Chiral confined space: Induction of stereochemistry in a M 4L4 metallosupramolecular container

Article abstract of DOI:10.1055/s-2008-1067245

A triangular triscatechol ligand with enantiomerically pure terminal amide groups is prepared and used for the diastereoselective self-assembly of enantiomerically pure metallosupramolecular M₄L₄ tetrahedra. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1067245

PAPER
2963
Chiral Confined Space: Induction of Stereochemistry in a M4L4
Metallosupramolecular Container
M
L
Container arkus Albrecht,*a Simon Burk,a Patrick Weisb
4
4
a
Institut für Organische Chemie, RWTH-Aachen, Landoltweg 1, 52074 Aachen, Germany
E-mail: markus.albrecht@oc.rwth-aachen.de
b
Institut für Physikalische Chemie, Universität Karlsruhe, Fritz-Haber-Weg, 76128 Karlsruhe, Germany
Received 29 May 2008; revised 5 June 2008
Those chiral units form in the interior of the compounds a
chiral confined space. In order to use the chiral informa-
tion, this ‘space’ has to be generated in an enantioenriched
or enantiopure form. Therefore, we now present the prep-
aration of a triscatecholimine ligand, which bears chiral
amide units at the terminus of the ligand and in a diaste-
reoselective assembly affords an enantiomerically pure
tetrahedron.
Abstract: A triangular triscatechol ligand with enantiomerically
pure terminal amide groups is prepared and used for the diastereo-
selective self-assembly of enantiomerically pure metallosupramo-
lecular M4L4 tetrahedra.
Key words: tetrahedron, chirality, coordination compound, self-
assembly, catechol
The amide linkage was chosen as the connecting unit, be-
cause a hydrogen bond is formed towards the catecholate
oxygen upon complexation, which forces the chiral unit
close to the coordination site. Therefore it is effective in
the induction of the configuration at this moiety
(Figure 1).6 This principle was already used to prepare
enantiomerically pure mononuclear complexes (like
siderophore analogues6) and triple stranded helicates.7
Container molecules are of general interest due to their
application as molecular flasks for the stabilization of re-
active intermediates and for the promotion of unusual re-
actions. The self-assembly of such compounds from
simple components can be achieved by utilization of non-
covalent bonds (like hydrogen bonds) or by metal coordi-
nation.1
In order to investigate the chiral recognition properties of
container molecules, it is important to prepare them in
enantiomerically pure form. This was realized for M4L6
tetrahedra. The first example was described by Stack, who
used ligands which contain chiral groups at the linker to
stereospecifically prepare enantiomerically pure contain-
er compounds.2 Later, Raymond reported the resolution of
a container compound in the presence of N-methylnicotin-
ium cations as templates. The cation can be substituted by
tetraethylammonium to leave the enantiomerically pure
complex, which due to the mechanical coupling of the co-
ordination sites at the corners possesses an impressive
configurational stability.3 Recently, Ward used linear
ligands bearing enantiomerically pure units at the termini,
which induce the twist at the ligand and thus the chirality
of the M4L6 cage.4
In 2003, we described the formation of big chiral, but ra-
cemic M4L4 tetrahedra. These show a pronounced host-
guest chemistry. Thus, it is possible to substitute internal-
ly bound alkali metal cations by appropriate organic cat-
ionic species.5
Figure 1 a) Schematic representation of a metallosupramolecular
M4L4 tetrahedron with triangular ligands. b) Rotation of the amide
unit at catechol occurs upon metal coordination and forces the chiral
moiety close to the complex unit.
A tetrahedron itself is not chiral. However, in the coordi-
nation compounds (M4L4 as well as M4L6) the overall tet-
rahedral structure is built up from chiral building blocks,
like the octahedral complex units at the corners and heli-
cally twisted connecting moieties (e.g., phenyl groups).
In the synthetic approach, 2,3-dimethoxybenzyl alcohol
(1) is deprotonated twice (n-BuLi, TMEDA). The first
deprotonation occurs at the OH group to generate an anion
that prevents proton abstraction at the benzylic position.
The second deprotonation takes place ortho to the meth-
oxy group.8 The dianion is quenched by addition of (S)-1-
phenylethyl isocyanate (Scheme 1). The low yield of 17%
for 2 is due to severe formation of side products and de-
manding purification steps. The alcohol unit of 2 is oxi-
dized by Swern reaction to afford the aldehyde 3 (88%)
SYNTHESIS 2008, No. 18, pp 2963–2967
x
x.
x
x
.
