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Self-Assembled Molecular CapsulesЧEven More Than Nano-Sized Reaction Vessels.

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Highlights
Supramolecular Chemistry
Self-Assembled Molecular Capsules—Even More Than
Nano-Sized Reaction Vessels
Arne Ltzen*
Keywords:
homogeneous catalysis · molecular capsules ·
photochemical sensitizers · self-assembly ·
supramolecular chemistry
The inclusion of molecules in defined
cavities implies the exciting possibility
of subsequent modification, since the
included molecule is greatly affected by
the specific steric conditions and arrangement of functional groups in the
cavity. In analogy to enzyme-catalyzed
reactions, such complexes can result in
accelerated reactions or even in completely new reactions. Container molecules like cryptophanes and (hemi-)carcerands can encapsulate small molecules and even facilitate the formation
of and studies on reactive compounds
such as cyclobutadiene, benzyne, cycloheptatetraene, and anti-Bredt bridgehead olefins within their interior.[1]
Although synthetic organic chemistry has had tremendous success in the
last years[2] the huge effort associated
with the fabrication of complicated
structures is a severe limitation. An
attractive alternative is the application
of synthetic strategies in which covalent
bonds are not formed, but rather reversible, self-assembly processes dictated by
“weak” intermolecular interactions such
as hydrogen bonds and metal coordination lead to complex structures.[3] This
strategy has a number of benefits: The
high degree of convergence usually
makes this synthesis easier than that of
the covalently assembled analogue (if
such a structure is indeed conceivable).
Depending on the reaction conditions,
[*] Priv.-Doz. Dr. A. Ltzen
Institute of Pure and Applied Chemistry
University of Oldenburg
P.O. Box 2503
26111 Oldenburg (Germany)
Fax: (+ 49) 441-798-3706
E-mail: arne.luetzen@uni-oldenburg.de
1000
the assembly is highly precise, because
these equilibrium-controlled processes
are self-controlling and self-repairing,
and the resulting thermodynamically
stable aggregates often show interesting
dynamic behavior. In addition, cooperative effects are often observed such
that after an initial rate-determining
nucleation, the assembly is rapid since
the formation of several bonds becomes
easier the than formation of just one.
Following this approach, capsular
aggregates could also be obtained.[4]
Figures 1–3 show some examples of
capsules that can not only stabilize
reactive species or molecules in uncommon conformations through encapsulation but also act as molecular reaction
vessels for encapsulated substrates. The
first examples were described in the late
1990s by the group of J. Rebek, Jr. with
their hydrogen-bonded assemblies[5]
(Figure 1). They demonstrated that
Diels–Alder reactions could be accelerated and even catalyzed within the socalled softball 1.[6] Several years later the
same group found that the cylindrical
capsule 2 can be used to control reactions like the formation of amides in a
very elegant manner. Small differences
in the encapsulation of single reactants
and reagents lead to a temporary compartmentalization of the species involved in this model transformation.[7c]
Similar results were obtained with
supramolecular metal complexes prepared by self-assembly.[8] Figure 2 shows
systems described by Fujita et al., which
were used as molecular reaction chambers.[8d, 9] Stoichiometric [2+2] cycloadditions could be performed in both
hexanuclear palladium(ii) complexes 3
and 4, leading to products that could not
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200462272
Figure 1. PM3-minimized structures of softball
1 and the cylindrical capsule 2 from the group
of J. Rebek, Jr. and some of the successful applications of these dimeric aggregates.[6, 7] The
long alkyl chains of the glycoluril units of 1
and of the lower rim of the resorcin[4]arenes
in 2 have been omitted.
be obtained without the capsules.[9b, c]
Furthermore, 3 could be employed as
an inverse phase-transfer reagent in
Wacker oxidations of styrene derivatives in aqueous solution.[9a] Recently,
M. Fujita and co-workers were able to
add another interesting feature to this
Angew. Chem. Int. Ed. 2005, 44, 1000 –1002
Angewandte
Chemie
Scheme 1. In the photochemically induced oxidation of adamantane encapsulated in 3, capsule
3 acts as a sensitizer. Although four molecules of adamantane are encapsulated in 3, only one
undergoes the reaction shown.
Figure 2. X-ray crystal structures of the hexanuclear
metal coordination compounds 3 and 4 synthesized by
M. Fujita et al. and some of their applications.[8d, 9]
Counterions, solvent molecules, and guest molecules
have been omitted. Complex 3 is shown with 2,2-bipyridine ligands; however, most of the studies were performed with ethylene diamine as the chelating ligand.
list of applications: they used capsule 3
as a sensitizer in photochemically induced oxidations of encapsulated adamantine molecules.[10] The assumption is
that the triazene ligand is photochemically excited in the first step. Subsequent
electron transfer from one of the four
encapsulated adamantane molecules to
the host cage leads to the formation of a
proton, an adamantyl radical, and a
deep-blue radical anion of 3 (in fact,
the former overall twelve-fold positively
charged complex is transformed into an
overall eleven-fold positively charged
radical). Finally, these reactive species
are then quenched by oxygen and water
to give regenerated capsule 3 and the
encapsulated
oxidation
products
(Scheme 1).
