Dehomologation of Aldehydes via Oxidative Cleavage of Silyl Enol Ethers with Aqueous Hydrogen Peroxide Catalyzed by Cetylpyridinium Peroxotungstophosphate under Two-Phase Conditions

Article abstract of DOI:10.1021/jo990477x

Dehomologation of aldehydes has been first successfully achieved via oxidative cleavage of silyl enol ethers, derived from aldehydes and trimethylchlorosilane, using aqueous hydrogen peroxide in the presence of a catalytic amount of peroxotungstophophate (PCWP) under phase-transfer conditions. For instance, the oxidation of 1-[(trimethylsilyl)oxy]-1-octene resulting from octanal and Me₃SiCl with 35% H₂O₂ catalyzed by PCWP in dichloromethane at room temperature afforded the one-carbon shorter aldehyde, heptanal, in 79% yield. A variety of silyl enol ethers were also converted into one-carbon shorter aldehydes in good yields. The oxidation under homogeneous conditions using tert-butyl alcohol gave hydrolysis products such as 2-oxooctanol and octanal. It is of interest that [1-(trimethylsilyl)oxy]-1,10-undecadiene involving an enol moiety and a terminal double bond afforded exclusively 9-decenal, in which the enol moiety was selectively oxidized. A plausible reaction path for the oxidative cleavage of silyl enol ethers by the present system has been suggested from the oxidation results of α-[(trimethylsilyl)oxy]styrene.

Full text of DOI:10.1021/jo990477x

5954
J . Org. Chem. 1999, 64, 5954-5957
Deh om ologa tion of Ald eh yd es via Oxid a tive Clea va ge of Silyl En ol
Eth er s w ith Aqu eou s Hyd r ogen P er oxid e Ca ta lyzed by
Cetylp yr id in iu m P er oxotu n gstop h osp h a te u n d er Tw o-P h a se
Con d ition s
Satoshi Sakaguchi, Yumiko Yamamoto, Takuma Sugimoto, Hiroyo Yamamoto, and
Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering & High-Technology Research Center,
Kansai University, Suita, Osaka 564-8680, J apan
Received March 17, 1999
Dehomologation of aldehydes has been first successfully achieved via oxidative cleavage of silyl
enol ethers, derived from aldehydes and trimethylchlorosilane, using aqueous hydrogen peroxide
in the presence of a catalytic amount of peroxotungstophophate (PCWP) under phase-transfer
conditions. For instance, the oxidation of 1-[(trimethylsilyl)oxy]-1-octene resulting from octanal
and Me3SiCl with 35% H2O2 catalyzed by PCWP in dichloromethane at room temperature afforded
the one-carbon shorter aldehyde, heptanal, in 79% yield. A variety of silyl enol ethers were also
converted into one-carbon shorter aldehydes in good yields. The oxidation under homogeneous
conditions using tert-butyl alcohol gave hydrolysis products such as 2-oxooctanol and octanal. It is
of interest that [1-(trimethylsilyl)oxy]-1,10-undecadiene involving an enol moiety and a terminal
double bond afforded exclusively 9-decenal, in which the enol moiety was selectively oxidized. A
plausible reaction path for the oxidative cleavage of silyl enol ethers by the present system has
been suggested from the oxidation results of R-[(trimethylsilyl)oxy]styrene.
In tr od u ction
Among these oxidants, aqueous hydrogen peroxide, a
cheap and environmentally benign oxidant, has attracted
much interest in recent years. Tungsten compounds such
as polyoxotungstophosphates and tungstic acid are ef-
ficient catalysts for the oxidative cleavage of olefinic
double bonds to carboxylic acids with aqueous hydrogen
peroxide.6 However, the transformation of alkenes to
aldehydes via oxidative cleavage is rarely explored
because of the difficulty in controlling the oxidation at
the stage of the formation of aldehydes, except for the
formation of glutaraldehyde by the oxidative cleavage of
cyclohexene with anhydrous hydrogen peroxide using H3-
PMo10W2O40 as a catalyst.10
The oxidation of silyl enol ethers to R-hydroxyketones
has been achieved by using various oxidation systems.11
In a previous paper, we showed that the silyl enol ether
1-[(trimethylsilyl)oxy]-1-octene (1) was first successfully
converted into the R-hydroxy ketone 2-oxooctanol (2)
using aqueous hydrogen peroxide as an oxidant in the
presence of a catalytic amount of cetylpyridinium per-
oxotungstophosphate (PCWP), [C5H5N+(CH2)15CH3]3{PO4-
Oxidative cleavage of carbon-carbon double bonds is
a fundamental and important transformation in synthetic
organic chemistry.1 Ozonolysis2 and oxidation by stoi-
chiometric oxidants such as osmiun tetraoxide,3 ruthe-
nium tetraoxide,4 and potassium permanganate5 are
frequently used for this purpose. However, because these
reagents have the obvious drawbacks of being toxic and
expensive, a number of catalytic systems using H2O2,6
NaOCl,7 NalO4,8 and peracetic acid9 as oxidants have
been examined.
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10.1021/jo990477x CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/14/1999

