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B. Krishnakumar et al. / Catalysis Communications 12 (2011) 375–379
Scheme 3. Proposed mechanism for the condensation of acetophenone with benzaldehyde catalyzed by TiO2–SO42−
.
acetophenones proceeded very cleanly at microwave condition and no
undesirable side-reactions were observed (Table 2, entries 8–11). 2-
Acetylnaphthalene gave moderate yield of 70.3% (Table 2, entry 12).
Acid–base and redox properties are the most important types of
surface chemical properties of metal oxide catalysts. Hence this
reaction follows the mechanism of acid-catalyzed condensation
reactions as shown in Scheme 3. For this reaction generally a base is
used as a catalyst. In the case of base catalyzed Claisen–Schmidt
condensation, the mechanism generally involves the formation of the
anion of acetophenone. But in our case TiO2–SO24− acts as a solid acid
catalyst, protonating the aldehyde. This protonated aldehyde is
attacked by the enolic form of acetophenone. This catalyst also
initiates the dehydration of the condensed product to form chalcone.
A similar mechanism has been proposed for this reaction with the acid
catalyst [21]. The possibility of recycling the catalyst (TiO2–SO24−) was
examined for the reaction of benzaldehyde and acetophenone. When
the reaction was complete, ethyl acetate was added to the solidified
mixture and the insoluble catalyst was separated by filtration. The
separated catalyst could be used five times without any treatment
and, no appreciable loss in its catalytic activity was observed up to
fifth run (Table 3).
Acknowledgements
B. Krishnakumar is thankful to CSIR, New Delhi, for the award of
Senior Research Fellowship. One of the authors M. S is thankful to
CSIR, New Delhi for financial support through research grant no. 21
(0799)/10/EMR-II.
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Table 3
Reusability of catalyst on condensation of acetophenone (1 mmol) and benzaldehyde
(1.1 mmol) under microwave irradiation.
Run
1
2
3
4
5
Yielda (TiO2–SO24–
)
N99
N99
98.8
98.5
97.5
a
Yields with respect to acetophenone catalyzed by TiO2–SO42–
.