Synthesis of phenols from hydroxymethylfurfural (HMF)

Article abstract of DOI:10.1039/c6gc01622g

This study describes the use of hydroxymethylfurfural (HMF) as a precursor to phenols that are accessible within a few simple catalytic steps. A key step is an efficient transformation of HMF into its propargyl ether derivative under flow conditions. The latter was subsequently functionalised and used for gold-catalysed conversion into aromatic phenol derivatives. Special attention was paid on performing all of the chemical transformations under mild and environmentally benign conditions.

Full text of DOI:10.1039/c6gc01622g

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DOI: 10.1039/C6GC01622G
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Synthesis of Phenols from
Hydroxymethylfurfural (HMF)
XReceived 00th January 20xx,
Accepted 00th January 20xx
Svetlana Tšupovaa, Frank Romingera†, Matthias Rudolpha, A.
Stephen K. Hashmia,b
DOI: 10.1039/x0xx00000x
This study describes the use of hydroxymethylfurfural (HMF) as a precursor
to phenols that are accessible within a few simple catalytic steps. A key step
is an efficient transformation of HMF into its propargyl ether derivative
under flow conditions. The latter was subsequently functionalised and used
for gold‐catalysed conversion into aromatic phenol derivatives. Special
attention was paid on performing all of the chemical transformations under
mild and environmentally benign conditions.
www.rsc.org/
Introduction
Hydroxymethylfurfural (HMF) as a platform chemical has
received enormous attention in the last few years. Numerous
reports on the synthesis of HMF1 and its transformation into
dimethylfuran (DMF),2 bishydroxymethylfuran,3 caprolactam,4
hexanediol,5 furandicarboxylic acid6 and ring opening
products,7 as potential bulk chemicals have been published.
Generally, etherification followed by reduction is the most
common way to transform HMF into DMF and other bio‐fuels.8
However, much less attention has been devoted to the
integration of HMF, as renewable building block, into the
synthesis of fine chemicals. There are only a few reports on the
introduction of HMF into structures of biologically active
compounds via reductive amination9 or for the synthesis of
molecular libraries.10 In addition HMF was transformed into
benzene‐1,2,4‐triol in 46% yield under very harsh conditions.11
Aromatic benzene derivatives, rather than heteroaromatic
furans, are the most valuable building blocks for the
pharmaceutical and fine chemical industry, they are typical
platform chemicals. Therefore, the demand for these
compounds from non‐fossil sources will undoubtedly emerge.
a.Institut für Organische Chemie, Universität Heidelberg, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany; Fax +49‐(0)6221‐544205; E‐mail:
hashmi@hashmi.de
b.Chemistry Department, Faculty of Science, King Abdulaziz University (KAU),
Jeddah 21589, Saudi Arabia
† Crystallographic investigation
Electronic Supplementary Information (ESI) available: [full catalysts screening
tables, detailed procedures and characterisation of the compounds]. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C6GC01622G
Scheme 2. Formation of propargyl‐HMF
Scheme 1. Gold‐catalysed isomerisation of furanynes into phenols.
We envisioned that the gold‐catalysed phenol synthesis
(Scheme 1), previously developed in our group,12 might serve
as an efficient tool for the transformation of simple HMF
derivatives into valuable aromatic compounds. This versatile
reaction can be performed with very low catalyst loadings
Entry
Eq.
propargyl
alcohol
3
Catalyst
(loading %)
Temp.,
oC
Solvent
Time
NMR
yielda
1
2
p‐TsOH (5)
p‐TsOH (5)
60
60
Ethyl
acetate
Propargyl
alcohol
16 h
16 h
33% b
(0.001%),13 as well as with heterogeneous gold catalysts.14
A
‐
Full
straightforward use of the propargyl ether derivative of HMF
for this reaction was disfavoured as the electron‐withdrawing
effect of an aldehyde moiety is not tolerated.12b Therefore our
focus was set on the search for easy available HMF derivatives
compatible with this gold‐catalysed reaction.
conv.
c
3
4
5
6
2.5
2.5
2.5
3
p‐TsOH (5)
p‐TsOH (5)
p‐TsOH (5)
FeCl3 (5)
60
60
60
rt
Toluene
1,2‐DCE
ACN
16 h
16 h
16 h
16 h
36% b
42% b
30% b
Full
ACN
conv.
