Synthesis and helical structure of spiroborate-based double-stranded helicate with oligophenol strands bearing bipyridine units

Article abstract of DOI:10.1002/chem.201102511

Double wrapping: A new spiroborate-based double-stranded helicate was prepared from tetraphenol strands with a bipyridine unit in the middle and NaBH₄. The X-ray single-crystallographic analysis revealed that the helicate adopts a double helica

Full text of DOI:10.1002/chem.201102511

DOI: 10.1002/chem.201102511
Synthesis and Helical Structure of Spiroborate-Based Double-Stranded
Helicate with Oligophenol Strands Bearing Bipyridine Units
Yoshio Furusho, Kazuhiro Miwa, Ryo Asai, and Eiji Yashima*[a]
A large number of synthetic double helices have been de-
veloped during the past few decades because of their signifi-
cant aesthetic and biomimetic appeal, and several structural
motifs for double helices are now available,[1–8] including the
peptide analogues of DNA,[2] transition metal complexes,
namely the helicates,[3,4] aromatic oligoamides,[5] amidinium–
carboxylate salt bridges,[6] and oligoresorcinols.[7] Among
them, the helicates have gained popularity due to their ap-
plications in many areas, such as synthetic receptors,[9] liquid
crystalline materials,[10] catalysis,[11] and DNA recognition.[12]
Most of them rely on the coordination of transition metal
cations to pyridine- or phenanthroline-containing molecular
strands, and their three-dimensional structures are deter-
mined by the coordination geometry requirements. In con-
trast, helicates assembled with typical metal cations are
quite rare and only a limited number of examples have been
reported for the Li+, Na+, K+, and B3+ ions.[13,14] In addi-
tion, only a few double-stranded helicates have been optical-
ly resolved into enantiomers despite the emerging interest
in their inherent helical chirality.[15]
We have recently reported the synthesis of a spiroborate-
based helicate (DH1BNaBÀ) from a hexaphenol ligand (1,
H6L1) and its optical resolution into enantiomers, which dis-
closed that the resolved enantiomers were stable toward rac-
emization (Scheme 1).[14a,16] We further designed and synthe-
sized the boron helicate (DH2BNaBÀ) containing tetraphenol
strands with a biphenylene unit in the middle (2, H4L2),
which was also optically resolved, and achieved precise con-
trol over the ion-triggered extension–contraction motion ac-
companied by anisotropic twisting by using the resolved
enantiomers.[14b] This study has clearly demonstrated that
the linkers connecting the two biphenol units can be ex-
changed for other units with a shape similar to that of the
biphenlyene linker. Bipyridines are obvious candidates for
the substitution and of particular importance since their
transition metal complexes, especially those of ruthenium,[17]
osmium,[18] and rhenium,[19] exhibit interesting photophysical
and photochemical properties. We now describe the design
and synthesis of
a
new spiroborate-based helicate
(DH3BNa2B) from a tetraphenol strand bearing a bipyridine
unit in the middle (3, H4L3), along with the elucidation of
the double helical structure and its optical resolution by
chiral HPLC.
The bipyridine-containing tetraphenol (3) was prepared
according to Scheme S1 (in the Supporting Information).
The 2,2’-dimethoxybiphenyl-based monoboronic acid[14a] (4)
and 6,6’-dibromo-2,2’-bipyridyl (5) were connected by
Suzuki coupling to give the tetramethoxy derivative (6), of
which the methyl groups were removed by treatment with
BBr3 to afford the tetraphenol (3). The bipyridine-contain-
ing helicate (DH3BNa2B) was synthesized in the same way as
those for DH1BNaBÀ·Na+ and DH2BNaBÀ·Na+ (Scheme 1).[14]
The tetraphenol 3 was treated with NaBH4 in 1,2-dichloro-
ethane/ethanol (6:1, v/v) at 808C for one week. The reaction
slowly proceeded and required a longer time to reach a rea-
sonable conversion than those for DH1BNaBÀ·Na+ and
DH2BNaBÀ·Na+. In contrast to DH1BNaBÀ·Na+ and
DH2BNaBÀ·Na+, DH3BNa2B was sufficiently stable under silica
gel chromatography conditions, therefore, DH3BNa2B could
be isolated in 28% yield by chromatographic purification.
