Copper-Catalyzed Twofold Silylmetalation of Alkynes

Article abstract of DOI:10.1055/s-0037-1611869

The first twofold silylmetalation across a C≡C triple bond was achieved. In the presence of a catalytic amount of copper cyanide, diarylacetylenes were converted into 1,2-dimetalated 1,2-disilyl-1,2-diarylethanes on treatment with silylpotassium species generated in situ from disilane and t -BuOK. The dimetalated species were subsequently protonated to yield a series of 1,2-disilyl-1,2-diarylethanes.

Full text of DOI:10.1055/s-0037-1611869

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–D
letter
A
en
H. Yamagishi et al.
Letter
Synlett
Copper-Catalyzed Twofold Silylmetalation of Alkynes
Hiroki Yamagishi
Si
Si
Si Si
H
H
H+
cat. CuCN, t-BuOK
Jun Shimokawa*
Hideki Yorimitsu*
0
0
0
0-0
0
0
2-
4
0
2
3-4
6
4
0
Ar
Ar
DME, 25 °C, 1.5 h
0
0
0
0-
0
0
0
2-0
1
5
3-1
8
8
8
Ar
Ar
Si
Si
Department of Chemistry, Graduate School of Science,
Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
shimokawa@kuchem.kyoto-u.ac.jp
K
K
11 examples
20 to 85%
Ar
Ar
yori@kuchem.kyoto-u.ac.jp
Received: 11.05.2019
(a) Typical silylmetalation of an alkyne
Accepted after revision: 29.05.2019
Published online: 25.06.2019
DOI: 10.1055/s-0037-1611869; Art ID: st-2019-u0259-l
Si
[M]
R’
Si
H
Si [M]
R
R’
R
R
R’
Abstract The first twofold silylmetalation across a C≡C triple bond
was achieved. In the presence of a catalytic amount of copper cyanide,
diarylacetylenes were converted into 1,2-dimetalated 1,2-disilyl-1,2-di-
arylethanes on treatment with silylpotassium species generated in situ
from disilane and t-BuOK. The dimetalated species were subsequently
protonated to yield a series of 1,2-disilyl-1,2-diarylethanes.
(b) Twofold silylmetalation of an alkyne (This work)
Si
[M]
Si
Si
Si
Si [M]
[M]
H
H
R
R’
R
R’
R
R’
Scheme 1 Mono- and disilylmetalation of alkynes
Key words silylmetalation, disilane, copper catalysis, ate complex,
disilyldiarylethanes
To develop standard reaction conditions, we performed
an extensive screening of various silicon sources, copper
species, bases, and solvents. This optimization eventually
revealed that treatment of diphenylacetylene (1a) with
copper(I) cyanide (20 mol%), 1,1,2,2-tetramethyl-1,2-di-
phenyldisilane (2, 3.0 equiv), and t-BuOK (3.2 equiv) in 1,2-
dimethoxyethane (DME) as the solvent at 25 °C for 1.5 h,
followed by aqueous workup, provided 1,2-disilyl-1,2-di-
phenylethane (3a) in 88% yield (66% isolated yield)4 as an
inseparable 1:1.2 diastereomeric mixture (Table 1, entry
1).5 In the absence of a copper catalyst, the reaction was
more complicated, and 3a was obtained in a much lower
yield (35%) (entry 2). Other copper species gave slightly re-
duced yields (entries 3 and 4). The reaction did not proceed
in the absence of a base (entry 5). Lithium or sodium tert-
butoxide did not mediate the reaction because they are in-
efficient in cleaving the Si–Si bond (entries 6 and 7). These
results underscore the importance of using a potassium
alkoxide to generate silylpotassium species in situ:6 these
species then mediate the formation of silylcopper species.
