Chemistry Letters 2001
227
The reaction mixture at 72 h contained 8% of sulfide, 54% of
sulfoxides with 30% ee, and 37% of sulfone under the present
reaction conditions. Asymmetric oxidation proceeded with (R)-
isomer excess. The recycle use of Ti-M41 (7 times) confirmed
that the present catalyst could be used repeatedly without any
degradation.
The extent of enantiomeric excess gradually increased with
the reaction time. Racemic mixture of methyl 4-methylphenyl
sulfoxides was here oxidized under the same conditions. (S)-
sulfoxide was consumed more rapidly than (R)-sulfoxide and
the degree of enantiomeric excess reached to 43% ee at 72 h. A
homogeneous catalyst10 has been reported to show similar
activity for the kinetic resolution of sulfoxides.
The kinetics of the asymmetric oxidation has been numeri-
cally analyzed by using following equations.
–d[sulfide]/dt = (k1R+k1S)[sulfide][H2O2]
geneous asymmetric oxidations.5,6,11 These would be attributa-
ble to the difference between the active sites on the surface and
in the solution: It is the future problem to be solved.
The present results conclude that the combination of Ti-
M41, optically active tartaric acid, and H2O2 is essential to the
heterogeneous asymmetric oxidation.
d[(R)-sulfoxide]/dt = k1R[sulfide][H2O2] – k2R[(R)-sulfoxide][H2O2]
d[(S)-sulfoxide]/dt = k1S[sulfide][H2O2] – k2S[(S)-sulfoxide][H2O2]
d[sulfone]/dt = k2R[(R)-sulfoxide][H2O2] + k2S[(S)-sulfoxide][H2O2]
where k1R and k1S are the rate constants of the formations of
(R)- and (S)-sulfoxide, and k2R and k2S of the formations of sul-
fone from (R)- and (S)-sulfoxide, respectively. The ratio of
k2R/k2S was determined to be 0.59 by using the data of the oxi-
dation of racemic methyl 4-methylphenyl sulfoxides. Then the
rate constants were calculated on the basis of the experimental
This work was partly supported by the Grant-in-Aid from
the Ministry of Education, Science, Culture, and Sports of
Japan.
data of Figure 1. The best-fitted results were obtained at k1R
=
References and Notes
0.19, k1S = 0.13, k2R = 0.083, and k2S = 0.14 dm3 mol–1 h–1. The
ratio of k1R/k1S was 1.5. It follows that first the present asym-
metric oxidation is composed of the asymmetric induction to
sulfoxide and the subsequent kinetic resolution of sulfoxides,
and secondly both of the reactions proceed to yield the (R)-iso-
mer preferentially.
The possibility of the homogeneous catalysis resulting
from the dissolved Ti ions has been studied strictly. First, the
filtrate after the first catalytic reaction did not show any activity
for the further oxidation reaction. Secondly, the Ti contents in
the filtrates were analyzed by a polarized Zeeman atomic
absorption spectroscopy and were below the limitation of detec-
tion (ppb order). Thirdly, the Si/Ti ratios of Ti-M41s recovered
after the reaction were in good agreement with those of fresh
Ti-M41s within experimental errors. It can be concluded that
there is no dissolved Ti ion in the solution and the observed
asymmetric oxidation apparently proceeds on the surface of Ti-
M41.
In Table 1, the chemical and optical yields of sulfoxides
were compared under several reaction conditions. Entry 2
revealed that the catalysis indeed proceeds on Ti-M41. Without
(R,R)-tartaric acid, there was no asymmetric oxidation (Entry
3). (S,S)-Tartaric acid gave almost the same results as those of
(R,R)-tartaric acid with counter stereoselectivity (Entry 4). This
clearly shows that the stereochemistry of the chiral ligands
determines that of sulfoxide produced. On the other hand, nei-
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that of H2O2 and diethyl (R,R)-tartarate (Entry 6) gave high
enantioselectivity. These are surprising in view of the fact that
TBHP or diethyl (R,R)-tartarate are effective reagents in homo-
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