# Pentafluoropropionic Acid: An Efficient and Metal-Free Catalyst for the One-Pot Synthesis of Tetrahydrobenzo[b]pyran Derivatives

Authors: Naser Montazeri; Taghva Noghani; Mona Ghorchibeigy; Rozita Zoghi
Journal: Journal of Chemistry
 (2014)

---

## Abstract

Pentafluoropropionic acid (PFPA) efficiently catalyzes the one-pot, three-component reaction of aromatic aldehyde, malononitrile, and dimedone to yield tetrahydrobenzo[b]pyran derivatives in high yields. This method is of great value because of its easy processing, short reaction time, environmentally, and high yields.


---

## Body

## 1. Introduction

Tetrahydrobenzo[b]pyran derivatives are important classes of heterocyclic compounds. Tetrahydrobenzo[b]pyran compounds are known to possess a variety of biological activities, such as anticoagulant, spasmolytic, diuretic, anticancer, and antianaphylactin properties [1–5]. Furthermore these compounds can be employed as pigments, and they constitute the structural unit of a series of natural products [6, 7]. In view of different biological and chemical applications of tetrahydrobenzo[b]pyran derivatives, the development of suitable synthetic methodologies for generation has been a topic of great interest in recent times [8]. A number of 2-amino-tetrahydropyran derivatives are useful as photoactive materials [9]. Several methods have been reported for the synthesis of tetrahydrobenzo[b]pyran derivatives from aromatic aldehydes, dimedone, and malononitrile, involving the use of catalysts such as ZnO-beta zeolite [10], Mw-NaBr [11], Na2SeO4 [12], Caro’s acide-SiO2 [13], trisodium citrate [14], PPA-SiO2 [15], KF-basic alumina under ultrasound irradiation [16], TEBA [17], 1-butyl-3-methylimidazolium hydroxide [18], sodium hypochlorite [19], 1,4-diazabicyclo[2.2.2] octane (DABCO) [20], triethylamine [21], Ce1MgxZr1-xO2 [22], silica gel supported polyamine [23], urea-choline chloride [24], 2,2,2-trifluoroethanol [25], tetramethyl ammonium hydroxide [26], ammonium acetate [27], and tetrabutylammonium bromide [28].However, most of these procedures have significant drawbacks such as harsh reaction conditions, difficult workup, and expensive reagents. These problems promoted us towards further investigation in search for a new catalyst, which will carry out the synthesis of tetrahydrobenzo[b]pyrans under simpler experimental setup. In continuation of our efforts to develop novel synthetic routes using metal-free catalysts in organic reactions [29–31], and due to our interest in the synthesis of heterocyclic compounds [32], herein we wish to report an efficient synthesis of tetrahydrobenzo[b]pyran derivatives by cyclocondensation reaction of aromatic aldehydes, dimedone, and malononitrile using pentafluoropropionic acid as a metal-free catalyst (Scheme 1).Scheme 1
Synthesis of tetrahydrobenzo[b]pyrans catalyzed by pentafluoropropionic acid.


## 2. Experimental

### 2.1. General

All chemicals were obtained from Merck or Fluka and were used without further purification. Melting points were recorded on an electrothermal type 9100 melting point apparatus and are uncorrected. Infrared spectra were obtained in KBr disks on shimadzu IR-470 spectrometer.1H NMR and 13C NMR spectra were recorded on a Bruker, DRX-400 Avance Bruker spectrometer, at 400.13 MHz and 100.22 MHz, respectively, in CDCl3 and chemical shifts are in ppm (δ) relative to internal TMS.


