Chemistry Reference
In-Depth Information
2.2 Alcohol Oxidation Catalysts Based
on the Copper-Nitroxyl Radical System
2.2.1 Homogeneous Catalysts
Stable nitroxyl radicals are widely used in oxidation catalysis. 2,2,6,6-Tetra-
methylpiperidine-1-oxyl (TEMPO) and its derivatives and other nitroxyl
radicals (Figure 2.2) have been used in combination with metal-free oxidants
for the oxidation of alcohols to the corresponding carbonyl compounds.
20,21
These methods are useful in synthetic chemistry but require stoichiometric
oxidants and produce salt wastes. To circumvent these problems, copper-
based catalytic systems incorporating organic radicals have been studied for
alcohol oxidation. Furthermore, the organic radicals with copper mimic the
copper-phenoxyl moiety present in the active site of GO.
In 1984, Semmelhack et al. reported the use of copper chloride (10 mol%)
and TEMPO (10 mol%) for the aerobic oxidation of alcohols in dimethyl-
formamide (DMF) under ambient conditions.
22
The method was practical
and ecient for benzylic and allylic primary alcohols. However, secondary
and sterically hindered alcohols were less susceptible to oxidation even with
excess catalyst. In the reaction pathway, oxidation of nitroxyl radical to
nitrosonium cation by copper(
II
) ion was proposed (Figure 2.3). The resulting
nitrosonium ion has been proposed as the active oxidant in the two-electron
d
n
4
r
4
n
g
|
0
OH
O
.
NHAc
NOH
N
N
O
N
O
O
O
A
B
C
D
O
O
COO
t
Bu
N
N
R
1
R
2
N
N
N
OH
N
N
F
N
NH
HN
N
N
N
N
N
O
R
1
R
2
N
N
N
O
O
O
E
G
Figure 2.2
Structures of different organic radicals or radical precursors used in
combination with copper salts for the aerobic oxidation of alcohols.
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