Tetrahedron Letters 53 (2012) 3149–3155

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Tetrahedron Letters

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Facile catalytic dehydration of fructose to 5-hydroxymethylfurfuralby Niobium pentachloride

Neha Mittal �, Grace M. Nisola �, Wook-Jin Chung ⇑Energy and Environment Fusion Technology Center, Department of Energy Science and Technology, Myongji University, San 38-2, Nam-dong, Cheoin-gu, Yongin-City, Gyeonggi-Do449-728, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 January 2012Revised 6 April 2012Accepted 11 April 2012Available online 17 April 2012

Keywords:5-HydroxymethylfurfuralFructoseNiobium pentachloride1-Butyl-3-methylimidozolium chloride

0040-4039/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.tetlet.2012.04.045

⇑ Corresponding author. Tel.: +82 31 336 8471; faxE-mail address: wjc0828@gmail.com (W.-J. Chung

� These authors contributed equally to this work.

Different metal chlorides in ionic liquids were examined as catalysts for fructose dehydration to 5-hydroxymethylfurfural (5-HMF). Niobium pentachloride (NbCl5) exhibited the highest 5-HMF yield.Reaction of 1.0 mmol fructose in 10.0 mmol 1-butyl-3-methylimidozolium chloride ([bmim]Cl) with0.20 mmol NbCl5 afforded 79% of 5-HMF at 80 �C after 30 min. No degradation products were formedunder these conditions. FTIR analysis indicates the moderate Lewis acidity of NbCl5, which was foundsuitable for the selective formation of 5-HMF. Other catalysts with higher Lewis acidities promoted 5-HMF side product formation which ultimately resulted in lower yields.

� 2012 Elsevier Ltd. All rights reserved.

Introduction

With a bi-functional moiety, 5-hydroxymethylfurfural (5-HMF)is a recognized novel base material for the synthesis of various di-substituted furanic compounds and an important precursor oftransportation fuels, pharmaceuticals, and other petroleum-de-rived chemicals.1–5 The synthesis of 5-HMF involves acid-catalyzedtriple dehydration of hexoses (Fig. 1),6 with initial works being per-formed in aqueous systems.7 But the inevitable conversion of 5-HMF to levulinic and formic acids as degradation products in thepresence of H2O decreases the selectivity of the reaction.8 Severalstrategies have been developed to minimize the decompositionproduct formations wherein careful selection of catalysts andsolvent systems as well as identification of optimal operatingconditions have been proven to be equally important.9

Organic and mineral acids were conventionally employed ascatalysts in which 5-HMF yields of 40–80% have beenachieved.10–13 While yields were satisfactory, product separationwas difficult to perform. To resolve this drawback, solid acids werealternatively used as heterogeneous catalysts. But homogenouscatalyst reactions (i.e., mineral acids) remained superior over het-erogeneous systems in terms of 5-HMF yield.8,14 Both types of cat-alysts (i.e., liquid and solid acids) are reacted at high reactiontemperatures to promote a faster 5-HMF production and to avoidthe side products which are eventually generated at prolonged

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reaction time.9 But the high acidities of mineral acids also createproblems as they promote 5-HMF decomposition.15 Alternatively,mild Lewis acid catalysts could minimize the problem and there-fore improve the reaction selectivity for 5-HMF.16 Particularly, me-tal chlorides are environmentally benign Lewis acid catalystswhich can be used instead of mineral acids. High conversions ofmono- and polysaccharides to 5-HMF using metal chlorides havebeen reported.16–19 So far, certain metal chlorides have success-fully demonstrated high 5-HMF yield but the potential of othershas not yet been fully elucidated.

For solvent selection, 5-HMF production from hexoses is typi-cally performed under sub- or supercritical conditions using polaraprotic solvents.12,20–24 However, their high water solubility, highboiling points, and necessity of high reaction temperatures couldinterfere with reaction selectivity, promote side product formation,and create difficulty in 5-HMF recovery.20,22,25 Alternatively, thehigh solvency power of ionic liquids (ILs) indicates their suitabilityas solvents for metal chlorides even at low reaction temperatures.ILs have high thermal and chemical stabilities while their negligi-ble vapor pressure makes them environment-friendly as they donot evolve toxic gases.26,27 Moreover, 5-HMF recovery and catalyst

Figure 1. Dehydration of D-fructose to 5-HMF.