### 2.2. General Procedure for the Synthesis of Tetrahydrobenzo[b]pyrans (4a–j)

A solution of dimedone 1 (1 mmol), an aromatic aldehyde2a–j (1 mmol), malononitrile 3 (1.2 mmol), and pentafluoropropionic acid (35 mol%) in H2O (10 mL) and EtOH (10 mL) was stirred at room temperature for the time period as indicated in Table 1. The progress of the reaction was monitored by TLC. After completion of reaction, the solid product was collected by filtration and recrystallized from ethanol to afford pure products4a–j (Table 2) in high yields. All the products were identified by comparison of spectral data (IR and 1H NMR) and m.p. with those reported.Table 1
Optimizing the reaction conditionsa.
Entry
Catalyst (mol%)
Solvent
Condition
Time (min)
Yield (%)b
1
—
EtOH
Reflux
150
30
2
—
H2O
Reflux
150
32
3
—
CHCl3
Reflux
180
25
4
—
CH3CN
Reflux
150
28
5
—
EtOH : H2O (1 : 1)
Reflux
120
34
6
—
EtOH
r.t
180
Trace
7
—
H2O
r.t
180
Trace
8
—
CHCl3
r.t
180
Trace
9
—
CH3CN
r.t
180
Trace
10
—
EtOH : H2O (1 : 1)
r.t
180
Trace
11
(PFPA) 30
—
r.t
100
55
12
(PFPA) 35
—
r.t
100
66
13
(PFPA) 40
—
r.t
100
64
14
(PFPA) 30
EtOH : H2O (1 : 1)
r.t
120
85
15
(PFPA) 35
EtOH : H2O (1 : 1)
r.t
80
90
16
(PFPA) 40
EtOH : H2O (1 : 1)
r.t
80
90
17
(PFPA) 35
EtOH
r.t
100
65
18
(PFPA) 35
H2O
r.t
100
68
19
(PFPA) 35
CHCl3
r.t
120
47
20
(PFPA) 35
CH3CN
r.t
100
56
21
(TFAA) 30
—
r.t
150
41
22
(TFAA) 35
—
r.t
150
52
23
(TFAA) 40
—
r.t
150
51
24
(TFAA) 30
EtOH : H2O (1 : 1)
r.t
150
73
25
(TFAA) 35
EtOH : H2O (1 : 1)
r.t
120
77
26
(TFAA) 40
EtOH : H2O (1 : 1)
r.t
120
79
27
(TFAA) 35
EtOH
r.t
120
36
28
(TFAA) 35
H2O
r.t
120
33
29
(TFAA) 35
CHCl3
r.t
180
24
30
(TFAA) 35
CH3CN
r.t
120
29
aBenzaldehyde (1 mmol), dimedone (1 mmol), and malononitrile (1.2 mmol).
bIsolated yields.Table 2
Synthesis of tetrahydrobenzo[b]pyrans using pentafluoropropionic acid as catalysta.
Entry
Ar
Productb
Time (min)
Yield (%)c
m.p., (°C) [reference]
1
C6H5
4a
80
90
227–229 [28]
2
3-NO2C6H4
4b
80
89
207–210 [28]
3
4-MeOC6H4
4c
60
90
197–199 [22]
4
4-MeC6H4
4d
60
92
216–218 [13]
5
4-BrC6H4
4e
60
90
206–208 [10]
6
3-MeOC6H4
4f
80
90
207–209 [—]
7
3-ClC6H4
4g
80
89
223–224 [13]
8
4-ClC6H4
4h
60
92
213–215 [11]
9
2-ClC6H4
4i
80
89
203–205 [14]
10
4-HOC6H4
4j
80
90
206–208 [22]
aAromatic aldehyde (1 mmol), dimedone (1 mmol), malononitrile (1.2 mmol), and PFPA (35 mol%) in the mixture of EtOH : H2O (1 : 1).
bThe products were characterized by comparison of their spectroscopic and physical data with those reported in the literature.
cIsolated yield.


