Fe3O4@SiO2-TEMPO as a magnetically recyclable catalyst for highly selective aerobic oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran under metal-and halogen-free conditions

Article abstract of DOI:10.1002/cctc.201301081

A highly selective aerobic oxidation of 5-hydroxymethylfurfural (5-HMF) into 2,5-diformylfuran (DFF) by employing a magnetically separable 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) catalyst (Fe₃O ₄@SiO₂-TEMPO) under metal-and halogen-free conditions is described. Quantitative yields of DFF with excellent selectivities as high as >99 % were achieved under several versatile reaction conditions in a catalytic system consisting of Fe₃O₄@SiO₂-TEMPO as a catalyst, tert-butyl nitrite as a co-catalyst, acetic acid as an additive, and toluene as a solvent. The results clearly demonstrate that the present catalytic system for selective conversion of 5-HMF into DFF is comparable or even superior to the many systems based on transition-metal catalysts. In addition, the catalyst can be successfully recovered and reused in five consecutive reaction runs by an external magnetic source with keeping its catalytic performance and selectivity. A simple pull-out: Highly selective aerobic oxidation of 5-hydroxymethylfurfural (5-HMF) into 2,5-diformylfuran (DFF) is achieved by using a magnetically separable 2,2,6,6- tetramethylpiperidine-N-oxide (TEMPO) catalyst (Fe₃O ₄@SiO₂-TEMPO) with tert-butyl nitrite (TBN) as cocatalyst under metal-and halogen-free conditions.

Full text of DOI:10.1002/cctc.201301081

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DOI: 10.1002/cctc.201301081
Fe3O4@SiO2–TEMPO as a Magnetically Recyclable Catalyst
for Highly Selective Aerobic Oxidation of
5-Hydroxymethylfurfural into 2,5-Diformylfuran under
Metal- and Halogen-Free Conditions
Babak Karimi,* Hamid M. Mirzaei, and Elham Farhangi[a]
A highly selective aerobic oxidation of 5-hydroxymethylfurfural
(5-HMF) into 2,5-diformylfuran (DFF) by employing a magneti-
cally separable 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO)
catalyst (Fe3O4@SiO2–TEMPO) under metal- and halogen-free
conditions is described. Quantitative yields of DFF with excel-
lent selectivities as high as >99% were achieved under several
versatile reaction conditions in a catalytic system consisting of
Fe3O4@SiO2–TEMPO as a catalyst, tert-butyl nitrite as a co-cata-
lyst, acetic acid as an additive, and toluene as a solvent. The re-
sults clearly demonstrate that the present catalytic system for
selective conversion of 5-HMF into DFF is comparable or even
superior to the many systems based on transition-metal cata-
lysts. In addition, the catalyst can be successfully recovered
and reused in five consecutive reaction runs by an external
magnetic source with keeping its catalytic performance and
selectivity.
obtained from biomass in biorefineries that in turn can be re-
placed with petroleum-derived chemical building blocks such
as terephthalic, isophthalic, and adipic acids, which are current-
ly used in polymer technology.[4] 2,5-Diformylfuran (DFF), is an-
other important platform derived from HMF that can be used,
for example, as a starting material for the synthesis of the
pharmaceuticals, macrocyclic ligands, and others.[5] Although
DFF is recognized as a top value-added chemical obtainable
from biomass, so far only limited attention has been paid to its
synthesis.[6] DFF can be prepared through partially oxidation of
the hydroxymethyl group in HMF, which behaves like primary
alcohols bearing an aromatic moiety. Thus, DFF can be com-
pared to a certain extent with benzyl or furfuryl alcohol
(Scheme 1).[7] However, the main challenge of this important
Over the last century, the world has concerned with crude-oil
replacement with other sources of energy as a result of the de-
pletion of fossil oil reservoirs and the awareness of climate
change. In this regard, biomass is the most fascinating alterna-
tive feedstock, as it is the only widely attainable carbon source
apart from oil and coal.[1] Production of fuels and chemicals
from biomass can be typically achieved through partially deox-
ygenating biomass derivatives, which primarily is derived from
hydrolysis and dehydration methods, to produce a variety of
fuels, fuel additives, and fine chemicals.[2] For example, hexose
monosaccharaides such as glucose and fructose and even
polysaccharides such as cellulose are able to be catalytically
dehydrated into 5-hydroxymethylfurfural (5-HMF), a versatile
biobased platform chemical with high potential. In particular, it
is well documented that the HMF itself is a key intermediate
for the production of 2,5-furandicarboxylic acid (FDCA) thor-
ough oxidation by employing various stoichiometric oxidizing
agents and catalysts.[3] The US Department of Energy biomass
program has identified FDCA as one of the twelve chemicals
Scheme 1. Selective oxidation of HMF into DFF rather than 2-hydroxyme-
thylfurancarboxylic acid (HFCA), FCA, or FDCA.
