An efficient and chemoselective procedure for acylal synthesis

Article abstract of DOI:10.3390/molecules15096493

A novel heterogeneous efficient procedure has been developed for the chemoselective synthesis of acylals (1,1-diacetates) under solvent-free conditions. A novel catalyst prepared by the sulfuric acid catalyzed copolymerization of p-toluenesulfonic acid and paraformaldehyde displays extremely high activities for the title reactions, affording average yields over 90% within several minutes. A comparative study showed that the novel catalyst has much higher activity than other catalysts used for this purpose. Besides, the novel catalyst displays chemoselectivity for the protection of aldehydes in the presence of ketones. In addition the high acidity (4.0 mmol/g), thermal stability (200 °C) and easy reusability make the novel catalyst one of the best choices for the process.

Full text of DOI:10.3390/molecules15096493

Molecules 2010, 15, 6493-6501; doi:10.3390/molecules15096493
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
An Efficient and Chemoselective Procedure for Acylal Synthesis
Da-He Fan 1, Hui Wang 1, Xing-Xing Mao 2 and Yong-Miao Shen 2,*
1
School of Chemical and Biological Engineering, Yancheng Institute of Technology, Yancheng
224003, China
2
Department of Chemistry, Shaoxing University, Shaoxing 312000, China
* Author to whom correspondence should be addressed; E-Mail:chemreagent@yahoo.cn;
Tel.: +86-575-88345682; Fax: +86-575-88345682.
Received: 23 June 2010; in revised form: 6 September 2010 / Accepted: 15 September 2010 /
Published: 16 September 2010
Abstract: A novel heterogeneous efficient procedure has been developed for the
chemoselective synthesis of acylals (1,1-diacetates) under solvent-free conditions. A novel
catalyst prepared by the sulfuric acid catalyzed copolymerization of p-toluenesulfonic acid
and paraformaldehyde displays extremely high activities for the title reactions, affording
average yields over 90% within several minutes. A comparative study showed that the
novel catalyst has much higher activity than other catalysts used for this purpose. Besides,
the novel catalyst displays chemoselectivity for the protection of aldehydes in the presence
of ketones. In addition the high acidity (4.0 mmol/g), thermal stability (200 ºC) and easy
reusability make the novel catalyst one of the best choices for the process.
Keywords: efficient; novel catalyst; heterogeneous; green process
1. Introduction
Acylals (1,1-diacetates) are widely utilized as one of the most useful protecting groups for carbonyl
compounds because of their stability under various reaction conditions and their easy conversion back
to the parent compounds [1]. Beyond their protecting role, acylals are also important precursors for the
synthesis of dienes, chiral allyclic esters and as cross linking reagents for the cellulose in cotton [2-5],
hence methods for their synthesis have received considerable attention. Some of the reported catalysts
for the preparation of 1,1-diacetates from aldehydes and acetic anhydride include Sc(OTf)3 [6], PCl3
[7], TMSCl (trimethylchlorosilane)-NaI [8], iodine [9], sulfuric acid [10], PVA-FeCl3 [11], NBS