### 2.3. Physical and Spectral Data for the Selected Compounds

2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-phenyl-4H-benzopyran
(4a). m.p. = 227–229°C; IR (KBr) cm−1 3410, 3330 (NH2), 3050 (C–H), 2233 (CN), 1681 (C=O), 1380 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.32–7.19 (m, 5H), 4.56 (s, 2H, NH2), 4.42 (s, 1H, CH), 2.47 (dd, 2H, J=19.2 Hz, CH2), 2.24 (dd, 2H, J=16 Hz, CH2), 1.13 (s, 3H, CH3), 1.06 (s, 3H, CH3).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(3-nitrophenyl)-5-oxo-4H-benzopyran
(4b). m.p. = 207–210°C; IR (KBr) cm−1 3420, 3300 (NH2), 3012 (C–H), 2257 (CN), 1710 (C=O), 1557, 1360 (NO2); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 8.11–7.48 (m, 4H), 4.80 (s, 2H, NH2), 4.53 (s, 1H, CH), 2.50 (dd, J=13.2 Hz, 2H, CH2), 2.25 (dd, J=16 Hz, 2H, CH2), 1.12 (s, 3H, CH3), 1.08 (s, 3H, CH3).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(4-methylphenyl)-5-oxo-4H-benzopyran
(4d). m.p. = 216–218°C; IR (KBr) cm−1 3421, 3297 (NH2), 2966 (C–H), 2194 (CN), 1674 (C=O), 1369 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.08–7.14 (m, 4H), 4.54 (s, 2H, NH2), 4.38 (s, 1H, CH), 2.46 (dd, J=5.2 Hz, 2H, CH2), 2.29 (s, 3H, CH3), 2.23 (dd, J=5.6 Hz, 2H, CH2), 1.12 (s, 3H, CH3), 1.06 (s, 3H, CH3).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(3-methoxyphenyl-5-oxo-4H-benzopyran
(4f). m.p. = 207–209°C; IR (KBr) cm−1 3397, 3291 (NH2), 2295 (CN), 1678 (C=O), 1384 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.29–6.76 (m, 4H), 4.57 (s, 2H, NH2), 4.40 (s, 1H, CH), 3.81 (m, 3H, CH3), 2.47 (dd, J=19.6 Hz, 2H, CH2), 2.26 (dd, J=18 Hz, 2H, CH2), 1.14 (s, 3H, CH3), 1.08 (s, 3H, CH3); 13C NMR (100.22 MHz, CDCl3) δ (ppm): 27.7 (CH3), 28.8 (CH3), 32.2 (C), 35.4 (CH2), 40.6 (CH2), 50.6 (CH3), 55.2 (CH), 63.5 (C), 112.3 (CH), 113.5 (CH), 113.9 (C), 118.6 (C), 119.9 (CH), 129.6 (CH), 144.8 (C), 157.4 (C), 159.7 (C), 161.6 (C), 195.8 (C).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(4-chlorophenyl)-5-oxo-4H-benzopyran
(4h). m.p. = 213–215°C; IR (KBr) cm−1 3421, 3105 (NH2), 2185 (CN), 1686 (C=O), 1356 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.25–7.02 (m, 4H), 4.57 (s, 2H, NH2), 4.41 (s, 1H, CH), 2.47 (s, 2H, CH2), 2.24 (dd, J=16.4 Hz, 2H, CH2), 1.14 (s, 3H, CH3), 1.05 (s, 3H, CH3).


## 2.1. General

All chemicals were obtained from Merck or Fluka and were used without further purification. Melting points were recorded on an electrothermal type 9100 melting point apparatus and are uncorrected. Infrared spectra were obtained in KBr disks on shimadzu IR-470 spectrometer.1H NMR and 13C NMR spectra were recorded on a Bruker, DRX-400 Avance Bruker spectrometer, at 400.13 MHz and 100.22 MHz, respectively, in CDCl3 and chemical shifts are in ppm (δ) relative to internal TMS.


## 2.2. General Procedure for the Synthesis of Tetrahydrobenzo[b]pyrans (4a–j)

A solution of dimedone 1 (1 mmol), an aromatic aldehyde2a–j (1 mmol), malononitrile 3 (1.2 mmol), and pentafluoropropionic acid (35 mol%) in H2O (10 mL) and EtOH (10 mL) was stirred at room temperature for the time period as indicated in Table 1. The progress of the reaction was monitored by TLC. After completion of reaction, the solid product was collected by filtration and recrystallized from ethanol to afford pure products4a–j (Table 2) in high yields. All the products were identified by comparison of spectral data (IR and 1H NMR) and m.p. with those reported.Table 1
Optimizing the reaction conditionsa.
Entry
Catalyst (mol%)
Solvent
Condition
Time (min)
Yield (%)b
1
—
EtOH
Reflux
150
30
2
—
H2O
Reflux
150
32
3
—
CHCl3
Reflux
180
25
4
—
CH3CN
Reflux
150
28
5
—
EtOH : H2O (1 : 1)
Reflux
120
34
6
—
EtOH
r.t
180
Trace
7
—
H2O
r.t
180
Trace
8
—
CHCl3
r.t
180
Trace
9
—
CH3CN
r.t
180
Trace
10
—
EtOH : H2O (1 : 1)
r.t
180
Trace
11
(PFPA) 30
—
r.t
100
55
12
(PFPA) 35
—
r.t
100
66
13
(PFPA) 40
—
r.t
100
64
14
(PFPA) 30
EtOH : H2O (1 : 1)
r.t
120
85
15
(PFPA) 35
EtOH : H2O (1 : 1)
r.t
80
90
16
(PFPA) 40
EtOH : H2O (1 : 1)
r.t
80
90
17
(PFPA) 35
EtOH
r.t
100
65
18
(PFPA) 35
H2O
r.t
100
68
19
(PFPA) 35
CHCl3
r.t
120
47
20
(PFPA) 35
CH3CN
r.t
100
56
21
(TFAA) 30
—
r.t
150
41
22
(TFAA) 35
—
r.t
150
52
23
(TFAA) 40
—
r.t
150
51
24
(TFAA) 30
EtOH : H2O (1 : 1)
r.t
150
73
25
(TFAA) 35
EtOH : H2O (1 : 1)
r.t
120
77
26
(TFAA) 40
EtOH : H2O (1 : 1)
r.t
120
79
27
(TFAA) 35
EtOH
r.t
120
36
28
(TFAA) 35
H2O
r.t
120
33
29
(TFAA) 35
CHCl3
r.t
180
24
30
(TFAA) 35
CH3CN
r.t
120
29
aBenzaldehyde (1 mmol), dimedone (1 mmol), and malononitrile (1.2 mmol).
bIsolated yields.Table 2
Synthesis of tetrahydrobenzo[b]pyrans using pentafluoropropionic acid as catalysta.
Entry
Ar
Productb
Time (min)
Yield (%)c
m.p., (°C) [reference]
1
C6H5
4a
80
90
227–229 [28]
2
3-NO2C6H4
4b
80
89
207–210 [28]
3
4-MeOC6H4
4c
60
90
197–199 [22]
4
4-MeC6H4
4d
60
92
216–218 [13]
5
4-BrC6H4
4e
60
90
206–208 [10]
6
3-MeOC6H4
4f
80
90
207–209 [—]
7
3-ClC6H4
4g
80
89
223–224 [13]
8
4-ClC6H4
4h
60
92
213–215 [11]
9
2-ClC6H4
4i
80
89
203–205 [14]
10
4-HOC6H4
4j
80
90
206–208 [22]
aAromatic aldehyde (1 mmol), dimedone (1 mmol), malononitrile (1.2 mmol), and PFPA (35 mol%) in the mixture of EtOH : H2O (1 : 1).
bThe products were characterized by comparison of their spectroscopic and physical data with those reported in the literature.
cIsolated yield.


