Spontaneous generation of chirality in simple diaryl ethers

Article abstract of DOI:10.1002/chir.22460

We studied the spontaneous formation of chiral crystals of four diaryl ethers, 3-phenoxybenzaldehyde, 1; 1,3-dimethyl-2-phenoxybenzene, 2; di(4-aminophenyl) ether, 3; and di(p-tolyl) ether, 4. Compounds 1, 3, and 4 form conformationally chiral molecules i

Full text of DOI:10.1002/chir.22460

CHIRALITY 27:425–429 (2015)
Spontaneous Generation of Chirality in Simple Diaryl Ethers
1
2
2
*
*
ANDERS LENNARTSON, ANNA HEDSTRÖM, AND MIKAEL HÅKANSSON
1Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Gothenburg, Sweden
2Department Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden
ABSTRACT
We studied the spontaneous formation of chiral crystals of four diaryl
ethers, 3-phenoxybenzaldehyde, 1; 1,3-dimethyl-2-phenoxybenzene, 2; di(4-aminophenyl) ether,
3; and di(p-tolyl) ether, 4. Compounds 1, 3, and 4 form conformationally chiral molecules in the
solid state, while the chirality of 2 arises from the formation of supramolecular helices. Com-
pound 1 is a liquid at ambient temperature, but 2–4 are crystalline, and solid-state CD-
spectroscopy showed that they could be obtained as optically active bulk samples. It should be
noted that the optical activity arise upon crystallization, and no optically active precursors were
used. Indeed, even commercial samples of 3 and 4 were found to be optically active, giving evi-
dence for the ease at which total spontaneous resolution may occur in certain systems. Chirality
27:425-429, 2015. © 2015 Wiley Periodicals, Inc.
KEY WORDS: CD-spectroscopy; Ullmann coupling; spontaneous resolution; optical resolution;
supramolecular chemistry
The optical activity displayed by drugs, catalysts, and
other important chemicals can always be traced back to
the so-called chiral pool, i.e., naturally occurring optically ac-
tive substances like L-amino acids, D-sugars, alkaloids, etc.1
The origin of this biomolecular homochirality is unknown.2,3
A strategy in the search for clues to this important question
is to study processes capable of transforming racemic or
achiral starting materials to optically active products without
chiral reagents or catalysts; such processes are generally
termed absolute asymmetric synthesis.4–6 Different strate-
gies for achieving absolute asymmetric synthesis include re-
actions under the influence of circularly polarized light,4
abrasion/grinding techniques,7,8 and total spontaneous
resolution.9 The latter is the subject of this article. When a
chiral compound crystallizes in one of the so-called Sohncke10
(or chirodescriptive)11 space groups, it will crystallize as a 1:1
mixture (a conglomerate) of enantiomerically pure crystals.
Such a substance is said to undergo spontaneous resolution,9
since the two enantiomers will crystallize separately. Sponta-
neous resolution was first observed in ammonium sodium
tartrate by Louis Pasteur.12 On crystallization of a stereo-
chemically labile substance one does not necessarily obtain
a 1:1 mixture of the two enantiomorphs. If crystallization
is initiated by a single nucleus, total spontaneous resolu-
tion (crystallization-induced asymmetric transformation) is
possible, meaning that a majority of the crystals will be
clones of this original nucleus and hence be of the same
enantiomorph. The bulk will then be obtained in an opti-
cally active state, and total spontaneous resolution means
that an excess (but not necessarily 100%) is obtained of
one enantiomorph. Over the past years, we have studied
total spontaneous resolution and absolute asymmetric syn-
thesis of a variety of molecules, ranging from pure organic
substances13 to organometallic reagents14–19 and coordina-
tion compounds.20–23 In this work we study four simple
diaryl ethers, 3-phenoxybenzaldehyde, 1; 1,3-dimethyl-2-
phenoxybenzene, 2; di(4-diaminophenyl) ether, 3; and di
(p-tolyl) ether, 4 (Scheme 1), all of which spontaneously
form chiral crystals.
MATERIALS AND METHODS
Commercial 3-phenoxybenzaldehyde (Aldrich, Milwaukee, WI) was
dissolved in hexane/dichloromethane 1:1. Cooling to –35°C afforded col-
orless crystals of 1 suitable for single-crystal X-ray diffraction and solid-
state CD-spectroscopy. Commercial di(4-aminophenyl) ether (Aldrich)
was recrystallized from boiling methanol to give crystals of 3. Crystals
of 4 were obtained by dissolving commercial di(p-tolyl)ether (Aldrich)
in 96% ethanol and cooling the solution to –20°C. Solid-state CD-spectra
were recorded in KBr-discs (1 mg sample, 100 mg KBr, 8 tons pressure)
on a Jasco (Tokyo, Japan) J-175 spectropolarimeter.
1,3-Dimethyl-2-Phenoxybenzene, 2
2,6-dimethylphenol (10.4 g, 0.085 mol) and potassium hydroxide (4.3 g,
0.072 mol) were fused, and copper(I) chloride (0.22 g) and bromoben-
zene (5.0 mL, 0.050 mol) were added. The mixture was heated under
reflux for 8 h and poured into a 2 M solution of sodium hydroxide in ice-
water (100 mL). The mixture was extracted with diethyl ether (3 × 50
mL). The red solution was washed with 2 M aqueous sodium hydroxide
(100 mL) and with small portions of water, until no colored material was
extracted. The ethereal solution was washed with brine (50 mL), dried
over sodium sulfate, and evaporated in vacuo. The residue was recrystal-
lized from hot ethanol, which afforded colorless crystals of 2. Yield: 1.4 g
(14%). Analytical data were consistent with previously published data.24
Crystal Structure Determination
Crystals were selected using a microscope and transferred to a Rigaku
R-AXIS IIc image plate system. Diffracted intensities were measured
using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) from a
RU-H3R rotating anode operated at 50 kV and 90 mA. Ninety oscillation
photographs with a rotation angle of 2° were collected and processed
using the CrystalClear software package.25 The structures were solved
by direct methods (SIR-92)26 and refined using full-matrix least-squares
*Correspondence to: Anders Lennartson, Department of Chemistry and
Chemical Engineering, Chalmers University of Technology, Gothenburg,
Sweden. E-mail: anle@chalmers.se.
Mikael Håkansson, Department Chemistry and Molecular biology. University
of Gothenburg. SE-412 96, Gothenburg, Sweden. E-mail: hson@cmb.gu.se
Present address for Anna Hedström: Borealis AB, SE-444 86 Stenungsund,
Sweden.
Received for publication 19 February 2014; Accepted 10 April 2015
DOI: 10.1002/chir.22460
Published online 31 May 2015 in Wiley Online Library
(wileyonlinelibrary.com).
© 2015 Wiley Periodicals, Inc.

