Preparation of alka-2,4-dienylsilanes and 3-cyclopropylprop-2-enylsilanes by the titanocene(II)-promoted reaction of 2,4-bis(phenylthio)but-3-enylsilanes

Article abstract of DOI:10.1016/S0040-4039(00)01477-5

Preparation of allylsilanes by the Cp₂Ti[P(OEt)₃]₂ (1)-promoted reaction of 2,4-bis(phenylthio)but-3-enylsilanes 5 is described. The olefination of carbonyl compounds 6 using the silanes 5 and the low-valent titanium speci

Full text of DOI:10.1016/S0040-4039(00)01477-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8377–8381
Preparation of alka-2,4-dienylsilanes and
3-cyclopropylprop-2-enylsilanes by the
titanocene(II)-promoted reaction of
2,4-bis(phenylthio)but-3-enylsilanes
Takeshi Takeda,* Yasuhiro Takagi, Naoko Saeki and Tooru Fujiwara
Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Koganei,
Tokyo 184-8588, Japan
Received 3 August 2000; accepted 1 September 2000
Abstract
Preparation of allylsilanes by the Cp2Ti[P(OEt)3]2 (1)-promoted reaction of 2,4-bis(phenylthio)but-3-
enylsilanes 5 is described. The olefination of carbonyl compounds 6 using the silanes 5 and the low-valent
titanium species 1 produced alka-2,4-dienylsilanes 2. 3-Cyclopropylprop-2-enylsilanes 3 were obtained by
the reaction of 5 with 1 in the presence of terminal olefins 9. © 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: carbonyl olefination; cyclopropanation; organosulfur compounds; titanium and compounds.
Allylsilanes are useful reagents for organic synthesis and their characteristic reactions are
employed for the construction of complex molecules.1 Although a variety of methods for the
preparation of allylsilanes have been developed, preparation and reactions of functionalized
allylsilanes have not been fully studied yet. Recently, we reported three different methods for the
synthesis of allylsilanes using titanium–carbene complexes formed by the reaction of thioacetals
with Cp2Ti[P(OEt)3]2 (1).2 We further studied the preparation of various allylsilanes using our
titanium–carbene chemistry, and here we wish to disclose convenient methods for the prepara-
tion of allylsilanes having a conjugated diene system or a cyclopropane moiety, 2 or 3, by the
reaction of vinylcarbene complexes 4 carrying a triorganosilyl group (Scheme 1).
* Corresponding author. Tel: +81 42 388 7034; fax: +81 42 388 7034; e-mail: takeda-t@cc.tuat.ac.jp
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01477-5

8378
Scheme 1.
The starting materials, 2,4-bis(phenylthio)but-3-enylsilanes 5, were easily synthesized by the
alkylation of 1,3-bis(phenylthio)allyllithium with trialkylsilylmethyl iodide (1.1 equiv./−78°C/4
h) and easily isolated by recrystallization from hexane (5a: R1=Me; 75%, 5b: R1=Ph; 71%). The
allyllithium was prepared by the treatment of 2-methoxy-1,3-bis(phenylthio)propane with LDA
(2.05 equiv./−78°C/2 h) similarly to the method reported by Corey et al.3 First, the preparation
of alka-2,4-dienylsilanes 2, vinylogues of allyltrialkylsilanes, was studied. Although the olefina-
tion of aldehydes was found to be messy, the dienylsilanes 2 were obtained in moderate to good
yields by the reaction of ketones 6 with 5 (Table 1). For example, after the silane 5a (1.2 equiv.)
was desulfurized with the titanocene(II) species 1 (3.6 equiv.) at room temperature for 20 min,
the resulting vinylcarbene complex was further treated with 1,5-diphenylpentan-3-one 6a to
produce the dienylsilane 2a in 58% yield (entry 1). An interesting application of this method is
the synthesis of 5-oxoalk-2-enylsilanes 7. After the olefination of esters under the usual reaction
conditions, the resulting vinyl ethers 8 were selectively hydrolyzed with hydrochloric acid under
argon to give 7, which would be difficult to prepare by the conventional methods (Scheme 2).
Preparation of (penta-2,4-dienyl)trimethylsilane by silation of the pentadienyl anion and its
addition to carbonyl compounds has been investigated.4 Its derivatives carrying a stereogenic
center have been studied as reagents for the synthesis of optically active compounds.5 A notable
feature of the present method is that the pentadienylsilanes bearing different substituents at the
5-position are readily prepared from ketones.
Since the vinylcarbene complexes react with terminal olefins 9 to produce vinylcyclopropanes,6
we expected that the g-cyclopropylallylsilanes 3 would be formed by the reaction of the silanes
5 with 9. As expected, the silanes 3 were produced in good yields by the treatment of 5 with the
low-valent titanium reagent 1 (2 equiv.) in the presence of excess 9 at room temperature for 2
h. It was found that the stereoselectivity of the double bond was higher than that of the
carbonyl olefination (Table 2).
Scheme 2.

