General Synthesis of Trialkyl-And Dialkylarylsilylboranes: Versatile Silicon Nucleophiles in Organic Synthesis

Article abstract of DOI:10.1021/jacs.0c03011

Compared to carbon-based nucleophiles, the number of silicon-based nucleophiles that is currently available remains limited, which significantly hampers the structural diversity of synthetically accessible silicon-based molecules. Given the high synthetic

Full text of DOI:10.1021/jacs.0c03011

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Article
General Synthesis of Trialkyl- and Dialkylarylsilylboranes:
Versatile Silicon Nucleophiles in Organic Synthesis
Ryosuke Shishido, Minami Uesugi, Rikuro Takahashi,
Tsuyoshi Mita, Tatsuo Ishiyama, Koji Kubota, and Hajime Ito
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03011 • Publication Date (Web): 22 Jul 2020
Downloaded from pubs.acs.org on July 22, 2020
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General Synthesis of Trialkyl- and Dialkylarylsilylboranes: Versatile
Silicon Nucleophiles in Organic Synthesis
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Ryosuke Shishido,† Minami Uesugi,† Rikuro Takahashi, Tsuyoshi Mita,‡ Tatsuo Ishiyama,† Koji
Kubota*, †, ‡ and Hajime Ito*, †, ‡
†Division of Applied Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
‡Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Sapporo, Hokkaido 001-0021,
Japan
ABSTRACT: Compared to carbon-based nucleophiles, the number of silicon-based nucleophiles that is currently available remains
limited, which significantly hampers the structural diversity of synthetically accessible silicon-based molecules. Given the high
synthetic utility and ease of handling of carbon-based boron nucleophiles, silicon-based boron nucleophiles, i.e., silylboranes, have
received considerable interest in recent years as nucleophilic silylation reagents that are activated by transition-metal catalysts or
bases. However, the range of practically accessible silylboranes remains limited. In particular, the preparation of sterically hindered
and functionalized silylboranes remains a significant challenge. Here, we report the use of rhodium and platinum catalysts for the
direct borylation of hydrosilanes with bis(pinacolato)diboron, which allows the synthesis of new trialkylsilylboranes that bear bulky
alkyl groups and functional groups as well as new dialkylarylsilylboranes that are difficult to synthesize via conventional methods
using alkali metals. We further demonstrate that these compounds can be used as silicon nucleophiles in organic transformations,
which significantly expands the scope of synthetically accessible organosilicon compounds compared to previously reported methods.
Thus, the present study can be expected to inspire the development of efficient methods for novel silicon-containing bioactive
molecules and organic materials with desirable properties. We also report the first 11B{1H} and 29Si{1H} NMR spectroscopic evidence
for the formation of i-Pr3SiLi generated by the reaction of i-Pr3Si–B(pin) with MeLi.
been applied in organic synthesis.1 Since the pioneering study
INTRODUCTION
of Suginome and Ito in 2000, silylboranes have been widely
Historically, organic synthesis has focused mainly on the
employed as useful reagents for the transition-metal- or base-
construction of carbon-based organic molecules. Although both
catalyzed nucleophilic introduction of silyl groups into organic
carbon and silicon are group-14 elements, exhibit a valency of
molecules.7–9 Whereas the benefits of these developed reactions
4, and form tetrahedral compounds, reported methods for
are well-established, the value of nucleophilic silylation
synthesizing silicon-based compounds have remained relatively
processes would most likely become even more apparent when
limited.1 One of the reasons for this limitation could be the
various types of silicon-based boron nucleophiles are easily
difficulties associated with obtaining synthetically useful
available.
silicon nucleophiles.1 Various types of carbon nucleophiles
have been developed and applied in organic synthesis. Among
Scheme 1. Carbon- and Silicon-based Boron Nucleophiles in
Organic Synthesis.
these, carbon-based boron nucleophiles have become
indispensable synthetic reagents for the construction of carbon–
carbon-bond-forming transformations, such as Suzuki-Miyaura
cross-coupling reactions (Scheme 1).2 In contrast, the number
of silicon-based nucleophiles currently available remains
unfortunately limited, which significantly hampers the
structural diversity of the synthetically accessible silicon-based
molecular frameworks.1 Given the high synthetic utility and
ease of handling of carbon-based boron nucleophiles,2,3 we
envisioned that the development of general methods for the
preparation of silicon-based boron nucleophiles, i.e.,
silylboranes, could expand the scope of synthetically accessible
silicon-based compounds, thus unlocking novel areas of
chemical space for the discovery of silicon-containing
pharmaceuticals and light-emitting materials (Scheme 1).4,5
carbon
nucleophiles
E
R
R
R
well-established approach
E
E
C
R
C
B
R
various type of carbon-based
boron nucleophiles are
available
reagent or
catalyst
R
E
R
×
×
under developed approach
R
Si
R
Si
B
R
limited availability of
silicon-based boron
nucleophiles
reagent or
catalyst
R
R
silicon
nucleophiles
The conventional method for accessing Si–B fragments
developed by Suginome and Ito involves a stoichiometric
reaction between a silyl anion and a boron electrophile to form
the corresponding silylboranes (e.g., 1a–c in Scheme 2a).7
Although a variety of protocols to synthesize silyl anions has
In 1960, Seyferth and Ryschkewitsch reported the first
compounds that contain a silicon–boron (Si–B) bond.6 Early
research focused mainly on the investigation of the physical
properties of these compounds, while more recently, they have
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been reported, the most widely used method is the reaction of a using previous methods, or only with substantial difficulty.
