Novel catalytic hydrogenolysis of trimethylsilyl enol ethers by the use of an acidic ruthenium dihydrogen complex

Article abstract of DOI:10.1002/(SICI)1521-3773(19991018)38:20<3047::AID-ANIE3047>3.0.CO;2-F

The heterolytic cleavage of H₂ is the key to the novel catalytic hydrogenolysis of trimethylsilyl enol ethers catalyzed by [RuCl(η²- H₂)(dppe)₂]OTf (dppe = 1,2-bis(diphenylphosphanyl)ethane, OTf = trifluoromethanesulfonate), which results in the formation of a ketone and Me₃SiH (see scheme). In addition, the stoichiometric, ruthenium-assisted protonation of a prochiral lithium enolate with H₂ gave a chiral ketone with high enantioselectivity (up to 75 % ee).

Full text of DOI:10.1002/(SICI)1521-3773(19991018)38:20<3047::AID-ANIE3047>3.0.CO;2-F

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diameter pinhole and focusing this light on to a single photon counting
silicon avalanche detector (EG&G SPCM-100).
[8] The switch was flipped by changing the atmosphere within the balloon
between argon or nitrogen and carbon dioxide. This was conveniently
accomplished by slowly purging and filling with the appropriate gas.
[9] The fact that the regenerated signal (Figure 3, bottom trace) originated
from 1 was verified by two observations. Firstly, the detected transients
displayed an identical diffusion time (td ˆ 55‡5 ms) with that of 1, as
given by their auto-correlated function (not shown). Secondly, the
reaction of 1 with carbon dioxide was reversible as indicated by
independent 1H NMR characterization of a scaled-up version of this
process.
Figure 3. Traces of single molecules of 1 diffusing through a 2.8 fL
(10 15 L) fluorescence element. a) Single molecules are readily detected in
n-heptane solution in an argon or nitrogen atmosphere. b) Transients from
these molecules disappear as the atmosphere is changed to carbon dioxide
and c) reappear upon returning to an argon or nitrogen atmosphere.
those from 2. While both materials share comparable
spectroscopic properties and solvent sensitivity, the number
of single molecule events was decreased but not eliminated
when 2 was subjected to the same conditions as those used in
Figures 2 and 3.
The fluorescence observed from the switching of 1 was
translated into a graphical display (Figure 3) by conversion of
the fluorescent photons into an electrical signal by means of a
single-photon counting photoavalanche detector. The relay of
the interaction between atmosphere and molecular photo-
physics to an electrical signal provides a novel approach for
the design of future optical mechanical and electronic devices.
Experimental Section
Measurements were conducted on a conventional confocal single molecule
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encased in a silver holder (2 Â 2 Â 0.6 cm) and loaded with 80 Æ 10 mL of a
100 pm solution of 1 in benzene or toluene was slowly brought into contact
with the cover-slip. The assembly was held in place by mounting the sample
holder on to the objective with four beams (see Figure 2). The droplet of 1
was open to the atmosphere. A balloon was placed over the entire sample
housing and filled with gas. Confocal single molecule examinations were
made repetitively on solutions of 1 in argon, nitrogen, and carbon dioxide
atmospheres.
Novel Catalytic Hydrogenolysis of
Trimethylsilyl Enol Ethers by the Use of an
Acidic Ruthenium Dihydrogen Complex**
Yoshiaki Nishibayashi, Izuru Takei, and
Masanobu Hidai*
Since the first report on a tungsten complex with an h2-
bound H2 ligand, W(h2-H2), by Kubas and co-workers in
1984,[1] a great number of this unique class of complexes have
Received: April 26, 1999
Revised version: July 5, 1999 [Z13318IE]
German version: Angew. Chem. 1999, 111, 3231 ± 3233
[*] Prof. Dr. M. Hidai, Dr. Y. Nishibayashi, I. Takei
Department of Chemistry and Biotechnology
Graduate School of Engineering
Keywords: atmospheric chemistry ´ charge transfer ´ fluo-
rescence ´ molecular switches ´ single molecule spectroscopy
The University of Tokyo, Hongo 7-3-1
Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (‡81)3-5841-7265
E-mail: hidai@chembio.t.u-tokyo.ac.jp
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31, 405; b) J. P. Sauvage, Acc. Chem. Res. 1998, 31, 611; c) B. Kenda, F.
