A convenient synthesis of α,α′-bis(substituted benzylidene)cycloalkanones catalyzed by Yb(OTf)3 under solvent-free conditions

Article abstract of DOI:10.1055/s-2004-834900

An array of aromatic aldehydes undergoes crossed aldol condensation with ketones in the presence of a catalytic amount of Yb(OTf)₃ under solvent-free conditions to afford the corresponding α,α′- bis(substituted benzylidene)cycloalkanones in excellent yields without occurrence of any side reactions. This method is very general, simple and environmental friendly in contrast with the existing methods, which use many classical Lewis acids. Furthermore, the catalyst can be recovered conveniently and reused efficiently without the loss of activity.

Full text of DOI:10.1055/s-2004-834900

3060
PAPER
A Convenient Synthesis of a,a¢-Bis(substituted benzylidene)cycloalkanones
Catalyzed by Yb(OTf)3 Under Solvent-Free Conditions
C
a
onvenientSyn
i
thesis
o
m
f
a
,
a
¢
-Bis(substitut
i
edbenz
n
ylidene)cycloalkan
W
ones ang,*a Jia Sheng,a He Tian,a Jianwei Han,a Zhaoyu Fan,a Changtao Qianb
Institute of Fine Chemicals, Lab for Advanced Materials, East China University of Science and Technology, 130 Meilong Lu, Shanghai
200237, P. R. China
Fax +86(21)64252758; E-mail: wanglimin@ecust.edu.cn; E-mail: wcathy@china.com
Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, P. R. China
b
Received 12 July 2004
a new type of Lewis acid, they have been applied in a wide
variety of reactions.18 The most characteristic features of
these rare earth metal triflates are that they act as water-
compatible Lewis acids in aqueous solvents, and that they
Abstract: An array of aromatic aldehydes undergoes crossed aldol
condensation with ketones in the presence of a catalytic amount of
Yb(OTf)3 under solvent-free conditions to afford the corresponding
a,a¢-bis(substituted benzylidene)cycloalkanones in excellent yields
without occurrence of any side reactions. This method is very gen- are strong Lewis acids because of the hard character of
eral, simple and environmental friendly in contrast with the existing
their metal cations and the strong withdrawing effect of
methods, which use many classical Lewis acids. Furthermore, the
the trifluoromethanesulfonyl group, so that it can activate
catalyst can be recovered conveniently and reused efficiently with-
the C–X bond to decrease the energy of transition state.18c
out the loss of activity.
Only catalytic amounts of the rare earth triflates
Key words: a,a¢-bis(substituted benzylidene)cycloalkanones,
crossed aldol condensation, Yb(OTf)3, solvent-free
[RE(OTf)3] are enough to complete the reactions in most
cases, Moreover, RE(OTf)3 can be easily recovered after
the reactions and reused without loss of activity.18c As part
of our program aiming at developing new, selective and
environmental friendly methodologies for the preparation
of fine chemicals and in continuation of our interest in lan-
thanide triflates-catalyzed organic reactions,19 we de-
scribe in this paper a novel, efficient, and high yielding
protocol for the synthesis of a,a′-bis(substituted ben-
zylidene)cycloalkanones.
Introduction
The a,a′-bis(substituted benzylidene)cycloalkanones are
very important precursors to potentially bioactive pyrimi-
dine derivates,1 intermediates of agrochemical, pharma-
ceuticals, and perfumes,2 new organic material for
nonlinear optical applications,3 cytotoxic analogous4 and
the units of liquid-crystalline polymers.5 Crossed aldol-
type reaction is available for these preparation and can be
operated with the aid of strong acid or base catalysts.
