C2-symmetric dibenzosuberane-based helicenes as potential chirochromic optical switches

Article abstract of DOI:10.1021/ja993297d

Three C₂-symmetric (10R,11R)-diethyl substituted dibenzosuberane (DBS)-based helicenes with varying steric and conjugation demands of their bottom fragments were synthesized. The X-ray analyses of their precursors-episulfides reveal the preferr

Full text of DOI:10.1021/ja993297d

7662
J. Am. Chem. Soc. 2000, 122, 7662-7672
C2-Symmetric Dibenzosuberane-Based Helicenes as Potential
Chirochromic Optical Switches
Chien-Tien Chen* and Y-Chen Chou
Contribution from the Department of Chemistry, National Taiwan Normal UniVersity,
Taipei, Taiwan, R.O.C.
ReceiVed September 13, 1999. ReVised Manuscript ReceiVed May 31, 2000
Abstract: Three C2-symmetric (10R,11R)-diethyl substituted dibenzosuberane (DBS)-based helicenes with
varying steric and conjugation demands of their bottom fragments were synthesized. The X-ray analyses of
their precursors-episulfides reveal the preferred conformation of the DBS skeleton possessing equatorially
oriented ethyl groups at C-8 and C-9 positions. In all cases, the absolute configuration in the episulfide moiety
is S, which can lead stereospecifically to the corresponding P helicenes after the reductive desulfurization of
the episulfides. Only the helicene-7a with the bottom part derived from R-tetralone was found photoswitchable
in reasonable time scale. Photoisomerization of the diastereomerically pure (10R,11R,P)-helicene (7a) at 280
nm led to virtually exclusive formation of the opposite M form-diastereomer 7a′ (7a′/7a ) 99.6/0.4). The
preferential return of 7a′ to 7a can be effected upon irradiation at 254 nm (7a′/7a ) 33/67) or thermally at 130
°C (7a′/7a ) 0/100). The photoinduced switching process amounts to a 133% difference in de (from 99.2%
to -34%). The concomitant change of helicene chirality between these two diastereomeric photostationary
states augurs well for their potential application as an optical switch in LC materials. To our knowledge, our
system serves as the best chirochromic optical switch as compared to the examples possessing similar
photochromic properties.
Liquid crystalline (LC) materials are consisted of relatively
flat or bar-like molecules whose alignments can be modulated
by electric and/or magnetic fields. It has been well recognized
that a common nematic LC phase with rough alignment along
one direction can be induced to a twisted nematic (i.e.,
cholesteric) one with helical continuum lacking layer boundaries
by the action of a LC-like chiral dopant, Figure 1.1 More
importantly, the enantiomeric chiral additives can effect the
corresponding cholesteric phases of opposite helical handedness.
In principle, a pair of enantiomeric molecules with bistability
that can be interconverted reversibly by an external mediator
may be utilized as a LC-based memory unit with binary logic.2
Unfortunately, the interchange of stereogenic, enantiomeric
molecules requires bond-breaking and bond-forming events.
Nevertheless, axially dissymmetric helicenes constitute one
major category of organic compounds meeting this requirement
due to their susceptibility to interchange by light.3 So far, there
are two main approaches to realize such contentions. The first
one involves photoresolution of enantiomeric helicenes4,5 by
Figure 1. Phase change from a nematic to a cholesteric one under the
influence of a chiral additive.
circularly polarized lights (approach I, Scheme 1). Research
endeavors toward this aspect has reached a preliminary success
despite the concentration of the helicene dopants could not be
reduced to a reasonable level (e.g., 1-5 wt %). Therefore, it
would not be practical for them to be used as LC-based optical
6
switches unless helicenes exhibiting higher eepss or twisting
powers can be accessed. The second approach is to transform
the enantiomeric helicenes into the corresponding geometrical
(diastereomeric) ones by incorporating different substituents onto
each geminal aromatic ring, Scheme 1.7 Despite a preliminary
report by Feringa and co-workers in 1991 using thioxanthene-
based helicenes exhibited a limited modulation behavior (from
+36% de to +28% de), a promising optical switch was
subsequently realized on similar helicenes with a donor-
acceptor pairing system.8 High switching selectivities of 10/90
and 70/30 (P/M′) were achieved (+40% de to -80% de).9
(1) Solladie´, G.; Zimmermann, R. G. Angew. Chem., Int. Ed. Engl. 1984,
23, 348.
(2) Feringa, B. L.; Jager, W. F.; de Lange, B. Tetrahedron 1993, 49,
8267.
(3) (a) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999,
38, 3418. (b) For a review of asymmetric photochemistry in solution, see:
Rau, H. Chem. ReV. 1983, 83, 535.
(4) (a) Udayakumar, B. S.; Schuster, G. B. J. Org. Chem. 1993, 58, 4165.
(b) Huck, N. P.; Jager, W. F.; de Lange, B.; Feringa, B. L. Science 1996,
273, 1686.
(5) For the photoresolution of molecules with axial chirality, see: (a)
Lemieux, R. P.; Schuster, G. B. J. Org. Chem. 1993, 58, 100. (b) Zhang,
M.; Schuster, G. B. J. Phys. Chem. 1992, 96, 3063. (c) Zhang, Y.; Schuster,
G. B. J. Org. Chem. 1995, 60, 7192. (d) Suarez, M.; Schuster, G. B. J. Am.
Chem. Soc. 1995, 117, 6732. (e) Burnham, K. S.; Schuster, G. B. J. Am.
Chem. Soc. 1999, 121, 10245. (f) see also: Li, J.; Schuster, G. B.; Cheon,
K.-S.; Green, M. M.; Selinger, J. V. J. Am. Chem. Soc. 2000, 122, 2603.
(6) eepss stands for the enantiomeric excess of a helicene under a
photostationary state at a given irradiation wavelength.
(7) (a) Huck, N. P. M.; Feringa, B. L. J. Chem. Soc., Chem. Commun.
1995, 1095. (b) Feringa, B. L.; Jager, W. F.; de Lange, B. J. Chem. Soc.,
Chem. Commun. 1993, 288. (c) Feringa, B. L.; Jager, W. F.; de Lange, B.;
Meijer, E. W. J. Am. Chem. Soc. 1991, 113, 5468.
10.1021/ja993297d CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/28/2000

Potential Chirochromic Optical Switches
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7663
Scheme 1
Scheme 2
Scheme 3a
As an extension of an ongoing program centered on the use
of C2-symmetric dibenzosuberanes (DBS) as chiral templates
in asymmetric catalysis,10 we have been recently engaged in
assessing the feasibility of incorporating our templates into the
helicenes on the basis of two considerations. First, the central
seven-membered ring in DBS makes the flanking aryl rings in
closer contact with that (or those) in the other end of the central
double bond. Due to the increasing steric encumbrance around
the central double bond, the energy barrier to thermal racem-
ization (or epimerization) for the DBS-based helicenes may be
significantly increased,11 Scheme 2. Second, desymmetrization
of the enantiomeric helicenes but still maintaining the enantio-
meric relationship of the helicene chirality can be achieved by
installing a C2 chirality in the DBS backbone (approach II,
Scheme 2). Under such circumstances, a chirochromic optical
switch may be attained if two photostationary states of opposite
diastereomeric preferences can be reached by photoisomeriza-
tion. Intrigued by the seminal study of Schuster on the
chirochromic phototriggers derived from BINOL-based diastereo-
meric acetals,12 we sought to examine the switching profiles of
our C2-symmetric DBS-based helicenes. We herein describe our
results toward this approach.
a (a) (i) TsCl/Et3N/CH2Cl2, cat. DMAP, (ii) Me2CuLi/ether, -10 °C;
(b) KMnO4, dicyclohexano-18-crown-6, benzene.
1, a common skeleton in various antiinflammatory drugs, in
scalemic form.13 This resolved (10R,11R)-1 was utilized as a
conceivable (10R,11R)-diethyl-dibenzosuberone precursor, whose
enantiomeric purity was determined to be >99% ee by HPLC
analysis on a chiral column (Chiralcel OJ), Scheme 3. To
derivatize the alcohol moieties into the corresponding ethyl
appendages, the diol-1 was tosylated smoothly in 96% yield
with p-toluenesulfonyl chloride (TsCl) in CH2Cl2 in the presence
of Et3N and catalytic 4-N,N-(dimethylamino)pyridine. Double
SN2 displacement of the ditosylate 2 with (CH3)2CuLi at -10
°C provided the diethyl-DBS 3 quantitatively.14 It should be
noted that the displacement reaction needs to be carried out in
pure ether despite the poor solubility of 2 in this solvent. Similar
reaction conditions in THF led to complete recovery of the
starting ditosylate 2. The C5-methylene flanked by the dibenzo
unit in 3 was readily oxidized at ambient temperature to the
corresponding ketone-4 in 91% yield by KMnO4 (2.5 equiv) in
benzene using dicyclohexano-18-crown-6 as a phase transfer
catalyst.15
Results and Discussions
Synthesis of (10R,11R)-Diethyl-dibenzosuberone 4. Platzek
and Snatzke have reported the synthesis of C2-symmetric diol-
(8) (a) Jager, W. F.; de Jong, J. C.; de Lange, B.; Huck, N. P. M.;
Meetsma, A.; Feringa, B. L. Angew. Chem., Int. Ed. Engl. 1995, 34, 348.
(b) Feringa, B. L.; Huck, N. P. M.; van Doren, H. A. J. Am. Chem. Soc.
1995, 117, 9927.
(9) For reviews, see: (a) Feringa, B. L.; Huck, N. P. M.; Schoevarrs, A.
M. AdV. Mater. 1996, 8, 681. (b) Feringa, B. L.; van Delden, R. A.;
Koumura, N.; Geertsema, E. M. Chem ReV. 2000, in press.
(10) (a) Chen, C.-T.; Chao, S.-D.; Yen, K.-C.; Chen, C.-H.; Chou, I.-
C.; Hon, S.-W. J. Am. Chem. Soc. 1997, 119, 11341. (b) Chen, C.-T.; Chao,
S.-D.; Yen, K.-C. Synlett 1998, 924. (c) Chen, C.-T.; Chao, S.-D. J. Org.
