Microwave accelerated the solvent-free synthesis of 4-aryl-3,4-dihydrocoumarin via the tandem reaction of cinnamic acids with phenols catalyzed by Amberlyst 15 resin

Article abstract of DOI:10.1080/00397911.2021.1925918

Amberlyst 15 resin supported the tandem reaction of cinnamic acids with phenols under solvent-free reaction condition has been introduced to afford 4-aryl-3,4-dihydrocoumarin (neoflavanone) derivatives. The efficiency of solid acidic sulfonic resin (A-15) has been illustrated in two reaction activation methods such as microwave irradiation and conventional heating. The important roles of Amberlyst 15 have been emphasized strongly through the high yields of 4-aryl-3,4-dihydrocoumarin in the shorter time under the assistance of microwave irradiation than of conventional heating, and its high recovery and reusability for six catalyst runs. The original catalyst as well as the recycled catalyst were characterized by XRD and FE-SEM to study the correlation of the surface of reused catalyst and its recyclability.

Full text of DOI:10.1080/00397911.2021.1925918

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Microwave accelerated the solvent-free synthesis
of 4-aryl-3,4-dihydrocoumarin via the tandem
reaction of cinnamic acids with phenols catalyzed
by Amberlyst 15 resin
Huu-Phuoc Le, Cong-Thang Duong, Xuan-Triet Nguyen & Thi Xuan Thi Luu
To cite this article: Huu-Phuoc Le, Cong-Thang Duong, Xuan-Triet Nguyen & Thi Xuan Thi
Luu (2021) Microwave accelerated the solvent-free synthesis of 4-aryl-3,4-dihydrocoumarin via
the tandem reaction of cinnamic acids with phenols catalyzed by Amberlyst 15 resin, Synthetic
Communications, 51:14, 2187-2203, DOI: 10.1080/00397911.2021.1925918
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2021, VOL. 51, NO. 14, 2187–2203
https://doi.org/10.1080/00397911.2021.1925918
Microwave accelerated the solvent-free synthesis of 4-aryl-
3,4-dihydrocoumarin via the tandem reaction of cinnamic
acids with phenols catalyzed by Amberlyst 15 resin
Huu-Phuoc Lea, Cong-Thang Duonga , Xuan-Triet Nguyena, and
Thi Xuan Thi Luua,b
b
aDepartment of Organic Chemistry, University of Science, Ho Chi Minh City, Vietnam; Department of
Chemistry, Vietnam National University, Ho Chi Minh City, Vietnam
ABSTRACT
ARTICLE HISTORY
Received 20 January 2021
Amberlyst 15 resin supported the tandem reaction of cinnamic acids
with phenols under solvent-free reaction condition has been intro-
duced to afford 4-aryl-3,4-dihydrocoumarin (neoflavanone) deriva-
tives. The efficiency of solid acidic sulfonic resin (A-15) has been
illustrated in two reaction activation methods such as microwave
irradiation and conventional heating. The important roles of
Amberlyst 15 have been emphasized strongly through the high
yields of 4-aryl-3,4-dihydrocoumarin in the shorter time under the
assistance of microwave irradiation than of conventional heating,
and its high recovery and reusability for six catalyst runs. The original
catalyst as well as the recycled catalyst were characterized by XRD
and FE-SEM to study the correlation of the surface of reused catalyst
and its recyclability.
KEYWORDS
Amberlyst 15; 4-aryl-3,4-
dihydrocoumarin; cinnamic
acid; microwave irradiation;
tandem reaction
GRAPHICAL ABSTRACT
Introduction
Coumarins (A) are one of the most well-known natural compounds presenting in plenty
of plants.[1–6] Their biological activities have been stated in anti-inflammatory,[7] anti-
oxidant,[7,8] anticoagulant,[9] anticancer,[9] and more. Consequently, a wide range of
organic compounds based on coumarin structure has been efficiently synthesized,
CONTACT Thi Xuan Thi Luu
ltxthi@hcmus.edu.vn
Department of Organic Chemistry, University of Science, Ho
Chi Minh City, Vietnam.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2021 Taylor & Francis Group, LLC

