Photoinversion of cisoid/transoid binaphthyls

Article abstract of DOI:10.1021/ol203053q

Axially chiral binaphthyl-azobenzene cyclic dyads in which the two moieties are connected by two linkers of different lengths were synthesized. In the case of benzylated-binaphthyl analogue 2b, photoirradiation resulted in a dramatic change of the CD spec

Full text of DOI:10.1021/ol203053q

ORGANIC
LETTERS
2012
Vol. 14, No. 1
276–279
Photoinversion of Cisoid/Transoid
Binaphthyls
Kazuto Takaishi,*,† Atsuya Muranaka,†,‡ Masuki Kawamoto,† and
Masanobu Uchiyama*,†,§
RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan, Japan Science and Technology
Agency (JST), PRESTO, Kawaguchi 332-0012, Japan, and Graduate School of
Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
ktakaishi@riken.jp; uchi_yama@riken.jp
Received November 12, 2011
ABSTRACT
Axially chiral binaphthyl-azobenzene cyclic dyads in which the two moieties are connected by two linkers of different lengths were synthesized. In
the case of benzylated-binaphthyl analogue 2b, photoirradiation resulted in a dramatic change of the CD spectrum and optical rotation.
Experimental and theoretical analyses indicated that the dihedral angle of the two naphthalene rings is strongly coupled to the azobenzene
photoisomerization; cis-azobenzene induces a transoid-binaphthyl structure, while trans-azobenzene induces a cisoid-binaphthyl structure.
Axially chiral binaphthyls have a wide chiral space
around their axes. Hence, they have been extensively used
as asymmetric catalysts, for specific molecular recognition,
and to induce helical twisting of liquid crystals. In most
cases, the dihedral angle between the two naphthalene
rings, θ, plays an important direct or indirect role in these
functions; moreover, some of the properties of cisoid- and
transoid-binaphthyl (Figure 1b) differ from each other.1
On the other hand, azobenzenes are widely used in light-
driven molecular switches and molecular machines, be-
cause they exhibit dramatic structural changes in response
to photoirradiation.2 In general, UV light isomerizes trans-
to cis-azobenzene, whereas visible light or heat isomerizes cis-
to trans-azobenzene. Optically active azobenzene adducts
have recently been developed as chiroptical switches.3ꢀ5
We have previously investigated the helical chirality of
cis-azobenzenes, the planar chirality of trans-azobenzenes,
and the switching of chiroptical properties, e.g., optical
rotation and circular dichroism (CD), using axially chiral
† RIKEN.
‡ JST, PRESTO.
§ The University of Tokyo.
(1) (a) Pu, L. Chem. Rev. 1998, 98, 2405. (b) Berthod, M.; Mignani,
G.; Woodward, G.; Lemaire, M. Chem. Rev. 2005, 105, 1801. (c) Akagi,
K. Chem. Rev. 2009, 109, 5354. (d) He, Y.; Bian, Z.; Kang, C.; Jin, R.;
Gao, L. New J. Chem. 2009, 33, 2073. (e) Raskatov, J. A.; Thompson,
A. L.; Brown, J. M. Tetrahedron: Asymmetry 2010, 21, 1737. (f)
Coluccini, C.; Dondi, D.; Caricato, M.; Taglietti, A.; Boiocchi, M.;
Pasini, D. Org. Biomol. Chem. 2010, 8, 1640. (g) Hashimoto, T.; Kimura,
H.; Nakatsu, H.; Maruoka, K. J. Org. Chem. 2011, 76, 6030.