However, this condensation process suffers from reverse
and side reactions (such as self-condensation of ketones
and aldehydes) giving the corresponding products in low
yields.6,7 It has been reported that various complexes of
metal(II) ions, such as Mn(II), Fe(II), Co(II), Ni(II),
Cu(II) and Zn(II), were used as catalysts,8 but all the re-
ported yields were less than 38%. In other case, bis(p-
ethoxyphenyl)telluroxide (BOMPTO),9 RuCl3,10 SmI3,11
Cp2ZrH2,7 Cp2TiPh2,12 TiCl3(SO3CF3),13 KF–Al2O3,14
Results and Discussion
Firstly, reaction of benzaldehyde (1a; 10 mmol) and cy-
clopentanone (2a; 5 mmol) was chosen as the model reac-
tion to detect whether the use of ytterbium triflate was
efficient and to investigate the optimized conditions
(Scheme 1). The results are summarized in Table 1. In the
absence of any catalyst, the desired a,a¢-dibenzylidene cy-
clopentanone (3a) was obtained in a very low yield (7%;
Table 1, entry1) even after being stirred for 2 days. The
conventional Lewis acids such as ZnCl2, AlCl3 and FeCl3
also showed poor effect. Just as mentioned in ref.7, they
prompted the self-condensations of ketones and alde-
hydes rather than the crossed aldol reaction. The yield of
3a was less than 45% (Table 1, entries 2–6). Under similar
reaction conditions but adding 0.1mol% Yb(OTf)3 to the
system, the phenomenon of self condensations was obvi-
ously depressed, but the product yield was still not satis-
factory (Table 1, entry 7). Further studies showed that an
increase in the dosage of Yb(OTf)3 improved the reaction
yield significantly. We changed the amount of catalyst
from 0.1% to 5%, and found that 0.5 mol% was good
enough to obtain satisfactory yields (Table 1, entry 7–12).
In addition, it could also be reused four times without
showing any loss of activity (Table 1, entry 13). The fact
FeCl3,15 BF3·OEt2,16 and InCl3 have also been used to
17
catalyze the cross-reaction. However, a good yield of
products can only be obtained at high temperature and the
purification operations are always complicated. It is,
therefore, important to find a more convenient method for
the preparations of these compounds.
Over the past few years, much effort has gone into devel-
oping rare earth metal triflate-catalyzed organic synthesis
especially the ones involving Yb(OTf)3 and Sc(OTf)3. As
SYNTHESIS 2004, No. 18, pp 3060–3064
Advanced online publication: 02.11.2004
DOI: 10.1055/s-2004-834900; Art ID: F10104SS
© Georg Thieme Verlag Stuttgart · New York
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x.
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x
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Convenient Synthesis of a,a¢-Bis(substituted benzylidene)cycloalkanones
3061
that lanthanide triflates are more soluble in water than in
common organic solvents allows easy recovery of the cat-
alysts from the aqueous phase in almost quantitative
yields. The catalyst can be reused without attenuated ac-
tivity. In view of ‘green chemistry’, recovery and reuse of
the catalyst is highly preferable.
Table 2 The Reaction of Benzaldehyde (1a) and Cyclopentanone
(2a): Effect of Solventsa
Entry
Solvent
Time (h)
Yield of 3a (%)b
1
2
3
4
5
6
7c
C2H5OH
CH3COOC2H5
Toluene
CH3CN
36
36
36
72
36
36
6
60
53
O
O
39
Cat.
PhCHO
+
trace
15
Ph
Ph
Cyclohexane
THF
3a
2a
1a
23
Scheme 1
Nonec
92
a Refluxed for the time given.
b Isolated yield.
Table 1 The Reaction of Benzaldehyde (1a) and Cyclopentanone
(2a): Effect of Catalystsa
c Refluxed at 90 °C under solvent-free conditions.
Entry Catalyst
Amount of
Time
Yield of 3ac
Catalyst (mol%)b (h)
(%)
the case of MeCN, no product was obtained even after
three days. The best results were achieved by carrying out
the reaction at 90 °C in the presence of catalytic amount
Yb(OTf)3 without any solvent (Table 2, entry 7).
1
2
none
48
7
ZnCl2
2.5
2.5
2.5
2.5
2.5
0.1
0.2
0.5
1
16
16
16
16
16
16
12
6
22
3
AlCl3
25
Based on the results obtained above, we turned our atten-
tions to apply the methodology to several other derivates
(Table 3) and found that in all cases, the reactions pro-
ceeded rapidly to afford the corresponding a,a¢-bis(substi-
tuted benzylidene)cycloalkanone derivatives. All the
yields were excellent, including the heteroaromatic alde-
hydes such as 2-furaldehyde, 2-thiaphenecarboxaldehyde
and 3-pyridinecarboxaddehyde (Table 3, entries 10, 11
and 20). It seems that the effect of substituted groups on
the aromatic aldehydes is not very strong; both the elec-
tron-donating [Me, MeO, N(CH3)2] and electron-with-
drawing (NO2, Cl) groups worked well, showing little
distinction. The purifications of these compounds were
easily performed by simple filtration and washing with al-
cohol.
