Zinc/Indium Bimetallic Lewis Acid Relay Catalysis for Dehydrogenative Silylation/Hydrosilylation Reaction of Terminal Alkynes with Bis(hydrosilane)s

Article abstract of DOI:10.1002/adsc.202000501

When mixed with two different Lewis acid catalysts of zinc and indium, terminal alkynes were found to react with bis(hydrosilane)s to selectively provide 1,1-disilylalkenes from among several possible products, by way of a sequential dehydrogenative silylation/intramolecular hydrosilylation reaction. Adding a pyridine base is crucial in this reaction; a switch as a catalyst of the zinc Lewis acid is turned on by forming a zinc?pyridine-base complex. A range of the 1,1-disilylalkenes can be obtained by a combination of aryl and aliphatic terminal alkynes plus aryl-, heteroaryl-, and naphthyl-tethered bis(hydrosilane)s. The 1,1-disilylalkene prepared here is available as a reagent for further transformations by utilizing its C?Si or C=C bond. The former includes Hiyama cross-coupling, bismuth-catalyzed ether formation, and iododesilylation; the latter includes double alkylation and epoxidation. Mechanistic studies clarified the role of the two Lewis acids: the zinc–pyridine-base complex catalyzes the dehydrogenative silylation as a first stage, and, following on this, the indium Lewis acid catalyzes the ring-closing hydrosilylation as a second stage, thus leading to the 1,1-disilylalkene. (Figure presented.).

Full text of DOI:10.1002/adsc.202000501

Title: Zinc/Indium Bimetallic Lewis Acid Relay Catalysis for
Dehydrogenative Silylation/Hydrosilylation Reaction of Terminal
Alkynes with Bis(hydrosilane)s
Authors: Tomohiro Tani, Yudai Sohma, and Teruhisa Tsuchimoto
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000501
Link to VoR: https://doi.org/10.1002/adsc.202000501

10.1002/adsc.202000501
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Zinc/Indium Bimetallic Lewis Acid Relay Catalysis for
Dehydrogenative Silylation/Hydrosilylation Reaction of
Terminal Alkynes with Bis(hydrosilane)s
Tomohiro Tani,a Yudai Sohma,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: When mixed with two different Lewis acid further transformations by utilizing its C–Si or C=C bond.
catalysts of zinc and indium, terminal alkynes were found to The former includes Hiyama cross-coupling, bismuth-
react with bis(hydrosilane)s to selectively provide 1,1- catalyzed ether formation, and iododesilylation; the latter
disilylalkenes from among several possible products, by way includes double alkylation and epoxidation. Mechanistic
of a sequential dehydrogenative silylation/intramolecular studies clarified the role of the two Lewis acids: the zinc–
hydrosilylation reaction. Adding a pyridine base is crucial in pyridine-base complex catalyzes the dehydrogenative
this reaction; a switch as a catalyst of the zinc Lewis acid is silylation as a first stage, and, following on this, the indium
turned on by forming a zinc–pyridine-base complex. A range Lewis acid catalyzes the ring-closing hydrosilylation as a
of the 1,1-disilylalkenes can be obtained by a combination of second stage, thus leading to the 1,1-disilylalkene.
aryl and aliphatic terminal alkynes plus aryl-, heteroaryl-,
and naphthyl-tethered bis(hydrosilane)s. The 1,1-
disilylalkene prepared here is available as a reagent for
Keywords: Addition; Alkenes; Alkynes; Dehydrogenation;
Hydrosilylation; Silicon
for the 1,1-type derived directly from alkynes, only
two rather specific reactions, also promoted by TMs,
have been reported: one is the stoichiometric or
catalytic addition of TM–disilyl complexes with the
carborane backbone (TM = Pt, Ni) to 1-hexyne
Introduction
As is well known, silicon is the second-most
abundant element after oxygen on Earth, and has
extensive industrial uses: wafers, ceramics, and alloys
of silicon, as well as glass and sand of silica, and also
silicone for implants, catheters, contact lenses, and
shampoos.[1] Therefore, continuous proper utilization
of silicon resources is an important issue for
sustainable growth of the silicon-based industry. In
terms of synthetic organic chemistry, one of the
primary roles of silicon should be the protection of
reactive functional groups in synthetic intermediates
for further elaboration.[2] Another important role is as
a reagent for various reactions, such as Hiyama cross-
coupling, Mukaiyama aldol, Diels–Alder, Hosomi–
Sakurai allylation, Tamao–Fleming oxidation,
Peterson olefination, and other reactions.[3] In this
regard, disilylalkenes with two C–Si bonds are also
expected to act as valuable reagents, and have two
structural isomers of 1,1- and 1,2-disilyl types.
Regarding the 1,2-type, a large number of practical
stereoselective synthetic methods, which, as can be
easily imagined, are the addition of a Si–Si bond to a
C≡C bond under transition metal (TM) catalysis,
have been developed (Scheme 1a).[4,5] However, as
[a]
Si
Si
cat. TM
R
R' +
Si Si
TM = Pd, Pt, Ni,
Fe, Au, Rh,...
R
R'
[b]
Bu
Me2
Si
Me2
•
•
•
•
H
Si
•
•
C
•
•
C
•
•
•
•
H
+ M
•
•
•
•
C
Me2
C
Si
Me2
•
•
•
•
Si
Bu
M = Pt, Ni
• = BH
[c]
H
R
Si
Si
cat. Pt
200 °C
Si
Si
R
H
+
R = mesityl, SiMe2Ph; Si = Si(i-Pr)2
[d] two-step strategy
Y
R
Si
Si
R
R
Si +
Y Si
Y = H, B, Si, acyl,
alkenyl, alkynyl,...
silylation
H
Scheme 1. Preceding routes from alkynes to 1,2- and 1,1-
disilylalkenes.
