5528
J . Org. Chem. 2000, 65, 5528-5530
Oxid a tion of Meth yl Tr im eth ylsilyl Keten e Aceta ls to
r-Hyd r oxyester s w ith Ur ea Hyd r ogen P er oxid e Ca ta lyzed by
Meth yltr ioxor h en iu m
Sasˇa Stankovic´ and J ames H. Espenson*
Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 50011
espenson@ameslab.gov
Received February 15, 2000
In the presence of catalytic amounts of MTO, methyltrioxorhenium, methyl trimethylsilyl ketene
acetals are oxidized with urea hydrogen peroxide to afford R-hydroxy and R-siloxy esters. On
treatment with potassium fluoride, the R-hydroxy esters are obtained in high yields.
In tr od u ction
active forms of the catalyst are the monoperoxo and
diperoxo complexes formed in reversible equilibria, eq 1.
Lead(IV) carboxylates,1 hypervalent iodine,2 m-chloro-
perbenzoic acid (m-CPBA),3 and dimethyldioxirane (DM-
DO)4 can be used to oxidize esters via their ketene acetals
to R-hydroxy carbonyl compounds. Catalytic reagents are
MnII(salen) complexes with various oxidants5 and cobalt6
or nickel(II)7 complexes with oxygen. Few reports of
hydrogen peroxide as the oxidant have appeared,8 pre-
sumably owing to the hydrolytic instability of ketene
acetals. The R-hydroxy esters are synthetic intermediates
of widespread use through the independent manipulation
of the hydroxy and ester groups. For example, they are
used in the preparation of esters and acids functionalized
at the 2-position, like 2-oxo esters and acids.9 R-Hydroxy
esters are also used in the synthesis of certain natural
products.10 We are unaware of any major application that
relies on the presence of the two functional groups; the
uses are those of the independent alcohol and ester
groups.
Water-labile silyl enol ethers form R-hydroxy ketones
with aqueous hydrogen peroxide and the MTO catalyst
in acetonitrile.16 We sought to extend this methodology
to ketene acetals, which are even more hydrolytically
sensitive owing to the presence of an additional alkoxy
functionality,17,18 because peroxide is such a convenient
laboratory reagent. For our study, we selected methyl-
trimethylsilyl ketene acetals. Herein, we report the
effectiveness of an optimized procedure that relies upon
the anhydrous material urea-hydrogen peroxide, UHP,
and quite importantly, the presence of pyridine.
Methyltrioxorhenium (CH3ReO3, abbreviated as MTO)
is a well-established catalyst for the reactions of hydrogen
peroxide,1,11 including the epoxidation of alkenes.12-15 The
Exp er im en ta l Section
Rea gen ts. The ketene acetals were prepared from the
parent esters and trimethylsilyl chloride using a published
procedure.19 The esters were purchased and used as such
except for methyl 2-phenylpropanoate, which was obtained
from 2-phenylpropanoic acid (10 g, 67 mmol) upon refluxing
for 7 days in methanol (50 mL in the presence of catalytic
amount of p-toluenesulfonic acid (0.6 g, 3.3 mmol). After
completion, the reaction mixture was dissolved in ether,
washed with saturated sodium bicarbonate solution, and dried
over anhydrous sodium sulfate. Solvent evaporation followed
by distillation afforded 8.64 g of methyl 2-phenylpropanoate.
Oxid a tion of Keten e Aceta ls. The ketene acetal (2.5
mmol) was introduced dropwise over 5 min into a cooled
mixture (0 °C) of UHP (0.35 g, 3.75 mmol), pyridine (0.05 g,
0.625 mmol), and MTO (0.031 g, 0.125 mmol) in 99:1 aceto-
nitrile/acetic acid (5 mL). After being stirred for an additional
5 min at room temperature, the reaction mixture was treated
with a minimal amount of saturated sodium bicarbonate
solution to neutralize acetic acid and destroy the catalyst. The
mixture was then dissolved in dichloromethane and the
organic layer separated and dried. After filtration and solvent
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10.1021/jo000212e CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/10/2000