A novel synthesis of silyl enol ethers from α-silylbenzylthiols and carboxylic acid derivatives via C-C bond formation; thermal rearrangement of S-α-silylbenzyl thioesters

Article abstract of DOI:10.1016/S0040-4039(01)02029-9

A new procedure for the synthesis of silyl enol ethers from S-α-silylbenzyl thioesters without need for either bases or catalysts via C-C bond formation is described. Solutions of S-α-silylbenzyl thioesters were simply heated at 180°C for 24 h in a sealed tube to give silyl enol ethers in good yields with high stereoselectivity. Cyclization of the dipoles generated by thermal rearrangement of the silyl group and elimination of sulfur afforded silyl enol ethers.

Full text of DOI:10.1016/S0040-4039(01)02029-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 9221–9223
A novel synthesis of silyl enol ethers from a-silylbenzylthiols and
carboxylic acid derivatives via CꢀC bond formation; thermal
rearrangement of S-a-silylbenzyl thioesters
Mitsuo Komatsu,* Choi Jinil, Eiichiro Imai, Yoji Oderaotoshi and Satoshi Minakata
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
Received 25 September 2001; revised 22 October 2001; accepted 26 October 2001
Abstract—A new procedure for the synthesis of silyl enol ethers from S-a-silylbenzyl thioesters without need for either bases or
catalysts via CꢀC bond formation is described. Solutions of S-a-silylbenzyl thioesters were simply heated at 180°C for 24 h in a
sealed tube to give silyl enol ethers in good yields with high stereoselectivity. Cyclization of the dipoles generated by thermal
rearrangement of the silyl group and elimination of sulfur afforded silyl enol ethers. © 2001 Elsevier Science Ltd. All rights
reserved.
It is well known that silyl enol ethers are very useful
species for carbonꢀcarbon bond formation via reactions
such as the Mukaiyama aldol condensation and the
Mannich reaction.1 Because of this, the preparation and
use of silyl enol ethers have been extensively studied
over the past decades.2 Silyl enol ethers are generally
prepared by thermodynamic reactions of carbonyl com-
pounds with silylating reagents in the presence of ter-
tiary amines via enolates, or by kinetic reactions of
metal enolates which are formed by treatment of car-
bonyl compounds with metal amides or alkoxides.3
While these methods required the addition of basic
reagents, alternative procedures for the preparation of
silyl enol ethers under neutral conditions would involve
the intramolecular rearrangement of silyl groups.4 For
example, the preparation of silyl enol ethers from b-
keto silanes via the 1,3-thermal rearrangement of the
silyl group represents a convenient route, but this
method is not generally used because of problems
encountered in the synthesis of b-keto silanes (Scheme
1).5 From these points of view, we wish to report on a
new method for the formation of silyl enol ethers from
S-a-silylbenzyl thioesters via silicon migration. The
method represents a new preparative method of silyl
enol ethers under completely neutral conditions, with
no need for any catalysts or additives, and involves a
Me3Si
R
Me3Si
Ph
O
R
Ph
Ph
1,3-silatropic shift
1,4-silatropic shift,
S
R
OSiMe3
O
C-C bond formation,
and elimination of sulfur
in a one-pot process
1
2
CuI
DCC or
base
Me3Si
Ph
O
+
Me3Si
Ph
O
+
MgCl Cl
R
SH
X
R
X = OH or Cl
Scheme 1.
Keywords: silyl enol ethers; thermal rearrangement; S-silylmethyl thioesters; CꢀC bond formation.
* Corresponding author. Tel.: +81-6-6879-7400; fax: +81-6-6879-7402; e-mail: komatsu@ap.chem.eng.osaka-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02029-9