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chlorosilane with an alkali metal (K, Na, or Li). However, this
method is limited to the preparation of aromatic-group-
functionalized silyl anions, Ar3-nRnSi–M (n = 0–2; M = K, Na,
or Li). Given the low reduction potentials of trialkyl
chlorosilanes and disilane intermediates for the anion precursor,
trialkylsilyl anions are much more difficult to prepare than aryl-
substituted silyl anions, which limits access to
trialkylsilylboranes such as 1d via this route (Scheme 2a).10–12
In addition, the synthesis of functionalized silyl anions suffers
from low functional-group compatibility due to the harsh
reduction conditions when using alkali metals. Therefore, only
a limited range of silylboranes can be prepared by this approach.
Furthermore, we investigated the preliminary application of
these new silylboranes in organic transformations and
demonstrated that they can be used as silicon nucleophiles,
which are either inaccessible via previously reported methods,
or accessible only with substantial difficulty. The present study
provides various silicon-based nucleophiles applicable to
organic synthesis, and would thus significantly expand the
scope of synthesizable organosilicon compounds with distinct
properties.
9
Specifically, we found that the Rh-based catalytic system is
particularly useful for the borylation of sterically hindered
trialkylsilanes, which provided for the first time an extremely
bulky i-Pr3Si– type silylborane. Such bulky silyl groups can
potentially tune the lipophilicity of drug molecules4 and also
provide steric protection that may be able to suppress
undesirable intermolecular interactions (e.g., π–π interactions),
which could enable the design of new solid-state light-emitting
materials.5 Furthermore, the bulky silyl groups are crucial for
controlling the conformational effect on the photophysical
properties of acyclic oligosilanes-based organic materials.15 As
such, the newly synthesized bulky trialkylsilylboranes can be
expected to become important building blocks for introducing
bulky silyl groups into valuable synthetic targets.
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Scheme 2. Synthetic Routes to Silylboranes.
a.
Reaction of boron electrophiles with silyl metals (Suginome and Ito)
alkali metal
(M = Li, Na, K)
Ar
Ar
Ar
B
X
(pin)
Si
B
(pin)
Si
Si
R
R
Cl
R
M
R
R
R
limited availability
limited scope
R = alkyl, aryl
Representative examples of silylboranes
Ph
Ph
Et
Si
B
Si
B
Si
B
Si
B
(pin)
Ph
(pin)
(pin)
Et2N
(pin)
Et
Et
Ph
Ph
1a
1b
1c
1d
We also discovered that the Pt-based catalyst shows
unprecedentedly high chemoselectivity that allows the
synthesis of benzyl-substituted silylboranes, which can be
changed to hydroxy groups by oxidation, and various
× trialkyl-substituted
aryl-substituted silylboranes
silylboranes
b.
Iridium-catalyzed borylation of hydrosilanes (Hartwig)
R
R
Ir cat./dtbpy
O
O
O
O
functional-group-containing
trialkylsilylboranes
and
B
B
Si
+
R
H
Si
B
(pin)
R
dialkylarylsilylboranes in good yield. The newly synthesized
functional-group-containing silylboranes can be expected to
provide rapid and efficient synthetic routes to novel silicon-
based compounds with interesting biological activity4 or
photophysical properties5,15 that are unavailable via the
conventional electrophilic silylation approach.1
cyclohex
R
R
R = Et, n-Pr, n-Bu etc.
limited scope
Representative examples of silylboranes
Et n-Pr
i-Pr
Me
Si
B
Si
B
Si
B
Si B
(pin)
Et
Et
(pin)
n-Pr
(pin)
i-Pr
i-Pr
(pin)
Bn
Me
n-Pr
1d
1e
1f
bulky
1g
aromatic
×
RESULTS AND DISCUSSION
simple trialkylsilylboranes
×
We initially investigated the reactivity of various transition-
metal catalysts in the Si–H borylation of trialkylsilanes with
B2(pin)2.16,17 We discovered that Rh- and Pt-based catalysts
effectively promote the borylation of the Si–H bond in
trialkylsilanes. Further optimization of the reaction conditions
c.
this work
)
Simple, general synthesis of silylboranes (
Rh or Pt cat.
R = i-Pr, t-Bu, Cy, i-Bu etc.
