In Situ Generation of Silylzinc by Si?B Bond Activation Enabling Silylzincation and Silaboration of Terminal Alkynes

Article abstract of DOI:10.1002/anie.201802887

A new protocol has been designed for the in situ generation of unstable Si?Zn species through the reaction of dialkylzinc, phosphine, and silylborane (Si?B). Successive reactions with various terminal alkynes using this protocol enabled highly controllable regio-/stereo-/chemoselective silylzincation and silaboration on demand without the need for a transition-metal catalyst.

Full text of DOI:10.1002/anie.201802887

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: In Situ Generation of Silylzinc by Si-B Bond Activation Enabling
Silylzincation and Silaboration of Terminal Alkynes
Authors: Yuki Nagashima, Daiki Yukimori, Chao Wang, and Masanobu
Uchiyama
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802887
Angew. Chem. 10.1002/ange.201802887
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http://dx.doi.org/10.1002/ange.201802887

10.1002/anie.201802887
Angewandte Chemie International Edition
COMMUNICATION
In Situ Generation of Silylzinc by Si–B Bond Activation Enabling
Silylzincation and Silaboration of Terminal Alkynes
Yuki Nagashima,* Daiki Yukimori, Chao Wang, and Masanobu Uchiyama*
Abstract: We designed a new protocol for the in-situ generation of
unstable Si–Zn species through the reaction of dialkylzinc,
phosphine, and silylborane (Si–B). Successive reaction with various
terminal alkynes using this protocol enabled highly controllable
regio-/stereo-/chemoselective silylzincation and silaboration on
demand without the need for a transition-metal catalyst.
successfully cleaved, formation of the Si–Zn species would be
thermodynamically favored over that of the corresponding B–Zn
species (Scheme 1). We disclose herein the in situ generation of
silylzinc species from silylboranes via a novel Si–B bond
activation reaction, which enables chemoselective cis-
silylzincation or cis-silaboration of various terminal alkynes.
Scheme 1. In situ generation of borylzinc or silylzinc species from silylborane,
Organozinc compounds are ubiquitous and versatile
Lewis base, and dialkylzinc.
intermediates in modern synthetic organic chemistry.[1]
A
number of preparation methods have been developed, which
permit the preparation of broad range of organozinc
Si
B
a
+
Zn
+
compounds.[2-5] In recent years, increasing attention has been
devoted to a new type of “heteroatom-zinc species” having Zn–
Het bond(s) (Het = B, Si, Sn, !),[6] but the unstable nature of the
Zn–Het bond(s) makes their stoichiometric preparation
problematic. Conventional methods to prepare stable
organozincs are often not appropriate or not effective: (1)
halogen-zinc exchange reaction of Het–X, and deprotonative
zincation of Het–H(hydrides) remain undeveloped,[7] and (2)
transmetalation by the reaction of organolithiums with zinc
halides suffers from difficulty in quantitative preparation of the
unstable Het–Li species, such as B–Li, Si–Li, or Sn–Li species.[6]
A recently developed protocol for activation of the B–B bond of
stable diborons and successive in situ transmetalation with
organozinc compounds to afford B–Zn species provides an
alternative approach for the utilization of unstable/reactive B–Zn
species for the borylation of aromatic halides and borylzincation
of unsaturated C–C bonds.[8] However, its simple application to
commercial Si–Si compounds for the preparation of Si–Zn
species has been unsuccessful due to low reactivity of the Si–Si
bond. Thus, we considered the use of the readily available,
storable, and easy-to-handle silylboranes,[9] even though it
initially appeared that activation of the Si–B bond would require
R
R
B
Zn
Si
Zn
Borylzinc species
Silylzinc species
Lewis Base
Based on our working hypothesis, we started with a screening
study to find a suitable activator (Lewis base) for Si–B bond
activation by using 1-octyne (1a), PhMe2Si–B(pin) (2, 1.1 eq)
and Me2Zn (1.1 eq) in THF (0.25 M) at 50°C as a fixed condition
(Table 1). In the absence of Lewis bases, only a trace amount of
the desired silylzincated product was detected (run 1). The use
of 1.1 equivalents of NaOMe promoted cis-silylzincation to afford
the desired products (3a and 4a) as a mixture of regioisomers
(45%, run 2), indicating the success of in situ generation of Si–
Zn species. The change to other alkoxy or fluoride anion did not
improve the yield or regioselectivity (runs 3 and 4). Interestingly,
n
however, addition of a phosphine base such as Bu3P or Ph3P
gave only the 1,2-cis-silylzincated product (3a, the silyl moiety is
installed at the terminal carbon), albeit in low yield (entries 5 and
6). After extensive experimentation, we found that the use of an
appropriate amount of phosphine (0.3 eq) is crucial for smooth
silylzincation (57%, run 7). Finally, the set of conditions shown in
run 8 was found to be optimal (94%, 3a:4a = >98:2).