3150 N. Mittal et al. / Tetrahedron Letters 53 (2012) 3149–3155

recyclability could be easier in IL system.28 Fructose dehydrationhas been performed in ILs with excellent yields and selectivity.29

Particularly, imidazolium based ILs like 1-butyl-3-methylimidazo-lium chloride ([bmim]Cl) were found to effectively dehydrate fruc-tose to 5-HMF with satisfactory yield of 63% after 50 min reactionat 120 �C.30

Several studies have already investigated certain IL/metal chlo-ride systems for this application but the basis for their selection re-mains vaguely explored.31 Thus in this work, several metalchlorides were screened for their catalytic abilities to dehydratefructose to 5-HMF. The appropriate solvent was selected fromtwo imidazolium-based ILs namely [bmim]Cl and 1-ethyl-3-meth-ylimidazolium chloride ([emim]Cl). To elucidate the reaction effi-ciency of each IL/metal chloride pair, Lewis acidities of metalchlorides in IL were examined using Fourier transform infraredspectroscopy (FTIR).32 The effects of different reaction parameterswere tested to determine the most efficient and mildest conditionfor 5-HMF production.

Results and discussion

Fructose dehydration by various metal chlorides in [bmim]Cl

Seven metal chlorides in [bmim]Cl were initially screened forthe production of 5-HMF at 80 �C16 as illustrated in Figure 2.Among the tested catalysts, five metal chlorides exhibited activi-ties for fructose dehydration. The yields of 5-HMF can be arrangedin the following order: NbCl5 > RuCl3 > CuCl2�2H2O > FeCl3 > CrCl3.Highest 5-HMF yield of 79% was attained from NbCl5 within theshortest reaction time of 30 min. No 5-HMF was produced in[bmim]Cl with LiCl and SnCl2�2H2O even after 70 min reaction.

As an acid catalyzed reaction, the relationship between theacidity of the reaction systems and their catalytic activities for 5-HMF production was examined. Earlier works used a weak Lewisbase, acetonitrile (ACN), as a probe molecule to observe the Lewisacidities in various IL-metal chloride pairs via FTIR analysis.32 It isknown that the interaction between the lone pair of nitrogen inACN and the Lewis acid sites of the metal chlorides in [bmim]Clwould affect the vibrational bands of C„N stretching.33 Thenitrogen orbitals would undergo re-hybridization, enhancing the

Figure 2. 5-HMF yield of various metal chlorides using 1.0 mmol fructose,0.2 mmol catalyst, and 10.0 mmol [bmim]Cl at 80 �C.

r bond component of C„N. In FTIR, this occurrence is indicatedby the blue shift in C„N band or formation of new peak at higherwave numbers.33 It was pointed out previously that the higher theblue shift, the higher the Lewis acidity strength of the metal chlo-ride.32 Other works also indicated that the higher peak intensitiesof the blue-shifted bands indicate a higher number of Lewis acidicsites.33,34 Moreover, the dipole-dipole interaction between theC„N group in ACN with a Lewis acid could result to charge densityrepulsion of the r bond component of the methyl (CH3) group inACN, resulting to C–H bond weakening. In FTIR, this is typicallyindicated by the red shift of CH3 frequency band.34 The changesin IR spectra were monitored between 2500 and 2000 cm�1.

In Figure 3, pure ACN features two characteristic peaks. Thestronger peak at 2252 cm�1 is attributable to the C„N bondstretching while the weaker band at 2292 cm�1 is due to the CH3

bending and C–C stretching.32,35 The spectrum of [bmim]Cl mixedwith ACN was similar except for the slight red shift from 2252 to2247 cm�1. Likewise, no new band formation at a higher wavenumber was observed from SnCl2�2H2O and only slight blue shiftwas observed at 2267 cm�1 for LiCl. The absence or slight blue shiftof C„N stretching indicates that pure [bmim]Cl, [bmim]Cl/LiCl,and [bmim]Cl/SnCl2�2H2O reaction systems have no Lewis acidcharacter. While evidence of 5-HMF production was observed fromCrCl3 (Fig. 2), no new peak was observed in Figure 3. This is due tothe difficulty encountered in dissolving the catalyst in ACN/[bmim]Cl during FTIR analysis.

