Highly stereoselective and efficient hydrosilylation of terminal alkynes catalyzed by [RuCl2(p-cymene)]2

Article abstract of DOI:10.1021/ol0059697

(Formula presented) With [RuCl₂(p-cymene)]₂ as a catalyst, extremely high regio-and stereoselectivity was observed in the hydrosilylation reaction of various terminal alkynes under mild conditions to afford β-(Z)-vinylsilanes in exce

Full text of DOI:10.1021/ol0059697

ORGANIC
LETTERS
2000
Vol. 2, No. 13
1887-1889
Highly Stereoselective and Efficient
Hydrosilylation of Terminal Alkynes
Catalyzed by [RuCl2(p-cymene)]2
Youngim Na and Sukbok Chang*
Department of Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea
sbchang@mm.ewha.ac.kr
Received April 19, 2000
ABSTRACT
With [RuCl2(p-cymene)]2 as a catalyst, extremely high regio- and stereoselectivity was observed in the hydrosilylation reaction of various
terminal alkynes under mild conditions to afford â-(Z)-vinylsilanes in excellent yields. A dramatic directing effect was also observed when
alkynes having a hydroxyl group at the â position to the triple bond were employed as a substrate, and in these cases regioisomeric r-vinylsilanes
were generated with excellent selectivity.
Organosilanes play an important role in chemistry as versatile
building blocks.1 Hydrosilylation of multiple bonds, there-
fore, represents a useful class of catalytic processes per-
formed on both laboratory and industry scales. Vinylsilanes,
widely used intermediates for organic synthesis, could be
efficiently prepared by the transition metal catalyzed addition
of silanes to alkynes.2,3 The major consideration in this
conversion is selectivity, the control of which, although
highly desirable, is not an easy task in most cases. For
example, hydrosilylation of monosubstituted alkynes may
give a primary mixture of three isomeric vinylsilanes
(Scheme 1).
tions. Although the regiocontrol reported is excellent in some
cases,4 the regioselectivity, in most metal-catalyzed hydrosi-
lylations, is capriciously affected by various factors such as
types of alkynes and silanes, catalytic metal species, and
reaction conditions including solvent or temperature. Con-
sidering its potential utility in synthetic organic chemistry,
an efficient catalytic system for the highly selective formation
of â-(Z)-vinylsilanes is strongly desirable. In this Letter we
report our results toward this goal.
Although ruthenium complexes are known to be generally
less reactive in hydrosilylation reactions compared to other
late transition metals,5 we envisioned that this property could
be developed into a highly selective catalyst system. Among
(1) (a) Ojima, I.; Li, Z.; Zhu, J. In The Chemistry of Organic Silicon
Compounds; Rappoport, S., Apeloig, Y., Eds.; Wiley: New York, 1998;
Chapter 29. (b) Colvin, E. W. Silicon Reagents in Organic Synthesis;
Academic Press: London, 1988; p 7. (c) Langkopf, E.; Schinzer, D. Chem.
ReV. 1995, 95, 1375 and references therein.
Scheme 1
(2) (a) Hiyama, T.; Kusumoto, T. In ComprehensiVe Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 8, p
763. (b) Reichl, J. A.; Berry, D. H. AdV. Organomet. Chem. 1998, 43, 197.
(c) Marciniec, B. ComprehensiVe Handbook on Hydrosilylation; Pergamon
Press: Oxford, 1992; p 130.
(3) For Lewis acids catalyzed hydrosilylations see: (a) Miura, K.;
Oshima, K.; Utimoto, K. Bull. Chem. Soc. Jpn. 1993, 66, 2356. (b) Buriak,
J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 1339. (c) Sudo, T.; Asao,
N.; Gevorgyan, V.; Yamamoto, Y. J. Org. Chem. 1999, 64, 2494.
(4) For example see: (a) Takeuchi, R.; Tanouchi, N. J. Chem. Soc.,
Perkin Trans. 1 1994, 2909. (b) Takeuchi, R.; Nitta, S.; Watanabe, D. J.
Org. Chem. 1995, 60, 3045.
â-(E)-Vinylsilanes, the thermodynamic products, are usu-
ally formed as a major isomer in most of the transition metal
catalyzed reactions developed. On the other hand, generation
of â-(Z)-isomers with a practical selectiVity is regarded much
more challenging in most user-friendly hydrosilylation reac-
10.1021/ol0059697 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/24/2000