Chem. 1999, 64, 1090.
(11) A barrier of 32 kcal/mol to racemization for tribenzocyclohepta-
triene-based helicenes has been documented: Tochtermann, W.; Ku¨ppers,
H.; Franke, C. Chem Ber. 1968, 101, 3808.
Synthesis of (10R,11R)-Diethyl-DBS-based Helicenes 7a-
c. By following a helicene assembly protocol similar to
Feringa’s,16 we have synthesized the desired triarylethylene-
type helicenes by employing (10R,11R)-diethyl-DBS-5-thione
5 and three different diazo fragments with varying degrees of
conjugation and steric congestion (i.e., a-mono, b-exo, and
c-endo), Scheme 4. The requisite thioketone 5 can be readily
(13) Platzek, J.; Snatzke, G. Tetrahedron 1987, 43, 4947 and references
therein.
(14) Posner G. H. Org. React. 1975, 22, 253.
(15) Sam, D. J.; Simmons, H. E. J. Am. Chem. Soc. 1972, 94, 4024.
(16) de Lange, B.; Jager, W. F.; Feringa, B. L. Mol. Cryst. Liq. Cryst.
1992, 217, 127.
(12) For the definition of a chirochromic optical switch and its realization,
see: Zhang, M.; Schuster, G. B. J. Am. Chem. Soc. 1994, 116, 4852.

7664 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Chen and Chou
Scheme 4a
a (a) Hydrazone, Ag2O/MgSO4, KOH/MeOH, CH2Cl2, -30 f 0 °C;
(b) Cu/xylene, reflux or HMPT/CH2Cl2.
generated by thiation of the corresponding ketone 4 with P4S10
or Lawesson’s reagent.17 The thioketone-5 is moisture sensitive
and tends to hydrolyze with time even at -20 °C. Therefore, it
had better be prepared and used immediately after purifica-
tion. Cycloaddition of the appropriately in situ-generated diazo
compounds arising from R-tetralone- and dihydrophenan-
threnone-derived hydrazones18 with thioketone-5 in a 1,3-dipolar
fashion and subsequent extrusion of molecular nitrogen led to
episulfides 6a-c in moderate yields (34-47%).19a It should be
noted that all three episulfides were furnished as single
diastereomers as evidenced by HPLC analyses of the crude
products on chiral columns (Chiralcel AD and Chiralpak OT).
Reductive desulfurization of the thirranes 6a-c by Cu powder
in refluxing xylene or by HMPT20 at ambient temperature led
to the respective dissymmetric helicenes 7a-c in essentially
quantitative yields (98%).
Figure 2. Chem-3D Presentations for X-ray Structures of 6a-c.
Table 1. Selected Dihedral Angles, φ (degrees), and Bond Angles,
R and R′ (degrees), from the X-ray Structures of 6a-c
Absolute Stereochemistry of Episulfides 6a-c and Heli-
cenes 7a-c. In all instances, the structural identities and absolute
stereochemistry of episulfides 6a-c were conformed by X-ray
crystallographic analyses, Figure 2. Several important structural
characteristics are summarized as follows: (1) The DBS skeleton
adopts a butterfly shape and both the ethyl groups are positioned
equatorially with the central seven-membered ring in a chairlike
conformation. (2) The central thirrane ring is slightly twisted
by 2-3° for the left dihedral angle (φ1) in 6a and 6b, Table 1.
This distortion is significantly larger (10.6°) in the endo-6c
presumably due to the increasing steric encumbrance of the
endo-naphthyl unit. The larger distortion with concomitant
release of eclipsed interactions around the episulfide unit might
be responsible for the larger dihedral angle (51.2°) observed
between the aryl ring A and ring C in 6c. On the other hand,
the right dihedral angles (φ2) around the episulfide units are in
the range of 4-8.2° with those in 6b and 6c being slightly larger
than that in 6a (3.9°). Therefore, the steric demand of the ring
C modulates the aryl torsions of both rings A and B. (3) In all
three cases, both bond angles (R and R′) remain essentially
constant with R′ uniformly larger by 8-10°. (4) The aryl ring
C in the lower fragment is placed syn to the proximal equatorial
mono-6a
exo-6b
endo-6c
φ1 (C25-C16-C1-C2)a
φ2 (C17-C16-C1-C15)
φ3 (C24-C25-C2-C3)
R (C2-C1-C15)
-2.0
3.9
41.2
106.3
115.4
3.0
7.1
41.2
107.6
115.5
10.6
8.2
51.2
105
R′ (C25-C16-C7)
115.3
a For clarity, the numberings consistent with those in the X-ray
structure for 6a are adopted.
ethyl group even in the endo-naphthyl case 6c.19b This confor-
mational preference may have to do with the crystal packing
effect since an opposite conformational preference (i.e., with
ring C syn to the proximal axial hydrogen) was found in the
achiral, parent system; (5) The absolute configuration of the
episulfide moiety in 6a-c (Figure 2) was determined to be S at
C16,21 which would lead to a P chirality in the resultant
helicenes 7a-c as inferred by the crystal structure of the endo
helicene-7c (vide infra).19b The absolute S stereo-control in 6a-c
implies an exclusive approach of the thioketone 5 from the si
face of the respective diazo compound, by which the aryl ring
in the diazo compound stays away from the proximal and distal
axial hydrogens at C8 and C9 in 5.
(17) (a) Safarzadeh-Amiri, A.; Verrall, R. E.; Steer, R. P. Can. J. Chem.
1983, 61, 894. (b) Cherkasov, R. A.; Kutyrev, G. A.; Pudovik, A. N.
Tetrahedron 1985, 41, 2567. (c) Mayer, R. Chem. Ber. 1957, 90, 2.
(18) (a) Newkome, G. R.; Fishel, D. L. J. Chem. Soc. 1966, 31, 677. (b)
Osterodt, J.; Nieger, M.; Windscheif, P.-M.; Vo¨gtle, F. Chem. Ber. 1993,
126, 6, 2331.
(19) (a) Their chemical yields were all ) 80% if based on recovered 1.
(b) See the Supporting Information for the X-ray data of 6a-c and 7c.
(20) (a) Neureiter, N. P.; Bordwell, F. G. J. Chem. Soc. 1959, 81, 578.
(b) Buynak, J. D.; Graff, N. M.; Jadhav, K. P. J. Org. Chem. 1984, 49,
1828.
So far, single crystals suitable for X-ray diffraction analysis
were available only for the endo-helicene 7c, Figure 3. The
structure of the helicene 7c shares a couple of the common
(21) The newly created stereocenter is carbon-16. Carbon-1 in 6a-c is
not a stereocenter since it is located in a C2 axis in the common starting
material-thioketone 5.

Potential Chirochromic Optical Switches
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7665
to accommodate the steric congestion around the helical turn
(i.e., between the endo-naphthyl ring and the A ring in DBS).
Finally, the absolute helicene chirality of 7c is P which
corresponds to a stereospecific chirality transfer from the
(10R,11R,S)-episulfide-6c. On the bases of (1) the unambiguous
structural determinations of 6a-c and 7c, (2) the stereospecific
chirality transfer from the episulfides to the corresponding
helicenes, and (3) same exciton chirality observed in circular
dichroism (CD) spectra for 7a-c,22,24 we can conclude that
7a-c all possess the same absolute helicene chirality-P.
Photochemistry of Helicenes 7a-c. To judiciously choose
irradiation wavelengths to effect photochemical shuttling be-
tween diatereomeric helicenes, both their UV-vis spectra with
fully calibrated extinction coefficients (ꢀ) are required. One can
easily assess their relative abundances under the pss at a certain
irradiation wavelength by their given ꢀ. The isomeric excess of
the pss from irradiation at a given wavelength is given by:
[de]pss ) (7a - 7a′)/(7a + 7a′) ) [(ꢀ7a′Φ7a′f7a - ꢀ7aΦ7af7a′)/
(ꢀ7a′Φ7a′f7a + ꢀ7aΦ7af7a′)]. The extinction coefficient difference
between the diastereomers at a given wavelength can directly
reflect the [de]pss as long as the photoisomerization quantum
yields (Φ7a′f7a and Φ7af7a′) for both processes stay similar. That
is to say, to effect efficient switching between them in a highly
selective manner (e.g., >90/10), irradiations are better targeted
at regions with significant changes in extinction coefficients.
At the onset, photoisomerization by irradiations at 260 and 300
nm was randomly selected for (10R,11R,P)-7a due to lack of
any UV spectrum information for the corresponding diastere-
omer (10R,11R,M)-7a′, which was unavailable by chemical
synthesis but subsequently obtained by photoisomerization at
280 nm (vide infra). Both experiments25a were conducted in
degassed hexane and were monitored both by CD spectroscopy
and HPLC analysis on Chiralpak OT until constant compositions
(i.e., under photostationary states; pss) were reached (up to 3
h). In both experiments, preferential formation of the opposite
7a′ was observed (7a′/7a ) 56/44 and 67/33, respectively). By
subtracting the UV spectrum of the diastereomeric helicenes
with a 67/33 (7a′/7a) composition from that of the pure 7a, we
were able to locate a couple of other wavelengths (250 and 290
nm) with distinct difference in extinction coefficients from the
difference spectrum. Photoisomerization of 7a near these two
irradiation wavelengths (25425b and 280 nm25a) led to two new
pss with complementary diastereoselectivities of 33/67 and 99.6/
0.4 (7a′/7a), respectively. Figure 5 shows a stacked plot of CD
spectra for monitoring the photoisomerization of 7a at 280 nm
with time. As revealed in the plot, there are two isodichroic
points26 at 214 and 271 nm, respectively. Their presence strongly
indicates the newly generated product (7a′) is the only homo-
geneous product in solution and has similar photochromic
properties as in 7a. The diastereomeric identity of 7a′ relative
to 7a was further secured by two independent lines of evidences.
First, the opposite sign of the Cotton effects in the range of
239 to 264 nm suggests a helicene chirality reversal for 7a′.
Second, a complete return of 7a′ to 7a can be effected upon
heating a solution of 7a′ in xylene at 130 °C for 2 h. The
Figure 3. Chem 3D Presentation of the X-ray Structure of 7c.
