Synthesis of sulfonated carbon-based solid acid as a novel and efficient nanocatalyst for the preparation of highly functionalized piperidines and acylals: a DFT study

Article abstract of DOI:10.1007/s11164-016-2448-4

Abstract: A novel carbon-based solid-acid nanocatalyst (Sta-SO₃H) was simply prepared for the first time by the thermal treatment of sulfuric acid with starch at 180?°C in a sealed autoclave. The catalytic activities of Sta-SO₃H as an efficient and reusable catalyst were investigated by the condensation reaction of aldehyde, amine and β-keto ester for the synthesis of functionalized piperidines under solvent-free conditions at room temperature in good to high yields. Density functional theory calculations were used to study the structure of methyl 1,2,6-triphenyl-4-(phenylamino)-1,2,5,6-tetrahydropyridine-3-carboxylate (MPPC) as well as the thermochemistry of the multicomponent reaction. The theoretically calculated infrared and ¹H nuclear magnetic resonance spectra of MPPC were compared to the experimental data. It was found that the synthesis of MPPC is exothermic accompanied by a decrease in entropy, internal energy and Gibbs free energy of reaction. Good consistency between the calculated and observed spectral data was found. Also, Sta-SO₃H has been developed for the synthesis of acylals (1,1-diacetate) in high yields through the reaction of aldehydes with acetic anhydride at room temperature under solvent-free conditions. The mild conditions, eco-friendliness, excellent yields, short reaction times and use of an inexpensive and reusable catalyst are important features of this method. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2448-4

Res Chem Intermed
DOI 10.1007/s11164-016-2448-4
Synthesis of sulfonated carbon-based solid acid
as a novel and efficient nanocatalyst for the preparation
of highly functionalized piperidines and acylals:
a DFT study
1
1,2
•
Zahra Hoseinabadi
1
Mahdi Zamani
•
Seied Ali Pourmousavi
Received: 16 October 2015 / Accepted: 16 January 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract A novel carbon-based solid-acid nanocatalyst (Sta-SO H) was simply pre-
3
pared for the first time by the thermal treatment of sulfuric acid with starch at 180 °C in a
sealed autoclave. The catalytic activities of Sta-SO H as an efficient and reusable cat-
3
alyst were investigated by the condensation reaction of aldehyde, amine and b-keto ester
for the synthesis of functionalized piperidines under solvent-free conditions at room
temperature in good to high yields. Density functional theory calculations were used to
study the structure of methyl 1,2,6-triphenyl-4-(phenylamino)-1,2,5,6-tetrahydropy-
ridine-3-carboxylate (MPPC) as well as the thermochemistry of the multicomponent
1
reaction. The theoretically calculated infrared and H nuclear magnetic resonance
spectra of MPPC were compared to the experimental data. It was found that the synthesis
of MPPC is exothermic accompanied by a decrease in entropy, internal energy and Gibbs
free energy of reaction. Good consistency between the calculated and observed spectral
data was found. Also, Sta-SO H has been developed for the synthesis of acylals
3
(1,1-diacetate) in high yields through the reaction of aldehydes with acetic anhydride at
room temperature under solvent-free conditions. The mild conditions, eco-friendliness,
excellent yields, short reaction times and use of an inexpensive and reusable catalyst are
important features of this method.
&
Seied Ali Pourmousavi
pourmousavi@du.ac.ir
1
2
School of Chemistry, Damghan University, 36716-41167 Damghan, Iran
Institute of Biological Sciences, Damghan University, 36716-41167 Damghan, Iran
123

Z. Hoseinabadi et al.
Graphical Abstract
Starch
+
H SO4
2
Hydrothermal
1
80 °C, 24h
HO3S
HO
OH
COOH
H
R3
N
N
O
NH2
CHO
OR2
O
O
COOH
+
+
R1
OR2
RT, Solvent Free
R1
R3
R1
R3
Keywords Multicomponent reaction Á DFT Á Thermochemistry Á Piperidine Á
,1-Diacetates
1
Introduction
The principles of green chemistry and increasing concerns about environmental
issues have stimulated the research for recyclable strong solid acids to replace
conventional toxic and corrosive acid catalysts, such as sulfuric acid [1]. Solid acid
catalysts have received much attention for the potential substitution of the
homogeneous acids with the advantages of easy separation and reusability [2].
Carbon-based catalysts have several advantages as solid catalysts because of the
2
3
unique properties of carbon possessing sp, sp and sp bondings resulting in the
ability to make different structures, such as layers, tubes and spheres. Carbon can be
easily functionalized in amorphous to crystalline structures especially in the
amorphous form [3–8]. Sulfonated (SO H-bearing) carbon materials have been
3
reported to act as strong solid-acid catalysts. Hara’s group first prepared sulfonated
carbon catalysts via the sulfonation and carbonization of polycyclic aromatic
hydrocarbons [9]. Recently, Hara et al. prepared a series of acidic carbon catalysts
through the direct carbonization of raw materials such as sugar and cellulose
followed by the sulfonation of the resulting carbons, and found that such acidic
carbon materials exhibited good catalytic behavior [10]. The solid acid catalysts
were generally prepared through two steps. In the first step, the carbonization of
biomass was carried out to form polycyclic aromatic carbon sheets. Then, the
aromatic carbon sheets were sulfonated to introduce the sulfonic acid groups. The
carbonization process was taken at high temperature for a long period and large
amount of wastes were produced during the process, which resulted in serious
pollution. Efficient procedures for the synthesis of carbon-based solid acid through
one-step hydrothermal carbonization have been developed [11].
1
23

