Self-assembly and chiroptical properties of chiral dendrimers consisting of a Hamilton receptor substituted porphyrin and depsipeptide cyanurates

Article abstract of DOI:10.1002/ejoc.200500951

The synthesis and characterisation of a new type of porphyrin-based tetrakis(Hamilton receptor) 1 is presented and the complexation of 1 with the chiral depsipeptide dendrons 7-12, with cyanuric acid functionalities as their focal points, is reported. The

Full text of DOI:10.1002/ejoc.200500951

FULL PAPER
DOI: 10.1002/ejoc.200500951
Self-Assembly and Chiroptical Properties of Chiral Dendrimers Consisting of a
Hamilton Receptor Substituted Porphyrin and Depsipeptide Cyanurates
Katja Maurer,[a] Kristine Hager,[a] and Andreas Hirsch*[a]
Keywords: Dendrimers / Self-assembly / Chirality / Depsipeptides / Porphyrins
The synthesis and characterisation of a new type of porphy- the porphyrin region are inversely proportional to the size of
rin-based tetrakis(Hamilton receptor) 1 is presented and the
complexation of 1 with the chiral depsipeptide dendrons 7–
12, with cyanuric acid functionalities as their focal points, is
reported. The resulting first- to third-generation chiral supra-
molecular dendrimers 13–18 were characterised by NMR,
UV/Vis and CD spectroscopy. Chirality transfer from the de-
psipeptide dendrons to the porphyrin core was demonstrated
by CD spectroscopy in the case of the second- and third-gen-
eration complexes 15–18, whereas no chirality transfer and
hence no diastereoselective formation of a chiral superstruc-
ture could be determined in the case of the first-generation
systems. The intensities of the complexes’ CD absorptions in
the dendrons, pointing to a size-dependent cooperativity of
the fourfold complexation of the dendrons with the more ef-
fective binding of the second-generation dendrons 9 and 10
relative to their bulkier third-generation counterparts 11 and
12. Pronounced cooperativity during the formation of the sec-
ond-generation 1:L4 complexes (L = 9, 10) is considered to be
the reason for the diastereoselective formation of a preferred
chiral conformation of the Hamilton receptor 1 in the com-
plexes 15 and 16.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
ently linked porphyrin–dendrimer hybrids have been syn-
thesised and studied as synthetic models for globular heme
proteins.[10] Experiments have established that the electro-
chemical properties of the porphyrin core are considerably
influenced by the size and type of the dendritic environ-
ment, and complexes of the dendritically functionalised FeII
species of these systems, containing 1,2-dimethylimidazole
(DiMeIm) as an axially coordinated ligand, have been in-
vestigated as model systems for T(tense)-state haemoglobin
and myoglobin.[10c] The (porphyrin)FeII derivatives, con-
taining secondary amide groups in their dendritic struc-
tures, showed stable and reversible complexation with O2
and CO. The same dendrimer derivatives containing (por-
phyrin)FeIII complexes as active site and imidazole as axial
ligand were investigated as heme oxygenase model com-
pounds.[10d] The catalytic activity of these systems, with re-
spect to epoxidation of alkenes and oxidation of sulfides,
was found to be independent of the size and character of
the peptidic shell. Because of the instability of the iron core
towards self-oxidation, these systems were found to be
rather unsuitable as catalysts. (Porphyrin)zinc cores con-
nected to different Fréchet-type dendrons have been inten-
sively studied with respect to their fluorescence proper-
ties.[11] Their derivatisation with up to eight boron-dipyrrin
pigments has been used to mimic natural photosynthetic
antenna complexes.[12] Recently, we reported on a new class
of chiral depsipeptide dendrons based on tartaric acid as
a branching juncture and ω-aminocapronic acid as spacer
units.[13–16] Investigations of the chiroptical properties of
Introduction
The rather new area of supramolecular chirality[1] repre-
sents a combination of molecular chirality and supramolec-
ular chemistry.[2] The self-assembly of chiral molecules[3] or
assembly between chiral and achiral molecules through
noncovalent interactions can result in the formation of chi-
ral superstructures.[4] As a consequence, transfer of chiral
information may not be restricted only to the induction of
chirality in the achiral building blocks, but can also cause
amplification of chirality in the whole supramolecular sys-
tem.[5] Chirality at the supramolecular level, termed supra-
molecular chirogenesis,[6] is abundant in biology and plays
an essential role in many natural systems. A large number
of natural supramolecular structures are stabilised by in-
terstrand hydrogen bonds: the DNA double helix[7] and its
handedness, for example, are influenced by the configura-
tions of chiral centres in the nucleotide backbone. Chiral
dendrimers are considered to be appealing candidates for
the investigation of chirality at the macromolecular level,[8]
and amino acids and peptides have frequently been used as
building blocks for the construction of chiral dendrimers.[9]
Because of the biological importance of porphyrins, coval-
[a] Institut für Organische Chemie, Friedrich-Alexander-Uni-
versität Erlangen-Nürnberg,
Henkestrasse 42, 91054 Erlangen, Germany
Fax: +49-9131-8522537
E-mail: andreas.hirsch@chemie.uni-erlangen.de
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Self-Assembly and Chiroptical Properties of Chiral Dendrimers
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depsipeptide dendrons in different solvents showed that for-
mation of chiral secondary structures in nonprotic solvents
such as CH3CN is possible.[14] In this context we also re-
ported the synthesis, characterisation and chiroptical prop-
erties of chiral RuII-coordinated dendrimers.[15] Each chiral
depsipeptide dendrimer was associated with a 2,2Ј-bipyri-
dine core. Threefold coordination of these bipyridine li-
gands with RuII resulted in the formation of octahedral ∆-
and Λ-configured diastereomers. Chiral depsipeptide den-
drimers containing ethylenediaminetetraacetic acid (EDTA)
ester derived cores have also been synthesized,[16] and chir-
optical investigations on ZnII and CuII complexes revealed
metal-induced diastereoselective chiral folding of these sys-
tems. We have recently been developing the self-assembly of
supramolecular dendrimers based upon the Hamilton re-
ceptor binding motif.[17] The biological importance and re-
markable photoelectronic properties of porphyrins[18] have
stimulated interest in the design and synthesis of dynamic
supramolecular systems. Porphyrins functionalised with the
Hamilton receptor coordination motif have been used as
model compounds for the study of enzymatic molecular re-
cognition[19] and for the construction of a noncovalent pho-
toactive system.[20] We now report on the synthesis and chi-
roptical properties of a new porphyrin building block, con-
taining four covalently bound Hamilton-type barbiturate or
cyanurate receptors, and its complexation with chiral depsi-
peptide dendrons possessing cyanuric acid functionalities.
