A Drastic Effect of TEMPO in Zinc-Catalyzed Stannylation of Terminal Alkynes with Hydrostannanes via Dehydrogenation and Oxidative Dehydrogenation

Article abstract of DOI:10.1002/adsc.201900540

With a system consisting of a catalytic zinc Lewis acid, pyridine, and TEMPO in a nitrile medium, terminal alkynes coupled with HSnBu₃, providing alkynylstannanes with structural diversity. The resulting alkynylstannane, without being isolated, could be directly used for Pd- and Cu-catalyzed transformations to deliver internal alkynes and more intricate tin-atom-containing molecules. Mechanistic studies indicated that TEMPOSnBu₃ formed in situ from TEMPO and HSnBu₃ works to stannylate the terminal alkyne in collaboration with the zinc catalyst, and that both of dehydrogenation and oxidative dehydrogenation processes are uniquely involved in a single reaction. (Figure presented.).

Full text of DOI:10.1002/adsc.201900540

Title: A Drastic Effect of TEMPO in Zinc-Catalyzed Stannylation of
Terminal Alkynes with Hydrostannanes via Dehydrogenation and
Oxidative Dehydrogenation
Authors: Yuichi Kai, Shinya Oku, Tomohiro Tani, Kyoko Sakurai, and
Teruhisa Tsuchimoto
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900540
Link to VoR: http://dx.doi.org/10.1002/adsc.201900540

10.1002/adsc.201900540
Advanced Synthesis & Catalysis
Very Important Publication
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
A Drastic Effect of TEMPO in Zinc-Catalyzed Stannylation of
Terminal Alkynes with Hydrostannanes via Dehydrogenation
and Oxidative Dehydrogenation
Yuichi Kai,a Shinya Oku,a Tomohiro Tani,a Kyoko Sakurai,a and Teruhisa
Tsuchimotoa,*
a
Department of Applied Chemistry, School of Science and Technology, Meiji University, 1-1-1 Higashimita, Tama-ku,
Kawasaki 214-8571, Japan
Fax: (+81)-44-934-7228
E-mail: tsuchimo@meiji.ac.jp
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract: With a system consisting of a catalytic zinc Lewis and HSnBu3 works to stannylate the terminal alkyne in
acid, pyridine, and TEMPO in a nitrile medium, terminal collaboration with the zinc catalyst, and that both of
alkynes coupled with HSnBu3, providing alkynylstannanes dehydrogenation and oxidative dehydrogenation processes
with structural diversity. The resulting alkynylstannane, are uniquely involved in a single reaction.
without being isolated, could be directly used for Pd- and Cu-
catalyzed transformations to deliver internal alkynes and more
intricate tin-atom-containing molecules. Mechanistic studies
indicated that TEMPOSnBu3 formed in situ from TEMPO
Keywords: Alkynes; Dehydrogenation; Lewis acids; Tin;
Zinc
friendly, in view of the functional group compatibility,
reagent usability, and only MeOH waste.
Introduction
On the other hand, making a molecular structure via
Organostannanes are a useful class of organometallic
dehydrogenation[10] or oxidative dehydrogenation[11]
compounds, due to their wide spectrum of applications, has attracted substantial attention in the last couple of
including industrial, agricultural, biological, and
synthetic uses.[1] Alkynylstannanes, which can be
converted to various targets, have also been used as
organometallic reagents.[2,3] The synthetic advantage
of organostannanes involving alkynylstannanes would
be ascribed to a good balance of the reactivity and
stability like no others. For example, bench-stable
alkynylstannanes participate in the Kosugi–Migita–
Stille cross-coupling (KMSCC) reaction without
external activators.[4] The most common way to obtain
alkynylstannanes consists of the following two stages:
deprotonation of terminal alkynes with strong metallic
bases, such as BuLi[5a] and (Me3Si)2NLi,[5b] and a
subsequent treatment of the resulting alkynylmetals
with tin halides. This strategy, however, requires
handling of the moisture-sensitive reagents, and also
coproduces stoichiometric wastes of metal salts and
protonated bases. Moreover, the base-sensitive
functionalities are incompatible with this strategy.
Terminal alkynes have also been known to undergo
stannylation with distannoxanes,[6] tin amides,[7] tin
cyanides,[8] and tin methoxides.[9] Among the methods,
the stannylation with the tin methoxide reported by the
Yasuda–Baba group is likely to be the most user-
decades. This would be due to requiring no time-
consuming pre-introduction of reactive functional
groups. In this regard, we have disclosed that, under
zinc–pyridine catalysis in a nitrile medium, alkynyl C–
H bonds couple with H–Si[12] and H–B(dan)[13] (dan =
1,8-diaminonaphtyl) in a dehydrogenative fashion.
The importance of our original zinc–pyridine–nitrile
system is that all inexpensive commercial sources
including the zinc Lewis acid based on an abundant
metal resource in nature are available, in contrast to the
general types catalyzed by rare, and thus, expensive
transition metals. To date, despite the well-studied
dehydrogenative C–Si[14,15] and C–B[16,17] bond-
forming couplings, the stannylation thereof has only
two precedents for the alkynyl C–H bond, once again,
with transition metal catalysts of Rh[18] and Ru.[19] We
herein report on the first case of non-transition-metal-
catalyzed stannylation of alkynes with hydrostannanes,
and also clarify that a unique reaction mechanism is
operative.