process is the existing both furan and aldehyde functionalities
in HMF, which makes it susceptible to undergoing many side
reactions such as overoxidation to FDCA and/or 5-formyl-2-fur-
ancarboxylic acid, decarbonylation, and cross-polymerization
to produce unwanted byproducts.
As claimed by several relevant research works, oxidation of
the hydroxyl group in HMF proceeds much slower than oxida-
tion of the aldehyde moiety.[8] Therefore, the development of
novel catalyst systems for efficient and selective oxidation of
HMF into DFF is a great challenge. In this way, the concomitant
use of molecular oxygen or air and heterogeneous oxidation
catalysts, which can be reused in liquid phase reactions, is
highly favored from both sustainable chemistry and economi-
cal point of views. To date, several metal-based catalytic sys-
tems such as Co/Mn/Zn/Br,[9] Pt–Bi/C,[10] various types of vana-
[a] Prof. Dr. B. Karimi, H. M. Mirzaei, E. Farhangi
Department of Chemistry
Institute for Advanced Studies in Basic Sciences (IASBS)
Zanjan 45137-6731 (Iran)
Fax: (+98)241-415-3232
E-mail: karimi@iasbs.ac.ir
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201301081.
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dium-based catalysts,[6,11] Ru/HT,[12] and KMn8O16·nH2O[13] have
been utilized with varied degrees of success in the selective
aerobic oxidation of HMF into DFF.
Several promising catalyst systems such as 2,2,6,6-tetrame-
thylpiperidine-1-oxide (TEMPO) in combination with stoichio-
metric halogen-based oxidants[14a] and/or halogen based sol-
vent,[14b] and TEMPO/CuCl/O2 either in the absence[14a] or in the
presence[15] of nitrogen-containing promoters have also been
employed for the same transformation. However, although the
selectivity toward DFF and product yields was addressed in the
above-mentioned pioneering efforts, there are several impor-
tant issues that should be still resolved. One of the important
drawbacks of current liquid-phase catalytic oxidation of HMF
relate to the use of transition-metal catalysts or cocatalysts,[9–15]
which are potentially difficult to separate from the products. In
addition, many of the present protocols require high reaction
temperatures (up to 1508C),[9,12] and/or high oxygen pressure
(up to 10 bar),[6,11] to ensure desirable DFF selectivities at high
HMF conversions. Acknowledging these important achieve-
ments, the quest for milder and greener methodologies by de-
signing metal-free catalyst systems for selective aerobic oxida-
tion of HMF into DFF remains a great challenge. In this regard,
the TEMPO radical seems to have an intriguing prospect in the
field of metal-free aerobic oxidation of alcohols.[16] Along the
line of this hypothesis and our continuous interest in develop-
ing novel systems for aerobic oxidation of alcohols,[17] we have
recently introduced a new system based on magnetic silica-
coated Fe3O4-nanoparticles-supported TEMPO (MNST) in the
aerobic oxidation of varied alcohols under metal- and halogen-
free conditions (Scheme 2). Considering the excellent catalytic
Figure 1. Vibrating sample magnetometer curve of MNST using KAVIR
magnetometer.
Table 1. Aerobic oxidation of HMF into DFF over MNST catalyst.
