A triruthenium carbonyl cluster bearing a bridging acenaphthylene ligand: An efficient catalyst for reduction of esters, carboxylic acids, and amides by trialkylsilanes

Article abstract of DOI:10.1021/jo025726n

An efficient reduction of carboxylic acids, esters, and amides with trialkylsilanes is accomplished using a triruthenium carbonyl cluster bearing a bridging acenaphthylene ligand, (μ₃,η²:η³:η⁵ -acenaphthylene)Ru₃(CO)₇, as the catalyst. Preactivation of the catalyst by hydrosilanes accelerates the reactions. Sterically small trialkylsilanes are effective in these reactions. Reduction of carboxylic acids and amides efficiently produces the corresponding silyl ethers and amines, respectively. Reduction of esters gives a mixture of silyl and alkyl ethers, but can be controlled by changing the silanes and solvents.

Full text of DOI:10.1021/jo025726n

catalyzedreduction of esters and amines with HSi(OEt)3,4
whereas Ohta5 and Ito6 published Rh-catalyzed hydrosi-
lylation of esters or reduction of amides with Ph2SiH2 or
PhSiH3. Selective reduction of esters to ethers was
developed by Cutler et al. by the manganese-catalyzed
reaction with PhSiH3.7 Fuchikami and co-workers have
recently demonstrated that Ru3(CO)12 and other ruthe-
nium complexes are active catalysts for the hydrosilyla-
tion of esters to silyl acetals and for the reduction of
amides.8 Although these ruthenium-catalyzed reactions
are attractive as synthetic methods to obtain aldehydes
or amines, the high reaction temperature (∼100 °C)
necessary to accomplish these reactions is a drawback.
We recently reported that a triruthenium carbonyl
cluster bearing a µ3-acenaphthylene ligand, (µ3,η2:η3:η5-
acenaphthylene)Ru3(CO)7 (1), is an active catalyst for the
hydrosilylation of ketones and aldehydes, the reduction
of acetals and cyclic ethers, and the ring-opening polym-
erization of cyclic ethers.9 The catalytic activity of 1 in
these reactions is much higher than that of Ru3(CO)12
under the same conditions;9 this prompted us to examine
reduction of carboxylic acids, esters, and amides with
trialkylsilanes using 1 as the catalyst. In this paper, we
wish to report that reduction of these compounds is
efficiently achieved by the catalysis of 1 preactivated by
hydrosilanes. By an appropriate choice of the hydrosi-
lanes, the production of silyl ethers from carboxylic acids,
that of silyl and alkyl ethers from esters, and that of
amines from amides proceed even at room temperature
within several hours, providing efficient methods to
prepare alcohols, alkyl ethers, and amines.
Scr een in g of th e Rea ction Con d ition s. As reported
previously, the hydrosilylation of ketones or aldehydes
with trialkylsilanes proceeds at room temperature in the
presence of 1.9 In a typical example, hydrosilylation of
acetophenone with EtMe2SiH (1.5 equiv) in the presence
of 1 (1 mol %) afforded the corresponding silyl ether in
over 95% yield after 18 h (method A, Scheme 1, eq 1). By
screening the reaction conditions, we have found that
prior activation of 1 with EtMe2SiH in dioxane followed
by addition of acetophenone in a benzene solution re-
sulted in the production of a highly active catalyst spe-
cies, which promoted rapid hydrosilylation of acetophe-
none with EtMe2SiH leading to the quantitative forma-
tion of the silyl ether within 1 h (method B, Scheme 1,
A Tr ir u th en iu m Ca r bon yl Clu ster Bea r in g
a Br id gin g Acen a p h th ylen e Liga n d : An
Efficien t Ca ta lyst for Red u ction of Ester s,
Ca r boxylic Acid s, a n d Am id es by
Tr ia lk ylsila n es
Kouki Matsubara,†,‡ Takafumi Iura,§
Tomoyuki Maki,§ and Hideo Nagashima*,†,‡,§
Institute of Advanced Material Study, Graduate School of
Engineering Sciences, and CREST, J apan Science and
Technology Corporation (J ST), Kyushu University,
Kasuga, Fukuoka 816-8580, J apan
nagasima@cm.kyushu-u.ac.jp
Received March 18, 2002
Abstr a ct: An efficient reduction of carboxylic acids, esters,
and amides with trialkylsilanes is accomplished using a
triruthenium carbonyl cluster bearing a bridging acenaph-
thylene ligand, (µ3,η2:η3:η5-acenaphthylene)Ru3(CO)7, as the
catalyst. Preactivation of the catalyst by hydrosilanes ac-
celerates the reactions. Sterically small trialkylsilanes are
effective in these reactions. Reduction of carboxylic acids and
amides efficiently produces the corresponding silyl ethers
and amines, respectively. Reduction of esters gives a mixture
of silyl and alkyl ethers, but can be controlled by changing
the silanes and solvents.
