Sulfonated nanohydroxyapatite functionalized with 2-aminoethyl dihydrogen phosphate (HAP@AEPH2-SO3H) as a reusable solid acid for direct esterification of carboxylic acids with alcohols

Article abstract of DOI:10.1007/s11164-015-2404-8

Sulfonated nanohydroxyapatite functionalized with 2-aminoethyl dihydrogen phosphate (HAP@AEPH₂-SO₃H) efficiently catalyzed direct esterification of carboxylic acids and alcohols with high selectivity toward the formation of esters in good to excellent yields. Our results clearly show that HAP@AEPH₂-SO₃H can be easily recovered by simple filtration and reused for subsequent five runs without any significant impact on yields of products. The main advantage of this methodology is easy and ecofriendly catalyst preparation, easy catalyst separation, practical simplicity, safe reaction conditions, recyclable catalyst and high products yields.

Full text of DOI:10.1007/s11164-015-2404-8

Res Chem Intermed
DOI 10.1007/s11164-015-2404-8
Sulfonated nanohydroxyapatite functionalized
with 2-aminoethyl dihydrogen phosphate
(HAP@AEPH2-SO3H) as a reusable solid acid
for direct esterification of carboxylic acids with alcohols
Narges Yousefi Siavashi1 Batool Akhlaghinia1
•
•
Monireh Zarghani1
Received: 4 October 2015 / Accepted: 21 December 2015
Ó Springer Science+Business Media Dordrecht 2016
Abstract Sulfonated nanohydroxyapatite functionalized with 2-aminoethyl dihy-
drogen phosphate (HAP@AEPH2-SO3H) efficiently catalyzed direct esterification
of carboxylic acids and alcohols with high selectivity toward the formation of esters
in good to excellent yields. Our results clearly show that HAP@AEPH2-SO3H can
be easily recovered by simple filtration and reused for subsequent five runs without
any significant impact on yields of products. The main advantage of this method-
ology is easy and ecofriendly catalyst preparation, easy catalyst separation, practical
simplicity, safe reaction conditions, recyclable catalyst and high products yields.
Keyword Sulfonated nanohydroxyapatite functionalized with 2-aminoethyl
dihydrogen phosphate (HAP@AEPH2-SO3H) Á Direct esterification Á
Carboxylic acid Á Alcohol
Introduction
Organic esters as key components in a variety of natural products are important
structural units having wide applications in a variety of areas such as perfumes,
flavors, pharmaceuticals, plasticizers, solvents and chemical intermediates [1, 2].
Accordingly, esterification of carboxylic acids constitutes a very important and
widespread process in organic synthesis. Consequently, numerous chemical
methods have been developed to construct ester linkage [3–25]. Particularly direct
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2404-8)
contains supplementary material, which is available to authorized users.
&
Batool Akhlaghinia
akhlaghinia@um.ac.ir
1
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad,
Mashhad 9177948974, Iran
123

N. Y. Siavashi et al.
coupling of carboxylic acids with alcohols is one of the simplest but yet highly
challenging transformations in organic chemistry from both academic and industrial
points of view. Although many strong homogeneous and conventional acid catalysts
can normally promote these transformations toward ester formation [26–33], there
are several crucial issues such as using corrosive, expensive, toxic reagents, harsh
reaction conditions, tedious workups, non-recyclability of catalysts, acidic waste
generation which constitutes real environmental problems. Therefore, in order to
remove the above mentioned drawbacks and also promote green chemistry and atom
efficiency by avoiding the use of large amounts of strong homogeneous and
conventional acid catalysts, the replacement of current chemical processes with
more environmentally benign alternatives is an increasingly attractive subject.
Solid acid catalysis as one of the economically and ecologically important fields
´
in catalysis, have many advantages over liquid BrØnsted acid catalysts such as
noncorrosive, effective separation of catalyst, waste reduction, and simplified
purification of the products. The solid acid catalysts employed in the esterification
reactions of carboxylic acids with alcohols include sulfonated polypyrene (S-PPR)
[34], sulfonic acid-functionalized periodic mesoporous organosilicas [35], polymer-
supported sulfonated magnetic resin [36] macroporous polymeric acid catalyst [37],
?
ß-cyclodextrin-SO3H [38], carbon material functionalized with NH2 and SO3H
groups [39], SBA-15-functionalized sulfonic acid [40], H3PO4/TiO2-ZrO2 [41].
Unfortunately, the mentioned methods have several drawbacks such as, the use of
expensive reagents, poor yields, long reaction times and tedious workup. In
addition, some of them do not promote green synthesis and atom efficiency. Hence,
development a new, simple, fast, efficient, and highly profitable direct esterification
reaction under mild reaction conditions is still highly desirable and demanded.
Hydroxyapatite (HAP) is one of the most attractive inorganic compounds due to its
excellent biocompatibility and surface active properties. Recently, there has been a
growing interest in the synthesis of heterogeneous catalyst based on the grafting of
various functional groups onto the surface of HAP because of good chemical stability,
high surface area and simple preparation methods [42–50]. Owing to the potential
advantages mentioned above, recently, we reported functionalized sulfonated
nanohydroxyapatite by 2-aminoethyl dihydrogen phosphate (HAP@AEPH2-SO3H)
as an efficient, eco-friendly and reusable heterogeneous solid acid catalyst in the
synthesis of 4,40-(aryl methylene)bis(1H-pyrazol-5-ol)s derivatives [51]. Herein we
decided to extend these approaches by using HAP@AEPH2-SO3H for direct
esterification of carboxylic acids with alcohols to affording high catalytic activity
and selectivity that can be reused properly at least 5 times.
Experimental
General
The purity determinations of the products were accomplished by TLC on silica gel
polygram STL G/UV 254 plates. The melting points of products were determined
with an Electrothermal Type 9100 melting point apparatus. The FT-IR spectra were
123

