Silylstannations of α,β-unsaturated carbonyl compounds via the generation of Bu3Sn- in ionic liquids

Article abstract of DOI:10.1039/b508400h

The tributylstannyl anion, Bu₃Sn, can be generated in imidazolium based ionic liquids from Me₃SiSnBu₃ and reacted with α,β-unsaturated carbonyl compounds to afford 3-tributylstannylated products in good yields. The Royal S

Full text of DOI:10.1039/b508400h

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Silylstannations of a,b-unsaturated carbonyl compounds via the
generation of Bu3Sn2 in ionic liquids
Steven Dickson, Darrell Dean and Robert. D. Singer*
Received (in Cambridge, UK) 15th June 2005, Accepted 13th July 2005
First published as an Advance Article on the web 4th August 2005
DOI: 10.1039/b508400h
The tributylstannyl anion, Bu3Sn2, can be generated in
with unsaturated substrates. The new C–Sn or C–Si bonds in these
adducts can then act as ‘‘handles’’ for further manipulation.
Useful tributylstannyl anions have been generated from
silylstannanes through nucleophilic displacement of the trialkylsilyl
moiety. This can be achieved through reaction of the silylstannane
with ‘‘naked’’ cyanide (i.e. NaCN, 18-Crown-6) or less successfully
with potassium tert-butoxide.12 The trialkyltin anion generated in
these systems reacts in a conjugate sense with enones to afford
3-triaklylstannylated silyl enol ethers. Benzyltriethylammonium
chloride can be used in place of the ‘‘naked’’ cyanide ion providing
a safer route to these useful adducts; however, this methodology
still utilizes hepatotoxic dimethylformamide, DMF, as a solvent.13
Owing to their well documented properties mentioned above,
ionic liquids have been deemed ‘‘green’’ solvents. We report herein
the use of ionic liquids as a means to reduce or eliminate the use of
DMF, an otherwise very toxic solvent, in the silylstannation of
a,b-unsaturated carbonyl compounds while at the same time
improving yields of isolated products. The amount of DMF used
in the silylstannation of enones has been reduced in one case
through the use of 1-butyl-3-methylimidazolium tetrafluoroborate,
[BMIM]BF4, while at the same time enhancing the isolated yield of
the stannylated product. When 1-hexyl-3-methylimidazolium
chloride, [HMIM]Cl, is used alone as the solvent not only has
the complete elimination of DMF in this reaction been
accomplished but also the benzyltriethylammoinium chloride has
been replaced by the solvent [HMIM]Cl (Scheme 1).
imidazolium based ionic liquids from Me3SiSnBu3 and reacted
with a,b-unsaturated carbonyl compounds to afford 3-tributyl-
stannylated products in good yields.
Ionic liquids have been established as viable alternative solvents for
a growing variety of applications.1 They have now seen wide use as
solvents in many chemical reactions,2 have been used in
separations3 and as lubricants,4 and have shown promise in
catalytic applications1,2,5 to mention but a few applications. Ionic
liquids are now quite well known for their rather unique set of
properties which make them attractive replacements for conven-
tional solvents. Among these properties are their inherent lack of
volatility, non-flammability, and polar, non-coordinating nature.
They are also excellent solvents for organic, inorganic, and
polymeric materials yet are insoluble in selected organic solvents
and in some cases water. These solubility characteristics enable
them to be easily recycled and used in biphasic applications and
typical extractions. Despite their growing popularity, ionic liquids
have been used in comparatively few reactions involving
organometallic reagents to form new covalent bonds (i.e.
carbon–carbon bonds). This undoubtedly stems from the acidity
of the proton on C-2 of the N,N9-dialkylimidazolium ring on
which the most commonly utilized ionic liquids are based. The
basicity of the metal–carbon bond in organometallic reagents most
frequently results in the abstraction of this proton affording a
highly reactive carbene species.6 Notable examples of organome-
tallic reagents used in ionic liquids to form new covalent bonds
include the reaction of phenyl magnesium bromide with
benzaldehyde in THF–tetradecyl(trihexyl)phosphonium chloride
mixtures7 and the reaction of organozinc reagents in 8-ethyl-1,8-
Our first attempt to improve this reaction through replacement
of DMF with the ionic liquid 1-butyl-3-methylimidazolium
tetrafluoroborate, [BMIM]BF4, resulted in a severe decrease in
the isolated yield of 3-tributylstannylcyclohexanone (Table 1,
Entry 2) even after extended reaction times and elevated
temperatures. We then surmised that if we used 1-butyl-3-
methylimidazolium chloride, [BMIM]Cl, in place of [BMIM]BF4
in a binary solvent system with DMF then we not only should be
able to reduce the amount of the noxious DMF used but should
also be able to eliminate the need to add benzyltriethylammonium
chloride to the reaction mixture since [BMIM]Cl is itself a
quaternary organic chloride. Hence, when a 1 : 1 ratio (v/v) of
DMF : [BMIM]Cl was used as the solvent system the isolated yield
diazabicyclo[5,4,0]-7-undecenium
trifluoromethanesulfonate,
[EtDBU]OTf.8 Further examples showing the potential of
organometallic reactions in ionic liquids are the Freidel–Crafts
acylation of ferrocene9 and the silylstannation of terminal
alkynes.10
Silylstannane reagents, R3SiSnR93, are versatile organometallic
reagents in organic synthesis.11 These compounds contain a
silicon–tin bond and are easily synthesized by generating a
trialkyltin anion followed by a quench with a trialkylsilyl chloride.
