Silylstannation of terminal alkynes using a recyclable palladium(0) catalyst immobilised in an ionic liquid

Article abstract of DOI:10.1039/b205999a

Silylstannanes can be regioselectively added across terminal alkynes in a quantitative fashion in the presence of a palladium(0) catalyst immobilised in the [bmim]PF₆ ionic liquid which can be recycled without loss of activity.

Full text of DOI:10.1039/b205999a

Silylstannation of terminal alkynes using a recyclable palladium(0)
catalyst immobilised in an ionic liquid
Ivan Hemeon and Robert D. Singer*
Department of Chemistry, Saint Mary’s University, Halifax, Canada B3H 3C3.
E-mail: Robert.Singer@stmarys.ca; Fax: (+)-902-420-5261; Tel: (+)-902-496-8189
Received (in Corvallis, OR, USA) 20th June 2002, Accepted 9th July 2002
First published as an Advance Article on the web 26th July 2002
tributyltin hydride with LDA followed by a quench with
chlorotrimethylsilane11 or was purchased from Aldrich. This
was stirred with one of three terminal alkynes containing
various functional groups—phenylacetylene, 1-decyne, and
5-hexyn-1-ol—in the [bmim]PF6 ionic liquid.13 1–5 mol%
Pd(PPh3)4 was immobilised in the dry ionic liquid, as this has
been shown to be the best palladium species to catalyse
silylstannations of terminal alkynes.11a The reaction was
generally carried out with diethyl ether as a co-solvent and
heated to 70 °C. When 5 mol% of the Pd(0) catalyst was used
the mixture of catalyst and ionic liquid appeared turbid and
underwent some colour change to a reddish-brown with
subsequent cycles of the reaction. The possibility that this is an
indication for the formation of some other active catalytic
species is under investigation. However, when 1 mol% of the
catalyst was used the bright yellow colour characteristic of the
Pd(0) catalyst was maintained. Products were easily isolated via
ether extraction of the ionic liquid, the yields of which are
presented in Table 1.†
The silylstannations of the three alkynes proceeded readily in
the [bmim]PF6 ionic liquid in the presence of both 5 mol% and
1 mol% Pd(PPh3)4. As expected, the reactions carried out with
1 mol% palladium required slightly longer reaction times than
those with 5 mol%; however, these are not inordinately long as
these reactions have generally been carried out at 70 °C for
24–48 h in refluxing tetrahydrofuran.11a All reactions pro-
ceeded quantitatively by gas chromatography and isolated
yields were also quite high. The isolated yield of the 5-hexyn-
1-ol adduct was the same as that previously reported for a
4-pentyn-1-ol adduct; although, this reaction shows that the
reagents tolerate a degree of alkyne functionality (it has been
shown that a wide variety of functional groups are tolerated by
this reaction in THF).11a A 91% yield was previously reported
for the reaction of phenylacetylene with Bu3SnSiMe311a and a
52% yield was reported for the reaction of 1-hexyne with the
same silylstannane,12a both reactions being performed in THF
with 1 mol% Pd(PPh3)4. In [bmim]PF6 99% isolated yields of
the adducts formed with phenylacetylene and 1-decyne were
obtained. This may be a result of the ionic liquid facilitating
product isolation as it is exhaustively extracted from the ionic
liquid with multiple ether washes. Since the reactants and
Silylstannanes can be regioselectively added across terminal
alkynes in a quantitative fashion in the presence of a
palladium(0) catalyst immobilised in the [bmim]PF6 ionic
liquid which can be recycled without loss of activity.
Ionic liquids have seen use in many applications owing to their
unique properties.1 Being ionic in nature, they provide a solvent
environment unobtainable with traditional molecular solvents.
They often contain poorly-coordinating anions and thus form a
new class of polar, non-coordinating solvents. As such, ionic
liquids have been used in several catalytic reactions involving
many different catalysts.2 Several catalysts have been im-
mobilised in ionic liquids and successfully recycled as a result
of the involatile nature of ionic liquids. These include palladium
catalysts for use in Stille,3 Suzuki,4 and Heck coupling
reactions,5 as well as several other coupling reactions,6
a
chromium catalyst for asymmetric epoxide openings,7 a chiral
manganese epoxidation catalyst,8 and a ruthenium catalyst for
asymmetric alkene hydrogenations.9 Few reactions involving
organometallic reagents have been performed in ionic liquids,
possibly stemming from the relatively high acidity of the proton
on C-2 of the N,NA-dialkylimidazolium ring on which the most
prevalent ionic liquids are based. One such ionic liquid is 1-n-
butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF6,
the ionic liquid used in this study (Fig. 1). The synthesis and
reactivity of organozinc reagents has been studied in a non-
imidazolium-based ionic liquid, showing the potential of some
organometallic reactions in ionic liquids.10
Silylstannanes are versatile organometallic reagents in or-
ganic synthesis. These compounds contain a silicon–tin bond
and are easily synthesized by generating a trialkyltin anion
followed by a quench with a trialkylsilyl chloride.11 The
resulting silylstannanes can be added across unsaturated organic
compounds via a palladium catalyst to afford a bifunctional
organic dianion equivalent containing both carbon–silicon and
carbon–tin bonds.11,12 These products can then be selectively
destannylated or desilylated, substitutions can be made for the
silyl or stannyl groups, or they can participate in such reactions
as Stille couplings. As the addition of silylstannanes across
unsaturated systems uses expensive palladium catalysts, im-
mobilisation of the palladium catalyst in an ionic liquid that
allows recycling of the catalyst without loss of activity was seen
as a potentially useful application of ionic liquids in synthesis.
This would also add to the growing number of organometallic
reactions shown to proceed in ionic liquids.
Table 1 Silylstannations of terminal alkynes
We report herein the regioselective addition of silylstannanes
to terminal alkynes in the presence of a palladium(0) catalyst.
This reaction proceeds cleanly, giving a terminally silylated,
internally stannylated alkene as a single cis-bismetallated
isomer.11a Yields obtained using the catalyst immobilised in an
ionic liquid affords equal to improved isolated yields of adducts.
This system also allows facile product isolation and easy
recycling of the immobilised catalyst without loss of activity.
The silylstannane employed, trimethyl(tributylstannyl)silane
(Bu3SnSiMe3), was easily synthesized via deprotonation of
Entry
R group
mol% Pd
Rxn time (h) Yield (%)a
1b
2
Ph
Ph
5
1
18
20
100 (78)
100 (99)
3
4
5
1
18
24
100 (89)
100 (87)
5
6
C8H17
C8H17
5
1
15
24
99 (97)
99 (99)
a GC yield (isolated yield in brackets). b Reaction performed without
ether.
Fig. 1 [bmim]PF6.
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CHEM. COMMUN., 2002, 1884–1885
This journal is © The Royal Society of Chemistry 2002

