COMMUNICATION
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Aerobic oxidation of thiols to disulfides using iron metal–organic
frameworks as solid redox catalystsw
Amarajothi Dhakshinamoorthy, Mercedes Alvaro and Hermenegildo Garcia*
Received 29th June 2010, Accepted 19th July 2010
DOI: 10.1039/c0cc02210a
Aerobic oxidation of thiols to disulfides has been carried out
using iron metal–organic frameworks (MOFs) as solid redox
catalysts with very high yield and selectivity in acetonitrile under
mild reaction conditions.
solvent gives higher initial rate measured at 15 min than
ethanol (51% conversion). Increasing the temperature from
40 to 70 1C with Fe(BTC) as catalyst in acetonitrile as
solvent resulted in a very high conversion of 1 in 1 h. On the
other hand, aerobic oxidation of 1 with Fe(BTC) in ethanol
exhibited similar activity of acetonitrile but toluene showed
only moderate conversion of 1. Further, we also wanted to
study the effect of added base (pyridine) on the Lewis acid
nature of Fe(BTC) but the influence of the presence of
pyridine on this conversion and selectivity of the aerobic
oxidation of 1 into 2 was negligible. This result suggests that
Fe(BTC) acts as a redox catalytic system. In order to under-
stand the nature of the active site, we did a control experiment
with iron nitrate as homogeneous catalyst for the aerobic
oxidation of 1 to 2. As it can be seen in Fig. 1, although the
initial reaction rate is similar in both systems, Fe(BTC)
exhibited long term better activity than iron nitrate due to
the deactivation of the latter catalyst. Therefore, the well
defined, crystalline, microporous structure of Fe(BTC) acts
as a single site heterogeneous redox catalyst for the aerobic
oxidation of 1 to 2 and maintains the catalytic activity during
the reaction (in contrast to dissolved iron nitrate).
Catalytic oxidation using oxygen as the terminal oxidant is a
hot topic in modern organic synthesis due to the environ-
1
mental advantages. The key point in aerobic oxidations is to
develop a highly selective catalyst able to promote at high
substrate conversion the formation of a single product among
all the possible oxidation products. While considerable progress
has been made in aerobic oxidation using noble metal nano-
2
particles such as Pt and Au, it would still be desirable
to develop catalysts based on less expensive metals. In this
3
,4
regard, novel metal–organic frameworks
appeared as promising heterogeneous catalysts for oxida-
(MOFs) have
5
,6
7
8
tions using organic peroxides and even molecular oxygen. In
the present work, we describe that commercially available
Fe(BTC) (BTC: 1,3,5-benzenetricarboxylate) is a suitable,
reusable redox catalyst to perform selectively the aerobic
oxidation of thiols to disulfides. Fe(BTC) is constituted by
the trimers of iron octahedra sharing a common vertex m -O
3
linked by the benzene-1,3,5-tricarboxylate moieties in such a
way that this leads to two types of mesoporous cages of free
˚
apertures of 25 and 29 A, accessible through microporous
3 2 2 3
Other MOFs namely Cu (BTC) and Al (BDC) (BDC: 1,4-
benzenedicarboxylate) showed poor catalytic activity for this
aerobic oxidation and the conversion achieved is 28 and 4%,
˚
windows of 5.5 and 8.6 A.
3 2
respectively. Particularly, Cu (BTC) exhibited poor stability
Oxidation of thiols to disulfides has important applications
in the preparation of biomolecules as well as ligands for metal
under the present experimental conditions as evidenced from
its colour change (Fig. 2) and the variation of the diffraction
pattern observed from optical spectroscopy and XRD. Even
9
–12
nanoparticles and has received considerable attention.
The
2
+
problem of thiol oxidation is the large variety of oxidation
products that can possibly be formed. Besides disulfides,
sulfones, sulfoxides and sulfonic acids are also typical products
formed in thiol oxidation. The target is to develop a selective
process for the transformation of thiols into disulfides avoiding
over oxidation to oxygenated sulfur products.
EPR spectroscopy shows disappearance of the Cu
signal
(see ESIw).
Heterogeneity of the reaction was established by the hot
filtration test. After achieving about 39% conversion of 1,
Fe(BTC) was removed from the reaction mixture by hot
In the first stage of our work, thiophenol (1) was selected as
the substrate. Aerobic oxidation of 1 to the corresponding
disulfide 2 in the absence of catalyst resulted in low conversion
2
À1
and with Fe(BTC) (840 m g BET specific surface area, used
as received) as the solid redox catalyst moderate to high con-
version was achieved at room temperature and at 70 1C under
air in acetonitrile as solvent. Acetonitrile (58% conversion) as
Instituto Universitario de Tecnologı ´a Quı´mica CSIC-UPV and
Departamento de Quı´mica, Universidad Polite´cnica de Valencia,
Av. De los Naranjos s/n, 46022 Valencia, Spain.
E-mail: hgarcia@qim.upv.es; Fax: +34 96387 7809;
Tel: +34 9638 7807
w Electronic supplementary information (ESI) available: Details of
experimental procedures and catalyst characterizations. See DOI:
Fig. 1 Time conversion plot for the aerobic oxidation of thiophenol
10.1039/c0cc02210a
with (a) iron nitrate and (b) Fe(BTC) as catalysts.
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476 Chem. Commun., 2010, 46, 6476–6478
This journal is c The Royal Society of Chemistry 2010