Optically active seleninic acid: Isolation, absolute configuration, stability, and chiral crystallization

Article abstract of DOI:10.1246/bcsj.78.710

Each optical isomer of methaneseleninic acid (1a) was isolated as chiral crystals by recrystallization from methanol/toluene. The absolute configuration of one of the enantiomers was determined by X-ray crystallographic analysis, and the relationship betw

Full text of DOI:10.1246/bcsj.78.710

710 Bull. Chem. Soc. Jpn., 78, 710–714 (2005)
Ó 2005 The Chemical Society of Japan
Optically Active Seleninic Acid: Isolation, Absolute Configuration,
Stability, and Chiral Crystallization
ꢀ
Yusuke Nakashima, Toshio Shimizu, Kazunori Hirabayashi, Masanori Yasui,1
Masaki Nakazato,1 Fujiko Iwasaki,1 and Nobumasa Kamigata
ꢀ
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University,
Minami-ohsawa, Hachioji, Tokyo 192-0397
1Department of Applied Physics and Chemistry, The University of Electro-Communications,
Chofu, Tokyo 182-8585
Received November 12, 2004; E-mail: kamigata-nobumasa@c.metro-u.ac.jp
Each optical isomer of methaneseleninic acid (1a) was isolated as chiral crystals by recrystallization from meth-
anol/toluene. The absolute configuration of one of the enantiomers was determined by X-ray crystallographic analysis,
and the relationship between the absolute configuration and the circular dichroism spectra of 1a was clarified. The op-
tically active seleninic acid (R)-1a was stable toward racemization in the solid state, although it racemized very rapidly
in solution. Each enantiomer of 1a was obtained in bulk by chiral crystallization in the presence of a seed crystal or
chiral solvents. The absolute configuration of optically active methanesulfinic acid (4) was also assigned by comparing
its circular dichroism spectra with those of 1a.
Tricoordinated chalcogen compounds have pyramidal struc-
tures around the chalcogen atoms.1,2 Therefore, chiral centers
exist on the chalcogen atoms of these compounds. Recently,
many chiral tricoordinated selenium and tellurium compounds,
such as oxides, onium salts, ylides, and imides, have been iso-
lated and their properties have been clarified.1 However, opti-
cally active chalcogenic acids, which are also tricoordinated
chalcogen compounds, have not been reported so far. Chiral
chalcogenic acids are difficult to isolate because they racemize
rapidly. Recently, we succeeded in the optical resolution of
areneseleninic acids with bulky substituents on a chiral col-
umn.3,4 However, the optically active areneseleninic acids
racemized in solution and the concentration of the eluates un-
der reduced pressure led to complete racemization. Thus, no
optically active seleninic acids have been isolated as stable sol-
ids so far. It is known that some racemic substances can be ob-
tained as chiral crystals (conglomerates), and that the race-
mates can be optically resolved by preferential crystallization.5
On the other hand, chiral crystallization is a well-known meth-
od for isolating optically active compounds in bulk that readily
racemize in solution.6 If seleninic acids crystallized in a chiral
space group, the isolation of enantiomers would be realized
and optically active seleninic acids could be obtained in bulk
by chiral crystallization.
(Chart 1) having various substituents, which were prepared
from the corresponding diselenides by oxidation with hydro-
gen peroxide or ozone and subsequent hydrolysis, were recrys-
tallized from several solvents, such as methanol, ethanol,
chloroform, acetone, acetonitrile, toluene, and their mixtures.
Circular dichroism spectra were measured in the solid state
to check whether an obtained crystal is chiral or not, because
seleninic acids racemize rapidly in solution. Each single crys-
tal obtained was pulverized with KBr and the mixture was
pressed into a disk for measurement. In the case of 1a, one
of the single crystals obtained by recrystallization from meth-
anol/toluene showed a positive Cotton effect at around 250
nm, and another one showed a negative Cotton effect in the
same region (Fig. 1), meaning that the crystals are chiral. How-
ever, the crystals of 1a that were recrystallized from methanol
or the other solvents did not show any Cotton effects, indicat-
ing that the crystals are racemic. In the cases of 1b–e, no crys-
tal that showed the Cotton effect was obtained, although a
number of recrystallization attempts were made.
Absolute Configuration and Crystal Structure of Opti-
cally Active Methaneseleninic Acid (1a). The X-ray crystal-
O
O
O
Me Se
OH
Se
OH
Se
OH
Therefore, we examined the chiral crystallization of various
seleninic acids and found that methaneseleninic acid can be
obtained as chiral crystals. We describe herein the isolation
of an optically pure seleninic acid as a stable solid, the abso-
lute configuration, and the methods for preparing the optically
active seleninic acid in bulk.7
1a
1b
1c
O
O
Se
OH
Ph Se
OH
1d
1e
Results and Discussion
Recrystallization of Seleninic Acids. Seleninic acids 1a–e
Chart 1.
