Green procedures for the chemoselective synthesis of acylals and their cleavage promoted by recoverable sulfonic acid based nanoporous carbon (CMK-5-SO3H)

Article abstract of DOI:10.1007/s12039-015-0884-0

A selective synthesis of gem-diacetates from the reaction of aldehydes and acetic anhydride in the presence of recyclable nanoporous solid sulfonic acid (CMK-5-SO₃H) under solvent-free reaction conditions is reported. The catalyst was also found to be highly active for deprotection of resulting acylals in water. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0884-0

c
J. Chem. Sci. Vol. 127, No. 7, July 2015, pp. 1229–1234. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0884-0
Green procedures for the chemoselective synthesis of acylals and their
cleavage promoted by recoverable sulfonic acid based nanoporous
carbon (CMK-5-SO3H)
DARYOUSH ZAREYEE∗, EHSAN MIRZAJANZADEH and MOHAMMAD ALI
KHALILZADEH
Department of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran
e-mail: zareyee@gmail.com
MS received 14 January 2015; revised 15 March 2015; accepted 28 March 2015
Abstract. A selective synthesis of gem-diacetates from the reaction of aldehydes and acetic anhydride in the
presence of recyclable nanoporous solid sulfonic acid (CMK-5-SO3H) under solvent-free reaction conditions
is reported. The catalyst was also found to be highly active for deprotection of resulting acylals in water.
Keywords. 1,1-Diacetates; acylals; solvent-free; water; CMK-5-SO3H; heterogeneous sulfonic acid
1. Introduction
mentioned catalysts are claimed to give protection as
well as deprotection. Therefore, the development of
efficient, recyclable and environmentally benign cat-
alytic methods for these transformations by using non-
polluting reagents and solvents is desirable. Along this
line, very recently, we used sulfonic acid nanoreactor
SBA-15-Ph-PrSO3H in the synthesis of gem-diacetates
under solvent-free reaction conditions. In continuation
of our studies toward the development of new and
cleaner methods for organic transformations,22–25 we
report herein the results of chemoselective preparation
of acylals under solvent-free reaction conditions and
deperotection of the corresponding acylals in water
as green solvent at room temperature using catalytic
amounts of heterogeneous recyclable sulfonic acid
CMK-5-SO3H (scheme 1).
From the green chemistry point of view, safety consid-
erations and the simplicity of the process, implementa-
tion of methods that avoid the use of harmful organic
solvents are fundamental strategies. In this context, with
an objective to develop environmentally benign reac-
tion media and conditions for organic reactions with
excellent efficiency, solvent-free conditions or using
water as the green solvent have aroused considerable
attention.1–5 Consequently, it is important to investigate
new catalysts for organic reactions using less hazardous
solvents.
Acylals have often been used as protecting groups for
carbonyl compounds because of their stability in neutral
and basic conditions and ease of chemoselective prepa-
ration in the presence of ketone.6,7 Hence, they play an
important role for the protection of aldehydes in multi-
step organic synthesis. Accordingly, methods for their
synthesis have received considerable attention.
2. Experimental
Generally, acylals can be prepared from aldehy-
des by treatment with acetic anhydride and Brønsted
acids,7–9 Lewis acids,10–14 or inorganic heterogeneous
catalysts.15,16,16–21 However, while acknowledging
the pioneering advances in this area, some of these
methods suffer from disadvantages such as the use of
hazardous organic solvents, harsh reaction conditions,
low yields, tedious work-up and effluent pollution.
Moreover, some of these procedures are not chemos-
elective in terms of aldehydes and keto carbonyl
functional groups. Additionally, a few of the above
2.1 General procedure for the preparation of
1,1-diacetates
To a mixture of aldehyde (1 mmol) and freshly dis-
tilled acetic anhydride (1.2 mmol) was added (0.023g,
1 mol%) CMK-5-SO3H, and the whole mixture was
stirred in a round bottomed flask at room temperature
for the appropriate time (table 1). The reaction progress
was followed by GC and TLC (eluent, n-hexane:ethyl
acetate, 4:1). After completion of the reaction, EtOAc
was added to the reaction mixture, and the resulting
mixture was filtered and then the extracts concentrated
∗For correspondence
1229

