Double helices with a controlled helicity composed of biphenol-derived phosphoric acid diesters

Article abstract of DOI:10.1002/chem.200901075

A new modular strategy to build artificial double helical oligomers and polymers with a controlled helicity composed of biphenol-derived phosphoric acid diesters through amidinium-carboxylate salt bridges was reported. The biphenyl derivative was dimerized as a linker and the dimer product was treated with phosphoryl chloride after hydrolysis of the acetoxy groups to afford the optically active phosphoric acid diesters. The calculated results show that the structure of homo-duplex of (S)-1 reveals a double-stranded helical structure, in which two aromatic rings of the linker moieties overlap with each other. The compound is also found to exhibit induced circular dichroism (ICD) in the π-conjugated chromophore regions above 300 nm. Optically active substituents introduced on the phenylene linker unit contribute to the bias in an excess of one helical sense.

Full text of DOI:10.1002/chem.200901075

DOI: 10.1002/chem.200901075
Double Helices with a Controlled Helicity Composed of Biphenol-Derived
Phosphoric Acid Diesters
Hidekazu Yamada,[a] Katsuhiro Maeda,*[a, b] and Eiji Yashima*[a]
Since the discovery of the DNA double helix,[1] the devel-
opments of artificial double helices have attracted signifi-
cant attention.[2] Nevertheless, double helices with a con-
trolled helix-sense are quite rare,[2–4] whereas a number of
single helical oligomers and polymers have been reported.[5]
Metal-directed self-assembly is
ported that chiral phosphoric acids can function as effective
asymmetric organocatalysts.[8] Herein, we report on the
design and synthesis of novel artificial double helices by uti-
lizing a specific self-assembly property of biphenol-derived
phosphoric acid diesters (Figure 1), whose helical sense can
the most widely used approach
to construct double-stranded
helices, helicates.[2a–c] Hydrogen-
bonding-driven self-assembly is
another useful approach to
build up supramolecular du-
plexes.[6] Among them, some ar-
omatic oligoamides were found
to fold into double helical struc-
tures through hydrogen-bond-
ing along with aromatic–aro-
Figure 1. Schematic illustration of self-assembled double helix formation.
matic interactions.[2d–f] Recently,
we reported a new modular
strategy to build artificial double helical oligomers[3a,c–e] and
polymers[3b,f] through amidinium–carboxylate salt bridges,
which assist intertwining the two crescent-shaped comple-
mentary strands, resulting in the optically active double heli-
ces with a controlled helix-sense induced by the chiral
groups introduced at the amidine residues.
be controlled by chiral substituents introduced onto the
phenylene unit as the linker moiety. In addition, chiral am-
plification during the double helix formation between chiral
and achiral strands is also described.
Novel chiral ((S)-1) and achiral (2) phosphoric acid die-
sters were synthesized according to Scheme 1.[9] The dibro-
mide 4 was obtained by the Sonogashira coupling of 2,2’-di-
acetoxy-3,3’,5,5’-tetrabromobiphenyl with triisopropylsilyla-
cetylene followed by hydrolysis of the ester groups. Lithia-
tion of 4 with nBuLi and subsequent quenching with water
afforded the monobromide, which was then coupled with
tri-n-butyl(trimethylsilylethynyl)tin using the Stille coupling
reagents to give 7 after the protection of the hydroxy
groups. The compound 7 was treated with NaOH and the
hydroxy groups were then acetylated to yield 9. The biphen-
yl derivative 9 was dimerized by the Sonogashira coupling
with (S,S)-2,5-bis(2-methylbutoxy)-1,4-diiodobenzene[10] as a
linker and the dimer product was then treated with phos-
phoryl chloride after hydrolysis of the acetoxy groups to
afford the optically active phosphoric acid diesters (S)-1.
On the other hand, phosphoric acids are known to form
dimers through hydrogen bonds.[7] Moreover, it has been re-
[a] H. Yamada, Dr. K. Maeda, Prof. E. Yashima
Department of Molecular Design and Engineering
Graduate School of Engineering
Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
Fax : (+81)52-789-3185
E-mail: maeda@t.kanazawa-u.ac.jp
yashima@apchem.nagoya-u.ac.jp
[b] Dr. K. Maeda
Present address: Graduate School of Natural Science and Technology
Kanazawa University, Kakuma-machi, Kanazawa 920-1192 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901075.