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(N-bromosuccinimide) [12], triflic acid [13], Cu(OTf)3 [14], CAN (ceric ammonium nitrate) [15],
ZrCl4 [16] and Bi(OTf)3 [17]. Although some of these methods offer convenient protocols with good to
high yields, the majority suffer from at least one of the following disadvantages: reactions under
oxidizing conditions, use of halogenated solvents, high temperatures, long reaction times, moisture
sensitivity of the catalyst, high cost and high toxicity. The well-documented advantages of solid
catalysts and solvent-free reactions are safety, economy, high yields, easy workup procedures and
short reaction times. Aiming for these advantages, many heterogenous catalysts such as zinc-
montmorillonite [18], SnCl4/SiO2 [19], zirconium sulfate tetrahydrate-silica gel [20], SO3H-
functionalized ionic liquids [21], silica-bonded S-sulfonic acid [22], silica-supported perchloric acid
(HClO4-SiO2) [23], copper p-toluenesulfonate/HOAc [24], H3PW12O40 supported MCM-41 [25] and
solid superacids [26] have been used for the synthesis of acylals. Here we report the development of a
solvent-free protocol for the conversion of aldehydes into acylals using a novel heterogeneous acid
catalyst that has been synthesized through the sulfuric acid catalyzed copolymerization of p-
toluenesulfonic acid (PTSA) and paraformaldehyde (Scheme 1). The results showed that the thus
prepared catalyst displays very high activity for the target reactions, affording average yields over 90%
within several minutes. A comparative study showed that the novel catalyst has much higher activity
than other catalysts ued for this reaction, with the additional advantage of reusability. The novel
catalyst also displays chemoselectivity for the protection of aldehydes in the presence of ketones.
Scheme 1. The synthetic route to the novel catalyst.
2. Results and Discussion
2.1. Characterization of the novel catalyst
The acidity of the novel catalyst was determined through a neutralization titration which gave the
results of 4.0 mmol/g, a little lower than that of pure PTSA (5.8 mmol/g). The catalyst showed much
higher acidity than other common heterogeneous acid catalysts such as Nafion and Amberlyst-15
(0.8 mmol/g). It can be seen from Scheme 1 that the acidity of the catalyst originated from the sulfonic
group of the PTSA molecule, so the acidic sites in the novel catalyst were strong acid sites, which was
confirmed by thermodesorption of chemisorbed ammonia (NH3-TPD). The result showed that the
catalyst had high acid strength in which ammonia desorbed in the range from 673 to 873 K a little
higher than that of the pure PTSA. NH3-TPD gave an acidity of 5.0 mmol/g, which indicated that
many acid sites were hiding inside the novel catalyst. The BET surface area using nitrogen adsorption
isotherms at the temperature of liquid nitrogen gave the result of 23 m2/g which also support the results
above. It can also be seen from Scheme 1 that almost every benzyl ring was attached with the sulfonic
groups. As a result, the catalyst should in theory display an acidity as high as 5.4 mmol/g. Here some
sulfonic groups might fall off from the PTSA molecule and not only the linear chains but also branches

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or cross-linked structures could be formed during the process. That was in accordance with the results
of the elemental analysis, which gave the result: C 54.3%; H 5.3%; S 16.2% compared to the
theoretical results: C 52.1%; H 4.3%; S 17.4%, indicating that almost all the S element existed in the
catalyst in the form of sulfonic acid groups, which also confirmed the strong acid sites equal to the
results of NH3-TPD. The carbon and hydrogen content was a little more than the theoretical value,
which indicated the presence of more CH2 groups in the catalyst, possibly in the form of some
branches or cross-linked structures. The thermal stability of the catalyst is an important property. The
TG curve for the catalyst is shown in Figure 1. The decrease in weight began at about 200 ºC,
indicating the decomposition of the catalyst. The covalent chemical bonds connection made the
catalyst highly thermally stable. This suggests that the catalyst could be used for high
temperature applications.
Figure 1. The TG curve of the novel catalyst.
16
14
12
10
Weight
/mg
100
200
300
400
500
600
700
800
T/ºC
The IR spectrum of the catalyst (Figure 2) showed the expected sulfonic group absorbtions at 1,040
and 1,195 cm-1 which confirmed the existence of the sulfonic acid groups.
Figure 2. The IR spectrum of the novel catalyst.
1040
1195
3500
3000
2500
2000
1500
1000
500
W avenumbers/cm-1

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2.2. Catalytic procedure for the synthesis of acylals
A wide variety of carbonyl compounds were successfully transformed into the corresponding
acylals using the novel catalyst under solvent-free conditions (Scheme 2). It can be seen from Table 1
that the catalyst was very efficient in these reactions, giving an average yield above 90% within several
minutes. It can also be deduced that the reactivity of aromatic aldehydes towards the formation of
acylals depends on the substituents present in the aromatic nucleus. Benzaldehyde or aromatic
aldehydes containing electron withdrawing substituents attached to the aromatic ring reacted very fast
to furnish acylals due to the better electrophilicity at the carbonyl center compared to the aromatic
aldehydes with electron donating substituents, which may reduce the electophilicity at the carbonyl
center by virtue of conjugation (entries 1-13). The yield of diethylaminobenzaldehyde is only 45% and
almost no side reactions occurred under the mild conditions (entry 13). In addition, the formation of
acylals from aliphatic aldehydes was also fast, but much slower than that of the aromatic aldehydes
(entries 16-18). We have found that both carbonyl and phenolic –OH groups were acylated. The
formation of triacetate was observed due to the acylation of phenolic –OH group (entries 9, 10).
Scheme 2. The reaction scheme.
Table 1. The results with various carbonyl compounds.
Entry
Substrate
Product
Reaction time/min
Yield/% a, b
1
3
98.9
2
3
4
6
97.0
94.0
Cl
CHO
4
5
5
2
95.0
99.4
6
7
3
4
99.2
99.0
8
15
98.0