1
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10.1002/adsc.202000501
Advanced Synthesis & Catalysis
(Scheme 1b);[6] the other is the addition of a strained
benzannulated-disilacyclobutene to bulky alkynes
under platinum catalysis at high temperature (Scheme
1c).[7] In contrast to the scarcity of the direct synthetic
route from alkynes, a two-step strategy, starting with
the silylation of terminal alkynes, followed by the
addition of various silylating agents to the resulting
silylalkynes, seems to be a more effective and reliable
avenue to reach the 1,1-type (Scheme 1d).[8,9]
However, the two-step strategy achieved in one pot
has no precedent, to the best of our knowledge. This
should be due to the difficulty of setting appropriate
reaction conditions to perform intrinsically different
styles of the two reactions. Besides these alkyne-
based methods,[10] trisilylmethanes (Si3CH) and their
derivatives,[11] dihalodisilylmethanes (Si1Si2CX2),[12]
halosilylalkenes (SiCX=CHR),[13] and 3,3-disilyl-1-
propenyl ether (Si2CHCH=CHOR)[14] have been used
to prepare the 1,1-type, though using Li- or Mg-based
reagents with high nucleophilicity and/or basicity.
Due to the outstanding utility of alkynes to be
platforms for various organic transformations,[15]
since 2000, we have been developing new Lewis-
acid-catalyzed alkyne-based reactions, which are
divided into two categories, depending on whether a
C(sp)–H or C≡C bond is functionalized. The former
includes zinc-Lewis-acid-catalyzed dehydrogenative
silylation (DS) of terminal alkynes;[16] the latter
includes addition reactions based on activation of the
C≡C bond by indium Lewis acid catalysts.[17] On the
basis of these original technologies, we envisioned
that mixing alkynes 1 and bis(hydrosilane)s 2 in the
presence of the two different Lewis acids could be a
new avenue to address 1,1-disilylalkenes 3 in one pot,
via the DS, followed by the intramolecular
hydrosilylation (HS) (Scheme 2). We disclose a
noticeable performance of the zinc/indium bimetallic
Lewis acid relay catalysis.
Table 1. Effect of zinc salts in zinc/indium-catalyzed
dehydrogenative silylation/hydrosilylation relay reaction.[a]
H H
Ph
Si
Si
+
1:1.5
H
1a
2a
Zn (10 mol%)
PrCN
pyridine (10 mol%) 120 ºC, 20 h
In(OTf)3 (10 mol%)
Ph
Ph
Si
Ph
Si
Ph
Si Si
Ph
Si
H
Si
Si
Si
+
+
+
3a
4a
5a
6a
Yield [%][b]
Entry Zn
3a + 4a (3a/4a)
5a
6a
1
2
3
4
5
Zn(OTf)2
Zn(ONf)2
Zn(NTf2)2
ZnBr2
ZnCl2
ZnF2
5 (96/4)
5 (99/<1)
16 (99/1)
2 (99/<1)
3 (99/<1)
3 (99/<1)
49
49
30
31
<1
20
11
10
11
11
16
16
[c]
6
[a]
Reagents: 1a (0.40 mmol), 2a (0.60 mmol), Zn (40
mol), pyridine (40 mol), In(OTf)3 (40 mol), PrCN (1.0
mL). [b] Determined by 1H NMR. [c] Nf = SO2C4F9.
Zn(NTf2)2 is more promising, giving 3a in higher
yield and selectivity (entries 2–6). Because almost all
of the previous methods with alkynes deliver 1,2-
types,[4,5] the almost exclusive formation of 3a in this
strategy is worthy of note.
Inspired by the above result, we methodically
explored the effect of other factors; results on the
effect of solvents are presented in Table 2. DMF was,
however, invalid, probably due to the strong
coordinating nature to the Lewis acids used (entry 2).
No improvements in the yield of 3a were made with
other solvents, except 5a, possibly leading to desired
3a, which was produced in the highest yield of 56%
when using PhCl (entries 3–6). Importantly, the
formation of 6a was efficiently suppressed in solvents
other than PrCN as a nitrile, which is the best
medium for the DS of terminal alkynes.[16]
Because of 5a being formed as a major product in
PhCl, it was expected that using an indium catalyst
able to more powerfully promote the stage of the
intramolecular HS of 5a would increase the yield of
3a. As shown in Table 3, indium salts with ONf,
NTf2, and Br ligands showed good performance for
enhancing the yield of 3a, albeit lowering the ratio of
3a/4a, and coproducing a small amount of 7a[18] by
double HS (entries 2–4). Compared to these three
indium salts, InCl3 and InF3 were less effective
(entries 5 and 6). As a result, we adopted InBr3 as an
indium catalyst, due to the higher yield of 3a, and
also due to 5a still remaining.
H Si
R
H
Si
Si
cat. Zn/In Lewis acid
R
H
+
H Si
3
1
2
H Si
cat. Zn
cat. In
intramolecular HS
DS
R
Si
Scheme 2. Working hypothesis: zinc/indium Lewis acid
relay catalysis leading to 1,1-disilylalkenes from alkynes.
Results and Discussion
We first tried to establish reaction conditions to
efficiently obtain 1,1-disilylalkene 3a by the reaction
of PhC≡CH (1a) with 1,2-bis(dimethylsilyl)benzene
(2a) (Table 1). Treating 1a and 2a with 10 mol%
each of Zn(OTf)2 (Tf = SO2CF3), pyridine, and
In(OTf)3 in PrCN at 120 ºC for 20 h gave 3a, albeit
along with its isomer 4a in low total yield of 5%
(entry 1). Besides these, single DS product 5a as a
major product, which is a possible intermediate for 3a
and/or 4a, and double DS product 6a were produced.