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COMMUNICATION
the linker moieties are stacked face-to-face, but slightly out
of alignment, so that the aromatic linker protons resonated
as four singlet peaks. The nonequivalency of these signals is
attributed to the formation of a self-assembled homo-duplex
of (S)-1, which was also supported by cold-spray ionization
mass spectrometry (CSI-MS) and matrix-assisted laser de-
sorption-ionization
time-of-flight
mass
spectrometry
(MALDI-TOF-MS) studies (see Figure S11, S26, and S27b
in the Supporting Information).
Repeated attempts failed to produce crystals suitable for
X-ray diffraction. Therefore, the energy-minimized structure
of the homo-duplex of (S)-1 was calculated based on the
hybrid density functional theory (DFT) method at the
B3LYP level with the 6-31G* basis set, in which the triiso-
propylsilylethynyl and (S)-2-methylbutoxy substituents of
(S)-1 were substituted with hydrogen atoms for simplicity.
The calculated structure of the homo-duplex of (S)-1 re-
vealed a double-stranded helical structure (Figure 3), in
which the two aromatic rings of the linker moieties over-
lapped with each other but were slightly out of alignment,
and the distance between the two central aromatic rings is
about 4 ꢁ. The phosphoric acid residues formed two identi-
cal hydrogen bonds, which enabled the two strands to be
held together, resulting in a helically twisted double helix
(Figure 3). This calculated double-stranded helical structure
can explain the results of NMR spectroscopic studies.
Circular dichroism (CD) measurements of (S)-1 were
then performed to explore whether (S)-1 formed a pre-
ferred-handed homo-double helix in CH2Cl2. Compound
(S)-1 exhibited an induced circular dichroism (ICD) in the
p-conjugated chromophore regions (above 300 nm) (Fig-
ure 4a), which is strong evidence that (S)-1 forms a homo-
double helix with an excess of either right- or left-handed-
ness. The ICD, however, almost disappeared upon the addi-
tion of two equivalents of pyridine at 258C; this was accom-
panied by a change in its absorption spectrum (see Figure S2
in the Supporting Information), resulting from the formation
of pyridinium salts with (S)-1 that likely have no regular
structure in solution.[11] In addition, the ICD intensity con-
siderably increased on decreasing the temperature and
reached an almost constant value at À108C (Figure 4a).
These ICD changes reversibly took place accompanied by
remarkable changes in the absorption spectra. In contrast,
the absorption spectrum of an achiral analogue 2 was not
temperature dependent (see Figure S13 in the Supporting
Information) and its spectral pattern was almost identical to
that of (S)-1 at À108C (Figure 4a). These results clearly in-
dicate that the optically active substituents introduced on
the phenylene linker unit contribute to the bias in an excess
of one helical sense, which increased with the decreasing
temperature, and the (S)-1 is supposed to adopt an almost
one-handed homo-double helical structure at around
À108C.
Scheme 1. Syntheses of chiral ((S)-1) and achiral (2) phosphoric acid die-
sters: a) triisopropylsilylacetylene, [Pd(PPh3)2Cl2], CuI, Et3N, 30 8C; b)
NaOH, MeOH, THF, rt; c) nBuLi, THF, H2O, À788C; d) acetyl chloride,
pyridine, rt; e) tributyl(trimethylsilylethynyl)tin, [Pd(PPh3)4], toluene,
reflux; f) K2CO3, MeOH, rt; g) acetyl chloride, pyridine, rt; h) (S,S)-2,5-
bis(2-methylbutoxy)-1,4-diiodobenzene for (S)-1, 2,5-dibutoxy-1,4-diiodo-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
benzene for 2, [PdACHTUNGTRENNUNG(PPh3)2Cl2], CuI, Et3N, rt ; i) K2CO3, MeOH, rt; j)
POCl3, pyridine, rt. A detailed version of the scheme including all the in-
termediate steps and compounds is given in Scheme S1 in the Supporting
Information.
The achiral 2 was also synthesized in a similar manner using
2,5-dibutoxy-1,4-diiodobenzene as a linker.[9]
The structure of (S)-1 in solution was first investigated by
1H NMR spectroscopy at 258C. In [D5]pyridine, the
1H NMR signals were sharp, and each signal was assigned to
a single strand as its pyridinium salt (Figure 2a). In contrast,
these signals became complicated ones in CD2Cl2 (Fig-
ure 2b). In particular, the signals attributed to the protons
for the phenylene linker moieties denoted by f, g, and g’ in
Figure 2b were separated into nonequivalent four peaks, re-
spectively, together with additional broad peaks probably
due to the diastereomers derived from helicity (see below),
and some of them significantly shifted upfield, suggesting a
self-assembled formation through hydrogen bonding and ar-
omatic–aromatic interactions in CD2Cl2.