R
R
B
2(pin)2
Si
R
H
Si
B
(pin)
R
Ph
Cl
iPrO2C
cyclohex
or DMF
3
3
R
R
revealed
that
[Rh(OMe)(cod)]2/ICy
(ICy:
1,3-
bulky
aromatic
broad scope
dicyclohexylimidazol-2-ylidene) and Pt/C (5 wt% of Pt on
activated carbon) show high catalytic activity (Scheme 3; Table
S1–S5).18 The Rh-based catalyst is especially effective for the
borylation of sterically hindered trialkylsilanes such as
triisopropylsilane (i-Pr3Si–H; 2f), while Hartwig’s catalyst
system [Ir(OMe)(cod)]2/dtbpy did not promote the reaction
(Scheme 3, top). The Pt-based catalyst enables the
new silylboranes: 17 examples
functional groups
In 2008, the group of Hartwig developed an alternative
iridium-catalyzed method for the direct borylation of
trialkylhydrosilanes with bis(pinacolato)diboron [B2(pin)2] to
prepare trialkylsilylboranes such as 1d and 1e (Scheme 2b).13
Although this approach is useful, sterically hindered
silylboranes such as 1f cannot be prepared. In addition,
aromatic-group-functionalized trialkylsilylboranes such as 1g
are inaccessible using this procedure due to the competing
undesired aromatic CH borylation reactions promoted by the
iridium catalyst.14 Thus, the development of a more general
synthetic route to trialkylsilylboranes is highly desirable.
unprecedented
benzyldimethylsilane (2g) without the formation of any
aromatic C–H borylation products. Conversely,
chemoselective
Si–H
borylation
of
[Ir(OMe)(cod)]2/dtbpy furnishes C–H borylation byproducts,
while the desired silylborane is not observed (Scheme 3,
bottom). We also tested the Rh- and Pt-based catalysts under
Hartwig's stoichiometry (silane: 4.0 equiv.; B2(pin)2: 1.0
equiv.)13 and found that both our catalysts exhibit superior
performance to those in Hartwig's study (Scheme S1). Notably,
silylboranes 1f and 1g show high stability toward air and
moisture and can be isolated by flash column chromatography
on silica gel.
Herein, we report the development of general synthetic routes
to silylboranes via rhodium- or platinum-catalyzed hydrosilane
borylation reactions (Scheme 2c). Compared to the
methodology developed by Hartwig, the present systems show
substantially broader substrate scopes and allow access to
seventeen new silylboranes that could either not be prepared
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Scheme 3. Discovery of Rh- and Pt-based Catalytic Systems
for the Si–H Borylation of Trialkylsilanes.a-c
R
R
O
O
O
O
conditions
B
B
Si
B
1
Si
2
R
(pin)
R
H
+
R
R
O
O
O
O
O
O
3
(2.5 equiv)
conditions
Si
B
Si
B B
H
+
[Rh]: [Rh(OMe)(cod)]2 (1 mol %), ICy·HCl/K(O-t-Bu) (4 mol %), DMF, 80 °C
[Pt]: Pt/C (5 wt %, 2 mol %), cyclohexane, 80 °C
[Ir]: [Ir(OMe)(cod)]2 (1 mol %), dtbpy (2 mol %), cyclohexane, 80 °C
3
(2.5 equiv)
2f
1f
sterically hindered trialkylsilylboranes
Hartwig's conditions
Our conditions (This work)
Ir cat.
Rh cat.
8
9
[Ir(OMe)(cod)]2 (2 mol %)
dtbpy (4 mol %)
[Rh(OMe)(cod)]2 (2 mol %)
ICy·HCl/K(O-t-Bu) (8 mol %)
DMF, 80 °C
Si
B
(pin)
Si B
(pin)
Si
B
(pin)
cyclohex, 80 °C
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58% yield
no reaction
1f
1h
1i
O
O
O
O
[Rh]: 30%b
conditions
[Rh]: 58%b, [Pt]: 20%d
[Ir]: 0%c
[Rh]: 78% (87%)
[Pt]: 56% (78%), [Ir]: (7)%
Si
B
B
Si
Bn
B
Bn
H
+
O
O
3
(2.5 equiv)
2g
1g
Si
Hartwig's conditions
Our conditions (This work)
Si
Si
Ir cat.
Pt cat.
Si
B
Si
B
(pin)
(pin)
Si
B
(pin)
[Ir(OMe)(cod)]2 (1 mol %)
dtbpy (2 mol %)
Pt/C (5 wt %, 2 mol %)
cyclohexane
80 °C
cyclohex, 80 °C
1j
1k
1l
63% yield
no desired product
[Rh]: 73%b, [Ir]: 0%c
[Rh]: 75%b, [Ir]: 0%c
[Rh]: 20%b
To explore the scope of the present Rh- and Pt-catalyzed Si–
H borylations, a variety of hydrosilanes were tested (Table 1).
The corresponding Ir-catalyzed borylations were also explored
to compare the reactivity of the Ir-, Rh-, and Pt-based
catalysts.19 Initially, we examined the borylation of sterically
hindered trialkylsilanes (Table 1, top row). The reaction of 2f
using the Rh-based catalyst proceeded smoothly to give the
desired silylborane 1f in 58% yield. Furthermore, we confirmed
that the reaction of 2f on the 5 mmol scale also produced 1f in
good yield.20 The use of Pt/C resulted in a lower yield of 1f
(20%), while the Ir-based catalyst did not transform 2f. The
sterically less hindered t-BuMe2Si–H (2h) is effectively
borylated using the Rh- and Pt-based catalysts to furnish the
corresponding products in high yield, while the Ir-based
catalyst furnishes only a trace amount of the product (Rh: 87%;
Pt: 78%; Ir: 7%) under these conditions. These results
demonstrate that [Rh(OMe)(cod)]2/ICy is especially effective
for the borylation of sterically hindered trialkylsilanes. The
reaction of tricyclohexylsilane (2i) with the Rh-based catalyst
also produced 1i (30%). The molecular structure of 1i was
confirmed unambiguously by a single-crystal x-ray diffraction
analysis (Figure 1, right side). The thus developed Rh-catalysis
conditions were also applied to trialkylsilanes bearing β-
branched alkyl groups (2j–2m) or a methoxy group (2n) (1j:
73%; 1k: 75%; 1l: 20%; 1m: 52%; 1n: 50%). The twofold Si–
H borylation of 2o provided the corresponding product (1o) in
excellent yield (86%). Additionally, we confirmed that the
bulky trialkylsilylboranes 1i–1m and 1o cannot be synthesized
using the Ir-based catalyst.