Entry MNST[a] Solvent
[mol%] [mL]
T
[8C]
Additive[b] TBN
t
DFF yield[c]
[mol%] [h] [%]
1
2
3
4
5
6
7
8
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
–
0.2
0.1
0.1
0.1
0.1
0.1
0.5
0.5
0.5
1
water (0.3) 50
water (0.3) 50
–
–
–
5
50
50
5
50
50
50
50
50
–
50
40
30
20
10
10
10
20
10
5
24
24
24
24
24
3
2
5
24
24
18
70
15
30
38
>99
>99
>99
7
water (2)
water (2)
water (2)
50
50 HOAc
50 HOAc
HOAc (0.5) RT
–
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) RT HOAc
toluene (2) 50 HOAc
toluene (2) RT HOAc
toluene (2) 50 HOAc
toluene (2) 50 HOAc
toluene (2) 50 HOAc
toluene (2) 50 HOAc
toluene(10) 50 HOAc
9
10
11
12
13
14
15
16
17
18
19
20
21
22[d]
23[e]
10
10 >99
12 >99
18 >99
24
24
24
24 >99
24 >99
12 >99
60
40
45
1
2
2
2
24
70
5
5
5
18 >99
18 >99
18 >99
[a] MNST loading ꢀ0.31 mmolgÀ1 according to CHN analysis. [b] (HOAc)
15 mg was added as an additive. [c] GC yield. [d] HMF 1 mmol was used.
[e] 5 mmol HMF was used (the amount of solvent was not optimized).
Scheme 2. MNST catalyst for aerobic oxidation of a variety of alcohols.
performance of MNST in performing alcohol oxidation to alde-
hydes[18a] and one-pot oxidative three-component Passerini re-
action,[19] we reasoned that this catalyst might be equally appli-
cable for accomplishing the selective oxidation of HMF into
DFF.
ing Information, Figure S9, S10). On the basis of the observa-
tion that MNST exhibited excellent catalytic activity under the
conditions demonstrated in Scheme 2, we began our investiga-
tions on the aerobic oxidation of HMF using essentially the
same reaction condition (Table 1, entry 1). To our surprise, the
initial results in pure water (0.3 mL) were rather disappointing
because DFF was obtained in poor yields of 18% after 24 h. Al-
though the yield of DFF was strongly improved upon employ-
ing large amounts of tert-butyl nitrite (TBN, up to 50 mol%),
the results were not reproducible because of the difficulty of
mixing the reaction mixture (entry 2).
Firstly, we aimed to prepare and re-characterize MNST by fol-
lowing our previous method.[18] The magnetic properties of
MNST catalyst were investigated by using a vibrating sample
magnetometer at room temperature by Meghnatis Daghigh
Kavir Company (Iran). As illustrated in Figure 1, the magnetiza-
tion curves of the prepared materials exhibited no hysteresis
loop, which demonstrates its superparamagnetic characteris-
tics. Also, N2 sorption analysis at 77 K revealed a type (III) iso-
therm specifying a nonporous material, in which BET surface
area calculation indicated a surface area of 50 m2 gÀ1 (Support-
In addition, the reaction did not reach completion even by
using such these high loadings of TBN. To solve the mixing
problem, the reaction was then performed by using a larger
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water volume of 2 mL under otherwise the same reaction con-
ditions. However, the oxidation reaction of 5-HMF proved to
be again unsatisfactory under these conditions (Table 1,
entry 3). In an attempt to improve the catalyst performance,
we examined the use of acetic acid as additive (entries 4 and
5), whereby we took into account our earlier findings[16u,20] of
a strong favorable influence of HOAc on the catalytic activity
of TEMPO in the aerobic oxidation of alcohols under metal-free
conditions. The beneficial effect of HOAc was immediately real-
ized, but the catalyst efficiency did not improve much because
the yield of DFF hardly reached to 38% even after prolonged
reaction times under these conditions. For further improving
the catalyst performance, we next examined several reaction
conditions by emphasizing the fact that the employment of
lower amounts of TBN would be more favorable from the
standpoint of both green and sustainable chemistry. We found
that the DFF yield was dramatically increased to >99% within
only 3 h if the reaction was conducted in pure HOAc as reac-
tion solvent (entry 6). The same level of the yields and also
DFF selectivities were also obtained in shorter reaction time of
2 h if toluene was employed in the presence of small quanti-
ties of HOAc (15 mg, entry 7). The reaction proceeded still well
at room temperature and selectively delivered DFF in quantita-
tive yields after somewhat longer reaction times (entry 8).
