Ionic liquid as catalyst and reaction medium: A Simple and Efficient Procedure for Paal-Knorr furan synthesis

Article abstract of DOI:10.1080/00397910902978049

The ionic liquid 1-butyl-3-methyl-imidazolium hydrogen sulfate, [bmim]HSO₄, efficiently catalyzes Paal-Knorr furan synthesis without any organic solvent. A wide range of aliphatic and aromatic 1,4-diketones easily undergo condensations to form furan derivatives, providing a general and convenient procedure. The Paal-Knorr reaction of ester-substituted 1,4-diketones is first reported. The ionic liquid can be recovered and reused for subsequent runs without any appreciable loss of efficiency.

Full text of DOI:10.1080/00397910902978049

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Ionic Liquid as Catalyst and Reaction
Medium: A Simple and Efficient
Procedure for Paal–Knorr Furan Synthesis
Gangqiang Wang a , Zhi Guan a , Rongchang Tang a & Yanhong He a
a School of Chemistry and Chemical Engineering, Southwest
University, Chongqing, China
Published online: 06 Jan 2010.
To cite this article: Gangqiang Wang , Zhi Guan , Rongchang Tang & Yanhong He (2010): Ionic Liquid
as Catalyst and Reaction Medium: A Simple and Efficient Procedure for Paal–Knorr Furan Synthesis,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 40:3, 370-377
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Synthetic Communications1, 40: 370–377, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902978049
IONIC LIQUID AS CATALYST AND REACTION
MEDIUM: A SIMPLE AND EFFICIENT PROCEDURE FOR
PAAL–KNORR FURAN SYNTHESIS
Gangqiang Wang, Zhi Guan, Rongchang Tang, and
Yanhong He
School of Chemistry and Chemical Engineering, Southwest University,
Chongqing, China
The ionic liquid 1-butyl-3-methyl-imidazolium hydrogen sulfate, [bmim]HSO4, efficiently
catalyzes Paal–Knorr furan synthesis without any organic solvent. A wide range of aliphatic
and aromatic 1,4-diketones easily undergo condensations to form furan derivatives, provid-
ing a general and convenient procedure. The Paal–Knorr reaction of ester-substituted
1,4-diketones is first reported. The ionic liquid can be recovered and reused for subsequent
runs without any appreciable loss of efficiency.
Keywords: Catalyst; furan derivatives; ionic liquid; Paal–Knorr reaction; reaction medium
The furan ring can be found in many pharmaceutical and natural products,[1,2] and it
is a useful building block in synthetic organic chemistry.[3] Among the various meth-
ods of furan synthesis, the Paal–Knorr reaction is the most widely used approach, in
which 1,4-dicarbonyl compounds are converted to furan derivatives via acid-
mediated dehydrative cyclization.[4–6] In recent years, with the development of many
excellent methods for the preparation of 1,4-diones,[7–18] the synthetic utility of
this century-old reaction has experienced rekindled interest. Generally, hazardous
organic solvents, strong acids such as H2SO4, P2O5, and p-toluenesulfonic acid
(p-TSA), have been employed for the Paal–Knorr furan synthesis,[19–21] and long
reaction times were often required for the condensation. These may not be the pre-
ferred choices in view of green chemistry. Hence, an efficient and environmentally
benign Paal–Knorr condensation is needed for contemporary chemical synthesis.
Ionic liquids have recently emerged as environmentally friendly alternatives for
organic synthesis. Use of ionic liquids in organic synthesis, particularly those based
on an imidazolium cation,[22–26] have experienced an impetus. However, their use in
Paal–Knorr furan synthesis remains largely unexplored. There is only one report by
Yadav et al. about the usage of Bi(OTf)3 immobilized in [bmim]BF4 as a catalytic
system for the Paal–Knorr furan synthesis.[27] Therefore, we report the facile and
Received January 21, 2009.
Address correspondence to Yanhong He, School of Chemistry and Chemical Engineering,
Southwest University, Chongqing 400715, China. E-mail: heyh@swu.edu.cn
370

