Direct synthesis of alkynylstannanes: ZnBr2 catalyst for the reaction of tributyltin methoxide and terminal alkynes

Article abstract of DOI:10.1002/anie.201104208

Metal hopping: Various alkynylstannanes were synthesized by the direct reaction of Bu₃SnOMe with terminal alkynes at room temperature in the presence of a ZnBr₂ catalyst. Rather than acting as a Lewis acid, ZnBr₂ was transmetalated with Bu₃SnOMe to give Zn(OMe)₂, which is key to the catalytic reaction. Copyright

Full text of DOI:10.1002/anie.201104208

DOI: 10.1002/anie.201104208
Stannanes
Direct Synthesis of Alkynylstannanes: ZnBr2 Catalyst for the Reaction
of Tributyltin Methoxide and Terminal Alkynes**
Kensuke Kiyokawa, Nodoka Tachikake, Makoto Yasuda, and Akio Baba*
A carbon–carbon triple bond is a highly valuable and versatile
functional group in many natural products, bioactive com-
pounds,[1] and organic materials.[2] Alkynylstannanes, which
have high stability, reactivity, and functional group tolerance,
are important reagents for introducing an alkynyl moiety into
organic molecules.[3] In particular, the Migita–Kosugi–Stille
of the strong basicity of a tin amide and the production of
basic amine by-products [Eq. (2), Scheme 1].[6] In contrast,
the direct condensation reaction between a tin alkoxide and a
terminal alkyne is regarded as a promising process that is mild
because no strong base is required and an alcohol is the only
by-product. Only alkynes bearing electron-withdrawing
groups (EWGs), however, have been reported to react
under reaction conditions requiring heat thus far [Eq. (3),
Scheme 1].[7] Activation of alkynes by Lewis acids, instead of
EWGs, was expected to achieve this direct coupling under
milder reaction conditions as a way to develop a more
versatile synthetic method of alkynylstannanes with various
types of functional groups. We report herein our serendipitous
discovery that a catalytic amount of ZnBr2 effectively
promoted a coupling reaction between Bu3SnOMe and
terminal alkynes at room temperature; the ZnBr2 was trans-
metalated with Bu3SnOMe rather than acting as a Lewis acid
[Eq. (4), Scheme 1]. This reaction system is applicable to
various types of aliphatic and aromatic terminal alkynes. In
addition, the mild reaction conditions, in which methanol is
the only waste, enables the one-pot synthesis of aryl alkynes
by the Migita–Kosugi–Stille coupling.
coupling using alkynylstannanes is widely used for the
2
À
construction of C(sp) C(sp ) bonds in the synthesis of aryl
alkynes or conjugated enynes.[4] Transmetalation between an
organotin halide and an alkynyllithium or alkynylmagnesium
compound is the most common route to alkynylstannanes
[Eq. (1), Scheme 1].[5] However, the method using those
alkynylmetals has some drawbacks such as poor functional
group tolerance and the production of an equimolar amount
of metal salts. The direct reaction of a tin amide with a
terminal alkyne is also employed for the synthesis of
alkynylstannanes, but its substrate scope is narrow because
Initially, the addition of weak Lewis acids, which were
expected to characteristically interact with alkynes,[8] was
examined in the reaction of Bu3SnOMe with 1-dodecyne (1a),
as partially summarized in Table 1. Only a trace amount of the
product 2a was formed in the absence of a catalyst even when
heated (Table 1, entry 1). In the presence of the transition-
metal catalysts PdCl2 and CuBr, 2a was obtained in modest
yields (Table 1, entries 2 and 3). While soft Lewis acids like
BiBr3 and InBr3 did not improve the yields (Table 1, entries 4
and 5),[9] Zn(OTf)2 produced a high product yield (Table 1,
entry 6).[10] In the search for more efficient catalysts, we were
delighted to find that inexpensive ZnBr2 was the most
practical catalyst employed (Table 1, entries 7 and 8). At
Scheme 1. Synthetic methods for alkynylstannanes.
Table 1: Effect of catalysts.[a]
[*] K. Kiyokawa, N. Tachikake, Dr. M. Yasuda, Prof. Dr. A. Baba
Department of Applied Chemistry, Graduate School of Engineering
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
E-mail: baba@chem.eng.osaka-u.ac.jp
[**] This work was supported by a Grant-in-Aid for Scientific Research on
Innovative Areas (No. 22106527, “Organic Synthesis Based on
Reaction Integration. Development of New Methods and Creation
of New Substances” and No. 23105525, “Molecular Activation
Directed toward Straightforward Synthesis”) and Challenging
Exploratory Research (No. 23655083) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (Japan). K.K. thanks
the Global COE Program of Osaka University.
Entry
Catalyst
Yield [%][b]
Entry
Catalyst
Yield [%][b]
1[c]
2
3
none
PdCl2
CuBr
BiBr3
<5
14
40
5
6
7
8
InBr3
Zn(OTf)2
ZnCl2
25
68
42
68
4
<5
ZnBr2
[a] Reaction conditions: Bu3SnOMe (1.2 mmol), 1a (1 mmol), catalyst
(0.05 mmol), THF (1 mL), RT, 3 h. [b] Determined by H NMR spec-
1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104208.
troscopy using 1,1,2,2-tetrachloroethane as the internal standard.
[c] Reaction was performed at 608C. THF=tetrahydrofuran.
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10393