The screening of other zinc Lewis acids showed that
With Zn(NTf2)2 and InBr3 as Lewis acid catalysts
and PhCl as a solvent, a proper amount of each of
Zn(NTf2)2, pyridine, and InBr3 was continuously
2
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10.1002/adsc.202000501
Advanced Synthesis & Catalysis
Table 2. Effect of solvents in zinc/indium-catalyzed
Table 4. Effect of amounts of Zn(NTf2)2, pyridine, and
InBr3 in zinc/indium catalyzed dehydrogenative
silylation/hydrosilylation relay reaction.[a]
dehydrogenative silylation/hydrosilylation relay reaction.[a]
1a
+
2a
1:1.5
1a
+
2a
1:1.5
Zn(NTf2)2 (10 mol%) solvent
pyridine (10 mol%)
In(OTf)3 (10 mol%)
120 ºC, 20 h
Zn(NTf2)2 (x mol%) PhCl
pyridine (y mol%)
InBr3 (z mol%)
120 ºC, 20 h
Ph
Si
Ph
Si
Ph
Si
Ph
Si Si
Ph
Si
Ph
H
Ph
Si Si
Ph
Si Si
Ph
Si
Si
Si
R
+
+
+
Si
Si
Si
+
+
+
3a
4a
5a
6a
Yield [%][b]
3a
4a
7a
5a (R = H)
6a (R = C CPh)
Entry Solvent
3a + 4a (3a/4a)
5a
6a
Yield [%][b]
1[c]
2
PrCN
16 (99/1)
<1
2 (99/<1)
8 (92/8)
1 (99/<1)
2 (99/<1)
30
<1
43
56
50
22
11
DMF[d]
<1
<1
2
2
2
3a + 4a
(3a/4a)
5a
6a
7a
3
4
1,4-dioxane
PhCl
Entry
1[c]
2
3
4
x
y
z
10 10 10 54 (86/14)
10 10 15 18 (90/10)
18
44
37
<1
<1
<1
<1
<1
<1
1
<1
<1
<1
<1
3
2
<1
13
4
5
PhMe
6
octane
10 20
15 20
15 30
15 35
15 40
5
5
5
5
5
38 (94/6)
[a]
Reagents: 1a (0.40 mmol), 2a (0.60 mmol), Zn(NTf2)2
(40 mol), pyridine (40 mol), In(OTf)3 (40 mol),
67 (87/13)
76 (90/10)
82 (89/11)
74 (89/11)
5
6
solvent (1.0 mL). Determined by 1H NMR. The same
[b]
[c]
2
1
as those in entry 3 of Table 1. [d] DMF = HCONMe2.
7
[a]
Reagents: 1a (0.40 mmol), 2a (0.60 mmol), Zn(NTf2)2
(40 or 60 mol), pyridine (40 mol, 80 mol, 0.12 mmol,
0.14 mmol or 0.16 mmol), InBr3 (20, 40 or 60 mol), PhCl
(1.0 mL). [b] Determined by 1H NMR. [c] The same as those
in entry 4 of Table 3.
Table 3. Effect of indium salts in zinc/indium-catalyzed
dehydrogenative silylation/hydrosilylation relay reaction.[a]
1a
+
2a
1:1.5
Zn(NTf2)2 (10 mol%) PhCl
pyridine (10 mol%)
In (10 mol%)
120 ºC, 20 h
of InBr3 from 10 to 5 mol% also lowered the yield of
3a + 4a to 38% (entry 3). However, adding 20 mol%
or more of pyridine in the presence of Zn(NTf2)2 (15
mol%) and InBr3 (5 mol%) was found to enhance the
yield (entries 4–7), which reached 82% when using
35 mol% of pyridine (entry 6).
Ph
Si
Ph
Si Si
Ph
Si Si
Ph
R
Si
Si
Si
+
+
+
To improve the ratio of 3a/4a, the effect of organic
bases was lastly examined (Table 5, see Table S1 in
the Supporting Information for further details). Using
pyrazine (B) and tertiary amines C–F negatively
affected this reaction, lowering both the yield and
selectivity. Then, pyridine bases with different
electronic and steric demands were tested, showing
that a 2-substituent tends to have an effect on raising
the selectivity of 3a, regardless of its electronic
property. As a result, only 3a was obtained in good
yield of 73% in using 2-(2,6-xylyl)pyridine (T); no
undesired products, 5a, 6a, and 7a, were favorably
detected under the reaction conditions.
Only the orderly progress of the DS, followed by
the intramolecular HS, guides a mixture of 1a and 2a
to desired product 3a; this relay style is shown as [1]
in Scheme 3. There can be 5 other relay styles in this
reaction, depending on the combination and order of
the DS and HS ([2]–[6]). Accordingly, it is worth
noting that the exclusive and efficient formation of 3a
is attributed to not only the closing of the route to 4a
by choosing pyridine base T, but also to the exquisite
direction to relay style [1] out of 6 possibilities, even
3a
4a
7a
5a (R = H)
6a (R = C CPh)
Yield [%][b]
Entry In
3a + 4a (3a/4a)
5a
6a
7a
1[c]
2
3
4
5
In(OTf)3
8 (92/8)
In(ONf)3 54 (77/23)
In(NTf2)3 50 (88/12)
56
<1
<1
18
25
3
2
<1
<1
<1
1
<1
3
3
3
<1
<1
InBr3
InCl3
InF3
54 (86/14)
38 (87/13)
40 (89/11)
6
1
[a]
Reagents: 1a (0.40 mmol), 2a (0.60 mmol), Zn(NTf2)2
(40 mol), pyridine (40 mol), In (40 mol), PhCl (1.0
[b]
1
[c]
mL). Determined by H NMR. The same as those in
entry 4 of Table 2.
examined (Table 4). An increased amount of InBr3
was expected to more efficiently promote the
cyclization of 5a to 3a, but failed to meet our
expectations (entry 2). Increasing the loading of
pyridine to 20 mol% as well as decreasing the loading
3
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10.1002/adsc.202000501
Advanced Synthesis & Catalysis
[1] DS intramolecular HS
Table 5. Effect of organic bases in zinc/indium-catalyzed
dehydrogenative silylation/hydrosilylation relay reaction.[a]
Ph
Ph
Ph
Ph
Ph
H
Zn(NTf2)2 (15 mol%)
Si Si
Si
Si
Si
Si
organic base (35 mol%)
InBr3 (5 mol%)
Si Si
Si
Si
1a
2a
+
+
PhCl, 120 ºC, 20 h
1:1.5
3a
5a
4a
3a
4a
yield [%] of 3a + 4a (3a/4a)[b]
H H
Ph
H
N
cat. Zn
cat. T
cat. In
Si
Si
N
+
+
N
N
N
D
A
B
C
82 (89/11)[c]
67 (88/12)
<1
61 (82/18)
1a
2a
[3] DS intermolecular HS
[4] intermolecular HS DS
N
CN
O
[2] DS DS
Ph
Ph
Ph
Ph
N
N
N
N
N
E
<1
F
G
H
Si
Si
Si
Si
6 (79/21)
38 (92/8)
66 (92/8)
6a
[5] intermolecular HS
[6] intermolecular HS
N
I
N
J
N
K
N
L
intramolecular HS
intermolecular HS
65 (93/7)
79 (94/6)
71 (97/3)
52 (96/4)
Ph
Ph
Si
Ph
Ph
Si Si
Si
Si
Si
N
M
Et
N
N
Bn
N
O
Br
N
P
I
65 (96/4)
75 (97/3)
74 (95/5)
68 (98/2)
7a
Scheme 3. Possible relay styles in zinc/indium catalysis.