Results and Discussion
Synthesis of the Hamilton Receptor Substituted Porphyrin 1
The target porphyrin 1 was synthesised as shown in
Scheme 1. Treatment of the diacid 2[21] with thionyl chloride
(SOCl2) at reflux, followed by in situ coupling of the dichlo-
ride with the aminopyridine derivative 3,[22] afforded the
benzyl-protected Hamilton receptor 4 in 33% isolated yield.
Deprotection of 4 with potassium hydroxide resulted in the
formation of compound 5, which was isolated in 61% yield.
The solubility of the deprotected receptor 5 was limited to
a few polar aprotic solvents such as tetrahydrofuran (THF),
dimethyl sulfoxide (DMSO), dimethylformamide (DMF)
and dioxane. The porphyrin tetraester 1 was obtained by
fourfold esterification of the commercially available porphy-
rin-tetraacid 6 with 5 under NЈ-(3-dimethylaminopropyl)-
N-ethylcarbodiimide (EDC) coupling conditions, the reac-
tion being performed in dry DMF with 4-(dimethylamino)-
Scheme 1. Synthesis of the Hamilton receptor substituted porphy-
rin 1: a) SOCl2, DMF, THF, room temperature, 16 h, 34%; b)
KOH, dioxane, room temperature, 48 h, 61%; c) EDC, DMAP,
DMF, room temperature, 7 d, 5%.
pyridine (DMAP) as a catalyst and the mixture stirred at out to be very difficult: even after repeated GPC column
room temp. for 7 d. chromatography (SX-3000; DMF) the separation of the
Because of its insolubility in most common organic sol- DCU by-product was not successful. It should be noted
vents except for DMF, THF and DMSO, together with its here that the formation of all possible porphyrin side prod-
high polarity, the purification of 1 was achieved by repeated ucts was observed by FAB-MS when either DCC or EDC
column chromatography with different solvents [silica gel were used in the esterification reaction. The target molecule
1
(SiO2); THF, THF/dichloromethane (CH2Cl2), 80:20] fol- 1 was characterized by H and 13C NMR, UV/Vis and IR
lowed by preparative HPLC (Nucleosil; THF/CH2Cl2, spectroscopy and its expected molecular weight was con-
80:20). The use of N,NЈ-dicyclohexylcarbodiimide (DCC) firmed by FAB mass spectrometry. The assignment of the
as coupling reagent is not recommended because the re- 1H NMR signals of the Hamilton receptor protons was car-
moval of the dicyclohexylurea by-product (DCU) turned ried out by analysis of the HETCOR and COSY spectra
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(Figure 1). In the range between δ = 0 and 3 ppm the while the signals of the aromatic protons of the porphyrin
COSY spectrum (Figure 1a) reveals the characteristic cross and the Hamilton receptor unit can be observed between δ
coupling pattern of the alkyl chain protons, which appear = 7.8 and 9.3 ppm. The signal of the aromatic protons 17-
as a well-resolved multiplet and two triplet signals. The H appears at δ = 7.92 ppm as a triplet, while the resonances
spectrum shows the expected coupling pattern of the aro- of 16-H and 18-H are covered by the signal of the solvent
matic protons of the pyridine fragment (16-H to 18-H) at residual peak at δ = 8.0 ppm. At δ = 8.5 ppm the spectrum
δ Ϸ 8 ppm. The HETCOR spectrum (Figure 1b) allowed displays a sharp singlet, which can be assigned to the ortho-
assignment of the corresponding C atoms. The resonances protons 20-H of the isophthalic unit. In the range between
of the porphyrin H atoms (21-H to 23-H) are located in δ = 8.6 and 8.7 ppm the spectrum shows the two character-
the downfield region between δ = 8.6 and 9.3 ppm. The istic doublets of the aromatic protons 21-H and 22-H, fol-
assignment of the C atoms was achieved by analysis of the lowed by the singlet of the para-protons 19-H of the iso-
correlation signals of the two characteristic doublets corre- phthalic unit. The β-pyrrolic protons 23-H resonate as a
sponding to 21-H and 22-H. The signals of the alkyl pro- broad singlet at δ = 9.1 ppm. In the region between δ =
tons are located in the range between δ = 0.9 and 1.8 ppm, 10.3 and 11.0 ppm the resonances of the NHa and NHb
protons can be observed as broad singlets, which is a char-
acteristic of Hamilton receptor derivatives. The pyrrolic
NHc protons of 1 resonate at δ Ϸ –3 ppm.
Formation of the Supramolecular Depsipeptide Dendrimers
13–18
The assembly of the supramolecular depsipeptide dendri-
mers 13–18 was achieved by a fourfold complexation of 1
with depsipetide cyanurates 7–12.[23] Each cyanurate 7–12
is bound to a complementary Hamilton receptor unit
through six hydrogen bonds.