a
high activation energy,[10] and that the transmetalation
With the optimized conditions in hand, we examined the 1,2-
silylzincation (3) of various terminal alkynes, as summarized in
Table 2. Not only primary aliphatic alkyl substituents (runs 1 and
4), but also secondary aliphatic substituents (runs 2 and 3) gave
the product in high yield with excellent selectivity (>98%).
Various polar functional groups, including alkyl chloride (run 5),
ether (run 6), protected hydroxyl group (run 7), amine (run 8)
and cyano group (run 9), are tolerated in the reaction. The
functional group specificity of this silylzincation is very high
because of the transition-metal-free condition (internal alkyne 1l
or terminal olefin 1m was intact), and the products 3l and 3m
were obtained chemoselectively (runs 10 and 11). Moreover, the
results of runs 2 and 11 provided some mechanistic insight, i.e.,
the radical mechanism is not plausible, since ring-opening of the
cyclopropyl group and 5-exo-trig cyclization by-products were
not detected at all.[12] Aryl alkynes were also reactive. The
electronic effect at acetylene carbons had little influence on the
selectivity would be low, leading to the formation of a mixture of
B–Zn and Si–Zn species.[11] We anticipated that if a direct in situ
activation-transmetalation method could be developed despite
these issues, it would open up a versatile new avenue for sila-
boration and element-zincation. We were encouraged by our
model calculations, which implied that if the Si–B bond could be
[*]
Y. Nagashima, D. Yukimori, Dr. C. Wang, Prof. Dr. M. Uchiyama
Graduate School of Pharmaceutical Sciences
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: yuki-nagashima@g.ecc.u-tokyo.ac.jp, uchiyama@mol.f.u-
tokyo.ac.jp
Dr. C. Wang, Prof. Dr. M. Uchiyama
Cluster of Pioneering Research (CPR), Advanced Elements
Chemistry Laboratory, RIKEN,
2-1 Hirosawa, Wako, Saitama 351-0198, Japan
[**]
This work was supported by JSPS Grant-in-Aid for Scientific
Research on Innovative Areas (No. 17H05430), JSPS KAKENHI
(S) (No.17H06173), and grants from Asahi Glass Foundation and
Daiichi-Sankyo Foundation (to M.U.).
reactivity/regioselectivity,
and
various
phenylacetylene
derivatives with an electron-donating group (o/p-MeO and Ph2N)
as well as an electron-withdrawing group (CF3, Br, and CO2Me)
on the phenyl ring were efficiently converted to the
Supporting information for this article is given via a link at the end
of the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201802887
Angewandte Chemie International Edition
COMMUNICATION
Me2Zn (1.1 eq)
nBu3P (0.3 eq)
corresponding products (runs 12-18). As the result of run 10
indicates, internal alkynes were not converted to the
corresponding silylzincated product at all (run 20). The resultant
silylzincated intermediates can be utilized as vinyl anion
equivalents (Scheme 2). For instance, the intermediate
generated by 1,2-silylzincation of 1-octyne (1a) or
phenylacetylene (1n) was treated with allyl bromide to give the
corresponding olefin (3ab or 3na) in high yield. The vinylzinc
intermediate also underwent copper-catalyzed methylation (3ab
and 3nb), and LiCl-mediated iodination (3ac).[13] Thus, the
present methodology for in situ generation of silylzinc species
enables versatile synthesis of densely functionalized vinylsilanes.
