Atom-economy click synthesis of tetrahydrobenzo[b]pyrans using carbon-based solid acid as a novel, highly efficient and reusable heterogeneous catalyst

Article abstract of DOI:10.1007/s11164-014-1536-6

Carbon-based solid acid has been used as a mild and efficient heterogeneous catalyst for the synthesis of tetrahydrobenzo[b]pyran derivatives via a one-vessel, three-component cyclocondensation of dimedone, aryl aldehydes, and malononitrile with high atom

Full text of DOI:10.1007/s11164-014-1536-6

Res Chem Intermed
DOI 10.1007/s11164-014-1536-6
Atom-economy click synthesis of tetrahydrobenzo[b]-
pyrans using carbon-based solid acid as a novel, highly
efficient and reusable heterogeneous catalyst
•
Maryam Moghaddas Abolghasem Davoodnia
Received: 14 October 2013 / Accepted: 9 January 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Carbon-based solid acid has been used as a mild and efficient hetero-
geneous catalyst for the synthesis of tetrahydrobenzo[b]pyran derivatives via a one-
vessel, three-component cyclocondensation of dimedone, aryl aldehydes, and mal-
ononitrile with high atom economy ranging from 94.2 to 95.4 %. The remarkable
features of this new method are short reaction times, high yields, clean reaction
profiles, and simple work-up procedure. The reactions were performed in ethanol
and the catalyst could be recycled after a simple work-up, and used for many runs
without significant loss of its activity. However, using aliphatic aldehydes, moderate
yields of the products were obtained. The structure of the catalyst was confirmed by
FT-IR spectroscopy, energy dispersive X-ray, X-ray diffraction, and the N2
adsorption/desorption analysis techniques.
Keywords Carbon-based solid acid Á Heterogeneous catalysis Á Reusable
catalyst Á One-vessel reaction Á Tetrahydrobenzo[b]pyrans
Introduction
Pyrans, benzo[b]pyrans, and tetrahydrobenzo[b]pyrans are of considerable interest
because of their wide range of biological properties, such as anticoagulant,
spasmolytic, diuretic, antiproliferative, anticancer, and antianaphylactin activities
[1–5]. A number of these compounds are useful as photoactive materials [6] or
constitute the structural unit of a series of natural products [7]. In addition, they can
be used as cognitive enhancers for the treatment of neurodegenerative disease,
M. Moghaddas Á A. Davoodnia (&)
Department of Chemistry, Faculty of Sciences, Mashhad Branch, Islamic Azad University,
Mashhad, Iran
e-mail: adavoodnia@yahoo.com; adavoodnia@mshdiau.ac.ir
123

M. Moghaddas, A. Davoodnia
including Alzheimer’s disease, amyotrophic lateral sclerosis, Huntington’s disease,
Parkinson’s disease, AIDS-associated dementia, and Down’s syndrome as well as
for the treatment of schizophrenia and myoclonus [8]. Hence, an easy access to
pyran derivatives is highly desirable as they possess unique pharmacological
properties.
The most straightforward synthesis of tetrahydrobenzo[b]pyrans involves a one-
vessel, three-component cyclocondensation of dimedone, aryl aldehydes, and
malononitrile in the presence of several catalysts such as SO3H-bearing solid
catalyst (PEG-SAC) [9], piperazine under solvent-free ball-milling conditions [10],
I2 [11], KF/Al2O3 under ultrasound irradiation [12], basic ionic liquid [13], nano-
mixed metal oxides [14], nano-ZnO [15, 16], PhB(OH)2 [17], tetrabutylammonium
bromide (TBAB) [18], Na2SeO4 [19], 1,4-diazabicyclo[2.2.2]octane (DABCO)
[20], ionic liquid [bmim][BF4] [21], and 2,2,2-trifluoroethanol (TFE) [22].
However, many of these procedures are not entirely satisfactory and suffer from
long reaction times, expensive reagents, unsatisfactory yields of products, harsh
reaction conditions, or tedious work-up using homogeneous liquid catalysts. These
problems prompted us towards further investigation in search of a new reusable
heterogeneous catalyst, which will carry out the synthesis of tetrahydrobenzo[b]py-
rans under simpler experimental set-up, and green and eco-friendly conditions.
The development of heterogeneous catalysts and how they affect specific
transformations in chemical synthesis has become a major area of research. The
potential advantages of these materials over homogeneous systems, in terms of their
simplified recovery and reusability, could potentially allow for the development of
environmentally benign chemical procedures in both academic and industrial
settings. Catalysts of this type have the potential to make the processes in which
they are applied cleaner, safer, higher-yielding, and relatively inexpensive [23–26].
During the course of our recent studies directed towards the development of
practical and environmentally friendly procedures for the synthesis of organic
compounds using reusable catalysts [27–32], we investigated the application of a
carbon-based solid acid (CBSA), which can be easily prepared by heating
naphthalene and concentrated sulfuric acid [33], as a catalyst for a series of
organic transformations. This reusable heterogeneous catalyst performed well and
showed a high level of catalytic activity in the Mannich [34] and Biginelli [35]
reactions, as well as in the synthesis of tetrasubstituted imidazoles [36]. This fact
prompted us to investigate the catalytic activity of this material in the synthesis of
tetrahydrobenzo[b]pyrans (Scheme 1).
Scheme 1 CBSA catalyzed synthesis of tetrahydrobenzo[b]pyrans
123

