Sequential aldol/Michael addition reaction in ionic liquid catalyzed by morpholine: A convenient synthesis of 1,3,5-triaryl-1,5-pentanedione

Article abstract of DOI:10.1080/00397910500514089

A series of 1,3,5-triaryl-1,5-pentanediones was obtained via a sequential aldol condensation and Michael addition reaction in ionic liquid catalyzed by morpholine, as a one-pot reaction. The significance of our findings relates to reducing the use of orga

Full text of DOI:10.1080/00397910500514089

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Sequential Aldol/Michael Addition
Reaction in Ionic Liquid Catalyzed by
Morpholine: A Convenient Synthesis of
1,3,5‐Triaryl‐1,5‐pentanedione
Leilei Lu a , Yang Shen a , Yu Wan a , Kaibei Yu c & Hui Wu a b
a Department of Chemistry , Xuzhou Normal University , Xuzhou, Jiangsu,
China
b Key Laboratory of Biotechnology on Medical Plants of Jiangsu Province ,
Xuzhou, Jiangsu, China
c Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences ,
Chengdu, Sichuan, China
Published online: 24 Feb 2007.
To cite this article: Leilei Lu , Yang Shen , Yu Wan , Kaibei Yu & Hui Wu (2006) Sequential Aldol/
Michael Addition Reaction in Ionic Liquid Catalyzed by Morpholine: A Convenient Synthesis of
1,3,5‐Triaryl‐1,5‐pentanedione, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 36:9, 1193-1200, DOI: 10.1080/00397910500514089
To link to this article: http://dx.doi.org/10.1080/00397910500514089
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Synthetic Communicationsw, 36: 1193–1200, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500514089
Sequential Aldol/Michael Addition
Reaction in Ionic Liquid Catalyzed by
Morpholine: A Convenient Synthesis of
1,3,5-Triaryl-1,5-pentanedione
Hui Wu
Department of Chemistry, Xuzhou Normal University, Xuzhou,
Jiangsu, China and Key Laboratory of Biotechnology on Medical Plants
of Jiangsu Province, Xuzhou, Jiangsu, China
Leilei Lu, Yang Shen, and Yu Wan
Department of Chemistry, Xuzhou Normal University, Xuzhou,
Jiangsu, China
Kaibei Yu
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences,
Chengdu, Sichuan, China
Abstract: A series of 1,3,5-triaryl-1,5-pentanediones was obtained via a sequential
aldol condensation and Michael addition reaction in ionic liquid catalyzed by morpho-
line, as a one-pot reaction. The significance of our findings relates to reducing the use of
organic solvents, potentially toxic and hazardous materials, as well as its simplicity,
good yields, mild conditions, and lower costs.
Keywords: Ionic liquid, morpholine, synthesis, 1,3,5-triaryl-1,5-pentanedione
INTRODUCTION
Krohnke-type pyridines[1] and other substituted pyridines including the terpyr-
¨
idines[2,3] are prominent building blocks in supramolecular chemistry, with
Received in Japan August 11, 2005
Address correspondence to Hui Wu, Department of Chemistry, Xuzhou Normal
University, Xuzhou, 221116, China. E-mail: wuhui72@yahoo.com.cn
1193