Results and Discussion
1
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10.1002/adsc.201900540
Advanced Synthesis & Catalysis
We began this study by evaluating whether our system
works for the reaction of PhCCH (1a) with HSnBu3
(2) (Scheme 1). Thus, the treatment of 1a and 2 with
Zn(OTf)2 (Tf = SO2CF3; 5 mol%) and pyridine (10
mol%) in EtCN at 70 °C for 8 h provided
tributyl(phenylethynyl)tin (3a) in 20% yield, but with
a considerable amount of undesired hydrostannylation
products 4a as an E/Z mixture (72:28). Radical-
induced hydrostannylation of 1a with 2 has been
reported to predominantly yield E-4a,[20] implying that
the E-enrichment in Scheme 1 is likely to be due to the
involvement of a radical species. To confirm this
conjecture and also to reduce the radical influence, the
reaction was conducted in the presence of 2,2,6,6-
tetramethylpiperidine 1-oxyl (TEMPO; 50 mol%)
known as a radical scavenger. Beyond our expectation,
the formation of 4a was completely suppressed, and,
instead, the efficiency of the desired route was
drastically improved, giving 3a as a sole product in
94% NMR yield. Inspired by this result, the effect of
other free radicals, such as TEMPO analogs as well as
galvinoxyl and 2,2-diphenyl-1-picrylhydrazyl (DPPH),
was investigated (Scheme 2). All of these effectively
suppressed the route for 4a, but, as a result, TEMPO
proved to be the most suitable choice in terms of both
the yield and selectivity of 3a.
With the promising reaction conditions in hand, we
explored the scope of alkynes in the reaction (Table 1).
Besides 1a,[21] aryl alkynes with different steric and
electronic natures reacted successfully with 2 to afford
alkynylstannanes 3a–3g in high yields. Heteroaryl
alkynes with the electron-rich thienyl ring and the
electron-poor pyridyl ring participated well in this
strategy (3h, 3i). A series of aliphatic alkynes having
Table 1. Zn-catalyzed stannylation of terminal alkynes with
HSnBu3.[a]
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
TEMPO (50 mol%)
R
H
R
SnBu3
+
H
SnBu3
EtCN, 70 ºC, 8 h
1
2
3
Ph
H
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
Ph
H
1a
+
SnBu3
SnBu3
S
N
Ph
SnBu3
+
X
EtCN, 70 ºC, 8 h
1a:2 = 1:1.2
H
SnBu3
H
SnBu3
4a
3h; 88% yield
3a (X = H); 90% yield, >99:1
3b (X = 2-Me); 91% yield, >99:1
3c (X = 3-Me); 92% yield, >99:1
3d (X = 4-Me); 82% yield, 99:1
3e (X = 4-OMe); 84% yield, 99:1
3f (X = 4-Br);[b] 72% yield, 99:1
3g (X = 4-F); 88% yield, 99:1
99:1
3a
20% yield
2
60% yield
E:Z = 72:28
SnBu3
with TEMPO (50 mol%); 94% yield
<1% yield
3i;[b] 54% yield
99:1
Scheme 1. Effect of TEMPO in Zn-catalyzed stannylation
of PhCCH with HSnBu3. Averages of yields determined
by 1H and 119Sn NMR are shown here.
Bu
SnBu3
Hex
SnBu3
SnBu3
3j; 73% yield
3k; 87% yield
3l; 84% yield
>99:1
>99:1
>99:1
Ph
Ph
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
free radical (50 mol%)
SnBu3
SnBu3
SnBu3
Ph
H
1a
Ph
H
3m; 57% yield
3n; 75% yield
3o; 82% yield
+
Ph
SnBu3
+
>99:1
>99:1
>99:1
EtCN, 70 ºC, 8 h
1a:2 = 1:1.2
H
SnBu3
H
SnBu3
4a
2
3a
tBuMe2SiO
HO
OH
O
SnBu3
SnBu3
3p;[c] (75%) 32% yield
3q; 74% yield
>99:1
>99:1
N
N
O
N
O
O
O
O
SnBu3
Cl
SnBu3
TEMPO
3a; 94% yield
4a; <1% yield
4-HO–TEMPO
3a; 76% yield
4a; 3% yield
4-oxo-TEMPO
3a; 81% yield
4a; 6% yield
3r; 78% yield
3s; 90% yield
>99:1
>99:1
O
N
SnBu3
SnBu3
CN
N
N
O
O
O
3t; (81%) 45% yield
3u; 71% yield
ABNO
3a; 83% yield
4a; <1% yield
AZADO
3a; 73% yield
4a; 1% yield
>99:1
>99:1
Fe
SnBu3
SnBu3
Et3Si
SnBu3
O2N
3v; 82% yield
3w; (88%) yield
3x; 79% yield
>99:1
N N
NO2
>99:1
99:1
O
O
[a] Reagents (unless otherwise noted): 1 (0.40 mmol), 2 (0.48
mmol), Zn(OTf)2 (20 µmol), pyridine (40 µmol), TEMPO
(0.20 mmol), EtCN (0.40 mL). Yields of isolated 3 based on
1 are shown here. Averages of yields determined by 1H and
119Sn NMR are given in parentheses. Ratios of 3:4
O2N
galvinoxyl
3a; 65% yield
4a; 5% yield
DPPH
3a; 25% yield
4a; <1% yield
[b]
determined by 119Sn NMR are shown after yields.
Scheme 2. Effect of free radicals in Zn-catalyzed
stannylation of PhCCH with HSnBu3. Averages of yields
determined by 1H and 119Sn NMR are shown here.
Performed with TEMPO (0.30 mmol). [c] Performed with 2
(0.96 mmol) and TEMPO (0.40 mmol).