Single crystals suitable for an X-ray analysis were grown
from an ethanol solution. The X-ray crystallographic analy-
sis unambiguously revealed that DH3BNa2B adopted a double
helical structure with a pseudo-D2 symmetry, similar to
those of DH1BNaBÀ and DH2BNaBÀ, in which the two tetraphe-
nol strands were intertwined with each other through two
spiroborate bridges and the two terminal benzene rings of
each tetraphenol strand were twisted by about 2808 (Fig-
ure 1A). The most striking difference from DH1BNaBÀ and
DH2BNaBÀ was that two Na+ ions were embraced in the
center of the complex coordinated by the bipyridine units
and the oxygen atoms of the spiroborate moieties, thereby
making the helicate electrically neutral. The two Na+ cat-
ions were located with a very short distance of 3.1 ꢀ, and
were stabilized by the two negatively charged spiroborate
groups.[20] In addition, the B–B distance of DH3BNa2B was
5.3 ꢀ, which was much shorter than those of DH1BNaBÀ
(6.3 ꢀ)[14a] and DH2BNaBÀ (6.0 ꢀ),[14b] as a result of the elec-
trostatic stabilization by the two Na+ ions (Figure 1B).
Electrospray ionization (ESI) mass measurements in the
positive mode supported the structure of the boron helicate.
These showed a strong signal due to the monocationic spe-
cies ([B2Na2(L3)2 +Na]+) at m/z 1580.20 along with a minor
[a] Dr. Y. Furusho, Dr. K. Miwa, R. Asai, Prof. Dr. E. Yashima
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
Fax : (+81)52-789-3185
E-mail: yashima@apchem.nagoya-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102511.
signal due to the dicationic species ([B2Na2(L3)2 +2Na]2+
)
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COMMUNICATION
Scheme 1. Synthesis of: A) DH1BNaBÀ·Na+ and DH2BNaBÀ·Na+, and B) DH3BNa2B
.
at m/z 801.60 (Figure S1 in the
Supporting Information). More-
over, no appreciable signals
were observed in the negative
mode measurements; this re-
flects the fact that the two Na+
ions were strongly bound by
the bipyridine nitrogen atoms
and the spiroborate oxygen
atoms. With the aim of remov-
ing the Na+ ions, cryptand
[2.2.1], which had been used to
remove the Na+ ion from
DH2BNaBÀ, was added to a solu-
tion of DH3BNa2B in CDCl3; this
failed to capture the Na+ ions
even at 508C for 24 h. This fail-
ure to remove the Na+ ions
also supported the strong chela-
tion of the Na+ ions by the hel-
icate.
1
The H NMR spectrum of the
helicate in CDCl3 also revealed
the pseudo-D2 symmetric struc-
ture as was determined by the
Figure 1. A) X-ray single crystal structure of DH3BNa2B; side view (left) and top view (right). B) Comparison of
[14a]
[14b]
the crystal structures of DH1BÀNaB
,
DH2ÀBNaB
,
and DH3BNa2B. Hydrogen atoms and solvent molecules are
X-ray analysis in the solid state.
omitted for clarity.
The two tBu signals and the ar-
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www.chemeurj.org
13955

E. Yashima et al.
omatic protons on the terminal benzene rings significantly
shifted up-field due to the ring current effect of the benzene
rings of the other strand, which was in good agreement with
the crystal structure (Figure 2). In addition, the 2D NOESY
Figure 3. A) UV (254 nm) detected HPLC chromatogram of DH3BNa2B
.
HPLC conditions: column, Chiralpak IB (Daicel, 2.0 fꢂ25 cm); eluent,
hexane/CHCl3 (8:2, v/v) containing diethylamine (0.1 vol%); flow rate,
8.0 mLminÀ1; column temperature, 258C. B) CD and UV/Vis spectra of
(+)-DH3BNa2B (f2) and (À)-DH3BNa2B (f1) in CHCl3 at 258C.