N
N
N
N
Ph
R
S
T
Q
80 (97/3)
72 (98/2)
73 (99/<1)
77 (91/9)
applied to other aliphatic alkynes with the phenyl
terminus and branched cyclohexyl group (1j and 1k).
The C=C bond of 1l was retained to lead to 3l without
suffering hydrosilylation.[19]
It was also found that bis(hydrosilane)s other than
2a can participate in this relay reaction: upon reacting
with 1a, the 1,1-types of 3m–3p with xylyl, C6H2F2,
thienyl, and naphthyl units were obtained selectively,
as similar to other entries.
[a]
Reagents: 1a (0.40 mmol), 2a (0.60 mmol), Zn(NTf2)2
(60 mol), organic base (0.14 mmol), InBr3 (20 mol),
PhCl (1.0 mL). [b] Determined by 1H NMR. [c] The same as
those in entry 6 of Table 4.
in the presence of the two Lewis acids with different
roles.
On the basis of the suitable reaction conditions, the
scope of the Zn/In-catalyzed relay reaction was next
explored (Table 6). Similar to 1a, a range of aryl
alkynes 1b–1g reacted with 2a to selectively furnish
five-membered 1,1-disilylalkenes 3b–3g, whereas co-
producing 1,2-types 4 in a small amount when using
electron-poor aryl alkynes with CF3 and Br groups.
The relay reaction of 3-ethynylthiophene (1h),
leading to 1,1-type 3h, also occurred exclusively. In
addition to these (hetero)aryl alkynes, aliphatic ones
were tested. However, as indicated separately in
Scheme 4, the standard reaction conditions with T as
a base was ineffective for 1-octyne (1i), giving only
13% yield of 3i + 4i, while ensuring a high selectivity
of 3i. Because of this, other pyridine bases were
retested. As Scheme 4 shows, the smaller was the size
that a 2-substituent on the pyridyl ring became, the
higher was the yield, which was greatly improved to
72% while maintaining the selectivity of 3i/4i; the
result of the isolated yield is given in Table 6. This
reaction conditions with simple pyridine (A) can be
A practical advantage can be demonstrated by
performing a scalable synthesis of 1,1-disilylalkene 3.
For example, the reaction of 1a with 2a on a 10-
mmol scale provided us with 1.80 g (61% yield) of 3a.
Different from the successful substrates in Table 6,
unfortunately, alkynes 1m–1p and bis(hydrosilane) 2f
in Figure 1 were incompatible with this objective. In
the use of alkynes 1m–1p, no desired relay reaction
occurred, despite their consumption.
With 3a synthesized in hand, we next looked into
its utility as a reagent, as shown in Scheme 5, where
each reaction was carried out under the conditions
established in each preceding study. Initially, Hiyama
cross-coupling with PhI was conducted, thereby both
of the two C–Si bonds being successfully reacted to
yield 8.[20,21] Next, according to a precedent,[22] the
treatment of 3a and diphenylmethanol with catalyst
BiBr3 was anticipated to provide trisubstituted alkene
9, but, in fact, it generated highly branched alkyl
ether 9’ in good yield. This could be due favorably to
4
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10.1002/adsc.202000501
Advanced Synthesis & Catalysis
Zn(NTf2)2 (15 mol%)
pyridine base (35 mol%)
InBr3 (5 mol%)
Hex
Table
6.
Zinc/indium-catalyzed
dehydrogenative
Hex
H
Hex
Si
silylation/hydrosilylation relay reaction.[a]
1i
+
Si Si
+
Si
R
PhCl, 120 ºC, 20 h
1i:2a = 1:1.5
R
Si
Zn(NTf2)2 (15 mol%)
H H
H H
Si Si
T (35 mol%)
InBr3 (5 mol%)
R
Si
Si
Si Si
Si
+
+
3i
4i
PhCl, 120 ºC, 20 h
1:2 = 1:1.5
Ar
Ar
2a
Ar
H
1
2
3
4
pyridine base
3a (1a + 2a; R = H); 71% yield, 3a/4a = 99/<1
R
3b (1b + 2a; R = 2-Me); 76% yield, 3b/4b = 99/<1
3c (1c + 2a; R = 4-Me); 64% yield, 3c/4c = 99/<1
N
N
N
T
K
A
3d (1d + 2a; R = 4-OMe);[b] 47% yield, 3d/4d = 99/<1
3e (1e + 2a; R = 4-CF3);[c] 83% yield, 3e/4e = 95/5
3f (1f + 2a; R = 4-Br); 66% yield, 3f/4f = 99/1
Si Si
yield [%] of 3i + 4i (3i/4i)
65 (96/4)
72 (97/3) (30 h)
13 (96/4)
46 (96/4)
S
Scheme 4. Effect of pyridine bases in zinc/indium-
catalyzed dehydrogenative silylation/hydrosilylation relay
reaction of 1-octyne (1i) with 2a. Yields and ratios were
determined by 1H NMR.
Si Si
Si Si
Si Si
O
3g (1g + 2a)
78% yield
3g/4g = 99/<1
3h (1h + 2a)
73% yield
3h/4h = 99/<1
3i (1i + 2a)[c,d]
71% yield
3i/4i = 97/3
O
O
H H
Si
O
S
Si
O
Fe
O
N
PhS
Si
Ph
1m
1n
1o
1p
2f
Si Si
Si Si
Si Si
Figure 1. Incompatible substrates with zinc/indium-
catalyzed dehydrogenative silylation/hydrosilylation relay
reaction.
