CHEMSUSCHEM
FULL PAPERS
DOI: 10.1002/cssc.201402059
SBA-15-Functionalized 3-Oxo-ABNO as Recyclable Catalyst
for Aerobic Oxidation of Alcohols under Metal-Free
Conditions**
Babak Karimi,*[a] Elham Farhangi,[a] Hojatollah Vali,[b] and Saleh Vahdati[a]
Dedicated to Prof. Dieter Enders for his scientific achievements, which we shall never forget, and on the occasion of his retirement.
The nitroxyl radical 3-oxo-9-azabicyclo [3.3.1]nonane-N-oxyl (3-
oxo-ABNO) has been prepared using a simple protocol. This or-
ganocatalyst is found to be an efficient catalyst for the aerobic
oxidation of a wide variety of alcohols under metal-free condi-
tions. In addition, the preparation and characterization of a sup-
ported version of 3-oxo-ABNO on ordered mesoporous silica
SBA-15 (SABNO) is described for the first time. The catalyst has
been characterized using several techniques including simulta-
neous thermal analysis (STA), transmission electron microscopy
(TEM), and nitrogen sorption analysis. This catalyst exhibits cat-
alytic performance comparable to its homogeneous analogue
and much superior catalytic activity in comparison with
(2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) for the aerobic
oxidation of almost the same range of alcohols under identical
reaction conditions. It is also found that SABNO can be con-
veniently recovered and reused at least 12 times without sig-
nificant effect on its catalytic efficiency.
Introduction
The selective oxidation of primary and secondary alcohols to
aldehydes and ketones is one of the most important reactions
in organic chemistry and industrial manufacturing.[1] In this
context, various oxidizing agents have been reported such as
stoichiometric CrVI salts,[2] DMSO-coupled reagents,[3] and hy-
pervalent iodine;[4] however, most of them are toxic and give
rise to hazardous wastes. On the other hand, many metal-
based catalysts are sometimes environmentally harmful and
leave toxic traces of heavy metals or are deactivated. Thus, it is
highly desirable to replace these catalysts with metal-free sys-
tems and greener alternatives. Developing new environmental-
ly benign and efficient catalytic methods for the selective aero-
bic oxidation of alcohols in the presence of molecular oxygen
has attracted increasing attention. Molecular oxygen as
a green oxidant has some advantages, such as low cost, abun-
dance, and no toxicity, and it water is produced as the only
by-product.[5] However, many of the reported catalyst systems
for the aerobic oxidation of alcohols require toxic or expensive
transition metals. To address these problems, the application
of stable nitroxyl radicals, especially 2,2,6,6-tetramethylpiperi-
dine-1-oxyl (TEMPO) and its derivatives, has received a great
deal of interest for the selective oxidation of alcohols under
metal-free conditions using molecular oxygen as a terminal ox-
idant.[6] In this context, the use of non-transition metal co-cata-
[8]
lysts such as tBuONO (TBN)[7] and NaNO2 is well documented
for the oxidation of primary and secondary alcohols in the
presence of TEMPO. However, many of these methods are inef-
ficient when using aliphatic and secondary alcohols; therefore,
their wide-spread applicability is hampered. The relatively high
steric hindrance arising from four methyl groups in TEMPO,
which gives rise to the additional advantage of selective oxida-
tion of primary alcohols,[9] is also a drawback as it often results
in TEMPO-based catalytic systems being unsuitable for the oxi-
dation of secondary and hindered alcohols. To circumvent this
limitation, several types of less hindered nitroxyl radicals such
as 2-azaadamantan-N-oxyl (AZADO), 1-Me-AZADO, 1,3-dimeth-
yl-AZADO, 5-F-AZADO, and Nor-AZADO have been introduced
as robust as well as more active alternatives to TEMPO for the
efficient catalytic oxidation of a wide range of alcohols.[10] How-
ever, although AZADO derivatives exhibited excellent catalytic
activity at low catalyst loading, they could be only obtained
through a long synthetic process consisting of up to ten steps.
To overcome this issue, 9-azabicyclo[3.3.1]nonane-N-oxyl
(ABNO) was introduced as a more readily available and yet
stable alternative to AZADO, which can be prepared through
a three-step synthetic route (Scheme 1).[11]
[a] Prof. Dr. B. Karimi, Dr. E. Farhangi, S. Vahdati
Department of Chemistry
Institute for Advanced Studies in Basic Sciences (IASBS)
Gava Zang, Zanjan 45137-6731 (Iran)
[b] Prof. H. Vali
Anatomy and Cell Biology and Facility for Electron Microscopy Research
McGill University
ABNO was then employed in the selective oxidation of
varied alcohols under standard Margarita[12] conditions.[11] The
critical stage toward the synthesis of ABNO is a double Man-
nich reaction of acetonedicarboxylic acid with glutaraldehyde
3450 University St, Montreal, Quebec, H3A 2A7 (Canada)
[**] 3-Oxo-ABNO=nitroxyl radical 3-oxo-9-azabicyclo [3.3.1]nonane-N-oxyl.
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ChemSusChem 2014, 7, 2735 – 2741 2735