Dehydrogenative silylation of terminal alkynes by iridium catalyst

Article abstract of DOI:10.1016/S0040-4039(99)02174-7

Dehydrogenative silylation of terminal alkynes with hydrosilanes proceeds in the presence of iridium catalyst to afford the corresponding silylacetylenes. When phenylacetylene and triethylsilane were heated in dry DME in the presence of Ir₄(CO)₁₂-PPh₃, (2-phenylethynyl)triethylsilane was obtained in 96% yield with little of hydrosilylated products. The present method is applicable for a variety of terminal alkynes and hydrosilanes to give the corresponding silylacetylenes in good yields with high selectivities. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02174-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 907–910
Dehydrogenative silylation of terminal alkynes by iridium
catalyst
Rie Shimizu and Takamasa Fuchikami ∗
Sagami Chemical Research Center, 4-4-1 Nishi-ohnuma, Sagamihara, Kanagawa229-0012, Japan
Received 25 October 1999; accepted 12 November 1999
Abstract
Dehydrogenative silylation of terminal alkynes with hydrosilanes proceeds in the presence of iridium catalyst
to afford the corresponding silylacetylenes. When phenylacetylene and triethylsilane were heated in dry DME
in the presence of Ir4(CO)12–PPh3, (2-phenylethynyl)triethylsilane was obtained in 96% yield with little of
hydrosilylated products. The present method is applicable for a variety of terminal alkynes and hydrosilanes to
give the corresponding silylacetylenes in good yields with high selectivities. © 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: iridium; iridium compounds; silicon; silicon compounds; alkynes; dehydrogenation; catalysis.
Hydrosilylation of carbon–carbon multiple bonds has been one of the most important laboratory and
industrial methods for the formation of silicon–carbon bonds, and many transition-metal catalysts have
been found to be effective for these reactions.1,2 On the other hand, there have been only a few reports on
transition-metal catalyzed dehydrogenative silylation of terminal alkynes with hydrosilanes to afford the
corresponding alkynylsilanes, selectively. For example, some cationic iridium complexes were reported
to have some catalytic activities toward this reaction, where a lot of the undesired alkenylsilanes were
formed together.3–5 Murai et al. also reported that a dehydrogenatively silylated compound was obtained
as a by-product in iridium catalyzed hydroformylation of terminal alkyne with hydrosilane under CO
pressure.6
(1)
Here, we wish to report that dehydrogenative silylation of terminal alkynes with hydrosilanes proceeds
in the presence of iridium catalysts to afford the corresponding silylacetylenes in good yields with high
selectivities (Eq. (1)).
∗
Corresponding author. Tel: +00 81 42 742 4791; fax: +00 81 42 749 7631; e-mail: tfuchika@n.cc.titech.ac.jp (T. Fuchikami)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02174-7
tetl 16102

908
First, we examined the reaction of phenylacetylene (1a) and triethylsilane (2a) (Eq. (2) and Table
1). When phenylacetylene (1a; 1.00 mmol) and triethylsilane (2a; 1.00 mmol) were heated in DME
in the presence of Ir4(CO)12 (1 mol%) at 100°C for 24 h, (2-phenylethynyl)triethylsilane (3aa) was
obtained only in 5% yield, in addition to a mixture of (phenylethenyl)triethylsilanes in 55% yield.7
[IrCl(cod)]2, IrCl(CO)(PPh3)2, IrH(CO)(PPh3)3, Rh/C, and Rh6(CO)16 were less effective for the present
dehydrogenative silylation. When the reaction was carried out in the presence of Ir4(CO)12–PPh3 catalyst,
3aa was obtained in good selectivity, with little of the hydrosilylated products. In this reaction, the
formation of styrene was also observed as a by-product, which indicates that phenylacetylene acts as a
trapping agent of hydrogen generated in situ. The employment of 2 equiv. of acetylene 1a in the presence
of Ir4(CO)12 (1 mol%)–PPh3 (12 mol%) gave the corresponding silylacetylene in 96% yield, as shown
in Table 1 (run 1). When decreasing the amount of additive phosphine, both yields and selectivities of
the desired 3aa were lower (runs 2 and 3). The silylated acetylene 3aa was obtained in excellent yields
not only in DME but also in hexane, toluene, THF, acetonitrile, and 1,2-dichloroethane as a solvent. The
reaction proceeded more slowly without any solvent.
(2)
Table 1
Dehydrogenative silylation of phenylacetylene (1a) with triethylsilanes (2a)a
The present method is applicable for a variety of terminal alkynes and hydrosilanes to give the
corresponding silylacetylenes with good to excellent results as summarized in Table 2 and Eq. (3). When
tert-butyldimethylsilane (2c), possessing a bulky tert-butyl group on the silicon atom, was employed as
a hydrosilane, a higher reaction temperature and greater amounts of catalyst were required (Table 2, run
2). Siloxy- and alkoxysilanes were also able to be submitted to this reaction (runs 3 and 4). Aliphatic
acetylenes also gave good to excellent results. Thus, 1-octyne (1b) reacted with triethylsilane (2a) at
100°C for 36 h to give (hexylethynyl)triethylsilane (3ba) in 93% yield. Cyclohexylacetylene (1c) and