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Table 1. Cont.
9
6
94.0
10
11
5
6
95.0
94.0
12
13
5
95.0
45.0
35
14
15
10
12
90.0
91.0
16
16
92.0
CHO
17
18
23
90.0
90.0
18
a) The reactions were carried out at R.T. (25 ºC) under solvent free condition: carbonyl compounds
0.02 mol, acetic anhydride 0.04 mol, catalyst 25 mg; b) The yields are isolated yields.
2.3. Comparative study on the catalytic activities of the novel catalyst and other catalysts
A comparative study on the catalytic activities of the novel catalyst with some of the reported
catalyst was carried out using benzaldehyde as a model substrate (Table 2). From this study it can be
concluded that the novel catalyst has much higher activity than others, furthermore it has the added
advantage of reusability (entries 1-10). The novel catalyst was also more efficient than the traditional
2-
solid acids such as HY zeolites and SO4 /TiO2 (entries 11 and 12) and the highest yield was achieved
with low catalyst usage and short reaction times. This clearly shows that the novel catalyst should be
considered as one of the best choices as an economical, convenient, user-friendly catalyst and for
scale-up purposes.
Table 2. Comparison of different catalysts.
Entry Catalyst
Catalyst loading/mol% Reaction time/min Yield/% a, b
1
2
3
4
H2SO4
PTSA
FeCl3
1
1
1
1
5
5
5
14
88.5
87.4
84.3
88.4
NH2SO3H
5
6
Bi(OTf)3
ZrCl4
1
1
15
8
90.3
85.4

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Table 2. Cont.
7
Iodine
NBS
1
25
80.5
83.2
85.6
87.8
82.3
90.3
98.9
8
1
13
10
15
30
15
3
9
Cu(OTf)3
CAN
1
10
11
12
13
1
HY
70 mg
80 mg
25 mg
2-
SO4 /TiO2
Novel catalyst
a) The reactions were carried out at R.T. (25 ºC) under solvent free condition: benzaldehyde
0.02 mol, acetic anhydride 0.04 mol; b) The yields are isolated yields.
2.4. The reuse of the catalyst
One interesting property of the novel catalyst is its heterogeneous catalytic process. Thus, recovery
of the catalyst is very convenient. After the reactions, the catalyst was recovered by filtration. The
activities of the recovered catalyst were carefully investigated through the reaction of benzaldehyde
and acetic anhydride (Figure 3). The yields and the sample composition remained unchanged even
after the catalyst had been recycled for a tenth time.
Figure 3. Catalyst reuse.
100
80
60
40
20
0
1 2 3 4 5 6 7 8 9 10
Recycled times
2.5. The chemoselectivity of the catalyst
It is noteworthy that ketones did not produce the corresponding acylals under the same reaction
conditions (Scheme 3). This result indicated that the present protocol could be applicable to the
chemoselective protection of aldehydes in the presence of ketones.