R
H
R
H
H
H2O
R
H
PhMe2Si B(pin)
2 (1.5 eq)
THF (0.25 M)
50ºC, 18 h
[Zn]
SiMe2Ph
SiMe2Ph
1
3
Run
Product[a]
Run
Product[a]
Run
Product[a]
F3C
1
8
15
H
O
N
H
H
SiMe2Ph
H
SiMe2Ph
H
SiMe2Ph
H
3j 59%
3q 70%
3a 92%
Br
CN
2[b]
9
16
H
H
H
H
SiMe2Ph
H
3k 37% (57%)
SiMe2Ph
H
SiMe2Ph
3r 89%
3b 72%
nBu
MeO2C
Table 1. Optimizing the conditions of silylzincation
3[b]
10
H
17
H
H
H
SiMe2Ph
Me2Zn (1.1 eq)
Lewis Base
H
SiMe2Ph
H
SiMe2Ph
H
H
D
3e 88%
3l 77%
3s 73%
PhMe2Si B(pin)
H
Ph2N
THF (0.25 M)
50ºC, 18 h;
D2O
D
SiMe2Ph
PhMe2Si
2 (x eq)
1a
3a
“1,2-silylzincation”
4a
“2,1-silylzincation”
4
5[b]
6
11
12
13
14
18
19
20
H
H
H
!"#ꢀ
9ꢀ
$%&'ꢀ
())%*+,ꢀ
-%,.)/012345ꢀ !4*6/0!"7#"2385ꢀ
H
H
H
SiMe2Ph
H
SiMe2Ph
H
SiMe2Ph
9:9/,;ꢀ
9:9/,;ꢀ
9:9/,;ꢀ
9:9/,;ꢀ
9:9/,;ꢀ
9:9/,;ꢀ
9:9/,;ꢀ
%&'()*ꢀ
&ꢀ
<=4>,ꢀ
CDꢀ
&ꢀ
3m 36% (50%)
3f 70%
3t 62%
?ꢀ
@4AB,/09:9/,;2ꢀ
@4A<'"/09:9/,;2ꢀ
GHI/09:9/,;2ꢀ
#'"FL/09:9/,;2ꢀ
LOFL/09:9/,;2ꢀ
#'"FL/0P:F/,;2ꢀ
+,-!.(/0&!()*1ꢀ
DE7CCꢀ
&ꢀ
Cl
Fꢀ
<=4>,ꢀ
FJꢀ
H
H
S
H
Cꢀ
KF79Jꢀ
MNK7?ꢀ
MNK7?ꢀ
MNK7?ꢀ
72$83ꢀ
SiMe2Ph
H
SiMe2Ph
H
SiMe2Ph
Dꢀ
FFꢀ
3g 80%
3n 91%
3u 81%
MeO
Eꢀ
Jꢀ
?Jꢀ
DJꢀ
MeO
H
H
$ꢀ
2#(/234561ꢀ
Me
SiMe2Ph
H
SiMe2Ph
H
SiMe2Ph
[a] NMR yield determined by 1H NMR analysis. [b] Ratio of 3a:4a determined
3h 72%
SiMe2tBu
3o 93%
3v n.d.
by 1H NMR analysis. [c] Isolated yield.
O
7
H
H
To gain insight into the mechanism of the present silylzincation,
we performed DFT calculations using model compounds
(Figures 1 and 2). The gas-phase calculations indicate that the
reaction pathway of generation of putative Si–Zn species shown
in Figure 1 is the most probable (see Supporting Information). In
this pathway, the addition of PMe3 to Me2Zn occurs first to form
an association complex CP1. The formation of this intermediate
activates the methyl transference aptitude of Me2Zn. Thus, one
of the methyl groups bound to the Zn atom can migrate smoothly
along the intrinsic reaction coordinate to the vacant orbital of the
boron atom of the silylborane with an activation energy of 13.6
kcal/mol. All attempts to locate a TS for the Me transfer to the Si
atom of the silylborane resulted in no-barrier rearrangement of
the geometry to generate CP2. This borate formation (CP2)
causes Si–B bond activation and the Si group migrates smoothly
to the Zn atom to produce the Si–Zn species (A). The
stabilization energy of the second step is very large (–26.8
kcal/mol) which can compensate for the energy loss of the first
step to afford the zwitterionic complex (CP2). This scenario is
consistent with the experimental detection of Me–B(pin) (B) by
GC-MS analysis in the present silylzincation. Having obtained
structural information for Si–Zn species A, which is likely to be
theoretically and experimentally relevant, we next performed
DFT calculations to investigate the silylzincation reaction of
terminal alkynes. We identified two plausible pathways for the
silylzincation reaction, through TSꢀ (2,1-silylzincation TS) and
TSꢁ (1,2-silylzincation TS), as shown in Figure 2. The activation
energy of the 1,2-silylzincation TS is lower by several kcal/mol
than that of the 2,1-silylzincation TS, probably because of the
steric factor, in good agreement with the experimental selectivity.