B
(pin)
Si
Si
Si
B
(pin)
Si
B
(pin)
Si
MeO
B
(pin)
1m
1n
[Rh]: 50%
phenyl- and functional-group-containing trialkylsilylboranes
1o
[Rh]: 52%b, [Ir]: 0%c
[Rh]: 86%, [Ir]: 0%
Si
B
(pin)
Si
B
O
Cl
(pin)
Si
B
(pin)
O(i-Pr)
1qe
[Pt]: 42 (62)%, [Ir]: 0%
1g
1p
[Pt]: 63%, [Rh]: trace
[Ir]: 0%
[Pt]: 50% (89%)
[Rh]: (31)%, [Ir]: (39)%
Si
B
(pin)
Si
B
(pin)
Cl
1g’
1p’
[Pt]: 52 (60)%, [Ir]: (22)%
[Pt]: 50% (68%), [Ir]: (9)%
simple trialkylsilylboranes
n-Pr
n-Hex
Et
Et
Si
B
Si B
(pin)
n-Pr
n-Pr
(pin) n-Hex
n-Hex
Si
B
(pin)
Et
Si
B
(pin)
1s
1d
1e
1r
[Rh]: 63%b
[Pt]: 42%
[Ir]: 55%c
[Rh]: 66%
[Pt]: 62%
[Ir]: 57%
[Rh]: 66%
[Pt]: 52%
[Ir]: 35%
[Rh]: 77%
[Pt]: 63%
[Ir]: 58%
dialkylarylsilylboranes
Table 1. Substrate Scope of SiH Borylations using the Rh-
Si
B
(pin)
Si
B
MeO
Si B
(pin)
(pin)
Cl
and Pt-based Catalysts.
1t
1u
[Pt]: 41%f
1v
[Pt]: 38%d,f
Pt]: 46% (57%)f
[Rh]: (13%), [Ir]: 0%
aConditions for the Rh-based catalytic system: 2 (0.5 mmol), 3
(1.25 mmol), [Rh(OMe)(cod)]2 (0.005 mmol), and ICy·HCl/K(O-
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t-Bu) (0.02 mmol) in DMF (0.5 mL) at 80 °C; conditions for the
an alkali metal. As the 4-MeO-C6H4Me2Si group is more
1
2
3
4
5
6
7
8
Pt-based catalytic system: 2 (0.5 mmol), 3 (1.25 mmol), and Pt/C
(5 wt% Pt, 0.01 mmol) in cyclohexane (0.5 mL) at 80 °C;
conditions for the Ir-based catalytic system: 2 (0.5 mmol), 3 (1.25
mmol), [Ir(OMe)(cod)]2 (0.005 mmol), and dtbpy (0.01 mmol) in
cyclohexane (0.5 mL) at 80 °C. Isolated yields are given. GC yields
are shown in parentheses. bThe reaction was carried out using
[Rh(OMe)(cod)]2 (2 mol %) and ICy·HCl/KOtBu (8 mo l%). cThe
reaction was carried out using [Ir(OMe)(cod)]2 (2 mol %) and
dtbpy (4 mol %). dThe reaction was carried out using 20 mol % of
Pt/C (5 wt% Pt). eThe borylation was performed using 2 (1.5 mmol,
reactive than the PhMe2Si group in Tamao-Flemming
oxidations,22 silylborane 1u will most likely find widespread
applications in organic synthesis.
Based on previous studies of Rh-catalyzed C–H borylations,
we propose a plausible reaction mechanism for the present Rh-
catalyzed Si–H borylation (Scheme 4A).14 The ICy-Rh(I)(OR)
complex generated in situ could react initially with B2(pin)2 to
produce borylrhodium(I) complex A as the catalytically active
species. Subsequent Si–H bond cleavage with A would proceed
to afford complex B, which would then produce complex C via
-bond metathesis. Finally, the reductive elimination of
silylborane 1 would regenerate borylrhodium(I) complex A. On
the basis of previous studies on Pt-catalyzed silylation reactions
using hydrosilanes, we propose a feasible Pt-catalyzed cycle
(Scheme 4A’).16 The oxidative addition of Pt on a metal cluster
in Pt/C to the Si–H bond in 2 would initially produce
silylhydridoplatinum(II) intermediate B′, which would
subsequently react with B2(pin)2 to form complex C′. Finally,
the reductive elimination of silylborane 1 would lead to the
regeneration of the active Pt(0) species.
9
f
3.0 equiv) and 3 (0.5 mmol, 1.0 equiv). The borylation was
performed at the 1.0 mmol scale.
