Facile Oxidation of Silyl Enol Ethers with Hydrogen Peroxide Catalyzed by Methyltrioxorhenium

Full text of DOI:10.1021/jo972315b

J . Org. Chem. 1998, 63, 4129-4130
4129
Ta ble 1. Syn th esis of r-Hyd r oxy Ca r bon yl Com p ou n d s
F a cile Oxid a tion of Silyl En ol Eth er s w ith
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
Received December 29, 1997
In the presence of catalytic amounts of MTO, silyl enol
ethers are oxidized with hydrogen peroxide to afford
R-hydroxy and R-siloxy ketones. On treatment of the
mixture with potassium fluoride, the former are obtained
in high yields.
Methyltrioxorhenium (CH3ReO3, abbreviated as MTO)
is a well-established catalyst for the reactions of hydrogen
peroxide,1,2 including the epoxidation of alkenes.3-6 The
active forms of the catalyst are the monoperoxo and
diperoxo complexes formed in reversible equilibria, eq 1:
We have devised a convenient and efficient method for
preparing R-hydroxy ketones from silyl enol ethers with
H2O2 as the oxidizing agent and MTO as the catalyst in
acetonitrile. The silyl enol ethers 1 were converted to
the R-hydroxy ketones 4, accompanied by the correspond-
ing R-hydroxy siloxy ketones 3, presumably through the
epoxide 2, which partitions by hydrolysis to 4 or silyl
rearrangement to 3, eq 2.7-9 Successive disilylation of
the crude reaction mixtures afforded 4 in high yields. The
compounds are presented in Table 1.
a
Combined GC/MS yields of both R-hydroxy and R-siloxy
ketones, the balance being the corresponding ketone formed by
the hydrolysis of the starting trimethylsilyl enol ether. In two
cases, entries 2 and 3, the GC/MS yield was confirmed from the
amount of the nonoxidized ketone, which was determined by the
b
method of standard addition. Isolated yield from a reaction on a
scale of ∼2 g. c Mixture of isomers, trans/cis 97:3 from GC-MS.
failed, giving only hydrolysis to ketones; apparently, MTO
or A or B are strong enough Lewis acids to catalyze
hydrolysis. Recently, it has been shown that pyridine is
able to suppress the Lewis acidity of MTO and its peroxo
adducts and prevent the hydrolysis of epoxides formed
by the oxidation of alkenes with a hydrogen peroxide/
MTO system.10 Also, pyridine accelerates peroxo complex
formation as in eq 1. The use of pyridine alone is not
satisfactory, since MTO is concurrently deactivated by
conversion to perrhenate.11 To stabilize the catalyst and
to allow higher levels of the enol ether and lower catalyst
concentrations, acetic acid was added along with pyridine
as a component of the solvent mixture. With 5% HOAc,
0.2 mol % MTO sufficed, with little or no hydrolysis of
the ether. Pyridine is necessary, however, as HOAc alone
gives only total hydrolysis. This system constitutes a
buffer, with each component having a separate role.
Pyridine reduces the Lewis acidity of the catalyst, thus
preventing the hydrolysis of 1; HOAc lowers the basicity
This conversion was best carried out in acetonitrile
solutions containing pyridine and acetic acid. The ethers
1 are moisture-sensitive compounds, especially when
acids or bases are present. Indeed, our initial attempts
with H2O2/MTO but lacking pyridine and acetic acid
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S0022-3263(97)02315-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/21/1998

4130 J . Org. Chem., Vol. 63, No. 12, 1998
Notes
of the solution, prolonging the catalyst’s lifetime. The
ratio HOAc/Py was ∼9:1.
methods because aqueous hydrogen peroxide, a cheap
and readily available oxidizing agent, was used; also,
MTO is commercially available and the procedure gives
high yields in reasonable reaction times. Recently, a
moderately successful oxidation of silyl enol ethers with
aqueous hydrogen peroxide catalyzed by peroxotungsto-
phosphate has been reported.21
As shown in eq 2, 3 accompanies the desired product
in an amount depending on the substrate. These materi-
als were not isolated, but were observed in the GC-MS.
Under the experimental conditions, slow hydrolysis of 3
to 4 was found when the potassium fluoride workup was
delayed.
The method works efficiently when 1 does not have an
electron-withdrawing group conjugated with the enol
ether double bond. In the case of 1-phenyl-1-(trimeth-
ylsiloxy)ethene (Table 1, entry 8), the yield is only 60%,
presumably due to its lower reactivity toward oxidation,
allowing hydrolysis to compete. The even less reactive
4-(trimethylsiloxy)-3-penten-2-one gave hydrolysis only,
resulting in 2,4-pentanedione.
The literature reports such reactions, utilizing reagents
such as peracids,7-9 chromyl chloride,12 hypervalent
iodine,13 dimethyldioxirane,14 sulfonyloxaziridines,15 os-
mium tetraoxide,16 triphenyl phosphite ozonide,17 lead
tetrabenzoate,18 and molecular oxygen.19 Optically active
R-hydroxy carbonyl compounds have been prepared from
silyl enol ethers using a number of oxidants with (salen)-
Mn(III) complex as a catalyst.20 The method described
here has the advantage over most of the mentioned
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r es. 1 (0.10 mmol) was
added to a rapidly stirred solution of MTO (0.2 mM), hydrogen
peroxide (0.2 M, added as a 30% solution in water), and pyridine
(0.1 M) in 1.0 mL of acetonitrile/HOAc, 95:5 by volume. After
15 min, most of the acetonitrile was removed by rotary evapora-
tion and the residue poured into 5 mL of saturated KF in
methanol. Stirring was continued for 2 h and the solution
dissolved in ether and washed first with saturated sodium
bicarbonate and then water. The ether layer was dried and the
product obtained by evaporation after column chromatography
(n-hexane/acetone). This procedure was successfully scaled up
by a factor of 100 to obtain the isolated yields reported in Table
1.
Ack n ow led gm en t is made to the donors of the
Petroleum Research Fund, administered by the ACS,
for partial support of this research. Some of this
research was conducted in facilities of the Ames Labo-
ratory, operated for the U.S. Department of Energy
under Contract No. W-7405-Eng-82.
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1976, 41, 1673.
Su p p or tin g In for m a tion Ava ila ble: 1H and 13C data of
4 (1 page). This material is contained in libraries on micro-
fiche, immediately follows this article in the microfilm version
of the journal, and can be ordered from the ACS; see any
current masthead page for ordering information.
J O972315B
(19) Takai, T.; Yamada, Y.; Rhode, O.; Mukaiyama, T. Catal. Lett.
1991, 281.
(20) Adam, W.; Fell, R. T.; Mock-Knoblauch, C.; Saha-Mo¨ller, C. R.
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