1230
Daryoush Zareyee et al.
the catalyst. The solvent was evaporated under reduced
pressure to afford the corresponding aldehydes.
SO3H
SO3H
SO3H
(2 mol%)
2.3 Preparation of CMK-5
Templated synthesis of CMK-5 has been achieved
using known procedure described by Ryoo and co-
workers.26,27 Initially, Al was incorporated into SBA-
1528 (molar ratio Si/Al = 20) by well dispersion of cal-
cined SBA-15 into an aqueous solution of AlCl3, fol-
lowed by removal of H2O by rotary evaporator and cal-
cination in air. Impregnation of Furfuryl alcohol (FA)
into Al-SBA-15 was achieved by incipient wetness
infiltration at room temperature. The mixture was then
heated up at 80◦C oven for 16 h for Al-catalyzed poly-
merization of FA. The obtained composite was recov-
ered by filtration to remove excess and unpolymerized
FA, and washed by EtOH and acetone. The compos-
ite was heated to 850◦C under vacuum at a ramp of
10◦C/min, and the carbonization was carried out at the
same temperature for 3 h under vacuum. Ordered meso-
porous carbon (CMK-5) was obtained by removal of
silica template by HF (10% in 1: 1 EtOH – H2O),
washed with copious water and EtOH, and finally dried
at 100◦C.
CMK-5
Ac2O (1.2 mmol), neat, r.t.
O
AcO
OAc
R
H
R
Cat (2 mol%), H2O, r.t.
H
Scheme 1. Synthesis of gem-diacetates and their deprotec-
tion using catalyst CMK-5-SO3H.
under vacuum. The pure product was isolated follow-
ing silica gel column chromatography to afford pure
acylals. Spectroscopic data for selected examples listed
below.
1
(Entry 1): H NMR (400 MHz; CDCl3): δH = 2.15
(s, 6H), 7.41–7.55 (m, 5H), 7.70 (s, 1H); 13C NMR (100
MHz, CDCl3): δC = 20.9, 89.7, 126.7, 128.6, 129.8,
135.5, 168.8.
2.4 Preparation of sulfonated ordered mesoporous
carbon (CMK-5-SO3H)
1
(Entry 3): H NMR (400 MHz; CDCl3): δH = 2.14
(s, 6H), 7.33–7.52 (m, 4H), 7.64 (s, 1H); 13C NMR (100
MHz, CDCl3): δC = 20.8, 88.8, 125.0, 126.8, 129.91,
129.97, 134.5, 137.2, 168.7.
In a typical modification, 1.2 g of CMK-5 was added in
a three-necked round bottom flask containing 12.0 g of
4-benzene-diazoniumsulfonate29 in 200 mL of distilled
water and 200 mL of ethanol. Subsequently, the mixture
was cooled down to 5◦C and 200 mL of 50 wt% H3PO2
aqueous solution was added. After stirring for 30 min,
another 200 mL of H3PO2 aqueous solution was added.
The mixture was stirred at 5◦C for another 30 min. The
sulfonic acid functionalized carbon material (denoted
as CMK-5-SO3H) was recovered by filtration, washed
thoroughly with distilled water and finally acetone, and
dried in an oven at 80◦C.
(Entry 14): 1H NMR (400 MHz; CDCl3): δH = 2.17
(s, 6H), 7.70 (s, 1H), 7.73 (d, 2H), 8.28 (d, 2H); 13C
NMR (100 MHz, CDCl3): δC = 20.8, 88.3, 123.9,
127.9, 141.8, 148.6, 168.6.
(Entry 20): 1H NMR (400 MHz; CDCl3): δH = 2.12
(s, 6H), δH = 2.36 (s, 3H), 7.13–7.15 (d, J = 8.4 Hz,
1H), 7.31–7.34 (t, J = 7.6 Hz, 1H), 7.43–7.47 (t, J = 7.6
Hz, 1H), 7.65-7.67 (d, J = 7.8 Hz, 1H), 7.92 (s, 1H).
2.2 General procedure for deprotection of
1,1-diacetates
3. Results and Discussion
To a stirred mixture of aldehyde (1 mmol) and acetic
anhydride (1.2 mmol), CMK-5-SO3H (0.023g, 1 mol%) For our initial protection studies, we carried out the
was added, and the mixture was stirred at room tem- reaction between 1 mmol benzaldehyde, 1 mmol Ac2O
perature for the appropriate time (table 1). The reaction and 1 mol% catalyst at room temperature under solvent
progress was followed by GC. After completion of the free reaction conditions to afford the desired acylal. It
protection reaction, water (3 mL) was added to the reac- was observed that the starting material was consumed
tion mixture to perform the deprotection reaction. After after long reaction time (90 min) as indicated by GC
completion of the cleavage reaction, Et2O (20 mL) was analysis. To optimize the reaction conditions, the reac-
added and the reaction mixture was filtered to separate tion was performed using 1.2 mmol acetic anhydride in