3j (1j + 2a)[c,d]
55% yield
3j/4j = 92/8
3k (1k + 2a)[c,d]
69% yield
3k/4k = 99/<1
3l (1l + 2a)[c,d]
44% yield
3l/4l = 99/<1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Si Si
Si Si
O
Ph
Si
Si
Si Si
Ph
Ph
9'; 61% yield
9
F
F
S
3o (1a + 2d)[e]
41% yield
3p (1a + 2e)
84% yield
3p/4p = 99/<1
I
I
Ph
Ph
Ph
Ph
3m (1a + 2b)
78% yield
3n (1a + 2c)
70% yield
3m/4m = 99/<1 3n/4n = 99/<1
3o/4o = 96/4
[b]
[a]
8; 59% yield
10; 31% yield
[a]
[c]
Reagents: 1 (0.20 mmol), 2 (0.30 mmol), Zn(NTf2)2 (30
mol), 2-(2,6-xylyl)pyridine (T) (70 mol), InBr3 (10
mol), PhCl (0.50 mL). Yields of isolated 3 based on 1 are
shown here. Ratios of 3/4 were determined by 1H NMR. [b]
Ph
Si
Si
[c]
[d]
[e]
[d]
2a (0.50 mmol) was used.
Performed for 30 h.
3a
Pyridine (70 mol) was used instead of T. [e] Performed for
Ph
Si
Bu
Hex
Ph
Si
Si
48 h.
O
Si
12; 91% yield
11; 80% yield
latent unique reactivity of 3, possibly arising from the
five-membered cyclic disilyl backbone. Upon being
reacted with I2/AgNO3, both of the Si moieties were
replaced with I atoms, albeit in 31% yield (10).[23]
This kind of 1,1-diiodoalkene is useful as a scaffold
towards alkene-based -systems.[24] In addition to
these C–Si bond transformations, 3a can be converted
to other disilyl motifs, inspired by the C=C bond
derivatization. Thus, multi-alkylated disilylcycle 11
was obtained in high yield by reacting 3a with BuLi,
and then adding hexyl bromide (HexBr).[25] With m-
chloroperbenzoic acid (mCPBA),[26] spiro-epoxide 12
Scheme 5. Synthetic applications of 3a. Yields of isolated
8–12 based on 3a are given here. Reagents and conditions:
[a] PhI (4.0 equiv.), Pd(dba)2 (20 mol%), KOH (4.0 equiv.),
MeOH, room temperature (rt), 24 h; [b] Ph2CHOH (3.0
equiv.), BiBr3 (5 mol%), ClCH2CH2Cl, rt, 24 h; [c] I2 (3.0
equiv.), AgNO3 (3.0 equiv.), MeCN, 0 ºC, 1 h; [d] BuLi
(1.2 equiv.), THF, 0 ºC, 15 min, and then HexBr (1.3
equiv.), rt, 1 h; [e] mCPBA (2.0 equiv.), CH2Cl2, rt, 18 h.
5
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000501
Advanced Synthesis & Catalysis
Table 7. Observation of 1H NMR signals of 2-(2,6-
xylyl)pyridine (T).[a]
was produced in high yield. These results, collected
in Scheme 5, show that 1,1-disilylalkene 3 is likely to
be a useful origin for molecular architecture.
Zn(NTf2)2
To get insight into the reaction pathway, we
explored whether, even in the co-presence of InBr3, a
T···ZnII complex could be formed (Table 7), as in the
DS of indoles catalyzed by the similar zinc–pyridine
system.[27] As shown in entries 1 and 2, the H peak
T
ZnX2
+
and/or
InBr3
Ha
N
C6D6, rt, 10 min
(T ZnII)
T
X = NTf2, Br
Entry Chemicals
included
Chemical shift [ppm] of H
1
of T found at  8.61 in a H NMR spectrum was
1
8.61
T
upfield-shifted to  8.29, upon measuring a mixture
of T, Zn(NTf2)2, and InBr3 in a 7:3:1 ratio, which is
the same as the relative amount of each used for the
standard reaction conditions. An upfield-shift of the
H signal was again recorded when mixing T and
Zn(NTf2)2 regardless of their ratio, but mostly not
when InBr3 was added instead of Zn(NTf2)2 (entries
3–5). These results exhibit that T would coordinate
preferentially to ZnII rather than InIII in the case of
entry 2, thus yielding a T···ZnII complex key for the
progress of the DS in the present relay reaction. The
smaller difference of 0.32 in entry 2 than 0.44 in
entry 3 may be due to partly forming Br-ligated zinc
salts, i.e. Zn(NTf2)Br and ZnBr2, with different Lewis
acidity from that of Zn(NTf2)2, via ligand exchange
between Zn(NTf2)2 and InBr3 in a NMR sample tube.
The roles of ZnII, T, and InIII in the present reaction
were also studied, as shown in Table 8, where the
result when using all of Zn(NTf2)2, T, and InBr3 is re-
given in entry 1 for a reference. Removing InBr3 from
this reduced the yield of 3a drastically, and, thereby,
5a remained uncyclized (entry 2). The reaction with
Zn(NTf2)2, but without T, lowered even the yield of
5a (entry 3). None of 3a, 4a, 5a, and 7a were formed
in the presence of InBr3 regardless of whether T
existed (entries 4 and 5). These results suggest that
the DS of the first stage is efficiently catalyzed by,
not a ZnII salt, but a T···ZnII complex,[28] and that the
intramolecular HS after the DS is catalyzed by, not
the T···ZnII complex, but likely an InIII salt. To
clarify the role of the InIII salt, 5a was heated with 5
mol% of InBr3 in PhCl, thereby giving 3a in high
yield in only 1 h (Scheme 6). Adding T here made
the intramolecular HS slower, but delivered high
yield of 3a after 16 h. Further adding Zn(NTf2)2
accelerated the reaction, implying that the in situ
formation of the T···ZnII complex may suppress the
negative effect of T slowing the progress of the
intramolecular HS when using InBr3 and T (see
reference 32 for further consideration). Only heating
with no catalysts generated almost no 3a. Therefore,
the results of Table 8 and Scheme 6 indicate the
followings: i) the InIII salt can promote the stage of
the intramolecular HS,[29] ii) 5a is an intermediate for
product 3a, and iii) Zn(NTf2)2, a pyridine base, and
InBr3 are all essential pieces for this relay reaction.