Synthesis of sulfonated carbon-based solid acid as a…
Reactions whereby three or more different components are combined in one
reaction vessel, leading to the formation of a single product, are summarized under
the term multicomponent reactions (MCRs) [12–15]. Recently, MCRs have
emerged as a powerful synthetic tool in organic synthesis due to their advantages
over conventional multi-step synthesis. In addition, MCRs are eco-friendly and have
superior atom economy, as well as avoiding costly purification processes and
protection–deprotection steps with minimum synthetic effort and time [16–20].
The piperidines and their analogues are important heterocycles that are present in
many naturally occurring alkaloids, biologically active synthetic molecules, and
organic fine chemicals [21–23]. Thus, the synthesis of highly substituted piperidines
has gained considerable attention, and a number of procedures have been developed
using several approaches [17, 24–36]. In recent times, very few methods have been
reported describing the one-pot multicomponent synthesis of functionalized
piperidines, based on catalysts such as L-proline/TFA [37], bromodimethylsulfo-
nium bromide (BDMS) [17], tetrabutylammonium tribromide (TBATB) [19],
molecular iodine [16], InCl [36], CAN [38], ZrOCl Á8H O [39], picric acid [40],
3
2
2
?
-
3
[
K PEG]Br [41], Bi(NO ) Á3H O [18], VCl [42], BF . SiO [43], AcOH [44],
3 3
2
3
3
2
Fe@Si–Gu–Prs [45], SSA [46] and tartaric acid [20]. However, some of these
procedures suffer from the drawbacks of multistep synthesis, long reaction times,
reaction work-ups, highly toxic catalysts, being difficult to prepare, having harsh
reaction conditions and expensive catalysts and procedures.
Here, we report for the first time a facile procedure for the synthesis of Sta-SO H
3
through the heat treatment of sulfuric acid and starch. These catalysts showed good
activity in the condensation reaction of aldehyde, amine and b-keto ester to produce
piperidine derivatives and the synthesis of acylals. Then, density functional theory
(DFT) calculations were used to study the structure of the general product as well as
the thermochemistry of the multicomponent reaction.
Experimental
Materials and instruments
All chemicals were purchased from Merck. All the reagents and chemicals were
obtained from Merck and used without further purification. The development of
reactions were monitored by TLC on Merck pre-coated silica gel 60 F254 aluminum
sheets, visualized by UV light. IR spectra were recorded on a Shimadzu FT-IR 8300
Spectrophotometer using the KBr pellets technique. The known products were
identified by comparison of their melting points and spectral data with those
1
reported in the literature. H NMR spectra were recorded at ambient temperature on
a BRUKER AVANCE DRX-400 MHz spectrophotometer using CDCl3 as the
solvent and TMS as an internal standard. XRD patterns were recorded by an X-ray
diffractometer (Philips-PW1800). The morphologies of the catalyst were studied by
a field emission scanning electron microscope (FESEM; Mira 3-XMU) with an
accelerating voltage at 10 kV.
123

Z. Hoseinabadi et al.
Catalyst preparation
The Sta-SO H was prepared via thermal treatment of the mixture of sulfuric acid/
3
starch at 180 °C. In a typical synthesis, 2 g of starch and 10 mL of H SO were
2
4
combined in a 100-mL Teflon-sealed autoclave and maintained at 180 °C for 24 h.
The obtained black products were filtered and washed with hot distilled water and
then oven-dried at 80 °C overnight. As a comparison, 2 g of starch without adding
H SO was also treated in the similar way and was denoted as Sta-180.
2
4
General procedure for the synthesis of piperidines
Aromatic aldehyde (2 mmol), amine (2 mmol), b-keto ester (1 mmol) and 0.035 g
of catalyst were eroded for 20 min at room temperature. The progress of the
reaction was monitored by TLC. The thick precipitate was filtered off and washed
with ethanol to give pure products. All products were characterized on the basis of
1
13
FT-IR, H-NMR and C-NMR spectral data, and by comparing the spectra to those
of authentic samples or reported data.
General procedure for preparation of 1,1-diacetates
To a mixture of aldehyde (1 mmol) and acetic anhydride (3 mmol), Sta-SO H
3
(
0.035 g) was added and the mixture was stirred at room temperature. The
progress of the reaction was monitored by TLC. After disappearance of the
starting material, CH Cl (10 mL) was added and the mixture was filtered off. The
catalyst was washed with CH Cl and then the organic layer was washed with
2
2
2
2
saturated NaHCO (10 mL) and water two times (2 9 10 mL) and dried with
3
anhydrous MgSO , filtered and the solvent was evaporated to give the pure
4
desired compound.
Spectral data of some selected products
Methyl-2,6-bis(4-methylphenyl)-1-phenyl-4-phenylamino-1,2,5,6-
tetrahydropyridine-3-carboxylate (Table 3, entry 1)
-
1 1
White solid; mp: 210–217 °C; IR (KBr): 1657, 1591 cm ; H-NMR (400 MHz,
CDCl ) d: 10.25 (1H, s, NH), 7.19 (2H, d, J = 8.0 Hz, ArH), 7.02 (11H, m, ArH),
3
6
6
2
.59 (1H, t, J = 7.2 Hz, ArH), 6.52 (2H, d, J = 8.0 Hz, ArH), 6.39 (1H, s, H-2),
.30 (2H, d, J = 8.0 Hz, ArH), 5.11 (1H, d, J = 3.2 Hz, H-6), 3.92 (3H, s, OMe),
0
00
.86 (1H, dd, J = 15.2, 5.6 Hz, H-5 ), 2.75 (1H, dd, J = 15.2, 2.4 Hz, H-5 ), 2.33
1
3H, s, Me), 2.32 (3H, s, Me); C-NMR (100 MHz, CDCl ) 168.8 (C=O), 156.5
3
(
3
(
C-4), 147.2, 141.1, 139.8, 138.1, 136.8, 136.0, 129.4, 129.1, 129.0, 128.9, 126.7,
1
26.5, 126.0, 125.8, 116.1, 113.0, 98.2 (C-3), 58.1 (C-6), 55.0 (C-2), 51.2, 33.8
C-5), 21.3, 21.2.
(
1
23