Published on the web April 8, 2005; DOI 10.1246/bcsj.78.710

Y. Nakashima et al.
Bull. Chem. Soc. Jpn., 78, No. 4 (2005)
711
Stability of Optically Active Seleninic Acid 1a. The cir-
cular dichroism and the IR spectra of optically active seleninic
acid (R)-1a (KBr disk made from a single crystal) were moni-
tored for investigating the thermal stability in the solid state.
No change was observed in the circular dichroism spectrum
ꢁ
or in the IR spectrum after heating the sample at 60 C for 6
days, indicating that neither racemization nor decomposition
of (R)-1a occurred under the given conditions. However, when
ꢁ
the sample was heated at 80 C for 1 day, the intensity of the
Cotton effect in the circular dichroism spectrum decreased to-
gether with a decrease in the intensity of the absorption bands
(3014, 821, and 682 cmꢂ1) of (R)-1a on the IR spectrum, indi-
cating that some decomposition of (R)-1a occurred under the
given conditions. The decomposition was completed after 4
days, and dimethyl diselenide and a small amount of selenium
were formed.
Fig. 1. Circular dichroism spectra of the optical isomers of
seleninic acid 1a in the solid state (KBr disk).
By contrast, no Cotton effect was observed in methanol so-
lution even within 5 s after the dissolution of (R)-1a at room
temperature, indicating that optically active 1a racemized very
rapidly in solution.
Chiral Crystallization of 1a. The chiral crystallization of
1a was attempted in order to obtain the optically active sele-
ninic acid in bulk. If only the seed crystals grew on recrystal-
lization when chiral seed crystals of (R)-1a were used, (R)-1a
would be obtained at higher than 50% yield from the racemic
1a, because 1a racemizes rapidly in the mother liquor and the
ratio of the enantiomers in solution is maintained at approxi-
mately 1:1 during recrystallization.
Table 1. Selected Atomic Distances and Bond Angles of 1a
Bond angle/ꢁ
ꢀ
Atomic distance/A
Se1–O1
Se1–O2
Se1–C1
1.753(2)
1.669(2)
1.914(4)
O1–Se1–O2
O1–Se1–C1
O2–Se1–C1
103.50(12)
93.59(15)
100.79(15)
A chiral single crystal (25 mg) of (S)-1a was pulverized and
seeded in a supersaturated methanol/toluene (1/1) solution of
1a (500 mg), which was prepared by cooling a hot methanol
(1.5 mL) solution of 1a to room temperature and adding tol-
uene (1.5 mL) to it. The mixture was allowed to stand for 1
day at room temperature, and the crystals precipitated were
collected by filtration (355 mg, 68% yield from racemic 1a).
Ten crystals were selected at random from the crystals ob-
tained, and their circular dichroism spectra were measured in
the solid state (KBr disk). As a result, all the 10 crystals
showed negative Cotton effects (S-form), which would suggest
that the absolute configuration of all the crystals obtained in
the pot is S. However, in another recrystallization using (S)-
1a as the seed crystal, 7 crystals showed negative Cotton ef-
fects and 3 showed positive Cotton effects. An additional 11
sets of recrystallization were carried out, and at least 6 of
the 10 crystals from one pot showed Cotton effects of the same
sign as the seed crystals in all cases. However, there were only
3 cases when all 10 crystals from one pot showed the same
sign of the Cotton effect as the seed crystals. The crystals orig-
inating from the surface of the flask may have caused the low
reproducibility. Thus, it is difficult to obtain chiral crystals of
the same configuration selectively by this method.
Fig. 2. Crystal packing of (S)-1a.
lographic analysis of 1a, which showed a negative Cotton ef-
fect, was carried out in order to determine the absolute config-
uration.7 The crystal is orthorhombic and belongs to the chiral
space group P212121. The selected atomic distances and bond
angles of 1a are shown in Table 1, and the crystal packing of
1a is shown in Fig. 2. The two oxygen atoms of the OH and
Se=O groups can be discriminated on the basis of the bond
distances between the selenium atom and the oxygen atoms
ꢀ
{1.753(2) and 1.669(2) A}, and the hydrogen atom of the
OH group was found on the D-map. Thus, the absolute config-
uration of the seleninic acid showing a negative Cotton effect
can be determined to be S.