Synthesis of tetrahydrobenzo[b]pyrans
Experimental
All the chemicals were purchased from Merck and Aldrich and used without
purification. Melting points of the compounds were recorded on a Stuart SMP3
melting point apparatus. The IR spectra were obtained using a Tensor 27 Bruker
1
spectrophotometer as KBr disks. The H NMR (500 MHz) spectra were recorded
with Bruker 500 spectrometer.
Preparation of CBSA catalyst
Naphthalene (20 g) was heated in concentrated sulfuric acid ([96 %, 200 ml) at
250 °C under a flow of N2. After heating for 15 h, excess sulfuric acid was removed
from the dark brown tar by vacuum distillation at 250 °C for 5 h, which resulted in a
black solid. The solid was then ground to a powder and was washed repeatedly in
boiling water until impurities such as sulfate ions were no longer detected in the
wash water. The density of the SO3H groups was measured using NaOH
(0.01 mol L-1) as titrant by acid–base potentiometric titration. The amount of
SO3H attached to the polycyclic aromatic carbon was 2.81 mmol/g. The resulting
black powder was insoluble in solvents such as water, methanol, ethanol, benzene,
and hexane even at their boiling temperatures [33].
General procedure for the synthesis of tetrahydrobenzo[b]pyrans 4a-o catalyzed
by CBSA
A mixture of dimedone 1 (2 mmol), an aromatic or aliphatic aldehyde 2a-o
(2 mmol), malononitrile 3 (2 mmol), and CBSA (0.07 g) in ethanol (5 ml) was
heated under reflux for 10–30 min. During the procedure, the reaction was
monitored by TLC. Upon completion, the catalyst was filtered under hot condition.
After cooling the fiterate, the precipitated solid was filtered and recrystallized from
ethanol to give products 4a-o in high yields.
1
Selected H NMR and FT-IR data
2-Amino-3-cyano-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-
tetrahydrobenzo[b]pyran 4e (Ar = 4-ClC6H4)
1H NMR (500 MHz, DMSO-d6): d 0.95 (s, 3H, CH3), 1.03 (s, 3H, CH3), 2.10 (d,
J = 16.0 Hz, 1H, one proton of diastereotopic protons in CH2), 2.24 (d,
J = 16.0 Hz, 1H, one proton of diastereotopic protons in CH2), 2.45–2.55 (m,
2H, diastereotopic protons in CH2 overlapped with solvent), 4.19 (s, 1H, CH), 7.05
(s br., 2H, NH2), 7.17 (d, J = 8.4 Hz, 2H, arom-H), 7.34 (d, J = 8.4 Hz, 2H, arom-
H); IR (KBr disc): t 3,380, 3,182, 2,958, 2,188, 1,675, 1,636, 1,491, 1,365,
1,216 cm-1
.
123