Sulfonic acid based nanoporous carbon
1231
Table 1. CMK-5-SO3H catalyzed solvent-free acylal formation, and acylal deprotection in water.
Preparation
Deprotection
Yield a,c (%)
Entry
1
1,1-Diacetates
Time (min)
10
Yield a,b (%)
Time (min)
10
AcO
OAc
97
98
H
AcO
OAc
2
3
35
20
97
93
15
15
98
H
Cl
AcO
OAc
H
96
Cl
AcO
OAc
4
5
20
40
94
89
15
25
99
92
H
Cl
AcO
OAc
H
Cl
Cl
OAc
AcO
AcO
6
35
98
15
92
H
Br
OAc
H
7
25
20
20
30
98
99
99
99
15
15
20
10
93
95
96
94
Br
AcO
OAc
8
H
Br
AcO
OAc
9
H
F
AcO
OAc
10
H
H3C
AcO
AcO
AcO
OAc
11
12
30
40
95
98
10
10
95
90
H
MeO
MeO
OAc
H
OMe
OAc
13
25
45
50
95
98
90
10
15
20
96
90
89
H
AcO
AcO
OAc
14d
15d
H
NC
OAc
H
O2N

1232
Daryoush Zareyee et al.
Table 1. continued.
Preparation
Deprotection
Yield a,c (%)
Entry
1,1-Diacetates
Time (min)
Yield a,b (%)
Time (min)
OAc
AcO
H
16
17
10
10
98
96
35
15
92
93
H
H
O
S
OAc
OAc
AcO
18
19
15
25
97
94
15
20
94
89
AcO
AcO
H
OAc
20
25
92
20
90
H
OAc
AcO
AcO
OAc
21
22
30
90
40
-
92e
-
H
OAc
AcO
OAc
CH3
24h
No reaction
aGC Yields.bReaction conditions: aldehyde (1 mmol), Ac2O (1.2 mmol), CMK-5-SO3H (2 mol%), room temperature,
solvent-free. cAll deprotection reactions were carried out at room temperature using water as solvent. d1.5 mmol Ac2O was
used. e2 mmol Ac2O was used.
O
the presence of 2 mol% catalyst (scheme 1), and consid-
AcO
AcO
OAc
H
erable improvement was observed in the yield of prod-
uct after 10 minute (table 1, entry 1). It is also important
to mention that when the same reaction was performed
without catalyst and no product was obtained.
H
95%
0%
CMK-5-SO3H (2 mol%)
O
Ac2O (1.2 mmol), solvent-free, r.t. 10 min.
OAc
CH3
CH3
GC monitoring
To generalize the scope and versatility of this pro-
tocol, different substituted aldehydes were used for
the synthesis of substituted acylals. Aldehydes bear-
ing both electron-donating and electron-withdrawing
substituents resulted in the corresponding acylals in
good to excellent yields (table 1, entries 1–15). The
catalyst was also found to be active for the prepa-
ration of acylal from sterically hindered aldehyde, 1-
naphtaldehyde (table 1, entry 16). Notably, furan-2-
carbaldehyde and thiophene-2-carbaldehyde as acid
sensitive substrates gave the related 1,1-diacetates in
excellent yields (table 1, entries 17–18). Also the
present protocol works well for aliphatic aldehydes to
give the corresponding geminal diacetates in excellent
yields (table 1, entries 19–20). When the reaction was
extended towards hydroxybenzaldehydes, the hydroxyl
group was also protected as acetate (table 1, entry 21).
Under the selected conditions, acylation of ketones
was not successful and the starting material was recov-
ered unchanged even when the reaction mixture was
Scheme 2. Chemoselectiveacylal formation from ben-
