Ultrasound-assisted synthesis of acylals from aldehydes using Mg(CH 3SO3)2-HOAC

Article abstract of DOI:10.1080/00397911.2010.523150

An efficient method for the synthesis of acylals from different aldehydes and acetic anhydride in the presence of magnesium methanesulfonate-acetic acid under ultrasound irradiation at room temperature is achieved. Only 1.5mol% of magnesium methanesulfonate and a small quantity of acetic acid are needed to effect the reaction. Selective conversion of aldehydes is observed in the presence of ketones. The catalyst remains active and exhibits no substantial loss of activity or selectivity in up to three reaction cycles.

Full text of DOI:10.1080/00397911.2010.523150

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Ultrasound-Assisted Synthesis of Acylals
from Aldehydes Using Mg(CH3SO3)2–HOAC
Qing Liu a , Hong-Mei Ai b & Shuai Feng a
a Department of Chemistry and Environmental Science , Taishan
University , Tai'an , China
b Shandong University of Science and Technology , Tai'an , China
Accepted author version posted online: 30 Jun 2011.Published
online: 14 Sep 2011.
To cite this article: Qing Liu , Hong-Mei Ai & Shuai Feng (2012) Ultrasound-Assisted Synthesis of
Acylals from Aldehydes Using Mg(CH3SO3)2–HOAC, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 42:1, 122-127, DOI:
10.1080/00397911.2010.523150
To link to this article: http://dx.doi.org/10.1080/00397911.2010.523150
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Synthetic Communications1, 42: 122–127, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.523150
ULTRASOUND-ASSISTED SYNTHESIS OF ACYLALS
FROM ALDEHYDES USING Mg(CH3SO3)2–HOAC
Qing Liu,1 Hong-Mei Ai,2 and Shuai Feng1
1Department of Chemistry and Environmental Science, Taishan University,
Tai’an, China
2Shandong University of Science and Technology, Tai’an, China
GRAPHICAL ABSTRACT
Abstract An efficient method for the synthesis of acylals from different aldehydes and acetic
anhydride in the presence of magnesium methanesulfonate–acetic acid under ultrasound
irradiation at room temperature is achieved. Only 1.5 mol% of magnesium methanesulfonate
and a small quantity of acetic acid are needed to effect the reaction. Selective conversion of
aldehydes is observed in the presence of ketones. The catalyst remains active and exhibits no
substantial loss of activity or selectivity in up to three reaction cycles.
Keywords Acylal; magnesium methanesulfonate; synthesis; ultrasound
INTRODUCTION
Ultrasound has increasingly been used in organic synthesis in the past three
decades. Ultrasound irradiation has been demonstrated as an alternative energy
source for organic reactions ordinarily accomplished by heating. A large number of
organic reactions can be carried out in greater yields, shorter reaction time, or milder
conditions under ultrasound irradiation. Many homogeneous and heterogeneous
reactions can be conducted smoothly by sonication to provide improved yields and
increased selectivities.[1–3]
Aldehydes react with acetic anhydride in the presence of acid catalysts to form
acylals, which are important precursors for the synthesis of acetoxy dienes and dihalo
vinyl acetates. Because of the remarkable stability of acylals toward a variety of
reaction conditions, they are also gaining importance in organic synthesis as an
alternative to acetals for the protection of aldehydes.[4,5] Generally, the preparation
of acylals has been achieved by the reaction of aldehydes with acetic anhydride under
the catalysis of strong protic acids or Lewis acids. Some of these reagents and
Received February 21, 2010.
Address correspondence to Qing Liu, Department of Chemistry and Environmental Science,
Taishan University, Tai’an 271021, China. E-mail: sdsslq@163.com
122

