International Journal of Organic Chemistry, 2011, 1, 143-147
doi:10.4236/ijoc.2011.14021 Published Online December 2011 (http://www.SciRP.org/journal/ijoc)
143
Solvent Free Synthesis of
α, ά-Bis(Substituted-benzylidene)cycloalkanones Using
Covalently Anchored Sulfonic Acid on Silica Gel
(SiO2-R-SO3H) as an Efficient and Reusable Heterogeneous
Catalyst
Azizollah Habibi1, Enayatollah Sheikhhosseini1*, Mohammadali Bigdeli1, Saeed Balalaie2,
Elinaz Farrokhi1
1Faculty of Chemistry, Tarbiat Moallem University, Tehran, Iran
2Peptide Chemistry Research Center, K. N. Toosi University of Technology, Tehran, Iran
Received August 2, 2011; revised September 8, 2011; accepted September 16, 2011
Abstract
We wish to report a mild and efficient Crossed-Aldol reaction for the synthesis of α, ά-bis(substituted-ben-
zylidene)cycloalkanones in the presence of catalytic amounts of covalently anchored sulfonic acid onto sil-
ica gel under heterogeneous and solvent-free conditions. The present methodology offers several advantages
such as excellent yields, simple procedure and work up steps, short reaction times and easy recovery of the
catalyst. We have also demonstrated that the catalyst can be reused successfully.
Keywords: Crossed-Aldol Condensation, Heterogeneous Catalysis,
α, ά-Bis(Substituted-benzylidene)cycloalkanones, SiO2-R-SO3H, Solvent-Free
1. Introduction
Heterogeneous catalysis has emerged as a process in whi-
ch catalysts can be recovered and reused to achieve high
turn-over numbers [1,2]. Incorporation of an active site
onto a large surface solid carrier is one strategy to gener-
ate a heterogeneous catalyst. Silica is one such support
which displays exceptional chemical and thermal quail-
ties [3,4]. Recently silica functionalized sulfonic acid has
been used for a variety of reactions [5-8].
Claisen-Schmidt condensation reaction followed by de-
hydration affords α-alkylidene or α-arylidene compounds.
Such an introduction of alkylidene or arylidene moieties
at the α-position of carbonyl compounds has been a use-
ful synthetic tool in natural product chemistry [9]. α, ά-
Bis(substituted-benzylidene)cycloalkanones exhibit nu-
merous biological activities [10-13]. They are also suit-
able as nonlinear optical materials [14], in preparation of
pyridine derivatives [15], 2,7-disubstituted tropones [16]
and in the total synthesis of natural products [17].
Such condensation processes are carried out with the
aid of strong acids or bases. The yields, however, are ge-
nerally low and cumbersome separation steps are requir-
ed [18,19]. It is reported that various complexes of metal
ions, such as Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn
(II) may be used as catalysts in condensation reactions
[20]. The reported yields, however, are less than 38%. In
other cases, amino-functionalized ionic liquid [21] and
copper (II) trifluoroacetate [22] have also been used to
catalyze the reaction. High yield of products, however,
can only be obtained at high temperatures and the purify-
cation operations are always complicated.
Such condensation reactions are also carried out in sol-
vent-free conditions [23]. Silica sulfonic acid, nanopor-
ous silica-based sulfonic acid and polymer supported sul-
fonic acid type catalysts are also employed for such con-
densations. Reaction times are reported to be in 3 - 12
hour range [24-26]. In the present work, we have used
covalently anchored sulfonic acid on silica gel (Catalyst 1,
Scheme 1), which its applications have been reported in a
few number of organic transformations [6,8,27,28] under
solvent-free conditions to prepare α, ά-bis(substituted-
benzylidene) cycloalkanones (Scheme 2) in excellent
yields, short reaction times and clean work up procedures.

A. HABIBI ET AL.
144
2. Result and Discussion
In order to find the optimum reaction conditions, various
parameters including solvent, temperature and molar ra-
tio of catalyst to starting material were examined. It was
found that the highest yields were obtainable under sol-
vent free conditions with 3.6 mol percent of catalyst to
the starting carbonyl compounds. The method is general
and encompasses a variety of aromatic aldehyde with
electron-donating or electron-withdrawing groups in high
yields (Table 1). We did expect some degree of selec-
tiveity depending upon the groups attached to the alde-
hyde moiety, since this could have bearing on the active-
ity of the aldehyde. It appears that the reaction under
these circumstances is rapid enough and thus isn’t affec-
ted by variations in the activity of carbonyl structure.