Figure 4. Chem-3D Presentations of Calculated Structures for
(10R,11R,P)-7c and (10R,11R,M)-7c′.
characteristics with those of the episulfides 6a-c. Namely, the
DBS skeleton adopts a butterfly shape and both the ethyl groups
are positioned equatorially with the central seven-membered ring
in a chairlike conformation. The aryl C ring in 7c is also placed
syn to the proximal equatorial ethyl group. Similar conforma-
tional preference (i.e., with C ring syn to the proximal equatorial
hydrogen) was also observed in the achiral, parent systems.22
The preferential equatorial disposition of the ethyl groups in
the helicene-7c is in harmony with its helicene chirality beside
a possible crystal packing effect (vide infra). In the molecular
simulation of (10R,11R)-7c the ethyl groups were predicted to
be placed axially in order to prevent the repulsive gauche-butane
interaction between the ethyl groups, Figure 4.23 Under such
circumstances, the (10R,11R,M)-7c′ was shown to be more stable
than (10R,11R,P)-7c by 4.3 kcal/mol in view of the severe steric
interaction between the endo-naphthyl ring and the proximal
axial ethyl group. Therefore, we surmise that (10R,11R)-diethyl-
DBS-based helicenes with M chirality would prefer to have a
DBS conformation with axially oriented ethyl groups. On the
other hand, the helicenes with P chirality call for an opposite
DBS conformation with equatorial ethyl groups as evidenced
by the X-ray crystal structure of 7c.
As revealed in the X-ray structure of 7c (Figure 3), there is
a small deviation of planarity around the central olefin unit
(∼2°). The phenyl torsional angle between the A ring and the
central double bond is 52.5° and that between the B ring and
the central double bond is 63.5°, which are also observed in all
the achiral parent helicenes.22 Apparently, the ethyl groups in
7c do not impose any significant torsional strain onto the DBS
template. On the other hand, the dihedral angle between the C
ring and the central double bond is 61.2°. This angle is 9° larger
than that of the corresponding achiral endo-helicene and is
increased by 28° as compared to that of the model mono-
helicene (33.1°). Apparently, the larger dihedral angle serves
(24) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Photochemistry; University Science Books: Mill
Valley, CA, 1983.
(25) (a) Irradiations were carried out with a 300 W Xe lamp equipped
with a monochromator with a slit size equivalent to 16 nm bandwidth. (b)
Irradiation was performed with a 100 W Hg lamp equipped with a 20-nm
bandwidth filter.
(22) Unpublished results from this laboratory.
(26) For a recent example on using the presence of isodichroic points in
CD spectra to support structure similarities of several oligo(phenylene
ethynylene)s, see: Gin, M. S.; Yokozawa, T.; Prince, R. B.; Moore, J. S.
J. Am. Chem. Soc. 1999, 121, 2643.
(23) Molecular modeling was carried out on an SGI-Indy Worksystem.
Global minimum conformations of the diastereomeric pair, 7c and 7c′, were
calculated with the Sybyl simulation package.

7666 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Chen and Chou
Figure 5. A stacked plot of CD spectra for the photoisomerization of
7a in hexane irradiated at 280 nm with time.
Figure 6. UV spectra of 7a and 7a′ and the UV difference spectrum
in hexane.
Table 2. Diastereoselective Photoisomerization by Irradiation of
(10R,11R,P)-7a in Different Solvents with Varying Wavelengths
entry
solvent
hexane
hexane
hexane
hexane
hexane
hexane
THF
THF
λ (nm)a
dr (7a:7a′)b
de,c %
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
254 ( 10d
260 ( 8
280 ( 8
300 ( 8
310 ( 8
330 ( 8
254 ( 10
280 ( 8
310 ( 8
254 ( 10
280 ( 8
310 ( 8
254 ( 10
280 ( 8
310 ( 8
67:33
44:56
0.4:99.6
33:67
70:30
no change
61:39
7:93
54:46
79:21
38:62
+34
-12
-99.2
-34
+40
+22
-86
+8
+58
-24
+64
-8
-81
+22
THF
cyclohexane
cyclohexane
cyclohexane
toluene
toluene
toluene
82:18
46:54
9.5:90.5
61:39
Figure 7. CD spectra of 7a and the pss resulted from irradiation at
254, 260, 280, and 300 nm in hexane.
a Irradiation time is 3 h unless otherwise stated. b Determined by
HPLC analysis on Chiralpak OT column. c Defined as (7a - 7a′)/(7a
+ 7a′). d Irradiation time is 30 s.
at its photophysical properties. As illustrated in the individual
and difference UV spectra of 7a and 7a′ in hexane, both isomers
display very strong absorption around 200 nm and an intense
absorption near 275 nm (ꢀ ) 80300), Figure 6. Unlike 7a, 7a′
exhibit much more unique absorptions at 255 and 325 nm. A
significant increase in extinction coefficient in the latter
absorption band (325 nm) in 7a′ suggests possible less phenyl
distortions and thus better conjugation for the (10R,11R,M)-
7a′ diastereomer. As evidenced in the UV difference spectrum,
well-defined extremes can be found at 230, 254, 273, 290, and
325 nm. Photoswitching of 7a with irradiation wavelengths close
to these extremes indeed led to distinctive pss and complemen-
tary pss particularly at 254 (or 310 nm) and 280 nm. Moreover,
the almost exclusiVe switching selectiVity obserVed at 280 nm
in hexane represents the most selectiVe switching process to
date by utilizing such a unfunctionalized helicene (compared
with the systems in approach II, Scheme 1).
structure of 7a′ was ultimately characterized by NMR and MS
spectrometric means.
The interconversion between these two pss (at 280 and 254
nm) is modestly reversible with slight decomposition (due to
free radical-mediated oxygenation reaction). On the other hand,
the analogous exo- and endo-helicenes (7b and 7c) are es-
sentially photochemically inert. No appreciable changes in
compositions were observed by irradiating 7b and 7c at several
wavelengths for 3 h presumably due to larger dihedral angles
and higher distortion between the central double bond and the
bottom parts.27 The results of several photoisomerization experi-
ments for 7a in various solvents are compiled in Table 2. Similar
switching profiles were observed in hexane, toluene, and THF
at 254 and 310 nm. The pss compositions are solvent dependent
particularly at 280 nm. The selectivity dropped from 99.6/0.4
to 93/7 and 91/9, respectively, when 7a was irradiated in THF
and in toluene (entries 8 and 14). Dramatically reduced
selectivity (62/38, 7a′/7a) was resulted in cyclohexane. How-
ever, more promising and opposite switching selectivities were
found at 254 (21/79) and 310 nm (18/82, 7a′/7a) in this solvent.
Figure 7 shows the stacked CD spectra of helicene-7a and
helicenes (7a/7a′) with diastereomeric compositions resulted
from the experiments in hexane as compiled in Table 2 (entries
1-4). In marked contrast to Feringa’s donor-acceptor-based
helicenes,8 the CD spectra of 7a and 7a′ displays opposite Cotton
effects only around 239-264 nm. Nevertheless, CD still serves
as a promising chiroptical technique for tracing the switching
profiles of our helicenes by signs and by intensity differences.
The significant changes in the CD spectra of 7a and 7a′ as well
as the 133% difference in de (i.e., from 99.2% to -34% de,
entries 3 and 1) between two complementary pss make our
helicenes potentially useful as an optical switch in LC materials.
With the essentially diastereomerically pure 7a′ in hand (from
the experiment in entry 3), we were able to take a closer look
(27) The endo helicene-7c exhibits very intriguing photophysical be-
haviors. A large increase in Stokes shift (50 nm) with a purplish-blue light
(λ ) 430 nm) emitting character and a 250-fold increase in Φf were observed
uniquely for 7c. Chen, C.-T.; Chang, I.-J.; Chou, Y.-C.; Kao, J.-I.; Lee,
T.-W.; Lin, J.-S.; Lee, I.-J.; Wang, C.-L. Manuscript in preparation.

Potential Chirochromic Optical Switches
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7667
Figure 9. Emission spectra of helicenes 7a and 7a′ in hexane.
The photochemical properties of the 10,11-diethyl-DBS-based
helicenes resemble those of other arylethylenes such as stilbenes,
triphenylethylenes, and tetraphenylethylenes. Their photophysi-
cal profiles have been examined theoretically and experimentally
by steady-state and time-revolved spectroscopic means.31 The
results indicated that the photoinduced isomerization involves
delicate motions on the excited state potential energy surface
imparted with at least three structurally well-defined states.32
Initial excitation of an arylethylene leads to a vertical (Franck-
Condon) singlet state. Its torsional relaxation about the aryl-
olefinic C-C bond furnishes a relaxed and emissive excited
state (Sv). Subsequent rotation around the central olefinic C-C
bond provides a nonemissive perpendicular (“phantom”) excited
state (Sp). Internal relaxation of the phantom state to a
perpendicular ground-state results in the final double bond
isomerization. The extent of partition for Sp highly depends on
the extent of coplanarity between the aryl and the olefinic
moieties in Sv. The larger the coplanarity (or the smaller the
torsional angle), the bigger the partition for Sp. Detailed
photophysical analyses of the 10,11-diethyl-DBS-based heli-
cenes are currently underway and it appears that the basic
principles behind the photoinduced process are the same as they
are for other arylethylenes.33
The extremely high switching selectivity of 7a at 280 nm
(except in cyclohexane) and the small difference in extinction
coefficients between 7a and 7a′ at 280 nm strongly suggest that
there exists a preferential difference in photoisomerization
quantum yields between Φ7af7a′ and Φ7a′f7a. An indirect piece
of evidence in support of this presumption is the observed large
difference in fluorescence quantum yields between 7a and 7a′
(Φ7a′/Φ7a ) ∼30) at 280 nm in hexane (Figure 9).34 Apparently,
a lot more of the energy in the excited state of 7a′ is dissipated
into fluorescent emission rather than double bond isomerization.
To gain insights into the origin of the predominant photo-
isomerization efficiency of 7a, molecular simulations of 7a and
Figure 8. Droplets of K-15 containing (top) 1% (10R,11R,P)-7a
(bottom) 1% (10R,11R,M)-7a′ suspended in glycerol viewed at 200×
magnification at room temperature. The distances between adjacent arms
of the spiral are 133 µm and 50 µm, respectively.