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Scheme 4. Proposed Reaction Mechanisms for the Rh- and
Pt-Catalyzed Si–H Borylation Reactions.
A
A’
Si
B
(pin)
R3
Si
R3
H
Si
3
R3
H
1
Rh
(I)
B
(pin)
Pt
(0)
3
Figure 1. Molecular structures of 1g and 1i with thermal ellipsoids
at 50% probability. Hydrogen atoms are omitted for clarity; color
code: gray = carbon; green = boron; blue = silicon; red = oxygen.
Si
R3
Si
R3
Rh
B
(pin)
Si Pt
B
Si Pt
R3
(pin)
R3
H
Rh
(III)
B
(pin)
(II)
B
(III)
(II)
(pin)
H
We then investigated the synthesis of functional-group-
containing trialkylsilylboranes (Table 1, middle row). Using
Pt/C, the aromatic-functionalized benzyldimethylsilane 2g was
efficiently converted into the desired silylborane (1g) without
any side reactions (63%). In contrast, [Ir(OMe)(cod)]2/dtbpy led
to C–H borylation of the phenyl group, while
[Rh(OMe)(cod)]2/ICy furnished a mixture of the Si–H and C–
H borylation products.13,21 The molecular structure of 1g was
determined by single-crystal x-ray diffraction analysis (Figure
1, left). We also confirmed that the reaction of 2g on the 5 mmol
scale produced 1g in good yield.20 These Pt-catalyzed
conditions were also applicable to trialkylsilanes containing
phenyl (2g′), chloro (2p, 2p′), and ester groups (2q), which
furnished the corresponding products in good yield (1g′: 60%;
1p: 89%; 1p′: 68%; 1q: 62%). Simple, small trialkylsilanes
provided the corresponding products in high yield using the Ir-,
Rh-, and Pt-based catalysts (Table 1, lower row), whereby
linear- or cyclic-alkyl-group-substituted hydrosilanes 2d, 2e, 2r,
and 2s effectively underwent the borylation. It should be noted
here that the silylboranes (1d–1s) shown in Table 1 can be
isolated by flash column chromatography on silica gel.
H
B
C
C’
B’
B
B
B
B
B B
(pin)
(pin)
(pin)
(pin)
H
(pin)
(pin)
2
2
(a) Rhodium catalytic cycle
(b) Platinum catalytic cycle
Subsequently, we turned our attention to the preliminary
investigation on application of the obtained trialkylsilylboranes
as silicon nucleophiles to demonstrate their utility in organic
synthesis (Scheme 5). Initially, we investigated several
transition-metal-catalyzed silylation reactions using the newly
synthesized trialkylsilylboranes (Scheme 5a5d).
A copper(I)-catalyzed conjugate addition of a silyl group to
2-cyclohexen-1-one (4), which was first reported by Hoveyda
using PhMe2Si–B(pin) (1a), proceeded effectively with bulky
trialkylsilylboranes to afford the respective silylation products
in high yield (Scheme 5a; 5a: 89%; 5b: 82%).23 The newly
synthesized functionalized trialkylsilylboranes 1g and 1p could
also be used as silylation reagents to produce the corresponding
products in high yield (5c: 82%; 5d: 90%). In this reaction, the
new trialkylsilylboranes showed high reactivity comparable to
that of 1a.
Furthermore, we found that the Pt-catalyzed reactions of
dimethylarylsilanes that bear phenyl (2t), 4-MeOC6H4 (2u), and
4-ClC6H4 (2v) groups proceeded smoothly to give the desired
dimethylarylsilylboranes (1t–1v) in moderate yield (1t: 46%;
1u: 41%; 1v: 38%). The lower yields of these
dialkylarylsilylboranes than the yields of trialkylsilylboranes
are mainly due to their instability in the isolation process. The
Ir-based catalyst did not provide the desired product (1t) due to
competing undesired aromatic CH borylation reactions.14
Notably, these are the first examples of dialkylarylsilylboranes
that bear methoxy- (1u) or chloro groups (1v); such compounds
cannot be directly prepared via the conventional method using
Then, we investigated the copper (I)-catalyzed radical
silylation of alkyl iodides, which has been reported by Oestreich
for 1a and Et3Si–B(pin) (1d).24 We examined the applicability
of our new functionalized and bulky trialkylsilylboranes in this
reaction (Scheme 5b). Iodocyclohexane 6 underwent silylation
with trialkylsilylboranes 1g or 1p to give the corresponding
silylation products in moderate yield (7a: 54%; 7b: 51%).
However, bulky trialkylsilylborane 1f could not be applied to
this reaction (Table S7).
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1
2
3
4
5
6
7
8
Scheme 5. Use of Trialkylsilylboranes as Silicon Nucleophiles in Organic Synthesis.a
a.
b.
Cu-catalyzed cross-coupling of alkyl iodide
Cu-catalyzed silyl conjugate addition
1g 1p
(1.1 equiv)
CuSCN (10 mol %)
dtbpy (10 mol %)
or
O
O
10 mol % CuCl
11 mol % IMes·HCl
R
R
R
Si
I
Si
Bn
or
+
Si
B
Si
R
R
(pin)
Cl
Li(O-t-Bu) (1.5 equiv)
THF/DMF (9:1), 30 ˚C
Na(O-t-Bu) (22 mol %)
MeOH (2.0 equiv)
THF, 30 °C
R
7a
7b
, 54%
, 51%
6
5
4
1
(1.1 equiv)
9
c.