Reduction of carboxylic acids, esters, and amides is an
important synthetic method of synthesizing aldehydes,
alcohols, or amines; however, reduction of these sub-
strates is generally more difficult than that of ketones
and aldehydes, and further investigation is required to
develop efficient protocols to reduce them under mild
conditions.1 In contrast to well-investigated catalytic
hydrosilylation of ketones and aldehydes,2 transition-
metal-catalyzed reduction of carboxylic acids, esters, and
amides with silanes has not been reported until re-
cently.3-8 Buchwald and co-workers reported titanocene-
† Institute of Advanced Material Study.
‡
J apan Science and Technology Corporation.
§ Graduate School of Engineering Sciences.
(1) (a) Hudlicky, M. Reductions in Organic Chemistry; Wiley: New
York, 1984. (b) Comprehensive Organic Synthesis: Selectivity, Strategy
and Efficiency in Modern Organic Chemistry, Vol 8, Reduction; Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, 1992. (c) Seyden-Penne,
J . Reductions by the Alumino- and Borohydrides in Organic Synthesis,
2nd ed.; Wiley: New York, 1997.
(2) Reviews for catalytic hydrosilylation reactions: (a) Comprehen-
sive Handbook on Hydrosilylation; Marciniec, B., Ed.; Pergamon
Press: Oxford, 1992. (b) Marciniec, B. In Applied Homogeneous
Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W.
A., Eds.; VCH: Weinheim, 1996; Vol. 1, Chapter 2.6, p 487. (c) Ojima,
I. In The Chemistry of Organosilicon Compounds; Patai, S., Rappoport,
Z., Eds.; Wiley: New York, 1989; p 1479. (d) Brook, M. A. Silicon in
Organic, Organometallic, and Polymer Chemistry; Wiley: New York,
2000; Chapter 12.8, p 401. (e) Spier, J . L. Adv. Organomet. Chem. 1978,
17, 407.
(4) (a) Berk, S. C.; Kreutzer, K. A.; Buchwald, S. L. J . Am. Chem.
Soc. 1991, 113, 5093. (b) Berk, S. C.; Buchwald, S. L. J . Org. Chem.
1992, 57, 3751. (c) Bower, S.; Kristina, K. A.; Buchwald, S. L. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1515. (d) Barr, K. J .; Berk, S. C.;
Buchwald, S. L. J . Org. Chem. 1994, 59, 4323.
(5) Rh-catalyzed reduction of esters by dihydrosilanes, see: Ohta,
T.; Kamiya, M.; Kusui, K.; Michibata, T.; Nobutomo, M.; Furukawa,
I. Tetrahedron Lett. 1999, 40, 6963. In the earlier reports, esters were
hardly reduced by the Rh-catalyzed hydrosilylation: Ojima, I.; Kuma-
gai, M.; Nagai, Y. J . Organomet. Chem. 1976, 111, 43. Ojima, I.;
Kogure, T.; Kumagai, M. J . Org. Chem. 1977, 42, 1671.
(6) Rh-catalyzed hydrosilylation of amides, see: Kuwano, R.; Ta-
kahashi, M.; Ito, Y. Tetrahedron Lett. 1998, 39, 1017.
(3) Earlier reports for the reduction of carboxylic acids or esters by
hydrosilanes which is not mediated by transition metal catalysts: (a)
Calas, R. Pure Appl. Chem. 1966, 13, 61 and references therein. (b)
Boyer, J .; Corriu, R. J . P.; Perz, R.; Poirier, M.; Reye´, C. Synthesis
1981, 558. (c) Chuit, C.; Corriu, R. J . P.; Perz, R.; Reye´, C. Synthesis
1982, 981. (d) Corriu, R. J . P.; Perz, R.; Reye´, C. Tetrahedron 1983,
39, 999. (e) Benkeser, R. A.; Ehler, D. F. J . Org. Chem. 1973, 38, 3660.
