Absolute helical arrangement of stacked benzene rings: Heterogeneous double-helical interaction comprising a hydrogen-bonding belt and an offset parallel aromatic-aromatic-interaction array

Article abstract of DOI:10.1002/anie.200352788

A resolution revolution? Self-assembled chiral structures resulting from the formation of two different helical interactions are found in the crystal structures of the achiral compound 1. The interactions give rise to Ar-H...π and OH...O=C arrays. The chi

Full text of DOI:10.1002/anie.200352788

Communications
Crystal Engineering
a single helical form upon ligation of a chiral ligand.[4b,c] In this
context, absolute asymmetric assembly of an achiral molecule
is especially intriguing. In the course of our investigation[11] of
spontaneous optical resolution,[12] we found a discotic com-
pound, tris(2-hydroxyethyl)-1,3,5-benzenetricarboxylate (1),
which crystallized as chiral crystals in which the nearly planar
molecules were helically arranged within individual single
crystals. The helicity of the triester 1 was derived from the
helical arrangement of the molecules, supported by an array
of hydrogen bonding through a single moiety and offset
aromatic stacking of the adjacent molecules.
The triester 1, which has an almost planar and highly
symmetrical primary structure, was synthesized and crystal-
lized from methanol/chloroform to give slender prismatic
single crystals. A preliminary X-ray crystallographic analysis
(Figure 1) revealed that the crystal belonged to the non-
Absolute Helical Arrangement of Stacked
Benzene Rings: Heterogeneous Double-Helical
Interaction Comprising a Hydrogen-Bonding Belt
and an Offset Parallel Aromatic–Aromatic-
Interaction Array**
Isao Azumaya,* Daisuke Uchida, Takako Kato,
Akihiro Yokoyama, Aya Tanatani, Hiroaki Takayanagi,
and Tsutomu Yokozawa*
Compounds that form helical structures in the crystalline
state or in solution[1] have attracted much attention because
living things utilize helical structures to store genetic infor-
mation, and because compounds with a helical-ordered
structure have provided new materials in solid- or liquid-
crystal engineering.[2] There are several types of helical
structure: a single strand that folds helically owing to
solvophobic effect[3] or guest ligation,[4] a double or multiple
helix of strands derived from cation or anion ligation,[5] or
hydrogen bonding,[6] and a helical super-structure formed by
conformationally helical or discotic small molecules through
columnar stacking supported by hydrogen bonds,[7] for other
examples see ref. [8]. In helical structures, a parallel aro-
matic–aromatic interaction[9] is often a very important driving
force to stabilize the structure, and sometimes hydrogen
bonding or other weak intermolecular interactions assist the
formation of the highly ordered helical arrangement. Such a
helical structure is intrinsically chiral, that is, it has a right-
handed helix (P-helix) or a left-handed helix (M-helix), which
are enantiomeric. Asymmetric synthesis to produce one
enantiomeric helix is important, especially in self-assembling
systems,[5–7] in foldamers,[1a] or in polymers with a helical
structure induced by catalytic chiral initiation.[10] There are
generally two kinds of driving forces to determine which
helicity a compound will adopt, they are, an internal or an
external chiral source. For example, the helicity of DNA is
derived from the internal chirality of the chiral carbon atom in
the deoxyribose chain. On the other hand, one of oligophe-
nylacetylenes prepared by Moore and co-workers folded into
Figure 1. ORTEP drawing of the crystal structure of 1. The thermal
ellipsoids are set at 50% probability.
centrosymmetric space group P61 (or P65). This result was
surprising, since the majority of achiral organic compounds
tend to pack into centrosymmetric crystals. The most
interesting feature is that the planar, discotic molecules
were arranged in a columnar stack with a progressive helical
twist between adjacent molecules to form helical super-
structures in the crystal lattice. The chirality of the crystals
was intrinsically derived from the helical arrangement of the
molecules, assisted by the twist of the terminal hydroxy
groups with respect to the plane of the central benzene ring,
which leads to the generation of conformational chirality.