On the other hand, three peaks were evident from ACN with[bmim]Cl/MClx�nH2O (M = Nb, Ru, Cu, Fe; x = 2, 3, 5; n = 0, 2). SlightCH3 red shifts were observed at 2291 cm�1 for FeCl3, at 2290 cm�1

for CuCl2�2H2O, and at 2289 cm�1 for NbCl5. No red shift was ob-served for the CH3 band in RuCl3 (2293 cm�1). These four metalchlorides also showed the original band of C„N stretching at2251 cm�1. For the same group of metal chlorides (MClx�nH2O,M = Nb, Ru, Cu, Fe; x = 2, 3, 5; n = 0, 2) new bands were formed athigher wave numbers: at 2316 cm�1 for NbCl5 and FeCl3, at2328 cm�1 for RuCl3, and at 2325 cm�1 with an additional peakappearance at 2360 cm�1 for CuCl2�2H2O. The changes in vibrationalfrequencies of CH3 (red shift) and C„N bonds (blue shift) in the fourmetal chlorides are indicative of Lewis acid interaction with ACN.Considering that the same group of metal chlorides exhibited Lewisacidities according to the FTIR results, it can be surmised that thecatalytic fructose dehydration activities of NbCl5, RuCl3, FeCl3, andCuCl2�2H2O in [bmim]Cl could be associated to this property.

According to FTIR data, RuCl3 and CuCl2�2H2O exhibited thewidest blue shifts and therefore, must have the strongest Lewisacidities.32 However, both catalysts showed inferior 5-HMF yieldsthan NbCl5. Earlier works have pointed out the promotion of 5-HMF by product formation by catalysts with strong Lewis acidi-ties.36–38 This is consistent with the remarkable formation ofdark-brown precipitates known as humins in RuCl3 andCuCl2�2H2O in [bmim]Cl systems.9,15 Thus, the lower reactionselectivities in [bmim]Cl/RuCl3 and [bmim]Cl/CuCl2�2H2O than in[bmim]Cl/NbCl5 ultimately resulted to lower 5-HMF yields.9,15,36,37

The FTIR results suggest that NbCl5 has moderate Lewis acidity rel-ative to RuCl3 and CuCl2�2H2O. On the other hand, the blue shift ofC„N 2316 cm�1 has the greatest intensity among all the testedmetal chlorides. This indicates the high number of Lewis acidicsites in NbCl5 and possibly explains its high 5-HMF yield.33,34

Thus herein, NbCl5 is determined as the most suitable Lewisacid catalyst among the tested metal chlorides for the dehydrationof fructose. The moderate Lewis acidity of NbCl5 resulted to a reac-tion with high selectivity toward 5-HMF; the major product was 5-HMF and interestingly, no degradation products (i.e., levulinic acidor formic acid) were detected.8 The dehydration of fructose withNbCl5 in [bmim]Cl was facile and rapid with remarkably high 5-HMF yield of 79% and fructose conversion of P95% at 80 �C.

Figure 4. Effect of reaction time and NbCl5 dosage on 5-HMF yield using 1.0 mmolfructose and 10.0 mmol of (a) [bmim]Cl and (b) [emim]Cl at 80 �C.

Figure 3. FTIR spectra of various metal chlorides in ionic liquid using ACN as IR probe at room temperature (1:5 mass ratio of ACN/sample; 0.5:1 mol ratio of IL/catalyst assample).

N. Mittal et al. / Tetrahedron Letters 53 (2012) 3149–3155 3151

Effect of IL, reaction time, and NbCl5 dosage on the 5-HMF yield

The production of 5-HMF was determined at different reactiontimes and NbCl5 catalyst dosages using ILs [bmim]Cl or [emim]Cl,reacted at 80 �C. The obtained 5-HMF yields are shown in Figure 4,wherein the highest conversions were achieved after 30- and60 min in [bmim]Cl/NbCl5 and [emim]Cl/NbCl5, respectively,regardless of the catalyst dosage.

