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Fourfold [2]Rotaxanes of Calix[4]arenes by Ring Closure.

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Angewandte
Chemie
Rotaxanes
DOI: 10.1002/anie.200602166
Fourfold [2]Rotaxanes of Calix[4]arenes by Ring
Closure**
Olena Molokanova, Myroslav O. Vysotsky,*
Yudong Cao, Iris Thondorf, and Volker Bhmer*
The synthesis of topologically interesting molecules[1] such as
(multiple) catenanes or rotaxanes is not only an intellectual
challenge. Mechanically connected assemblies of these types,
in which the molecular substructures (interlocking rings or
wheels and axles) have a certain mobility with respect to each
other, have been frequently proposed as ?molecular
machines?.[2] Various potential applications ranging from
two-stage[3] and multi-stage ?molecular switches?[4] to nanomechanical devices[5] and information storage[6] have been
discussed. Various examples for topologically complex molecules are known also from nature.[7, 8]
Recently, we showed[9?12] that the preorganization of two
molecules into homo- and heterodimers of tetraureacalix[4]arenes 1 (Figure 1 a) can be used for the selective covalent
connection of reactive groups that are attached to them, for
example by metathesis reaction[13] of alkenyl groups.[14]
The exclusive formation of heterodimers in a 1:1 mixture
of tetra(aryl urea)- (1) and tetra(tosylurea)calix[4]arenes (2)
has long been known.[15] Metathesis between alkenyl groups
in heterodimers of 1 a or 1 b with 2 as template furnished diand tetraloop derivatives 3 and 4 in high yields.[9] For steric
reasons 3 and 4 do not form homodimers, which would
require an unfavorable overlap of the loops. Consequently
they form heterodimers with 1 (or 2), since this is the only way
to get all urea groups ?saturated? by hydrogen bonds. This
process could be used to synthesize multiple catenanes in
excellent yields by metathesis and subsequent hydrogenation.[10, 11] On the other hand, heterodimers of 4 with 1 c were
easily converted to fourfold [2]rotaxanes by introduction of
bulky stoppers by Diels?Alder cycloaddition of a tetraalkoxy
anthracene residue,[12] a reaction often dubbed ?stoppering?.
[*] O. Molokanova, Dr. M. O. Vysotsky, Dr. Y. Cao, Dr. V. B hmer
Abteilung f(r Lehramtskandidaten der Chemie
Fachbereich Chemie, Pharmazie und Geowissenschaften
Johannes Gutenberg-Universit4t Mainz
Duesbergweg 10?14, 55099 Mainz (Germany)
Fax: (+ 49) 6131-39-25419
E-mail: vysotsky@mail.uni-mainz.de
vboehmer@mail.uni-mainz.de
Priv.-Doz. Dr. I. Thondorf
Institut f(r Biochemie
Fachbereich Biochemie/Biotechnologie
Martin-Luther-Universit4t Halle-Wittenberg
Kurt-Mothes-Strasse 3, 06099 Halle (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(Bo 523/14).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 8051 ?8055
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8051
Communications
Figure 1. a) Hydrogen-bonded dimer of tetraureacalix[4]arenes as seen
from one narrow rim and its schematic representation. b) Schematic
representation of the synthesis of fourfold [2]rotaxanes by the attachment of bulky stoppers (?stoppering?) or by fourfold ring closure
(?clipping?).
As shown schematically in Figure 1 b, there should also be the
possibility to obtain such fourfold [2]rotaxanes by a ?clipping? (ring closure) procedure. We describe herein the first
examples, which we discovered serendipitously.