Effective binding of 1 with 7–12 can only be achieved in
nonpolar aprotic solvents such as chloroform (CHCl3) and
CH2Cl2 (Scheme 2). However, 1 is insoluble in these sol-
vents but it successively dissolves upon addition of 7–12,
forming the complexes 13–18. The stepwise solubilisation
can be monitored by the deepening of the colour of the
solution as shown in Figure S1 (see Supporting Infor-
mation).
The formation of 13–18 was demonstrated by UV/Vis,
1
CD and NMR spectroscopy. Figure 2 shows the H NMR
spectra of the porphyrin derivative 1, compound 7 and its
corresponding complex 13 in the diagnostic δ = 4–11 ppm
region. The alkyl chain protons 2-H resonate as a multiplet
at δ = 4 ppm followed by five characteristic doublet signals
of the benzyl protons 3-H and 9-H. The resonances of the
protons 7-H and 8-H, which are located at the chiral centre
of 7, are two doublets at δ = 6 ppm. In the δ = 7–8 ppm
region the spectrum shows the resonances of the aromatic
protons, and at δ = 9 ppm the signal of the amide protons
1-H appears as a sharp singlet. After complexation with 1
the characteristic resonances and splitting patterns of the
dendrimer protons barely change, with the exception of the
signals of 1-H and 2-H. The alkyl protons 2-H in the com-
plex 13 appear as a broad and slightly downfield-shifted
(compared to the free dendron) multiplet. In addition to
1
the resonances of the dendron protons 2-H to 15-H the H
NMR spectrum of the complex 13 shows a set of new sig-
nals, due to the Hamilton receptor substituted porphyrin 1.
In the downfield region the resonances of the NHa and
NHb protons are observed as broad singlets at δ = 9.51
Figure 1. a) COSY and b) HETCOR NMR spectra of compound
1 (400 MHz, room temperature, [D7]DMF).
and 10.13 ppm. These chemical shifts are characteristic of
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Scheme 2. General procedure for the synthesis of 13–18: a) CHCl3, room temperature, 16 h.
a Hamilton receptor containing a hydrogen-bonded cyan- protons NHa and NHb. As a consequence, NMR titration
urate,[23] which clearly confirms the successful complex- experiments allow determination of the association con-
ation. The signal patterns and the chemical shifts of the stants. This is only possible, however, if all components are
protons 16-H to 23-H within the complexes 13–18 are com- soluble in appropriate solvents such as chloroform or
parable to those in the free receptor 1. In Figure 2c and in dichloromethane, so because of the insolubility of 1 in these
1
all H NMR spectra of the compounds 13–18, the signals solvents it was not possible to determine the association
of the amide protons 1-H of the dendrons are missing, constants and binding cooperativitities of the 1:4 complexes
which is the result of the dynamic character of the associa- 13–18 by NMR spectroscopy. On the other hand, NMR
tion of the complexes. As shown by temperature-dependent titration studies on 1:3 complexes of the homotritopic,
1H NMR experiments on complexes of 19 with 7–12, the chloroform-soluble Hamilton receptor 19 with the chiral
temperature range between 0–50 °C represents a coalesc- depsipeptide dendrons 7–12 have been carried out success-
ence regime.[23] This results in very broad and thus unde- fully and can serve as a suitable model case for the complex-
tectable resonances of the amide 1-H protons of the bound ation of 1 with the same dendrons.[23]
and free depsipeptide cyanurates 7–12 at room tempera-
The clearly separated electronic absorptions of the por-
ture.[23] The pyrrolic NHc protons of the complexes typi- phyrin receptor 1 are well suited for the determination of
cally appear at δ Ϸ –3 ppm as a broad singlet.
chirality transfer, caused by the complexation of the chiral
It has been reported before,[23,24] and has also been dem- depsipeptide dendrons 7–12, by CD spectroscopy. The UV/
onstrated in this study, that complexation through hydrogen Vis spectrum of 1 (Figure 3a and d) is characterised by its
bonding in similar systems results in pronounced downfield two most intensive absorptions at λmax = 300 and 420 nm,
shifts of the resonances of the Hamilton receptor amide due to the bis(butyrylamino)pyridyl moieties (Hamilton re-
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Soret band is much broader and has a lower intensity than
in THF (Figure 3d). In THF 1 is very soluble and no inter-
molecular hydrogen bonding takes place. The correspond-
ing λmax values, however, are independent of the solvent.
The absorption spectra of the free dendrons 7–12 are char-
acterised by the transitions of the aromatic units at 270 nm
only (Figure 3b). The electronic absorption spectra of the
complexes 13–18 each represent a superposition of those of
the constituting components (Figure 3c) and are dominated
by the absorptions of the porphyrin receptor 1. Unlike in
the case of the spectrum of the receptor 1 in chloroform, the
Soret band is now the most intensive one: a consequence of
the formation of the complexes 13–18, accompanied by the
disaggregation of 1 and the breaking of the intermolecular
hydrogen bonds.
The CD spectra of the dendrons 7–12 are shown in Fig-
ure 4. Compounds 8, 10 and 12 each display a positive Cot-
ton effect in the area below 280 nm, while their correspond-
ing enantiomers 7, 9 and 11 each show perfect mirror image
behaviour in the same region (Figure 4).[23] The intensities
of the CD absorptions rise with the increasing generation
number of the depsipeptide dendrons.