Ni-catalyzed cross-coupling of aryl methyl ether
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
O
O
Si
Cl
Si
Si
Bn
Si
Si
9a
9b
, 52%
1f 1k
Ni(cod)2 (15 mol %)
K(O-t-Bu) (3.4 equiv)
or
(2.0 equiv)
OMe
or
5a
5b
5c
5d
, 90%
, 89%
, 82%
, 82%
toluene, 95 ˚C
Si
, 53%
d.
Pd-catalyzed silylation of aryl halide
8
1g
Si
Br
(1.5 equiv)
Bn
Pd(PPh3)4 (5 mol %)
K2CO3 (3.0 equiv)
Et2N
Et2N
e.
NHC-catalyzed silyl conjugate addition
O
O
toluene, 80 ˚C
1g
(1.1 equiv)
SIMes·HCl (5 mol %)
DBU (20 mol %)
11
, 85%
10
O
O
Cl
OH
N
N
Si
Bn
Mes
Mes
1) TBAF, THF, rt
THF/H2O (1:1), 30 ˚C
Et2N
SIMes·HCl
2) H2O2, KHCO3,
MeOH, rt
O
5c
, 57%
4
12
, 94%
f.
Introduction of various functionalized trialkylsilyl groups
1q
(1.1 equiv)
O
O
NaI
(1.2 equiv)
SIMes·HCl (5 mol %)
DBU (20 mol %)
1p
(1.1 equiv)
Si
I
Si
O
Si
O
Si
I
acetone, 60 °C
i-PrO
THF/H2O (1:1), 40 ˚C
silyl conjugate
addition
O
silyl anion
equivalent
silyl anion
equivalent
5g
5e,
5d
, 71%
46%
, 90%
O
O
KN
O
4
O
O
NaN3
(1.5 equiv)
(1.2 equiv)
1p
(1.1 equiv)
1p
(1.1 equiv)
O
Si
5f
N3
N
Si
Si
N3
Si
Phth
DMF, 80 °C
DMF, 100 °C
silyl conjugate
addition
silyl conjugate
addition
O
silyl anion
equivalent
silyl anion
equivalent
5d
5h
, 99%
, 90%
, 96%
5d
, 90%
aIsolated yields are given. For details of the reaction conditions, see the Supporting Information.
Sterically hindered trialkylsilylboranes 1f and 1k were
successfully applied to the Ni-catalyzed silylation of aryl
methyl ethers, which has been reported by Martin using less
bulky 1d (Scheme 5c).8b The reaction of 2-methoxynaphthalene
(8) with 1f or 1k furnished the corresponding silylation
products in moderate yield (9a: 52%; 9b: 53%); these yields are
lower than that achieved using 1d due to steric hindrance. The
use of benzyl-functionalized 1g resulted in a complex product
mixture (Table S8).
converted into a hydroxy group by a Tamao-Fleming oxidation,
which quantitatively affords the phenol derivative 12 (94%).26
The newly synthesized trialkylsilylboranes can also be
applied in base-mediated silylation reactions. NHC-catalyzed
silyl conjugate addition to 2-cyclohexen-1-one 4, which was
reported by Hoveyda and co-workers using 1a, proceeded
effectively with trialkylsilylborane 1g to produce the
corresponding product (5c) in good yield (57%; Scheme 5e).27
We then investigated the synthesis of multi-functionalized
organosilicon compounds that have been difficult to access
using previously reported procedures. Ester-functionalized
trialkylsilylborane 1q successfully afforded silylation product
5e in moderate yield (46%; Scheme 5f). Subsequently, further
transformation of the chloro group in 5d, which was obtained
The Pd-catalyzed cross-coupling of aryl bromide 10 with 1g
proceeded effectively to give the corresponding product in good
yield (Scheme 5d; 11: 85%).25 However, bulky
trialkylsilylboranes 1f and 1k did not provide any product in
this reaction (Table S9). The BnMe2Si group in 11 can be
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Page 6 of 10
R
R
R
(2.0 equiv)
via copper(I)-catalyzed silyl conjugate addition to 4 with 1p,
was investigated (Scheme 5f). Compound 5d was easily
converted into phthalimide 5f in 96% yield by reaction with the
phthalimide potassium salt. The reaction of 5d with sodium
iodide produced the corresponding iodination product (5g) in
71% yield, and 5d underwent azidation with sodium azide to
quantitatively furnish 5h. Thus, the present procedures provide
Si
R3 -X
MeLi (1.5 equiv)
Si
B
Si Si
13
R
(pin)
R
R3
1
2
3
4
5
6
7
8
–78 °C, 10 min –78 °C to rt, 1 h
(X = Cl or OTf)
R
1
Et
Si Si
Si Si
Si Si
Et
Si Si
unprecedented access to silyl anion equivalents that bear
Et
–
various
functionalized
groups,
including
"
"
–
SiCH2CH2CH2CO2R",
"–SiCH2CH2CH2I",
9
SiCH2CH2CH2NPhth", and "–SiCH2CH2CH2N3" (Scheme 5f).
