Production of biobased HMF derivatives by reductive amination

Article abstract of DOI:10.1039/c002340j

5-(Hydroxymethyl)furfural (HMF) has recently attracted a significant amount of revived attention as a renewable building block for conversion to a wide range of useful derivatives. A simple procedure for the conversion of HMF to (5-alkyl- and 5-arylaminomethyl-furan-2-yl)methanol has now been developed. Reactions were conducted without the use of a catalyst and under very mild conditions. As a proof of concept, a small library of derivatives was produced from HMF and several aliphatic and aromatic amines in high yields and requiring only minimal purification. This route presents a novel way for the production of furan-based renewable building blocks. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002340j

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Production of biobased HMF derivatives by reductive amination†
Ana Cukalovic and Christian V. Stevens*
Received 3rd February 2010, Accepted 1st April 2010
First published as an Advance Article on the web 18th May 2010
DOI: 10.1039/c002340j
5-(Hydroxymethyl)furfural (HMF) has recently attracted a significant amount of revived attention
as a renewable building block for conversion to a wide range of useful derivatives. A simple
procedure for the conversion of HMF to (5-alkyl- and 5-arylaminomethyl-furan-2-yl)methanol
has now been developed. Reactions were conducted without the use of a catalyst and under very
mild conditions. As a proof of concept, a small library of derivatives was produced from HMF
and several aliphatic and aromatic amines in high yields and requiring only minimal purification.
This route presents a novel way for the production of furan-based renewable building blocks.
than ketohexoses since an alkaline medium is required for
Introduction
the formation of the enol and for their further conversion to
HMF. Under the conditions that work well for ketohexoses,
aldoses form a stable pyranose ring and the enolisation rate is
low.1
With the upcoming depletion of petrochemical reserves on
Earth, the chemical community is focussing on renewable
resources to find or improve procedures for their conversion
to useful chemicals. 5-(Hydroxymethyl)furfural (HMF) is one
of the renewable building blocks with a high potential for
further chemical modifications and it has recently attracted
much attention by a number of research groups. An extensive
review on HMF production was published by Kuster;1 two
reviews by Lewkowski2 and Cottier and Descotes3 incorporated
both HMF production and chemistry.
Agents promoting the dehydration reaction include various
organic and inorganic protic acids, Lewis acids, different salts,
but also heterogeneous catalysts such as ion-exchange resins
and zeolites (ref. 2 and references therein). Some novel work
on HMF production includes the use of different ionic liquids
as promoting media for the dehydration,4-12 the use of super-
critical solvents,13-15 two-phase optimized solvent systems,16-19 or
microwave heating.20,21 However, in most of the published pro-
cedures that claim an improved HMF production, its separation
and final purification still remain an issue. HMF is not easy to
extract since the distribution coefficient between the organic and
the aqueous phase is not very favourable; in accordance, the yield
of HMF in the above cited work is mostly reported in solution
as analysed by HPLC of both phases. In some work,5,10,11 batch
extraction was reported to be 100% efficient, however these
results were not reproducible in our lab. High vacuum distillation
is another option but this technique is rather energy demanding.
Furthermore, under the conditions needed for its production,
HMF tends to further degrade to levulinic and formic acid and
other side products and to polymerize to soluble and insoluble
humins. These undesirable reactions are observed primarily in
aqueous medium and in concentrated solutions, and eventually
cause problems during HMF purification22 as well as during
its handling and storage. The difficulties in isolating and in
manipulating this interesting molecule are the major obstacles
in its large-scale production and conversion.
HMF is obtained by acid-catalysed dehydration of hexoses,
mainly fructose. Two mechanisms for HMF formation have been
suggested: the cyclic A and alicyclic route B (Scheme 1). Aldoses
mostly give lower yields of HMF under the same conditions
An interesting concept of HMF isolation from the reaction
mixtures is based on the formation of derivatives: a production
process patented by Avantium (NL) starts from glucose and
continues to HMF ethers and esters in a continuous process.23,24
The authors suggested the use of the obtained derivatives as
starting materials for the production of polymers and fuel
additives. Apart from these structures, there is a range of
molecules that can easily be produced from HMF and have
applications in the chemical and in other industries. For example,
Scheme 1 Proposed routes of HMF formation from glucose and
fructose.1
SynBioC Research Group, Department of Organic Chemistry, Faculty of
Bioscience Engineering, Coupure links 653, B-9000, Ghent, Belgium.