For the chiroptical investigation of the complexes 13–18,
1 equiv. of the porphyrin receptor 1 was mixed with 4 equiv.
of the corresponding depsipeptide-dendron 7–12 in HPLC-
grade chloroform. Because of the insolubility of parent 1,
all solutions were stirred overnight in order to guarantee
quantitative formation of the soluble complexes 13–18. The
first-generation complexes 13–14 showed no detectable Cot-
ton effects in the region of the Hamilton receptor and por-
phyrin absorptions. This is due to very weak chirality trans-
fer, attributed only to the two stereogenic centres in each
depsipeptide dendron 7–8. Even more importantly, the dia-
stereoselective formation of a chiral superstructure can be
ruled out. However, the CD spectra of the complexes 15–
18, involving the second- and third-generation dendrons 9–
12, revealed pronounced chirality transfer: two absorptions
in the regions of the Hamilton receptor and the Soret band
of the porphyrin at λmax = 302 and 420 nm, respectively,
were observed (Figures 5 and 6). Similarly to the absorption
of the free dendrons (Figure 4) the CD spectra of 15–18
also display the Cotton effects of the chiral depsipeptide-
dendrons 9–12 at λmax = 260 nm. The spectra of the com-
plexes of 15 and 17 show positive Cotton effects at 300 nm,
together with negative Cotton effects in the Soret band re-
gion at 420 nm. On the other hand, complexes 16 and 18,
incorporating the corresponding enantiomeric dendrons of
generation two and three, gave rise to CD spectra with op-
posite Cotton effects.
Figure 2. 1H NMR spectra (δ = 3.6–11.0 ppm region, 400 MHz,
room temperature): a) porphyrin derivative 1 ([D7]DMF); b) depsi-
peptide dendron 7 (CDCl3); c) its corresponding complex 13
(CDCl3).
Significantly, the intensities of the CD absorptions in the
regime of 1 at λmax = 302 and 420 nm do not correlate with
the generation number: the corresponding intensities of the
second-generation systems 15 and 16 (Figure 5) are higher
ceptor) and the Soret band of the porphyrin core, respec- than those of the third-generation complexes 17 and 18
tively. The relative intensities and the widths of these ab- (Figure 6). This is especially true for the absorption at λmax
sorption bands strongly depend on the nature of the sol- = 302 nm, caused by the chirality transfer to the Hamilton
vent: because of pronounced intermolecular hydrogen receptor unit of 1. On the other hand, the optical activity
bonding in chloroform, in which 1 is almost insoluble, the in the dendron region at λmax = 260 nm remains unaffected
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Figure 3. UV/Vis spectra: a) compound 1 in CHCl3 (qualitative); b) compound 9 in CHCl3; c) complex 13 in CHCl3; d) compound 1 in
THF.
Figure 4. Circular dichroism (CD) spectra of the chiral depsipep-
tide dendrons 7–12 in CHCl3 at 25 °C.
Figure 5. Circular dichroism (CD) spectra of the receptor 1 with
4–8 equiv. of the second-generation dendrons 9 and 10 in CHCl3
at 25 °C.
and increases with the number of stereogenic centres in the
same way as for the free dendrons 9–12 (Figure 5). The in- third-generation dendrons 9/10 and 11/12, respectively. As
tensities of the CD absorptions at λmax = 302 and 420 nm demonstrated for the complexation of these dendrons with
barely increase when excesses of the second- and third-gen- the receptor 19, containing three instead of four binding
eration dendrons 15–18 are used for the complexation with sites, the association constants and the positive cooperativ-
1 (Figures 5 and 6). A possible explanation for this behav- ity for threefold complexation are much more pronounced
iour is the different steric requirements of the second- and for the second-generation dendrons 9 and 10 than for the
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As the enantiomerically pure dendrons 9 and 10 with
(all-R) and (all-S) configurations, respectively, were utilised,
the formation of two diastereoisomers such as Λ-1:(all-R)-
104 and ∆-1:(all-R)-104 is to be expected. One of these dia-
stereomers will have a lower energy than the other and will
be formed preferably. The presence of an excess of one dia-
stereoisomer involving a chiral supramolecular motif will
cause an increased intensity of the CD absorptions, which
is actually observed in the complexes 15 and 16 in relation
to the third-generation analogues 17 and 18. Obviously, this
phenomenon is much less pronounced for the complexation
of the third-generation dendrons because the corresponding
1:L4 complexes are less favoured and the diastereoselective
formation of distinct chiral superstructures is less preferred.
The scenario suggested here for the complexation of the
second-generation dendrons 9 and 10 represents a case of
supramolecular chirogenesis and has a variety of pre-
cedents: examples include chirality induction in achiral bis-
[(octaethylporphyrin)zinc] (ZnD) by complexation of enan-
tiomerically pure amino acids and other chiral amines and
alcohols,[26] whilst related behaviour was also observed with
a free-base porphyrin, covalently bound to eight peripheral
(porphyrin)zinc units through enantiomerically pure nucle-
oside linkers[27] and a self-assembling system consisting of
(porphyrin)zinc-appended foldamers and a chiral C60 ad-
duct incorporating histidine moieties.[28] In the former case
the optical activity of the nonaporphyrin system was inter-
preted in terms of the diastereoselective preference for a
helical tetraphenylporphyrin conformation. A Cotton effect
was observed in the Soret band region, resembling those of
the complexes 15–18 (Figure 3), but much more pro-
nounced. In the case of the supramolecular structure re-
ported in ref.[28] the Cotton effect in the Soret band region
is much less pronounced than those in 15–18. Also in this
case the preference for a specific helical conformation of
the entire supramolecular construct involving phenylpor-
phyrine building blocks was suggested.
Figure 6. Circular dichroism (CD) spectra of the receptor 1 with
4–8 equiv. of the third-generation dendrons 11 and 12 in CHCl3 at
25 °C.
third-generation dendrons 11 and 12,[23] due to the fact that
the third-generation systems are too bulky to allow an effec-
tive binding of the third dendron. As a consequence, only
50% of the core 19 is involved in a 19:L3 complex (L = 11,
12) at a stoichiometric 1:3 ratio of 19 and L. Under the
same conditions, on the other hand, about 90% of 19 is
bound as 19:L3 complex when the second-generation den-
drons 9 and 10 are used. Although the benzene core of 19
is significantly smaller than the porphyrin core of 1, the
required fourfold binding may also cause overcrowding
when the third-generation dendrons are allowed to react
with 1 and the fraction of 1:L4 (L = 11, 12) in a 1:4 mixture
of the components can be considerably lower than 100%.