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Scheme 3. The chemoselectivity of the catalyst.
3. Experimental
3.1. General
All organic reagents were commercial products with the highest purity available (>98%) and used
for the reactions without further purification.
3.2. Synthesis of the catalyst
The catalyst was synthesized through the polymerization of p-toluenesulfonic acid (PTSA) and
paraformaldehyde catalyzed by sulfuric acid (Scheme 1). In the typical procedure a mixture of p-
toluenesulfonic acid (10 g), paraformaldehyde (2 g) and sulfuric acid (0.2 g) was heated in a three
necked round bottomed flask equipped with a magnetic stirrer, a thermometer and a funnel. The
reaction mixture was kept in the range of 100-110 ºC for 48 h to form a black solid. Sulfuric acid is
absolutely necessary here and the polymerization does not take place efficiently without it. The solid
2-
was filtered and washed with hot water (>80 ºC) until no SO4 was detectable in the filtrate. The
catalyst was obtained after drying at 120 ºC in an oven overnight. The titration was carried out as
follows: 40 mg novel catalyst and 2 N 4 mL aqueous NaCl were stirred at room temperature for 24 h.
The solids were filtered off and washed with water four times and 2 mL each time. The combined
filtrate was titrated with 0.01 N NaOH using phenol red as indicator [27].
3.3. Preparation of acylals
Typical procedure for the synthesis of acylals (Scheme 2): The novel catalyst (25 mg) was added to
a stirred solution of carbonyl compound (0.02 mol) and acetic anhydride (0.04 mol), then the reaction
mixture was stirred at room temperature for the time as shown in table 1 and 2. The reaction was
monitored by TLC. After completion, the mixture was diluted with ethyl acetate and filtered. The
organic layer was washed with 10% NaHCO3 solution and saturated solution of NaHSO3 and then
dried over anhydrous Na2SO4. The solvent was evaporated under reduced pressure and the product
purified by column chromatography on silica gel to give the corresponding products. All the
compounds were characterized by GC–MS (Agilent 6890N GC/5973N MS, DM-5MS) and
1H-NMR(BRUKER, AVANCE 500 MHZ).

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4. Conclusions
In conclusion, a novel heterogeneous strong acid catalyst has been found to be highly efficient for
the formation of 1,1-diacetates from aliphatic and aromatic aldehydes. Operational simplicity without
need of any solvent, exceptionally fast reactions, low cost of the catalyst used, high yields, excellent
chemoselectivity, and applicability to large-scale reactions are the key features of this methodology.
Acknowledgements
We greatly appreciate the support of Zhejiang Provincial Natural Science Foundation (Y4080395)
and Yancheng Scientific Project (Grant No. YK2009032)..
References and Notes
1. Greene, T.W.; Wuts, P.G.M. Protective Groups in Organic Synthesis, 3rd ed.; John Wiley and
Sons: New York, NY, USA, 1999.
2. Frick, J.G.; Harper, R.J. Acetals as Crosslinking Reagents for Cotton. J. Appl. Polym. Sci. 1984,
1433-1447.
3. Shelke, K.; Sapkal, S.; Kategaonkar, A.; Shingate, B.; Shingare, M.S. An efficient and green
procedure for the preparation of acylals from aldehydes catalyzed by alum [KAl(SO4)212H2O].
South Afr. J. Chem. 2009, 62, 109-112.
4. VanHeerden, F.R.; Huyser, J.J.; Bardley, D.; Williams, G.; Holzapfel, C.W. Palladium-catalysed
substitution reactions of geminal allylic diacetates. Tetrahedron Lett. 1998, 39, 5281-5283.
5. Andberg, M.; Sydness, L.K. The chemistry of acylals. Part II. Formation of nitriles by treatment
of acylals with trimethylsilyl azide in the presence of a Lewis acid. Tetrahedron Lett. 1998, 39,
6361-6364.
6. Yadav, J.S.; Subba Reddy, V.B.; Shrihari, P. Scandium triflate catalyzed allylation of acetals and
gem-diacetates: a facile synthesis of homoallyl ethers and acetates. Synlett 2001, 673-675.
7. Michie, J.K.; Miller, J.A. Phosphorus trichloride as a catalyst in the preparation of 1,1-diacetates
from aldehydes. Synthesis 1981, 824.
8. Daka, N.; Borah, R.; Kalita, D.J.; Sarma, J.C. Synthesis of 1,1-Diacetates from Aldehydes using
Trimethylchlorosilane and Sodium Iodide as Catalyst. J. Chem. Res. 1998, (S), 94-95.
9. Deka, N.; Kalita, D.J.; Borah, R.; Sarma, J.C. Iodine as Acetylation Catalyst in the Preparation of
1,1-Diacetates from Aldehydes. J. Org. Chem. 1997, 62, 1563-1564.
10. Kochhar, K.S.; Bal, B.S.; Deshpande, R.P.; Rajadhyaksha, S.N.; Pinnick, H.W. Protecting groups
in organic synthesis. Part 8. Conversion of aldehydes into geminal diacetates. J. Org. Chem. 1983,
48, 1765-1567.
11. Li, Y.-Q. A rapid and convenient synthesis of 1,1-diacetates from aldehydes and acetic anhydride
catalyzed by PVC-FeCl3 catalyst. Synth. Commun. 2000, 30, 3913-3917.
12. Karimi, B.; Seradj, H.; Ebrahimian, R.G. Mild and efficient conversion of aldehydes to 1,1-
diacetates catalyzed with N-bromosuccinimide (NBS). Synlett 2000, 623-624.
13. Firouzabadi, H.; Iranpoor, N.; Amani, K. Heteropoly acids as heterogeneous catalysts for
thioacetalization and transthioacetalization reactions. Synthesis 2002, 59-62.