MeO
H
SiMe2Ph
H
SiMe2Ph
3p 89%
3i 71%
[a] Isolated yield (NMR yield, determined by 1H NMR analysis, is shown in
parentheses). In all cases, the ratio of regioselectivity (1,2-silylzincation:2,1-
silylzincation) was >98:2, as determined by 1H NMR analysis. [b] Run at 75°C.
Scheme 2. In situ electrophilic trapping of the silylzincated intermediate
Me2Zn (1,1 eq)
nBu3P (0.3 eq)
R
H
PhMe2Si B(pin)
R
H
THF (0.25 M)
50ºC, 18 h
[Zn]
SiMe2Ph
2 (1.5 eq)
1
a
a
c
b
b
H
H
H
H
H
Me
SiMe2Ph
I
SiMe2Ph
SiMe2Ph
Me
SiMe2Ph
SiMe2Ph
3na
89%
3nb
72%
3aa
77%
3ab
71%
3ac
64%
[Conditions] [a] allyl bromide (3 eq), 75°C, 18 h. [b] MeI (5 eq), CuCN (5
mol%), 75°C, 18 h. [c] N-Iodosuccinimide (2 eq), LiCl (2 eq), r.t., 30 min.
Ph
Si
Me
Si Ph
Me
Me
O
O
O
O
Me
Si
Me
Me
Me
Me
B
Me
–
B
Ph
Me
B
B
Si Ph
Me
Me
O
+
+
+13.6
+0.8
O
Zn
O
O
Me Zn Me
+
PMe3
+
Zn
PMe3
PMe3
Zn
–5.6
+4.1
–26.8
Me
Si-Zn (A)
+
Me-B (B)
PMe3
Me
PD
RT
CP1
CP2
Figure 1. Energy profile for in situ generation of silylzinc species
(M06/631SVP (SVP for Zn, and 6-31+G* for others)). For each step, the
energy change ("E) is shown below the arrow and values in italics above the
arrow are activation barriers ("E‡) in kcal/mol.
Table 2. 1,2-Silylzincation of terminal alkynes
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10.1002/anie.201802887
Angewandte Chemie International Edition
COMMUNICATION
hydrosilylated product (3n), probably because the intermediary
vinylzinc species is too stabilized both electronically and
sterically for the borylation step to occur (run 12). The
silaboration products (5) are versatile platforms for further
chemical elaborations. For instance, 1,2-silaborated product 5a,
generated by 1,2-silaboration of 1-octyne (1a), was treated
under cross-coupling conditions to give the arylated vinylsilane
(5aa, eq 1) in 83% yield. 5a also underwent selective oxidation
in high yield to give 1-silylketone (5ab, eq 2), which is normally
synthesized via a complex multi-step synthetic procedure. In
contrast, chemoselective desilyliodination of 5a proceeded
smoothly with N-iodosuccimide to give the boryliodinated
product (5ac, eq 3). The present strategy enables versatile
synthesis of various vinylsilanes and vinylborons as platforms for
TS"
TS!
Me
H
H
Me
Me
Zn
Zn
Me
Si
Si
!G‡ = +23.9
+21.0
Figure 2. Energy profile for silylzincation of 1-propylene (M06/631SVP; energy
in kcal/mol). The structures of TS# and TS$ are shown in the dotted boxes.
Scheme 3. Proposed mechanism of Zn-catalyzed silaboration reaction
Me2Zn
R
H
PnBu3
PhMe2Si B(pin)
(pin)B
SiMe2Ph
D
obtaining
regio-controlled
trisubstituted
olefins
and
functionalized materials.
