Facile synthesis of 1,1-diacetates from aldehydes using environmentally benign solid acid catalyst under solvent-free conditions

Article abstract of DOI:10.1081/SCC-120034166

Variety of aromatic and aliphatic aldehydes are converted into their 1,1-diacetates in the presence of environmentally benign Mo/TiO ₂-ZrO₂ solid acid catalyst in good to excellent yields under solvent-free conditions.

Full text of DOI:10.1081/SCC-120034166

SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 10, pp. 1839–1845, 2004
Facile Synthesis of 1,1-Diacetates from
Aldehydes Using Environmentally
Benign Solid Acid Catalyst Under
Solvent-Free Conditions
*
Benjaram M. Reddy, Pavani M. Sreekanth, and
Ataullah Khan
Inorganic and Physical Chemistry Division, Indian Institute of Chemical
Technology, Hyderabad, India
ABSTRACT
Variety of aromatic and aliphatic aldehydes are converted into their 1,1-
diacetates in the presence of environmentally benign Mo/TiO2–ZrO2
solid acid catalyst in good to excellent yields under solvent-free
conditions.
Key Words: 1,1-Diacetates; Carbonyl group; Solid acid Catalyst;
Solvent-free conditions.
*
Correspondence: Benjaram M. Reddy, Inorganic and Physical Chemistry Division,
Indian Institute of Chemical Technology, Hyderabad 500 007, India; E-mail:
bmreddy@iict.res.in/mreddyb@yahoo.com.
1839
DOI: 10.1081/SCC-120034166
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Reddy, Sreekanth, and Khan
INTRODUCTION
The carbonyl group plays a predominant role in organic synthesis due to
its electrophilic nature and one of the most common problems in multi-step
synthetic processes is protecting the carbonyl group from nucleophilic attack
until its electrophilic property could be utilized. Therefore, protection and
deprotection of carbonyl group is an area of interest in synthetic organic chem-
istry.[1] Protecting group is also very useful in separation and purification of
products particularly in multi-step liquid phase methodologies.
Various procedures including acetalization and thioacetalization are
known for protection of carbonyl groups.[1–3] In recent years, gem-diacetates
are gaining more attention in the protection of aldehydes. The main advantage
in diacetalization of aldehydes is that the diacetates formed are quite stable in
neutral, acidic, and basic media than the corresponding O,O-acetals.[1,2] Dia-
cetates are important precursors in the synthesis of dienes for Diels–Alder
reactions.[1,2,4]
In presence of acid catalysts, aldehydes react with acetic anhydride
to form gem-diacetates. Many reagents are reported in the literature for the
synthesis of 1,1-diacetates from aldehydes which include protic acids such
as sulfuric and phosphoric acid, Lewis acids like zinc chloride and phos-
phorous(III) chloride, LiBr, InCl3, montmorillonite clay, zeolites, metal triflates,
and N-bromosuccinimide.[5–8] In view of environmental concern, there is a
global effort to replace the conventional catalysts by new solid acids which
are easily separable from the products, less toxic, and reusable. Although,
zeolites and clays are better alternatives than homogeneous catalysts, they
are highly susceptible to deactivation due to pore blocking and hydration.
On those lines, promoted metal oxides offer several advantages over zeolites
and clay-based catalysts. They are active over a wide range of temperature and
more resistant to thermal excursions. Among promoted metal oxide catalysts,
sulfate ion doped ZrO2 exhibits very strong solid acidity and catalyzes many
reactions even under very mild conditions.[9] However, the sulfated zirconia
undergoes rapid deactivation due to loss of sulfate ions during activation
and catalytic runs. Recent studies reveal that Mo- or W-oxide doped zirconia
also exhibits similar solid acidity and excellent catalytic properies.[9–14] It is
known that mixed oxides often show enhanced acidity and thermal stability
than their constituent single oxides.[15] This is primarily due to charge imbal-
ance generated upon the minor component oxide by imposition of the bond
matrix of the major component oxide.[15–17] Therefore, we undertook inves-
tigations to make solid acid catalysts without sulfate ions by extending the
mixed oxide analogy to titania–zirconia and other combinations.[18] In this
communication, we report an efficient and simple methodology for the
protection of aldehydes under mild and solvent-free conditions catalyzed by

