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2.7% ee in the sol state (0.2 mg mLÀ1) but antipodal (R)-1E in 9.5%
ee in the gel state (0.625 mg mLÀ1), while g-CDNS 5 behaved
differently to give racemic product throughout the suspension,
flowing gel and rigid gel regions.
Switching the chiral sense of the product in asymmetric synthesis
may seem infeasible without using an antipodal chiral source, but
has been demonstrated to occur in chiral photoreactions by manip-
ulating the entropy-related environmental variants, such as tempera-
ture, pressure and solvent.1a,2a,7 Similar switching behavior has
rarely been reported for a supramolecular system, most probably
due to the well ordered, but less dynamic nature of the framework.
One of the available strategies to alter the chiral environment in a
supramolecular system is the phase change associated with supra-
molecular aggregation, which was proven effective in this study.
Without changing the chemical structure of the chiral source
employed (i.e. CD), the chirality switching was achieved through
the change in aggregation mode. This strategy is promising as a new
tool for manipulating the stereochemical outcomes in supramole-
cular (photo)chirogenesis.
This work was supported by the grants from JST (PRESTO for
CY), JSPS (No. 21245011, 23350018 and 23750129 for YI, TM
and GF, respectively). WL acknowledges the financial support
by the ‘‘Global 30’’ Program (MEXT). We thank Prof. Mitsuru
Akashi and Dr Takami Akagi of Osaka University for the use of
their DLS instrument. CY thanks Prof. Jason Chruma of
Sichuan University for the valuable suggestions and discussion.
Fig. 2 The ee profile of 2EZ as a function of the concentration of 3 (top), 4
(middle) and 5 (bottom) upon enantiodifferentiating photoisomerization of 2ZZ.
The pink, yellow, blue and violet regions represent the solution, suspension,
flowing gel and rigid gel states, respectively.
gel state (1.0 g mLÀ1) gave 2EZ in 13.3% ee, which is the highest
value ever reported for the supramolecular photosensitization of
2ZZ.5 This result suggests that the photochirogenic environment,
which is more effective than the conventional CD cavity, is created
in the CDNS gel. We deduce that the hydrophobic void surrounded
by the outside walls of CD created in the gel becomes more
compact at higher CDNS concentration to form a chiral sensitizing
site suitable for the enantiodifferentiating photoisomerization of 1Z
and 2ZZ (Fig. 3). This idea is supported by the high resolution
magic angle spinning (HRMAS) NMR spectra of 2ZZ dissolved in
gel 5. HRMAS and T2-filtered 1H NMR spectra showed sharp peaks
of 2ZZ protons (Fig. S14 and S15, ESI†), indicating that the
substrate is freely moving around on the NMR time-scale in the
void spaces of the gel despite the high viscosity.
The product’s ee was highly phase-dependent also in the photo-
sensitization of 1Z. As shown in Table S6 and Fig. S16 (ESI†), the
photoisomerization of 1Z sensitized by 3 gave (S)-1E in 7.5% ee in
the sol state (0.2 mg mLÀ1), which however rapidly faded out to
almost zero by increasing the concentration to 1.5 mg mLÀ1 and
stayed low throughout the suspension region (1.5–40 mg mLÀ1), but
revived at higher concentrations of 3 to give antipodal (R)-1E in
7.6% ee in the gel state (2 g mLÀ1). A very similar chirality inversion
by altering phase was observed for b-CDNS 4, affording (S)-1E in
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c
3512 Chem. Commun., 2013, 49, 3510--3512
This journal is The Royal Society of Chemistry 2013