This direct formation of silylcopper species from disilanes
differentiates the current method from previous silylmeta-
lations of alkynes.3 KHMDS was ineffective as a base (entry
8), indicating the importance of a nucleophilic alkoxide for
Organosilicon compounds have attracted much atten-
tion as both synthetic intermediates1 and functional mate-
rials.2 The development of methods for the formation of C–
Si bonds is therefore considered to be an important re-
search topic in organic chemistry. Among such methods,
transition-metal-catalyzed silylation of C–C multiple bonds
represents a straightforward method. In particular, silyl-
metalation of alkynes is useful for the synthesis of highly
functionalized alkenylsilanes (Scheme 1a)3 and, conse-
quently, has been thoroughly investigated by using various
combinations of a copper species and a silyl alkali metal
species. All the reported silylmetalation reactions of
alkynes occur only once to yield the corresponding monosi-
lylmetalated alkenes. There are no previous reports of a
twofold addition of silylmetals across the triple bond of an
alkyne to yield a doubly silylmetalated alkane. Here we re-
port the first such twofold silylmetalation of alkynes with
the aid of a copper catalyst and a disilane as a precursor of a
silylpotassium species (Scheme 1b). The resulting doubly
silylmetalated species can be protonated to provide access
to 1,2-disilylethanes.
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–D
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
H. Yamagishi et al.
Letter
Synlett
this reaction. The reaction was relatively sluggish in THF,
and none of the desired product was obtained in 1,4-diox-
ane or toluene (entries 9–11).
PhMe2Si
H
SiMe2Ph
H
PhMe2Si
H
SiMe2Ph
H
50%
(dr 1:1.6)
70%
(dr 1:2.2)
Table 1 Optimization of Conditionsa
3b
MeO
OMe
3c
PhMe2Si SiMe2Ph (2)
(3.0 equiv)
PhMe2Si
H
SiMe2Ph
H
PhMe2Si
H
SiMe2Ph
H
PhMe2Si
H
SiMe2Ph
H
CuCN (20 mol%)
t-BuOK (3.2 equiv)
61%
(NMR yield)
(dr 1:3.3)
0%
DME, 25 °C, 1.5 h
then H2O
1a
3d
3e
3a
Entry
Deviations from the standard conditions
Yielda (%)
PhMe2Si
H
SiMe2Ph
H
PhMe2Si
H
SiMe2Ph
H
1
2
–
88 (66)b
82%
(dr 1:1.2)
85%
(dr 1:1)
no CuCN
35
79
76
–
3
Cu(IPr)Cl instead of CuCN
CuI instead of CuCN
no t-BuOK
3g
3f
NMe2
OMe
4
5
PhMe2Si
H
SiMe2Ph
H
PhMe2Si
H
SiMe2Ph
H
85%
(dr 1:1.3)
27%
(dr 1:3.3)
6
t-BuOLi instead of t-BuOK
t-BuONa instead of t-BuOK
KHMDS instead of t-BuOK
THF instead of DME
1,4-dioxane instead of DME
toluene instead of DME
–
7
–
OMe
F
8
–
3h
3i
9
74
–
10
11
PhMe2Si
H
SiMe2Ph
H
PhMe2Si
H
SiMe2Ph
H
–
a Determined by 1H NMR analysis with 1,3,5-trimethoxybenzene as an in-
ternal standard.
0%
0%
b Isolated yield; dr = 1:1.2.
F3C
CF3
C(O)NMe2
3k
3j
3l
With the optimized conditions in hand, we surveyed
the scope of this double silylmetalation with respect to the
alkyne (Figure 1). The di-p-tolyl, di-p-methoxyphenyl, and
di-2-naphthyl adducts 3b–d, respectively, were obtained in
good yields. The di-o-tolyl adduct 3e was not obtained,
probably due to the steric effect of the o-methyl groups.
Less-symmetrical p-(dimethylamino)phenyl-, p-methoxy-
phenyl-, and m-methoxyphenyl-substituted products 3f–h,
respectively, were obtained in high yields. Whereas an elec-
tron-withdrawing m-fluorophenyl substituent was barely
tolerated to give a 27% yield of product 3i, products 3j and
3k were not formed when the para-position was substitut-
ed with an electron-withdrawing group. Alkylacetylenes
were not good substrates for this transformations; the de-
sired products 3l and 3m were not obtained. With terminal
alkynes, the reaction rapidly gave an unidentified precipi-
tate, probably through the formation of an acetylide.