We ware so far unable to demonstrate the feasibility of 7a
as an optical switch in a UV transparent LC material like ZLI-
116728a due to its poor solubility in ZLI-1167 at ambient
temperature and the difficulty for determining the diastereomeric
compositions after photoisomerization experiments.29 Neverthe-
less, photoisomerization of 7a in a 1:1 mixture of hexane and
ZLI-1167 at 280 nm seems to display a similar diastereomeric
preference as in pure cyclohexane, albeit with a longer irradia-
tion time (5 h). Although the photoswitching selectivity for 7a
at 280 nm doped with LC materials such as ZLI-1167 and
K-1528b could not be determined, they would be expected to
display similar switching profiles as in cyclohexane and toluene,
respectively, in view of their similarity in core structures.
The helical twisting powers (âM) of 7a and 7a′ were
determined by the droplet method30 in a nematic LC K-15. As
shown in Figure 8 (top), droplets of K-15 doped with 1% of 7a
suspended in glycerol were viewed microscopically at 200×
magnification at ambient temperature. The helical pitch equals
twice the distance between adjacent spirals. The pitch (p) of a
cholesteric LC formed by addition of an optically active dopant
to a nematic phase depends on its optically purity (γ), its helical
twisting power (âM), and its concentration (C, mol of dopant/
mol of solution) according to p ) [âMCγ]-1. This measurement
provides a âM value of 1.2 µm-1 (p ) 133 µm) for 7a in K-15.
Similar measurement (bottom in Figure 8) for 7a′ (99.2% de)
comes to a âM value of 3.2 µm-1 (p ) 50 µm).
(31) (a) Pederson, S.; Banares, L.; Zewail, A. H. J. Chem. Phys. 1992,
97, 8801. (b) Saltiel, J.; Sun, Y.-P. In Photochromism: Molecules and
Systems; Durr, H.; Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990;
p64. (c) Gegiou, D.; Muszkat, K. A.; Fischer, E. J. Am. Chem. Soc. 1968,
90, 13. (d) Schilling, C. L.; Hilinsky, E. F. J. Am. Chem. Soc. 1988, 110,
2296. (e) Shultz, D. A.; Fox, M. A. J. Am. Chem. Soc. 1989, 111, 6311. (f)
Udayakumar, B. S.; Devados, C.; Schuster, G. B. J. Phy. Chem. 1993, 97,
8712 and references therein.
(28) (a) ZLI-1167: 4-n-alkyl-4-cyano-1,1′-bicyclohexyls. (b) K-15: 4-cy-
ano-4′-pentoxy-biphenyl.
(29) The retention time of ZLI-1167 happens to coincide with that of
7a on HPLC chromatogram.
(30) (a) Seuron, P.; Solladie, G. Mol. Cryst. Liq. Cryst. 1979, 56, 1. (b)
Candau, S.; LeRoy, P.; Debeauvais, F. Mol. Cryst. Liq. Cryst. 1973, 23,
283.
(32) (a) Zimmt, M. B. Chem. Phys. Lett. 1989, 160, 564. (b) Morais, J.;
Ma, J.; Zimmt, m. B. J. Phys. Chem. 1991, 95, 3885. (c) Ma, J.; Zimmt,
M. B. J. Am. Chem. Soc. 1992, 114, 9723.
(33) Chen, C.-T.; Chang, I.-J.; Chou, Y.-C.; Kao, J.-I.; Lee, T.-W.; Lin,
J.-S.; Lee, I.-J.; Wang, C.-L. unpublished results from these laboratories.
(34) Huck, N. P. M.; Feringa, B. L. J. Chem. Soc., Chem. Commun.
1995, 1095.

7668 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Chen and Chou
Table 3. Minimized Energies and Three Appropriate Phenyl
Torsional Angles for 7a and 7a′
sentially constant (38.1-39.6°). Apparently, the phenyl torsion-
φ1 around the central double bond that can be modulated by
the conformation of the DBS plays a pivotal role in the structural
stability of the resultant helicene.
As shown in Scheme 5, 7a preferentially adopts a DBS
conformation (7aP-eq, E ) 36.0 kcal/mol) with equatorially
disposed ethyl groups. The conformation (7aP-ax, E ) 38.3 kcal/
mol) with the flipped DBS ring is clearly less favored (by 2.3
kacl/mol) in view of the larger φ1 (increased by 7.4°) and the
increased steric repulsion between the C ring and the proximal
axial ethyl group. On the other hand, adoption of the same DBS
conformation for 7a′ (7a′M-eq, E ) 38.7 kcal/mol) would impose
a larger phenyl distortion for φ1 (-73.5°) by 11.5° beside the
steric interaction between ring-C and the proximal axial
hydrogen. The unfavorable phenyl and steric strains can be
relieved in 7a′M-eq by a conformational inversion leading to
7a′M-ax (E ) 34.8 kcal/mol) in which the φ1 is reduced by almost
18° and the ring C is now anti to the proximal, axial ethyl group.
Under such circumstances, the extent of conjugation in 7a′M-ax
becomes slightly better than that in 7aP-eq as supported by the
slight increase in extinction coefficient in the UV spectrum of
7a′. Therefore, we surmise that 7aP-eq and 7a′M-ax are the
predominant species in solution (i.e., hexane) for 7a and 7a′,
respectively. On the basis of this presumption, photoisomer-
ization of 7aP-eq would involve its initial double bond rotation
in a counterclockwise fashion leading to 7a′M-eq followed by a
facile conformational flip leading to 7a′M-ax (pathway I). Since
planarization of the DBS skeleton is a prerequisite step both
for the double bond isomerization in the singlet excited states
of 7a and 7a′ and for the conformational flip of the DBS, the
intermediacy of 7a′M-eq (pathway I) for the photoinduced
interconversion of 7aP-eq and 7a′M-ax might not be necessary.
Therefore, an alternative route (pathway II) would involve its
7a
7a′
P-eq
P-ax
M-eq
M-ax
energy, kcal/mol
36.0
61.9
-79.3
39.3
38.3
69.3
-61.5
38.1
38.7
-73.5
68.5
34.8
-55.4
75.7
φ1 (A and C1dC16)
φ2 (B and C1dC16)
φ3 (C and C16dC1)
-39.3
-39.6
Scheme 5
double bond isomerization with a concomitant conformational
36
flip of the DBS leading directly to 7a′M-ax
.
The relative
inertness of 7a′M-ax toward photoisomerization can be rational-
ized in terms of an increased energy of activation to bring the
DBS into planarity by which the torsional angle between ethyl
appendages changes from 180° (with 0 strains) to 120° (with
two pairs of Et-H torsional and steric strains). On the other
hand, to bring the DBS into planarity in the case of 7aP-eq would
involve a torsional angle change from 60° (with one pair of
Et-Et gauche strain) to 120°. The eroded switching selectivity
at 280 nm along with the improved selectivities at 254 and 310
nm when 7a was irradiated in cyclohexane suggest that different
conformational balances might be involved specifically in this
solvent system.
7a′ with conformational permutations of the DBS were carried
out both at the molecular mechanics (MM3) and the semi-
empirical levels (PM3).35 The minimized energies and three
appropriate phenyl torsional angles for 7a and 7a′ are compiled
in Table 3. It should be noted that the DBS with the eqatorially
and axially oriented ethyl groups constitute the more stable
conformations for 7a and 7a′, respectively (Table 3 and Scheme
5). In addition, the relative stability for either conformations of
7a and 7a′ seems to correlate well with the phenyl torsion (φ1)
between ring A and the central double bond. Namely, the larger
the torsional angle, the higher the energy. Interestingly, in both
conformations of 7a and 7a′ the phenyl torsions between the
bottom fragment (ring C) and the central olefin remain es-
Summary and Conclusions
We have developed C2-symmetric DBS-based helicenes as a
favorable chirochromic optical switch. The requisite episulfides
6a-c can be synthesized stereoselectively by coupling of the
DBS-derived thioketone 5, prepared in four steps from the
resolved 10,11-bis(hydroxymethyl)-DBS 1, and the respective
in situ-generated diazo compounds. The X-ray structures of all
episulfides 6a-c share a common DBS conformation with
equatorially disposed ethyl groups and with the aryl ring in each
bottom fragment syn to the proximal equatorial ethyl group.
The preferred conformation in the solid state was rationalized
in terms of crystal packing effect. Their absolute stereochemistry
(35) (a) Simulations were performed in CAChe Worksystem. (b) The
X-ray structure of the corresponding achiral, parent compound was employed
as the initial template for the structure construction and minization of
(36) Photoisomerization with a concomitant conformational flip for
1,1′,2,2′,3,3′,4,4′-octahydro-3,3′-dimethyl-4,4′-biphenanthrylidenes as mono-
directional molecular rotors has been recently documented, see: Feringa,
B. L. Nature 1999, 401, 152.
7aP-eq
.

Potential Chirochromic Optical Switches
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7669
67-100%), m (medium 33-67%), or w (weak 0-33%). Mass spectra
were recorded on a Finnigon TCQ-700 spectrometer with ionization
voltages of 70 eV. Data are reported in the form m/e (intensity relative
to base ) 100%). Combustion analyses were performed on a Perkin-
Elmer 2400-CHN analyzer by the Northern Instrument Center of
Taiwan. High-resolution mass spectra were recorded on a JEOL JMX-
SX/SX 102A spectrometer by the Central Instrument Center of Taiwan.
Melting points were determined on a Hargo MP-2D melting point
apparatus and are uncorrected. Analytical TLC was performed on Merck
silica gel plates with QF-254 indicator. Visualization was accomplished
with UV light, PMA, and KMnO4. Column (flash) chromatography
was performed using 32-63 µm silica gel. Solvents for extraction and
chromatography were reagent grade. Solvents used for recrystallization
were spectral grade and are shown under mp unless otherwise indicated.