If, on the other hand, the fourfold binding shows strong
cooperativity, which is very likely to be the case in the com-
plexation of the second-generation dendrons 9 and 10, the
1:L4 species can be considered predominant. It can be as-
sumed that the most stable conformation of such a 1:L4
complex should have a chiral propeller-like shape with two
enantiomeric left-handed (Λ) or right-handed (∆) confor-
mations (Figure 7). X-ray crystal structures of a series of
meso-aryl-substituted porphyrins showed that the aryl rings
are in most cases not oriented perpendicularly to the por-
phyrin plane but exhibit typical angles in the range between
65 and 80°.[25] For this reason, the tetraphenylporphyrin
moiety adopts chiral C4-symmetrical conformations. This
clearly demonstrates that such propeller-like conformations
are preferred, which is also confirmed by quantum mechan-
ical calculations (Figure 7).
Conclusions
In this work we present the synthesis of a new type of
porphyrin-based tetrakis(Hamilton receptor) capable of
forming chiral supramolecular dendrimers 13–18 with de-
psipeptide dendrons 7–12 incorporating cyanuric acid func-
tionalities as their focal points. Chirality transfer from the
depsipeptide dendrons to the porphyrin core was demon-
strated by CD spectroscopy in the cases of the second- and
third-generation complexes 15–18. The intensities of the
CD absorptions in the porphyrin region of the complexes
are inversely proportional to the size of the dendrons, which
points to a size-dependent cooperativity of the fourfold
complexation of the dendrons, with more effective binding
of the second-generation dendrons 9 and 10 than of their
bulkier third-generation counterparts 11 and 12. Pro-
nounced cooperativity during the formation of the second-
generation 1:L4 complexes (L = 9, 10) is considered to be
the reason for the diastereoselective formation of a pre-
Figure 7. PM3-calculated[29] model of the C4-symmetric propeller-
shaped tetraphenylporphyrin with ∆ and Λ configurations.
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3
(s, 4 H, Bz, Bn), 8.00 (d, JH,H = 7.3 Hz, 2 H, Py), 8.10 (s, 1 H,
Bn), 8.55 (br., 2 H, CONH), 8.66 (br., 2 H, CONH) ppm. 13C
NMR (100.5 MHz, CDCl3, 25 °C): δ = 13.7 (CH3), 18.7, 39.3
(CH2), 109.7, 110.3 (Py), 122.9, 125.0 (Bn), 128.1, 128.7, 130.3,
134.3 (Bz), 135.9 (Bn), 140.7, 148.9, 150.0 (Py), 151.3 (Bn), 163.7,
ferred chiral conformation of the Hamilton receptor 1, such
as a propeller-like Λ or ∆ conformation of the TPP unit in
the complexes 15 and 16. Such a scenario corresponds to a
case of supramolecular chirogenesis.
165.2, 172.1, (C=O) ppm. React-IR (thin film): ν
= 3243, 2954,
˜
max
2937, 1743, 1689, 1586, 1530, 1454, 1321, 1285, 1103, 1095, 1046,
1023, 980, 854, 782, 730 cm–1. MS (FAB): m/z = 609 [M]+.
C33H32N6O6 (608.7): calcd. C 65.12, H 5.30, N 13.81; found C
64.80, H 5.42, N 13.67.
Experimental Section
General Remarks: Commercially available chemicals were pur-
chased from Aldrich, Fluka, Sigma and Acros Organics.
5,10,15,20-Tetrakis(p-carboxyphenyl)porphyrin (6) was purchased
from Porphyrin Systems. Compound 2 and 3 were synthesised ac-
cording to literature procedures.[24,25] The preparation of the chiral
depsipeptide dendrimers 7–12 has been described previously.[23]
Solvents were dried by standard techniques, dry DMF was ob-
tained from Acros. HPLC-grade solvents were purchased from
Acros Organics or SDS. Prior to use, HPLC-grade CHCl3 was
freshly distilled from potassium carbonate to avoid protonation of
the free-base porphyrin 1. Reactions were monitored by thin-layer
chromatography (TLC) with Riedel–de Haën silica gel 60 F254 alu-
minium foils, detection by UV lamp. 1H and 13C NMR spectra
were recorded with JEOL JNM EX 400, JEOL JNM GX 400 and
JEOL A 500 instruments. The chemical shifts are given in ppm
relative to tetramethylsilane (TMS) or the solvent peak as a stan-
dard reference. The resonance multiplicities are indicated as s (sing-
let), d (doublet), t (triplet), q (quartet) and m (multiplet), unre-
solved signals as broad (br.) or very broad (v. br.). Mass spectra
were measured with a Micromass Lab Spec (FAB) or a Finnigan
MAT 900 spectrometer with 3-nitrobenzyl alcohol as a matrix. IR
spectra were recorded with a React IR®-1000 ASI Applied Systems
(ATR-DiComp-Detector) instrument on a diamond crystal. Ana-
lytical HPLC was performed with a Shimadzu LC-10 HPLC system
(Nucleosil, 200×4 mm, particle size 5 µm, Macherey–Nagel), de-
tection by photo diode array. Compound 1 was purified by prepar-
ative high pressure liquid chromatography with a Shimadzu LC-8A
HPLC system (Nucleosil, 21×250 mm, particle size 5 µm, Mach-
erey–Nagel), detection by UV/Vis. Circular dichroism (CD) mea-
surements were carried out with a Jasco J 810 spectrometer with
optical grade solvents and quartz glass cuvettes with a 2 mm path
length. UV spectroscopy was performed with a Shimadzu UV-3102
spectrophotometer. Elementary analysis was carried out by com-
bustion and gas chromatographic analysis with an EA 1110 CHNS
analyzer (CE Instruments). Products were isolated by flash column
chromatography (FC) (silica gel 60, particle size 0.04–0.063 nm,
Merck).