2D COSY and ROESY spectra of (S)-1 revealed that the
eight peaks (g1–g4 and g1’–g4’ in Figure 2b) were divided into
four pairs of the geminal protons bonded to the individual
carbon atoms, and the four singlet peaks (f1–f4 in Figure 2b)
were composed of two pairs of protons exchanging each
other (see Figures S4–S10 in the Supporting Information).
These observations suggest that the two aromatic rings of
This assumption was supported by the variable-tempera-
ture 1H NMR measurements of (S)-1 in CD2Cl2. As de-
scribed previously in Figure 2b, the minor peaks (denoted
by asterisks in Figure 4b (i) and Figure S1 in the Supporting
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K. Maeda, E. Yashima, and H. Yamada
handed double helices (Fig-
ure 4b (ii) and Figure S1 in the
Supporting Information), result-
ing in a set of correlated peaks
for enantiomeric double helices,
as supported by 2D NMR
measurements
(see
Figur-
es S14–S20 in the Supporting
Information).[13] On the basis of
the changes in the relative peak
intensities between the major
and minor peaks with tempera-
ture, the diastereomeric excess
(de) values of the homo-double
helical (S)-1 were calculated
and plotted versus temperature
(see Figure S3 in the Support-
ing Information). We note that
the relative changes in the CD
intensity at 347 nm and absorb-
ance at 378 nm of (S)-1 (Fig-
ure 4a) plotted versus tempera-
ture were almost proportional
to those in the de values with
temperature (see Figure S3 in
the Supporting Information),
indicating that (S)-1 exists as a
mixture of diastereomeric right-
Figure 2. Partial 1H NMR (500 MHz, 258C, 6.6 mm) spectra of (S)-1 in [D5]pyridine (a) and CD2Cl2 (b). The
asterisks denote the signals for solvents.
and left-handed homo-double helices under equilibrium in
solution, and one of the helices becomes predominant at
low temperatures, which could be monitored by 1H NMR
and CD spectroscopies.
Next, we examined if chiral amplification could take place
during the hetero-double helix formation between the opti-
cally active (S)-1 and achiral 2, by which a preferred-handed
hetero-double helix would be formed. First, the 1H NMR
spectrum of an equimolar mixture of (S)-1 and 2 was mea-
sured in CD2Cl2 to estimate the de value of the resulting
hetero-double helix of (S)-1·2. However, this proved difficult
because of the overlap of the proton resonances of the
homo- and hetero-double helices composed of (S)-1 and/or
2, even at temperatures as low as À108C (see Figure S25 in
the Supporting Information). The CD spectral changes of
mixtures of (S)-1 and 2 were then measured under the con-
ditions of constant [(S)-1 and 2] with varying [(S)-1] in
CH2Cl2 at À108C (Figure 5a). The Cotton effect intensities
at 347 nm of the mixtures monotonically increased on in-
creasing the mole fraction of (S)-1 (Figure 5b, black bold
line), whereas their absorption spectra remained unchanged
at À108C (Figure 5a).
Figure 3. DFT calculated structures (front view (a) and side view (b)) of
homo-double helix of (S)-1. Triisopropylsilylethynyl and (S)-2-methylbu-
toxy groups were substituted with hydrogen atoms for simplicity.
Information), which probably correspond to the opposite
diastereomeric homo-double helix, were clearly observed at
358C; the intensities of these peaks gradually decreased
with the decreasing temperature and the peaks almost disap-
peared at À108C. These changes are also reversible.[12] The
analogous achiral strand 2 with no chiral linker units
During the double helix formation, three duplex species,
two homo-double helices (S)-1·(S)-1 and 2·2, and a hetero-
double helix (S)-1·2 are at equilibrium in solution. Among
them, the (S)-1·(S)-1 and (S)-1·2 are optically active, thus
contributing to the CD, and the (S)-1·(S)-1 likely forms a
one-handed double helix at À108C (approximately 100%
de) according to the variable-temperature NMR and CD re-
1
showed a similar H NMR spectrum to that of (S)-1 at 258C,
but no such minor peaks due to diastereomeric helices were
observed even at low temperatures (see Figure S12 in the
Supporting Information) because 2 self-assembles to form
most likely an equal mixture of enantiomeric right- and left-
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Double Helices with a Controlled Helicity
COMMUNICATION
Figure 4. Temperature-dependent spectral changes of (S)-1. a) CD and absorption spectra (0.60 mm, CH2Cl2) of (S)-1 at various temperatures (colored
1
lines). Absorption spectrum (0.60 mm, CH2Cl2) of 2 at 258C (black line) is also shown. b) Partial H NMR (500 MHz, 6.6 mm, CD2Cl2) spectra of (S)-1 at
various temperatures (i) and 2 at 258C (ii). The asterisks denote the signals for the diastereomers.