2
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10.1002/adsc.201900540
Advanced Synthesis & Catalysis
Table 2. Zn-catalyzed stannylation of terminal alkynes and
primary, secondary, and tertiary alkyl groups with or
without a functional group underwent stannylation as
well, thereby giving 3j–3u in good to high yields. The
C=C bond of 1v was tolerated without suffering
hydrostannylation.[22] The stannylation of 1w with the
ferrocenyl group gave 3w, which is reportedly useful
to make, for instance, a photo-switching material.[23]
Silylethynyltin 3x with the two transformable units for
further elaboration can also be obtained. The results in
Table 1 reveal accordingly that 1) various functional
groups involving Br–C(sp2), F–C(sp2), pyridyl, HO–
C2H4, tBuMe2SiO, Cl–C(sp3), acetoxy, phthalimidoyl,
NC, C=C, ferrocenyl, and Et3Si are compatible with
this method [note: despite the successful coupling of 1p
with the OH group, propargyl alcohol (HOCH2CCH)
failed to react with 2.]; 2) excellent selectivity of the
stannylation over the hydrostannylation is always
achievable.
There is an additional note to be aware of when
isolating products 3. Due to their decomposition on
silica gel, yet having good thermal stability,[9]
distillation will be the most suitable way to purify 3; in
fact, almost all 3 in Table 1 were distilled safely in pure
form (see Supporting Information for details).
However, complete decomposition occurred in
distilling 3p, 3t, and 3w.[24] Among these, pure 3p and
3t were partly obtained by column chromatography on
alumina.
When a product is difficult to be isolated efficiently,
its one-pot transformation via no purification is
expected to be a smart choice. We therefore explored
whether 3 being not isolated could be used directly as
reagents for the Pd-catalyzed KMSCC reaction (Table
2). Thus, 3a derived from 1a and 2 was treated in one-
pot with 4-bromobenzaldehyde (5a) under the reaction
conditions given in Table 2,[2a] providing 6aa in good
yield of 74%.[25] Similarly, 3 based on a range of aryl
and aliphatic alkynes 1 also coupled with 5a to give
6ea, 6ga, 6ka, 6pa, 6ta, and 6wa, in which internal
alkynes prepared from 3p, 3t, and 3w with difficulty
of being isolated are included. Moreover, BrPh (5b),
2-bromothiphene (5c), and alkenyl bromides 5d and 5e
became the coupling partner of choice for this purpose
(6tb, 6wc, 6ed, 6ae). As a result, the one-pot KMSCC
reaction proved to proceed without being negatively
affected by the catalysts and solvent used for the 1st
step on the Zn-catalyzed stannylation.
To further enhance the practicality of this reaction,
we kept on executing the one-pot application, and next
focused on alkynylstannylation of CC bonds for
preparing more complex organostannanes. Firstly, the
Pd–IP-catalyzed addition of 3a to PhCCH was
examined (Scheme 3). In this case, a procedure
consisting of removing EtCN in vacuo after preparing
3a, introducing THF instead, and conducting the next
stage in THF is effective for obtaining 7a as a single
regioisomer.[26] Of note is that the yield and
regioselectivity observed here are identical to those in
the original report.[3a] Other than the palladium-based
transformations, 3 added to the CC bond of benzyne
derived from 8 under copper catalysis (Table 3).[3g]
Here again, removing volatiles after the 1st step is
Pd-catalyzed KMSCC reaction in one-pot.[a]
Br–R' 5 (1.5 equiv.)
Pd2(dba)3 (1.5 mol%)
P(tBu)3 (3.3 mol%)
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
TEMPO (50 mol%)
R
H
1
+
[ 3 ]
R
R'
EtCN, 70 ºC, 8 h
rt, t h
H
SnBu3
6
2
O
O
X
6ka; 67% yield (t = 5)
6aa (X = H); 74% yield (t = 5)
6ea (X = OMe); 78% yield (t = 5)
6ga (X = F); 82% yield (t = 5)
O
O
HO
6pa;[b] 66% yield (t = 5)
O
O
N
N
Fe
O
6ta; 56% yield (t = 5)
6wa; 60% yield (t = 5)
O
S
Fe
O
6tb; 71% yield (t = 12)
6wc; 71% yield (t = 12)
MeO
6ed;[c] 78% yield (t = 12)
6ae; 64% yield (t = 12)
[a] Reagents (unless otherwise noted): 1 (0.40 mmol), 2 (0.48
mmol), Zn(OTf)2 (20 µmol), pyridine (40 µmol), TEMPO
(0.20 mmol), EtCN (0.40 mL), and 5 (0.60 mmol), Pd2(dba)3
(6.0 µmol), P(tBu)3 (13 µmol). Yields of isolated 6 based on
[b]
1 are shown here.
Performed with 2 (0.96 mmol) and
[c]
TEMPO (0.40 mmol).
Performed with Pd2(dba)3 (12
µmol) and P(tBu)3 (26 µmol).
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
TEMPO (50 mol%)
Ph
H
1a
+
removal of volatiles
10 Pa, 1 h
[ 3a ]
EtCN, 70 ºC, 8 h
1a:2 = 1:1.2
H
SnBu3
2
Ph
Ph
H (3 equiv.)
Cy
[PdCl(p-C3H5)]2 (2.5 mol%)
N
SnBu3
H
IP (5 mol%)
PPh2
IP
THF, 50 ºC, 12 h
Ph
7a; 74% yield
Scheme 3. Zn-catalyzed stannylation of PhCCH and Pd-
catalyzed phenylethynylstannylation of PhCCH in one-pot.