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Synthesis of 1,1-Diacetates Using Solid Acid Catalyst
1841
Mo/TiO2–ZrO2 solid acid catalyst. The reported method is very useful
especially when both ketone and aldehyde groups are present in the same
molecule.
The Mo/TiO2–ZrO2 catalyzed synthesis of 1,1-diacetates from various
aldehydes and acetic anhydride under solvent-free conditions is summarized
in Table 1. As presented in Table 1, aldehydes bearing ring-deactivating
groups (electron withdrawing groups) can also be smoothly converted into
diacetates in good yields (entries 4 and 5). The a-, b-unsaturated aldehydes
are also converted into diacetates without affecting the double bond (entries
8 and 11). In the case of molecules having both ketone and aldehyde groups,
only aldehyde group is selectively converted into diacetate (entry 10). A com-
petitive experiment between benzaldehyde and acetophenone was carried out
by taking a mixture of them to confirm the chemoselectivity of the catalyst.
Only benzaldehyde was found to convert selectively into diacetate. It can
be inferred from this study that the present methodology is highly useful for
chemoselective protection of aldehydes from ketones. No appreciable change
in the activity of the catalyst was noticed even after four to five cycles. The use
of a reusable heterogeneous catalyst in achieving selective protection of car-
bonyl compounds is very significant from the point of view of its commercial
implications.
In conclusion, the present results demonstrate that Mo/TiO2–ZrO2 is
an efficient catalyst for chemoselective diacetalization of aldehydes under
solvent-free conditions. The main advantage of this catalyst is easy operation
and simplicity in the work-up, which involves mere filtration of the catalyst
and reusability.
EXPERIMENTAL
Preparation of Catalyst
The titania–zirconia (1 : 1 mole ratio) mixed oxide was prepared by a
homogeneous coprecipitation method. In a typical experiment, the required
quantities of titanium tetrachloride and zirconium oxychloride were dissolved
separately in deionized water and mixed together (pH ¼ 2). The resulting sol-
ution was neutralized by adding dilute NH4OH drop-wise from a burette up to
pH 8. The obtained precipitate was washed several times until free from chlor-
ide ions and dried at 393 K for 16 hr. To dope molybdenum oxide (10 wt.), the
desired quantity of ammonium heptamolybdate was dissolved in excess water
and to which the powdered hydrous titania–zirconia mixed oxide was added.
The excess water was evaporated on a water-bath with vigorous stirring. The
obtained sample was oven dried at 383 K for 12 hr and calcined at 1023 K for

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Reddy, Sreekanth, and Khan
Table 1. Diacetalization of aldehydes using Mo/TiO2–ZrO2 solid acid catalyst.
Entry
1
Substrate
Product
Yield
94
Ref.
[5–7]
2
3
91
89
[5,7]
[5,6]
4
92
[5–7]
5
6
90
89
[6]
[7]
(continued)

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Synthesis of 1,1-Diacetates Using Solid Acid Catalyst
1843
Table 1. Continued.
Entry
7
Substrate
Product
Yield
81
Ref.
[5–7]
8
9
95
96
[5–7]
[5,7]
10
11
92
79
[7]
[5]
12
13
86
—
[6]
—
No reaction
6 hr in air atmosphere. The prepared catalyst was characterized by means of
BET surface area, x-ray diffraction, x-ray photoelectron spectroscopy, and
Raman techniques.[18]
General Reaction Procedure
A mixture of 1 : 1.5 molar amounts of aldehyde and acetic anhydride
along with the catalyst (0.2 g) were taken into a round bottom flask and
refluxed at 323 K for 1–2 hr. Completion of the reaction was monitored by

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Reddy, Sreekanth, and Khan
TLC. After completion of the reaction the catalyst was filtered off. The wet
catalyst was used for recycling and no appreciable change in the activity
was noted. The product was recovered from the filtrate. An aqueous
NaHCO3 solution was added to remove excess acetic anhydride and CH2Cl2
to separate aqueous and organic layers. The organic layer was decanted into
another flask by using separating funnel and concentrated. NMR and mass
spectrometry techniques were utilized along with their melting/boiling points
to analyze the products and GC for quantification.
ACKNOWLEDGMENT
Support from Department of Science and Technology, New Delhi is
gratefully acknowledged (SERC Scheme: SP/S1/H-20/98). PMS is the reci-
pient of senior research fellowship of CSIR, New Delhi.
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Received in India December 31, 2003

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