peaks derived from (S)-1·(S)-1, (S)-1·2, and 2·2 duplexes was
approximately 1:2:1 (see Figure S27 in the Supporting Infor-
mation), 2) (S)-1 and 2 may exist as a homo- or hetero-
double helix and free (S)-1 and 2 could be excluded, be-
cause their dimerization constants seem to be extremely
high over 105 mÀ1 (see Figure S23 in the Supporting Informa-
tion),[14] 3) a hetero-double helix (S)-1·2 may have the iden-
tical CD pattern to that of (S)-1·(S)-1 and its maximum CD
intensity may be the same as that of the one-handed double
helical (S)-1·(S)-1 at À108C, since the absorption spectrum
of (S)-1·(S)-1 at À108C, at which the homo-double helix
exists in an almost completely one-handedness excess
(100% de), was nearly identical to that of the 2·2.
Based on the above assumptions, we simulated the
changes in the CD intensity of the mixtures of (S)-1 and 2 at
347, 378, and 403 nm by varying the de value of the (S)-1·2
hetero-double helix (from 0 to 100% de at 10% intervals)
versus the mole fraction of (S)-1 (Figure 5b and Figure S28
in the Supporting Information). The experimental CD inten-
sity changes fit well with the simulation curves for the de
values of the hetero-double helix of (S)-1·2 (70–80%). Con-
sequently, chirality is significantly amplified during the
hetero-double helix formation between (S)-1 and 2, leading
to an excess of the one-handed hetero-double helix of ap-
proximately 70–80% de.
In summary, we have successfully prepared the artificial
homo- and hetero-double helices through the assemblies of
biphenol-derived phosphoric acid diesters. A bias in the
Figure 5. a) CD and absorption spectra (CH2Cl2, À108C) of (S)-1 with 2.
The total concentration is 0.60 mm and mole fractions of (S)-1 are 0, 0.25,
twist sense of the double helices can be achieved by incor-
porating chiral substituents into the linker unit. Further-
more, a strong chiral amplification was observed during the
double helix formation between chiral (S)-1 and achiral 2.
We believe that this novel hydrogen-bond-driven double
helix with a controlled helix-sense may provide a new
design strategy to develop a variety of discrete supramolec-
ular helical assemblies with a controlled helicity, and novel
chiral materials for possible applications in asymmetric cat-
alysis and enantioselective selectors.
0.50, 0.75, and 1. b) The approximation curve for the experimental CD
intensities at 347 nm (black bold line) and the simulated CD intensity
changes on varying the diastereomeric excess (de) of (S)-1·2 (0–100% de
at 10% intervals) as a function of the mole fraction of (S)-1.
sults, whereas 2·2 is totally optically inactive. We then esti-
mated the excess of one-handedness of the hetero-double
helix (S)-1·2 on the basis of the following assumptions: 1)
The dimerization of (S)-1 and 2 takes place at random, be-
cause the MALDI-TOF mass spectrum of an equimolar
mixture of (S)-1 and 2 showed that the intensity ratio of the
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K. Maeda, E. Yashima, and H. Yamada
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Acknowledgements
We thank Dr. Ivan Huc for his invaluable comments and fruitful discus-
sion. We are deeply grateful to Dr. Y. Furusho (Nagoya University) for
fruitful discussions. This work was supported in part by Grant-in-Aid for
Scientific Research (S) from the Japan Society for the Promotion of Sci-
ence (JSPS).
Keywords: chiral amplification
·
chirality
·
helical
structures · self-assembly · supramolecular chemistry
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1
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(S)-11 (Dꢀ 6.0ꢃ10À–10 m2 sÀ1) (see Figure S21 and S22 in the Sup-
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Received: April 22, 2009
Published online: June 16, 2009
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