4
SnCl2
19
5
FeCl3
20
6
YCl3
41
7
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
40
8
72
9
92
10
11
12
13
6
93
1.5
5
6
92
6
92
d
1
6
92, 90, 89, 89
O
O
a Refluxed at 90 °C under solvent-free conditions.
b Based on the amount of ketone.
c Isolated yield.
Yb(OTf)3
90 °C
ArCHO
+
Ar
Ar
n = 1,2
(
)
( )n
n
d Catalyst was reused four times.
1
2
3
Scheme 2
Having established the advantages of Yb(OTf)3 as cata-
lyst, bearing in mind that solvents can also effect the
chemical reaction, we continued to optimize the process
by varying the solvents (Scheme 1). Several classical or-
ganic solvents, such as EtOH, toluene and THF (shown in
Table 2) were chosen as the medium for a comparison. In
each case, 1a (10 mmol) and 2a (5 mmol) were mixed to-
gether with Yb(OTf)3 (0.5 mol%) and solvent (5 mL) un-
der the reflux conditions. It was found that solvent-free
condition gave the highest yield of the desired product 3a.
When the organic solvents were used, the reaction was not
complete even after refluxing for several days and in ad-
dition, some unidentified by-products were detected. In
In summary, it can be concluded that Yb(OTf)3 is an effi-
cient and excellent catalyst in the crossed aldol condensa-
tions of different ketones with various aromatic aldehydes
to prepare the a,a′-bis(substituted benzylidene)cycloal-
kanone derivatives under solvent-free and mild condi-
tions. The catalyst can be easily recovered from the
aqueous layer after the reaction is complete and can be re-
used with no loss of activity. Furthermore, the reaction is
a green process avoiding the discharge of toxic solvents
and acid liquids.
Synthesis 2004, No. 18, 3060–3064 © Thieme Stuttgart · New York

3062
L. Wang et al.
PAPER
3b
Table 3 Preparation of a,a¢-Bis(substituted benzylidene)cycloal-
kanones Catalyzed by Yb(OTf)3 at 90° C Under Solvent-free Condi-
tionsa (Scheme 2)
Mp 243–244 °C.
1H NMR (CDCl3): d = 7.43–7.50 (m, 6 H), 7.17–7.18 (m, 4 H), 3.03
(s, 4 H), 2.32 (s, 6 H).
13C NMR (CDCl3): d = 188.0, 144.2, 142.5, 136.9, 131.8, 128.9,
126.1, 23.6, 20.6.
Entry Ar
n
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
Time (h) Product Yield (%)b
1
2
C6H5
6
6
3a
3b
3c
3d
3e
3f
92
95
93
92
93
94
97
95
92
92
88
94
96
92
93
92
93
95
93
90
p-MeC6H4
p-MeOC6H4
p-HOC6H4
p-Me2NC6H4
p-ClC6H4
3c
Mp 211–212 °C.
1H NMR (CDCl3): d = 6.89–7.57 (m, 10 H), 3.84 (s, 6 H), 3.07 (s, 4
H).
13C NMR (CDCl3): d = 187.0, 161.2, 143.6, 142.3, 127.2, 114.8,
56.5, 23.3.
3
6
4
6
5
6
6
6
3d
7
2,4-Cl2C6H3
C6H5CH=CH
p-NO2C6H4
2-furyl
4
3g
3h
3i
Mp >300 °C (dec).
1H NMR (DMSO-d6): d = 9.99 (s, 2 H), 6.82–7.49 (m, 10 H), 2.97
(s, 4 H).
13C NMR (CDCl3): d = 187.4, 156.2, 143.6, 142.1, 127.6, 115.8,
23.4.
8
6
9
12
7
10
11
12
13
14
15
16
17
18
19
20
3j
3-pyridyl
7
3k
3l
3e
Mp 270–273 °C.
IR(KBr): 2899, 1672, 1588, 1524, 1365, 1162, 810 cm–1.
1H NMR (CDCl3): d = 7.55 (s, 2 H), 7.51–6.72 (m, 8 H), 3.05 (s, 4
H), 3.01 (s, 12 H).