A common problem in the application of molecular capsules as reaction
vessels is that the products themselves
are typically good guests. Consequently,
the product inhibits further reaction,
and the host does usually not show
catalytic activity. This was also the case
Angew. Chem. Int. Ed. 2005, 44, 1000 –1002
for the C H activation mediated
by an iridium complex encapsulated by the tetranuclear supramolecular gallium compound 5
(Figure 3), which was reported
by Bergman, Raymond et al.
only a few months ago.[11] More
recently they have circumvented
this problem in an extremely elegant
manner. Again they employed the highly charged anionic assembly 5, whose
spectacular properties, for example, its
special affinity to sterically complementary ammonium ions and its “structural
memory”, were already demonstrated in
the past.[12] In this new account they
describe another important application:
aza-Cope rearrangements can be accelerated within this capsule by a factor of
almost 1000 and can even be performed
in a catalytic fashion (Scheme 2).[13] This
reaction is not necessarily one of the
most important chemical transformations, but the authors were clever to
choose it because the previously reported properties of the capsule fit this
reaction almost perfectly. The substrate,
the quaternary ammonium ion 6, is an
excellent guest for the anionic capsule.
The confining cavity of 5 forces 6 to
adopt a conformation that is barely
populated in bulk solution but very
favorable for the desired rearrangewww.angewandte.org
ment. The reversible rearrangement
gives iminium ion 7, which, like 6, is a
good guest. Subsequently the included
Figure 3. X-ray crystal structure of the tetranuclear gallium complex (D,D,D,D)-5 synthesized by Raymond et al. and some of the applications achieved with these supramolecular
assemblies.[8a, 11, 12] Counterions, solvent molecules, and the guest molecule have been omitted for clarity.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1001
Highlights
Scheme 2. Catalytic cycle of the aza-Cope rearrangement within 5. Since 5 was used in racemic
form, 8 was also obtained as a racemate, even though only one enantiomer is shown above.
product equilibrates with the free product in the bulk solution. Free 7 undergoes irreversible hydrolysis in the aqueous solution to furnish neutral aldehyde
8, which is not a good guest for 5 and
thus does not compete with 6 for
encapsulation. Alternatively, one could
also consider that 7 is hydrolyzed inside
the cavity almost instantaneously, because water is able to enter the cavity
rather easily. The resulting aldehyde 8 is
then subsequently exchanged for 6. In
both cases, the catalytic cycle is completed.
It is possible to create molecular
capsules with distinct properties that
meet the requirements of a given reaction—even though the exact mechanism
of guest exchange is not yet fully understood and the activity of the catalyst is
far less than that of enzymes. It will be
very interesting to see which reaction
will be studied next within anionic,
cationic, or neutral self-assembled molecular capsules. The topic of stereochemical control of reactions within
chiral aggregates will certainly be addressed in future work in this area.
[1] Some recent reviews: a) A. Jasat, J. C.
Sherman, Chem. Rev. 1999, 99, 931 – 967;
b) R. Warmuth, M. A. Marvel, Eur. J.
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[2]
[3]
[4]
[5]
[6]
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Weinheim, 1996; b) K. C. Nicolaou,
S. A. Snyder, Classics in Total Synthesis
II, Wiley-VCH, Weinheim, 2003.
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Chem. 1990, 102, 1347 – 1362; Angew.
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T. Jiang, M. Levett, Chem. Rev. 1995, 95,
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Angew. Chem. 1996, 108, 1242 – 1286;
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h) L. M. Greig, D. Philp, Chem. Soc.
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a) J. Rebek, Jr., Chem. Soc. Rev. 1996,
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a) J. Kang, J. Rebek, Jr., Nature 1997,
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[7] a) T. Heinz, D. M. Rudkevich, J. Rebek, Jr., Angew. Chem. 1999, 111, 1206 –
1209; Angew. Chem. Int. Ed. 1999, 38,
1136 – 1139; b) S. Krner, F. C. Tucci,
D. M. Rudkevich, T. Heinz, J. Rebek, Jr., Chem. Eur. J. 2000, 6, 187 –
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D. M. Rudkevich, J. Rebek, Jr., Nature
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e) A. Shivanyuk, J. Rebek, Jr., Angew.
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[8] Some recent reviews: a) D. L. Caulder,
K. N. Raymond, Acc. Chem. Res. 1999,
32, 975 – 982; b) S. Leininger, B. Olenyuk, P. J. Stang, Chem. Rev. 2000, 100,
853 – 908; c) G. F. Swiegers, T. J. Malefetse, Chem. Rev. 2000, 100, 3483 – 3537;
d) M. Fujita, K. Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa, K. Biradha,
Chem. Commun. 2001, 509 – 518; e) B. J.
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[9] a) H. Ito, T. Kusukawa, M. Fujita, Chem.
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[10] M. Yoshizawa, S. Miyagi, M. Kawano, K.
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Angew. Chem. Int. Ed. 2005, 44, 1000 –1002
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