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[37] A solution of copper complex (0.01 mmol) in 20 mL acetonitrile was added to the
solution of substrate (1 mmol) and H5IO6 (1 mmol). The mixture was stirred at
70 °C. At the requisite times aliquots of the reaction mixture were removed and
the alcohol and aldehyde/ketone extracted with ether. The ether solution was
then analyzed by GC.
[38] The reaction product analysis was carried out using gas chromatography (GC)
(Shimadzu 2014, Japan); the instrument has a 5% diphenyl and 95% dimethyl
siloxane Restek capillary column (30 m length and 0.25 mm diameter) and a
flame ionization detector (FID). Nitrogen gas was used as the carrier gas. The
retention times for different compounds were determined by injecting commer-
cially available compounds under identical gas chromatography conditions. The
oxidation products are commercially available, and were identified by GC co-
injection with authentic samples.
A mechanism has been proposed for the above described results
(Fig. 3). The mechanistic studies suggest that an initial hydrogen-
transfer reaction within the copper-alkoxide, generates the carbonyl-
bound copper species [39]. Upon reaction with oxidant, this copper
complex then produces the carbonyl compound. Homolytic cleavage
followed by hydrogen atom abstraction from the complexed Cu(I)
species. With concomitant loss of a water molecule, regenerates the
loaded catalyst and initiates a second catalytic cycle.
In conclusion a five coordinated complexes of Cu(II) were
investigated as catalysts for the oxidation of primary and secondary
alcohols in the presence of periodic acid as co-oxidant. The catalyst
can be easily prepared and is therefore extremely cost effective. High
conversions and selectivity were observed for all alcohol substrates.
Nevertheless, the conversions and selectivities are comparable to
those reported with the most active homogeneous metal Schiff-base
complexes. The design and synthesis of well-characterized Schiff base
complexes containing triphenylphosphine may provide a new class of
catalysts for oxidation reactions.
Acknowledgements
Authors are thankful to Technical Education Quality Improvement
Programme (TEQIP), and NITK, for financial support. Authors also
thank the SAIF, Indian Institute of Technology, Bombay, Mumbai, for
ESR analysis.
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