conditions. The results of these experiments are presented in
Table 1. Primary benzyl alcohols afforded the corresponding
aldehydes selectively without any evidence for the formation of
acids due to further oxidation. However, among the various
secondary alcohols studied, those having aromatic substituents
were found to be more reactive than alicyclic alcohols (Table 1,
entry 8–9). The presence of 2-methylpropanal was found to be vital
for this transformation, in its absence the oxidation of alcohols to
carbonyl compounds did not occur. The reactions were found
to be slow at room temperature, however 50 ◦C was found to
be optimum temperature for this transformation. Likewise, in a
controlled blank experiment, i.e. the stirring of a reaction mixture
containing benzhydrol (1 mmol), 2-methylpropanal (1.5 mmol)
and acetonitrile (3 ml) at 50 ◦C, no reaction occurred in the absence
of catalyst even after prolonged reaction time (3.5 h). Similarly,
the oxidation of benzhydrol did not occur when the reaction was
performed with 2-methylpropanal alone in the absence of catalyst
and molecular oxygen under otherwise similar reaction condition.
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Conclusions
In summary, we have demonstrated the successful application of
the copper catalyzed [3 + 2] azide–alkyne cycloaddition (CuAAC)
“click reaction” for grafting cobalt(II) Schiff base complexes
to mesoporous silica supports. Either step-wise synthesis of
silica immobilized Schiff bases followed by their subsequent
complexation with cobalt ions or direct immobilization of already
prepared homogeneous complexes onto the support via “click
reaction” could be successfully achieved. The key features of
the “click reaction” such as its simplicity in use, versatility and
high efficiency with respect to the catalyst loading without using
excess reagents, makes it an attractive and superior tool for the
preparation of immobilized catalysts. The so obtained silica-
immobilized cobalt(II) Schiff base complexes could readily be
recycled for several runs with consistent activity and without
observable metal or ligand leaching to the aerobic oxidation
of alcohols to give the corresponding aldehydes or ketones in
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Acknowledgements
We kindly acknowledge the Director, IIP for his kind permission to
publish these results. We thank the analytical division of Institute
for providing analysis. BS thanks CSIR for his fellowship. DST,
New Delhi is acknowledged for research funding.
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