B. Krishnakumar, M. Swaminathan / Catalysis Communications 16 (2011) 50–55
55
Table 2
time. Another attractive feature of this green process is the reusability
of solid acid catalyst, sulfated titania.
Reusability of catalyst on condensation of benzophenone hydrazone (1 mmol) and
acetophenone (1 mmol) under grinding at room temperature.
Run
1
2
3
4
5
Acknowledgements
Yielda (TiO2–SO42−
)
98.0
98.0
98.0
97.0
97.0
a
One of the authors M.S is thankful to CSIR, New Delhi for financial
support through research grant No. 21(0799)/10/EMR-II. GC-MS facil-
ity provided by University Grants Commission, New Delhi under Spe-
cial Assistance Program is gratefully acknowledged. One of the
authors B.K is thankful to CSIR, New Delhi, for the award of Senior Re-
search Fellowship.
Yields with respect to benzophenone hydrazone (mol%).
However, in the case of benzophenone, no reaction was observed
when it was ground with benzophenone hydrazone at room temper-
ature for 10 min in the presence of TiO2–SO42−. No product was
obtained even by refluxing the mixture in alcohol (Scheme 2). This
may be due to steric effect of two phenyl groups present in the ben-
zophenone. This steric effect is also reflected with benzaldehyde
and acetophenone. Benzaldehyde reacts faster than acetophenone
with benzophenone hydrazone. Hence, synthesis of benzophenone
azine was carried out under microwave irradiation. The use of micro-
waves in organic synthesis has attracted considerable attention in re-
cent years due to short reaction time and improved product yield
[27–29].
When a mixture of benzophenone hydrazone (1 mmol) and ben-
zophenone (1 mmol) without solvent was irradiated in microwave
oven (480 W) for 10 min, no reaction was observed. But addition of
a 0.1 g of TiO2–SO42− to this mixture initiated condensation reaction
producing 98 mol% 1,2-bis(diphenylmethylene)hydrazine in 8 min
under microwave irradiation (Scheme 2) (Table 1, entry 15).
Since the sulfated titania has more acidic sites, acid catalyzed
mechanism is proposed for this reaction (Scheme 3). This mechanism
involves the protonation of hydrazone with acidic TiO2–SO42−. This
protonated hydrazone 1 condenses with acetophenone forming an
intermediate 2 which on dehydration and deprotonation produces
the product azine 3. Solid acid catalyst TiO2–SO42− promotes dehydra-
tion and deprotonation. The possibility of recycling the catalyst was
examined for the reaction of benzophenone hydrazone with aceto-
phenone. When the reaction was complete, ethanol was added to
the solidified mixture and the insoluble catalyst was separated by fil-
tration. The separated catalyst could be used five times without any
treatment and, no appreciable loss in its catalytic activity was ob-
served up to fifth run (97.0 mol%) (Table 2, entry 5).
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