Regio- and stereospecific cleavage of α,β-epoxysilanes with lithium phenylsulfide

Article abstract of DOI:10.1016/S0040-4039(99)02242-X

Trimethyl- or dimethylphenylsilylepoxides react with lithium phenylsulfide to give regio- and stereodefined vinyl sulfides resulting from α-ring opening and Peterson elimination. When the epoxide bears the bulky tert-butyldiphenylsilyl group the reaction

Full text of DOI:10.1016/S0040-4039(99)02242-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1111–1114
Regio- and stereospecific cleavage of α,β-epoxysilanes with
lithium phenylsulfide
Purificación Cuadrado and Ana M. González-Nogal ∗
Departamento de Química Orgánica, Universidad de Valladolid, 47011 Valladolid, Spain
Received 4 November 1999; accepted 29 November 1999
Abstract
Trimethyl- or dimethylphenylsilylepoxides react with lithium phenylsulfide to give regio- and stereodefined
vinyl sulfides resulting from α-ring opening and Peterson elimination. When the epoxide bears the bulky tert-
butyldiphenylsilyl group the reaction is more puzzling. Depending on the β-substitution and the presence of alu-
minium chloride, we obtained silyl enol ethers, α-silylaldehydes or α-hydroxy-β-phenylthiosilanes, all resulting
from β-opening. © 2000 Elsevier Science Ltd. All rights reserved.
The considerable importance of regio- and stereospecificity to organic synthesis makes the continued
search for such methodology a high priority challenge. α,β-Epoxysilanes are interesting synthons
because they can serve as versatile Z or E vinyl cation equivalents. They undergo regio- and stereospecific
α-opening by a variety of nucleophiles to give diastereomerically pure β-hydroxysilanes,1,2 which
experience syn or anti β-elimination3 providing a convenient route to a number of heterosubstituted
olefins of known stereochemistry.4
We have recently reported5 that dimethylphenyl- and tert-butyldiphenylsilylepoxides react with lith-
ium diphenylphosphide and then with methyl iodide giving regio- and stereodefined vinylphosphonium
iodides resulting from α-opening and subsequent Peterson elimination. When the methylation was
omitted vinylphosphines were isolated. Otherwise, when an α-hindered tert-butyldiphenylsilylepoxide
bore a β-phenyl group we obtained stereospecifically the corresponding silyl enol ether by β-opening
followed by the Brook rearrangement with simultaneous anti-elimination of methyldiphenylphosphine.
We have now found that silylepoxides prepared by epoxidation6,7 of vinylsilanes, obtained, in turn,
by dimethylphenylsilyl-8 and tert-butyldiphenylsilylcupration7 from alkynes, react with lithium phenyl-
sulfide in THF9 to give different products depending on the nature of the silyl group and also of the
substitution pattern of the epoxides (Scheme 1). All new compounds showed satisfactory spectroscopic
and analytical data.10
Trimethyl- or dimethylphenylsilylepoxides 1a–d and 1g react under mild conditions (−78°C→0°C,
when R=H; −78°C→rt, when R=Bu, Ph), and in short time reactions (1–4 h), providing regio- and
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02242-X
tetl 16170

1112
Scheme 1.
stereodefined11 vinyl thioethers 2a–c and 2f, in one step12 and with good-to-excellent yields. α,β-Bis-
silylepoxides 1e and 1f are shown to be less reactive. In order to obtain good yields of interesting β-
silylvinyl thioethers 2d and 2e, it was necessary to heat the reaction mixture in THF at reflux for longer
reaction times (15–20 h). Fortunately, under these conditions the E geometry of 2d and 2e is retained.
The regio- and stereochemical outcome is consistent with an α-opening with inversion of configuration
followed by a syn β-elimination process (Scheme 2).
Scheme 2.