PAAL–KNORR FURAN SYNTHESIS
371
high-yielding Paal–Knorr reaction using [bmim]HSO4 ionic liquid alone as an
efficient and recyclable catalyst as well as a green reaction medium.
In the present method, dehydrative cyclization of 1,4-diketones in ionic liquid
[bmim]HSO4 at 150ꢀC afforded furans in good to quantitative yields. To assess the
generality of the method, aryl and alkyl, di-, tri-, and tetra-substituted 1,4-dicarbonyl
compounds were transformed to the furan derivatives (Table 1). The method is
efficient and easy to carry out. The reactions were complete within 2–5 h, and the
products were easily isolated by simple extraction with diethyl ether. All the non-
ester-containing aromatic 1,4-diketones (2a and 4a–9a, entry 2 and entries 4–9)
provided corresponding furan products in quantitative yields after simple extraction
and removal of the solvents. However, alkyl-substituted 1,4-diketones (1a and 3a,
entries 1 and 3) gave furan products in lesser yields (84% and 80% respectively).
For 1,4-diketone hexane-2,5-dione (1a, entry 1), in consideration of the volatility,
the reaction was first carried out at 60ꢀC in ionic liquid, which furnished furan in
only 21% yield with the starting material recovered after 24 h. From thin-layer chro-
matography (TLC) no side products were observed. Then, the reaction temperature
was raised to 150ꢀC in a sealed tube within an oven without stirring, which gave the
product in 84% yield after 4 h. From TLC, the reaction was clean and the starting
material has been consumed, so the poor yield is probably due to the volatility of
the product. In the case of 1-phenylpentane-1,4-dione (3a, entry 3), it provided the
product furan in 80% yield. No starting material was remained, and some unidenti-
fied side products were observed.
Interestingly, when trisubstituted 1,4-diketo-2-esters (10a–18a, entries 10–18)
were submitted to the cyclization, the corresponding furan esters were obtained,
accompanied by decarboxylated furan products. We found that when C-1 was sub-
stituted by CH3 or Ph, the furan esters were obtained as the major products (entries
10–15), but when C-1 was substituted by the strong electron-withdrawing group
(p-)NO2Ph, the decarboxylated furans were received as the major products (entries
16–18). It seems that the strong electron-withdrawing group on C-1 of diketones
facilitates the decarboxylation of C-2 ester. However, no decarboxylation was
observed for tetrasubstituted 1,4-diketo-2-esters (19a–21a, entries 19–21), which
provided corresponding furan esters as the only products in quantitative yields,
regardless of whether there was an electron-donating (CH3) or electron-
withdrawing [Ph or (p)NO2Ph] group on C-1. When 1,4-dimethyl-2,3-diester
butane-1,4-dione (22a, entry 22) was submitted to the cyclization reaction, the dec-
arboxylated product monoester furan 22c was obtained in 53% yield as the only
product without diester furan detected. The reaction was clean; the poor yield of
22c probably due to its volatility. Nevertheless, the cyclization of 1,4-diphenyl-2,3-
diester butane-1,4-dione (23a) showed the contrasting results. The diester furan 23b
was received as the major product, and the decarboxylated product monoester
furan 23c was obtained as the minor one. It can be seen that the substituents can
strongly influence the decarboxylation of ester-containing 1,4-diketones. The
Paal–Knorr reaction of ester-substituted 1,4-diketones has not been reported, to
our best knowledge.
A proposed mechanism for the conversion of 1,4-diketones into furans using
ionic liquid is given in Scheme 1. The ionic liquid [bmim]HSO4 provides protons
as a Brønsted acid to catalyze the reaction. First, the protonation of one carbonyl

372
G. WANG ET AL.
Table 1. Cyclization of 1,4-diketones into furans using ionic liquid at 150ꢀCa
Entry
1
1,4-Diketone
Furan
Time (h) Yield (%)
4
3
2
2
2
2
2
2
2
84c
2
3
4
5
6
7
8
9
100b
80c
100b
100b
100b
100b
100b
100b
10b: 60c
10c: 28c
10
11
12
13
14
15
3
3
3
4
4
4
11b: 71c
11c: 14c
12b: 80c
12c: 7c
13b: 55c
13c: 34c
14b: 71c
14c: 14c
15b: 70c
15c: 23c
(Continued )

PAAL–KNORR FURAN SYNTHESIS
373
Table 1. Continued
Entry
16
1,4-Diketone
Furan
Time (h) Yield (%)
16b: 7c
4
16c: 90c
17b: 5c
17
18
4
17c: 91c
18b: 10c
4
18c: 85c
19
20
21
22
23
3
4
4
5
5
19b: 100b
20b: 100b
21b: 100b
22c: 53c
23b: 63c
23c: 14c
aThe reaction was carried out in a sealed tube at 150ꢀC within an oven without stirring.
bYield of crude product of > 95% purity by NMR.
cYield of isolated product after flash chromatography.
group generates protonated species, which undergoes enolization. Then, intramol-
ecular nucleophilic attack of the enol oxygen upon the electrophilic carbon of
another protonated carbonyl group takes place. The final step is the dehydration
to furnish furan derivatives.
Finally, to verify that the ionic liquid could be reused, it was recovered from
the cyclization of 7a, washed with diethyl ether (2 ꢁ 10 mL) to remove any organic
impurity and dried in vacuo for use in subsequent reactions.[28,29] The reaction
was performed three times using the same ionic liquid without any appreciable loss
of efficiency.
In summary, we have developed a facile and efficient method for the most
widely used Paal–Knorr furan synthesis. In comparison with reported protocols,
the method presented herein avoids the use of metals, mineral acids, organic
solvents, and other hazardous chemicals and instead employs ionic liquid

