Butane-1,4-diyl dications stabilized by steric factors: Electrochiroptical response systems based on reversible interconversion between dihydro[5]helicene-type electron acceptors and electron-donating 1,1′-binaphthyls

Article abstract of DOI:10.1039/b506435j

Incorporation in the dihydro[5]helicene framework prevents deprotonation of the title dications by steric factors, thus allowing their isolation as deeply colored stable salts. Based on the reversible interconversion with the electron-donating binaphthyli

Full text of DOI:10.1039/b506435j

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A R T I C L E
Butane-1,4-diyl dications stabilized by steric factors:
electrochiroptical response systems based on reversible
interconversion between dihydro[5]helicene-type electron acceptors
and electron-donating 1,1ꢀ-binaphthyls
Eisuke Ohta, Hiroki Higuchi, Hidetoshi Kawai, Kenshu Fujiwara and Takanori Suzuki*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo, 060-0810,
Japan. E-mail: tak@sci.hokudai.ac.jp; Fax: 81-11-706-2714; Tel: 81-11-706-2714
Received 9th May 2005, Accepted 9th June 2005
First published as an Advance Article on the web 7th July 2005
Incorporation in the dihydro[5]helicene framework prevents deprotonation of the title dications by steric factors, thus
allowing their isolation as deeply colored stable salts. Based on the reversible interconversion with the
electron-donating binaphthylic diolefins, they constitute a new class of electrochromic systems, in which C–C bond
making/breaking is accompanied by two-electron transfer. Optically pure (R)-binaphthylic donors are
interconvertible with the 1,4-dications with the R,R-configuration. The very large molar ellipticity makes it possible
for them to be used as electrochiroptical response systems, by which the electrochemical input is transduced into two
spectral outputs, i.e. UV-Vis and circular dichroism. Structurally related push–pull-type bis(quinonemethide)s also
exhibit a similar multi-output electrochemical response.
Introduction
The butane-1,4-diyl dications (A) belong to
a peculiar
class of carbocations that have been postulated as reactive
intermediates1,2 but not isolated or even detected spectroscopi-
cally due to their instability against rapid deprotonation to give
the corresponding 1,3-dienes (B) (Scheme 1). Since protonation
of dienes generally affords allyl cations (C),3 dimerization of the
olefin cation radicals (D) seems the only effective way to produce
these intriguing species.
Scheme 3
effective enough to stabilize 4b2+ against deprotonation, which
was thus transformed into the conjugated diene derivative in
situ upon oxidation of 3b.7 The different behavior between 4a2+
and 4b2+ is in line with the much smaller pKR value of (4-
+
MeOC6H4)2CPh+ (−1.24)8a than (4-Me2NC6H4)2CPh+ (6.90).8b
We envisaged that deprotonation from 1,4-dications (A) could
be suppressed not only by electronic effects but also by steric
factors. That is, the bulky substituents may kinetically protect
the acidic C–H groups against the base. Furthermore, the
deprotonated dienes may be thermodynamically destabilized by
severe deformation due to steric repulsion between substituents.
We have succeeded here in isolating new members of stable
butane-1,4-diyl dications incorporated in the dihydro[5]helicene
skeleton having two bis(4-methoxyphenyl)methylium (6b2+) or
two xanthenylium (6c2+) units (Scheme 4). Here we report the
preparation, redox properties, and X-ray structure of 6b,c2+ and
the electron-donating binaphthylic precursors 5b,c. It is note-
worthy that the optically active (R)-5b,c and (R,R)-6b,c2+ can
Scheme 1
During the course of our continuing studies4 on novel
electrochromic systems2,5 that undergo reversible C–C bond
making/breaking upon electron transfer,6 we found that strong
electron-donating groups such as the dimethylaminophenyl
group stabilize this type of dication effectively. Thus, upon
=
oxidative dimerization of (4-Me2NC6H4)2C CHPh 1, the salt
of 1,4-dication 22+ was obtained as the first isolable member of
the general formula A (Scheme 2).4a Similarly, another dication
4a2+ with the 9,10-dihydrophenanthrene skeleton was isolated
upon two-electron oxidation of the biphenyl-type donor 3a with
two bis(4-dimethylaminophenyl)ethenyl units (Scheme 3).4b In
contrast, the less electron-donating methoxyphenyl group is not
Scheme 2
Scheme 4
3 0 2 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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constitute novel electrochiroptical response systems, by which
the electrochemical input can be transduced into two different
spectroscopic outputs, i.e. UV-Vis and circular dichroism (CD).
Results and discussion
Preparation and X-ray structures of new electron donors 5b,c
The reaction of rac-2,2ꢀ-dimethyl-1,1ꢀ-binaphthyl with BuLi in
the presence of TMEDA gave 2,2ꢀ-bis(lithiomethyl) derivative,4b
which was then reacted with 4,4ꢀ-dimethoxybenzophenone to
give diol rac-7b. Dehydration to rac-5b proceeded smoothly by
treating with a catalytic amount of TsOH in refluxing benzene
(Dean–Stark) in a two-step yield of 78%. By starting with
optically pure (R)-dimethylbinaphthyl, (R)-5b was obtained as
a colorless solid by the same procedures in 66% yield over two
steps.
Fig. 2 (a) CD spectra of (R)-5b and (R,R)-6b2+ measured in MeCN.
(b) CD spectra of (R)-5c and (R,R)-6c2+ measured in MeCN.
According to the X-ray analysis on rac-5b (Fig. 1a), the
binaphthyl skeleton is twisted by 81.1(2)◦, and the two bis(4-
methoxyphenyl)ethenyl moieties are arranged close to each
other. The intramolecular separation between methine carbons
Compound rac-5c with two 9-xanthenylidene units as
electron-donating groups was prepared from rac-dimethyl-
binaphthyl and xanthone via diol rac-7c in 80% overall yield
as sparingly soluble pale yellow crystals. Solubility is much
higher for (R)-5c prepared from (R)-dimethylbinaphthyl in 43%
yield. The X-ray structure determined on rac-5c has several
common features to 5b: the binaphthyl skeleton is twisted almost
perpendicularly (dihedral angle 91.5(1)◦), and the separation
˚
of the 2,2-diarylethenyl units is 3.77 A, which is marginally larger
9
˚
than the sum of van der Waals radii (3.40 A). Optically pure (R)-
5b exhibits a strong negative couplet (A = 173) around 340 nm
in the CD spectrum (Fig. 2a), which can be accounted for by
assuming the similar conformation in solution to realize effective
exciton coupling10 between diarylethenyl chromophores.