9222
M. Komatsu et al. / Tetrahedron Letters 42 (2001) 9221–9223
thermal 1,4-shift of the silyl group onto the oxygen of
the carbonyl function, followed by CꢀC bond forma-
tion. Since S-a-silylbenzyl thioesters 16 are easily pre-
pared by the condensation of a-silylbenzylthiols7 and
carboxylic acid derivatives, silyl enol ethers can be
produced by this procedure from readily available start-
ing materials with simple manipulation as shown in
Scheme 1.
tivity of the silyl enol ether 2d prepared from aliphatic
thioester 1d was slightly improved. It is noteworthy that
the reactions of a,b-unsaturated thioesters 1e, f gave
dienol silyl ethers 2e, f along with small amounts
of 4-substituted 5-phenyl-2-trimethylsiloxythiophenes
(entries 5 and 6). When 1g, having a dimethylphenyl-
silyl group, was employed, the reaction was accelerated
in spite of its bulkiness, since the partial negative
charge developing on the silicon atom in the transition
state can be delocalized by the phenyl group on the silyl
group (entry 7). In the case where a,a-bissilylated
thioester 1h was used, the reaction was complete within
1 h, and gave a single isomer of the corresponding enol
ether 2h (entry 8). The observed acceleration of the
reaction of 1h may be due partly to the stabilization of
the positive charge on the sulfur atom during the
rearrangement step by a b-effect of another silyl group
and partly to the relaxation of steric repulsion between
the two bulky silyl groups. On the other hand, the
reaction of a-trimethylsilylbenzyl benzoate did not
occur, even at 200°C, probably because the interaction
between the silicon and the oxygen of the carbonyl
group is weaker than that of the corresponding
thioester due to the greater electronegativity of the
oxygen of alkoxy group vis-a`-vis that of sulfur.
S-a-Trimethylsilylbenzyl thiobenzoate (1a, 0.05 mmol)
in benzene (0.5 ml) was heated in a sealed tube at
180°C for 24
h to give 1,2-diphenyl-1-trimethyl-
siloxyethylene (2a) in 93% yield with high Z-selectivity.
In a similar manner, silyl enol ethers 2b–h could be
efficiently prepared from thioesters 1b–h, and the
results are summarized in Table 1. The structures and
Z–E ratios of the products were determined by 1H
NMR spectral analysis.8
In cases where aromatic thioesters were used as the
starting materials (entries 1–3, 7, and 8), high yields as
well as high Z-selectivity of the corresponding silyl enol
ethers were observed. The reaction of 1c, bearing an
electron-donating group on the aromatic ring of the
benzoyl function, was complete within a shorter reac-
tion time than those of 1a, b containing no substituents
or electron-withdrawing groups.
We propose a mechanism for the present silyl enol ether
forming reaction as shown in Scheme 2 based on the
generation of 1,3-dipoles from N-(a-silylbenzyl)amides
via 1,4-silatropy and their cycloaddition as outlined in
our previous studies.11 Initially, the thermal 1,4-shift of
the silyl group onto the oxygen in thioesters 1 gives
1,3-dipolar intermediates 3 (thiocarbonyl ylides). Thi-
iranes 4 are formed by the intramolecular cyclization of
dipoles 3, followed by the elimination of sulfur from 4
to give the silyl enol ethers 2. When a,b-unsaturated
thioesters 1e, f are employed in the reactions, a,b-unsat-
urated thiocarbonyl ylides 3e, f undergo 1,3- and 1,5-
cyclization to give silyl enol ethers 2 and thiophenes,
It would be expected that the reaction of 1c occurs at
an accelerated rate due to an increase in the interaction
between the silicon and the oxygen electron-donated
from the 4-methoxyphenyl group in the rate-limiting
rearrangement step. The Z-selectivity of the products
from aromatic thioesters 1a–c, g, and h is higher than
that of the silyl enol ethers prepared by treatment of
benzyl phenyl ketone with LDA and trimethylsilyl chlo-
ride at −78°C.9 Furthermore, compared to the thermo-
dynamic reaction of benzyl methyl ketone with
triethylamine and trimethylsilyl chloride,10 the Z-selec-
Table 1. Synthesis of silyl enol ethers 2 from S-a-silylbenzyl thioesters 1
R2
Me2R1Si R2
O
R3
Ph
benzene,
sealed tube,
180 °C
R3
Ph
S
OSiMe2R1
1
2
Entry
Thioestera
R1
R2
R3
Time (h)
Yield (%)b
Ratio Z:Eb
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
Me
Me
Me
Me
Me
Me
Ph
H
H
H
H
H
H
H
Me3Si
Ph
24
24
8
64
12
2
93
92
91
91:9
91:9
91:9
75:25
85:15
78:22
88:12
SIf
p-NO2C6H4
p-MeOC6H4
Me
(E)-PhCHꢁCH
(E)-CH3CHꢁCH
Ph
44c
76d,e
68d
87
1g
1h
5
1
Me
Ph
91
a All reactions were carried out with 0.05 mmol of thioesters in sealed tubes.
b Determined by 1H NMR in the presence of mesitylene as an internal standard.6
c Starting material (45%) remained in the reaction mixture.
d 4-Substituted 5-phenyl-2-trimethylsiloxythiophene was detected.
e A mixture of E- and Z-isomers of 2e and a small amount of 1,4-dipheyl-3-buten-2-one which was produced by the hydrolysis of 2e were isolated
by preparative TLC.
f Product 2h was a single isomer, but the geometry was not determined.