## 2.3. Physical and Spectral Data for the Selected Compounds

2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-phenyl-4H-benzopyran
(4a). m.p. = 227–229°C; IR (KBr) cm−1 3410, 3330 (NH2), 3050 (C–H), 2233 (CN), 1681 (C=O), 1380 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.32–7.19 (m, 5H), 4.56 (s, 2H, NH2), 4.42 (s, 1H, CH), 2.47 (dd, 2H, J=19.2 Hz, CH2), 2.24 (dd, 2H, J=16 Hz, CH2), 1.13 (s, 3H, CH3), 1.06 (s, 3H, CH3).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(3-nitrophenyl)-5-oxo-4H-benzopyran
(4b). m.p. = 207–210°C; IR (KBr) cm−1 3420, 3300 (NH2), 3012 (C–H), 2257 (CN), 1710 (C=O), 1557, 1360 (NO2); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 8.11–7.48 (m, 4H), 4.80 (s, 2H, NH2), 4.53 (s, 1H, CH), 2.50 (dd, J=13.2 Hz, 2H, CH2), 2.25 (dd, J=16 Hz, 2H, CH2), 1.12 (s, 3H, CH3), 1.08 (s, 3H, CH3).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(4-methylphenyl)-5-oxo-4H-benzopyran
(4d). m.p. = 216–218°C; IR (KBr) cm−1 3421, 3297 (NH2), 2966 (C–H), 2194 (CN), 1674 (C=O), 1369 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.08–7.14 (m, 4H), 4.54 (s, 2H, NH2), 4.38 (s, 1H, CH), 2.46 (dd, J=5.2 Hz, 2H, CH2), 2.29 (s, 3H, CH3), 2.23 (dd, J=5.6 Hz, 2H, CH2), 1.12 (s, 3H, CH3), 1.06 (s, 3H, CH3).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(3-methoxyphenyl-5-oxo-4H-benzopyran
(4f). m.p. = 207–209°C; IR (KBr) cm−1 3397, 3291 (NH2), 2295 (CN), 1678 (C=O), 1384 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.29–6.76 (m, 4H), 4.57 (s, 2H, NH2), 4.40 (s, 1H, CH), 3.81 (m, 3H, CH3), 2.47 (dd, J=19.6 Hz, 2H, CH2), 2.26 (dd, J=18 Hz, 2H, CH2), 1.14 (s, 3H, CH3), 1.08 (s, 3H, CH3); 13C NMR (100.22 MHz, CDCl3) δ (ppm): 27.7 (CH3), 28.8 (CH3), 32.2 (C), 35.4 (CH2), 40.6 (CH2), 50.6 (CH3), 55.2 (CH), 63.5 (C), 112.3 (CH), 113.5 (CH), 113.9 (C), 118.6 (C), 119.9 (CH), 129.6 (CH), 144.8 (C), 157.4 (C), 159.7 (C), 161.6 (C), 195.8 (C).2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-4-(4-chlorophenyl)-5-oxo-4H-benzopyran
(4h). m.p. = 213–215°C; IR (KBr) cm−1 3421, 3105 (NH2), 2185 (CN), 1686 (C=O), 1356 (C–O); 1H NMR (400.13 MHz, CDCl3) δ (ppm): 7.25–7.02 (m, 4H), 4.57 (s, 2H, NH2), 4.41 (s, 1H, CH), 2.47 (s, 2H, CH2), 2.24 (dd, J=16.4 Hz, 2H, CH2), 1.14 (s, 3H, CH3), 1.05 (s, 3H, CH3).