In Figure 3, the spectrum of NbCl5 in [emim]Cl (with ACN)showed the appearance of the new peak at the same region withthat of NbCl5 in [bmim]Cl. But the peak intensity of NbCl5 in[emim]Cl is lower than that of NbCl5 in [bmim]Cl.34 Thus, the lower5-HMF yield of NbCl5 in [emim]Cl can be associated to its lower Le-wis acidic sites than NbCl5 in [bmim]Cl. Moreover, previous workindicated the slightly lower pKa value of [bmim]Cl than [emim]Cl39

and the acidity differences between ILs could have also influencedthe production of 5-HMF.

The 5-HMF yields declined after achieving the maximum valuesin both IL systems, concomitant with the observed humin forma-tion.9,15 The self-polymerization of 5-HMF or the cross-polymeri-zation between 5-HMF and fructose, are promoted at a longerreaction period, which resulted in the eventual decrease of 5-HMF yields.9,15

At optimum reaction periods, both ILs (i.e., without NbCl5

added), were unable to produce 5-HMF at 80 �C. But addition ofNbCl5 effectively promoted 5-HMF formation, which gradually im-proved when the catalyst dosage was increased to some extent.Particularly, when NbCl5 loading was increased from 0.05 to0.20 mmol (for 1 mmol fructose), the maximum 5-HMF yields in-creased from 67% to 79% in [bmim]Cl (at 30 min) and from 59%to 70% in [emim]Cl (at 60 min). However, 5-HMF yields declinedwith further increase in catalyst dosage (>0.20 mmol). It is possiblethat the high catalyst dosage resulted to high acidity in the systemwhich might have accelerated humin formation.9,15,36–38

From these results, [bmim]Cl exhibited better 5-HMF yield than[emim]Cl. Regardless of the IL used, the optimum NbCl5 dosage of

0.20 mmol for 1 mmol fructose was determined and subsequentlyused for the rest of the experiments, if not otherwise stated.

3152 N. Mittal et al. / Tetrahedron Letters 53 (2012) 3149–3155

Effect of reaction temperature on the 5-HMF yield

The 5-HMF yields at different reaction temperatures from 50 �Cto 180 �C were also investigated using 0.2:1 mole ratio of NbCl5

with respect to fructose, at optimum reaction periods. In Table 1,it is revealed that the reaction temperature is a critical parameterin obtaining satisfactory yields of 5-HMF. For example, low 5-HMFyields were obtained from [bmim]Cl/NbCl5 (24%) at 50 �C and[emim]Cl/NbCl5 (59%) at 55 �C. Increasing the temperature to80 �C showed the maximum 5-HMF yields of 79% and 67% from[bmim]Cl/NbCl5 (30 min) and [emim]Cl (60 min), respectively. Fur-ther increase in temperatures (100 �C) resulted to drastic declinesin 5-HMF yields which may be attributed to the increased inci-dence of humin formation. Nonetheless, no formation of levulinicacid or formic acid as side products was observed even at hightemperatures. Thus, reaction at 80 �C was found optimal for bothIL systems. Compared to previous works reported,30,40 the presentsystem features a facile and more energy-efficient method of pro-ducing 5-HMF from fructose dehydration.

Comparison among efficiency of group V metal chlorides on the5-HMF yield

Using [bmim]Cl as the appropriate solvent, the catalytic poten-tial of other metal chlorides belonging to the same group with Nio-bium was also explored. The 5-HMF yields of other naturallyoccurring group V metal chlorides, vanadium trichloride (VCl3),and tantalum pentachloride (TaCl5), are shown in Figure 5a. Com-pared to other metal chlorides tested (see Fructose dehydration byvarious metal chlorides in [bmim]Cl), group V catalysts showed thehighest 5-HMF yields. But within the group, highest 5-HMF yieldwas still attained by NbCl5. Group V metal chlorides have similarFTIR spectra as shown in Figure 5b. The band at 2316 cm�1 indi-cates their comparable Lewis acidities but the peak intensities de-creased in the order of NbCl5 > TaCl5 > VCl3, which is consistentwith the trend of their catalytic activities. Moreover, minor forma-tions of humins were evident in VCl3 and TaCl5, which could alsoexplain their lower 5-HMF yields than NbCl5.

