J. Am. Chem. Soc. 2002, 124, 12932.