Analytical high-pressure liquid chromatography (HPLC) was performed
on a Jasco Liquid Chromatograph equipped with PU-980 pumps, UV-
975 detector, and 807-IT integrator. The columns used were Dicel
Chiralpak OT and AD columns with the detector wavelength at 254
nm. The flow rate and solvent systems were as denoted. Optical rotation
were obtained on a Jasco DIP-1000 Digital Polarimeter at room
temperature and reported as follows: [λ]wavelength, concentration (c )
g/100 mL), and solvent. UV-vis spectra were recorded on a HP-8453
Diode Array UV-visible spectrometer in hexane solution unless
otherwise noted. Fluorescence spectra were recorded on an Aminco-
Browman series-2 Luminescence spectrometer. CD spectra were
recorded on a Jasco model J-817 spectropolarimeter. Microscopic
analyses were performed with an Olympus BX50-P polarizing micro-
scope equipped with a PM10 automatic photomicrographic system. All
reactions were run under nitrogen.
is all S indicating a predominant approach of the thioketone 5
from the si face of the respective diazo compound. Careful
examination of the dihedral angles around the central thirrane
ring led us to conclude that the steric interaction between ring
A and ring C in the episulfide dictates the torsional strain in
the central episulfide unit. Reductive desulfurization of
(10R,11R,1′S)-episulfides 6a-c was achieved in a stereospecific
manner furnishing respective helicenes 7a-c of P chirality. The
X-ray structure of the endo-helicene 7c reveals a delicate steric
match between the DBS conformation and the helicene absolute
stereochemistry. The P helicene favors, in an energetic sense,
the DBS conformation with equatorially oriented ethyl groups.
On the opposite, the analogous M helicene would prefer an
inverted DBS conformation with axial ethyl groups as suggested
by molecular simulation.
The helicenes are thermally stable toward epimerization even
for the one (7a) with the lower fragment derived from
R-tetralone. Initial photoisomerization experiments for 7a
performed at 260 and 300 nm allowed us to sort out other
irradiation wavelengths (254, 290, and 310 nm) exhibiting
distinctive difference in extinction coefficients in the UV
difference spectrum before and after experiments. Photoisomer-
ization of 7a at 254 (or 310) nm and 280 nm led to two pss
with opposite diastereoselectivities in four different solvents
examined. In all solvents except cyclohexane, the best switching
selectivities were attained at 280 nm in a range of 91/9 and
99.6/0.4 (7a′/7a). The identity of the photoisomerization product
7a′ was secured by the observed isodichroic points in the circular
dichroism kinetic experiments, by its thermo-chemical return
to 7a, and by its 1H NMR spectrum. The photo-inertness of 7b
and 7c were attributed to their larger deformation between the
central olefin and the respective bottom aryl moiety. The high
switching selectivity for 7a at 280 nm in hexane was attributed
to a large difference in photoiosmerization quantum yields
between their interconversion in view of the minor difference
in ꢀ between both diastereomers. Molecular simulations of the
diatereomeric helicenes 7a and 7a′ with both DBS conforma-
tions suggest that the preponderant species of 7a′ in solution
adopts a flipped DBS conformation with axial ethyl groups in
order to minimize steric encumbrance around the helicity. The
sluggish photoresponse of 7a′ in hexane at 280 nm was
rationalized in terms of its increased energy of activation toward
planarization in its singlet excited-state en-route to a perpen-
dicular singlet excited state.
(10R,11R)-11-({[(4-Methylphenyl)sulfonyl]oxy}methyl)-10,11-di-
hydro-5H-dibenzo[a,d]cycloheptene-10-yl)methyl 4-methyl-1-ben-
zenesulfonate (2). In a 100-mL, three-necked, round-bottomed, flask
fitted with a glass stopper, N2-inlet, and septum was placed diol 1
(1.5053 g, 5.92 mmol) in anhydrous CH2Cl2 (55 mL). Anhydrous Et3N
(4.95 mL, 35.5 mmol, 6 equiv) was added at 0 °C followed by addition
of p-toluenesulfonyl chloride (3.724 g, 19.5 mmol, 3.3 equiv) and
4-(dimethylamino)pyridine (51 mg, 0.41 mmol, 7 mol %) in anhydrous
CH2Cl2 (18 mL) over 10 min. The resulting reaction mixture was
warmed to room temperature and stirred overnight. The reaction mixture
was poured into a saturated aqueous NH4Cl-ice mixture (75 mL, 5/3).
The aqueous layer was separated and extracted with CH2Cl2 (2 × 40
mL). The combined organic extracts were washed with saturated
aqueous NH4Cl (25 mL) and H2O (20 mL), dried (MgSO4), and
evaporated. The crude yellowish solid was purified by column
chromatography (hexane/EtOAc, 2/1) to give 3.196 g (96%) of
ditosylate 2 as a white solid: mp 120.5-121.5 °C (CH2Cl2/hexane);
1H NMR (400 MHz) 7.59 (d, J ) 8.3, 4H, 4 × HC(2′)), 7.23 (d, J )
8.2, 4H, 4 × HC(3′)), 7.15-6.96 (m, 8H, Ar), 4.17 (dd, J ) 9.9, 5.3,
2H, 2 × CHaHbOTs), 3.97 (s, 2H, H2C(5)), 3.87 (dd, J ) 9.8, 8.2, 2H,
2 × CHaHbOTs), 3.52 (quintet, J ) 5.4, 2H, 2 × HC(10)), 2.42 (s, 6H,
2 × CH3); 13C NMR (100 MHz) 144.71 (C(4′)), 137.33 (C(4a), C(6a)),
134.02 (C(1a), C(9a)), 132.46 (C(1′)), 131.95 (C(4), C(6)), 130.17 (C(2),
C(8)), 129.74 (C(3′)), 127.66 (C(2′)), 127.52 (C(1), C(9)), 126.88 (C(3),
C(7)), 70.97 (CH2OTS), 44.43 (C(10), C(11)), 41.88 (C(5)), 21.57
(CH3C6H4SO3); IR (CCl4) 2926 (w), 1717 (m), 1497 (w), 1456 (w),
1372 (s, SO2, asym.), 1264 (w), 1217 (w), 1190 (s, C-O), 1179 (s,
SO2, sym.), 1097 (w), 968 (m), 818 (s, S-O), 810 (w); MS (70 eV)
390 (M-C7H7SO3H+, 11), 219 (24), 218 (100), 217 (28), 205 (40),
204 (14), 203 (27), 202 (12), 192 (13), 191 (21), 179 (10), 178 (22),
172 (14), 92 (58), 65 (11); TLC Rf 0.4 (hexane/EtOAc, 2/1); [R]D +40.5
(c ) 0.74, CH2Cl2); Anal. Calcd for C31H30O6S2 (562.7): C, 66.17; H,
5.37; S, 11.40. Found: C, 66.14; H, 5.36; S, 11.38.
To our knowledge, our system serVes as the best chirochromic
optical switch as compared to the examples possessing similar
photochromic properties.7c,12 A maxium of 133% difference in
de and the opposite helicene chirality between these two
photostationary states (at 254 and 280 nm) in hexane augur well
for its potential application as an optical switch in LC materials.
Investigations toward improving the compatibility and the
twisting power of our helicene system in common LC phases
and toward incorporating a donor-acceptor pair onto the vicinal
aryl rings across the central double bond in both dissymmetric
triaryl- and tetraaryl-ethylenes are underway.
Experimental Section
(10R,11R)-10,11-Diethyl-10,11-dihydro-5H-dibenzo[a,d]cyclohep-
tene (3). In a 500 mL, three-necked, round-bottomed, flask equipped
with an addition funnel, N2-inlet, and septum was placed CuI (8.364
g, 43.9 mmol, 6 equiv) suspended in anhydrous ether (37 mL). The
reaction flask was cooled on ice and methyllithium (1.6 M in ether, 55
mL, 87.8 mmol, 12 equiv) was added via the addition funnel over 40
min. The solution became clear and yellowish. The resulting (CH3)2-
CuLi was cannulated to a precooled (-78 °C) solution of ditosylate-2
(4.1187 g, 7.32 mmol) in anhydrous ether (150 mL). The resulting
1
General. H NMR and 13C NMR were recorded on JEOL JVM-
EX400 (400 MHz H, 100 MHz 13C) and Varian Gemini-2000 (200
1
MHz 1H, 50 MHz 13C) spectrometers in deuterochloroform with
tetramethylsilane (TMS) or chloroform as an internal reference unless
otherwise stated. Chemical shifts are reported in ppm (δ), coupling
constants, J, are reported in Hz. Infrared spectra were recorded on a
Perkin-Elmer Paragon-500 FTIR spectrometer. Peaks are reported in
units of cm-1 with the following relative intensities: br (broad), s (strong

7670 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Chen and Chou
heterogeneous mixture was gradually warmed to -10 °C over 1 h and
stirred at this temperature for 28 h. Saturated aqueous NH4Cl (200 mL)
was added, and the reaction mixture was warmed to room temperature
for 30 min. The aqueous layer was separated and extracted with ether
(3 × 60 mL). The combined organic layers were washed with brine (2
× 30 mL) and H2O (2 × 20 mL), dried (MgSO4), and evaporated. The
crude yellowish oil was purified by column chromatography (hexane/
CH2Cl2) by microsyringe. Evolution of nitrogen was observed, and the
deep red solution slowly decolored. The thioketone was added until
the evolution of nitrogen had subsided. A total of 204 mg (0.72 mmol)
of 5 was added. After having been stirred for 16 h, the resulting reaction
mixture was quenched by saturated aqueous NaHCO3 (15 mL). The
aqueous layer was separated and extracted with ether (3 × 15 mL).