Figure 2. Partial 1H NMR spectra (500 MHz) of 3 (H4L3, upper spec-
trum) and DH3BNa2B (lower spectrum) in CDCl3 at 258C.
Acknowledgements
experiments showed strong negative cross-peaks between
the phenol rings A and C (Figures S5–S7 in the Supporting
Information; for complete peak assignments, see 2D COSY,
TOCSY, and NOESY spectra (Figures S2–S7)), which were
attributed to the interstrand NOEs (distance approximately
2.5 ꢀ); this indicates that the helicate retained the double-
stranded helical tetranuclear structure in solution.
This work was supported in part by Grant-in-Aids for Scientific Research
(S) from the Japan Society for the Promotion of Science (JSPS) and for
Scientific Research on Innovative Areas, “Emergence in Chemistry”
from the MEXT (21111508).
Keywords: borates
· helical structures · oligophenols ·
The optical resolution of (Æ)-DH3BNa2B was successfully
achieved by chiral HPLC (Chralpak IB, Daicel, Co., Ltd.)
by using hexane/CHCl3 (8:2, v/v) containing diethylamine
(0.1 vol%) as the eluent (Figure 3A). The resolved two frac-
tions showed the CD spectra that are mirror images of each
other, which reflect their enantiomerism (Figure 3B). The
resolved enantiomers were stable and no racemization was
observed even after the samples were stored in the eluent at
ambient temperature for two weeks or in 1,2-dichloroethane
at 808C for 24 h.
In conclusion, we have successfully prepared the spirobo-
rate-based helicate with oligophenol strands bearing bipyri-
dine units in the middle. Due to its neutral feature, both
enantiomers of the racemic helicate were separated by
chiral HPLC. The present results suggest that further design
of the spiroborate-based helicates with functional units in
the middle or at the ends is possible. The application of the
helicates to chiral discrimination and asymmetric catalysis
are ongoing in our laboratories.
optical resolution
[1] For reviews on synthetic double helices, see: a) I. Huc, Eur. J. Org.
Chem. 2004, 17–29; b) R. Amemiya, M. Yamaguchi, Org. Biomol.
Chem. 2008, 6, 26–35; c) D. Haldar, C. Schmuck, Chem. Soc. Rev.
2009, 38, 363–371; d) Y. Furusho, E. Yashima, Macromol. Rapid
Commun. 2011, 32, 136–146.
[2] a) P. E. Nielsen, M. Egholm, R. H. Berg, O. Buchardt, Science 1991,
254, 1497–1500; b) M. Egholm, O. Buchardt, L. Christensen, C.
Behrens, S. M. Freier, D. A. Driver, R. H. Berg, S. K. Kim, B.
Norden, P. E. Nielsen, Nature 1993, 365, 566–568.
[3] For reviews on helicates, see: a) E. C. Constable, Tetrahedron 1992,
48, 10013–10059; b) J.-M. Lehn, Supramolecular Chemistry: Con-
cepts and Perspectives, Wiley-VCH, Weinheim, 1995; c) C. Piguet, G.
Bernardinelli, G. Hopfgartner, Chem. Rev. 1997, 97, 2005–2062;
d) M. Albrecht, Chem. Rev. 2001, 101, 3457–3497.
[4] a) J.-M. Lehn, A. Rigault, J. Siegel, J. Harrowfield, B. Chevrier, D.
Moras, Proc. Natl. Acad. Sci. USA 1987, 84, 2565–2569; b) J.
Libman, Y. Tor, A. Shanzer, J. Am. Chem. Soc. 1987, 109, 5880–
5881; c) U. Koert, M. M. Harding, J.-M. Lehn, Nature 1990, 346,
339–342; d) E. C. Constable, S. M. Elder, J. Healy, M. D. Ward, J.
