Thermal behaviors of ionic liquids under microwave irradiation and their application on microwave-assisted catalytic Beckmann rearrangement of ketoximes

Article abstract of DOI:10.1081/SCC-120021511

Microwave-assisted Beckmann rearrangements of ketoximes in room temperature ionic liquids have been accomplished successfully using only 5 mol% of H₂SO₄ as a catalyst.

Full text of DOI:10.1081/SCC-120021511

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Thermal Behaviors of Ionic Liquids Under Microwave
Irradiation and Their Application on Microwave-
Assisted Catalytic Beckmann Rearrangement of
Ketoximes
a
b
a
a
Jae Kyun Lee , Dok-Chan Kim , Choong Eui Song & Sang-gi Lee
a
Life Sciences Division , Korea Institute of Science and Technology , Cheongryang, Seoul,
Korea
b
Department of Chemical Engineering , The University of Seoul , Seoul, Korea
Published online: 17 Aug 2006.
To cite this article: Jae Kyun Lee , Dok-Chan Kim , Choong Eui Song & Sang-gi Lee (2003) Thermal Behaviors of Ionic Liquids
Under Microwave Irradiation and Their Application on Microwave-Assisted Catalytic Beckmann Rearrangement of Ketoximes,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:13,
2301-2307, DOI: 10.1081/SCC-120021511
To link to this article: http://dx.doi.org/10.1081/SCC-120021511
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 13, pp. 2301–2307, 2003
Thermal Behaviors of Ionic Liquids Under
Microwave Irradiation and Their Application on
Microwave-Assisted Catalytic Beckmann
Rearrangement of Ketoximes
1
Jae Kyun Lee, Dok-Chan Kim, ChoongEui Song,
2
1
1
,
and Sang-gi Lee *
1
Life Sciences Division, Korea Institute of Science and
Technology, Cheongryang, Seoul, Korea
2
Department of Chemical Engineering, The University
of Seoul, Seoul, Korea
ABSTRACT
Microwave-assisted Beckmann rearrangements of ketoximes in room
temperature ionic liquids have been accomplished successfully using
only 5 mol% of H SO asa cataly st .
2
4
*
Correspondence: Sang-gi Lee, Life Sciences Division, Korea Institute of
Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea;
E-mail: sanggi@kist.re.kr.
2
301
DOI: 10.1081/SCC-120021511
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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2
302
Lee et al.
Microwave-assisted organic reactions have been recently received a
great deal of attention and are quickly developing area in synthetic chem-
[1]
istry. The reactionsunder microwave irradiation so metimesproceeded
much faster with higher yields compared to the conventional heating.
Despite of several advantages of this technology, one of the drawbacks
is its limitation to the use of harmful organic solvents. Moreover, explo-
sions caused by irradiation of microwave to organic solvent are some-
times unavoidable. Therefore, the finding of ideal replacements, less
toxic, and environmentally benign, for conventional solvent offers new
opportunities in microwave-assisted organic reactions. In this context,
room temperature ionic liquidsmay have a great potential for attaining
this goal. Ionic liquids, particularly those based on 1-n-butyl-3-methyli-
midazolium salts, [bmim] [X ] where X ¼ BF , PF , SbF , and OTf, have
4
6
6
[
2]
received recently much attention as novel green solvents. During our on
[
3]
going study on organic reaction using ionic liquids, we interested in the
combination of ionic liquids with microwave (MW)-assisted organic
reactions. Ionic liquids consisting entirely of ions could absorb the micro-
wave energy efficiently. Moreover, the risk of an explosion caused by
rapid increase of vapor pressure of the solvent can be avoided since the
ionic liquids have no measurable vapor pressure. Very recently, upon
[
4]
[5]
taking advantage of these properties, we and others utilized the
ionic liquids for microwave-assisted organic reactions. Here we wish to
report the extension of utility of ionic liquids as reaction medium for
microwave-assisted organic reaction to the catalytic Beckmann
rearrangement of ketoximes.
Beckmann rearrangement of a ketoxime to the corresponding amide
isan important sy nthetic method. Generally the reaction requiresa large
amount of Brønsted acids such as sulfuric acid, which cause a serious
[
6]
drawback by forming ammonium sulfate as a by product. To overcome
this, various catalytic systems for liquid-phase Beckmann rearrangement
[
7]
of ketoximeshave been developed. However, the efficiency isnot very
high, and the turn over number (TON) based on acid catalysts is gener-
ally less than 5. The application of ionic liquids in Beckmann rearrange-
mentsof ketoximesmay provide a beneficial effect on st abilization of the
positively charged iminium intermediates. Acidity of the Brønsted acid
may be enhanced due to the solvophobicity of ionic liquids, and thus, the
reactivity of the reaction could be increased. Recently, it has been
reported that the Beckmann rearrangementsof ketoximesin ionic liquids
[
8]
using 10–20 mol% of PC15 by Peng and Deng and 10 mol% of P O or
2
5
[
9]
Eaton’sreagent by Ren et al. However, to our best knowledge, there is
no report on microwave-assisted Beckmann rearrangement of ketoximes
in ionic liquids.