8379
Table 1
Carbonyl olefination of ketones 6 using the silanes 5-titanocene(II) 1
The following is a typical experimental procedure. Finely powdered molecular sieves 4 A (100
mg), magnesium turnings (29 mg, 1.2 mmol; purchased from Nacalai Tesque Inc., Kyoto,
Japan) and Cp2TiCl2 (249 mg, 1 mmol) were placed in a flask and dried by heating with a heat
gun under reduced pressure (2–3 mmHg). During this procedure, care is taken not to sublime
Cp2TiCl2. After cooling, THF (3 ml), P(OEt)3 (0.34 ml, 2 mmol), and styrene (9a) (0.12 ml, 1
mmol) were added successively with stirring at room temperature under argon. After 3 h, a THF

8380
Table 2
Titanocene(II) 1-promoted reaction of silanes 5 with terminal olefins 9
(1.5 ml) solution of 1,3-bis(phenylthio)-4-(trimethylsilyl)but-1-ene (5a) (172 mg, 0.5 mmol) was
added and the reaction mixture was stirred for 2 h. The reaction was quenched by addition of
1 M NaOH and the insoluble materials were filtered off through Celite. The filtrate was
extracted with ether and the organic phase was dried over Na2SO4. After removal of the solvent,
the residue was purified by PTLC (hexane) to yield 97 mg (84%) of 1-phenyl-2-[3-(trimethyl-
silyl)prop-1-enyl]cyclopropane (3a).
Although the preparation and reactions of cyclopropylcarbinylsilanes have been studied,7 to
the best of our knowledge, a synthetic route to its vinylogue has yet not appeared. It should be
noted that the cyclopropanation of olefins using the silanes 5-titanocene(II) system provides a
convenient tool for the synthesis of such compounds. Further study on the reactions of the
functionalized allylsilanes described above is currently under way.

8381
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of
Education, Science, Sports and Culture, Japan (No. 11440213).
References
1. Hosomi, A. Acc. Chem. Rec. 1988, 21, 200–206. Sarkar, T. K. Synthesis 1990, 969–983. Sarkar, T. K. Synthesis
1990, 1101–1111. Fleming, I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063–2192.
2. Fujiwara, T.; Takamori, M.; Takeda, T. Chem. Commun. 1998, 51–52. Takeda, T.; Nozaki, N.; Fujiwara, T.
Tetrahedron Lett. 1998, 39, 3533–3536. Takeda, T.; Watanabe, M.; Rahim, M. A.; Fujiwara, T. Tetrahedron Lett.
1998, 39, 3753–3756.
3. Corey, E. J.; Erickson, B. W.; Noyori, R. J. Am. Chem. Soc. 1971, 93, 1724–1729.
4. Seyferth, D.; Pornet, J. J. Org. Chem. 1980, 45, 1721–1722. Hosomi, A.; Saito, M.; Sakurai, H. Tetrahedron Lett.
1980, 3783–3786.
5. Hayashi, T.; Konishi, M.; Okamoto, Y.; Kabeta, K.; Kumada, M. J. Org. Chem. 1986, 51, 3772–3781. Fleming,
I.; Sarkar, A. K.; Doyle, M. J.; Raithby, P. R. J. Chem. Soc. Perkin Trans. 1 1989, 2023–2030. Fleming, I.; Leslie,
C. P. J. Chem. Soc. Perkin Trans. 1 1996, 1197–1203.
6. Horikawa, Y.; Nomura, T.; Watanabe, M.; Fujiwara, T.; Takeda, T. J. Org. Chem. 1997, 62, 3678–3682.
7. Grignon-Dubois, M.; Pillot, J. P.; Dunogues, J.; Duffaut, N.; Calas, R.; Henner, B. J. Organomet. Chem. 1977,
124, 135–142. Chan, T. H.; Fleming, I. Synthesis 1979, 761–786.
.
.