The yield of isolated 7a based on 1a is shown here. Cy =
cyclohexyl.
effective,[27] giving (o-alkynylphenyl)stannanes 9a, 9f,
9t, and 9w in good to high yields. Interestingly, for
example, 9a was obtained in higher yield of 91%,
compared to 70% yield of the original reaction starting
with 3a.[3g] To address the possible reason for the
higher yield, how those used in the 1st step of the one-
pot reaction affect the 2nd step was examined (Scheme
4). The Cu-catalyzed addition of 3a to benzyne was
thus carried out by adding Zn(OTf)2, pyridine,
TEMPO, or HSnBu3 (2) to the reported conditions,[3g]
3
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10.1002/adsc.201900540
Advanced Synthesis & Catalysis
Table 3. Zn-catalyzed stannylation of terminal alkynes and
51% yield, even when using only 50 mol% of TEMPO
(entries 2–4). However, using 100 mol% of TEMPO
led to no quantitative yield of 3a (entry 5). Combining
TEMPO (50 mol%) with Zn(OTf)2 (5 mol%) enhanced
the yield of 3a drastically (entry 6). This result may be
acceptable enough, but the result of entry 1 reveals that
the additional participation of pyridine is important for
the complete selectivity and higher yield of 3a (entries
1 and 6). None of the three led to exclusive
hydrostannylation (entry 7). These results show that at
least TEMPO and Zn(OTf)2 is necessary for the
selective and high-yield formation of 3a.
Cu-catalyzed alkynylstannylation of benzyne in one-pot.[a]
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
TEMPO (50 mol%)
+
[ 3 ]
R
H
H
SnBu3
2
EtCN, 70 ºC, 8 h
1
SiMe3
R
OTf 8 (1.2 equiv.)
CuCN (5 mol%)
removal of
volatiles
10 Pa, 1 h KF (2.4 equiv.)
18-crown-6 (1.2 equiv.)
THF, 65 ºC, t h
SnBu3
9
Based on the above, we further examined the role of
TEMPO and Zn(OTf)2. Thus, treating HSnBu3 (2) and
TEMPO in EtCN at 70 °C for only 10 min provided
TEMPOSnBu3 (10)[28] in 81% yield as a major product
along with Bu3SnSnBu3 (11) in 12% yield (Scheme 5-
A), as observed previously in benzene.[29] Besides 10
and 11, Bu3SnOSnBu3 (12) was formed in a small
amount. To ascertain the origin of the oxygen atom in
12, as shown in Scheme 5-B, 2 was treated with H2O
but without TEMPO under O2, thereby giving only 11.
This shows that the oxygen atom of 12 comes not from
Br
SnBu3
SnBu3
9a; 91% yield (t = 27)
9f;[b] 70% yield (t = 24)
O
N
Fe
O
SnBu3
SnBu3
9t; 77% yield (t = 20)
9w; 74% yield (t = 30)
Table 4. Effect of Zn(OTf)2, pyridine and TEMPO in Zn-
catalyzed stannylation of PhCCH with HSnBu3.[a]
[a] Reagents (unless otherwise noted): 1 (0.40 mmol), 2 (0.48
mmol), Zn(OTf)2 (20 µmol), pyridine (40 µmol), TEMPO
(0.20 mmol), EtCN (0.40 mL), and 8 (0.48 mmol), CuCN
(20 µmol), KF (0.96 mmol), 18-crown-6 (0.48 mmol), THF
(4.0 mL). Yields of isolated 9 based on 1 are shown here. [b]
Performed with TEMPO (0.30 mmol).
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
TEMPO (50 mol%)
Ph
H
1a
+
Ph
H
+
SnBu3
Ph
EtCN, 70 ºC, 8 h
H
SnBu3
4a
H
SnBu3
3a
2
Yield (%)[b]
Entry Zn(OTf)2 pyridine TEMPO
3a
94
18
5
51
19
90
<1
4a
<1
65
67
1
1
1
64
Ph
SiMe3
1
2
3
4
5
6
7
○
○
×
×
×
○
×
○
×
○
×
×
×
×
○
×
×
○
○
○
×
+
1:1.2
additive
(mol%)
Yield (%)
of 9a
OTf
8
SnBu3 3a
70 (70)[a]
none
additive
KF (2.4 equiv.)
CuCN (5 mol%) 18-crown-6 (1.2 equiv.)
[c]
88
37
70
86
Zn(OTf)2 (5)
pyridine (10)
TEMPO (50)
HSnBu3 (20)
THF, 65 ºC, 27 h
Ph
[a]
Reagents (unless otherwise noted): 1a (0.40 mmol), 2
(0.48 mmol), Zn(OTf)2 (20 µmol), pyridine (40 µmol),
9a
SnBu3
[b]
TEMPO (0.20 mmol), EtCN (0.40 mL).
Averages of
yields determined by 1H and 119Sn NMR are shown here. [c]
TEMPO (100 mol%) was used.
Scheme 4. Effect of additives in Cu-catalyzed
phenylethynylstannylation of benzyne. Averages of yields
1
[a]
determined by H and 119Sn NMR are shown here. The
yield reported in reference 3g is given in parentheses.
N
(1 equiv.)
indicating that Zn(OTf)2 and 2 favorably act to
enhance the yield of 9a. Accordingly, the one-pot
recipe disclosed herein, which requires no pre-
synthesis of 3 and gives 9 in higher yield, would be a
better choice when 9 and also compounds derived
from 9 are necessary.
O
H
SnBu3
+
+
A:
N
EtCN, 70 ºC, 10 min
under Ar
OSnBu3
10; 81% yield
2
O
Bu3Sn SnBu3
Bu3Sn
SnBu3
11; 12% yield
12; 7% yield
Some experimental observations are available for
mechanistic studies. We first examined the effect of
each of Zn(OTf)2, pyridine, and TEMPO, as shown in
Table 4, where the result on the most suitable set is
given in entry 1 for a reference. The independent use
of Zn(OTf)2 or pyridine resulted in the predominant
hydrostannylation, but 3a was selectively formed in
H2O (0.5 equiv.)
O
H
SnBu3
Bu3Sn SnBu3
+
B:
Bu3Sn
SnBu3
EtCN, 70 ºC, 10 min
under O2
2
11; 92% yield
12; <1% yield
Scheme 5. Reaction of HSnBu3 with TEMPO (A) or H2O
(B). 119Sn NMR yields based on 2 are shown here.