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Beckmann Rearrangement of Ketoximes
2303
Figure 1. TGA (thermogravimetric analysis) of ionic liquids, [bmim][X ], and
ꢀ
DMF (10 C/min, N
2
).
It isgenerally known that the ionic liquidsare thermally ts able.
However, there isno reported data on the thermal ts ability of ionic
liquidsin the literature. Therefore, the thermal ts ability of the ionic
liquids, [bmim] [X ], has been investigated first using thermogravimetric
analysis (TGA) (Fig. 1). The TGA showed that all of the ionic liquids
exhibited similar thermal stability regardless of the counter anion and
ꢀ
decomposed drastically in the temperature range 380–400 C. We also
examined the thermal behaviorsof the ionic liquidsunder microwave
irradiation. The heating ratesof the ionic liquid,s [bmim] [ X ], are
faster than DMF, and largely affected by the counter anion, X. The
[
bmim] [OTf ] showed the fastest heating rate, thus, the temperature
ꢀ
reached 200 C after 2 min. An interest finding is that the temperature
of [bmim] [PF ] and [bmim] [BF ] were reached thermal stationary
states after 100 s (ca. 115–120 C) (Fig. 2).
6
4
ꢀ
With these thermal data in hand, we examined the utility of ionic
liquids as solvents for microwave-assisted catalytic Beckmann rearrange-
mentsof ketoxime u si ng H SO asa cataly st . H SO isone of the mo st
2
4
2
4
commonly used Brønsted acid catalysts, but normally required excess
amounts. In the first time, the effects of counter anion of [bmim] [X ]
on the reactivity have been examined using 25 mol% of H SO . To pre-
2
4
vent the localized superheating and improve the homogeneity of the
reaction mixture, we conducted the reaction with intermittent heating
and mixing at a full power of domestic oven (2450 MHz, 850 W). Thus,
a 1 M solution of propiophenone oxime in [bmim] [X ] in the presence of
25 mol% of H SO wasirradiated for 20 s, then the reaction mixture was
2 4

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Lee et al.
Figure 2. Thermal behaviorsof ionic liquidsand DMF under MW irradiation.
ꢀ
Temperature isthe mean average ( Æ5 C error range) of three measurements.
taken out, mixed again for 5 s and then irradiated again for 20 s. This step
was repeated until completion of the reaction. The results are summar-
ized in Table 1.
As shown in Table 1, all of the reactions using 25 mol% of H SO
2
4
were completed within 60 sto give phenyl group migrated product quan-
titatively indicating that the rearrangementswere not much affected on
the counter anion of ionic liquids. The hydrophobic ionic liquids, [bmim]
[SbF ], [bmim] [BF ], [bmim] [PF ] wasalmo ts retained even in u is ng
6 4 6
5
mol% of catalyst (Entries 1–3). In contrast with microwave irradiation,
the rearrangementsof propiophenone oxime under conventional heating
condition using 25 mol% of H SO were not efficient. After 3 h heating at
2
4
ꢀ
120 C, 76% yield of the phenyl group migrated product wasobtained by
using [bmim] [PF ] as a solvent, however, less than 30% of product was
6
formed in other ionic liquids.
The efficiency of thisnovel protocol con ists ing of H
SO catalyst-
4
2
ionic liquid-microwave for Beckmann rearrangement were tested for
other ketoximesu si ng 5 mol% of H 2SO in [bmim] [PF ] and the results
4
6
are summarized in Table 2. All of the acyclic ketoximes rearranged highly
efficiently within 60 smicrowave irradiation (Entries1–5). However, the
cyclic ketoxime, a-tetralone oxime, wasnot rearranged effectively, thus,
only 30% of the rearranged product wasformed under 120 smicrowave
irradiation (Entry 6).
In summary, the utility of environmentally benign room temperature
ionic liquids as solvents for microwave-assisted catalytic Beckmann
rearrangement of ketoximeshasbeen proved. The ketoximeswere