˚
of two methine carbons is 3.79 A (Fig. 1b). This solid-state
structure is also suitable for exciton coupling to induce strong
CD signals as observed (Fig. 2b).
Redox properties of 5b and reversible interconversion with
1,4-dication 6b2+
According to the voltammetric study in MeCN (Fig. 3),
the binaphthylic donor 5b with four 4-methoxyphenyl groups
undergoes electrochemical oxidation at a peak potential of
ox
+1.07 V, which is close to that of biphenyl analogue 3b (Ep
+1.03 V).7 The corresponding reduction wave appeared in the
far cathodic region (Ep +0.25 V). Such a large shift of redox
red
peaks is indicative of the chemical process such as C–C bond
making that follows electron transfer.
Fig. 3 Cyclic voltammogram of 5b measured in MeCN containing
0.1 mol dm−3 Et4NClO4 (E/V vs SCE, scan rate 300 mV s−1, Pt electrode).
Upon treatment of colorless rac-5b with 2 equivalents of (4-
Fig. 1 (a) X-Ray structure of 5b determined on the benzene solvate at
153 K. (b) X-Ray structure of 5c determined at 300 K.
•
−
BrC6H4)3N+ SbCl6 in CH2Cl2, a deep red solid was obtained
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1
3 0 2 5

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in 82% isolated yield, which was assigned to the 1,4-dication salt
of rac-6b2+ on the basis of the 1H NMR spectrum with a sharp
singlet at d 6.16 for acidic methine protons. Its structure was
finally confirmed by the X-ray analysis (vide infra). Treatment
of this salt with Zn dust in MeCN gave the starting diolefin
rac-5b in quantitative yield. Such a high-yield interconversion
indicates that 5b and 6b2+ can be considered as a sort of reversible
redox pair (dynamic redox pair),6 for which the C–C bond
making/breaking is accompanied by 2e-transfer.
vacant p orbital of the methylium center (dihedral angle of ca.
60◦) also disfavors deprotonation of 5b to the conjugated diene.
In fact, the 1,4-dication 6b2+ is definitely stable under ambient
conditions, and several attempts at deprotonation even by using
small bases such as F− or H− caused recovery of 6b2+ or forma-
tion of intractable material. When the salt of rac-6b2+ was treated
with aqueous NaOH solution, the bis(quinonemethide) deriva-
tive rac-8b was obtained in quantitative yield, which is the hy-
drolysis product at the methoxy terminus of the diarylmethylium
dyes.11a These results clearly show that steric bulkiness11b,c by
the dihydro[5]helicene framework in 6b2+ suppresses nucleophilic
attack on the methylium centers or abstraction of acidic C–H
protons by base, showing the validity of our molecular design
concept for the sterically stabilized 1,4-dications. Upon similar
alkaline hydrolysis of rac-6a2+ with quadruple 4-dimethylamino
substitution4b rac-8a was also obtained in quantitative yield.
X-Ray structure of dication 6b2+ and alkaline hydrolysis to
bis(quinonemethide) 8b
The dication 6b2+ is located on the crystallographic 2-fold axis,
and the newly formed central 6-membered ring adopts the half-
chair conformation (Fig. 4a). The bulky diarylmethylium units
are located on the pseudo-equatorial positions. Absence of meso-
isomer of 6b2+ upon oxidation of 5b can be accounted for by its
larger steric hindrance caused by positioning one of the two
bulky dye units at the pseudo-axial position. The acidic methine
protons of 6b2+ are located at the pseudo-axial positions and
concealed deeply inside the aryl p cloud. So that, the bases are
prevented from approaching these protons. At the same time,
the largely twisted arrangement of the C–H r orbital and the
Redox properties of 5c and reversible interconversion with
1,4-dication 6c2+
In view of the success in isolating the 1,4-dication 6b2+ with four
4-methoxyphenyl groups, less sterically hindered 6c2+ with two
xanthenylium units was chosen as the next target. As shown
+
by comparisons of the pKR values of 9-phenylxanthenylium
(0.81)8c and (4-MeOC6H4)2CPh+, the former is the thermody-
namically more stable cation. Thus, 6c2+ may still have a chance
to be a persistent species despite the lesser degree of kinetic
protection than for 6b2+.
The oxidation potential of diolefin 5c (Epox +0.94 V) is slightly
less positive than that of 5b, and the corresponding reduction
red
peak was again observed in the far cathodic region (Ep
+0.22 V). Upon treatment of pale yellow rac-5c with 2 equiv-
•
alents of (4-BrC6H4)3N+ SbCl6 in CH2Cl2, a greenish yellow
−
solid of the 1,4-dication salt rac-6c2+ (SbCl6−)2 was obtained
in good isolated yield of 82%, which regenerated the starting
diolefin rac-5c in 97% yield upon treatment with Zn dust in
MeCN. The X-ray analysis of the salt indicates that 6c2+ adopts
a similar geometry to 6b2+ (Fig. 4b).
In this way, we could demonstrate that the 1,4-dication with
xanthenylium units is also a stable and persistent species that
does not undergo facile deprotonation. When rac-6c2+ (SbCl6
)
−
2
was treated with aqueous NaHCO3 in MeCN, tetrahydrofuran
derivative rac-9c was obtained in 84% yield. Its structure was
unambiguously determined by X-ray analysis (Fig. 5), and the
two methine protons are located in the trans manner at the
pseudo-axial positions. In contrast to the reaction of tetrakis(4-
methoxyphenyl) derivative 6b2+, hydroxide ion did attack the
cationic center of 6c2+. This is in accord with the less hindered
structure of 6c2+ than 6b2+. It is noteworthy that the adjacent
methine protons remain intact. These results indicate that the
E1 process of 6c2+ to form the corresponding diene is completely
suppressed due to the severe steric hindrance in the deprotonated
product.
Electrochiroptical response of chiral redox pairs: (R)-5b,c and
6b,c2+
−
The high yield interconversion between 5b,c and 6b,c2+ indicates
that they can be considered as a sort of reversible redox pair.
The C–C bond making/breaking accompanied by electron
Fig. 4 (a) X-Ray structure of 6b2+ determined on the SbCl6 − salt
at 300 K. (b) X-Ray structure of 6c2+ determined on the SbCl6 salt
(dichloromethane solvate) at 153 K.
3 0 2 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1

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reducing the chance of side reactions from the labile open-shell
species.2b
When a similar electrochemical oxidation was conducted by
using optically pure (R)-5b and followed by CD spectroscopy,
drastic changes with several isosbestic points were observed as
shown in Fig. 6b. Thus, the redox pair of (R)-5b and (R,R)-
6b2+ provides a new entry into electrochiroptical systems,4b,12 the
less well-developed category of multi-output response systems.