The combined organic layers were dried (MgSO4), filtered, and
evaporated. The bluish residue was purified by column chromatography
(hexane) to get 140 mg (47%) of 6a as a white solid: mp 154-156 °C
(hexane); 1H NMR (400 MHz, CDCl3) 8.01-7.96 (m, 1H, HC(4)), 7.46
(dd, J ) 7.2, 1.4, 1H, HC(6)), 7.24-7.18 (m, 2H, HC(5′), HC(8′)),
7.12-6.96 (m, 6H, Ar), 6.81 (t, J ) 8.0, 1H, HC(2)), 6.70 (t, J ) 8.0,
1H, HC(8)), 3.17-3.03 (m, 1H, HeqC(4′)), 2.90-2.82 (m, 2H, HaxC(4′),
HC(10)), 2.42-2.28 (m, 2H, HeqC(2′), HC(11)), 2.04-1.81 (m, 5H,
H2C(3′), H2C(12), HaxC(2′)), 1.54-1.41 (m, 1H, HaHbC(13)CH3)), 1.03
(t, J ) 7.4, 3H, H3C(14)), 0.64-0.44 (m, 1H, HaHbC(13)CH3)), 0.32
(t, J ) 7.4, 3H, H3C(15)); 13C NMR (100 MHz, CDCl3) 145.83 (C(4a)),
140.62 (C(6a)), 139.56 (C(1a)), 138.37 (C(9a)), 136.93 (C(8′a)), 136.57
(C(4′a)), 131.92 (C(1)), 128.81 (C(9)), 128.79 (C(5′)), 128.72 (C(6′)),
127.87 (C(7′)), 127.27 (C(2)), 126.99 (C(8)), 126.71 (C(3)), 125.91
(C(7)), 125.81 (C(4)), 125.62 (C(6)), 125.32 (C(8′)), 71.57 (C(1′)), 58.60
(C(5)), 51.72 (C(10)), 44.15 (C(11)), 39.74 (C(2′)), 31.42(C(4′)), 29.50
(C(3′)), 23.04 (C(12)), 22.41 (C(13)), 12.67 (C(14)), 11.51 (C(15));
IR (CCl4) 2958 (m), 2929 (m), 2871 (m), 2289 (w), 1731 (m), 1550
(s), 1457 (w), 1376 (w), 1253 (m), 1218 (m), 1115 (w), 1070 (w), 1005
(m), 979 (m), 784 (s); MS (70 eV) 411 (27), 410 (M+, 100), 381 (33),
347 (42), 326 (52), 305 (56), 303 (13), 247 (30), 219 (63), 178 (55),
165 (11), 129 (33), 91 (25); TLC Rf 0.42 (hexane); [λ]D -30 (c )
0.03, i-PrOH); Anal. Calcd For C29H30S (410.6): C, 84.93; H, 7.36.
Found: C, 85.21; H, 7.03.
1
ether, 100/1) to give 1.7995 g (98%) of 3: bp 125 °C (0.1 Torr); H
NMR (400 MHz) 7.22-7.07 (m, 8H, Ar), 4.22 (s, 2H, H2C(5)), 2.89
(quintet, J ) 5.5, 2H, HC(10), HC(11)), 1.66-1.42 (m, 4H, 2 ×
CH2CH3), 0.86 (t, J ) 7.4, 6H, 2 × CH2CH3); 13C NMR (100 MHz)
141.30 (C(4a), C(6a)), 137.56 (C(1a), C(9a)), 131.54 (C(4), C(6)),
129.79 (C(2), C(8)), 125.90 (C(1), C(9)), 125.79 (C(3), C(7)), 50.44
(C(10), C(11)), 42.63 (C(5)), 28.08 (CH2CH3), 12.63 (CH2CH3); IR
(neat) 3058 (m), 3017 (m), 2959 (s), 2928 (s), 2870 (s), 1698 (w),
1651 (w), 1557 (w), 1539 (w), 1507 (w), 1491 (s), 1455 (s), 1420 (w),
1375 (m), 1063 (w); MS (70 eV) 250 (M+, 32), 222 (11), 221 (56),
207 (10), 191 (16), 180 (16), 179 (100), 178 (37), 165 (10), 91 (24),
43 (41); TLC Rf 0.45 (hexane/ether, 100/1); [R]D +144 (c ) 0.60,
CH2Cl2); Anal. Calcd for C19H22 (250.4): C, 91.14; H, 8.86. Found:
C, 90.98; H, 8.82.
(10R,11R)-10,11-Diethyl-10,11-dihydro-5H-dibenzo[a,d]cyclohep-
ten-5-one (4). In a 50-mL, round-bottomed, flask was placed potassium
permanganate (77.6 mg, 0.49 mmol, 2.5 equiv) suspended in benzene
(6 mL). A solution of dicyclohexano-18-crown-6 (182.53 mg, 0.49
mmol, 2.5 equiv) in benzene (6 mL) was added. The solution became
homogeneous and purple. To this mixture was added a solution of
10,11-diethyldibenzosuberane 3 (49.2 mg, 0.20 mmol) in benzene (5
mL). The resulting reaction mixture was stirred at ambient temperature
for 2 h and then passed through a silica gel column (20 mm × 3 cm)
using benzene (30 mL) and ether (30 mL) as eluents. The filtrate was
concentrated, and the crude oil was purified by column chromatography
(hexane/EtOAc, 20/1) to afford 48 mg (92%) of 4 as an yellowish
solid: mp 100.5-101 °C (hexane); 1H NMR (400 MHz) 8.13 (dd, J )
8.1, 1.5, 2H, HC(4), HC(6)), 7.45 (dt, J ) 7.3, 1.5, 2H, HC(2), HC(8)),
7.34 (dt, J ) 7.8, 1.2, 2H, HC(3), HC(7)), 7.21 (dd, J ) 7.6, 1.2, 2H,
HC(1), HC(9)), 3.02 (m, 2H, HC(10), HC(11)), 1.55-1.37 (m, 4H, 2
× CH2CH3), 0.84 (t, J ) 7.3, 6H, 2 × CH2CH3); 13C NMR (100 MHz)
193.55 (C(5)), 143.04 (C(1a), C(9a)), 137.08 (C(4a), C(6a)), 132.31
(C(2), C(8))), 131.34 (C(4), C(6)), 131.28 (C(1), C(9)), 126.57 (C(3),
C(7)), 50.99 (C(10), C(11)), 29.59 (CH2CH3), 12.48 (CH2CH3); IR
(CHCl3) 3068 (m), 3028 (m), 3010 (m), 2964 (s), 2935 (m), 2875 (m),
1639 (s, CdO), 1599 (s, CdC), 1481 (m), 1463 (m), 1454 (m), 1379
(w), 1294 (s), 1226 (m), 1218 (m), 1213 (w), 931 (m); MS (70 eV)
264 (M+, 39), 263 (16), 236 (21), 235 (91), 217 (11), 194 (19), 193
(100), 178 (24), 165 (23), 115 (10), 91 (14); TLC Rf 0.45 (hexane/
EtOAc, 20/1); [R]D 43.9 (c ) 0.54, CH2Cl2); Anal. Calcd for C19H20O
(264.4): C, 86.32; H, 7.63. Found: C, 86.27; H, 7.64.
(10R,11R,1′S)-10,11-Diethyl-5-(1,2,3,4,-tetrahydro-1-phenanthren-
yliden-2′-thirrane)-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (6b).
In a 50-mL, two-necked, round-bottomed, flask fitted with a Schlenk
filtration tube and septum was placed hydrazone (315 mg, 1.5 mmol)
in anhydrous CH2Cl2 (10 mL). The solution was cooled to -30 °C,
whereupon MgSO4 (0.98 g), Ag2O (518 mg, 2.25 mmol, 1.5 equiv),
and a saturated solution of KOH in methanol (1.18 mL) were
successively added. After having been stirred for 40 min, the resulting
deep red solution was filtered into another ice-cooled flask, and the
remaining residue was washed with CH2Cl2 (5 mL). To this clear
solution was added dropwise a solution of thioketone 5 (1.0 M in
CH2Cl2) by microsyringe. Evolution of nitrogen was observed, and the
deep red solution slowly decolored. The thioketone was added until
the evolution of nitrogen had subsided. A total of 188 mg (0.67 mmol)
of 5 was added. After having been stirred for 16 h, the resulting reaction
mixture was quenched by saturated aqueous NaHCO3 (15 mL). The
aqueous layer was separated and extracted with ether (3 × 15 mL).
The combined organic layers were dried (MgSO4), filtered, and
evaporated. The bluish residue was purified by column chromatography
(hexane) to get 139 mg (45%) of 6b as a white solid: mp 188-190
(10R,11R)-10,11-Diethyl-10,11-dihydro-5H-dibenzo[a,d]cyclohep-
ten-5-thione (5). In a 50- mL, two-necked, round-bottomed, flask was
placed P4S10 (444.5 mg, 1 mmol, 4.0 equiv) suspended in anhydrous
toluene (5 mL). To the reaction flask was added a solution of 4 (264
mg, 1 mmol) in anhydrous toluene (10 mL). The resulting reaction
mixture was stirred at 100 °C for 15 h and then passed through a short
column of neutral alumina (15 mm × 5 cm) using n-hexane as eluent.
The filtrate was collected and concentrated to give 280 mg (100%) of
5 as a blue oil: 1H NMR (200 MHz, CDCl3) 7.93 (dd, J ) 7.8, 1.4,
2H, HC(4), HC(6)), 7.41-7.06 (m, 6H, Ar), 2.96 (q, J ) 6.2, 2H,
HC(10), HC(11)), 1.65-1.40 (m, 4H, 2 × CH2CH3), 0.86 (t, J ) 7.6,
6H, 2 × CH2CH3)); TLC Rf 0.32 (hexane).
