Seventeen-membered water cluster resulting from recognition of solvated anions on brucinium corrugated layers

Article abstract of DOI:10.1021/cg500310n

Formation of diastereomeric salts remains the most important method for the separation of racemic acids and bases. Selection of a suitable resolving agent in this method is a key for successful resolution. There are primary, secondary, and tertiary chiral amines among frequently used resolving agents for the separation of racemic acids. Cations of most of them and anions of resolved acids are linked to each other by a characteristic system of hydrogen bonds resulting in common cationic-anionic self-assemblies. In this respect, brucine and strychnine are unique, because incorporation of anionic species into a crystal lattice of their salts usually does not affect common cationic self-assembly. The uniqueness of both resolving agents is also reflected in a high frequency of solvated salt formation. In this paper, we show that the presence of water molecules incorporated into the crystal lattice of the brucinium salt may result from recognition of the resolved compound together with its closest aqueous environment on the common brucinium corrugated layers. Performing racemic resolution of model compounds and studying structural relations between succeeding crystalline fractions, we also point out factors responsible for the successful separation of N-(4-nitrobenzoyl)alanine by fractional crystallization of brucinium diastereomeric salts.

Full text of DOI:10.1021/cg500310n

Article
pubs.acs.org/crystal
Seventeen-Membered Water Cluster Resulting from Recognition of
Solvated Anions on Brucinium Corrugated Layers
Agata Białonska* and Zbigniew Ciunik
́
Faculty of Chemistry, University of Wrocław, 14. F. Joliot-Curie, 50-383 Wrocław, Poland
S
* Supporting Information
ABSTRACT: Formation of diastereomeric salts remains the
most important method for the separation of racemic acids and
bases. Selection of a suitable resolving agent in this method is a
key for successful resolution. There are primary, secondary, and
tertiary chiral amines among frequently used resolving agents for
the separation of racemic acids. Cations of most of them and
anions of resolved acids are linked to each other by a
characteristic system of hydrogen bonds resulting in common
cationic−anionic self-assemblies. In this respect, brucine and
strychnine are unique, because incorporation of anionic species into a crystal lattice of their salts usually does not affect common
cationic self-assembly. The uniqueness of both resolving agents is also reflected in a high frequency of solvated salt formation. In
this paper, we show that the presence of water molecules incorporated into the crystal lattice of the brucinium salt may result
from recognition of the resolved compound together with its closest aqueous environment on the common brucinium corrugated
layers. Performing racemic resolution of model compounds and studying structural relations between succeeding crystalline
fractions, we also point out factors responsible for the successful separation of N-(4-nitrobenzoyl)alanine by fractional
crystallization of brucinium diastereomeric salts.
It indicates that driving forces, leading to molecular
recognition of an acid by brucine or strychnine, may be
different from those leading to molecular recognition of the
acid by other resolving agents. Previously, we showed that both
enantiomers of resolved N-(4-nitrobenzoyl)- or N-(3,5-
dinitrobenzoyl)amino acids (alanine, serine, asparagine, and
aspartic acid) are recognized in a similar way by hydrophobic
interactions of the N-protected group in deep rows of common
strychninium corrugated layers.11,12 In these cases, diastereo-
meric diversification has been realized by different hydrogen
bond patterns observed in both strychninium diastereomeric
salts. On the other hand, in cases in which both diastereomeric
salt cations and anions are linked to each other by a similar
hydrogen bond network, as has been observed in many crystal
structures of salts of many other resolving agents, diastereo-
meric diversification is often related to differences in hydro-
phobic interactions in both diastereomeric salts.13,14
The different driving forces leading to recognition in
strychninium or brucininium salts and in salts of many other
resolving agents are reflected in the frequency of solvates
formation. Contrary to the others, brucine and strychnine
reveal a strong tendency to form solvated salts. Nearly 82% of
strychninium, 89% of brucinium, and only 6% of 1-
arylethylammonium salts are solvates. Among brucinium and
strychninium salts, 61 and 64% are hydrates, respectively. If
INTRODUCTION
■
Pasteur’s (1853) resolution of racemic compounds via
formation and fractional crystallization of diastereomeric salts
of chiral resolving agent remains the most practically important
method for the preparation of enantiopure compounds.1,2
During such resolution, any chiral amine can be used as a
resolving agent. However, for some reasons, such as
accessibility or price, some chiral amines are more frequently
used for the separation of racemic acids than others. Among
them, there are primary, secondary, and tertiary chiral amines.