Synthesis of sulfonated carbon-based solid acid as a…
Methyl-1,2,6-triphenyl-4-phenylamino-1, 2, 5, 6-tetrahydropyridine-3-carboxylate
(
Table 3, entry 5)
-
1 1
White solid; mp: 195–196 °C; IR (KBr): 1663, 1591 cm ; H-NMR (400 MHz,
CDCl ) d: 10.27 (1H, s, NH), 7.24–7.32 (8H, m, ArH), 7.15 (2H, d, J = 6.4 Hz,
3
ArH), 7.04–7.09 (5H, m, ArH), 6.67 (1H, t, J = 7.3 Hz, ArH), 6.49 (2H, d,
J = 8.2 Hz, ArH), 6.43 (2H, d, J = 7.6 Hz, ArH), 6.38 (1H, s, H-2), 5.11–5.12 (1H,
0
d, H-6), 3.93 (3H, s, OMe), 2.85 (1H, dd, J = 15.0, 5.8 Hz, H-5 ), 2.77 (1H, dd,
0
0
13
J = 15.0, 2.1 Hz, H-5 ); C-NMR (100 MHz, CDCl3) 168.7 (C=O), 156.4 (C-4),
1
1
47.1, 144.0, 142.9, 137.9, 129.0, 128.9, 128.8, 128.4, 127.3, 126.8, 126.5, 126.0,
25.9, 116.3, 113.0, 98.0 (C-3), 58.3 (C-6), 55.2 (C-2), 51.2, 33.8 (C-5).
Ethyl-2,6-bis(4-methylphenyl)-1-phenyl-4-phenylamino-1,2,5,6-tetrahydropyridine-
-carboxylate (Table 3, entry 6)
3
-
1 1
White solid; mp: 229–231 °C; IR (KBr): 1649, 1592 cm ; H-NMR (400 MHz,
CDCl ) d:10.33 (1H, s, NH), 7.28 (2H, d, J = 8.0 Hz, ArH), 7.13–7.08 (11H,m,
3
ArH), 6.63 (1H, t, J = 7.2 Hz, ArH), 6.57 (2H, d, J = 8.8 Hz, ArH), 6.45(1H, s,
H-2), 6.34 (2H, d, J = 7.6 Hz, ArH), 5.15 (1H, d, J = 2.4 Hz, H-6), 4.53–4.45 (1H,
0
m, OCH H ), 4.40–4.32 (1H, m, OCH H ), 2.90 (1H, dd, J = 15.2, 5.6 Hz, H-5 ),
a
b
a b
00
2.8 (1H, dd, J = 15.2, 2.4 Hz, H-5 ), 2.37 (3H, s, Me), 2.36 (3H, s, Me), 1.50 (3H, t,
1
J = 7.2 Hz, OC–CH3); C-NMR (100 MHz,CDCl ) 168.4 (C=O), 156.2 (C-4),
3
3
1
1
2
47.2, 141.2, 139.9, 138.2, 136.8, 135.9, 129.4, 129.1, 129.0, 128.9, 126.7, 126.5,
25.9, 125.7, 116.1, 113.1, 98.5 (C-3), 59.8, 58.1 (C-6), 55.0 (C-2), 33.8 (C-5), 21.3,
1.2, 15.0.
Ethyl-2,6-bis(4-chlorophenyl)-1-phenyl-4-phenylamino-1,2,5,6-tetrahydropyridine-
-carboxylate (Table 3, entry 7)
3
-
1 1
White solid; mp: 206–208 °C; IR (KBr): 1652, 1597 cm ; H-NMR (400 MHz,
CDCl ) d:10.32 (1H, s, NH), 7.21–7.25 (8H, m, ArH), 7.03–7.16 (5H, m, ArH), 6.67
3
(
1H, t, J = 6.8 Hz, ArH), 6.49 (2H, d, J = 8.2 Hz, ArH), 6.44 (2H, d, J = 7.3 Hz,
ArH), 6.35 (1H, s, H-2), 5.12 (1H, d, H-6), 4.44–4.52 (1H, m, OCH H ), 4.31–4.39
a
b
0
1H, m, OCH H ), 2.86 (1H, dd, J = 15.1, 6.0 Hz, H-5 ), 2.77 (1H, dd, J = 15.1,
a b
(
0
0
13
2
1
1
5
.4 Hz, H-5 ), 1.48 (3H, t, J = 7.3 Hz, OC–CH ); C-NMR (100 MHz, CDCl3)
3
68.4 (C=O), 156.2 (C-4), 146.6, 142.5, 141.0, 137.7, 133.0, 132.3, 129.2, 128.9,
28.6, 128.2, 127.9, 126.2, 125.9, 116.9, 113.1, 97.6 (C-3), 57.5 (C-6), 54.8 (C-2),
1.3, 33.8 (C-5).
Computational details
In this study, the recently developed M06-2X/6-311G(d,p) meta hybrid density
functional which is one of the recommended DFT methods for applications
involving main-group thermochemistry [47] was used for the structural, spectro-
scopic and the thermochemical analysis of MPPC. The vibrational frequencies were
calculated to check the nature of the minimum structure, the prediction of IR
123