Sulfonated nanohydroxyapatite functionalized with 2…
recorded on an Avatar 370 FT-IR Therma Nicolet spectrometer. The NMR spectra
were provided on Bruker Avance 300 and 400 MHz instruments in CDCl3.
Elemental analyses were performed using a Thermo Finnegan Flash EA 1112 Series
instrument. Mass spectra were recorded with a CH7A Varianmat Bremen
instruments at 70 eV; in m/z (rel %). Elemental compositions were determined
with a Leo 1450 VP scanning electron microscope equipped with an SC7620 energy
dispersive spectrometer (SEM–EDS) presenting a 133 eV resolution at 20 kV.
Transmission electron microscopy (TEM) was performed with a Leo 912 AB
(120 kV) microscope (Zeiss, Germany). BET surface area and pore size distribution
were measured on a Belsorp mini II system at -196 °C using N2 as adsorbate.
All of the known products were characterized by the FT-IR spectroscopy, mass
spectrometry and comparison of their melting points with authenic samples. The
1
structure of selected products was further confirmed by H NMR and 13C NMR
spectroscopy. The catalyst was prepared and characterized according the method
reported previously [51]. All yields refer to isolated products after purification by
recrystallization or thin layer chromatography.
Preparation of nanoHAP functionalized with AEPH2 (HAP@AEPH2)
To a solution of Ca(NO3)2Á4H2O (50 mL, 1.08 M, at pH adjusted to 10 with
NH4OH) in a three-necked 500 mL round-bottomed flask equipped with condenser,
argon gas inlet tube and dropping funnel, solution of (NH4)2HPO4 (50 mL, 0.65 M
at pH adjusted to 10 with NH4OH) was added at 90 °C with stirring. AEPH2 (0.8 g,
5 mmol) was added to the resulting suspension. The precipitate was maintained in
contact with the reaction solution for 5 h at 90 °C under stirring. Then suspension
was centrifuged at 10,000 rpm for 10 min and repeatedly washed with CO2-free
distilled water (3 9 20 mL). The product (nanoHAP functionalized with AEPH2)
was dried at 50 °C overnight [51].
Preparation of HAP@AEPH2-SO3H
To a magnetically stirred mixture of HAP@AEPH2 (1 g) in 20 mL of dry
dichloromethane, 0.5 mL (0.75 mmol) of chlorosulfuric acid was added drop by
drop at room temperature. After 20 min the suspension was filtered and washed with
20 mL of dichloromethane and dried at room temperature to afford HAP@AEPH2-
SO3H as a white powder [51].
Characterization of HAP@AEPH2-SO3H
HAP@AEPH2-SO3H as a novel, eco-friendly, reusable heterogeneous solid acid
catalyst was synthesized according to the procedure presented in our previous
report. The structure of the prepared catalyst was characterized by various
spectroscopic analyses; including Fourier transform infrared spectroscopy (FT-IR),
thermogravimetric analysis (TGA), scanning electron microscopy (SEM), elemental
analysis (CHN), transmission electron microscopy (TEM) and X-ray powder
123

N. Y. Siavashi et al.
diffraction (XRD) which confirmed the successful preparation of the new catalyst
[51].
The morphology and size of the HAP@AEPH2-SO3H were evaluated using
scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
(Fig. 1). SEM images are shown in Fig. 1a, b. It can be seen that HAP@AEPH2-
SO3H nanoparticles have uniform nanorods morphology. Moreover, the size of
HAP@AEPH2-SO3H was observed by TEM as shown in Fig. 1c, d. TEM analysis
of the HAP@AEPH2-SO3H revealed rod-like particles with a mean size range of
10–100 nm.
Furthermore energy dispersive spectrometer (EDS) was recorded at random area
on the surface of HAP@AEPH2-SO3H for qualitative analysis (Fig. 2). The EDS
spectrum shows the presence of C, O, P, S and Ca elements which confirmed
successful modification surface of nanoHAP.
Additionally, FT-IR spectroscopy as an appropriate technique to approve the
successful synthesis of catalyst was used. The FT-IR spectrum of HAP@AEPH2-
Fig. 1 a, b SEM images of HAP@AEPH2-SO3H, c, d TEM images of HAP@AEPH2-SO3H
123

Sulfonated nanohydroxyapatite functionalized with 2…
Fig. 2 The EDS analysis of HAP@AEPH2-SO3H
SO3H is given in Fig. 3a. As can be seen from Fig. 3a, the absorption band at
3571 cm-1 could be attributed to hydroxyl ions in hydroxyapatite. Moreover, the
broad band with a maximum at around 3450 cm-1 (due to stretching vibration) and
the weak band at around 1637 cm-1 (due to bending vibration) are assigned to the
crystal water and surface adsorbed water in hydroxyapatite. Another bands
appearing at 1098, 1031, 962, 601, 571, and 471 cm-1 are related to the
3-
asymmetric, symmetric stretching and vibrational bending of the PO4
ions
respectively. Along with the primary absorption bands of nanoHAP, appearance of
absorption bands at 2917, 2851, 1325 and 1155 cm-1 which are attributed to
asymmetric and symmetric vibrational frequencies of –CH2 and the asymmetric and
symmetric stretching frequencies of S = O,respectively, revealed that new ligands
(AEPH2 and –SO3H group) have been successfully attached onto the surface of
nanoHAP.
120
100
80
b
a
60
40
20
0
-20
-40
-60
3900
3400
2900
2400
1900
1400
900
400
Wavenumbers (cm-1)
Fig. 3 FT-IR spectrum of a HAP@AEPH2-SO3H, b fifth reused HAP@AEPH2-SO3H
123

N. Y. Siavashi et al.
Table 1 Elemental analysis of the HAP@AEPH2-SO3H and the fifth reused catalyst
Samples
Elemental analysis (w/w %)
C
N
S
HAP@AEPH2-SO3H
2.419 (1.008 mmol/g) 1.409 (1.007 mmol/g)
3.32 (1.040 mmol/g)
Fifth reused HAP@AEPH2-
SO3H
2.396 (0.998 mmol/g) 1.365 (0.975 mmol/g) 3.098 (0.970 mmol/g)
Table 2 BET surface area, pore volume and pour size of HAP@AEPH2-SO3H
Sample
Morphology
Nanoparticle
SBET (m2/g)
Pore volume (cm3/g)
Pore size (nm)
12.011
HAP@AEPH2-SO3H
130.2
0.391
To further investigate, the contents of organic fragments attached onto the
surface of catalyst were determined by elemental analysis (Table 1). The elemental
analysis of the catalyst revealed that carbon, nitrogen and sulfur contents of
HAP@AEPH2-SO3H were estimated to be 2.419, 1.409 and 3.320 % respectively,
which indicated that 1.008 mmol of AEPH2 and 1.040 mmol of sulfonic acid were
embedded on to 1.000 g of nanoHAP.
Additionally, nitrogen adsorption–desorption isotherm of the HAP@AEPH2-
SO3H was investigated. Table 2 illustrates the average surface area, pore volume
and pore size of the HAP@AEPH2-SO3H. By considering the BET results and
comparing with literature, it can be seen that the surface area of HAP@AEPH2-
SO3H is a little smaller than the pure HAP NPs [52].
Typical procedure for the preparation of benzhydryl benzoate in the presence
of HAP@AEPH2-SO3H
HAP@AEPH2-SO3H (0.08 g, 8 mol %) was added to a solution of benzoic acid
(1 mmol, 0.12 g) in dry dichloromethane (2 mL) at 41 °C. Benzhydrol (2 mmol,
0.368 g) was dissolved in dichloromethane (2 mL) and added to the reaction
mixture dropwise during 1 h. Progress of the reaction was monitored by TLC. After
completion of the reaction, the catalyst was removed by filtration. The filtrate was
washed with distilled water (3 9 4 mL), dried over Na2SO4, and concentrated in
vacuo to give the crude product which was then purified by thin layer
chromatography using EtOAc/n-hexane (2:8) to afford benzhydryl benzoate
(0.264 g, 95 %).
Benzhydryl benzoate [53] (Table 2, entry 1) White solid; yield 95 %; mp
83–85 °C (Lit. 83.5–85.5 °C); FT-IR (KBr): tmax/cm-1 3056, 3023, 2917, 2851,
1713 (C=O), 1597, 1585, 1494, 1453, 1316, 1268 (C–O), 1176, 1110, 1022, 970,
894, 845, 754, 708, 598, 578; 1H NMR (300 MHz, CDCl3, ppm) d: 8.19 (d,
J = 7.2 Hz, 2H, Ph), 7.61 (t, J = 7.2 Hz, 1H, Ph), 7.52–7.29 (m, 12H, Ph), 7.16 (s,
1H, Ph2CH); MS (EI): m/z (%) 288 (5) [M?], 181 (61) [M?–PhCO], 164 (100)
[M?–PhCO2], 104 (87) [M?–Ph2CHO].
123