E-mail: Chris.Stevens@UGent.be; Fax: +32 9 264 6243; Tel: +32 9 264
5957
† Electronic Supplementary Information (ESI) available: Product char-
acterisation. See DOI: 10.1039/c002340j
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2,5-furandicarboxylic acid can be successfully used as a
substitute for petrochemically produced terephthalic and
isophthalic acid in the production of polymers;25,26 2,5-
di(hydroxymethyl)furan is already used in the production of
polyurethane foams.25 2,5-Furandicarboxaldehyde is a starting
material for the production of 2,5-di(hydroxymethyl)furan and
of the Schiff bases used in further conversions.25 Novel vinyl-
based polymers made from HMF have recently been reported,27
and 2,5-dimethylfuran, a potential fuel additive with very
promising features has recently attracted much attention.28,29
In addition to this, 2-alkyl- and 2-arylaminomethylfurans are
known for their wide range of pharmacological activities: they
are used in medicinal chemistry as antihistaminics,30-32 glutamate
modulators,33 glycine antagonists,34 muscarinic agonists,35 renin
inhibitors,36 antifungal agents,37 kinase inhibitors38,39 etc. These
structures are generally produced from furfural; however, the
production procedures usually include long reaction times and
rather drastic temperature/pressure conditions.40,41 The reason
for this is the deactivating effect of the C2 carbaldehyde group
that makes the addition to the C5 position of the furan ring fairly
difficult.
purification was performed routinely but simple extraction and
solvent evaporation already gave a product purity >95%).
Results and discussion
Our work was based on a one-pot, two-step reductive amina-
tion, thus without the purification of the intermediate imines
(Scheme 2). By the choice of reaction media we have attempted
to increase the sustainability of this process. Water is the
most abundant, the cheapest and environmentally most benign
solvent,44 whereas ethanol and methanol can be obtained in
a sustainable way by biomass fermentation.45,46 Of course, a
somewhat negative aspect of using water as reaction medium is
the difficult and costly purification, which may lead to additional
costs.
Scheme 2 Schematic production route of the HMF based amines.
One example of obtaining these compounds from HMF was
published by Gupta et al.42 Using a template directed approach
and solid-phase catalysis, a library of similar heterocyclic
compounds was synthesized starting from resin-immobilized
HMF. In an automated synthesizer, more than fifty compounds
were produced with purities of 70–92% but in rather modest
yields.
The mild reaction conditions are essential to prevent break-
down of HMF and to obtain a reasonable yield of the end
product.
Reductive amination is a widely used tool in organic syn-
thesis and is mostly performed in two steps: the production
of aldimines followed by the in situ reduction with sodium
borohydride.47
Production of similar compounds from HMF was mentioned
in two patents.33,39 However, production processes involved
prolonged heating in several organic solvents, addition of several
equivalents of resin and/or extensive purification by preparative
HPLC.
We hereby report a very simple protocol for the production
of 5-aminomethyl-2-furfuryl alcohols using HMF as starting
material. By using this facile and straightforward method, a
small library of compounds containing a hydroxymethyl and an
aminomethyl moiety on the furan ring was produced (Fig. 1).
Schiff bases of aromatic aldehydes are easily obtained by
condensation with amines. The chemistry of this process involves
the elimination of water from an intermediate amino alcohol.
Water elimination is thus usually aided by various means, such as
magnesium or calcium sulfate,48 molecular sieves49 or azeotropic
distillation with a suitable organic solvent.50 On the contrary,
Simion et al.51 reported a simple procedure for the reaction
between various aldehydes and amines to form a range of imines
in aqueous medium. Reactions were conducted in the absence of
a catalyst during short reaction times at room temperature and
gave excellent purities and generally good yields of the resulting
imines.
Following this approach, a number of reactions were per-
formed using aqueous solutions of HMF to yield several imines.
The imines are then reduced to the corresponding amines
with sodium borohydride in a one-pot reaction. Generally this
procedure gave very good results in reactions with aliphatic
amines (Table 1). Complete conversion to the imines was
obtained within several hours and the imine reduction was
generally completed after one hour.
However, conversions of aromatic imines did not proceed
easily in aqueous medium: conversions up to 50% were observed
after several days. Therefore, other reaction media were tested.