To confirm the involvement of a dehydrogenation
(–H2) process in this reaction, we tried to trap H2 gas
(Scheme 7). Thus, with the intention of capturing the
H2 gas that could be released from the relay reaction
of 1a with 2a, trans-stilbene (13) was mixed with
Pd/C as a hydrogenation catalyst, expectedly giving
PhCH2CH2Ph (14) in high yield; an experimental
2[b]
3
T/Zn/In (7:3:1)
T/Zn (7:3)
T/Zn (7:1)
8.29 (0.32)[c]
8.17 (0.44)[c]
8.34 (0.27)[c]
8.57 (0.04)[c]
4
5[b]
T/In (7:1)
[a] 1H NMR spectra were recorded in C6D6 at 30 ºC using
Me4Si as an internal standard. Zn = Zn(NTf2)2. In = InBr3.
[b]
1
Before measuring a H NMR spectrum, the mixture in
C6D6 was heated at 50 ºC for 10 min, due to the low
solubility of InBr3. [c] The value in parentheses shows the
difference from the H signal (8.61 ppm) given by
measuring only T.
Table 8. Effect of catalysts in zinc/indium-catalyzed
dehydrogenative silylation/hydrosilylation relay reaction.[a]
H H
Si Si
Ph
+
1:1.5
H
1a
2a
Zn(NTf2)2 (15 mol%) PhCl
T (35 mol%)
InBr3 (5 mol%)
120 ºC, 20 h
Ph
Ph
Si Si
Ph
Si Si
Ph
Si
H
Si
Si
Si
+
+
+
3a
4a
5a
7a
Conv.
[%]
Yield [%][b]
3a + 4a
5a 7a
Entry Zn
1[c]
2
3
4
T
In
of 1a (3a/4a)
○
○
○
×
×
○
○
×
○
×
○
×
×
○
○
>99
>99
>99
11
73 (99/<1)
6 (99/<1)
8 (91/9)
<1
<1
<1 <1
50 <1
10 65
<1 <1
<1 <1
5
1
[a]
Reagents: 1a (0.20 mmol), 2a (0.30 mmol), Zn(NTf2)2
(30 mol), T (70 mol), InBr3 (10 mol), PhCl (0.50 mL).
Zn = Zn(NTf2)2. In = InBr3. Determined by 1H NMR.
[b]
[c]
The same as those obtained when using T in Table 5.
procedure including glassware used is presented in
the Supporting Information.
Based on the results of the mechanistic studies, and
on previous findings reported by us and others, we
propose a plausible reaction mechanism for this
reaction in Scheme 8. First up is the complexation of
ZnX2 with T to form a T···ZnII {[ZnII]} complex,
which most likely catalyzes the DS of 1 with 2. Thus,
6
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000501
Advanced Synthesis & Catalysis
Ph
H
H
T + ZnX2
T
ZnX2
[ZnII]
Ph
Si Si
Si
Si
H
Si
Si
PhCl, 120 ºC
2
R
5a
3a
d–[ZnII]
with InBr3 (5 mol%)
>99% conv.
H
H
H
DS
96% yield (1 h)
d+ Si
Si
1
with InBr3 (5 mol%) and T (35 mol%)
20% conv.
>99% conv.
d– [ZnII]
15% yield (1 h)
95% yield (16 h)
15
H
H
with InBr3 (5 mol%), T (35 mol%), and Zn(NTf2)2 (15 mol%)
d+ T Si
Si
>99% conv.
93% yield (1 h)
1% yield (16 h)
T
R
without catalysts
1% conv.
H
R
16
InX3
H
+
H2
+ T
Si Si
Si
Si
Scheme
6.
Indium-catalyzed
intramolecular
1
hydrosilylation of 5a. Yields determined by H NMR are
provided here.
5
3
R
10% Pd/C (1 mol% on Pd)
Ph
Ph
R
d+
H2
d–
X2In
Ph
13 (0.20 mmol)
Ph
intramolecular
HS
PrCN, 120 ºC, 20 h
InX2
Si
H
H
14; 96% yield
X
X
Si
Si
Si
R
Ph
H
Zn(NTf2)2 (15 mol%)
T (35 mol%)
Ph
X2In
17
18
H
1a (0.80 mmol)
X
InBr3 (5 mol%)
Si
Si
H2
Si Si
+
+
PhCl, 120 ºC, 20 h
1a:2a = 1:1.5
R
b
H H
R
b
d+
d–
X3In
InX2
Si
Si
18
H
H
a
Si
a
Si
X
3a; 65% yield
Si
Si
2a (1.2 mmol)
Scheme 7. Trapping experiment of H2 gas. Yields of
isolated 14 based on 13 and 3a based on 1a are shown here.
a-anion stabilization
b-cation stabilization
Scheme 8. Proposed reaction mechanisms. X = NTf2, Br.
[ZnII] activates one H–Si bond of 2 to form 15 with a
more electrophilic Si, to which T may coordinate for
stabilization, as previously demonstrated.[27] The
nucleophilic terminal carbon of 1 then reacts with the
electrophilic Si of 16[30] in a –H2 fashion to yield 5,
and also to regenerate [ZnII] and T.[31] Continuously,
5 is relayed to InX3, which activates the C≡C bond[17]
to induce a nucleophilic attack of the other Si–H, as
shown in 17,[32] thereby giving 18.[8c,33] Lastly,
desired 3 is formed via the cyclization of 18,
accompanied by releasing InX3. The selective
generation of 3 over 4 should be due in part to the
well-known -cation[34] and -anion[35] stabilizing
effects of silicon affecting in the process from 17 to
18. However, at present, how bulky base T
contributes to enhancing the selectivity of 3 in the
reaction of aryl alkynes, and why small base A is
useful in using aliphatic alkynes are unclear.
promotes the DS/intramolecular HS relay reaction of
terminal alkynes 1 with bis(hydrosilane)s 2 to deliver
1,1-disilylalkenes 3. This is the first demonstration of
constructing the 1,1-disilyl structure directly from
alkynes under Lewis acid catalysis. Our strategy
features the high selectivity of 3 over 4 as well as
products being possibly formed through sequences
other than that of the DS/intramolecular HS. Product
3 proved to be promising as a reagent, which can be
variously transformed to, for instance, triarylalkenes,
branched ethers, 1,1-diiodoalkenes, and other 1,1-
disilyl motifs. Mechanistic analysis exhibited that the
successful progress of this relay reaction is ascribed
to the proper behavior of each Lewis acid in the
proper situation: the zinc–pyridine complex acts as a
catalyst for the DS as a first stage, which is followed
by the intramolecular HS catalyzed by the indium
Lewis acid.