Proposed fructose dehydration mechanism in [bmim]Cl/NbCl5

Niobium (V) chloride is a renowned Lewis acid catalyst whichhas been used in various organic transformations such as tetrahy-dropyranylation, allylation, Diels–Alder reaction, ring-opening ofepoxides, Mukaiyama aldol reaction, Biginelli reaction, dealkyla-tion of alkyl aryl ethers, and C–H insertion reaction.41 But so far,no study has been reported regarding its use in fructose dehydra-tion for 5-HMF production.

As the first report on [bmim]Cl/NbCl5 system for fructose dehy-dration, tentative mechanisms are proposed in Figure 6. Figure 6Ashows the possible interaction between NbCl5 and [bmim]Cl.42

Niobium is highly electrophilic, oxophilic, and capable of forminghexacoordinates with halides, oxygen, and other nucleophiles.43

In Figure 6B, fructofuranosyl oxocarbenium ion is initially formed

Table 1Effect of reaction temperature on 5-HMF yield using 1.0 mmol fructose, 0.20 mmolNbCl5, and 10.0 mmol ionic liquid

Temperature (oC) 5-HMF yield (%)

[bmim]Cl, 30 min [emim]Cl, 60 min

50 24 59 (55oC)80 79 70

100 60 57150 47 20180 15 1

when NbCl5 attacks the hydroxyl (OH) at the hemiketal carbon.6

From here, the reaction may proceed in two possible directions.First, the ‘nucleophile’ pathway reported by Binder and Raines con-siders the formation of 2-deoxy-2-halo intermediate from thenucleophilic attack on the oxocarbenium ion.44 The IL [bmim]Clis a good source of Cl� anions which could participate as nucleo-philes in the reaction.45 The subsequent deprotonation at C1 leadsto the formation of an enol which could rearrange into the corre-sponding aldehyde.44,45 The other plausible route regards theinteraction of the primary OH with the oxocarbenium ion whichleads to the formation of an epoxide intermediate, followed byits rearrangement via NbCl5-promoted C–O bond cleavage.46 Thesubsequent release of two moles of H2O in both pathways affords5-HMF.

Potential use of [bmim]Cl/NbCl5 for other hexoses

As for the substrate scope of [bmim]Cl/NbCl5 system, glucosedehydration was also performed using the optimum conditionsfor fructose. 5-HMF yield of 42% was obtained after 50 min reac-tion, at 80 �C. This result clearly indicates that the [bmim]Cl/NbCl5

system is worthy of further investigation on its applicability for thedehydration of other sugars aside from fructose.

Conclusion

A new reaction system for fructose dehydration to 5-HMF hasbeen successfully demonstrated. The system, [bmim]Cl/NbCl5, in-volves a rapid and energy-efficient reaction wherein the maximum5-HMF yield was achieved only after 30 min of reaction, at 80 �C.Likewise, the other group V metal chlorides like VCl3 and TaCl5

showed promising yields but the minor formation of humins re-sulted in their slightly lower yields than NbCl5. Reaction tempera-ture, NbCl5 dosage, and reaction time were found critical for theminimization of humin formation and for attaining high yields of5-HMF. From these results, it is considered that NbCl5 in [bmim]Clis a cheap and competent reaction system for 5-HMF production.