In the preparative scale experiments, (R)-5b was converted to
the salt of (R,R)-6b2+ in 82% yield, which regenerated (R)-5b
in quantitative yield. These transformations did not cause any
deterioration of optical purity.
Compared with bis(4-methoxyphenyl)methylium, the xan-
thenylium chromophore does not exhibit strong absorption in
the long-wavelength region. So that, the electrolysis of rac-
5c [kmax 346 (log e 4.57)] to rac-6c2+ [363 (4.61), 457 (3.80),
597 (3.37)] did not induce spectacular changes in the UV-Vis
spectrum (Fig. 7a). However, huge changes were observed in the
CD spectrum again with several isosbestic points. For example,
molar ellipticity at 255 nm is +18 for (R)-5c which becomes −231
in the oxidized form (R,R)-6c2+, thus the electrochemical input
causes DDe of ca. 250 for the electrochiroptical pair of (R)-5c
and (R,R)-6c2+ at this wavelength. This is one of the largest CD
outputs ever reported upon redox reactions. In the preparative
scale experiments, (R)-5c and (R,R)-6c2+ could be interconverted
without decrease in optical activity.
Fig. 5 X-Ray structure of 9c determined on the chloroform solvate at
300 K. The dihedral angle of two naphthalene units is 48.3(1)◦. The
central 6-membered ring adopts the half-chair conformation, to which
the tetrahydrofuran ring is fused in a trans fashion.
transfer (dynamic redox properties)6 endows the pairs with high
electrochemical bistability. A drastic color difference between
5b,c and 6b,c2+ prompted us to explore their electrochemical
response in solution, which was first followed by UV-Vis
spectroscopy.
When the electrolysis of rac-5b [kmax 325 nm (log e 4.63)] was
monitored, smooth and clean oxidation to rac-6b2+ [506 (4.81)]
was observed with a vivid change in color from colorless to deep
red. The spectroelectrogram exhibits several isosbestic points, in-
dicating the negligible concentration of the intermediary cation
radical (Fig. 6a). Such behavior has been commonly observed
in dynamic redox pairs,6 and provides favorable conditions to
realize molecular response systems with high reversibility by
Fig. 7 (a) Changes in the UV-Vis spectrum and (b) the CD spectrum of
optically pure (R)-5c (3 mL, 1.12 × 10−5 mol dm−3 in MeCN containing
0.05 mol dm−3 Bu4NBF4 as a supporting electrolyte) to (R,R)-6c2+ upon
constant-current electrochemical oxidation (30 lA, 5 min intervals).
Electrochiroptical response of bis(quinonemethide) 8a,b
As in the cases of p-quinonemethide derivatives substituted with
the electron-donating unit at C7 position,2b,11b,c 8a [kmax 460
red
(log e 4.53); Ep −1.08 V] and 8b [375 (4.69); −1.14] exhibit
remarkable absorption in the visible region. Although they are
weaker electron acceptors than the corresponding dications
6a2+ (Ep −0.43 V) or 6b2+ (+0.25), their reduction may
red
induce C–C bond cleavage to give the binaphthylic bisphenol
10a,b (Scheme 5). In fact, treatment of rac-8a,b with SmI2
in THF gave the bisphenol rac-10a,b in respective isolated
Fig. 6 (a) Changes in the UV-Vis spectrum of rac-5b (3 mL, 3.94 ×
10−5 mol dm−3 in MeCN containing 0.05 mol dm−3 Bu4NBF4 as a sup-
porting electrolyte) to rac-6b2+ upon constant-current electrochemical
oxidation (30 lA, 5 min intervals). (b) Changes in the CD spectrum of
optically pure (R)-5b (3 mL, 1.07 × 10−5 mol dm−3 in MeCN containing
0.05 mol dm−3 Bu4NBF4 as a supporting electrolyte) to (R,R)-6b2+ upon
constant-current electrochemical oxidation (30 lA, 5 min intervals).
Scheme 5
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1
3 0 2 7

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yields of 85 and 91% (isomer mixture), which regenerated
rac-8a,b upon oxidation with DDQ in 67% and quantitative
yield, respectively. In this way, 8a,b and 10a,b are shown to
constitute the interconvertible redox pair13 that undergo C–C
bond making/breaking upon 2e-transfer.
redox pairs, 5b–6b2+ and 8b–10b, can be coupled by chemical
transformation. This example provides unique a prototype to
study the multi-input response systems.
Conclusion
Since bisphenols are only faintly colored, interconversion
between 8a,b and 10a,b causes a vivid color change. Fig. 8
shows the spectroelectrogram obtained upon electrochemical
reduction of the quinonemethide rac-8a,b. It is evident that ab-
sorption bands in the visible region for the push–pull compound
8a,b diminished gradually with isosbestic point. Furthermore,
optically pure (S,S)-8a,b obtained by hydrolysis of (R,R)-6a,b2+
exhibited drastic changes in CD spectrum upon electrochemical
reduction to (R)-10a,b (Fig. 9). These results indicate that
newly prepared bis(quinonemethide)s 8a,b can also serve as
electrochiroptical materials.
As described in this paper, we have succeeded in stabilizing the
intrinsically labile butane-1,4-diyl dication (A) by incorporation
into the dihydro[5]helicene framework. The newly isolated
dications 6b,c2+ are interesting dyes that serve as electrochromic
systems on the basis of reversible interconversion with the
binaphthylic donors 5b,c. Configurational stability in terms
of axial chirality of 5b,c as well as point chiralities (helicity)
of 6b,c2+ endow the present systems with the controllable
chiroptical properties by electron transfer. Electrochiroptical
response was also realized upon electrolysis of novel push–pull-
type quinonemethides-type acceptors 8a,b.
Experimental
Preparation of 2,2ꢀ-bis[2,2-bis(4-methoxyphenyl)ethenyl]-
binaphthyl (5b)
To a solution of rac-2,2ꢀ-dimethylbinaphthyl (959 mg, 3.4 mmol)
in dry TMEDA (1.70 mL) was added dropwi◦se BuLi
(1.58 mol dm−3 in n-hexane, 4.73 mL, 7.5 mmol) at 23 C under
Ar, and the mixture was stirred for 20 h at 70 ◦C. Then, to
the resultant suspension of 2,2ꢀ-bis(lithiomethyl)binaphthyl was
added a suspension of 4,4ꢀ-dimethoxybenzophenone (1.81 g,
7.5 mmol) in dry THF (100 mL) at −20 ◦C, and the suspension
was kept at this temperature for 2 h. The whole mixture was
allowed to warm and stirred for 4 h at 23 ◦C. After addition
of 30 mL of water, the suspension was extracted with 1,2-
dichloroethane. The organic layer was washed with brine and
dried over MgSO4. Evaporation of solvent followed by chro-
matographic separation (Al2O3, benzene–EtOAc) gave rac-2,2ꢀ-
bis[2,2-bis(4-methoxyphenyl)-2-hydroxyethyl]binaphthyl rac-7b
(2.19 g) as a yellow solid.