Dehomologation of Aldehydes
J . Org. Chem., Vol. 64, No. 16, 1999 5955
[W(O)(O2)2]4}3- in a mixed solvent of ethanol and dichlo-
Ta ble 1. Oxid a tion of 1 by th e P CWP -H2O2 System
Un d er Selected Con d ition sa
romethane (eq 1).12 However, under two-phase conditions
using dichloromethane as a solvent, the reaction gave
oxidatively cleaved products such as the aldehyde hep-
tanal (3). Kaneda et al. have shown that treatment of
silyl enol ethers with tert-butylhydroperoxide using a
molybdenum complex leads to the corresponding car-
boxylic acids but not to aldehydes.13 Because silyl enol
ethers can be easily prepared by the reaction between
the corresponding aldehydes and Me3SiCl,14 we are
interested in the PCWP-catalyzed dehomologation of
aldehydes to one-carbon shorter aldehydes.15 The syn-
thesis of odd-numbered long-chain aldehydes, which are
difficult to obtain by conventional methods, using this
methodology seems to be very useful. In this paper, we
would like to report further details of the dehomologation
of aldehydes via oxidative cleavage of silyl enol ethers
with aqueous hydrogen peroxide catalyzed by PCWP
under two-phase conditions.
Ta ble 2. Oxid a tion of Va r iou s Silyl En ol Eth er s by th e
P CWP -H2O2 System a
Resu lts
At first, to obtain further information on the oxidation,
1-[(trimethylsilyl)oxy]-1-octene (1) was allowed to react
with aqueous hydrogen peroxide (35%) in the presence
of a catalytic amount of PCWP under selected conditions.
Representative results are shown in Table 1. Although
excess H2O2 (6 equiv) with respect to 1 was employed,
the amounts of 3 and heptanoic acid (4) were almost the
same as those of run 1 except for 2 (run 2). These results
suggest that the aldehyde 3 was not further oxidized to
carboxylic acid 4 by the PCWP-H2O2 system under these
conditions. In refluxing temperature (ca. 40 °C), however,
a considerable amount of heptanoic acid 4 was formed.
Under homogeneous conditions using tert-butyl alcohol
as a solvent, the oxidative cleavage to 3 was found to take
place in competition with the hydrolysis to octanal (5)
(run 4).
On the basis of these results, a variety of silyl enol
ethers, derived from aldehydes and Me3SiCl, were al-
lowed to react with 35% H2O2 in the presence of PCWP
in dichloromethane at room temperature (Table 2).
Various types of silyl enol ethers were successfully
oxidized to one-carbon shorter aldehydes in satisfactory
yields (runs 1-5). The odd-numbered aldehydes such as
nonanal and undecanal were produced by the oxidation
of the even-numbered silyl enol ethers obtained from
decanal and dodecanal, respectively (runs 1 and 2).
3-Phenyl-1-[(trimethylsilyl)oxy]-1-propene (7) derived from
3-phenylpropionaldehyde was oxidized to phenylaceto-
aldehyde (8) (run 4). In the case of the oxidation of
1-[(trimethylsilyl)oxy]-1,10-undecadiene (11) involving
enolic and terminal double bonds, the enolic double bond
was preferentially oxidized to give 9-decenal (12) in 58%
yield along with small amounts of 2-oxo-10-undecen-1-
ol, 9,10-epoxydecenal, 10,11-epoxyundecanal, and 10-
undecenal in <5% yields, respectively; 9,10-epoxy-1-
[(trimethylsilyl)oxy]-1-decene formed by the epoxidation
of the terminal double bond was not detected. This is
(12) Yamamoto, H.; Tsuda, M.; Sakaguchi, S.; Ishii, Y. J . Org. Chem.
1997, 62, 7174.
(13) Kaneda, K.; Kii, N.; J itsukawa, K.; Teranishi, S. Tetrahedron
Lett. 1981, 22, 2595.
(14) House, H. O.; Czuba, L. L.; Gall, M.; Olmstead, H. D. J . Org.
Chem. 1969, 34, 2324.
(15) Very recently, dehomologation of R-substituted aldehydes to the
corresponding ketones via oxidative cleavage of enamine with transi-
tion metal oxides such as CrO3 and KMnO4 has been reported: (a)
Harris, C. E.; Lee, L. Y.; Dorr, H.; Sigaram, B. Tetrahedron Lett. 1995,
36, 2921. (b) Harris, C. E.; Chrisman, W.; Bickford, S. A.; Lee, L. Y.;
Torreblanca, A. E.; Sigaram, B. Tetrahedron Lett. 1997, 38, 981. (c)
Sreekumar, R.; Padmakumer, R. Tetrahedron Lett. 1997, 38, 5143.