Diederich, Angew. Chem. 1998, 110, 3162; Angew. Chem. Int. Ed. 1998,
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Nature 1998, 396, 60; e) C. D. Mao, W. Q. Sun, Z. Y. Shen, N. C.
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[**] This work was supported by a Grant-in-Aid for Specially Promoted
Research (09102004) from the Ministry of Education, Science, Sports,
and Culture, Japan. We thank Dai Masui and Shin Takemoto for
assistance with NMR analysis.
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/angewandte/ or from the author.
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been found and their structures and reactivities have been
delineated.[2, 3] Dihydrogen complexes allow the design of a
simple fascinating pathway for the heterolytic activation of
molecular dihydrogen (H2) on a transition metal center.
Actually, the heterolytic cleavage of coordinated H2 has been
achieved by treatment with a variety of bases.[2, 3] This has led
to systematic investigation of ligand effects on the reactivity
of coordinated H2 and to the synthesis of highly acidic M(h2-
H2) complexes.[2, 3] However, the catalytic reactions that
proceed through heterolytic activation of H2 by acidic M(h2-
H2) complexes are limited.[3f, 4±7] Very recently, we have
succeeded in the formation of NH3 by the protonation of
the coordinated N2 on a tungsten atom with an acidic Ru(h2-
H2) complex under mild conditions.[8] This finding has led us
to develop a novel catalytic reaction in which the heterolytic
activation of H2 is catalyzed by an acidic Ru(h2-H2) complex.
Transition metal catalyzed hydrogenation of a variety of
unsaturated substrates is an important and conventional
method in preparative organic chemistry.[9] The Wilkinson
complex, [RhCl(PPh3)3], is probably one of the most widely
used catalysts for the homogeneous hydrogenation of car-
bon ± carbon double bonds in the laboratory.[9] For example,
trimethylsilyl enol ethers are converted into saturated tri-
methylsilyl ethers in the presence of [RhCl(PPh3)3] under an
atmospheric pressure of H2.[10, 11] In this case, the addition of
H2 to the carbon ± carbon double bond occurs. In sharp
contrast to the conventional hydrogenation, we have now
found that employment of an acidic Ru(h2-H2) complex as
catalyst results in hydrogenolysis of the Si O bond. In this
relatively high acidity,[14] 1a is considered to be protonated at
the oxygen atom by the coordinated H2 in 4 to give 3a, while
the Me3Si group in 1a eventually binds with the remaining
hydride to form Me3SiH (vide infra). Interestingly, the
reaction between 1a and [RuH(h2-H2)(dppe)2]OTf with low
acidity[15] did not occur.
The direct proton transfer from H2 to 1a catalyzed by
complex 2 was confirmed by the experiment employing D2
gas. The reaction of 1a was carried out under 1 atm of D2 at
508C for 48 h in the presence of 5 mol% of 2 in anhydrous
dichloroethane. This reaction resulted in 3a' in >95% yield
(GLC) with monodeuteration at the a-position (Scheme 2).
O
OSiMe3
H
D
95% D
H
5 mol % 2
+ D2 (1atm)
1a
3a'
D
99% D
H
H
H
H
Ph
Ph
OSiMe3
O
1b
3b'
OSiMe3
Me
O
D
93% D
Me
Me
1c
3c'
‡
reaction, H2 is heterolytically cleaved into H and H on the
Ru center and transferred to the enol oxygen[12] and the
trimethylsilyl silicon atom, respectively, affording a ketone
and Me3SiH. This provides a conceptually new type of
catalytic reaction. Preliminary results on this catalytic reac-
tion are described here.
OSiMe3
Me
O
D
85% D
Me
Me
Me
Me
1d
3d'
Scheme 2. Hydrogenolysis of 1a ± d with D2.
Treatment of 1-trimethylsilyloxy-1-cyclohexene (1a) in the
presence of a catalytic amount of [RuCl(dppe)2]OTf (2)
(10 mol%) (dppe ˆ 1,2-bis(diphenylphosphanyl)ethane, Tf ˆ
trifluoromethanesulfonyl) under 1 atm of H2 in anhydrous
C6D6 at 508C for 3 h in an NMR tube afforded cyclohexanone
(3a) and Me3SiH in almost quantitative yields by NMR
spectroscopy (Scheme 1). This novel reaction proceeded
Other trimethylsilyl enol ethers (1b ± d) were also protonated
with D2 to give ketones (3b' ± d') in >95% yields (GLC) with
monodeuteration at the a-position. Incorporation of D at the
a-position of 3a' ± d' was quantitatively analyzed by 1H NMR
spectroscopy and GC-MS (see Scheme 2 and Supporting
Information for experimental details). These results indicate
that one of the deuterium atoms of the coordinated D2 is
catalytically transferred to the oxygen atom, and the resultant
enol then tautomerizes to form the corresponding ketone with
monodeuteration at the a-position. When THF or benzene
was used as solvent in place of dichloroethane, almost the
same results were obtained.