The molecules of 1a are connected lengthwise via intermo-
lecular hydrogen bonds between the hydrogen atom of OH and
the oxygen atom of Se=O (Fig. 2), whereas the molecules of
seleninic acid 1e are known to form dimers via two strong hy-
drogen bonds in the racemic crystal.8 The distance between the
oxygen of Se=O and the oxygen of OH of the neighboring
Therefore, the preparation of a large single crystal was
attempted in order to obtain a lump of enantiomerically pure
1a. A chiral single crystal of (R)-1a (4 mg) was tied with a
string and suspended in a supersaturated methanol solution
(2 mL) of 1a (1 g), and allowed to stand for 2 h. A crystal that
grew was suspended again in the supersaturated solution that
was prepared by dissolving other precipitates in the mother liq-
uor by heating. Finally, the crystal developed into a large chi-
ꢀ
molecule is 2.548(3) A, which is slightly shorter than that of
9
ꢀ
acetic acid (2.63 A), and is almost the same as that of 1e
8
ꢀ
(2.52 A).

712 Bull. Chem. Soc. Jpn., 78, No. 4 (2005)
Chiral Crystallization of Seleninic Acid
O
Me
S
OH
4
Chart 3.
Fig. 3. A lump of crystal of optically active seleninic acid
(R)-1a obtained by chiral crystallization (753 mg).
HO
OH
OH
OH
Cl
OH
Cl
OH
(2R, 3R)-2
(R)-3
(S)-3
Chart 2.
Fig. 4. Circular dichroism spectra of optical isomers of
methanesulfinic acid (4) in the solid state (neat; powder).
Table 2. Chiral Crystallization of 1a in the Presence of
Chiral Solvents
the optically active alcohol was finally concentrated under
reduced pressure.
Optically active
alcohol
Recovery
yield/%
Absolute configuration of
crystals of 1a obtained
Absolute Configuration of Methanesulfinic Acid. The
crystals of methanesulfinic acid (4) (Chart 3), which is the
sulfur analogue of 1a, have been reported to belong to a chiral
space group.10 However, neither determination of the absolute
configuration nor measurement of the chiroptical property was
successful.11 If the circular dichroism spectra of the chiral
crystals of 4 could be measured, the absolute configurations
of the optical isomers of 4 would be determined on the basis
of the relationship between the absolute configuration and
the circular dichroism spectrum of 1a.
Chiral crystals of 4 were obtained by recrystallization from
ether/hexane. Measurement of the circular dichroism spectrum
of a single crystal was attempted in the solid state (KBr disk).
However, 4 decomposed during the preparation of the KBr
disk, perhaps because it is unstable in air.10 Therefore, the
sample for the measurement was prepared at low temperature
to prevent decomposition. A single crystal of 4 was sand-
wiched between two pieces of quartz glass, cooled at ꢂ196
ꢁC, and pulverized. Then, the circular dichroism spectrum
was measured. The circular dichroism spectrum of one sample
showed a negative Cotton effect at around 240 nm, as shown in
Fig. 4, and the shape of the spectrum was similar to that of (S)-
1a. On the other hand, another crystal showed a positive
Cotton effect in the same region. On the basis of the similarity
in the circular dichroism spectra of 4 and 1a, the absolute con-
figuration of 4 that showed a negative Cotton effect was deter-
mined to be S and that of the crystal with a positive Cotton ef-
fect was determined to be R.
(2R,3R)-2
(R)-3
(S)-3
65
65
70
R
R
S
ral single crystal of enantiomerically pure (R)-1a. The optical
purity of the crystal was confirmed by measuring the circular
dichroism spectra (KBr disk) of several crushed pieces of the
crystal. As a result, 753 mg of (R)-1a was obtained as a single
crystal in 75% yield from racemic 1a (Fig. 3). In a similar
manner, 701 mg of (S)-1a was obtained as a single crystal in
69% yield by using a seed crystal of (S)-1a (11 mg).
Another method for the chiral crystallization of 1a, which
uses not seed crystals but chiral solvents, was examined. Opti-
cally active alcohol (2R,3R)-2, (R)-3, or (S)-3 (Chart 2) (0.2–
0.5 mL) was added to a methanol solution (0.5 mL) of 1a
(200–300 mg) and the solution was allowed to stand for a
few days. It was confirmed from the IR spectra that the depos-
ited crystals were not mixed crystals that included the chiral
solvents but the resulting crystals of 1a. Ten crystals were se-
lected at random from the crystals obtained in one pot, and the
absolute configurations were checked by measuring the circu-
lar dichroism spectra (KBr disk). The results are shown in
Table 2. In the case of adding (2R,3R)-2, the yield was 65%
from racemic 1a and all of the 10 crystals were R-form. In
the case of 3, only the crystals of (R)-1a were obtained by
recrystallization in methanol/(R)-3, and (S)-1a was obtained
selectively by recrystallization from methanol/(S)-3 at higher
than 50% yield. The optically active alcohols can be recovered
after recrystallization. In fact, 92% of (R)-3 can be recovered
from the mother liquor with ease according to the following
procedure: ether was added to the mother liquor and precipitat-
ed crystals of 1a were filtered off, and the filtrate containing
Conclusion
The optical isomers of methaneseleninic acid (1a) were iso-
lated as stable crystals and the relationship between the abso-
lute configuration and the circular dichroism spectra of 1a was
determined by X-ray crystallographic analysis. Optically ac-

