Bronsted acidic ionic liquids as efficient and recyclable catalysts for protection of carbonyls to acetals and ketals under mild conditions

Article abstract of DOI:10.1080/00397910500214409

A series of acidic ionic liquids have been used as efficient catalysts for the protection of various carbonyl compounds at room temperature. The features of mild conditions, satisfactory isolated yield, simple workup, and the recyclability of the catalyst were present in this process. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500214409

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Brønsted Acidic Ionic Liquids
as Efficient and Recyclable
Catalysts for Protection of
Carbonyls to Acetals and Ketals
Under Mild Conditions
Yuying Du a & Fuli Tian a
a Department of Chemistry, College of Chemistry
and Chemical Engineering, Inner Mongolia
University, Hohhot, China
Published online: 22 Aug 2006.
To cite this article: Yuying Du & Fuli Tian (2005) Brønsted Acidic Ionic Liquids as
Efficient and Recyclable Catalysts for Protection of Carbonyls to Acetals and Ketals
Under Mild Conditions, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 35:20, 2703-2708
To link to this article: http://dx.doi.org/10.1080/00397910500214409
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Synthetic Communicationsw, 35: 2703–2708, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500214409
Brønsted Acidic Ionic Liquids
as Efficient and Recyclable Catalysts
for Protection of Carbonyls to Acetals
and Ketals Under Mild Conditions
Yuying Du and Fuli Tian
Department of Chemistry, College of Chemistry and Chemical
Engineering, Inner Mongolia University, Hohhot, China
Abstract: A series of acidic ionic liquids have been used as efficient catalysts for the
protection of various carbonyl compounds at room temperature. The features of mild
conditions, satisfactory isolated yield, simple workup, and the recyclability of the
catalyst were present in this process.
Keywords: Acetals, ionic liquid, ketals, protection of carbonyls
INTRODUCTION
Protection of carbonyls as acetals (or ketals) is one of the most important
reactions in organic chemistry and many methods have been developed for
these reactions.[1] Although a lot of conventional catalysts including acidic
catalysts,[2,3] Sc(NTf)3,[4] and decaborane[5] have been reported for the protec-
tion of carbonyls as dimethyl acetals, many of these procedures are associated
with one or more disadvantages such as toxic and expensive catalysts, poor
chemoselectivity and yield, limited examples, and nonrecovery of the
catalyst.[3] Therefore, the search for a new catalyst is still actively pursued.
Ionic liquids (ILs) are a new class of solvents entirely composed of ions.
Their use as an environmentally friendly alternative to conventional solvents
Received in the USA April 28, 2005
Address correspondence to Fuli Tian, Department of Chemistry, College of
Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021,
China. Tel.: þ 86-471-4993443; E-mail: ceac@imu.edu.cn
2703

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2704
Y. Du and F. Tian
has gained much attention recently.[6–9] We are specifically interested in the
designability of ILs because it gives us the ability to manipulate IL structure
(with respect to the organic cation, inorganic anion, and the length of the side
chain attached to the organic cation) as well as their properties. When
an alkane sulfonic acid group is covalently attached to the imidazolium
cation, the IL would be a strong Brønsted acidic material.[10] These SO3H-
functionalized ILs have exhibited great potential as replacements for conven-
tional homogenous and heterogeneous acidic catalysts because they are
flexible, nonvolatile, noncorrosive, and immiscible with many organic
solvents.[11–13] However, these ILs are very expensive and have a tedious
preparation procedure. To extend the scope and decrease the cost of ILs,
a new and inexpensive class of acidic ILs was synthesized by simple
protonation of 1-alkylimidazoles with protic acids. From the recent reports,
these ILs could be applied in esterification, Mannich reaction, Biginelli con-
densation, Pechmann reaction, and protection of carbonyl compounds using
alcohols as protection reagents.[14–18] Herein, we describe the protection of
carbonyls as acetals (or ketals) using orthoformate as a protection reagent
over a catalytic amount of acidic ILs (Scheme 1).
EXPERIMENTAL
Typical Reaction Procedure
ILs were synthesized according to the literature.[14–16] A catalyst (0.5 mmol)
was added to a stirred mixture of a carbonyl compounds (10 mmol) and
triethyl orthoformate (15 mmol, 2.22 g) in a reaction flask. The mixture was
stirred at room temperature (258C), and the progress of the reaction was
monitored using a Hewlett-Packard 6890/5793 GC/MS. After completion
of the reaction, the catalyst was recovered from the mixture by filtration.
The filtrate was dried over anhydrous Na2SO4 and then evaporated to give
almost pure product. Further purification was achieved through a column of
silica gel (20 cm in length, 1 cm in diameter) with petroleum ether. Purity
analyses of organic products were conducted with a HP 1790 GC equipped
Scheme 1.