Experimental

Materials

Ionic liquids [bmim]Cl (98%) and [emim]Cl (97%), metal chlo-rides CrCl3 (99%, anhydrous), and RuCl3 (35–40% Ru, hydrate) aswell as 5-HMF (98%, reagent grade) were purchased from AcrosOrganics (USA). Acetonitrile (anhydrous, 99.8%), fructose (>99%),NbCl5 (99.9%), TaCl5 (99.9%), and VCl3 (97%) were purchased fromSigma–Aldrich (USA). LiCl (98.2%), and SnCl2.2H2O (>95%) wereprocured from Samchun Chemical Co., Ltd (South Korea), and Dae-jung Chemicals & Metals Co., Ltd (South Korea), respectively. Othermetal chlorides like CuCl2�2H2O (>95%) was supplied by DuksanPure Chemicals Co., Ltd (South Korea) whereas FeCl3 (97%) wasfrom Showa Chemical Co., Ltd (Japan). All chemicals were directlyused without further purification. Ionic liquids were dried at 60–70 �C before reactions in a vacuum oven. The reactions were per-formed in glass vials heated in a temperature-controlled oil bathwith magnetic stir bars.

FTIR characterization

The FTIR analysis was performed using Varian 2000 (ScimitarSeries) FTIR spectrophotometer at room temperature. Prior to anal-ysis, samples were prepared by mixing ACN with IL or IL/metalchloride by 1:5 weight ratio. The prepared IL/metal chloride sam-ples contained 67 mol % of metal chloride according to the method

Figure 5. (a) 5-HMF yield of group V metal chlorides using 1.0 mmol fructose, 0.2 mmol catalyst, 10.0 mmol [bmim]Cl at 80 �C in 30 min. (b) FTIR spectra of group V metalchlorides in [bmim]Cl using ACN as IR probe at room temperature (1:5 mass ratio of ACN/sample; 0.5:1 mol ratio of IL/catalyst as sample).

N. Mittal et al. / Tetrahedron Letters 53 (2012) 3149–3155 3153

of Yang and Kou.32 Samples were then spread into liquid films onCaF2 windows (25 � 4 mm, PIKE Technologies, USA) in a dry,anaerobic environment.

5-HMF analysis

All reactions were monitored by Waters HPLC system equippedwith binary 1525 pumps, a 2707 auto sampler, and 2414 refractiveindex detector, maintained at 40 �C. Analytes were separated inBio-Rad Aminex HPX-87H (300 � 7.8 mm) ion-exclusion column,maintained at 60 �C using 5 mM H2SO4 as mobile phase with0.6 mL/min flow rate. The HPLC system was controlled and pro-cessed by Empower software. Standard 5-HMF, levulinic, and for-mic acid solutions were prepared for calibration. Each productsample was diluted with a known volume of ultra pure deionizedwater before analysis to prevent the overloading of the column.All experiments were done in triplicates, and the average valueswere reported; standard deviations of triplicates were <2.0%.

General procedure for fructose dehydration

Approximately 10 mmol of IL was placed in a 10 mL thickwalled glass vial. The vial was capped and heated at 150 �C to meltthe ionic liquid with continuous stirring for 20 min. After heating,

the IL was cooled to room temperature. Fructose (1.0 mmol) andcatalyst (amount as indicated in the discussion) were added tothe molten IL. The samples were then reacted at a constant tem-perature (as indicated in the discussion) for a given period of time.Aliquots of reaction mixture were collected at different time inter-vals and diluted with deionized water (5 mL). The solution wasshaken well and after the settling of insoluble precipitates (hu-mins), the upper liquid was filtered through 0.2 lm Nylon syringefilters. The filtered sample was analyzed using HPLC.

5-HMF quantification method

An aliquot from reaction mixture was diluted with 5 mL deion-ized water. After shaking the samples and settling of precipitates,the upper clear supernatant was collected for analysis. The 5-HMF concentration was measured using HPLC (Water Series) withRI detector. The 5-HMF yield was calculated using Eq. 1 whereinMHMF (mg) is the total mass of 5-HMF formed in the reaction mix-ture, MF (mg) is the mass of fructose as substrate while MW (g/mol)is the molecular weight of fructose or 5-HMF. On the other hand,MHMF was estimated using (Eq. (2)) where CHMF is the 5-HMF con-centration (mg/mL) from HPLC analysis, V is the volume of wateradded (mL) while WRM and WAL are the masses of total reactionmixture and aliquot, respectively.

