Hydrogen-bonded helices in 2-aminothiazole derivatives: Generation of chiral crystals from small achiral molecules

Article abstract of DOI:10.1016/j.tetasy.2011.07.012

The hydrobromide and nitrate of 4-(tert-butyl)-5-(4-chlorobenzyl)thiazol-2- amine have been synthesized, and their chiral symmetry breaking in the solid-state has been investigated. It is shown that each molecule is connected by continuous N-H...Br/N-H...

Full text of DOI:10.1016/j.tetasy.2011.07.012

Tetrahedron: Asymmetry 22 (2011) 1332–1336
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Hydrogen-bonded helices in 2-aminothiazole derivatives: generation
of chiral crystals from small achiral molecules
Aixi Hu a, , Gao Cao b,
⇑
⇑
a College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
b School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Guangzhou 510006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The hydrobromide and nitrate of 4-(tert-butyl)-5-(4-chlorobenzyl)thiazol-2-amine have been synthe-
sized, and their chiral symmetry breaking in the solid-state has been investigated. It is shown that each
molecule is connected by continuous N–HꢀꢀꢀBr/N–HꢀꢀꢀO hydrogen bonds to afford left- or right-handed
helical assemblies in the crystal packing. Mirror-image CD spectra were obtained for the right- and
left-handed helical-type chiral crystals.
Received 27 May 2011
Accepted 13 July 2011
Available online 10 August 2011
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Chiral molecules are sufficient for chiral crystals but are not
necessary, that is, chiral crystals can be formed through self-
assembly even if they are non-chiral molecules.1 In supramolecular
systems, some chiral crystals are composed of achiral molecules by
supramolecular assembly through its own special three-dimen-
sional structure.2 Recently, much attention has been paid to the
supramolecular chirality, while the chiral crystallization of achiral
compounds in the absence of a chiral source remains a great chal-
lenge and an infrequent and unpredictable phenomenon.3
1-(4-Chþlorophenyl)-4,4-dimethylpentan-3-one was brominated
ylpentan-3-one.6 The
a-bromoketone reacted with thiourea to give
the title compound 1 without additional separation. The relatively
high melting points (175.8–176.5 °C) and IR spectra patterns sug-
gested the formation of salt bonding in the crystals. Compound 1
reacted with AgNO3 in an EtOH solution to give the nitrate salts
of 4-(tert-butyl)-5-(4-chlorobenzyl)thiazol-2-amine 2 (Scheme 1).
Both compounds have no fixed asymmetric element.
by ½Bmimꢁ Brꢂ3 to give 2-bromo-1-(4-chlorophenyl)-4,4-dimeth-
On a supramolecular level, chirality is usually achieved by non-
covalent interactions, for example, electrostatic interactions,
X-ray crystallographic analysis and solid-state CD measure-
ments were performed in order to reveal the molecular packing
arrangement and to study the chirality generation mechanism. A
single crystal of the test compounds obtained by spontaneous
crystallization in EtOH was cut into two pieces. One half was used
for solid-state CD spectra analysis, and the other was subjected to
X-ray structure analysis followed by an absolute configuration
determination. Figure 1 shows the ORTEP drawings of 1 and 2.