(2) (a) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema,
E. M. Chem. Rev. 2000, 100, 1789. (b) Cornelissen, J. J. L. M.; Rowan,
A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem. Rev. 2001, 101,
4039. (c) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim,
2001. (d) Balzani, V.; Venturi, M.; Credi, A. Molecular Devices and
Machines; Wiley-VCH: Weinheim, 2003. (e) Hoeben, F. J. M.; Jonkheijm,
P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491. (f)
Caruso, F. J.;M., M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.;
White, S. R.; Jeffrey, S.; Moore, J. S. Chem. Rev. 2009, 109, 5755. (g)
(3) For examples of axially chiral compounds, see: (a) Muraoka, T.;
Kinbara, K.; Aida, T. Nature 2006, 440, 512. (b) Green, L.; Li, Y.; White,
T.; Urbas, A.; Bunning, T.; Li, Q. Org. Biomol. Chem. 2009, 7, 3930. (c)
Chen, W.-C.; Lee, Y.-W.; Chen, C.-T. Org. Lett. 2010, 12, 1472. (d)
Mathews, M.; Zola, R. S.; Hurley, S.; Yang, D.-K.; White, T. J.;
Bunning, T. J.; Li, Q. J. Am. Chem. Soc. 2010, 132, 18361. (e) Reuter,
R.; Wegner, H. A. Org. Lett. 2011, 13, 5908.
€
Stoll, R. S.; Peters, M. V.; Kuhn, A.; Heiles, S.; Goddard, R.; Buhl, M.;
Thiele, C. M.; Hecht, S. J. Am. Chem. Soc. 2009, 131, 357.
r
10.1021/ol203053q
Published on Web 12/13/2011
2011 American Chemical Society

binaphthyl-azobenzene cyclic dyads, mainly1a, 1c, 2a, and
2c (Figure 1a).6 Among the chiroptical properties, the CD
spectrum is useful to identify the conformation and con-
figuration of all or part of a chiral compound. In the CD
spectra of our dyads, the signal in the shorter-wavelength
region (around 240 nm) is derived from the 1Bb transition
of the naphthalene rings, which reflects mainly the value of
θ (C(9)ꢀC(1)ꢀC(10)ꢀC(90), Figure 1b),7 while the signal
in the longer-wavelength region (over 350 nm) reflects
the helical chirality of cis-azobenzene6d and the twisting
direction of trans- azobenzene.6e The CD spectra in the
indicating a value of θ of ca. 90°, representing a somewhat
cisoid conformation. In contrast, the spectral shapes of
benzylated (R)-2a and (R)-2c were greatly different from
each other; that is, the first strong Cotton effects at around
240 nm were negative for (R)-2a and positive for (R)-2c
(Figure S2). These results indicated that the value of θ of
the benzylated analogues depends strongly on the length
of the linkers. Based on this finding, we speculated that
it might be possible to achieve reversible photoinversion,
i.e., cisoid/transoid transformation, of the benzylated bi-
naphthyl-azobenzene dyads. Compounds in which θ could
be controlled by photoirradiation would potentially have
wide application.
First, we attempted to predict θ of (R)-2a and (R)-2c by
calculation. Inoue et al. reported calculations on the
relationship between the θ, energy difference, and CD
spectra of 2,20-dimethoxy-1,10-binaphthyl.8 With reference
to their work, we examined the relationship between the
CD spectra and θ of model binaphthyl (R)-3 by calculation
at the DFT-D-B97D/TZVP level9 (Figure S5).10 The pre-
dicted CD spectra suggested that the binaphthyl skeleton
of (R)-2a favors the cisoid form, while that of (R)-2c favors
the transoid form, whether the azobenzene structure is in
cis- or trans-form. Therefore, we considered that (R)-2b,
in which the two moieties are connected by two linkers
of different lengths, might exhibit novel behavior; i.e., it
might be possible to achieve cisoid/transoid binaphthyl
conversion by means of photoirradiation.
Figure 2a shows the optimized structures of the cis and
trans forms of (R)-2b. Dihedral angles θ of the optimized
structures were 110° for (R)-cis-2b and 81° for (R)-trans-
2b. However, it should be noted that the permissible
conformations of each compound may include these opti-
mized conformations but do not seem to be limited to
them, because of metastable states due to molecular flex-
ibility. Therefore, the relationship between energy differ-
ence and θ was analyzed (Figure 2b). Energy minima of
both (R)-cis- and (R)-trans-2b appeared to be localized to a
Figure 1. Binaphthyl-azobenzene cyclic dyads (R)-1aꢀc, (R)-
2aꢀc, and a model binaphthyl (R)-3. (b) View of the frame
format of cisoid- and transoid-binaphthyls.
shorter-wavelength region of (R)-1a and (R)-1c were very
similar (Figure S1) and showed a typical negative split CD,
(4) For examples of planar-chiral compounds, see: (a) Jousselme, B.;
Blanchard, P.; Allain, M.; Levillain, E.; Dias, M.; Roncali, J. J. Phys.
