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M. Hajimohammadi et al. / Tetrahedron Letters 51 (2010) 4061–4065
Table 4 (continued)
Entry
Aldehyde
Acid
Time (h)
48
Conversionb (%)
96
TONc
960
O
O
15
16
H
OH
O
O
48
72
100
96
1000
960
OH
O
H
O
17
H
OH
a
1 Â 10À6 mol H2TMP and 1 Â 10À3 mol aldehydes as substrates.
%Selectivity was >99% in all cases.
Turnover number of the catalyst.
b
c
O
H
O
O
O
H
R
O
O
R
H
1O2
OH
+
R
O
2
R
OH
R
H
O
R
O
1
2
4
3
Scheme 3. A possible mechanism for the formation of the carboxylic acids.
Eds.; Elsevier Science: Oxford, 1995; Vol. 5, p 23; a Hudlicky, M.. Oxidations
in Organic Chemistry. In ACS Monograph Series 186; American Chemical
To check the generality of this method, the oxidation of a vari-
ety of aromatic and aliphatic substrates was studied. As shown in
Table 4, this catalytic system was applicable to a wide range of aro-
matic and aliphatic substrates. The aldehydes were converted into
the corresponding carboxylic acids in good isolated conversions in
reasonable times with a high turnover number. Aromatic alde-
hydes possessing electron-withdrawing groups on the phenyl ring
(Table 4, entries 2, 3, and 9–12) were more reactive than those
with electron-donating substituents (Table 4, entries 4–8). Porphy-
rin degradation was accelerated in the presence of less reactive
substrates. This observation shows that there is competition be-
tween the porphyrins and the substrates for 1O2.
We have explored a new porphyrin catalyst for the aerobic oxi-
dation of aldehydes.32–35 Our method requires low catalyst con-
centrations, gave the highest turnover numbers (TON), has low
cost, safety, and avoids using a heavy metal. The catalyst showed
good activity in the aerobic oxidation of aldehydes.36
A possible mechanism for the formation of the products is
shown in Scheme 3. Thus, in the presence of singlet oxygen, there
may be a new photochemical pathway that involves the overall
insertion of molecular oxygen into the C–H bond of aldehyde 1
to form a peracid 2.37 In the final stage of the reaction, aldehyde
1 and peracid 2 generate the adduct 3. Compound 3 then decom-
poses to give the final product 4.38
In conclusion, we have developed a new approach for the aero-
bic oxidation of aldehydes to the corresponding carboxylic acids
using a catalytic amount of a porphyrin in the presence of visible
light and O2 in acetonitrile under very mild conditions. This proce-
dure is very simple and works efficiently at room temperature
affording good to excellent conversions.
Society: Washington, DC, 1990.
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Acknowledgement
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We gratefully acknowledge the financial support from the
Research Council of Shahid Beheshti University.
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