Communications
ambient temperature, 5 mol% of ZnBr2 afforded the desired
alkynylstannane 2a in 68% yield.
entries 16 and 17). In addition, alkynes directly connected by
ester and silyl moieties are suitable for coupling to produce
the corresponding alkynylstannanes 2r and 2s, respectively
(Table 2, entries 18 and 19).
The synthesis of tributyl(3-bromopropynyl)stannane (2t)
was examined, because the general reaction using ethyl-
magnesium bromide, propargyl bromide (1t), and Bu3SnCl
resulted in a mixture of 2t (29%) and 3 (25%) even under
controlled reaction conditions (Scheme 2).[12] The generation
of 3-bromo-1-propynylmagnesium bromide and propargyl-
Under the optimized reaction conditions, reactions with
various terminal alkynes were carried out. As summarized in
Table 2, a wide range of functional groups were compatible
with the reaction conditions.[11] Aliphatic terminal alkynes,
including base-labile ones bearing a cyano or carbonyl group,
afforded the corresponding products 2a–2e in high yields
(Table 2, entries 1–5). The products 2 f, 2g, and 2h were also
obtained effectively from propargyl chloride (1 f), the prop-
argyl ether 1g, and propargyl ester 1h, respectively (Table 2,
entries 6–8). Unfortunately, the reaction of propargyl alcohol
(1i) was suppressed, probably because of the hydroxy proton
(Table 2, entry 9). This method was also applicable to
aromatic alkynes bearing an electron-donating or electron-
withdrawing group (Table 2, entries 10–15). Heteroaromatic
compounds 1p and 1q gave high yields, as well (Table 2,
Table 2: Catalytic synthesis of alkynylstannanes 2 from Bu3SnOMe and
terminal alkynes 1.[a]
Scheme 2. Synthesis of tributyl(3-bromopropynyl)stannane (2t). Yields
were determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloro-
ethane as the internal standard. The value in parenthesis is the yield of
the isolated product.
Entry
Alkyne 1
2
Yield [%][b]
magnesium bromide in the first step led to the formation of
the mixture.[13] However, our method provided the desired
reaction and produced 2t in 89% yield with no side reactions.
One possible reason might have been that the catalytic
amount of ZnBr2 was sufficient and no base stronger than
Bu3SnOMe appeared in the system.
To gain insight into the reaction mechanism, a mixture of
Bu3SnOMe and ZnBr2 was monitored by 13C NMR spectros-
copy (Figure 1). When ZnBr2 and 2 equivalents of Bu3SnOMe
were mixed in [D8]THF at room temperature, the generation
of Bu3SnBr (5; d(13C) = 30.0, 27.6, 18.3, and 13.8 ppm) and the
complete consumption of the starting Bu3SnOMe were
1
2
1a
1b
2a
2b
68 (61)
78 (70, 97[c])
3[d]
4
1c
1d
1e
2c
2d
2e
72 (75)
73 (77)
5
68 (39, 79[c])
6
7
8
9
1 f
1g
1h
1i
2 f
2g
2h
2i
75 (62)
56 (46, 92[c])
76 (47)
n.d.
10
1j (X=H)
2j
75 (77)
11
1k (X=4-MeO)
1l (X=4-tBu)
1m (X=3-Me)
1n (X=3-Cl)
1o (X=2-F)
2k
2l
2m
2n
2o
79 (80)
78 (72)
12[d]
13
80 (69, 94[c])
84 (61, 84[c])
88 (74)
14
15
16
1p
2p
72 (74, 92[c])
17
1q
2q
80 (79)
18[d]
1r
2r
84 (58, 77[c])
65 (49)
19[d,e]
1s
2s
[a] Reaction conditions: Bu3SnOMe (1.2 mmol), 1 (1 mmol), ZnBr2
(0.05 mmol), THF (1 mL), RT, 3 h. [b] Yields of crude products
determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as
the internal standard. Values in parentheses are yields of isolated
1
products. [c] Purity of the products as determined by H NMR spec-
Figure 1. 13C NMR spectra in [D8]THF: a) Bu3SnOMe. b) The mixture
of ZnBr2 and 2 equivalents of Bu3SnOMe. c) Just after the addition of
alkyne 1j (2.5 equiv) to the mixture (b). See the Supporting Informa-
tion for the experimental details.
troscopy using 1,1,2,2-tetrachloroethane as the internal standard.
[d] MeCN was used instead of THF. [e] Bu3SnOMe (1 mmol) and 1s
(2 mmol) were used.
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Angew. Chem. Int. Ed. 2011, 50, 10393 –10396