Sulfonated nanohydroxyapatite functionalized with 2…
Benzhydryl 4-methylbenzoate [54] (Table 2, entry 2) White solid; yield 92 %; mp
108–110 °C (Lit. 110 °C); FT-IR (KBr): tmax/cm-1 3088, 3060, 3019, 2923, 2847,
1710 (C=O), 1608, 1503, 1450,1331, 1269 (C–O), 1179,1102, 979, 894, 841, 750,
696, 573, 469; MS (EI): m/z (%) 302 (5) [M?], 301 (47) [M?–1], 182 (41) [M?–(p-
MePhCO)], 165 (100) [M?–(p-MePhCO2)], 118 (90) [M?–Ph2CHO].
Benzhydryl 4-methoxybenzoate [55] (Table 2, entry 3) White solid; yield 95 %;
mp 96–97 °C (Lit. 96–98 °C); FT-IR (KBr): tmax/cm-1 3064, 3027, 2966, 2937,
2839, 1707 (C=O), 1603, 1509, 1493, 1451, 1334, 1315, 1257 (C–O), 1165, 1098,
1025, 967, 894, 853, 820, 769, 742, 699, 616, 571, 506; MS (EI): m/z (%) 318 (10)
[M?], 317 (29) [M?–1], 182 (82) [M?–(p-MeOPhCO)], 166 (100) [M?–(p-
MeOPhCO2)], 134 (93) [M?–Ph2CHO].
Benzhydryl 4-nitrobenzoate [56] (Table 2, entry 4) White solid, yield 85 %, mp
131–132 °C (Lit. 131–132 °C); FT-IR (KBr): tmax/cm-1 3117, 3060, 2925, 2851,
1723 (C=O), 1693, 1604, 1521, 1491, 1433, 1347, 1311, 1279 (C–O), 1259, 1184,
1099, 1053, 976, 878, 741, 717, 699, 587, 510; 1H NMR (400 MHz, CDCl3, ppm) d:
8.33 (s, 4H, p-NO2Ph), 7.47–7.35 (m, 10H, Ph), 7.17 (s, 1H, Ph2CH); MS (EI): m/z
(%) 333 (7) [M?], 182 (93) [M?–(p-NO2PhCO)], 166 (100) [M?–(p-NO2PhCO2)],
151 (68) [M?–Ph2CHO].
Benzhydryl 3-nitrobenzoate [57] (Table 2, entry 5) White solid; yield 80 %; mp
93–95 °C (Lit. 95 °C); FT-IR (KBr): tmax/cm-1 3088, 3064, 3027, 2953, 2925,
2855, 1729 (C=O), 1617, 1527, 1494, 1452, 1349, 1290 (C–O), 1253,1191, 1126,
1094, 1065, 969, 912, 753, 719, 699, 600, 582; 1H NMR (400 MHz, CDCl3, ppm) d:
8.96-8.92 (m, 1H, m-NO2Ph), 8.47–8.40 (m, 2H, m-NO2Ph), 7.70–7.63 (m, 1H, m-
NO2Ph), 7.46–7.30 (m, 10H, Ph), 7.29–7.17 (m, 1H, Ph2CH); MS (EI): m/z (%) 333
(5) [M?], 182 (80) [M?–(m-NO2PhCO)], 165 (100) [M?–Ph2CH], 105 (70) [M?–
(Ph2CHO ? NO2)].
Benzhydryl 4-bromobenzoate [58] (Table 2, entry 6) White solid; yield 85 %; mp
90–92 °C; FT-IR (KBr): tmax/cm-1 3060, 3023, 2953, 2913, 2851, 1719 (C=O),
1587, 1494, 1482, 1452, 1395, 1267 (C–O), 1171, 1114, 1101, 1009, 970, 844, 747,
697, 600, 579, 461; MS (EI): m/z (%) 367 (67) [M?], 182 (77) [M?–(p-BrPhCO)],
166 (100) [M?–(p-BrPhCO2)], 105 (66) [M?–(Ph2CHO ? Br)].
1-Phenylethyl benzoate [59] (Table 2, entry 7) Oil; yield 85 %; FT-IR (neat):
max/cm-1 3087, 3063, 3033, 2980, 2930, 2854, 1717 (C=O), 1601, 1584, 1494,
t
1451, 1375, 1315, 1271 (C–O), 1209, 1176, 1109, 1067, 1027, 994, 936, 912, 868,
805, 760, 712, 597, 549; MS (EI): m/z (%) 226 (17) [M?], 121 (65) [M?–
PhCH3CH], 105 (100) [M?–PhCO2], 77 (82) [M?–PhCO2 CHCH3].
1-Phenylethyl 4-methylbenzoate [60] (Table 2, entry 8) Oil; yield 90 %; FT-IR
(neat): tmax/cm-1 3092, 3055, 3033, 2980, 2928, 2867, 1713 (C=O), 1612, 1580,
1494, 1452, 1380, 1330, 1270 (C–O), 1177, 1104, 1063, 1016, 910, 840, 753, 697,
549, 465; MS (EI): m/z (%) 240 (11) [M?], 118 (100) [M?–PhCH3CHO], 105 (95)
[M?–(p-MePhCO2)], 91 (82) [M?–PhCH3CHOCO].
123

