Titanocene-catalyzed formation of allylsilanes from allyl ethers and chlorosilanes

Article abstract of DOI:10.1016/j.tetlet.2003.12.101

A new method for silylation of allyl ethers with chlorosilanes has been developed by the use of Cp₂TiCl₂ as a catalyst. This reaction proceeds efficiently at -20°C in THF using ⁿBuMgCl. A plausible reaction pathway via allyltitanocene intermediate was proposed.

Full text of DOI:10.1016/j.tetlet.2003.12.101

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1699–1702
Titanocene-catalyzed formation of allylsilanes from allyl ethers
and chlorosilanes
Shinsuke Nii, Jun Terao* and Nobuaki Kambe*
Department of Molecular Chemistry and Science and Technology Center for Atoms, Molecules and Ions Control, Osaka University,
Suita, Osaka 565-0871, Japan
Received 21 November 2003; revised 13 December 2003; accepted 17 December 2003
Abstract—A new method for silylation of allyl ethers with chlorosilanes has been developed by the use of Cp2TiCl2 as a catalyst.
n
This reaction proceeds efficiently at )20 °C in THF using BuMgCl. A plausible reaction pathway via allyltitanocene intermediate
was proposed.
Ó 2003 Elsevier Ltd. All rights reserved.
1. Introduction
yield along with 94% yield of PhOSinPr3 (2) (Fig. 1). The
product was obtained in pure form in 94% yield by
HPLC.
Allyl alcohols and their derivatives are useful synthetic
intermediates as readily available reagents for intro-
duction of allylic moieties into organic molecules.1
There have been developed a number of catalytic reac-
tions employing late transition metals2 that allowed the
use of a varied of allyl alcohol derivatives such as allyl
ethers, acetates, carbonates, sulfonates, etc. As for early
transition metal catalysts, it is known that Zr3 and Ti4
complexes catalyze carbon–carbon bond forming reac-
tion of allyl ethers with ethyl Grignard reagent, how-
ever, allyl ethers are still rarely used. We have recently
established new methods for regioselective silylation of
alkenes and 1,3-butadienes with chlorosilanes by the
combined use of Grignard reagents and titanocene5 or
zirconocene6 catalyst. Here we disclose the transforma-
tion of allyl ethers and a thioether to the corresponding
allylsilanes by the aid of titanocene catalyst using chloro-
When the reaction was carried out at 0 or 25 °C, the
yields of 1 decreased in 68% or 35%, respectively. The
use of EtMgCl instead of nBuMgCl resulted in poor
t
yield (28%)7 and no reaction took place with BuMgBr
or PhMgCl. When Ti(OiPr)4 was used instead of
Cp2TiCl2, only a 10% yield of 1 was obtained. Cp2ZrCl2
was ineffective under the same conditions.
Results obtained using some other allyl ethers and
chlorosilanes are shown in Table 1. Alkyl and silyl allyl
ethers also afforded 1 in 71% and 78% yields, respec-
tively (runs 1 and 2). When cinnamyl phenyl ether 5 was
used, 6 was obtained regioselectively in good yield (run
3).8 Allyl ether 7 possessing a Ph group at the b-carbon
gave the corresponding allylsilane 8 in good yield (run
4). It should be noted that 6 was also formed from allyl
ether 9 (run 5). The evidence that allyl ethers 5 and 9
gave the identical product 6 implies that those reactions
involve the same intermediate. Two silyl groups could be
n
silanes and BuMgCl.
For example, to a THF solution of phenyl allyl ether
(1 mmol), nPr3SiCl (2.0 mmol), and nBuMgCl (2.5 mmol)
was added a catalytic amount of Cp2TiCl2 (0.05 mmol)
at )20 °C. After stirring for 15 h, the reaction was
quenched with H2O. NMR analysis of the crude mixture
indicated the formation of allyltripropylsilane (1) in 98%
Cp2TiCl2 (5 mol%)
nBuMgCl (2.5 mmol)
SinPr3
nPr3SiCl
OPh
+
THF, -20 oC, 15 h
1, 98%
1 mmol
2 mmol
+
PhOSinPr3
Keywords: Titanocene dichloride; Grignard reagent; Allyl ether; Chloro-
silane; Allylsilane.
2, 94%
* Corresponding authors. Tel.: +81-6-6879-7388; fax: +81-6-6879-7390;
e-mail: kambe@chem.eng.osaka-u.ac.jp
Figure 1. Titanocene-catalyzed formation of allylsilane.
0040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.12.101

