Drastic change in racemization barrier upon redox reactions: Novel chiral-memory units based on dynamic redox systems

Article abstract of DOI:10.1039/c0cc00026d

The helical configuration of dication dyes 2²⁺ with a dihydrodibenzoxepin unit remained unchanged even at high temperature, whereas the corresponding neutral electron donors 1 with a tetrahydrophenanthroxepin skeleton easily underwent racemization. Due to their electrochemical bistability, electron exchange between 1 and 2²⁺ is prohibited. Thus, the above electrochromic pairs can serve as novel chiral-memory units where redox reactions trigger switching between an "erasable/writable"-state (1) and a "memorizing"-state (2²⁺).

Full text of DOI:10.1039/c0cc00026d

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Drastic change in racemization barrier upon redox reactions: novel
chiral-memory units based on dynamic redox systemsw
Takanori Suzuki,*a Kazuhisa Wada,a Yusuke Ishigaki,a Yasuyo Yoshimoto,a Eisuke Ohta,a
Hidetoshi Kawaiab and Kenshu Fujiwaraa
Received 19th February 2010, Accepted 30th March 2010
First published as an Advance Article on the web 13th April 2010
DOI: 10.1039/c0cc00026d
The helical configuration of dication dyes 22+ with a dihydro-
dibenzoxepin unit remained unchanged even at high temperature,
whereas the corresponding neutral electron donors 1 with a
tetrahydrophenanthroxepin skeleton easily underwent racemization.
Due to their electrochemical bistability, electron exchange
between 1 and 22+ is prohibited. Thus, the above electrochromic
pairs can serve as novel chiral-memory units where redox
reactions trigger switching between an ‘‘erasable/writable’’-state
(1) and a ‘‘memorizing’’-state (22+).
The intermolecular transmission of asymmetric information is
one of the most important events in natural biological systems
and in the field of supramolecular chirality.1 Chiral-memory
units play important roles in the above phenomenon, where
they act as asymmetric-information receptors and store chiral
information transmitted from the source.2 To ensure this
Scheme 1
function, the chiral unit should be able to adopt an easily-
racemized state (‘‘erasable/writable’’-state), but must also be
Such separation of redox potentials would suppress electron-
exchange between 1 and 22+, which should satisfy the requirement
intentionally interconvertible with the non-racemized state
(‘‘memorizing’’-state) under certain conditions. Folded poly-
mer chains with a helical structure3 and some macrocyclic
for a novel chiral-memory unit that works under redox
molecules4 are important compounds that exhibit such switching
conditions (Scheme 2).6a,b
By reacting a dilithio compound derived from the known
dibromide rac-38,9 with 4,40-dimethoxybenzophenone, we
by heating/cooling. However, much less has been reported on
compounds whose interstate-switching5 is achieved by electron
obtained rac-diol 4,w which gave the stable dication salt
transfer (‘‘chiral redox memory’’).
rac-2a2+(BF4ꢀ)2w quantitatively under acidic dehydrating
In the present study, we designed and prepared 5,7-dihydro-
dibenz[c,e]oxepin derivatives 22+ attached to two cationic
chromophores at the 1,11-positions. These dications undergo
reversible C–C bond formation (‘‘dynamic redox properties’’)6
to give electron donors
1 with a 4,6,10,11-tetrahydro-
phenanthr[4,5-cde]oxepin skeleton (Scheme 1). Colorless donors
1 would exist as an inseparable mixture of enantiomers by a
facile ring flip,6c,d whereas the racemization of 22+ would be
completely suppressed7 by the large steric repulsion between
the bulky Ar2C+ units at the bay region. In this way, only the
neutral donors
1 would racemize (‘‘erasable/writable’’)
whereas the helicity in dications 22+ would be configurationally
stable (‘‘memorizing’’). Furthermore, the oxidation of 1 and
reduction of 22+ would occur at quite different potentials.
a Department of Chemistry, Faculty of Science, Hokkaido University,
Sapporo 060-0810, Japan. E-mail: tak@sci.hokudai.ac.jp;
Fax: +81 11 7062714; Tel: +81 11 7062714
b PRESTO, Japan Science and Technology Agency, Japan
w Electronic supplementary information (ESI) available: X-Ray structure
of 1b, VT-NMR data of 1a, spectral data of new compounds, and
another route to dibromine 3. CCDC 728166, 728167 and 762284. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc00026d.
Scheme 2
ꢁc
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Fig. 2 ORTEP drawings of (a) (M)-1a in rac-1 crystal and (b) (M)-
2a2+ (mol-1) in rac-2a2+(SbCl6ꢀ)2 crystal.
