Chemoselective synthesis of geminal diacetates (acylals) using eco-friendly reusable propylsulfonic acid based nanosilica (SBA-15-Ph-PrSO3H) under solvent-free conditions

Article abstract of DOI:10.1016/j.molcata.2013.07.001

An expeditious method for the acetylation of aldehydes by mesoporous solid sulfonic acid (SBA-15-Ph-PrSO₃H) under solvent-free reaction conditions was described. The route furnished selectively and in excellent yields the corresponding 1,1-diacetates starting from aldehydes and acetic anhydride using the environmentally friendly catalyst. The catalyst was found to be highly active and selective and could be recycled several times.

Full text of DOI:10.1016/j.molcata.2013.07.001

Accepted Manuscript
Title: Chemoselective synthesis of geminal diacetates (acylals)
using eco-friendly reusable propylsulfonic acid based
nanosilica (SBA-15-Ph-PrSO3H) under solvent-free
conditions
Author: Daryoush Zareyee Sayed M. Moosavi Alireza
Alaminezhad
PII:
S1381-1169(13)00251-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.07.001
MOLCAA 8798
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
20-2-2013
15-6-2013
1-7-2013
Please cite this article as: D. Zareyee, S.M. Moosavi, A. Alaminezhad,
Chemoselective synthesis of geminal diacetates (acylals) using eco-friendly
reusable propylsulfonic acid based nanosilica (SBA-15-Ph-PrSO3H) under
solvent-free conditions, Journal of Molecular Catalysis A: Chemical (2013),
http://dx.doi.org/10.1016/j.molcata.2013.07.001
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Chemoselective synthesis of geminal diacetates (acylals) using eco-friendly reusable propylsulfonic
acid based nanosilica (SBA-15-Ph-PrSO3H) under solvent-free conditions
Daryoush Zareyee1a, Sayed M. Moosavib and Alireza Alaminezhadb
aDepartment of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran
bDepartment of Chemistry, Imam Hossein University, Tehran, Iran
Abstract: An expeditious method for the acetylation of aldehydes by mesoporous solid sulfonic acid
(SBA-15-Ph-PrSO3H) under solvent-free reaction conditions was described. The route furnished
selectively and in excellent yields the corresponding 1,1-diacetates starting from aldehydes and acetic
anhydride using the environmentally friendly catalyst. The catalyst was found to be highly active and
selective and could be recycled several times.
Keywords: Acylals; 1,1-Diacetates; Protecting groups; Solvent-free; SBA-15.
1. Introduction
Acylal formation plays a pivotal role in selective protection of carbonyl groups because of their stability
in neutral and basic media as well as towards aqueous acids [1]. Hence, protection of aldehydes as
acylals plays an important role in multi-step organic synthesis. Accordingly, methods for their synthesis
have received considerable attentions. Usually the formation of 1,1-diacetates is catalyzed by strong
protic acids, such as H2SO4 [2,3], CH3SO3H [4], NH2SO3H [5], KHSO4 [6] and Lewis acids such as PCl3
[7], NBS [8], Bi(OTf)3 [9], In(OTf)3 [10], AlCl3 [11], CAN [12] and ZrCl4 [13]. From the environmental
and economical standpoints, various inorganic heterogeneous catalysts, such as Nafion-H [14], K-10
[15], AlPW12O40 [16], HClO4.SiO2 [17], silica sulfuric acid [18], H3PMo12O40 supported MCM-41 [19],
BEA-SO3H [20], and N-sulfonic acid poly(4-vinylpyridinum) chloride [21] were also reported.
However, except a few, many of these methods have not been entirely satisfactory, because the use of
conventional Bronsted or Lewis acid catalysts as well as excess organic solvents usually entails the
1 Corresponding author. Tel.:+98 123 214 5048; Fax: +98 123 214 5050.
E-mail address: zareyee@gmail.com, zareyee.d@qaemshahriau.ac.ir
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problems of corrosiveness, tedious work-up and effluent pollution. Moreover, some of these methods are
not chemoselective in terms of aldehydes and keto carbonyl functional groups. Therefore, a great deal of
research focused on development of efficient, recyclable and environmentally benign catalytic methods
for this transformation by using inexpensive and nonpolluting reagents and solvents. We have recently
initiated studies into new catalytic properties of solid sulfonic acid SBA-15-Ph-PrSO3H as a reusable,
heterogeneous and easily recyclable catalyst for the von Pechmann synthesis of coumarin [22], Strecker
syntheses of α-amino nitriles [23] and silylation of hydroxyl groups using HMDS [24]. In further
extension to our interest on solvent-free catalytic method and our ongoing green organic chemistry
program, herein we describe a simple, convenient and chemoselective method for the synthesis of
acylals from aldehydes using recoverable solid sulfonic acid SBA-15-Ph-PrSO3H (Scheme 1).
Scheme 1. Schematic synthesis of gem-diacetate using catalyst SBA-15-Ph-PrSO3H (1 mol%) under solvent-free reaction
conditions.
