1366
S. Kim et al. / Tetrahedron Letters 52 (2011) 1363–1367
Figure 3. Schematic diagram of recycling of osmium catalyst using IPA as a reducing agent.
Finally, to test the substrate scope of the optimal recycling
conditions, a repetitive oxidative cleavage of various olefins was
run with recycled catalyst upto six times and the results are shown
in Table 518 Oxidation of trans-stilbene, styrene, and trans-b-meth-
ylstyrene proceeded smoothly to provide aldehydes upon the 6th
recycling (entries 1, 2, and 4). However, recycling reactions of
Table 5
Substrate scope under the optimal recycling conditions
a-methylstyrene, allyloxybenzene, and allylbenzene showed
somewhat decreased yields upon recycling (entries 3, 5, and 6).
With cyclic olefins, such as cyclohexene, 1-phenylcyclohexene,
and norbonene, desired aldehydes were obtained in only moderate
yields (entries 7–9). The low yields of aldehydes in these cases are
presumably due to slow conversion of diols to aldehydes.
In summary, we have developed a new chemoentrapment strat-
egy for recycling osmium in the catalytic olefin cleavage reaction.
IPA–KOH treated aqueous layer was able to successfully trap os-
mium catalyst as a water soluble form, and allowed for an efficient
recycling of osmium in the oxidative cleavage reactions of olefins
with 1 mol % of OsO4 without any significant side reactions in good
yields. Among several solvent systems examined, acetonitrile–
water pair worked the best for efficient recycling reactions. Mono-
and di-substituted olefins are the best substrates for this recyclable
oxidative cleavage.
Entry
Olefin
Yielda (%)
1st
2nd
3rd
4th
5th
6th
1
2
3
4
5
6
7
8
9
trans-Stilbene
99
99
77
99
99
55
74
40
40
99
99
50
98
99
90
47
70
51
93
99
27
98
88
75
42
70
48
99
99
40
86
85
75
41
67
43
99
99
57
92
74
65
39
61
19
98
78
42
88
66
47
26
51
23
Styreneb
a-Methylstyrene
trans-b-Methyl-styrene
Allyloxybenzene
Allylbenzene
1-Phenyl cyclohexene
Cyclohexene
Norbonylene
a
Yields of isolated products.
Yields were determined with HPLC integration with mesitylene as the internal
standard.
b
Acknowledgment
concentration of NaIO4. It was observed that at least 0.5 equiv of
NaIO4 is needed to produce aldehyde in high yields, and no in-
crease of yield was observed even with excess NaClO2 (entries 9–
12). Therefore, we chose a combinaiton of 0.5 equiv of NaIO4 and
2 equiv of NaClO2 (entry 10) for optimal olefin cleavage conditions
in t-BuOH and water.
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2009-
0073645).
With the optimal combination of NaIO4 and NaClO2 for an oxi-
dant pair, we applied the reaction conditions to recycling experi-
ments in tert-BuOH/H2O system with IPA–KOH protocol.
Oxidation judged by the disappearance of stilbene was maintained
upon recycling, however, yield of benzaldehyde decreased notice-
ably from the second cycle leaving some diol products (Table 4, en-
try 1), which may be due to slow oxidation rate. In order to speed
up the oxidation, we invesitagted the influence of solvents on the
reactivity. In the case of aqueous t-BuOH, THF or dioxane system,
although stilbene disappeared completely in 14 h until the 6th cy-
cle, products in each cycle contained about 20% of diol (entries 1–
3). We then examined various solvent–water combinations and
found that reaction in acetonitrile–water pair exhibited the most
optimal reactivity showing clean oxidation to aldehydes in 14 h
upto the 6th recycle (entry 4). The tri-solvent system, CCl4–
CH3CN–water often employed in ruthenium-mediated oxidation17
provided no significant advantage over the CH3CN–water system
(entry 5). When the reaction was run in water only, only a minimal
amount of oxidation product was formed (entry 6).
References and notes
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A
diagram showing the recycling process using all the
optimized reaction conditions is shown in Figure 3.