CHEMSUSCHEM
COMMUNICATIONS
DOI: 10.1002/cssc.201300289
A Sustainable Process for Catalytic Oxidative Bromination
with Molecular Oxygen
[
a]
[a]
[a, b]
[a, b]
[a, b]
Zhijun Huang, Fengbo Li,* Bingfeng Chen,
Tao Lu ,
Yin Yuan,
and
[a]
Guoqing Yuan*
The twelve principles of green chemistry are valuable bench-
marks when developing chemical products and processes that
genation catalysts, based on metal centers such as iron, vana-
dium, and molybdenum.
[
1]
are more ecofriendly and sustainable. The sustainability of
a chemical process, as promoted by the European Technology
Herein, we report the development of a sustainable oxida-
tive halogenation process based on a well-designed palladium
catalyst. The catalytic reaction proceeds in microemulsions sus-
pended in an aqueous solution, offering high activity and se-
lectivity under mild reaction conditions. The catalysts are dis-
persed over the interface between the lipophilic droplets and
the aqueous solution. Molecular oxygen is used as oxidant.
The products in the lipophilic phase are easily separated by de-
stroying the microemulsion, causing the reaction mixture to
form three phases: an organic phase, containing products; the
used catalyst; and an aqueous phase. The catalyst can be re-
covered and recycled into the next batch without much effort.
The hybrid multifunctional catalyst was prepared according
to the procedure illustrated in Figure 1. A rigid tripodal anion
receptor, based on a 1,3,5-trisubstituted benzene spacer
(1; 1,3,5-tris[(3-methylimidazolio) methyl]-2,4,6-trimethylben-
[
2]
Platform on Sustainable Chemistry (ETP SusChem), is relevant
to process intensification. A core technology of process intensi-
fication is the development of new catalysts. Such develop-
ments can allow improvements in yields/productivity, abate
process costs through longer catalyst life, enable milder reac-
tion conditions, and reduce costs associated with separation
[
3]
and environmental requirements.
The importance of halogenated organic compounds in
chemistry is evident from the fact that they are essential start-
ing compounds and intermediates in organic synthesis, and
are also widely applied as building-block molecules in
materials science, industrial chemistry, and medicinal com-
[
4]
pounds. Most often, brominated compounds are used be-
cause they have a relatively high activity and acceptable costs.
However, the synthesis of brominated molecules generally re-
quires hazardous, toxic, and corrosive bromine in chlorinated
solvents. Other synthesis processes involve modifying bromina-
tion reagents (N-bromosuccinimide; NBS), bromine-carrying
agents (mainly derivatives of pyridinium perbromides), and oxi-
dative bromination. If hydrogen peroxide or oxygen is selected
as oxidant, one bromine atom can be incorporated into the
molecule while bromine can be regenerated from the residual
[10]
zene tribromide ) was used as the organic structure-directing
agent. 1 was precipitated from the aqueous solution by intro-
2À
ducing [PdCl ] . After reduction, palladium nanoparticles sta-
4
bilized by 1 were dispersed in water, resulting in a black sus-
5À
pension. Polyoxometalate anion [PV Mo O ]
was then
2
10 40
added and the hybrid organic–inorganic nanocomposites grad-
ually precipitated. The solids were collected by centrifugation.
The transmission electron microscopy (TEM) image in Figure 1
reveals the morphology of the as-synthesized nanocomposites.
The porous nanostructures result from self-assembly between
the rigid tripodal ligands and polyoxometalate anions, directed
by electrostatic interactions. The palladium nanoparticles are
embedded in the nanostructure, and kept in the monodis-
persed state. The chemical composition of the nanocomposite
was characterized by X-ray photoelectron spectroscopy (XPS;
Figure S1), allowing to detect the chemical elements compris-
[
5]
HBr by oxidation, and water is the only byproduct. An excel-
lent Review by Iskra et al. describes “green” oxidative halogen-
[
6]
ation. Approximately 4500 naturally occurring organohalogen
compounds have been reported, the biological production of
[
7]
which involves halogenating enzymes. Processes based on
[
8]
[9]
haloperoxidases and halogenases are potentially the most
effective and ecofriendly routes. In halogenations by enzymes,
hydrogen peroxide and molecular oxygen serve as oxidants.
Such biological halogenation processes offer a refreshing per-
spective on developing sustainable, biomimetic oxidative halo-
ing
the
tripodal
ligand,
polyoxometalate
anion
5À
([PV Mo O ] ), and palladium nanoparticles, and identify
2
10 40
their chemical states. The binding energy of the palladium spe-
cies was 334.9 eV (Pd 3d5/2), which was indexed to zero-valent
palladium nanoparticles (Figure S1c). The palladium nanoparti-
cles were further characterized by high-resolution TEM (Fig-
ure 2a). The particle size was about 10 nm. Porous substruc-
tures surround the nanoparticles.
[
a] Z. Huang, Dr. F. Li, B. Chen, T. Lu, Y. Yuan, Prof. G. Yuan
Beijing National Laboratory of Molecular Science
Laboratory of New Materials
Institute of Chemistry, Chinese Academy of Sciences
Beijing (PR China)
Fax: (+86)10-62559373
E-mail: lifb@iccas.ac.cn
Oxidative bromination experiments were then performed by
mixing organic substrate and catalyst in an aqueous buffer so-
lution (MeCOONa/MeCOOH) containing NaBr. Dibutyl ether
was used as organic solvent for the microemulsion system.
Molecular oxygen was provided by using a balloon. The reac-
tion mixture was stirred gently to form the microemulsion.
[
b] B. Chen, T. Lu, Y. Yuan
University of Chinese Academy of Sciences
Beijing, 100049 (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300289.
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2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 2 – 4
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