1194
H. Wu et al.
their p-stacking ability, directional H-bonding, and coordination. They are
also the useful intermediates in the syntheses of pesticides, desiccants,
surfactants, and so forth.[4,5]
In general, 1,3,5-triaryl-1,5-pentanedione was used as the important
precursor in their synthesis. But the general methods used in the synthesis
of 1,3,5-triaryl-1,5-pentanedione always use volatile organic solvents and
display only moderate to low yields.[2,6,7] To adopt the principles of green
chemistry,[8] Yang[9] and coworkers obtained the target molecule in water
catalyzed by NaOH through two steps. Gareth[10] and coworkers developed
a solvent-free route to such compounds, but the strong base NaOH was also
used as catalyst.
In these syntheses, acetophenone substituted by an electron-withdrawing
group (such as 4-acetylpyridine) and p-substituted ones (such as 4-methylace-
tophenone), which have smaller steric effects, were used. Probably because of
the deactivation and steric effect of hydroxyl group on the carbonyl group, this
kind of reaction (including 20-hydroxyacetophenone) is difficult and has never
been reported. As one hydroxyl group is added on the benzene ring, the yield
will reduce 30%–40%.[11]
But we have to conquer this difficulty because the hydroxyl group is a
general group in the structure of most natural products and often displays bio-
logical activity.[12] Our basic idea in this communication is to develop a green,
cheap, conventional synthesis of 1,3,5-triaryl-1,5-pentanedione including
hydroxyl groups.
Recently, there have been several reports on the new versions of Michael
additions in ionic liquids; for example, Xuesen Fan and coworkers[13] reported
the Knoevenagel and Michael reactions using the ionic liquid [bmim][BF4],
which was also used in this article.
Ionic liquids have been the subject of considerable current interest as
environmentally benign reaction media in organic synthesis because of their
unique properties of nonvolatility, nonflammability, recyclability, and
ability to dissolve a wide range of materials.[14]
We discovered that diethylamine (pKb ¼ 2.89) can promote the reaction
of p-cyanbenzaldehyde and 20-hydroxyacetophenone to produce 1,3,5-triaryl-
1,5-pentanedione. Do other weak bases also have this kind of ability? Morpho-
line, a reagent that had never been used as a catalyst, was sought out for the
contrasting experiment.
We first established that using morpholine, a cheap and facile weak base
(pKb ¼ 5.51), as the catalyst involving sequential aldol and Michael addition
reactions of aromatic aldehydes with 20-hydroxyacetophenone in one pot,
and the ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate
([bmim][BF4]), a colorless, flavorless, nonvolatile liquid,[15] was used as the
solvent with 58–80% yield. Overall, this versatile new approach can be
applied to the synthesis of a range of pyridines in general bearing aryl
groups on the 2, 4, and 6 positions. The significance of our finding also
relates to reducing the use of organic solvents, potentially toxic and

1,3,5-Triaryl-1,5-pentanedione
1195
hazardous materials, as well as its simplicity, mild conditions, and lower costs.
Moreover, the findings are further evidence that it is possible to synthesize
activated natural products and analogous natural products.
The signal crystal structure of product 3a (Figure 1 and Scheme 1) was
established on the basis of spectroscopic data and confirmed by X-ray diffrac-
tion studies. (The crystallographic data are given in the Experimental section.)
We used seven other amines as the catalysts in the reaction of p-cyanbenz-
aldehyde and 20-hydroxyacetophenone, and the yields were lower than the
reaction catalyzed by morpholine (Table 1). We chose morpholine as the
catalyst throughout. Why didn’t the Mannish reaction take place in this
reaction? We speculated that the reason was that the ionic liquid weakened
the nucleophilicity of the amines. That proves that ionic liquid has good
selectivity and superiority, as it is used as the reaction medium. The effect
of different ionic liquids on the yields was investigated in the reaction of
p-cyanbenzaldehyde and 20-hydroxyacetophenone (Table 2).
Table 1 indicates that there was no direct relationship between yields and
the pKb of amines. Even weaker bases such as aniline can catalyze this
reaction. We used morpholine as the catalyst throughout. Table 2 indicates
that with increasing basicity of the anion (increasing pKa of the corresponding
Figure 1. X-ray crystal structure of 3a.

1196
H. Wu et al.
Scheme 1.
acid), there is a progressive increase in the yield, so we chose [bmim][BF4]
throughout. The yields of the products are shown in Table 3.
The results shown in Table 3 indicate that the steric hindrance of the
adjacent group of aromatic aldehydes influenced the yields more than the
other ones, and the electronic effect of the substitutes almost has no
influence on the yields.
EXPERIMENTAL
Melting points were measured with a Fisher-Johns melting-point apparatus
without correction. IR spectra were recorded on a Nicolet Nexus 670 spec-
trometer in KBr. The proton nuclear magnetic resonance (1H NMR) spectra
were measured on a Bruker AM-400 spectrometer with Me4Si (TMS) as the
internal reference and CDCl3 as solvent. X-Ray diffraction was measured
on a Siemens P4 diffractometer with graphite monochromated MoKna
radiation.
Silica gel (200–400 mesh), from Qingdao Ocean Chemical Co. Ltd.
(China), was used for column and thin-layer chromatography. The other
reagents were all analytical pure.
The ionic liquid [bmim][BF4] was synthesized according to the
literature.[16]
Table 1. Effect of amines on the yields of the corresponding
reactions
Amines
pKb (25 8C)
Yield (%)
Aniline
p-Methyl-aniline
Morpholine
9.40
8.92
5.51
4.70
4.50
3.38
3.35
2.89
59.0
63.0
78.6
48.7
53.9
60.4
52.6
51.3
Ammonium acetate
Cholamine
Methylamine
Cyclopentamine
Diethylamine