We measured the solid-state CD spectrum (in KBr) of
several crystals of 1.[13] The Cotton effect was characteristic of
a helical structure of a chromophore.[14] As expected, we
found two enantiomeric crystals which showed mirror-image
curves in the region between 200–320 nm (Figure 2); one (red
line in Figure 2) showed a negative Cotton effect at around
210 nm, a large positive one at 224 nm, a negative one at
248 nm, and two faint positive ones at 288 and 298 nm, and
the other showed Cotton effects with the opposite sign at the
corresponding wavelengths. Because one torsion angle of
OC–aromatic ring unit was À2.7(4)8 and the others were
0.1(4)8 and 1.2(4)8, the Cotton effects of the crystal could be
derived from the helical arrangement of the benzene rings of
the molecule as well as torsion of one carbonyl group with
respect to the central benzene ring.
[*]Dr. I. Azumaya, T. Kato, Prof. Dr. H. Takayanagi
Faculty of Pharmaceutical Sciences
Kitasato University
5-9-1 Shirokane, Minato-ku, Tokyo 108-8641 (Japan)
Fax: (+81)334-42-5707
E-mail: azumayai@pharm.kitasato-u.ac.jp
D. Uchida, Dr. A. Yokoyama, Dr. A. Tanatani, Prof. Dr. T. Yokozawa
Department of Applied Chemistry
Kanagawa University
Kanagawa-ku, Yokohama 221-8686 (Japan)
Fax: (+81)45-413-9770
E-mail: yokozt01@kanagawa-u.ac.jp
[**]I.A. acknowledges partial funding of this work by The Asahi Glass
Foundation.
The absolute structure could be determined[15] by com-
parison of the Flack parameter[16] for each space group (P61 or
P65) using the least-squares method. For example, if the
Supporting information (view of the left and right handed helixes)
for this article is available on the WWW under http://www.ange-
wandte.org or from the author.
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DOI: 10.1002/anie.200352788
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Angewandte
Chemie
Figure 2. CD spectra of two enantiomeric crystals of 1 (P61: red line;
P65: blue line) in KBr. A mixture of 250 mg of each crystal and 100 mg
of KBr was well ground and formed into a transparent disk with a
radius of 5 mm.
Figure 4. Top view (a) and tilted view (b) of the crystal packing of 1
showing the helical arrangement of the central benzene rings of the
molecules. Neighboring benzene rings are stacked in an offset mode.
crystal that showed a positive Cotton effect at 224 nm was in
the space group P61, the Flack parameter was 0.003 and in
P65, it was 0.942, so the crystal was assigned to P61. On the
other hand, if the crystal that showed a negative Cotton effect
at 224 nm was in the space group P61, the Flack parameter
was 1.036 and in P65, it was À0.041, so the crystal was assigned
to P65. This correspondence between the Flack parameter and
the sign of the Cotton effect was confirmed by examining at
least three crystals for each enantiomer.
A further investigation of the crystal structure of 1
revealed that the arrangement of the molecule showed
unique features. Only one of the three hydroxyethyloxycar-
bonyl groups formed a hydrogen bonding array between
adjacent molecules, that is, the hydroxy group of a hydroxy-
ethyloxycarbonyl group formed a hydrogen bond with the
carbonyl group of a hydroxyethyloxycarbonyl group of the
next molecule (Figure 3). This single-hydrogen-bond belt
c axis, which gives rise to a complete turn in 21.0 Š, the
distance between adjacent benzene rings is 3.50 Š. The
exterior width of the array of molecules is about 12.5 Š,
which is just over half of that of DNA (ca. 20 Š).