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Beckmann Rearrangement of Ketoximes
2305
rearranged in the presence of only 5 mol% of H SO in hydrophobic
2
4
ionic liquids us ch as[bmim] [PF 6] to give the corresponding amides
in high yieldswithin 60 smicrowave irradiation. Further application
Table 1. The effect of counter anion of [bmin] [X] on Beckmann
rearrangement of ketoxime.
a
Yield (%)
Thermal heating
ꢀ
MW
25 mol% H
MW
5 mol% H
(120 C)
Entry Ionic liquid
2
SO
4
2
SO
4
2 4
25 mol% H SO
1
2
3
4
[bmin] [SbF6]
>99 (60 s)
>99 (60 s)
>99 (40 s)
>99 (60 s)
>99 (100 s)
>99 (60 s)
>99 (60 s)
>99 (120 s)
25 (3 h)
<5 (3 h)
76 (3 h)
30 (3 h)
[bmin] [BF
[bmin] [PF
4
]
]
6
[bmin] [OTF]
a
Isolated yield.
Table 2. Microwave-assisted Beckmann rearrangement of
ketoximesin [bmin] [PF
6 2 4
] using 5 mol% H SO .
0
a
Entry
R
R
Time (s)
Yield (%)
1
2
3
4
5
6
7
4-Cl-C
4-Me-C
4-MeO-C H
6
H
4
C
C
2
H
H
5
5
5
5
5
5
60
40
81
91
80
99
99
99
30
6 4
H
2
C H
2
60
60
6
4
C
6
H
5
C
C
6
H
H
4-MeO-C
C H
6
H
4
6
60
60
C H
2
2
5
a-Tetralone oxime
Isolated yield.
120
a

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2
306
Lee et al.
of ionic liquids as solvents for microwave-assisted organic reactions are
under investigation.
EXPERIMENTAL
Synthesis of [bmim] [X ]
[
10]
All ionic liquidswere prepared from [bmim] [Cl].
Thus, to a solu-
tion of [bmim] [C1] (1 equiv.) in anhydrousacetone at room temperature
was added 1 equiv. of sodium hexafluorophosphate for [bmim] [PF6],
sodium tetrafluoroborate for [bmim] [BF ] or sodium hexafluoroantimo-
4
ꢀ
nate for [bmim] [SbF ]. The reaction mixture was st irred for 24 h at 20
6
C
and wasthen filtered through a plug of Celite. The volatile wasremoved
in vacuo, and the residue was dissolved in methylene chloride and washed
with water several times. After evaporation of the methylene chloride,
ionic liquid wasvacuum dried for 2 daysand st ored under an atmo sp here
of dry argon. All ionic liquidscontained lesthan 10 ppm of chlorine in
chlorine analysis at Analysis Center in Korea Institute of Science and
Technology and less than 15 ppm of water in Karl Fischer analysis
(
Metrolm Model: 756KF Coulometer).
Thermal Behavior of Ionic Liquid
ꢀ
Thermographic analyses, (TGA) with 10 C/min heating rate have
been carried out using TGA 2050 model from TA instrument Inc.
under N atmosphere. For thermal behavior of ionic liquids under micro-
2
wave irradiation, an ionic liquid (1 mL) in a Pyrex vessel (10 mL) was
irradiated in the absence of insulation during given period of time and the
temperature was measured using digital thermometer equipped with type
K-2 thermocouple.
General Procedure for Beckmann Rearrangement
A solution of a ketoxime (1 mmol) and catalytic amount of H SO4
2
(
0.5 mmol) in an ionic liquid (1 mL) wasplaced in a flat bottom fla sk and
mixed thoroughly on a vortex mixer (Scientific Inc., Model G-560).
The mixture wasirradiated intermittently in an unmodified dome ts ic
microwave oven for 60 s(20 þ 20 þ 20 s), and loaded on silica column
chromatography directly to isolate product.

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Beckmann Rearrangement of Ketoximes
ACKNOWLEDGMENT
2307
This work was supported from MOST (National Research
Laboratory Program), KIST, and CMDS at KAIST.
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1. Recent selected reviews for MW-assisted reactions. See (a)
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¨
Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225.
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Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772.
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6. Gawley, R.E. Org. React. 1988, 35, 1.
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1999, 2133.
Received in the NetherlandsOctober 25, 2002

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