1
°C (hexane); H NMR (400 MHz, CDCl3) 8.14 (dd, J ) 8.0, 4.0, 1H,
HC(5′)), 8.09 (d, J ) 8.0, 1H, HC(8′)), 7.69 (d, J ) 8.0, 1H, HC(1)),
7.58-7.54 (m, 2H, HC(9′), HC(8)), 7.50-7.46 (m, 1H, HC(6′)), 7.32-
7.15 (m, 7H, Ar), 6.87 (d, J ) 8.0, 1H, HC(10′)), 3.56 (dt, J ) 8.0,
4.0, 1H, HeqC(4′)), 3.19-3.09 (m, 2H, HaHbC(4), HC(10)), 2.56 (td, J
) 8.0, 4.0, 1H, HaxC(2′)), 2.36 (td, J ) 8.0, 4.0, 1H, HC(11)), 2.23-
2.10 (m, 2H, HaHbC(12), HeqC(2′)), 2.03-1.95 (m, 3H, H2C(3′),
HaHbC(12)), 1.44-1.38 (m, 1H, HaHbC(13)), 1.10 (t, 3H, J ) 8.0,
H3C(14)), 0.52-0.45 (m, 1H, HaHbC(13)), 0.23 (t, 3H, J ) 8.0,
H3C(15)); 13C NMR (100 MHz, CDCl3) 146.02 (C(6a)), 140.53 (C(4a)),
138.59 (C(9a)), 136.97 (C(1a)), 135.43 (C(10′a)), 134.14 (C(4′a)),
132.20 (C(4′ab)), 132.02 (C(8′a)), 131.92 (C(1)), 131.75 (C(9)), 127.99
(C(9′)), 127.82 (C(6′)), 127.27 (C(7′)), 126.99 (C(8′)), 126.03 (C(2)),
126.00 (C(8)), 125.93 (C(3)), 125.80 (C(7)), 125.63 (C(5′)), 25.50
(C(4)), 125.04 (C(6)), 123.33 (C(10′)), 71.54 (C(1′)), 60.18 (C(5)), 51.76
(C(10)), 44.24 (C(11)), 39.01 (C(2′)), 29.39 (C(4′)), 27.17 (C(12)), 23.02
(C(13)), 22.07 (C(3′)), 12.76 (C(14)), 11.34 (C(15)); IR (CCl4) 2929
(m), 2292 (w), 1733 (w), 1684 (w), 1653 (w), 1550 (s), 1253 (m), 1218
(m), 1110 (w), 1070 (w), 1006 (m), 979 (m); MS (70 eV) 460 (M+,
34), 428 (28), 399 (40), 355 (13), 305 (13), 265 (14), 247 (100), 219
(45), 178 (51), 141 (29), 91 (14); TLC Rf 0.33 (hexane); [λ]D -43 (c
(10R,11R,1′S)-10,11-Diethyl-5-(1,2,3,4,-tetrahydro-1-naphtha-
lenyliden-2′-thirrane)-10,11- dihydro-5H-dibenzo[a,d]cycloheptene
(6a). In a 50-mL, two-necked, round-bottomed, flask fitted with a
Schlenk filtration tube and septum was placed hydrazone (272 mg, 1.7
mmol) in anhydrous CH2Cl2 (10 mL). The solution was cooled to -30
°C, whereupon MgSO4 (1.11 g), Ag2O (588 mg, 2.55 mmol, 1.5 equiv),
and a saturated solution of KOH in methanol (1.33 mL) were
successively added. After having been stirred for 40 min, the resulting
deep red solution was filtered into another ice-cooled flask, and the
remaining residue was washed with CH2Cl2 (5 mL). To this clear
solution was added dropwise a solution of thioketone 5 (1.0 M in

Potential Chirochromic Optical Switches
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7671
) 0.03, i-PrOH); Anal. Calcd For C33H32S (460.68): C, 86.04; H, 7.00.
(m), 1450 (w), 1353 (w), 1240 (w), 1070 (w), 770 (s); MS (70 eV)
378 (M+, 100), 349 (67), 289 (16), 247 (46), 219 (33), 190 (25), 145
(10); High-resolution MS calcd for C29H30: 378.2347, found: 378.2345;
TLC Rf 0.38 (hexane); [λ]D +62 (c ) 0.1, i-PrOH); HPLC tR 8.87 min
(Chiralpak OT, i-PrOH/hexane, 10/90, -20 °C, 0.5 mL/min, λ ) 254
nm).
Found: C, 85.56; H, 6.72.
(10R,11R,1′S)-10,11-Diethyl-5-(1,2,3,4,-tetrahydro-4-phenanthren-
yliden-2′-thirrane)-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (6c).
In a 50-mL, two-necked, round-bottomed, flask fitted with a Schlenk
filtration tube and septum was placed hydrazone (420 mg, 2.0 mmol)
in anhydrous CH2Cl2 (10 mL). The solution was cooled to -30 °C,
whereupon MgSO4 (1.30 g), Ag2O (691 mg, 3.0 mmol, 1.5 equiv),
and a saturated solution of KOH in methanol (1.57 mL) were
successively added. After having been stirred for 40 min, the resulting
deep red solution was filtered into another ice-cooled flask, and the
remaining residue was washed with CH2Cl2 (5 mL). To this clear
solution was added dropwise a solution of thioketone 5 (1.0 M in
CH2Cl2) by microsyringe. Evolution of nitrogen was observed, and the
deep red solution slowly decolored. The thioketone was added until
the evolution of nitrogen had subsided. A total of 224 mg (0.8 mmol)
of 5 was added. After having been stirred for 16 h, the resulting reaction
mixture was quenched by saturated aqueous NaHCO3 (15 mL). The
aqueous layer was separated and extracted with ether (3 × 15 mL).
The combined organic layers were dried (MgSO4), filtered, and
evaporated. The bluish residue was purified by column chromatography
(hexane) to get 126 mg (34%) of 6c as a white solid: mp 154-156 °C
(hexane); 1H NMR (400 MHz, CDCl3) 9.17 (d, J ) 8.0, 1H, HC(10′)),
8.08 (d, J ) 8.0, 1H, HC(7′)), 7.66 (d, J ) 8.0, 1H, HC(6′)), 7.62 (d,
J ) 8.0, 1H, HC(8′)), 7.57 (d, J ) 8.0, 1H, HC(6)), 7.37-7.12 (m,
6H, Ar), 6.97 (t, J ) 8.0, 1H, HC(2)), 6.78 (t, J ) 8.0, 1H, HC(8)),
6.47 (d, J ) 8.0, 1H, HC(1)), 3.24 (td, J ) 8.0, 4.0, 1H, HC(10)),
3.21-3.12 (m, 1H, HaHbC(4′)), 2.88 (m, 1H, HaHbC(4′)), 2.48 (td, J
) 8.0, 4.0, 1H, HaxC(2′)), 2.14-1.95 (m, 5H, H2C(3′), HeqC(2′),
HC(11)), 1.69-1.51 (m, 1H, HaHbC(12)), 1.22-108 (m, 1H,
HaHbC(12)), 1.10 (t, J ) 8.0, 3H, H3C(14)), 1.02-0.94 (m, 1H,
HaHbC(13)), 0.03 (t, J ) 8.0, 3H, H3C(15)), -0.69- -0.80 (m, 1H,
HaHbC(13)); 13C NMR (100 MHz, CDCl3) 147.25 (C(4a)), 141.48
(C(6a)), 139.44 (C(10′a)), 138.59 (C(6′a)), 136.29 (C(9a)), 133.34
(C(1a)), 133.08 (C(4′a)), 131.78 (C(10′ab)), 129.77 (C(9)), 128.63
(C(1)), 128.14 (C(5′)), 127.99 (C(2)), 126.91 (C(8)), 126.83 (C(9′)),
126.74 (C(8′)), 126.55 (C(7′)), 126.41 (C(6′)), 126.00 (C(7)), 125.83
(C(3)), 125.23 (C(10′)), 124.29 (C(4)), 123.10 (C(6)), 71.35 (C(1′)),
60.31 (C(5)), 51.77 (C(10)), 46.65 (C(11)), 45.35 (C(2′)), 34.15 (C(4′)),
28.39 (C(12)), 23.25 (C(13)), 21.16 (C(3′)), 12.93 (C(14)), 12.74
(C(15)); IR (CCl4) 2291 (w), 1550 (s), 1253 (m), 1218 (m), 1108 (w),
1068 (w), 1006 (m), 979 (m), 810 (s); MS (70 eV) 460 (M+,100), 428
(26), 397 (10), 355 (12), 318 (10), 305 (26), 247 (24), 221 (15), 191
(19), 179 (52), 141 (36), 91 (14); TLC: Rf 0.32 (hexane); [λ]D -48 (c
) 0.06, i-PrOH); Anal. Calcd For C33H32S (460.68): C, 86.04; H, 7.00.
Found: C, 86.01; H, 7.29.
(10R,11R,P)-10,11-Diethyl-5-(1,2,3,4,-tetrahydro-1-phenanthren-
yliden)-10,11-dihydro-5H-dibenzo [a,d] cycloheptene (7b). In a 25-
mL, two-necked, round-bottomed, flask fitted with a condenser topped
with an N2-inlet and septum was placed episulfide 6b (82 mg, 0.18
mmol) and copper powder (115 mg, 1.8 mmol, 10 equiv) in anhydrous
xylene (0.5 mL). The resulting reaction mixture was refluxed for 2 h
and then quenched with saturated aqueous NaHCO3 (5 mL). The
aqueous layer was separated and extracted with ether (3 × 15 mL).
The combined organic layers were dried (MgSO4), filtered, and
evaporated. The crude oil was purified by column chromatography
(hexane) to get 45 mg (58%) of 7b as a white solid: 1H NMR (400
MHz, CDCl3) 8.04 (d, 1H, J ) 8.0, HC(8′)), 7.65 (d, 1H, J ) 8.0,
HC(5′)), 7.52-7.36 (m, 2H, Ar), 7.28-7.00 (m, 6H, Ar), 6.84-6.80
(m, 4H, HC(2), HC(4), HC(6), HC(8′)), 3.60-3.15 (m, 4H, HC(11),
HaHbC(4′), HaxC(2′), HC(10)), 2.82-2.63 (m, 1H, HaHbC(4′)), 2.30-
1.72 (m, 5H, HeqC(2′), H2C(3′), H2C(12)), 1.23-0.75 (m, 2H, H2C(12)),
1.08 (t, J ) 8.0, 3H, H3C(15)), 0.80 (t, J ) 8.0, 3H, H3C(14)); 13C
NMR (50 MHz, CDCl3) 144.23 (C(5)), 142.45 (C(1′)), 140.62 (C(6a)),
139.57 (C(4a)), 136.55 (C(9a)), 134.53 (C(1a)), 133.65 (C(10′a)), 133.30
(C(4′a)), 131.45 (C(4′ab)), 128.33 (C(8′a)), 128.27 (C(1)), 128.15 (C(9)),
127.99 (C(9′)), 127.07 (C(6′)), 126.63 (C(7′)), 126.45 (C(8′)), 126.37
(C(2)), 125.87 (C(8)), 125.82 (C(3), 125.67 (C(7)), 125.51 (C(5′)),
125.45 (C(4)), 124.31 (C(6)), 123.47 (C(10′)), 50.42 (C(11)), 46.38
(C(10)), 28.92 (C(12)), 28.66 (C(13)), 26.42 (C(2′)), 26.05 (C(4′)), 24.35
(C(3′)), 12.85 (C(15)), 10.47 (C(14)); IR (CCl4) 3065 (w), 3018 (w),
2965 (m), 2931 (m), 2874 (w), 1598 (w), 1509 (w), 1480 (w), 1462
(w), 1379 (w), 1191 (w), 1105 (w), 1030 (w); TLC Rf 0.31 (hexane);
[λ]D +83 (c ) 0.03, i-PrOH); Anal. Calcd For C33H32 (428.6): C, 92.47;
H, 7.53. Found: C, 92.22; H, 7.59
(10R,11R,P)-10,11-Diethyl-5-(1,2,3,4,-tetrahydro-4-phenanthren-
yliden)-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (7c). In a 25-
mL, two-necked, round-bottomed, flask fitted with a condenser topped
with an N2-inlet and septum was placed episulfide 6c (20 mg, 0.04
mmol) and copper powder (28 mg, 0.4 mmol, 10 equiv) in anhydrous
xylene (0.5 mL). The resulting reaction mixture was refluxed for 2 h
and then quenched with saturated aqueous NaHCO3 (5 mL). The
aqueous layer was separated and extracted with ether (3 × 15 mL).