426
LENNARTSON ET AL.
Scheme 1
TABLE 1. Data collection, structure solution, and refinement parameters for 1-4
Parameter
1
2
3
4
Empirical formula
M
Crystal system
Space group
a/Å
b/Å
c/Å
C
13H10O2
C14H14
O
C12H12N2O
200.24
Othorhombic
P212121
5.6959(16)
7.705(2)
22.771(6)
90
999.3(5)
4
C14H14O
198.21
monoclinic
P21
8.185(2)
6.4760(15)
9.722(3)
104.760(10)
498.3(2)
2
198.25
Othorhombic
P212121
6.9970(11)
7.6398(14)
20.983(4)
90
1121.6(3)
4
1.174
198.25
Othorhombic
P212121
5.7985(13)
7.6626(16)
24.464(5)
90
1087.0(4)
4
1.211
β/°
V/Å3
Z
Dc/g cm-3
1.321
0.089
1.331
0.087
298(2)
μ(Mo Kα)/mm-1
T/K
0.072
100(2)
0.074
100(2)
100(2)
Crystal size/mm
θ Range/°
Reflections collected
Unique reflections, Rint
Reflections with I > 2σ(I)
No. of parameters
GOF on F2
0.3 × 0.3 × 0.3
2.91 to 27.5
3808
1185, 0.0271
1145
176
1.034
0.0293, 0.0302
0.0770, 0.0775
0.3 × 0.3 × 0.3
2.84 to 25.48
7398
1215, 0.0372
1181
192
1.065
0.0279, 0.0287
0.0731, 0.0735
0.2 × 0.25 × 0.25
3.19 to 26.00
6940
1924, 0.0600
1718
184
1.001
0.0386, 0.0871
0.0468, 0.0905
0.2 × 0.3 × 0.4
1.66 to 28.00
8605
1520, 0.0487
1432
192
1.089
0.0340, 0.0374
0.0862, 0.0872
Final R1(F)a(I > 2σ(I))/ wR2(F2)b
a
R1 / wR2(F2)b (all data)
aR1 (F) = Σ(||Fo| - |Fc||)/Σ|Fo|
½
bwR2(F2) = {Σ[w(Fo2 - Fc2)2] /Σ [w(Fo2)2]}
calculations on F2 (SHELXL-97)27 on all reflections. Anisotropic thermal
displacement parameters were refined for all nonhydrogen atoms. Struc-
tural illustrations have been drawn with ORTEP-3 for Windows28 and
PLUTON.29 SIR-92, SHELXL-97, ORTEP-3, and PLUTON were included
in the WinGX software package.30 The crystal and refinement parameters
are summarized in Table 1.
of the Sohncke space groups. Compound 2 (Fig. 1) is a crys-
talline solid at ambient temperature, and was also found to
crystallize in a Sohncke space group (P212121).
From a stereochemical point of view, the two ethers are
very different. Molecules of 1 are conformationally chiral in
the solid state (Fig. 2), while molecules of 2 were found to
accommodate a virtually achiral conformation (pseudo-Cs-
symmetry) in the crystalline state. The chirality of 2 is
therefore to be sought in the supramolecular structure.
Well-known examples of simple achiral inorganic compounds
crystallizing in Sohncke space groups include quartz, magne-
sium sulfate heptahydrate, strontium formate, and sodium
RESULTS AND DISCUSSION
Of the four compounds, 1, 3, and 4 are commercially avail-
able, while compound 2 was prepared through an Ullmann
coupling reaction31 between 2,6-dimethylphenol and bromo-
benzene. The yield was low, but the reaction conditions were
by no means optimized. Several methods for the synthesis of
2 have been reported previously, e.g., by Niu et al.24
Molecular Structures and Chirality
Compound 1 (Fig. 1) is a liquid at ambient temperature,
but crystallizes on cooling in space group P21, which is one
Fig. 2. Diagram of the molecular conformations of 1 and 2, viewed along
the plane of the nonsubstituted phenyl ring and the ether link. Molecules of
1 appear as two enantiomers (left) while molecules of 2 displays a mirror
plane bisecting the molecule.
Fig. 1. The molecular structures of 1 (left) and 2 (right) displaying the
crystallographic numbering schemes. Displacement ellipsoids are drawn at
the 50% probability level. All H atoms have been omitted.
Chirality DOI 10.1002/chir