2
0
0
8
Advanced online publication: 22.08.2008
DOI: 10.1055/s-2008-1067245; Art ID: Z13008SS
© Georg Thieme Verlag Stuttgart · New York

2964
M. Albrecht et al.
PAPER
and the methoxy groups are finally cleaved by reaction J = 7.4, 6.9 Hz) as well as the methyl protons (1.57 ppm,
with BBr3. The catechol building block 4 is obtained in d, J = 6.9 Hz) of the phenylethyl group.
78%. This is coupled to the triphenylamine backbone by
Reaction of the ligand L-H6 with titanoyl bisacetylace-
triple imine condensation with tris(4-aminophenyl)amine
tonate and alkali metal carbonates in DMF afford tetra-
in methanol. The ligand L-H6 does not precepitate from
nuclear titanium(IV) complexes M8[L4Ti4] (M = Li, Na,
this solvent and has to be purified by repeated recrystalli-
zation from dichloromethane–pentane (1:1), dichlo-
romethane–cyclohexane (1:1) and dichloromethane–
diethyl ether–cyclohexane (1:1:1). This intense purifica-
tion procedure leads to a low yield of 35% for L-H6 in the
final step.
K) in quantitative yield. The 1H NMR spectrum of the so-
dium salt in DMSO-d6 shows the amide proton which is
hydrogen bonded to a catecholate oxygen at d = 10.05 and
the signal of the imine CH=N at d = 8.88. The aromatic re-
gion is not well resolved. It reveals overlapping signals at
d = 7.4–6.8. As mentioned before, the NMR resonances
of the ethyl group of the amide side chain are informative.
They appear at d = 5.20 (CH) and 1.30 (CH3). The obser-
vation of one set of signals for this group indicates the
presence of only one stereoisomer (or at least a dominat-
ing one).
The composition of the [L4Ti4]8– tetrahedron is confirmed
by negative ESI FT-ICR MS. The sodium salt shows the
corresponding peaks at m/z = 1550.1 ([Na5L4Ti4]3–),
1543.1 ([HNa4L4Ti4]3–), 1535.8 ([H2Na3L4Ti4]3–), 1156.8
Ph
O
N
1. 2 n-BuLi,
H
TMEDA
OMe
2. (S)-PhCH(Me)NCO
OMe
17%
OMe
OMe
OH
1
2
OH
Ph
O
N
O
([Na4L4Ti4]4–),
1151.6
([HNa3L4Ti4]4–),
([HNa2L4Ti4]5–),
1146.1
H
([H2Na2L4Ti4]4–), 1140.6 ([H3NaL4Ti4]4–), 920.9
OMe
BBr3, CH2Cl2
78%
(COCl)2, DMSO, Et3N
88%
([Na3L4Ti4]5–),
916.5
912.3
([H2NaL4Ti4]5–), 907.9 ([H3L4Ti4]5–), 763.6 ([Na2L4Ti4]6–),
759.9 ([HNaL4Ti4]6–), 756.4 ([H2L4Ti4]6–). No species
with a composition of the central complex unit other than
[L4Ti4]8– are observed.
OMe
3
Ph
N
O
O
Ph
H
H
N
It has not been possible to obtain X-ray quality crystals of
M8[L4Ti4]. However, the solid state structure of the corre-
sponding complex without the chiral amide substituents is
known.5a Based on this we calculated the structure of the
octaanion [L4Ti4]8– by MMFF (molecular mechanics
force field) methods.9 The result of the modeling is shown
in Figure 2a. Figure 2b depicts one of the ‘corners’ of the
tetrahedron. The chiral amide substituents are bound ‘on
top’ of the triscatechol titanium(IV) unit, forming hydro-
gen bonds to the catecholate oxygens. From investigations
by Raymond, it is known that the (S)-phenylethylamide
induces a L-configuration at the complex units as shown
in the figure. He used mono-, di-, and tritopic ligands,
which contain the (S)-phenylethylamide as terminal group
bound to the catechol unit and prepared different com-
plexes with iron(III) or gallium(III) cations, which adopt-
ed exclusively a L-configuration. Based on the solid state
structure, Raymond could show that the preferred trans
configuration of an amide proton to a methine proton in
combination with the described hydrogen bonding pro-
motes the interaction among the terminal chiral groups.