Z. Hoseinabadi et al.
spectrum and the calculation of thermochemical data for the multicomponent
reaction (Scheme 1).
The difference of the sum of electronic energy and thermal energy for the
reactants and products was used for the calculation of the internal energy of the
reaction (D U°). The difference of the sum of electronic energy and thermal
r
enthalpy for the reactants and products was used for the calculation of the standard
enthalpy of the reaction (D H°). The entropy of the reaction (D S°) was evaluated on
r
r
the basis of thermodynamic functions obtained by vibrational analysis results and
the statistical thermodynamic method. The Gibbs free energy of reaction (D G°) was
calculated via Eq. (1), where T is 298.15 K.
r
ꢀ
ꢀ
ꢀ
DrG ¼ DrH À TDrS
ð1Þ
1
H NMR chemical shifts (d, ppm) versus tetramethylsilane (TMS) were
calculated by the gauge-independent atomic orbitals (GIAO) method [48, 49] using
the more convenient B3LYP/6-311?G(2d,p) density function. All calculations were
performed using the Gaussian 09 program package [50].
Results and discussion
Characterization of the novel catalyst
The Sta-SO H sample has a broad and very weak XRD pattern (shown in Fig. 1),
3
confirming that the Sta-SO H is still amorphous and far from the graphitic carbon
3
[51]. Two broad and diffuse peaks at 2 angles of 20–30° and 40–50° are due to the
carbon (0 0 2) and (1 0 0) reflections, respectively. These diffraction patterns reflect
that Sta-SO H is an amorphous carbon composed of aromatic carbon sheets oriented
3
in a random fashion. Structurally, the carbon framework of Sta-SO H is
3
approximately consistent with the others reported in the literatures [52, 53].
Nevertheless, the peaks of the 0 0 2 and 1 0 0 reflections are changed slightly in their
position and shape. This may be due to the differences in carbonization mechanisms
H
NH2
N
N
O
CHO
O
O
OR2
Sta-SO3H
RT. Solvent Free
+
+
OMe
MPPC
Scheme 1 Synthesis of MPPC
1
23

Synthesis of sulfonated carbon-based solid acid as a…
3
Fig. 1 The XRD patterns of the Sta-SO H
of starch and other carbohydrates, which impact the size and orientation degree of
graphite crystallite.
The FT-IR spectra of Sta-180 and Sta-SO H are shown in Fig. 2. Compared to
3
sample Sta-180, the distinguished features of Sta-180 were the presence of new
-1
-
1
-1
absorption bands at 1032 cm , 1007 cm and 1118 cm , which are attributed to
SO H groups. This indicates that the SO H groups were successfully incorporated
into the carbon framework by adding H SO in the synthesis system. On the other
3
3
2
4
-
1
-1
hand, the bands due to –OH stretching at 3420 cm , C=O stretching at 1710 cm
-
C=C bonds stretching at 1620 cm , and C–C bonds stretching at 1250 cm were
observed for both samples independent of the sulfonation.
Structural analyses of catalyst were carried out using FESEM, and representative
,
1
-1
images are illustrated in Fig. 3. Sta-SO H was found to be composed of
3
nonspherical and irregular grains of \ 20 nm in size. The sample was gold-coated
before scanning.
The acidity of the novel carbon-based solid acid was 8 mmol/g, which were
determined by acid–base back titration. The elemental analysis gave the results:
C 61.79 %; H 1.634 %; S 1.6 %. The results indicated that the almost all the
element S existed in the catalyst in the form of sulfonic groups.
If all the sulfur element in the catalysts was assumed to be –SO H groups, the
3
?
acid site densities of the catalysts were 0.5–2 mmol H /g depending on the
synthesis conditions. But the acid titration experiments demonstrated much higher
acid site densities than the estimations based on sulfur elemental analysis. The
reason is that abundant phenolic –OH and –COOH groups were generated in the
process of partial oxidation by concentrated sulfuric acid. The strong sulfonation
agent may also oxidize aliphatic CH /CH groups to carboxylic acid groups which
3
2
may further explain the significant increase in total acid density after sulfonation.
According to the reported carbon-based solid acid, the proposed carbon structure is
schematically illustrated in Scheme 2 [54].
123