4
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10.1002/adsc.201900540
Advanced Synthesis & Catalysis
TEMPO + 2
EtCN, 70 ºC, 10 min
+
N
N
O
N
H
EtCN, 70 ºC, 2 h
OH
13
Zn(OTf)2 (5 mol%)
EtCN, 70 ºC, 8 h
A: 1a
+
1:1.2
Ph
SnBu3
14
32% yield
N
3a
67% yield
OSnBu3
10
+ 11 + 12
70% yield
15% yield
85% yield
without Zn(OTf) ;
with pyridine (10 mol%);
2
Scheme 7. Reaction of TEMPOH. GC yields are shown here.
Zn(OTf)2 (5 mol%)
Ph
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
TEMPO (50 mol%)
B:
+
1:0.6
Bu3Sn SnBu3
SnBu3
1a
EtCN, 70 ºC, 8 h
Ph
H
11
3a
1a (1.6 mmol)
<1% yield
6% yield
H2
+
A:
+
Ph
SnBu3
without Zn(OTf) ;
2
EtCN, 70 ºC, 8 h
H
SnBu3
3a; 75% yield
2 (1.9 mmol)
Zn(OTf)2 (5 mol%)
O
C: 1a
+
Ph
SnBu3
Bu3Sn
SnBu3
10% Pd/C
EtCN, 70 ºC, 8 h
1:x
3a
12
(1 mol% on Pd)
Ph
Ph
Ph
H2
45% yield (x = 0.6)
54% yield (x = 1)
without Zn(OTf)2; 51% yield (x = 0.6)
Ph
EtCN, 70 ºC, 8 h
15
16
(0.40 mmol)
from A; 98% yield
from B; 90% yield
Scheme 6. Stoichiometric reaction of PhCCH (1a) with
TEMPOSnBu3 (A), (Bu3Sn)2 (B) or (Bu3Sn)2O (C).
N
1
Averages of yields determined by H and 119Sn NMR are
10; 83% yield
O
+
(3.2 mmol)
shown here.
H2
+
11; 3% yield
+
12; 2% yield
B:
+
EtCN, 70 ºC, 8 h
H
SnBu3
2 (3.2 mmol)
H2O and O2 but TEMPO, and thus that the formation
of 12 would originate in breaking of the N–O bond, as
reported before.[30] We next explored whether 10, 11
or 12 has the ability to stannylate PhCCH (1a)
(Scheme 6, in which 10 prepared in situ was used).[28]
With 10 albeit including 11 and 12 as minors (see
Scheme 5-A), 3a was formed in good yield of 70%,
but in low yield in the absence of Zn(OTf)2 (Scheme
6-A). With both of Zn(OTf)2 and pyridine, the yield of
3a was improved, indicating that pyridine is an
important supporter for the stannylation of alkynes, as
shown in Table 4. In contrast, 11 was totally inactive,
especially in the presence of Zn(OTf)2 (Scheme 6-B).
Although the stannylation proceeded moderately in the
use of 12,[31] Zn(OTf)2 had no positive effect in this
case (Scheme 6-C). Therefore, based on the results of
Table 4 and Schemes 5 and 6, a key stannylating agent
of the present reaction would be 10, which is formed
mainly from the reaction of TEMPO with 2, and then
stannylates 1a more powerfully when Zn(OTf)2 exists.
The contribution of 12 is not negligible, but would be
restrictive due to the small amount produced.
After the stannylation of 1a by 10, TEMPOH (13)
is assumed to be formed along with 3a. On the basis of
this supposition, how 13 changes was next examined
(Scheme 7). Upon simply heating 13 in EtCN for 2 h
without the zinc and pyridine catalysts, a mixture of
TEMPO and 2,2,6,6-tetramethylpiperidine (14) was
quantitatively produced in a 2:1 ratio. Whereas 13 was
partly reduced to 14, the regeneration of TEMPO in
the amount of two-thirds of 13 is likely to be sufficient
for yielding 3a in a high efficiency.[32]
Scheme 8. Trapping experiments of hydrogen gas. Yields
of isolated 3a based on 1a and of isolated 16 based on 15,
and also 119Sn NMR yields of 10 and 12 based on TEMPO
and of 11 based on 2 are shown here.
stannylation of 1a with 2, thereby giving 1,2-
diphenylethane (16) in 98% yield (Scheme 8-A).
However, just mixing TEMPO and 2 proved to release
H2 gas for the reduction of 15 (Scheme 8-B), thus
suggesting that the source of H2 gas is only 2.
Taking all of the results from the mechanistic
studies into consideration, a proposed mechanism is
illustrated in Scheme 9-A. First up is the encounter of
TEMPO and H–Sn (2) to give TEMPOSn (10) and
minor SnOSn (12) accompanied by the release of 1/2
H–H. Next, RCCH (1) undergoes stannylation by 10
with the aid of Zn to give desired RCCSn (3) along
with TEMPOH (13), which successively leads to
regenerating TEMPO, along with the formation of 14
and H2O. This would be a major pathway depicted as
cycle A, and a detailed route from 13 to TEMPO is
also proposed in Scheme 9-B, which starts with
disproportionation of two molecules of 13 to give
TEMPO, H2O and piperidyl radical 17. The
subsequent abstraction of H• from another 13 by 17
provides TEMPO and 14. Thus, it can be considered
that, among the three molecules of 13, two are
oxidized to TEMPO and one is reduced to 14 via the
redox disproportionation.[33] Considering in this way,
the result of Scheme 7 in which TEMPO and 14 were
formed in the 2:1 ratio would be rationally understood.
