90
A. S. Amarasekara, A. Razzaq / Carbohydrate Research 386 (2014) 86–91
study further confirmed the mechanism proposed in Figure 6. The
C-2 carbons in the anomeric forms of -glucose, -glucofuranose
and open chain -fructose (8) can be identified in the spectrum
shown in Figure 5 by comparison with literature values.27 The
13C peaks at 75.25 and 72.80 ppm can be assigned to
and b- -glu-
cose C-2 carbons, respectively; while peaks at 80.35 and 72.21 ppm
can be assigned to and b- -glucofuranose C-2 carbons, respec-
tively. The low field peak at 205.54 ppm was assigned to the C-2
carbon in the open chain
-fructose (8).27 The remaining two low
catalyzed decomposition of D-glucose were recorded using a Carey
D
D
50 UV–vis spectrophotometer and 1 cm quartz cells. Reaction
tubes were heated in a mineral oil bath placed on a VWR VMS-C7
hot-plate attached to a VWR VT-5 temperature controller with
0.1 °C accuracy.
D
a
D
a
D
4.2. 1-(1-Propylsulfonic)-3-methylimidazolium chloride
catalyzed decomposition of
study
D
-glucose in DMSO-d6—1H NMR
D
field peaks at 136.30 and 152.16 were assigned to the C-2
carbons of the intermediate 10 and the HMF (12) product. The
A solution of D-glucose (45.0 mg, 0.25 mmol) and 1-(1-propyl-
2-13
C
D-glucose study also supports the possibility of two isomeric
sulfonic)-3-methylimidazolium chloride (3.3 mg, 0.0137 mmol,
and 5.48 mol % catalyst) in 0.60 mL of anhydrous DMSO-d6 was
prepared in a 5-mm NMR tube. The sample was allowed to stabi-
lize at room temperature for 4 h, then, 1H NMR (rd = 1 s, NS = 8)
spectrum was recorded as the initial baseline data. Afterward the
NMR tube was heated in a thermostated oil bath at 150 0.1 °C
for 6.0 min, and then the reaction was quenched by immersing
the NMR tube in an ice-water bath. The tube was immediately
transferred to the NMR spectrometer, 1H NMR spectrum was
recorded using conditions identical to the first t = 0 spectrum. Fur-
ther spectra were recorded after heating 18.0, 30.0, 48.0, 90.0,
150.0, 210.0, 290, 410, 530, and 710.0 min of total heating time
structures for imidazolium cation complexed open chain form 7,
where peaks at 189.88 and 192.40 are assigned to C-2 carbons of
isomeric forms of 7.
To further support the mechanism, we have studied the decom-
position of
D-fructose in 1-(1-propylsulfonic)-3-methylimidazo-
lium chloride in DMSO-d6 at 150 °C, under similar catalyst
loading described in the experiment 4.2. In this experiment all
D-fructose disappeared without a trace after 1.0 min at 150 °C,
and could not observe the (4R,5R)-4-hydroxy-5-hydroxymethyl-
4,5-dihydrofuran-2-carbaldehyde (11) intermediate seen in earlier
experiments.5 Additionally, HMF was the only identifiable product
formed in 90% yield. Therefore, we believe that life-time of fructose
and this intermediate is very short in strongly acidic medium at
150 °C used here, and could be the reason that a standing concen-
at 150 0.1 °C.
D-Glucose a/b anomer compositions and the
amount of HMF produced at different time intervals were mea-
sured by manual integration of the 1H NMR peaks. The imidazole
ring C4, C5 proton NMR peak areas of 1-(1-propylsulfonic)-3-
methylimidazolium chloride were used as an internal standard
tration of D-fructose is not observed in the present experiment.
for the calculation of HMF yields. The variations in the
a/b D-glu-
3. Conclusion
cose anomer ratios were monitored up to 48.0 min of the reaction
and these data are shown in Table 1. The changes in the 1H NMR
spectra during the course of the reaction are shown as a selected
sample of spectra in Figure 1. The calculated percent yields of
HMF produced after different times of heating are shown in the
plot in Figure 2.