Scheme 3
is 1.631(2) A. The latter value is much greater than the standard
Csp3–Csp3 bond length (1.54 A) due to steric repulsion, which can
explain its facile cleavage upon 2e-oxidation to 2a2+. Two of the
four methoxyphenyl groups are located at pseudo-axial positions,
whereas the other two occupy pseudo-equatorial positions. They
do not interconvert in the crystal, but do so in solution. VT-NMR
analysis (C6D6, 300 MHz) of 1a showed that two sharp
resonances for the methoxy protons observed at 10 1C (3.09
and 3.28 ppm) coalesced upon heating to 40 1C (Fig. S1, ESIw),
which corresponds to a DGz of 15.6 kcal molꢀ1 for the ring flip
(k = 22.5 sꢀ1 at 25 1C). Thus, 1a is proven to act as an easily-
racemized ‘‘erasable/writable’’-state. Similarly, the energy barrier
for the racemization of 1b was determined to be 12.2 kcal molꢀ1
(k = 7580 sꢀ1 at 25 1C in CD2Cl2); the smaller value can be
explained by considering the higher planarity of the helical
tetracyclic core [dihedral angle of the biphenyl unit: 29.58(4)1]
as determined by the X-ray analysisz of rac-1b (Fig. S2).w
Scheme 4
conditions (Scheme 3). The deep red dication salt was then
transformed into colorless donor 1aw upon reduction with Zn
dust, which regenerated starting dication 2a2+ when treated
with two equivalents of (4-BrC6H4)3N+ꢂSbCl6ꢀ. Their redox
interconversion gave the products in good isolated yields. On
the other hand, upon the bis(quaternization) of helical diacridine
rac-5 with TfOMe, the fluorescent yellow dye rac-2b2+ (Ff =
0.35 in CH3CN) was obtained as a (TfOꢀ)2 saltw (Scheme 4),
which gave colorless dispiro donor 1b upon reduction.
Again, 1b was reoxidized to give 2b2+(SbCl6ꢀ)2 salt,w which
confirmed the reversible interconversion of 1b and 2b2+
.
In the crystal of 2a2+(SbCl6ꢀ)2, there are two crystallo-
graphically independent molecules (mol-1, mol-2) with similar
geometries. The separation between the two cationic centers is
3.52(4) or 3.53(3) A, and the dihedral angle for the biphenyl
unit in the condensed heterocycle is 56(3)1 or 51(4)1, respectively,
for mol-1 and mol-2. The p-conjugated system in 2a2+ forms a
helix-like structure with 1.5 turns (Fig. 2b), which makes the
racemization of 2a2+ very difficult. In fact, the NMR spectrum
of 2a2+ remained C2-symmetric even at 150 1C in 1,2-dichloro-
benzene-d4, and thus the configuration of the ‘‘memorizing’’-
state would be quite stable. The same is true for bis(10-methyl-
According to a voltammetric analysis (E/V vs. SCE), 2a/b2+
undergoes irreversible 2e-reduction at +0.29(CH2Cl2)/
ꢀ0.20(MeCN) V (Ered), respectively, and the return peak in the
far anodic region corresponds to the oxidation process of 1a/b
[Eox = +1.46(CH2Cl2)/+0.14(MeCN) V] (Fig. 1). The observed
separation of redox potentials provides the present pairs with high
electrochemical bistability to prevent the loss of chiral infor-
mation stored in 22+ even in the coexistence of 1 and 22+. Such
characteristics were later confirmed experimentally (vide infra).
According to an X-ray analysis,z the condensed heterocycle
in 1a adopts a helical structure, as expected (Fig. 2a). The
biphenyl unit in the condensed heterocycle makes a dihedral
angle of 32.83(5)1, and the polyarylated C10–C11 bond length
acridinium) dye 2b2+
.
In fact, optically pure salts of dication 2a/b2+ were obtained as
follows. We succeeded in the optical resolution of the precursor
diol rac-4 by using chiral HPLC at 25 1C (Sumichiral OA-2000,
AcOEt : CH2Cl2 : hexane = 1 : 2 : 4 with 0.5% Et3N,
recycled) (Fig. S3, ESIw). Optically pure (ꢀ)-4 {first fraction,
[a]D23 = ꢀ186 (c = 0.3 in CHCl3)} was converted to the chiral
ꢀ
dication salt 2a2+(BF4
)
2
as in the case of the racemic
compound. In the circular dichroism (CD) spectrum, it showed
a very strong positive couplet in the visible region [lext 559 nm
(De +182), 518 (ꢀ133) in CH2Cl2] (Fig. 3), which is due to
effective exciton coupling between the two cationic chromophores
[An2CPh+: UV-Vis (CH2Cl2): lmax 525 nm (log e 4.71)] arranged
in a helical configuration.10 The CD signals did not show any
decay over several days at 25 1C (DGz 4 25 kcal molꢀ1), which
confirmed the long-lasting chiral memory of this unit.