54%
Results and discussion
7
8
‐
‐
‐
‐
Amberlyst
15, (100)
Amberlyst
15, (100)
Amberlyst
15, (100)
Amberlyst
15, (100)
rt
45
Propargyl
alcohol
Propargyl
alcohol
Propargyl
alcohol
Propargyl
alcohol
1 h
1 h
1 h
1 h
Formation of propargyl ether of HMF. As the phenol
synthesis requires both a furan core and a terminal alkyne in
the molecule, the first aim was to establish a green method
for the synthesis of HMF propargyl ether. HMF ethers are
mostly obtained in situ, via acid‐catalysed etherifications of
HMF, upon formation from fructose.15 Recent advances in this
field include the use of magnetic supported nanocatalysts,16
sulfonated‐functionalised ionic liquids,17 carbon materials18
and graphene oxides.19 The vast majority of the literature
reports formation of HMF ethyl ether (EMF) as a by‐product of
the HMF formation, as ethanol is often used as a solvent.
However, reports on the synthesis of longer chained ethers are
rare. These were either synthesised by using TiO2 as catalyst20
or by starting from bromomethylfurfural.21
69%
80%
3%
9
75
10
100
Table 1. Formation of propargyl‐HMF under batch conditions (See ESI for full table): 0.1
mmol of HMF in 0.5 ml of solvent, a. 1,3,5‐trimethoxybenzene used as internal
standard. b. dimerisation of HMF as side‐reaction. c. in NMR experiment 88%
yield was achieved, isolated yields in upscale to 0.5‐1.0 g of HMF: 28‐52%.
Initially, the formation of HMF propargyl ether was attempted
from HMF and propargyl alcohol by using p‐TsOH as a catalyst
(Table 1, entries 1‐5). In propargyl alcohol as a solvent, full
conversion and exclusive formation of the desired product was
1
monitored by H NMR. However, scaling up of the reaction
turned out to be problematic and the prolonged reaction times
that were required led to lower yields (28‐52%) of propargyl‐
HMF (likely due to thermal decomposition and
polymerisation). A similar trend was observed with FeCl3 as
catalyst. These problems are most probably caused by the
instability of the aldehyde functionality of HMF and propargyl‐
HMF that initiates polymerisation upon heating. Our attempts
to use other Lewis acids (Fe(OTf)3, Al(OTf)3, InBr3, BiBr3) as
catalysts for this transformation gave no or only minor
conversions (see Table 1 in ESI). Therefore, we turned to a
solid supported acid in a high catalytic loading (entries 7‐10).
The high catalyst to HMF ratio in this case shortens the
reaction time dramatically, therefore by‐product formation is
suppressed and higher yields (at an optimum temperature of
75oC as in entry 9) can be achieved.
Figure 1. Dependence of the HMF to alkyl‐HMF ratio on the solvent. The reaction were
carried out with 200 mg of HMF initially dissolved in 2 ml of solvent at 1.5 ml/min flow
rate at 70 oC. Full consumption of HMF was achieved.
Based on these promising results we considered flow
conditions as ideal tool for the generation of larger quantities
of product within a shorter time frame.
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mixture was flushed through the column at 1.5 ml/min rate with ethyl acetate at 70 oC.
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Full consumption of HMF was observed. The yields refer to combined isolated yields
DOI: 10.1039/C6GC01622G
and product ratios were determined from 1H NMR.