Fig. 8 (a) Changes in the UV-Vis spectrum of rac-8a (3 mL, 2.71 ×
10−6 mol dm−3 in MeCN containing 0.05 mol dm−3 Bu4NBF4 as a
supporting electrolyte) upon constant-current electrochemical reduction
(0.1 mA, 5 min intervals). (b) Changes in the UV-Vis spectrum of rac-8b
(3 mL, 6.87 × 10−6 mol dm−3 in MeCN containing 0.05 mol dm−3
Bu4NBF4 as a supporting electrolyte) upon constant-current electro-
chemical reduction (0.1 mA, 5 min intervals).
To a solution of rac-7b in benzene (150 mL) was added a
catalytic amount of TsOH (17.5 mg). After the mixture was
refluxed for 5 h under dehydrating conditions (Dean–Stark),
the mixture was washed with water and brine, and dried over
MgSO4. Evaporation of solvent and chromatographic separa-
tion on Al2O3 (benzene) gave rac-5b (1.96 g) as a pale yellow solid
in 78% yield over two steps from rac-2,2ꢀ-dimethylbinaphthyl.
An analytical sample was obtained by recrystallization from
benzene as a solvated crystal (5b·benzene).
Optically pure (R)-5b was similarly obtained in 66% yield over
two steps by starting with (R)-2,2ꢀ-dimethylbinaphthyl.
Data for rac-5b. Mp 248–251 ◦C (decomp.); 1H NMR
(300 MHz, CDCl3) d/ppm 7.77 (2H, d, J = 7.8 Hz), 7.49 (2H,
d, J = 8.7 Hz), 7.40 (2H, ddd, J = 7.8, 7.8, 1.5 Hz), 7.22 (2H,
ddd, J = 7.8, 7.8, 1.5 Hz), 7.09 (2H, d, J = 7.8 Hz), 7.05 (2H, d,
J = 8.7 Hz), 6.99 (4H, AAꢀXXꢀ), 6.72 (4H, AAꢀXXꢀ), 6.36 (4H,
AAꢀXXꢀ), 6.36 (2H, s), 3.76 (6H, s), 3.67 (6H, s); IR (KBr) 1604,
1510, 1249, 1173, 1033, 833, 680 cm−1; FD-MS m/z 730 (M+,
BP), 731 (M+ + 1); UV-vis (CH3CN) kmax 325 (log e 4.63), 285
sh (4.61), 255 sh (4.68), 238 (4.77), 215 (4.91) nm. Anal. Calcd.
for C52H42O4: C, 85.45; H, 5.79. Found: C, 85.62; H, 6.00%.
Data for (R)-5b: mp 143–153 ◦C (decomp.); [a]2D7 +792 (c 0.114
in CH3CN); CD (CH3CN) k 361 (De −42), 319 (131), 282 (−8),
258 (21), 243 (−26), 217 (150) nm.
Fig. 9 (a) Changes in the CD spectrum of optically pure (S,S)-8a (3 mL,
2.71 × 10−6 mol dm−3 in MeCN containing 0.05 mol dm−3 Bu4NBF4
as a supporting electrolyte) upon constant-current electrochemical
reduction (0.1 mA, 5 min intervals). (b) Changes in the CD spectrum
of optically pure (S,S)-8b (3 mL, 5.89 × 10−6 mol dm−3 in MeCN
containing 0.05 mol dm−3 Bu4NBF4 as a supporting electrolyte) upon
constant-current electrochemical reduction (0.1 mA, 5 min intervals).
Preparation of 2,2ꢀ-bis(9-xanthenylidenemethylidene)binaphthyl
(5c)
To a solution of rac-2,2ꢀ-dimethylbinaphthyl (554 mg, 2.0
mmol) in dry TMEDA (1.5 mL) was added dropwise BuLi
◦
It is interesting to note that bisphenol rac-10b was easily
reconverted into the tetramethoxy derivative rac-5b in 76% yield
by treatment with NaH–MeI in THF. In this way, the two
(1.59 mol dm−3 in n-hexane, 3.00 mL, 4.8 mol) at 24 C under
Ar, and the mixture was stirred for 20 h at 70 ◦C. Then,
to the resultant suspension of 2,2ꢀ-bis(lithiomethyl)binaphthyl
3 0 2 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1

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−
was added a solution of xanthone (935 mg, 4.8 mmol) in dry
THF (50 mL) at −20 ◦C, and the suspension was kept at
this temperature for 2 h. The whole mixture was allowed to
Oxidation of diolefin 5c to dication salt 6c2+(SbCl6
)
2
To a solution of diolefin rac-5c (103 mg, 0.16 mmol) in dry
•
CH2Cl2 (70 mL) was added (4-BrC6H4)3N+ SbCl6 (263 mg,
−
◦
warm and stirred for 1 h at 24 C. After addition of 30 mL of
0.32 mmol) under Ar, and the mixture was stirred for 26 h at
water, the suspension was extracted with 1,2-dichloroethane.
The organic layer was washed with brine and dried over
MgSO4. Evaporation of solvent followed by chromatographic
separation (Al2O3, hexane–EtOAc) gave rac-2,2ꢀ-bis(9-hydroxy-
9-xanthenylmethyl)binaphthyl 7c (1.13 g) as a pale yellow solid.
To a solution of the diol 7c in benzene (100 mL) was added
a catalytic amount of TsOH (10.0 mg). After refluxing for
13 h under dehydrating conditions (Dean–Stark), the mixture
was washed with water and brine, and dried over MgSO4.
Evaporation of solvent and chromatographic separation on SiO2
(CHCl3) gave rac-5c (856 mg) as a pale yellow solid in 80% yield
over two steps from rac-2,2ꢀ-dimethylbinaphthyl. An analytical
sample was obtained by recrystallization from benzene.
23 ◦C. Addition of dry ether (70 mL) and removal by filtration of
a dark brown powder gave dication salt rac-6c2+(SbCl6−)2 in 82%
yield. Similarly, (R)-5c was transformed into (R,R)-6c2+(SbCl6
)
−
2
in 72% yield.