Single crystal X-ray structural analysis revealed that 1 crystallized
in the asymmetrical orthorhombic space group P212121, which is
the most common space group for small and irregularly shaped
chiral molecules.7 The absolute configuration of the compound
molecule was successfully determined by refining the Flack param-
eter [0.007(5)].8 In the molecular structure, the benzene ring is
nearly perpendicular with the thiazole ring, making a dihedral
angle of 81.6(2)°. The most salient structural feature of 1, however,
is the chiral assembly of the achiral components due to the
presence of a hydrogen bonding helix: the molecules are arranged
in a left-handed helical (M-helix) conformation through continu-
ous N–Hꢀ ꢀ ꢀBr hydrogen bonds (Fig. 2). The N atoms of the amino
group act as hydrogen-bond donors and the Br anion as the
hydrophobic interactions,
p–p stacking, and especially hydrogen
bonding.4 It has been deduced that the appearance of such chirality
could be ascribed to chiral symmetry breakage, since the helicity in
different spatial manifestations is a predominant driving force.5
Herein, we report that the chiral helicity of the hydrobromide 1
and nitrate 2 of 4-(tert-butyl)-5-(4-chlorobenzyl)thiazol-2-amine
induces a self-assembled helix in each individual crystal. In the chi-
ral crystallization of 1 and 2, the key component is the amino group
acting as an electron donor, which plays a critical role in forming
continuous ꢀ ꢀ ꢀBrꢀ ꢀ ꢀH–N–Hꢀ ꢀ ꢀBrꢀ ꢀ ꢀH–N–Hꢀ ꢀ ꢀBrꢀ ꢀ ꢀ or ꢀ ꢀ ꢀOꢀ ꢀ ꢀH–N–
Hꢀ ꢀ ꢀOꢀ ꢀ ꢀH–N–Hꢀ ꢀ ꢀOꢀ ꢀ ꢀ hydrogen bonding helical assemblies. To
the best of our knowledge, this appears to be the first report on a
helical structure built only on continuous N–Hꢀ ꢀ ꢀBr or N–Hꢀ ꢀ ꢀO
hydrogen bonds by a single component system of achiral small
molecules.
⇑
Corresponding authors. Tel.: +86 731 88823734; fax: +86 731 88713642.
E-mail addresses: axhu@hnu.edu.cn (A. Hu), g.cao@mail.scut.edu.cn (G. Cao).
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.07.012

A. Hu, G. Cao / Tetrahedron: Asymmetry 22 (2011) 1332–1336
1333
S
O
O
-
[Bmim]+Br3
H2N
NH2
Br
Cl
Cl
H
.
H
.
N+
EtOH
N+
Br
NH2
NO3
NH2
Cl
Cl
AgNO3
S
S
1
2
Scheme 1. Synthesis of compounds 1 and 2.
Figure 1. ORTEP drawings of 1 and 2 with thermal ellipsoids at 30% probability.
hydrogen-bond acceptors, resulting in helical hydrogen bonding
Solid state CD spectrometry is a powerful tool for determining
whether a single crystal is chiral or not.9 The results confirmed
the chiral nature of 1. As shown in Figure 3 (higher curve), a single
crystal of M-1 shows three negative dichroic signals at 209, 248,
and 281 nm, respectively. As expected, no CD bands were observed
for a solution of the enantiomeric crystals of 1 due to the achiral
nature of the compounds. In the UV–vis spectrum (lower curve
ꢀ ꢀ ꢀBrꢀ ꢀ ꢀH–N–Hꢀ ꢀ ꢀBrꢀ ꢀ ꢀH–N–Hꢀ ꢀ ꢀBrꢀ ꢀ ꢀ [N2—H2B ꢀ ꢀ ꢀ Bri 2.50 Å, 150°;
1
N2–H2Aꢀ ꢀ ꢀBr1 2.59 Å, 147°; N1—H1A ꢀ ꢀ ꢀ Brii 2.54 Å, 146°, with sym-
ii
1
metry codes (i) x, y, z ꢂ 1; (ii) x ꢂ 1/2, ꢂy + 1/2, ꢂz + 1] in the
direction of the a axis. As shown in Figure 2, the screw-pitch of
the M-helix is 9.6 Å.
of Fig. 3), the first absorption at 223 nm is the
the benzene ring, while the other absorption at 264 nm is due to
the
p⁄ transition of the 2-aminothiazole unit.
p–
p⁄ transition of
p–
Figure 2. View illustrating the N–Hꢀ ꢀ ꢀBr hydrogen-bonding interactions between
adjacent molecules that lead to the formation of a hydrogen-bonded helice in M-1.
Figure 3. Solid-state CD and UV–vis (lower curve) spectra of M-1.