2. Results and discussion
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In the beginning, efforts were made towards the catalytic evaluation of SBA-15-Ph-PrSO3H for the
synthesis of acylal from benzaldehyde. In the initial catalytic activity experiment, a reaction was carried
out using 1mmol benzazldehyde, 1.2mmol Ac2O and 0.5mol% catalyst. These were stirred at ambient
temperature in solvent-free reaction conditions and 55% of the corresponding geminal diacetate was
obtained after 2h. Considerable improvement was observed in the yield of the related product when
amount of catalyst increased to 1mol%. As shown in Table 1 (entry 1), the corresponding product was
obtained in quantitative yield after 5min.
Having established the preferred reaction conditions, we varied aldehydes while retaining other reactants
(Ac2O and SBA-15-Ph-PrSO3H). The reactions took place efficiently in good to excellent yields for all
studied aldehydes. The electronic effects and the nature of substituents on the aromatic ring did not
show strongly obvious effects in terms of yields under the present reaction conditions (Table 1, entries
1-14). Noteworthy, this catalyst was not able to prepare acylal from p-(N,N-
dimethylamino)benzaldehyde (Table 1, entry 15). It should be noted that furfural as an acid sensitive
substrate gave the related 1,1-diacetate in excellent yield (Table 1, entry 16). The tolerance of various
functional groups under the present reaction conditions is also worthy of mention in that acid sensitive
groups, such as chloro, bromo, methoxy and nitro do survive under such conditions. Aliphatic aldehydes
also underwent the reaction producing the corresponding geminal diacetate in excellent yields (Table 1,
entries 17-20). The generality of this reaction was tested by subjecting 2-hydroxy benzaldehyde which
both the hydroxyl and carbonyl groups were acetylated (Table 1, entry 21).
Table 1. Solvent-free acylal formation from aldehydes using Ac2O in the presence of SBA-15-Ph-PrSO3H
Time
(min)
Yielda,b
(%)
Entry
1
Product
AcO
OAc
5
100
98
H
AcO
OAc
2
15
H
Cl
3
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AcO
OAc
H
3
10
96
Cl
AcO
OAc
4
5
10
15
98
92
H
Cl
AcO
AcO
OAc
H
Br
OAc
H
6
10
97
Br
AcO
AcO
AcO
OAc
7
8
10
10
15
15
98
90
90
88
H
Br
F
OAc
H
OAc
9
H
F3C
AcO
AcO
OAc
10
H
OMe
OAc
H
11
15
92
OMe
AcO
OAc
12
13
14
15
15
60
95
88
H
MeO
AcO
OAc
H
NO2
AcO
OAc
60
100
H
O2N
AcO
OAc
No
reaction
24h
H
Me2N
H
16
17
18
15
20
25
95
89
99
S
OAc
AcO
H
OAc
AcO
AcO
H
OAc
H
19
20
35
40
89
90
OAc
AcO
H
OAc
AcO
4
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AcO
AcO
OAc
21
35
89
H
OAc
OAc
No
reaction
22
24h
CH3
a GC Yields.
b All reaction were carried out at room temperature using aldehyde (1 mmol), Ac2O (1.2 mmol) and SBA-15-Ph-PrSO3H (1 mol%).
As shown in Table 1, Ketone could not be converted into the corresponding gem-diacetate even when
the reaction mixture was stirred for 24 h (Table 1, entry 22). Encouraged by this result, we studied
competitive reactions for acylation of benzaldehyde in the presence of acetophenone under the same
reaction conditions. We found that benzaldehyde was converted to the related gem-diacetate while the
acetophenone remained unaffected (Scheme 2). This observation suggested that chemoselective acylal
formation from aldehydes in the presence of ketones could be achieved with this catalytic system.
O
AcO
AcO
OAc
H
H
100%
0%
SBA-15-Ph-PrSO3H (1 mol%)
O
solvent-free, Ac2O (1.2 mmol), 5 min.
OAc
CH3
CH3
GC monitoring
Scheme 2. Competitive acylal formation from benzaldehyde in the presence of acetophenone.
In addition to these results, we further studied the possibility of deprotection of resulting acylals in this
catalytic system by addition of water as a green solvent. Indeed, when the formation of 1,1-diacetate
o
from benzaldehyde was completed, water was added and the reaction temperature increased to 100 C.
By this procedure, related acylal has been completely transformed into benzaldehyde in short reaction
time (Scheme 3).
O
AcO
OAc
H
H
SBA-15-Ph-PrSO3H (1 mol%)
H2O, reflux, 15 min., 100%
Scheme 3. Cleavage of acylal to benzaldehyde in water catalyzed by SBA-15-Ph-PrSO3H.
Our attention next turned to the recycling performance of the catalyst in the synthesis of 1,1-diacetate
from benzaldehyde. After the completion of the first run, ethyl acetate was added and then catalyst was
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isolated from the reaction mixture by filtration. As shown in Figure 1, the recovered SBA-15-Ph-
PrSO3H could be directly reused in nine successive runs without significant loss of activity.