1,3,5-Triaryl-1,5-pentanedione
Table 2. Effect of ionic liquid on the yield of 3a
1197
IL
pKaa
Isolated yield (%)
[bmimb][Br]
[bmim][BF4]
[bmim][ClO4]
[bpc] [Br]
29
0.5
44.0
78.6
9.1
211
29
0.5
211
23.6
33.6
8.1
[bp][BF4]
[bp][ClO4]
aThe pKa value of the parent acid of the anions.
b[bmim] ¼ 1-n-butyl-3-methylimidazolium.
c[bp] ¼ 1-n-butylpyridinium.
General Procedure
A dry 50-mL flask was charged with aromatic aldehydes (1) (2 mmol), 20-
hydroxyacetophenone (2) (4 mmol), morpholine (1 mmol), and [bmim][BF4]
(2 mL). The mixture was stirred at 808C for 8–20 h. Then the system was
poured into the water, and the precipitate was washed with water 2–3 times
and purified by recrystallization from absolute EtOH and DMF to give (3).
Data
1
3a: IR (KBr, n, cm21): 3320, 3025, 2950, 1680, 1600, 1265, 2223 (C N); H
;;
NMR (CDCl3, d, ppm): 11.63 (s, 2H, –OH), 7.87–7.89 (dd, 2H, 3J ¼ 8.0 Hz,
4J ¼ 1.6 Hz, Ar-H), 7.71–7.73 (d, 2H, J ¼ 8 Hz, Ar-H), 7.56–7.58 (d, 2H,
J ¼ 8.4 Hz, Ar-H), 7.48–7.52 (m, 2H, Ar-H), 6.93-6.96 (m, 4H, Ar-H),
3.95–4.03 (m, 1H, –CH), 3.54–3.60 [m, 4H, –CH(CH2)2].
Table 3. Physical properties and yields of the products (IL: [bmim][BF4])
Molecular
formula
Entry
Ar
Mp (8C)
Yield (%)
Time (h)
3a
3b
3c
3d
3e
3f
4-CNC6H4
4-ClC6H4
C24H19NO4
C23H19O4Cl
C23H19O4Br
C24H22O4
162.7–163.2
205.2–205.9
110.5–111.7
205.9–207.0
195.0–199.4
114.5–118.6
135.1–138.2
143.4–146.5
158.4–160.4
162.9–163.4
78.6
80.3
79.5
68.7
75.6
70.4
60.4
65.8
57.6
58.9
8.5
18.0
17.5
19.0
9.5
4-BrC6H4
4-CH3C6H4
4-NO2C6H4
C23H19NO6
4-OCH3C6H4 C24H22O5
20.0
13.5
17.0
18.5
20.0
3g
3h
3i
3-NO2C6H4
2-ClC6H4
2-BrC6H4
C23H19NO6
C23H19O4Cl
C23H19O4Br
3j
2-OCH3C6H4 C24H22O5

