Rapid conversion of sorbitol to isosorbide in hydrophobic ionic liquids under microwave irradiation

Article abstract of DOI:10.1002/cssc.201402655

Sorbitol was effectively converted to isosorbide by treatment with [TMPA][NTf₂] in the presence of catalytic amounts of TsOH under microwave heating at 180°C. The reaction completed within 10 min and isosorbide was isolated to about 60%. Ionic liquids were readily recovered by an extraction treatment and reused several times.

Full text of DOI:10.1002/cssc.201402655

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DOI: 10.1002/cssc.201402655
Rapid Conversion of Sorbitol to Isosorbide in Hydrophobic
Ionic Liquids under Microwave Irradiation
Akio Kamimura,*[a] Kengo Murata,[b] Yoshiki Tanaka,[a] Tomoki Okagawa,[a]
Hiroshi Matsumoto,[a, c] Kouji Kaiso,[a, c] and Makoto Yoshimoto[a]
Sorbitol was effectively converted to isosorbide by treatment
with [TMPA][NTf2] in the presence of catalytic amounts of TsOH
under microwave heating at 1808C. The reaction completed
within 10 min and isosorbide was isolated to about 60%. Ionic
liquids were readily recovered by an extraction treatment and
reused several times.
achieved the efficient depolymerization of polyesters and cellu-
lose by treating polymers in ionic liquids at high tempera-
tures.[8] Moreover, ionic liquids have an advantage in heating
when microwaves are used.[9]
We expected that the high polarity and low solvation to the
hydroxyl group of ionic liquids may accelerate the reaction
rate of the conversion of sorbitol into isosorbide, by enhancing
the nucleophilicity and reactivity of the hydroxyl group in sor-
bitol. The use of hydrophobic ionic liquids affords ready sepa-
ration of ionic liquid and hydrophilic product by simple extrac-
tion methods. In addition, the ionic liquids are readily recover-
able in pure form, allowing their repeated use. Herein, we
show rapid conversion of sorbitol to isosorbide within 10 min
of heating with microwaves in the presence of catalytic
amounts of acid.[10]
The recent developments in the efficiency of the conversion of
biomass into chemical materials have been of interest in sus-
tainable chemistry.[1] Cellulose is a biomass source produced
on a massive scale every year, and its conversion to glucose
and its derivatives has been actively explored since the discov-
ery that some ionic liquids dissolve cellulose.[2] Sorbitol is one
of the derivatives of glucose,[3] and its conversion to isosorbide,
a material useful for polymer synthesis, is important for the
preparation of chemical feedstocks from biomass.[4] The con-
version has been extensively developed and several good
methodologies have been reported.[5] For example, Shirai re-
ported that efficient conversion can be achieved in subcritical
water (a benign solvent); and they also successfully performed
a kinetic analysis of the reaction.[6] Although all methodologies
reported so far are simple and useful, most of them require
high temperatures and long heating times. Further, equipment
that can handle high pressures is occasionally necessary for
these reactions. However, the ideal conditions for the reaction
would involve short heating times at low temperatures under
normal pressure and the use of ordinary glassware.
Sorbitol 1 was mixed with N,N,N-trimethyl-N-propylammoni-
um bis(trifluoromethylsulfonyl)amide ([TMPA][NTf2]) and the re-
sulting mixture was heated under microwave irradiation condi-
tions (Scheme 1). The results are summarized in Table 1. When
a mixture of sorbitol 1 in [TMPA][NTf2] was heated by micro-
Scheme 1. Conversion of sorbitol to isosorbide in ionic liquids.
Ionic liquids are regarded as next-generation solvents and
possess unique properties such as high polarity and high solu-
bility.[7] Low volatility at high temperatures is another impor-
tant property, and allows to perform reactions at such temper-
atures without fear of vaporization. Recently, we successfully
wave irradiation in the presence of 5 mol% of TsOH, sorbitol
disappeared within 10 min and two products were detected.
One was isosorbide 2, with a yield estimated to be 16% by
HPLC analysis using an XBridgeAmide reversed-phase column.