Data for rac-6c2+(SbCl6−)2. Mp 183 ◦C (decomp.); 1H NMR
(300 MHz, CD3CN) d/ppm 8.58 (2H, dd, J = 8.7, 1.2 Hz),
8.52 (2H, dd, J = 8.7, 1.2 Hz), 8.38 (2H, ddd, J = 8.7, 6.9, 1.2
Hz), 8.31 (2H, ddd, J = 8.7, 6.9, 1.2 Hz), 8.07 (2H, dd, J = 8.7,
1.2 Hz), 8.07 (2H, dd, J = 8.7, 1.2 Hz), 8.03 (2H, dd, J = 8.7,
1.2 Hz), 7.85 (2H, d, J = 8.4 Hz), 7.73–7.62 (8H, m), 7.51 (2H,
ddd, J = 8.7, 6.9, 1.5 Hz), 7.39 (2H, s) 6.85 (2H, d, J = 8.4 Hz);
IR (KBr) 1596, 1576, 1538, 1512, 1365, 750 cm−1; FD-MS m/z
635 (M+ − 3, BP); UV-vis (CH3CN) kmax 597 (log e 3.37), 457
(3.80), 363 (4.61), 255 (5.08), 222 (5.15) nm. Anal. Calcd. for
C48H30O2Cl12Sb2: C, 44.09; H, 2.31; Cl, 32.53. Found: C, 44.20;
Optically pure (R)-5c was similarly obtained in 43% yield over
two steps by starting with (R)-2,2ꢀ-dimethylbinaphthyl.
◦
H, 2.56; Cl, 32.35%. Data for (R,R)-6c2+(SbCl6−)2: mp 174 C
Data for rac-5c. Mp 282 ◦C (decomp.); 1H NMR (300 MHz,
CDCl3) d/ppm 7.95 (2H, d, J = 8.4 Hz), 7.82 (2H, d, J = 8.7
Hz), 7.55 (2H, d, J = 8.7 Hz), 7.51–7.46 (4H, m), 7.29–7.26 (4H,
m), 7.17–7.12 (2H, m), 7.04 (2H, ddd, J = 8.4, 7.8, 1.5 Hz), 7.01
(2H, d, J = 8.7 Hz), 6.97 (2H, dd, J = 8.4, 1.2 Hz), 6.91–6.89
(4H, m), 6.52 (2H, s) 6.42 (2H, ddd, J = 7.5, 7.2, 1.5 Hz); IR
(KBr) 1452, 1281, 1248, 766, 754 cm−1; FD-MS m/z 638 (M+,
BP), 639 (M+ + 1); UV-vis (CH3CN) kmax 346 (log e 4.57), 284
(4.51), 222 (5.16) nm. Anal. Calcd. for C48H30O2: C, 90.17; ◦H,
4.61. Found: C, 90.38; H, 4.70%. Data for (R)-5c: mp 160 C
(decomp.); [a]2D2 +657 (c 0.0310 in CH3CN); CD (CH3CN) k 381
(De −18), 340 (82), 310 (48), 277 (−64), 256 (18), 232 (38), 223
(41), 211(86) nm.
(decomp.); CD (CH3CN) k 389 (De +34), 364 (−29), 323 (−13),
268 (+78), 255 (−231), 228 (−34), 219 (+123) nm.
Reduction of dication salt 6c2+(SbCl6−)2 to diolefin 5c
To a suspension of rac-6c2+(SbCl6
) (7.7 mg, 5.9 lmol) in
−
2
CH3CN (5 mL) was added Zn powder (7.7 mg, 0.12 mmol),
and the mixture was stirred overnight at 23 ◦C. The mixture was
diluted with water and extracted with benzene. The organic layer
was washed with brine and dried over MgSO4. Evaporation of
solvent gave a pale yellow solid of diolefin rac-5c (3.7 mg) in
97% yield. Similarly, (R,R)-6c2+(SbCl6−)2 was transformed into
(R)-5b in quantitative yield.
Hydrolysis of dication salt 6a2+(I3−)2 to quinonemethide 8a
Oxidation of diolefin 5b to dication salt 6b2+(SbCl6
)
−
2
To a solution of rac-6a2+(I3
)
(92 mg, 60 lmol) in CH3CN
−
4b
2
To a solution of diolefin rac-5b benzene solvate (74.4 mg,
(50 mL) was added aqueous NaOH (0.1 mol dm−3, 5.0 mL,
0.5 mmol), and the mixture was stirred for 6 h at 23 ◦C. The
mixture was diluted with aqueous Na2S2O3 and extracted with
EtOAc. The organic layer was washed with brine and dried over
MgSO4. Filtration through an Al2O3 column and evaporation
of solvent gave deep red solid of quinonemethide rac-8a (43 mg)
in quantitative yield. An analytical sample was obtained by
recrystallization from CH3CN. Similarly, (R,R)-6a2+(I3−)2 was
transformed into (S,S)-8a in 96% yield.
•
92 lmol) in dry CH2Cl2 (10 mL) was added (4-BrC6H4)3N+
−
SbCl6 (149 mg, 0.18 mmol) under Ar, and the mixture was
stirred for 14 h at 23 ◦C. Addition of dry ether (30 mL) and
removal by filtration of dark green powder gave dication salt rac-
6b2+(SbCl6−)2 in 82% yield. Similarly, (R)-5b was transformed
into (R,R)-6b2+(SbCl6−)2 in 82% yield.
Data for rac-6b2+(SbCl6−)2. Mp 189–190 ◦C; 1H NMR
(300 MHz, CD3CN) d/ppm 8.07 (2H, d, J = 8.1 Hz), 7.96
(2H, d, J = 8.1 Hz), 7.83 (4H, AAꢀXXꢀ), 7.63 (2H, ddd, J =
8.1, 6.6, 1.2 Hz), 7.57 (2H, d, J = 8.4 Hz), 7.43 (2H, ddd, J =
8.1, 6.6, 1.2 Hz), 7.32 (4H, AAꢀXXꢀ), 7.20 (4H, AAꢀXXꢀ), 7.02
(4H, AAꢀXXꢀ), 7.01 (2H, d, J = 8.4 Hz), 6.16 (2H, s), 4.17
(6H, s), 3.95 (6H, s); IR (KBr) 1606, 1576, 1399, 1314, 1284,
1161 cm−1; FD-MS m/z 730 (M+, BP), 731 (M+ + 1); UV-vis
(CH3CN) kmax 506 (log e 4.81), 339 (4.43), 316 (4.43), 272 (4.77),
220 (5.01) nm. Anal. Calcd. for C52H42O4Cl12Sb2: C, 44.62; H,
3.02; Cl, 30.39. Found: C, 45.30; H, 3.23; Cl, 29.69%. Data for
(R,R)-6b2+(SbCl6−)2: mp 169–171 ◦C; CD (CH3CN) k 538 (De
−24), 497 (−22), 362 (−15), 334 (+21), 256 (−115), 228 (−72),
218 (121) nm.