2188
H.-P. LE ET AL.
(A)
(B)
Figure 1. The structure of courmarin (A) and 4-aryl-3,4-dihydrocourmarin (B, called neoflavanone).
especially 4-aryl-3,4-dihydrocoumarin as known as neoflavanone (B) (Figure 1). This
system embraced a distinct spot where the neoflavanone has played an important role
as an agent for bactericidal, molluscicidal, insecticidal, anti-inflammatory, anti-fungal,[10]
immunomodulatory, antioxidant,[11] antiherpetic agent affecting on aldose reductase
and protein kinases.[12,13] Especially, 6-methyl-4-phenyl-3,4-dihydrocoumarin was a
common reagent to synthesis tolterodine tartrate, a compound to treat overactive urin-
ary bladder syndrome.[14]
Although the uncommon occurrence of 4-aryl-3,4-dihydrocoumarin in nature has
been accounted for, there are a great number of neoflavanone derivatives found in
Aloe vera,[11] fern Pityrogramma Calomelanos,[15,16] fern Pityrogramma Tartarea,[17]
and Prunus mahaleb.[18] Due to rarely natural existence, several synthetic works of
4-aryl-3,4-dihydrocoumarin framework have been developed with various categories of
reactions, for instance the chiral hydrogenation of 4-arylcoumarins,[19] the direct oxida-
tive cyclization of 3-arylpropionic acids using phenyliodine(III) bis(trifluoroacetate) or
oxone as an oxidant,[20] the three-component reaction of 2-hydroxybenzaldehydes,
Meldrum’s acid and isocyanides,[21] the annulation reactions of phenols with 5-alkyli-
dene Meldrum’s acids catalyzed by Yb(OTf)3,[22] aluminum chloride mediated the reac-
tion of acrylonitrile with phenol or the para-substituted phenols,[23] aluminum chloride
catalyzed the coupling of a-hydroxyketene-S,S-acetals with phenols or naphthol,[24] the
domino conjugate addition-cyclization between arylboroxines and ortho-hydroxycinna-
mate catalyzed by [Rh(OH)(cod)]2,[25] alkenyl carbene chromium(0) complexes reacting
with ketene acetals,[26] Friedel-Crafts alkylation-cycloaddition between tert-butyl
3-hydroxyl-3-(2-hydroxyphenyl)propanoate and arenes catalyzed by HClO4,[27] the
inverse-electron-demand hetero-Diels–Alder reaction with ortho-quinone methides and
oxazolones catalyzed by Sc(III)-complex[28] or organocatalyst,[29] the cycloaddition
ortho-quinone methides and 3-nitro-3,4-dihydrocoumarin with organocatalyst,[30] the
[4 þ 2] annulation of ortho-hydroxy benzhydryl alcohols with oxazolones under phos-
phoric acid catalyst,[31] carbene and acid combination accelerated the cycloaddition of
a-chloro aldehyde and ortho-hydroxy benzhydryl amine,[32] the a-addition/transesterifi-
cation reaction with butenolides and 2-(1-tosylalkyl)phenols under alkaline condition
catalyzed by organocatalyst[33], the 1,6-addition/transesterification of a-isocyanoacetates
and para-quinone methides catalyzed by organocatalyst[34] and the tandem reaction of
cinnamic acid/cinnamate and phenols regarded as the most popular synthetic pathway
of 4-aryl-3,4-dihydrocoumarins. In this route, numerous catalysts such as FeCl3,[35]