The most favored aryl/aryl and methyl/aryl interaction
leads to the stereospecific formation of the complexes. In
the modeled structure, the amide substitutents adopt a
conformation, which is related to the one in Raymond’s
complexes.6 Therefore, we expect that the (S)-phenyleth-
ylamide induces a L-configuration at the metals which
transfer the chiral information from the outside of the tet-
rahedron to the interior. The chirality might be enhanced
by induction of the twisting of the triphenyl amine propel-
ler.
tris(4-aminophenyl)-
amine, MeOH
OH
OH
O
35%
OH
4
OH
N
HO
OH
N
O
Ph
N
N
H
N
L-H6
HO
H
HO
N
O
Ph
Scheme 1 Preparation of the enantiomerically pure triscatechol
ligand L-H6 (TMEDA = N,N,N¢,N¢-tetramethylethylenediamine).
L-H6 shows in its 1H NMR spectrum in CDCl3 the singlet
of the imine CH at d = 8.56 (3 H). The aromatic units give
rise to resonances at d = 7.35–7.12 (m, 24 H), 7.12 (d,
J = 8.8 Hz, 1 H), 7.08 (d, J = 8.5 Hz, 6 H), 6.87 (d, J = 8.5
Hz, 6 H). One OH proton is found at 10.60 ppm, while the
other is hidden under aromatic signals at 7.35–7.25 ppm.
The amide NH appears at d = 7.01 (d, J = 7.4 Hz). As spe-
cial spectroscopic probes act the methine (5.28 ppm, dq,
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M4L4 Container
2965
In order to support our stereochemical considerations, we
performed CD-measurements in methanol at room tem-
perature. The ligand L-H6 shows only a slight Cotton ef-
fect at 260 nm (probably p → p* transition) (Figure 3).
The complexes M8[L4Ti4] (M = Li, Na) lead to more pro-
nounced spectra, which are both similar to each other. The
region of LMCT bands is especially informative. Follow-
ing earlier investigations, the band around 450 nm (posi-
tive Cotton effect) might be assigned to the p → Ti(dx2-y2,
dxy) transition and the band around 400 nm (negative Cot-
ton effect) to the p → Ti(dx2-y2, dxy, dz2) transition. Ac-
cording to the results of the earlier theoretical
investigations, the signs of the observed Cotton effects
support the conclusion that the metal complex moieties
are L-configured.10
In summary, we have reported the preparation of an enan-
tiomerically pure triangular ligand, which forms the first
enantiomerically pure M4L4 tetrahedron. Due to our ratio-
nalization considering earlier results by us and others and
to CD spectroscopic investigations the most probable con-
formation at the metal complex units is L. In future stud-
ies chiral organic cations will be introduced and it will be
investigated if our chiral cavity is able to discriminate be-
tween different stereoisomers.
Figure 2 (a) Calculated model of the octaanion [L4Ti4]8– (MMFF,
Spartan O2). The structure of the central tetranuclear unit corresponds
to the one found by X-ray crystal structure analysis. The orientation
of the phenylethyl group is similar to the one observed in related mo-
nonuclear triscatcholate complexes. (b) Zoom-in of the structure,
showing the coordination site at one titanium(IV) ion.
NMR spectra were recorded on a Varian Mercury 300 or Inova 400
spectrometer. FT-IR spectra were recorded on a Bruker IFS spec-
trometer. ESI FT-ICR mass spectra were measured on a Bruker
APEX IV Fourier-transform ion cyclotron resonance (FT-ICR)
mass spectrometer with an Apollo electrospray ion source. Elemen-
tal analyses were obtained with a Heraeus CHN-O-Rapid analyzer.
Melting points: Büchi B-540 apparatus (uncorrected).
(S)-4-(Hydroxymethyl)-2,3-dimethoxy-N-(1-phenylethyl)benz-
amide (2)
2,3-Dimethoxybenzylic alcohol (1; 1.00 g, 5.92 mmol) was dis-
solved in anhyd hexane (60 mL) under N2. TMEDA (2.06 g, 17.8
mmol) and n-BuLi (1.6 n in hexane, 11.2 mL, 17.8 mmol) were
added and the mixture was stirred for 4 h at r.t. At 0 °C, (S)-1-phe-
nylethyl isocyanate (1.99 g, 11.8 mmol) was added and the mixture
was stirred for 2 h at this temperature before it was allowed to warm
to r.t. and stirred overnight. The solvent was removed and aq 2 N
HCl (1 mL) and brine (10 mL) were added. The mixture was ex-
tracted with Et2O (3 × 20 mL) and the combined organic phases
were dried (Na2SO4). The solvent was removed in vacuum and the
residue purified by column chromatography on silica gel (pentane–
EtOAc, 2:3) to obtain a yellow oil; yield: 0.320 g (17%).
