Chirality transcription and amplification by [2]pseudorotaxanes

Article abstract of DOI:10.1039/c2cc38758a

Chirality transcription and amplification by the formation of chiral [2]pseudorotaxanes by an achiral crown ether having the 2′, 2′′-quaterphenyl group and chiral sec-ammonium ions are reported. It was revealed that the absolute configurations of the chiral sec-ammonium ions can be detected directly from the CD spectra of the chiral [2]pseudorotaxanes.

Full text of DOI:10.1039/c2cc38758a

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Shunsuke Kuwahara–Yoichi Habata’s laboratory,
Toho University, Japan
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Chirality transcription and amplification by [2]pseudorotaxanes
A chirality transcription and amplification by the forming of chiral
[2]pseudorotaxanes by an achiral crown ether having
2’,2’’-quaterphenyl group and chiral sec-ammonium ions are
reported. It was revealed that the absolute configurations of the
chiral sec-ammonium ions can be detected directly from the CD
spectra of the chiral [2]pseudorotaxanes.
See Yoichi Habata et al.,
Chem. Commun., 2013, 49, 2186.
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Chirality transcription and amplification by
[2]pseudorotaxanes†
Shunsuke Kuwahara,ab Rie Chamura,a Sho Tsuchiya,a Mari Ikedaab and
Yoichi Habata*ab
Cite this: Chem. Commun., 2013,
49, 2186
Received 6th December 2012,
Accepted 2nd January 2013
DOI: 10.1039/c2cc38758a
www.rsc.org/chemcomm
Chirality transcription and amplification by the formation of chiral
[2]pseudorotaxanes by an achiral crown ether having the 20,200-
quaterphenyl group and chiral sec-ammonium ions are reported.
It was revealed that the absolute configurations of the chiral sec-
ammonium ions can be detected directly from the CD spectra of
the chiral [2]pseudorotaxanes.
Chirality recognition in supramolecular interaction plays an
important role in many natural systems such as the DNA
double helix and secondary a-helix structure of proteins.1
Recently various artificial and biomimetic supramolecular
systems for chirality recognition have been extensively studied.1–7
Crown ethers having chiral centers are widely used for chirality
recognition of chiral amines, ammonium salts, amino acids,
and so on.8–10
Chiral crown ethers recognize chiral guests by the difference
in stabilities between diastereomeric complexes such as
(R)-host–(R)-guest and (R)-host–(S)-guest complexes. When we
looked at the recognition system from a different angle, we
came up with an idea for a supramolecular system. That is, an
achiral crown ether having a functional group which can invert
Fig. 1 Conceptual diagram of the chiral transcription by the formation of
[2]pseudorotaxanes.
the chirality of an atropisomer depending on the chirality of into the spatial arrangement of two biphenyl chromophores in
guests could achieve chirality transcription to the crown ether the 20,200-quaterphenyl group. It is well known that the circular
from the guest. We designed a new benzo-20,200-quaterphenyl- dichroism (CD) exciton chirality method provides valuable
26-crown-8 ether (1). The 20,200-quaterphenyl group can flip like information on the spatial arrangement of the chromophores.12–14
a co-axis rotor blade, and the macrocyclic ring can form Therefore, information on the absolute configuration is detected
[2]pseudorotaxanes11 with cationic organic guests such as sec- directly by CD spectroscopy. In addition, information on the
ammonium salts. We expected that information on the chirality chirality would be amplified by the drastic conformational
of the ammonium salts is transcribed into the 20,200-quaterphenyl changes of the 20,200-quaterphenyl group which has two large
unit of 1 (Fig. 1). Concretely, information on the absolute electronic transition moments. Recently, some research groups
configurations of sec-ammonium ions would be transcribed have reported the supramolecular chirality transcription
system of amines,15–18 amino acids,19–22 and peptides23 using
a Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama,
Funabashi, Chiba 274-8510, Japan. E-mail: habata@chem.sci.toho-u.ac.jp
b Research Center for Materials with Integrated Properties, Toho University,
2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
CD spectroscopy. However, their systems require complicated
molecules or need chemical conversion of the target molecules,
and it is difficult to obtain the information about their absolute
configurations. Here we report chiral transcription and ampli-
fication by the formation of [2]pseudorotaxanes.
The new benzo-20,200-quaterphenyl-26-crown-8 ether (1) was pre-