Figure 6. Tentative fructose dehydration mechanism in [bmim]Cl/NbCl5 system. (A) NbCl5 complexation with [bmim]Cl, (B) fructose dehydration in [bmim]Cl/NbCl5.

3154 N. Mittal et al. / Tetrahedron Letters 53 (2012) 3149–3155

5-HMF yield ð%Þ ¼ MHMF

MF

� �MWF

MWHMF

� �� 100% ð1Þ

MHMF ¼ CHMF � VWRM

WAL

� �ð2Þ

Acknowledgments

This work is the outcome of the Priority Research Centers Pro-gram through the National Research Foundation of Korea (NRF)funded by the Ministry of Education, Science and Technology(2011-0022968). The authors would like to thank Professor SanghoKoo of the Department of Chemistry at Myongji University, SouthKorea for his suggestions and comments.

References and notes

1. Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem., Int. Ed. 2007, 46, 7164.2. Gupta, P.; Singh, S. K.; Pathak, A.; Kundu, B. Tetrahedron 2002, 58, 10469.3. Carlini, C.; Patrono, P.; Galletti, A. M. R.; Sbrana, G.; Zima, V. Appl. Catal., A: Gen.

2005, 289, 197.4. Casanova, O.; Iborra, S.; Corma, A. J. Catal. 2009, 265, 109.5. Amarasekara, A. S.; Green, D.; McMillan, E. Catal. Commun. 2008, 9, 286.6. Antal, M. J.; Mok, W. S. L.; Richards, G. N. Carbohydr. Res. 1990, 199, 91.7. Asghari, F. S.; Yoshida, H. Ind. Eng. Chem. Res. 2006, 45, 2163.

8. Qi, X.; Watanabe, M.; Aida, T. M.; Smith, R. L., Jr. Green Chem. 2008, 10, 799.9. Tong, X.; Ma, Y.; Li, Y. Appl. Catal., A: Gen. 2010, 385, 1.

10. Leshkov, Y. R.; Chheda, J. N.; Dumesic, J. A. Science 2006, 312, 1933.11. Leshkov, Y. R.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A. Nature 2007, 447, 982.12. Bicker, M.; Kaiser, D.; Ott, L.; Vogel, H. J. Supercrit. Fluids 2005, 36, 118.13. Chheda, J. N.; Leshkov, Y. R.; Dumesic, J. A. Green Chem. 2007, 9, 342.14. Qi, X.; Watanabe, M.; Aida, T. M.; Smith, R. L., Jr. Catal. Commun. 2009, 10, 1771.15. Corma, A.; Iborra, S.; Velty, A. Chem. Rev. 2007, 107, 2411.16. Zhao, H.; Holladay, J. E.; Brown, H.; Zhang, C. Science 2007, 316, 1597.17. Kim, B.; Jeong, J.; Lee, D.; Kim, S.; Yoon, H.; Lee, Y.; Cho, J. K. Green Chem. 2011,

13, 1503.18. Hu, S.; Zhang, Z.; Song, J.; Zhou, Y.; Han, B. Green Chem. 2009, 11, 1746.19. Yong, G.; Zhang, Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2008, 47, 9345.20. Chheda, J. N.; Dumesic, J. A. Catal. Today 2007, 123, 59.21. Kuster, B. F. M. Carbohydr. Res. 1977, 54, 177.22. Matras, C. L.; Moreau, C. Catal. Commun. 2003, 4, 517.23. Szmant, H. H.; Chundury, D. D. J. Chem. Technol. Biotechnol. 1981, 31, 135.24. Asghari, F. S.; Yoshida, H. Carbohydr. Res. 2006, 341, 2379.25. Kim, D. W.; Song, C. E.; Chi, D. Y. J. Org. Chem. 2003, 68, 4281.26. Brennecke, J. F.; Maginn, E. J. AIChE J. 2001, 47, 2384.27. Weingartner, H. Angew. Chem., Int. Ed. 2008, 47, 654.28. Ray, D.; Mittal, N.; Chung, W.-J. Carbohydr. Res. 2011, 346, 2145.29. Zhao, D.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002, 74, 157.30. Cao, Q.; Guo, X.; Yao, S.; Guan, J.; Wang, X.; Mu, X.; Zhang, D. Carbohydr. Res.