909
3,3-dimethyl-1-butyne (1d) afforded the desired alkynylsilanes 3ca and 3da in high yields with good
selectivities, respectively.
(3)
Table 2
Dehydrogenative silylation of acetylenes with hydrosilanesa
Our method may be accompanied with the generation of molecular hydrogen in situ. Thus, we exami-
ned the reaction in the presence of unsaturated compounds which were expected to act as a hydrogen-
scavenger (Eq. (4) and Table 3). An equimolar reaction of phenylacetylene (1a) and triethylsilane (2a) in
the absence of any hydrogen-scavenger afforded (phenylethynyl)triethylsilane (3aa) and styrene in 55%
and 38% yields, respectively, in addition to 39% of unchanged 2a. When the reaction was carried out in
the presence of 1.1 equiv. of diethyl maleate, the desired product 3aa was obtained in 79% yield with
57% yield of diethyl succinate. Diphenylacetylene and diethyl fumarate are also effective for trapping
molecular hydrogen which is generated in situ, preventing phenylacetylene (1a) from hydrogenation.
In conclusion, the dehydrogenative silylation of terminal alkynes with hydrosilanes was effectively
catalyzed by Ir4(CO)12–PPh3 to give the corresponding silylacetylenes in good yields with high selectiv-
ities.

910
(4)
Table 3
Reaction of phenylacetylene (1a) with triethylsilane (2a) in the presence of H2-scavengersa
References
1. Speier, J. L. Homogeneous catalysis of hydrosilation by transition metals. In Advances in Organometallic Chemistry, Vol.
17: Catalysis and Organic Synthesis; Academic Press: New York, 1979; pp. 407–447.
2. Ojima, I. The hydrosilylation reaction. In The Chemistry of Organic Silicon Compounds: Part 2; John Wiley & Sons:
Chichester, 1989; pp. 1479–1526.
3. Jun, C.-H.; Crabtree, H. J. Organomet. Chem. 1993, 447, 177–187.
4. Esteruelas, M. A.; Nürnberg, O.; Oliván, M.; Oro, L. A.; Werner, H. Organometallics 1993, 12, 3264–3272.
5. Esteruelas, M. A.; Oliván, M.; Oro, L. A.; Tolosa, J. I. J. Organomet. Chem. 1995, 487, 143–149.
6. Chatani, N.; Yamaguchi, S.; Fukumoto, Y.; Murai, S. Organometallics 1995, 14, 4418–4420.
7. General procedure. A mixture of phenylacetylene (1a, 2.3 mmol), triethylsilane (2a, 1.0 mmol), Ir4(CO)12 (0.01 mmol), PPh3
(0.12 mmol), and dry DME (1.0 ml) was stirred at 100°C for 24 h under Ar in a Pyrex tube. GLC analysis of the reaction
mixture revealed that (phenylethynyl)triethylsilane (3aa) was formed in 96% yield. The mixture was evaporated to be freed
of solvent in vacuo. 3aa was isolated by column chromatography on silica gel eluted with hexane, and had spectroscopic data
identical with reported data.