The resulting silylstannanes can be added across unsaturated
organic compounds such as alkynes and enones. These reagents
are particularly interesting since one or both metals may be
incorporated into the product generated through their reaction
Scheme 1 Silylstannation of cyclohex-2-en-1-one in 1-hexyl-3-methyl-
Department of Chemistry, Saint Mary’s University, Halifax,
Nova Scotia, Canada B3H 3C3
imidazolium chloride, [HMIM]Cl.
4474 | Chem. Commun., 2005, 4474–4476
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We believe that the reaction proceeds by initial attack of the
silicon atom in Me3SiSnBu3 by the chloride ion of [HMIM]Cl
affording a hypervalent silicon species which then gives rise to
TMSCl and the tributylstannyl anion. The tributylstannyl anion
then reacts in a 1,4-conjugate sense with the a,b-unsaturated
carbonyl compound that further reacts with the TMSCl formed
in situ to form the silyl enol ether. In forming the silyl enol
ether the chloride ion is regenerated and hence can be regarded as
a catalytic species. The silyl enol ether is destroyed during
aqueous work-up to afford the observed 3-stannylated product
(Scheme 2).
Table 1 Silylstannation of cyclohex-2-en-1-one
Entry
Conditions
Yield (%)a
1
2
3
4
5
6
7
8
a
DMF, BnEt3NCl, room temp., 26 h
[BMIM]BF4, BnEt3NCl, 60–70 uC, 8 d
1 : 1 DMF, [BMIM]Cl, room temp., 3d
1 : 3 DMF, [BMIM]Cl, room temp., 4d
[BMIM]Cl, 60–70 uC, 2 d
[BMIM]BF4, [BMIM]Cl, 60–70 uC, 2d
[BMIM]PF6, [BMIM]Cl, 60–70 uC, 4d
[HMIM]Cl, room temp., 2d
60
13
75
80
39
50
33
69
Isolated yield.
Evidence supporting this mechanism was obtained through
observation of a [HMIM]Cl–Me3SiSnBu3 mixture using electro-
spray mass spectrometry run in the negative mode (Fig. 1). A series
of peaks are observed centred around m/z 291 which corresponds
to the Bu3Sn2 anion containing the most abundant isotope of tin
(i.e. 120Sn). The isotopic pattern observed in the ESI-MS is
consistent with the isotopic distribution for tin hence confirming its
presence. Confirmation of the presence of the anion comes from
the fact that the ESI-MS was run under negative mode.
of the 3-stannylated product increased to 75% isolated yield
(Table 1, Entry 3) compared to a 60% isolated yield when using
DMF alone (Table 1, Entry 1).13 When the proportion of DMF
was reduced further to a 1 : 3 ratio of DMF : [BMIM]Cl the yield
increased to an 80% isolated yield of the 3-stannylated product,
albeit after extended reaction times (Table 1, Entry 4).
When DMF was totally removed from the reaction mixture and
only [BMIM]Cl was present the reaction had to be heated to
between 60 and 70 uC since the melting point of [BMIM]Cl falls in
this range. The yield dropped to a disappointing 39% isolated yield
under these conditions (Table 1, Entry 5). Our attempts to improve
the solution characteristics of the system by adding equal volumes
of either [BMIM]BF4 or [BMIM]PF6 to [BMIM]Cl also did not
result in improved yields when compared to the reaction run in
DMF alone (Table 1, Entries 6 & 7). However, these systems did
allow the elimination of both DMF and the benzyltriethylammo-
nium chloride additive.
Hence, we have demonstrated that features of the silylstannation
of a,b-unsaturated carbonyl compounds can be improved through
use of ionic liquids. The overall isolated yield of 3-stannylated
product can be improved either by reducing the amount of DMF
used through replacement with an ionic liquid, [BMIM]BF4, or
through the use of an ionic liquid alone, [HMIM]Cl. From a green
The optimum results for the silylstannation of cyclohex-2-en-1-
one were obtained when a room temperature ionic liquid was used.
We chose 1-hexyl-3-methylimidazolium chloride, [HMIM]Cl, since
it is a liquid well below room temperature (i.e. 285 uC) allowing us
to easily conduct the reaction at room temperature.{ We could
therefore avoid using DMF in this reaction. Furthermore, since
the solvent is also a chloride salt we would not require the addition
of any additives such as benzyltriethylammonium chloride. Thus,
when tributyl(trimethylsilyl)stannane was reacted with cyclohex-2-
en-1-one at room temperature in [HMIM]Cl we obtained a 69%
isolated yield of 3-tributylstannylcyclohexanone (Table 1, Entry 8).