The stacked structure of 1 is reminiscent of the layered
crystal structures of N,N’,N’’-tris(2-methoxyethyl)benzene-
1,3,5-tricarboxamide (2) which was reported to crystallize as
chiral crystals belonging to space group P21.[7a] However, the
triamide 2 showed straight stacking of the central benzene
ring along the b axis, with complete overlap, and the distance
between the central benzene rings is 3.60 Š. Further, the
helical structure is derived from triple helices formed by
hydrogen bonding between the secondary amide hydrogen
and the carbonyl oxygen of the neighboring molecule. On the
other hand, the helicity of the triester 1 was derived from a
heterogeneous double-helical interaction, that is, a relatively
weak, single hydrogen-bonding belt involving one of the
hydroxyethyloxycarbonyl groups and an offset parallel aro-
À
matic–aromatic interaction between Ar H and the center of
the neighboring benzene ring to give the offset stacking. Our
triester is the first example of an absolute helical arrangement
of a single benzene ring in a parallel offset-stacking array that
undergoes spontaneous resolution in the crystal, assisted by
progressive hydrogen bonding; that is, the spontaneous
resolution is driven by two different weak helical interactions
À
=
which result in the formation of Ar H···p and OH···O C
arrays. Such helical chirality induction of an achiral com-
pound is of great interest, since such a system could be a
simple model for spontaneous helical structure formation in
nature.
Figure 3. Schematic representation of the relative position of adjacent
molecules in the crystal packing of 1; red: benzene separations, blue: -
hydrogen-bonding distances.
Experimental Section
forms a helical strand, which was right-handed for P61 and
left-handed for P65.[17] Another remarkable feature is that the
central benzene rings of the molecules exhibit circular offset
stacking to form an enantiomeric single helix (Figure 4).
These heterogeneous helical weak interactions have the same
helicity. The attractive interaction between two aromatic rings
in this structure would be strongest among various types of
parallel stacking because an aromatic hydrogen center is
located just above the center of the adjacent benzene
ring.[9d,g–i] Because the unit cell has six molecules along the
1: Preparation of 2-(tert-Butyldimethylsilyloxy)ethanol: dropwise,
over 1 h,
a solution of tert-butyldimethylsilyl chloride (15.1 g,
0.10 mol) in dry THF (500 mL) was added to a stirred solution of
ethylene glycol (37.2 g, 0.60 mol) and imidazole (6.81 g, 0.10 mol) in
dry THF (500 mL) at room temperature, and the mixture was stirred
at room temperature for 6 h. After addition of water, THF was
removed by evaporation under reduced pressure, and the residue was
extracted with ethyl acetate. The organic layer was washed with
water, dried over anhydrous MgSO4, and concentrated under reduced
pressure. The residue was purified by silica gel column chromatog-
raphy (elution solvents: ethyl acetate/hexane = 1/4) to give 2-(tert-
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Communications
butyldimethylsilyloxy)ethanol as
a
colorless oil (14.1 g, 80%):
Tanatani, K. Oh, J. S. Moore,J. Am. Chem. Soc. 2002, 124, 5934 –
5935.
1H NMR (200 MHz, CDCl3): d = 3.74–3.40 (m, 4H), 2.47–2.33 (m,
1H), 0.85 (s, 9H), 0.03 ppm (s, 6H); 13C NMR (50 MHz, CDCl3): d =
64.2, 63.7, 25.9, À5.4 ppm.