We have proposed a mechanism for the Brönsted acidic ionic
liquid 1-(1-propylsulfonic)-3-methylimidazolium chloride cata-
lyzed transformation of
DMSO. The proposed mechanism involves the isomerization of
-glucose to -fructose via the complexation of the open chain
D-glucose to 5-hydroxymethylfurfural in
D
D
sugar with the imidazolium cation of the acid catalyst. Key inter-
mediates in the proposed pathway could be identified by studying
the changes in the 13C NMR spectra of acidic ionic liquid catalyzed
4.3. 1-(1-Propylsulfonic)-3-methylimidazolium chloride
catalyzed decomposition of
spectroscopy study
D-glucose in DMSO—visible
transformations of C-1 and C-2 13C labeled
D-glucose under compa-
rable conditions. HMF yield rapidly increases in the first 100 min of
reaction at 150 °C, but then the rate of increase in the yield de-
creases beyond 100 min and levels off to a maximum yield of about
15.7% around 600 min. The visible spectroscopy study of the reac-
tion mixture suggests that rate of HMF formation slows down after
100 min due to increase in the rate of humin formation after this
period.
A solution of D-glucose (450.0 mg, 2.50 mmol) and 1-(1-propyl-
sulfonic)-3-methylimidazolium chloride (33.0 mg, 0.137 mmol,
5.48 mol % catalyst) in 6.00 mL of anhydrous DMSO was prepared
in a round bottom flask. The sample was allowed to stabilize at
room temperature for 4 h, then, 100 lL of the solution was with-
drawn and diluted with 3.00 mL of DMSO and visible spectrum
was recorded in the 400–800 nm range against a pure DMSO blank.
Afterward the reaction mixture was heated in a thermostated oil
4. Experimental
bath at 150 0.1 °C for 5.0 min, and then 100 lL of the solution
was withdrawn and diluted with 3.00 mL of DMSO and visible
spectrum was recorded using conditions identical to the first
t = 0 spectrum. Further visible spectra were recorded after heating
20.0, 40.0, 60.0, 80.0, 100.0, 130.0, 160, and 190.0 min of total heat-
ing time at 150 0.1 °C. A stack plot of visible spectra showing the
progress of the reaction is shown in Figure 3.
4.1. Instrumentation and materials
D
-Glucose (99.9%), 1-13C- -glucose (99% atom 1-13C), 2-13C-
D D-
glucose (99% atom 2-13C), anhydrous DMSO-d6 (99.9% atom D)
were purchased from Aldrich Chemical Co. Brönsted acidic ionic
liquid catalyst 1-(1-propylsulfonic)-3-methylimidazolium chloride
was prepared by condensation of 1-methylimidazole with 1,3-pro-
panesultone and acidification of the resulting salt with concd HCl
according to the literature procedure.35,36 1H NMR spectra were re-
corded in DMSO-d6 on a Varian Mercury Plus spectrometer operat-
ing at 400 MHz. Chemical shifts are given in ppm downfield from
TMS (d = 0.00). 13C NMR spectra in DMSO-d6 were recorded on
the same spectrometer operating at 100 MHz. Chemical shifts are
reported relative to DMSO-d6 and converted to d (TMS) using d
(DMSO) = 39.51. Visible spectra of the reaction mixtures produced
during the 1-(1-propylsulfonic)-3-methylimidazolium chloride
4.4. 1-(1-Propylsulfonic)-3-methylimidazolium chloride
D C
catalyzed decomposition of 1-13C- -glucose in DMSO-d6—13
NMR study
A solution of 1-13C-
D-glucose (45.0 mg, 0.25 mmol) and 1-(1-
propylsulfonic)-3-methylimidazolium chloride (3.3 mg, 0.0137
mmol, 5.48 mol % catalyst) in 0.60 mL of anhydrous DMSO-d6
was prepared in a 5-mm NMR tube. The sample was allowed to
stabilize at room temperature for 4 h, then, 13C NMR (rd = 2 s,
NS = 128) spectrum was recorded as the initial baseline data.