PhMe2Si
H
SiMe2Ph
PhMe2Si
H
SiMe2Ph
H
H
0%
0%
3m
Figure 1 Scope of the reaction with respect to the alkyne
might be hampered by steric hindrance by the Ph2MeSi
group.
To probe the reaction mechanism, the reaction mixture
was treated with D2O to afford the corresponding doubly
deuterated 1,2-disilylated diphenylethane 4 in 85% yield
with 91% deuterium incorporation (Scheme 2).7 This con-
firmed that a dianionic intermediate 5 was generated in si-
tu. This dianionic intermediate 5 could be oxidized in situ
with iodine8 to afford (E)-1,2-bis[dimethyl(phenyl)silyl]-
Next, we examined the effect of the organosilicon
source (Figure 2). Hexamethyldisilane and 1,2-dimethyl-
1,1,2,2-tetraphenyldisilane showed lower reactivities,
yielding 3n and 3o in yields of 20 and 24%, respectively. The
lower yield of 3n might be due to the elevated temperature
required to generate the trimethylsilylcopper species by
cleaving the less-reactive Si–Si bond. The formation of 3o
Me3Si
H
SiMe3
H
Ph2MeSi
H
SiMePh2
H
20%
(dr 1:2.7)
(50 °C, 2 h)
24%
(dr 1:1)
3n
3o
Figure 2 Scope of the reaction with respect to the disilane
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–D

C
H. Yamagishi et al.
Letter
Synlett
1,2-diphenylethene (6) selectively in 44% yield. Notably, the
overall transformation represents a rare intermolecular
anti-disilylation of an alkyne.9 The reaction with other elec-
trophiles such as BuCl or ClCH2OMe did not provide a dou-
bly alkylated product, probably due to the bulkiness of the
metalated species 5.
In conclusion, we have developed the first twofold silyl-
metalation across a C≡C triple bond. The reaction is con-
ducted in the presence of a copper cyanide catalyst and a
silylpotassium species generated from a disilane and t-
BuOK. Protonation of the 1,2-dimetalated species provided
a series of 1,2-disilyl-1,2-diarylethanes. Further applica-
tions of the cuprate species and a detailed mechanistic
study will be reported in due course.
PhMe2Si
D
SiMe2Ph
D
R
D2O
1
0 to 25 °C
1 h
R = OMe
Funding Information
PhMe2Si SiMe2Ph
(3.0 equiv)
2
4
OMe
CuCN (20 mol%)
t-BuOK (3.2 equiv)
DME, 25 °C, 1.5 h
85%
(dr 1:2.2, 91%D)
This work was supported by JSPS KAKENHI Grant Numbers
JP15H05641,
JP16H04109,
JP18H04254,
JP18H04409,
and
JP19H00895. H.Y. thanks The Asahi Glass Foundation for financial
support.
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PhMe2Si
K
SiMe2Ph
K
SiMe2Ph
Supporting Information
I
2 (4.0 equiv)
PhMe2Si
44%
0 to 25 °C
1 h
R = H
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611869.
5
R
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6
Scheme 2 Derivatization of twofold silylmetalated species
References and Notes
(1) (a) Handbook of Reagents for Organic Synthesis: Reagents for
Silicon-Mediated Organic Synthesis; Fuchs, P. L., Ed.; Wiley:
Chichester, 2011. (b) Chemistry of Organosilicon Compounds,
Vol. 3; Rappoport, Z.; Apeloig, Y., Ed.; Wiley-VCH: New York,
2001. (c) Colvin, E. W. Silicon Reagents in Organic Synthesis 1988.
(2) (a) Silicon Polymers; Muzafarov, A. M., Ed.; Springer: Heidelberg,
2011. (b) Silicon-Containing Polymers: The Science and Technol-
ogy of Their Synthesis and Applications; Jones, R. G.; Ando, W.;
Chojnowski, J., Ed.; Kluwer Academic: Dordrecht, 2000.