Conclusion
Experimental Section
We have disclosed that a system consisting of
Zn(NTf2)2, a pyridine base, and InBr3 catalytically
7
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000501
Advanced Synthesis & Catalysis
Zinc/Indium-Catalyzed
Dehydrogenative
0 ºC for 15 min, followed by adding 1-bromohexane (21.5
mg, 0.130 mmol). After further stirring at room
temperature for 1 h, H2O (1.0 mL) was added to the tube,
and the aqueous phase was extracted with Et2O (3 mL × 3).
The combined organic layer was washed with brine (1 mL),
and then dried over anhydrous sodium sulfate. Filtration
through a cotton plug and evaporation of the solvent
followed by column chromatography on silica gel (n-
hexane) gave 11 as a white solid.
Silylation/Hydrosilylation Relay Reaction of Terminal
Alkynes with Bis(hydrosilane)s: A General Procedure
for Table 6
Zn(NTf2)2 (18.8 mg, 30.0 μmol) and InBr3 (3.55 mg, 10.0
μmol) were placed in a 20 mL Schlenk tube, which was
heated at 80 ºC in vacuo for 15 min. The tube was cooled
down to room temperature and filled with argon. PhCl
(0.50 mL) was added to the tube, and the mixture was
stirred at room temperature for 3 min. Terminal alkyne 1
(0.200 mmol), bis(hydrosilane) 2 (0.300 or 0.500 mmol)
and 2-(2,6-xylyl)pyridine (T) or pyridine (70.0 μmol) were
added to the tube, and the resulting mixture was stirred at
120 ºC. After the time specified in Table 6, and then
cooling down to room temperature, a saturated NaHCO3
aqueous solution (0.5 mL) was added, and the aqueous
phase was extracted with EtOAc (5 mL × 3). The
combined organic layer was washed with brine (1 mL),
and then dried over anhydrous sodium sulfate. Filtration
through a cotton plug and evaporation of the solvent
followed by purification gave product 3, along with a small
amount of 4 in some cases.
A
Procedure to Synthesize 1,3-Dihydro-1,1,3,3-
tetramethyl-3’-phenyl-spiro[2,2’-oxirane-2H-1,3-
disilaindene] (12):[26] Under an argon atmosphere, a 20
mL Schlenk tube was charged with 3a (29.5 mg, 0.100
mmol), ≥ 65.0 wt% of mCPBA with H2O (53.1 mg, 0.200
mmol), and CH2Cl2 (1.0 mL). After stirring at room
temperature for 18 h, a saturated NaHCO3 aqueous
solution (1.0 mL) was added, and the aqueous phase was
extracted with Et2O (3 mL × 3). The combined organic
layer was washed with brine (1 mL), and then dried over
anhydrous sodium sulfate. Filtration through a cotton plug
and evaporation of the solvent followed by column
chromatography on silica gel (n-hexane/CHCl3 = 2/1),
gave 12 as a colorless oil.
Synthetic Applications of 3a (Scheme 5)
Characterization data of, and NMR spectra of products
are collected in the Supporting Information.
A Procedure to Synthesize Triphenylethene (8):[20]
Under an argon atmosphere, a 20 mL Schlenk tube was
charged with 3a (29.5 mg, 0.100 mmol), PhI (81.6 mg,
0.400 mmol), KOH (22.4 mg, 0.400 mmol), and MeOH
(0.70 mL). After degassing by three freeze-pump-thaw
cycles, to this was added Pd(dba)2 (11.5 mg, 20.0 μmol),
and the resulting mixture was stirred at room temperature
for 24 h. H2O (1.0 mL) was added to the mixture, and the
aqueous phase was extracted with Et2O (3 mL × 3). The
combined organic layer was washed with brine (1 mL),
and then dried over anhydrous sodium sulfate. Filtration
through a cotton plug and evaporation of the solvent
followed by column chromatography on silica gel (n-
hexane) gave 8 as a white solid.
Acknowledgements
Financial support by KAKENHI (Grants-in-Aid for Scientific
Research) No. 24550123 and 15K05505 from Japan Society for
the Promotion of Science is highly acknowledged. We thank Mr.
Mikio Tanabe for experimental assistance.
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8
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000501
Advanced Synthesis & Catalysis
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[15] B. M. Trost, C.-J. Li, Modern Alkyne Chemistry:
Catalytic and Atom-Economic Transformations, Wiley-
VCH, Weinheim, 2015.
[16] T. Tsuchimoto, M. Fujii, Y. Iketani, M. Sekine, Adv.
Synth. Catal. 2012, 354, 2959–2964.
[17] In 2000, we disclosed for the first time that an indium
Lewis acid works as a catalyst to effectively activate a
C≡C bond of alkynes, which successively accept
addition of arenes to give alkenylarenes, see: a) T.
Tsuchimoto, T. Maeda, E. Shirakawa, Y. Kawakami,
Chem. Commun. 2000, 1573–1574. Since then, we
have reported related studies, based on the same
concept, which is “activation of hydrocarbon functional
groups classified into CC, C=C, C–C, and C–H by
Lewis acids, mainly InX3 (X = OTf, ONf, NTf2, ...)”.
For representative reports, see: b) T. Tsuchimoto, K.
Hatanaka, E. Shirakawa, Y. Kawakami, Chem.
Commun. 2003, 2454–2455; c) T. Tsuchimoto, H.
Matsubayashi, M. Kaneko, Y. Nagase, T. Miyamura, E.
Shirakawa, J. Am. Chem. Soc. 2008, 130, 15823–
15835; d) T. Tsuchimoto, T. Wagatsuma, K. Aoki, J.
Shimotori, Org. Lett. 2009, 11, 2129–2132; e) T.
Tsuchimoto, M. Kanbara, Org. Lett. 2011, 13, 912–
[9] Although silylalkynes are used as substrates, unique
reactions are collected here. For selected recent reports,
see: a) M. Lamač, A. Spannenberg, W. Baumann, H.
