A recyclable fluoroalkylated 1,4-disubstituted [1,2,3]-triazole organocatalyst for aldol condensation of aldehydes and ketones

Article abstract of DOI:10.1016/j.jfluchem.2010.11.004

A new fluoroalkylated 1,4-disubstituted [1,2,3]-triazole was prepared and acted as an organocatalyst for aldol condensation of different ketones with various aldehydes. Aldol condensation proceeded efficiently in the presence of catalytic amount of fluoroalkylated [1,2,3]-triazole. The catalyst could be easily recovered by fluorous solid-phase extraction with excellent purity and reused for three runs with slightly decrease in its activity.

Full text of DOI:10.1016/j.jfluchem.2010.11.004

Journal of Fluorine Chemistry 132 (2011) 71–74
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Journal of Fluorine Chemistry
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Short communication
A recyclable fluoroalkylated 1,4-disubstituted [1,2,3]-triazole organocatalyst
for aldol condensation of aldehydes and ketones
*
Yi-Wei Zhu, Wen-Bin Yi , Chun Cai
School of Chemical Engineering, Nanjing University of Science and Technology, 200 Xiao Ling Wei Street, Nanjing, Jiangsuang 210094, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history:
A new fluoroalkylated 1,4-disubstituted [1,2,3]-triazole was prepared and acted as an organocatalyst for
aldol condensation of different ketones with various aldehydes. Aldol condensation proceeded
efficiently in the presence of catalytic amount of fluoroalkylated [1,2,3]-triazole. The catalyst could be
easily recovered by fluorous solid-phase extraction with excellent purity and reused for three runs with
slightly decrease in its activity.
Received 17 September 2010
Received in revised form 18 October 2010
Accepted 9 November 2010
Available online 16 November 2010
Crown Copyright ß 2010 Published by Elsevier B.V. All rights reserved.
Keywords:
Fluoroalkylated 1,4-disubstituted triazole
Aldol condensation
Fluorous solid-phase extraction
1. Introduction
In the course of our extensive synthetic and physical
investigations involving substituted [1,2,3]-triazole 1 (Fig. 1),
Currently, due to low toxicity, operational simplicity and
efficiency, compared to traditional metal-based catalysts, there is
much interest in organocatalysts [1]. However, their drawbacks
also have been realized. The major limitations using organocatalyst
catalyzed reactions are high catalyst loadings (up to 20 mol%) and
the difficulty of recovering the catalyst [2]. An alternative strategy
is to design recyclable and subsequently reusable versions of
organocatalysts [3].
In 1997, Curran’s group reported the first example of
application of solid–liquid separations based on fluorous silica
gel [4]. They found that these separations were operationally
convenient and were applicable to substances that contain only
relatively few fluorine atoms such as light-fluorous reagent. This is
a key finding since light-fluorous reagents and catalysts have
chemical and physical properties that are similar to their non-
fluorous counterparts. Recently, fluorous organocatalysts have
emerged as attractive tools in Michael addition, Diels–Alder
reaction as well as 1,3-dipolar cycloaddition with high recyclabili-
ty [5]. Compared to recyclable heterogeneous organocatalysts, the
fluorous organocatalysts are soluble in common reaction solvents,
yet they can be easily separated and recovered from the reaction
mixture by fluorous solid-phase extraction (F-SPE) [6–8].
also, our group inspired by fluorous tag idea [9] and recent reports
on the N-methylimidazole as a Lewis base catalyst for the aldol
condensation of trimethylsilyl enolate with aldehyde [10,11]. As in
the continuation of our work on the fluorous catalysts for C–C bond
formation reaction, we tried to examine whether the substituted
[1,2,3]-triazoles could catalyze this type reaction efficiently. So we
have applied the fluoroalkylated [1,2,3]-triazole
2 (Fig. 2)
organocatalyst to one of the most fundamental and important
C–C bondforming reactions—the aldol condensation of ketones
with aldehydes, which are usually catalyzed by strong acids or
bases, and various Lewis acids [12]. To our delight, the catalyst
exhibited excellent catalytic activity in aldol condensation.