Methanol was suitable for several reasons: it is a good solvent for
both starting materials, a preferred solvent for reductions with
borohydride and is easily produced in a sustainable way from
renewable materials. Unfortunately, reactions in dry methanol
with aromatic amines also gave poor results, similar to the
reactions in water. On the other hand, the required reaction times
of aliphatic amines with HMF were drastically improved: e.g.
imine formation from allylamine and HMF took 45 minutes
Fig. 1
Surprisingly and to the best of our knowledge, most of
these simple structures were never described before. These
amines can serve as starting materials for a wide range of
conversions43 and be particularly useful in the above mentioned
production of pharmaceuticals. This method is also based on a
renewable molecule (HMF instead of furfural) but, (a) it does
not necessarily involve any particular conditions of pressure,
temperature or catalyst, (b) it is based on readily available
reagents and reaction media and (c) demands a minimal work-
up. Water, ethanol and methanol were used as reaction media
and they worked well without the addition of catalysts. Beside
the starting materials, a reasonably mild and cheap reducing
agent (sodium borohydride) was used. Mild conditions of room
temperature and atmospheric pressure were initially applied and
minimal purification of the product was necessary (additional
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Table 1 Overview of the reductive aminations
Entry
1
R
Solvent
H2O
Reaction time step 1
5h
Reaction time step 2
1h
Yield
92%
Chromatographical purification
MeOH–CH2Cl2
10/90 Rf 0.42
2
3
H2O
H2O
4h
4h
1h
1h
80%
97%
MeOH–CH2Cl2
10/90 Rf 0.12
MeOH–CH2Cl2
10/90 Rf 0.55
4
H2O
4h
1h
77%
EtOAc–PE
5/95 Rf 0.45
5
6
H2O
H2O
6h
6h
1h
1h
82%
88%
MeOH–CH2Cl2
10/90 Rf 0.24
MeOH–CH2Cl2
10/90 Rf 0.26
6¢
MeOH
2h
30¢
94%
see above
7
MeOH
MeOH
1h
1h
99%
93%
MeOH–CH2Cl2
10/90 Rf 0.30
see above
2¢
45¢
30¢
8
EtOH
30h
1h
96%
MeOH–CH2Cl2
2/98 Rf 0.36
9
EtOH
EtOH
30h
2h
1h
1h
86%
96%
MeOH–CH2Cl2
2/98 Rf 0.22
4¢
see above
see above
5¢
EtOH
EtOH
1.5h
50h
1h
1h
92%
83%
10
MeOH–CH2Cl2
2/98 Rf 0.18
11
EtOH
50h
1h
91%
MeOH–CH2Cl2
2/98 Rf 0.30
10¢
EtOH
EtOH
30h
50h
1h
1h
83%
97%
MeOH–CH2Cl2
2/98 Rf 0.18
12
MeOH–CH2Cl2
10/90 Rf 0.61
in methanol whereas reaction in water needed 4 hours for
completion (entries 2 and 2¢, Table 1).
time was enhanced already by the solvent change and that the
addition of the sieves did not have a significant influence on the
reaction with an aliphatic amine (entry 4¢ compared to entry
5¢). By comparison of entries 8 and 9, it can also be seen that
the addition of sieves did not significantly influence the reaction
performance. On the other hand, in entries 10 and 10¢, addition
of the sieves proved to substantially reduce the reaction time
from 50 to 30 hours.
Using this procedure, a small library was produced and is
presented in Table 1. Reactions were followed on TLC and
1H-NMR and the identity of the final products was confirmed
by LC-MS, 1H- and 13C-NMR, IR spectroscopy and elemental
analysis where possible.
Dry ethanol was the next solvent of choice. Ethanol is,
similar to methanol, a good solvent for the starting materials
and reagents (sodium borohydride has a limited solubility of
4 g/100 ml at 20 ◦C, which was still enough to perform the
reactions). Acting as a water scavenger, ethanol is also assumed
to increase the rate of imine formation. The reactions were
repeated with aliphatic amines in ethanol as reaction medium,
which again resulted in a decrease of the required reaction time:
a couple of hours required for formation of the imines (entries
4¢ and 5¢, Table 1). Aromatic amines, however, demanded longer
reaction times (up to 50 hours, entries 9-12, Table 1). Addition
of molecular sieves was tested to improve the efficiency of the
For comparison, tetrahydrofuran was also tested as a medium
for the reaction with aromatic amines but the conversion to
the imine was poor compared to ethanol (up to 40% vs. >99%
conversion).