The structures of compounds 4a-y were deduced from
their elemental analysis and their IR, 1H NMR, and 13C
NMR spectra. The 1H NMR spectrum of 4t exhibited a
doublet signal (δ = 0.92 ppm) for the methyl group, mul-
tiple signals (δ = 1.86, 2.45, 2.93 ppm) for the methylene
groups and methine protons, a complex signal (δ = 7.45 -
8.09 ppm) for the aromatic protons along with a singlet
(δ = 8.45 ppm) due to the two olefin protons. The proton-
decoupled 13C NMR spectrum of 4t exhibited 23 distinct
resonances in agreement with the bis(substituted-ben-
zylidene) structure.
SiO
2
OH
OH
OH (MeO)3Si SH
Toluene, Reflux, 24h
SiO
2
O
O
OSH
i. 30% H2O2
ii. 0.5 M H2SO4
iii. Distilled H2O
Si O2
O
O
OSO3H
C
atal
y
s
t1
Scheme 1. Preparation of SiO2-R-SO3H catalyst.
n = 0, 1
n
O
X
X = H, Me, Et, t-But
ArCHO Catalyst 1
Solvent free
90
o
C
3
O
X
ArAr
24
p
-
y
Scheme 2. Synthesis of bis(substituted-benzylidene)cycloa-
lkanones.
Table 1. Preparation of α,ά-bis(substituted benzylidene)cycloalkanones catalyzed by SiO2-R-SO3H at 90˚C under solvent-free
conditions.
Entry Ar n, x Time (h) Product Yielda (%) Mp (˚C)
Found Reported
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
C6H5
p-MeC6H4
p-MeOC6H4
p-ClC6H4
p-NO2C6H4
o-ClC6H4
α-Naphttyl
β-Naphttyl
C6H5
p-MeC6H4
p-MeOC6H4
p-ClC6H4
p-NO2C6H4
o-ClC6H4
α-Naphthyl
C6H5
p-MeC6H4
p-MeOC6H4
p-ClC6H4
α-Naphthyl
β-Naphthyl
α-Naphthyl
β-Naphttyl
α-Naphttyl
β-Naphttyl
n = 0, x = H
n = 0, x = H
n = 0, x = H
n = 0, x = H
n = 0, x = H
n = 0, x = H
n = 0, x = H
n = 0, x = H
n = 1, x = H
n = 1, x = H
n = 1, x = H
n = 1, x = H
n = 1, x = H
n = 1, x = H
n = 1, x = H
n = 1, x = Me
n = 1, x = Me
n = 1, x = Me
n = 1, x = Me
n = 1, x = Me
n = 1, x = Me
n = 1, x = Et
n = 1, x = Et
n = 1, x = t-But
n = 1, x = t-But
1.3
1.5
1.4
1.0
1.1
1.6
1.3
1.5
1.2
1.6
1.4
1.2
1.2
1.8
1.4
1.5
1.7
2.0
1.4
1.0
1.1
2.0
2.0
1.0
1.0
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
4u
4v
4w
4x
4y
85
85
84
83
80
75
82
79
80
84
85
81
79
76
89
80
80
79
83
75
81
80
84
84
93
189 - 191 188 - 190 [23]
241 - 243 243 - 244 [23]
212 - 214 211 - 212 [23]
227 - 228 228 - 229 [23]
227 - 230 229 - 231 [23]
154 - 157 152 - 153 [22]
173 - 174 ----
274 - 275 ----
115 - 116 117 - 118 [23]
168 - 171 164 - 165 [23]
162 - 163 161 - 163 [23]
146 - 148 147 - 148 [23]
162 - 163 161 - 162 [23]
102 - 104 102 - 104 [23]
199 - 201 ----
97 - 99 97 - 99 [23]
124 - 126 125 - 127 [23]
136 - 138 137 - 139 [23]
153 - 157 156 - 160 [23]
151 - 153 ----
160 - 162 ----
146 - 148 ----
139 - 140 ----
189 - 192 ----
182 - 185 ----
a Isolated yield.

145 A. HABIBI ET AL.
Shortening of reaction times and the use of an eco-
friendly catalyst are two major merits of this work. Re-
usability of the catalyst is also an added advantage in its
own right.
