Micelle-based nanoreactors containing Ru-porphyrin for the epoxidation of terminal olefins in water

Article abstract of DOI:10.1016/j.molcata.2016.02.033

This contribution introduces a strategy to use Ru(II)-porphyrin complexes as catalysts for the epoxidation of alkenes in water. The design is based on shell cross-linked micelle-based nanoreactors with hydrophobic cores and hydrophilic shells as supports

Full text of DOI:10.1016/j.molcata.2016.02.033

Journal of Molecular Catalysis A: Chemical 417 (2016) 122–125
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Micelle-based nanoreactors containing Ru-porphyrin for the
epoxidation of terminal olefins in water
∗
Jie Lu, Linus Liang, Marcus Weck
Molecular Design Institute and Department of Chemistry, New York University, 100 Washington Square East, New York, NY 10003, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
This contribution introduces a strategy to use Ru(II)-porphyrin complexes as catalysts for the epoxidation
of alkenes in water. The design is based on shell cross-linked micelle-based nanoreactors with hydropho-
bic cores and hydrophilic shells as supports for the Ru(II) porphyrin complexes. The supported complexes
are efficient catalysts for oxidation reactions, specifically the selectively oxidation of alkenes to epoxides.
The presented strategy is the first system that allows for running Ru-catalyzed epoxidation reactions in
water.
Received 11 December 2015
Received in revised form 26 February 2016
Accepted 27 February 2016
Available online 2 March 2016
Keywords:
©
2016 Elsevier B.V. All rights reserved.
Epoxidation
Supported catalysis
Ruthenium
1
. Introduction
catalytic systems, in which the active catalyst is located in the
hydrophobic micelle core, allow for efficient and selective catal-
ysis in water due to the shielding effect of the hydrophilic micelle
shell [15–26]. In recent years, micelle-bound catalysts have gar-
nered increased attention, as they feature properties such as local
concentration effects, shielding, and substrate selectivity, often in
conjunction with greater reusability over traditional homogeneous
analogs [24]. Additionally, the use of water as the solvent makes the
reaction not only environmentally friendlier but can also improve
the rate and selectivity of the desired transformation [27,28]. We
hypothesized that such a polymeric micelle-supported catalytic
systems should be able to carry out the epoxidation of terminal
olefins in aqueous environments. One drawback of micelle-bound
catalysts is the stability of the self-assembled micelle structure
which is dynamic in nature. Shell cross-linked micelles (SCMs) are
able to overcome this disadvantage and provide a more robust poly-
meric assembly [29–33]. This contribution reports SCM-supported
Ru-porphyrin complexes as the first example for carrying out Ru-
porphyrin catalyzed epoxidations in water.
Catalytic oxidations of organic substrates play a key role in
the pharmaceutical and fine chemical industry. In Nature, a well-
known monooxygenase for aerobic oxidation is Cytochrome P-450,
which features an iron porphyrin core. This motivated inves-
tigations into the use of synthetic metalloporphyrin-catalyzed
oxidation systems [1,2]. Ruthenium porphyrins, in particular, show
high activity and stability [3–6] and have been studied for the
epoxidation of alkenes for about 35 years [7]. First explored by
Groves and co-workers, a ruthenium porphyrin-based oxidation
system usually consists of terminal oxidants (e.g. iodosylben-
zene, sodium hypochlorite, or 2,6-dichloropyridine-N-oxide) and
a solvent such as chloroform or benzene [8,9]. Traylor and co-
workers proposed using H O as an environmentally clean oxidant
2
2
[
10,11]. This modification, however, only works with electron-
deficient metalloporphyrins as ligands and solvent systems such
as CH Cl /CH OH/H O or CH CN/alcohol [12]. Protic solvents like
2
2
3
2
3
alcohols function as a general-acid catalyst that facilitates O
O
bond heterolysis, thus generating a high-valent oxometal por-
phyrin intermediate [13,14]. While water would be the ideal green
solvent, it cannot be used as a majority component in a CH CN/H O
2. Results and discussion
3
2
mixture due to the insolubility of the alkene substrate and the Ru-
porphyrin catalyst in water.
Our strategy to overcome the solubility problem is by supporting
the catalyst on a polymeric micelle. Polymeric micelle-supported
Our cross-linked micelle support is formed from amphiphilic
ABC-triblock copolymers based on poly(2-oxazoline) s (Scheme 1)
containing a hydrophobic block (A) and a hydrophilic block (C).
Cross-linking block (B) is introduced as the middle layer. The 2-
substituted 2-oxazoline monomer A was synthesized following
literature procedures [34]. Monomer B was synthesized in a two-
step one-pot reaction in 72% yield (see SI) [35]. Poly(2-oxazoline)
triblock copolymers were synthesized via cationic ring-opening
∗ Corresponding author.
E-mail address: marcus.weck@nyu.edu (M. Weck).
http://dx.doi.org/10.1016/j.molcata.2016.02.033
1
381-1169/© 2016 Elsevier B.V. All rights reserved.