ABSOLUTE ASYMMETRIC SYNTHESIS
427
chlorate32 and more recently we reported chiral crystals of
the achiral reagent pyridinium dichromate.33 Especially the
chirality of sodium chlorate crystals has attracted conside-
rable interest over the years.34–38 With achiral molecules,
there are no enantiomers and the crystallization of 2 is per-
haps better described as a spontaneous generation of
chirality rather than spontaneous resolution. The crystalliza-
tion of achiral organic substances as chiral crystals has been
reviewed.39
Why are crystals of 2 chiral? There is one set of CH…π
interactions40 in 2 that is of particular interest: the shortest
contact, 2.85(2) Å, is between H3 and C13i [symmetry code:
(i) -1/2+x, 1/2-y, 1-z], and the distance to the centroid of the
aromatic ring is 2.80(2) Å. These interactions give rise to
chains extended parallel to the a-axis, and these chains are
folded into helices. The resulting helices are best visualized
by allowing the centroid of the phenyl ring to be a member
in the helix, as shown in Figure 3.
Having found that 1 and 2 crystallize in Sohncke space
groups, we turned to the Cambridge Structural Database
(CSD)41 to find additional candidates for total spontaneous res-
olution. The simplest diaryl ether, diphenyl ether, crystallizes
in space group P212121,42 but its low melting point and a com-
peting racemic polymorph is a disadvantage. Instead, we turn-
ed our attention to di(4-aminophenyl) ether, 3, and di(p-tolyl)
ether, 4.
The crystal structure of 3 was previously published (except
for the coordinates of most of the H atoms),43 while only the
cell parameters and the space group have been reported for
compound 4.44 Both compounds have been reported to crys-
tallize in space group P212121, which we could confirm. Like
the molecules of 1, the individual molecules of 3 and 4 are
conformationally chiral in the solid state (Fig. 4).
Fig. 5. Scheme displaying total spontaneous resolution of 1. If primary nu-
cleation is slow, one enantiomorph is likely to form a crystal nucleus before
the other. Secondary nucleation, i.e., the emission of small fragments from a
crystal present in the solution, may ensure that all new crystals are of the
same enantiomorph as the first crystal formed, if the two enantiomers inter-
convert rapidly in solution.
not possible to determine the absolute structure of 1–4 and
it is therefore not possible to discriminate between the
enantiomorphs. In our previous work on total spontaneous
resolution, we relied on solid-state CD-spectroscopy,45 and it
was found that single-crystals of 2 displayed circular dichro-
ism in the solid state. This is noteworthy, since the molecules
are essentially achiral and the solid-state circular dichroism
has to be attributed to the crystal packing. This fact may per-
haps explain the comparably low magnitude of the circular
dichroism. Grinding a bulk sample homogenous and analyz-
ing it by solid-state CD spectroscopy can provide proof for
total spontaneous resolution. Indeed, when 2 was crystallized
by slow evaporation of a solution in ethanol, the bulk product
displayed circular dichroism of similar magnitude as single
crystals, indicating substantial “crystal enantiomeric excess-
es” in the samples (Fig. 6). We previously reported a method
for quantitative solid-state CD-spectroscopy,20 but since crys-
tals of 1–4 have no heavy atoms, no reliable Flack parame-
ter46,47 can be obtained by single-crystal X-ray diffraction.
Total Spontaneous Resolution
In solution or the liquid state, fast interconversion between
different chiral and achiral conformations of compounds 1–4
are expected, and no chirality is expected to persist. There-
fore, these four compounds are all suitable candidates for to-
tal spontaneous resolution (Fig. 5).
The problem is to measure optical activity in the bulk
samples obtained by crystallization, since only solid-state
methods are useful. Using Mo-Kα or Cu-Kα radiation, it is
4
2
Fig. 3. Plot showing CH…π interactions (dashed lines) in 2, generating
supramolecular helices. The helix is viewed along the crystallographic b-axis.
0
-2
-4
400
300
Fig. 4. Molecular structure of 3 (left) and 4 (right) displaying the crystal-
lographic numbering scheme. Displacement ellipsoids are drawn at the 50%
probability level. The different sizes of the ellipsoids for 3 and 4 is due to
the fact that data were recorded at ambient temperature for 3, but at 100 K
in the case of 4. All H atoms have been omitted.
Wavelength (nm)
Fig. 6. Solid-state CD spectra (KBr-matrix) for both enantiomorphs of 2.
Chirality DOI 10.1002/chir

