Cascade reactions in crystals through cation-π-controlled reorientation on exposure to HCl gas

Article abstract of DOI:10.1039/c2cc17019a

Exposure of 4-azachalcones to HCl gas produced the corresponding HCl salts with a head-to-tail stacked alignment, irradiation of which produced the corresponding syn-HT dimers with high regio- and stereoselectivities, thus showing the effectiveness of the cascade process in crystals.

Full text of DOI:10.1039/c2cc17019a

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COMMUNICATION
Cascade reactions in crystals through cation–p-controlled reorientation
on exposure to HCl gasw
Shinji Yamada,* Yoko Tokugawa, Yuka Nojiri and Eri Takamori
Received 12th November 2011, Accepted 13th December 2011
DOI: 10.1039/c2cc17019a
Exposure of 4-azachalcones to HCl gas produced the corresponding
HCl salts with a head-to-tail stacked alignment, irradiation of
which produced the corresponding syn-HT dimers with high
regio- and stereoselectivities, thus showing the effectiveness of
the cascade process in crystals.
The importance of cascade reactions in synthetic organic
chemistry has been recognized in both economic and environ-
mental terms.1 Most cascade reactions are carried out in
Fig. 1 Schematic diagram of the reorientation/photodimerization
solution, and there has been little application of the cascade
cascade reactions.
process2,3 to solid-state reactions4,5 due to intrinsic difficulties
such as contamination by the reagent residues and the lower
crystallinity of the produced solid. If the resultant solid
possesses high crystallinity, it can be applied directly to the
next solid-state reaction, thereby providing a cascade process
in crystals. A gas–solid reaction6 affords a possible candidate
Scheme 1 Formation of 2a and 2b on exposure of 1a and 1b,
for solid-state cascade reactions due to the easy removal of the
respectively, to HCl gas.
gas after the reactions.
Recently, we have reported that a cation–p interaction7,8
between a pyridinium and an aromatic ring is effective for the
alignment of molecules in a crystal.9,10 We presumed that the
pyridinium–p interaction would be of significant utility for
making crystals by a gas–solid reaction. In this communication,
we report that exposure of 4-azachalcones to HCl gas gave the
corresponding HCl salts with a head-to-tail alignment (Fig. 1).
Furthermore, the utility of a cascade process in crystals was shown
by conversion of the produced crystals to the corresponding
syn-HT dimers with high regio- and stereoselectivities (Fig. 1).
4-Azachalcone (1a) and 40-methoxy-4-azachalcone (1b)
were employed as we have previously confirmed the existence
of significant differences between the packing diagrams of
1a and 1b and their corresponding HCl salts 2a and 2b
(Scheme 1).10 Compound 1a, as a powdered crystal, was kept
in a desiccator filled with dried HCl gas for 10 min. The
colorless powder immediately turned pale yellow, although the
solid-state was preserved during the reaction. The powder
Fig. 2 The simulated PXRD patterns for (a) 1a and (d) 2a and the
actual PXRD patterns of 1a after exposure to HCl gas for (b) 10 s and
(c) 10 min.
Department of Chemistry, Faculty of Science, Ochanomizu University,
2-2-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan.
E-mail: yamada.shinji@ocha.ac.jp
w Electronic supplementary information (ESI) available: Experimental
details, thermogravimetric analysis data of 2a, 1H NMR spectra for 3a
and 3b before purification and for 3bꢀ2HCl before workup produced
through the cascade reactions, photomicrographs of 2b before and
after irradiation. See DOI: 10.1039/c2cc17019a
X-ray diffraction pattern of the resultant 2a clearly differed
from that of 1a, indicating the completion of the reaction
within 10 min (Fig. 2a and c). Surprisingly, the PXRD pattern
was very close to that simulated from the single crystal X-ray
data of the recrystallized 2a (Fig. 2c and d).10 Thermogravimetric
analysis of 2a shows a 3.4 wt% loss of water at 325–385 K,
c
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Chem. Commun., 2012, 48, 1763–1765 1763

