Supramolecular reactor from self-assembly of rod-coil molecule in aqueous environment

Article abstract of DOI:10.1021/ja048264z

We synthesized an amphiphilic coil-rod-coil triblock molecule consisting of hexa-p-phenylene as a rod block and poly(ethylene oxide) with the number of repeating units of 17 as coil blocks and investigated aggregation behavior in aqueous environment. The rod-coil molecule was observed to aggregate into discrete micelles consisting of hydrophobic disklike rod bundles encapsulated by hydrophilic poly(ethylene oxide) coils. The aromatic bundles of the micelles were demonstrated to be used as an efficient supramolecular reactor for the room temperature Suzuki cross-coupling reaction of a wide range of aryl halides, including even aryl chlorides with phenylboronic acids in aqueous environment. These results demonstrate that self-assembly of amphiphilic rod-coil molecules can provide a useful strategy to construct an efficient supramolecular reactor for aromatic coupling reaction. Copyright

Full text of DOI:10.1021/ja048264z

Published on Web 06/12/2004
Supramolecular Reactor from Self-Assembly of Rod-Coil Molecule in
Aqueous Environment
Myongsoo Lee,* Cheong-Jin Jang, and Ja-Hyoung Ryu
Center for Supramolecular Nano-Assembly and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
Received March 26, 2004; E-mail: mslee@yonsei.ac.kr
Rod-coil molecules, consisting of a flexible and a rigid block,
represent a unique class of block copolymers, where the anisotropic
orientation of the rod segments and the repulsion between the
covalently connected segments lead to self-organization into a wide
1
variety of aggregation structures. The supramolecular structures
can be tuned by careful selection of the type and relative length of
the respective blocks. Another interesting feature of rod-coil
molecules is their amphiphilic characteristic that shows a tendency
of their lipophilic and lipophobic segments to separate in space
2
into distinct nanodomains. Depending on the solvent content and
polarity, rod-coil molecules self-assemble into a wide variety of
different aggregation structures through mutual interactions between
block segments and solvent.
One can envision that rod-coil amphiphilic molecules based on
a hydrophilic poly(ethylene oxide) as a flexible chain self-assembles
3
into micellar aggregates in aqueous environment. The resulting
hydrophobic aromatic core of the aggregates would provide a
nanoenvironment suitable for the confinement of nonpolar aromatic
guest molecules through intermolecular interactions including
hydrophobic interactions and π-π interactions. Accordingly, mi-
cellar aggregates of amphiphilic rod-coil molecules based on poly-
Figure 1. Schematic representation of supramolecular reactor.
(ethylene oxide) coils raise the possibility of the use as containers
for aromatic coupling reactions in aqueous media (Figure 1).
With this in mind, we prepared rod-coil triblock molecule 1
consisting of poly(ethylene oxide) with the number of repeating
units of 17 as coil segments and hexa-p-phenylene as a rod segment
and investigated the Suzuki aromatic coupling reactions in aqueous
environment at room temperature. The rod-coil molecule was
synthesized using similar procedures reported previously from our
Figure 2. (a) Dynamic laser light scattering. (b) Scanning electron
Hm 2i cO r )o .graph. (c) Transmission electron micrograph of 1 (2 × 10 g/mL in
-
4
4
laboratory. All of the analytical data, including NMR, GPC, and
-
3
elemental analysis, are in full agreement with the chemical structure
presented. This amphiphilic molecule based on an elongated rod
shows aggregation behavior in aqueous media. Dynamic light
scattering studies of aqueous solutions showed that the rod-coil
molecule self-assembles into micellar aggregates (Figure 2a). The
catalyst and 2 equiv of NaOH in a 2.5 × 10 M aqueous solution
of 1 to give the corresponding products with nearly quantitative
conversion after 12 h. Under similar conditions, the coupling
reactions of a wide range of bromobenzenes including electron-
rich and electron-deficient substrates also underwent with quantita-
tive conversion (entries 4-6), clearly demonstrating that the cross-
coupling reaction in the presence of 1 is highly effective and allows
for the room temperature cross-coupling reaction in aqueous media.