2. Results and discussion
The fluoroalkylated 1,4-disubstituted [1,2,3]-triazoles 2, 3 and 4
(Fig. 2) were synthesized via the 1,3-dipolar cycloaddition of
fluoroalkylated azides with terminal alkynes in the presence of
Cu(I) salt as catalyst at room temperature (Scheme 1). Similar
article was described by Yong-Ming Wu and co-workers, while we
did not obtain what we want in this way [13,14]. The fluorous
catalyst is easily separated by fluorous solid-phase extraction (F-
SPE) and can be reused for several times without a significant loss
of catalytic activity.
We first carried out a model reaction involving benzaldehyde,
acetophenone, and fluorous catalysts to optimize the reaction
conditions. As could be seen from Table 1, both the structure of
* Corresponding author. Tel.: +86 25 84315514; fax: +86 25 84315030.
E-mail address: yiwenbin@mail.njust.edu.cn (W.-B. Yi).
0022-1139/$ – see front matter. Crown Copyright ß 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2010.11.004

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Y.-W. Zhu et al. / Journal of Fluorine Chemistry 132 (2011) 71–74
R1
Table 1
Exploration of fluoroalkylated 1,4-disubstituted [1,2,3]-triazole organocatalyst
promoted aldol reaction of benzaldehyde with acetophenonea.
[DT$INLE]
R2
N
N
N
1
Fig. 1. Fluoroalkylated 1,4-disubstituted [1,2,3]-triazole.
[()TD$FIG]
Entry
Catalyst (mol%)
Temperature (8C)
Time (h)
Yield (%)b
1
2
3
4
5
6
2(10)
3(10)
4(10)
2(5)
100
100
100
100
120
80
16
16
16
16
16
16
91
87
82
85
90
84
2(10)
2(10)
a
The reaction condition: benzaldehyde, 4 mmol; acetophenone, 5 mmol;
ethanol, 6 mL.
b
Isolated yield based on benzaldehyde.
Table 2
Fluoroalkylated 1,4-disubstituted [1,2,3]-triazole 2 catalyzed aldol condensationa.
[TD$INLINE]
Scheme 1. Preparation of fluoroalkylated 1,4-disubstituted [1,2,3]-triazole
organocatalysts 2, 3, 4.
catalyst and the temperature had a pronounced effect on the yield.
The catalysts 2, 3 and 4 were investigated (Table 1, entries 1–3). As
a result, 2 was found to be the most effective structure. The effect of
catalyst loading was also evaluated (Table 1, entries 4 and 5), When
5 mol% of catalyst 2 was used, the reaction was accelerated and
afforded the product in 85% yield, while 10 mol% of catalyst was
found to be most efficient. Next, different reaction temperature
was studied (Table 1, entries 4–6), the best temperature was found
at 100 8C. Thus we selected 2 as the catalyst, 100 8C as the reaction
temperature and 10 mol% of catalyst which are the best condition
for the aldol condensation.
Entry
R3
R4
R5
Time (h)
Yield (%)b
1
2
Ph
H
H
H
H
4-Cl-Ph
4-CH3O-Ph
4-CH3-Ph
4-NO2-Ph
Ph
16
16
16
16
20
20
20
10
10
20
20
16
16
91
66
74
94
91
84
95
96
90
70
92
79
74
Ph
3
Ph
4
Ph
5
(CH2)6
(CH2)6
(CH2)6
(CH2)5
(CH2)5
4-NO2-Ph
4-CH3O-Ph
CH3
6
4-CH3O-Ph
4-NO2-Ph
4-NO2-Ph
4-CH3O-Ph
Ph
7
8
9
10
11
12
13
H
H
H
H
To demonstrate the scope of the reaction, a series of aldehydes
and ketones were used for aldol condensation. As revealed in Table
2, all the reactions proceeded efficiently to furnish the products in
good to excellent yields (66–96%). The condensation products were
Ph
Ph
CH3
4-CH3O-Ph
a
The reaction condition: aldehyde, 4 mmol; ketone, 5 mmol; ethanol, 6 mL;
isolated and identified as
a,b-unsaturated ketones, and no side-
100 8C.
b
reactions were observed. Based on 1H NMR and GC–MS data, the
reaction was found to give the E-stereoisomer as the sole product.