˚
reaction i.e. to shorten the reaction time; molecular sieves of 4 A
˚
in entries 4¢ and 8, and 3 A in entry 10¢, Table 1. This approach
was moderately successful as it was estimated that the reaction
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Table 2 Reactions conducted under conventional and microwave heating
Entry
5a
R
Solvent
H2O
Time
5¢
Temperature
Heat source
Microwave
Conversion to imines
50 ◦
50 ◦
50 ◦
C
C
C
100%
100%
100%
5b
H2O
15¢
15¢
Conventional
Conventional
6a
H2O
11a
11b
11b
11c
EtOH
EtOH
EtOH
H2O
1.5 h
6h
40 ◦
C
Microwave
50%
65%
85%
70%
100 ◦
C
Microwave
6h+24 h
2h
100→-20 ◦
100→200 ◦
C
C
Microwave→none
Microwave
12a
12a
12b
EtOH
EtOH
EtOH
20¢
40 ◦
C
Microwave
Microwave
None
65%
70%
80%
1 h
40→100 ◦
C
72 h
-20 ◦
C
In the first step of the reaction (the imine formation), a
significant difference in the reactivity of amines was noticed.
A decreasing reactivity order can be determined: aliphatic
amines > aromatic amines with electron donating groups >
aromatic amines > aromatic amines with electron withdrawing
groups (the experiments with aromatic amines containing NO2
and Cl substituents did not yield more than 30% conversion
compared to the corresponding imines in 24 h and therefore the
further procedure was abandoned).
In an additional attempt to reduce the reaction time necessary
for imine formation, reactions were conducted under microwave
heating, as microwaves were shown to enhance this type of
reaction52,53 (Table 2).
Reactions were followed by 1H-NMR and HPLC. It was noted
that microwave heating indeed resulted in a drastic improvement
of the iminations with aliphatic amines (down to 5 minutes at
50 ◦C, entry 5a). However, conventional heating gave similar
results—complete conversion to imines in a matter of minutes
(entries 5b and 6a). On the other hand, mild microwave heating
did not give such positive results in trials with aromatic amines
(entry 11a). Nevertheless, the pressurised vessel in the microwave
synthesizer allowed reactions at temperatures higher than the
boiling point of the solvent. Reactions performed at 100 ◦C (in
water and ethanol) and 200 ◦C (in water) resulted in improved
conversions (entries 11b and 11c). It is noted that, due to the
high absorbing nature◦of ethanol, the energy required to reach
this temperature (100 C) was only 1-2 W during the reaction.
For reactions in water, temperatures of 100 ◦C were achieved
and retained by the use of 10-20 W power. Maximal power of
the reaction setup (200 W) was used only during heating of the
reactions to 200 ◦C in water.
Microwave heating did improve the reaction rate of the imine
formation; however, using the times mentioned in Table 2,
no complete conversions were observed. Moreover, under the
prolonged heating, HMF tends to degrade to humic substances
(these were noticed as black insoluble accumulations in the
reaction mixture).
It is also important to notice the formation of imines on
simple standing at -20 ◦C in the freezer for prolonged periods. In
entry 11b, a 6 hours microwave irradiated reaction was kept for
24 hours in the freezer which resulted in an additional conversion
to imine, up to 85% (estimated by HPLC). Similarly, in entry 12b,
a significant conversion was noted without any prior heating, on
standing at -20 ◦C during 72 hours. Obviously, the reaction time
is a very important factor in these conversions, apart from the
reaction temperature.
Considering the production issues elaborated in the first part
of the article and following some unpublished work previously
performed in our lab, the feasibility of performing the reductive
amination in situ from HMF precursors was tested. Motivated
by the fact that HMF can be produced in acidic media from
hexoses, preliminary experiments were performed on HMF in
acidic media (aqueous solutions acidified by citric or levulinic
acid to a pH of 2–4). Under these conditions, conversion to the
imine took a longer time (85% conversion in 5 hours for a test
reaction with benzylamine) but was successful. Unfortunately,
when the production of HMF from fructose in acidic aqueous
media was followed by a one-pot conversion to the imine, the
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resulting reaction mixture contained many side-products which
made the purification difficult and led to a significantly lower
yield.
gave purities of 95% of the resulting amines (determined by
1H-NMR). Additional purification was performed by column
chromatography, resulting in high yields of isolated material in
all cases.