products form an upper layer in those reactions conducted
without ether and also since products can be efficiently isolated
from this system via ether extraction it follows that when a
biphasic system is used the majority of the reactants and
products reside in the upper ethereal layer. Small portions of the
reactants partition into the catalyst/ionic liquid layer where the
bismetallation occurs and then the products, once formed,
partition predominantly back into the upper ethereal layer
leaving behind the regenerated Pd(0) catalyst in the ionic liquid
layer. It is also possible that the reaction is proceeding further to
completion in the ionic liquid than when it is performed neat or
in THF. Ether was not used as a cosolvent in the first addition
of Bu3SnSiMe3 across phenylacetylene (Table 1, entry 1) and
the isolated yield was only 78%. Ether was used from this point
forward since phenylacetylene was partially evaporating and
condensing onto the flask walls, thus lowering the isolated
yields of adducts. Evaporation of the ether was avoided through
use of a dried reflux condenser attached to the reaction flask.
Temperature was maintained at a constant 70 °C using an oil
bath heated by a thermally controlled hotplate. All subsequent
isolated yields from reactions using ether as a cosolvent were
higher.
The identities of the isolated compounds were confirmed by
GC/MS as well as by 1H and 13C NMR. The 1H NMR spectra
were especially useful as they helped confirm the ster-
eochemistry of the silylstannylated alkene product. Coupling
constants for the Sn–H coupling to the lone vinylic proton are
known to be around 180 Hz for trans coupling but are around
100 Hz for cis and geminal coupling.11a The Sn–H vinylic
coupling constant for all isolated adducts was 160–178 Hz,
indicating trans coupling and that only cis-bismetallated
adducts were formed. As well, only a single peak was present in
the GC trace, indicating only one isomer.
Ionic liquids have been described as being potential “green
solvents” mainly due to their involatility, since they cannot
contaminate the atmosphere and can be reused. The silylstanna-
tion of alkynes is also a green reaction as it exhibits total “atom
economy” in that all atoms of the starting reagents are
incorporated into the products and no by-products are produced.
Recycling of the Pd(PPh3)4/[bmim]PF6 catalyst/ionic liquid
system was relatively straight forward. After a reaction was
performed in 1.0 mL of [bmim]PF6 containing 1 or 5 mol%
Pd(PPh3)4 and the products were extracted under an inert
atmosphere, a further amount of Bu3SnSiMe3 and the desired
alkyne were injected followed by 5.0 mL of dry ether and the
reaction was repeated. The results of these recycling experi-
ments are presented in Table 2.
equilibrate with the possible hexaalkyldistannane and hexaalk-
yldisilane compounds through a disproportionation reaction
catalysed by Pd(PPh3)4.11a Use of equimolar amounts of
silylstannanes and alkynes should avoid the formation of the
distannane adduct and give the expected product cleanly.
In summary, we have shown that the ionic liquid [bmim]PF6
can be used as a solvent for the palladium-catalysed reaction of
silylstannanes with terminal alkynes to afford 1-trimethylsilyl-
2-tributylstannylalk-1-enes in quantitative yields. We have
shown that the palladium catalyst can be immobilised in the
ionic liquid phase and reused at least four times without
appreciable loss in activity. The reaction proceeds cleanly and
the use of the ionic liquid allows for easy product/catalyst
separation and product isolation. It is likely that the system can
be recycled many more than four times, allowing extensive,
simple recycling of expensive palladium catalysts to form the
highly synthetically useful silylstannylated alkene products.
These aspects of this system as well as the use of other
palladium catalysts and kinetic studies are currently underway
and will be a topic of a subsequent full account of this
research.
