Titanocene-Catalyzed Regioselective syn-Hydrosilation of Alkynes

Article abstract of DOI:10.1021/ol035277t

Equation presented. The titanium catalyst, which was generated in situ from titanocene dichloride and 2 equiv of butyllithium, was found to catalyze hydrosilation of a variety of alkynes with excellent regio- and syn-selectivity.

Full text of DOI:10.1021/ol035277t

ORGANIC
LETTERS
2
003
Vol. 5, No. 19
479-3481
Titanocene-Catalyzed Regioselective
syn-Hydrosilation of Alkynes
3
Tamotsu Takahashi,* Fengyu Bao, Guohua Gao, and Masamichi Ogasawara
Catalysis Research Center and Graduate School of Pharmaceutical Sciences,
Hokkaido UniVersity, and CREST, Science and Technology Corporation (JST),
Kita-ku, Sapporo 060-0811, Japan
tamotsu@cat.hokudai.ac.jp
Received July 11, 2003
ABSTRACT
The titanium catalyst, which was generated in situ from titanocene dichloride and 2 equiv of butyllithium, was found to catalyze hydrosilation
of a variety of alkynes with excellent regio- and syn-selectivity.
Alkenylsilanes have been used as important building blocks
in organic synthesis. In addition to classical organic trans-
formation of alkenylsilanes, novel applications of these
catalyzed reactions have encountered difficulty in controlling
stereochemistry of alkenylsilane products. For instance,
hydrosilation of terminal alkyne RCtCH often afforded a
mixture of three isomers I-III and complete control of
stereochemistry in the hydrosilation of alkynes remains to
be resolved.
4
1
compounds to transition metal catalyzed reactions have been
developed recently2 that have enhanced synthetic usefulness
of alkenylsilanes.
,3
However, the utility of alkenylsilanes has been somewhat
reduced by a lack of regio- and stereoselective synthetic
methods of these compounds. Among the synthetic methods
of alkenylsilanes, hydrosilation of alkynes is one of the most
straightforward methods and the most attractive one from
an atom-economical point of view. Late transition metal
catalysts of Pt, Pd, Rh, etc. have been frequently used for
hydrosilation of alkynes; however, the late-transition metal
4
In this paper, we wish to report highly regioselective syn-
hydrosilation of alkynes catalyzed by titanocene species.
Although titanocene derivatives have been utilized for
5
6
7
8
hydrosilation of olefins, ketones, imines, and pyridines,
to the best of our knowledge, this is the first example of the
alkyne hydrosilation catalyzed by a group 4 metal catalyst
(
1) (a) Colvin, E. W. Silicon in Organic Synthesis; Butterworth: London,
1
981; p 44. (b) Weber, W. P. Silicon Reagents for Organic Synthesis;
Springer: Berlin, 1983; p 79. (c) Colvin, E. W. Silicon Reagents in Organic
Synthesis; Academic: London, 1988; p 7.
except Lewis acid-catalyzed trans-hydrosilation of alkynes
9
(
2) For application of alkenylsilanes to palladium-catalyzed cross-
coupling reactions, see: (a) Hatanaka, Y.; Hiyama, T. Synlett 1991, 845.
b) Hiyama, T. Organosilicon Compounds in Cross-Coupling Reactions.
using MCl
4
(M ) Ti, Zr, and Hf). This paper also represents
a rare example of hydrosilation of alkynes using early
(
1
0
In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P., Eds.;
Wiley-VCH: Weinheim, 1998.
transition metal catalysts.
(
3) For application of alkenylsilanes to rhodium-catalyzed nucleophilic
(5) (a) Harrod, J. F.; Yun, S. S. Organometallics 1987, 6, 1381. (b) Kesti,
M. R.; Abdulrahman, M.; Waymouth, R. M. J. Organomet. Chem. 1991,
417, C12. (c) Corey, J. Y.; Zhu, X.-H. Organometallics 1992, 11, 672. (d)
Kesti, M. R.; Waymouth, R. M. Organometallics 1992, 11, 1095.
(6) Halterman, R. L.; Ramsey, T. M.; Chen, Z. J. Org. Chem. 1994, 59,
2642.
addition reactions, see: (a) Fujii, T.; Koike, T.; Mori, A.; Osakada, K. Synlett
002, 295. (b) Fujii, T.; Koike, T.; Mori, A.; Osakada, K. Synlett 2002,
98. (c) Oi, S.; Honma, Y.; Inoue, Y. Org. Lett. 2002, 4, 667. (d) Oi, S.;
2
2
Taira, A.; Honma, Y.; Inoue, Y. Org. Lett. 2003, 5, 97.
4) (a) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai,
(
S., Rappoport, Z., Eds.; John Wiley: Chichester, 1989; p 1479. (b) Hiyama,
T.; Kusumoto, T. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 8, p 763. (c)
Marciniec, B. ComprehensiVe Handbook on Hydrosilylation; Pergamon
Press: Oxford, 1992; p 130.
(7) (a) Verdaguer, X.; Lange, U. E. W.; Reding, M. T.; Buchwald, S. L.
J. Am. Chem. Soc. 1996, 118, 6784. (b) Tillack, A.; Lefeber, C.; Peulecke,
N.; Thomas, D.; Rosenthal, U. Tetrahedron Lett. 1997, 38, 1533.
(8) Hao, L.; Harrod, J. F.; Lebuis, A.-M.; Mu, Y.; Shu, R.; Samuel, E.
Angew. Chem., Int. Ed. Engl. 1998, 37, 3126.
1
0.1021/ol035277t CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/26/2003