observed (Figure 1b).[14] These results indicate that trans-
metalation between Bu3SnOMe and ZnBr2 occurred to give
Zn(OMe)2 (4; d = 56.5 ppm; Figure 1b).[15] The addition of
phenylacetylene (1j) to the mixture furnished the corre-
sponding alkynylstannane 2j (Figure 1c). In contrast, when
Zn(OTf)2 instead of ZnBr2 was treated with Bu3SnOMe, no
transmetalation was observed (see the Supporting Informa-
tion). Apparently, an alternative mechanism should be
considered.
On the basis of the NMR study, a plausible reaction
mechanism is shown in Scheme 3. First, the transmetalation
between Bu3SnOMe and ZnBr2 gives the zinc methoxide 6,
Scheme 3. Plausible reaction mechanism.
which should be Zn(OMe)2 because Bu3SnOMe is in large
excess of ZnBr2 in the reaction mixture.[16] Next, an abstrac-
tion of the terminal proton from alkyne 1 by 6 provides the
alkynylzinc species 7.[17] Finally, the reaction of 7 with
Bu3SnOMe affords the alkynylstannane 2 with the regener-
ation of 6. The mechanism using a Zn(OTf)2 catalyst may be
the usual one (Table 1, entry 6), whereby the reaction would
be started from the activation of the alkyne 1 by coordination
to Zn(OTf)2.[18]
Scheme 4. One-pot synthesis of aryl alkynes by the Migita–Kosugi–
Stille coupling. See the Supporting Information for experimental
details. Yields were determined by 1H NMR spectroscopy using
1,1,2,2-tetrachloroethane as the internal standard. Values in parenthe-
ses are yields of isolated products.
This catalytic method allowed the one-pot synthesis of
various functionalized aryl alkynes by the Migita–Kosugi–
Stille coupling (Scheme 4). The ZnBr2-catalyzed formation of
alkynylstannanes 2 was directly followed by palladium-
catalyzed coupling with aryl bromides to furnish the corre-
sponding aryl alkynes 8 in good to high yields.[19] In most
cases, the yields of coupling products 8 paralleled those of
alkynylstannanes 2, as shown in Table 2. These results
indicate that in situ generated alkynylstannanes were fully
converted into coupling products without suppression by a
zinc catalyst or the by-products MeOH and Bu3SnBr.
To further expand the utility of this reaction, the synthesis
of a diyne compound was investigated.[20] After the ZnBr2-
catalyzed reaction of Bu3SnOMe with 1e, the resulting 2e
(unpurified) was subjected to the coupling with the aryl
bromide 9 bearing a terminal alkyne moiety to give the
corresponding product 10 in 51% yield (Scheme 5). On the
contrary, when 1e was treated with 9 under the standard
Sonogashira conditions, no product 10 was obtained
(Scheme 6).[21,22] The zinc-catalyzed synthesis of alkynylstan-
nanes/Migita–Kosugi–Stille coupling sequence is expected to
be a helpful tool in the synthesis of more elaborate molecules.
In summary, the ZnBr2-catalyzed synthesis of alkynyl-
stannanes with a wide range of functional group compatibility
was achieved. As far as can be ascertained, this is the first
Scheme 5. Synthesis of diyne compound 10. [a] Yield was determined
by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as the inter-
nal standard. The value in parenthesis is the yield of the isolated
product.
Scheme 6. Sonogashira reaction of 1e with 9.
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www.angewandte.org 10395