R
H
Table 4. 1,2-silaboration of terminal alkynes and transformations of the
intermediate (5a)
Me B(pin)
MeZn
SiMe2Ph
MeZn SiMe2Ph
Me B(pin)
B
C
A : Silylzinc
Me2Zn (0.3 eq)
Ph3P (0.3 eq)
R
H
R
H
R
H
PhMe2Si B(pin)
MTBE (0.5 M)
75ºC, 18 h
(pin)B
SiMe2Ph
Table 3. Optimizing the conditions of silaboration
2 (1.5 eq)
1
5
Run
1
Product[a]
Run
Product[a]
Run
9
Product[a]
CN
Me2Zn (0.3 eq)
Lewis Base
H
H
PhMe2Si B(pin)
H
solvent
75ºC, 18 h;
D2O
(pin)B
SiMe2Ph
PhMe2Si
B(pin)
6a
“2,1-silaboration”
5
H
H
H
2 (2 eq)
1a
5a
“1,2-silaboration”
(pin)B
SiMe2Ph
(pin)B
SiMe2Ph
(pin)B
SiMe2Ph
5f 67% (96:4)[b]
5k 56% (65%)
!"#ꢀ
8ꢀ
$%&'(#)ꢀ
9:;/0<=>?/@2ꢀ
*++,-'(ꢀ
.,(&+/012345ꢀ !4-%/0!"6#"2375ꢀ
5a 83%
nBu
#A"BC/0<=B/(D2ꢀ
#A"BC/0<=B/(D2ꢀ
#A"BC/0<=B/(D2ꢀ
CLBC/0<=B/(D2ꢀ
0120+,-.2+34/ꢀ
EBꢀ
JBꢀ
FGH6>ꢀ
FGH6>ꢀ
FGH6>ꢀ
FGH6>ꢀ
7859:ꢀ
Cl
>ꢀ
@9AI/0<=>?/@2ꢀ
@9AI/0<=?/@2ꢀ
@9AI/0<=?/@2ꢀ
'()*+,-.!+'/ꢀ
2[c]
6
10
11
12
H
H
H
Bꢀ
HKꢀ
(pin)B
(pin)B
SiMe2Ph
(pin)B
SiMe2Ph
(pin)B
SiMe2Ph
Eꢀ
GEꢀ
5b 47% (62%)
5l 46% (68%)
5g 75%
$%&ꢀ
!
5#+,52$6&/ꢀ
3[c,d]
[a] NMR yield determined by 1H NMR analysis. [b] Ratio of 5a:6a determined
by 1H NMR analysis. [c] 1.5 eq of 2 was employed. [d] Isolated yield.
7
H
MeO
(pin)B
H
H
SiMe2Ph
SiMe2Ph
(pin)B
SiMe2Ph
5c 70% (95:5)[b]
5h 41% (75%)
5m 78%
Next, we envisioned silaboration reaction via Zn–B exchange
reaction of the in-situ-generated silylzincation intermediate C
(Scheme 3) to afford the corresponding silaboration product D
with regeneration of dialkylzinc. In fact, the combination of a
catalytic amount of Me2Zn (0.3 eq) with an excess amount of
silylborane (2, 2 eq) at higher temperature (75°C) gave the cis-
1,2-silaborated product (5a) in 43% yield accompanied with the
cis-1,2-silylzincated product (26% yield) (Table 3, run 1),
indicating that silaboration proceeds via a stepwise mechanism,
as expected. To our knowledge, this reaction is the first example
of an ionic silaboration of unactivated acetylene without any
transition-metal catalyst.[9,14,15] The use of MTBE (methyl tert-
butyl ether), instead of THF, and a higher concentration (0.5 M)
promoted the borylation step without reducing the
regioselectivity (runs 2 and 3). The change of nBu3P to Ph3P
gave the best yield of 1,2-silaboration (94%, 5a:6a = >98:2, run
4), and in this case, reducing the amount (1.5 eq) of PhMe2Si–
B(pin) (2) also gave comparable results in terms of reactivity and
selectivity (run 5).
