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M. B. Ansari et al.
Electron-withdrawing groups such as –Cl and –NO2 on the the aromatic ring
(Table 3, entries 2–4) increases the conversion due to the -I effect (electron-
withdrawing inductive effect). A slight decrease in conversion was observed for the
4-hydroxybenzaldehyde, 4-methoxybenzaldehyde, and 3-phenoxybenzaldehyde
(Table 3, entries 6, 8 and 10) due to the cumulative role of ?M (electron-donating
mesomeric effect) and the -I effect. In the case of salicylaldehyde, the
intramolecular hydrogen bonding between carbonyl ‘‘O’’ and hydroxyl ‘‘H’’
restricts the reaction. The electron-donating group such as 4-isopropyl-(Table 3,
entry 9) on the aromatic ring decreases the yields due to the existence of ?I
(electron-donating inductive effect) effect. These reaction results are in agreement
with those reported earlier [16–19]. The catalyst recyclability was tested for three
cycles (Fig. 1) in Knoevenagel condensation between benzaldehyde and ethyl-
cyanoacetate, and no significant loss in catalytic activity was observed.
Plausible mechanism
As mentioned earlier, the TMP is nitrogen-enriched which plays a role as a Lewis
base site for Knoevenagel condensation. Figure 2 depicts a plausible mechanism for
the TMP-catalyzed Knoevenagel condensation of benzaldehyde with ethylcyanoac-
etate. The amine nitrogen from the TMP network abstracts the acidic proton from
the active methylene group of ethylcyanoacetate. The generated anion makes a
nuleophilic attack on the carbonyl-carbon atom of the benzaldehyde to form
oxyanion. The oxyanion abstracts H? from the protonated amine group of TMP
network and the catalyst is regenerated. The formed hydroxyl molecule eliminates
the water molecule, leading to the formation of Knoevenagel product [20].
Conclusion
In summary, we have developed a new metal-free heterogeneous catalyst for
microwave-mediated Knoevenagel condensation operative under water as a green
solvent media. The TMP catalyst exhibited a wide substrate scope for Knoevenagel
condensation reaction with high to optimum conversions and selectivities. The
catalyst can be easily recovered and reused without loss of its catalytic activity and
selectivity.
Acknowledgments This work was supported by the National Research Foundation of Korea[NRF]
grant funded by the Korea government [MSIP] [No.20090083525] and INHA University, Korea.
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