A starting solution of HMF dissolved in a mixture of propargyl
alcohol and ethyl acetate (1:1 v/v) was loaded into a
preheated column. Then at 70oC ethyl acetate was pumped
through the column. Ethyl acetate was chosen due to its green
and renewable nature,22 following a screening: in a blank
experiment, the use of butyl acetate and iso‐propyl acetate
were tested as alternatives to ethyl acetate, but no reduction
of the rate of trans‐etherification was obtained (Figure 1). The
major product resulted from the etherification of HMF to the
propargyl ether, a minor product (EMF obtained by trans‐
etherification from ethyl acetate) was also observed. A
screening for the optimal flow rate showed that the best
propargyl‐HMF to EMF ratio at full conversion was obtained at
a flow rate of 1.5 ml/min (Figure 2). An excess of 10
equivalents of propargyl alcohol delivered the best results ‐ full
consumption of HMF was achieved. The amount of EMF did
not correlate with the amount of alcohol used and staid
between 12‐16% (Figure 3). It is noteworthy that 10 runs on
the column were possible without any change in catalytic
activity (Figure 4). This enabled the conversion of 2.5 grams of
HMF into 2.47 grams of propargyl‐HMF by using 2.2 grams of
Amberlyst 15. Each run took around 20 minutes and 25‐30 ml
Figure 2. Dependence of the yields of propargyl‐HMF and EMF on the flow rate
employing 10 eq. of propargyl alcohol. 1.5 M solution of HMF in ethyl acetate was
mixed with propargyl alcohol and flushed through the column at 1.5 ml/min rate with
ethyl acetate at 70 oC. The yields refer to combined isolated yields after filtration over
silica gel with ether and product ratios were determined from 1H NMR.
Preparation of potential phenol synthesis substrates. Having
devised
a
simple synthesis of propargyl‐HMF, its
transformation into starting materials for the phenol synthesis
was investigated (Scheme 3). The main goal here was to
transfer the aldehyde moiety of propargyl‐HMF into a
functional group tolerated by the gold catalyst. La(NO3)3
catalysed the formation of the acylal smoothly,23 furnishing
the protected compound
2 in quantitative yield. Attempts to
Figure 3. Dependence of the yields of propargyl‐HMF and EMF on the equivalents of
propargyl alcohol employed. 1.5 M solution of HMF in ethyl acetate was mixed with
propargyl alcohol and flushed through the column at 1.5 ml/min rate with ethyl acetate
at 70 oC. The yields refer to combined isolated yields after filtration over silica gel with
ether and product ratios were determined from 1H NMR.
protect the aldehyde as an acetal by heating in the presence of
ethylene glycol failed and only polymerisation of the starting
material took place. Interestingly, the rather seldom used
acylal synthesis could be performed under conditions that are
very simple and environmentally friendly. Employing only 2%
La(NO3)3∙6H2O and
2
equivalents of Ac2O at room
temperature, followed by a dilution of the reaction mixture
with ether and filtration over celite after completion furnished
the pure product.
Alternatively,
1 can be “protected” by a AuCl3‐catalysed two‐
fold addition of methylfuran,24 a renewable and cheap
compound easily accessible from pentoses.25 A further one‐pot
transformation to a phenol product was not observed, even
when a catalytic amount of IPrAuNTf2 was added. Green
conditions, namely Amberlyst 15 in ethyl acetate, can be used
alternatively, however only 25% of
substrate was prepared from ethyl ether
synthesis was previously reported in the literature.26 Attempts
to realize the transformation of to by using our flow
3
were obtained. Finally,
6
5
for which a green
Figure 4. Recyclability study under optimised flow conditions. 250 mg of HMF were
dissolven in 1.3 ml og ethyl acetate and 10 eq. of propargyl alcohol were added. The
5
6
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was observed. mostly the deprotection and polymerisation Vioewf ArticlewOnelirnee
conditions failed and only polymerisation of
5
2
DOI: 10.1039/C6GC01622G
Thus a more conventional route was employed to synthesise
6.