(7) Mn-catalyzed hydrosilylation of esters, see: Mao, Z.; Gregg, B.
T.; Cutler, A. R. J . Am. Chem. Soc. 1995, 117, 10139.
(8) (a) Igarashi, M.; Mizuno, R.; Fuchikami, T. Tetrahedron Lett.
2001, 42, 2149. (b) Igarashi, M.; Fuchikami, T. Tetrahedron Lett. 2001,
42, 1945.
(9) Nagashima, H.; Suzuki, A.; Iura, T.; Ryu, K.; Matsubara, K.
Organometallics 2000, 19, 3579.
10.1021/jo025726n CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/22/2002
J . Org. Chem. 2002, 67, 4985-4988
4985

TABLE 1. Hyd r osilyla tion of Eth yl Hyd r ocin n a m a te
w ith EtMe2SiH u n d er Va r iou s Con d ition s
SCHEME 1
eq 1). Similarly, reduction of methyl hydrocinnamate
with EtMe2SiH (2.5 equiv) under the conditions of
method A resulted in formation of the ethyldimethylsilyl
ether of 3-phenylpropanol (11%) after 15 h with recovery
of the starting material (Scheme 1, eq 2). Application of
method B to the reduction of methyl hydrocinnamate
with EtMe2SiH accelerated the reaction leading to com-
plete consumption of the ester after 15 h (Scheme 1, eq
2). Furthermore, the reaction was over within 1 h when
neat methyl hydrocinnamate instead of its benzene
solution was added to the solution of the activated
catalyst as shown in Scheme 1, eq 2 (method C). These
results showed that method C can be the most efficient
method for the hydrosilylation of methyl hydrocinnamate
and can be applicable to the reduction of carboxylic acids,
esters, and amides at room temperature.
As described later, the reduction of carboxylic acids,
esters, and amides is actually accomplished by
EtMe2SiH according to method C. The reaction of car-
boxylic acids affords the corresponding silyl ethers as a
single product, whereas that of amides gives amines. In
contrast, the reaction of esters generally gives two
products, silyl ethers and alkyl ethers. Alkyl silyl acetals
were detected at the initial stage of the reaction as
intermediates, and the selectivity of the subsequent
reductive cleavage of either the C-OR bond or the C-OSi
bond in the silyl acetal determines the ratio of the silyl
ether and the alkyl ether in the products. In contrast to
the fact that the reaction of methyl hydrocinnamate with
EtMe2SiH gave the corresponding silyl ether as the single
product, ethyl hydrocinnamate gave a 4:1 mixture of the
silyl ether and the ethyl ether under the same reaction
conditions. To understand the effect of silanes, catalysts,
and the activation methods of the catalyst on the rate
and the selectivity of the reaction, the reaction of ethyl
hydrocinnamate with various organosilanes was inves-
a
b
Concentration of 1 was 0.05 mol/L. The ratio of the products
was determined on the basis of integral ratios of the 1H NMR
spectra of the crude mixture using dichloroethane as an internal
standard.
tigated. In Table 1 are summarized the results obtained
by changing the reaction conditions when using ethyl
hydrocinnamate. The yields and the ratios of the products
were determined after hydrolysis of the formed silyl
ethers or of the alkyl silyl acetals. As shown in entries
1-7 (Table 1), the rate and the product ratio are
dependent on the hydrosilane used. The rate is in the
order EtMe2SiH > HSiMe2(CH2)2SiMe2H > Et2MeSiH >
i
PhMe2SiH > Ph2MeSiH . Et3SiH > Pr3SiH. The ratio
of the alcohol to the alkyl ether is 80:20-83:17 with
EtMe2SiH, HSiMe2(CH2)2SiMe2H, PhMe2SiH, and
Ph2MeSiH, whereas the ethyl ether predominantly forms
when Et2MeSiH is used as the silane. Monitoring the
reaction by NMR revealed that the reaction is stepwise;
the silyl acetal is formed first, followed by the production
of a mixture of the silyl ether and the alkyl ether. In fact,
slow hydrosilylation with Et3SiH gave the silyl acetal as
a single product in low yield, which in turn gave hydro-
cinnamaldehyde after hydrolysis.