5956 J . Org. Chem., Vol. 64, No. 16, 1999
Sakaguchi et al.
believed to be due to the fact that the electron density of
the enolic double bond is higher than that of the terminal
bond. In fact, electron densities of C1, C2, C9, and C10 were
estimated to be +0.059, -0.278, -0.137, and -0.168,
respectively, by the PM3 semiempirical method as imple-
mented in the MOPAC 93.16 Unfortunately, the oxidation
of the cyclic silyl enol ether 1-[(trimethylsilyl)oxy]-1-
cyclohexene resulted in the formation of adipic acid and
2-hydroxycyclohexanone in 48% and 21% yields, respec-
tively (run 7). Similarly, 1-[(trimethylsilyl)oxy]-1-cyclo-
octene was oxidized to 2-hydroxyoctanone as a major
product (run 8).
conversion (eq 2). It is interesting that 13 bearing an
R-silyloxy group gave 14 and 15, in contrast to the
oxidation of 9 bearing a â-silyloxy group, for which
benzaldehyde 10 was formed as a principal product
(Table 2, run 5). The formation of 12 and 16 from R- and
â- [(trimethylsilyl)oxy]styrenes 9 and 13, respectively,
suggests an alternative oxidation path of the silyl enol
ethers (Scheme 2). The formation of 10 from 9 may be
Discu ssion s
The dehomologation of aldehydes was performed via
the oxidative cleavage of silyl enol ethers, synthetic
equivalents of aldehydes, upon treatment with the
PCWP-H2O2 system under two-phase conditions. The
successful achievement of this transformation is believed
to be due to the fact that the PCWP possesses both
powerful oxidizing ability and phase-transfer catalysis.17
Another important factor is that the reaction could be
carried out under the two-phase system using dichlo-
romethane. As a consequence, the hydrolysis of silyl enol
ethers to the original aldehydes could be restricted to a
considerable extent even in the presence of aqueous
hydrogen peroxide. In addition, the reaction proceeded
smoothly at room temperature as a result of the higher
reactivity toward the peroxo species of enolic double
bonds bearing a trimethylsilyloxyl group compared to
that of normal olefinic double bonds.
Sch em e 2
rationalized by the further oxidation of R-trimethylsilyl-
oxy epoxide (D), which is a primary product generated
by the epoxidation of silyl enol ether, to the one-carbon
shorter aldehyde (Scheme 2). In the same sense as the
formation of 10, the carbon-carbon bond cleavage of the
epoxide D derived from 13 leads to the formation of
benzoic acid precursor 18, although the hydrolysis of D
to R-hydroxyketone 14 is a preferred over the bond
cleavage to 18.
In general, the oxidation of silyl enol ether is thought
to proceed via the formation of an R-trimethylsilyloxy
epoxide A, which then is converted into an R-hydroxy
ketone C through silyl rearrangement or hydrolysis
(Scheme 1).11,18 It is noteworthy, however, that the
Sch em e 1
In conclusion, a variety of silyl enol ethers obtained
from aldehydes and Me3SiCl were first successfully
oxidized to one-carbon shorter aldehydes with aqueous
hydrogen peroxide in the presence of a catalytic amount
of PCWP. The oxidation apparently proceeds via the
formation of a trimethylsilyloxy epoxide, which then is
converted into an R-trimethylsilyloxy aldehyde whose
oxidation results in the one-carbon shorter aldehyde.
Because silyl enol ethers are easily prepared from alde-
hydes and Me3SiCl, the present reaction provides a novel
route for the dehomologation of aldehydes.
present oxidation of silyl enol ethers, having a trimeth-
ylsilyloxy group at the terminal position, produced the
oxidatively cleaved products, aldehydes, rather than
R-hydroxy ketones as major products as shown in Table
2 (runs 1-6).
The oxidation of R-[(trimethylsilyl)oxy]styrene (13)
with 35% H2O2 catalyzed by PCWP (2 mol %) at room
temperature for 0.5 h afforded 2-hydroxyacetophenone
(14) (51%), 2-[(trimethylsilyl)oxy]acetophenone (15) (10%),
and benzoic acid (16) (7%) together with acetophenone
(17) (17%), hydrolysis product of the starting 13, in >99%
Exp er im en ta l Section
Gen er a l P r oced u r es. All silyl enol ethers except for 9,
1-[(trimethylsilyl)oxy]-1-cyclohexene, and 12 were synthesized
according to the literature procedures and purified by distil-
lation under reduced pressure.14 Compounds 9, 1-[(trimethyl-
silyl)oxy]-1-cyclohexene, and 12 were commercially available
and used without further purification. PCWP was prepared
by the method reported previously.13 GC analysis was per-
formed with a flame ionization detector using a 0.2 mm × 25
m capillary column (OV-1). 1H and 13C NMR spectra were
measured at 270 or 400 MHz and 67.5 or 100 MHz, respec-
tively, in CDCl3 with Me4Si as the internal standard. Infrared
(IR) spectra were measured using a NaCl plate. GC-MS spectra
were obtained at an ionization energy of 70 eV. The product
yields were estimated from the peak areas on the basis of the
internal standard technique.
(16) Stewart, J . J . J . Comput. Chem. 1989, 10, 209.
(17) Sakaguchi, S.; Nishiyama, Y.; Ishii, Y. J . Org. Chem. 1996, 61,
5307 and references therein.
(18) (a) Clark, R. D.; Heathcock, C. H. J . Org. Chem. 1976, 41, 1396.
(b) Rubottom, G.; Gruber, J . M. J . Org. Chem. 1978, 43, 1599.