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Scheme 1. Solvent-free synthesis of 4-aryl-3,4-dihydrocourmarin via the tandem reaction of cinnamic
acids with phenols.
trifluoroacetic acid (TFA),[36,37] p-toluenesulfonic acid (p-TSA),[38,39] I2,[40,41] Mont
K10,[42,43] silica-supported Wells-Dawson heteropoly acid (H6P2W18O62.24H2O),[44]
Preyssler heteropolyacid (H14P5NaW30O110),[45] BF3ꢀEt2O,[46] H-Y zeolite,[47] and ionic
liquid [H-MNP][HSO4] have been investigated.[48]
With the large and useful applications of 4-aryl-3,4-dihydroxycourmarin group and
our ambitiousness for reaction improvement based on the principles of green chemistry,
we found the interest to synthesize 4-aryl-3,4-dihydroxycoumarin derivatives via the
solvent-free reaction of cinnamic acids with phenols catalyzed by Amberlyst-15 under
the microwave irradiation with high demand for the efficient and eco-friendly method,
preferably recyclable and reusable catalyst (Scheme 1). Amberlyst-15 is a macro reticular
polystyrene based ion exchange resin with strongly acidic sulfonic group. It has been
used as a green and solid acidic catalyst due to safe to use, easy to handle, easy storage,
quick removal of catalyst at the reaction work up with high recyclability and reusability
in several organic reactions, for instance, the esterification of simple alcohols and car-
boxylic acid, the transesterification, the condensation, halogenation, Friedel–Crafts reac-
tion, Michael addition reaction, aza-Michael addition reaction, opening epoxide ring
reaction, multicomponent reaction, etc.[49–51]
Result and discussion
To test our strategy, several categories of acidic catalysts, e.g. traditional Lewis acid
(zinc chloride), Brønsted acid (p-toluenesulfonic acid, p-TSA), acidic solid support
(montmorillonite K-10), acidic ionic liquid 1-methyl-3-(4-sulfobutyl)imidazolium hydro-
gen sulfate [(HSO3)4C4C1Im]HSO4) and acidic polymer (Amberlyst 15 resin, A-15)
were selected incidentally for the reaction of cinnamic acid with phenol at the beginning
of this work as in Table 1. Among the used catalysts, 1-methyl-3-(4-sulfobutyl)imidazo-
lium hydrogen sulfate was regarded as less efficient than zinc chloride, Mont K-10 and
p-TSA; while A-15 resin was stood out as the most efficient catalyst owing to better
yield of desired product obtained 65% (Entry 5, Table 1) under microwave irradiation
at 120 ꢁC for 5 minutes.

2190
H.-P. LE ET AL.
With our experiences on the esterification,[52] the factor on reaction temperature of
the esterification mediated by A-15 firstly selected in the range from 80 to 130 ꢁC was
investigated under the microwave irradiation (Figure 2). The results in Figure 2 showed
that increasing the reaction temperature led to increasing the product selectivity as well
as the reaction yield. Finally, the highest product selectivity and yield of desired product
alternately achieved 100 and 69% at 130 ꢁC.
In the further investigation, the molar ratios of cinnamic acid, phenol and the
amount of A-15 were paid attention owing to their considerable influences on the opti-
mized reaction condition as in Figures 3 and 4. Consequently, increasing the amount of
phenol from 1.5 to 2.0 mmol or the amount of A-15 from 0.1 to 0.2 g led to the increas-
ing of desired product significantly; however, the excess amount of A-15 (used over
Table 1. Nature of catalyst influenced on the formation of 4-aryl-3,4-dihydrocourmarin.a
Yield (%)b
Entry
Catalyst (g)
(3a)
(4a)
1
2
3
4
5
ZnCl2 (0.17)
1
<1
2
3.5
7
32
26
48
4
Mont K10 (0.4)
p-TSA (0.22)
[(HSO3)4C4C1Im]HSO4 (0.6)
Amberlyst 15 (0.2)
65
aThe reaction of cinnamic acid (1.5 mmol) with phenol (1.5 mmol) was performed under microwave irradiation at 120oC
for 5 min.
bYield was calculated based on GC/MS analyses.
100
90
80
69
65
70
60
50
40
30
20
10
0
62
48
36
24
20
10
7
0
80
100
Temperature (°C)
(3a) (4a)
110
120
130
Figure 2. Influence of temperature on the tandem reaction of cinnamic acid (1.5 mmol) with phenol
(1.5 mmol) catalyzed by A-15 (0.2 g) under microwave irradiation for 5 minutes.