Molecules 2010, 15
6501
14. Lawesson, S.O.; Berglund, C.; Gronwall, S. Peroxy compounds. VIII. Preparation of acylals and
related compounds by Cu salt catalyzed reaction of tert-butyl perbenzoate with simple ethers and
sulfides. Acta Chem. Scand. 1961, 15, 249-259.
15. Roy, S.C.; Banerjee, B. A mild and efficient method for the chemoselective synthesis of acylals
from aldehydes and their deprotection catalyzed by ceric ammonium nitrate. Synlett 2002,
1677-1678.
16. Smitha, G.; Reddy, C.S. A facile and efficient ZrCl4 catalyzed conversion of aldehydes to
geminal-diacetates and dipivalates and their cleavage. Tetrahedron 2003, 59, 9571-9576.
17. Aggen, D.H.; Arnold, J.N.; Hayes, P.D.; Smoter, N.J.; Mohan, R.S. Bismuth compounds in
organic synthesis. Bismuth nitrate catalyzed chemoselective synthesis of acylals from aromatic
aldehydes. Tetrahedron 2004, 60, 3675-3679.
18. Nagy, N.M.; Jakab, M.A.; Konya, J.; Antus, S. Convenient preparation of 1,1-diacetates from
aromatic aldehydes catalyzed by zinc-montmorillonite. Appl. Clay Sci. 2002, 21, 213-216.
19. Li, Y.Q.; Cheng, L.H. A rapid and convenient synthesis of acylals from aldehydes and acetic
anhydrides catalyzed by SnCl4/SiO2. Chin. Chem. Lett. 2001, 12, 565-568.
20. Jin, T.; Feng, G.; Yang, M.; Li, T. An efficient and practical procedure for synthesis of 1,1-
diacetates from aldehydes catalyzed by zirconium sulfate tetrahydrate-silica gel. Synth. Commun.
2004, 34, 1645-1651.
21. Wang, Y.Y.; Xu, Y.N.; Wang, Z.Z.; Dai, L.Y. A new approach to the synthesis of pentaerythritol
diacetals (diketals) catalyzed by SO3H-functionalized ionic liquids. Chin. Chem. Lett. 2010, 21,
524-528.
22. Niknam, K.; Saberi, D.; Sefat, M.N. Silica-bonded S-sulfonic acid as a recyclable catalyst for
chemoselective synthesis of 1,1-diacetates. Tetrahedron Lett. 2009, 50, 4058-4062.
23. Khan, A.T.; Choudhury, L.H.; Ghosh, S. Silica-supported perchloric acid (HClO4-SiO2): A highly
efficient and reusable catalyst for geminal diacylation of aldehydes under solvent-free conditions.
J. Mol. Catal. A: Chem. 2006, 255, 230-235.
24. Wang, M.; Song, Z.; Gong, H.; Jiang, H. Synthesis of 1,1-diacetates using a new combined
catalytic system, copper p-toluenesulfonate/HOAc. Synth. Commun. 2008, 38, 961-966.
25. Rabindran, J.B.; Pandurangan, A. Synthesis of geminal diacetates (acylals) using heterogeneous
H3PW12O40 supported MCM-41 molecular sieves. Catal. Commun. 2008, 9, 577-583.
26. Lermontov, S.A.; Yurkova, L.L. Solid superacids as catalysts for the synthesis of acylals from
aldehydes and acetic anhydride. Russ. Chem. Bull. 2008, 57, 2561-2563.
27. Margelefsky, E.L.; Anissa, B.; Zeidan, R.K.; Dufaud, V.; Davis, M.E. Nanoscale Organization of
Thiol and Arylsulfonic Acid on Silica Leads to a Highly Active and Selective Bifunctional,
Heterogeneous Catalyst. J. Am. Chem. Soc. 2008, 130, 13442–13449.
Sample Availability: Samples of the compounds are available from the authors.
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