Dehomologation of Aldehydes
J . Org. Chem., Vol. 64, No. 16, 1999 5957
Gen er a l P r oced u r e for th e Oxid a tion of Silyl En ol
Eth er s to Ald eh yd es. To a stirred solution of substrate (1
mmol) and PCWP (40 mg, 2 mol %) in CH2Cl2 (0.25 mL) was
added 35% H2O2 (1 mmol). The reaction mixture was stirred
at room temperature for 16 h and extracted with dichlo-
romethane. The extract was dried over anhydrous MgSO4 and
evaporated under reduced pressure. The products were puri-
fied by column chromatography on silica gel with hexane/ethyl
acetate (10-3/1 v/v). All products except for 3,12 12, 2-hydroxy-
cylcohexanone,12 2-hydroxycyclooctanone,12 and 15 were com-
mercially available and were identified through the compari-
son of the isolated products with authentic samples. Compound
15 was identified through the comparison of the isolated
products with the compound prepared by the reaction of 14
with TMSCl.
Oxid a tion of 1-[(Tr im eth ylsilyl)oxy]-1-Cycloh exen e. To
a stirred solution of substrate (1 mmol) and PCWP (40 mg, 2
mol %) in CH2Cl2 (0.25 mL) was added 35% H2O2 (1 mmol).
The reaction mixture was stirred at room temperature for 16
h and extracted with dichloromethane. The extract was dried
over anhydrous MgSO4. After evaporation of the extract under
reduced pressure, a white precipitate was obtained. The
precipitate was purified by recrystallization with ethyl acetate
to give adipic acid in 48% isolated yield.
Ack n ow led gm en t. This work was partially sup-
ported by Research for the Future program J SPS.
Su p p or t in g In for m a t ion Ava ila b le: Copies of 1H and
13C NMR spectra for the compounds 3, 12, 2-hydroxycylco-
hexanone, 2-hydroxycyclooctanone, and 15. This information
available free of charge via the Internet at http://pubs.acs.org.
9-Decen a l (12): 1H NMR (CDCl3, 270 MHz) δ 9.69 (s, 1 H),
5.77-5.65 (m, 2 H), 4.94-4.83 (m, 2 H), 2.34 (t, J ) 7.3 Hz, 2
H), 1.97-1.93 (m, 2 H), 1.58-1.53 (m, 2 H), 1.28-1.20 (m, 8
H); 13C NMR (CDCl3, 67.5 MHz) δ 202.8, 139.0, 114.1, 43.8,
33.6, 29.1, 29.0, 28.8, 28.7, 22.0; IR 2928, 1730, 1658, 1253,
1164, 1083, 845, 757.
J O990477X