N. Y. Siavashi et al.
1-Phenylethyl 4-methoxybenzoate [59] (Table 2, entry 9) Oil; yield 95 %; FT-IR
(neat): tmax/cm-1 3068, 3031, 2979, 2934, 2838, 1709 (C=O), 1606, 1511, 1455,
1421, 1317, 1255 (C–O), 1167, 1100, 1062, 1029, 914, 848, 768, 698, 620, 551,
514; 1H NMR (300 MHz, CDCl3, ppm) d: 8.10 (dd, J1 = 6.9 Hz, J2 = 2.7 Hz, 2H,
p-MeOPh), 7.52–7.29 (m, 5H, Ph), 6.97 (dd, J1 = 6.9 Hz, J2 = 2.7 Hz, 2H, p-
MeOPh), 6.17 (q, J = 6.6 Hz, 1H, OCH(Ph)CH3), 3.88 (s, 3H, p-MeOPh), 1.72 (d,
J = 6.6 Hz, 3H, OCHCH3); MS (EI): m/z (%) 256 (5) [M?], 151 (73) [M?–
PhCH3CH], 134 (100) [M?–PhCH3CHO], 105 (82) [M?–(p-MeOPhCO2)].
1-Phenylethyl 4-nitrobenzoate [56] (Table 2, entry 10) Oil; yield 85 %; FT-IR
(neat): tmax/cm-1 3112,3059, 3031, 2982, 2937, 2859, 1727 (C=O), 1612, 1527,
1457, 1404, 1347, 1273 (C–O), 1212, 1098, 1061, 1008, 873, 848, 762, 720, 699,
1
554, 501, 423; H NMR (400 MHz, CDCl3, ppm) d: 8.32–8.29 (m, 2H, p-NO2Ph),
8.27–8.24 (m, 2H, p-NO2Ph), 7.49–7.34 (m, 5H, Ph), 6.18 (q, J = 6.4 Hz, 1H,
OCH(Ph)CH3), 1.74 (d, J = 6.4 Hz, 3H, OCHCH3); MS (EI): m/z (%) 271 (5)
[M?], 166 (18) [M?–PhCH3CH], 149 (91) [M?–PhCH3CHO], 121 (76) [M?–(p-
NO2PhCO)], 104 (100) [M?–(p-NO2PhCO2)].
1-Phenylethyl 3-nitrobenzoate [61] (Table 2, entry 11) Oil; yield 80 %; FT-IR
(neat): tmax/cm-1 3088, 3033, 2933, 2868, 1725 (C=O), 1616, 1533, 1495, 1453,
1351, 1292 (C–O), 1263, 1209, 1137, 1064, 1029, 994, 918, 843, 762, 719, 699,
596, 541; MS(EI): m/z (%) 271(5) [M?], 150 (88) [M?–PhCH3CHO], 121 (75)
[M?–(m-NO2PhCO)], 104 (100) [M?–(m-NO2PhCO2)].
1-Phenylethyl 4-bromobenzoate [59] (Table 2, entry 12) Oil; yield 85 %; FT-IR
(neat): tmax/cm-1 3088, 3062, 3028, 2973, 2927, 2870, 1721 (C=O), 1589, 1492,
1451, 1370, 1268 (C–O), 1209, 1172, 1099, 1028, 1011, 951, 906, 853, 758, 699,
637, 544, 469; MS (EI): m/z (%) 305 (9) [M?], 182 (73) [M?–PhCH3CHO], 121
(73) [M?–(p-BrPhCO)], 105 (100) [M?–(p-BrPhCO2)].
Benzhydryl 2-phenylacetate [62] (Table 2, entry 13) Oil; yield 95 %; FT-IR
(neat): tmax/cm-1 3087, 3063, 3031, 2927, 2859, 1742 (C=O), 1602, 1586, 1495,
1452, 1341, 1245 (C–O), 1144, 1077, 984, 916, 742, 698, 597, 549, 531; MS (EI):
m/z (%) 302 (5) [M?], 182 (47) [M?–PhCH2CO], 165 (100) [M?–PhCH2CO2], 90
(77) [M?–Ph2CHCO2].
Benzhydryl 2,2-diphenylacetate [63] (Table 2, entry 14) White solid; yield 85 %;
mp 105–107 °C (Lit. 107–108 °C); FT-IR (KBr): tmax/cm-1 3088, 3064, 3029,
2929, 2851, 1734 (C=O), 1597, 1495, 1449, 1346, 1304, 1278 (C–O), 1187, 1141,
1
1078, 1003, 977, 914, 784, 741, 695, 603, 549, 473; H NMR (300 MHz, CDCl3,
ppm) d: 7.35–7.26 (m, 20H, Ph), 7.00 (s, 1H, Ph2CHO), 5.25 (s, 1H, Ph2CH); MS
(EI): m/z (%) 378 (3) [M?], 180 (10) [M?–Ph2CHCO], 166 (100) [M?–
Ph2CHCO2].
((E)-Benzhydryl 3-(3-nitrophenyl)acrylate) (Table 2, entry 15) White solid; yield
90 %; mp 100–102 °C; FT-IR (KBr): tmax/cm-1 3105, 3076, 3035, 2949, 2851,
1725 (C=O), 1639, 1613, 1523, 1491, 1446, 1356, 1328, 1213 (C–O), 1161, 998,
972, 918, 865,808, 743, 699, 661, 563; 1H NMR (300 MHz, CDCl3, ppm) d: 8.43 (s,
123