13956
www.chemeurj.org
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Spiroborate-Based Double-Stranded Helicates
COMMUNICATION
Am. Chem. Soc. 1990, 112, 4590–4592; e) R. Kramer, J.-M. Lehn,
A. Marquis-Rigault, Proc. Natl. Acad. Sci. USA 1993, 90, 5394–
5398.
Tsang, W.-Y. Wong, H.-L. Kwong, Chem. Commun. 2007, 5203–
5205; c) T. Hasegawa, Y. Furusho, H. Katagiri, E. Yashima, Angew.
Chem. 2007, 119, 5989–5992; Angew. Chem. Int. Ed. 2007, 46, 5885–
5888; d) H.-L. Yeung, K.-C. Sham, C.-S. Tsang, T.-C. Lau, H.-L.
Kwong, Chem. Commun. 2008, 3801–3803; e) K.-C. Sham, H.-L.
Yeung, S.-M. Yiu, T.-C. Lau, H.-L. Kwong, Dalton Trans. 2010, 39,
9469–9471.
[5] a) V. Berl, I. Huc, R. G. Khoury, M. J. Krische, J.-M. Lehn, Nature
2000, 407, 720–723; b) C. Dolain, C. Zhan, J.-M. Leger, L. Daniels,
I. Huc, J. Am. Chem. Soc. 2005, 127, 2400–2401; c) C. Zhan, J.-M.
Leger, I. Huc, Angew. Chem. 2006, 118, 4741–4744; Angew. Chem.
Int. Ed. 2006, 45, 4625–4628; d) Q. Gan, C. Bao, B. Kauffmann, A.
Grelard, J. Xiang, S. Liu, I. Huc, H. Jiang, Angew. Chem. 2008, 120,
1739–1742; Angew. Chem. Int. Ed. 2008, 47, 1715–1718; e) Y. Fer-
rand, A. M. Kendhale, J. Garric, B. Kauffmann, I. Huc, Angew.
Chem. 2010, 122, 1822–1825; Angew. Chem. Int. Ed. 2010, 49, 1778–
1781; f) Q. Gan, Y. Ferrand, C. Bao, B. Kauffmann, A. Grelard, H.
Jiang, I. Huc, Science 2011, 331, 1172–1175.
[6] For artificial double helices based on amidinium–carboxylate salt
bridges, see: a) Y. Tanaka, H. Katagiri, Y. Furusho, E. Yashima,
Angew. Chem. 2005, 117, 3935–3938; Angew. Chem. Int. Ed. 2005,
44, 3867–3870; b) Y. Furusho, Y. Tanaka, E. Yashima, Org. Lett.
2006, 8, 2583–2586; c) M. Ikeda, Y. Tanaka, T. Hasegawa, Y. Fur-
usho, E. Yashima, J. Am. Chem. Soc. 2006, 128, 6806–6807; d) Y.
Furusho, Y. Tanaka, T. Maeda, M. Ikeda, E. Yashima, Chem.
Commun. 2007, 3174–3176; e) H. Ito, Y. Furusho, T. Hasegawa, E.
Yashima, J. Am. Chem. Soc. 2008, 130, 14008–14015; f) T. Maeda,
Y. Furusho, S.-I. Sakurai, J. Kumaki, K. Okoshi, E. Yashima, J. Am.
Chem. Soc. 2008, 130, 7938–7945; g) H. Iida, M. Shimoyama, Y.
Furusho, E. Yashima, J. Org. Chem. 2010, 75, 417–423; h) H.
Yamada, Y. Furusho, H. Ito, E. Yashima, Chem. Commun. 2010, 46,
3487–3489; i) Z.-Q. Wu, Y. Furusho, H. Yamada, E. Yashima,
Chem. Commun. 2010, 46, 8962–8964; j) H. Ito, M. Ikeda, T. Hase-
gawa, Y. Furusho, E. Yashima, J. Am. Chem. Soc. 2011, 133, 3419–
3432.
[7] For poly- and oligo(m-phenylene)-based double helices, see: a) H.
Goto, H. Katagiri, Y. Furusho, E. Yashima, J. Am. Chem. Soc. 2006,
128, 7176–7178; b) H. Goto, Y. Furusho, E. Yashima, J. Am. Chem.