Usually the tetraloop compound 4 (formed after metathesis and hydrogenation of the double bonds) is detached
from the template 2 under hydrogen-bond-breaking conditions, for example, in THF as solvent, and can be isolated and
purified by column chromatography. However, for reactions
carried out with the dialkenyl compounds 1 b (m = 4, 5) this
detachment was not possible. In these cases chromatographic
purification furnished in remarkably high yields (55?88 %) a
product that was still a dimer containing a guest, as proved by
1
H NMR spectroscopy and mass spectrometry. Obviously the
expected tetraloop compounds 4 with six and eight methylene
groups (n = 2 m 2 = 6, 8) in the connecting ether chains
cannot slip from the tosylurea arms, and we obtained the
rotaxanes 5 a?d (Figure 2). A potential reason may be found
in the tetrahedral arrangement of the substituents at the
sulfur atom (N-S-C angle ca. 1098; compare Figure 3).
Although we obtained several single crystals of rotaxanes
5 a?d, their structures could not be solved. Structural information was provided by molecular dynamics (MD) simulations[16] of 5 b and 5 d. The time-averaged conformation of 5 b
Figure 2. Synthesis of fourfold [2]rotaxanes 5 by metathesis and
subsequent hydrogenation. (OY partly omitted for clarity).
8052
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 8051 ?8055
Angewandte
Chemie
(Figure 3) demonstrates that the aliphatic chains -(CH2)nconnecting the aryl residues of 4 are ?hooked? on the tosyl
groups of 2, which by their volume alone are not too big to slip
through the loops (as demonstrated by tolyl and other aryl
Figure 3. Molecular shape of rotaxane 5 b obtained by MD simulation
of an assembly of 4 (red, stick presentation) and 2 (space-filling
model; C gray, H white, S orange, O red, N blue) including two
dichloromethane guest molecules (green); top: side view; bottom:
view from the narrow rim of 4.
residues). In contrast to the capsules composed of two
molecules of 1, in 5 b and 5 d additional intermolecular
hydrogen bonds can be formed between the urea protons of 4
and the sulfonyl oxygen atoms of 2. These hydrogen bonds are
stronger as judged from the N H贩稯 separations and angles
than the usual ones involving the urea carbonyl group. The
different hydrogen-bonding pattern causes an enlargement of
the capsule by about 30?35 % (260?270 B3) as compared to
the capsules 1�and enables the inclusion of more than one
guest molecule (see below). The interaction energies of the
two calixarenes are nearly equal in both cases (5 b: DE =
227.7 4.8 kcal mol 1; 5 d: DE = 229.4 5.2 kcal mol 1),
thus indicating that the two molecules forming the fourfold
rotaxane are more or less ?unstrained?.
Interestingly, the rotaxanes 5 c,d with loops of eight
methylene groups contain ethyl acetate as a guest (as shown
by MALDI-TOF MS), although the metathesis reaction was
carried out in dichloromethane. Obviously, the initial guest
Angew. Chem. Int. Ed. 2006, 45, 8051 ?8055
was exchanged during the chromatographic work up, which
was carried out using mixtures of ethyl acetate and hexane as
eluant. The inclusion of ethyl acetate can be seen also in the
1
H NMR spectrum in [D8]THF. For 5 d it is exchanged against
the solvent in a clear first-order reaction (t1/2 = 32 min) while
the capsule itself proves (relatively) stable under these
conditions owing to the fourfold [2]rotaxane structure (see
below).[17]
For rotaxanes 5 a,b (n = 6) a guest exchange is obviously
not possible under similar conditions, and the 1H NMR
spectra in [D8]THF do not change with time.[18] Compound
5 a contains two molecules of dichloromethane per capsule, as
shown by the respective peak in the MS. This is the first
example for the inclusion of two guest molecules in the cavity
of a tetraurea dimer.[19] For 5 b an additional peak with lower
intensity corresponds to a species with one encapsulated
CH2Cl2 molecule. The presence of two rotaxane complexes is
clearly shown by the 1H NMR spectrum in C6D6. Integration
of suitable signals revealed a ratio of [5 b@2 CH2Cl2]/[5 b@
CH2Cl2] = 3:1.