zaldehyde in the presence of acetophenone.
stirred for 24 h (table 1, entry 22). Encouraged by this
result, in a competitive experiment, equimolecular mix-
ture of benzaldehyde and acetophenone was allowed
to react with 1.2 mmol of acetic anhydride under the
same reaction condition, gave only 1,1-diacetates of
benzaldehyde while the ketone functionality remained
unaffected (scheme 2). Hence, chemoselective acylal
formation from aldehydes in the presence of ketones
could be achieved with this catalytic system.
The feasibility of recyclable properties of the cata-
lyst was examined through a series of sequential syn-
thesis of acylal from benzaldehyde and Ac2O as model
substrates. The catalyst was easily separated by filtra-
tion after completion of each run, with addition of ethyl
acetate. Interestingly, the recycled catalyst was used for

Sulfonic acid based nanoporous carbon
1233
Table 2. Comparison of various methods for acylal synthesis from 4-nitrobenzaldehyde.
Entry
Catalyst (mol%)
Equiv. of Ac2O
Solvent
T (◦C)
Time (min/[h])
Yield (%)
1
2
3
4
5
6
7
CMK-5-SO3H (2)
CAN (10)
1.2
2
2
5–8
3
Solvent free
Solvent free
Solvent free
Solvent free
CH2Cl2
rt
rt
rt
rt
rt
rt
rt
50
[24]
90
[15]
[4]
45
90a
96 [12]b,c,d
96 [27]b,c
94 [28]b,c,d,e
94[29]c,d,f
89 [30]C,f
99[11]C,f
KHSO4 (10)
LiOTf (20)
Cu(OTf)2 (2.5)
AlPW12O40
In(OTf)3
1
CH2Cl2
CH2Cl2
1.2–1.5
15
aThis work. bHigh catalyst loading. cNon-recoverable catalyst. dLong reaction time. eExcess amount of Ac2O. fToxic solvent
(CH2Cl2).
eight consecutive runs without any appreciable change Acknowledgements
in catalytic activity.
The authors acknowledge the Islamic Azad University
In order to show the efficiency of the present method,
of Qaemshahr Research Councils for support of this
we have compared our results obtained from the acety-
work.
lation of 4-nitrobenzaldehyde with acetic anhydride cat-
alyzed by CMK-5-SO3H with other results reported in
the literature. As can be seen in table 2, this method
avoids some disadvantages of other procedure such
as long reaction times12,31,32 toxic solvents,32 excess
reagents,31,32 and non-recoverable catalyst.11,12,29–33
In addition to these results, to evaluate the cleavage
of resulting acylals, we also investigated the deprotec-
tion procedure by addition of water as a green solvent
to the reaction mixture. Indeed, when the formation of
1,1-diacetates were completed, water was added. All
reactions were performed at room temperature in good
to high yields (scheme 1, table 1).
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CMK-5-SO3H (figure S6), TEM image for recovered
catalyst (figure S7) and copy of 1H and 13C spectra for
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