Chem. A 2006, 110, 3488. (b) Tamaoki, N.; Mathews, M. J. Am. Chem.
Soc. 2008, 130, 11409. (c) Basheer, M. C.; Oka, Y.; Mathews, M.;
Tamaoki, N. Chem.;Eur. J 2010, 16, 3489. (d) Hashim, P. K.; Thomas,
R.; Tamaoki, N. Chem.;Eur. J. 2011, 17, 7304.
(8) Nishizaka, M.; Mori, T.; Inoue, Y. J. Phys. Chem. Lett. 2010, 1,
1809. They pointed out that the DFT calculations with common density
functionals (e.g. B3LYP, PBE0) were not suitable for this type of
calculation, but the DFT-D-B97D or RI-CC2 level should be used.
€
(9) (a) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97,
€
2571. (b) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1992, 100,
5829. (c) Calculations of the CD spectra of these dyads with this basis set
were infeasible due to the large molecular size.
(5) For examples of compounds with helical chirality, see: (a) Haberhauer,
G.; Kallweit, C. Angew. Chem., Int. Ed. 2010, 49, 2418. (b) Zhang, W.;
Yoshida, K.; Fujiki, M.; Zhu, X. Macromolecules 2011, 44, 5105.
(6) (a) Kawamoto, M.; Aoki, T.; Wada, T. Chem. Commun. 2007,
930. (b) Takaishi, K.; Kawamoto, M.; Tsubaki, K.; Wada, T. J. Org.
Chem. 2009, 74, 5723. (c) Kawamoto, M.; Shiga, N.; Takaishi, K.;
Yamashita, T. Chem. Commun. 2010, 46, 8344. (d) Takaishi, K.;
Kawamoto, M.; Tsubaki, K.; Furuyama, T.; Muranaka, A.; Uchiyama,
M. Chem.;Eur. J. 2011, 17, 1778. (e) Takaishi, K.; Muranaka, A.;
Kawamoto, M.; Uchiyama, M. J. Org. Chem. 2011, 76, 7623.
(7) (a) Harada, N.; Nakanishi, K. Acc. Chem. Res. 1972, 5, 257. (b)
Bari, L. D.; Pescitelli, G.; Salvadori, P. J. Am. Chem. Soc. 1999, 121,
7998. (c) Bari, L. D.; Pescitelli, G.; Marchetti, F.; Salvadori, P. J. Am.
Chem. Soc. 2000, 122, 6395. (d) Wang, C.; Zhu, L.; Xiang, J.; Yu, Y.;
Zhang, D.; Shuai, Z.; Zhu, D. J. Org. Chem. 2007, 72, 4306. (e)
Niezborala, C.; Hache, F. J. Am. Chem. Soc. 2008, 130, 12783. (f)
Polavarapu, P. L.; Jeirath, N.; Walia, S. J. Phys. Chem. A 2009, 113,
5423. (g) Takaishi, K.; Kawamoto, M.; Tsubaki, K. Org. Lett. 2010, 12,
1832. (h) Nishizaka, M.; Mori, T.; Inoue, Y. J. Phys. Chem. A 2011, 115,
5488. (i) Pescitelli, G.; Bari, L. D.; Berova, N. Chem. Soc. Rev. 2011, 40,
4603.
(10) Calculations were done by Gaussian 09, revision B.01. Frisch,
M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson,
G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov,
A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari,
K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.;
Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin,
R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
€
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman,
J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc.: Wallingford,
CT, 2009. The theoretical analysis of (R)-3 indicated that the sign of the
Cotton effect at 240ꢀ270 nm, which is characteristic, reversed between
80°ꢀ90°.
Org. Lett., Vol. 14, No. 1, 2012
277

small region, i.e., the transoid-binaphthyl region for (R)-
cis-2b, and the cisoid-binaphthyl region for (R)-trans-2b.
Essentially, the binaphthyl skeleton, (R)-3, was more
stable in the transoid than the cisoid form, and the plot
pattern was in agreement with that of (R)-cis-2b (FigureS6).