The other products were monodehydrated compounds, re-
ferred to globally as 3; 13C NMR spectroscopy revealed that
1,5-anhydrosorbitol was the main product. To our surprise, 1,4-
anhydrosorbitol, which is an expected intermediate to yield
isosorbide, was not detected. The yield of 3 was estimated to
be 25% (entry 1). To improve the yield of 2, we prolonged the
reaction time to 30 min, but the yield of 2 only slightly in-
creased, to 34% (entry 2). The use of large amounts of TsOH
did not change the yield of 2 (entry 3). A reaction performed
at 1508C progressed much more smoothly and the desired 2
was obtained in 39% yield (entry 4). A prolonged reaction time
was not effective, and the yield of 2 decreased to 29%
(entry 5).
[a] Dr. A. Kamimura, Y. Tanaka, T. Okagawa, H. Matsumoto, Dr. K. Kaiso,
Dr. M. Yoshimoto
Department of Applied Molecular Bioscience
Graduate School of Medicine, Yamaguchi University
Ube 755-8611 (Japan)
E-mail: ak10@yamaguchi-u.ac.jp
[b] K. Murata
Department of Applied Chemistry
Faculty of Engineering, Yamaguchi University
Ube 755-8611 (Japan)
[c] H. Matsumoto, Dr. K. Kaiso
Ube Laboratory, Ube Industries Ltd.
Ube 755-8633 (Japan)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402655.
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ered [TMPA][NTf2] was useful for
the reaction, and isosorbide 2
was obtained in about 50–60%
yield. Recovery of the ionic liq-
uids was always about 90% and
was effectively performed by
a simple extraction procedure.
Thus, recycling and re-use of
[TMPA][NTf2] was successfully
performed at least five times.
Table 1. Conversion of sorbitol to isosorbide 2 in [TMPA][NTf2].
Entry
Temp.
[8C]
Time
[min]
TsOH
[mol%]
Yield of 2[a]
[%]
Yield of 3[a]
[%]
Recovery of 1[a]
[%]
Amount of [TMPA][Ntf2][b]
recovered [%]
1
2
3
4
5
6
7
8
130
130
130
150
150
180
180
180
180
180
200
200
10
30
30
10
30
10
30
10
10
30
10
10
5
5
7
6
6
5
5
1
3
1
5
0
16
34
31
39
29
61
49
42
39
38
60
0
25
40
49
29
14
5
0
0
0
0
0
0
0
0
0
0
0
78
93
94
96
85
91
96
83
91
97
91
95
94
0
19
20
4
0
6
The present dramatic accelera-
tion of the reaction is assumed
to be attributable to the high
polarity and nonsolvating effect
of ionic liquids. Thus, the reac-
tion progresses by the OH group
9
10
11
12
[a] HPLC yield. [b] Isolated yield.
Table 2. Iterative use of ionic liquids.
Run
Yield of 2 [%]
Recovery of IL [%]
1
2
3
4
5
60
56
55
57
48
91
87
92
90
88
Scheme 2. Examination of various hydrophobic ionic liquids.
at C4 attacking the C1 carbon to yield 1,4-anhydrosorbitol,
which undergoes a second intramolecular SN2 reaction be-
tween the OH groups at the C3 and C6 carbons. Under the ex-
perimental conditions, the nucleophilic OH group at C4 has no
external solvation because no polar protic solvent such as
water is used; its nucleophilicity is therefore enhanced. Catalyt-
ic amounts of TsOH then protonate the OH group at C1 for ac-
tivating the intramolecular SN2 reaction. The high polarity of
ionic liquids also activates such a polar reaction. We postulate
that the acceleration of the reaction is due to these properties
of the ionic liquids (Scheme 3).
When the reaction was performed at 1808C for 10 min in
the presence of 5 mol% of TsOH, the yield of 2 improved to
61%, and the yield of 3 decreased to 5% (Table 1, entry 6). A
prolonged reaction time was not effective because reaction
products 2 and 3 decompose under these conditions (entry 7).
The use of smaller amounts of TsOH lowered the reaction rate
and the yield of 2 remained at moderate levels (entries 8–10).
Heating at 2008C was also effective for the reaction (entry 11).
The reaction in the absence of TsOH failed to achieve conver-
sion and 78% of the starting material 1 was recovered
(entry 12). We examined the reaction catalyzed by 5 mol% of
TsOH with conventional heating at 1808C for 10 min, but the
yield of isosorbide 2 was only 8% along with the for-
mation of 1,5-anhydrosorbitol 3 in 22% yield. Thus,
microwave irradiation offers very effective heating
for the present reaction.