pared by the reaction of 1,10:30,100:300,100 0-quaterphenyl-40,600-diol24
† Electronic supplementary information (ESI) available: Experimental procedures
and physicochemical properties, X-ray structures, and molecular calculations.
CCDC 914474. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c2cc38758a
c
2186 Chem. Commun., 2013, 49, 2186--2188
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and triethylene glycol ditosylate25 in 20% yield. Chiral ammonium
salts (R)- and (S)-2(a and b)-HꢀPF6 were prepared from (R)- or
(S)-phenylethylamine and the corresponding aldehyde in good
yield. The structures of the chiral ammonium salts were con-
firmed by 1H and 13C NMR, MS, and elemental analysis. Single
crystals of (R)-2a-HꢀPF6 were obtained, and X-ray structure was
determined. (For further information see ESI.†)
Crown ether 1 was treated with equimolar amounts of
(R)-2a-HꢀPF6, (R)-2b-HꢀPF6, (S)-2a-HꢀPF6, and (S)-2b-HꢀPF6 to give
complexes [1ꢀ(R)-2a-H][PF6], [1ꢀ(R)-2b-H][PF6], [1ꢀ(S)-2a-H][PF6],
and [1ꢀ(S)-2b-H][PF6] in quantitative yield, respectively. The
structure of [1ꢀ(R)-2a-H][PF6] was confirmed by 1H NMR,
13C NMR and NOESY. When equimolar amounts of 1 and
(R)-2a-HꢀPF6 were mixed in CDCl3, the ethyleneoxy protons
(H2–H7) of 1 and the methyl protons of (R)-2a-HꢀPF6 exhibited
peak broadening in the 1H NMR spectrum, indicative of
exchange occurring on the 1H NMR time scale between the
[2]pseudorotaxane and free species (Fig. 2).26,27 ‡ In the
13C NMR, remarkable chemical shift changes of the ethyleneoxy
carbons (C2–C7) were observed (Fig. S12, ESI†). In addition, the
Fig. 3 CSI-MS (CHCl3, 298 K, [1] = [ammonium salts] = 30 mM) of a 1 : 1 mixture
of 1 and (R)-2a-HꢀPF6 (a), 1 and (S)-2a-HꢀPF6 (b), 1 and (R)-2b-HꢀPF6 (c), 1 and
(S)-2b-HꢀPF6 (d).
NOESY spectrum of the mixture exhibits
a NOE cross
peak between the ethyleneoxy protons (H4 or H5) of 1 and
the methyl protons (Hd) of (R)-2a-H+ (Fig. S13, ESI†). From
these results, it was confirmed that the combination of the
crown ether 1 and the chiral sec-amine salts forms the
[2]pseudorotaxanes.
In the Cold Spray Ionization-MS (CSI-MS) of [1ꢀ(R)-2a-H][PF6]
and [1ꢀ(R)-2b-H][PF6], fragment ion peaks arising from [1ꢀ(R)-2a-H]+
and [1ꢀ(R)-2b-H]+ were observed at m/z = 888 and 841, respec-
tively (Fig. 3). The CSI-MS measurements of the enantiomers
[1ꢀ(S)-2a-H][PF6] and [1ꢀ(S)-2b-H][PF6] showed the same frag-
ment ion peaks. The CSI-MS data also support the formation of
the [2]pseudorotaxanes.
Fig. 4 CD spectra (CHCl3, 273 K, [1] = [ammonium salts] = 3.0 mM) (a) and UV
spectra (CHCl3, 293 K, [1] = [ammonium salts] = 0.20 mM) (b) of 1 (red), (R)-2a-HꢀPF6
(black), [1ꢀ(R)-2a-H][PF6] (pink), [1ꢀ(S)-2a-H][PF6] (purple), [1ꢀ(R)-2b-H][PF6] (green),
and [1ꢀ(S)-2b-H][PF6] (blue). The preferred conformation of [1ꢀ(R)-2a-H][PF6] (c).
Optimized structure of [1ꢀ(R)-2a-H] by ab initio B3LYP/6-31G* (d).
To investigate the chirality transcription and amplification
by the [2]pseudorotaxanes, the UV-vis and CD spectra of the
crown ether 1, ammonium salts, and [2]pseudorotaxanes were
measured (Fig. 4a and b). Crown ether 1 has a lmax arising from
p–p* transition around 260 nm in the UV-vis spectrum. This
absorption is due to the polarization along the long axis of the
biphenyl chromophore moieties in the 20,200-quaterphenyl
group. Since 1 is an achiral molecule having a flexible 20,200-
quaterphenyl group, no Cotton effect was observed. Although
the chiral ammonium salt (R)-2a-HꢀPF6 shows CD Cotton effects
around 260 nm (aromatic 1Lb band), the amplitude is very weak.
On the other hand, the [2]pseudorotaxane [1ꢀ(R)-2a-H][PF6]
exhibits significant CD Cotton effects due to the exciton
coupling between the two biphenyl chromophores in the
20,200-quaterphenyl group; lext = 263.4 nm (y = ꢁ8700), lext
=
243.1 nm (y = +18 700). The negative first and positive second
Cotton effects indicate the negative exciton chirality12–14 between
the two chromophores. Therefore, the two long axes of the two
chromophores of [1ꢀ(R)-2a-H][PF6] constitute a counterclockwise
screw sense (Fig. 4c). In contrast, [2]pseudorotaxane [1ꢀ(S)-2a-H]-
[PF6] shows the mirror image of the Cotton effects of [1ꢀ(R)-2a-H]-
[PF6] in the CD spectrum (lext = 261.6 nm (y = +12 100), lext
=
Fig. 2 Partial 1H NMR spectra (400 MHz, CDCl3, 298 K, [1] = [ammonium salts] =
14 mM) of (R)-2a-HꢀPF6 (a), 1 (b), and [1ꢀ(R)-2a-H][PF6] (c).
242.8 nm (y = ꢁ20 800)). The CD spectra of [1ꢀ(R)-2b-H][PF6]
and [1ꢀ(S)-2b-H][PF6] show the same sign of Cotton effects with
c
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Chem. Commun., 2013, 49, 2186--2188 2187