Based on the calculated energies, it appeared that photo-
inversion of cisoid/transoid binaphthyls with Δθ of ca. 30°
might be feasible using (R)-2b. To test this idea, we
synthesized and analyzed (R)-2b.
Scheme 1. Synthesis of Binaphthyl-azobenzene Cyclic Dyads
(R)-1b and (R)-2b
and is useful as an indicator of isomerization. Meanwhile,
there are major and global differences in CD spectra between
the isomers (Figure 3a). The longer wavelength region
(over 350 nm) reflects the twisting of azobenzene.6d,e,12
Theoretical analysis indicated that (R)-cis-(P)-2b and
(R)-trans-(pS)-2b were favored over other configurations.13
Next, at shorter wavelengths, the CD spectra were drasti-
cally altered upon photoirradiation (Δε = þ70 at 235 nm
after 365 nm irradiation; Δε = ꢀ41 at 242 nm after 436 nm
irradiation). These changes suggested that θ was altered as
a result of the photoirradiation. To eliminate the influence
of azobenzene, the CD spectra of the binaphthyl moiety
(i.e., (R)-3) extracted from each optimized (R)-2b were cal-
culated (Figure 4). These theoretical CD spectra were in
good agreement with the experimental spectra and strongly
supported the occurrence of cisoid-transoid photoinversion
of (R)-2b as shown in Figure 2.14 Compounds showing such
a major photoinduced CD change have potential for the
development of characteristic optical switches.15 In con-
trast, the absorption and CD spectra of (R)-1b showed
similar changes to those of (R)-1a and (R)-1c (Figure S3).
Regarding the conformation of our dyads, especially
(R)-trans-2b, we suggest that the two different short linkers
result in differential interaction between the benzyl groups
and the azobenzene moiety, and suppress the twisting
motion of the binaphthyl structure. This is supported by
the splitting patterns and the chemical shifts of the benzyl
Figure 2. (a) Optimized structures of (R)-cis-2b and (R)-trans-2b
at the DFT-D-B97D/TZVP level. Azobenzene moieties are
shown in orange, and benzyl groups in black. Hydrogen atoms
are not shown. (b) Relationship between energy difference and
binaphthyl dihedral angle of (R)-cis-2b (blue line) and (R)-trans-
2b (red line) at the DFT-D-B97D/TZVP level.
Dyads (R)-2b and (R)-1b, a reference compound, were
synthesized as shown in Scheme 1. Compound 5 was
prepared in moderate yield by the coupling of 2,20-dihy-
droxyazobenzene (4) and 1 equiv of 2-bromo-1-ethanol.
Subsequently, compound 6 was obtained from 5 and
3-bromo-1-propanol. Usual dimesylation of 6 gave 7 in
highyield. (R)-1bwas preparedby tandem etherification of
(R)-BINOL (8) and dimesylate 7. (R)-2b was synthesized
similarly, using benzylated binol (R)-9.11 As is typical,
transfcis isomerization was efficiently induced by irradia-
tion at 365 nm, whereas cisftrans isomerization was induced
by irradiation at 436 nm. The isomerization ratio of the
cis-form in CDCl3 after irradiation at 365 nm was 0.88,
and that of the trans-form after irradiation at 436 nm was
0.82 for (R)-2b.
Absorption spectra of (R)-2b after photoirradiation are
shown in Figure 3b. The spectra at shorter wavelength are
similar for the two isomers. But, there was an enormous
change in the absorption in the longer wavelength region
over 350 nm, which is due to the azobenzene moiety, since
the absorption wavelength of the binaphthyl skeleton is
under 340 nm.6,7 Absorption at about 350 nm is derived
from the allowed πꢀπ* transition of the trans-azobenzene
(12) Toro, C.; Passier, R.; Diaz, C.; Tuuttila, T.; Rissanen, K.;
ꢀ
Huuskonen, J.; Hernandez, F. E. J. Phys. Chem. A 2011, 115, 1186.
(13) See Figures S10 and S11 in the Supporting Information. The
results of calculation at the B97D and B3LYP levels were essentially the
same. In all calculations on (R)-2b in the manuscript, these configura-
tions were employed.
(14) The calculated CD of optimized (R)-3 did not completely
correspond to that of (R)-3 extracted from (R)-2b because of the
differences in the position of benzyl groups, linkers, and the molecular
symmetry.
