A new approach for the generation and reaction of organotin hydrides: The development of reactions catalytic in tin

Full text of DOI:10.1021/jo982332g

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J . Org. Chem. 1999, 64, 342-343
A New Ap p r oa ch for th e Gen er a tion a n d
Rea ction of Or ga n otin Hyd r id es: Th e
Develop m en t of Rea ction s Ca ta lytic in Tin
Sch em e 1
Ina Terstiege and Robert E. Maleczka, J r.*
Department of Chemistry, Michigan State University,
East Lansing, Michigan 48824
drawbacks. For example, the use of NaBH is not compatible
4
with functional groups susceptible to borohydride or borane
Received November 25, 1998
reductions. As for the silanes, they do not reduce tin halides,
and therefore, their utilization has been limited to the
recycling of tin alkoxides. Given such limitations, we be-
lieved a mild method that allowed the recycling of tin halides
back to tin hydrides would be highly desirable.
As fluoride had already been shown to heighten the
reducing properties of PMHS,12 we wondered whether poly-
methylhydrosiloxane (PMHS) made hypervalent by the
action of KF13 could efficiently convert tin halides to tin
hydrides (Scheme 1). In fact, we found that simply stirring
an ethereal solution of Bu3SnCl with 1.1 equiv of PMHS and
Organotin compounds are versatile reagents in organic
1
chemistry. Among the most commonly used tin reagents is
tributyltin hydride.2 One of tributyltin hydride’s major
3
applications is in free-radical reactions such as dehaloge-
nations of alkyl, vinyl, or aryl halides, often followed by
intra- or intermolecular C-C coupling. These chain reactions
allow the formation of quite complex ring systems and the
installation of multiple new stereocenters in one pot. Since
these reactions proceed under very mild conditions, a large
variety of functional groups is tolerated, avoiding laborious
protection and deprotection sequences. Tributyltin hydride
has also been used in the reduction of halides, tosylates (via
the iodide), thiols, isonitriles, nitrates, and R,â-unsaturated
2
.2 equiv of an aqueous KF solution for 3.5 h, followed by
NaOH workup, extraction, and removal of the ether provides
Bu3SnH in nearly quantitative yield, albeit with approxi-
mately 2-3 mol % of residual PMHS. Distillation of this
aldehydes (1,4-reduction).4,5 The reagent is also employed
“
crude” material affords analytically pure Bu3SnH in 82%
yield.
We assume the active species in this reaction to be a
in the generation of vinyl stannanes, which are important
and useful building blocks in organic synthesis.6,7
Despite the broad applications of organotin hydrides, their
utilization is not unproblematic. Tributyltin hydride is
relatively expensive, unstable, and toxic. Furthermore, the
1
3
hypervalent silane species, as neither Bu3SnCl or Bu3SnF
reacts with PMHS in the absence of KF.14 However, we have
not ruled out the involvement of highly coordinated orga-
notin species in the process.15 In either event, we were
interested in determining whether the tin hydride generated
under theses conditions could be employed in subsequent
in situ chemical transformations.
tin byproducts of its reactions are not always easy to
separate and represent an ever increasing disposal problem.8
The invention and development of new methods allowing
for the catalytic employment of organostannanes in such
reactions would significantly reduce the amount of tin waste
and thereby be of great value with respect to both environ-
mental and practical concerns. As such, a specific aim of this
research program is the development of reaction procedures
that will allow for the in situ generation of organotin
hydrides and their subsequent use in organic reactions.
Several research groups have investigated reaction meth-
odologies that allow the in situ formation of tin hydride from
cheaper starting materials or the employment of catalytic
amounts of tin. An early method developed for the in situ
generation of tin hydride involved the reduction of tributyltin
We were pleased to find that our combination of Bu3SnCl,
aqueous KF, and PMHS performs well in “classical” tin
hydride reactions, such as free-radical dehalogenations.