Y. Nakashima et al.
Bull. Chem. Soc. Jpn., 78, No. 4 (2005)
713
dd, J ¼ 12:2, 4.75 Hz), 4.84 (1H, s); 13C NMR (125 MHz,
CD3OD) ꢀ 23.5, 25.1, 66.1; 77Se NMR (95 MHz, CD3OD) ꢀ
1314; MS (EI, 70 eV) m=z 171 (Mþ þ 1, 80Se), 169 (Mþ þ 1,
78Se), 154, 152; IR (KBr) 2960, 2900 (br), 2380 (br), 1680 (br),
tive 1a was found to be stable in the solid state at 60 ꢁC,
whereas it decomposed at 80 C. Optically active 1a was also
ꢁ
found to racemize very rapidly in solution. Two methods for
obtaining each enantiomer of 1a in bulk by chiral crystalliza-
tion were developed by taking advantage of the rapid racemi-
zation in solution. One method involves the use of a seed crys-
tal; a chiral seed crystal of 1a tied with a string developed into
a large chiral single crystal by recrystallization. The other
method involves the use of chiral solvents; optically active
1a was obtained in bulk by recrystallization in the presence
of chiral solvent at higher than 50% yield from racemic 1a.
The absolute configuration of optically active methanesulfinic
acid (4) was clarified by comparing its circular dichroism spec-
tra with those of 1a.
1460, 1319, 844 (Se=O), 669 cmꢂ1
.
2,ꢁ2-Dimethylpropane-1-seleninic Acid (1c). Yield 23%; mp
1
123 C (colorless film from acetonitrile; decomp.); H NMR (500
MHz, CD3OD) ꢀ 1.16 (9H, s), 2.85 (1H, d, J ¼ 12:1 Hz), 3.20
(1H, d, J ¼ 12:1 Hz), 4.82 (1H, s); 13C NMR (125 MHz, CD3OD)
ꢀ 30.7, 32.0, 72.8; 77Se NMR (95 MHz, CD3OD) ꢀ 1316; MS (EI,
70 eV) m=z 185 (Mþ þ 1, 80Se), 183 (Mþ þ 1, 78Se), 168
(Mþ ꢂ O, 80Se), 166 (Mþ ꢂ O, 78Se), 159, 157; IR (KBr) 2963,
2850 (br), 2410 (br), 1680 (br), 1467, 1364, 1322, 842 (Se=O),
670 cmꢂ1; Anal. Calcd for C5H12O2Se: C, 32.80; H, 6.61%.
Found: C, 32.69; H, 6.61%.
ꢁ
Cyclohexaneseleninic Acid (1d). Yield 64%; mp 77–79 C
Experimental
(colorless film from acetonitrile); 1H NMR (500 MHz, CD3OD)
ꢀ 1.25–1.50 (3H, m), 1.55–1.80 (3H, m), 1.81 (2H, dd,
J ¼ 9:50, 5.20 Hz), 2.06 (2H, dd, J ¼ 13:1, 3.95 Hz), 3.03 (1H,
m), 4.84 (1H, s); 13C NMR (125 MHz, CD3OD) ꢀ 25.7, 25.9,
26.9, 27.1, 27.2, 66.8; 77Se NMR (95 MHz, CD3OD) ꢀ 1322;
MS (EI, 70 eV) m=z 180 (Mþ ꢂ O, 80Se), 178 (Mþ ꢂ O, 78Se);
IR (KBr) 2927, 2900 (br), 2450 (br), 1446, 1292, 814 (Se=O),
658 cmꢂ1; Anal. Calcd for C6H12O2Se: C, 36.93; H, 6.20%.
Found: C, 36.60; H, 5.89%.