The reuse of the catalyst is a major factor in a new
synthetic procedure. To test this, a series of ten consecu-
tive runs of the reaction of benzaldehyde and cyclohex-
none with catalyst 1 were carried out. The results, how-
ever, demonstrated no significant change in the activity
of the catalyst. The results in Table 2 show a 12.5% loss
of activity over ten recycles.
3. Experimental
3.1. Instruments and Characterization
Melting points were measured on an Electrothermal 9200
apparatus and were uncorrected. 1H NMR spectra were
recorded on a Bruker DRX-300 AVANCE spectrometer
at 300.13 MHz. IR spectra were recorded on a Bomem MB-
Series FTIR. Elemental analyses were carried out on a
Heraeus CHN-O-Rapid analyzer.
3.2. General Procedure for the Preparation of
Catalyst 1
For the preparation of sulfonic acid covalently anchored
onto the surface of silica gel (SiO2-R-SO3H), activated
silica was refluxed with trimethoxy (3-sulfanylpropyl)
silane in toluene for 24 h. This gave 3-sulfanylpropyl-
substituted silica, which was then oxidized with 30% hy-
drogen peroxide in the presence of concentrated sulfuric
acid, to give SiO2-R-SO3H (Scheme 1). The detailed ex-
perimental procedure for the preparation and characteri-
zation of this silica gel/sulfonic acid catalyst is reported
elsewhere [8].
3.3. General Procedure for the Synthesis of α, ά-
Bis(Substituted-benzylidene)cycloalkanones
A mixture of a substituted benzaldehyde (8 mmol), cy-
cloalkanone (4 mmol) and catalyst (0.3 g, 3.6 mol% of
SO3H) was stirred at 90˚C for an appropriate time. The
reaction mixture was diluted with CHCl3 (15 ml), cataly-
st was filtered off, washed with acetonitrile and distilled
water, dried at 110˚C for 3 h and was used for the next
run directly. The filtrate was evaporated and the resulting
Table 2. Reusability of catalyst 1 at 90˚C.
Run 1 2 3 4 5 6 7 8 910
Yield (%) 80 80 79 77 77 75 75 72 7070
solid product was recrystallized from ethanol. The results
are shown in Table 1.
3.4. The Spectral Data of Unreported
Compounds Is Given Below
(2E,5E)-2,5-Bis((naphthalen-1-yl)methylene)cyclopentan-
one (4g): Mp = 173˚C - 174˚C. IR (KBr, cm–1): 1703,
1629. 1H NMR (300 MHz, CDCl3): δ = 3.01 (s, 4H, 2CH2),
7.51 - 7.64 (m, 6H, H-Ar), 7.67 (d, 2H, J = 7.0 Hz, H-Ar),
7.90 (dd, 4H, J = 9.0, 2.2 Hz, H-Ar), 8.27 (d, 2H, J = 8.0
Hz, H-Ar), 8.44 (s, 2H, H-olefin). 13C NMR (75 MHz,
CDCl3) δ: 27.0, 124.0, 125.1, 126.2, 126.7, 127.1, 127.5,
128.7, 129.8, 130.4, 130.5, 132.3, 132.4, 133.6, 139.8,
195.7. Anal. Calc. for C27H20O (360.15): C, 89.97; H,
5.59. Found: C, 89.96; H, 5.57.
(2E,5E)-2,5-Bis((naphthalen-6-yl)methylene)cyclopent-
anone (4h): Mp = 274˚C - 275˚C. IR (KBr, cm–1): 1695,
1683, 1615. 1H NMR (300 MHz, CDCl3): δ = 3.30 (s, 4H,
2CH2), 7.53 - 7.56 (m, 4H, H-Ar), 7.73 - 7.80 (m, 4H,
H-Ar), 7.85 - 7.93 (m, 6H, H-Ar), 8.11 (s, 2H, H-olefin).