On the other hand, the bulky tert-butyldiphenylsilyl group changes the regioselectivity and intro-
duces useful differences in the chemistry of the silylepoxides. The behaviour and reactivity of tert-
butyldiphenylsilylepoxides 1h–k toward lithium phenylsulfide depends on the β-substitution type. Thus,
the β-unsubstituted silylepoxide 1h is predominantly attacked by sulfide at the β-position.13 In its
reaction with lithium phenylsulfide it affords a 1:2 mixture of the vinyl thioether 2a, resulting from
α-attack (Scheme 2) and the silyl enol ether 3. The latter is presumably formed by β-opening followed
by Brook rearrangement of the intermediate 6 with anti-elimination of the β leaving group (phenylthio)14
(Scheme 3, path a). Although the silyl enol ether 3 has no stereochemistry, an anti stereochemistry
was first demonstrated by Hudrlik et al.15 for this elimination. Moreover, the same mechanism and
stereochemistry has been proposed by us5 to explain the formation of the Z silyl enol ether resulting
from β-opening of trans 1-tert-butyldiphenylsilyl-2-phenyloxirane with lithium diphenylphosphide.
Likewise, the formation of α-silylaldehyde 4 resulting from β-phenylsilylepoxide 1i may be logically
explained via the same intermediate 6 by silicon migration to the β-carbon with concomitant loss of the
β-leaving group16 (Scheme 3, path b).
Both cis and trans β-butylsilylepoxides 1j and 1k are recovered untransformed even after prolonged
heating with lithium phenylsulfide in THF at reflux. Nevertheless, when aluminium chloride is added to
the mixture of 1k and lithium phenylsulfide, the erythro α-hydroxysilane 5, resulting from β-attack on
the backside, is isolated. In this case, ring β-opening would be expected to be governed by the relative

1113
Scheme 3.
stability of an incipient β-silyl carbocation generated by the initial electrophile coordination. In these
conditions, the intermediate similar to 6 (probably an aluminium α-alkoxysilane) does not undergo Brook
rearrangement and affords 5 in the final hydrolysis.
In summary, since epoxysilanes have been obtained by epoxidation of vinylsilanes, the overall process
using trimethyl- or dimethylphenylsilyl derivatives provides a general regio- and stereospecific method
for converting vinylsilanes into vinylsulfides with retention of configuration. Other methods for preparing
vinylsulfides, such as addition of thiols17 to alkynes or reactions of carbonyl compounds with sulfur-
modified Wittig or related reagents,18 are not stereospecific. Apart from the known applications of vinyl
thioethers19 we should point out the special utility of sulfur and silicon bifunctionalized olefins 2d and 2e
in organic synthesis.20 For example, they serve as reagents for thiophenyl-functionalized cyclopentenone
annulations via Nazarov cyclizations.21
On the other hand, all β-opening products obtained starting from tert-butyldiphenylsilylepoxides
are of great synthetic utility. The silyl enol ether is one of the more versatile functional groups.22
The formation of α-tert-butyldiphenylsilyl aldehyde 4 is particularly noteworthy since attempts to
isolate α-trimethylsilyl and α-dimethylphenylsilyl aldehydes were unsuccessful.23 Moreover, they are
useful reagents for the stereoselective synthesis of α-vinylcarbonyl compounds.24 Finally, the synthetic
possibilities of β-sulfur gem-hydroxysilanes as precursors of silyl enol ethers, β-sulfur acylsilanes, β-
silyl sulfones, etc, will be the subject of later research.
Acknowledgements
We thank the Spanish Junta de Castilla y León for financial support. (Grant VA73/99).
References
1. (a) Hudrlik, P. F.; Hudrlik, A. M.; Rona, R. J.; Misra, R. N.; Withers, G. P. J. Am. Chem. Soc. 1977, 99, 1993. (b) Hudrlik, P.
F.; Peterson, D.; Rona, R. J. J. Org. Chem. 1975, 40, 2263.