Table 1. Titanium-Catalyzed Hydrosilation of Alkynesa
Scheme 1
It was found that hydrosilation of alkynes was efficiently
catalyzed by a titanium species, which was generated in situ
from titanocene dichloride and 2 equiv of butyllithium
(
Scheme 1). Generally, reactions proceeded smoothly at room
temperature. A representative procedure for the Ti-catalyzed
hydrosilation of alkyne is as follows. To a solution of Cp
TiCl (50.0 mg, 0.200 mmol) in THF (5 mL) was added
2
-
2
n-BuLi hexane solution (1.60 mol/L, 0.25 mL, 0.40 mmol)
at -78 °C. After the solution was stirred for 1 h at this temp-
erature, 4-octyne (1a; 110 mg, 1.00 mmol) and diphenylsilane
(2a; 203 mg, 1.10 mmol) were added subsequently by means
of syringe. The reaction mixture was warmed to room temp-
erature and stirred for 1 h. After aqueous workup, GC ana-
lysis of the reaction mixture showed formation of 4-(diphe-
nylsilyl)-4-hexene 3a in 87% yield (Table 1, entry 1).
The geometry of 3a was determined to be (E) by
a
The reaction was carried out with 1 (1.0 mmol) and 2 (1.1 mmol) in
THF at room temperature in the presence of titanocene catalyst (20 mol %
to 1) generated in situ from Cp2TiCl2 and n-BuLi (2 equiv to Cp2TiCl2)
unless otherwise noted. GC yields. Isolated yields are given in parentheses.
At 50 °C with 2c (0.55 mmol).
b
1
13
comparison of the H and C NMR spectra with those
c
11
reported previously. No formation of the corresponding (Z)-
isomer was detected by NMR and GC analysis. The (E)-
geometry in 3a indicates that the reaction proceeded through
a syn-addition of the hydrosilane to the carbon-carbon triple
bond in 1a. Analogous highly selective syn-addition was also
observed for the reaction of 1b and 2a (entry 2). With
hydrosilane 2b, the reaction proceeded as well giving an (E)-
product 3c exclusively (entry 3).
titanocene-catalyzed hydrosilation is highly stereoselective
in the reactions of other terminal alkynes 1d-f as well
(entries 5-7). All the products were 1-silylalkenes of (E)-
isomers exclusively, and no other regio- or geometrical
isomers were detected.
When a reaction was carried out using sterically compact
phenylsilane 2c at room temperature, a product was obtained
as a complex mixture, which contained a single-hydrosilation
product 3h (15% GC yield), a hydrogenated product 4 (18%
GC yield), and other uncharacterized products (Scheme 2).
However, a double-hydrosilation product 5 was prepared in
The titanium-catalyzed hydrosilation could be applied to
terminal alkyne substrates. A reaction of 1-hexyne (1c) and
2
a in the presence of 20 mol % of the titanocene catalyst
1
afforded an alkenylsilane 3d in 78% yield (entry 4). The H
and 13C NMR analysis of the product revealed that 3d
consisted of a single isomer. A mode of the addition reaction
was found to be 2-hydro-1-silation, and the large coupling
3
constant between the two olefinic hydrogens in 3d ( JHH
)
Scheme 2
18.3 Hz) indicates the product being an (E)-isomer. The
(
9) (a) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654.
b) Sudo, T.; Asao, N.; Yamamoto, Y. J. Org. Chem. 1999, 64, 2494. (c)
Sudo, T.; Asao, N.; Yamamoto, Y. J. Org. Chem. 2000, 65, 8919.
10) (a) Molander, G. A.; Retsch, W. H. Organometallics 1995, 14, 4570.
b) Schumann, H.; Keitsch, M. R.; Winterfeld, J.; Muhle, S.; Molander, G.
(
(
(
A. J. Organomet. Chem. 1998, 559, 181. (c) Dash, A. K.; Wang, J. Q.;
Eisen, M. S. Organometallics 1999, 18, 4724. (d) Dash, A. K.; Wang, J.
X.; Berthet, J. C.; Ephritikhine, M.; Eisen, M. S. J. Organomet. Chem.
2
000, 604, 83. (e) Dash, A. K.; Gourevich, I.; Wang, J. Q.; Wang, J.; Kapon,
M.; Eisen, M. S. Organometallics 2001, 20, 5084.
11) (a) Tillack, A.; Pulst, S.; Baumann, W.; Baudisch, H.; Kortus, K.;
(
Rosenthal, U. J. Organomet. Chem. 1997, 532, 117. (b) Lee, S. J.; Park,
M. K.; Han, B. H. Silicon Chem. 2002, 1, 41.
3480
Org. Lett., Vol. 5, No. 19, 2003