374
G. WANG ET AL.
Scheme 1. Plausible mechanism for the Paal–Knorr furan synthesis catalyzed by [bmim]HSO4 ionic liquid.
[bmim]HSO4 as an efficient and reusable catalyst as well as a green reaction medium,
which is easy to separate from the product mixture by simple extraction. The reac-
tion is high-yielding and easy to perform.
EXPERIMENTAL
Typical Procedure for the Synthesis of Substituted Furans
A solution of 1,4-dione 15a (0.353 g, 0.7 mmol) in ionic liquid [bmim]H-
SO4(1.6 g, 6.7 mmol) was stirred at 150ꢀC for 4 h in oil bath. After cooling to rt,
the reaction mixture was thoroughly extracted with diethyl ether. The combined
extracts were washed with water and brine, dried (Na2SO4), and filtered. The sol-
vents were removed, and the residue was purified by flash chromatography (pet-
roleum ether=dichloromethane 2:1) to get 15b (240 mg, 70%) as yellow crystals
and 15c (43 mg, 23%) as yellow crystals.
Compound 15b. Mp: 130–131ꢀC 1H NMR (300 MHz, CDCl3): d ¼ 8.52–7.46
(m, 9H), 7.24 (s, 1H), 4.34 (q, 2H, J ¼ 7.1 Hz), 1.37 (t, 3H, J ¼ 7.1 Hz); 13C NMR
(75 MHz, CDCl3): d ¼ 163.0, 157.5, 149.6, 148.7, 131.3, 129.9, 129.8, 129.3, 129.1,
128.4, 128.2, 122.3, 118.6, 116.0, 110.2, 60.8, 14.2. HRMS (EI) calcd. for
C19H15NO5 337.095; found 337.0953.
Compound 15c. Mp: 134–135ꢀC; 1H NMR (300 MHz, CDCl3): d ¼ 8.52–7.31
(m, 9H), 6.88 (s, 1H), 6.77 (s, 1H); 13C NMR (75 MHz, CDCl3): d ¼ 154.7, 150.7,
148.7, 132.2, 130.1, 129.7, 129.0, 128.8, 128.0, 123.9, 121.5, 118.2, 109.5, 107.4.
HRMS (EI) calcd. for C16H11NO3 265.0739; found 265.0738.
Spectroscopic Data of Selected Compounds
Compound 12b. Yellow crystals. 1H NMR (300 MHz, CDCl3): d ¼ 8.47–7.53
(m, 4H), 7.05 (s, 1H), 4.34 (q, 2H, J ¼ 7.1 Hz), 2.68 (s, 3H), 1.38 (t, 3H, J ¼ 7.1 Hz);