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2191
100
90
80
70
60
50
40
86
86
86
84
83
69
57
1.6:1.5 1.5:1.5 1.5:2 1.5:2.5 1.5:2.7 1.5:2.8 1.5:3
Cinnamic acid : phenol (mmol:mmol)
Figure 3. The influence of molar ratio between cinnamic acid and phenol on the yield of 4-aryl-3,4-
dihydrocourmarin formed from the tandem reaction catalyzed by A-15 (0.2 g) under microwave irradi-
ationat 130ꢁC for 5 minutes.
100
90
83
81
75
80
70
60
50
40
30
70
44
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Catalyst amount (g)
Figure 4. Influence of catalytic amount on the tandem reaction of cinnamic acid (1.5 mmol) with
phenol (2.0 mmol) catalyzed by A-15 under microwave irradiation at 130oC for 7 minutes.
0.2 g) could cause the obstacles for reactant interactions leading to the decreasing of
product yield. Therefore, the optimized amount of reagents for the excellent formation
of 4-aryl-3,4-dihydrocourmarin were achieved at the amount of phenol (2.0 mmol), cin-
namic acid (1.5 mmol) and A-15 (0.2 g). In order to increase the reaction conversion as
well as the yield of reaction, the reaction time was also tested for each minute from
5 minutes up to longer time at 130 ꢁC under microwave irradiation. Finally, the max-
imum yield of 4-aryl-3,4-dihydrocourmarine via the tandem reaction was up to 83% at
the optimized reaction time, seven minutes (Entry 1, Table 2).
In order to understand the influences of substituents linked aromatic rings of phenols
and cinnamic acids, altogether twelve phenols, six cinnamic acids and two naphthols were
subjected for further series of experiments by fixing the optimized conditions consisting of
reaction temperature, the molar ratio of reagents and the amount of catalyst and only

2192
H.-P. LE ET AL.
Table 2. The optimized yields of 4-aryl-3,4-dihydrocourmarin derivatives from the hydroarylation
catalyzed by Amberlyst 15 under two activation methods.a
Isolated yield (%)(Time)b
Entry
1
R1
H
R2
H
R3
H
R4
H
R’
H
R’’
H
Product
Method Ac
Method Bd
83 (7)
81 (1.5)
4a
2
3
4
5
6
H
H
H
H
H
H
H
H
H
H
F
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
92 (17)
90 (25)
87 (25)
88 (18)
76 (15)
83 (2.0)
81 (2.0)
80 (2.5)
82 (3.0)
76 (2.0)
4b
4c
4d
Br
OCH3
CH3
4e
4f
7
8
C6H5
H
H
H
H
H
H
H
H
H
47 (18)
40 (9.0)
25e (8.0)
4g
4h
H
NO2
27e (17)
9
H
CH3
CH3
H
H
H
94 (13)
95 (4.0)
(continued)

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2193
10
H
CH3
Cl
H
H
H
94 (36)
92 (3.5)
11
12
H
H
CH3
Cl
H
CH3
H
H
H
93 (12)
93 (7)
91 (4.0)
90 (3.0)
H
H
OCH3
13
H
H
H
H
H
H
H
H
H
CH3
95 (7)
89 (3.0)
14
15
OCH3
OCH3
89 (11)
92 (25)
78 (5.0)
89 (8.0)
H
H
H
H
H
H
H
H
H
Cl
16
17
18
H
H
H
NO2
0 (20)
85 (18)
95 (19)
0 (8.0)
80 (8.0)
92 (9.0)
H
H
aThe reactions were performed with the amount of cinnamic acid derivatives (1.5 mmol), phenols (2.0 mmol) and
Amberlyst 15 (0.2 g) at temperature 130 oC. bReaction time in minutes for method A and in hours for method B.
cMethod A: the reaction mixture was assisted by microwave irradiation.dMethod B: the reaction mixture was assisted
f
by conventional heating. eYields were calculated based on the GC/MS analyses. Ratio of isomers were reported based
on the GC/MS analysis results.
varying a period of appreciate time for each substance tended to reach a noteworthy yield
under two activation methods: microwave irradiation and conventional heating (Table 2).
The results also illustrated that the scope of phenol derivatives was applied for the
tandem reaction in a wide range of substituents from the electron donating groups to