(3) (a) Fleming, I.; Roessler, F. J. Chem. Soc., Chem. Commun. 1980,
276. (b) Fleming, I.; Newton, T. W.; Roessler, F. J. Chem. Soc.,
Perkin Trans. 1 1981, 2527. (c) Hayami, H.; Sato, M.; Kanemoto,
S.; Morizawa, Y.; Oshima, K.; Nozaki, H. J. Am. Chem. Soc. 1983,
105, 4491. (d) Okuda, Y.; Morizawa, Y.; Oshima, K.; Nozaki, H.
Tetrahedron Lett. 1984, 25, 2483. (e) Okuda, Y.; Wakamatsu, K.;
Tückmantel, W.; Oshima, K.; Nozaki, H. Tetrahedron Lett. 1985,
26, 4629. (f) Wakamatsu, K.; Nonaka, T.; Okuda, Y.; Tückmantel,
W.; Oshima, K.; Utimoto, K.; Nozaki, H. Tetrahedron 1986, 42,
4427. (g) Cuadrado, P.; González-Nogal, A. M.; Sánchez, A. J. Org.
Chem. 2001, 66, 1961. (h) Liepins, V.; Karlström, A. S.; Bäckvall,
J.-E. J. Org. Chem. 2002, 67, 2136. (i) Barbero, A.; Pulido, F. J. Acc.
Chem. Res. 2004, 37, 817. (j) Auer, G.; Oestreich, M. Chem.
Commun. 2006, 311. (k) Wang, P.; Yeo, X.-L.; Loh, T.-P. J. Am.
Chem. Soc. 2011, 133, 1254. (l) Fujihara, T.; Tani, Y.; Semba, K.;
Terao, J.; Tsuji, Y. Angew. Chem. Int. Ed. 2012, 51, 11487.
(m) Hazra, C. K.; Fopp, C.; Oestreich, M. Chem. Asian J. 2014, 9,
3005. (n) García-Rubia, A.; Romero-Revilla, J. A.; Mauleón, P.;
Gómez Arrayás, R.; Carretero, J. C. J. Am. Chem. Soc. 2015, 137,
6857. (o) Shintani, R.; Kurata, H.; Nozaki, K. J. Org. Chem. 2016,
81, 3065.
The plausible reaction mechanism shown in Figure 3
was accordingly proposed. Copper cyanide reacts with two
equivalents of the silylpotassium species generated in situ
from disilane and t-BuOK.6,10 The resulting copper–ate com-
plex 7 should be susceptible to silylcupration with the dia-
rylacetylene to form adduct 8.11 The remaining silyl group
on the copper atom migrates to the adjacent carbon to give
carbanion 9. With the aid of two equivalents of the silylpo-
tassium species, the copper atom in 9 is replaced with po-
tassium to afford 1,2-dimetalated species 5. The copper–ate
species 7 is concomitantly regenerated to close the catalytic
cycle. The efficient double silylmetalation might be as-
cribed to smooth 1,2-silyl migration from 8 with the aid of
potassium as the countercation.12
+
CuCN
2Si-K
Si
Si
Ar
Ar
K
K
1
Ar
Ar
5
Si2Cu(CN)K2
7
2
Si–K
1,2-addition
Cu
Ar
(CN)K2
Si
Si
Cu(CN)K
Ar
Si
K
Si
Ar
9
8
Ar
1,2-migration
(4) The reduction in the isolated yield is due to the difficulty in
chromatographically separating the product from the t-BuO-
SiMe2Ph generated during the formation of the silyl potassium
species.
Figure 3 Proposed reaction mechanism
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–D

D
H. Yamagishi et al.
Letter
Synlett
(5) 1,2-Bis[dimethyl(phenyl)silyl]-1,2-diphenylethane
(3a);
M.; Hara, M.; Koike, Y.; Kitahara, T.; Nagai, Y. J. Chem. Soc., Chem.
Commun. 1977, 534. (d) Buncel, E.; Venkatachalam, T. K.;
Edlund, U. J. Organomet. Chem. 1992, 437, 85.