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Tris[2-(tert-butyldimethylsilyloxy)ethyl]-1,3,5-benzenetricarbox-
ylate: dropwise, over 3 h, a solution of 1,3,5-benzenetricarbonyl
trichloride (5.88 g, 22 mmol) in dry CH2Cl2 (100 mL) was added to a
stirred solution of 2-(tert-butyldimethylsilyloxy)ethanol (14.1 g,
80 mmol; see above) and triethylamine (8.06 g, 80 mmol) in dry
CH2Cl2 (200 mL) at room temperature, and the mixture was stirred at
room temperature for 3 days. After addition of water, the mixture was
extracted with CH2Cl2, and the organic layer was washed with water,
dried over anhydrous Na2SO4, and concentrated under reduced
pressure. The residue was purified by silica gel column chromatog-
raphy (elution solvents: ethyl acetate/hexane = 1/4) to give tris-
[2-(tert-butyldimethylsilyloxy)ethyl]-1,3,5-benzenetricarboxylate as a
yellow oil (13.6 g, 90%): 1H NMR (200 MHz, CDCl3): d = 8.89 (s,
3H), 4.45 (t, J = 5.0 Hz, 6H), 3.96 (t, J = 5.0 Hz, 6H), 0.90 (s, 27H),
0.09 ppm (s, 18H); 13C NMR (125 MHz, CDCl3): d = 164.9, 134.7,
131.3, 66.7, 61.1, 25.8, 18.3, À5.31 ppm; IR (neat) 2954, 2930, 2884,
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2857, 1733, 1472 cmÀ1
.
Tris(2-hydroxyethyl)-1,3,5-benzenetricarboxylate (1): A mixture
of tris[2-(tert-butyldimethylsilyloxy)ethyl]-1,3,5-benzenetricarboxy-
late (13.6 g, 20 mmol; see above), 6m hydrochloric acid (1 mL), and
THF (200 mL) was stirred at room temperature for 6 h, and
concentrated under reduced pressure. The crude solid obtained was
washed with hexane and dried in vacuo to give 1 as a colorless solid
(6.11 g, 90%): 1H NMR (600 MHz, [D6]DMSO): d = 8.74 (s, 3H), 4.99
(t, J = 5.4 Hz, 3H), 4.39 (t, J = 4.9 Hz, 6H), 3.75 ppm (t, J = 5.0 Hz,
6H); 13C NMR (151 MHz, [D6]DMSO): d = 164.5, 133.8, 131.3, 67.5,
59.1 ppm; IR (KBr): n˜ = 3327, 2955, 1722, 1441, 1242, 1078, 1025 cmÀ1
Elemental analysis calcd (%)for C15H18O9: C 52.63, H 5.30; found: C
52.60, H 5.34.
;
Received: September 4, 2003
Revised: October 27, 2003 [Z52788]
Keywords: chiral resolution · chirality · crystal engineering ·
.
helical structures · hydrogen bonds
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[15] Crystal data of 1a (P61): C15H18O9, Mr = 342.30, hexagonal, space
group P61, a = 11.304(1), c = 20.997(4) Š, V= 2323.5(6) Š3, Z =
6, 1calcd = 1.468 gcmÀ3
, T= 296 K, crystal size 0.20 ” 0.20 ”
0.20 mm3, R = 0.043, Rw = 0.055, GOF = 1.23 for 2817 reflections
with I > 0.0s(I). The Flack absolute structure parameter has a
value of 0.003(0.161), while the inverted stereochemistry
requires a value of 0.942(0.154). Crystal data of 1b (P65):
C15H18O9, Mr = 342.30, hexagonal, space group P65, a =
11.3048(9), c = 21.011(3) Š, V= 2325.5(5) Š3, Z = 6, 1calcd
=
1.466 gcmÀ3, T= 296 K, crystal size 0.20 ” 0.20 ” 0.20 mm3, R =
0.047, Rw = 0.067, GOF = 1.29 for 2781 reflections with I >
0.0s(I). The Flack absolute structure parameter has a value of
À0.041(0.174), while the inverted stereochemistry requires a
value of 1.036(0.164). CCDC-218393 (1a) and CCDC-218394
(1b) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[16] H. D. Flack, Acta Crystallogr. Sect. A 1983, 39, 876 – 881.
[17] Only one of the hydroxy groups contributes to the formation of a
À =
single-helical hydrogen-bonding array of C O···HOCH2-
=
À
CH2OC O···HO (see Supporting Information). The two
other hydroxy groups form weak hydrogen bonds with carbonyl
groups on the next molecule, but do not form a hydrogen-
bonding array.
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