By contrast, the catalytic hydrogenation of trimethylsilyl
enol ethers using [RhCl(PPh3)3] under the same reaction
conditions led to the formation of the corresponding tri-
methylsilyl ethers. For example, treatment of 1b in the
presence of 5 mol% of [RhCl(PPh3)3] under 1 atm of H2 in
anhydrous C6H6 at 508C for 24 h afforded (1-phenyl-1-
trimethylsilyloxy)ethane in >95% yield (GLC).
OSiMe3
O
10 mol % 2
+
H2
+
Me3SiH
C6D6, 50 °C
1a
3a
Scheme 1. Hydrogenolysis of 1a into 3a and Me3SiH. 29Si{1H} NMR of 1a
(C6D6): d ˆ 14.6; 29Si{1H} NMR of Me3SiH (C6D6): d ˆ 16.4.
smoothly at 508C, however, very slowly at room temperature.
The formation of Me3SiH was confirmed by the 1H and
29Si{1H} NMR spectra of the reaction mixture. No other
products were observed by NMR spectroscopy, gas ± liquid
chromatography (GLC), and gas chromatography mass
spectrometry (GC-MS). Under 1 atm of H2, complex 2 was
quantitatively converted into [RuCl(h2-H2)(dppe)2]OTf (4)
within 5 min at ambient temperature. Since complex 4 has
Figure 1 shows the unequivocal effect of H2 on the
conversion of 1a into 3a catalyzed by complex 2. Under
1 atm of N2 the reaction of 1a in the presence of 1 mol% of 2
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H2
(1 atm)
H
H
OTf
H
P
P
P
P
P
P
P
P
+
Me3SiH
Ru
+
Ru
Me3SiOTf
C6D6
Cl
Cl
4
5
Scheme 4. Conversion of 5 into 4. 31P{1H} NMR of 4 (C6D6): d ˆ 51.6 (s);
31P{1H} NMR of 5 (C6D6): d ˆ 62.7 (s).
Based on these findings we propose a mechanism for the
formation of 3a and Me3SiH from 1a and H2 (Scheme 5). The
initial step is the protonation of the trimethylsilyl enol ether
Figure 1. Effect of the reaction atmosphere on the conversion of 1a into
3a. y ˆ yield.
OSiMe3
OTf
H H
in THF at 508C did not occur at all. When the N2 atmosphere
was replaced by 1 atm of H2 after 45 min, the reaction began
and 3a was rapidly produced. However, the reaction was
again stopped after 60 min when the reaction atmosphere was
changed back to 1 atm of N2. Further production of 3a started
when the N2 atmosphere was again replaced by 1 atm of H2
after 150 min. This result demonstrates that H2 is essential to
this catalytic reaction.
To elucidate the mechanism of this novel catalytic reaction,
the following stoichiometric and catalytic reactions were
investigated. First, treatment of lithium enolate 6a, which was
prepared from 1a and MeLi, with one equivalent of [RuCl(h2-
D2)(dppe)2]OTf (4') under 1 atm of D2 in anhydrous THF at
room temperature afforded 3a' with monodeuteration at the
a-position in >95% yield (GLC) and a deuteride complex
[RuClD(dppe)2] (5') in 85% yield of isolated product
(Scheme 3). This provides the direct evidence of heterolytic
P
P
P
P
Ru
1a
O
+
Cl
4
H2
3a
Me3SiOTf
H
OTf
PPh2
Ru
P
P
P
P
Ph2P
Ph2P
Ru
Cl
Cl
5
PPh2
2
Me3SiH
Scheme 5. A mechanism for the hydrogenolysis of 1a.
Me3SiOTf
probably at the oxygen atom with H2 coordinated on the Ru
atom; this yields 3a and Me3SiOTf together with 5. Subse-
quent reaction of Me3SiOTf with 5 under 1 atm of H2 results
in the formation of the starting Ru(h2-H2) complex 4 via 2,
concurrent with Me3SiH. We believe that a delicate balance of
the acidity of the Ru(h2-H2) complex 4 and the nucleophilicity
of the hydride complex 5 might rationalize this novel hydro-
genolysis.
