Synthesis and optical resolution of a double helicate consisting of ortho-linked hexaphenol strands bridged by spiroborates

Article abstract of DOI:10.1002/anie.200504326

(Figure Presented) Double Twist: The first spiroborate-based helicate was synthesized and shown to be stable in the solid state as well as in solution. The double-stranded structure (see picture) was characterized by ¹H NMR spectroscopy, ESI MS

Full text of DOI:10.1002/anie.200504326

Angewandte
Chemie
As part of our program to develop synthetic helical
polymers and oligomers,[2] we started constructing helical
foldamers by utilizing poly-m-phenylene as a new key
structural motif. The poly- and oligo-m-phenylenes are
known to adopt the 51-helical conformation in the solid
state, but take a random conformation in solution.[3] In studies
of the helix induction on 2-hydroxylated oligo-m-phenylene
derivatives, that is, ortho-linked oligophenols,[4] we serendip-
itously obtained a boron-containing helicate comprised of an
ortho-linked hexaphenol. In general, helicates are con-
structed from oligodentate strands and transition metals,[5]
and only a few helicates that contain of typical metal cations,
such as lithium, sodium, and potassium, are known.[6] How-
ever, to the best of our knowledge, boron has never been
employed as an assembling center to generate helicates.
Boron helicates should be promising for new chiral receptors
and asymmetric catalysts owing to the Lewis basic feature of
the borate moieties along with their inherent helical chir-
ality.[7,8] We report herein the synthesis and characterization
of a double helicate consisting of ortho-linked hexaphenol
strands bridged by spiroborates and its optical resolution by
the formation of a diastereomeric salt.
Helical Structures
Single crystals of the boron complex [B2Na(H2L)2]ÀH+
suitable for X-ray crystallographic analysis were obtained
unexpectedly from a solution of a crude ortho-linked
hexaphenol H6L in CDCl3 (Scheme 1)[9] containing a small
amount of boric acid and toluene. The X-ray single-crystal
analysis revealed the double-stranded helical structure of the
complex,[10] which has D2 symmetry with three mutually
perpendicular C2 axes. The two strands are C2 symmetric,
structurally identical, and are bridged by the spiroborates
formed from the terminal biphenol units and the boron atoms
(Figure 1).[11] Interestingly, an octacoordinated sodium cation
is found at the center of the complex.[12]
DOI: 10.1002/anie.200504326
Synthesis and Optical Resolution of a Double
Helicate Consisting of ortho-Linked Hexaphenol
Strands Bridged by Spiroborates
Hiroshi Katagiri, Toyoharu Miyagawa,
Yoshio Furusho,* and Eiji Yashima*
Not surprisingly, the search for new structural motifs for
helical architectures has been an intensive research area to
date, as the helix is the central structural motif for biomac-
romolecules in nature.[1] In particular, much effort has
recently focused on foldamers, synthetic molecules that fold
into an ordered conformation by noncovalent interactions.[1b]
For the successful design of foldamers, the structural motifs
should be as simple as possible because of the predictability of
higher-order structures, the tunability of the structures, etc.
Next, we prepared the boron complex [B2Na(H2L)2]ÀNa+
by heating a mixture of sodium borohydride and H6L in 1,2-
dichloroethane/ethanol (1:1 v/v) at 808C (Scheme 1). The
formation of the boron complex was also confirmed by
ESI MS and NMR spectroscopic studies of solution of the
complex in acetone.[9] Only one intense peak was observed at
m/z 1819 (negative mode) and 1865 (positive mode), which
correspond to the [B2Na(H2L)2]À and [B2Na(H2L)2]À2Na+
species, respectively; both spectra agree well with the
1
simulated pattern. The H NMR spectrum of the complex in
[D6]acetone is also consistent with the structure determined
by X-ray diffraction analysis in the solid state; each strand is
C2 symmetric and the central biphenol units have two free
phenolic protons.[13] Moreover, the NOESY spectrum of the
complex showed strong negative cross-peaks between the
phenol rings Aand C (Figure 2). These cross-peaks were
attributed to the interstrand NOE interactions (distance
ꢀ 2.5 Š), thus indicating that the complex adopts a double-
stranded helical trinuclear structure in solution.
To date, only a few examples of the optical resolution of
racemic helicates have been reported, and they are usually
based on the formation of diastereomer ion pairs.[14] The
optical resolution of the helicate (Æ )-[B2Na(H2L)2]ÀNa+ was
carried out by the formation of a diastereomeric salt
(Scheme 1). The cation exchange of the racemate was
[*] Dr. H. Katagiri, Dr. T. Miyagawa, Dr. Y. Furusho, Prof. E. Yashima
Yashima Superstructured Helix Project
Exploratory Research for Advanced Technology (ERATO)
Japan Science and Technology Agency (JST)
Creation Core Nagoya 101, 2266-22 Anagahora, Shimoshidami,
Moriyama-ku, Nagoya 463-0003 (Japan)
Fax: (+81)52-739-2081
E-mail: furusho@yp-jst.jp
yashima@apchem.nagoya-u.ac.jp
Prof. E. Yashima
Institute for Advanced Research
Nagoya University
Chikusa-ku, Nagoya 464-8601 (Japan)
Fax: (+81)52-789-3185
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 1741 –1744
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1741