2194
H.-P. LE ET AL.
Table 3. Comparison of previous methods for the synthesis of 3,4-dihydrocoumarin from the reac-
tion of cinnamic acids and phenols promoted by several catalysts.
Catalyst cost (USD)
For 100
For
Method
(ꢁC)
Reusability Yieldg
Catalyst (g)
gramb experiment
Solvent Time (h) (run times)
(%)
Ref.
[35]
FeCl3 (0.24)
TFA (1 mL)
TFA (1 mL)
p-PSA (0.86)
p-PSA (0.25)
I2 (0.33)
I2 (0.33)
Mont K10 (1.0)
Mont K10 (2.0)
SiO2@ H6P2W18O62 (0.44)
H14P5NaW30O110 (0.037)
BF3ꢀEt2O (2.5 mL)
H-Y Zeolite (1.0)
[H-NMP][HSO4] (0.59)
Amberlyst 15 (0.2)
44.50
0.11
Stirring (rt.) CH2Cl2 6 ꢂ 12
None
54–85
54–99
20–100
67–99h
20–83
60–85
50–85
19–81
65–69
56–82
61–77
54–75
68–82
82–95
[36]
89.80c 0.90
D (100)
Stirring (rt.)
D (125)
D (140)
D (130)
D (130)
–
–
–
–
–
–
16 ꢂ 72 None
[37]
89.80c 0.90
24
3
4
None
None
None
None
None
3
[38]
34.30
34.30
91.90
91.90
30.70
30.70
–
0.29
0.08
0.30
0.30
0.31
0.61
[39]
[40]
1 ꢂ 4
1 ꢂ 4
5f
[41]
[42]
M.W (160) PhCl
[43]
M.W (640e)
M.W (110)
D (130)
–
–
–
10f
None
None
4
None
None
None
6
[44]a
[45]a
[46]
–
–
5–15f
2
–
38.00c 0.95
Stirring (rt.)
D (110)
D (120)
–
CH3Ph
–
4 ꢂ 12
[47]a
[48]a
–
–
–
–
4
2 ꢂ 5
7–25f
106.00d 0.85
M.W (130)
–
47–95 Our work
aSynthesized catalysts which could not be estimated their economic potential.
bThe price was consulted in www.sigmaaldrich.com.
e
f
cPrice 100 mL; dPrice 250 gram; Watt; Minutes; gIsolated yield; hGC yield.
the electron withdrawing groups. Owing to the acidity of phenol, the nucleophilic add-
ition of phenol into carbon atom of C ¼ O bond of carboxylic acid often occurs diffi-
cultly. Therefore, the electron donating groups on phenolic ring via inductive effect or
resonance effect have certainly caused the reaction transformation toward the advan-
tages; for instances, the halogen substitutents (e.g. fluoro, chloro and bromo) as well as
methoxy group with electron releasing resonance increased the yields of products, espe-
cially under microwave irradiation (Entry 2–5, Table 2). Similarly, several electron-
donating substituents in phenolic ring accounted for a bit higher yield than the yield
obtained from one electron-donating substituent (Entry 6,9–11, Table 2). Exceptionally,
although the presence of phenyl group located in the ortho position of phenolic ring
caused electron-donating inductive and resonance, a steric hindrance has influenced
predominately on the decrease of the desired product unexpectedly (Entry 7, Table 2).
Moreover, strong electron withdrawing group in phenolic ring as nitro substituent
increases the acidity and decreases the nucleophilicity of phenol group remarkably, con-
sequently the formation of 6-nitro-4-phenyl-3,4-dihydrocoumarin slowly occurred with
low product selectivity and low yield detected by thin layer chromatography and gas
chromatography-mass spectrometry (Entry 8, Table 2). Furthermore, the effects of sub-
stituents in aromatic ring of cinnamic acids have been studied. Subsequently, nitro
group in aromatic ring of cinnamic acid also affected on reaction toward a disadvantage
(Entry 16, Table 2), while the electron donating groups as methyl or methoxy promoted
the better conversion (Entry 12–14, Table 2).
Additionally, in Table 2, the first series of reactions were performed under microwave
irradiation at 130 ꢁC (Method A) and the next series were carried out under conven-
tional heating (Method B). Consequently, the yields of the products were not achieved
much different in both above methods, however the reaction time was reduced remark-
ably. Microwave irradiation is proved that it is useful for the fast and homogeneous