The factors determining the rate and the selectivity
other than the structure of the silanes described above
are the catalyst used, the activation method of the
catalyst, and the reaction temperature. The ruthen-
ium complexes, (µ2,η2:η3-4,6,8-trimethylazulene)Ru3(CO)7
(2), (µ2,η2:η3-4,6,8-trimethylazulene) Ru2(CO)5 (3), and
(µ2,η2:η3-4,6,8-trimethylazulene)Ru4(CO)9 (4)10 were found
to be catalytically active in the reduction of ethyl hydro-
4986 J . Org. Chem., Vol. 67, No. 14, 2002

TABLE 2. Hyd r osilyla tion of Ester s w ith EtMe2SiH
Ca ta lyzed by 1
cinnamate with EtMe2SiH. Although the reactions were
slower than that of 1, the ethyl ether was obtained as a
single product. Ru3(CO)12 used for the Fuchikami hy-
drosilylation was totally inactive at room temperature.
As solvent for the activation of 1, benzene, Et2O, and
tetrahydropyran can be used instead of dioxane. The
activation of 1 in oxepane is less effective, whereas
pyridine causes deactivation of the catalyst. Use of THF
resulted in its polymerization.8 Interestingly, the ratios
of ether to silyl ether were somewhat higher in these
solvents than in dioxane. At higher reaction temperature,
the reaction was more rapid, but the alcohol/alkyl ether
ratio did not differ much with temperature. All of these
results suggest that the reduction of ethyl hydrocin-
namate is achieved successfully under mild conditions
by using 1, preactivated by hydrosilanes and sterically
small hydrosilanes as the catalyst and the reducing
reagents. The ratio of the silyl ether to the alkyl ether is
affected by various factors; however, good to high selec-
tivity can be attained by an appropriate choice of the
hydrosilanes as described below.
Th e Red u ction of Ester s, Ca r boxylic Acid s, a n d
Am id es. In Tables 2 and 3 are summarized the reduction
of esters, lactones, carboxylic acids, and amides with
EtMe2SiH using 1 as the catalyst. As shown in entries
1, 2, and 4 in Table 2, the reaction rate with EtMe2SiH
is sensitive to the bulkiness of the alkyl group of the
t
ester: Me > Et . Bu. The product ratios also depend
on the alkyl group of the esters; the ethyl and tert-butyl
esters gave the corresponding alkyl ethers as byproducts.
Aromatic and aliphatic esters including those containing
a Br atom or a carbon-carbon double bond underwent
the reaction with EtMe2SiH as shown in entries 7-9
(Table 2), in which a mixture of alcohols and ethers was
obtained after hydrolysis of the reaction mixture in ratios
of 93:7-59:41. Improvement of the selectivity to form the
alkyl ether as a single product is possible by appropriate
choice of the silane and the solvent used for the activation
of 1. As typical examples, ethyl hydrocinnamate or ethyl
laurate was reduced to the corresponding ethyl ether as
the major product by Et2MeSiH, when 1, activated by
Et2MeSiH in tetrahydropyran, was used as the catalyst
as shown in entries 3 and 6 (Table 2). Selective for-
mation of cyclic ethers was achieved from lactones with
EtMe2SiH as shown in entries 10 and 11 (Table 2).
As shown in Table 3, similar to the reduction of esters,
carboxylic acids were easily reduced to the corresponding
silyl ethers in the presence of 3.5 equiv of EtMe2SiH
based on the carboxylic acid, subsequent hydrolysis of
which afforded the corresponding alcohols as a single
product in good yields (Table 3, entries 1-3). The initial
step of the reaction is a dehydrogenative silylation of the
carboxylic acids to the corresponding silyl esters, which
could be confirmed by spectral data of the products
formed in the reaction of hydrocinnamic acid with 1.2
equiv of EtMe2SiH. Application of method C to the
reduction of tertiary amides also proceeded smoothly to
give the corresponding amines in good yields (Table 3,
entries 4-6). Reaction of secondary amides only re-
sulted in dehydrogenative silylation to give the corre-
a
The ratios of A (alcohol) and B (ether) were determined by 1H
NMR spectroscopy of the crude mixture using dichloroethane as
b
an internal standard. All products were purified by column
chromatography. c Et2MeSiH and tetrahydropyran were used as
d
the silane and solvent. HSiMe2Et (4 equiv) was used.
sponding silyl amides, and addition of the H-Si bond of
EtMe2SiH to the CdO group did not take place under
the conditions used.