Z. Hoseinabadi et al.
6
6
6
6
6
5
5
5
5
5
5
4
3
2
1
0
9
8
7
6
5
4
3
900
3400
2900
2400
1900
1400
900
400
-1
Wavenumber (cm )
3
3
3
3
3
3
3
3
2
2
7
6
5
4
3
2
1
0
9
8
3
900
3400
2900
2400
1900
1400
900
400
-1
Wavenumber (cm )
Fig. 2 The IR spectra of the Sta-SO
3
H (top) and the Sta-180 (bottom)
Catalytic procedure for the synthesis of piperidines
Characterizations showed that the acid site densities of the catalyst were
?
mmol H /g and, in continuation of our work in the application of heterogeneous
8
catalyst [55–58], the catalytic performances were also investigated in the synthesis
of highly substituted piperidines as multicomponent reactions (Scheme 3).
A series of trial reactions were performed with a combination of 4-methylben-
zaldehyde, aniline, and methyl acetoacetate to obtain reaction conditions. Several
solvents were screened prior to concluding that reactions carry out without any
solvent (Table 1).
Thus, the optimum conditions for synthesis of highly substituted piperidines was
achieved by treatment of 1 mmol of aldehyde, 2 mmol aniline and 1 mmol b-
ketoester in the presence of Sta-SO H (0.35 g) at room temperature under solvent-
3
free conditions. Therefore, we employed the above conditions for the conversion of
various aldehydes and amines to the highly substituted piperidines under solvent-
1
23

Synthesis of sulfonated carbon-based solid acid as a…
3
Fig. 3 The SEM image of Sta-SO H
HO S
3
OH
Hydrothermal
80 °C, 24h
1
Starch
+
H SO4
2
COOH
HO
COOH
3
Scheme 2 Synthesis of Sta-SO H and proposed structure for the catalyst
free conditions (Table 2). The structures of all the compounds were characterized
by a comparison of their IR and NMR spectra with authentic samples. Also, the
relative stereochemistry of these piperidines has been confirmed by single X-ray
123

Z. Hoseinabadi et al.
H
R3
N
N
O
NH2
CHO
O
O
OR2
Sta-SO3H
+
+
R1
OR2
RT. Solvent Free
R3
R1
R1
R3
Scheme 3 Synthesis of functionalized piperidines
Table 1 Optimization of the
Entry
Solvent
Catalyst (g)
Time
Yield (%)
reaction conditions for the
synthesis of functionalized
a
1
2
3
4
5
6
7
8
9
CH OH
3
0.035
0.035
0.035
0.035
0.035
Neat
5 h
5 h
5 h
5 h
5 h
63
61
61
62
61
28
48
67
67
piperidine
ETOH
Ethyl acetate
2 2
CH Cl
n-Hexane
Neat
a
Conditions:
20 min
30 min
20 min
20 min
4
-methylbenzaldehyde
2 mmol), aniline (2 mmol), and
methyl acetoacetate (1 mmol)
and Sta-SO H (0.035 g) at RT
under solvent-free conditions
Neat
0.0175
0.035
0.07
(
Neat
3
Neat
crystallography analysis in previously reported literature [16–19, 38, 39, 43, 59–61],
and the relative stereochemistry of the products in the present work was proved by
comparison of spectroscopic data of some products with those authentic samples.
Using the optimal reaction conditions, the reaction of benzaldehyde with aniline
and methyl acetoacetate was studied and the product was obtained in good yield
(
substituents in the aromatic ring such as OMe, Cl, Br, and NO with aniline and
Table 2, entry 5). The reactions of various aromatic aldehydes containing
2
methyl acetoacetate were performed under the same reaction conditions. The
reaction time and the percentage yield of the products are shown in Table 2.
However, in the case of 3- and 4-nitrobenzaldehydes the products were obtained in
low yield (Table 2, entries 4 and 8). This may be attributed to the formation of more
stable imine having an extra conjugation in the presence of nitro group. This
stable imine is less reactive.
Various anilines with substituents such as Me, OMe, Br, and NO were treated
2
with 4-methylbenzaldehyde and methyl acetoacetate under identical reaction
conditions. All these reactions underwent smoothly to provide the corresponding
piperidine derivatives, in moderate to good yields (Table 2, entries 8–14).
The reaction was further examined with ethyl acetoacetate as a 1, 3-dicarbonyl
compound and the desired piperidine derivative was obtained in good yields
1
23