1334
A. Hu, G. Cao / Tetrahedron: Asymmetry 22 (2011) 1332–1336
formation of the helix as shown in Figure 4 (N1–H1ꢀ ꢀ ꢀO1 2.06 Å,
i
163°; N2–H2Aꢀ ꢀ ꢀO2 1.98 Å, 165°; N2–H2Bꢀ ꢀ ꢀO2 2.33 Å, 140°; N2–
i
H2Bꢀ ꢀ ꢀO3 2.21 Å, 149°, symmetry code: (i) x ꢂ 1/2, ꢂy + 5/2,
ꢂz + 1). On the other hand, the benzene ring is also approximately
vertical to the thiazole ring with a dihedral angle of 81.4(2)°, and
the combination of two N–Hꢀ ꢀ ꢀO hydrogen bonds generates a cyc-
lic centrosymmetric R22 (8).12
We also observed that the CD signals of 2 in a KBr matrix could
be opposite in different deposition batches, suggesting that 2
should have both single right- and left-handed helix structures in
each individual crystal. The enantiomorphous crystals were easily
discriminated by measuring their solid state CD spectra, the two
curves of the enantiomorphous crystals are in a good mirror image
relationship, thus being easy to discriminate between (Fig. 5). A sin-
gle crystal of P-2 showed three positive dichroic signals at 207, 231,
and 273 nm, respectively. The single crystal of M-2 exhibited a CD
spectrum similar to that of P-2, with three negative dichroic signals
at 207, 231, and 272 nm, respectively. On the contrary, such chiral
helicity was not observed in 2-amino-4-(tert-butyl)-5-(4-chloro-
benzyl)thiazol-3-ium chloride.13
Figure 4. Views of the right-handed (P-helix) helical chains of 2 in a perpendicular
arrangement. tert-Butyl moieties are omitted for clarity; the hydrogen bonds are
indicated by dotted lines, screw-pitch of the hydrogen bonding helix: 9.9 Å.
3. Conclusion
The helix is an old and ever fascinating example of conforma-
tional chirality. Herein, we have reported a new type of chiral
supramolecular assembly from achiral 2-aminothiazole deriva-
tives. The array of molecules built by the N–Hꢀ ꢀ ꢀO or N–Hꢀ ꢀ ꢀBr
hydrogen bonding interaction forms an enantiomeric helical
superstructure in each individual crystal. This type of helical motif
arising from hydrogen bonds alone, free from the influences of a
metal atom and other interactions, is rare.14 This is significant
because chiral crystallization of achiral molecules by itself always
gives both enantiomorphous (left and right) crystals. These results
are helpful in understanding why achiral molecules can form chiral
molecular assemblies. Furthermore, this type of chiral crystalliza-
tion may also be useful in the development of new chiral materials
for applications in multidisciplinary areas such as nonlinear optics,
asymmetric catalysis, structural biology and the chiral separation
of drugs.
4. Experimental section
4.1. General
Figure 5. CD spectra of the two enantiomeric crystals of 2.
All melting points were measured on an X-4 electrothermal dig-
ital melting point apparatus and are uncorrected; 1H NMR spectra
were recorded on a Varian VARIAN INOVA-400 instrument with
TMS as an internal standard, at 399.970 MHz; IR were recorded
on AVATAR 360FT infrared spectrometer, using KBr disks; ESI-MS
were recorded on a Finngan LCQ LC–MS spectrometer; TLC was
performed on E-Merck pre-coated 60 F254 plates and the spots
were rendered visible by exposing to UV light or iodine; each start-
ing material was obtained from commercial suppliers and was
purified according to the literature procedures.
In many cases, right- and left-handed helices are obtained in
equal amounts as a racemate (meso-helix) when achiral building
blocks are used.10 The results of the single crystal X-ray diffraction
analysis reveal that these three crystals that were randomly se-
lected in a different beaker show that they are the same enantio-
mer crystals (M-helix). We speculate that the precipitation from
an ethanol solution of 1 can only form chiral crystals, and the crys-
tallization is a crystal-controlled process.11 The chiral crystals of an
absolute configuration may be randomly distributed. It is difficult
to predict which helical-type crystals will crystallize in a supersat-
urated solution.