A selected series of a,b-unsaturated carbonyl compounds was
then reacted under the same conditions at room temperature using
only [HMIM]Cl as the solvent to perform a cursory survey of the
scope of the reaction conducted in an ionic liquid alone. Only a
few examples have been reported under conventional conditions
(i.e. without ionic liquids) for the conjugate addition of
tributylstannyl anions generated from silylstannanes with the best
reported yield being ca. 60%.13,14 As can be seen in Table 2
superior yields compared to conventional systems can be obtained
when using [HMIM]Cl as the solvent.
Scheme 2 Suggested mechanism for the silylstannation of cyclohex-2-en-
1-one in 1-hexyl-3-methl-imidazolium chloride, [HMIM]Cl.
Table 2 Silylstannation of selected a,b-unsaturated carbonyl com-
pounds in [HMIM]Cl
Starting material
Yield (%)a
Cyclohex-2-en-1-one
Cyclopent-2-en-1-one
trans-4-Phenyl-3-buten-2-one
a
Isolated yields.
69
69
72
Fig. 1 Electrospray mass spectrum (negative mode) of [HMIM]Cl +
Me3SiSnBu3.
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chemistry15 point of view we have dramatically improved this
process by eliminating a harmful solvent and also by eliminating
the need to add benzyltriethylammonium chloride to the reaction.
In essence we have demonstrated that [HMIM]Cl is in fact a task
specific ionic liquid, TSIL, in that it acts not only as a solvent but
also as a catalyst for this reaction. Noteworthy is the fact that the
intermediate species, [HMIM]SnBu3, represents a new ionic liquid
in and of itself. Such an ionic liquid will be useful as both a solvent
and a reagent. Such a dual role for [HMIM]SnBu3 is possible since
this compound is not only a liquid but also contains the reactive
anion Bu3Sn2; a very useful synthon in organic chemistry.
[HMIM]SnBu3 is one of the first, if not the first, 1,3-
dialkylimidazolium ionic liquids that contains an anion basic
enough to extract a proton from water and other protic sources
such as alcohols but which is not basic enough to remove the C-2
proton from the imidazolium cation.
through a septum via a syringe. The mixture was stirred at room
temperature for 2 d. 10 mL of distilled water was added to the reaction and
the reaction mixture was extracted with diethyl ether (4 6 15 mL). The
combined organic extracts were dried over anhydrous magnesium sulfate,
filtered and concentrated in vacuo to afford a clear oil. The oil was purified
by flash column chromatography (hexane–ethyl acetate) to afford a clear,
colourless oil (265 mg, 69% yield). IR (neat): nCO 1709 cm21
.
1H NMR
(CDCl3): d 0.79–0.95 (m, 15H, –Sn(CH2CH2CH2CH3)3); 1.25–1.35 (sextet,
6H, Sn(CH2CH2CH2CH3)3
J 5 7.3 Hz); 1.44–1.58 (m, 6H,
Sn(CH2CH2CH2CH3)3); 1.60–2.50 (m, 9H, H2, H3, H4, H5, H6). 13C
NMR (CDCl3): d 8.0, 13.6, 25.2, 24.4, 29.2, 30.1, 31.2, 42.3, 46.5, 212.4.
MS: m/z 331 (M+ 2 Bu), 275 (M+ 2 Bu2), 217 (M+ 2 Bu3), 177 (SnBu+).
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Efforts are underway in our laboratory to more comprehen-
sively study the scope and limitations of this ionic liquid system
and others used for the silylstannation of a,b-unsaturated carbonyl
compounds. We are currently characterizing [HMIM]SnBu3
formed from the reaction between Me3SiSnBu3 and [HMIM]Cl.
These results will be the topic of a forthcoming publication
describing the full details of the study.
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (Discovery Grant to
R.D.S. and NSERC-CRD to Saint Mary’s University) and by
Saint Mary’s University, Faculty of Graduate Studies and
Research. NMR spectra were obtained at the Atlantic Regional
Magnetic Resonance Centre.
Notes and references
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K. Naotake, I. Naohiro and M. Shibasaki, Tetrahedron Lett., 1991, 32,
6139–6142.
{ Representative Procedure: [HMIM]Cl (2.00 g) was added to an oven-
dried, vacuum-cooled 25 mL round bottom flask equipped with a magnetic
stirbar. The flask was placed under vacuum at 70 uC for 4 h to ensure
dryness of the ionic liquid. After cooling the flask to room temperature the
vacuum was released under an argon atmosphere. Cyclohex-2-en-1-one
(0.10 mL, 1.0 mmol) and Bu3SnSiMe3 (0.70 mL, 2.0 mmol) were injected
15 P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, Oxford, 1998.
4476 | Chem. Commun., 2005, 4474–4476
This journal is ß The Royal Society of Chemistry 2005