Blank runs using either MNST (0.2 mol%) or TBN (50 mol%)
alone gave no conversion, indicating the key importance of co-
existence of these materials for driving the successful aerobic
oxidation of 5-HMF (entries 9,10). In addition, by reducing the
amount of MNST from 0.2 mol% to 0.1 mol%, the same excel-
lent yields of DFF were still obtained in the presence of
50 mol% TBN at room temperature (entry 11). This system also
reached completion in somewhat longer reaction times of the
reaction (typically 12–18 h) in the presence of lower TBN load-
ing (up to 30 mol%), but the yields of DFF were significantly
reduced to 40% upon further decreasing the TBN loading to
10–20 mol% (entries 12–15). Although further increasing of
MNST loading from 0.1 mol% to 0.5 mol% offered no advan-
tages in the presence of 10 mol% TBN at room temperature,
the same reaction system led to DFF with excellent conversion
and selectivity at 508C (entries 16 and 17). Further studies re-
vealed that the same high DFF yields and selectivities were
also obtained by employing 20 mol% TBN, 0.5 mol% MNST at
room temperature (entry 18). It was also found that a small in-
crease in MNST loading to 1–2 mol% could significantly im-
prove the efficiency of the reaction at >99% conversion and
selectivity in the presence of as low as 5 mol% TBN (en-
tries 19–21). Furthermore, to show the potential application of
this catalytic system for large-scale operation, the oxidation of
HMF was studied on either 1 or 5 mmol scale under the opti-
mized conditions demonstrated in entry 21. Interestingly, no
detectable change in DFF yield and selectivity was observed
over MNST catalytic system compared to the case of the small-
scale experiments (entries 22 and 23). On the basis of several
relevant studies underlying the overall mechanism for metal-
free aerobic oxidation of alcohols using homogeneous TEMPO
in the presence of various NOx sources,[16v–z] a possible reaction
pathway was proposed and is depicted in Scheme 3, in which
Scheme 3. Proposed reaction pathway for the aerobic oxidation of HMF into
DFF using MNST catalyst.
the mediation of electron transfer from alcohol to O2 by NO/
NO2 couples and immobilized nitroxyl radical (MNST) through
a combination of two redox cycles is highlighted.
After oxidation of 5-HMF under the conditions obtained in
Table 1 entry 22, MNST was quantitatively separated from the
reaction mixture by employing an external magnetic field,
thoroughly washed with ethanol, and reutilized in subsequent
runs under identical reaction conditions. The results indicate
that no efficiency loss was observed in 5-HMF oxidation in up
to four successive runs (Figure 2).[21]
Figure 2. Reusability of MNST catalyst in the oxidation of HMF into DFF;
HMF (1 mmol), MNST (2 mol%), toluene (2 mL), TBN (5 mol%), HOAc
(75 mg), 508C, 18 h.
It was also noticed that the catalyst exhibited little (almost
no) morphological changes as evidenced by comparison of
TEM images before and after catalysis (Figure 3). The preferen-
tial formation of DFF with excellent selectivity in all reactions
demonstrated in Table 1 and also during the recycling experi-
ments is particularly noteworthy, because no detectable
amounts of either FDCA or 5-formyl-2-furancarboxylic acid
were observed in the products.
In conclusion, we have developed a new aerobic oxidation
system for the selective oxidation of 5-hydroxymethylfurfural
(5-HMF) into 2,5-diformylfuran (DFF) using a magnetically sepa-
rable organocatalyst (Fe3O4@SiO2–TEMPO, MNST). This reaction
was performed under transition-metal- as well as halogen-free
conditions, which is suitable from the viewpoint of green and
sustainable chemistry. Notably, the superparamagnetic nature
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Keywords: alcohols
properties · oxidation
· biomass · heterocycles · magnetic
[1] R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J.