observed. Only sulfolane afforded some conversion into
phenol . In comparison, reactions performed in DCM gave
8
significantly improved results (80% combined yield of
(Table 2)). Finally, we could conduct the reaction with
7
1
and
gram
8
of
25 ml DCM as solvent. Following this protocol 40% of
37% of were obtained. Following this method, the formation
of 0.6 mol of phenols from 1 mol of HMF is estimated. The
2
by using only 0.5 mol% of t‐BuXPhosAuNTf2 as catalyst in
7
and
8
structure of
5).27
8 was confirmed by X‐ray crystallography (Figure
Compound
using IPrAuNTf2 as catalyst in 81% yield (Scheme 4). Like in the
case of , full elimination of the protected aldehyde fragment
3 was cleanly transformed into phenol 8 in DCM
2
was observed. Similar behaviour has been reported before:
cation‐stabilising and bulky groups attached in 5‐position of
the furan favour elimination of the cationic fragment, shifting
the selectivity towards the formation of phenols without
migration of the furan oxygen.28 This structural feature can
also be found in
2 and 3 and therefore the excellent selectivity
in the formation of such phenols seems to be a characteristic
feature of the “protected” propargyl HMF cycloisomerisation.
Scheme 3. Preparation of phenol synthesis substrates. TBAHS – tetrabutylammonium
hydrogensulfate.
Phenol synthesis. Finally, we examined the transformation of
compounds
2,
3
and
6
into phenols.
Scheme 4. Gold‐catalysed cyclisation of 3.
The gold‐catalysed transformation of
6 in DCM delivered a
mixture of three different phenols 11 (Scheme 5). Phenol 9
9‐
was isolated in 48% yield and phenols 10 and 11 in 36%
combined yield as a 5 : 2 mixture. For this substrate the
reaction could be performed in ethyl acetate as a renewable
1
Catalyst
Time
Conversion of
Yield of 7,
Yield of 8,
and green solvent. In this case
product in 59% yield. In addition, phenol
9
was obtained as major
was formed in the
2, %
69
38
76
100
100
%
%
16
0
23
34
32
IPrAuNTf2
Ph3PAuNTf2
TEDIIAuNTf2
BrettPhosAuNTf2
1 h
24 h
1 h
1 h
1 h
19
36
29
46
45
8
reaction in 14% yield, with the combined 10 and 11
contributing for another 7% yield (phenols 10 and 11 in a ratio
2:1). Additionally, the dependency of the phenol product ratios
on the applied ligands was explored (Table 3). An increase in
steric hindrance at the NHC ligand did not significantly affect
the ratio of phenols (entry 1 vs. 3). When an N‐acyclic carbene
t‐BuXPhosAuNTf2
Table 2. Catalyst screening for the cyclisation of 2. 0.1 mmol of 2 in 0.5 ml of CDCl3,
1% of catalyst, room temperature. Mesitylene was used as internal standard,
NMR yields are reported. Only catalysts that gave the desired phenols are shown,
for a full table see ESI Table 2. TEDII – 1‐(2,6‐diisopropylphenyl)‐3‐
cyclopentadecylimidazo‐2‐ylidene.
was employed the formation of
4). A switch to bulky phosphine ligand also induced preferable
formation of , but ortho phenols were still observed (entry 6).
The cleanest result in CDCl3 was achieved with AuCl3 which
furnished 72% of . Therefore, the reaction was also
9 was highly prevalent (entry
9
The reaction of compound
2 resulted in a mixture of phenols 7
and . After a preliminary screening of the catalyst in CDCl3 we
8
9
found that BrettPhosAuNTf2 and t‐BuXPhosAuNTf2 gave the
best results (Table 2). Then the possibilities of carrying out the
reaction in renewable solvents were probed (Table 3 in the
supporting information). Unfortunately, in all of the screened
green solvents (ethyl acetate, diethyl ether, ethanol, acetone,
anisol, BMIMPF6) only trace amounts of phenols were formed,
conducted in ethyl acetate. Although the reaction was not
complete after 24 hours whereas the reaction in CDCl3 reached
completion within 4 hours, 72% of
Therefore, selective route to phenol
to the selective formation of from
9
were still obtained.
was devised, in contrast
and
9
8
2
3.
