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mmol) and in THF (3.0 mL) was refluxed for 10 min and then H2O (0.04
mL) was added. The resulting mixture was stirred for 10 min, followed by
addition of K2CO3 (0.2760 g, 2.0 mmol). After being stirred for 1 min, to
this suspension was added a mixture of aldehyde (1.0 mmol), tert-butyl
bromoacetate (0.12 mL, 0.8 mmol) and water (0.03 mL) in THF (1.0 mL)
in portions in 3.5 h. The reaction was quenched by anhydrous MgSO4 after
the reaction was complete (monitored by TLC). The resulting mixture was
filtered rapidly through a glass funnel with a thin layer of silica gel and
washed with ethyl acetate. The combined filtrate was concentrated and the
residue was purified by flash column chromatography to afford the desired
product.
was used as the cocatalyst. Both aromatic aldehydes and
aliphatic aldehydes could react with tert-butyl bromoacetate to
afford the corresponding products in high yields with high
stereoselectivity (enties 2, 4, 6, 8, 10, 12, 14, 16 and 18) in the
presence of sodium bisulfite. It was noteworthy that phthalalde-
hyde could react with bromoacetate under these conditions and
both aldehyde groups could be olefinated in high yield (entry 19
in Table 1). g,d-Epoxy-a,b-unsaturated ester, a useful building
block, could also be synthesized by the current method in
moderate yield with excellent stereoselectivity (entry 20 in
Table 1).
This modification simplified the purification greatly and
realized a phosphorus-free catalytic reaction. One could get the
product just by filtering off the solid inorganic compounds and
precipitating the catalyst after the reaction was completed.
Further study showed that a,b-unsaturated esters can be
mass-produced under these reaction conditions and the catalyst
could be recovered in quantity but lost its activity partially
through multiple cycles. 2-Furaldehyde (10.85 mmol) was
reacted with tert-butyl bromoacetate in the presence of sodium
bisulfite and the desired product was obtained in 90% yield
when 2 mol% of PEG-telluride was used. The catalyst was
recovered in 100% yield by filtering off the solid of the reaction
mixture, followed by addition of ether and collection of the
precipitate. The recovered catalyst could be used in the second
run but only 69% yield was obtained probably due to the
decomposition of PEG-telluride during the catalytic olefina-
tion.12
Despite the advantages of easy separation and purification of
products, the uses of polymer-supported catalysts suffered from
lowered catalytic activity and stereoselectivity due to the
restriction of polymer matrix that resulted in limited mobility
and the accessibility of the active site. The PEG-telluride for
catalytic ylide olefination reported here represents a novel,
highly efficient polymer-supported catalyst that shows higher
catalytic activity as compared to the free corresponding
catalysts. Thus, we have developed an effective catalytic ylide
olefination, which involves a simple procedure, mild reaction
conditions, the use of catalytic PEG-telluride, and in particular,
the use of sodium bisulfite as cocatalyst. The high catalytic
efficiency, together with the ability to easily purify the product,
demonstrates our method to be practical for the synthesis of a,b-
unsaturated esters. The extension of our method to other
olefination, epoxidation, cyclopropanation and aziridinations is
in progress in our laboratory.
1 S. D. Burke and R. L. Danheiser, Oxidizing and Reducing Agents, in
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This research was supported by ‘Hundred Scientist Program’
from Chinese Academy of Sciences and State Key Project of
Basic Research (PROJECT 973, No.2000 48007).
Notes and references
11 PEG-supported telluride 2 was prepared readily from PEG 4,000 (MW
Av. 3,000) in two steps by the sequential tosylation of PEG and the
substitution of tosylate 1 with lithium butyltelluride in high yield as
shown in the following scheme.
† Typical procedure for the synthesis of a,b-unsaturated esters. A,
P(OPh)3 as the cocatalyst: a mixture of catalyst 2 (0.0680 g, 2 mol%), ethyl
bromoacetate (0.06 mL, 0.5 mmol), P(OPh)3 (0.36 mL, 1.4 mmol) in
toluene (3.0 mL) was stirred at 80 oC for 10 min and then K2CO3 (0.1796
g, 1.3 mmol) was added. The resulting suspension was stirred for 1 min,
followed by addition of a mixture of aldehyde (1.0 mmol) and ethyl
bromoacetate (0. 1 mL, 0.90 mmol) in toluene (1.0 mL) in portions in 3.5 h.
After the reaction was completed (monitored by TLC), the mixture was
filtered rapidly through a glass funnel with a thin layer of silica gel and
washed with ethyl acetate. The filtrate was concentrated and the residue was
purified by flash column chromatography to afford the desired product.
B, NaHSO3 as the cocatalyst: a mixture of catalyst 2 (0.0680 g, 2 mol%),
tert-butyl bromoacetate (0.06 mL, 0.4 mmol), NaHSO3 (0.1664 g, 1.6
12 M. D. Detty, J. Org. Chem., 1980, 45, 560; H. D. K. Drew, J. Chem.
Soc., 1929, 560.
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