Communications
Scheme 1. Synthesis of the boron complex [B2Na(H2L)2]ÀNa+ and subsequent optical resolution by formation of a diastereomeric salt by using
(À)-DMEph+BrÀ followed by replacement with the achiral ammonium salt DMAmm+BrÀ.
Figure 1. Stereoview of the crystal structure (top) and schematic
representation of the boron complex [B2Na(H2L)2]ÀH+ (bottom). The
hydrogen atoms bonded to carbon atoms, the tert-butyl groups, and
solvent molecules are omitted for clarity.
performed by adding a tenfold excess of (À)-N-dodecyl-N-
methylephedrinium bromide ((À)-DMEph+BrÀ) to a solution
of the helicate in acetonitrile, and a white microcrystalline
Figure 2. Partial NOESY spectrum of the boron complex [B2Na-
(H2L)2]ÀNa+ in [D6]acetone at 258C (mixing time=500 ms). The cross-
peaks attributable to the interstrand NOE interactions are labeled with
blue circles.
solid was precipitated within 24 h. The crystals of (+)313
-
[B2Na(H2L)2]À(À)-DMEph+ were collected by filtration
1
(ꢀ100% de based on H NMR spectrum (CD2Cl2, 258C)).[9]
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Angewandte
Chemie
The mother liquor was then evaporated to afford a white solid
consisting of the opposite diastereomer (À)313-[B2Na-
(H2L)2]À(À)-DMEph+ as a major component and the excess
helicates to chiral recognition and asymmetric synthesis will
be the subject of a forthcoming investigation.
1
(À)-DMEph+BrÀ (66% de based on H NMR spectrum).[15]
Received: December 6, 2005
The circular dichroism (CD) spectra of both diastereomers
showed almost all mirror-image Cotton effects in their
patterns in the range of 240–330 nm except for the intensities,
because the CD signals due to (À)-DMEph+BrÀ are negli-
gibly weak (Figure 3).
Keywords: borates · boron · chiral resolution · helical structures ·
.
oligophenols
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Figure 3. CD and UV/Vis spectra (CH3CN, 258C) of the boron com-
plexes of (+)313-[B2Na(H2L)2]À(À)-DMEph+ with 100% de (red), the
corresponding (+)313-[B2Na(H2L)2]ÀDMAmm+ (dashed black line),
(À)313-[B2Na(H2L)2]À(À)-DMEph+ obtained from the filtrate (blue), and
(À)-DMEph+BrÀ (green).
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Finally, the resulting diastereomeric double-stranded
helicates were successfully converted into the corresponding
enantiomers when the optically active ammonium cation (À)-
DMEph+ was exchanged with dodecyltrimethylammonium
bromide (DMAmm+BrÀ) as the achiral source. Thus, the
complex (+)313-[B2Na(H2L)2]ÀDMAmm+, which exhibits hel-
ical chirality without any other chiral factors, was obtained.
The Cotton effects and UV/Vis spectra were perfectly
identical to those of (+)313-[B2Na(H2L)2]À(À)-DMEph+, sug-
gesting that the helicate is really stereoinert and enantiomer-
ically pure (Figure 3). The complex (À)313-[B2Na(H2L)2]À(À)-
DMEph+ (66% de). was similarly converted into the complex
(À)313-[B2Na(H2L)2]ÀDMAmm+ (66% de based on CD spec-
trum).[9] Furthermore, neither racemization nor decomposi-
tion of the complex (+)313-[B2Na(H2L)2]ÀDMAmm+ was
observed after heating in acetonitrile at 808C for 24 h or in
methanol at 608C for 12 h.
[4] H. Goto, Y. Furusho, E. Yashima, Polym.Prepr.Jpn. 2005, 54,
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In summary, we have successfully prepared the first
spiroborate-based double-helical complex by using a simple
ortho-linked hexaphenol as the ligand, and the helicate is
stable in the solid state as well as in solution. Both
enantiomers of the racemic helicates were resolved by the
formation of a diastereomeric, optically active ammonium
salt. We believe that various other optically active spirobo-
rate-based double helicates can be prepared by the same
strategy. Further applications of the present optically pure
[8] For synthetic catalysts, see: a) K. Ishihara, H. Yamamoto, J.Am.
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[9] See the Supporting Information for details of the synthesis,
structures, and characterization of the ligand H6L and the boron
complex [B2Na(H2L)2]ÀNa+ and its optical resolution.
Angew. Chem. Int. Ed. 2006, 45, 1741 –1744
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1743