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phenol synthesis. The gold catalysis of these substrates
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DOI: 10.1039/C6GC01622G
7 and 8 with unusual
delivered selective routes to phenols
ortho‐substitution and phenol 9 in good efficiency.
Acknowledgements
We are grateful for a Kekulé fellowship of the Fonds der
Chemischen Industrie for ST. Gold salts were donated by
Umicore.
Scheme 5. Gold‐catalysed cyclisation of 6.
Experimental
Entry
Catalyst
Solvent
Time
9,
%
10 and
11, %
(ratio)
8,
%
∑
phenol,
%
84
80
5-((prop-2-yn-1-yloxy)methyl)furan-2-carbaldehyde
(1).
Stainless steel HPLC column was packed with dry Amberlyst
15 (2.3 g), connected to pump, washed with ethyl acetate and
heated to 70°C. HMF (200 mg, 1.6 mmol) was dissolved in
propargyl alcohol (1 ml, 17 mmol) and ethyl acetate (1 ml)
was added. Resulted mixture was introduced into column at
the flow rate 1.5 ml/min and ethyl acetate was used to pump it
through the column. Volatiles were removed from the obtained
solution resulting in yellowish oil. If necessary, purification
might be achieved by filtration through silica gel using diethyl
1a
2b
IPrAuNTf2
IPrAuNTf2
CH2Cl2
Ethyl
1 h
1 h
48 36 (5:2)
59
‐
14
7
(2:1)
acetate
CDCl3
CDCl3
CDCl3
CDCl3
3 c
4c
5c
6c
IPr*AuNTf2
NACAuNTf2
Ph3PAuNTf2
t‐ Bu
XPhosAuNTf2
AuCl3
2 h
2 h
2 h
2 h
42 38 (2:1)
59
‐
‐
‐
‐
‐
80
59
‐
trace
trace
59 15 (3:2)
74
7c
8c
CDCl3
Ethyl
acetate
1 h
1 h
72
72
trace
trace
‐
‐
72
72
ether as eluent yielding
1 (228 mg, 78% titled compound and
AuCl3
15% of EMF) as yellowish oil. Rf (petroleum ether/ethyl
acetate 3:2) = 0.62. FTIR (neat) cm-1: 3283, 3122, 2905, 2856,
2117, 1770, 1693, 1585, 1522, 1444, 1402, 1351, 1276, 1193,
1016, 970, 943, 910, 809, 784. 1H NMR (300 MHz, CDCl3) δ =
9.56 (s, 1H), 7.15 (d, J = 3.5 Hz, 1H), 6.50 (d, J = 3.5 Hz, 1H),
4.58 (s, 2H), 4.16 (d, J = 2.4 Hz, 2H), 2.44 (t, J = 2.3 Hz, 1H).
13C NMR (75 MHz, CDCl3) δ = 177.74 (s), 157.31 (s), 152.73
(s), 121.66 (d), 111.77 (d), 78.60 (d), 75.48 (s), 63.27 (t), 57.72
(t). HRMS (EI+) [M+] calcd: 164.0473; found: 164.0461.
Table 3. Formation of phenols from 6. a. isolated yields starting from 100 mg of 6. b.
isolated yields starting from 200 mg of 6. c. NMR experiment: 0.1 mmol of 6 in
0.5 ml of CDCl3, using 1% of catalyst and mesitylene as internal standard. IPr* ‐
1,3‐bis(2,6‐bis(diphenylmethyl)‐4‐methylphenyl)imidazo‐2‐ylidene, NAC ‐ N,N‐
diethyl‐N’‐tertbutyldiaminomethylidene.