Besides cycle A as a major route, cycle B, where 3 is
formed from 1 and 12, may participate as only a minor
pathway.[34] At present, the exact role of pyridine is
unclear, but could be to form an active species with Zn,
as previously clarified in the zinc–pyridine–nitrile
Next, we confirmed whether hydrogen (H2) gas is
evolved via the progress of the stannylation reaction
(Scheme 8). trans-Stilbene (15) was thus mixed with
the palladium catalyst with the intention of capturing
H2 gas that is possibly formed from the Zn-catalyzed
5
This article is protected by copyright. All rights reserved.

10.1002/adsc.201900540
Advanced Synthesis & Catalysis
system.[14f] When viewed as a whole, interestingly, a
unique mechanism is operative here; both processes of
the dehydrogenation (–H2) and the oxidative
dehydrogenation (–H2O), which would be unlikely to
coexist, are involved in a single reaction.
with argon. EtCN (0.40 mL) was added to the tube (NOTE:
EtCN over P2O5 in a distillation apparatus should not be
used over 6 days to ensure reproducible results; the same
applies hereinafter.), and the mixture was stirred at room
temperature for 3 min. To this were added terminal alkyne
1 (0.400 mmol), tributyltin hydride (2) [(140 mg, 0.480
mmol) or (279 mg, 0.960 mmol)] (NOTE: the use of freshly
distilled 2 is recommended to ensure reproducible results;
the same applies hereinafter.), pyridine (3.16 mg, 40.0
µmol) and TEMPO [(31.3 mg, 0.200 mmol), (46.9 mg,
0.300 mmol) or (62.5 mg, 0.400 mmol)]. After stirring at
70 °C for 8 h, a saturated NH4F aqueous solution (0.5 mL)
was added to the mixture, and the aqueous phase was
extracted with EtOAc (5 mL × 3). The combined organic
layer was dried over anhydrous sodium sulfate. Filtration
through a pad of Celite and evaporation of the solvent
provided a crude mixture including desired product 3, which
A:
+ H2O
N
H 14
N
R
R
H
OH 13
1
cycle A (major)
Zn
N
O
1
was analyzed by H and 119Sn{1H} NMR spectroscopy in
Sn
H Sn 2
3
N
CDCl3. In the case of successively isolating product 3, the
following operations were performed. Thus, after
evaporation of solvent CDCl3 followed by diluting with
EtOAc (15 mL), a 10 wt% NH4F aqueous solution (10 mL)
was added, and the resulting mixture was stirred at room
temperature for 10 min. The aqueous phase was extracted
with EtOAc (5 mL × 3), and the combined organic layer was
dried over anhydrous sodium sulfate. Filtration through a
pad of Celite and evaporation of the solvent followed by
purification gave product 3.
1
2
–
H H (+ 11)
OSn 10
+
SnOSn
1
–
H2O
2
12
cycle B (minor)
R
H
SnOH
1
B:
+
+ H2O
3
2
Zn-Catalyzed Stannylation of Terminal Alkynes and Pd-
Catalyzed KMSCC Reaction in One-Pot: A General
Procedure for Table 2
N
N
O
N
H
OH 13
14
Zn(OTf)2 (7.27 mg, 20.0 µmol) was placed in a 20 mL
Schlenk tube, which was heated at 150 ºC in vacuo for 1.5
h. The tube was cooled down to room temperature and filled
with argon. EtCN (0.40 mL) was added to the tube, and the
mixture was stirred at room temperature for 3 min. To this
were added terminal alkyne 1 (0.400 mmol), tributyltin
hydride (2) [(140 mg, 0.480 mmol) or (279 mg, 0.960
mmol)], pyridine (3.16 mg, 40.0 µmol) and TEMPO [(31.3
mg, 0.200 mmol) or (62.5 mg, 0.400 mmol)]. After stirring
at 70 ºC for 8 h and then cooling down to room temperature,
aryl or alkenyl bromide 5 (0.60 mmol), Pd2(dba)3 [(5.49 mg,
N
N
N
O H 13
H O 13
O
+
N
N
O H
17
N
N
13
H 14
O + H2O
Scheme 9. Proposed reaction mechanisms. The H atom
derived from 1 is shown in italic. Zn = Zn(OTf)2. Sn = SnBu3. 6.00 µmol) or (11.0 mg, 12.0 µmol)], and a 10 wt% P(tBu)3
solution in hexanes (43.2 µL, 13.2 µmol) or (86.4 µL, 26.4
µmol) were added to the tube, and, under an argon
atmosphere, the resulting mixture was stirred at room
Conclusion
temperature for 5 or 12 h. A saturated NH4F aqueous
solution (0.5 mL) was added, and the aqueous phase was
extracted with EtOAc (5 mL × 3). The combined organic
In closing, we have disclosed herein that the zinc–
layer was dried over anhydrous sodium sulfate. Filtration
pyridine–nitrile system combined with TEMPO
efficiently catalyzes the coupling of terminal alkynes
with HSnBu3. This is the first stannylation of the C–H
bond under non-transition metal catalysis. This
method is a reliable tool capable of offering a broad
range of alkynyl–SnBu3 with various functional
groups, which are successfully available for further
elaboration based on the KMSCC reaction as well as
the alkynylstannylation of alkynes and arynes in one-
pot. Mechanistic studies showed that TEMPOSnBu3
formed in situ works as a key player with the
cooperation of a zinc catalyst for the exclusive
formation of the alkynyl–SnBu3.
through a pad of Celite and evaporation of the solvent
followed by column chromatography on silica gel gave
product 6.