Significant structural variation of aldehydes, which possess both
electron withdrawing and donating groups could efficiently
participate in the process. The condensations of p-substituted
benzaldehyde with p-substituted acetophenones yielded the
corresponding chalcones and the product yields were remarkably
affected by the substituent groups of either aldehydes or ketones:
reactants having electron-withdrawing substituents gave high
yields and those having electron-donating ones gave low yields.
The cross-condensations of cyclopentanone and cyclohexanone
with different aromatic aldehydes were finished within 10–20 h
and could give good yields (84–96%). Reactions of acetone with
different aromatic aldehydes were also been studied, good yields
could be obtained of 74–79%.
Isolated yield based on the aldehyde.
After 16 h, condensation product was obtained in 91% isolated
yield by the reaction of benzaldehyde and acetophenone. As
summarized in Fig. 3, good yields could maintaine for 3 cycles.
Benzaldehyde and acetophenone were combined in ethanol at
room temperature. The sample was then heated to 100 8C and
became homogeneous. After stirring for 16 h, the mixture was
concentrated and then loaded onto a FluoroFlash1 silica gel
cartridge (2 g) for F-SPE, eluted by 75% methanol at first for non-
fluorous component, methanol was then added onto the fluorous
silica gel column continuously for obtaining the elutant of
fluoroalkylated triazole organocatalyst 2. When the bulk of solvent
was removed and product was dried in vacuo at 50 8C for 8 h, the
[()TD$FIG]
Fig. 2. Fluoroalkylated triazole organocatalysts.

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Y.-W. Zhu et al. / Journal of Fluorine Chemistry 132 (2011) 71–74
73
fluoroalkylated 1,4-disubstituted [1,2,3]-triazole 2 as a white
solid (1.099 g, 1.86 mmol, 62%).
100
80
60
40
20
0
4.2.1. 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluorodecyl)-
4-phenyl-1H-1,2,3-triazole
White solid m.p. 147–150 8C; 1H NMR (500 MHz, DMSO)
d
91
89
84
7.84 (d, J = 8 Hz, 2H, Ar-H), 7.82 (s, 1H, Tr-H), 7.42 (t, J = 8 Hz, 2H,
Ar-H), 7.35 (t, J = 8 Hz, 1H, Ar-H), 4.71 (t, J = 7 Hz, 2H, Tr.CH2),
2.88 (m, 2H, CH2CF2); 13C NMR (125 MHz, DMSO) 148.2 (C),
130.2 (C), 129.2 (CH), 128.4 (CH), 126.1 (CH), 120.2 (CH), 42.6
(CH2), 32.2 (CH2) (t, 2JFC = 21 Hz); IR (KBr) 3123, 3094, 1339,
1
2
3
1206, 1146, 1115, 1098, 986, 970, 768, 706, 693, 678, 663 cmꢁ1
;
Run
MS (EI, 70 eV) m/z (rel intensity) 591 (M+, 20), 572 ([M-F]+, 15)
563 ([M-N2]+, 62), 144 ([M-C8F17 CH2.CH2]+, 30), 116 ([M-
C8F17.CH2.CH2-N2]+, 100).
Fig. 3. Recycling and reuse of organocatalyst 2 in promoting the reaction of
benzaldehyde and acetophenone.