Sulfonated nanohydroxyapatite functionalized with 2…
1H, m-NO2Ph), 8.268 (dd, J1 = 8.1 Hz, J2 = 2.1 Hz, 1H, m-NO2Ph), 7.87–7.80 (m,
2H, m-NO2Ph, HC = CH), 7.60 (t, J = 8.1 Hz, 1H, m-NO2Ph), 7.47–7.29 (m, 10H,
Ph), 7.08 (s, 1H, Ph2CH), 6.74 (d, J = 15.9 Hz, 1H, HC = CHCO2); 13C NMR
(75 MHz, CDCl3, ppm) d: 121.2, 122.5, 124.6 127.2, 128.1, 128.4, 128.6, 130.04,
133.7, 136.07, 139.9, 142.5, 148.7, 165.1; MS (EI): m/z (%) 359 (66) [M?], 183
(79) [M?–(m-NO2PhCH = CHCO)], 166 (100) [M?–(m-NO2PhCH = CHCO2)],
102 (72) [M?–(Ph2CHCO2 ? NO2)]; CHN (C22H17NO4) calc. (%) C (73.53), H
(4.77), N (3.90); found (%) C (72.75), H (4.89), N (3.74).
1-Phenylethyl 2-phenylacetate [64] (Table 2, entry 16) Oil; yield 90 %; FT-IR
(neat): tmax/cm-1 3088, 3063, 3031, 2980, 2931, 2872, 1732 (C=O), 1609, 1495,
1453, 1344, 1254 (C–O), 1213, 1157, 1063, 1029, 1009, 952, 849, 759, 722, 698,
547; 1H NMR (300 MHz, CDCl3, ppm) d: 7.44-7.37 (m, 10H, Ph), 6.04 (q,
J = 6.6 Hz, 1H, OCH(Ph)CH3), 3.76 (s, 2H, PhCH2), 1.64 (d, J = 6.6 Hz, 3H,
OCH(Ph)CH3); MS (EI): m/z (%) 240 (7) [M?], 122 (25) [M?–PhCH2CO], 105
(100) [M?–PhCH2CO], 91 (73) [M?–PhCH3CHOCO].
4-Methoxybenzyl benzoate [15] (Table 2, entry 17) Oil; yield 70 %; FT-IR (neat):
max/cm-1 3067, 3035, 3006, 2961, 2927, 2855, 1717 (C=O), 1613, 1585, 1515,
t
1452, 1374, 1272 (C–O), 1175, 1109, 1069, 1030, 959, 926, 825, 711, 575, 524; MS
(EI): m/z (%) 242 (3) [M?], 136 (20) [M?–PhCO], 121 (100) [M?–PhCO2], 100
(94) [M?–(p-MeOPhCH2CO)].
4-Methoxybenzyl hexanoate [65] (Table 2, entry 18) Oil; yield 70 %; FT-IR
(neat): tmax/cm-1 3060, 3031, 2999, 2953, 2929, 2835, 1736 (C=O), 1610, 1583,
1510, 1462, 1439, 1300, 1246 (C–O), 1175, 1111, 1036, 809, 747, 578, 538; MS
(EI): m/z (%) 236 (10) [M?], 121 (92) [M?–CH3(CH2)4CO2], 115 (42) [M?–(p-
MeOPhCH2)].
Results and discussion
Following our interest in application of solid acids in organic transformation [51,
66–73] we speculated on the possibility of using HAP@AEPH2-SO3H to activate
carboxylic acids for a nucleophilic substitution at the carbonyl groups to achieve
ester linkage (Scheme 1).
The optimization of the reaction conditions was carried out for the reaction of
benzoic acid with benzhydrol in the presence of HAP@AEPH2-SO3H under various
reaction parameters in order to achieve the maximum chemical yield at the lowest
reaction time and lowest reaction temperature. The representative results are shown
in Table 3. On the basis of our previous reports [66, 67] we found that in acidic
media, the yield of esterification reaction was influenced by the rate of addition of
benzhydrol to the reaction mixture. Quick addition of benzhydrol leads to the
formation of excess amount of symmetric ether which was obtained from self-
condensation of benzhydrol. To minimize self-condensation of benzhydrol in all of
the following reactions, addition of benzhydrol should be performed drop by drop,
which consumes more times. In this manner, the corresponding ester was obtained
123

N. Y. Siavashi et al.
Scheme 1 Direct esterification of different structurally carboxylic acids with different alcohols in the
presence of HAP@AEPH2-SO3H
as the chief product. In the absence of any catalyst, there was no conversion to
benzhydryl benzoate (Table 3, entry 1). Applying 1/1 molar ratio of benzoic acid/
benzhydrol in the presence of 4 mol % of catalyst in refluxing CH2Cl2 leads to
formation of desired product and (benzhydryloxy)diphenylmethane as by product
(Table 3, entry 2). Low yield of corresponding product was obtained even after long
reaction time, when the same reaction condition was performed at room temperature
(Table 3, entry 3). In an effort to develop better reaction conditions, different molar
ratios of reactants were examined on model reaction (Table 3, entries 4–7). The
maximum yield of benzhydryl benzoate with trace amount of by product was
obtained by using 1/2 molar ratio of benzoic acid/benzhydrol. To improve direct
esterification reaction the effect of catalyst loading was investigated on model
reaction (Table 3, entries 8–10). According to this study, the best result of
benzhydryl benzoate was produced in the presence of 8 mol % of catalyst with
lowest yield of by product (Table 3, entry 9). The catalytic effect of HAP@AEPH2-
SO3H was studied in different solvents as well as solvent free conditions. No
product was obtained when the reaction was performed in refluxing H2O and THF
and also in solvent free condition (Table 3, entries 11–12, 22). As shown in Table 3,
the catalytic effect of HAP@AEPH2-SO3H was efficiently decreased in CHCl3,
CH3CN, 1, 2-dichloroethane, CH3NO2, 1,4-dioxane and toluene (Table 3, entries
13–21).
After optimizing the reaction parameters, generality of the method was tested
with other carboxylic acids and alcohols. The results are summarized in Table 4.
From this Table, it is clear that, in the presence of HAP@AEPH2-SO3H, the desired
esters can be obtained in good to excellent yields. In comparison, benzhydrol
condensed with aromatic carboxylic acids more quickly than 1-phenylethanol due to
more stability of diphenylmethylium cation than 1-phenylethan-1-ylium cation
123

Sulfonated nanohydroxyapatite functionalized with 2…
Table 3 Direct esterification of benzoic acid with benzhydrol in the presence of HAP@AEPH2-SO3H
under different reaction conditions
Entry Molar ratio of Catalyst Solvent
benzoic acid/
benzhydrol
Temperature Time Conversiona Selectivity
(mol %)
(°C)
(h)
(%)
A/B (%)
1
1/1
1/1
1/1
1/1.2
1/1.5
1/2
1/2.5
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
0
4
4
4
4
4
4
6
8
2
8
8
8
8
8
8
8
8
8
8
8
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
H2O
Reflux
Reflux
R.T
24
2
0
70
30
80
90
100
100
100
100
90
0
0:0
2
90:10
20:10
90:10
90:10
90:10
85:15
95:5
3
24
4
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
40
2.5
5
3
6
3.5
4
7
8
3.5
2.5
4.5
9
95:5
10
11
12
13
14
15
16
17
18
19
20
21
22
a
80:20
0:0
24
THF
24
5
0
0:0
CHCl3
95
80
50
100
45
95
70
75
65
0
85:15
75:25
10:40
80:20
25:15
70:30
60:40
70:30
40:60
0:0
CH3CN
CH3CN
1,2-DCE
1,2-DCE
CH3NO2
8
24
3
Reflux
40
24
12
20
7
Reflux
1,4-Dioxane Reflux
Toluene
CHCl3
Reflux
40
24
24
Solvent free 40
The corresponding data referred to the conversion of benzoic acid
(compare entries 1–6 with 7–12). Electron density of aromatic carboxylic acids
plays an essential role in the reaction rate. Comparatively electron–donating groups
on aromatic rings accelerates the esterification reaction whilst esterification of
aromatic carboxylic acids bearing electron withdrawing substituents was completed
in longer reaction times (compare entries 2–3 with 4–6 and entries 8–9 with 10–12).
Aliphatic carboxylic acids also followed this manner in direct esterification reaction
in the presence of HAP@AEPH2-SO3H (Table 4, entries 13–16). Difference in
reactivity between diphenylacetic acid and the other aliphatic carboxylic acids can
be rationalized by the steric effects of two phenyl groups. On the basis of the results
obtained from Table 4 and according to the proposed mechanism in Scheme 2, the
123

