Catalytic Decarboxylation of Silyl Alkynoates to Alkynylsilanes

Article abstract of DOI:10.1021/acs.organomet.0c00433

Herein, we describe a decarboxylative approach to the preparation of alkynylsilanes. Treatment of a silyl alkynoate in N,N-dimethylformamide (DMF) at 80 °C in the presence of catalytic amounts of CuCl and PCy3 produced the corresponding alkynylsilane in excellent yield. The copper-catalyzed decarboxylation proceeded smoothly with low catalyst loadings (0.5 mol % of CuCl and 1.0 mol % of PCy3) under mild reaction conditions and is easily scalable to gram quantities.

Full text of DOI:10.1021/acs.organomet.0c00433

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Catalytic Decarboxylation of Silyl Alkynoates to Alkynylsilanes
Takahiro Kawatsu,† Keiya Aoyagi,† Yumiko Nakajima, Jun-Chul Choi, Kazuhiko Sato,
and Kazuhiro Matsumoto*
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ABSTRACT: Herein, we describe a decarboxylative approach to
the preparation of alkynylsilanes. Treatment of a silyl alkynoate in
N,N-dimethylformamide (DMF) at 80 °C in the presence of
catalytic amounts of CuCl and PCy3 produced the corresponding
alkynylsilane in excellent yield. The copper-catalyzed decarboxyla-
tion proceeded smoothly with low catalyst loadings (0.5 mol % of
CuCl and 1.0 mol % of PCy3) under mild reaction conditions and is
easily scalable to gram quantities.
Alkynylsilanes are important building blocks used in transition-
metal-catalyzed cross-coupling reactions and related trans-
formations.1 A conventional synthetic approach to an
alkynylsilane involves the nucleophilic substitution of a
halosilane with a metal acetylide, which is readily generated
by the deprotonation of a terminal alkyne with a strong base,
such as an alkyllithium or a Grignard reagent (Scheme 1a).2
and terminal alkynes at 150 °C in the presence of a catalytic
amount of CuCl.3a Moreover, Yamaguchi and co-workers
demonstrated that AgCl, AgNO3, and CuCl promote the
catalytic alkynylation of Me3SiCl in the presence of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) under mild reaction
conditions.3b In addition, iodosilanes4 and silyl triflates5 are
silicon electrophiles that can be alkynylated under {Ir(μ-
Cl)(CO)2}2 and Zn(OTf)2 catalysis, respectively. Moreover,
dehydrogenative alkynylations using terminal alkynes and
hydrosilanes have been achieved,6 and a unique alkynylation
of vinylsilanes that includes elimination of ethylene was
reported by Marciniec and co-workers.7
Scheme 1. Synthetic Approaches to Alkynylsilanes
As discussed above, condensation reactions, such as
nucleophilic substitutions and cross-coupling reactions, are
general approaches for introducing carbon substituents onto
silicon atoms, irrespective of alkynylation. On the other hand,
another possible approach involves the decarboxylation of a
silyl ester, as some kinds of silyl esters have been reported to
undergo decarboxylation in the presence of a base; however,
substrates that can be used under these decarboxylation
conditions are highly limited to particular silyl esters, such as
trichloroacetates and pentafluorobenzoates.8−10 While bis-
(trimethylsilyl) acetylenedicarboxylate, a silyl alkynoate, also
undergoes decarboxylation in the presence of an amine base, to
our knowledge no other examples of silyl alkynoates suitable
for such a decarboxylation have been reported in the
literature.11 In this communication, we apply transition-metal
catalysis to decarboxylate various silyl alkynoates and produce
the corresponding alkynylsilanes (Scheme 1c). We hypothe-
However, the coproduction of stoichiometric amounts of
metal-based wastes and the low functional-group tolerance
associated with the highly reactive metal acetylide are
sometimes problematic in terms of practicality and sustain-
ability. Consequently, catalytic cross-coupling reactions involv-
ing terminal alkynes and silicon electrophiles have been
developed as alternative synthesis routes to alkynylsilanes
(Scheme 1b).3−5 For example, Calas and co-workers reported
that alkynylsilanes can be prepared by heating chlorosilanes
Received: June 25, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00433
© XXXX American Chemical Society
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sized that a metal alkynoate generated from a silyl alkynoate
would undergo decarboxylation to afford a metal acetylide
intermediate, after which the metal acetylide would react with
the silyl alkynoate to give the desired alkynylsilane, with
regeneration of the metal alkynoate.
Table 2. Silyl Alkynoate Scope and Limitations
As CuCl was reported to be an effective catalyst for the
decarboxylation of 2-alkynoic acids,12−14 and copper acetylides
are potentially nucleophilic,3,15 we initially examined the
decarboxylation of trimethylsilyl 3-phenylpropiolate (1a) in
the presence of a catalytic amount of CuCl (Table 1). As
Table 1. Ligand Optimization
a
entry
ligand
x (mol %)
yield (%)
1
2
3
4
5
none
PPh3
PCy3
PtBu3
PCy3
−
23
30
0.5
0.5
0.5
1.0
b
64 (65)
40
c
>95 (>95)
a
b
c
Isolated yield. With 0.25 mol % of [CuCl(PCy3)]2. With 0.5 mol %
of [CuCl(PCy3)2].
expected, decarboxylation occurred to give the desired
alkynylsilane 2a when 0.5 mol % of CuCl was used as the
catalyst, albeit in low yield (23%, entry 1). Since the addition
of 0.5 mol % of PPh3 improved the yield slightly to 30% (entry
2), other phosphine ligands were briefly screened. While PCy3