(5-((prop-2-yn-1-yloxy)methyl)furan-2-yl)methylene
(2). 5-((prop-2-yn-1-yloxy)methyl)furan-2-carbaldehyde
diacetate
(2.51
1
g, 15.3 mmol) was dissolved in acetic anhydride (2.8 ml, 30
mmol) and La(NO3)3*6H2O (132 mg, 0.31 mmol) was added.
The obtained mixture was stirred overnight. Then 50 ml of
diethyl ether were added and the obtained mixture was filtered
through celite. Volatiles were removed yielding to 2 as
brownish oil (4.2 g, quanititive yield). Rf (petroleum ether/ethyl
acetate 3:2) = 0.61. FTIR (neat) cm-1: 1935, 1764, 1682, 1563,
1
1524, 1441, 1373, 1239, 1203, 1080, 1018, 969, 810. H NMR
(300 MHz, CDCl3) δ = 7.67 (s, 1H), 6.48 (d, J = 3.2 Hz, 1H),
6.36 (d, J = 3.2 Hz, 1H), 4.56 (s, 2H), 4.18 (d, J = 2.2 Hz, 2H),
2.47 (t, J = 1.9 Hz, 1H), 2.13 (s, 6H). 13C NMR (75 MHz,
CDCl3) δ = 168.38 (s), 152.19 (s), 148.21 (s), 110.55 (d),
110.46 (d), 83.39 (d), 79.05 (s), 75.01 (t), 63.08 (t), 57.09 (t),
20.70 (q). HRMS (EI+) [M+] calcd: 266.0790; found: 266.0816
Figure 5. X‐Ray structures of 8 and 9.
Conclusions
1,3-dihydroisobenzofuran-5-yl acetate (8). (5-((prop-2-yn-1-
In conclusion, we have developed novel synthetic routes for
the synthesis of aromatic phenols from heteroaromatic
renewable HMF. We show that the ether formation from HMF
can be successfully performed under flow conditions. The
obtained propargyl ether can be transformed with easy
manipulations to starting materials for the gold‐catalysed
yloxy)methyl)furan-2-yl)methylene diacetate
2 (1.00 g, 3.76
mmol) was dissolved in DCM (25 ml) and t-BuXPhosAuNTf2
(17 mg, 0.02 mmol) was added. The resulting mixture as stirred
at room temperature overnight. Volatiles were removed and the
residue was purified by column chromatigraphy on silica gel
(eluent petroleum ether/ethyl acetate 5:1).
8 as colorless oil
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7
(a) C. R. Waidmann, A. W. Pierpont, E. R. Batista, J. C.
(267 mg, 40% yield) and
9 as white solid (190 mg, 37% yield)
Gordon, R. L. Martin, L. A. Silks, R. M. DWOeI:s1t0a.1n0d3V9Ri/eC.wW6AGrutCic,0leC1O6an2tl2ainGle.
were isolated. Rf (petroleum ether/ethyl acetate 5:1) = 0.44.
FTIR (neat) cm-1: 3496, 3288, 3060, 3029, 3856, 2674, 2386,
2084, 1940, 1750, 1682, 1654, 1622, 1597, 1485, 1434, 1365,
1317, 1287, 1180, 1136, 1099, 1012, 949. 1H NMR (400 MHz,
CDCl3) δ = 7.22 (d, J = 8.8 Hz, 2H), 7.00 – 6.94 (m, 3H), 5.09
(s, 4H), 2.30 (s, 3H). 13C NMR (101 MHz, CDCl3) δ = 169.63
(s), 150.21 (s), 140.78 (s), 136.60 (s), 121.68 (d), 120.66 (d),
114.54 (d), 73.36 (t), 73.23 (t), 21.06 (q). HRMS (EI+) [M+]
calcd: 178.0630; found: 178.0615.
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yloxy)methyl)furan.
yloxy)methyl)furan
of
2-(ethoxymethyl)-5-((prop-2-yn-1-
2-(ethoxymethyl)-5-((prop-2-yn-1-
6
(200 mg, 1.03 mmol) was dissolved in
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HMF-derived substrates were converted to highly substituted phenols by gold catalysis.