N. Y. Siavashi et al.
stability of carbocation II and the nucleophilicity of carboxylic acid are thought to
be two effective factors in the esterification reaction. The electron-rich aryl ring
increases the nucleophilicity of carboxylic acid and accelerates the formation of
intermediate III which in turn facilitates the formation of desired product. We also
tested the direct esterification reaction of carboxylic acids with tertiary alcohols
(triphenyl methanol, t-butyl alcohol), secondary and primary aliphatic alcohols (2-
heptanol, 3-phenyl 1-propanol) in the presence of HAP@AEPH2-SO3H. Unfortu-
nately, the catalyst was not effective in esterification of carboxylic acids with the
above mentioned alcohols even under forced conditions (16 mol % of catalyst).
Although, the catalyst was effective in promoting the reaction of carboxylic acids
with electron-rich primary benzylic alcohol (Table 4, entry 17), but the esterifica-
tion reaction of carboxylic acids with primary benzylic alcohols bearing electron
withdrawing groups were not successful even after long period of time.
The present protocol, in its entirety, simply involves the reaction of carboxylic
acids with primary/or secondary benzylic alcohol in the presence of a catalytic
amount of HAP@AEPH2-SO3H, and warming the reaction mixture to afford the
desired product. The reactions are reasonably fast, but because of acidic media, to
prevent formation of any by-products (symmetrical ethers or alkenes in the case of
1-phenyl ethanol), the addition of benzylic alcohols should be carried out drop by
drop, which prolonged the reaction times.
In all of experiments, the completion of the reaction was confirmed by the
disappearance of the carboxylic acids on TLC followed by the disappearance of
acidic OH stretching frequency at 3400–2400 cm-1 in FT-IR spectra. Also,
absorption bands at 1742–1707 and 1292–1213 cm-1 due to carbonyl and C–O
group of esters in FT-IR spectra confirmed the ester formation. In the 13C NMR
spectrum, a signal at 165 ppm is assigned to the quaternary carbonyl carbon. The
structures of all synthesized compounds were confirmed spectroscopically and
found to be comparable in all respects with pure samples.
According to these observations we propose the mechanism reported in
Scheme 2. It is speculated that in the presence of HAP@AEPH2-SO3H the
protonated species I was produced initially which subsequently generates the
corresponding stable benzylic carbocation II. This idea is supported by performing
the reaction in the absence of catalyst. Without any catalyst, the esterification
reaction is not performed even after a long period of time (Table 3, entry 1). The
formation of benzylic carbocation II was also confirmed by the fact that the more
stable carbocation reacts more quickly with carboxylic acid (compare the reaction
rate of benzhydrol with 1-phenyethanol). Nucleophilic attack of carboxylic acid to
benzylic carbocation II produced the protonated ester III which was followed by
releasing of the acidic catalyst. Then the solid acid catalyst re-enters to the catalytic
cycle. As the addition of benzylic alcohol to the reaction mixture was performed
drop by drop the formation of any symmetric ether IV as by product was prohibited.
Also, as the reaction was performed at low temperature (41 °C) dehydration of
I through benzylic cation II which leads to alkene V, did not happen in the
esterification reaction in the presence of HAP@AEPH2-SO3H [74–76]. Further
studies to elucidate the details of the mechanism are ongoing.
123

Sulfonated nanohydroxyapatite functionalized with 2…
Table 4 Direct esterification of various carboxylic acids with alcohols catalyzed by HAP@AEPH2-SO3H
Entry
Carboxylic acid
Alcohol
Product
Time
(h)
Isolateda
Yield
(%)
1
2
benzhydrol
benzhydrol
2.5
95
2
92
95
3
4
benzhydrol
benzhydrol
1.5
3.5
85
5
6
benzhydrol
benzhydrol
3.5
80
85
4
7
1-phenylethanol
3
85
123

N. Y. Siavashi et al.
Table 4 continued
Entry
Carboxylic acid
Alcohol
Product
Time
(h)
Isolateda
Yield
(%)
10
1-phenylethanol
4
85
11
12
13
1-phenylethanol
1-phenylethanol
benzhydrol
4.5
5
80
85
95
2.5
14
15
benzhydrol
3.5
85
90
benzhydrol
3
16
17
1-phenylethanol
3
3
90
75
4-methoxy
benzyl alcohol
4-methoxy
benzyl alcohol
18
4.5
70
a
The data referred to isolated yield of the corresponding ester
123

Sulfonated nanohydroxyapatite functionalized with 2…
Scheme 2 Proposed mechanism for direct esterification of carboxylic acid with alcohol in the presence
of HAP@AEPH2-SO3H
Catalyst re-usability was assessed in the direct esterification of benzoic acid with
benzhydrol. To this end, the reaction was stopped after 2.5 h (i.e. at 100 % conversion of
benzoic acid) and the catalyst removed by centrifugation and washed with ethylacetate three
times. Table 5shows the results obtained after five re-use cycles. As can be seen, no significant
loss of activity of the catalyst was observed after 5 runs. In another word the heterogeneous
catalyst can be used for five successive times in the new experiments without any significant
impact on yields of products with purity similar to that obtained in the first run.
In order to study the stability of the catalyst in the reaction mixture, the FT-IR
spectrum and elemental analysis are employed to give detail investigation of the
fifth reused catalyst. Figure 3b represents the FT-IR spectrum of the fifth reused
HAP@AEPH2-SO3H nanoparticles. As shown in Fig. 3b, there was no significant
leaching of organic segments (AEPH2 and -SO3H) from the catalyst surface after
five runs of the reaction. Moreover, elemental analysis of the reused catalyst is
presented in Table 1. Based on this analysis (C 2.396 %, N 1.365 % and S 3.098 %)
no considerable leaching of organic segments occurred in the reaction mixture away
from the surface of HAP@AEPH2-SO3H.
Table 5 Reaction of benzoic
acid with benzhydrol in the
presence of reused catalyst
Run
Time (h)
Conversion (%)
Isolated yield (%)
1
2.5
100
95
95
95
90
90
2
2.5
100
3a
4a
5
2.5/3
2.5/3
3
95/100
95/100
95
a
The second numbers in the
third columns correspond to
conversion after 3 h
123