Synthesis of sulfonated carbon-based solid acid as a…
3
Table 2 Synthesis of functionalized piperidines using Sta-SO H
a
b
Yield (%)
Entry
Substrate
Product
Time (min)
m.p. (°C) [Ref.]
Found
Reported
NH
2
CHO
1
20
67
210–217
215–217 [16]
NH
N
O
OCH3
CH
3
O
O
OCH
3
NH
2
CHO
2
3
4
5
6
7
8
20
20
40
50
50
50
40
68
180–182
223–225
181–183
195–196
229–231
206–208
252–255
181 [62]
NH
O
OCH3
OCH3
OCH3
N
O
O
H3CO
OCH3
NH2
CHO
71
223–225 [42]
NH
O
OCH3
Cl
O
O
N
OCH3
Cl
Cl
NH2
CHO
47
182 [62]
NH
N
O
NO2
OCH3
O
O
O2N
NO2
OCH3
CHO
NH2
62
195–196 [42]
228–231 [16]
206–208 [42]
253–255 [16]
NH
N
O
OCH3
O
O
OCH3
CHO
NH
2
65
NH
N
O
OC2H5
O
O
2 5
OC H
NH2
CHO
73
NH
N
O
OC2H5
Cl
O
O
OC2H5
Cl
Cl
NH2
CHO
O2N
Trace
NH
O
OCH3
NO2
O
CH3
O
N
OCH3
H3C
CH3
NO2
NH2
CHO
Cl
9
50
55
180–181
181–183 [42]
NH
N
O
OCH3
Cl
O
CH3
O
H3C
CH3
OCH3
Cl
123

Z. Hoseinabadi et al.
Table 2 continued
Entry Substrate
a
b
Yield (%)
Product
Time (min)
m.p. (°C) [Ref.]
Found
Reported
–
NH2
CHO
H3CO
1
0
1
40
66
67
223–225
NH
N
O
OCH3
OCH3
O
O
OCH3
OCH3
NH2
CHO
H3CO
1
45
225–227
223–225 [42]
NH
N
O
OCH3
OCH3
O
O
OCH3
OCH3
NH2
CHO
1
1
1
2
3
4
45
40
40
65
69
66
206–208
213–215
223–225
205–207 [42]
213–215 [62]
220–222 [62]
NH
O
OCH3
N
O
O
OCH3
NH2
CHO
NH
N
O
OCH3
Cl
O
O
Cl
Cl
OCH3
NH2
CHO
NH
N
O
OCH3
O
O
OCH3
Conditions: aldehyde (2 mmol), amine (2 mmol), methyl acetoacetate (1 mmol), Sta-SO3H (0.035 g), at
roomtemperature under solvent free conditions
a
All compounds were characterized by 1H-NMR, 13C-NMR, IR
b
Isolated yield
(
Table 2, entries 7 and 8). This confirms that the alkoxy (–OR) moiety present in the
ester functionality does not have any major role in determining the course of the
reaction.
In order to show the merit of the present work in comparison with recently
reported protocols, we compared the results of condensation of 4-chlorobenzalde-
hyde, aniline and methyl acetoacetate in the presence of various catalysts (Table 3).
The results show that Sta-SO H promotes the reactions effectively as far the amount
3
of catalyst and reaction times are concerned.
The reusability of the catalyst is of great importance for commercial feasibility;
therefore, the recovery and reusability of Sta-SO H was investigated in the reaction
3
of 4-methylbenzaldehyde, aniline and methyl acetoacetate for 20 min, at r.t. under
1
23

Synthesis of sulfonated carbon-based solid acid as a…
Table 3 Comparison of different catalysts for synthesis of piperidines
Entry
Catalyst
Solvent/condition
Reaction time
% Yield
Ref.
1
2
3
4
5
6
7
8
9
Sta-SO
ZrOCl
TBATB (10 mol%)
3
H (0.035 g)
None/rt
20 min
4 h
71
82
82
76
90
78
75
85
76
87
2
ÁH O (20 mol%)
2
EtOH/Reflux
EtOH/rt
EtOH/rt
EtOH/rt
None/rt
[39]
[19]
[18]
[41]
[44]
[42]
[16]
[17]
[20]
10 h
18 h
8 h
Bi(NO
?
3
)
3
Á3H
2
O (10 mol%)
-
[K PEG]Br
AcOH (5 mL)
VCl (10 mol%)
(10 mol%)
3
(10 mol%)
2 h
3
EtOH/rt
MeOH/rt
7 h
I
2
6 h
BDMS (10 mol%)
CH
3
CN/rt
6 h
1
a
0
Tartaric acid (0.075 g)
MeOH/rt
7 h
Reaction conditions: 4-chloro benzaldehyde (1 mmol), aniline (1 mmol), methyl acetoacetate (1 mmol)
Isolated yields
b
100
90
80
70
60
50
40
30
20
10
0
6
7
67
67
67
1
2
3
4
Run
Fig. 4 The reuse of the catalyst
solvent-free conditions, and the results are displayed in Fig. 3. The separated
catalyst can be reused after washing with ethyl acetate and drying at 100 °C. The
catalyst was recovered in good yields and catalyst was used in the mentioned
reaction for four successive reactions without significant loss of activity (Fig. 4).
Acylals are a useful carbonyl-protecting group because of their stability under
both neutral and basic media and under acidic conditions [63]. Generally, acylals
have been prepared by the reaction of aldehydes with acetic anhydride catalyzed by
several reagents or catalysts such as Bi(NO ) Á5H O [64], silica-bonded S-sulfonic
3
3
2
acid [65], saccharin sulfonic acid [66], cyanuric chloride [67], silica phosphoric acid
[
68], and SiO /B(SO H) [69]. However, in these, some reported methods suffer
2 4 3
from one or more drawbacks such as prolonged reaction times, use of
environmentally unfavorable solvents and frequently low yields. Thus, the
development of a new method for the synthesis of acylal derivatives would be
highly desirable.
123