IR (CHCl3): 3368, 3061, 3028, 2974, 2936, 2869, 1735, 1644, 1528,
1453, 1407, 1266, 1016, 839, 762, 702 cm–1.
1H NMR (300 MHz, CDCl3): d = 8.18 (d, J = 7.4 Hz, 1 H, NH), 7.80
(d, J = 8.1 Hz, 1 Harom), 7.35–7.20 (m, 5 Harom), 7.13 (d, J = 8.1 Hz,
1 Harom), 5.29 (quint, J = 7.0 Hz, 1 H, CH), 4.65 (s, 1 H, OH), 3.84
(s, 3 H, OCH3), 3.79 (s, 3 H, OCH3), 1.54 (d, J = 7.0 Hz, 3 H, CH3).
MS (EI): m/z = 315.2 (M+), 316.2 (M + H+).
Anal. Calcd for C18H21NO4 (315.36): C, 68.55; H, 6.71; N, 4.44.
Found: C, 68.02; H, 6.93; N, 4.96.
(S)-2,3-Dimethoxy-N-(1-phenylethyl)benzamide-4-carbalde-
hyde (3)
At –78 °C, DMSO (3.61 g, 46.1 mmol) was added to oxalyl chloride
(4.10 g, 32.3 mmol) in anhyd CH2Cl2. After 5 min, compound 2
Figure 3 Top: CD spectrum of the ligand L-H6 in methanol. Bot-
tom: CD-spectrum of Na8[L4Ti4] in methanol.
Synthesis 2008, No. 18, 2963–2967 © Thieme Stuttgart · New York

2966
M. Albrecht et al.
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(1.46 g, 4.60 mmol) in CH2Cl2 (80 mL) was added, followed by
DIPEA (5.96 g, 46.1 mmol) after another 5 min. After warming to
r.t., the mixture was filtered over silica gel and the residue was
washed with CH2Cl2 (50 mL) and pentane–Et2O (1:1, 50 mL). The
solvent was removed, the residue dissolved in H2O (100 mL) and
extracted with EtOAc (2 × 30 mL). Drying (Na2SO4) and removal
of the solvent afforded a yellow oil; yield: 1.34 g (93%).
ESI-MS (negative, MeOH): m/z = 1090.8 (M – H–), 823.9
(C50H43N6O6 ).
–
Anal. Calcd for C66H57N7O9·3.5 H2O (1155.3): C, 68.62; H, 5.58; N,
8.49. Found: C, 68.25; H, 5.32; N, 8.81.
Metal Complexes with L-H6; General Procedure
L-H6 (0.025 g, 0.023 mmol), TiOacac2 (6.0 mg, 0.023 mmol) and
the respective alkali metal carbonate (0.023 mmol) were dissolved
in DMF (40 mL) and the mixture stirred overnight at r.t. The solvent
was distilled off and the residue dried in vacuum. An orange solid
was obtained as the product. Elemental analyses show a high con-
tent of solvent in the crystal, which probably fills up the cavity of
the tetrahedron and the pores in the structure as it was observed ear-
lier for a related complex by X-ray diffraction.5a
IR (CHCl3): 3368, 2977, 2939, 1692, 1650, 1530, 1455, 1408, 1386,
1256, 1107, 1039, 1011, 910, 828, 766, 733, 701 cm–1.
1H NMR (300 MHz, CDCl3): d = 12.16 (s, 1 H, CHO), 8.12 (d,
J = 7.7 Hz, 1 H, NH), 7.78 (dd, J = 8.2, 0.7 Hz, 1 Harom), 7.51 (dd,
J = 8.2 Hz, 1 Harom), 7.34–7.14 (m, 5 Harom), 5.25 (dq, J = 7.7, 6.9
Hz, 1 H, CH), 4.65 (s, 2 H, CH2), 3.91 (s, 3H, OCH3), 3.80 (s, 3 H,
OCH3), 1.52 (d, J = 6.9 Hz, 3 H, CH3).
MS (EI): m/z = 313.0 (M+), 314.0 (M + H+).