4.3. Typical procedure for organocatalyst 2 catalyzed the aldol
solid could be reused for the next reaction. As would be expected,
the fluorous catalyst can be reused for three times without a
significant loss of yield.
reaction
Acetophenone (5 mmol) and fluoroalkylated 1,4-disubstituted
[1,2,3]-triazole organocatalyst 2 (0.236 g, 0.4 mmol) in 6 mL of
ethanol was stirred at room temperature for 20 min. Then
benzaldehyde (4 mmol) was added and the resulting mixture
was stirred at 100 8C for 16 h. The crude product was purified by
column chromatography on silica gel, eluted by 75% methanol then
methanol to give 0.757 g (91%) of the desired product as a
yellowish solid.
3. Conclusions
In summary, a new fluoroalkylated 1,4-disubstituted [1,2,3]-
triazole organocatalyst has been developed. The catalyst shows
high activity in aldol condensation. Also, the catalyst can be easily
recovered by fluorous solid-phase extraction with excellent purity.
Application of this catalyst for other reactions is under investiga-
tion and will be reported in due course.
4.3.1. Chalcone
A yellowish solid; m.p. 56–58 8C (lit. [15] 57–58 8C). IR (KBr)
3231, 2932, 1830, 1731, 1654, 1286, 752, 681 cmꢁ1 1H NMR
.
4. Experimental
(500 MHz, CDCl3) d 6.12 (d, J = 16.0 Hz, 1H), 7.24 (d, J = 16.0 Hz,
1H), 7.09–7.31 (m, 5H), 7.39–7.92 (m, 5H). MS (EI) m/z 207 (M+).
4.1. General remarks
4.3.2. 4-Chlorochalcone
Melting points were obtained with Shimadzu DSC-50 thermal
analyzer. 1H NMR, 13C NMR and 19F NMR spectra were character-
ized with a Bruker Advance RX500 spectrometer. IR spectra were
recorded in KBr disks with a SHIMADZU IRPrestige-21 FT-IR
spectrometer. Mass spectra were recorded on a Saturn 2000GC/MS
instrument. All chemicals were reagent grade and used as
purchased without further purifications.
A yellow solid; m.p. 114–115 8C (lit. [15] 114–117 8C). IR (KBr)
3080, 2908, 1780, 1713, 1651, 1205, 913, 748 cmꢁ1 1H NMR
.
(500 MHz, CDCl3)
d 6.20 (d, J = 16.0 Hz, 1H), 7.12 (d, J = 16.0 Hz,
1H), 6.23 (d, 2H), 7.10–7.34 (m, 4H), 7.36–8.01 (m, 5H). MS (EI) m/z
244 (M++2), 242 (M+).
4.3.3. 4-Methoxychalcone
A russety solid; m.p. 75–76 8C (lit. [15] 75–77 8C). IR (KBr) 3302,
4.2. General procedure for the fluoroalkylated 1,4-disubstituted
[1,2,3]-triazole organocatalyst
2979, 1830, 1722, 1661, 1185, 929, 815 cmꢁ1 1H NMR (500 MHz,
.
CDCl3)
d 3.72 (s, 3H), 5.96 (d, J = 16.0 Hz, 1H), 6.85 (d, J = 16.0 Hz,
1H), 6.92–7.21 (m, 4H), 7.32–7.99 (m, 5H). MS (EI) m/z 238 (M+).
Fluoroalkyl tosylate (I, 3.098 g, 5 mmol) was added to a
solution of NaN3 (0.650 g, 10 mmol) in a mixed solvent of DMF
(15 mL) and benzene (15 mL) at 70 8C. The mixture was stirred
for 20 h, then cooled down to 50 8C, poured into 100 mL ice
water, extracted with ether (3ꢀ 40 mL). The combined organic
layer was washed with saturated brine (2ꢀ 50 mL) and dried by
magnesium sulfate, after removal of the solvent, the crude
product was purified by distillation under atmospheric or
reduced pressure. So the fluoroalkylazide (II, 2.125 g, 71.3%)
was in hand.