Data for rac-8a. Mp 223 ◦C (decomp.); 1H NMR (300 MHz,
CDCl3) d/ppm 7.94 (2H, d, J = 8.4 Hz), 7.85 (2H, d, J = 8.7 Hz),
7.52 (2H, ddd, J = 8.4, 7.8, 0.9 Hz), 7.51 (2H, d, J = 8.7 Hz),
7.42 (2H, dd, J = 9.9, 3.0 Hz), 7.36 (2H, d, J = 8.4 Hz), 7.31
(2H, ddd, J = 8.4, 7.8, 0.9 Hz), 6.77 (4H, AAꢀXXꢀ), 6.73 (2H, dd,
J = 10.2, 3.0 Hz), 6.43 (4H, AAꢀXXꢀ), 6.34 (2H, dd, J = 9.9, 2.1
Hz), 5.94 (2H, dd, J = 10.2, 2.1 Hz), 5.18 (2H, s), 2.92 (12H, s);
IR (KBr) 3429, 1629, 1595, 1503, 1172 cm−1; FD-MS m/z 728
(M+ + 1, BP); UV-vis (CH3CN) kmax 460 (log e 4.53), 331 (4.51),
246 (4.73), 221 (5.00) nm. Anal. Calcd. for C52H42N2O2·H2O:
C, 83.84; H, 5.95, N; 3.76. Found: C, 83.55; H, 5.88; N, 3.76%.
Data for (S,S)-8a: mp 209 ◦C (decomp.); CD (CH3CN) k 465 (De
−29), 365 (−14), 336 (+31), 285 (+28), 258 (−210), 226 (−58),
216 (180) nm.
Reduction of dication salt 6b2+(SbCl6−)2 to diolefin 5b
Hydrolysis of dication salt 6b2+(SbCl6−)2 to quinonemethide 8b
To a solution of rac-6b2+(SbCl6−)2 (30 mg, 21 lmol) in CH3CN (5
mL) was added Zn powder (20 mg, 0.31 mmol), and the mixture
was stirred overnight at 24 ◦C. The mixture was diluted with
water and extracted with benzene. The organic layer was washed
with brine and dried over Na2SO4. Evaporation of solvent gave a
pale yellow solid of diolefin rac-5b (16 mg) in quantitative yield.
Similarly, (R,R)-6b2+(SbCl6−)2 was transformed into (R)-5b in
quantitative yield.
To a solution of rac-6b2+(SbCl6−)2 (130 mg, 86 lmol) in CH3CN
(100 mL) was added aqueous NaOH (0.1 mol dm−3, 5.0 mL,
0.5 mmol), and the mixture was stirred for 5 min at 23 ◦C. The
mixture was diluted with water and extracted with EtOAc. The
organic layer was washed with brine and dried over MgSO4.
Filtration through a SiO2 column and evaporation of solvent
gave a deep yellow solid of quinonemethide rac-8b (60.6 mg)
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1
3 0 2 9

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in quantitative yield. An analytical sample was obtained by
recrystallization from CH3CN as a solvated crystal (8b·CH3CN).
Similarly, (R,R)-6b2+(SbCl6−)2 was transformed into (S,S)-8b in
quantitative yield.
Reduction of quinonemethide 8b to bisphenol 10b
A solution of rac-8b CH3CN solvate (36 mg, 48 lmol) in dry
THF (20 mL) was degassed by bubbling with Ar, then a solution
of SmI2 in THF (0.1 mol dm−3, 100 mL, 10 mmol) was added
at 23 ◦C. After stirring at this temperature for 5 h, the mixture
was diluted with water and extracted with EtOAc. The organic
layer was washed with brine and dried over MgSO4. Filtration
through a Florisil column and evaporation of solvent gave a
pale yellow solid of bisphenol 10b (31 mg) in 91% yield. This
material is an inseparable mixture of three isomers in terms of
E- and Z-diarylethenyl units.
Data for rac-8b. Mp 207 ◦C (decomp.); 1H NMR (300 MHz,
CDCl3) d/ppm 7.96 (2H, d, J = 8.1 Hz), 7.88 (2H, d, J = 8.4 Hz),
7.54 (2H, dd, J = 8.1, 7.8 Hz), 7.47 (2H, d, J = 8.4 Hz), 7.37–
7.29 (6H, m), 6.82 (4H, br), 6.70 (4H, AAꢀXXꢀ), 6.64 (2H, dd,
J = 9.9, 3.0 Hz), 6.35 (2H, dd, J = 10.2, 2.1 Hz), 6.06 (2H,
dd, J = 9.9, 2.1 Hz), 5.05 (2H, s) 3.73 (6H, s); IR (KBr) 3438,
1632, 1604, 1505, 1254, 1168 cm−1; FD-MS m/z 700 (M+, BP),
701 (M+ + 1); UV-vis (CH3CN) kmax 375 (log e 4.69), 310 (4.51),
240 (4.69), 221 (5.01) nm. Anal. Calcd. for C50H36O4·CH3CN:
C, 84.19; H, 5.30. Found: C, 84.55; H, 5.25%. Data for (S,S)-8b:
mp 197 ◦C (decomp.); CD (CH3CN) k 419 (De + 20), 375 (−70),
344 (+10), 292 (+21), 255 (−180), 228 (−125), 218 (+138) nm.
Data for 10b. 1H NMR of major isomer (300 MHz, CDCl3)
d/ppm 7.77 (2H, d, J = 7.5 Hz), 7.49 (2H, d, J = 8.7 Hz), 7.40
(2H, ddd, J = 7.5, 7.5, 1.2 Hz), 7.22 (2H, ddd, J = 7.5, 7.5,
1.2 Hz), 7.08 (2H, d, J = 7.5 Hz), 7.02 (2H, d, J = 8.7 Hz),
6.78 (4H, AAꢀXXꢀ), 6.95 (4H, AAꢀXXꢀ), 6.64 (4H, AAꢀXXꢀ),
6.37 (4H, AAꢀXXꢀ), 6.35 (2H, s), 3.68 (6H, s); IR (KBr) 3413,
1606, 1510, 1245, 1171, 1105, 1032, 833, 820 cm−1; FD-HR-MS
Calcd. for C50H38O4: 702.2770 Found: 702.2771.