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S. Nii et al. / Tetrahedron Letters 45 (2004) 1699–1702
Table 1. Titanocene-catalyzed formation of allylsilanea
Run
1
Substant
R in R3Si–Cl
nPr
Product
Yield (%)b
71
n
OctO
1
3
Me SiO
3
2
3
nPr
Et
1
78
4
PhO
Ph
Et Si
Ph
3
87 (83)
6
5
Ph
Ph
4
Et
82 (78)
PhO
PhO
Et Si
3
7
8
5
6
Et
6
94 (85)
Ph
9
n
Pr Si
3
nPr
45 (42) [E/Z ¼ 78/22]
O
n
11
Si Pr
3
10
PhS
7
nPr
1
79
12
a Allyl ether (1.0 mmol), chlorosilane (2.0 mmol), nBuMgCl (2.5 mmol), Cp2TiCl2 (0.05 mmol), )20 °C, 15 h.
b NMR yield. Isolated yield is given in parentheses.
introduced at terminal carbons of the 2-butene skeleton
of 2,5-dihydrofuran (10) (run 6). Phenylthio group could
also be replaced with a silyl group to give allylsilane in
79% yield (run 7).
butane
+
Cp Ti
2
butenes
14
A plausible reaction pathway of this reaction is outlined
in Scheme 1. Titanocene dichloride reacts with 2 equiv of
nBuMgCl at low temperatures to generate dibutyltit-
anocene (13),9 which readily decomposes to Ti(II) com-
plex 14 along with butane and butenes.9b Thus formed 14
reacts with allyl ether to afford allyltitanocene complex
15.10 Subsequent transmetallation of 15 with 2 equiv of
nBuMgCl gives allyl Grignard reagent 1611 and alkoxy-
magnesium compound (17) along with regeneration of
13. Then 16 and 17 react with chlorosilane to give
allylsilane 18 and alkoxysilane 19, respectively.
Cp TiCl
2
2
n
RO
BuMgCl
n
Cp Ti Bu
2
2
n
2 BuMgCl
13
OR
Cp Ti
MgCl
ROMgCl
2
+
16
17
In order to confirm the validity of the proposed pathway,
we carried out several control experiments focusing on
the active species of the C–Si bond forming process.
Since it is known that (iPrO)2Ti(II) reacts with allyl
ethers to form (iPrO)2TiOR(allyl), which react with
aldehydes to give homoallyl alcohols,10 we first examined
whether a similar allyltitanocene(IV) complex can be
15
R SiCl
3
SiR
ROSiR
3
3
+
18
19
Scheme 1. A plausible reaction pathway.

