Facile Synthesis of 1,1-Diacetates from Aldehydes Using Montmorillonite K-10 Clay under Microwave Irradiation

Article abstract of DOI:10.1039/a801594e

Selective and easy synthesis of acylals in dry media using montmorillonite K-10 clay in an Erlenmeyer flask under microwave activation in remarkably reduced reaction time is described.

Full text of DOI:10.1039/a801594e

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382 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 382±383$
Facile Synthesis of 1,1-Diacetates from Aldehydes
Using Montmorillonite K-10 Clay under Microwave
Irradiation$
Dipankar Karmakar, Dipak Prajapati and Jagir S. Sandhu*
Regional Research Laboratory, Jorhat-785 006, Assam, India
Selective and easy synthesis of acylals in dry media using montmorillonite K-10 clay in an Erlenmeyer flask under micro-
wave activation in remarkably reduced reaction time is described.
Acylals or diacetates are synthetically useful as aldehyde
protecting groups1 as alternatives to acetals and are
important building blocks for the synthesis of dienes
for Diels±Alder cycloaddition reactions.2 Usually the
formation of 1,1-diacetates is catalysed by strong acids3
such as sulfuric, phosphoric or methanesulfonic, or by
In a typical case, p-chlorobenzaldehyde (10 mmol),
acetic anhydride (20 mmol) and montmorillonite K-10 clay
(0.3 g) were mixed without solvent in an Erlenmeyer ¯ask,
placed in a commercial microwave oven and irradiated
for 5 min. The mixture was allowed to reach room tem-
perature and after completion of reaction the corres-
ponding p-chlorobenzylidienic diacetate, mp 80±81 8C (lit.,8
80±81 8C) was obtained in 98% yield. Similarly, other
aldehydes gave the corresponding 1,1-diacetates in good
purity. The excellent yields demonstrate the eciency of
the montmorillonite K-10 clay catalyst (Table 1). It is
remarkable that p-nitrobenzaldehyde and cinnamaldehyde,
gave 85±90% yields, in contrast to yields of 4 and 30%
respectively in an earlier report.7 Also worthy of mention
in our case is that a,b-unsaturated aldehydes (entries 7,8)
gave no isomerization of double bonds under the reaction
conditions. The reverse reaction, i.e. selective deprotection
of the diacetates to give aldehydes, has been reported13 to
occur under microwave irradiation using a neutral alumina
surface as catalyst, while montmorillonite K-10 induces the
formation of 1,1-diacetates under microwave irradiation
in excellent yields. The generality of this acylal-forming
reaction was tested by subjecting polycyclic aromatic
aldehydes to similar conditions. The aldehydes examined
were 2-naphthaldehyde and 9-phenanthraldehyde, which
yielded the corresponding 1,1-diacetates in 78 and 75%
respectively (Table 1).
4
Lewis acids such as ZnCl2 and FeCl3.5 These methods6
have not been entirely satisfactory, owing to such drawbacks
as low yields, long reaction time, corrosiveness and e‚uent
pollution. Olah Mehrotra7 reported the use of Na®on-II
as a catalyst where the yields of products are moderate
and reaction times are generally 3±5 h. Phosphorus(II)
chloride8 and b-zeolite9 have also been employed for this
conversion.10 However, in most cases either a long reaction
time (up to 120 h in the case of 2-furfuraldehyde with
8
PCl3 ) or a low yield of product (4% in the case of p-nitro-
benzaldehyde7) is observed. Therefore, there is merit in
developing
a truly catalytic process using inexpensive
and non-polluting reagents. Herein we disclose the ®rst
example that montmorillonite K-10 clay can be utilized
for the formation of acylals from aldehydes using micro-
wave irradiation11 under solvent-free conditions12 in almost
quantitative yields. The reaction proceeds eciently at ambi-
ent pressure within minutes, in the absence of solvent con-
ditions12 in almost quantitative yields. The reaction
proceeds eciently at ambient pressure within minutes, in
the absence of solvent.
In conclusion, the present new method for the formation
of acylals, without any solvent under microwave irradiation,
o€ers signi®cant improvements over the existing procedures
and constitutes a useful and important addition to the
Scheme 1
Table 1 1,1-Diacetates from aldehydes using montmorillonite K-10 under microwave
activation
Yield
reported
(%)
Reaction
time
reported/h
Yieldb
(%)
Reaction
time/min
Entry
Substratea
1
2
3
4
5
6
7
8
p-Chlorobenzaldehyde
m-Chlorobenzaldehyde
p-Methoxybenzaldehyde
p-Nitrobenzaldehyde
p-Tolualdehyde
Benzaldehyde
Cinnamaldehyde
Crotonaldehyde
2-Furfuraldehyde
98
90
95
85
85
96
90
82
80
90
78
75
888
86.53
928
48
808
988
308
758
708
908
676c
586c
6
7
8
10
8
10
15
12
15
10
15
20
48
0.75
12
24
6
12
24
43
120
2.5
24
9
10
11
12
Isobutyraldehyde
2-Naphthaldehyde
9-Phenanthraldehyde
168
aProducts were identified by the comparison of IR, NMR spectra and melting points with
those of authentic samples. bRefers to the yield of pure isolated products.
*To receive any correspondence.
present methodologies. The main advantages are mild reac-
tion conditions, reduced reaction time, lack of side-products
and excellent yields.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).

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J. CHEM. RESEARCH (S), 1998 383
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Experimental
Mps were taken in open capillary tubes on a Buchi apparatus
and were uncorrected. IR spectra were recorded as KBr discs on a
Perkin-Elmer 237B IR spectrometer. Microanalyses were performed
on a Perkin-Elmer 240C analyser. The 1H NMR spectra were
recorded on a Varian T-60 machine using tetramethylsilane (TMS)
as the internal standard. The chemical shifts are recorded as
d values. The montmorillonite K-10 was of commercial grade from
Aldrich Chemical Company. All other chemicals were puri®ed by
distillation or crystallization prior to use.
General Procedure for the Preparation of 1,1-Diacetates in the
Absence of Solvent under Microwave Irradiation.ÐA mixture of
p-chlorobenzaldehyde (1.41 g, 10 mmol), acetic anhydride (2.04 g,
20 mmol) and montmorillonite K-10 clay (0.3 g) in an Erlenmeyer
¯ask at room temperature was placed in a commercial microwave
oven (operating at 2450 MHz frequency) and irradiated for 5 min.
Upon completion, the reaction mixture was allowed to reach room
temperature (monitored via TLC), water was added and the K-10
clay was ®ltered o€. The catalyst was washed with dichloromethane
(2Â 20 ml) and then the ®ltrate was extracted with dichloromethane
(3Â 20 ml). The combined extract was washed with 10% HCl
solution (20 ml) and brine (2Â20 ml) dried over anhydrous sodium
sulfate and distilled. The p-chlorobenzylidene diacetate thus
obtained was puri®ed by column chromatography using dichloro-
methane-light petroleum (1:1) as the eluent, mp 80±81 8C (lit.,8
80±81 8C) (Table 1). Similarly other aldehydes gave the corres-
ponding 1,1-diacetates (entries, 2 to 12) in 75±96% yields. All the
products were characterized by infrared and 1H NMR spectroscopy
and ®nally by comparison with authentic samples.
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Received, 25th February 1998; Accepted, 12th March 1998
Paper E/8/01594E
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