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (PGS-A to I. H. and
Discovery Grant to R. D. S.) and by Saint Mary’s University,
Senate Research. NMR spectra were obtained at the Atlantic
Regional Magnetic Resonance Centre.
Notes and references
† Representative silylstannation procedure: 1.0 mL [bmim]PF6 was dried
under vacuum at 60 °C for 2 h. Pd(PPh3)4 (0.05 or 0.01 mmol) was added
in an argon glove box and stirred to dissolve. Bu3SnSiMe3 (1.2 mmol) was
injected under nitrogen via needle and syringe while stirring, followed by
the desired alkyne (1.0 mmol). 5.0 mL freshly dried and distilled diethyl
ether was injected and the reaction vessel was fitted with a dry reflux
condenser. The biphasic reaction mixture was heated to 70 °C under
nitrogen for 18–24 h and was monitored by gas chromatography. Upon
completion, the ether layer was removed via needle and syringe and the
ionic liquid was washed with dry ether (8 3 10 mL). The products were
separated from excess Bu3SnSiMe3 by flash chromatography with hexanes
in the cases of phenylacetylene and 1-decyne and with 5+1 hexanes/ethyl
acetate in the case of 5-hexyn-1-ol to afford products in near quantitative
yield as clear oils. The ionic liquid containing the catalyst could then be
reused.
1 (a) P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39,
3772; (b) T. Welton, Chem. Rev., 1999, 99, 2071.
2 R. Sheldon, Chem. Commun., 2001, 3, 2399.
The recycling experiments were monitored by gas chroma-
tography and showed that the catalyst/ionic liquid system could
be recycled at least four times with negligible loss in activity
(Table 2, entries 1–4), even with only 1 mol% Pd(PPh3)4 (Table
2, entries 5 and 6). Gas chromatography performed on the final
ether extracts after each cycle showed no traces of either starting
materials or products. The only products formed were the
bismetallated alkene, unreacted Bu3SnSiMe3, and a small
amount of Bu3SnSnBu3. The appearance of the distannane
product was not surprising as silylstannanes are known to
3 S. T. Handy and X. Zhang, Org. Lett., 2001, 3, 233.
4 C. J. Matthews, P. J. Smith and T. Welton, Chem. Commun., 2000,
1249.
5 (a) H. Hagiwara, Y. Shimizu, T. Hoshi, M. Ando, K. Ohkubo and C.
Yokoyama, Tetrahedron Lett., 2001, 42, 4349; (b) A. J. Carmichael, M.
J. Earle, J. D. Holbrey, P. B. McCormac and K. R. Seddon, Org. Lett.,
1999, 1, 997.
6 (a) E. Mizushima, T. Hayashi and M. Tanaka, Green Chemistry, 2001,
3, 76; (b) S. Toma, B. Gotov, I. Kmentova and E. Solcaniova, Green
Chemistry, 2000, 2, 149; (c) W. Chen, L. Xu, C. Chatterton and J. Xiao,
Chem. Commun., 1999, 1247.
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1743.
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Jessop, J. Am. Chem. Soc., 2001, 123, 1254.
Table 2 Recycling of Pd(PPh3)4/[bmim]PF6 for silylstannations of terminal
alkynes with Bu3SnSiMe3 for 16–20 h
Entry
Alkyne
mol% Pd
Cycle
Yield (%)a
10 T. Kitazume and K. Kasai, Green Chemistry, 2001, 3, 30.
11 (a) B. L. Chenard and C. M. Van Zyl, J. Org. Chem., 1986, 51, 3561; (b)
I. Hemeon and R. D. Singer, Silyltin Reagents, in Houben-Weyl Methods
of Molecular Transformations, Science of Synthesis, ed. I. Fleming,
Georg Theime Verlag, New York, 2002, p. 205.
12 (a) T. N. Mitchell, R. Wickenkamp, A. Amamria, R. Dicke and U.
Schneider, J. Org. Chem., 1987, 52, 4868; (b) I. Beletskaya and C.
Moberg, Chem. Rev., 1999, 99, 3435; (c) M. Suginome and Y. Ito,
Chem. Rev., 2000, 100, 3221; (d) M. Lautens and J. Mancuso, Synlett,
2002, 3, 394.
1b
2b
3b
4b
5
6
7
8
9
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
5-Hexyn-1-ol
5-Hexyn-1-ol
1-Decyne
5
5
5
5
1
1
5
5
5
5
1
2
3
4
1
2
1
2
1
2
100
100
100
100
100
97
100
97
99
10
1-Decyne
100
13 Preparation of [bmim]PF6 and its precursor [bmim]Cl: J. G.
Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser and R. D.
Rogers, Chem. Commun., 1998, 1765.
a GC yield. b Reaction performed without ether.
CHEM. COMMUN., 2002, 1884–1885
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