In summary, an efficient catalytic reduction of car-
boxylic acids, esters, and amides with trialkylsilanes
could be established by using 1 as the catalyst. Prior
activation of 1 by hydrosilanes dramatically accelerated
the reactions. The reaction rate and selectivity of the
products are dependent on hydrosilanes, solvents, sub-
strates, and catalysts. Sterically small trialkylsilanes,
such as HSiMe2Et, HMe2Si(CH2)2SiMe2H, and HSiMeEt2,
are effective in the catalytic reduction of the carboxylic
acid derivatives, whereas bulky hydrosilanes and dihy-
dro- and trihydrosilanes were ineffective. Reduction of
esters generally gives a mixture of silyl and alkyl ethers.
The product ratios are mainly dependent on the structure
of the esters, but can be controlled by changing the
silanes and solvents in some cases. Reduction of carboxy-
lic acids and amides efficiently produce the corresponding
(10) (a) Nagashima, H.; Suzuki, A.; Nobata, M.; Aoki, K.; Itoh, K.
Bull. Chem. Soc. J pn. 1998, 71, 2441. (b) Matsubara, K.; Ryu, K.; Maki,
T.; Iura, T.; Nagashima, H. Organometallics, in press.
J . Org. Chem, Vol. 67, No. 14, 2002 4987

temperature. Stirring was continued for 1 h. The 1H NMR
measurement of the crude mixture showed the quantitative
formation of 3-phenylpropyl dimethylethylsilyl ether. After the
solvent was removed in vacuo, distillation of the residual liquid
under reduced pressure (70 °C, 102 Pa) gave the silyl ether in
57% yield (115 mg, 0.52 mmol). Alternatively, silyl ethers were
prepared in high yields by reduction of the carboxylic acids with
dimethylethylsilane. In a typical example, a solution of the
mixture of 1 (7.5 mg, 11.5 µmol) and dimethylethylsilane (0.62
mL, 4.7 mmol) in dioxane (0.20 mL) was stirred for 0.5 h at room
temperature, and hydrocinnamic acid (172.6 mg, 1.15 mmol) was
added. The solution was stirred for 1 h, and the desired
3-phenylpropyl dimethylethylsilyl ether was subsequently puri-
fied by distillation at 150 °C (215 mg, 0.96 mmol, 84% yield):
1H NMR (CDCl3) δ 7.18-7.31 (m, 5H), 3.62 (t, J ) 6.4 Hz, 2H),
2.67 (dd, J ) 7.6, 7.9 Hz, 2H), 1.85 (m, 2H), 0.96 (t, J ) 7.9 Hz,
3H), 0.58 (q, J ) 7.9 Hz, 2H), 0.09 (s, 6H); 13C NMR (CDCl3) δ
142.1, 128.4, 128.3, 125.7, 62.0, 34.3, 32.1, 8.0, 6.7, -2.7; 29Si
NMR (CDCl3) δ 18.5; EI-MS m/z 222 [M+], 223 [M+ + 1], 224
[M+ + 2], 225 [M+ + 3]; HRMS calcd for C13H22OSi 222.1440,
found 222.1447.
TABLE 3. Ca ta lytic Red u ction of Ca r boxylic Acid s a n d
Am id es w ith EtMe2SiH Ca ta lyzed by 1
Typ ica l P r oced u r e for th e Red u ction of Ester s. To a
solution of 1 (7.7 mg, 12 µmol) in dioxane (0.20 mL) was added
dimethylethylsilane (0.38 mL, 2.9 mmol). The mixture was
stirred for 30 min at room temperature, and the color of the
solution turned from red to dark brown. Methyl hydrocin-
namate (0.185 mL, 1.2 mmol) was added dropwise at ambient
temperature, and an exothermic reaction took place. The mixture
was stirred for 0.5 h. The reaction was quenched by adding
aqueous hydrochloric acid. Then, the mixture was extracted with
ether. The combined organic layers were washed with aqueous
NaHCO3 and brine and dried over MgSO4. After the solvent was
removed in vacuo, the residual mixture was purified by column
chromatography by eluting with 7-10% ethyl acetate in hexane
to give 3-phenylpropyl alcohol (128 mg, 0.94 mmol, 97%).