O
OLi
OTf
D
D
88% D
+
D
96% D
D
D
2 (1 atm)
P
P
P
P
P
P
P
P
+
Ru
Ru
+
LiOTf
THF
Cl
4'
Cl
5'
6a
3a'
Finally, the present hydrogenolysis has been extended to
the asymmetric protonation of silyl enol ether 1c and the
corresponding lithium enolate 6c with [RuCl(h2-H2)((S)-
BINAP)2]OTf (7) (BINAP ˆ (2,2'-bis(diphenylphosphanyl)-
1,1'-binaphthyl). Treatment of 1c with one equivalent of 7 at
788C in CH2Cl2 under 1 atm of H2 afforded 3c in >95%
yield (GLC) with no enantioselectivity. On the other hand,
the reaction of lithium enolate 6c with 1 equiv of 7 at 788C
in CH2Cl2 under 1 atm of H2 produced 3c with 75%ee (S) and
8[17] in >95% yields (Scheme 6).[18] The enantioselective
protonation of prochiral enolates has attracted much atten-
tion,[19] thus providing a new approach to the asymmetric
protonation of enol ethers. Further work on the effect of
chiral ligands around the Ru atom is currently in progress.
In summary, we have found novel catalytic hydrogenolysis
of trimethylsilyl enol ethers with H2 catalyzed by [RuCl(h2-
H2)(dppe)2]OTf (4). In this reaction, the Si O bond is
heterolytically cleaved by coordinated H2 to form a ketone
and Me3SiH. We have also demonstrated the stoichiometric,
but enantioselective ruthenium-assisted protonation of a
= dppe
P
P
Scheme 3. Reaction between D2 complex 4' and lithium enolate 6a.
‡
cleavage of D2 coordinated on the ruthenium atom: D is
probably transferred to the sp2 carbon of 6a to form 3a' (vide
infra), while D remains at the ruthenium atom as a deuteride.
Furthermore, treatment of 2-cyclohexen-1-one in the pres-
ence of a catalytic amount of 2 under 1 atm of H2 at 508C did
not give 3a at all. This result indicates that the hydrogenolysis
of 1a does not proceed via 2-cyclohexen-1-one, which might
be formed from dehydrosilylation of 1a.[16] Second, the
reaction of [RuClH(dppe)2][14a] (5) with one equivalent of
Me3SiOTf under 1 atm of H2 in anhydrous C6D6 at room
temperature rapidly gave 4 in quantitative NMR yield
together with Me3SiH (Scheme 4). In this stoichiometric
reaction Me3SiOTf behaves as a hydride acceptor. Thus,
complex 2 may be initially formed from complex 5 by reaction
with Me3SiOTf, which is immediately converted into the
starting Ru(h2-H2) complex 4 under 1 atm of H2.
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COMMUNICATIONS
[1] G. J. Kubas, R. R. Ryan, B. I. Swanson, P. J. Vergamini, H. J. Wasser-
man, J. Am. Chem. Soc. 1984, 106, 451.
H
H
H
OTf
P
P
P
P
P
P
P
P
Ru
Ru
[2] For recent examples, see: a) A. C. Ontko, J. F. Houlis, R. C. Schnabel,
D. M. Roddick, T. P. Fong, A. J. Lough, R. H. Morris, Organometallics
1998, 17, 5467; b) D. H. Lee, B. P. Patel, E. Clot, O. Eisenstein, R. H.
Crabtree, Chem. Commun. 1999, 297; c) Z. Xu, I. Bytheway, G. Jia, Z.
Lin, Organometallics 1999, 18, 1761.
[3] For recent reviews, see: a) P. G. Jessop, R. H. Morris, Coord. Chem.
Rev. 1992, 121, 155; b) D. M. Heinekey, W. J. Oldham, Chem. Rev.
1993, 93, 913; c) R. H. Morris, Can. J. Chem. 1996, 74, 1907; d) R. H.
Crabtree, J. Organomet. Chem. 1998, 577, 111; e) S. Sabo-Etienne, B.
Chaudret, Chem. Rev. 1998, 98, 2077; f) M. A. Esteruelas, L. A. Oro,
Chem. Rev. 1998, 98, 577.