2. Fleming, I.; Lawrence, N. J. J. Chem. Soc., Perkin Trans. 1 1992, 3309.
3. For reviews, see: Ager, D. J. J. Org. React. 1990, 38, 1, and Synthesis 1984, 384.
4. Hudrlik, P. F.; Peterson, D. Tetrahedron Lett. 1974, 1133; J. Am. Chem. Soc. 1975, 97, 1464.
5. Cuadrado, P.; González-Nogal, A. M. Tetrahedron Lett. 1997, 38, 8117.
6. Fleming, I.; Newton, T. W. J. Chem. Soc., Perkin Trans. 1 1984, 119.
7. Barbero, A.; Cuadrado, P.; Fleming, I.; González-Nogal, A. M.; Pulido, F. J.; Sánchez, A. J. Chem. Soc., Perkin Trans. 1
1995, 1525.
8. Fleming, I.; Newton, T. W.; Roessler, F. J. Chem. Soc., Perkin Trans. 1 1981, 2527.
9. General procedure: To a stirred THF solution of lithium phenylsulfide [prepared from benzenethiol (0.1 ml, 1 mmol) and
butyllithium (0.625 ml, 1.6 M solution in hexane, 1 mmol) in THF (5 ml) at −78°C under an N2 atmosphere for 10 min]
was added dropwise a solution of epoxysilane (1 mmol) in THF (5 ml). Depending on the reactivity of the silylepoxide, the

1114
resulting solution was allowed to warm up to 0°C for 1a and 1b, to room temperature for 1c, 1d, 1g and 1i, or heated at
reflux for 1e and 1f. The reaction mixture was stirred at this temperature until TLC indicated complete reaction. Ammonium
chloride solution was added and the mixture was extracted with ether, washed with 5% aqueous sodium hydroxide solution,
dried (MgSO4) and chromatographed.
10. Selected spectroscopy data: compound 2e:1H NMR (CDCl3) δ 7.54–7.51 (m, 2H, PhSi), 7.43–7.27 (m, 8H, PhSi and PhS),
6.75 (d, 1H, J=18.0, _CHS), 5.99 (d, 1H, J=18.0, _CHSi), 0.36 (s, 6H, Me2Si); 13C NMR (CDCl3) δ 140.70 ( _CHS),
139.74, 129.28, 127.45, 127.12 (PhS), 137.03, 133.01, 129.06, 127.74 (PhSi), 126.05 (_CHSi), 0.92 (Me2Si). Compound
1
3: H NMR (CDCl3) δ 7.83–7.19 (m, 10H, PhSi), 6.44 (dd, 1H, J=13.5, 5.8, _CHOSi), 4.56 (d, 1H, J=13.5, _CH cis to
OSi), 4.14 (d, 1H, J=5.8, _CH trans to OSi), 1.18 (s, 9H, Me3CSi); 13C NMR (CDCl3) δ 146.47 (_CHOSi), 135.75, 132.45,
129.79, 127.41 (PhSi), 94.74 (_CH2), 26.49 (MeCSi), 19.05 (CSi). Compound 4: IR (CCl4)/cm−1 1700 (C_O), 1100 (SiPh);
1H NMR (CDCl3) δ 9.86 (d, 1H, J=3.5, HC_O), 7.68–6.96 (m, 15H, PhC, Ph2Si), 4.48 (d, 1H, J=3.5, CHCHO), 0.97 (s,
9H, tBuSi); 13C NMR (CDCl3) δ 200.09 (HC_O), 136.69, 132.27, 131.52, 129.90, 129.76, 127.79, 127.62 (Ph2Si), 134.11,
t
129.47, 128.39, 126.18 (Ph), 53.68 (CHCHO), 27.72, 19.61 (tBu); MS m/z 358 (M+, 17%), 301 (M− Bu, 18), 281 (M−Ph,
1
59), 239 (tBuPh2Si+, 100). Compound 5: IR (film)/cm−1 3450 br (OH), 1100 (SiPh); H NMR (CDCl3) δ 7.82–7.35 (m,
15H, Ph2Si, PhS), 4.28 (d, 1H, J=3.8, CHOH), 3.72 (m, 1H, CHS), 1.67 (s br, 1H, OH), 1.35 (m, 4H, CH2–CH2), 1.16 (s,
9H, tBuSi), 1.06 (m, 2H, CH2), 0.71 (t, 3H, J=7.1, Me); 13C NMR (CDCl3) δ 136.19, 132.73, 129.77, 127.48 (PhSi), 136.67,
132.03, 129.61, 127.87 (PhS), 72.73 (CHOH), 55.38 (CHS), 34.06, 27.90, 22.18, 13.81 (Bu), 28.54, 18.85 (tBuSi).