13C NMR (75 MHz, CDCl3) δ: 26.9, 124.1, 125.1, 126.4,
126.8, 127.3, 127.6, 127.9, 128.4, 128.8, 129.9, 130.2,
130.6, 133.0, 133.1, 133.5, 136.4, 136.6, 137.0, 137.4,
195.4. Anal. Calc. for C27H20O (360.15): C, 89.97; H,
5.59. Found: C, 89.93; H, 5.55.
(2E,6E)-2,6-Bis((naphthalen-1-yl)methylene)cyclohe-
xanone (4o): Mp = 200˚C. IR (KBr, cm–1): 1663, 1606,
1578. 1H NMR (300 MHz, CDCl3): δ = 1.71 (m, 2H,
CH2), 2.81 (m, 4H, 2CH2), 7.46 - 7.60 (m, 8H, H-Ar), 7.86
- 7.92 (m, 4H, H-Ar), 8.07 - 8.09 (m, 4H, H-Ar), 8.45 (s,
2H, H-olefin). 13C NMR (75 MHz, CDCl3) δ: 23.6, 28.7,
124.1, 124.8, 125.0, 126.1, 126.4, 126.9, 128.1, 128.5,
128.9, 129.3, 132.0, 133.2, 133.5, 134.9, 135.2, 135.3,
138.3, 190.1. Anal. Calc. for C28H22O (374.17): C, 89.81;
H, 5.92. Found: C, 89.79; H, 5.90.
(2E,6E)-4-Methyl- 2,6-bis((naphthalen-1-yl)methylene)
cyclohexanone (4t): Mp = 151˚C - 153˚C. IR (KBr, cm–1):
1700, 1671, 1614, 1588. 1H NMR (300 MHz, CDCl3): δ
= 0.92 (d, 3H, J = 6.5 Hz, Me), 1.86 (m, 1H, CH), 2.45
(m, 2H, CH2), 2.93 (dd, 2H, J = 15.5, 3.4, CH2), 7.45 -
7.60 (m, 6H, H-Ar), 7.87 - 7.93 (m, 4H, H-Ar), 8.06 -
8.09 (m, 2H, H-Ar), 8.45 (s, 2H, H-olefin). 13C NMR (75
MHz, CDCl3) δ: 21.3, 29.9, 37.0, 123.9, 124.8, 125.1,
125.4, 125.5, 126.2, 126.4, 127.0, 127.6, 128.5, 128.9,
129.3, 129.5, 132.0, 133.2, 133.5, 135.5, 135.6, 137.4,
189.9. Anal. Calc. for C29H24O (388.18): C, 89.66; H,
6.23. Found: C, 89.61; H, 6.20.
(2E,6E)-4-Methyl-2-((naphthalen-2-yl)methylene)-6-
((naphthalen-3-yl)methylene)cyclohexanone (4u): Mp =
160˚C. IR (KBr, cm–1): 1667, 1657, 1601, 1590. 1H
NMR (300 MHz, CDCl3): δ = 1.11 (d, 3H, J = 6.8 Hz,
Me), 1.95 (m, 1H, CH), 2.67 (m, 2H, CH2), 3.02 (dd, 2H,

A. HABIBI ET AL.
146
J = 15.5, 3.5, CH2), 7.51 - 7.61 (m, 6H, H-Ar), 7.84 -
7.95 (m, 6H, H-Ar), 8.00 (s, 2H, H-olefin). 13C NMR (75
MHz, CDCl3) δ: 21.6, 29.5, 36.6, 126.4, 126.8, 127.6,
127.9, 128.4, 130.2, 133.0, 133.1, 133.5, 135.6, 137.3,
190.1. Anal. Calc. for C29H24O (388.18): C, 89.66; H,
6.23. Found: C, 89.61; H, 6.20.
(2E,6E)-4-Ethyl-2,6-bis((naphthalen-1-yl)methylene)
cyclohexanone (4 v): Mp = 146˚C - 148˚C. IR (KBr,
cm–1): 1669, 1607, 1589. 1H NMR (300 MHz, CDCl3): δ
= 0.70 (t, 3H, J = 7.4 Hz, CH3), 1.29 (m, 2H, CH2-Me),
1.62 (m, 1H, CH), 2.46 (m, 2H, CH2), 2.98 (dd, 2H, J =
15.6, 3.3 Hz, CH2), 7.46 - 7.60 (m, 8H, H-Ar), 7.87 -
7.93 (m, 4H, H-Ar), 8.07 - 8.11 (m, 2H, H-Ar), 8.47 (s,
2H, H-olefin). 13C NMR (75 MHz, CDCl3) δ: 11.3, 28.1,
34.2, 36.1, 124.8, 125.0, 126.1, 126.4, 126.9, 127.6,
127.9, 128.1, 128.5, 128.9, 132.0, 133.1, 133.5, 135.2,
135.6, 137.3, 190.2. Anal. Calc. for C30H26O (402.2): C,
89.51; H, 6.51. Found: C, 89.46; H, 6.47.