M. Moghaddas, A. Davoodnia
2-Amino-3-cyano-4-(4-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-
tetrahydrobenzo[b]pyran 4i (Ar = 4-MeOC6H4)
1H NMR (500 MHz, DMSO-d6): d 0.94 (s, 3H, CH3), 1.04 (s, 3H, CH3), 2.08 (d,
J = 16.0 Hz, 1H, one proton of diastereotopic protons in CH2), 2.24 (d,
J = 16.0 Hz, 1H, one proton of diastereotopic protons in CH2), 2.43–2.57 (m,
2H, diastereotopic protons in CH2 overlapped with solvent), 3.71 (s, 3H, OCH3),
4.12 (s, 1H, CH), 6.83 (d, J = 8.7 Hz, 2H, arom-H), 6.93 (s br., 2H, NH2), 7.04 (d,
J = 8.7 Hz, 2H, arom-H); IR (KBr disc): t 3,378, 3,185, 2,953, 2,198, 1,685, 1,655,
1,509, 1,370, 1,249, 1,213, 1,034 cm-1
.
2-Amino-3-cyano-4-(4-methylphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-
tetrahydrobenzo[b]pyran 4j (Ar = 4-MeC6H4)
1H NMR (500 MHz, DMSO-d6): d 0.95 (s, 3H, CH3), 1.05 (s, 3H, CH3), 2.09 (d,
J = 16.0 Hz, 1H, one proton of diastereotopic protons in CH2), 2.24 (d,
J = 16.0 Hz, 1H, one proton of diastereotopic protons in CH2), 2.25 (s, 3H,
CH3), 2.43–2.57 (m, 2H, diastereotopic protons in CH2 overlapped with solvent),
4.13 (s, 1H, CH), 6.95 (s br., 2H, NH2), 7.02 (d, J = 8.0 Hz, 2H, arom-H), 7.08 (d,
J = 8.0 Hz, 2H, arom-H); IR (KBr disc): t 3,427, 3,331, 2,958, 2,192, 1,675, 1,602,
1,510, 1,368, 1,249, 1,206, 1,140, 1,033 cm-1
.
2-Amino-3-cyano-4-(3-nitrophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-
tetrahydrobenzo[b]pyran 4 k (Ar = 3-O2NC6H4)
1H NMR (500 MHz, DMSO-d6): d 0.96 (s, 3H, CH3), 1.04 (s, 3H, CH3), 2.11 (d,
J = 16.1 Hz, 1H, one proton of diastereotopic protons in CH2), 2.27 (d,
J = 16.1 Hz, 1H, one proton of diastereotopic protons in CH2), 2.55 (s, 2H,
diastereotopic protons in CH2), 4.42 (s, 1H, CH), 7.17 (s br., 2H, NH2), 7.55–7.70
(m, 2H, arom-H), 7.97 (t, J = 1.7 Hz, 1H, arom-H), 8.05-8.10 (m, 1H, arom-H); IR
(KBr disc): t 3,432, 3,321, 2,957, 2,186, 1,664, 1,599, 1,530, 1,375, 1,350, 1,251,
1,211, 1,138, 1,038 cm-1
.
Results and discussion
Characterization of the catalyst
The catalyst was characterized by FT-IR spectroscopy, energy dispersive X-ray
(EDX), X-ray diffraction (XRD), and the N2 adsorption/desorption analysis (BET)
techniques.
The FT-IR spectrum of the catalyst exhibits the SO2 symmetric and asymmetric
stretching modes in 1,100–1,250 cm-1. The spectrum also shows a broad OH
stretching absorption around 2,500–3,600 cm-1 (Fig. 1).
The EDX spectrum of CBSA is shown in Fig. 2. The EDX pattern shows the
clear presence of C, O, and S in the sample which indicates the SO3H groups are
123

Synthesis of tetrahydrobenzo[b]pyrans
Fig. 1 FT-IR spectrum of CBSA catalyst
Fig. 2 EDX spectrum of CBSA catalyst
successfully attached to the aromatic carbon atoms. The mass percentages of C, O
and S atoms are 59, 24, and 14 %, respectively.
The XRD pattern exhibits two broad, weak diffraction peaks (2h = 10–308,
35–508) attributable to amorphous carbon (Fig. 3).
The N2 adsorption–desorption isotherm of the catalyst is shown in Fig. 4. The
majority of isotherms have been grouped into six types by IUPAC classification (I,
123

M. Moghaddas, A. Davoodnia
Fig. 3 XRD pattern of CBSA catalyst
Fig. 4 N2 adsorption–desorption isotherm of CBSA catalyst
II, III, IV, V, and VI) [37]. The shape of the isotherm of the CBSA catalyst in this
paper is type II. A Type II isotherm is the form of isotherm that is normally obtained
with non-porous or macroporous adsorbents. Average pore diameter measured by
the BET method is 10.172 nm.
123