Representative results illustrating the scope of the present
silaboration are summarized in Table 4. Various aliphatic alkyl
substituents gave the desired products in high yield with high
regio-/stereoselectivity (runs 1-5); various polar functional
groups (runs 6-9), an internal C–C triple bond, and a terminal
olefin (runs 10 and 11) were tolerated. On the other hand,
aromatic alkynes such as phenylacetylene (1n) did not
effectively undergo 1,2-silaboration (5n), but gave the 1,2-
SiMe2tBu
O
4[c]
8[c]
H
H
H
(pin)B
SiMe2Ph
(pin)B
SiMe2Ph
(pin)B
SiMe2Ph
5i 83% (95:5)[b]
5d 51% (93%) (92:8)[b]
5n n.d.[e]
PhI (1.1 eq)
KOHaq. (3 eq), PdCl2dppf (5 mol%)
H
5aa
83%
(1)
(2)
THF (0.2 M), 50ºC, 8 h
Ph
SiMe2Ph
H2O2•urea (1 eq)
NaOAc (1 eq)
H
H
5ab
77%
THF (0.2 M), 0ºC, 18 h
(pin)B
SiMe2Ph
O
SiMe2Ph
5a
H
5ac
43%
(86%)
NIS (1.1 eq)
(3)
MeCN (0.08 M), dark, r.t., 18 h
(pin)B
I
[a] Isolated yield (NMR yield, determined by 1H NMR analysis, is shown in
parentheses). [b] The ratio of 5:6 (1,2-:2,1-silaboration) was determined by 1H
NMR analysis. Unless otherwise noted, all ratios were >98:2. [c] 2 eq of 2 was
employed. [d] Run at 100°C. [e] 1,2-Hydrosilylated product (3n) was obtained.
During this silaboration study, we fortuitously found that
removal of phosphine from the present 1,2-silaboration reaction
mixture resulted in completely opposite regioselectivity of
silaboration (Table 5). Although further theoretical and
spectroscopic studies of the present 2,1-silaboration are needed
to elucidate the reaction pathway, it seems that a distinct
species is generated that facilitates the selectivity.
Table 5. 2,1-Silaboration of terminal alkynes
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10.1002/anie.201802887
Angewandte Chemie International Edition
COMMUNICATION
R
H
Me2Zn (0.3 eq)
Oestreich, Chem. Commun. 2006, 42, 311. i) S. Nakamura, M.
Uchiyama, J. Am. Chem. Soc. 2007, 129, 28. j) C. Fopp, E. Romain, K.
Isaac, F. Chemla, F. Ferreira, O. Jackowski, M. Oestreich, A. Perez-
Luna, Org. Lett. 2016, 18, 2054. Sn–Zn species: k) J. Hibino, S.
Matsubara, Y. Morizawa, K. Oshima, H. Nozaki, Tetrahedron Lett. 1984,
25, 2151. l) S. Matsubara, J. Hibino, Y. Morizawa, K. Oshima, H.
Nozaki, J. Organomet. Chem. 1985, 285, 163. m) T. Nonaka, Y. Okuda,
S. Matsubara, K. Oshima, K. Utimoto, H. Nozaki, J. Org. Chem. 1986,
51, 4716. n) A. Krief, L. Provins, W. Dumont, Angew. Chem., Int. Ed.
1999, 38, 1946. o) D.-Y. Wang, C. Wang, M. Uchiyama, J. Am. Chem.
Soc. 2015, 137, 10488.
R
H
PhMe2Si B(pin)
THF (0.5 M)
75ºC, 18 h
PhMe2Si
B(pin)
2 (2 eq)
1
6
Run
1
Product[a]
Run
2
Product[a]
Run
3
Product[a]
SiMe2tBu
Cl
O
H
H
H
PhMe2Si
B(pin)
PhMe2Si
B(pin)
PhMe2Si
B(pin)
6g 41% (68%) (8:92)[b]
6i 60% (76%) (7:93)[b]
6a 68% (83%) (6:94)[b]
[a] Isolated yield (NMR yield, determined by 1H NMR analysis, is shown in
parentheses) [b] The ratio of 5:6 (1,2-:2,1-silaboration) was determined by 1H
NMR analysis.
[7]
Since heteroatoms (B, Si, Sn, ...) have Lewis acidity, attempts to
abstract a proton from a Het-H bond should result in the formation of a
Lewis acid-base adduct rather than deprotonation to form “heteroatom
anions”.