ULTRASOUND-ASSISTED SYNTHESIS OF ACYLALS
123
Scheme 1. Synthesis of acylals catalyzed by magnesium methanesulfonate–acetic acid under ultrasound
irradiation or through the conventional method.
catalysts that have been employed include H2SO4, H3PO4, CH3SO3H,[6] amino-
sulfonic acid,[7] indium(III) chloride,[8] ruthenium(III) chloride,[9] bismuth triflate,[10]
iodine,[11] and other types of catalysts such as sulfurated silica gel,[12] [Hmim]
HSOÀ4 ,[13] and HY-zeolite.[14] Although some improvements have been observed in
these methods, there are still some limitations, such as the long reaction time and
highly corrosive and expensive catalysts. Hence, a practical and more efficient alter-
native using an inexpensive and environmentally friendly reagent is still of interest.
Superacidic systems that may be formed by mixing appropriate Lewis and
Brønsted acids have been used efficiently in many types of electrophilic substitutions.
We have shown that copper benzenesulfonate–acetic acid is an excellent catalyst for
diacetoxylation.[15] In continuation of our work on superacidic systems, the mixture
of magnesium methanesulfonate (MgMS) with acetic acid as a new catalytic system
was applied in the synthesis of acylals under ultrasound irradiation or through the
conventional method (Scheme 1).
RESULTS AND DISCUSSION
When aldehydes 1 were treated with acetic anhydride in the presence of MgMS
and HOAc under ultrasound irradiation or through a conventional method, the
corresponding acylals 3 were obtained. All of the results are shown in Table 1.
According to the results we have obtained in Table 1, what satisfied us was that
the reaction time was remarkablely reduced and the yields improved under ultrasound
irradiation. Side-product formation was not observed in any reaction. Aromatic
aldehydes with an electron-withdrawing group were converted to their corresponding
acylals under these conditions in good yields after a short time (such as 1b–1e). On the
other hand, 1f proceeded with difficulty. Furthermore, aliphatic aldehydes (such as
1g), acid-sensitive aldehydes (such as 1h), and a,b-unsaturated aldehydes (such as
1i and 1j) reacted well without any decomposition or polymerization under the
reaction conditions. The aromatic aldehydes with electron-donating group afforded
the corresponding acylals in good yields and purity (1k). Hydroxy groups were
also acetylated to afford the corresponding triacetates (1l–n). Additionally, 1o was
deactivated and remained unaffected after 12 h.
To show the merit of the present work, the results for benzaldehyde (1a) can be
compared with the catalyst amount, the reaction time, and the yields recently reported
in other protocols (Table 2). Mg(CH3SO3)2–HOAC is much more efficient than
Cu(CH3SO3)2=HOAC[16] and Co(CH3SO3)2=HOAC[17] as a catalyst for this conver-
sion assisted by ultrasound irradiation. Only 1.5 mol% of Mg(CH3SO3)2 Á 2H2O and
a lesser amount of HOAc (6 mmol) are needed to effect the reaction.
To evaluate the chemoselectivity of this method, we also investigated compete-
tive reactions for acylation of aldehydes in the presence of ketones using MgMS-HOAc

124
Q. LIU, H.-M. AI, AND S. FENG
Table 1. Preparation of acylals in the presence of MgMS-HOAc under ultrasound irradiation or through
conventional methoda
Time (h)
Yield (%)b,c
Mp (ꢀC)
Reported [Ref.]
Entry
Substrate
Benzaldehyde (1a)
A
B
A
B
Found
1
2
2.0 0.5
4.0 0.5
4.0 1.0
7.0 2.0
3.0 1.5
12.0 2.0
7.0 2.0
3.0 0.5
4.0 1.5
2.0 1.0
7.0 1.0
6.0 2.0
12.0 2.0
10.0 2.0
65
70
60
67
63
55
53
75
65
80
52
57
62
65
0
97
95
91
92
98
75
70
93
91
94
95
85
81
87
0
44–46
55–56
79–80
99–100
90–92
120–122
Oil
44–46[7]
56–57[6]
79–80[11]
101–102[7]
91–92[6]
124–125[7]
Oil[14]
52–53[11]
84–86[8]
Oil[14]
64–65[6]
101–102[13]
89–90[9]
90–91[18]
–
2-Chlorobenzaldehyde (1b)
4-Chlorobenzaldehyde (1c)
2, 4-Dichlorobenzaldehyde (1d)
4-Bromocbenzaldehyde (1e)
4-Nitrobenzaldehyde (1f)
Propionaldehyde (1g)
Furfural (1h)
3
4
5
6
7
8
50–52
83–85
Oil
9
Cinnamaldehyde (1i)
Crotonaldehyde (1j)
10
11
12d
13d
14d
15
4-Methoxybenzaldehyde (1k)
Salicylal (1l)
63–64
101–103
91–93
91–92
–
4-Hydroxybenzaldehyde (1m)
Vanillin (1n)
4-Dimethylaminobenzaldehyde (1o) 12.0 3.0
aA: without ultrasound; B: under ultrasound.
bIsolated yield.
cThe products were characterized from the spectral analysis (1H NMR and IR).
d4 equiv. of Ac2O were used.
as a catalyst under ultrasound irradiation at room temperature. Employing this
catalytic system, the highly selective conversion of aldehydes in the presence of ketones
was observed, (Table 3). Smooth conversion of the aldehyde to the acylal was
observed, while the ketone functionality remained unaffected. This observation
indicates that aldehydes are more reactive than ketones, and this may be due to greater
electrophilicity of aldehyde in comparison to ketones.
Catalyst reusability was assessed in the reaction of benzaldehyde with acetic
anhydride. Figure 1 showed the results obtained after three reuse cycles. MgMS
was filtered after the product was extracted by CH2Cl2, dried at 120 ꢀC, and then
reused for three cycles without distinct loss of activity.
We also investigated the catalytic activity of different magnesium salts such
as MgCl2 . 6H2O, MgSO4 . 7H2O, Mg(NO3)2 . 2H2O, and Mg(OAc)2 . 4H2O in the dia-
cetylation of benzaldehyde under the same conditions. Whether they were used alone or
combined with HOAc, they were inert in this reaction in spite of ultrasound irradiation.
Table 2. Comparison of the effect of catalyst for acylal synthesisa
Catalyst
Mol%
HOAc (mmol)
Time (h)
Yield (%)b
Cu(CH3SO3)2 Á 4H2O
Co(CH3SO3)2 Á 4H2O
Mg(CH3SO3)2 Á 2H2Oc
2
24
18
6
1.3
1.3
0.5
94
98
97
2
1.5
aReaction conditions: benzaldehyde, 30 mmol; Ac2O, 60 mmol.
bIsolated yield.
cUnder ultrasound.