PAAL–KNORR FURAN SYNTHESIS
375
13C NMR (75 MHz, CDCl3): d ¼ 163.5, 159.8, 149.2, 148.8, 131.6, 129.8, 129.0,
122.0, 118.3, 115.9, 107.9, 60.4, 14.3, 13.9.
Compound 12c. Yellow crystals. Mp: 71–72ꢀC; 1H NMR (300 MHz,
CDCl3): d ¼ 8.43–7.47 (m, 4H), 6.69 (s, 1H), 6.11 (s, 1H), 2.39 (s, 3H); 13C NMR
(75 MHz, CDCl3): d ¼ 153.5, 149.8, 148.7, 132.6, 129.5, 128.6, 121.0, 117.9, 108.2,
13.7. HRMS (EI) calcd. for C11H9NO3 203.0582; found 203.0583.
Compound 14b. Yellow crystals. Mp: 174–175ꢀC; 1H NMR (300 MHz,
CDCl3): d ¼ 8.30–7.48 (m, 9H), 7.32 (s, 1H), 4.35 (q, 2H, J ¼ 7.1 Hz), 1.37 (t, 3H,
J ¼ 7.1 Hz); 13C NMR (75 MHz, CDCl3): d ¼ 162.9, 158.2, 149.8, 146.8, 135.3,
130.0, 129.0, 128.5, 128.2, 124.4, 124.1, 116.3, 111.8, 60.9, 14.2. HRMS (EI) calcd.
for C19H15NO5 337.095; found 337.0962.
Compound 16b. Yellow crystals. Mp: 128–129ꢀC; 1H NMR (300 MHz,
CDCl3): d ¼ 8.32–7.32 (m, 9H), 7.09 (s, 1H), 4.35 (q, 2H, J ¼ 7.1 Hz), 1.39 (t, 3H,
J ¼ 7.1 Hz); 13C NMR (75 MHz, CDCl3): d ¼ 162.9, 153.7, 152.9, 147.3, 135.2
128.9, 128.8, 128.7, 128.4, 124.1, 123.3, 118.6, 108.5, 61.1, 14.2. HRMS (EI) calcd.
for C19H15NO5 337.095; found 337.0953.
Compound 17b. Yellow crystals. Mp: 240–241ꢀC 1H NMR (300 MHz,
CDCl3): d ¼ 8.35–7.90 (m, 8H), 7.38 (s, 1H), 4.38 (q, 2H, J ¼ 7.1 Hz), 1.41 (t, 3H,
J ¼ 7.1 Hz); 13CNMR (75 MHz, CDCl3): d ¼ 162.5, 155.0, 151.4, 148.0, 147.4,
134.7, 129.1, 124.6, 123.6, 119.0, 112.1, 61.4, 14.2. HRMS (EI) calcd. for
C19H14N2O7 382.0801; found 382.0796.
Compound 18b. Yellow crystals. Mp: 177–178ꢀC; 1H NMR (300 MHz,
CDCl3): d ¼ 8.55–7.63 (m, 8H), 7.31 (s, 1H), 4.39 (q, 2H, J ¼ 7.1 Hz), 1.42 (t, 3H,
J ¼ 7.1 Hz); 13CNMR (75 MHz, CDCl3): d ¼ 162.5, 154.3, 151.2, 148.8, 147.8,
134.8, 130.6, 130.1, 129.6, 123.5, 123.0, 118.9, 110.7, 61.4, 14.2. HRMS (EI) calcd.
for C19H14N2O7 382.0801; found 382.0802.
Compound 18c. Yellow crystals. Mp: 158–159ꢀC; 1H NMR (300 MHz,
CDCl3): d ¼ 8.57–7.60 (m, 8H), 6.98 (s, 1H), 6.97 (s, 1H); 13C NMR (75 MHz,
CDCl3): d 152.8, 152.2, 148.8, 146.7, 135.6, 131.5, 130.0, 129.5, 124.4, 124.1,
122.5, 118.7, 111.3, 110.0. HRMS (EI) calcd. for C16H10N2O5 310.0590; found
310.0593.
1
Compound 19b. White crystals. H NMR (300 MHz, CDCl3): d ¼ 7.58–7.29
(m, 5H), 4.30 (q, 2H, J ¼ 7.1 Hz), 2.60 (s, 3H), 2.39 (s, 3H), 1.36 (t, 3H, J ¼ 7.1 Hz);
13C NMR (75 MHz, CDCl3): d 164.7, 158.3, 147.6, 130.9, 128.4, 127.1, 126.0, 116.8,
115.2, 59.8, 14.3, 10.8.
Compound 21b. 1H NMR (300 MHz, CDCl3): d ¼ 8.28–7.30 (m, 9H), 4.12
(q, 2H, J ¼ 7.1 Hz), 2.36 (s, 3H), 0.98 (t, 3H, J ¼ 7.1 Hz); 13C NMR (75 MHz,
CDCl3): d 164.3, 150.4, 147.0, 135.7, 132.2, 129.4, 128.1, 127.4, 123.9, 123.6,
117.8, 61.0, 13.5, 12.1. HRMS (EI) calcd. for C20H17NO5 351.1107; found 351.1105.
Compound 22c. 1H NMR (300 MHz, CDCl3): d ¼ 6.21 (1H, s), 4.26 (2H, q,
J ¼ 7.1 Hz), 2.52 (3H, s), 2.36 (3H, s), 1.33 (3H, t, J ¼ 7.1 Hz); 13C NMR (75 MHz,
CDCl3) d ¼ 164.4, 157.6, 150.0, 114.2, 106.3, 60.0, 14.5, 13.7, 13.2.

376
G. WANG ET AL.
ACKNOWLEDGMENTS
Financial support from the Scientific Research Foundation for the Returned
Overseas Chinese Scholars of the State Education Ministry, the Natural Science
Foundation Project of Chongqing, Chongqing Science & Technology Commission
(CQ CSTC) (2006BB5377), the High-Tech Training Fund of Southwest University
(XSGX0601), and the Doctoral Fund of Southwest University (SWNUB2005012)
is gratefully acknowledged.
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