Scheme 3
Scheme 4
moderate yield (52%) at higher temperature (50 °C) with a
molar ratio of 1f/2c ) 2.0/1.1 (Table 1, entry 8). Each step
of the hydrosilation reactions giving 5 also took place in a
stereoselective manner, and thus 5 was obtained as a single
isomer. Analogous double-hydrosilation was not observed
with the disubstituted dihydrosilanes 2a or 2b, and no triple-
hydrosilation was detected with 2c.
The highly regio- and syn-selective hydrosilation could
be realized for a certain unsymmetrically substituted internal
alkyne. The titanocene-catalyzed reaction of 1-(trimethyl-
silyl)propyne (1g) with 2b gave an 1,2-disilyl olefin 3i of
Two possible paths have been suggested for the zir-
conocene-catalyzed olefin hydrosilation (Scheme 4, top).15
One is oxidative addition of an H-Si bond to a divalent Zr
center, and the other is via a zirconium-olefin-silane
(
(
E)-geometry in 82% yield as a sole hydrosilation product
Table 1, entry 9). With this internal alkyne 1g, the mono-
2
2
complex Cp
2 3
Zr(η -olefin)(η -H-SiR ). It is reasonable to
assume that the present titanocene-catalyzed hydrosilation
of alkynes also proceeds through a reaction mechanism
similar to either of the two. Recently, Rosenthal and co-
phenyltrihydrosilane 2c reacted cleanly to give a single-
hydrosilation product 3j in 68% yield (entry 10).
The present catalytic hydrosilation of alkynes is charac-
teristic to titanocene species among group 4 metals. Zir-
conocene and hafnocene species do not catalyze the reactions
under the analogous conditions, although zirconium and
hafnium complexes have been known to catalyze hydrosi-
2
workers reported a trans-η -alkyne complex of titanocene
2
(
B) in which strong (η -Si-H)-Ti agostic interaction was
1
6
evident. Although we have no evidence supporting either
of the two (or other) possible reaction paths, Rosenthal’s
complex could be regarded as a stabilized intermediate of
the latter reaction path (A).
5
b-d,12
lation of olefins.
A transient divalent zirconocene Cp
2
-
II
Zr , which is generated in situ by thermal decomposition of
reacts with 2 equiv of alkynes to form a
stable zirconacyclopentadiene species.
In summary, we have developed a novel catalytic hydrosi-
lation reaction of alkynes using the titanocene derivative as
a catalyst. The reaction proceeded with excellent regiose-
lectivity, and syn-addition products were obtained exclusively
for a variety of substrates.
1
3
Cp
2
Zr(n-Bu)
2
,
13,14
The zirconacy-
clopentadiene is inert to hydrosilanes, and thus a catalytic
cycle of alkyne hydrosilation is not operative with the
zirconocene species. Indeed, aqueous workup of a reaction
2 2
mixture of Cp Zr(n-Bu) , 1a, and 2a afforded a substituted
butadiene derivative 6 with unreacted 2a, but no 3a was
detected (Scheme 3).
Supporting Information Available: Detailed experi-
mental procedures and compound characterization data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(
12) Takahashi, T.; Hasegawa, M.; Suzuki, N.; Saburi, M.; Rousset, C.
J.; Fanwick, P. E.; Negishi, E. J. Am. Chem. Soc. 1991, 113, 8564.
13) Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron Lett. 1986,
7, 2829.
OL035277T
(
2
(
14) (a) Takahashi, T.; Swanson, D. R.; Negishi, E. Chem. Lett. 1987,
(15) Ura, Y.; Hara, R.; Takahashi, T. Chem. Lett. 1998, 233.
(16) (a) Ohff, A.; Kosse, P.; Baumann, W.; Tillack, A.; Kempe, R.; G o¨ rls,
H.; Burlakov, V. V.; Rosenthal, U. J. Am. Chem. Soc. 1995, 117, 10399.
(b) Peulecke, N.; Ohff, A.; Kosse, P.; Tillack, A.; Spannenberg, A.; Kempe,
R.; Baumann, W.; Burlakov, V. V.; Rosenthal, U. Chem. Eur. J. 1998, 4,
1982. (c) Fan, M.-F.; Lin, Z. Organometallics 1997, 16, 494.
6
1
23. (b) Nugent, W. A.; Thorn, D. L.; Harlow, R. L. J. Am. Chem. Soc.
987, 109, 2788. (c) Negishi, E.; Holms, S. J.; Tour, J.; Miller, J. A.;
Cederbaum, F. E.; Swanson, D. R.; Takahashi, T. J. Am. Chem. Soc. 1989,
11, 3336. (d) Van Wagenen, B. C.; Livinghouse, T. Tetrahedron Lett.
989, 30, 3495.
1
1
Org. Lett., Vol. 5, No. 19, 2003
3481