Synthesis of Compound 5: A solution of KOH (1.84 g, 32.8 mmol)
in water (200 mL) was added dropwise at room temp. to a solution
of 4 (1.00 g, 1.64 mmol) in dioxane (400 mL). The reaction was
allowed to proceed for 48 h and the dioxane was evaporated. The
pH of the remaining solution was adjusted to 2 by the addition of
concentrated HCl and the solvents were evaporated to dryness. The
residue was suspended in water (25 mL) and filtered. The filter resi-
due was washed with water (80 mL) and cold EtOAc (10 mL) and
dried under reduced pressure. Yield: 500 mg (61%) of a brownish-
yellow solid, m.p. 154 °C (decomposition). 1H NMR (400 MHz,
3
[D6]DMSO, 25 °C): δ = 0.94 (t, JH,H = 7.4 Hz, 6 H, CH3), 1.64
3
(m, 4 H, CH2), 2.42 (t, JH,H = 7.3 Hz, 4 H, CH2), 7.62 (s, 2 H,
3
Bn), 7.70 (m, 2 H, Py), 7.96 (d, JH,H = 5.1 Hz, 4 H, Py), 8.09 (d,
4JH,H = 1.0 Hz, 1 H, Bn), 10.55 (br., 2 H, CONH), 10.97 (br., 2 H,
CONH) ppm. 13C NMR (100.5 MHz, [D6]DMSO, 25 °C): δ = 13.5
(CH3), 18.2, 38.0 (CH2), 109.9, 110.3 (Py), 118.4 (Bn), 118.8 (Py),
135.5, 141.9 (Bn), 149.4, 149.8 (Py), 158.3 (Bn), 166.2, 173.4 (C=O)
ppm. React-IR (thin film): ν
= 2968, 1652, 1586, 1532, 1444,
˜
max
1324, 1293, 1243, 1216, 1154, 1000, 876, 799, 710, 668 cm–1. MS
(FAB, NBA): m/z = 1009 [2 M]+, 505 [M]+. C26H28N6O5 ×1.5H2O
(532.24): calcd. C 60.57, H 6.06, N 16.30; found C 60.94, H 5.69,
N 16.07.
Synthesis of Compound 1: EDC (407 mg, 2.13 mmol) and DMAP
(260 mg, 2.13 mmol) were added at room temp. to a solution of
compound 5 (957 mg, 1.88 mmol) and 5,10,15,20-tetrakis(p-car-
boxyphenyl)porphyrin (6) (300 mg, 0.38 mmol) in dry DMF
(8 mL). The reaction was allowed to proceed for 7 d in the dark
and the crude reaction product was precipitated with water. The
precipitate was collected by filtration, washed with water
(3×20 mL) and air-dried overnight. The reaction product was
dried under reduced pressure at 50 °C for 1 d and purified by re-
peated column chromatography (SiO2; THF, THF/CH2Cl2, 80:20;
Rf = 0.95) followed by preparative HPLC (Nucleosil; THF/CH2Cl2,
80:20). Yield: 52 mg (5%), purple solid, m.p. 300 °C (decomposi-
1
tion). H NMR (400 MHz, [D7]DMF, 25 °C): δ = –2.69 (br., 2 H,
Synthesis of Compound 4: Diacid 2 (7.49 g, 15.69 mmol) was sus-
pended in SOCl2 (149 mL), and dry DMF (1 mL) was added under
N2. The suspension was heated at reflux, and after 2 h the solid
had entirely dissolved and the reaction mixture was heated at reflux
for a further 3 h. The excess of SOCl2 was distilled off and the oily
residue was kept under high vacuum for 60 min. The crude dichlo-
ride was used without further purification. A solution of the dichlo-
ride in dry THF (60 mL) was added dropwise at room temp. to a
solution of 3 (6.19 g, 34.54 mmol) and triethylamine (3.51 g,
34.54 mmol) in dry THF (60 mL). The reaction mixture was stirred
at room temp. for 30 min and heated at reflux for another 12 h.
After cooling to room temp., the solution was filtered, concentrated
to dryness, and purified by flash column chromatography (SiO2;
EtOAc/hexane, 60:40; Rf = 0.46). Yield 3.2 g (34%), slightly yellow
3
NH), 0.96 (t, JH,H = 7.3 Hz, 24 H, CH3), 2.74 (m, 16 H, CH2),
2.78 (t, 3JH,H = 7.4 Hz, 16 H, CH2), 7.94 (m, 8 H, Py), 8.02 (m, 16
4
3
H, Py), 8.50 (d, JH,H = 0.8 Hz, 8 H, Bn), 8.67 (d, JH,H = 8.1 Hz,
3
8 H, Bn), 8.77 (d, JH,H = 8.3 Hz, 8 H, Bn), 8.84 (s, 8 H, Bn), 9.11
(s, 8 H, pyrrole β-H), 10.39 (br., 8 H, CONH), 10.92 (br., 8 H,
CONH) ppm. 13C NMR (100.5 MHz, [D7]DMF, 25 °C): δ = 13.93
(CH3), 19.4, 39.1 (CH2), 110.4 (Py), 120.2 (Bn), 125.8 (Py), 126.1,
129.5, 135.8, 137.2, 140.7, 151.5 (Bn), 152.0 (Py), 163.0, 165.7,
173.0 (C=O) ppm. React-IR (thin film): ν
= 1671, 1584, 1293,
˜
max
1241, 1177, 1067, 797, 729 cm–1. UV/Vis (THF): λmax (ε) = 301
(142000), 420 (429000), 515 (19600), 547 (9320), 590 (6150) nm.
MS (FAB): m/z = 2736 [M]+.
