Supported choline hydroxide (ionic liquid) as heterogeneous catalyst for
aldol condensation reactions
Sònia Abelló,a Francisco Medina,*a Xavier Rodríguez,a Yolanda Cesteros,a Pilar Salagre,a Jesús E.
Sueiras,a Didier Tichitb and Bernard Coqb
a Dept. de Química i Enginyeria Química, Universitat Rovira i Virgili, 43007, Tarragona, Spain.
E-mail: fmedina@etse.urv.es; Fax: 34 977559621; Tel: 34 977559787
b Laboratoire des Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM, 8
rue de l’Ecole Normale, 34296 Montpellier, Cedex 5, France
Received (in Cambridge, UK) 2nd February 2004, Accepted 11th March 2004
First published as an Advance Article on the web 1st April 2004
Choline hydroxide was used as a basic catalyst for aldol
condensation reactions to produce new C–C bonds between
several ketones and aldehydes. Choline supported on MgO
exhibits higher TOF values than other well known basic
catalysts in these reactions.
and NaOH. Several condensation reactions of a variety of carbonyl
compounds were carried out by using these catalysts.†
Most of the obtained products are interesting in pharmacological
and flavour and fragrance industries. The results are summarised in
Table 1. The two isomers of citral (neral and geranial) (entry 1) can
be converted (93%) in one hour to the corresponding pseudoionone
PS (2 isomers) with excellent selectivity, when using CH. After 1
hour of reaction, and comparing the amount of OH2 for CH (4.4
mmol OH2), CHMgO (1.1 mmol OH2) and a classical solution of
NaOH (2.2 mmol OH2), better results are obtained for CHMgO.
The self-aldol condensation of acetone (entry 2) was carried out at
1 °C to avoid the dehydrated product; the thermodynamic
equilibrium to diacetonealcohol was achieved only in 0.4 hours for
the CHMgO catalyst. At 2.5 h of reaction time, citral can be coupled
to 2-butanone (entry 3), for the synthesis of methyl-pseudoionones
with a high conversion around 90%, and a selectivity of 94% using
both CH and CHMgO as catalysts. When comparing to HTreh (35
mmol OH2, around 35 times higher than in CHMgO), only a 57%
of conversion is obtained. A TOF value around 70 times higher is
obtained for CHMgO than for HTreh. The formation of two
different carbanions in the 2-butanone molecule results in the
production of four products (iso- and n-methyl PS isomers from
neral and geranial). Higher conversions and selectivities are
obtained in the production of benzylideneacetone (entry 4) from the
aldol condensation between acetone and benzaldehyde with CH,
CHMgO with respect to MgO. Besides, CHMgO shows the highest
activity (a TOF five times higher than for CH), indicating that there
is a synergic effect between choline and MgO. However, at total
conversion (0.10 h of reaction) and due to the production of
dibenzylideneacetone, the selectivity to benzylideneacetone de-
creases to around 77% because of the double attack of acetone to
two benzaldehyde molecules. From the condensation (entry 5) of
benzaldehyde 1 and 2A-hydroxyacetophenone 2, (see also Fig. 3) 2A-
hydroxychalcone 3 is produced and directly isomerised to flava-
none 4. After 2.5 hours of reaction, 99% of the 2A-hydrox-
yacetophenone was condensed to benzaldehyde. Selectivity is
expressed as 2A-hydroxychalcone together with its flavanone
isomer. When the reaction begins, high yields of the two products
are achieved, but for longer times, selectivity is drastically reduced
from 90% at a conversion level of 56% to 58% when the conversion
of 2A-hydroxyacetophenone is complete. This fact can be explained
due to the formation of other by-products, mainly, product 5. This
major side reaction occurs through the condensation of flavanone
and the remaining benzaldehyde.
It is an exciting challenge to find new solid Brønsted-type basic
catalysts able to perform with high activities and selectivities
condensation reactions for the synthesis of pharmaceutical and fine
chemicals. These reactions are indeed known to be catalysed by
Brønsted sites and industrially are carried out in the homogeneous
phase with KOH or NaOH.1 The use of these kinds of bases has
numerous disadvantages such as waste production, corrosion and
no catalyst recovery. In recent years, different solid base catalysts,
such as hydrotalcites have been in the spotlight for a large number
of reactions. They have emerged as heterogeneous catalysts, and
have been used as precursors in the self-aldol condensation of
acetone,2 benzaldehyde–acetone condensation,3 synthesis of flava-
nones4 and the condensation of citral and ketones, into pseudo-
ionones, which are precursors for vitamin A.5 Ionic liquids have a
large nature of physical and chemical properties which make them
useful for different applications, mainly as solvents in homoge-
neous catalysis.6 They are composed of organic cations and
inorganic or organic anions. The resulting salts are non volatile,
thermally and chemically stable and their miscibility can be altered
by varying the alkyl chain length of the cation or the nature of the
anions.7 Most of common ionic liquids are based in neutral
1,3-dialkylimidazolium cations, and [PF6]2 or [BF4]2 anions, and
are used in catalytic applications.8 They are also good solvents for
transition metal complexes, and can be recycled easily. They have
been used as solvents for several reactions involving C–C bond
formation, such as Michael additions,9 Knoevenagel condensa-
tions10 or aldol reactions.11 Recently, a process12 for the synthesis
of aldol condensation products is described with the use of a neutral
ionic liquid medium comprising an imidazolium cation together
with a basic catalyst, KOH (Fig. 1). This shows that a hydroxylated
ionic liquid would act as a basic catalyst. Therefore, in this
communication we report, for the first time, the activity of choline
hydroxide (CH) (Fig. 2). Moreover, a heterogeneous catalyst has
been tentatively performed by impregnation of CH on MgO support
(CHMgO). The activity of this new catalyst was compared to other
well known ones, such as rehydrated hydrotalcites (HTreh), MgO
For the synthesis of jasminaldehyde (entry 6), both catalysts, CH
and CHMgO, lead to excellent conversions in a relatively short time
and selectivities around 84%. The undesired product comes from
the self-condensation of heptanal to form 2-n-pentyl-2-nonenal, but
it could be inhibited to some extent by maintaining a low
concentration of heptanal relative to benzaldehyde in the reaction
mixture, or by a slow addition of heptanal. The results from the
condensation of piperonal (heliotropine) with propionaldehyde
(entry 7) show that a conversion of 79% can be achieved in 5 hours,
and a selectivity of 85% to the main product (methylenediox-
yphenyl methacrolein). A complete piperonal conversion is
Fig. 1 Basic ionic liquid derived from an imidazolium salt.
Fig. 2 Choline hydroxide molecule.
1096
C h e m . C o m m u n . , 2 0 0 4 , 1 0 9 6 – 1 0 9 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
Abello, Sonia
