on the IR (KBr) spectrum. These results may be due to
intermolecular interactions among tellurinic acids both in
solution and in the solid state.
When tellurinic acid 1 was subjected to chromatography
on two types of chiral column (4.6 mm × 250 mm), one
packed with amylose carbamate derivative-silica gel and the
other with cellulose carbamate derivative-silica gel, using
hexane/2-propanol as the eluent, the chromatograms showed
only one peak. However, we surmised that the ratio of the
enantiomers differs between the former portion and the latter
portion of the peak. Then tellurinic acid 1 was subjected to
chromatography on a larger column (10 mm × 250 mm)
that was packed with cellulose carbamate derivative-silica
gel using hexane as the eluent, and the eluates were divided
into several fractions. The fraction corresponding to the
first half of the peak showed a positive specific rotation
Figure 2. First-order rate plots for the racemization of optically
active tellurinic acid (+)-1 (ca. 0.02 mM): (a) in hexane/2-propanol
(
2 2
99/1); (b) in 2-propanol/H O (4/1); (c) in 2-propanol/D O (4/1).
2
8
3
{
[R] 2.5 × 10 (c 0.0012, hexane)}, as well as a positive
lives are 98.2 and 5.34 min, respectively. The rate constant
4
35
first Cotton effect and a negative second Cotton effect at
70 and 238 nm (Figure 1), respectively, on the circular
in 2-propanol/H O is much larger than that in hexane/2-
2
2
propanol, indicating that a small amount of water in the
distilled 2-propanol may have caused the racemization of
the optically active tellurinic acid. Two mechanisms of the
racemization in which water participates are proposed. One
involves the formation of an achiral tellurane by the addition
of water to tellurinic acid, and the other involves the
formation of an achiral tellurinate anion by deprotonation
of tellurinic acid by water. When racemic tellurinic acid 1
1
8
18
was dissolved in 2-propanol/H
2
O (4/1, 95 atom % O) and
1
6
allowed to stand for 2 h, the ratio of 2,4,6-i-Pr
3 6 2 3 6 2 2
,4,6-i-Pr C H Te O OH:2,4,6-i-Pr C H Te O H was 5:3:5
3
18
C H
6 2
Te O
2
H:
16
18
2
based on the peak intensities on the MS spectrum, meaning
that the oxygen atoms of the tellurinic acid were exchanged
via an achiral tellurane formed by the addition of water. The
rate constant for the racemization was also measured in
Figure 1. Circular dichroism spectrum of (+)-1 in hexane.
2
2
-propanol/D O (4/1) (Figure 2c). The rate constant (7.87
-4 -1
×
10 s ) is approximately one-third of that in 2-propanol/
O, indicating that there is a primary kinetic isotope effect
H
2
dichroism spectrum, whereas the following fractions showed
no Cotton effect. In the case of optically active seleninic
acids, concentration of the eluates caused complete racem-
of the racemization and the rate-controlling step is the
protonation to tellurinic acid. Vertex inversion and edge
inversion are also first-order mechanisms for the racemization
of tricoordinated optically active chalcogen compounds.5
However, the barriers for vertex inversion and edge inversion
of benzenetellurinic acid were estimated to be 82 and 26
1
,2
ization. By contrast, concentration of the hexane solution
of (+)-1 did not reduce the molar ellipticity, indicating that
no racemization of the optically active tellurinic acid took
place by concentrating the eluate under reduced pressure.
To our knowledge, this is the first report of the isolation of
an optically active tellurinic acid, although the enantiomeric
excess was not determined.
-
1
6
kcal mol , respectively, by MO calculations (MP2 /
7
8
LANL2DZ ) using the Gaussian 98 program, which are too
(5) (a) Andersen, K. K.; Folly, J. W.; Perkins, T. I.; Gaffield, W.;
The change of the circular dichroism spectrum of (+)-1
in various solvents was monitored at room temperature to
investigate the stability toward racemization of (+)-1. No
change was observed in the ellipticity at 270 nm on the
circular dichroism spectrum of (+)-1 in hexane even after 3
days, indicating (+)-1 is stable toward racemization under
the given conditions. However, the ellipticity decreased with
time in hexane/2-propanol (99/1) and showed a good linear
relationship in the first-order rate plots (Figure 2a). Race-
Papanikolau, N. E. J. Am. Chem. Soc. 1964, 86, 5637. (b) Mislow, K.;
Green, M. M.; Laur, P.; Melillo, J. T.; Simmons, T.; Ternary, A. L., Jr. J.
Am. Chem. Soc. 1965, 87, 1958. (c) Zhao, S.; Samuel, O.; Kagan, H. B.
Tetrahedron 1987, 43, 5135. (d) Darwish, D.; Tomilson, R. L. J. Am. Chem.
Soc. 1968, 90, 5938. (e) Campbell, S. J.; Darwish, D. Can. J. Chem. 1974,
52, 2953. (f) Moriyama, M.; Oae, S.; Numata, T.; Furukawa, N. Chem.
Ind. 1976, 163. (g) Andersen, K. K.; Caret, R. L.; Ladd, D. L. J. Org.
Chem. 1976, 41, 3096. (h) The Chemistry of the Sulphonium Group; Stirling,
C. J., Patai, S., Eds.; Wiley: New York, 1981. (i) Day, J.; Cram, D. J. J.
Am. Chem. Soc. 1965, 87, 4398. (j) Moriyama, M.; Yoshimura, T.;
Furukawa, N.; Numata, T.; Oae, S. Tetrahedron 1976, 32, 3003. (k) Shimizu,
T.; Matsuhisa, A.; Kamigata, N.; Ikuta, S. J. Chem. Soc., Perkin Trans. 1
1
1
2
995, 1805. (l) Shimizu, T.; Kamigata, N. ReV. Heteroat. Chem. 1998, 18,
1. (m) Shimizu, T.; Kamigata, N.; Ikuta, S. J. Chem. Soc., Perkin Trans.
1999, 1469.
mization of (+)-1 was also observed in 2-propanol/H
2
O
(Figure 2b). The first-order rate constants for the racemization
(6) (a) Moeller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618. (b) Hehre,
of (+)-1 in hexane/2-propanol (99/1) and in 2-propanol/H
2
O
W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecular Orbital
Theory; Wiley: New York, 1986; Chapter 2.9.3.
-
4
-3 -1
(4/1) are 1.18 × 10 and 2.16 × 10 s , and the half-
2576
Org. Lett., Vol. 6, No. 15, 2004
Nakashima, Yusuke