Jiao, C. Fischer, S. Hansen, P. Arndt, U. Rosenthal, J.
Am. Chem. Soc. 2010, 132, 4369–4380; b) A.
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28, 280–285; d) N. L. Lampland, A. Pindwal, K. Yan,
A. Ellern, A. D. Sadow, Organometallics 2017, 36,
4546–4557.
9
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10.1002/adsc.202000501
Advanced Synthesis & Catalysis
915; f) Y. Nagase, H. Shirai, M. Kaneko, E. Shirakawa,
T. Tsuchimoto, Org. Biomol. Chem. 2013, 11, 1456–
1459.
[29] Despite the results of Scheme 6, those of entries 4
and 5 in Table 8 where the low conversion of 1a as
well as no formation of 7a were observed imply that
InIII has no ability to catalyze intermolecular HS
reaction of 1a with 2a.
[18] Byproduct 7a was not detected in the reactions
conducted for Tables 1 and 2.
[30] Instead of 16, an ionic pair complex, {[ZnII]––H}[T–
Si+–C6H4SiH], may be another possible form in the DS
cycle, as previously presented in the DS reaction of
terminal alkynes catalyzed by a combination of a Lewis
acid and a Lewis base, as similar to our system. For a
selected example, see: Y. Ma, S.-J. Lou, G. Luo, Y.
Luo, G. Zhan, M. Nishiura, Y. Luo, Z. Hou, Angew.
Chem. Int. Ed. 2018, 57, 15222–15226.
[19] For an example of Lewis acid-catalyzed
hydrosilylation of dienes with bis(hydrosilane)s, see: K.
Shin, S. Joung, Y. Kim, S. Chang, Adv. Synth. Catal.
2017, 359, 3428–3436.
[20] For the reaction conditions of the Hiyama cross-
coupling reaction, see: H. F. Sore, D. T. Blackwell, S. J.
F. MacDonald, D. R. Spring, Org. Lett. 2010, 12,
2806–2809.
[31] We have reported that PhMe2SiH, being structurally
similar to 2, dehydrogenatively reacts with an alkyne
under zinc–pyridine catalysis to give an alkynylsilane
(see reference 16 for further details), suggesting that
the involvement of InX3 to this stage should be
unnecessary to be considered.
[21] Although Hiyama cross-coupling reaction of 3p with
PhI was also examined, almost 3p was recovered
unreacted under essentially the same reaction
conditions as those for 3a.
[22] For the reaction conditions of the bismuth catalysis,
see: Y. Nishimoto, M. Kajioka, T. Saito, M. Yasuda, A.
Baba, Chem. Commun. 2008, 6396–6398.
[32] The results of entries 1 and 2 in Table 8 indicate that
InBr3 is necessary for the efficient intramolecular HS of
5a leading to product 3a. This observation is well
consistent with the result that the intramolecular HS of
5a to 3a proceeds with only InBr3 as a catalyst (Scheme
6). However, as also shown in Scheme 6, the
[23] For the reaction conditions to iodinate a C–Si bond of
silylalkenes, see: a) R. Nakajima, C. Delas, Y.
Takayama, F. Sato, Angew. Chem. Int. Ed. 2002, 41,
3023–3025; b) W. Geng, W.-X. Zhang, W. Hao, Z. Xi,
J. Am. Chem. Soc. 2012, 134, 20230–20233.
intramolecular HS is much faster when using InBr3, T,
R
and Zn(NTf2)2 (1 h) than when using
InBr3 and T (16 h), suggesting that the
[ZnII] complex may participate in the
stage of the intramolecular HS to
increase the reaction rate. As suggested
by one reviewer, one possibility for this
consideration may be 17’.
[ZnII]
[24] For examples, see: a) A. Jayaraman, S. Lee, Org. Lett.
2019, 21, 3485–3489; b) W. Wu, E. Cui, Y. Zhang, C.
Zhang, F. Zhu, C.-H. Tung, Y. Wang, ACS Catal. 2019,
9, 6335–6341; c) A. Jayaraman, S. Lee, Org. Lett. 2019,
21, 7923–7927.
X3In
H
Si
Si
17'
[25] For the reaction conditions to doubly alkylate 1,1-
disilylalkenes, see: A. Inoue, J. Kondo, H. Shinokubo,
K. Oshima, Chem. Commun. 2002, 114–115.
[33] For a selected recent report on indium-mediated
nucleophilic attack of silyl nucleophiles to alkynes and
its mechanistic studies, see: a) K. Kang, Y. Nishimoto,
M. Yasuda, J. Org. Chem. 2019, 84, 13345–13363 and
related references therein.
[26] For the reaction conditions to epoxidize 1,1-
disilylalkenes, see: K. D. Safa, K. Ghorbanpour, A.
Hassanpour, S. Tofangdarzadeh, J. Organomet. Chem.
2009, 694, 1907–1911.
[34] For example, see: J. B. Lambert, Y. Zhao, R. W.
Emblidge, L. A. Salvador, X. Liu, J.-H. So, E. C.
Chelius, Acc. Chem. Res. 1999, 32, 183–190.
[27] K. Yonekura, Y. Iketani, M. Sekine, T. Tani, F.
Matsui, D. Kamakura, T. Tsuchimoto, Organometallics
2017, 36, 3234–3249.
[35] For example, see: E. A. Brinkman, S. Berger, J. I.
Brauman, J. Am. Chem. Soc. 1994, 116, 8304–8310.
[28] The result of entry 3 shows that the ability of ZnII to
catalyze the DS reaction is low in the absence of T.
Accordingly, ZnII without being coordinated by T may
have mainly catalyzed a different pathway, double HS
reaction of 1a leading to 7a.
10
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10.1002/adsc.202000501
Advanced Synthesis & Catalysis
FULL PAPER
Zinc/Indium Bimetallic Lewis Acid Relay
Catalysis for Dehydrogenative
Silylation/Hydrosilylation Reaction of Terminal
Alkynes with Bis(hydrosilane)s
H
R
HH
H H
Si Si
R
H
cat.
cat.
Si Si
Zn
In
+
Adv. Synth. Catal. Year, Volume, Page – Page
+
cat.
Ar
Ar
pyridine base
Tomohiro Tani, Yudai Sohma, Teruhisa
Tsuchimoto*
11
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