Conclusions
Procedures in methanol and ethanol
A simple and convenient procedure was developed for the
production of several novel as well as known aminomethylfurans
starting from HMF, a promising renewable building block. This
procedure can be utilised to produce a number of building blocks
for the chemical and pharmaceutical industry. Use of water as
the reaction medium, as well as the biobased solvents ethanol
and methanol improves the sustainability of the reaction.
Conventional and microwave heating resulted in an increase
of the reaction rate but increased the energy demand of the
process. In most cases a simple extraction provided derivatives of
sufficient purity. The formation of these derivatives from sugars
(precursors of HMF) was unfortunately not successful.
In a 50 ml round-bottom flask, 1.1 eq of primary amine was
added to ~300 mg of HMF (2.3 mmol) in 10 ml of alcohol,
and the reaction mixture was stirred under a nitrogen flow at
room temperature (exact values are reported in Table 1). In
˚
entries 4¢ and 8, ~1 g of molecular sieves 4 A was added to
the reaction mixture. In entry 10¢, ~1 g of molecular sieves
˚
3 A was added to the reaction mixture. After the reaction
was complete, the resulting imine was reduced by addition
of 1.5 eq of sodium borohydride for generally one hour. The
alcohol was then evaporated and to the resulting dry mixture
1 ml of water was added. The final product was extracted
with 2 ¥ 15 ml of dichloromethane. The organic phase was
washed with water and brine, dried over MgSO4, filtered and
evaporated under reduced pressure. This procedure gave purities
over 95% of the resulting amines (determined by 1H-NMR). The
products were additionally purified by column chromatography
and characterised.
Experimental section
˚
Methanol was dried over molecular sieves (4 A) and fractionally
distilled. Ethanol was dried over CaO, refluxed and fractionally
distilled.
The reaction materials, including organic solvents for the
reactions or for the work-up were purchased from Sigma-
Aldrich and Acros Organics and used as such, except for these
mentioned above.
Reactions performed under microwave heating
In a 10 mL sealed vessel 1.1 eq of primary amine was added
to ~150 mg of HMF (~1.16 mmol) in 5 ml of solvent (water or
ethanol). Reactions were followed by HPLC and 1H-NMR.
Microwave-assisted reactions were performed on a CEM
Discover BenchMate synthesizer with a IR temperature sen-
1
sor. High-resolution H-NMR (300 MHz) and 13C-NMR (75
Reactions performed under conventional heating
MHz) spectra were recorded on a Jeol JNM-EX 300 NMR
spectrometer. HPLC analysis of the reaction mixtures and
the final products was performed on an Agilent 1200 series
HPLC equipped with a UV/Vis DAD detector at 254 and
280 nm, on a Zorbax Eclipse XBD-C18 4.6 ¥ 150 ¥ 5 mm RP
column. Low-resolution mass spectra were recorded with an
Agilent 1100 Series VS (ES = 4000 V) mass spectrometer. IR
spectra were recorded on a Perkin–Elmer Spectrum BX FT-
IR spectrometer. All compounds were analysed in neat form
with an ATR (Attenuated Total Reflectance) accessory. Melting
points of the solid compounds were measured with a Bu¨chi
540 apparatus and have not been corrected. Elemental analysis
was performed on a Perkin-Elmer 2400 Elemental Analyzer.
Additional purification of the reaction mixtures was performed
by column chromatography in a glass column on silica gel
(Acros, particle size 0.035–0.070 mm, pore diameter ca. 6 nm).
In a 25 ml round-bottom flask, 1.1 eq of primary amine was
added to ~150 mg of HMF (1.16 mmol) in 5 ml of solvent
(water or ethanol). Reaction mixtures were heated in an oil bath
and followed by HPLC and 1H-NMR.
Acknowledgements
We would like to thank Prof. G. Marin (UGent) and the
Methusalem finance to finalize this work. The authors wish
to thank their colleagues Ann De Blieck and Bart Roman for
providing help and constructive remarks during this work.
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