Since tin halides are the byproducts of these reactions, we
hoped to be able to recycle them, allowing us to perform
these reactions with catalytic amounts of tin. Our results
summarized in Scheme 2 demonstrate that we are indeed
able to perform these reactions with catalytic amounts of
tin. Importantly, we could show in control experiments that
the halides are not reduced by the reagent combination
9
PMHS/potassium fluoride, which is known to act as a
halides by NaBH4. In fact, in one of the first examples of a
reducing agent.1
2,13
Apparently the hypervalent silane alone
1
0
reaction catalytic in tin, Corey et al. used sodium borane
requires more polar solvents or anhydrous conditions in
order to serve as an effective reducing agent.
to successfully recycle the tin halide byproduct of a dehalo-
genation reaction. More recently, Fu et al.11 have developed
several elegant methodologies to perform “classical” reac-
tions of tin hydride with only catalytic amounts of tin by
using silanes such as polymethylhydrosiloxane (PMHS) to
regenerate the tin. However, both of these methods have
Given our success at free-radical hydrodehalogenations
with catalytic amounts of tin hydride, we decided to inves-
tigate the reaction of more complex substrates and the
possible formation of carbon-carbon bonds. All such reac-
(
1) Pereyre, M.; Quintard, J .-P.; Rahm, A. In Tin in Organic Synthesis;
Butterworth: Toronto, 1987.
2) RajanBabu, T. V. In Encyclopedia of Reagents for Organic Synthesis;
Paquette, L., Ed.; Wiley: New York, 1995; Vol. 7, pp 5016-5023.
(10) Corey, E. J .; Suggs, J . W. J . Org. Chem. 1975, 40, 2554-2555.
(11) (a) Hays, D. S.; Fu, G. C. J . Org. Chem. 1996, 61, 4-5. (b) Hays, D.
S.; Scholl, M.; Fu, G. C. J . Org. Chem. 1996, 61, 6751-6752. (c) Lopez, R.
M.; Hays, D. S.; Fu, G. C. J . Am. Chem. Soc. 1997, 119, 6949-6950. (d)
Hays, D. S.; Fu, G. C. J . Org. Chem. 1997, 62, 7070-7071. (e) Hays, D. S.;
Fu, G. C. J . Org. Chem. 1998, 63, 2796-2797.
(
(
3) (a) Curran, D. P. Synthesis 1988, 417-439 and 489-513. (b) Curran,
D. P Synlett 1991, 63-72. (c) Giese, B. Radicals in Organic Synthesis:
Formation of Carbon-Carbon Bonds; Pergamon: New York, 1986.
(
(
(
(12) (a) Chuit, C.; Corriu, R. J . P.; Perz, R.; Reye, C. Synthesis 1982,
981-984. (b) Drew, D. M.; Lawrence, N. J .; Fontaine, D.; Sehkri, L. Synlett
1997, 989-991.
4) Neumann, W. P. Synthesis 1987, 665-683.
5) Keinan, E.; Gleize, P. A. Tetrahedron Lett. 1982, 23, 477-480.
6) (a) Zhang, H. X.; Guib e´ , F.; Balavoine, G. Tetrahedron Lett. 1988,
(13) Chuit, C.; Corriu, R. J . P.; Reye, C.; Young, J . C. Chem. Rev. 1993,
93, 1371-1448.
2
5
9, 619-622. (b) Zhang, H. X.; Guib e´ , F.; Balavoine, G. J . Org. Chem. 1990,
5, 1857-1867. (c) Ichinose, Y.; Oda, H.; Oshima, K.; Utimoto, K. Bull.
(14) The employment of less than 1 equiv of KF significantly diminishes
the production of tributyltin hydride. This fact along with preliminary 1
-
19
Chem. Soc. J pn. 1987, 60, 3468-3470.
(
7) (a) Stille, J . K.; Groh, B. L. J . Am. Chem. Soc. 1987, 109, 813-817.
3 3
Sn NMR data suggests a rapid conversion of Bu SnCl into Bu SnF (ref 8)
prior to reduction. However, the mechanism of this reaction needs further
study.
(15) Kawakami, T.; Shibata, I.; Baba, A. J . Org. Chem. 1996, 61, 82-87
and references cites therein.
(
b) Farina, V.; Krishnamurthy, V.; Scott, W. J . Org. React. 1997, 50, 1-652.