Synthesis of tetrahydrobenzo[b]pyrans
Table 1 Effect of CBSA catalyst amount, solvent, and temparature on the model reaction
Entry
Catalyst (g)
Solvent
T (°C)
Time (min)
Yielda (%)
1
–
H2O
Reflux
Reflux
Reflux
Reflux
120
120
120
120
120
120
60
–
2
–
CHCl3
CH3CN
EtOH
–
–
3
–
–
4
–
Trace
–
5
–
6
0.05
0.05
0.05
0.05
0.05
0.07
0.07
0.07
0.07
0.07
0.10
0.10
0.10
0.10
0.10
0.15
0.10
H2O
Reflux
Reflux
Reflux
Reflux
120
30
Trace
20
65
20
55
20
45
95
30
57
25
46
95
36
96
20
7
CHCl3
CH3CN
EtOH
–
120
70
8
9
30
10
11
12
13
14
15
16
17
18
19
20
21
22
60
H2O
Reflux
Reflux
Reflux
Reflux
120
30
CHCl3
CH3CN
EtOH
–
90
30
15
30
H2O
Reflux
Reflux
Reflux
Reflux
120
30
CHCl3
CH3CN
EtOH
–
100
20
15
30
EtOH
EtOH
Reflux
r.t.
20
120
Reaction conditions: dimedone 1 (2 mmol), 4-chlorobenzaldehyde 2e (2 mmol), and malononitrile
(2 mmol)
a
Isolated yields
Synthesis of tetrahydrobenzo[b]pyrans using CBSA catalyst
The CBSA catalyst is amorphous carbon consisting of polycyclic aromatic carbon
sheets with attached SO3H groups that functions as a strong solid acid with a high
density of acid sites and therefore can promote the reactions. To search for the
optimal conditions for the synthesis of tetrahydrobenzo[b]pyrans, the synthesis of
compound 4e was selected as a model reaction. The reaction was carried out by
heating a mixture of dimedone (2 mmol), 4-cholorobenzaldehyde (2 mmol), and
malononitrile (2 mmol) in the absence or presence of CBSA as catalyst in different
solvents, including H2O, CHCl3, CH3CN, EtOH, and also under solvent-free
conditions. The results are summarized in Table 1. No or a trace product was
obtained in the absence of the catalyst at reflux temperature in solvent or 120 °C
under solvent-free conditions even after reaction for 120 min (entries 1–5), whereas
good results were obtained in the presence of a catalytic amount of CBSA. Also, as
shown, the reaction in EtOH gave better results in terms of yield as well as reaction
time than others. The reaction temperature also strongly influenced the reaction.
123

M. Moghaddas, A. Davoodnia
Table 2 CBSA catalyzed synthesis of tetrahydrobenzo[b]pyran derivatives 4a-o
Entry
R
Producta
Time
(min)
Yieldb (%) m.p. (°C)
Found
Reported
230–232 [14]
1
C6H5
18
90
229–231
2
3-BrC6H4
15
93
230–232
227–229 [18]
3
4-BrC6H4
15
98
217–218 215 [19]
4
2-ClC6H4
15
93
217–219
218–220 [18]
5
4-ClC6H4
15
95
218–219
215–216 [22]
123

Synthesis of tetrahydrobenzo[b]pyrans
Table 2 continued
Entry
R
Producta
Time
(min)
Yieldb (%) m.p. (°C)
Found
Reported
176–178 [18]
6
4-FC6H4
15
90
180–182
7
3-HOC6H4
20
85
227–229 225 [19]
8
4-HOC6H4
17
90
210–212
209–211 [14]
9
4-MeOC6H4
18
85
198–199
193–195 [15]
10
4-MeC6H4
17
90
216–218
215–218 [17]
123

M. Moghaddas, A. Davoodnia
Table 2 continued
Entry
R
Producta
Time
(min)
Yieldb (%) m.p. (°C)
Found
Reported
210–212 [17]
11
3-O2NC6H4
10
90
212–214
12
4-O2NC6H4
10
93
180–182
183–185 [13]
13
2-thienyl
15
86
220–222
224–226 [21]
14
15
Me
30
30
58
53
176–177
195–197
177–179 [17]
193–194 [17]
Et
Reaction conditions: dimedone 1 (2 mmol), aromatic or aliphatic aldehyde 2a-o (2 mmol), malononitrile
3 (2 mmol), CBSA (0.07 g), EtOH (5 ml), reflux
a
All the products were characterized by their IR spectral data and a comparision of their melting points
with those of authentic samples. The structures of some products were also confirmed by 1H NMR
analysis
b
Isolated yields
Only a low yield of the product 4e was obtained in EtOH at room temperature in the
presence of 0.10 g catalyst in the model reaction (entry 22). The best result was
obtained when the reaction was conducted in refluxing EtOH in the presence of
123