ULTRASOUND-ASSISTED SYNTHESIS OF ACYLALS
125
Table 3. Competitive acylal formation of aldehydes using Ac2O in the presence of MgMS-HOAca
Entry
1
Substrate
Yield (%)b
2
3
aReaction conditions: Ac2O, 60 mmol; MgMS, 0.45 mmol; HOAc, 6 mmol for 1.0 h.
bIsolated yield.
Figure 1. Reusability of MgMS for synthesis of acylal.
CONCLUSION
In conclusion, we have shown the mixing of MgMS and HOAc is a new
catalytic system, which leads to a powerful synergistic effect for the chemoselective
conversion of aldehydes to acylals under ultrasound irradiation. Ultrasound
improved the activity of the catalytic system greatly, not only for aromatic aldehydes

126
Q. LIU, H.-M. AI, AND S. FENG
but also aliphatic aldehydes to give the corresponding acylals in good yields at room
temperature. This method is chemoselective for the preparation of acylals from
aldehydes in the presence of ketones. The catalyst can be reused at least three times
with no appreciable loss of activity. The use of inexpensive catalyst, the solvent-free
condition, and excellent yields of the products make this procedure more economical
and environmental.
EXPERIMENTAL
All reagents were purchased and used without further purification. Melting
points were determined by using an XRC-1 micromelting point apparatus and were
uncorrected. GC analysis was carried out on a Shanghai GC-7890 II gas chromato-
graph. Ultrasonication was performed in a KQ-300VDE ultrasound cleaner with a
frequency of 45 kHz and an output power of 300 W.
General Procedure for the Preparation of Acylals Method A
A mixture of aldehyde (30 mmol), Ac2O (60 mmol), MgMS (0.45 mmol), and
HOAc (6 mmol) were put in a flask and stirred at room temperature for an appro-
priate time (monitored by GC). After the reaction, most of the mixture solidified
gradually, and 20 mL CH2Cl2 was added to dissolve the solid product. The organic
layer was washed twice with saturated Na2CO3 solution (20 mL), dried (Na2SO4),
and evaporated to yield the almost pure product. The product was purified further
by crystallization from cyclohexane or by column chromatography on silica gel
(ethyl acetate=hexane, 1:9 as the eluent).
General Procedure for the Preparation of Acylals Method B
A mixture of aldehyde (30 mmol), Ac2O (60 mmol), MgMS (0.45 mmol), and
HOAc (6 mmol) were put in a flask. The flask was located at the maximum energy
area in the cleaner and addition or removal of water was used to control the tempera-
ture of the water bath. After completion of the reaction (monitored by GC), the
subsequent steps were the same as in method A. The products were identified by
1
comparing the melting point, infrared, and H NMR data with those observed for
authentic samples.
ACKNOWLEDGMENTS
We gratefully thank H. Jiang for his kind help and Taishan University for
financial support.
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