cis-hydrosilylation of terminal alkynes. However, the Pt-
catalyzed reaction of unsymmetrical internal alkynes, par-
ticularly those with one or two aromatic rings, has received
very little attention probably because of the difficulty in
controlling the regioselectivity of the H-Si bond addition.
To the best of our knowledge, only one example regarding
the Pt/C-catalyzed hydrosilylation of 1-phenyl-1-propyne has
been described, and in this case, a mixture of regioisomers
was obtained.7 Therefore, the search for new selective
procedures and more selective catalysts presents an interest-
ing challenge.
Scheme 1
Previously, we reported the effect of ortho substituents
on the regioselectivity in the palladium-catalyzed hydrostan-
nylation of aryl-substituted alkynes.8 We demonstrated that
this ortho-substituent regiocontrol concept was especially
well suited to the synthesis of stannylated stilbene derivatives
via selective addition of tributyltin hydride to unsymmetrical
diaryl- and heteroarylarylalkynes.9 The results have been
rationalized in terms of electronic polarization across the
alkyne bond, induced by the ortho substituent whatever its
electronic nature. It occurred to us that this unprecedented
ortho-directing effect (ODE) could be extended to control
the regiochemistry of the platinum-catalyzed hydrosilylation
of internal arylalkylalkynes as well as diarylalkynes to
provide trisubstituted vinylsilanes of defined stereochemistry.
Herein, we report our first efforts realizing this goal and we
also compare the hydrosilylation reaction selectivity of
various para-substituted substrates with their corresponding
ortho-substituted derivatives.
Table 1. Platinum-Catalyzed Hydrosilylation of Para- and
Ortho-Substituted Alkynes 1a,b with Triethylsilanea
entry alkynei
catalyst
solvent ratiob 2/3/4 yieldc (%)
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1b
1b
1b
1a
1a
1a
H2PtCl6
H2PtCl6
H2PtCl6
Pt(PPh3)4
Pt/C
PtCl2
PtO2
PtCl2
PtO2
THFd
neat
neat
neat
neat
neat
neat
neat
neat
neat
82/18/0
84/16/0
82/0/18g
65e
87e
70f
0
53/0/47
100/0/0
100/0/0
83/17/0
83/17/0
80/20/0
nd
72h
90h
88e
92e
91e
10
Pt/C
a All reactions were conducted with triethylsilane (1.5 equiv) in the
presence of 5 mol % of platinum catalyst at 60 °C for 1 h. b Ratio was
1
determined by H NMR in the crude reaction mixture. c Isolated yields of
Since hexachloroplatinic acid (Speier’s catalyst) is cur-
rently considered to be the catalyst of choice for cis-
hydrosilylation of terminal alkynes with high regioselectiv-
ity,10 we decided to examine its catalytic activity with Et3SiH
in the case of internal para- and ortho-substituted aryl alkynes
1a,b as model systems (Scheme 1). At first, we studied the
hydrosilylation of arylalkylalkyne 1a bearing a para π-electron-
withdrawing substituent and an ethoxycarbonyl group, and
the results are summarized in Table 1.
Reaction of 1a with triethylsilane (1.5 equiv) in THF (4
M) in the presence of H2PtCl6 (5 mol %) at 60 °C for 1 h
afforded stereoselectively an inseparable 82:18 mixture of
the vinylsilanes 2a and 3a in 65% yield (entry 1, Table 1).
Performing the reaction without solvent increased the yield
of the hydrosilylation reaction but had no significant change
on the regioselectivity (entry 2). To evaluate the influence
of the ortho substituent on the reaction selectivity (ODE),
we next examined the platinum-catalyzed hydrosilylation of
alkyne 1b with an ortho π-electron-withdrawing group. In
this case, when using Speier’s catalyst, no â-isomer 3b could
the vinylsilane mixture after column chromatography. All of the reported
compounds exhibited spectral data in agreement with the assigned structures.
d A 4 M solution was used; other solvents such as toluene, MeCN, or CH2Cl2
could also be employed without affecting selectivity. e Isolated yields of
an inseparable mixture of R and â isomers. f Isolated yield of pure R-isomer
2b after flash column chromatography. g Mixture of E/Z isomers in a 64/
36 ratio. h Exclusively cis-addition of H-Si bond occurred. i Alkynes were
prepared according to ref 11.
be detected, but the reaction gave an 82/18 mixture of
R-isomer 2b and olefin 4b (E/Z 64/36) which were easily
separated by flash chromatography on silica gel (entry 3).
Notwithstanding the hydrosilylation being regioselective, a
notable amount of the side product 4b was formed from a
competing H2PtCl6-catalyzed direct reduction of alkyne 1b.12
This side reaction encouraged us to examine other platinum
catalysts to avoid the reduction. In the presence of Pt(PPh3)4
(entry 4), no reaction took place, probably due to the fact
that platinum does not catalyze the hydrosilylation in the
presence of phosphine ligands.13 Pt/C catalyzed this hydrosi-
lylation reaction, but now the side reduction became domi-
nant and a 53:47 mixture of R-isomer 2b and 4b was
obtained (entry 5). The use of PtO2 or PtCl2 as catalysts
(7) (a) Chauhan, M.; Hauck, B. J.; Keller, L. P.; Boudjouk, P. J.
Organomet. Chem. 2002, 645, 1-13. (b) For other metal-catalyzed
hydrosilylations of 1-phenyl-1-propyne, see: (b) Field, L. D.; Ward, A. J.
J. Organomet. Chem. 2003, 681, 91-97.
(8) Liron, F.; Le Garrec, P.; Alami, M. Synlett 1999, 246-248.
(9) (a) Alami, M.; Liron, F.; Gervais, M.; Peyrat, J. F.; Brion, J. D.
Angew. Chem., Int. Ed. 2002, 41, 1578-1580. (b) Liron, F.; Gervais, M.;
Peyrat, J. F.; Alami, M.; Brion, J. D. Tetrahedron Lett. 2003, 44, 2789-
2794.
(11) Alami, M.; Ferri, F.; Linstrumelle, G. Tetrahedron Lett. 1993, 34,
6403-6406.
(12) When exposing pure 2b to Et3SiH (1.5 equiv) in the presence of
H2PtCl6 (5 mol %) at 70 °C for a prolonged time (3 h), no trace of 4b was
detected. This experiment exclude the formation of 4b from a protodesi-
lylation process. A similar result was obtained from a mixture of R- and
â-regioisomers.
(10) Speier’s catalyst is well-known to provide cis-addition processes
for internal alkynes; see: Tsipis, C. A. J. Organomet. Chem. 1980, 187,
427-446.
(13) Shimada, T.; Mukaide, K.; Shinohara, A.; Han, J. W.; Hayashi, T.
J. Am. Chem. Soc. 2002, 124, 1584-1585.
5626
Org. Lett., Vol. 7, No. 25, 2005
Hamze, Abdallah
