Table 4 Reusability of OsO4 and (DHQD)2PHAL in PEG
To substantiate our results and check versatility, other olefins
were subjected to dihydroxylation. For instance, trans-stilbene
(entry 3) produced 1,2-diphenyl-1,2-ethanediol in 95% yield.
Electron deficient olefins viz., trans-ethyl cinnamate (entry 5),
ethyl-4-methoxy cinnamate (entry 6) and the conjugated ester
obtained on Wittig olefination between Garner aldehyde and
(carboethoxymethylene)triphenylphosphorane (entry 10) pro-
duced diols in 93, 95 and 92% yield with ease. Furanose
derivative (entry 11) yielded the hexose sugar in 88% yield and
greater than 90% de. To check the reusability of OsO4, styrene
was subjected to dihydroxylation after each extraction with
ether and we found that even after the fifth run we could isolate
diol in 90% yield (Table 2)! This experiment proved to us
beyond doubt that OsO4 is never extracted into ether and
succesfully recycled for dihydroxylation. Attempts to identify
osmium tetroxide in the ether layer by atomic absorption
spectroscopy (AAS) showed the presence of osmium to an
extent of less than 2 ppm.
Run
Time/h
Yield (%)
ee (%)
1
2
3
4
5
2
2
2
2
2
95
91
92
92
91
94
92 (81)a
95 (83)a
92 (80)a
90 (65)a
a The ee in parentheses is for the run without additional ligand added.
h, the product was separated by ether extraction and fresh trans-
stilbene was added. To our great surprise, in the second run diol
was obtained with more than 80% ee, which indicates that
ligand loss during the extraction was minimal. Further addition
of 0.5 mol% ligand helped us to obtain diol with more than 95%
ee. Fourth and fifth runs with additional ligand (0.25 mol%) and
no addition of OsO4 consistently furnished products in over
90% yield and ee.
In summary, we have developed for the first time, PEG as a
natural encapsulation medium for OsO4 and also for ligand to
some extent. Application of other metal catalyzed reactions is
currently pursued.
Table 2 Dihydroxylation of styrene using OsO4 in PEG (recycling
studies)
Run
1
2
3
4
5
Isolated yield (%)
94
94
92
93
90
IICT Communication no. 030120. C. N., S. S. S. and N. R. K.
thank CSIR, New Delhi for financial support.
Encouraged by these promising results, we then attempted
the asymmetric dihydroxylation of olefins according to the
Sharpless procedure. We chose trans-stilbene as an initial
substrate (entry 1, Table 3) and subjected it to dihydroxylation
as follows. To (DHQD)2PHAL (2 mol%) and OsO4 (0.5 mol%)
trans-stilbene (2 mmol) in PEG (2 g) was added and stirred for
2 h. The reaction mixture was extracted with ether. The ether
layer was washed with 10% HCl and then with water and brine.
The ether layer yielded 1,2-diphenyl-1,2-ethanediol in 94% ee
and 95% chemical yield. Conventionally, the reaction required
longer reaction hours (up to 24 h) and addition of 1 eq. of
methane sulfonamide for dihydroxylation of internal double
bonds. Interestingly the present protocol is very rapid and no
additional reagents like methane sulfonamide were required.
Similarly three more experiments were carried out on olefins,
namely ethyl cinnamate (entry 2, Table 3), ethyl-4-methoxy
cinnamate (entry 3, Table 3) and 1-tetradecene (entry 4, Table
3). With the exception of 1-tetradecene, the chemical and
optical purities were all very high. Again to check the
recyclability of OsO4 and possibly ligand as well trans-stilbene
was subjected to asymmetric dihydroxylation (Table 4). After 2
Notes and references
§ Typical experimental procedure: To styrene (208 mg, 2 mmol) in
poly(ethylene glycol) (400 MW, 2 g) was added OsO4 (2.5 mg, 0.5 mol%)
and NMO·H2O (351 mg, 2.6 mmol) under an inert atmosphere and stirred
at ambient temperature (25 °C). The reaction mixture was diluted with ether
(5 mL) and stirred for five minutes. The ether layer was decanted and the
procedure repeated (4 3 5 mL). The combined ether layer was washed with
water and brine then dried over Na2SO4. Volatiles were removed on a rotary
evaporator and subsequent column chromatography (30% EtOAc in
hexane) gave styrene diol (260 mg) in 94% yield. The PEG layer was reused
for dihydroxylation.
1 M. Schroder, Chem. Rev., 1980, 80, 187.
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Table 3 Asymmetric dihydroxylation using (DHQD)2PHAL, OsO4 and
NMO·H2O in PEG
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Product
Time/h
Yield (%) ee (%)a
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1
2
95
92
94
91
2
2
3
4
2
2
94
96
96
44
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a The products ee was determined by chiral GC analysis (Cyclosil-B).
CHEM. COMMUN., 2003, 1716–1717
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