Complexation of
1
with 7: Dendron
7
(4 equiv., 1.49 mg,
(1.54 mg,
solid, m.p. 169 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 0.93 22.52·10–4 mmol) was added to compound
1
(t, JH,H = 7.3 Hz, 6 H, CH3), 1.70 (m, 4 H, CH2), 2.30 (t, JH,H 5.63·10–4 mmol), suspended in CHCl3 (5 mL, HPLC grade). The
3
3
1
= 7.3 Hz, 4 H, CH2), 7.38 (m, 2 H, Py), 7.50 (m, 2 H, Bz), 7.58 (t,
solution was stirred at room temp. for 13 h. H NMR (400 MHz,
3JH,H = 7.5 Hz, 1 H, Bz), 7.71 (d, JH,H = 7.8 Hz, 2 H, Py), 7.83
CDCl3, 25 °C): δ = –2.81 (br., 2 H, pyrrrole NH), 0.83–2.43 (m, 88
3
Eur. J. Org. Chem. 2006, 3338–3347
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3345

K. Maurer, K. Hager, A. Hirsch
FULL PAPER
2
H, CH2, CH3), 3.90 (m, 8 H, CH2N), 5.00 (d, JH,H = 12.1 Hz, 4
8.0 Hz, 8 H, Py) 8.21 (s, 8 H, Bn), 8.24, (br., 8 H, Bn), 8.65 (br., 8
H, Bn), 8.81 (s, 4 H, Bn), 8.87 (br., 8 H, pyrrole β-H), 9.48 (br., 8
H, CONH), 10.02 (br., 8 H, CONH) ppm. UV/Vis (CHCl3): λmax
(ε) = 302 (53000), 422 (117000), 518 (6000), 552 (3000), 592 (2000),
648 (2000) nm.
H, Bn-CH2), 5.07 (d, 2JH,H = 12.1 Hz, 4 H, Bn-CH2), 5.14 (d, 2JH,H
2
= 12.0 Hz, 4 H, Bn-CH2), 5.20 (d, JH,H = 12.1 Hz, 4 H, Bn-CH2),
3
3
5.80 (d, JH,H = 2.81 Hz, 4 H, CH*), 5.89 (d, JH,H = 2.81 Hz, 4
H, CH*), 7.07–7.39 (m, 48 H, Bn, Bz), 7.49 (m, 4 H, Bz), 7.85 (t,
3JH,H = 8.1 Hz, 8 H, Py), 7.87 (d, JH,H = 8.1 Hz, 8 H, Bz), 8.02
3
Complexation of
1 with 11: Dendron 11 (4 equiv., 7.42 mg,
3
3
(d, JH,H = 8.1 Hz, 8 H, Py), 8.14 (d, JH,H = 8.2 Hz, 8 H, Py),
8.21 (s, 8 H, Bn) 8.37 (br., 8 H, Bn), 8.54 (br., 8 H, Bn), 8. 61 (s, 4
H, Bn-CH), 8.87 (br., 8 H, pyrrole β-H), 9.51 (br., 8 H, CONH),
10.03 (br., 8 H, CONH) ppm. UV/Vis (CHCl3): λmax (ε) = 302
(98000), 421.5 (142000), 518.5 (7000), 552.5 (4000), 591.5 (2000),
647.5 (2000) nm.
22.52·10–4 mmol) was added to compound
1
(1.54 mg,
5.63·10–4 mmol), suspended in CHCl3 (5 mL, HPLC grade). The
1
solution was stirred at room temp. for 13 h. H NMR (400 MHz,
CDCl3, 25 °C): δ = –2.82 (br., 2 H, pyrrole NH), 0.83–2.45 (m, 280
H, CH2, CH3), 3.18 (m, 56 H, CH2N), 3.84 (br., 16 H, CONH),
5.18 (m, 64 H, Bn-CH2), 5.87 (m, 56 H, CH*), 6.62 (br., 8 H,
CONH), 7.07–8.01 (m, 324 H, Bn, Bz, Py), 8.10 (d, 8 H, Py), 8.21
(s, 8 H, Bn), 8.32 (br., 8 H, Bn), 8.54 (br., 12 H, Bn), 8.84 (br., 8
H, pyrrole β-H), 9.50 (br., 8 H, CONH), 9.96 (br., 8 H, CONH)
ppm. UV/Vis (CHCl3): λmax (ε) = 277.5 (55000), 284 (55000), 302
(53000), 421.5 (109000), 517 (6000), 553 (3000), 592 (2000), 648
(1000) nm.
Complexation of
1
with 8: Dendron
8
(4 equiv., 1.49 mg,
(1.54 mg,
22.52·10–4 mmol) was added to compound
1
5.63·10–4 mmol), suspended in CHCl3 (5 mL, HPLC grade). The
1
solution was stirred at room temp. for 13 h. H NMR (400 MHz,
CDCl3, 25 °C): δ = –2.81 (br., 2 H, pyrrole NH), 0.83–2.43 (m, 88
2
H, CH3, CH2), 3.93 (m, 8 H, CH2N), 5.09 (d, JH,H = 12.0 Hz, 4
H, Bn-CH2), 5.16 (d, 2JH,H = 12.0 Hz, 4 H, Bn-CH2), 5.22 (d, 2JH,H
Complexation of
1 with 12: Dendron 12 (4 equiv., 7.42 mg,
2
= 12.0 Hz, 4 H, Bn-CH2), 5.28 (d, JH,H = 12.0 Hz, 4 H, Bn-CH2),
22.52·10–4 mmol) was added to compound
1
(1.54 mg,
3
3
5.80 (d, JH,H = 2.81 Hz, 4 H, CH*), 5.88 (d, JH,H = 2.81 Hz, 4
H, CH*), 7.06–7.44 (m, 48 H, Bn, Bz), 7.51 (m, 4 H, Bz), 7.84 (t,
5.63·10–4 mmol), suspended in CHCl3 (5 mL, HPLC grade). The
1
solution was stirred at room temp. for 13 h. H NMR (400 MHz,
3JH,H = 7.9 Hz, 8 H, Py), 7.9 (d, JH,H = 7.9 Hz, 8 H, Bz), 8.11 (d,
3
CDCl3, 25 °C): δ = –2.83 (v. br., 2 H, pyrrole NH), 0.84–2.46 (m,
280 H, CH2, CH3), 3.15 (m, 56 H, CH2N), 5.17 (m, 64 H, Bn-
CH2), 5.84 (m, 56 H, CH*), 6.60 (br., 8 H, CONH), 7.06–8.02 (m,
324 H, Bn, Bz; Py), 8.09 (d, 8 H, Py), 8.21 (s, 8 H, Bn), 8.25 (br. 8
H, Bn), 8.53 (m, 12 H, Bn), 8.83 (br., 8 H, pyrrole β-H), 9.44 (br.,
8 H, CONH), 9.95 (br., 8 H, CONH) ppm. UV/Vis (CHCl3): λmax
(ε) = 276.5 (55000), 284.5 (55000), 302.5 (53000), 422 (109000), 517
(6000), 552.5 (3000), 592 (2000), 648.5 (1000) nm.