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Fig. 3 Crystal structures of 1a viewed along the (a) a- and (b) b-axes,
and (c) the crystal structure of 2a viewed along the b-axis.
confirming the inclusion of 0.5 equiv. of water molecules in the
2a crystal. When the exposure was stopped after 10 s, the
PXRD patterns of both 1a and 2a were observed (Fig. 2b).
To understand the reorientation process from 1a to 2a in the
crystals on exposure to HCl, the crystal structures were
compared (Fig. 3). The molecules in the unit cell are indicated
in different colours to better demonstrate the changes in the
packing structures. Fig. 3a and b show packing diagrams of 1a
along the a- and b-axes, respectively, and Fig. 3c shows the
crystal structure of 2a. 1a crystallizes in space group P21/n in
a zigzag arrangement. The intermolecular distances of the
neighbouring two-ring centroids are 4.777 A and 4.893 A.
The molecule appears in a twisted conformation with a tilt
angle of 23.71 between the pyridine and the phenyl rings.
These features indicate that there are no stacking interactions
in the neighbouring molecules (Fig. 3a and b). On the other
hand, 2a crystallizes in C2/c, in which the molecules of 2a are
arranged in a stacked head-to-tail fashion, thereby forming
four sets of dimers with four water molecules in the unit cell
(Fig. 3c). The separation between the pyridinium and the aryl
rings is 3.954 A, indicating the contribution of a cation–p
interaction in this crystal structure. These observations provide
an insight into the changes in the orientation mode from 1a to
2a; the neighbouring molecules of 1a in the same column
approached each other so as to produce cation–p-stabilized
head-to-tail dimers.
Fig. 4 The simulated PXRD patterns for (a) 1b and (c) 2b, and (b)
the actual PXRD pattern of 1b after exposure to HCl gas for 10 min.
Fig. 5 Crystal structures of (a) 1b and (b) 2b viewed along the a-axis.
which are related by the center of symmetry. The separation
between the pyridinium and phenyl rings is 3.654 A (Fig. 5b).
Although the precise mechanism for the remarkable phase
transition from 1b to 2b while preserving the crystalline state is
still unclear, the formation of ionic crystals on exposure to
HCl gas may be a key to these observations. The HCl salt
formation produces repulsion between the produced pyridinium
cations and an attractive cation–p interaction between the
pyridinium and aromatic rings, which promotes reorientation
from head-to-head to head-to-tail. Although this process
would proceed through transient phases, the Coulomb attrac-
tion between the pyridinium ions and the chloride anions of
the HCl salts may restrict the movement of the molecules,
which would result in the crystal-to-crystal transformation. A
model for the possible reorientation process is as follows
(Fig. 4S, ESIw): the repulsive and attractive forces described
above trigger the molecules to rotate clockwise or anticlockwise
to form two sets of anti-parallel columns oriented perpendi-
cular to the initial parallel columns through a restricted
transient phase.
There have been reports of several examples in which ordered
crystals were formed by the reaction with gases through the
interconversion of hydrogen bond11 or coordination bond12
networks. On the other hand, the reorientation described above
is controlled by a cation–p interaction, providing a new pathway
for making crystals from crystals.13
Next, 1b was exposed to HCl gas in the same manner as for
1a. During the reaction, the solid-state was retained, as in 1a.
A comparison between the PXRD pattern of 1b after exposure
to HCl for 10 min and the simulated pattern of 2b clarified that
the crystal structures of the resultant salt and the recrystallized
2b are identical (Fig. 4). This means that exposure of 1b to
HCl gas leads to significant changes in the orientation from
head-to-head to head-to-tail.
Irradiation of the resultant HCl salts 2a and 2b for 24 h gave
syn-HT dimers 3a and 3b in 88% and 99% yields, respectively
(Scheme 2). These results are in agreement with those previously
obtained from the photodimerization reactions of the recrystallized
HCl salts,10 indicating the high crystallinity of the resultant
HCl salts on the microscale level.
%
Compound 1b crystallized in P1, the crystal structure of
which shows two independent molecules in the asymmetric
unit,10 indicated in green and red (Fig. 5a). These molecules
are stacked in a head-to-head fashion to form columns along
the a-axis with a separation of 3.857 A. On the other hand,
2b crystallized in P21/n, and the molecules are arranged in
an anti-head-to-tail fashion, forming two types of columns,
Scheme 2 Photodimerization of the resultant HCl salt 2a or 2b.
c
1764 Chem. Commun., 2012, 48, 1763–1765
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This work was supported by a Grant-in-Aid for Scientific
Research (C) (No. 21550097) from the Japan Society for the
Promotion of Science.
Notes and references
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3 The term ‘‘cascade’’ is used here in accordance with the definition
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Fig. 6 Photomicrographs of a single crystal of 1b (a) before and
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irradiation of the resultant HCl salt shown in Fig. 6c for 24 h.
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the photomicrographs of a single crystal of 1b before and after
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the surface becoming slightly uneven (Fig. 6b). After exposure
for 10 min, the unevenness and cracking became more obvious
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Fig. 6d shows a photomicrograph taken after irradiation of
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