Copper Catalysts for Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 46

Copper( I ) is oxidized to copper( II ) under the experimental conditions. The
catalytic system is very useful for the oxidation of allylic and benzylic alco-
hols exhibiting up to 250 turnovers after 5 h. A mechanism similar to that
proposed by Sheldon and co-workers has been implicated for the oxidation
reaction. 38 In 2007, Repo and co-workers succeeded in the selective and ef-
ficient catalytic oxidation of primary and secondary benzylic alcohols using a
Cu-TEMPO-diimine system (diimine ¼ phen, bpy) in an alkaline aqueous
solvent (pH 12.6-13.5) under 10 bar pressure at 80 1C. The use of an aqueous
solvent and dioxygen as the oxidant makes it an ecient and environ-
mentally viable oxidation process. 39 Gartshore and Lupton reported a new
copper-based catalytic system using 2,2,4,4-tetramethyloxazolidine-N-oxyl
(TOXYL) derivatives (G in Figure 2.2). 40 Bifunctional TOXYL derivatives with
pyridyl and 2,2 0 -bipyridyl substituents on the TOXYL scaffold have been
utilized as co-catalysts in catalytic oxidation using CuBr 2 and base in
acetonitrile-water solvent mixture. The catalyst was found to be ecient for
a range of benzylic and allylic alcohols. Unfortunately, the oxidation of
unactivated primary and secondary alcohols was not possible under similar
experimental conditions.
Kumpulainen and Koskinen studied the effects of different components of
the Cu-TEMPO catalytic system on the aerobic oxidation of various aliphatic
and allylic alcohols. 41 It was reported that the amount of base was critical for
the outcome of the oxidation. A comparative study with a range of bases
revealed that amine bases such as N-methylimidazole (NMI), N,N-dimethyl-
aminopyridine (DMAP) and triethylamine (TEA) show moderate catalytic
activity whereas high activity is exhibited by 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) and 1,4-diazabicyclo[2.2.2]octane (DABCO). Kinetic studies with a
CuBr 2 -bpy-base system in acetonitrile showed a first-order kinetic depend-
ence of the reaction on TEMPO and second-order on copper. The oxidation
reactions follow a second-order dependence on the alcohol substrate at low
concentrations and a first-order dependence on oxygen at high alcohol
concentrations. Use of copper( II ) trifluoromethanesulfonate instead of
copper( II ) bromide increased the eciency of the catalyst for challenging
alcohol substrates.
The use of an air-microbubble strategy to increase the gas concentration
in the liquid or gas phase in homogeneous catalysis has been experimentally
supported to promote catalytic conversion in the aerobic oxidation of alco-
hols. 42 Although the exact role of the microbubbles is not clear, it provides
an ecient method to increase the eciency of the Cu-TEMPO catalyst.
Recently, Stahl and co-workers reported a highly practical method for
the aerobic oxidation of alcohols using (bpy)Cu( I )-TEMPO catalyst and
N-methylimidazole (NMI) base in acetonitrile solvent with ambient air as
the oxidant. 43,44 The catalytic system exhibits high chemoselectivity towards
primary alcohols including aliphatic substrates. Moreover, the catalytic
method is compatible with alcoholic substrates containing different func-
tional groups. Highly chemoselective oxidations of diols can also be carried
out without any protection (Figure 2.6a).

d n 4 r 4 n g | 0

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