13a
13b
13c
13d
, 77%
, 73%
, 92%
, 91%
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Oligosilanes have attracted research interest due to their
unique optical, electronic, and photoreactive properties, which
originate from their silicon–silicon bonds.15 Thus, we decided
to focus on silicon–silicon cross-coupling reactions of silyl
electrophiles using the newly synthesized trialkylsilylboranes in
the presence of an activating nucleophile (Scheme 6 and Table
S10).28 Silicon–silicon bonds are generally formed by Wurtz-
type condensations of a halosilane in the presence of an alkali
metal or the reaction of a silyl anion with a silyl electrophile.29
However, in these methods, the synthesis of all-alkyl-
substituted unsymmetrical oligosilanes is especially
challenging due to the limitations in the generation of silyl
anions. On the other hand, reactions of silylboranes with
activating nucleophiles can easily produce various silyl anion
equivalents. Therefore, we envisioned that various
asymmetrically substituted oligosilanes that are difficult to
access using previously reported methods could be synthesized
using trialkylsilylboranes prepared by Rh- or Pt-catalyzed Si–H
borylation reactions. Indeed, the reactions of bulky
trialkylsilylboranes 1f, 1k, and 1h proceeded effectively to
afford the corresponding desired disilanes (13a: 73%; 13b:
92%; 13c: 91%; 13d: 77%) in high yield. Moreover, the chloro-
functionalized silylborane 1p could be applied to the Si–Si
coupling reaction with i-Pr3Si–OTf to furnish 13e in 78% yield.
Dichlorosilanes and -disilanes also engaged in this reaction to
give the corresponding tri- or tetrasilanes in good yield. The
unsymmetrical trisilane 13f was obtained in low yield (35%)
using 1f and 1h. Two i-Pr3Si- or tricyclohexylsilyl-groups could
be introduced into dichlorodisilane or -silane to give 13g and
13h in 61% and 55% yield, respectively, when 2.0 equivalents
of 1f or 1i were used. These di-, tri-, and tetrasilanes have not
been synthesized before, and their controlled synthesis via
previously reported methods should be very difficult. In
addition, the sterically hindered alkyl substituents are known to
be important for controlling the conformational effect on the
photophysical properties of acyclic oligosilanes-based organic
materials, which suggests high potential utility of such
trialkylsilylboranes with bulky alkyl substituents.5,15 The
molecular structure of 13h was confirmed by single-crystal x-
ray diffraction analysis.
Et
Si Si
Cl
Si Si Si
Si Si Si Si
Et
b
c
13e
, 78%
13f
13g
, 61%
, 35%
Et
Si Si Si
Et
c
13h
, 55%
13h
X-ray structure of
aConditions: 1 (0.2 mmol), MeLi (1.1 M in Et2O, 0.27 mL), silyl
electrophile (X = Cl or OTf, 0.4 mmol) in THF (1.0 mL).
Percentage values refer to isolated yields. bConditions: 1f (0.2
mmol), 1h (0.2 mmol), MeLi (1.1 M in Et2O, 0.44 mL), Et2SiCl2
(0.2 mmol) in THF (3.0 mL). cConditions: 1 (0.4 mmol), MeLi (1.1
M in Et2O, 0.41 mL), silyl electrophile (0.2 mmol) in THF (2.0
mL).
Finally, we conducted in situ 11B{1H} and 29Si{1H} NMR
experiments to confirm the formation of a trialkylsilyl anion
equivalent in the reaction of a silylborane with methyl lithium
(MeLi) (Figure 2). Kawachi and Tamao have reported the
formation of Ph3SiLi from the reaction of Ph3Si–B(pin) with
MeLi,28 whereas our group and that of Shibata have reported
that the reaction of PhMe2Si–B(pin) (1a) or Et3Si–B(pin) (1b)
with K(O-t-Bu) produces the corresponding adduct with an sp3-
boronate structure.8a,30 To the best of our knowledge, the
generation of a trialkylsilyllithium via the reaction of a
trialkylsilylborane with an alkyl lithium compound has not been
reported. In the present study, NMR results revealed that i-
Pr3SiLi (15) is the main product generated in the reaction of 1f
with MeLi (Figure 2A and B). Treatment of 1f with 1.5
equivalents of MeLi in THF-d8 led to two new 11B signals: A
large signal consistent with Me–B(pin) (15,  33.2 ppm), which
was assumed to be formed via a heterolytic cleavage of the Si–
B bond in 1f, and a small signal ( 8.2 ppm) that was attributed
to the sp3 boron atom of 14, which adopts a tetrahedral
coordination geometry.8a, 28-32 Furthermore, a new 29Si signal (
14.7 ppm), which was attributed to silyllithium 15,33 was
detected in the 29Si{1H} NMR spectrum. The 29Si–7Li coupling
of 15 was observed at –100 °C ( 11.8 ppm, quartet, J [29Si–7Li]
= 52 Hz) (Figure 2B’).12 The 29Si{1H} NMR signal of the i-
Pr3Si–B(pin)/MeLi ate complex was not observed, probably
due to the presence of the quadrupolar boron atom.31 This signal
is not consistent with that of i-Pr3Si–H ( 12.1 ppm), which can
Scheme 6. Oligosilane Synthesis by Silicon–Silicon Cross-
Coupling Using Trialkylsilylboranes.a
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be formed by quenching 15 with H2O. These results indicate These results demonstrate that the developed methodology
1
2
3
4
5
6
7
8
that 15 is generated in situ, which is in agreement with
Kawachi’s report on the heterolytic cleavage and the formation
of silyl anion species for Ph3SiLi.28 This is the first observation
of the generation of the i-Pr3SiLi (15), which is not accessible
by any other methods.