Usually, they form common hydrogen-bonded cationic−
anionic self-assemblies.3−7 For example, cations and anions
are linked to each other by an ionic hydrogen bond system
8,9
3
defined by an R4 (10) graph set in 72% of 324 chiral crystal
structures of 1-arylethylammonium salts (CSD 5.35).10 The
2
1
C2 (6) or C2 (4) graph sets define ionic hydrogen bond
patterns between cations and anions in 67% of 46 crystal
structures of ephedrinium, pseudoephedrinium, or deoxyephe-
drinium salts. Characteristic of chininium, chinidinium,
cinchoninium, or cynchonidinium salts (54% of 66 structures)
is the presence of cationic−anionic chains defined by the
2
C2 (9) graph set. In this respect, brucine and strychnine are
unique, because incorporation of anionic species into the crystal
lattice of their salts usually does not affect common cationic
self-assembly. Similar, stabilized by hydrophobic interactions
only, cationic self-assemblies are observed in 79% of 34 crystal
structures of strychnium salts and 70% of 102 crystal structures
of brucinium salts.
Received: March 3, 2014
Revised: March 21, 2014
Published: April 11, 2014
© 2014 American Chemical Society
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Scheme 1
with occupancy factors of <0.5 were not found in the Δρ map. All
other H atoms were found in Δρ maps or placed at calculated
positions. Before the last refinement cycle, all H atoms were fixed and
allowed to ride on their parent atoms. The absolute structures were
chosen on the basis of the known absolute configuration of brucine.24
Crystal data for brucinium N-(4-nitrobenzoyl)-D-alaninate methanol
only hydrates are taken into account, these values are far from
the frequency of hydrates that resulted from database analysis,
which for the presence of a carboxylate group is equal to 35%.15
It is worth mentioning that the percentage of solvated 1-
arylethylammonium salts is comparable to the frequency of
solvated cocrystals.16
Recognition on the surface of a resolving agent self-assembly
supplies interesting information about fruitful or fruitless
racemic resolution, like two possible mechanisms of double-
salt formation17 or factors governing the kinetics of salt
precipitation.18 However, it also can supply information about
the resolved compound itself. Governed by the dielectric
constant of the solvent, the ability of spontaneous racemic
resolution of N-(3,5-dinitrobenzoyl)asparagine is the best
example of it.12 In this paper, we present racemic resolution
of N-(4-nitrobenzoyl)alanine by formation of brucinium
diastereomeric salts (Scheme 1).19
+
−
disolvate [B4NBDA(1)]: C23H27N2O4 C10H9N2O5 2CH4O, M =
696.74, orthorhombic, P212121, a = 7.668(2) Å, b = 12.341(3) Å, c =
35.548(4) Å, V = 3363.9(13) Å3, Z = 4, Dc = 1.376 Mg m−3, T =
100(2) K, R = 0.059, wR = 0.107 [4773 reflections with I > 2σ(I)] for
451 variables. Crystal data for brucinium N-(4-nitrobenzoyl)-L-
+
−
alaninate 4.8-hydrate [B4NBLA(2)]: C23H27N2O4 C10H9N2O5
4.8H2O, M = 719.13, monoclinic, C2, a = 31.826(6) Å, b =
12.510(3) Å, c = 35.668(3) Å, β = 104.98(3)°, V = 13718(5) Å3, Z =
16, Dc = 1.393 Mg m−3, T = 100(2) K, R = 0.095, wR = 0.155 [16741
reflections with I > 2σ(I)] for 2097 variables.
RESULTS
■
B4NBDA(1) is quasi-isomorphous with the previously
reported crystal structure of brucinium N-(4-nitrobenzoyl)-D-
serinate methanol disolvate and the solid solution of brucinium
N-(4-nitrobenzoyl)serinate methanol disolvate.25 Brucinium
cations form common corrugated layers, stabilized by hydro-
phobic interactions (Figure 1).26−28 N-(4-Nitrobenzoyl)-D-
EXPERIMENTAL SECTION
■
N-(4-Nitrobenzoyl)alanine was synthesized in a reaction of 4-
nitrobenzoyl chloride with DL-alanine in water by applying standard
procedures.20 Racemic resolution of the alanine derivative was
performed by fractional crystallization of brucinium salts. Anhydrous
brucine (100 mg, commercially available) and an equimolar amount of
N-(4-nitrobenzoyl)alanine were dissolved in 10 mL of methanol (p.a.).
