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
Supporting information for this article is available on the WWW
signal due to the dicationic species ([B2Na2(L3)2 +2Na]2+
)
13954
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 13954 – 13957