N. Y. Siavashi et al.
Table 6 Comparison efficiency of various catalysts in the direct esterification of benzoic acid with
1-phenylethanol
Entry Catalyst
Solvent
Temperature Time Yield Reference
(°C)
(h)
(%)
1
2
3
TiO(acac)2
ClHC = N? MeCl-a
5,50-Dimethyl-3,30-azoisoxazole/ CH3CN
PPh3
Xylene
THF
138.5
R.T
82
36
16
11
97
66
55
[77]
[78]
[79]
4
5
6
7
a
PS-SIb
RHAPhe–Ruc
Et–PMO–Me– PrSO3Hd
H2O
80
48
12
6
90.7 [80]
Solvent-free 85
Solvent-free 60
82
[99
85
[81]
[35]
HAP@AEPH2-SO3H
CH2Cl2
41
3
Present study
(Chloromethylene)dimethylammonium chloride
b
Polymer-supported sulfonamide, the corresponding data referred to the direct esterification of n-butyric
acid with n-butanol
c
L-(N-a-acetylphenylalanine)–Ru(III) complex, the corresponding data referred to the direct esterifica-
tion of acetic acid with ethyl alcohol
d
Sulfonic acid-functionalized periodic mesoporous organosilicas, the corresponding data referred to the
direct esterification of acetic acid with benzyl alcohol
To further evaluate the overall utility of the current methodology, we compared
our results with those of the other methods reported for direct esterification of
carboxylic acids with alcohols (Table 6). As shown in Table 6, this method avoids
disadvantages of the other procedures such as long reaction times, expensive
catalysts, toxic reagents, high temperature and low yield [35, 77–81]. Consequently,
HAP@AEPH2-SO3H can be considered as a novel, eco-friendly and reusable solid
acid catalyst for this reaction.
Conclusion
In conclusion, we have developed an efficient process, for preparation of esters
using HAP@AEPH2-SO3H as a recyclable catalyst. The HAP@AEPH2-SO3H is
able to promote the esterification of a variety of carboxylic acids with benzylic
alcohols in high yields under mild reaction conditions. The yields for the
esterification reaction were excellent in the range of 78–99 %. After completion
of the reaction the HAP@AEPH2-SO3H were removed by filtration, which could be
reused five times without any loss of activity. Stability and recyclability of
HAP@AEPH2-SO3H and the easy workup make the present system an alternative
way to construct ester bonds from carboxylic acids and alcohols in future.
Acknowledgments The authors gratefully acknowledge the partial support of this study by Ferdowsi
University of Mashhad Research Council (p/3/33830).
123

Sulfonated nanohydroxyapatite functionalized with 2…
References
1. J. Lilja, D.Y. Murzin, T. Salmi, J. Aumo, P. Maki Arvela, M. Sundell, J. Mol. Catal. A: Chem.
182–183, 555 (2002)
2. K. Weissermel, H.J. Arpe, Industrial Organic Chemistry, 3rd edn. (VCH, New York, 1997),
pp. 1–400
3. T.W. Greene, P.G.M. Wuts, Protective Groups in Organic Synthesis, 3rd edn. (Wiley, New York,
1999), pp. 50–58
4. K. Ishihara, S. Ohara, H. Yamamoto, Tetrahedron 58, 8179 (2002)
5. N. Bhatt, A. Patel, P. Selvam, K. Sidhpuria, J. Mol. Catal. 275, 14 (2007)
6. N. Sanchez, M. Martinez, J. Aracil, A. Corma, J. Am. Oil Chem. Soc. 69, 1150 (1992)
7. E. Heykants, W.H. Verrelst, R.F. Parton, P.A. Jacobs, Stud. Surf. Sci. Catal. 105, 1277 (1997)
8. J. Inanaga, K. Hirata, H. Saeki, T. Katsuki, M. Yamaguchi, Bull. Chem. Soc. Jpn 52, 1989 (1979)
9. O. Mitsunobu, M. Yamada, Bull. Chem. Soc. Jpn 40, 2380 (1967)
10. K.C.K. Swamy, N.N.B. Kumar, E. Balaraman, K.V.P.P. Kumar, Chem. Rev. 109, 2551 (2009)
11. T. Yuen, S. Sze, But, P. H. Toy. J. Am. Chem. Soc. 128, 9636 (2006)
12. E.A. Veliz, P.A. Beal, Tetrahedron Lett. 47, 3153 (2006)
13. A.M. Balint, A. Bodor, A. Gomory, K. Vkey, D. Szabo, J. Rabai, J. Fluorine Chem. 126, 1524 (2005)
14. D. Szabo, A.M. Bonto, I. Kovesdi, A. Gomory, J. Rabai, J. Fluorine Chem. 126, 641 (2005)
15. A.M. Harned, H.S. He, P.H. Toy, D.L. Flynn, P.R. Hanson, J. Am. Chem. Soc. 127, 52 (2005)
16. S. Dandapani, D.P. Curran, Chem. Eur. J. 10, 3130 (2004)
17. R. Dembinski, Eur. J. Org. Chem. 13, 2763 (2004)
18. S. Dandapani, D.P. Curran, Tetrahedron 58, 3855 (2002)
19. A.P. Dobbs, C. Mcgregor, Johnson. Tetrahedron Lett. 43, 2807 (2002)
20. J.C. Pelletier, S. Kincaid, Tetrahedron Lett. 41, 797 (2000)
21. N. Iranpoor, H. Firouzabadi, D. Khalili, S. Motevalli, J. Org. Chem. 73, 4882 (2008)
22. J.S. Pizey, Synthetic Reagents (Horwood, New York, 1974), p. 65
23. S.W. Fenton, A.E. DeWald, R.T. Arnold, J. Am. Chem. Soc. 77, 979 (1955)
24. C.G. Overberger, J.P. Anselme, J. Org. Chem. 28, 592 (1963)
25. S.T. Heller, R. Sarpong, Org. Lett. 12, 4572 (2010)
26. C.S. Cho, D.T. Kim, H.J. Choi, T.J. Kim, S.C. Shim, Bull. Korean Chem. Soc. 23, 539 (2002)
27. M. Nakayama, A. Sato, K. Ishihara, H. Yamamoto, Adv. Synth. Catal. 346, 1275 (2004)
28. J. Mcnulty, V. Krishnamoorthy, A. Robertson, TetrahedrLett. 49, 6344 (2008)
29. K. Kenichi, O. Akiyoshi, I. Noboru, S. Yoshihiro, Synthesis 21, 3407 (2008)
30. T. Taniguchi, D. Hirose, H. Ishibashi, ACS Catal. 1, 1469 (2011)
31. S.P. Morcillo, L.A. Cienfuegos, A.J. Mota, J. Justicia, R. Robles, J. Org. Chem. 67, 2277 (2011)
32. A. Sakakura, Y. Koshikari, M. Akakura, K. Ishihara, Org. Lett. 14, 30 (2012)
33. M.K. Reilly, R.P. King, A.J. Wagner, S.M. King, J. Chem. Educ. 91, 1706 (2014)
34. K. Tanemura, T. Suzuki, Tetrahedron Lett. 54, 1927 (2013)
35. B. Karimi, H.M. Mirzaei, A. Mobaraki, H. Vali, Catal. Sci. Technol. 5, 3624 (2015)
36. A.K. Purohit, A. Kumar, V. Singh, R. Goud, R. Jain, D.K. Dubey, Tetrahedron Lett. 55, 6844 (2014)
37. M. Minakawa, H. Baek, Y.M.A. Yamada, J.W. Han, Y. Uozumi, Org. Lett. 15, 5798 (2013)
38. R.S. Thombal, A.R. Jadhav, V.H. Jadhav, RSC Adv. 5, 12981 (2015)
39. S. Zhan, X. Tao, L. Cai, X. Liua, T. Liu, Green Chem. 16, 4649 (2014)
40. B. Karimi, M. Vafaeezadeh, Chem. Commun. 48, 3327 (2012)
41. R.J. Kalbasi, A.R. Massah, Z. Barkhordari, Bull. Korean Chem. Soc. 31, 2361 (2010)
42. B.M. Chowdary, C. Sridhar, M.L. Kantam, B. Sreedhar, Tetrahedron Lett. 45, 7319 (2004)
43. E. Boanini, M. Gazzano, K. Rubini, A. Bigi, Adv. Mater. 19, 2499 (2007)
44. A. Indra, C.S. Gopinath, S. Bhaduri, G.K. Lahiri, Catal. Sci. Technol. 3, 1625 (2013)
45. G. Wang, Y. Zhao, J. Tan, S. Zhu, K. Zhou, Trans. Nonferrous Met. Soc. China 25, 490 (2015)
46. Y. Daniels, N. Lyczko, A. Nzihou, S.D. Alexandratos, Ind. Eng. Chem. Res. 54, 585 (2015)
47. R. Chakraborty, S.K. Das, Ind. Eng. Chem. Res. 51, 8404 (2012)
48. R. Chakraborty, D. Roychowdhury, Indian J. Sci. Technol. 6, 3933 (2013)
49. R. Chakraborty, D. RoyChowdhury, Chem. Eng. J. 215–216, 491 (2013)
50. R. Chakraborty, D. RoyChowdhury, J. Taiwan Inst. Chem. Eng. 45, 92 (2014)
51. M. Zarghani, B. Akhlaghinia, RSC Adv. 5, 87769 (2015)
123