As usual for dimeric capsules of tetraureacalix[4]arenes[20]
(and common for cages with aromatic walls[21]) the 1H NMR
signals for the included guest are shifted upfield because of
the shielding by the p-electron systems. In the present case we
are able to report such Dd values for the rotaxanes 5 in THF, a
solvent in which (hetero)dimeric capsules such as 4�do not
exist. For the encapsulated CH2Cl2 molecule in 5 b one singlet
is observed for the 1:1 complex (Dd = 3.23 ppm) as well as
for the 1:2 complex (Dd = 3.30 ppm). Obviously the filling
with two molecules causes a ?closer? contact to the aromatic
walls, but still both encapsulated molecules are ?freely?
rotating. For the incorporated ethyl acetate molecule in 5 d
three values are found: Dd = 4.54 (C(O)CH3), Dd = 3.47
(CH2CH3), and Dd = 2.13 ppm. This suggests that the CH2
groups mainly occupy an ?equatorial? position (oriented
towards the urea belt),[20] while the acetyl methyl groups of
the incorporated ethyl acetate molecules are mainly oriented
towards the aromatic rings/p electrons. Time-averaged, the
CH2CH3 protons assume an intermediate position.
The rotaxanes were usually isolated only after hydrogenation to avoid complications by cis/trans isomers, but it is
also worth having a look at the original reaction product of
the metathesis reaction. In the case of 5 d? (precursor of 5 d)
six singlets (of different intensity) for the C(O)CH3 group of
the incorporated ethyl acetate molecule are observed
(1H NMR in CD2Cl2), which would correspond to the six
isomers 5 d? expected by all possible combinations of cis/trans
double bonds. Although not strictly proved, this does not
seem unreasonable. For the nonhydrogenated rotaxane 5 b? (a
3:1 mixture of two complexes; see above) the major set of
signals corresponds to a C4-symmetric species, consistent with
the formation of only one isomer with four identical double
bonds (presumably in trans configuration). This would mean
that the small loop size determines the stereochemistry of the
double bonds formed.
The slippage of stopper groups in rotaxanes, and hence the
stability of a rotaxane, can be solvent-dependent.[22] Dramatic
differences in the stability of 5 a,b versus 5 c,d were found with
other hydrogen-bond-breaking solvents. Dissolving 5 a or 5 b
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8053
Communications
in [D5]pyridine or [D6]DMSO does not affect their rotaxane
structure. No change occurs in the 1H NMR spectrum of 5 b in
[D5]pyridine over three weeks at 25 8C, while 5 a loses one of
the two incorporated CH2Cl2 molecules with an estimated
half-life of 12 days, thus indicating that the capsule with
methoxy groups is slightly more flexible. For 5 c,d, on the
other hand, only two sets of signals corresponding to the
?monomeric? 4 and to 2 were observed immediately after
dissolution.[23] Although pyridine and DMSO are more
powerful hydrogen-bond acceptors than THF, an overall
change of the conformation and/or flexibility of the whole
assembly owing to different solvation may be responsible for
the rapid dissociation.
In conclusion, we have shown that fourfold [2]rotaxanes
can be prepared by fourfold ring closure (metathesis) using
the preorganization available in heterodimers of octaalkenylsubstituted tetraureacalix[4]arenes 1 b with tetra(tosylurea)calix[4]arenes 2. In contrast to (schematically) identical
fourfold [2]rotaxanes obtained by ?slipping and stoppering?[12] the novel pathway allows for shorter axles and smaller
rings (wheels). Thus, not only the whole assembly is stable in
solvents which break hydrogen bonds, but also the guest
exchange can be completely suppressed. Fourfold [2]rotaxanes 5 a,b may be regarded as mechanically interlocked
equivalents to (hemi)carcerands. Among various possible
applications, this opens up an easy way to have incorporated
guests also in polar, protic solvents where they could be
liberated on demand.