Cl
Cl
8
7
H
2 (1 atm)
+
+
–78 °C
OLi
O
H
Me
Me
*
3c
75% ee (S)
6c
= (S)-BINAP
P
P
[4] a) R. T. Hembre, S. McQueen, J. Am. Chem. Soc. 1994, 116, 2141;
b) R. T. Hembre, J. S. McQueen, V. W. Day, J. Am. Chem. Soc. 1996,
118, 798.
Scheme 6. Asymmetric protonation of 6c.
prochiral lithium enolate with H2 to give a chiral ketone with
high enantioselectivity (up to 75%ee).
[5] a) W. C. Chan, C. P. Lau, Y. Z. Chen, Y. Q. Fang, S. M. Ng, G. Jia,
Organometallics 1997, 16, 34; b) H. S. Chu, C. P. Lau, K. Y. Wong,
W. T. Wong, Organometallics 1998, 17, 2768.
[6] V. I. Bakhmutov, E. V. Vorontsov, D. Y. Antonov, Inorg. Chim. Acta
1998, 278, 122.
Experimental Section
[7] The first examples of the direct involvement of M(h2-H2) complexes in
catalytic hydrogenation are found in the following articles: a) C.
Bianchini, A. Meli, M. Peruzzini, P. Frediani, C. Bohanna, M. A.
Esteruelas, L. A. Oro, Organometallics 1992, 11, 138; b) C. Bianchini,
C. Bohanna, M. A. Esteruelas, P. Frediani, A. Meli, L. A. Oro, M.
Peruzzini, Organometallics 1992, 11, 3837; c) M. A. Esteruelas, J.
Herrero, A. M. Lopez, L. A. Oro, M. Schulz, H. Werner, Inorg. Chem.
1992, 31, 4013.
[8] Y. Nishibayashi, S. Iwai, M. Hidai, Science 1998, 279, 540.
[9] ªReductions in Organic Chemistryº: M. Hudlicky, ACS Symp. Ser.
1996, 188.
[10] For examples, see a) M. Tanaka, Y. Watanabe, T. Mitsudo, Y.
Yasunori, Y. Takegami, Chem. Lett. 1974, 137; b) R. Kuwano, S.
Okuda, Y. Ito, J. Org. Chem. 1998, 63, 3499.
Preparation of 2 ´ (CH2Cl2)0.5
: This complex was prepared from cis-
[RuCl2(dppe)2] and NaOTf by a procedure similar to that for reported
for [RuCl(dppe)2]PF6.[14a] A solution of NaOTf (2.13 g, 12.4 mmol) and
cis-[RuCl2(dppe)2] (10.0 g, 10.3 mmol) in THF (100 mL) and EtOH
(50 mL) was stirred at room temperature for 12 h under 1 atm of Ar.
After evaporation of the solvents, the residue was extracted with CH2Cl2
(20 mL). Addition of hexane to the concentrated solution gave 2 ´
(CH2Cl2)0.5 (8.96 g, 7.97 mmol, 77%) as dark red crystals; 1H NMR
(CDCl3): d ˆ 1.65 (br.s, 4H), 2.56 (br.s, 2H), 2.65 (br.s, 2H), 6.78 ± 7.76
(m, 40H); 31P{1H} NMR (CDCl3): d ˆ 55.6 (br.t, J ˆ 12 Hz), 83.7 (brt, J ˆ
12 Hz); elemental analysis (%) calcd for C53H48ClF3O3P4SRu ´ (CH2Cl2)0.5
C 57.12, H 4.39; found: C 57.13, H 4.57.
:
7: To a solution of 8 (1.152 g, 0.83 mmol) in dichloromethane (10 mL) and
THF (10 mL) was added HOTf (80 mL) by syringe under 1 atm of H2. The
reaction mixture was stirred at room temperature for 30 min. The color of
the solution turned from yellow to red during the reaction. Hexane (50 mL)
was then added to the reaction mixture to give a pale red solid 7, which was
collected by filtration, washed with hexane (3 Â 20 mL), and dried under
reduced pressure (1.050 g, 0.68 mmol, 82%); 1H NMR (CD2Cl2): d ˆ 9.11
(br., 2H), 5.2 ± 8.8 (m, 64H); a minimum T1 value of 21 ms (400 HMz) at
243 K was obtained for the broad signal at d ˆ 9.11, assignable to h2-H2;
31P{1H} NMR (CD2Cl2): d ˆ 2.5 (t, J ˆ 27 Hz), 26.3 (t, J ˆ 27 Hz); elemental
analysis (%) calcd for C89H66ClF3O3P4SRu: C 69.73, H 4.34; found: C 69.74,
H 4.38.