Heeres, J. G. de Vries, Chem. Rev. 2013, 113, 1499–1597.
[2] D. M. Alonso, J. Q. Bond, J. A. Dumesic, Green Chem. 2010, 12, 1493–
1513.
[3] C. Moreau, M. N. Belgacemb, A. Gandinib, Top. Catal. 2004, 27, 11–30.
[4] T. Werpy, G. Petersen, Top Value Added Chemicals from Biomass, Vol. 1
2004, 26–28; available at: http://www.osti.gov/bridge.
[5] a) K. T. Hopkins, W. D. Wilson, B. C. Bender, D. R. McCurdy, J. E. Hall, R. R.
Tidwell, A. Kumar, M. Bajic, D. W. Boykin, J. Med. Chem. 1998, 41, 3872–
3878; b) A. S. Benahmed-Gasmi, P. Frere, M. Jubault, A. Gorgues, J.
Cous-seau, B. Garrigues, Synth. Met. 1993, 56, 1751–1755; c) D. T. Richt-
er, T. D. Lash, Tetrahedron Lett. 1999, 40, 6735–6738; d) D. W. Sheibley,
M. A. Manzo, O. D. Gonzalezsanabria, J. Electrochem. Soc. 1983, 130,
255–259.
Figure 3. TEM micrographs of MNST a) before catalysis (scale bar 100 nm)
and b) after five reaction cycles (scale bar: 20 nm) of the oxidation of HMF
into DFF; HMF (1 mmol), MNST (2 mol%), toluene (2 mL), 5 mol% TBN,
75 mg HOAc, 508C, 18 h.
[6] J. Ma, Z. Du, J. Xu, Q. Chu, Y. Pang, ChemSusChem 2011, 4, 51–54.
[7] J. Lewkowski, ARKIVOC (Gainesville, FL, U.S.) 2001, 17–54.
[8] a) Y. Y. Gorbanev, S. K. Klitgaard, J. M. Woodley, C. H. Christensen, A. Riis-
ager, ChemSusChem 2009, 2, 672–675; b) O. Casanova, S. Ibora, A.
Corma, ChemSusChem 2009, 2, 1138–1144; c) B. Saha, S. Dutta, M. M.
Abu-Omar, Catal. Sci. Technol. 2012, 2, 79–81; d) N. K. Gupta, S. Nishi-
mura, A. Takagaki, K. Ebitani, Green Chem. 2011, 13, 824–827; e) S. E.
Davis, B. N. Zope, R. J. Davis, Green Chem. 2012, 14, 143–147.
[9] W. Partenheimer, V. V. Grushin, Adv. Synth. Catal. 2001, 343, 102–111.
[10] A. Giroir-Fendler, P. Denton, A. Boreave, H. Praliaud, M. Primet, Top.
Catal. 2001, 16–17, 237–241.
[11] a) G. A. Halliday, R. J. Young, V. V. Grushin, Org. Lett. 2003, 5, 2003–2005;
b) C. Carlini, P. Patrono, A. M. Raspolli Galletti, G. Sbrana, V. Zima, Appl.
Catal. A 2005, 289, 197–204; c) O. C. Navarro, S. Iborra Chornet, Top.
Catal. 2009, 52, 304–314; d) I. Sꢁdaba, Y. Y. Gorbanev, S. Kegnæs, S. S. R.
Putluru, R. W. Berg, A. Riisager, ChemCatChem 2013, 5, 284–293.
[12] A. Takagaki, M. Takahashi, S. Nishimura, K. Ebitani, ACS Catal. 2011, 1,
1562–1565.
of the MNST catalyst facilitates easy separation of the catalyst
from reaction media by simply employing an external magnet-
ic field. The quantitative yields of DFF was achieved in excel-
lent chemoselectivity (>99%) under several versatile reaction
conditions as demonstrated in Table 1. In this way, the catalyst
can be successfully recovered and reused in five consecutive
reaction runs, keeping its catalytic performance and selectivity.