1198
H. Wu et al.
3b: IR (KBr, n, cm21): 3323, 3046, 2975, 1685, 1612, 1260, 1092 (CAr-Cl); H1
NMR (CDCl3, d, ppm): 11.46 (s, 2H, –OH), 8.12–8.14 (d, 2H, J ¼ 8.0 Hz,
Ar-H), 7.86–7.78 (dd, 2H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, Ar-H), 7.66–7.68
(d, 2H, J ¼ 8.0 Hz, Ar-H), 7.49–7.52 (m, 2H, Ar-H), 6.93–6.97 (m, 4H,
Ar-H), 4.00–4.10 [m, 1H, –CH(CH2)2], 3.62–3.64 [m, 4H, –CH(CH2)2].
3c: IR (KBr, n, cm21): 3300, 3025, 2970, 1663, 1597, 1253, 1073 (CAr-Br); H1
NMR (CDCl3, d, ppm): 11.52 (s, 2H, –OH), 8.10–8.12 (d, 2H, J ¼ 8.0 Hz,
Ar-H), 7.85–7.87 (dd, 2H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, Ar-H), 7.64–7.66
(d, 2H, J ¼ 8.0 Hz, Ar-H), 7.48–7.51 (m, 2H, Ar-H), 6.91–6.95 (m, 4H,
Ar-H), 3.90–4.00 [m, 1H, –CH(CH2)2], 3.62–3.64 [m, 4H, –CH(CH2)2].
3d: IR (KBr, n, cm21): 3296, 3030, 2980, 1665, 1590, 1254, 1600 (CAr-Cal); H1
NMR (CDCl3, d, ppm): 12.35 (s, 2H, –OH), 7.92–7.94 (d, 2H, J ¼ 8.0 Hz, Ar-
3
4
H), 7.85–7.87 (dd, 2H, J ¼ 8.0 Hz, J ¼ 1.2 Hz, Ar-H), 7.62–7.64 (d, 2H,
J ¼ 8.0 Hz, Ar-H), 7.46–7.49 (m, 2H, Ar-H), 6.89–6.93 (m, 4H, Ar-H), 4.00–
4.10 [m, 1H, –CH(CH2)2], 3.38–3.40 [m, 4H, –CH(CH2)2], 2.51 (s, 3H, –CH3).
3e: IR (KBr, n, cm21): 3325, 3035, 2963, 1680, 1584, 1255, 1341 (CAr-NO2);
H1 NMR (CDCl3, d, ppm): 11.63 (s, 2H, –OH), 8.11–8.13 (d, 2H, J ¼ 8.8 Hz,
Ar-H), 7.87–7.90 (dd, 2H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, Ar-H), 7.65–7.67 (d, 2H,
J ¼ 8.8 Hz, Ar-H), 7.48–7.50 (m, 2H, Ar-H), 6.93–6.96 (m, 4H, Ar-H),
4.00–4.11 m, 1H, –CH(CH2)2, 3.62–3.65 [m, 4H, –CH(CH2)2].
3f: IR (KBr, n, cm21): 3315, 3022, 2960, 1696, 1575, 1258, 1055 (CAr-
OCH3); H1 NMR (CDCl3, d, ppm): 12.01 (s, 2H, –OH), 7.84–7.86 (d, 2H,
J ¼ 8.0 Hz, Ar-H), 7.76–7.78 (dd, 2H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, Ar-H),
7.62–7.64 (d, 2H, J ¼ 8.0 Hz, Ar-H), 7.44–7.47 (m, 2H, Ar-H), 6.87–6.91
(m, 4H, Ar-H), 3.92–4.03 [m, 1H, –CH(CH2)2], 3.56–3.58 [m, 4H, –
CH(CH2)2], 3.34 (s, 3H, –OCH3).
3g: IR (KBr, n, cm21): 3310, 3018, 2978, 1678, 1577, 1263, 1351 (CAr-NO2);
H1 NMR (CDCl3, d, ppm): 12.12 (s, 2H, –OH), 7.85–7.87 (dd, 2H,
4
3J ¼ 8.0 Hz, J ¼ 1.2 Hz, Ar-H), 7.41–7.51 (m, 2H, Ar-H), 7.39–7.42 (dd,
1H, 3J ¼ 8.0 Hz, 4J ¼ 1.2 Hz, Ar-H), 7.33–7.35 (dd, 1H, 3J ¼ 8.0 Hz,
4J ¼ 1.2 Hz, Ar-H), 7.21–7.24 (dd, 2H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, Ar-H),
7.18–7.20 (dd, 1H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, Ar-H), 6.70–6.98 (d, 3H,
J ¼ 8.4 Hz, Ar-H), 6.90–6.94 (t, 1H, Ar-H), 4.56–4.59 (m, 1H, –
CH(CH2)2), 3.50–3.56 (m, 4H, –CH(CH2)2).
3h: IR (KBr, n, cm21): 3306, 3037, 2969, 1673, 1600, 1248, 1036 (CAr-Cl);
H1 NMR (CDCl3, d, ppm): 11.67 (s, 2H, –OH), 7.90–7.93 (dd, 2H,
3J ¼ 8.0 Hz, 4J ¼ 1.6 Hz; Ar-H), 7.54–7.56 (dd, 2H, 3J ¼ 8.0 Hz,
4J ¼ 1.2 Hz, Ar-H), 7.48–7.53 (m, 2H, Ar-H), 7.37–7.39 (dd, 2H,
4
3J ¼ 8.0 Hz, J ¼ 1.2 Hz, Ar-H), 7.24–7.28 (m, 2H, Ar-H), 7.18–7.21 (dd,