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2195
100
90
80
70
60
50
40
30
20
76
69
67
66
66
64
62
48
7
0
1
2
3
4
5
6
Number of catalyst cycles
Figure 5. Reusability of Amberlyst (0.2 g) used for the tandem reaction of cinnamic acid (1.5 mmol)
with p-cresol (2.0 mmol) under microwave irradiation at 130oC for 15 minutes.
Figure 6. XRD patterns of (a) original Amberlyst 15 (0A-15), (b) first-cycled Amberlyst 15 (1A-15) and
(c) sixth-cycled Amberlyst 15 (6A-15)
heating in any kind of reactions which need heating activation as the
esterification.[53–55]
With advantages of Amberlyst-15 on enhanced reactivity and mildness, its recovery
and reusability were paid attention to be examined following the straightforward work-
up procedure and the optimized reaction conditions. The Amberlyst-15 particles col-
lected after filtration from the previous reaction were washed with methanol (3 ꢃ 5 mL),
and then impregnated with 0.1% aqueous solution of hydrochloric acid for two hours.
Subsequently, Amberlyst-15 particles after filtration to remove the solution of hydro-
chloric acid were washed with methanol (3 ꢃ 5 mL) until pH ¼ 5–6, dried at 100 ꢁC for
3 hours, stored in desiccator overnight and obtained in 85% of recycled yield. The
recycled Amberlyst-15 particles were used for the solvent-free tandem reaction between
p-cresol and cinnamic acid under microwave irradiation at 130 ꢁC for 15 minutes as
that of the optimized experiment presented in Entry 6, Table 2. The catalytic efficiency
of Amberlyst-15 did not decrease drastically even after six cycles of catalyst recovery
and reuse (Figure 5).
The structures of recovered catalysts at the first and sixth recycle times were analyzed
by X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscope (FE-
SEM) in comparison with the structure of the fresh A-15 in terms of surface and activ-
ity. The positions and relative intensities in XRD images of the fresh catalyst (0 A-15),

2196
H.-P. LE ET AL.
Figure 7. SEM images of catalyst Amberlyst 15 (a) original, (b) 1st cycle, (c) 6th cycle at the magnifi-
cation of (1) 40 times, (2) 5,000 times, (3) 100,000 times.
O
COOH
COOH
0.2 g A-15
MW, 130 o
+
No reaction
C, 5 min
1.5 mmol
1.5 mmol
1.5 mmol
S
S
0.2 g A-15
MW, 130 o
H
S
+
C, 5 min
p-Tolyl disulfide
1.5 mmol
(66%)
O
O
O
O
O
COOH
0.2 g A-15
MW, 120 o
H
+
+
C, 5 min
1.5 mmol
1.5 mmol
Phenyl cinnamate 4-Phenyl-3,4-dihydrocoumarin
(7%) (65%)
Scheme 2. Several controlling experiments for reaction mechanism.

