molybdenum,10 nickel,11 osmium,12 palladium,13 ruthenium,14
and vanadium15 have been used as catalysts for alcohol
oxidation with molecular oxygen as the terminal oxidant.
As an alternative, the nitroxy radical TEMPO (2,2,6,6-
tetramethyl-piperidyl-1-oxy) has emerged to be the catalyst
of choice for the mild and selective alcohol oxidation.16,17
TEMPO-catalyzed aerobic alcohol oxidation in the absence
of transition metal has also been recently reported.18 Of the
particular interest are the catalytic systems consisting of
TEMPO and an inexpensive transition-metal compound.
However, TEMPO is a rather expensive chemical agent,
and efficient recycling of TEMPO is a very desirable goal.
Several groups have addressed this problem by involving
heterogeneous variants of TEMPO.19 Recently, we reported
a commercially available acetamido-TEMPO that can be
recycled and reused in an ionic liquid, 1-butyl-3-methylimid-
azolium hexafluorophosphate ([bmim]PF6), for selective
oxidation of benzylic alcohols to aldehydes with hydrogen
peroxide as the terminal oxidant.20 In continuation of our
interest in exploring green oxidation of alcohols in ionic
liquids, we herein report a mild and effective procedure for
the selective aerobic oxidation of primary alcohols to
aldehydes using a three-component catalytic system consist-
ing of acetamido-TEMPO, copper(II) salt, and DMAP with
an ionic liquid as the solvent (Scheme 1). Furthermore, we
demonstrate that the catalytic system can be recycled and
reused for five runs without any significant loss of catalytic
activity.
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Scheme 1
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The initial study was carried out using 4-methoxybenzyl
alcohol as the substrate to optimize the reaction conditions,
and the results are summarized in Table 1. At first, three
Table 1. Optimization of the Reaction Conditions for
Oxidizing 4-Methoxybenzyl Alcohol to
4-Methoxybenzaldehydea,b
conversion
(%)c
yield
(%)d
entry
copper salt
ionic liquid
1
2
3
4
5
6
7
8e
9f
10g
CuCl2
CuCl2
CuCl2
CuCl2
Cu(OAc)2
CuBr2
Cu(ClO4)2
Cu(ClO4)2
Cu(ClO4)2
-
[bmim]PF6
[bmim]BF4
[mmim]OSO3Me
[bmpy]PF6
[bmpy]PF6
[bmpy]PF6
[bmpy]PF6
[bmpy]PF6
[bmpy]PF6
[bmpy]PF6
52
47
44
87
66
91
99
4
43
45
32
81
59
88
91
-
0
0
-
-
a 2 mmol 4-methoxybenzyl alcohol, 5 mol% acetamido-TEMPO, 5 mol%
copper(II) salt, 10 mol% DMAP, 1 atm O2, 0.50 g ionic liquid, room
1
temperature for 5 h. b Selectivity is over 99% determined by H NMR of
the crude product mixture. c Conversion by 1H MNR of the crude product
mixture. d Isolated yield by flash chromatography. e No DMAP was added.
f No acetamido-TEMPO was added. g No copper salt was added.
imidazolium-type ionic liquids, [bmim]PF6, [bmim]BF4, and
[mmim]OSO3Me (1,3-dimethylimidazolium methyl sulfate),
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126, 4112. (b) Liu, R.; Dong, C.; Liang, X.; Wang, X.; Hu, X. J. Org.
Chem. 2005, 70, 729.
(20) Jiang, N.; Ragauskas, A. J. Tetrahedron Lett. 2005, 46, 3323.
3690
Org. Lett., Vol. 7, No. 17, 2005