1,3,5-Triaryl-1,5-pentanedione
1199
3
4
2H, J ¼ 8.0 Hz, J ¼ 1.6 Hz, Ar-H), 6.92–6.96 (m, 4H, Ar-H), 4.35–4.45
[m, 1H, –CH(CH2)2), 3.53–3.55 (m, 4H, –CH (CH2)2].
3i: IR (KBr, n, cm21): 3295, 3024, 2958, 1683, 1592, 1267, 1038 (CAr-Br); H1
NMR (CDCl3, d, ppm): 12.11 (s, 2H, –OH), 7.85–7.87 (dd, 2H, 3J ¼ 8.0 Hz,
4J ¼ 1.6 Hz, Ar-H), 7.59–7.61 (dd, 2H, 3J ¼ 8.0 Hz, 4J ¼ 1.2 Hz, Ar-H),
7.47–7.51 (m, 2H, Ar-H), 7.30–7.33 (m, 2H, Ar-H), 7.09–7.13 (m, 2H,
Ar-H), 6.97–7.00 (dd, 2H, 3J ¼ 8.4 Hz, 4J ¼ 1.2 Hz, Ar-H), 6.90–6.94
(m, 2H, Ar-H), 4.56–4.60 (m, 1H, –CH(CH2)2), 3.46–3.59 [m, 4H, –
CH(CH2)2].
3j: IR (KBr, n, cm21): 3315, 3033, 2960, 1680, 1584, 1270, 1063
(CAr-OCH3); H1 NMR (CDCl3, d, ppm): 11.97 (s, 2H, –OH), 7.85–7.87
(dd, 2H, 3J ¼ 8.4 Hz, 4J ¼ 1.6 Hz, Ar-H), 7.38–7.46 (m, 3H, Ar-H),
7.28–7.30 (dd, 1H, 3J ¼ 8.0 Hz, 4J ¼ 1.2 Hz, Ar-H), 7.21–7.25
(m, 1H, Ar-H), 6.95–7.00 (m, 1H, Ar-H), 6.86–6.90 (m, 4H, Ar-H),
3.89–3.96 [m, 1H, –CH(CH2)2], 3.48–3.52 [m, 4H, –CH(CH2)2], 3.54
(s, 3H, –OCH3).
Crystallographic Data of 3a
Empirical formula: C24H19NO4; formula weight: 385.40; shape/color:
˚
dimetric/brown; temperature: 292 (2) K; wavelength: 0.71073 A; crystal
system, space group: monoclinic, P2(1)/c; unit cell dimensions: a ¼ 9.114
˚
˚
˚
3
(2) A; a ¼ 90.008, b ¼ 21.023 (4) A, b ¼ 106.988 (2), c ¼10.628 (2) A,
3
˚
g ¼ 90.008; volume: 1947.4 (7) A ; Z, calculated density: 4, 1.314 g/cm ;
absorption coefficient: 0.090 mm21; F (000): 808; crystal size: 0.50 ꢀ 0.38
ꢀ0.16 mm; u range for data collection: 1.94 to 25.508; limiting indices:
0 ꢁ h ꢁ 11, 0 ꢁ k ꢁ 25, 212 ꢁ l ꢁ 12; reflections collected/unique: 4125/
1500 [R (int) ¼ 0.0273]; absorption correction: none; refinement method:
full-matrix least-squares on F2; data/restraints/parameters: 3620/2/271;
goodness of fit on F2: 0.839; final R indices [I . 2s (I)]: R1 ¼ 0.0399,
wR2 ¼ 0.0512; final R indices [I . 2s (I)]: R1 ¼ 0.1221, wR2 ¼ 0.0591;
23
˚
largest diff. peak and hole: 0.123 and 20.129 e. A
.
CONCLUSION
In summary, we have demonstrated the application of a multicomponent
reaction for the synthesis of a series of 1,3,5-triaryl-1,5-pentanedione deriva-
tives in good yields, which have developed a facile synthesis of 2,4,6-triphe-
nylpyridine derivatives. We first used the amines in this sequential aldol and
Michael reactions in ionic liquid as a mild condition.

1200
H. Wu et al.
ACKNOWLEDGMENT
We are grateful to the foundation of the Natural Science Research Project of
University in Jiangsu Province (No. JH03-038) and Hi-Tech Development
Project of University in Jiangsu Province (No. 03KJD150213) for financial
support.
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