To a 25 mL 3-necked round flask, fluoroalkylazide (3 mmol),
phenylacetylene (3.3 mmol), triethylamine (3.3 mmol), acetoni-
trile (4 mL), water (8 mL) and CuI (6 mg, 0.03 mmol) were added
successively. The mixture was stirred at room temperature for
20 h. Then 10 mL water was added, the mixture was extracted
with ether (3ꢀ 10 mL). The combined organic layer was washed
with saturated brine and dried with sodium sulfate. After
removal of solvent under reduced pressure, the crude product
was purified by flash chromatography on a silicon gel column
with hexane/ethyl acetate (v/v: 5:1) as eluent to give
4.3.4. 4-Methylchalcone
A yellowish solid; m.p. 98 8C (lit. [15] 97–98 8C). IR (KBr) 3210,
2915, 1760, 1696, 1651, 1086, 942, 842, 722 cmꢁ1 1H NMR
.
(500 MHz, CDCl3)
d 2.31 (s, 3H), 6.01 (d, J = 16.4 Hz, 1H), 7.04 (d,
J = 16.4 Hz, 1H), 7.05–7.36 (m, 4H), 7.38–7.82 (m, 5H). MS (EI) m/z
221 (M+).
4.3.5. 4-Nitrochalcone
A brown solid; m.p. 158–159 8C (lit. [15] 158–160 8C). IR
(KBr) 3235, 2908, 1871, 1673, 1660, 1476, 922, 834 cmꢁ1 1H
.
NMR (500 MHz, CDCl3)
J = 16.1 Hz, 1H), 7.36–7.69 (m, 3H), 7.70–8.31 (m, 4H). MS (EI)
m/z 252 (M+).
d 6.35 (d, J = 16.1 Hz, 1H), 7.41 (d,
4.3.6. 2,6-Dibenzylidenecyclohexanone
A yellow solid; m.p. 116 8C (lit. [15] 116–117 8C). IR (KBr) 3020,
2921, 1663, 1612, 1570, 1269, 1139, 768, 689 cmꢁ1 1H NMR
.
(500 MHz, CDCl3)
d 1.74–1.86 (m, 2H), 2.94 (t, J = 6.4 Hz, 4H), 7.26–
7.46 (m, 10H), 7.80 (s, 2H). MS (EI) m/z 273 (M+).

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Y.-W. Zhu et al. / Journal of Fluorine Chemistry 132 (2011) 71–74
4.3.7. 2,6-Di(p-methoxybenzylidene)cyclohexanone
4.4. Procedure for recycling and reusing organocatalyst 2 in the aldol
reaction
A yellow solid; m.p. 202–203 8C (lit. [15] 203–204 8C). IR (KBr)
3022, 2920, 1658, 1605, 1568, 1265, 1140, 781, 687 cmꢁ1. 1H NMR
(500 MHz, CDCl3)
d
1.80–1.84 (m, 2H), 2.96 (t, J = 6.0 Hz, 4H), 3.84
Acetophenone (5 mmol) and fluoroalkylated 1,4-disubstituted
[1,2,3]-triazole organocatalyst 2 (0.236 g, 0.4 mmol) in 6 mL of
ethanol was stirred at room temperature for 20 min. Then
benzaldehyde (4 mmol) was added and the resulting mixture
was stirred at 100 8C for a specified reaction time period. The
mixture was concentrated and then loaded onto a FluoroFlash1
silica gel cartridge (2 g), eluted by 75% methanol at first for non-
fluorous component, methanol was then added onto the fluorous
gel column continuously for obtaining the elutant of fluoroalky-
lated triazole organocatalyst 2. When the bulk of solvent was
removed and product was dried in vacuo at 50 8C for 8 h, the
(s, 6H), 6.92–7.38 (m, 8H), 7.79 (s, 2H). MS (EI) m/z 333 (M+).
4.3.8. 2,6-Di(p-nitrobenzylidene)cyclohexanone
A russety solid; m.p. 158 8C (lit. [15] 159 8C). IR (KBr) 3086,
2932, 1670, 1612, 1581, 1524, 1340, 805 cmꢁ1. 1H NMR (500 MHz,
CDCl3)
d 1.86–1.93 (m, 2H), 2.99 (t, J = 5.6 Hz, 4H), 7.60–8.24 (m,
8H), 8.32 (s, 2H). MS (EI) m/z 363 (M+).