Communications
[10] The peak corresponding to the countercation could not be
located from difference Fourier maps, and the counterproton
was missing probably due to the disordered configurations.
Crystal data for the complex (C137H164B2Cl9NaO14): Mr =
2398.45, dimensions 0.20 ” 0.20 ” 0.10 mm3, monoclinic, space
group C2/c, Z = 8, 1calcd = 1.296 gcmÀ3, m(MoKa) = 0.363 mmÀ1
,
F(000) = 5624, 2qmax = 52.78 were a = 17.9772(8), b =
27.6706(12), c = 27.8814(13) Š, b = 96.7810(10)8, V=
13772.3(11) Š3. Atotal of 43299 reflections were collected, of
which 14091 reflections were independent (Rint = 0.075). The
structure was refined to final R1 = 0.0800 for 14091 data [I >
2s(I)] with 901 parameters, wR2 = 0.2482 for all data, GOF =
0.912, and residual electron density max./min. = 0.94/
À0.80 eŠÀ3. CCDC-291892 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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moieties, see: a) M. Periasamy, L. Venkatraman, S. Sivakumar,
N. Sampathkumar, C. R. Ramanathan, J.Org.Chem. 1999, 64,
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Chem.Soc.Perkin Trans.1 2000, 4403 – 4408; c) D. Liu, Z. Shan,
F. Liu, C. Xiao, G. Lu, J. Qin, Helv.Chim.Acta 2003, 86, 157 –
163.
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56; b) M. Albrecht, M. Schneider, H. Röttele, Angew.Chem.
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c) M. Albrecht, O. Blau, R. Frölich, Proc.Natl.Acad.Sci.USA
2002, 99, 3867 – 4872.
[13] The signals were assigned based on COSY, HSQC, and
HMBC NMR experiments.[9]
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[15] “(+)313” and “(À)313” denote the signs of the Cotton effect at
313 nm.
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