Z. Hoseinabadi et al.
In order to check the catalytic activity of Sta-SO H as a solid acid, the synthesis
3
of acylals was investigated. The optimum conditions for the acylation of
benzaldehyde was achieved by treatment of benzaldehyde (1 mmol) with acetic
anhydride (3 mmol) in the presence of Sta-SO H at r.t. under solvent-free
3
conditions (Scheme 4). By following this reaction procedure, benzaldehyde
derivatives were converted to the corresponding acylal in high yields. The results
are summarized in Table 4.
Thermochemistry of reactions and spectroscopic analysis of products
Figure 3 shows the optimized geometry of the MPPC at M06-2X/6-311G(d,p) level
of theory. This structure is minimal on the potential energy surface since there is no
imaginary frequency. In this compound; the –NH of aniline fragment orients to the
side of –C=O group of methyl acetoacetate part to make favored intramolecular
˚
hydrogen bonding (1.840 A). Other important structural parameters of MPPC are
indicated in Fig. 5.
Figure 6 shows the electrostatic potential (ESP) map and the electron density of
highest and lowest molecular orbitals (HOMO and LUMO) for MPPC calculated by
the M06-2X/6-311G(d,p) method. The red and blue positions in the ESP map
O
O
O
Sta-SO H
3
AcO
R
OAc
H
+
H3C
O
CH3
R
H
Solvent Free Conditions, RT
R: Aryl
Scheme 4 Synthesis of acylals
3
Table 4 Conversion of aldehydes to the corresponding 1,1-diacetates by Sta-SO H as catalyst at r.t. and
under solvent-free conditions
a
b
Entry Substrate
Product
Time (min) Yield (%) m.p. °C [Ref.]
Found
41–44
Reported
44–45 [70]
CHO
CHO
1
2
5
7
92
94
NO2
CHO
CHO
Cl
123–126 125–127 [70]
Cl
CH(OAc)2
CH(OAc)2
3
4
8
6
91
87
81–83
57–60
82–83 [70]
59 [71]
NO2
CH(OAc)2
CH(OAc)2
Cl
Cl
a
1
The products were characterized from their spectral (IR and H-NMR) and comparison to authentic
samples
b
The yields refer to the isolated pure products
1
23

Synthesis of sulfonated carbon-based solid acid as a…
H9
H7
C11
C9
H8
C10
C7
H18
H5
H14
H12
C8
C6
H19
C22
C23
C17
C15 H13
C16
H6
C20 H16
1.436
C21
H17
C13
H2
H10
C14
C12
N1 1.473
C19
C18
H22
H24
H11 1.476
.526
1
.515
C2
C1
1.389
H15
1
C27
C5
1.528
C29
C28
1
.375
1.511 H3
.411
1
.391
C3
H4 1.526 C4
H1
1.396
1
1
.362
C25
C30
1.391
H25
N2
1
.459
1
.343
C24
H20 1.018
1.396
C26
O2
1
.221 O1
1
.840
H26
1
.425
1.388
H27
C31
H21
H23
H28
Fig. 5 The optimized structure of MPPC by M06-2X/6-311G(d,p) level of theory
Fig. 6 ESP, HOMO and LUMO electron density maps for MPPC calculated by the M06-2X/6-
11G(d,p) method (from left to right)
3
indicate the most negative and the most positive electrostatic potentials, respectively
unit for the legend is in kcal/mol). The calculated orbital energies of HOMO and
LUMO and the energy separation between them which so calls DEgap are -6.9,
0.1, 7.0 eV, respectively. As seen in this figure that the electron density of HOMO
(
-
and LUMO are mostly delocalized on the piperidine ring and the aniline component
of MPPC.
Table 5 lists the calculated thermochemical properties for the synthesis of MPPC
using the multicomponent reaction of benzaldehyde, aniline and methyl
123