[11] In the catalytic hydrogenation of trimethylsilyl enol ethers, the
corresponding ketones are sometimes observed as by-products. This is
explained by the reaction of trimethylsilyl enol ethers with adventi-
tious water in the solvent.
[12] Since no enantioselectivity was observed in the stoichiometric
reaction of 1c (see Scheme 2) with 7 (see Scheme 6), we consider
that the hydrogenolysis of trimethylsilyl enol ethers catalyzed by 4
(see Scheme 4) proceeds through O-protonation by the activated H2
ligand.[13] Thus, in this novel reaction an enol product is initially
produced, which isomerizes to the corresponding keto form.
[13] A study on protonolysis of several silyl enol ethers showed that
C-protonation occurred in the case of tert-butyldimethylsilyl enol
ethers while no conclusion was made for trimethylsilyl enol ethers;
M. H. Novice, H. R. Seikaly, A. D. Seiz, T. T. Tidwell, J. Am. Chem.
Soc. 1980, 102, 5835.
[14] a) B. Chin, A. J. Lough, R. H. Morris, C. T. Schweitzer, C. DꢁAgostino,
Inorg. Chem. 1994, 33, 6278. b) The pKa value of [RuCl(h2-
H2)(dppe)2]PF6 was estimated to be 6.0.
[15] a) E. P. Cappellani, S. D. Drouin, G. Jia, P. A. Maltby, R. H. Morris,
C. T. Schweitzer, J. Am. Chem. Soc. 1994, 116, 3375. b) The pKa value
of [RuH(h2-H2)(dppe)2]PF6 was estimated to be 15.0.
[16] The formation of 2-cyclohexen-1-one by the PdII-catalyzed dehydro-
silylation of 1a was reported; Y. Ito, T. Hirao, T. Saegusa, J. Org.
Chem. 1978, 43, 1011.
Asymmetric protonation of 6c with 7 (Scheme 6): A solution of 6c was
prepared by lithiation of 1c (25.0 mg, 0.10 mmol) with methyllithium
(0.10 mL of 1.02n solution in diethyl ether, 0.10 mmol) in Et2O (3 mL) at
room temperature for 2 h under 1 atm of N2. A solution of complex 7
(150 mg, 0.10 mmol) in dry dichloromethane (5 mL) was then added to the
above solution of 6c at 788C under 1 atm of H2. The mixture was stirred
at that temperature for 4 h under 1 atm of H2. Then the reaction mixture
was gradually warmed up to room temperature and stirred at room
temperature for 12 h. GLC analysis showed the formation of 3c (>95%).
The solvent was removed under reduced pressure, and the residue was
extracted with Et2O (3 Â 5 mL). The extract was purified by TLC (SiO2,
hexane/EtOAc ˆ 7/3 as eluent) to afford 3c as a pale yellow liquid (12 mg,
0.075 mmol, 75%). The remaining residue was recrystallized from CH2Cl2/
Et2O to give 8 as a yellow solid (95 mg, 0.069 mmol, 69%). The absolute
[17] H. Kawano, T. Ikariya, Y. Ishii, M. Saburi, S. Yoshikawa, Y. Uchida, H.
Kumobayashi, J. Chem. Soc. Perkin Trans. 1 1989, 1571.
[18] We suspect that the reaction of lithium enolate 6c with 7 favors
C-protonation by the activated H2 ligand because high enantioselec-
tivity was achieved in this case, in contrast to the reaction of 1c.[12]
[19] a) A. Yanagisawa, K. Ishihara, H. Yamamoto, Synlett 1997, 411, and
references therein; b) C. Fehr, Angew. Chem. 1996, 108, 2726; Angew.
Chem. Int. Ed. Engl. 1996, 35, 2566, and references therein.
configuration of (S)-3c was determined by its optical rotation; [a]D19
ˆ
30
(c ˆ 0.40 in dioxane); the 75%ee value of (S)-3c was determined by GLC
(helium carrier gas, 1208C column temperature, 20:1 split ratio) on a
cyclodextrin phase (Chiraldex GT-A, 30 m); retention time of (R)-3c ˆ
22.87 min (12.6%); retention time of (S)-3c ˆ 24.01 min (87.4%).
Received: May 18, 1999 [Z13438IE]
German version: Angew. Chem. 1999, 111, 3244 ± 3247
Keywords: asymmetric synthesis ´ enols ´ protonations ´
ruthenium ´ silicon
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