Li4[L6Ti4]
Yield: 0.038 g (quant).
Anal. Calcd for C18H19NO4·H2O (331.4): C, 65.24; H, 6.39; N, 4.23.
Found: C, 65.79; H, 6.62; N, 4.70.
IR (KBr): 3797, 3679, 3419, 2966, 2929, 2873, 2374, 2345, 1658,
1499, 1430, 1384, 1321, 1214, 1102, 1026, 831, 776, 686, 540, 501
cm–1.
(S)-2,3-Dihydroxy-N-(1-phenylethyl)benzamide-4-carbalde-
hyde (4)
1H NMR (400 MHz, DMSO-d6): d = 10.2–10.0 (m, 12 H, 12 × NH),
8.82 (s, 12 H, 12 × N=CH), 7.4–6.8 (m, 132 Harom), 5.2–5.0 (m, 12
H, 12 × CH), 1.5–1.2 (m, 3J = 7.4 Hz, 36 H, 12 × CH3).
BBr3 (2 mL, 5.12 g, 20.4 mmol) was slowly added to the protected
compound 3 (0.80 g, 2.6 mmol) in CH2Cl2 (50 mL) at –78 °C. After
12 h, MeOH (5 mL) was added and the solution was concentrated
in vacuum. The oily residue was dissolved in EtOAc (50 mL) and
washed with acidified H2O (pH 4–5, 30 mL). The solvent was re-
moved and the residue dissolved in CH2Cl2 (50 mL). On addition of
pentane, the product precipitated as an orange oil; yield: 0.61 g
(83%).
ESI-MS (negative): m/z = 1140.9 (M – 4 Li4–), 911.3 (M – 5 Li5–),
758.4 (M – 6 Li6–).
Anal. Calcd for C264H204Li8N28O36Ti4·42 H2O·17 DMF (6590.85):
C, 57.40; H, 6.22; N, 9.56. Found: C, 57.44; H, 6.24; N, 9.36.
IR (CHCl3): 3372, 3022, 2977, 2932, 1657, 1610, 1539, 1447, 1390,
1292, 1217, 1104, 833, 757, 700, 666 cm–1.
Na4[L6Ti4]
Yield: 0.041 g (quant).
1H NMR (400 MHz, CDCl3): d = 11.41 (s, 1 H, OH), 10.68 (s, 1 H,
OH), 9.88 (s, 1 H, CHO), 7.31–7.23 (m, 5 Harom), 7.10 (d, J = 8.4
Hz, 1 H, NH), 7.02 (d, J = 8.4 Hz, 1 Harom), 6.93 (m, 1 Harom), 5.25
(dq, J = 7.3, 6.9 Hz, 1 H, CH), 1.56 (d, J = 6.9 Hz, 3 H, CH3).
IR (KBr): 3430, 3269, 3056, 2968, 2927, 2864, 2345, 1664, 1543,
1500, 1427, 1383, 1321, 1288, 1205, 1096, 1034, 830, 781, 699,
666, 500 cm–1.
1H NMR (400 MHz, DMSO-d6): d = 10.1–10.0 (m, 12 H, 12 × NH),
8.88 (s, 12 H, 12 × N=CH), 7.4–6.8 (m, 132 Harom), 5.3–5.1 (m, 12
H, 12 × CH), 1.4–1.2 (m, 3J = 7.4 Hz, 36 H, 12 × CH3).
ESI-MS (positive, MeOH): m/z = 286.3 (MH+), 331.9
(C18H21NO5H+ dimethoxyacetal of 4).
Anal. Calcd for C16H15NO4·2/3 H2O (297.30): C, 64.64; H, 5.54; N,
4.71. Found: C, 64.17; H, 5.03; N, 4.11.
ESI-MS (negative): m/z = 1550.1 (M – 3 Na3–), 1543.1 (M – 4 Na +
H3–), 1535.8 (M – 5 Na + 2 H3–), 1156.8 (M – 4 Na4–), 1151.6 (M –
5 Na + H4–), 1146.1 (M – 6 Na + 2 H4–), 1140.6 (M – 7 Na + 3 H4–
), 920.9 (M – 5 Na5–), 916.5 (M – 6 Na + H5–), 912.3 (M – 7 Na + 2
H5–), 907.9 (M – 8 Na + 3 H5–), 763.6 (M – 6 Na6–), 759.9 (M – 7
Na + H6–), 756.4 (M – 8 Na + 2 H6–).