Additionally, for better investigation of catalyst recyclability, loading of catalyst decreased to 0.5 mol%
in another performed experiment. The yield of corresponding product reached to 35% after 55 min under
new reaction conditions. Interestingly, the catalyst reused again in nine successive runs with no
significant decrease in its efficiency, providing same yield (35%) of the respective product.
Figure 1. Recovery of SBA-15-Ph-PrSO3H for the formation of gem-diacetate from benzaldehyde
The TEM images of SBA-15-Ph-PrSO3H after nine times recovery showed that the hexagonal array of
uniform channels of SBA-15 remained unchanged after recovery (Figure 2).
Figure 2. TEM image of catalyst after the 9th reaction cycle.
In order to compare the catalytic potentiality of SBA-15-Ph-PrSO3H with some recently reported
protocols, we shown the results of the gem-diacetate synthesis from 4-nitrobenzaldehyde in the presence
of KHSO4, CAN, LiOTf, InCl3, Cu(OTf)2, K-10, H3PW12O40/MCM-41 and SSA with respect to the
reusability and amounts of catalysts, amounts of Ac2O, reaction times and yields of the products (Table
2). A glance survey on the results in Table 2 clearly elucidate that some protocols took much longer time
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[12, 25-27], some used non-recovered catalyst [6, 12, 25-28], or some used dangerous materials with
reduced isolated yields [19, 25]. The higher efficacy of SBA-15-Ph-Pr-SO3H in solvent-free synthesis of
gem-diacetate from aldehydes might likely be attributed to the high surface area of ordered SBA-15 and
hydrophobic character of phenyl groups in close proximity to sulfonic acid groups.
Table 2. Comparison of the efficiency of catalysts for gem-diacetate synthesis from 4-nitrobenzaldehyde
Equiv. of
Time
(min/[h])
60
Entry
Catalyst (mol %)
Solvent
T (oC)
Yield (%)
Ac2O
1.2
2
1
2
3
4
5
6
7
8
9
SBA-15-Ph-PrSO3H (1)
KHSO4 (10)
Solvent free
Solvent free
Solvent free
H2O
rt
rt
100 a
90
96 6,b,c
96 12,b,c,d
88 27,b,c,d
94 28,b,c,d
94 26,c,d,e
85 29,c,g
61 19,f
CAN (10)
2
rt
[24]
[4]
InCl3 (10)
2
rt
LiOTf (20)
5-8
3
Solvent free
CH2Cl2
rt
[15]
[4]
Cu(OTf)2 (2.5)
rt
Montmorillonite K-10 (0.03 g)
H3PW12O40/MCM-41 (0.03 g)
2
Solvent free
Solvent free
rt
10
0.5
20
60
rt
[2]
SSA (0.06 g)
Solvent free
50s
99 18,h
a
b
c
d
e
f
Present work. High catalyst loading. Non-recoverable catalyst. Long reaction time. Toxic solvent (CH2Cl2). Low yield of product and high
temperature. g Microwave irradiation. h Excess amount of Ac2O.
3. Conclusions
In summary, we have demonstrated that SBA-15-Ph-Pr-SO3H is a highly efficient and chemoselective
catalyst for gem-diacetate synthesis from aldehydes under solvent-free reaction conditions at room
temperature. The excellent catalytic capacity and outstanding recyclability of catalyst, the simple
workup and solvent –free reaction conditions add to the versatility of this method.
4. Experimental section
General procedure for the synthesis of 1,1-diacetates: In a round bottom flask, SBA-15-Ph-PrSO3H
(6mg, 1mol%) was added to the mixture of aldehyde (1mmol) and freshly distilled acetic anhydride
(1.2mmol) and the reaction mixture stirred at room temperature for the appropriate time (Table 1). The
progress of reaction was monitored by GC and TLC (eluent, n-hexane:ethyl acetate, 4:1). After
completion of the reaction, a mixture of water and EtOAc (1:1) was added to the reaction mixture, and
the resulting mixture filtered and then extracted with EtOAc (3*10ml). The extracts were combined,
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dried over Na2SO4 and concentrated in vacuo. Compounds with purity below 95% were further purified
using column chromatography on silica gel to afford pure acylals. Characterization of some products is
achieved by 1H NMR and 13C NMR.
5. Supplementary data
Experimental procedure and characterization data (TGA, BET, BJH, and N2 adsorption-desorption
isotherm) for SBA-15-Ph-PrSO3H and general procedure for the synthesis of acylals are available.
Acknowledgements
The authors acknowledge the Islamic Azad University of Qaemshahr Research Councils for support of
this work.
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Graphical Abstract
Research highlights
ꢀ Chemoselective synthesis of acylals from aldehydes in the presence of ketones
ꢀ Solvent-free reaction conditions
ꢀ Exceedingly simple workup and uncomplicated recovery (nine reaction cycles)
ꢀ Deprotection of resulting acylals in water as a green solvent.
Outstanding thermal stability (up to 200 °C) of SBA-15-Ph-Pr-SO3H
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