11. Although the overall process is stereospecific, the initial Z or E vinyl thioether undergoes postisomerization in solution in
the presence of catalytic amounts of benzenethiol. To avoid this problem, the hydrolyzed reaction mixtures were washed
with 5% aqueous sodium hydroxide solution.
12. Unpublished work by Hudrlik and Kulkarni (Hudrlik, P. F.; Hudrlik, A. M.; In Advances in Silicon Chemistry; Larson, G.
L., Ed. α,β-Epoxysilanes. JAI Press: London, 1993; Vol. 2, p. 55) involving two steps (ring opening by PhSH/alumina and
elimination by KH/THF or BF3·Et2O). We have found only one example of the formation of a trans vinyl sulfide resulting
from the α-opening of a trans trimethylsilylepoxide with lithium phenylsulfide: Okamoto, S.; Yoshino, T.; Tsujiyama, H.;
Sato, F. Tetrahedron Lett. 1991, 32, 5793.
13. Regiospecific β-opening of tert-butyldiphenylsilylepoxides has been observed by us in their reactions with methyllithium
at −25°C (Ref. 7) and with lithium diphenylphosphide (Ref. 5).
14. This result is different from the single example described of β-opening using lithium phenylsulfide: Lipshutz, B. H.;
Lindsley, C. Susfalk, R.; Gross, T. Tetrahedron Lett. 1994, 35, 8999. They obtained the β-sulfur α-silyl alcohol resulting
from β-opening of α-epoxytriisopropylsilane without Brook rearrangement.
15. Hudrlik, P. F; Hudrlik, A. M.; Kulkarni, A. K. J. Am. Chem. Soc. 1985, 107, 4260.
16. This pathway has also been observed in the reactions of acylsilanes with diazomethane: Brook, A. G.; Limburg, W. W.;
MacRae, D. M.; Fieldhouse, S. A. J. Am. Chem. Soc. 1967, 89, 4384, and in the reactions of α-chloroacylsilanes with
Grignard reagents: Sato, T.; Abe,T.; Kuwajima, I. Tetrahedron. Lett. 1978, 259.
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A.; Court, A. S. J. Org. Chem. 1972, 37, 939. (d) Trost, B. M.; Keeley, D. E. J. Am. Chem. Soc. 1976, 98, 248.
19. For a description of the synthetic chemistry of vinylsulfides, see: (a) Trost, B. M.; Lavoie, A. C. J. Am. Chem. Soc. 1983,
105, 5075, and references cited therein. (b) Rigby, J. H.; Sage, J. M. J. Org. Chem. 1983, 48, 3591. (c) Morris, T. H.; Smith,
E. H.; Walsh, R. J. Chem. Soc. Chem. Commun. 1987, 964.
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21. Magnus, P.; Quagliato, D. J. Org. Chem. 1985, 50, 1621.
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in Organic Synthesis; Butterworths: London, 1981; pp. 198–287. (d) Weber, W. P. Silicon Reagents for Organic Synthesis;
Springer-Verlag: Berlin, 1983; pp. 206–272.
23. Hudrlik, P. F.; Hudrlik, A. M.; Misra, R. N.; Peterson, D.; Withers, G. P.; Kulkarni, A. K. J. Org. Chem. 1980, 45, 4444.
24. Hudrlik, P. F.; Kulkarni, A. K. J. Am. Chem. Soc. 1981, 103, 6251.