M. Komatsu et al. / Tetrahedron Letters 42 (2001) 9221–9223
9223
(c) Bassindale, A. R.; Brook, A. G.; Harris, J. J. Organomet.
Chem. 1975, 90, C6.
5. (a) Larson, G. L.; Lo´pez-Cepero, I. M.; Torres, L. E.
Tetrahedron Lett. 1984, 25, 1673–1676; (b) Brook, A. G.;
MacRae, D. M.; Limburg, W. W. J. Am. Chem. Soc. 1967,
89, 5493–5495.
Me3Si
Ph
O
∆
R
Ph
S
R
OSiMe3
1
2
– S
6. Thioesters1werepreparedbyreactionofa-silylbenzylthiols
to acyl chlorides. For example, a procedure of synthesis of
S-a-trimethylsilylbenzyl thiobenzoate 1a is as follows. To
a solution of a mixture of a-trimethylsilylbenzylthiol (2.03
g, 10.0 mmol) and triethylamine (1.47 g, 14.0 mmol) in 40
ml of diethyl ether was added dropwise benzoyl chloride
(1.48 g, 10.0 mmol) at 0°C. The mixture was stirred for 2
h at 0°C, and the reaction was quenched with water. The
aqueous layer was extracted twice with diethyl ether (50 ml),
and the combined extracts were dried with anhydrous
MgSO4. The solution was concentrated in vacuo. Purifica-
tion by recrystallization in benzene/hexane was afforded
S-a-trimethylsilylbenzyl thiobenzoate 1a (2.94 g, 95%).
7. Geiß, K.-H.; Seebach, D.; Seuring, B. Chem. Ber. 1977, 110,
1833–1851.
OSiMe3
R
R
Ph
–
OSiMe3
Ph
S
+
S
3
4
Scheme 2.
respectively. The formation of thiophene derivatives in
the reactions of 1e, f provides us with additional per-
suasive evidence for thiocarbonyl ylides as intermedi-
ates in the reactions.
In summary, we report on the successful development
of a simple new method for the preparation of silyl enol
ethers from S-a-silylbenzyl thioesters via the thermal
1,4-silatropy and cyclization of dipoles. In this method,
the starting materials, S-a-silylbenzyl thioesters, can be
readily prepared by esterification of carboxylic acid
derivatives with a-silylbenzylthiols, and the reactions
proceed under completely neutral conditions. We are
currently investigating the mechanism for this reaction
in more detail, as well as dipolar cycloaddition utilizing
in situ generated intermediates 3.
8. Structures and E/Z ratios of silyl enol ethers 2a, d, g, and
h were determined by comparison of their 1H NMR spectra
with known data. (a) Anders, E.; Stankowiak, A.; Riemer,
R. Synthesis 1987, 931–934; (b) Davis, F.; Sheppard, A. C.;
Chen, B.-C.; Haque, M. S. J. Am. Chem. Soc. 1990, 112,
6679–6689; (c) Fleming, I.; Roberts, R. S.; Smith, S. C. J.
Chem. Soc., Perkin. Trans. 1 1998, 1209–1214; (d) Fu¨rstner,
A.; Seidel, G.; Gabor, B.; Kopiske, C.; Kru¨ger, C.; Mynott,
R. Tetrahedron 1995, 51, 8875–8888. Structures and/or E/Z
ratios of silyl enol ethers 2b, c, e, and f were determined
by1HNMR;Z-2b:1HNMR(C6D6):l−0.07(s, 9H, SiMe3),
1
6.10 (s, 1H, PhCH), 7.0–8.0 (m, 9H, Ar); E-2b: H NMR
(C6D6): l 0.11 (s, 9H, SiMe3), 6.23 (s, 1H, PhCH), 7.0–8.0
(m, 9H, Ar); Z-2c: 1H NMR (C6D6): l 0.05 (s, 9H, SiMe3),
3.30 (s, 3H, OMe), 6.19 (s, 1H, PhCH), 6.7–7.8 (m, 9H,
Acknowledgements
1
Ar); E-2c: H NMR (C6D6): l 0.19 (s, 9H, SiMe3), 3.19
This work was partially supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
(s, 3H, OCH3), 6.26 (s, 1H, PhCH), 6.6–8.0 (m, 9H, Ar);
Z-2e: 1H NMR (C6D6): l 0.13 (s, 9H, SiMe3), 5.81 (s, 1H,
PhCHꢁCOSiMe3-), 7.8–8.2 (m, 12H, Ar and -CHꢁCHPh);
E-2e: 1H NMR (C6D6): l 0.27 (s, 9H, SiMe3), 6.22 (s, 1H,
PhCHꢁCOSiMe3-), 7.8–8.2 (m, 12H, Ar and -CHꢁCHPh);
Z-2f: 1H NMR (CDCl3): l 0.09 (s, 9H, SiMe3), 1.82 (d, 3H,
J=5.7 Hz, Me), 5.64 (s, 1H, PhCHꢁ), 5.94 (dq, 1H, J=15.4
and 5.7 Hz, -CHꢁCHMe), 6.03 (d, 1H, J=15.4 Hz,
-CHꢁCHMe), 7.1–7.5(m, 5H, Ph);E-2f:1HNMR(CDCl3):
l 0.28 (s, 9H, SiMe3), 1.79 (dd, 3H, J=7.0 and 1.1 Hz, Me),
5.88 (s, 1H, PhCHꢁ), 6.14 (dq, 1H, J=15.1 and 7.0 Hz,
-CHꢁCHMe), 6.40 (dq, 1H, J=15.1 and 1.1 Hz,
-CHꢁCHMe), 7.1–7.5 (m, 5H, Ph).
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