13C NMR (CDCl3): d = 187.6, 143.7, 143.2, 142.3, 127.1, 113.8,
43.4, 23.5.
MS: m/z = 346 [M+], 331, 302, 286, 226.
C6H5
6
p-MeC6H4
p-MeOC6H4
p-HOC6H4
p-Me2NC6H4
2,4-Cl2C6H3
C6H5CH=CH
p-NO2C6H4
2-thienyl
6
3m
3n
3o
3p
3q
3r
3s
3t
6
6
9
3f
4
Mp 228–229 °C.
IR (KBr): 2911, 1692, 1620, 1605, 1580 cm–1.
1H NMR (CDCl3): d = 7.45–7.47 (m, 6 H), 7.33–7.35 (m, 4 H), 3.01
(s, 4 H).
5
12
7
13C NMR (CDCl3): d = 187.8, 143.8, 143.6, 133.3, 128.8, 23.5.
a Refluxed at 90 °C for the time given under solvent-free conditions.
b Isolated yield.
3g
Mp 206–207 °C.
IR (KBr): 3071, 2911, 1692, 1614, 1576 cm–1.
Synthesis of 3: General Procedure
1H NMR (CDCl3): d = 7.82 (s, 2 H), 7.48–7.30 (m, 6 H), 2,98 (s, 4
Ketone 1 (5 mmol), aldehyde 2 (10 mmol) and Yb(OTf)3 (0.025
mmol, 0.5 mol%) were mixed. The mixture was stirred at 90 °C for
a certain period of time to complete the reaction (monitored by
TLC). After the reaction was complete, the system was cooled to
r.t., then H2O was added and the pure product 3 was obtained by fil-
tration and washing with a little amount of alcohol. The catalyst re-
maining in the aqueous phase can be recovered by removing the
water by heating and then drying under reduced pressure at 100 °C
for 2 h.
H).
13C NMR (CDCl3): d = 187.3, 143.5, 142.6, 134.3, 132.8, 129.7,
23.6.
MS: m/z = 398 [M+], 363, 327, 291, 255.
3h
Mp 215–216 °C.
IR (KBr): 3026, 1669, 1618, 1584 cm–1.
1H NMR (CDCl3): d = 7.52 (d, J = 7.2Hz, 4 H), 7.23–7.46 (m, 8 H),
6.98–7.01 (m, 4 H), 2.93 (s, 4 H).
3a
Mp 188–190 °C.
13C NMR (CDCl3): d = 187.2, 147.5, 143.6, 134.9, 128.4, 127.7,
IR(KBr): 3052, 3017, 2910, 1688, 1625, 1600 cm–1.
1H NMR (CDCl3): d = 7.58–7.60 (m, 6 H), 7.35–7.45 (m, 6 H), 3.12
(s, 4 H).
13C NMR (CDCl3): d = 188.1, 144.4, 142.3, 133.3, 128.5, 127.5,
126.7, 22.6.
MS: m/z = 260 [M+], 129, 115.
24.6.
3i
Mp 229–231 °C.
IR (KBr): 3106, 2844, 1706, 1608, 1522 cm–1.
1H NMR (CDCl3): d = 8.30–8.32 (m, 4 H), 7.65–7.76 (m, 6 H), 3.21
(s, 4 H).
Synthesis 2004, No. 18, 3060–3064 © Thieme Stuttgart · New York

PAPER
Convenient Synthesis of a,a¢-Bis(substituted benzylidene)cycloalkanones
3063
13C NMR (CDCl3): d = 187.7, 147.6, 143.6, 142.1, 141.0, 127.4,
123.5, 23.6.
13C NMR (CDCl3): d = 187.7, 144.3, 143.7, 141.5, 127.1, 113.0,
43.6, 27.6.
3j
3q
Mp 163–164 °C.
Mp 163–164 °C.
IR (KBr): 3133, 2917, 1682, 1624, 1600, 1290–1238, 755 cm–1.
IR (KBr): 3096, 2968, 1658, 1593, 1575, 1464, 1139, 827 cm–1.
1H NMR (CDCl3): d = 7.60 (s, 2 H), 7.36 (s, 2 H), 6.71 (d, J = 3.3
Hz, 2 H), 6.55 (s, 2 H), 3.09 (s, 4 H).