Soc. 2007, 129, 109–112; c) H. Goto, Y. Furusho, E. Yashima, J. Am.
Chem. Soc. 2007, 129, 9168–9174; d) T. Ben, Y. Furusho, H. Goto,
K. Miwa, E. Yashima, Org. Biomol. Chem. 2009, 7, 2509–2512;
e) H. Goto, Y. Furusho, K. Miwa, E. Yashima, J. Am. Chem. Soc.
2009, 131, 4710–4719.
[8] For other examples of double helices, see: a) H. Sugiura, Y. Nigori-
kawa, Y. Saiki, K. Nakamura, M. Yamaguchi, J. Am. Chem. Soc.
2004, 126, 14858–14864; b) H.-C. Yang, S.-Y. Lin, H.-C. Yang, C.-L.
Lin, L. Tsai, S.-L. Huang, I. W.-P. Chen, C.-h. Chen, B.-Y. Jin, T.-Y.
Luh, Angew. Chem. 2006, 118, 740–744; Angew. Chem. Int. Ed.
2006, 45, 726–730; c) J. Li, J. A. Wisner, M. C. Jennings, Org. Lett.
2007, 9, 3267–3269; d) T. Sugimoto, T. Suzuki, S. Shinkai, K. Sada,
J. Am. Chem. Soc. 2007, 129, 270–271; e) H. Abe, H. Machiguchi, S.
Matsumoto, M. Inouye, J. Org. Chem. 2008, 73, 4650–4661; f) H.-J.
Kim, E. Lee, M. G. Kim, M.-C. Kim, M. Lee, E. Sim, Chem. Eur. J.
2008, 14, 3883–3888.
[12] a) A. Oleksi, A. G. Blanco, R. Boer, I. Uson, J. Aymami, A. Rodger,
M. J. Hannon, M. Coll, Angew. Chem. 2006, 118, 1249–1253;
Angew. Chem. Int. Ed. 2006, 45, 1227–1231; b) L. Cerasino, M. J.
Hannon, E. Sletten, Inorg. Chem. 2007, 46, 6245–6251; c) J. Malina,
M. J. Hannon, V. Brabec, Nucleic Acids Res. 2008, 36, 3630–3638.
[13] a) T. W. Bell, H. Jousselin, Nature 1994, 367, 441–444; b) E. Psilla-
kis, J. C. Jeffery, J. A. McCleverty, M. D. Ward, Chem. Commun.
1997, 479–480; c) C. Dietrich-Buchecker, J.-P. Sauvage, Chem.
Commun. 1999, 615–616.
[14] a) H. Katagiri, T. Miyagawa, Y. Furusho, E. Yashima, Angew. Chem.
2006, 118, 1773–1776; Angew. Chem. Int. Ed. 2006, 45, 1741–1744;
b) K. Miwa, Y. Furusho, E. Yashima, Nat. Chem. 2010, 2, 444–449.
[15] a) B. Hasenknopf, J.-M. Lehn, Helv. Chim. Acta 1996, 79, 1643–
1650; b) C. R. Woods, M. Benaglia, F. Cozzi, J. S. Siegel, Angew.
Chem. 1996, 108, 1977–1980; Angew. Chem. Int. Ed. Engl. 1996, 35,
1830–1833; c) G. Rapenne, B. T. Patterson, J.-P. Sauvage, F. R.
Keene, Chem. Commun. 1999, 1853–1854; d) R. Annunziata, M.
Benaglia, M. Cinquini, F. Cozzi, C. R. Woods, J. S. Siegel, Eur. J.
Org. Chem. 2001, 173–180; e) T. E. Wood, A. C. Ross, N. D. Dalgle-
ish, E. D. Power, A. Thompson, X. Chen, Y. Okamoto, J. Org.
Chem. 2005, 70, 9967–9974; f) Q. Sun, Y. Bai, G. He, C. Duan, Z.