General. All solvents used in recrystallization were of prac-
tical grade. Melting points were determined on a Yamato
MP-21 Melting point apparatus. IR spectra were measured on a
PERKIN ELMER Spectrum GX FT-IR system. 1H, 13C, and
77Se NMR spectra were recorded on a JEOL JNM-EX-500 FT
NMR System. Mass spectra (MS) were determined on a JEOL
JMS-GCMATE System. Elemental analysis was performed using
a PERKIN-ELMER 240-C. Circular dichroism spectra were mea-
sured on a JASCO J-725 Spectropolarimeter. Methaneseleninic
acid (1a) was prepared by oxidation of dimethyl diselenide, ac-
cording to the literature.12 Benzeneseleninic acid (1e) is commer-
cially available.
Procedure for Measurement of Circular Dichroism Spectra
in Solid State. KBr disk: A mixture of a single crystal of 1a (ca.
5 mg) and 70 mg of KBr was ground and formed into a disk with a
radius of 6.5 mm. Neat: A single crystal of 4 was pinched between
a pair of pieces of quartz glass and this combination was cooled by
liquid nitrogen for a few seconds. Rubbing the glasses together
gave powder of 4, and which was used for measurement of circu-
lar dichroism spectrum.
Typical Procedure for Chiral Crystallization of 1a. Method
A: Compound 1a (500 mg) was dissolved in hot methanol (1.5
mL) and the solution was cooled slowly to room temperature,
and then toluene (1.5 mL) was added. The powder of (S)-1a, pre-
pared by pulverizing the single crystal, was scattered into the so-
lution and the mixture was allowed to stand for 1 day. The crystals
that precipitated (68–99%) were filtered and dried in air.
Method B: Compound 1a (1.00 g) was dissolved in hot metha-
nol (2 mL) and the solution was cooled slowly to room tempera-
ture. A single crystal of (S)-1a (ꢄ10 mg) tied with string was hung
in the solution. The crystal that grew was pulled up and then hung
again in a solution that was prepared by dissolution of other pre-
cipitates in the mother liquor by heating. Repetition of this proce-
dure afforded a lump of enantiomerically pure (S)-1a.
Methaneseleninic Acid (1a; Racemate). Mp 124–125 ꢁC
1
(colorless prisms from methanol); H NMR (500 MHz, CD3OD)
ꢀ 2.68 (3H, s), 4.86 (1H, s); 13C NMR (125 MHz, CD3OD) ꢀ
41.3; 77Se NMR (95 MHz, CD3OD) ꢀ 1294; MS (EI, 70 eV)
m=z 128 (Mþ, 80Se), 126 (Mþ, 78Se), 111 (Mþ ꢂ OH, 80Se),
109 (Mþ ꢂ OH, 78Se); IR (KBr) 3014, 2800 (br), 2350 (br),
1750 (br), 988, 821 (Se=O), 682 cmꢂ1; UV (2-propanol) ꢁmax
230 (" 1:09 ꢃ 103), 200 (" 2:44 ꢃ 103) nm.
(S)-1a. Mp 128–130 ꢁC (colorless prisms from methanol/
toluene); IR (KBr) 3014, 2800 (br), 2350 (br), 1750 (br), 989,
820 (Se=O), 681 cmꢂ1
.
(R)-1a. Mp 128–130 ꢁC (colorless prisms from methanol/
toluene); IR (KBr) 3014, 2800 (br), 2350 (br), 1750 (br), 990,
820 (Se=O), 680 cmꢂ1
.
General Procedure for Preparation of Seleninic Acids 1b–d.
To a DMF (20 mL) solution of sodium diselenide (10 mmol) pre-
pared according to the literature,13 alkyl bromide (20–25 mmol)
was added slowly, and the mixture was stirred until the color of
solution changed from violet into green or yellow. To the mixture
was added water (20 mL); this combination was extracted with di-
chloromethane (40 mL ꢃ 2); the organic components was washed
with 6 M hydrochloric acid (60 mL) and water (50 mL ꢃ 2), and
dried over anhydrous magnesium sulfate. Evaporation of the sol-
vent afforded crude diselenide, which was dissolved in dichloro-
methane (50 mL); ozone was then bubbled into the solution at
ꢂ40 ꢁC. After disappearance of color for diselenide, water (10
mL) was added to the solution, and the mixture was stirred vigo-
rously for 1 h at room temperature. Evaporation of solvent and re-
crystallization of the residue afforded seleninic acid.
Method C: Compound 1a (200–300 mg) was dissolved in hot
methanol (0.5 mL) and the solution was cooled slowly to room
temperature. Optically active alcohol (0.2–0.5 mL) was added to
the solution, and the mixture was allowed to stand for a few days.
The crystals that deposited were filtered and dried in air.
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714 Bull. Chem. Soc. Jpn., 78, No. 4 (2005)
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7
Some of our preliminary results in this area have been