The sample was left for solvent evaporation at room temperature. Pale
yellow crystals of brucinium N-(4-nitrobenzoyl)-D-alaninate methanol
disolvate [B4NBDA(1)] precipitated as a first fraction in a few hours.
Crystals of B4NBDA(1) were separated from the mother liquid.
Yellow crystals of brucinium N-(4-nitrobenzoyl)-L-alaninate 4.8-
hydrate [B4NBLA(2)] precipitated from the filtrate as a further
fraction after 1 day.
X-ray data for both crystalline fractions were collected on a Kuma
KM4CCD diffractometer (Mo Kα radiation; λ = 0.71073 Å). X-ray
data for each compound were collected at 100 K using an Oxford
Cryosystems device. Data reduction and analysis were conducted with
the CrysAlis “RED” program.21 Space groups were determined using
XPREP.22 Structures were determined by direct methods using
SHELXS-2013 and refined using all F2 data, as implemented in
SHELXL-2013.23 Non-hydrogen atoms were refined with anisotropic
displacement parameters. In B4NBLA(2), one of four crystallo-
graphically unrelated anions, the carboxylate O atom of another, and
eight of 19 crystallographically unrelated water molecules reveal two-
position disorder. Preliminary, occupancy factors were calculated
separately for each of the crystallographically unrelated components. It
allowed us to group particular components of the disorder into two
groups, depending on occupancy factors. The occupancy factor of the
major components is equal to 0.70, and the occupancy factor of the
minor components is equal to 0.30. Besides, two maxima on the Δρ
map were interpreted as an additional two water molecules, both with
occupancy factors equal to 0.10. Displacement parameters of the
disordered atoms with occupancy factors of <0.5 were restrained to be
approximately isotropic (ISOR). For partially overlapping atoms, the
SIMU restraint was applied. H atoms of a majority of water molecules
Figure 1. Orientation of N-(4-nitrobenzoyl)-D-alaninate anions and
methanol molecules to the surface of the common corrugated
brucinium layer (blue) in B4NBDA(1).30,31
alaninate anions are bonded to the layer by ionic N−H+···O−
hydrogen bond and hydrophobic C−H···O interactions. The
nitro group of the anions, located in shallow holes in the
brucinium layer, participates in the C−H···O interactions.
Consecutive anions, bonded to one brucinium layer, are linked
to each other by C−H···O hydrogen bonds. In turn, anions,
bonded to neighboring brucinium layers, are linked to each
other by N−H···O− hydrogen bonds. Methanol molecules are
located in narrow channels formed between the corrugated
cationic and relatively flat anionic layer. Methanol molecules are
involved in O−H···O hydrogen bonds, in which the O4 amide
atom of cations and the O7 amide atom of the anions are
acceptors.
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In B4NBLA(2), there are four crystallographically unrelated
brucinium N-(4-nitrobenzoyl)-L-alaninate ion pairs and 19
water molecules (the major components of the disorder) in an
asymmetric part of the unit cell. Brucinium cations form two
crystallographically unrelated corrugated layers. The cationic
layers in B4NBLA(2) are somehow modified as compared to
the common corrugated brucinium layers. Two corrugated
brucinium ribbons are assembled in a way in which concave
parts of one meet concave parts of the other (like in the
common corrugated layers). However, the two consecutive
ribbons are shifted, so their concave parts meet convex parts of
the former. Anions, linked by ionic N−H+···O− hydrogen
bonds to one brucinium layer, form N4(nitro)···O4(amide)
and other hydrophobic interactions with the neighboring
brucinium layer (Figure 2). Crystallographically unrelated
cluster, and their amide O atom is bonded to neighboring water
clusters or the water dimer. The water dimer forms hydrogen
bonds with the carboxylate and the amide O atoms of three
anions bonded to the same 17-membered water cluster.
DISCUSSION
■
The crystal structures described above result from fruitful
racemic resolution of N-(4-nitrobenzoyl)alanine by fractional
crystallization of brucinium diastereomeric salts. It seems that
the most important factor driving the separation of both the
brucinium diastereomeric salts is related to recognition of the
carboxylate and the nitro groups of N-(4-nitrobenzoyl)-D-
alaninate anions by the same brucinium corrugated layer. It
results in a relatively flat surface and in a fast precipitation of
brucinium N-(4-nitrobenzoyl)-D-alaninate methanol disolvate.