(11) Tsubaki, K.; Morikawa, H.; Tanaka, H.; Fuji, K. Tetrahedron:
Asymmetry 2003, 14, 1393.
(15) Suk, J.; Naidu, V. R.; Liu, X.; Lah, M. S.; Jeong, K-. S. J. Am.
Chem. Soc. 2011, 133, 13938 and references cited therein.
278
Org. Lett., Vol. 14, No. 1, 2012

located further from azobenzene, while the other benzyl
group, which may interact with azobenzene owing to the
shorter linker, shows the AB quartet. These signals indi-
cated that benzyl substituents have an important influence
on the overall conformation of dyads of this type. We were
unable to detect NOEs between the benzyl groups, linkers,
and azobenzene. (R)-2b should also be available as an
optical rotationswitch, aswellasa CDswitch. Thevalue of
Figure 5. 1H NMR spectra of benzyl protons of (R)-2b ([cis]/[trans] =
1:2.4, CDCl3, 1 ꢁ 10ꢀ3 M, 295 K, 300 MHz).
Figure 3. (a) CD spectra of (R)-2b; (b) absorption spectra of (R)-
2b after irradiation at 365 nm (blue, rich in cis-form) and after
irradiation at 436 nm (red, rich in trans-form). The inset shows
an expanded version of part of the spectra. Conditions: 1,4-
dioxane 1.0 ꢁ 10ꢀ5 M, 20 °C, light path length = 10 mm,
irradiation at 10 mW/cm2 for 100 s.
optical rotation, [R]D (CHCl3, c 0.10, 20 °C), after irradia-
tion of (R)-2b at 365 nm was ꢀ246°, whereas it was þ710°
after irradiation at 436 nm; i.e., there was a large, reversible
change, with sign inversion. These changes derive from
the anomalous dispersion of helical cis-azobenzene and
twisted trans-azobenzene.6d,e The half-life of the cis-form
of (R)-2b is over 300 h at 298 K.
In summary, axially chiral binaphthyl-azobenzene cyclic
dyads connected by two different short linkers were
synthesized. The azobenzene moieties were photoisome-
rized easily and efficiently. In the benzyl-substituted deri-
vative (R)-2b, the cis-azobenzene structure induced the
transoid-binaphthyl structure, and the trans-azobenzene
structure induced the cisoid-binaphthyl structure. These
structural changes were induced by photoirradiation at
distinct wavelengths and were accompanied by changes of
the CD spectrum and [R]D, including sign inversion. These
results are relevant to the development of photocontrolled
organocatalysts,17 chiral dopants for liquid crystals, chir-
optical switches, and other intelligent chiral materials.
Figure 4. Calculated CD spectra of (R)-3 extracted from opti-
mized (R)-cis-2b (blue line) and from (R)-trans-2b (red line) at
the DFT-D-B97D/TZVP level. Gaussian bands with a half-
bandwidth of 2500 cmꢀ1
.
Acknowledgment. We are grateful to members of the
molecular characterization team at RIKEN for the spec-
tral measurements. The calculations were performed by
using the RIKEN Integrated Cluster of Clusters (RICC).
protons of (R)-2b in the 1H NMR spectra (Figure 5). The
benzyl proton signals of (R)-cis-2b appeared as two sing-
lets, one of which overlapped with a signal of (R)-trans-
2b.16 But, in (R)-trans-2b, one of the benzyl proton signals
appeared as a singlet, while the other appeared as an AB
quartet with a Δν of 0.18 ppm (53 Hz). The singlet peak
may be that of the free rotatable benzyl group, which is
Supporting Information Available. Synthesis, spectra,
computational details, and thermodynamic parameters
of novel compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
(17) Wang, J.; Feringa, B. L. Science 2011, 331, 1429. (b) Berryman,
ꢀ
O. B.; Sather, A. C.; Lledo, A.; Rebek, J., Jr. Angew. Chem., Int. Ed.
2011, 50, 9400.
(16) It is also possible that the overlapped peaks of (R)-cis- and (R)-
trans-2b are AB quartets with small Δν.
Org. Lett., Vol. 14, No. 1, 2012
279