4.2. Synthetic procedures and characterization
4.2.1. 1-Butyl-3-methylimidazolium bromide
In order to find the right-handed helical-type crystals of these
2-aminothiazole derivatives, we prepared its nitrate salts. We
successfully obtained the crystal structure of P-2. The X-ray crys-
tallographic analysis confirmed the chiral nature of P-2 whose
space groups are also P212121 with a high degree of certainty by
using the Flack parameter ꢂ0.02(13). Similar to M-1, the molecules
in the crystal packing were connected by continuous ꢀ ꢀ ꢀOꢀ ꢀ ꢀH–N–
Hꢀ ꢀ ꢀOꢀ ꢀ ꢀH–N–Hꢀ ꢀ ꢀOꢀ ꢀ ꢀ hydrogen bonding, which supported the
An oil bath with a stirred flask containing equimolar amounts
of commercially available 1-methylimidazole and 1-bromobutane
in toluene was heated at 70 °C for 24 h. After cooling to room tem-
perature, the reaction mixture was separated and the upper tolu-
ene phase was recycled. The lower phase was washed with ethyl
acetate and chloroform, and the residue was dried under vacuum
to give the product (white crystals), yield: 96.3%, mp: 77–78 °C. 1H

A. Hu, G. Cao / Tetrahedron: Asymmetry 22 (2011) 1332–1336
1335
Table 1
Crystal data and structure refinement for 1 and 2
Compound
1
2
Empirical formula
Formula weight
Crystal system
Space group
C
14H18BrClN2S
C14H18ClN3O3S
343.82
Orthorhombic
P212121
361.72
Orthorhombic
P212121
Unit cell dimensions
a = 9.6239(4) Å,
a
= 90°
a = 9.9374(7) Å, a = 90°
b = 11.3115(5) Å, b = 90°
b = 11.3849(8) Å, b = 90°
c = 14.6077(6) Å,
c
= 90°
c = 14.9855(10) Å, c = 90°
Volume
1590.21(12) Å3
1695.4(2) Å3
Z
4
4
Density (calculated) (Mg/m3)
1.511
2.873
736
1.347
0.363
720
Absorption coefficient (mmꢂ1
F(0 0 0)
Crystal size
)
0.47 ꢃ 0.42 ꢃ 0.41 mm3
0.45 ꢃ 0.44 ꢃ 0.21 mm3
Crystal shape
Block
Block
Theta range for data collection
Index ranges
Reflections collected
2.28–27.00°
2.25–25.99°
ꢂ11 6 h 6 12, ꢂ14 6 k 6 14, ꢂ16 6 l 6 18
ꢂ10 6 h 6 12, ꢂ13 6 k 6 13, ꢂ11 6 l 6 18
10,535
6173
Independent reflections
Completeness to theta = 27.00°
Absorption correction
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
3429 [R(int) = 0.0212]
100.0%
Semi-empirical from equivalents
0.308 and 0.276
3429/0/175
1.023
3128 [R(int) = 0.0343]
94.3%
Semi-empirical from equivalents
0.9277 and 0.8537
3128/18/227
1.078
Final R indices [I > 2
r
(I)]
R1 = 0.0205, wR2 = 0.0460
R1 = 0.0244, wR2 = 0.0471
0.007(5)
R1 = 0.0621, wR2 = 0.1403
R1 = 0.0794, wR2 = 0.1514
ꢂ0.02(13)
R indices (all data)
Absolute structure parameter5
Largest diff. peak and hole
0.215 and ꢂ0.292 e Åꢂ3
0.400 and ꢂ0.242 e Åꢂ3
CCDC 832550 and 832551 contains the supplementary crystallographic data for this article. Copies of this information may be obtained free of charge from the Director, CCDC,
12 Union Road, Cambridge, CBZ IEZ, UK. Facsimile (44) 01223 336 033, E-mail: deposit@ccdc.cam.ac.uk or http//www.ccdc.com.ac.uk/deposit.
NM (D2O, 400 MHz) d: 1.68 (t, J = 14.8 Hz, 3H, CH3), 2.06 (m, 2H,
CH2), 2.61 (m, 2H, CH2), 4.95 (t, J = 14.4 Hz, 2H, NCH2), 5.54 (s,
3H, NCH3), 8.16–8.25 (m, 2H, C3H3N2 4,5-H), 9.48 (s, 1H, C3H3N2
2-H).