Scheme 4. Summary of conditions for Zn-mediated silylmetalations
Me2Zn (1.1 eq)
[8]
[9]
Y. Nagashima, R. Takita, K. Yoshida, K. Hirano, M. Uchiyama, J. Am.
Chem. Soc. 2013, 135, 18730.
nBu3P (0.3 eq)
R
H
For reviews on silylborane and Si–B bond activation, see: a) T. Ohmura,
M. Suginome, Bull. Chem. Soc. Jpn. 2009, 82, 29. b) M. Oestreich, E.
Hartmann, M. Mewald, Chem. Rev. 2013, 113, 402. For reviews on
Het–Het (Het = B, Si, Sn, ...) bond activation, see: c) I. Beletskaya, C.
Moberg, Chem. Rev. 1999, 99, 3435. d) M. Suginome, Y. Ito, Chem.
Rev. 2000, 100, 3221. e) I. Beletskaya, C. Moberg, Chem. Rev. 2006,
106, 2320. f) M. B. Ansell, O. Navarro, J. Spencer, Coord. Chem. Rev.
2017, 336, 54.
THF (0.25 M), 50ºC, 18 h
[Zn]
SiMe2Ph
1,2-silylzincation (3)
Me2Zn (0.3 eq)
Ph3P (0.3 eq)
R
H
R
H
PhMe2Si B(pin)
2
MTBE (0.5 M), 75ºC, 18 h
(pin)B
SiMe2Ph
1
1,2-silaboration (5)
R
H
Me2Zn (0.3 eq)
PhMe2Si
B(pin)
THF (0.25 M), 75ºC, 18 h
[10] Activation of the Si–B bond by Lewis basic compounds and its
utilization for several silaboration reactions have been reported.
However, base-catalyzed silaboration of unactivated acetylenes has not
been developed. For reviews, see: a) J. Cid, H. Gulyás, J. J. Carbó, E.
Fernández, Chem. Soc. Rev. 2012, 41, 3558. b) A. B. Cuenca, R.
Shishido, H. Ito, E. Fernández, Chem. Soc. Rev. 2017, 46, 415.
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activation of Si–B bond with Lewis bases and Cu-catalysts: a) P. Wang,
X.L. Yeo, T.P. Loh, J. Am. Chem. Soc. 2011, 133, 1254. b) T. Fujihara,
Y. Tani, K. Semba, J. Terao, Y. Tsuji, Angew. Chem., Int. Ed. 2012, 51,
11487. c) F. Meng, H. Jang, A. H. Hoveyda, Chem. Eur. J. 2013, 19,
3204. d) H. Yoshida, Y. Hayashi, Y. Ito, K. Takaki, Chem. Commun.
2015, 51, 9440. No Cu-catalyzed silaboration of unactivated terminal
alkynes has developed.
2,1-silaboration (6)
In summary, we have developed a new method for in-situ
generation of highly reactive silylzinc species (Si–Zn) via a novel
activation of the stable Si–B bond with dialkylzinc and phosphine.
This method can provide highly controllable regio-/stereo-
/chemo-selective silylmetalations, i.e., 1,2-silylzincation, 1,2-
silaboration and 2,1-silaboration, on demand (Scheme 4). We
believe that the present protocols might also offer a new
approach for the activation of other inert bonds and provide
access to a wide variety of hetero element-zinc species. Further
work to expand the scope of this methodology and to apply this
approach to a variety of reactions is in progress.
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1989, 111, 275.
Keywords: zinc • silicon • silylmetalation • silylborane • alkyne
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10.1002/anie.201802887
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
In-situ generation of silylzinc species
was achieved by reaction of dialkylzinc,
phosphine, and silylborane, based on model
DFT calculations indicating that Si–B bond
cleavage would afford the Si–Zn species.
Using this protocol, successive reaction with
various terminal alkynes enabled highly
Yuki Nagashima,* Daiki Yukimori,
Chao Wang, and Masanobu
Uchiyama*
Page No. – Page No.
controllable
silylzincation and silaboration of terminal
alkynes on demand, affording synthetically
regio-/stereo-/chemoselective
In Situ Generation of Silylzinc by
Si–B Bond Activation Enabling
Silylzincation and Silaboration of
Terminal Alkynes
versatile
and
densely
functionalized
vinylsilane and vinylboron derivatives.
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