Synthesis of tetrahydrobenzo[b]pyrans
Table 3 Comparison of the efficiencies of different catalysts for the synthesis of
tetrahydrobenzo[b]pyrans
Catalyst
Conditions
Solvent
Time
(min)
Yield
(%)
Ref.
T (°C) Other
Piperazine
I2
–
r.t.
Ball-milling
20–120
88–96 [10]
DMSO
EtOH
120
–
180–240 80–92 [11]
KF/Al2O3
27-34
Reflux
Reflux
Ultrasound irradiation 20–240
81–98 [12]
50–94 [13]
90–94 [14]
Basic ionic liquid EtOH/H2O
–
–
5–240
35–45
Nano mixed
metal oxides
EtOH
Nano ZnO
Nano ZnO
PhB(OH)2
TBAB
H2O
80
–
–
–
–
–
–
–
–
–
20–180
50–98 [15]
EtOH/H2O
EtOH/H2O
EtOH
r.t.
180–240 79–91 [16]
Reflux
Reflux
Reflux
Reflux
30
61–88 [17]
87–95 [18]
80–98 [19]
75–97 [20]
85–95 [21]
80–95 [22]
85–98 This work
20–140
40–210
120
Na2SeO4
DABCO
[bmim][BF4]
TFE
EtOH/H2O
H2O
[bmim][BF4] 80
30–120
300
TFE
Reflux
Reflux
CBSA
EtOH
10–20
Fig. 5 Reusability of CBSA for the synthesis of compound 4e
0.07 g of the CBSA catalyst (entry 14). No significant improvement in yield was
observed using a higher amount of the catalyst (entries 19 and 21).
Encouraged by this success, we extended the reaction of dimedone, and
malononitrile with a range of aromatic aldehydes under the optimized reaction
conditions. In all cases, the expected products were obtained in high yields in short
reaction times. The results are summarized in Table 2. As shown, aromatic
aldehydes bearing either electron-donating or electron-withdrawing substituents
reacted successfully with dimedone and malononitrile to give the corresponding
123

M. Moghaddas, A. Davoodnia
Scheme 2 Plausible mechanism for the formation of tetrahydrobenzo[b]pyrans in the presence of
CBSA : HA as catalyst
tetrahydrobenzo[b]pyran products in high yields over short reaction times. The
results also show that the aromatic aldehydes with electron-withdrawing groups
react faster and the yields are more than the aromatic aldehyde with electron-
donating groups. These reactions have high atom economy ranging from 94.2 to
95.4 %. The reaction of dimedone and malononitrile with aliphatic aldehydes under
the optimized reaction conditions was also tested. As shown in Table 2 (entries 14
and 15), in these cases, the moderate yields of the products were obtained.
To show the merit of the present methodology, we compared the obtained results
using CBSA as a heterogeneous catalyst with other reported results for the synthesis
of tetrahydrobenzo[b]pyrans in the presence of various homogeneous, heteroge-
neous, and supported catalysts. This comparison is shown in Table 3. As can be
seen, our reaction conditions showed a shorter reaction time with high yields of the
desired products.
Reusability of the catalyst was also investigated. After the completion of the
reaction, the catalyst was readily recovered from the reaction mixture by a simple
filtration. In this catalyst, SO3H groups have been covalently bound to the
polycyclic aromatic carbon sheets. Thus, the catalyst has high stability and could
behave as a recyclable solid catalyst without the danger of leaching that is observed
in most supported catalysts. The separated catalyst was washed with cold ethanol
and subsequently dried at 60 °C under vacuum for 1 h before being reused in a
similar reaction. We found that the catalyst could be used at least five times with
only a slight reduction in activity (Fig. 5).
A plausible mechanism may proceed as depicted in Scheme 2. The reaction
occurs via initial formation of the dicyano olefin [I], prepared by Knoevenagel
condensation of aryl aldehydes 2a-o and malononitrile 3, which reacts with
intermediate [II] obtained from dimedone 1 to give the intermediate [III] which
123

Synthesis of tetrahydrobenzo[b]pyrans
subsequently cyclizes to afford the desired compounds 4a-o via the intermediate
[IV]. The catalyst CBSA : HA facilitates the formation of the intermediates [I],
[II], [III], and [IV].
Conclusions
In conclusion, we describe a mild and efficient procedure for the preparation of
tetrahydrobenzo[b]pyrans through the one-vessel, three-component reaction of
dimedone, aryl aldehydes, and malononitrile using a catalytic amount of CBSA as a
heterogeneous catalyst in refluxing EtOH. The reactions have high atom economy
ranging from 94.2 to 95.4 %. This procedure offers several advantages including
clean reactions, high yields, short reaction times, low cost of the catalyst, and
environmentally non-harmful media, as well as a simple experimental and isolated
procedure, which makes it a useful and attractive protocol for the synthesis of these
compounds. However, using aliphatic aldehydes, the moderate yields of the
products were obtained. The catalyst can be recycled after simple handling, and
used at least five times without any substantial reduction in its catalytic activity.
Acknowledgment The authors express their gratitude to the Islamic Azad University, Mashhad Branch
for its financial support.
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