## 3. Result and Discussion

In order to optimize the reaction conditions, including solvents and temperature, the reaction was conducted under various conditions and the results are listed in Table1. In an optimized reaction condition, benzaldehyde (1 mmol), dimedone (1 mmol), and malononitrile (1.2 mmol) in H2O (10 mL) and EtOH (10 mL) were mixed in the presence of pentafluoropropionic acid (35 mol%) as catalyst for 60–80 min. The reaction proceeds very cleanly at room temperature and was free of side products. After completion of the reaction (monitored by TLC), a simple workup affords the products in high yields (Scheme 1). Among the solvents tested, the reaction in H2O, EtOH, CHCl3, and CH3CN using 35 mol% of the catalyst gave a moderate yield of the desired product at room temperature. However in the mixture of EtOH and H2O relatively high yield of the product is obtained at room temperature after 80 min. Without catalyst, in refluxing EtOH, H2O, CHCl3, CH3CN, and mixture of EtOH-H2O or at room temperature in this solvents the reaction times are prolonged and the yields are poor. In the solvent-free conditions, even in the presence of 40 mol% of the catalyst at room temperature, the yields are moderate. The results are summarized in Table 1. For comparison, we also investigated the efficiency of trifluoroacetic acid (TFAA) as catalyst in this model reaction. As shown in Table 1, it can be seen that PFPA proved to be a better catalyst than TFAA in terms of reaction time and yield.We also evaluated the amount of pentafluoropropionic acid required for this transformation. It was found that the yield of product was affected by the catalyst amount. Increasing the amount of the catalyst up to 35 mol% in the mixture of EtOH and H2O at room temperature increased the yield of the product. Further increase in the catalyst amount did not increase the yield noticeably. In order to show generality and scope of this new protocol, we used various substituted aromatic aldehydes and the results obtained are summarized in Table 2.In all cases, aromatic aldehydes with substituents carrying either electron-donating or electron-withdrawing groups reacted successfully and gave the expected products in high yields and short reaction times. The type of aldehyde had no significant effect on the reaction. The efficiency of pentafluoropropionic acid as a catalyst for the synthesis of the 2-amino-3-cyano-5,6,7,8-tetrahydro-7, 7-dimethyl-5-oxo-4-phenyl-4H-benzopyran(4a), was compared with that of other catalysts reported in the literature. Some of the results are summarized in Table 3. It is clear from this table that pentafluoropropionic acid is an efficient and environmentally benign catalyst which could be useful in the synthesis of a series of tetrahydrobenzo[b]pyran derivatives.Table 3
Comparison of efficiency of various catalysts in synthesis of tetrahydrobenzo[b]pyrans.
Entry
Catalyst
Conditions
Time (min)
Yield (%)
Reference
1
Trisodium citrate
EtOH–H2O; reflux
5–120
80–96
[14]
2
Na2SeSO4
EtOH–H2O; reflux
30–180
85–98
[12]
3
PPA-SiO2
H2O; reflux
8–15
77–93
[15]
4
Caro’s acid-SiO2
EtOH–H2O; reflux
15–20
92–95
[13]
5
ZnO-beta zeolite
EtOH; reflux
35–52
86–95
[10]
6
NaBr
MW; 70–80°C
10–15
85–95
[11]
7
TBAB
EtOH; reflux
20–140
87–95
[28]
8
NaOCl
Grinding
10–30
80–88
[19]
9
Ce1MgxZr1-xO2
EtOH; reflux
35–45
90–94
[22]
10
Supported polyamine
EtOH–H2O; reflux
120–220
88–95
[23]
11
Urea-ChCl
80°C
60–240
75–95
[24]
12
TMAH
H2O; r.t.
30–120
80–92
[26]
13
PFPA
EtOH–H2O; r.t.
60–80
89–92
This work


## 4. Conclusion

In conclusion, a mild and efficient method is proposed for the one-pot three-component reactions of aromatic aldehydes, dimedone, and malononitrile using pentafluoropropionic acid catalyst for synthesis of tetrahydrobenzo[b]pyran derivatives. Some attractive features of this protocol are high yields, easy workup, and the simplicity of the procedure.



---

*Source: 596171-2014-04-23.xml*