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2197
Scheme 3. Plausible mechanism of the reaction between cinnamic acid and phenol.
the first recycled catalyst (1 A-15) and the sixth-recycled catalyst (6 A-15) are all com-
patible. The XRD patterns of three samples have been similar in a broad and wide bot-
tom of a peak to stand for amorphousness of Amberlyst 15 (Figure 6). As shown in
Figure 7, the FE-SEM images at forty-time magnification illustrated that the original
Amberlyst 15 were spherical shape particles with an average diameter in the range of
400–650 mm (1a, Figure 7); however, the resin particles with spherical shape after the
first recycling became bigger in size varying within the range of 550–800 mm (1 b,
Figure 7) and the spherical-shape particles after the sixth-time recycling were larger in
the range of 600–900 mm with long cracks on the surface (1c, Figure 7). At the magnifi-
cation of 5,000 times, three images showed the differences that the small and shallow
cracks appeared on the surface of resin particles after the first recycle (2 b, Figure 7);
while several rougher and deeper cracks were recognized on the surface of resin after
the sixth recycle (2c, Figure 7). Undoubtedly, Amberlyst 15 particles were illustrated to
be porous structure with multiple units linked homogeneously, however the porous
structure of resin particles were found un-homogeneous gradually after the sixth recycle
(3c, Figure 7). The cracks on the surface obviously demonstrated that the catalyst struc-
ture had been destructed after several times recycling, consequently the cohesion inside
spherical-shaped particles was gradually collapsed to make the size of catalyst par-
ticle bigger.
Based on previous investigations on the formation of 4-aryl-3,4-dihydrocoumarin from
the reaction of cinnamic acids and phenols, the reaction mechanism was proposed in two
pathways, either via the esterification followed by the hydroarylation or via the hydroaryla-
tion followed by the esterification.[36,37,40,48] Moreover, the products from the hydroaryla-
tion of cinnamic acids and anisole derivatives mediated by p-toluenesulfonic acid were
obtained in high yields at 125 ꢁC.[38] In order to understand the tandem reaction mechanism
of cinnamic acids and phenols mediated by A-15, several reactions were investigated by per-
forming the reaction of cinnamic acid and anisole or thiophenol catalyzed by A-15 as

2198
H.-P. LE ET AL.
described in Scheme 2. Consequently, either the hydroarylation or the esterification cata-
lyzed by A-15 has not been occurred at 130 ꢁC under the assistance of microwave irradiation
for 5 minutes. While the experiments described in Figure 2 showed that the intermediate
product, phenyl cinnamate was formed and obtained with low yield (7%).
The results of several controlling experiments illustrated that a possible mechanism
for the tandem reaction of cinnamic acids and phenols catalyzed by A-15 under
the assistance of microwave irradiation was proposed firstly via the esterification to
produce phenyl cinnamate, and subsequently the hydroarylation to afford 4-phenyl-
3,4-dihydrocoumarin as described in Scheme 3.
The introduced protocol for the synthesis of 4-aryl-3,4-dihydrocoumarin offers many
advantages in terms of high product yield, green, efficient and economic catalyst, solvent-
free and shorter reaction time under microwave irradiation, compared with the previous lit-
erature reported the syntheses with different reaction conditions as well as catalysts.
Conclusion
Comprehensive experiments on the solvent-free tandem reaction of cinnamic acids and
phenols utilizing the solid acidic catalyst Amberlyst 15 resin under the assistance of
microwave irradiation or conventional heating have made us to introduce and get down
to some considerations. Microwave irradiation has efficiently activated the formation of
4-aryl-3,4-dihydrocoumarin derivatives with less time-consumption (in minutes) than
conventional heating (in hours). Furthermore, Amberlyst 15 resin is eco-friendly, effi-
cient and economic acidic catalyst for this solvent-free tandem reaction irradiated by
means of microwave oven. The catalyst has been proved to successfully promote in a
wide range of phenols and cinnamic acids with moderate to high yields of desired prod-
ucts. Interestingly, the structure and efficiency of the reused catalyst have not impacted
the product yield even after several recycles.
Experimental
Chemicals and instrumentation
All commercially available chemicals used were from Aldrich Sigma/Acros and analyzed
for authenticity and purity by GC/MS (gas chromatography/mass spectrometry) before
€
being used. The melting point was determined by Buchi melting point. The microwave
irradiation was performed by means of microwave oven CEM – Discover, the irradiation
program was applied to get the most efficient reaction conditions. NMR spectra were
recorded on a Bru€ker Advance DPX 500 MHz spectrometer at 500 MHz (1H) and
125 MHz (13C); deuterated solvent was purchased from Cambridge Isotope Laboratories
Inc. (CDCl3 and CD3COCD3). GC/MS Hewlett Packard 6890GC Series II with TX – DP5
column was employed to monitor reaction progress. HRMS-ESI (electrospray ionization)
€
analyses were performed on a microTOF – QII (Bruker) with UV/Vis and MS detector,
the heated capillary of ion trap mass spectrometer was set to 350 ꢁC, reverse column ACE
3C18 (5 mm ꢃ 4.6ꢃ 150 mm). The structure of Amberlyst 15 particles before and after
usage were characterized by D2 Phaser ꢂ Bru€ker X-ray diffraction (XRD) (Cu Ka radiation,