Remarkably, the cross-coupling reaction of aryl chloride, which is
generally unreactive under the conditions employed to couple aryl
bromides and aryl iodides,8 also proceeded under similar room-
temperature conditions to afford the corresponding biphenyl in 72%
yield (entry 7). It should be noted that, in the presence of
conventional non-ionic amphiphile (entry 10) or organic solvent
instead of 1 (entry 11), no reactions take place under the
aforementioned reaction conditions, indicating that the presence of
a bundle-like aromatic core of the aggregate plays a key role in
the room-temperature coupling reaction in aqueous environment.
Although the definitive mechanistic details of the reactions still
remain to be solved, we believe that these results indicate clearly
that the amphiphilic rod-coil system is well-suited as a supra-
H
average hydrodynamic radius (R ) of the aggregate was observed
to be approximately 6 nm, suggesting that the micelles consist of
hydrophobic disklike rod bundles encapsulated by hydrophilic poly-
5,6
(
ethylene oxide) coils, as proposed by rod-coil theories. The
formation of micellar aggregates of 1 was also confirmed by FE-
SEM and TEM experiments, which show spherical entities. As
shown in Figure 2, parts b and c, the micrographs show spherical
aggregates that are roughly 9-15 nm in diameter and are thus
consistent with the results obtained from dynamic light scattering
experiments.
,9
The use of the resulting aggregate as a supramolecular reactor
for an aromatic coupling reaction in aqueous media was investigated
with the Suzuki cross-coupling reaction of aryl halides and aryl
7
boronic acids at ambient temperature. As shown in Table 1, the
cross-coupling reactions of iodobenzenes with 1.2 equiv of phen-
ylboronic acids took place in the presence of 0.5 mol % palladium
8082
9
J. AM. CHEM. SOC. 2004, 126, 8082-8083
10.1021/ja048264z CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Table 1. Suzuki Cross-Coupling Reaction in Watera
fluorescence intensities of the aqueous rod-coil solutions containing
the aromatic substrates were showed to be significantly suppressed
because of fluorescence quenching,11 demonstrating that they are
effectively entrapped within the aromatic bundle of micelle
composed of rod-coil building blocks. These results strongly
support that the Suzuki coupling reaction occurs within the confined
environment of the aromatic bundle of the aggregates. The results
described here represent a significant example that self-assembly
of amphiphilic rod-coil molecules can provide a useful strategy
to construct an efficient supramolecular reactor for aromatic
coupling reaction.
In conclusion, we have demonstrated that self-assembly of
amphiphilic rod-coil molecules in aqueous solution can be used
as a supramolecular reactor for the room temperature Suzuki
coupling of a wide range of aryl halides, including even aryl
chloride with phenyl boronic acids in the absence of organic
solvents, giving rise to an environmentally friendly reaction system.
We believe that this supramolecular approach to reactor in aqueous
environment will be broadly applicable to a wide variety of catalytic
coupling reactions.
entry
X
Y
Z
R
yield (%)b
1
2
3
4
5
6
7
8
9
I
I
I
NO2
H
OCH3
NO2
H
OCH3
NO2
H
OCH3
OCH3
OCH3
H
OCH3
H
H
OCH3
H
H
OCH3
H
H
H
1
1
1
1
1
1
1
1
99
99
99
99
99
99
72
17
15
Br
Br
Br
Cl
Cl
Cl
Br
Br
1
c
10
1
non-ionic surfactant
no reaction
no reaction
d
1
THF
a
Reaction conditions: aryl halide (0.1 mmol), aryl boronic acid (0.12
mmol), Pd(OAc)2 (0.5 mol %), PPh3 (1.0 mol %), NaOH (0.2 mmol), R
50 mg, 0.025 mmol), and H2O (10 mL), stirred at room temperature for
(
1
b
2 h. Percent yields are calculated on the basis of GC analysis using
c
xylene as the internal standard. Polyethylene-block-poly(ethylene glycol)
d
(Mn ) 920, 50 wt % ethylene oxide). THF 5 mL.
Acknowledgment. We are grateful to Prof. Chul-Ho Jun for
helpful discussion. This work was supported by the National
Creative Research Initiative Program of the Korean Ministry of
Science and Technology.
Supporting Information Available: Detailed synthetic procedures,
characterization, and UV-vis spectra (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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4
Figure 3. Emission spectra of 1 (2 × 10 g/mL in H2O) by (a) added or
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