4.3.9. 2,6-Di(p-nitrobenzylidene)cyclopentanone
A russety solid; m.p. 230–233 8C (lit. [15] 230–231 8C). IR (KBr)
3108, 2850, 1708, 1602, 1525, 1344, 821 cmꢁ1. 1H NMR (500 MHz,
fluoroalkylated triazole organocatalyst
2 could be recycled
CDCl3)
d
3.05 (t, 4H), 7.61–8.13 (m, 8H), 8.27 (s, 2H). MS (EI) m/z
effectively. Furthermore, the recoverable organocatalyst could
be used directly for the next run.
349 (M+).
4.3.10. 2,6-Di(p-methoxybenzylidene)cyclopentanone
Acknowledgments
A green solid; m.p. 211–212 8C (lit. [15] 210–211 8C). IR (KBr)
2965, 2841, 1649, 1592, 1506, 1247, 1025, 830 cmꢁ1 1H NMR
.
The authors gratefully acknowledge the financial support of the
project of ‘‘NUST Excellence Initiative’’, NUST Research Funding
(2010ZYTS022), the Doctoral Fund of Ministry of Education of
China (200802881024), Jiangsu Provincial Natural Science Foun-
dation of China (sbk201040048) and National Natural Science
Foundation of China (20902047).
(500 MHz, CDCl3)
d 3.11 (t, 4H), 3.86 (s, 6H), 6.96–7.59 (m, 8H),
7.60 (s, 2H). MS (EI) m/z 319 (M+).
4.3.11. 40-Nitrochalcone
A yellow solid; m.p. 150–153 8C (lit. [15] 151–152 8C). IR
(KBr) 3010, 2895, 1708, 1681, 1642, 1468, 920, 837 cmꢁ1 1H
NMR (500 MHz, CDCl3) 6.33 (d, J = 16.1 Hz, 1H), 7.35 (d,
.
d
J = 16.1 Hz, 1H), 7.14–7.38 (m, 5H), 7.48–8.24 (m, 4H). MS (EI)
m/z 253 (M+).
References
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4.3.12. 40-Methoxychalcone
A russety solid; m.p. 109–111 8C (lit. [15] 109–110 8C). IR (KBr)
3210, 2880, 1832, 1725, 1670, 1210, 854, 730 cmꢁ1 1H NMR
.
(500 MHz, CDCl3)
d 3.78 (s, 3H), 6.10 (d, J = 16.0 Hz, 1H), 7.24 (d,
J = 16.0 Hz, 1H), 7.08–7.61 (m, 9H). MS (EI) m/z 238 (M+).
4.3.13. Benzylideneacetone
A yellowish solid; m.p. 40–42 8C (lit. [15] 41–42 8C). IR (KBr)
3108, 2960, 1835, 1720, 1663, 1180, 943, 817 cmꢁ1 1H NMR
.
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(500 MHz, CDCl3)
d 2.38 (s, 3H), 6.65 (d, J = 16.5 Hz, 1H), 7.45 (d,
J = 16.5 Hz, 1H), 7.35–7.72 (m, 5H). MS (EI) m/z 147 (M+).
4.3.14. 4-Methoxybenzylideneacetone
A yellowish solid; m.p. 44–45 8C (lit. [15] 44–46 8C). IR (KBr)
3202, 2946, 1780, 1723, 1588, 1182, 920, 826, 764 cmꢁ1 1H
.
NMR (500 MHz, CDCl3)
J = 16.0 Hz, 1H), 7.45 (d, J = 16.0 Hz, 1H), 7.09–7.32 (m, 4H). MS
(EI) m/z 178 (M+).
d 2.26 (s, 3H), 3.81 (s, 3H), 6.50 (d,