An optically pure salt of (M)-2b2+(OTfꢀ)2 was prepared
from the resolved precursor (M)-511 by N-methylation with
Fig. 1 Cyclic voltammogram of 2b2+(BF4ꢀ)2 salt in MeCN containing
0.1 mol dmꢀ3 Et4NClO4 (E/V vs. SCE, Pt electrode, scan rate 100 mV sꢀ1).
ꢁc
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facile process, the CD signals should have shown current-
independent decay in the middle of electrolysis, but this was not
the case (Fig. S4, ESIw).
The intermolecular transmission of asymmetric information
into these chiral memory units is the next step in our project,
and the catalytic asymmetric induction of 1 (biasing of the
P/M ratio) followed by oxidation to configurationally stable
22+ is now being studied to demonstrate the ‘‘writing’’-process
for the present chiral memory unit.
Notes and references
Fig. 3 CD spectrum of optically pure 2a2+(BF4ꢀ)2 prepared from (ꢀ)-4.
z Crystal data of 1a: C44H38O5,
M 646.78, monoclinic, P21/n,
a = 12.331(2), b = 20.121(4), c = 13.178(2) A, b = 94.0081(9)1,
V = 3261.6(10) A3, Z = 4, Dc 1.317 g cmꢀ3, independent reflection
TfOMe. It exhibits a large negative couplet in the CD spectrum
[lext 270 nm (De ꢀ36.4), 258 (+33.0) in CH2Cl2], and again the
Cotton effects remained unchanged over several days at 25 1C.
This material was subjected to electrochemical reduction, and the
changes were followed by three kinds of spectroscopy at 25 1C.
The UV-Vis spectral changes of 2b2+ into neutral donor 1b
exhibit several isosbestic points with a loss of absorptions in the
visible region, which were regenerated upon reoxidation (Fig. 4a).
Fluorescence of (M)-2b2+ was also lost upon reduction to 1b, yet
recovered completely upon reversal of the polarity of the electro-
des (Fig. 4b), which demonstrates clean interconversion between
1b and 2b2+. In contrast, the CD signals of (M)-2b2+ disappeared
gradually upon reduction, and were not reproduced upon
reoxidation (Fig. 4c), which demonstrates that the resulting
(M)-1b was easily racemized to lose chiral information. These
findings successfully demonstrate the electrochemical response of
a novel chiral memory unit (‘‘erasing’’-process of chiral memory).
If the electron-exchange between 1b and 2b2+ (Scheme 2) was a
7479 (all), 4304 (2s), T = 153 K, R = 3.6%, CCDC 728167. Crystal data
of rac-2a2+(SbCl6ꢀ)2: C44H38Cl12O5Sb2,
M
1315.72, triclinic, P1,
ꢀ
a = 12.073(4), b = 15.599(6), c = 27.406(11) A, a = 81.941(18)1,
b
=
80.772(18)1,
g
=
79.240(18)1,
V
=
4972(3) A3,
Z =
4,
Dc 1.757 g cmꢀ3, independent reflection 23827 (all), 2969 (2s), T = 153
K, R = 5.8%, CCDC 728166. Crystal data of 1b benzene solvate (1 : 1):
C48H38N2O, M 658.84, monoclinic, C2/c, a = 23.560(4), b = 11.900(2),
c = 13.927(2) A, b = 120.2315(5)1, V = 3373.5(9) A3, Z = 4, Dc 1.297 g
cmꢀ3, independent reflection 3776 (all), 3289 (2s), T = 153 K, R = 7.0%,
CCDC 762284.
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Fig. 4 (left) Changes in the (a) UV-Vis, (b) fluorescence (lexc 370 nm),
and (c) CD spectra of (M)-2b2+ (9.0 ꢃ 10ꢀ6 M) in MeCN (containing
0.05 M Et4NClO4) upon constant-current electrochemical reduction
(37 mA at 2.5 min interval, at 25 1C). Figures on the right show the
changes upon reoxidation of the as-prepared 1b (37 mA at 3 min interval).
ꢁc
This journal is The Royal Society of Chemistry 2010
4102 | Chem. Commun., 2010, 46, 4100–4102