various easily available ruthenium complexes examined for
hydrosilylation of alkynes, [RuCl2(p-cymene)]2 (1)6 exhibited
remarkably high â-(Z)-selectivity under mild reaction condi-
tions (Table 1). The stereochemistry of the double bond in
identical conditions. Although CH2Cl2 was most conveniently
used as a solvent in the reaction, other solvents such as
benzene, 1,2-dichloroethane, or toluene could also be em-
ployed without affecting either activity or selectivity. Em-
ployment of other silanes such as triethylsilane as a silylating
source resulted in similar regio- and stereoselectivity,
although with slightly lower isolated yields compared to the
reactions with triphenylsilane (entries 2 and 4). The ex-
tremely high selectivity for the formation of the â-(Z)-isomer
was maintained to a very similar extent in the hydrosilylation
of various other alkynes examined. The presence of func-
tional groups such as chloro, alkoxy, or ester in the alkyl
chain did not affect the â-(Z) selectivity in the reactions. It
is especially noteworthy that the triple bond is exclusively
hydrosilylated in the presence of an olefinic bond. For
example, reaction of 3-butenyl-4-pentynoate with triphenyl-
silane afforded the â-(Z)-adduct with extremely high chemo-,
regio-, and stereoselectivity (entry 10).
Table 1. Hydrosilylation of Terminal Alkynes Catalyzed by 1a
In the course of the present study, we observed a very
interesting and dramatic directing effect of a hydroxyl group,
if it is suitably positioned to the triple bond, on the
regioselectivity. For example, hydrosilylation of 3-butyn-1-
ol with a trialkylsilane gave the R-isomeric product instead
of â-adducts with excellent selectivity (R-:â-(Z) ) 98:2) in
the presence of 1 (5 mol %) at 45 °C (entry 1 in Table 2).
No trace of the corresponding â-(E)-vinylsilane was detected
by NMR spectroscopy in this reaction. However, the same
reaction with the O-protected alkyne provided the regioiso-
meric product, â-(Z)-vinylsilane, as a major isomer (entry
2). This implies that the dramatically changed selectivity for
the formation of the Markovnikov’s product in the reaction
of 3-butyn-1-ol was caused by the directing effect of the
hydroxyl group, presumably through the coordination of the
carbinol oxygen to a ruthenium metal intermediate.9 The
rather low isolated yields from the reaction of alkynes having
a hydroxyl group in Table 2 were mainly due to the
competitive O-silylation.10
a All reactions were carried out at 0.5 M concentration under N2
atmosphere. b Ratios were determined by 1H NMR integration of the reaction
mixture, and no R-isomer was observed in all cases. c Isolated yields of
pure â-(Z)-products after column chromatography on silica gel.
Although the selectivity for the R-regioisomer was moder-
ate in the reaction of a secondary propargylic alcohol (entry
3), an alkyne substrate having a hydroxyl group â to the
triple bond was hydrosilylated to afford the R-product with
excellent selectivity (entry 4). Upon increasing the chain
length between the hydroxyl group and the triple bond, the
products was unambiguously determined on the basis of the
1
coupling constants (J) of H NMR spectra.7
Reaction of phenylacetylene with triphenylsilane (1.1
equiv) in CH2Cl2 was effectively catalyzed by 1 (5 mol %),
and the alkyne was completely consumed within 2 h at 45
(8) Representative Experimental Procedure. To a solution of the
p-cymene Ru complex (30.6 mg, 0.05 mmol) in CH2Cl2 (2 mL) was added
triphenylsilane (286.5 mg, 1.1 mmol), and the mixture was stirred for 15
min at 45 °C before addition of phenylacetylene (102.1 mg, 1.0 mmol).
The reaction mixture was stirred at the same temperature for 2.5 h under
N2 atmosphere. After evaporation of the solvent under reduced pressure,
the crude mixture was chromatographed on silica gel to afford the
analytically pure (Z)-1-triphenylsilyl-2-phenylethene (341 mg, 94%): 1H
NMR (250 MHz, CDCl3) δ 7.82 (1H, d, J ) 15.3 Hz), 7.66-7.33 (15H,
m), 7.26-6.99 (5H, m), 6.43 (1H, d, J ) 15.3 Hz); 13C NMR (CDCl3, 62.5
MHz) δ 150.8, 138.4, 136.2, 135.5, 129.8, 128.3, 127.9, 125.7; HRMS (EI)
calcd for C26H22Si 362.1492 (M+), found 362.1490.
(9) For precedent examples of directional effects by an existing carbonyl
group on hydrosilylation see: (a) Stork, G.; Jung, M. E.; Colvin, E.; Noel,
Y. J. Am. Chem. Soc. 1974, 96, 3684. (b) Murai, T.; Kimura, F.; Tsutsui,
K.; Hasegawa, K.; Kato, S. Organometallics 1998, 17, 926.
(10) The relative rate between the triple-bond addition vs O-silylation
was ca. 2:1 in 1H NMR competitive experiments using equivalent molecular
amounts of 1-hexyne, 1-butanol, and triphenylsilane in the catalyst system
employed here. Those of secondary and tertiary alcohols vs terminal alkynes
were ca. 5:1 and >10:1, respectively.
1
°C.8 Analysis of the crude reaction mixture by H NMR
indicated that the â-(Z)-adduct was formed with significantly
high selectivity over the other isomers (â-(Z):â-(E):R- ) 96:
4:0). Other silyl derivatives such as fully silylated alkanes
or silylalkynes were not detected in this reaction. The reaction
rate was slowed at lower temperatures, and ca. 30%
conversion was observed after 12 h at 25 °C under otherwise
(5) (a) Esteruelas, M. A.; Herrero, J.; Oro, L. A. Organometallics 1993,
12, 2377. (b) Adams, R. D.; Barnard, T. S. Organometallics 1998, 17, 2567.
(6) Bennett, M. A.; Smith, A. K. J. Chem. Soc., Dalton Trans. 1974,
233.
(7) Values for J were obtained in the range of 13-15 Hz for the double
bond of â-(Z), 18-20 Hz for â-(E), and 1.1-2.6 Hz for R-isomers, which
are in good agreement with those reported in the literature.2-4
1888
Org. Lett., Vol. 2, No. 13, 2000