2011, 346, 956.31. Guan, J.; Cao, Q.; Guo, X.; Mu, X. Comput. Theor. Chem. 2011, 963, 453.32. Yang, Y.; Kou, Y. Chem. Commun. 2004, 226.33. Platero, E. E.; Mentruit, M. P.; Morterra, C. Langmuir 1999, 15, 5079.34. Cao, F.; Tian, L.; Fang, D.; Ying, W.; Wang, J.-A. Catal. Commun. 2009, 10, 1310.35. Elabd, Y. A.; Sloan, J. M.; Barbari, T. A. Polymer 2000, 41, 2203.

N. Mittal et al. / Tetrahedron Letters 53 (2012) 3149–3155 3155

36. Sievers, C.; Valenzuela-Olarte, M. B.; Marzialetti, T.; Musin, I.; Agrawal, P. K.;Jones, C. W. Ind. Eng. Chem. Res. 2009, 48, 1277.

37. Sievers, C.; Musin, I.; Marzialetti, T.; Valenzuela-Olarte, M. B.; Agrawal, P. K.;Jones, C. W. ChemSusChem 2009, 2, 665.

38. Takagaki, A.; Ohara, M.; Nishimura, S.; Ebitani, K. Chem. Commun. 2009, 6276.39. Chu, Y.; Deng, H.; Cheng, J. P. J. Org. Chem. 2007, 72, 7790.40. Tong, X.; Ma, Y.; Li, Y. Carbohydr. Res. 2010, 345, 1698.41. (a) Nagaiah, K.; Reddy, B. V. S.; Sreenu, D.; Narsaiah, A. V. ARKIVOC 2005, iii,

192; (b) Andrade, C. K. Z.; Azevedo, N. R. Tetrahedron Lett. 2001, 42, 6473; (c)Guo, Q.; Miyaji, T.; Hara, R.; Shen, B.; Takahashi, T. Tetrahedron 2002, 58, 7327;(d) Yadav, J. S.; Reddy, B. V. S.; Naidu, J. J.; Sadashiv, K. Chem. Lett. 2004, 33, 926;(e) Arai, S.; Sudo, Y.; Nishida, A. Synlett 2004, 6, 1104; (f) Kobayashi, S.;Busujima, T.; Nagayama, S. Chem. Eur. J. 2000, 6, 3491; (g) Andrade, C. K. Z.;Azevedo, N. R.; Oliveira, G. R. Synthesis 2002, 7, 928; (h) Andrade, C. K. Z.;

Matos, R. A. F. Synlett 2003, 8, 1189; (i) Sudo, Y.; Arai, S.; Nishida, A. Eur. J. Org.Chem. 2006, 3, 752; (j) Hernández, H.; Bernes, S.; Quintero, L.; Sansinenea, E.;Ortiz, A. Tetrahedron Lett. 2006, 47, 1153; (k) Yadav, J. S.; Reddy, B. V. S.;Eeshwaraiah, B.; Reddy, P. N. Tetrahedron 2005, 61, 875.

42. Alves, M. B.; Santos, V. O., Jr.; Soares, V. C. D.; Suarez, P. A. Z.; Rubim, J. C. J.Raman Spectrosc. 2008, 39, 1388.

43. Hubert-Pfalzgraf, L. G. Niobium & Tantalum Inorganic & CoordinationChemistry. In Encyclopedia of Inorganic Chemistry; John Wiley & Sons Ltd, 2006.

44. (a) Binder, J. B.; Raines, R. T. J. Am. Chem. Soc. 2009, 131, 1979; (b) Defaye, J.;Gadelle, A. Carbohydr. Res. 1985, 136, 53; (c) Stahlberg, T.; Sorensen, M. G.;Riisager, A. Green Chem. 2010, 12, 321.

45. Boovanahalli, S. K.; Kim, D. W.; Chi, D. Y. J. Org. Chem. 2004, 69, 3340.46. Marouka, K.; Murase, N.; Bureau, R.; Ooi, T.; Yamamoto, H. Tetrahedron 1994,

50, 3663.