Hydrolysis of dication salt 6c2+(SbCl6−)2 to tetrahydrofuran
derivative 9c
To a solution of rac-6c2+(SbCl6−)2 (32.0 mg, 24 lmol) in CH3CN
(25 mL) was added 0.5 mL of saturated aqueous NaHCO3, and
the mixture was stirred for 30 min at 23 C. The mixture was
Oxidation of bisphenol 10b to quinonemethide 8b
◦
To a solution of rac-10b (1.1 mg, 1.4 lmol) in CH2Cl2 (1.0 mL)
and water (1.0 mL) was added DDQ (1.2 mg, 5.3 lmol), and
the mixture was stirred for 5 min at 23 ◦C. The mixture was
diluted with water and extracted with CH2Cl2. The organic
layer was washed with brine and dried over MgSO4. Filtration
through a SiO2 column and evaporation of solvent gave 1.1 mg
of quinonemethide rac-8b in quantitative yield.
diluted with water and extracted with benzene. The organic layer
was washed with brine and dried over MgSO4. Evaporation
of solvent and chromatographic separation on SiO2 (hexane–
CHCl3) gave rac-9c (13.4 mg) as a colorless solid in 84% yield.
An analytical sample was obtained by recrystallization from
benzene–hexane as a solvated crystal (9c·benzene).
Data for rac-9c. Mp 261–264 ◦C; 1H NMR (300 MHz,
CDCl3) d/ppm 7.79 (2H, d, J = 8.1 Hz), 7.60 (2H, d, J =
8.4 Hz), 7.42–7.29 (12H, m), 7.27–7.12 (8H, m), 7.06 (2H, ddd,
J = 7.1, 7.0, 1.8 Hz), 6.82 (2H, ddd, J = 7.5, 7.5, 1.1 Hz), 5.18
(2H, s); IR (KBr) 3427, 1598, 1477, 1448, 1321, 1302, 1261, 1246,
1230, 905, 820, 761, 751, 744, 685 cm−1; FD-MS m/z 654 (M+,
BP), 655 (M+ + 1); UV-vis (CH3CN) kmax 355 (log e 4.08), 341
(4.14), 327 (4.02), 287 (4.15), 243 sh (4.82), 224 (5.20) nm. Anal.
Calcd. for C48H30O3·C6H6: C, 88.50; H, 4.95. Found: C, 88.28;
H, 4.91%.
Methylation of bisphenol 10b to tetrakis(methoxyphenyl)
derivative 5b
To a solution of rac-10b (2.4 mg, 3.4 lmol) in dry THF (5.0 mL)
was added NaH (60% oil dispersion, 1.0 mg, 25 lmol) under
Ar at 0 ◦C, and the mixture was stirred at this temperature for
30 min. CH3I (50 lL, 0.80 mmol) was added to the mixture.
Then, the resultant mixture was allowed to warm and stirred for
30 min at 23 ◦C. After addition of 8 mL of water, the suspension
was extracted with benzene. The organic layer was washed with
brine and dried over MgSO4. Evaporation of solvent followed
by chromatographic purification (SiO2, benzene) gave 1.9 mg of
rac-5b in 76% yield.
Reduction of quinonemethide 8a to bisphenol 10a
A solution of rac-8a (29 mg, 39 lmol) in dry THF (40 mL)
was degassed by bubbling with Ar, then a solution of SmI2 in
THF (0.1 mol dm−3, 5 mL, 0.5 mmol) was added at 23 ◦C. After
stirring at this temperature for 2 h, the mixture was diluted
with water and extracted with EtOAc. The organic layer was
washed with brine and dried over MgSO4. Filtration through a
Florisil column and evaporation of solvent gave a pale yellow
solid of bisphenol 10a (25 mg) in 91% yield. This material is
an inseparable mixture of three isomers in terms of E- and Z-
diarylethenyl units.
Data for 10a. 1H NMR of major isomer (300 MHz, CDCl3)
d/ppm 7.75 (2H, d, J = 7.8 Hz), 7.45 (2H, d, J = 8.7 Hz), 7.37
(2H, dd, J = 7.8, 7.8 Hz), 7.17 (2H, dd, J = 7.8, 7.8 Hz), 7.00
(2H, d, J = 7.8 Hz), 7.00 (2H, d, J = 8.7 Hz), 6.83 (4H, AAꢀXXꢀ),
6.79 (4H, AAꢀXXꢀ), 6.32 (2H, s), 6.27 (4H, AAꢀXXꢀ), 6.23 (4H,
AAꢀXXꢀ), 4.62 (2H, s), 2.78 (12H, s); IR (KBr) 3417, 1608, 1510,
1356, 1261, 1225, 1197, 1168, 817 cm−1; FD-HR-MS Calcd. for
C54H44N2O2: 728.3403 Found: 728.3403.
Redox potential measurements
Redox potentials (Eox and Ered) were measured by cyclic
voltammetry in dry MeCN containing 0.1 mol dm−3 Et4NClO4
as a supporting electrolyte. Ferrocene undergoes one-electron
oxidation at +0.38 V under the same conditions. All of the values
shown in the text are in E/V vs SCE. All the voltammograms
are irreversible because of the redox switching of single bonds,
so that, the peak potentials (Ep) were given in the text.
Crystallographic study†
Crystal data for rac-5b benzene solvate. C58H48O4, M 809.01,
colorless prism, 0.2 × 0.2 × 0.2 mm, monoclinic P21/c, a =
˚
10.54(2), b = 25.92(2), c = 16.25(1) A, b = 100.15(2), V =
3
−1
˚
4367.7(6) A , q(Z = 4) = 1.230 g cm . A total of 9960 unique
data (2hmax = 55◦) were measured at T = 153 K by a Rigaku
˚
Mercury CCD apparatus (Mo-Ka radiation, k = 0.71069 A, 50
kV 50 mA). Absorption correction was applied (l = 0.76 cm−1).
The structure was solved by the direct method (SIR92) and
refined by the full-matrix least-squares method on F2 (all data)
with anisotropic temperature factors for non-hydrogen atoms.
Oxidation of bisphenol 10a to quinonemethide 8a
To a solution of rac-10a (8.2 mg, 11 lmol) in CH2Cl2 (5.0 mL)
and water (1.0 mL) was added DDQ (11 mg, 48 lmol), and the
mixture was stirred for 5 min at 23 ◦C. The mixture was diluted
with water and extracted with EtOAc. The organic layer was
washed with brine and dried over MgSO4. Filtration through
an Al2O3 column and evaporation of solvent gave 5.5 mg of
quinonemethide rac-8a in 67% yield.
