J.R. Bernardo et al.
MolecularCatalysis465(2019)87–94
the synthesis of LA is still rare in comparison to homogeneous catalysts
mesitylene as internal standard.
Due to the high importance of the conversion of carbohydrates into
biofuels and value added chemicals, the research for efficient catalysts
is a key target in organic chemistry and also in the chemical industry.
In the last years, oxo-rhenium complexes have been applied in the
deoxygenation of many organic compounds [28] such as sulfoxides
[29–35], epoxides [36–42], aromatic nitro compounds [43], N-oxides
[44], alcohols [45–48] and carbonyl compounds [49,50]. The synthesis
of EL, EMF, and LA has never been studied using oxo-rhenium com-
plexes as catalysts.
2.4. General procedure for the conversion of carbohydrates into HMF
To a Schlenk flask equipped with a J. Young tap containing a so-
lution of carbohydrate (0.180 g, 1.0 mmol of hexose) in DMSO (5 mL)
was added HReO4 (0.025 g, 10 mol%). The reaction mixture was stirred
in a closed Schlenk at 140 °C during 1 h. The yields were determined by
spectroscopy 1H NMR using mesitylene as internal standard.
In this work, we report the first methodology for the direct and
selective conversion of carbohydrates into potential fuels and value
added chemicals such as EL, EMF, HMF and LA catalyzed by oxo-rhe-
nium complexes.
2.5. General procedure for the conversion of carbohydrates into LA
To a Schlenk flask equipped with a J. Young tap containing a so-
lution of carbohydrate (0.180 g, 1.0 mmol of hexose) in 1,4-dioxane
(5 mL) was added HReO4 (0.025 g, 10 mol%). The reaction mixture was
stirred in a closed Schlenk at 140 °C during 1 h. The yields were de-
termined by spectroscopy 1H NMR using mesitylene as internal stan-
dard.
2. Methods
2.1. General information
All the reactions were carried out under air atmosphere, without
using any dry solvent. Carbohydrates and HReO4 (75–80 wt.% in H2O),
MTO, NaReO4 were obtained from commercial suppliers and were used
without further purification. ReIO2(PPh3)2 [51], ReOCl3(PPh3)2 [52]
and [ReOCl3(SMe2)(OPPh3)] [53] were prepared according to literature
procedures. Flash chromatography was performed on MN Kieselgel
60 M 230–400 mesh. 1H NMR and 13C NMR spectra were measured on a
Bruker Avance II+ 400 MHz and 300 MHz spectrometers. Chemical
shifts are reported in parts per million (ppm) downfield from an in-
ternal standard.
3. Results and discussion
3.1. Synthesis of alkyl levulinates
Initially, we optimized the conversion of carbohydrates into ethyl
levulinate using different reaction conditions.
3.1.1. Effect of the catalyst
The conversion of fructose was studied using 10 mol% of different
oxo-rhenium complexes as catalysts (Fig. 1). The best result was ob-
tained in the presence of the Re(VII) complex HReO4 in ethanol at
160 °C, producing selectively EL with 80% yield after 16 h. The reac-
tions catalyzed by [ReOCl3(OPPh3)(SMe2)] and MTO also gave selec-
tively EL with 53% and 16% yields, respectively. In the presence of
ReOCl3(PPh3)2, EL was afforded in 50% yield along with 12% yield of
EMF. A mixture of EL (44%), EMF (5%) and HMF (8%) was obtained in
the reaction catalyzed by the oxo-complex ReIO2(PPh3)2. Among the Re
(V) complexes, the electronic properties of the halide ligands Cl and I
can influence the activity of the catalysts, leading to a slight higher
activity for the catalyst containing the Cl ligand. Finally, the weak
chelate character of the ligands SMe2 and OPPh3 also enhances the
catalytic activity of [ReOCl3(SMe2)(OPPh3)]. The conversion of fructose
using NaReO4 as catalyst afforded only 11% of EL and 22% of EMF,
confirming the importance of the presence of the Brønsted acid. Finally,
no reaction was observed in the absence of catalyst.
2.2. Synthesis of EL
2.2.1. General procedure for the conversion of carbohydrates into EL
To a Schlenk flask equipped with a J. Young tap containing a so-
lution of carbohydrate (0.180 g, 1.0 mmol of hexose) in ethanol (5 mL)
was added HReO4 (0.025 g, 10 mol%). The reaction mixture was stirred
in a closed Schlenk at 160 °C during 16 h. The yields of the products
were determined by spectroscopy 1H NMR using mesitylene as internal
standard.
2.2.2. Use of catalyst HReO4 in several cycles
To a Schlenk flask equipped with a J. Young tap containing a so-
lution of fructose (0.09 g, 0.5 mmol) in ethanol (5 mL) was added
HReO4 (0.0126 g, 10 mol%). The reaction mixture was stirred in a
closed Schlenk at 160 °C during 16 h. The reaction mixture was cooled
and the yield was determined by 1H NMR spectroscopy using mesyti-
lene (0.5 mmol) as internal standard. In the next catalytic cycles,
fructose (0.5 mmol) and mesytilene (0.5 mmol) were added to the re-
action mixture and stirred for 16 h at 160 °C. The reaction mixture was
cooled and the yields were determined by 1H NMR spectroscopy.
2.2.3. Synthesis of EL in gram scale
To a Schlenk flask equipped with a J. Young tap containing a so-
lution of fructose (1.80 g, 10 mmol) in ethanol (50 mL) was added
HReO4 (0.25 g, 10 mol%). The reaction mixture was stirred in a closed
Schlenk at 160 °C during 16 h. The yields were determined by spec-
troscopy 1H NMR using mesitylene as internal standard.
2.3. General procedure for the conversion of carbohydrates into EMF
To a Schlenk flask equipped with a J. Young tap containing a so-
lution of carbohydrate (0.180 g, 1.0 mmol of hexose) in a mixture
ethanol/THF (7/3) (Vtotal = 5 mL) was added HReO4 (0.025 g, 10 mol
%). The reaction mixture was stirred in a closed Schlenk at 140 °C
during 1 h. The yields were determined by spectroscopy 1H NMR using
Fig. 1. Reaction of fructose catalyzed by different oxo-rhenium complexes: 1.
HReO4; 2. [ReOCl3(SMe2)(OPPh3)]; 3. ReOCl3(PPh3)2; 4. ReIO2(PPh3)2; 5.
MTO; 6. NaReO4. Reaction conditions: Fructose (1 mmol), catalyst (10 mol%),
EtOH (5 mL), 160 °C, 16 h. Conversions and yields determined by 1H NMR
spectroscopy.
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