Similar recognition of the carboxylate and the nitro group of
anions on the same brucinium corrugated layers was observed
in solid solutions of brucinium diastereomeric salts with N-(3-
nitrobenzoyl)amino acids (alanine, serine, and threonine) or N-
(4-nitrobenzoyl)serine. On the basis of the crystal structures of
related brucinium salts with N-(4-nitrobenzoyl)serinate meth-
anol disolvate, it seems that N-(4-nitrobenzoyl)-L-alaninate
anions could be recognized in a similar way and form a solid
solution as was observed for the serine analogue. However, if
the statistical analysis is taken into account (CSD 5.35), the
conformation of N-(4-nitrobenzoyl)-L-serinate [here the H−
C(α)−N−H torsion angle] in the solid solution of brucinium
N-(4-nitrobenzoyl)-(D0.85 0.15)-serinate methanol disolvate is
L
unfavorable. Unlike the alanine analogue, the intermolecular
O−H···O hydrogen bond, in which the hydroxyl group of the
serine derivative is involved, may stabilize the unfavorable
conformation. The lack of such a stabilizing factor leads to
racemic resolution of N-(4-nitrobenzoyl)alanine by fractional
crystallization of brucinium salts.
Another consequence of the difference between N-(4-
nitrobenzoyl)serine (containing the hydroxyl group) and N-
(4-nitrobenzoyl)alanine (without the hydroxyl group) is the
formation of various solvates of the brucinium salts with the L
enantiomer of the alanine or the serine derivative. In two
solvates of brucinium N-(4-nitrobenzoyl)-L-serinate (methanol
1.66-solvate 1.66-hydrate and methanol tetrasolvate) obtained
from a methanol solution, water and methanol, or methanol
molecules form short, at most 7-membered, hydrogen-bonded
chains. In B4NBLA(2), water molecules form 17-membered
water clusters built up of three five-membered conjugated rings
and one four-membered conjugated ring (Scheme 2). If the
carboxylate group of the anions, hydrogen bonded to water
molecules, is taken into account, the cluster is extended on four
Figure 2. Packing of B4NBLA(2). Corrugated brucinium layers (blue)
are separated by N-(4-nitrobenzoyl)alaninate anions bonded to the
water dimer and the 17-membered water cluster.30,31
water molecules are assembled as a dimer and a 17-membered
water cluster (see Scheme 2). The carboxylate O and the amide
N atoms of the anions are bonded to the 17-membered water
Scheme 2. Representation of the 17-Membered Water
Cluster (red) Presented in B4NBLA(2) and the Hydrogen-
Bonded Carboxylate O and Amide N Atoms of the Anions
(R is a 4-nitrobenzoyl group)19
5
additional five-membered rings [two defined by R5 (10) and
4
two defined by R4 (10) graph sets]. In turn, if the fact that the
carboxylate O and amide N atoms of the anions are bonded to
the same water cluster is taken into account, one additional
2
5
R2 (7) ring and three additional R5 (13) rings are formed with
the 17-membered water cluster. The water dimer, hydrogen
bonded to the carboxylate and to the amide O atoms, also
forms rings with the 17-membered water cluster [among them
4
5
R5 (13) and R6 (17)]. B4NBLA(2) seems to reflect a
recognition of the anions together with their closest aqueous
environment on the surface of a brucinium corrugated layer.
Among the anions with their closest aqueous environment, we
found one in which even the methyl group is surrounded by
water molecules. Thirteen of the 19 unrelated water molecules,
bonded to each other as well as to both the carboxylate O and
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the amide O and N atoms of the anion, participate in the
solvation of its methyl group (Figure 3). Such connected water
molecules are like a basket with the methyl group directed
inward.
AUTHOR INFORMATION
Corresponding Author
*Faculty of Chemistry, University of Wrocław, 14. F. Joliot-
Curie, 50-383 Wrocław, Poland. E-mail: agata.bialonska@chem.
uni.wroc.pl.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Ministry of Science and Higher Education of
Poland for the financial support.
■
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CONCLUSIONS
■
We showed that, contrary to the unsuccessful racemic
resolution of N-(3-nitrobenzoyl)amino acids (alanine, serine,
and threonine) or N-(4-nitrobenzoyl)serine resulting in the
formation of a solid solution of brucinium salts, brucine is a
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nitrobenzoyl)alanine. Recognition of the carboxylate and the
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