(2E,6E)-4-Ethyl-2-((naphthalen-2-yl)methylene)-6-((n
aphthalen-3-yl)methylene)cyclohexanone (4w): Mp =
139˚C - 140˚C. IR (KBr, cm–1): 1696, 1663, 1601, 1565.
1H NMR (300 MHz, CDCl3): δ = 0.87 (t, 3H, J = 7.4 Hz,
CH3), 1.45 (m, 2H, CH2-Me), 1.71 (m, 1H, CH), 2.67 (m,
2H, CH2), 3.23 (dd, 2H, J = 15.6, 3.3 Hz, CH2), 7.51 -
7.55 (m, 4H, H-Ar and H-olefine), 7.60 (dd, 2H, J = 8.5,
1.3 Hz, H-Ar), 7.84 - 7.90 (m, 6H, H-Ar), 7.95 - 8.0 (m,
4H, H-Ar and H-olefin). 13C NMR (75 MHz, CDCl3) δ:
11.5, 28.5, 34.2, 35.7, 126.4, 126.8, 127.7, 127.9, 128.4,
128.8, 130.2, 133.1, 133.1, 133.6, 135.6, 137.4, 190.4.
Anal. Calc. for C30H26O (402.2): C, 89.51; H, 6.51.
Found: C, 89.48; H, 6.49.
(2E,6E)-4-Tert-butyl-2,6-bis((naphthalen-1-yl)methyle
ne)cyclohexanone (4x): Mp = 189˚C - 192˚C IR (KBr,
cm–1): 1659, 1595, 1575, 1566. 1H NMR (300 MHz,
CDCl3): δ = 0.78 (s, 9H, 3Me), 1.52 (m, 1H, CH), 2.38 (t,
2H, J = 15 Hz, CH2), 3.07 (d, 2H, J = 15.1 Hz, CH2),
7.47 - 7.60 (m, 8H, H-Ar), 7.86 - 7.93 (m, 4H, H-Ar),
8.08 - 8.12 (m, 2H, H-Ar), 8.44 (s br, 2H, H-olefin). 13C
NMR (75 MHz, CDCl3) δ: 27.0, 27.1, 29.7, 30.0, 32.5,
44.7, 44.9, 124.8, 125.0, 126.1, 126.4, 126.7, 127.5,
127.6, 128.0, 128.5, 128.9, 130.4,132.1, 133.1, 133.5,
135.2, 137.5, 138.1, 190.3. Anal. Calc. for C32H30O
(430.23): C, 89.26; H, 7.02. Found: C, 89.21; H, 6.97.
(2E,6E)-4-Tert-butyl-2-((naphthalen-2-yl)methylene)-
6-((naphthalen-3-yl)methylene)cyclohexanone (4y): Mp =
182˚C - 185˚C. IR (KBr, cm–1): 1695, 1662, 1600, 1563.
1H NMR (300 MHz, CDCl3): δ = 0.67 (s, 9H, 3Me), 1.51
(m, 1H, CH), 2.57 (t, 2H, J = 15 Hz, CH2), 3.27 (d, 2H, J
= 15.1 Hz, CH2), 7.46 - 7.57 (m, 6H, H-Ar), 7.60 - 8.05
(m, 8H, H-Ar), 8.10 (s br, 2H, H-olefin). 13C NMR (75
MHz, CDCl3) δ: 27.3, 29.6, 32.5, 44.5, 126.4, 126.8,
127.5, 127.6, 128.0, 128.4, 130.3, 130.6, 133.0, 133.1,
133.6, 136.4, 136.6, 137.0, 137.4, 190.5. Anal. Calc. for
C32H30O (430.23): C, 89.26; H, 7.02. Found: C, 89.23; H,
7.01.
4. Conclusions
We are reporting a new method for preparation of α, ά-
bis(substituted-benzylidene)cycloalkanones using SiO2-
R-SO3H. The catalyst is reusable with only small loss of
activity over ten trials. Simple experimental procedure,
use of small quantities of catalyst, no toxicity, no corro-
siveness, short reaction times, inexpensive solid catalyst,
high yields, mild reaction conditions and clean work up
steps are significant merits of this work.
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