428
LENNARTSON ET AL.
40
20
Therefore, the possibility of twinning-by-inversion cannot be
excluded, and it cannot be proved that a reference crystal
actually is enantiomerically pure.
No special precautions (such as stirring36) are needed to
obtain optically active samples, and the enantiomorph ob-
tained in excess is randomly distributed between different
batches. Occasionally (about two batches out of ten), racemic
samples were also observed. Since the distribution of enan-
tiomorphs is stochastic, it is unlikely that external influences
such as cryptochirality5,21,48 induce the optical activity. Since
compound 1 is a liquid at ambient temperature, no solid-
state CD-spectra were obtained. Instead, we attempted to
trap the conformational chirality of 1 by a solid state nucleo-
philic addition to the carbonyl group. This would give rise a
chirogenic center, and any enantiomeric excess could be de-
termined by classical methods. We have previously shown
that the reaction between solid methyllithium and chiral
crystals of tris{2,6-diphenylphenolato-(p-tolylaldehyde)}-alu-
minum gave 1-p-tolylethanol with an enantiomeric excess of
17%.49 To test this hypothesis, the reaction between 1 and
solid dimethylmagnesium was investigated. This reaction,
however, was found to be difficult to control. At low temper-
ature, the reaction between carefully mixed powders of 1
and dimethylmagnesium was very slow. Four weeks at 195
K gave only traces of (racemic) product along with several
unidentified by-products. Performing the reaction at 243 K
did not improve the results. If the temperature was slowly
increased, the reaction mixture would suddenly turn black
with evolution of heat and smoke. Even at 195 K, such vio-
lent reactions could take place on vigorous grinding.
0
-20
-40
300
400
500
600
Wavelength (nm)
Fig. 8. Solid-state CD spectra for both enantiomorphs of 4.
active, one was racemic). All attempts to react optically active
2-benzoyl pyridine or antracen-9-carboxaldehyde with solid
dimethylmagnesium gave racemic products.
In conclusion, we studied four simple diaryl ethers, three of
which readily underwent total spontaneous resolution. In fact,
two of them were actually obtained in an optically active form
in the commercial products, showing that total spontaneous
resolution indeed is very spontaneous in certain cases.
ACKNOWLEDGMENTS
Financial support from the Swedish Research Council is
gratefully acknowledged.
SUPPORTING INFORMATION
When 3 is recrystallized from methanol without stirring, a
racemic product was obtained, but on stirring the product
obtained is optically active, as demonstrated by solid-state
circular dichroism (Fig. 7). More noteworthy is that the
commercial bulk product showed a low, but still detectable,
optical activity. In commercial 4, the solid state circular
dichroism was of the same magnitude as for single crystals,
indicating a substantial enantiomeric excess. Upon recrystal-
lization (with or without stirring) 4 was always obtained in
an optically active form (Fig. 8) and excess of both enantio-
mers was obtained with similar frequency.
It has recently been reported that benzil, diphenyl sul-
fide, benzophenone, tetraphenylethylene, guanidine carbon-
ate, 2,6-di-tert-butyl-4-methylphenol, hipuric acid, cytosine, and
ninhydrin may be optically active in commercial bulk sam-
ples.50 In addition to 3 and 4, we found another three exam-
ples of “achiral” compounds commercially available in an
optically active form: 2,6-diphenylphenol, 2-benzoyl pyridine,
and antracene-9-carboxaldehyde (two bottles were optically
Additional supporting information may be found in the
online version of this article at the publisher’s web-site.
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Chirality DOI 10.1002/chir