Z. Hoseinabadi et al.
Table 5 Thermodynamic properties for synthesis of MPPC using the multicomponent reaction of
benzaldehyde, aniline and methyl acetoacetate calculated at M06-2X/6-311G(d,p) level of theory
Property
D
r
D
r
D
r
D
r
U°
H°
S°
-14.6
-15.2
-0.02
-8.4
G°
Energies are in kcal/mol and entropy is in kcal/mol K
acetoacetate (Scheme 1) at M06-2X/6-311G(d,p) level of theory. As the synthesis of
highly functionalized piperidines in this work was performed at room temperature,
gas phase enthalpy, internal energy, entropy and Gibbs free energy of reactions were
calculated at 298.15 K. According to the results, all the thermodynamic properties
of the multicomponent reaction at this temperature are negative. Formation of
MPPC is an exothermic process (D H° = -15.2 kcal/mol) accompanied by a
r
decrease in internal energy (D U° = -14.6 kcal/mol), entropy (D S° =
r
r
-
0.02 kcal/mol K) and Gibbs free energy of reaction (D G° = -8.4 kcal/mol).
r
The negative value of the change in Gibbs free energy during the reaction indicates
that the reaction can occur spontaneously under ambient temperature. As the
number of product molecules is less than the number of molecules of the starting
materials, the entropy of the reaction is also negative.
All the highly functionalized piperidines listed in Table 2 were known
1
13
compounds and characterized by IR, H NMR, and C NMR spectra and well
matched with the literature-reported compounds. In the following, these results for
one of the general compound entry 5 are discussed. A broad range of IR spectrum of
-
1
this compound (400–3600) cm , is covered by bending and stretching vibrations
-
1
(
Fig. 7, top). The peaks observed around 3200 cm
stretching mode. The wide peaks around 3000–3100 cm are assigned to the
corresponds to the N–H
1
-
2
stretching sp = C–H vibrations. The peaks appearing below 3000 cm are related
-1
3
-1
to the sp –C–H stretching modes. The sharp peak observed at 1660 cm is
attributed to the C=O stretching mode, which is shifted to lower frequencies than the
-
1
normal position of the C=O group in esters, i.e. 1730–1750 cm
conjugation with C=C and intramolecular hydrogen bonding to N–H). The C=C
(because
-
stretch adsorptions are seen at 1618, 1596 and 1498 cm . The peak around
1
-
200 cm is related to the C–O stretching mode. Using the scaling factor of 0.94
1
1
for the calculated data at M06-2X/6-311G(d,p) level of theory, a good consistency
between the calculated and observed spectra was found. For example the N–H,
C=O, C=C and C–O stretching modes appear at 3256, 1654, (1575, 1554, 1401) and
-
1
1
238 cm ,respectively (Fig. 7, bottom).
1
Table 6 lists the calculated H NMR chemical shifts (d, ppm) for MPPC in the
gas phase and in chloroform as solvent via B3LYP/6-311 ? G(2d,p) level of theory
(
vs. TMS) (see also Fig. 8). The experimental spectrum recorded in CDCl is shown
3
in Fig. 8. Due to intramolecular NHÁÁÁO hydrogen bonding, the hydrogen atom
1
23

Synthesis of sulfonated carbon-based solid acid as a…
1
Table 6 H NMR chemical shifts (d, ppm) for MPPC calculated via B3LYP/6-311 ? G(2d,p) level of
theory in comparison to experimental data
H atoms
Calculated
Gas phase
Experimental
In solvent
In solvent
Explanation
–
NH (H20)
11.2
6–8.2
6.3
11.2
6–8.2
6.3
10.3
Singlet, 1H
Phenyl rings (H5–H19, H21–25)
6.4–7.3
6.4
Multiplet, 20H
Singlet, 1H
–
–
–
–
CH (H4)
CH (H1)
4.8
4.8
5.1
doublet, 1H
CH
2
(H2, H3)
2.4, 3.4
3.7
2.4, 3.4
3.7
2.8, 2.9
3.9
Doublet of doublet, 2H
Singlet, 3H
OCH
3
(H26–H28)
3600
3200
2800
2400
2000
1600
1200
800
400
-1
Frequency (cm )
3600
3200
2800
2400
2000
1600
1200
800
400
-1
Frequency (cm )
Fig. 7 Experimental FTIR spectrum of MPPC (top) in comparison to the calculated data using M06-2X/
-311G(d,p) level of theory with scaling factor of 0.94 (bottom)
6
attached to nitrogen (H20) markedly shifts downfield and appears close to
0.3 ppm. The hydrogen atoms attached to aromatic rings are seen at 6.4–7.3 ppm.
The single peaks at 5.1 and 6.4 are related to benzylic-allylic –CH (H4) and
1
123

Z. Hoseinabadi et al.
1
3
Fig. 8 H NMR spectrum of MPPC recorded in CDCl as solvent
benzylic –CH (H1), respectively. The hydrogen atoms of –CH (H2, H3) are
2
diasterotopic and appear in the area of 2.8 and 2.9 as a doublet of doublet. The
hydrogen atoms of OCH are singlet and they resonance at 3.9 ppm. An accept-
3
able agreement was found between the calculated and observed spectroscopic data
Table 6).
(
Conclusion
A novel carbon-based solid acid has been synthesized through the simple heat
treatment of sulfuric acid and starch. The novel process was more environmentally
friendly in the mild condition with little waste emissions and high yield. Here, we
have found that the formation of highly functionalized piperidines is possible in the
presence of Sta-SO H as catalyst via a one-pot five-component reaction at room
3
temperature from readily available starting materials. Some advantages of this
MCRs protocol are good yields, mild reaction conditions, environmentally benign
catalyst, absence of tedious separation procedures, superior atom economy, and low
cost. The catalyst has the advantages of high acidity, low cost and high thermal and
chemical stability, which made the novel heterogeneous catalyst have great
potential for the replacement of the homogeneous catalysts in the green process.
Density functional theory (DFT) calculations were used to study the structure of
MPPC as well as the thermochemistry of the multicomponent reaction. The
1
theoretically calculated infrared (IR) and H nuclear magnetic resonance (NMR)
spectra were compared to the experimental data. It was found that the synthesis of
highly functionalized piperidines is exothermic accompanied by a decrease in
entropy, internal energy and Gibbs free energy of reaction. Also, good consistency
between the calculated and observed spectral data was found. On the other hand, an
efficient and rapid catalytic method was achieved for the preparation of acylals
1
23

Synthesis of sulfonated carbon-based solid acid as a…
using this catalyst at room temperature and under solvent-free conditions. The mild
condition, eco-friendliness, good to high yields, short reaction times and use of an
inexpensive and reusable catalyst are important features of this method.
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