Lin, Q. Meng, Chem. Commun. 2006, 2777–2779; g) T. Hashimoto,
T. Nishimura, J. M. Lim, D. Kim, H. Maeda, Chem. Eur. J. 2010, 16,
11653–11661; h) C. Diebold, P. Mobian, C. Huguenard, L. Alloun-
che, M. Henry, Inorg. Chem. 2010, 49, 6369–6371.
[16] Boronic acids are used as building blocks constructing supramolec-
ular architectures due to a variety of interactions through covalent
or non-covalent bonds, see: a) K. Hiratani, M. Albrecht, Chem. Soc.
Rev. 2008, 37, 2413–2421; b) K. Severin, Dalton Trans. 2009, 5254–
5264; c) R. Nishiyabu, Y. Kubo, T. D. James, J. S. Fossey, Chem.
Commun. 2011, 47, 1124–1150.
[17] a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von
Zelewsky, Coord. Chem. Rev. 1988, 84, 85–277; b) S. Torelli, S. De-
lahaye, A. Hauser, G. Bernardinelli, C. Piguet, Chem. Eur. J. 2004,
10, 3503–3516; c) G. Canard, C. Piguet, Inorg. Chem. 2007, 46,
3511–3522.
[18] a) E. M. Kober, J. V. Caspar, R. S. Lumpkin, T. J. Meyer, J. Phys.
Chem. 1986, 90, 3722–3734; b) T. Riis-Johannessen, N. Dupont, G.
Canard, G. Bernardinelli, A. Hauser, C. Piguet, Dalton Trans. 2008,
3661–3677; c) S. A. Miller, A. M. Moran, J. Phys. Chem. A 2010,
114, 2117–2126.
[19] a) J. V. Caspar, T. J. Meyer, J. Phys. Chem. 1983, 87, 952–957; b) L.
Yang, A.-M. Ren, J.-K. Feng, X.-J. Liu, Y.-G. Ma, M. Zhang, X.-D.
Liu, J.-C. Shen, H.-X. Zhang, J. Phys. Chem. A 2004, 108, 6797–
6808; c) J. S. Gancheff, R. Q. Albuquerque, A. Guerrero-Martꢃnez,
T. Pape, L. De Cola, F. Ekkehardt Hahn, Eur. J. Inorg. Chem. 2009,
27, 4043–4051.
[9] a) C. J. Baylies, T. Riis-Johannessen, L. P. Harding, J. C. Jeffery, R.
Moon, C. R. Rice, M. Whitehead, Angew. Chem. 2005, 117, 7069–
7072; Angew. Chem. Int. Ed. 2005, 44, 6909–6912; b) S. Goetz, P. E.
Kruger, Dalton Trans. 2006, 1277–1284; c) A. V. Davis, D. Fiedler,
M. Ziegler, A. Terpin, K. N. Raymond, J. Am. Chem. Soc. 2007, 129,
15354–15363.
[10] a) A. El-Ghayoury, L. Douce, A. Skoulios, R. Ziessel, Angew.
Chem. 1998, 110, 2327–2331; Angew. Chem. Int. Ed. 1998, 37, 2205–
2208; b) R. Ziessel, L. Douce, A. El-Ghayoury, A. Harriman, A.
Skoulios, Angew. Chem. 2000, 112, 1549–1553; Angew. Chem. Int.
Ed. 2000, 39, 1489–1493.
[20] The reported Na+–Na+ distances incorporated in host molecules in
the crystal were 3.3–3.4 ꢀ. See: a) K. Rissanen, J. Huuskonen, P.-M.
Windscheif, F. Vçgtle, Supramol. Chem. 1993, 2, 247–250; b) J. Shi-
bayama, H. Sakiyama, M. Yamasaki, Y. Nishida, Anal. Sci. 2008, 24,
x177–x178.
[11] a) H.-L. Kwong, H.-L. Yeung, W.-S. Lee, W.-T. Wong, Chem.
Commun. 2006, 4841–4843; b) C.-T. Yeung, H.-L. Yeung, C.-S.
Received: August 12, 2011
Published online: November 7, 2011
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