displayed superior catalytic performance to give alkynylsilane
2a in 64% yield (entry 3), the bulkier PtBu3 gave an inferior
yield (40%, entry 4). Dramatic improvement was observed
when PCy3 was loaded at twice the level of CuCl, to afford 2a
in quantitative yield within 6 h (entry 5). A comparable yield
was obtained in the reaction with the isolated [CuCl(PCy3)2]
complex.16
With the optimized reaction conditions in hand, the scope
and limitations of the silyl alkynoate were explored (Table 2).
Triethylsilyl 3-phenylpropiolate (1b) and dimethylphenylsilyl
3-phenylpropiolate (1c) underwent smooth decarboxylation to
give the corresponding alkynylsilanes 2b (>95%) and 2c
(95%), respectively. However, decarboxylation hardly pro-
ceeded with the more sterically demanding triisopropylsilyl
group (2e). Vinyl and allyl substituents on the silicon atom
were tolerated under the decarboxylation conditions to give
the corresponding alkynylsilanes 2f,g in excellent yields.
Adding a substituent to the benzene ring of 1a had little
effect on the product yield. ((Trimethylsilyl)ethynyl)benzene
derivatives with electron-donating methyl (2h) and methoxy
(2i) groups and electron-withdrawing chloro (2j) and nitro
(2k) groups at the para position were obtained in quantitative
yields in all cases. A normal ester group (OAc) was tolerated
under the reaction conditions, to afford 2l in >95% yield.
Dimethylsilanediyl bis(3-phenylpropiolate) (1m) underwent
2-fold decarboxylation efficiently to give dialkynylsilane 2m in
high yield (92%). Bis((trimethylsilyl)ethynyl)benzene (2n)
was also obtained in quantitative yield by 2-fold decarbox-
ylation. Aliphatic alkynoates were also examined, with the
propyl-bearing alkynylsilane 2o obtained in >95% yield. Other
silyl esters, such as benzoate and cinnamate, gave no
decarboxylation products under the optimized conditions.
Since we hypothesized that copper acetylides are active
intermediates in the catalytic cycle, we next investigated the
catalytic activity of copper(I) phenylacetylide in the decarbox-
ylation of silyl alkynoate 1a (Scheme 2a); the reaction with
copper acetylide was observed to proceed more quickly than
that with CuCl and was complete within 2 h. While the details
are unclear at present, this study suggests that copper acetylide
is an active intermediate or precatalyst of the decarboxylation.
Scheme 2. (a) Decarboxylation with Copper Acetylide and
(b) Decarboxylation on the Gram Scale
B
https://dx.doi.org/10.1021/acs.organomet.0c00433
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Notes
The copper-catalyzed decarboxylation was readily scaled up
without difficulty (Scheme 2b); 1.04 g of highly pure
alkynylsilane 2a (>95% yield) was isolated by simply passing
the reaction mixture through a short silica gel pad.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
In summary, we developed a facile method for the
preparation of alkynylsilanes through the decarboxylation of
silyl alkynoates. The CuCl/2PCy3 system effectively catalyzed
the decarboxylation of an array of silyl alkynoates with low
catalyst loadings under mild reaction conditions to afford the
corresponding alkynylsilanes in quantitative yields in most
cases. The copper-catalyzed decarboxylation is scalable to the
gram scale without any loss of efficiency. In addition, the
alkynylsilane products are easily purified because gaseous
carbon dioxide is the sole byproduct of decarboxylation.
Experimental studies suggested that copper acetylide is an
active intermediate or precatalyst in the decarboxylation
process. Our laboratory is further investigating catalytic
systems for decarboxylating silyl esters other than alkynoates.
This work was supported by the “Development of Innovative
Catalytic Processes for Organosilicon Functional Materials”
project (Project Leader: K.S.) from the New Energy and
Industrial Technology Development Organization (NEDO).
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00433.
Experimental procedures, characterization data, and
NMR spectra (1H, 13C{1H}, and 29Si{1H}) of the
alkynylsilane products (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
Kazuhiro Matsumoto − Interdisciplinary Research Center for
Catalytic Chemistry (IRC3), National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba 305-8565,
Ibaraki, Japan; orcid.org/0000-0003-1580-8822;
Email: kazuhiro.matsumoto@aist.go.jp
Authors
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Takahiro Kawatsu − Interdisciplinary Research Center for
Catalytic Chemistry (IRC3), National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba 305-8565,
Ibaraki, Japan
Keiya Aoyagi − Interdisciplinary Research Center for Catalytic
Chemistry (IRC3), National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba 305-8565, Ibaraki,
Japan
Yumiko Nakajima − Interdisciplinary Research Center for
Catalytic Chemistry (IRC3), National Institute of Advanced
Industrial Science and Technology (AIST), Tsukuba 305-8565,
Ibaraki, Japan; orcid.org/0000-0001-6813-8733
Jun-Chul Choi − Interdisciplinary Research Center for Catalytic
Chemistry (IRC3), National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba 305-8565, Ibaraki,
Japan; orcid.org/0000-0002-7049-5032
Kazuhiko Sato − Interdisciplinary Research Center for Catalytic
Chemistry (IRC3), National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba 305-8565, Ibaraki,
Japan; orcid.org/0000-0002-4929-4973
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