3J (H,H) = 8.2 Hz, 8 H, Py), 8.13 (d, JH,H = 8.2 Hz, 8 H, Py),
3
8.21 (s, 8 H, Bn), 8.32 (br., 8 H, Bn), 8.52 (br., 8 H, Bn), 8.56 (br.,
4 H, Bn), 8.83 (br., 8 H, pyrrole β-H) 9.48 (br., 8 H, CONH), 9.99
(br., 8 H, CONH) ppm. UV/Vis (CHCl3): λmax (ε) = 302 (98000),
422 (142000), 519 (7000), 552 (4000), 592.5 (2000), 649 (2000) nm.
Complexation of
1
with 9: Dendron
9
(4 equiv., 3.47 mg,
(1.54 mg,
22.52·10–4 mmol) was added to compound
1
5.63·10–4 mmol), suspended in CHCl3 (5 mL, HPLC grade). The
Supporting Information (see footnote on the first page of this arti-
cle): Solubilisation studies concerning the stepwise formation of
complex 18.
1
solution was stirred at room temp. for 13 h. H NMR (400 MHz,
CDCl3, 25 °C): δ = –2.88 (br., 2 H, pyrrole NH), 0.84–2.42 (m, 152
H, CH3, CH2), 3.15 (m, 24 H, CH2N), 3.84 (br., 4 H, CONH), 5.01
2
2
(d, JH,H = 3.1 Hz, 4 H, Bn-CH2), 5.03 (d, JH,H = 3.1 Hz, 4 H,
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Bn-CH2), 5.09 (d, 4 H, Bn-CH2), 5.10 (d, 4 H, Bn-CH2), 5.16 (d,
2JH,H = 1.9 Hz, 4 H, Bn-CH2), 5.19 (d, JH,H = 2.7 Hz, 8 H, Bn-
2
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3
CH2), 5.22 (d, JH,H = 2.8 Hz, 4 H, Bn-CH2), 5.69 (d, JH,H
=
3
3.7 Hz, 4 H, CH*) 5.77 (d, JH,H = 2.8 Hz, 8 H, CH*), 5.79 (d,
3JH,H = 2.8 Hz, 4 H, CH*), 5.88 (d, JH,H = 2.9 Hz, 4 H, CH*),
3
3
5.90 (d, JH,H = 2.8 Hz, 4 H, CH*), 7.06–7.56 (116 H, Bn, Bz),
7.89 (t, 8 H, Py), 7.97 (m,16 H, Bz), 8.00 (m, 24 H, Bz, Py-CH),
3
8.12 (d, JH,H = 7.9 Hz, 8 H, Py-CH), 8.21(s, 8 H, Bn-CH), 8.34
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(br., 8 H, Bn-CH), 8.52 (br., 12 H, Bn-CH), 8.85 (br., 8 H, pyrrole
β-H), 9.48 (br., 8 H, CONH), 10.02 (br., 8 H, CONH) ppm. UV/
Vis (CHCl3): λmax (ε) = 303 (53000), 421.5 (117000), 517.5 (6000),
552.5 (3000), 592 (2000), 648 (2000) nm.
Complexation of
1 with 10: Dendron 10 (4 equiv., 3.47 mg,
22.52·10–4 mmol) was added to compound
1
(1.54 mg,
5.63·10–4 mmol), suspended in CHCl3 (5 mL, HPLC grade). The
solution was stirred at room temp. for 13 h. H NMR (400 MHz,
1
CDCl3, 25 °C): δ = –2.88 (br., 2 H, pyrrole NH), 0.85–2.42 (m, 152
H, CH2, CH3), 3.18 (m, 24 H, CH2N), 3.84 (br., 4 H, CONH), 5.01
2
2
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2
2
Bn-CH2), 5.09 (d, JH,H = 3.0 Hz, 4 H, Bn-CH2), 5.12 (d, JH,H
=
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2
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(d, JH,H = 3.8 Hz, 8 H, Bn-CH2), 5.23 (d, JH,H = 3.8 Hz, 4 H,
3
3
Bn-CH2), 5.69 (d, JH,H = 3.7 Hz, 4 H, CH*), 5.78 (d, JH,H
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3
2.8 Hz, 8 H, CH*), 5.81 (d, JH,H = 2.8 Hz, 4 H, CH*), 5.89 (d,
3JH,H = 2.8 Hz, 4 H, CH*), 5.92 (d, JH,H = 2.8 Hz, 4 H, CH*),
3
6.22 (br., 4 H, CONH), 7.10–7.53 (116 H, Bn, Bz), 7.90 (t, 8 H,
3
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=
3346
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 3338–3347

Self-Assembly and Chiroptical Properties of Chiral Dendrimers
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Received: December 6, 2005
Published Online: June 1, 2006
Eur. J. Org. Chem. 2006, 3338–3347
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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