† CCDC reference numbers 270835–270838 and 273466. See http://
dx.doi.org/10.1039/b506435j for crystallographic data in CIF or other
electronic format.
3 0 3 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1

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Hydrogen atoms were located at the calculated positions. The
final R and Rw values are 0.044 and 0.109 for 7473 reflections
with I > 2rI and 559 parameters. Estimated standard deviations
at the calculated positions. The final R and Rw values are 0.068
and 0.176 for 5013 reflections with I > 2rI and 532 parameters.
˚
Estimated standard deviations for rac-9c are 0.003–0.005 A for
◦
bond lengths and 0.2–0.3◦ for bond angles.
˚
are 0.002–0.003 A for bond lengths and 0.1–0.2 for bond angles.
Crystal data for rac-5c. C48H30O2, M 638.76, yellow needle,
0.1 × 0.03 × 0.03 mm, monoclinic P21/n, a = 11.977(3), b =
References
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˚
˚
24.465(6), c = 12.250(4) A, b = 110.328(2), V = 3365.8(1) A ,
q(Z = 4) = 1.260 g cm−1. A total of 7457 unique data (2hmax
=
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Miyashi, Chemistry of Nanomolecular Systems. Chapter 1: Dynamic
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55◦) were measured at T = 293 K by a Rigaku Mercury
˚
CCD apparatus (Mo-Ka radiation, k = 0.71069 A, 50 kV,
200 mA). Absorption correction was applied (l = 0.75 cm−1).
The structure was solved by the direct method (SIR92) and
refined by the full-matrix least-squares method on F2 (all data)
with anisotropic temperature factors for non-hydrogen atoms.
Hydrogen atoms were located at the calculated positions. The
final R and Rw values are 0.062 and 0.164 for 2329 reflections
with I > 2rI and 451 parameters. Estimated standard deviations
◦
˚
are 0.005–0.01 A for bond lengths and 0.4–0.7 for bond angles.
Crystal data for rac-6b2+(SbCl6−)2. C52H42Cl12O4Sb2, M
1399.84, red platelet, 0.2 × 0.2 × 0.05 mm, monoclinic C2/m,
˚
a = 18.425(7), b = 34.50(1), c = 10.674(4) A, b = 115.457(3),
3
−1
˚
V = 6125.4(3) A , q(Z = 4) = 1.518 g cm . A total of 7050
unique data (2hmax = 55◦) were measured at T = 293 K by
a Rigaku Mercury CCD apparatus (Mo-Ka radiation, k =
˚
0.71069 A, 50 kV, 200 mA). Absorption correction was applied
(l = 14.43 cm−1). The structure was solved by the direct method
(SIR92) and refined by the full-matrix least-squares method on
F2 (all data) with the anisotropic temperature factors for non-
hydrogen atoms. Hydrogen atoms were located at the calculated
positions. The final R and Rw values are 0.054 and 0.160 for
1874 reflections with I > 2rI and 322 parameters. Estimated
standard deviations for 6b2+ are 0.008–0.010 A for bond lengths
˚
and 0.4–0.9◦.
−
Crystal data for rac-6c2+(SbCl6
) dichloromethane solvate.
2
C50.5H30Cl15O2Sb2, M 1444.09, black platelet, 0.2 × 0.05 ×
0.01 mm, orthorhombic Fdd2, a = 57.17(1), b = 35.880(9), c =
3
−1
˚
˚
10.483(3) A, V = 21504(8) A , q(Z = 16) = 1.789 g cm . A total
of 6324 unique data (2hmax = 55◦) were measured at T = 153 K
by a Rigaku Mercury CCD apparatus (Mo-Ka radiation, k =
˚
0.71069 A, 50 kV, 200 mA). Absorption correction was applied
(l = 17.89 cm−1). The structure was solved by the direct method
(SIR92) and refined by the full-matrix least-squares method on
F2 (all data) with the anisotropic temperature factors for non-
hydrogen atoms. Hydrogen atoms were located at the calculated
positions. The final R and Rw values are 0.056 and 0.2134 for
1582 reflections with I > 2rI and 611 parameters. Estimated
standard deviations for 6c2+ are 0.02–0.03 A for bond lengths
˚
and 1–2◦.
12 (a) L. Zelikovich, J. Libman and A. Shanzer, Nature, 1995, 374,
790; (b) C. Westermeier, H.-C. Gallmeier, M. Komma and J. Daub,
Chem. Commun., 1999, 2427; (c) G. Beer, C. Niederalt, S. Grimme
and J. Daub, Angew. Chem., Int. Ed., 2000, 39, 3252; (d) A. Rajca,
A. Safronov, S. Rajca and J. Wongsriratanakul, J. Am. Chem. Soc.,
2000, 122, 3351; (e) J. Nishida, T. Suzuki, M. Ohkita and T. Tsuji,
Angew. Chem., Int. Ed., 2001, 40, 3251; (f) T. Suzuki, R. Yamamoto,
H. Higuchi, E. Hirota, M. Ohkita and T. Tsuji, J. Chem. Soc., Perkin
Trans. 2, 2002, 1937; (g) H. Higuchi, K. Ichioka, H. Kawai, K.
Fujiwara, M. Ohkita, T. Tsuji and T. Suzuki, Tetrahedron Lett., 2004,
45, 3027.
Crystal data for rac-9c chloroform solvate. C50H32N3Cl6, M
¯
893.52, colorless rod, 0.3 × 0.3 × 0.3 mm, triclinic P1, a =
˚
11.209(4), b = 12.480(5), c = 16.557(7) A, a = 86.02(1), b
◦
3
˚
= 89.83(1), c = 64.63(1) , V = 2088.6(1) A , q(Z = 2) =
1.421 g cm−1. A total of 8353 unique data (2hmax = 55◦) were
measured at T = 293 K by a Rigaku Mercury CCD apparatus
˚
(Mo-Ka radiation, k = 0.71069 A, 50 kV, 200 mA). Absorption
correction was applied (l = 4.55 cm−1). The structure was solved
by the direct method (SIR92) and refined by the full-matrix least-
squares method on F2 (all data) with anisotropic temperature
factors for non-hydrogen atoms. Hydrogen atoms were located
13 H. Kurata, T. Shimoyama, K. Matsumoto, T. Kawase and M. Oda,
Bull. Chem. Soc. Jpn., 2001, 74, 1327.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 3 0 2 4 – 3 0 3 1
3 0 3 1