In conclusion, we develop a new effective method to con-
vert sorbitol to isosorbide in hydrophobic ionic liquids. The re-
action progresses efficiently at a relatively low temperature
We next examined the reaction using other hydro-
phobic ionic liquids (Scheme 2). Other ionic liquids
that have [NTf2] as counter anion allowed the reac-
tion to progress smoothly, and the desired isosor-
bide 2 was produced in about 60% yield. All ionic
liquids were readily recovered (more than 90%) by
simple extraction methods.
We next examined the iterative use of [TMPA]-
[NTf2]. The results are summarized in Table 2. Recov-
Scheme 3. Plausible reaction mechanism.
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[1] J. J. Bozell, G. R. Petersen, Green Chem. 2010, 12, 539–554.
(1808C) within a very short period (10 min). The yield of isosor-
bide is about 60%, which is similar to yields achieved by meth-
ods reported earlier. The use of hydrophobic ionic liquids such
as [TMPA][NTf2] affords ready separation of ionic liquid and the
product isosorbide by simple extraction procedures. The recov-
ery of ionic liquids is effective; we realized five iterative uses.
The present method contributes to the development of new,
greener methods for biomass conversion. Investigations on fur-
ther details of the reaction are currently underway in our labo-
ratory.
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Experimental Section
Typical procedure for the conversion of sorbitol to isosorbide:
(Table 1, entry 6) A mixture of sorbitol 1 (202.1 mg, 1.11 mmol) and
TsOH·H2O (11.1 mg, 0.058 mmol) in [TMPA][NTf2] (2.673 g) was
stirred and heated to 1808C by microwave irradiation for 10 min.
CH2Cl2 (100 mL) was added to the mixture and the resulting solu-
tion was filtered. The residue was dissolved in MeOH (10 mL) and
the MeOH solution was combined with the filtrate. The resulting
organic solution was concentrated in vacuo. The filtrate was con-
centrated and water (30 mL) was added. The water solution was
extracted with CH2Cl2 (10ꢁ30 mL). The organic phase was dried
over Na2SO4. Filtration and concentration gave [TMPA][NTf2] in
96% recovery (2.5647 g). The water phase was concentrated in
vacuo. Fructose (201.3 mg) was added to the residue as the inter-
nal standard and the mixture was dissolved in water (10 mL). This
solution was analyzed by HPLC (Waters X-bridgeAmide column,
3.5 mm, 4.6 mm idꢁ100 mm, CH3CN/water 75:25, 3.0 mLminÀ1, de-
tected by JASCO RI2031plus RI detector), which estimated the
yield of isosorbide 2 as 61% (98.6 mg) and that of 1,5-anhydrosor-
1
bitol as 5% (9.3 mg). H NMR (500 MHz, D2O) d=4.51 (t, J=4.6 Hz,
1H), 4.36 (d, J=4.2 Hz, 1H), 4.27 (td, J=6.9, 5.0 Hz, 1H), 4.20 (d,
J=2.8 Hz, 1H), 3.84–3.77 (m, 2H), 3.74 (dd, J=10.5, 3.2 Hz, 3H),
3.36 ppm (dd, J=9.0, 7.5 Hz, 1H); 13C NMR (126 MHz, D2O) d=87.3,
81.4, 75.5, 75.1, 71.8, 71.1 ppm.
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Acknowledgements
This work was partially supported by Grant-in-Aid for Scientific
Research (A) (24241023).
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2012; b) A. Kamimura, K. Kaiso, H. Matsumoto, JP 2014101327, 2014.
Received: July 10, 2014
Revised: August 10, 2014
Published online on && &&, 0000
Keywords: biomass · ionic liquids · isosorbide · microwave
chemistry · sorbitol
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A. Kamimura,* K. Murata, Y. Tanaka,
T. Okagawa, H. Matsumoto, K. Kaiso,
M. Yoshimoto
&& – &&
Rapid Conversion of Sorbitol to
Isosorbide in Hydrophobic Ionic
Liquids under Microwave Irradiation
As easy as cooking instant food! Only
10 min of microwave irradiation effi-
ciently converts sorbitol in hydrophobic
ionic liquids to isosorbide. A catalytic
amount of acid accelerates the reaction.
Separation of ionic liquids and isosor-
bide is easily achieved by an extraction
method and recovered ionic liquids can
be used in several iterations of the reac-
tion.
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