4,4¢,4¢¢-(1E,1¢E,1¢¢E)-[4,4¢,4¢¢-Nitrilotris(4,1-phenylene)tris(azan-
1-yl-1-ylidene)]tris(methan-1-yl-1-ylidene)tris(2,3-dihydroxy-N-
[(S)-1-phenylethyl]benzamide) (L-H6)
N1,N1-Bis(4-aminophenyl)phenylene-1,4-diamine (0.058 g, 0.2
mmol) and 4 (0.22 g, 0.8 mmol) were dissolved in MeOH (20 mL)
and stirred overnight at r.t. The solvent was removed and the residue
dissolved in CH2Cl2 (40 mL), and filtered. The product obtained af-
ter evaporation of the solvent was repeatedly recrystallized from
CH2Cl2–pentane (1:1), CH2Cl2–cyclohexane (1:1), and CH2Cl2–
Et2O–cyclohexane (1:1:1). The product was obtained as a red solid;
yield: 0.081 g (37%).
Anal. Calcd for C264H204N28Na8O36Ti4·36 H2O·25 DMF (7194.89):
C, 56.58; H, 6.32; N, 10.32. Found: C, 56.65; H, 6.37; N, 10.21.
K4[L6Ti4]
Yield: 0.039 g (quant).
IR (KBr): 3273, 1642, 1535, 1504, 1462, 1322, 1286, 1241, 1207,
971, 832, 783, 700, 673, 539 cm–1.
1H NMR (400 MHz, DMSO-d6): d = 9.9–9.7 (m, 12 H, 12 × NH),
8.92 (s, 12 H, 12 × N=CH), 7.6–7.0 (m, 132 Harom), 5.3–5.1 (m, 12
H, 12 × CH), 1.4–1.2 (m, 3J = 7.4 Hz, 36 H, 12 × CH3).
ESI-MS (negative): m/z = 1552.7 (M – 6 K + Na + 2 H3–), 1539.8
(M – 8 K + 3 Na + 2 Li3–), 1527.4 (M – 8 K + 2 Na + 3 H3–), 1513.4
(M – 8 K + 5 H3–), 1154.1 (M – 6 K + 2 H4–), 1144.6 (M – 7 K + 3
H4–), 1138.8 (M – 8 K + 4 H + H2O4–), 1135.1 (M – 8 K + 4 H4–),
907.9 (M – 8 K + 3 H5–).
IR (KBr): 3389, 1613, 1535, 1502, 1445, 1293, 1206, 832, 700, 539
cm–1.
1H NMR (400 MHz, CDCl3): d = 10.60 (s, 3 H, 3 × OH), 8.56 (s, 3
H, 3 × N=CH), 7.35–7.12 (m, 24 H, 3 × OH + 21 Harom), 7.08 (d,
J = 8.5 Hz, 6 Harom), 7.01 (d, J = 7.4 Hz, 3 H, 3 × NH), 6.87 (d,
J = 8.5 Hz, 6 Harom), 5.28 (dq, J = 7.4, 6.9 Hz, 3 H, 3 × CH), 1.57 (d,
J = 6.9 Hz, 9 H, 3 × CH3).
ESI-MS (positive, MeOH): m/z = 1092.3 (M + H+), 825.4
(C50H44N6O6H+), 558.4 (C34H31N5O3H+).
Anal. Calcd for C264H204K8N28O36Ti4·52 H2O·13 DMF (6735.9): C,
54.03; H, 5.97; N, 8.53. Found: C, 54.06; H, 5.98; N, 8.41.
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M4L4 Container
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(3) (a) Terpin, A. J.; Ziegler, M.; Johnson, D. W.; Raymond, K.
N. Angew. Chem. Int. Ed. 2004, 40, 157; Angew. Chem.
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15354.
Acknowledgment
This work was supported by the Deutsche Forschungsgemeinschaft
(SPP 1118) and the Fonds der Chemischen Industrie. We thank
Prof. Dr. Gerhard Raabe for the measurement of the CD spectra and
Professor Dr. M. Kappes as well as the Nanotechnology Institute,
Forschungszentrum Karlsruhe for facilitating the ESI-MS measure-
ments.
(4) Argent, S. P.; Riis-Johannessen, T.; Jeffery, J. C.; Harding,
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(5) (a) Albrecht, M.; Janser, I.; Meyer, S.; Weis, P.; Fröhlich, R.
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