N. Y. Siavashi et al.
52. L.E. Hammari, H. Marroun, A. Laghzizil, A. Saoiabi, C. Roux, J. Livage, T. Coradin, J. Solid State
Chem. 181, 848 (2008)
53. E.H. White, M. Ribi, L.K. Cho, N. Egger, P.M. Dzadzic, M.J. Todd, J. Org. Chem. 49, 4866 (1984)
54. W. Tochtermann, R.G.H. Kirrstetter, Chem. Ber. 111, 1228 (1978)
55. J. Milton Harris, A. Becker, J.F. Fagan, F.A. Walden, J. Am. Chem. Soc. 96, 4484 (1974)
56. H. Rouhi-Saadabad, B. Akhlaghinia, J. Braz. Chem. Soc. 25, 253 (2014)
57. C. Perusquia-Hernandez, G.R. Lara-Issasi, B.A. Frontana-Uribe, E. Cuevas-Yanez, Tetrahedron Lett.
54, 3302 (2013)
58. K. Bowden, K.D.F. Ghadir, J. Chem. Soc., Perkin Trans. 2: Physical Organic Chemistry 8, 1329
(1990)
59. W. Shiue-Shien, C. Fong-Kuang, K. Chih-Shueh, Synth. Commun. 43, 2615 (2013)
60. G. Majji, S. Guin, A. Gogoi, S.K. Rout, B.K. Patel, Chem. Commun. 49, 3013 (2013)
61. F. Jie, L. Shuai, C. Shan-Yong, Z. Ji, F. Song-Sen, Y. Xiao-Qi, Adv. Synth. Catal. 354, 1287 (2012)
62. R. Paredes, F. Agudelo, G. Taborda, Tetrahedron Lett. 37, 1965 (1996)
63. A.A. Adhikari, J.P. Shah, K.T. Howard, C.M. Russo, D.R. Wallach, M.R. Linaburg, J.D. Chisholm,
Synlett 25, 283 (2014)
64. P. Barton, A.P. Laws, M.I. Page, J. Chem. Soc., Perkin Trans. 2: Physical Organic Chemistry. 9, 2021
(1994)
65. G.V. Madhava Sharma, C.G. Reddy, P.R. Krishna, J. Org. Chem. 68, 4574 (2003)
66. S. Khafjeh, B. Akhlaghinia, S. Rezazadeh, H. Eshghi, J. Chem. Sci. 126, 1903 (2014)
67. B. Nazari Moghadam, B. Akhlaghinia, S. Rezazadeh, Res. Chem. Intermed. (2015). doi: 10.1007/
s11164-015-2098-y
68. N. Razavi, B. Akhlaghinia, RSC Adv. 5, 12372 (2015)
69. M. Zarghani, B. Akhlaghinia, Appl. Organometal. Chem. 29, 683 (2015)
70. S.S.E. Ghodsinia, B. Akhlaghinia, RSC Adv. 5, 49849 (2015)
71. Z. Zareie, B. Akhlaghinia, Chem. Pap. 69, 1421 (2015)
72. N. Razavi, B. Akhlaghinia, New J. Chem. (2016). doi:10.1039/C5NJ02123E
73. R. Jahanshahi, B. Akhlaghinia, RSC Adv. 5, 104087 (2015)
74. A. Sakakura, S. Nakagawa, K. Ishihara, Tetrahedron 62, 422 (2006)
75. S. Zhan, X. Tao, L. Cai, X. Liu, T. Liu, Green Chem. 16, 4649 (2014)
76. A.D. Todda, C.W. Bielawski, Catal. Sci. Technol. 3, 135 (2013)
77. C.T. Chen, Y.S. Munot, J. Org. Chem. 70, 8625 (2005)
78. H. Nagao, M. Michida, T. Mukaiyama, Synth. Commun. 38, 3208 (2008)
79. N. Iranpoor, H. Firouzabadi, D. Khalili, Org. Biomol. Chem. 8, 4436 (2010)
80. Z. Zhang, S. Zhou, J. Nie, J. Mol. Catal. A Chem. 265, 9 (2007)
81. F. Adam, K.M. Hello, S.J. Chai, Chem. Eng. Res. Des. 90, 633 (2012)
123