Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 144

burning off the support. 7 However, hydrophilic supports are preferable
when water is the solvent, because the dependence on a basic promoter is
reduced, as in the case of ethanol oxidation to acetic acid catalyzed by Au/
MgAl 2 O 4 . This reaction, conducted under forced conditions (3.5 atm of air,
150-180 1C) with notable yields up to the 92% in acetic acid, represents the
first example of a gold-catalyzed oxidation proceeding in water in the ab-
sence of a base. 84
Concerning the oxidizing agent, hydrogen peroxide represents an e cient
oxidant for performing environmentally friendly oxidations of organic
compounds in addition to air or molecular oxygen. As an example,
Kholdeeva et al. developed new clean and ecient methods for the pro-
duction of 2-methyl-1,4-naphthoquinone (MNQ, menadione, vitamin K 3 )
based on the oxidation of 2-methyl-1-naphthol (MNL) with cheap and en-
vironmentally benign oxidants, aqueous hydrogen peroxide, tert-butyl
hydroperoxide and molecular oxygen, using three types of heterogeneous
catalysts, namely mesoporous titanium-silicate Ti-MMM-2, silica-supported
iron phthalocyanine and supported gold nanoparticles. Interestingly, on
performing blank tests, it was also found that this reaction proceeds fairly
well without any catalyst with oxygen at 3 atm in non-polar solvents. 50
Until a few years ago, catalytic methods for the oxidation of amino al-
cohols were almost unknown in organic synthesis, owing to the inhibiting
effect of the amino group on traditional metal catalysts, but over the last
5 years a number of papers have appeared on this topic. 11,12,32 Rossi and
co-workers first found gold to be an exception when compared with pal-
ladium and platinum under similar conditions, displaying notable and
stable catalytic performance in the aerobic oxidation of amino alco-
hols. 12,32 By reacting different kinds of amino alcohols at 70 1C, with O 2
under a slight oxygen pressure (3 atm) and a substrate-to-NaOH molar ratio
of 1:1, it was found that alkali increased the oxidation rate and alumina
was a better supporting material than carbon. In the case of N-substituted
amino alcohols, the oxidation took place exclusively at the nitrogen atom,
leading to the corresponding N-oxide without any amino acid formation.
Therefore, starting from 3-dimethylamino-1-propanol, 100% regio-
selectivity to the corresponding N-oxide was reached. In principle, the
oxidation of the amino group is possible both in the absence and in the
presence of alkali: without any alkali, 100% selectivity was observed with
different metal catalysts, but only gold-containing catalysts allowed 100%
conversion, whereas Pt/C was inert and Rh/C led to only 20% conversion to
unidentified compounds. 32
An effective alternative to liquid-phase reactions, but suitable only for
thermally stable and volatile alcohols, is to perform selective oxidation in the
gaseous phase. This procedure has the advantages over liquid-phase re-
actions of being solventless and alkali free and allowing the oxidation to be
stopped at the corresponding aldehyde or ketone. Furthermore, gold-based
catalysts have displayed good performance in terms of activity and selectivity
also in gaseous-phase processes. One of the first examples was the selective

d n 4 r 4 n g | 3

.