448
S. Bose et al. / Catalysis Communications 12 (2011) 446–449
Table 2
Oxidation of alcohols by t-BuOOH catalyzed by manganese(III) corroles.
It is observed from Table 2 that substituted benzyl alcohols are
transformed into the corresponding benzaldehyde derivatives
(entries 1–4) with good yields and 100% selectivity. The secondary
alcohols gave the corresponding ketones (entries 5–6). No over
oxidation by-products, carboxylic acids, were detected in the present
work. The presence of an electron-donating or an electron-with-
drawing group in the substrates has no appreciable effect on the
reaction time or yield.
Method Aa
Method Bb
Substrate
Catalyst Time
Conversion Yield
Conversion Yield
(%)
c
d
c
e
(%)
(%)
(%)
Mn-1
Mn-2
Mn-3
12hrs. 56
12hrs. 63
12hrs. 65
54 (97)
62 (98)
64 (98)
32
36
38
76 (96)
87 (97)
90 (97)
OH
Mn-1
Mn-2
Mn-3
12hrs. 60
8hrs. 64
8hrs. 68
58 (97)
63 (98)
66 (98)
39
42
42
92 (97)
100 (98)
100 (98)
The influence of electronegative substituents on the catalytic activity
of the manganese(III) corroles has been examined. The performances of
the manganese(III) corrole catalysts towards the oxidation of different
alcohols have been shown in Fig. 1. Manganese(III) corroles used in the
present oxidizing system steadily maintain the catalytic efficiency in the
order of Mn-3NMn-2NMn-1 for each substrate (Table 2), although the
effect is not so dramatic. Therefore, manganese(III) corroles with greater
electron-withdrawing substituents have been found to be better
catalysts in oxidizing alcohols. However, complete bleaching of all the
three catalysts has been observed at the end of the reactions.
OH
OH
MeO
Mn-1
Mn-2
Mn-3
12hrs. 52
12hrs. 60
8hrs. 64
50 (96)
58 (96)
62 (97)
35
39
40
84 (97)
94 (97)
99 (97)
O2
N
Mn-1
Mn-2
Mn-3
12hrs. 38
12hrs. 44
12hrs. 50
36 (94)
42 (96)
48 (96)
29
33
34
70 (95)
78 (96)
82 (96)
OH
HO
Mn-1
Mn-2
Mn-3
12hrs. 50
12hrs. 54
12hrs. 58
48 (96)
52 (97)
56 (97)
34
36
38
80 (96)
85 (97)
91 (97)
OH
OH
12hrs. 39
12hrs. 45
12hrs. 48
37 (94)
43 (96)
46 (96)
24
26
29
56 (92)
61 (93)
68 (93)
Mn-1
Mn-2
Mn-3
4. Conclusion
To the best of our knowledge, this is the first report on manganese(III)
corrole catalyzed selective and efficient oxidation of alcohols at room
temperature with t-BuOOH as terminal oxidant. In conclusion, the present
work demonstrates that manganese(III) corroles together with environ-
mentally benign t-BuOOH are an efficient, safe and low-cost system for
the oxidation of benzylic alcohols. Electron-withdrawing groups on the
meso-phenyl rings of the corrole ligands enhance the catalytic activity of
the manganese catalysts. The mild and room temperature oxidation of
various electron rich alcohols seems promising for practical purposes.
a
b
c
Method A: the molar ratio of substrate/t-BuOOH/catalyst was 100:800:1.
Method B: the molar ratio of substrate/t-BuOOH/catalyst was 200:80:1.
Percentage conversion of the substrate.
d
Yields are reported with respect to the initial concentration of the substrate. Yields in
parentheses are reported with respect to the consumed substrate.
e
Yields are reported with respect to the concentration of the oxidant. Yields in parentheses
are reported with respect to the consumed substrate.
The previously mentioned results (Table 1) reveal that a non-polar
medium like benzene is most effective in terms of selectivity and yield
of benzaldehyde. The molar ratio of benzyl alcohol, t-BuOOH and the
catalyst was 100:800:1 and the yields were reported with respect to
the substrate concentration. Hereafter, this reaction condition is
described as ‘Method A’. However, it has been found that using benzyl
alcohol 2.5 times more than the oxidant results in almost a
quantitative conversion of the substrate to benzaldehyde based on
the concentration of the oxidant. Thus the molar ratio of benzyl
alcohol, t-BuOOH and the catalyst has been optimized to 200:80:1 for
further reactions (Method B). Oxidation of different alcohols under
both condition A and condition B has been performed and the results
are summarized in Table 2.
Acknowledgements
The financial support (SR/S1/IC-08/2007) from DST, Government
of India, is gratefully acknowledged. We thank UGC (India) for the
award of fellowships to SB and AP.
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