Zn-Catalyzed Stannylation of PhCCH and Pd-
Catalyzed Phenylethynylstannylation of PhCCH in
One-Pot: A Procedure for Scheme 3
Zn(OTf)2 (7.27 mg, 20.0 µmol) was placed in a 20 mL
Schlenk tube, which was heated at 150 ºC in vacuo for 1.5
h. The tube was cooled down to room temperature and filled
with argon. EtCN (0.40 mL) was added to the tube, and the
mixture was stirred at room temperature for 3 min. To this
were added phenylacetylene (1a) (40.9 mg, 0.400 mmol),
tributyltin hydride (2) (140 mg, 0.480 mmol), pyridine (3.16
mg, 40.0 µmol) and TEMPO (31.3 mg, 0.200 mmol). After
stirring at 70 ºC for 8 h and then cooling down to room
temperature followed by concentration under reduced
pressure of 10 Pa for 1 h, THF (3.6 mL), phenylacetylene
(123 mg, 1.20 mmol), [PdCl(π-C3H5)]2 (3.66 mg, 10.0
µmol) and IP (7.43 mg, 20.0 µmol) were added to the tube,
and, under an argon atmosphere, the resulting mixture was
stirred at 50 ºC for 12 h. A saturated NH4F aqueous solution
(0.5 mL) was added, and the aqueous phase was extracted
with EtOAc (5 mL × 3). The combined organic layer was
dried over anhydrous sodium sulfate. Filtration through a
pad of Celite and evaporation of the solvent gave a crude
Experimental Section
Zinc-Catalyzed Stannylation of Terminal Alkynes with
HSnBu3: A General Procedure for Table 1
Zn(OTf)2 (7.27 mg, 20.0 µmol) was placed in a 20 mL
Schlenk tube, which was heated at 150 ºC in vacuo for 1.5
h. The tube was cooled down to room temperature and filled
6
This article is protected by copyright. All rights reserved.

10.1002/adsc.201900540
Advanced Synthesis & Catalysis
product, which was successively purified by recycling GPC
after filtration through a pad of silica gel (EtOAc) to give
tributyl[(1Z)-2,4-diphenyl-1-buten-3-ynyl]stannane (7a) as
a brown oil.
J. Am. Chem. Soc. 1999, 121, 10221–10222; b) H.
Yoshida, E. Shirakawa, T. Kurahashi, Y. Nakao, T.
Hiyama, Organometallics 2000, 19, 5671–5678; c) H.
Yoshida, Y. Honda, E. Shirakawa, T. Hiyama, Chem.
Commun. 2001, 1880–1881; d) A. M. González-Nogal,
M. Calle, P. Cuadrado, R. Valero, Tetrahedron 2007, 63,
224–231; e) B. Wrackmeyer, P. Thoma, S. Marx, T.
Bauer, R. Kempe, Eur. J. Inorg. Chem. 2014, 2103–
2112; f) N. Kinashi, K. Sakaguchi, S. Katsumura, T.
Shinada, Tetrahedron Lett. 2016, 57, 129–132; g) H.
Yoshida, T. Kubo, H. Kuriki, I. Osaka, K. Takaki, Y.
Ooyama, ChemistrySelect 2017, 2, 3212–3215.
Zn-Catalyzed Stannylation of Terminal Alkynes and
Cu-Catalyzed Alkynylstannylation of Benzyne in One-
Pot: A General Procedure for Table 3
Zn(OTf)2 (7.27 mg, 20.0 µmol) was placed in a 20 mL
Schlenk tube, which was heated at 150 ºC in vacuo for 1.5
h. The tube was cooled down to room temperature and filled
with argon. EtCN (0.40 mL) was added to the tube, and the
mixture was stirred at room temperature for 3 min. To this
were added terminal alkyne 1 (0.400 mmol), tributyltin
hydride (2) (140 mg, 0.480 mmol), pyridine (3.16 mg, 40.0
µmol) and TEMPO [(31.3 mg, 0.200 mmol) or (46.9 mg,
0.300 mmol)], and the resulting mixture was then stirred at
70 ºC for 8 h. After cooling down to room temperature, the
reaction mixture was concentrated under reduced pressure
of 10 Pa for 1 h. In another 50 mL Schlenk tube were placed
KF (55.8 mg, 0.960 mmol) and 18-crown-6 (127 mg, 0.480
mmol), which were evacuated at room temperature for 1 h
and filled with argon. To the 50 mL Schlenk tube were
added THF (2.0 mL) and CuCN (1.79 mg, 20.0 µmol) and
was then transferred the solution in the 20 mL Schlenk tube
through a cannula. THF (0.70 mL) was added to the 20 mL
Schlenk tube, the inside of which was rinsed with the added
THF, and the THF was transferred again through a cannula
into the 50 mL Schlenk tube, and this operation was
repeated a further twice with THF (0.70 mL + 0.60 mL). To
the 50 mL Schlenk tube was added 2-(trimethylsilyl)phenyl
trifluoromethanesulfonate (8) (143 mg, 0.480 mmol), and
the resulting mixture was stirred at 65 ºC. After the time
specified in Table 3, H2O (0.5 mL) was added to the reaction
mixture, and the aqueous phase was extracted with EtOAc
(5 mL × 3). The combined organic layer was dried over
anhydrous sodium sulfate. Filtration through a pad of Celite
and evaporation of the solvent followed by column
chromatography on silica gel gave product 9.
[4] For a review on the mechanism of the KMSCC reaction,
see: P. Espinet, A. M. Echavarren, Angew. Chem. Int. Ed.
2004, 43, 4704–4734.
[5] For selected reports, see: a) M. W. Logue, K. Teng, J.
Org. Chem. 1982, 47, 2549–2553 (BuLi); b) A.
Beauchard, V. A. Phillips, M. D. Lloyd, M. D.
Threadgill, Tetrahedron 2009, 65, 8176–8184
[(Me3Si)2NLi]. Conventionally, alkynylstannanes have
been also prepared from other alkynylmetals (metal = Na,
MgX), see: c) C. Beermann, H. Hartmann, Z. Anorg.