J. Lu et al. / Journal of Molecular Catalysis A: Chemical 417 (2016) 122–125
123
Scheme 1. Schematic representation for the synthesis of the micelle supported metal catalyst.
Table 1
polymerization using methyl triflate as the initiator. The poly-
merization process was monitored by H NMR spectroscopy and
1
Solvent system tests of unsupported Ru(II)-porphyrin complex catalyzed epoxida-
tion of styrene.
gel-permeation chromatography (GPC) (see SI). The disappearance
of the monomer peak in the 1H NMR spectrum at 3.8 ppm and
Entry
CH CN: H O
Time (h)
Conv. (%)
3
2
4
.2 ppm and clear shifts of the GPC traces after each block formation
1
2
3
4
5
100:0
80:20
50:50
40:60
20:80
24
24
24
24
24
95
92
86
20
0
indicated the stepwise growth of the block copolymer. The disper-
sity (Ð) and apparent molecular weight (Mn ) of the final triblock
copolymer 1 are 1.23 and 7700 g mol respectively, as determined
by GPC using poly(styrene) standards.
app
−
1
The terminal ester groups of the hydrophobic block A were
hydrolyzed and converted into carboxylic acids. They serve as func-
tional handles to attach the hydroxyl-functionalized Ru-porphyrin
complexes. The hydroxyl-functionalized unsymmetrical porphyrin
ligand was synthesized following Lindsey’s method and ruthenium
metallation was carried out by refluxing the porphyrin ligand and
RuCl3 in decalin to yield the target complex 3 [36,37]. The Ru(II)-
porphyrin containing amphiphilic block copolymer 4 was prepared
by the esterification of polymer 2 and complex 3. Micelle formation
was induced by dissolving 4 in water at 5 mg/mL. Dynamic light
of 30 ± 5 nm that is in agreement with the hydrodynamic radius of
32 ± 6 nm obtained by DLS analysis.
Epoxidation of styrene with H O as the oxygen source using
2
2
terminal alkenes as substrates in an aqueous environment was
investigated using an unsupported Ru(II)-porphyrin complex as
control experiment as well as nanoreactors 4 and 5. For the small
molecule catalysts, different CH CN and water solvent mixtures
3
were tested. Poor conversions were observed when the water con-
tent was higher than 50% (Table 1) most likely due to the insolubility
of the Ru(II)-porphyrin complex.
scattering (DLS) indicated the presence of micelle aggregates with
app
Rh
= 47 ± 4 nm.
To form the SCM-supported Ru-porphyrin nanoreactor 5, the
Nanoreactors 4 and 5 both worked in aqueous environments
for the epoxidation step, demonstrating 95% conversions over 24 h,
thus validating the hypothesis that polymeric micelles can be used
as matrices to carry out epoxidations under aqueous conditions.
Our working hypothesis is that hydrophobic molecules, such as
styrene, can easily diffuse into the micellar core. Additionally,
the presence of amide bonds and carboxylic acid groups in the
hydrophobic block enables small amounts of water and H O to dif-
thiol-yne reaction was chosen for the cross-linking step between
a dithiol linker and the carbon-carbon triple bonds in block B [38]
(Scheme 1). Using 2,2-dimethy-2-phenylacetophenone (DMPA) as
the radical initiator, the thiol-yne reaction was completed via a
◦
2
4 h UV-irradiation of the micelle solution at 4 C. The success-
ful cross-linking step was confirmed by the stable hydrodynamic
radius of 5 in different solvents as analyzed by DLS (see SI). The size
of nanoreactor 5 was further characterized via SEM to give a radius
2
2
fuse into the core and serve as the general-acid to facilitate the O
O
bond heterolysis. [39] With all reagents and the Ru(II)-porphryin

1
24
J. Lu et al. / Journal of Molecular Catalysis A: Chemical 417 (2016) 122–125
Table 3
1
00
a
Recycling data of 5 for the epoxidation of styrene in water .
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
cycle
time (h)
conv. (%)b
1
2
3
24
24
24
96
95
95
a
Conditions: 0.5 mmol styrene, 0.75 mmol H2O2, 0.5 mol% catalyst loading in 1 mL
H O at room temperature.
2
b
Determined by GC analyses.
3. Conclusion
Synthetic amphiphilic block copolymers based on poly(2-
0
3
6
9
12
15
18
21
time (h)
oxazoline) with functional handles in the hydrophobic block
were designed and synthesized to engineer micelle-based
nanoreactors with hydrophilic and hydrophobic compartments.
Ru(II)-porphyrin complexes were attached to the micelle core
that then can serve as catalytic site for the epoxidation of ter-
minal alkenes. The epoxidation reaction proceeds in water. This
micelle-supported catalyst strategy combines easy recycling, high
efficiency and product selectivity in an environmentally friendly
aqueous system.
Fig. 1. Kinetic evaluation of the epoxidation of styrene for the micelle supported
Ru-porphyrin catalysts 4 (blue diamond) and 5 (red square). Conditions: 0.05 mmol
styrene, 0.075 mmol H2O2, 0.35 mol% catalyst loading in 1 mL D2O at room temper-
ature. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
Table 2
a
Substrate scope for the epoxidation with nanoreactor 4 .
Acknowledgements
entry
substrate
cat. loading (mol%)
time (h)
conv. (%)b
1
2
3
4
5
styrene
0.5
1
1
0.5
1
24
48
48
24
48
99
39
94
99
94
Funding provided by theU.S. Department of Energy, Office of
Basic Energy Sciences, through Catalysis Science Contract DE-FG02-
2-bromostyrene
4-vinylbenzylchloride
1-hexene
0
3ER15459, is gratefully acknowledged.
vinylcyclohexane
a
Appendix A. Supplementary data
Conditions: 0.05 mmol substrate, 0.075 mmol H2O2, 0.35 mol% catalyst loading
in 1 mL H2O at room temperature.
b
Determined by GC analyses.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.02.
0
33.
in the confined space, the epoxidation proceeded similarly to an
organic/water solvent mixture.
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