Typical Procedure
An oven-dried Schlenk tube was charged with diphenylacety-
lene (1a; 53.5 mg, 0.300 mmol), CuCN (5.4 mg, 0.060 mmol),
and t-BuOK (108 mg, 0.960 mmol). DME (2.0 mL) and 1,1,2,2-
tetramethyl-1,2-diphenyldisilane (2, 244 mg, 0.900 mmol) were
added sequentially to the mixture, which was then stirred at
25 °C for 1.5 h. H2O (30 mL) was added at 0 °C, and the resulting
biphasic solution was extracted with Et2O (3 × 15 mL). The com-
bined organic layer was washed with brine, dried (Na2SO4), and
concentrated under reduced pressure. The resulting residue was
purified by column chromatography (silica gel, hexane) and
then by GPC (CHCl3 eluent) to give a white solid; yield: 89.2 mg
(0.198 mmol, 66%) (minor/major = 1:1.2 as an inseparable mix-
ture).
(7) A different isomeric ratio was observed when intermediate 5
was quenched with D2O. So far, the reason for this phenomenon
is unclear.
(8) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc. 2003, 125,
13662.
(9) The oxidation of dianionic species 5 with iodine probably pro-
ceeds through the formation of a radical intermediate, resulting
in loss of stereochemical information. Thus, the less stereo-
chemically congested 6 is selectively obtained. See: Garst, J. F.;
Roberts, R. D.; Pacifici, J. A. J. Am. Chem. Soc. 1977, 99, 3528.
(10) Even though a free silylpotassium species seems to function in
the reaction, the formation of a silicate species generated from
disilane and tert-butoxide cannot be excluded.
1H NMR (CDCl3):  = 7.38–7.36 (m, 2 H × 0.55), 7.34–7.30 (m, 8
H × 0.55), 7.22 (ddd, J = 7.2, 7.2, 1.2 Hz, 2 H × 0.45), 7.14–7.11
(m, 6 H × 0.55 + 4 H × 0.45), 7.07–7.01 (m, 6 H × 0.45), 6.98 (dd,
J = 7.2, 1.2 Hz, 4 H × 0.45), 6.92 (d, J = 7.2 Hz, 4 H × 0.45), 6.84–
6.83 (m, 4 H × 0.55), 2.88 (s, 2 H × 0.45), 2.75 (s, 2 H × 0.55), 0.10
(s, 6 H × 0.55), –0.17 (s, 6 H), –0.22 (s, 6 H × 0.45). 13C NMR
(CDCl3):  = 142.5, 141.6, 138.5, 138.4, 134.1, 134.0, 131.5,
129.5, 128.8, 128.2, 127.7, 127.6, 127.3, 127.0, 125.1, 125.1,
38.5, 36.8, –2.36, –2.42, –3.36, –4.04. HRMS: m/z [M+] calcd for
(11) Ref. 3b reports an example of monosilylcupration by
(R3Si)2Cu(CN)Li2 of a C≡C triple bond however, no report of any
type of addition of (R3Si)2Cu(CN)K2 is known.
(12) In the presence of smaller amounts of disilane (1.0 equiv) and t-
BuOK (1.2 equiv), we obtained recovered alkyne 1a (32%) and
disilylated product 3a (19%), as well as the monosilylated alkene
(25%), after protic workup. This suggests that a monosilylated
copper species is formed just before the the silylpotassium
species is consumed. The monosilylated copper species might
also add to the alkyne to provide the monosilylated alkene
eventually.
C30H34Si2: 450.2194; found: 450.2197.
(6) (a) Sakurai, H.; Okada, A.; Kira, M.; Yonezawa, K. Tetrahedron
Lett. 1971, 12, 1511. (b) Sakurai, H.; Kondo, F. J. Organomet.
Chem. 1975, 92, C46. (c) Watanabe, H.; Higuchi, K.; Kobayashi,
© 2019. Thieme. All rights reserved. — Synlett 2019, 30, A–D