Typ ica l P r oced u r e for t h e R ed u ct ion of Ca r b oxylic
Acid s. To a solution of 1 (16.5 mg, 25.2 µmol) in dioxane (0.45
mL) was added dimethylethylsilane (0.84 mL, 6.3 mmol). The
mixture was stirred for 30 min at room temperature. Hydrocin-
namic acid (380 mg, 2.55 mmol) was added at ambient temper-
ature. Then, stirring continued for 0.5 h. A vigorous gas evolution
occurred. The reaction was quenched by adding aqueous hydro-
chloric acid, and then the mixture was extracted with ether. The
combined organic layers were washed with aqueous NaHCO3
and brine, and dried over MgSO4. After the solvent was removed
in vacuo, the residual mixture was separated by column chro-
matography by eluting with 7-10% ethyl acetate in hexane to
give 3-phenylpropyl alcohol (248 mg, 1.82 mmol, 72%).
a
HSiMe2Et (4 equiv) was treated with carboxylic acids.
b
HSiMe2Et (2.5 equiv) was treated with amides.
silyl ethers and amines, respectively, in good yields. By
means of preactivation of 1 with hydrosilanes to produce
the highly active catalyst species, the reactions appar-
ently proceed at lower temperatures compared with the
conventional ruthenium-catalyzed reduction of esters and
amides which proceeds at 100 °C to afford alkyl silyl
acetals and amines, respectively.8 Application of this
highly catalytic active species to the reduction of other
organic substrates with hydrosilanes is actively in
progress.
Exp er im en ta l Section
Typ ica l P r oced u r e for th e Red u ction of Am id es. To a
solution of 1 (6.4 mg, 9.8 µmol) in dioxane (0.18 mL) was added
dimethylethylsilane (0.32 mL, 2.4 mmol). The mixture was
stirred for 30 min at room temperature. N,N-diethylhydrocin-
namamide (0.168 mL, 0.99 mmol) was added at ambient tem-
perature. Stirring was continued for 1 h. The 1H NMR mea-
surement of the crude mixture showed the quantitative formation
of N,N-diethyl-2-phenylpropylamine. After removal of the solvent
in vacuo, distillation of the residual liquid gave N,N-diethyl-2-
phenylpropylamine (121 mg, 0.74 mmol, 75%).
All manipulations were carried out under
a dinitrogen
atmosphere unless otherwise noted. The solvents, benzene,
diethyl ether, tetrahydropyran, oxepane, and benzene-d6 were
distilled from benzophenone ketyl and stored under a dinitrogen
atmosphere. Dioxane and pyridine were dried over CaH2 and
distilled before use. The hydrosilanes HSiMe2Et, Me2HSi(CH2)2-
SiHMe2, HSiMeEt2, HSiMe2Ph, HSiEt3, and HSiiPr3 were
distilled just before use. Other reagents were used as received.
Column chromatography was carried out using silica gel (Merck
No. 1.07734.9025). The ruthenium complexes 1-4 were prepared
according to published methods.9,10 In the reduction of esters
and carboxylic acids, silyl ethers were produced. These could be
isolated as shown below as the representative. However, in most
of the cases, the product determination was carried out after
hydrolysis of the silyl ethers. The products were identified by
comparison of their spectral data with those of authentic samples
or published data. Spectral data of these products are in the
Supporting Information.
Ack n ow led gm en t. A part of this study was finan-
cially supported by the J apan Society for the Pro-
motion of Science (Grants-In-Aid for Scientific Research
12750768, 13450374).
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of 3-phenylpropyl dimethylethylsilyl ether and the
NMR spectral data of the alcohols, ethers, and amines. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Typ ica l P r oced u r e To Syn th esize Silyl Eth er s. To a
solution of 1 (6.0 mg, 9.3 µmol) in dioxane (0.18 mL) was added
dimethylethylsilane (0.30 mL, 2.3 mmol). The mixture was
stirred for 0.5 h at room temperature, and methyl hydrocin-
namate (0.16 mL, 0.91 mmol) was added dropwise at ambient
J O025726N
4988 J . Org. Chem., Vol. 67, No. 14, 2002