Experimental Section
5 a: Compounds 2 (0.294 g, 0.190 mmol) and 1 b (0.200 g, 0.127 mmol)
were dissolved in dichloromethane (400 mL) and stirred for 48 h (to
ensure complete formation of the heterodimer) at room temperature.
Nitrogen was bubbled through the solution for 30 min, then a solution
of GrubbsG catalyst (0.042 g, 0.051 mol) in dichloromethane (5 mL)
was added, and the nitrogen stream was continued for 30 min. After
48 h in the dark, the solution was evaporated, and the residue was
dissolved again in dichloromethane (5 mL). The catalyst was removed
by column chromatography (silica gel, ethyl acetate/hexane 2:1). The
product was dissolved in toluene (20 mL), PtO2 (0.052 g) was added,
and the suspension was stirred under hydrogen (1 atm) for 24 h. After
evaporation the residue was dissolved again in dichloromethane
(5 mL) and purified by column chromatography (ethyl acetate/
hexane 1:3) to remove the last traces of the catalysts. Yield: 0.321 g
(79 %), characterized as the complex with dichloromethane.
1
H NMR (400 MHz, [D6]benzene, 295 K): d = 11.25 (s, 4 H, NH),
8.89 (s, 4 H, NH), 8.35 (d, 4J(H,H) = 2.4 Hz, 4 H, CHcalix), 8.27 (s, 4 H,
NH), 8.15 (d, 3J(H,H) = 8.5 Hz, 8 H, CHtolyl), 7.84 (s, 4 H, NH), 7.69 (d,
4
J(H,H) = 2.4 Hz, 4 H, CHcalix), 7.60 (br t, 4 H, CH), 7.16 (d, overlap
with the solvent signal, 4 H, CHcalix), 7.07 (br t, 4 H, CH), 6.88 (d,
3
J(H,H) = 8.5 Hz, 8 H, CHtolyl), 6.28 (br t, 4 H, CH), 5.25 (d, 4J(H,H) =
2.4 Hz, 4 H, CHcalix), 4.46 (d, 2J(H,H) = 11.7 Hz, 4 H, ArCH2Ar), 4.26
(d, 2J(H,H) = 11.6 Hz, 4 H, ArCH2Ar), 4.23 (m, 4 H, OCH2), 4.00 (dt,
2
J(H,H) = 12.1 Hz, 3J(H,H) = 5.1 Hz, 4 H, OCH2 (loop)), 3.76 (d,
2
J(H,H) = 12.5 Hz, 4 H, ArCH2Ar), 3.73 (s and m, 20 H, OCH3,
OCH2), 3.44 (br m, 4 H, CH2 (loop)), 3.04 (br m, 4 H, CH2 (loop)), 2.84
(d, 2J(H,H) = 11.7 Hz, 4 H, ArCH2Ar), 2.61 (s, 4 H, encapsulated
CH2Cl2), 2.33 (br m, 4 H, CH2 (loop)), 1.96 (s and m, 20 H, ArCH3,
CH2), 1.51 (br m, 4 H, CH2 (loop)), 1.36 (quin, 3J(H,H) = 7.0 Hz, 8 H,
CH2), 1.28 (m, 8 H, CH2), 0.94 (t, 3J(H,H) = 7.0 Hz, 12 H, CH3), 0.80
(br m, 16 H, CH2 (loop)), 0.59 (br m, 4 H, CH2 (loop)), 0.13 ppm (br m,
4 H, CH2 (loop)).
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MALDI-TOF: m/z calcd: 3305.39 [M+Ag+2(CH2Cl2)]+; found:
3305.03.
Received: May 31, 2006
Revised: July 20, 2006
Published online: October 25, 2006
.
Keywords: calixarenes � hydrogen bonding � metathesis �
molecular capsules � rotaxanes
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 8051 ?8055
Angewandte
Chemie
[17] Dissociation in THF occurs with an estimated half-life of 10?
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Angew. Chem. Int. Ed. 2006, 45, 8051 ?8055
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