The combined organic layers were dried (MgSO4), filtered, and
evaporated. The crude oil was purified by column chromatography
(hexane) to get 18 mg (98%) of 7c as a white solid: mp 142-146 °C
1
(hexane); H NMR (400 MHz, CDCl3) 7.75-7.64 (m, 2H, HC(6′),
(10R,11R,P)-10,11-Diethyl-5-(1,2,3,4,-tetrahydro-1-naphthalen-
yliden)-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (7a). In a 25-
mL, two-necked, round-bottomed, flask fitted with a condenser topped
with an N2-inlet and septum was placed episulfide 6a (54 mg, 0.13
mmol) and copper powder (84 mg, 1.3 mmol, 10 equiv) in anhydrous
xylene (5 mL). The resulting reaction mixture was refluxed for 2 h
and then quenched with saturated aqueous NaHCO3 (5 mL). The
aqueous layer was separated and extracted with ether (3 × 15 mL).
The combined organic layers were dried (MgSO4), filtered, and
evaporated. The crude oil was purified by column chromatography
(hexane) to get 49 mg (100%) of 7a as a colorless oil: 1H NMR (400
MHz, CDCl3) 7.19-7.08 (m, 5H, Ar), 7.06-7.01 (m, 3H, Ar), 6.88 (t,
J ) 8.0, 1H, HC(2)), 6.82 (d, J ) 8.0, 1H, HC(6)), 6.79-6.71 (m, 2H,
HC(4), HC(8′)), 3.30-3.22 (m, 1H, HC(11)), 3.08-2.91 (m, 3H,
H2C(4′), HaxC(2′), H2C(1′), HC(11)), 2.78-2.72 (m, 1H, HC(10)),
2.13-1.73 (m, 5H, HeqC(2′), H2C(13), H2C(3′)), 1.65-1.51 (m, 1H,
HaHbC(12)), 1.48-1.35 (m, 1H, HaHbC(12)), 1.06 (t, J ) 8.0, 3H,
H3C(15)), 0.78 (t, 3H, J ) 8.0, H3C(14)); 13C NMR (100 MHz, CDCl3)
144.72 (C(5)), 142.27 (C(1′)), 140.62 (C(1a)), 139.07 (C(9a)), 138.85
(C(1′a)), 136.64 (C(4′a)), 131.69 (C(4)), 131.32 (C(6)), 128.69 (C(4)),
128.53 (C(1)), 127.76 (C(9)), 127.48 (C(7′)), 127.26 (C(5′)), 126.38
(C(6′)), 126.35 (C(8′)), 126.32 (C(6a)), 126.27 (C(2)), 126.19 (C(8)),
125.28 (C(3)), 124.36 (C(7)), 50.59 (C(11)), 46.01 (C(10)), 29.96
(C(4′)), 29.00 (C(2′)), 28.96 (C(13)), 25.57 (C(12)), 23.99 (C(3′)), 12.96
(C(15)), 10.49 (C(14)); IR (CCl4) 2929 (m), 2918 (m), 2890 (m), 1754
HC(7′)), 7.59 (d, J ) 8.0, 1H, HC(5′)), 7.38 (d, J ) 8.0, 1H, HC(10′)),
7.28-6.95 (m, 6H, Ar), 6.77 (t, J ) 8.0, 1H, HC(2)), 6.31 (t, J ) 8.0,
1H, HC(3)), 6.23 (d, J ) 8.0, 1H, HC(4)), 3.49 (m, 1H, HC(11)), 3.12-
2.83 (m, 4H, H2C(4), HC(10), HeqC(2′)), 2.49-2.35 (m, 1H,
HaHbC(13)), 2.33-1.61 (m, 6H, HaHbC(13), HaxC(2′), H2C(3′), H2C(12)),
1.13 (t, J ) 8.0, 3H, H3C(15)), 0.98 (t, J ) 8.0, H3C(14)); 13C NMR
(100 MHz, CDCl3) 142.80 (C(5)), 142.45 (C(1′)), 141.87 (C(1a)),
140,41 (C(9a)), 139.66 (C(6′a)), 138.16 (C(4′a)), 134.83 (C(10′a)),
132.90 (C(10′ab)), 132.07 (C(4)), 130.99 (C(6)), 129.67 (C(4a)), 129.21
(C(1)), 128.37 (C(9)), 128.19 (C(3)), 127.52 (C(8′)), 127.36 (C(7′)),
126.99 (C(6a)), 126.81 (C(5′)), 126.73 (C(6′)), 126.17 (C(10′)), 126.07
(C(2)), 125.24 (C(8)), 124.67 (C(3)), 124.07 (C(7)), 50.81 (C(11)), 45.81
(C(10)), 29.10 (C(4′)), 28.51 (C(13)), 28.87 (C(2′)), 26.90 (C(12)), 21.55
(C(3′)), 12.36 (C(15)), 10.01 (C(14)); IR (CCl4) 2293 (m), 1550 (s),
1253 (m), 1218 (m), 1108 (w), 1068 (w), 1006 (m), 979 (m), 774 (s);
MS (70 eV) 428 (M+, 23), 247 (38), 229 (16), 219 (100), 203 (50),
200 (22); [λ]D -82 (c ) 0.08, i-PrOH); TLC Rf 0.35 (hexane); Anal.
Calcd For C33H32 (428.6): C, 92.47; H, 7.53. Found: C, 92.40; H,
7.53.
(10R,11R,M)-10,11-Diethyl-5-(1,2,3,4,-tetrahydro-1-naphthalen-
yliden)-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (7a′). A solution
of 7a (3.5 mg, 9.2 µmol) in degassed hexane (0.5 mL) was irradiated
with 300 W Xe-lamp equipped with a monochromator. The irradiation
wavelength was 280 nm with a slit size equivalent to 16 nm bandwidth.

7672 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Chen and Chou
The experiment was carried on for 4 days till a constant composition
(99.6/0.4, 7a′/7a) was observed by HPLC analysis on Chiralpak OT
column. The solution was concentrated to give 7a′ quantitatively as a
white solid: 1H NMR (200 MHz, CDCl3) 8.57 (t, J ) 10.0, 1H, HC(3)),
7.46 (t, J ) 10.2, 1H, HC(7′)), 7.37 (t, J ) 6.8, 1H, HC(6′)), 7.27-
7.01 (m, 9H, Ar), 3.21-2.71 (m, 7H, HC(11), HC(10), H2C(4′),
HaxC(2′), H2C(12)), 2.16-1.78 (m, 5H, HeqC(2′), H2C(13), H2C(3′)),
0.89 (t, J ) 6.2, 3H, H3C(14)), 0.83 (t, 3H, J ) 7.4, H3C(15)); 13C
NMR (100 MHz, CDCl3) 142.31 (C(5)), 141.18 (C(1a)), 139.53 (C(9a)),
138.21 (C(7′a)), 137.44 (C(8′)), 135.36 (C(4′a)), 133.99 (C(4)), 133.83
(C(6)), 132.47 (C(1)), 130.75 (C(9)), 130.42 (C(6′)), 127.56 (C(4′)),
127.34 (C(5′)), 126.85 (C(7′)), 126.45 (C(4a)), 125.96 (C(6a)), 125.36
(C(2)), 124.73 (C(8)), 121.79 (C(3)), 121.34 (C(7)), 53.99 (C(11)), 49.56
(C(10)), 32.18 (C(13)), 31.05 (C(12)), 24.03 (C(1′)), 23.20 (C(3′)), 21.99
(C(2′)), 12.24 (C(15)), 11.80 (C(14)); MS (70 eV) 378 (M+, 7), 377
(24), 376 (92), 347 (45), 305 (100), 289 (24), 276 (11), 239 (7), 194
(7), 167 (8), 145 (11); High-Resolution MS calcd for C29H30: 378.2347,
found: 378.2342; TLC Rf 0.38 (hexane); [λ]D +55 (c ) 0.01, i-PrOH);
HPLC tR 10.40 min (Chiralpak OT, i-PrOH/hexane, 10/90, -20 °C,
0.5 mL/ min, λ ) 254 nm).
Acknowledgment. We are indebted to Professor I-Jy Chang
for helpful discussion on the photophysical aspects of our
helicenes and Professor Chong-Mou Wang for the generous
sharing of his photochemistry equipment with us. We thank
Professor Chuen-Her Ueng and Professor Yu Wang for X-ray
crystallographic analyses of 6a-c and 7c. We are grateful to
the National Science Council of the Republic of China for a
generous support of this research.
Supporting Information Available: X-ray position param-
eters, full bond distances and angles, and ORTEP drawings for
6a-c and 7c as well as chemical equations and numberings of
products in the Experimental Section are included (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA993297D