1H NMR (CDCl3): d = 7.76 (s, 2 H), 7.40–7.19 (m, 6 H), 2.68 (t,
J = 5.6 Hz, 4 H), 1.72–1.69 (m, 2 H).
13C NMR (CDCl3): d = 187.8, 155.3, 149.6, 142.1, 127.7, 111.8,
20.8.
13C NMR (CDCl3): d = 187.6, 144.3, 134.7, 133.4, 129.2, 126.8,
27.8.
3k
3r
Mp 222–224 °C.
Mp 177–178 °C.
1H NMR (CDCl3): d = 8.87 (s, 2 H), 8.62 (s, 2 H), 7.92 (d, J = 7.7
IR (KBr): 3052, 2900, 1690, 1600 cm–1.
1H NMR (CDCl3): d = 7.44–7.40 (m, 6 H), 7.30–6.90 (m, 10 H),
2.74–2.72 (t, J = 5.6 Hz, 4 H), 1.75 (m, 2 H).
13C NMR (CDCl3): d = 187.3, 148.3, 142.7, 134.8, 128.2, 127.8,
126.1, 125.3, 27.8.
Hz, 2 H), 7.59 (s, 2 H), 7.42–7.44 (m, 4 H), 3.18 (s, 2 H).
13C NMR (CDCl3): d = 187.0, 155.7, 149.6, 149.4, 136.7, 122.5,
121.1, 23.4.
HRMS: m/z calcd for C17H14N2O: 262.3110; found: 262.3112.
MS: m/z = 326 [M+], 235, 222, 207, 131, 115, 91.
3l
Mp 117–118 °C.
3s
IR (KBr): 3024, 2926, 1657, 1609, 1570, 1440, 1268, 1144, 773
cm–1.
1H NMR (CDCl3): d = 7.81 (s, 2 H), 7.33–7.48 (m, 10 H), 2.96 (t,
J = 6.4 Hz, 4 H), 1.83–1.76 (m, 2 H).
13C NMR (CDCl3): d = 187.2, 144.3, 141.5, 134.7, 128.6, 127.5,
27.8.
Mp 161–162 °C.
IR (KBr): 3107, 2930, 1705, 1600, 1515, 1342, 850 cm–1.
1H NMR (CDCl3): d = 8.3 (s, 2 H), 8.2–7.58 (m, 8 H), 2.94 (t, J =
6.0 Hz, 4 H), 1.87–1.85 (m, 2 H).
13C NMR (CDCl3): d = 187.8, 147.6, 144.3, 141.5, 141.0, 127.1,
123.5, 27.6.
MS: m/z = 273.1 [M+], 217, 115.
3t
3m
Mp 142–143 °C.
Mp 164–165 °C.
IR (KBr): 2942, 2918, 1660, 1600 cm–1.
1H NMR (CDCl3): d = 7.78 (s, 2 H), 7.18–7.39 (m, 8 H), 2.92 (t,
J = 5.6 Hz, 4 H), 2.39 (s, 6 H), 1.75–1.79 (m, 2 H).
13C NMR (CDCl3): d = 187.1, 144.3, 141.6, 131.9, 136.9, 129.1,
27.8; 20.9.
1H NMR (CDCl3): d = 7.99 (s, 2 H), 7.54 (d, J = 4.6 Hz, 2 H), 7.39
(d, J = 2.5 Hz, 2 H), 7.16 (t, J = 3.7 Hz, 2 H), 2.94 (t, J = 4.7 Hz, 4
H), 2.00–1.97 (m, 2 H).
13C NMR (CDCl3): d = 187.3, 150.6, 141.5, 136.6, 130.3, 127.8,
126.4, 27.4.
Acknowledgement
3n
Mp 161–163 °C.
IR (KBr): 2937, 1661, 1595, 1552, 1505, 1248, 1021, 833 cm–1.
1H NMR (CDCl3): d = 7.77 (s, 2 H), 7.48–6.96 (m, 8 H), 3.78 (s, 6
H), 2.88 (t, J = 5.4 Hz, 4 H), 1.73–1.75 (m, 2 H).
We thank the National Nature Science Foundation of China and the
Science Foundation of Lab for Advanced Materials for their finan-
cial support.
13C NMR (CDCl3): d = 187.4, 161.2, 144.3, 141.5, 127.2, 114.0,
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Synthesis 2004, No. 18, 3060–3064 © Thieme Stuttgart · New York