ing â-(Z)-vinylsilane as an almost sole product (entry 8).
The significantly reduced directing effect in this case can
be attributed to the hindrance of the tertiary hydroxyl group
for coordination to a ruthenium metal intermediate, presum-
ably due to steric reasons.
Table 2. Hydroxyl Group Directed Hydrosilylation of
Alkynesa
As suggested in most reported transition metal catalyzed
reactions, the present hydrosilylation seems to proceed via
initial activation of the H-Si bond of a trialkylsilane by the
catalyst 1.11 Mixing of a trialkysilane with a stoichiometric
amount of the ruthenium complex 1 in CDCl3 produces a
singlet peak at -10.0 ppm in the 1H NMR spectrum (in less
than 10 min at 45 °C), which corresponds to a ruthenium
hydride.12 Upon addition of an alkyne to the generated
ruthenium hydride solution, vinylsilanes are produced im-
mediately and quantitatively with the same selectivity as in
the catalytic case. No isomerization of one vinylsilane to
other isomers occurred by the catalyst 1 under the reaction
conditions employed here. Although the origin of the
extremely high selectivity is not clear now, the catalytic
hydrosilylation protocol described here is characterized by
its excellent chemo-, regio-, and stereoselectivity, functional
group compatibility, and mild reaction conditions, and
therefore it should be an attractive and practical alternative
for the selective preparation of vinylsilanes.
Acknowledgment. We thank the Center for Molecular
Design and Synthesis (CMDS) at KAIST for support of this
research.
Supporting Information Available: 1H and 13C NMR,
IR, and mass spectroscopic data for all vinylsilane com-
pounds obtained in this study. This material is available free
of charge via the Internet at http:/pubs.acs.org.
a Alkynes (1.0 equiv), silane (1.3 equiv), 5 mol % of catalyst 1, CH2Cl2
(0.5 M), 45 °C, 3 h (N2 atmosphere). b Determined by 1H NMR integration
of the reaction mixture. c Isolated yields of the major isomer after column
chromatography on silica gel.
OL0059697
(11) For discussions on mechanistic aspects of hydrosilylation reaction
see: (a) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16. (b)
Jun, C.-H.; Crabtree, R. H. J. Organomet. Chem. 1993, 447, 177. (c)
Hofmann, P.; Meier, C.; Hiller, W.; Heckel, M.; Riede, J.; Schmidt, M. U.
J. Organomet. Chem. 1995, 490, 51. (d) Uozumi, Y.; Tsuji, H.; Hayashi,
T. J. Org. Chem. 1998, 63, 6137. (e) Dash, A. K.; Wang, J. Q.; Eisen, M.
S. Organometallics 1999, 18, 4724 and ref 5a.
(12) (a) Ayllon, J. A.; Sayers, S. F.; Sabo-Etienne, S.; Donnadieu, B.;
Chaudret, B. Organometallics 1999, 18, 3981. (b) Grumbine, S. K.; Mitchell,
G. P.; Straus, D. A.; Tilley, T. D. Organometallics 1998, 17, 5607.
directing effect induced by the hydroxyl group was, as
expected, significantly diminished and the high selectivity
for the formation of â-(Z)-adducts recovered completely
(entries 5-7). On the other hand, a tertiary hydroxyl group
exhibited little directing effect on the regioselectivity regard-
less of its position relative to the triple bond. For example,
hydrosilylation of 3-ethyl-6-heptyn-3-ol gave the correspond-
Org. Lett., Vol. 2, No. 13, 2000
1889