To our knowledge, this method is the first report on the selec-
tive aerobic oxidation of 5-HMF using magnetically separable
catalysts under metal- as well as halogen-free conditions. It is
believed that this finding will stimulate new areas in designing
novel solid catalysts in a designated pathway with appropriate
catalytic performance and recycling properties to control the
selectivity of this important transformation, addressing eco-
nomic and environmental issues.
[13] Z. Z. Yang, J. Deng, T. Pan, Q. X. Guo, Y. Fu, Green Chem. 2012, 14,
2986–2989.
[14] a) L. Cottier, G. Descotes, E. Viollet, J. Lewkowski, R. ScowroÇski, J. Heter-
ocycl. Chem. 1995, 32, 927–930; b) C. Aellig, D. Scholz, S. Conrad, I. Her-
mans, Green Chem. 2013, 15, 1975–1980.
Experimental Section
[15] T. S. Hansen, I. Sꢁdaba, E. J. Garcꢂa-Suꢁrez, A. Riisager, Appl. Catal. A
2013, 456, 44–50.
A typical catalytic procedure for 5-HMF oxidation to the desired
2,5-diformylfuran: In a 10 mL two-necked flask, 5-HMF (1 mmol),
MNST (2 mol%), HOAc (75 mg), and toluene (2 mL) were charged.
To this mixture, TBN (5 mol%) was added. Then the flask was evac-
uated and recharged with pure oxygen for three times and finally
was charged with pure oxygen (balloon filled, PO2 ꢀ1 atmos-
phere). The resulting mixture was stirred (800 rpm) at 508C for
18 h. After this time, the catalyst was simply separated by employ-
ing an external magnetic field and the resulting solution was ana-
lyzed by GC for conversion and selectivity determination by using
standard addition methods. Moreover, after completion of the
aerobic oxidation and magnetic separation of the catalyst, the sol-
vent was evaporated under reduced pressure and the remaining
white solid was analyzed by NMR spectroscopy (see the Support-
ing Information).
[16] a) A. E. J. de Nooy, A. C. Besemer, H. van Bekkum, Synthesis 1996, 1153–
1174; b) W. Adam, C. R. Saha-Moller, P. A. Ganeshpur, Chem. Rev. 2001,
101, 3499–3548; c) R. A. Sheldon, I. W. C. E. Arends, G. J. ten Brink, A.
Dijksman, Acc. Chem. Res. 2002, 35, 774–781; d) R. A. Sheldon, I. W. C. E.
Arends, Adv. Synth. Catal. 2004, 346, 1051–1071; e) P. L. Anelli, C. Biffi, F.
Montanari, S. Quici, J. Org. Chem. 1987, 52, 2559–2562; f) P. L. Anelli, S.
Banfi, F. Montanari, S. Quici, J. Org. Chem. 1989, 54, 2970–2972; g) P. L.
Anelli, F. Montanari, S. Quici, Org. Synth. 1990, 61, 212–219; h) C. Bolm,
T. Fey, Chem. Commun. 1999, 1795–1796; i) M. Shibuya, M. Tomizawa, I.
Suzuki, Y. Iwabuchi, J. Am. Chem. Soc. 2006, 128, 8412–8413; j) M. Zhao,
J. Li, E. Mano, Z. Song, D. M. Tschaen, E. J. J. Grabowski, P. J. Reider, J.
Org. Chem. 1999, 64, 2564–2566; k) L. Huang, N. Teumelsan, X. Huang,
Chem. Eur. J. 2006, 12, 5246–5252; l) T. Inokuchi, S. Matsumoto, T. Nish-
iyama, S. Torri, J. Org. Chem. 1990, 55, 462–466; m) A. de Mico, R. Mar-
garita, L. Par-lanti, A. Vescovi, G. Piancatelli, J. Org. Chem. 1997, 62,
6974–6977; n) S. D. Rychnovsky, R. Vaidyanathan, J. Org. Chem. 1999,
64, 310–312; o) L. de Luca, G. Giacomelli, A. Porcheddu, Org. Lett. 2001,
3, 3041–3043; p) C. Bolm, S. Angelika, S. Magnus, J. P. Hildebrand, Org.