(
8) Leibner, J . E.; J acobus, J . J . Org. Chem. 1979, 44, 449-450.
(9) Birnbaum, E. R.; J avora, P. H. J . Organomet. Chem. 1967, 9, 379-
3
82.
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0.1021/jo982332g CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/05/1999

Communications
J . Org. Chem., Vol. 64, No. 2, 1999 343
Sch em e 2
Sch em e 5
Finally, the catalytic Bu3SnCl/PMHS/KF combination is
not limited to reactions that produce tin halide byproducts.
We have applied our tin hydride recycling methodology to
the reduction of R,â-unsaturated carbonyls.18 Keinan et al.
have shown that the tin hydride reduction of the π-allyl
palladium species is a very mild way for the exclusive 1,4-
reduction of such compounds. However, their method re-
quired 1.5- to 3-fold excess of the hydride. By recycling the
tin with aqueous KF and PMHS, we were able to generate
the desired saturated aldehydes in good yields, using only
Sch em e 3
5
2
0 or 10 mol % of tin, respectively. The reduction of cinnamyl
aldehyde proceeds within 30 min at room temperature, while
citral required several hours to generate citronellal in 51%
yield, which corresponds to a tin turnover of 5 (Scheme 5).
Control experiments showed that no reduction of the R,â-
1
9
carbonyl compound occurs in absence of the tin halide.
Furthermore, no reduction of the carbonyl functionality was
observed, although Kobayashi et al.20 have shown that
PMHS/TBAF reduces cinnamyl aldehyde to the correspond-
ing alcohol in high yields.
In summary, we have developed a methodology that
allows the in situ generation of tin hydride from inexpensive
starting materials that can be applied to the recycling of
tributyltin hydride in reactions catalytic in tin. Furthermore,
since carbonyl compounds are not reduced under these
reaction conditions it would appear this methodology is
inherently more chemoselective than the borohydride method
of recycling tin halides back to tin hydride. Current studies
are aimed at increasing our mechanistic understanding of
tin hydride formation, as well as broadening the scope and
efficiency of the reactions discussed herein. The results of
these studies will be reported in due course.
Sch em e 4
tions studied to date (Scheme 3) proceeded well with the
successful recycling of tributyltin hydride and no observable
reduction of starting material.
We have also secured evidence that it is possible to carry
out multiple transformations in the reaction pot. Namely,
we have employed this method to our recently described
protocol for a one-pot palladium-mediated hydrostannyla-
tion/Stille cross-coupling with catalytic amounts of tin.16
Stirring a THF solution of 3,5-dimethyl-1-hexyn-3-ol and
iodobenzene, in the presence of PMHS, aqueous KF, and
catalytic amounts of Pd2dba3, trifurylphosphine, and tributyl
tin chloride, for 48 h produced the anticipated 1,3-diene
Ack n ow led gm en t. Generous support was provided by
the NIH (HL-58114), Abbott Laboratories (Diagnostics
Division), and MSU through an All University Research
Initiation Grant and startup funds for R.E.M.
Su p p or tin g In for m a tion Ava ila ble: Spectroscopic data for
all new compounds pictured as well as selected experimental
procedures.
J O982332G
(
Scheme 4). While the overall yield and turnovers for this
1
7
sequence were low, the results of this experiment clearly
showed that the tin is indeed recycled.
(18) Lipshutz, B. H.; Keith, J .; Papa, P.; Vivian, R. Tetrahedron Lett.
1
998, 39, 4627-4630 and references therein.
19) Stirring cinnamaldehyde in the presence of 20 mol % Bu
PMHS, Pd (cat.), H O, and THF (no KF) for 2 days did give a 50% yield of
(
3
SnH,
2
(
16) Maleczka, R. E., J r.; Terstiege, I. J . Org. Chem. 1998, 63, 9622-
623.
17) The overall stepwise yield for this reaction is 59%.
hydrocinnamaldehyde, 10% cinnamyl alcohol, and 40% starting material.
(20) Kobayashi, Y.; Takahisa, E.; Nakano, M.; Watatani, K. Tetrahedron
1997, 53, 1627-1634.
9
(