Green Chemistry
Cite this: Green Chem., 2011, 13, 1659
COMMUNICATION
Solvent free aerobic oxidation of alcohols with 1-methyl-2-azaadamantane
N-oxyl as a recyclable catalyst through phase separation
Yongbo Kuang, Yuta Nabae, Teruaki Hayakawa and Masa-aki Kakimoto*
Received 20th January 2011, Accepted 6th April 2011
DOI: 10.1039/c1gc15076f
An expedient, non-metallic green protocol for aero-
bic oxidation of alcohols was established. 1-Methyl-2-
azaadamantane N-oxyl was used as the core catalyst due
to its superior chemical stability and catalytic performance.
The catalyst can be easily reused through phase separation
by taking advantage of its solubility feature, which varies
with its oxidation state.
This research was inspired by the catalytic mechanism of
nitroxide radical catalyzed oxidation of alcohols.28 The mech-
anism involves an intermediate cation, which is oxidized from
the starting nitroxide radical and serves as the real oxidant for
alcohols. To complete the catalytic cycle, the cation oxidizes
an alcohol to a carbonyl compound with itself reduced to
a hydroxyamine, which is then oxidized by a bulk oxidant
to regenerate the nitroxide radical. We noted that during the
process, the catalyst first has limited solubility in water in its
radical form, then it becomes soluble in water and insoluble in
organic phase when oxidized to a cation, and finally it reverts to
water insoluble upon reduction to the hydroxyamine form. And
we postulated that this unique feature could be used to facilitate
the catalyst recycling, promising a much simpler solution than
immobilization. Therefore, we designed a two phase oxidation
system with diluted HNO3 solution as the aqueous phase and
the alcohol substrate as the organic phase, without use of any
organic solvents. Because diluted nitric acid is not able to oxidize
hydroxyamines, a catalytic amount of NaNO2 was added to
generate stronger oxidants, i.e., HNO2 and further NO2, which
is also a key component for O2 activation.
To find an appropriate nitroxide radical for this system, two
main factors were taken into consideration. One is, of course,
the catalytic performance, and the other is the stability of the
corresponding cation. This is because an important step in
the recycling process is the separation of the cations with the
aqueous phase. This requires the catalysts to be able to remain
stable in the ion form for a long enough time. If the cation is
a short-lived intermediate, then a considerable amount of the
catalyst will be decomposed during recycling, leading to the
failure of this method. Three nitroxide radicals were chosen
to be evaluated, i.e., TEMPO, 1-Me-AZADO, and ABNO.
TEMPO is the most commonly used one in this field. 1-Me-
AZADO has been recently reported to show remarkably higher
performance than TEMPO.29 ABNO is reported to possess
activity comparable to 1-Me-AZADO; moreover, its synthetic
route is much simpler and shorter than that of 1-Me-AZADO,
indicating a lower cost if adopted on a large scale.30 To examine
the stabilities of the corresponding cations, we prepared the
ionic solutions by dissolving the three nitroxide radicals into
30% nitric acid, where the radicals were immediately oxidized
to cations, and the changes in cationic concentrations with time
were determined using UV-Vis spectroscopy. As shown in Fig. 1,
Selective oxidation of alcohols to the corresponding aldehydes
or ketones is one of the most fundamental transformations
in organic synthesis.1,2 Molecular oxygen is obviously superior
to other conventional stoichiometric oxidants, from both the
viewpoints of green chemistry and economy.3,4 In the past
decades, numerous catalytic systems, mostly based on noble
metal catalysts, have been developed to harness the power of
molecular oxygen for the oxidation of alcohols. Notably, there
is another category, which is non-metallic and uses carbon
materials5,6 or organic compounds, e.g. IBX7,8 and TEMPO9–12
as core catalysts. As typical stable nitroxide radicals, TEMPO
and its derivatives have been found to be very active towards the
specific transformation of hydroxyl groups to carbonyl groups.
However, the homogeneous nature and relatively high prices
make them less attractive for large scale applications. Thus,
many efforts have been devoted to improve their reusability.
Most endeavours were focused on changing their homogeneous
nature to heterogeneous via immobilization on certain sup-
porting materials, such as organic polymers13–18 and silicas,19,20
particularly mesoporous structured silicas.21–23 Other attempts
include tagging TEMPO with fluorous24,25 or ionic26,27 chains
of small molecular weights to facilitate catalyst separation. We
are also interested in developing green, economical and efficient
oxidation systems. Herein, we report a green protocol for aerobic
alcohol oxidation, which also utilizes a nitroxide radical 1-Me-
AZADO as a homogenous catalyst, and the recycling of 1-Me-
AZADO by taking advantage of the change of its solubility with
oxidation state.
Department of Organic and Polymeric Materials, Tokyo Institute of
Technology, Meguro-ku, Tokyo, 152-8552, Japan.
E-mail: mkakimot@o.cc.titech.ac.jp.ccc; Fax: +81 3-5734-2875;
Tel: +81 3-5734-2433
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 1659–1663 | 1659
©