J = 8.4 Hz, 2H, benzene ring 2,6-H), 7.42 (d, J = 8.4 Hz, 2H, benzene
ring 3,5-H), 8.93 (br, 2H, NH2), 13.10 (br, 1H, N+H), IR (KBr) /cmꢂ1
3261 (NH2), 2972 (CH2), 1631 (C@N), 1558 (C@C), 1086 (C@S), 727
(C–Cl), MS (m/z): 280 (the base group, M+), 265 (100%), 237, 223,
155, 125, 111, 101, 89, 77, 60, 41.
m
:
4.2.2. 1-Butyl-3-methylimidazolium tribromide
The combination of 1-butyl-3-methylimidazolium bromide
with an equimolar amount of bromine in a 1:1 molar ratio pro-
duced the 1-butyl-3-methylimidazolium tribromide ionic liquid
[Bmim]Br3 at room temperature. The untreated products were
used directly for the following bromination.
4.2.5. 4-(tert-Butyl)-5-(4-chlorobenzyl)thiazol-2-amine
nitrate 2
Silver nitrate was added to a solution of 2-amino-4-(tert-butyl)-
5-(4-chlorobenzyl)thiazol-3-ium bromide 1 in ethanol, and the
mixture was stirred at 35 °C for 0.5 h. The mixture was filtered to
remove the solid AgBr, and the filtrate was concentrated. The crys-
tals were collected on a filter to give 4-(tert-butyl)-5-(4-chloroben-
zyl)thiazol-2-amine nitrate 2. Yield: 87.7%, the results indicated
that the sublimation temperature of 2 is 135–137 °C.
4.2.3. 2-Bromo-1-(4-chlorophenyl)-4,4-dimethylpentan-3-one
At first, [Bmim]Br3 0.02 mol (7.56 g) was added to 1-(4-chloro-
phenyl)-4,4-dimethylpentan-3-one 0.02 mol (4.48 g) with continu-
ous stirring for 10 min. After the reaction was completed, the solid
crude product was extracted with ethyl acetate (30 mL ꢃ 3). The
crude product was purified by silica gel column chromatography
to give the corresponding pure product as white crystals (5.62 g).
Yield: 92.7%, mp: 53–54 °C. 1H NMR (CDCl3, 400 MHz) d: 1.01 (s,
9H, C(CH3)3), 3.15 (dd, J = 13.6 Hz, J = 5.6 Hz, 1H, CH2), 3.43 (dd,
J = 13.6 Hz, J = 9.6 Hz, 1H, CH2), 4.71 (dd, J = 5.6 Hz, J = 9.6 Hz, 1H,
CHBr), 7.09 (d, J = 8.4 Hz, 2H, C6H4 2,6-H), 7.25 (d, J = 8.4 Hz, 2H,
C6H4 3,5-H).
4.3. CD and UV–vis measurements
Solid-state CD spectra were measured on a JASCO J-810 circular
dichroism spectrometer (Jasco, Inc., Easton, MD, USA) in a KBr ma-
trix for M-1, P-2 and M-2. A mixture of the test compound and
150 mg KBr was ground and pressed into a disk with a radius of
10 mm. The disk was rotated around the optical axis and the CD
recordings were made in several positions in order to check the
reproducibility of the spectra. UV–vis spectra were measured on
a Shimadzu (model UV-160A) spectrophotometer in MeOH or so-
lid-state in KCl matrix.
4.2.4. 2-Amino-4-(tert-butyl)-5-(4-chlorobenzyl)thiazol-3-ium
bromide 1
4.4. X-ray crystallographic data
A mixture of 2-bromo-1-(4-chlorophenyl)-4,4-dimethylpentan-
3-one (0.01 mol) and thiourea (0.01 mol) in ethanol (50 mL) was
refluxed for 3 h. Next, the excess solvent was evaporated to give
a white solid, which was recrystallized from ethanol to give com-
pound 1 in 89.5% yield, mp: 175.8–176.5 °C. 1H NMR (400 MHz,
CD3SOCD3) d: 1.34 (s, 9H, C(CH3)3), 4.16 (s, 2H, CH2), 7.27 (d,
Single crystal X-ray diffraction experiment for M-1 and P-2
were performed on a Bruker AXS SMART 1000 CCD diffractometer
equipped with graphitemono chromated Mo
K
radiation
a
(k = 0.71073 Å). Crystals suitable for X-ray diffraction analysis were
grown by slow evaporation of ethanol. The data set was recorded

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A. Hu, G. Cao / Tetrahedron: Asymmetry 22 (2011) 1332–1336
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