.
Angewandte
Communications
DOI: 10.1002/anie.201302662
Supramolecular Catalysis
Supramolecular Assemblies of Amphiphilic l-Proline Regulated by
Compressed CO2 as a Recyclable Organocatalyst for the Asymmetric
Aldol Reaction**
Long Qin, Li Zhang,* Qingxian Jin, Jianling Zhang,* Buxing Han, and Minghua Liu*
Dedicated to Professor Chunli Bai
Self-assembled nano/micro structures formed by amphiphiles
in water, such as micelles, vesicles, and emulsions, have been
well-studied and have provided a wide number of applications
in the fields of biochemistry,[1] drug delivery,[2] oil recovery,[3]
catalysis,[4] and so forth. In terms of the structural regime,
these assemblies could serve as nanoreactors with some
unique merits. For example, they offer boundaries that
separate the reaction space from the environment.[5] They
could also assist the preorganization of the reactants in
a favored conformation, creating a space where reaction can
be selectively controlled.[6] Among these systems, controlling
and regulating the assembled nano/microstructures become
an important issue to efficiently facilitate applications.
Although various ways of regulating the assembled structures,
such as temperature,[7] ionic strength,[8] pH value,[9] and
additives[10] are generally employed, it still remains a great
challenge to regulate the structures dynamically, reversibly,
and circularly.
In recent years, with the progress in sustainable chemistry,
supercritical or compressed CO2 has been widely used in
many aspects.[11] Compressed CO2 can trigger the formation
of nanoemulsions reversibly,[12] and switch the micelle-to-
vesicle transition (MVT)[13] and the liquid-crystal-to-micelle
transition.[14] Furthermore, by regulating the pressure, com-
pressed CO2 favors the separation of the products.[15] Thus,
compressed CO2 is expected to be used in the regulation of
the nanostructures as well as reactions.[16] Herein, by combin-
ing the merits of the triggering self-assembled nanostructures
and the regulation ability of compressed CO2, we applied
compressed CO2 in a catalytic asymmetric aldol reaction.
While metal-assisted catalysts are generally used,[17] supra-
molecular organocatalysts are attracting great interest
recently,[18] and there is still a need to develop a low-cost,
safe, and environmentally benign catalyst. In the present case,
by aid of compressed CO2, we have performed the asym-
metric aldol reaction in water with great efficiency
(Scheme 1). First, although the amphiphilic proline could
not be dissolved in water and catalyze the reaction in pure
water, by introducing compressed CO2, the nanostructures
can be induced and applied in the catalysis of the aldol
reaction. Furthermore, the nanostructure can be regulated by
the pressure of CO2 and subsequently the reaction selectivity,
thus providing a dynamic regulation of the nanostructure and
reaction. Second, with the aid of compressed CO2, the
products can be separated easily. Third, after simple separa-
tion, the amphiphilic proline can be reused for catalyzing the
reaction with the formation of nanostructure. Such a process
can be repeated several times, thus making the catalyst
recyclable. to date, although there are several reports on the
regulation of vesicle structures through CO2,[19] this is the first
report that compressed CO2-induced nanostructures are used
to the asymmetric catalysis. Furthermore, such a process is
essentially sustainable and recyclable.
The phase behavior of PTC12 with a concentration of 8 mm
in water was investigated in situ in the presence of com-
pressed CO2 at 258C under different pressure, as shown in
Figure 1. The PTC12 is practically insoluble in water. Interest-
ingly, as compressed CO2 was charged into the system to
2.0 MPa, a transparent solution was obtained (Figure 1A,a),
suggesting the formation of a PTC12 assembly. A subsequent
increase of the CO2 pressure caused the solution to show
a gradual transition from colorless to a slightly bluish color
(Figure 1A,b–e), indicating the increase of the size of
assemblies. This process was simultaneously monitored by
the turbidity change of the system by UV/Vis spectra
(Figure 1B). Consistent with the continuous phase transition,
the transmittance at 500 nm shows dramatic decrease after
CO2 pressure exceeded 2 MPa, and then drops slowly when
CO2 pressure beyond about 6 MPa. The assemblies obtained
at all pressures we tested were stable enough and there was no
precipitation, even after more than two weeks.
[*] L. Qin, Dr. L. Zhang, Dr. Q. X. Jin, Prof. J. L. Zhang, Prof. B. X. Han,
Prof. M. H. Liu
Beijing National Laboratory for Molecular Science (BNLMS)
CAS Key Laboratory of Colloid, Interface and Chemical Thermody-
namics
Institute of Chemistry, Chinese Academy of Sciences
Beijing, 100190 (P.R. China)
E-mail: zhangli@iccas.ac.cn
[**] We gratefully acknowledge funding of this research by the Basic
Research Development Program (2010CB833305), the National
Natural Science Foundation of China (Nos. 91027042, 21021003,
21227802).
To further understand such changes, the in situ emission
spectra of PTC12 were detected (Figure 2A). PTC12 dispersed
in pure water shows a weak emission at 333 nm. Upon
introducing compressed CO2, the fluorescence of PTC12
increases obviously and shows a shoulder band at 410 nm,
Supporting information for this article is available on the WWW
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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