Allg. Chem. 1954, 276, 20–32; d) H. G. Viehe, Chem.
Ber. 1959, 92, 1270–1276.
[6] a) W. P. Neumann, F. G. Kleiner, Tetrahedron Lett.
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[8] Y. Obora, A. S. Baleta, M. Tokunaga, Y. Tsuji, J.
Organomet. Chem. 2002, 660, 173–177.
Characterization data of and NMR spectra of products
are collected in Supporting Information.
[9] K. Kiyokawa, N. Tachikake, M. Yasuda, A. Baba,
Angew. Chem. Int. Ed. 2011, 50, 10393–10396.
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7
This article is protected by copyright. All rights reserved.

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Advanced Synthesis & Catalysis
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[21] The stannylation of 1a with HSnMe3 (2’) instead of 2
as a stannylating agent was also carried out under the
standard and modified reaction conditions, as shown
below. However, the use of 2’ resulted in lower yields of
3’a, compared to that of 3a when using 2. Accordingly,
further investigation on the scope of alkynes was
conducted with 2.
Zn(OTf)2 (5 mol%)
pyridine (10 mol%)
TEMPO (50 mol%)
1:1.2
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Ph
H
+
Ph
SnMe3
H
SnMe3
EtCN, 70 ºC, 8 h
1a
2'
3'a
20% yield[a]
variation from the standard conditions
for 24 h; 22% yield[a]
32% yield[a]
35% yield[a]
with 2' (2 equiv.);
with 2' (3 equiv.);
[a]Average yield determined by 1H and 119Sn NMR.
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[24] The decomposition of 3t and 3w is likely to be
responsible for their high boiling points, and that of 3p
might be due to the proto-destannylation by the internal
HO group.
[25] To identify whether the corresponding alkyne,
PhCCH (1a), can couple with 5a in the manner of the
Sonogashira–Hagihara cross-coupling (SHCC) reaction
under the reaction conditions, the same reaction except
without using 2 was conducted but gave 6aa in only 3%
NMR yield, thus indicating that proto-destannylation of
3a to give 1a followed by the SHCC reaction with 5a is
not the major route for providing the 74% yield of 6aa,
in other words, the reaction conditions used for the 2nd
step does not work for the SHCC reaction.
[26] One-pot reaction performed in EtCN without
removing volatiles provided a 92:8 regioisomeric
mixture of stannylenynes in 69% yield. The major
isomer is 7a, and the structure of the minor isomer is as
follows:
in
which
articles
on
hydrogen-acceptorless
dehyderogenative borylation are included, see: j) T.
Ishiyama, N. Miyaura, J. Organomet. Chem. 2003, 680,
3–11; k) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J.
M. Murphy, J. F. Hartwig, Chem. Rev. 2010, 110, 890–
8
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10.1002/adsc.201900540
Advanced Synthesis & Catalysis
Ph
regenerated at a second cycle, and this sequence
continues further. However, by summing up these, the
total amount of TEMPO leading to TEMPOSn 10 that
can react with 1 is already over 100% (50% + 33% +
22% = 105%) at this stage.
Bu3Sn
Ph
H
[27] For example, one-pot reaction of 1a in EtCN
conducted without removing volatiles gave 9a in lower
yield of 70%.
[33] The same disproportionation of 13 has been proposed,
see: A. Dijksman, A. Marino-González, A. M. i Payeras,
I. W. C. E. Arends, R. A. Sheldon, J. Am. Chem. Soc.
2001, 123, 6826–6833.
[28] Due to the instability under air, 10 could not be isolated
but was detected by HRMS. See Supporting Information.
[34] We could not find out a 119Sn NMR spectrum of
Bu3SnOH (SnOH) in CDCl3 or C6D6 in the literature. On
the other hand, since SnOH in 1-methyl-2-pyrrolidone
(NMP) has been reported to have a signal at ca. 48 ppm,
a signal of SnOH in CDCl3 or C6D6 should be predicted
to appear in a similar region. However, we have not
observed any peak at the region in 119Sn NMR spectra
after stannylation reactions. Due to this, SnOH is shown
as a transient species with brackets in Scheme 9-A. For
the 119Sn NMR spectrum of Bu3SnOH in NMP, see: V.
Farina, S. Kapadia, B. Krishnan, C. Wang, L. S.
Liebeskind, J. Org. Chem. 1994, 59, 5905–5911.
[29] M. Lucarini, E. Marchesi, G. F. Pedulli, J. Org. Chem.
1998, 63, 1687–1693.
[30] G. H. Spikes, Y. Peng, J. C. Fettinger, J. Steiner, P. P.
Power, Chem. Commun. 2005, 6041–6043. See also
reference 29 for the cleavage of the N–O bond of
TEMPOSi(SiMe3)3 to afford a disiloxane.
[31] (R3Sn)2O has been reported to work as a stannylating
agent in benzene and THF. See reference 6.
[32] Assuming that all the processes where TEMPO is
involved proceeds theoretically, at a first cycle, 33% of
TEMPO is regenerated from 50% of TEMPO, which is
the amount used first to alkyne 1. Continuously, 22% of
TEMPO from the resulting 33% of TEMPO is
9
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10.1002/adsc.201900540
Advanced Synthesis & Catalysis
FULL PAPER
A Drastic Effect of TEMPO in Zinc-Catalyzed
Stannylation of Terminal Alkynes with
Hydrostannanes via Dehydrogenation and
Oxidative Dehydrogenation
H H
R
H
Zn
pyridine
TEMPO
+
R
Sn
EtCN
H Sn
Adv. Synth. Catal. Year, Volume, Page – Page
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H2O
Yuichi Kai, Shinya Oku, Tomohiro Tani, Kyoko
Sakurai, Teruhisa Tsuchimoto*
N
OSn
10
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