Lett. 2000, 2, 1173–1175; q) H. R. Bjørsvik, L. Liguori, F. Cos-tantino, F.
Minisci, Org. Process Res. Dev. 2002, 6, 197–200; r) R. A. Miller, R. S.
Hoerrner, Org. Lett. 2003, 5, 285–287; s) A. Cecchetto, F. Fontana, F. Min-
isci, F. Recupero, Tetrahedron Lett. 2001, 42, 6651–6653; t) W. A. Herr-
mann, J. P. Zoller, R. W. Fischer, J. Organomet. Chem. 1999, 579, 404–
407; u) B. Karimi, A. Biglari, J. H. Clark, V. Budarin, Angew. Chem. 2007,
119, 7348–7351; Angew. Chem. Int. Ed. 2007, 46, 7210–7213; v) R. H.
Liu, X. M. Liang, C. Y. Dong, X. Q. Hu, J. Am. Chem. Soc. 2004, 126, 4112–
4113; w) X. L. Wang, R. H. Liu, Y. Jin, X. M. Liang, Chem. Eur. J. 2008, 14,
Acknowledgements
The authors gratefully acknowledge the IASBS Research Council
and Iran National Science Foundation (INSF) for supporting this
work.
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2679–2685; x) Y. Xie, W. M. Mo, D. Xu, Z. L. Shen, N. Sun, B. X. Hu, X. Q.
Hu, J. Org. Chem. 2007, 72, 4288–4291; y) X. J. He, Z. L. Shen, W. M. Mo,
N. Sun, B. X. Hu, X. Q. Hu, Adv. Synth. Catal. 2009, 351, 89–92; z) R. H.
Liu, C. Y. Dong, X. M. Liang, X. J. Wang, X. Q. Hu, J. Org. Chem. 2005, 70,
729–731.
[20] B. Karimi, E. Badreh, Org. Biomol. Chem. 2011, 9, 4194–4198.
[21] In a separate experiment, we have also employed the conditions de-
scribed in entry 7 of Table 1 [HMF (0.2 mmol), TBN (50 mol%), MNST
(0.2 mol%), toluene (2 mL), AcOH (15 mg), at RT] for the catalyst recy-
cling. We noticed that at RT the catalyst can be recycled in 14 succes-
sive reaction cycles without any significant loss of catalytic activity
giving consistently excellent yields of DFF (>99%). Such an impressive
stability of immobilized TEMPO is certainly owing to the fact that all re-
cycling reactions proceeded at low reaction temperature (RT) and low
reaction times (2 h). However, this recycling could be only attained at
the expense of employing large amounts of TBN (50 mol%) in each
run, which is not certainly attractive form a practical point of view.
[17] a) B. Karimi, A. Zamani, J. H. Clark, Organometallics 2005, 24, 4695–
4698; b) B. Karimi, S. Abedi, J. H. Clark, V. Budarin, Angew. Chem. 2006,
118, 4894–4897; Angew. Chem. Int. Ed. 2006, 45, 4776–4779; c) B.
Karimi, A. Zamani, S. Abedi, J. H. Clark, Green Chem. 2009, 11, 109–119;
d) B. Karimi, F. Kabiri Esfahani, Chem. Commun. 2009, 5555–5557; e) B.
Karimi, D. Elhamifar, J. H. Clark, A. J. Hunt, Org. Biomol. Chem. 2011, 9,
7420–7426; f) B. Karimi, F. Kabiri Esfahani, Adv. Synth. Catal. 2012, 354,
1319–1326; g) B. Karimi, H. Behzadnia, M. Bostina, H. Vali, Chem. Eur. J.
2012, 18, 8634–8640; h) B. Karimi, D. Elhamifar, O. Yari, M. Khorasani, H.
Vali, J. H. Clark, A. Hunt, Chem. Eur. J. 2012, 18, 13520–13530.
[18] B. Karimi, E. Farhangi, Chem. Eur. J. 2011, 17, 6056–6060.
[19] B. Karimi, E. Farhangi, Adv. Synth. Catal. 2013, 355, 508–516.
Received: December 16, 2013
Published online on February 12, 2014
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