S. Verma et al. / Tetrahedron xxx (2016) 1e4
3
Table 1
corresponding carbonyl derivatives (Table 2, entries 5e8). The
important feature of this reaction was the use of aerial oxygen and
solar energy to accomplish the oxidation of alcohol without the
formation of any by-product, which is an unprecedented finding.
Results of screening experimentsa
Entry
Solvent
Time
Yieldb
1
2
3
4
H2O
Toluene
DMF
12 h
12 h
12 h
8 h
45%
48%
16%
97%
3. Conclusion
CH3CN
a
Reaction conditions: Benzyl alcohol (1 mmol), Pd@TiC (25 mg), Solvent (2 mL),
20 W domestic bulb, Air.
We have demonstrated the synthesis of highly dispersed
palladium nanoparticles grafted on titanium cluster. The developed
Pd@TiC catalyst is found to be a highly effective in the aerial oxi-
dation of alcohols under visible light and its activity is attributed to
the reduced band gap, which makes it active under visible light
thus harnessing solar energy for the chemical reactions. The
Pd@TiC is found to be highly stable and could be recycled several
times without any loss in its activity. The most important feature of
this catalyst is its ability to bind with the atmospheric oxygen
which serves as an oxidant in the reaction cycle.
b
Isolated yields.
reaction outcome in different solvents clearly indicate that aceto-
nitrile is the best solvent for this reaction as it gives quantitative
yield of the desired product; conventionally used polar solvents
(water and DMF) and non-polar solvent (toluene) were in effective
in the visible light mediated aerial oxidation of alcohol.
After uncovering the appropriate conditions, the scope of the
developed Pd@TiC catalyst was studied in the oxidation of a wide
range of alcohols and the results are depicted in Table 2. They
clearly demonstrate the excellent activity and selectivity towards
the formation of corresponding carbonyl compounds (Table 2;
entries 1e8) where electron donating and electron with drawing
substituents apparently have trifling effect on the rate of the re-
action (Table 2, entries 2e4). The secondary alcohols and hetero-
cyclic alcoholic derivatives were also readily converted to
4. Experimental section
4.1. Synthesis of Pd@TiC catalyst
Titanium (IV) isopropoxide (0.518 mL) was added to a solution
of 4-aminobenzoic acid (0.96 g, 7.0 mmol) in 2-propanol (30.0 mL).
The reaction mixture was stirred at room temperature for 30 min;
an orange-colored slurry was obtained. The slurry was heated at
120 ꢀC for 72 h in a pressure reactor when bright yellow crystals of
titanium cluster ensued. The catalyst was isolated by decanting and
centrifugation, washed with 2-propanol and dried under vacuum
for 2 h. The Ti cluster (100 mg) was then suspended in isopropanol
(10 mL) and treated with palladium nitrate (5 mol %) at 80 ꢀC. 2-
propanol reduced the Pd metal into Pd-nanoparticle which were
deposited over the titanium cluster. The palladium nanoparticle
grafted titanium cluster (Pd@TiC) was isolated and characterized
using SEM, XRD, and ICP-AES.
Table 2
Pd@TiC catalyzed oxidation of alcoholsa
Entry
1
Substrate
Product
Yieldb
97%
OH
O
OH
O
2
3
95%
96%
OH
O
4.2. General procedure for the oxidation of alcohols
O
O
A 10 mL side-armed round bottomed flask equipped with
a magnetic stirring bar and a balloon filled with air was charged
with alcohol (1 mmol), catalyst Pd@TiC (25 mg) and acetonitrile
(2 mL). The reaction mixture was exposed to visible light irradiation
using 20 W domestic bulb (Fig. 3) and the progress of reaction was
monitored using TLC. After the completion of the reaction, the
Pd@TiC catalyst was separated using a centrifuge and the product
was isolated by extracting with ethyl acetate, dried over sodium
sulfate, concentrated under reduced pressure and characterized.
OH
O
4
5
96%
94%
O2N
O2N
O
OH
OH
O
6
7
94%
97%
97%
O
O
OH
O
S
O
8
OH
S
a
Reaction conditions: Substrate (1 mmol), Pd@TiC (25 mg), CH3CN (2 mL), 20 W
domestic bulb, Air, 8 h.
b
Isolated yield.
Fig. 3. Pictorial representation of the reaction set-up.
Verma, Sanny