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This research was supported by Grants-in-Aids (11011761)
and Supported program for the Strategic Research Foundation
at Private Universities (2012–2016) from MEXT of Japan for Y.H.
Notes and references
‡ The association constant Ka of [1ꢀ(R)-2a-H][PF6] was not determined
because of the peak broadening in the 1H NMR spectrum. The Ka value
of [1ꢀ(R)-2b-H][PF6] is calculated using 1H NMR titration experiments at
273 K to be 370 Mꢁ1 (Fig. S14, ESI†). The value is smaller than the
[2]pseudorotaxane of dibenzo[24]crown-8 and dibenzylammonium
(27 000 Mꢁ1 at 298 K)11 and is larger than that of chiral crown ether
and benzyl ammonium (3.3 Mꢁ1 at 283 K; Y. Tachibana, N. Kihara,
Y. Ohga, T. Takata, Chem. Lett., 2000, 29, 806).
Fig. 5 CD signals at 261.6 nm of [1ꢀ(R)-2a-H][PF6] and [1ꢀ(S)-2a-H][PF6] with
varying %ee values. [1] = [ammonium salts] = 3.0 mM.
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that the [2]pseudorotaxanes containing the chiral sec-ammonium
salts with the same absolute configuration show the same Cotton
effects in CD spectra.
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This journal is The Royal Society of Chemistry 2013