R
SYNTHETIC COMMUNICATIONSV
2199
scanning speed 0.02ꢁ minꢂ1 from 5ꢁ to 70ꢁ) and were investigated about their dimension
by means of S4800 HITACHI Scanning electron microscopy (SEM).
General procedure
The tandem reaction of cinnamic acid with phenol catalyzed by amberlyst-15 under
microwave irradiation (method A)
Suitable amounts of cinnamic acid (1.5 mmol, 0.222 g), phenol (2.0 mmol, 0.188 g) and
Amberlyst 15 (0.2 g) were added successively into the test tube (U ¼ 10 mm, h ¼ 100 mm)
placed into CEM oven. The irradiation program set up at 130 ꢁC for 7 minutes was
applied to the reaction mixture. After cooling, the product mixture was extracted with
diethyl ether (4ꢃ 15 mL) and filtrated by porous filter funnel to separate and collect the
acidic catalyst A-15 for recycling. Subsequently, the filtrate was stirred magnetically with
10mL of aqueous sodium hydroxide 5% (to remove all excess or unreacted cinnamic acid
and phenol) and then washed with water until neutralization. Organic layer was dried by
anhydrous sodium sulfate and removed solvent by rotary evaporation. The remaining
crude product was analyzed by GC/MS to check the composition of crude product. The
desired product was purified by silica gel column chromatography, and then was identi-
1
fied their structure by HRMS, H and 13C NMR spectroscopy.
The tandem reaction of cinnamic acid with phenol catalyzed by amberlyst-15 under
conventional heating (method B)
A test tube (U ¼ 10 mm, h ¼ 200 mm) containing a pertinent quantity of cinnamic acid
(1.5 mmol, 0.222 g), phenol (2.0 mmol, 0.188 g) and Amberlyst 15 (0.2 g) was placed in
an oil bath heated to the suitable temperature (130 ꢁC) for 1.5 hours. After cooling, the
reaction mixture was worked up as described in Method A.
Full spectral data of all compounds can be found in Supporting Information via the
“Supplementary Content” section of this article’s webpage.
Acknowledgement
We acknowledge Van Cong Le, Xuan-Huyen Thi Ngo, Thanh Van Tran, Hoai-Linh Thi Tran
(Ho Chi Minh University of Science) and Prof. Fritz Duus (Roskilde University) for technical
assistance and chemical support.
Disclosure statement
No potential conflict of interest was reported by the author(s).
ORCID
Cong-Thang Duong
Thi Xuan Thi Luu
http://orcid.org/0000-0002-3753-5389
http://orcid.org/0000-0002-9984-6633

2200
H.-P. LE ET AL.
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