S. Nii et al. / Tetrahedron Letters 45 (2004) 1699–1702
1701
References and notes
n
2 BuLi
PhO
—
Ph
n
Cp TiCl
2
Cp Ti Bu
2
2
2
o
20 oC, 1 h
OH
1. Godleski, S. A. In Comprehensive Organic Synthesis; Trost,
B. M., Ed.; Pergamon: Tokyo, 1991; Vol. 4, Chapter 3.3.
2. (a) Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; John Wiley and Sons: New
York, 2002; Vol. 2, Chapter 1, and references cited therein;
(b) Franco, D.; Wenger, K.; Antonczak, S.; Cabrol-Bass,
D.; Dunach, E.; Rocamora, M.; Gomez, M.; Muller, G.
Chem. Eur. J. 2002, 8, 664–672, and references cited
therein.
—78 C
OPh
PhCHO
Cp Ti
2
Ph
25 oC, 1 h
Ph
Ph
21, 63%
20
3. (a) Visser, M. S.; Harrity, J. P. A.; Hoveyda, A. H. J. Am.
Chem. Soc. 1996, 118, 3779–3780; (b) Takahashi, T.;
Kondakov, D. Y.; Suzuki, N. Oraganometallics 1994, 13,
3411–3412; (c) Suzuki, N.; Kondakov, D. Y.; Takahashi,
T. J. Am. Chem. Soc. 1993, 115, 8485–8486; (d) Morken, J.
P.; Didiuk, M. T.; Hoveyda, A. H. J. Am. Chem. Soc.
1993, 115, 6697–6698.
4. Kulinkovich, O. G.; Epstein, O. L.; Isakov, V. E.;
KhmelÕnitskaya, E. A. Synlett 2001, 49–52.
5. Terao, J.; Kambe, N.; Sonoda, N. Tetrahedron Lett. 1998,
Et SiCl
3
n
1) 2 BuMgCl
Et Si
Ph
3
2) 2 Et SiCl
3
6, 58%
ClMg
Ph
22
39, 9697–9698.
6. Terao, J.; Torii, K.; Saito, K.; Kambe, N.; Baba, A.;
Sonoda, N. Angew. Chem., Int. Ed. 1998, 37, 2653–2656.
7. The reaction using EtMgCl was slow and 59% of allyl
ether was recovered under the identical conditions. This
can be explained by a hypothesis that degradation of
Cp2TiEt2 gives Cp2Ti(ethylene), which is stable and does
not readily afford active species Cp2Ti. The Cp2Ti(ethyl-
ene) complex formed by a reaction of Cp2Ti(PMe3)2 with
ethylene was identified by NMR; Alt, H. G.; Schwind, K.;
Rausch, M. D.; Thewalt, U. J. Organomet. Chem. 1988,
349, C7–C10; (C5Me5)2Ti(ethylene) complex has been
isolated; Cohen, S. A.; Auburn, P. R.; Bercaw, J. E. J. Am.
Chem. Soc. 1983, 105, 1136–1143.
Scheme 2. Transmetallation of allyltitanocene.
formed in our reaction system. To a THF solution of
Cp2Ti(II), generated by the reaction of Cp2TiCl2 with
2 equiv of nBuLi,9 was added a stoichiometric amount of
cinnamyl phenyl ether 5 at )20 °C. After stirring for 1 h
benzaldehyde (1.5 equiv) was added and the solution was
stirred for another 1 h at 25 °C. NMR and GC analysis
indicated the formation of homoallyl alcohol in 63%
yield suggesting that 20 was generated. However, similar
reaction using Et3SiCl (2 equiv) instead of PhCHO under
the identical conditions did not afford the expected
product 6. On the other hand, 6 was obtained in 58%
yield when a reaction was performed in the presence of
nBuMgCl (2 equiv). These results suggest that allyltit-
anocene(IV) species (20) is generated in this reaction
system but inert toward chlorosilanes and that 6 is
obtained by the reaction of chlorosilane with 22 formed
8. Typical experimental procedure. (E)-1-Phenyl-3-(triethyl-
silyl)prop-1-ene (6). To a mixture of cinnamyl phenyl ether
5 (210 mg, 1 mmol), Et3SiCl (310 mg, 2.0 mmol), and a
n
THF solution of BuMgCl (2.8 mL, 2.5 mmol) was added
a catalytic amount of titanocene dichloride (11.4 mg,
0.05 mmol) at )20 °C. After stirring for 15 h, H2O was
added to the solution at )20 °C, and the mixture was
warmed to 20 °C. A saturated aqueous NH4Cl solution
(50 mL) was added, and the product was extracted with
Et2O (50 mL), dried over MgSO4, and evaporated to give a
crude product (87% NMR yield). Purification by HPLC
with CHCl3 as an eluent afforded 181 mg (83%) of
allylsilane 6. Registry number 63522-98-5: The products
1, 8, 11 are also known compounds. Registry numbers: 1,
18105-48-1; 8, 104014-97-3; 11, 349125-14-3.
n
by transmetallation of 20 with BuMgCl (Scheme 2).12
In summary, a new method for preparation of allyl-
silanes from allyl ethers and chlorosilanes has been
developed by the aid of a titanocene catalyst. The present
reaction involves (i) oxidative addition of allyl ethers to
Cp2Ti(II), (ii) transmetallation of allyltitanocenes with
nBuMgCl to afford allyl Grignard reagents, and (iii)
electrophilic trapping of allyl Grignard reagents with
chlorosilanes in the carbon–silicon bond forming step.
There are many catalytic reactions using allyl ethers as
precursors of allyl anions or their synthetic equivalents.
In these reactions, the late transition metals have been
employed.2b;13 The present study provides the first
example of this type catalyzed by early transition metals.
9. (a) McDermott, J. X.; Wilson, M. E.; Whitesides, G. M.
J. Am. Chem. Soc. 1974, 96, 947–948; (b) McDermott,
J. X.; Wilson, M. E.; Whitesides, G. M. J. Am. Chem. Soc.
1976, 98, 6529–6536.
10. Kasatkin, A.; Nakagawa, T.; Okamoto, S.; Sato, F. J. Am.
Chem. Soc. 1995, 117, 3881–3882.
n
11. As for transmetallation of allyltitanocene with BuMgCl
giving rise to allyl Grignard reagent, see: Nii, S.; Terao, J.;
Kambe, N. J. Org. Chem. 2000, 65, 5291–5297; Nii, S.;
Terao, J.; Kambe, N. J. Org. Chem., in press.
12. Cinnamylmagnesium chloride reacted with trimethyl-
chlorosilane at a terminal carbon to give trimethyl-3-
phenyl-2-propenyl-silane, see: Robert, R. M.; Kaissi, F. E.
J. Organomet. Chem. 1968, 12, 79–88.
Acknowledgements
13. (a) Kimura, M.; Shimizu, M.; Shibata, K.; Tazoe, M.;
Tamaru, Y. Angew. Chem., Int. Ed. 2003, 42, 3392–3395;
(b) Araki, S.; Kamei, T.; Hirashita, T.; Yamamura, H.;
Kawai, M. Org. Lett. 2000, 2, 847–849; (c) Kimura, M.;
Tomizawa, T.; Horino, Y.; Tanaka, S.; Tamaru, Y.
This research was financially supported in part by the
Ministry of Education, Culture, Sports, Science, and
Technology of Japan. S.N. is grateful to JSPS for the
Research Fellowship Program for Young Scientists.

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S. Nii et al. / Tetrahedron Letters 45 (2004) 1699–1702
Tetrahedron Lett. 2000, 41, 3627–3629; (d) For reviews,
see: Tamaru, Y. J. Organomet. Chem. 1999, 576, 215–231;
(e) Tamaru, Y. In Handbook of Organopalladium Chem-
istry for Organic Synthesis; Negishi, E., Ed.; John Wiley
and Sons: New York, 2002; Vol. 2, Chapter 3.4, and
references cited therein.