Communications
[7] a) W. P. Neumann, F. G. Kleiner, Tetrahedron Lett. 1964, 5, 3779;
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example of the versatile synthesis of alkynylstannanes from
Bu3SnOMe and terminal alkynes under very mild reaction
conditions. The transmetalation between Bu3SnOMe and
ZnBr2 to generate Zn(OMe)2 is proposed as a key process to
complete the catalytic cycle. Moreover, aryl alkynes were
synthesized using a one-pot protocol that included the
Migita–Kosugi–Stille coupling. Additional investigations will
focus on the reaction mechanism and synthetic applications of
this catalytic method.
[8] Y. Yamamoto, J. Org. Chem. 2007, 72, 7817.
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Experimental Section
Typical procedure (Table 2): Bu3SnOMe (1.2 mmol) was added to a
solution of ZnBr2 in THF (0.05m, 1 mL) and alkyne 1 (1 mmol). The
mixture was stirred for 3 h at room temperature, and then quenched
by H2O (10 mL). The mixture was extracted with diethyl ether (3 ꢀ
10 mL). The collected organic layers were dried (MgSO4), and
evaporation of volatiles gave the crude product, which was analyzed
by 1H NMR spectroscopy. The crude product was diluted with AcOEt
(30 mL) and washed with NH4F (aq) (10%, 20 mL). The obtained
white precipitate was filtered off, and the filtrate was dried (MgSO4).
Evaporation of volatiles gave the product.
[11] The products, alkynylstannanes 2, were decomposed during
isolation by column chromatography using silica gel.
[12] Even though nBuLi or NaH was used instead of EtMgBr, the
yields of 2t were very low (nBuLi: 17% and NaH: 13%). The
by-products have not been fully identified.
[13] A. Boaretto, D. Marton, G. Tagliavini, J. Organomet. Chem.
1985, 297, 149.
[14] The generation of Bu3SnBr was also confirmed by 119Sn NMR
spectroscopy (see the Supporting Information).
Received: June 18, 2011
Revised: July 29, 2011
Published online: September 13, 2011
[15] The transmetalation between a tin alkoxide and zinc halide has
been proposed; see: a) M. Yasuda, S. Tsuji, I. Shibata, A. Baba,
J. Org. Chem. 1997, 62, 8282; b) M. Yasuda, S. Tsuji, Y.
Shigeyoshi, A. Baba, J. Am. Chem. Soc. 2002, 124, 7440.
[16] We used Zn(OMe)2, which was prepared according to the
literature (R. C. Mehrotra, M. Arora, Z. Anorg. Allg. Chem.
1969, 370, 300), as a catalyst instead of ZnBr2 in the reaction of
Bu3SnOMe with phenylacetylene 1j. The corresponding product
2j was obtained in 11% yield. This result supports the catalytic
cycle in Scheme 3 including Zn(OMe)2. The solid zinc species
employed was not soluble in the reaction mixture, and it might
be a reason for the low yield. In situ generated Zn(OMe)2 may
have high reactivity owing to low aggregation. The detail of the
zinc species will be investigated.
[17] The generation of 6 has not been directly observed yet. The
detail investigation of the mechanism is now underway.
[18] Zn(OTf)2 is an effective catalyst for the synthesis of alkynylzinc
species, and the mechanism involving an activation of an alkyne
by the coordination to Zn(OTf)2 has been proposed; see: R.
Fassler, C. S. Tomooka, D. E. Frantz, E. M. Carreira, Proc. Natl.
Acad. Sci. USA 2004, 101, 5843.
Keywords: alkynes · homogeneous catalysis · stannanes · tin ·
.
zinc
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