significantly expands the boundaries of silylborane chemistry
and the scope of accessible organosilicon compounds. Beyond
the immediate utility of these protocols, the newly synthesized
silicon nucleophiles could inspire the development of efficient
methods for providing new silicon-containing bioactive
molecules and organic materials with distinct properties.4,5,15
Further applications of these silylboranes and investigations
directed toward the elucidation of the reaction mechanism are
in progress and will be reported in due course.
Si
Li
O
O
MeLi
O
B
Si
+
Si
B
O
9
d8-THF
Li
B
Me
(pin)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Me
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the
internet at http://pubs.acs.org.
1f
14
15
11B NMR: δ 33.2 ppm
both detected
11B NMR: δ 34.5 ppm
11B NMR: δ 8.2 ppm
Experimental procedures and data (PDF)
X-ray crystallographic data for 1g, 1i, and 13h (CIF)
AUTHOR INFORMATION
Corresponding Author
*hajito@eng.hokudai.ac.jp
*kbt@eng.hokudai.ac.jp
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was financially supported by the Japan Society for the
Promotion of Science (JSPS) via KAKENHI grants 18H03907,
17H06370, and 19K15547, by JST CREST grant JPMJCR19R1,
the Program for Leading Graduate Schools, and the Institute for
Chemical Reaction Design and Discovery (ICReDD), which has
been established by the World Premier International Research
Initiative (WPI), MEXT, Japan. We are grateful to Dr. M. Jin and
T. Mashimo (Hokkaido University) for their assistance with the x-
ray crystallographic analyses. We also thank Dr. Y. Kumaki (High-
resolution NMR Laboratory, Faculty of Science, Hokkaido
University) for his assistance with NMR measurements. Parts of
the computational calculations were performed at the Research
Center for Computational Science, Okazaki, Japan.
Figure 2. 11B{1H} and 29Si{1H} NMR spectra of the bulky
silyllithium compounds obtained from the reaction of
trialkylsilylborane 1f with MeLi: (A) 1f; (B) 1f (0.1 mmol) with
MeLi (0.15 mmol) in THF-d8 (0.14 M) after stirring for 30 min at
–78 °C. 11B{1H} and 29Si{1H} NMR analyses were conducted at
room temperature. BF3·OEt2 was used as an external standard to
calibrate the 11B{1H} NMR spectra, while Me4Si was used as an
external standard to calibrate the 29Si{1H} NMR spectra. The
29Si{1H} NMR spectrum of the mixture of 1f (0.1 mmol) with MeLi
(0.15 mmol) in THF-d8 (0.14 M) analyzed at –100 °C is shown in
inset B’.
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In summary, we have developed new methods for the
synthesis of trialkylsilylboranes via rhodium- or platinum-
catalyzed direct borylations of hydrosilanes with
bis(pinacolato)diboron. The developed conditions provide
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Tani, Y.; Yamaguchi, T.; Fujiwara, T.; Terao, J.; Tsuji, Y. Copper-
catalyzed silylative allylation of ketones and aldehydes employing
allenes and silylboranes. Chem. Lett. 2015, 44, 271–273.
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Suginome, M. Pyridine-based organocatalysts for regioselective syn-
1,2-silaboration of terminal alkynes and allenes. Asian J. Org. Chem.
2019, 8, 1092–1096; (b) Kojima, K.; Nagashima, Y.; Wang, C.;
Uchiyama, M. In situ generation of silyl anion species through Si–B
bond activation for the concerted nucleophilic aromatic substitution of
fluoroarenes. ChemPlusChem 2019, 84, 277–280; (c) Gu, Y.; Shen, Y.;
Zarate, C.; Martin, R. A mild and direct site-selective sp2-C–H
silylation of (poly)azines. J. Am. Chem. Soc. 2019, 141, 127–132; (d)
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C(sp2)–F and C(sp3)–F bonds. Angew. Chem., Int. Ed. 2019, 58, 2064–
2068; (e) Uematsu, R.; Yamamoto, E.; Maeda, S.; Ito, H.; Taketsugu,
T. Reaction mechanism of the anomalous formal nucleophilic
borylation of organic halides with silylborane: combined theoretical
and experimental studies. J. Am. Chem. Soc. 2015, 137, 4090–4099; (f)
Yamamoto, E.; Ukigai, S.; Ito, H. Boryl substitution of functionalized
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selective
formation
of
(2-alkoxydisilanyl)lithium
vs
(dialkoxysilyl)lithium. Organometallics 1996, 15, 4653–4656; (d)
Kawachi, A.; Tamao, K. Structures of [(amino)phenylsilyl]lithiums
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theoretical aspects of the conformational dependence of UV absorption
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(16) Our investigation was inspired by transition-metal-catalyzed C–
H borylation chemistry. For selected reviews, see: (a) Ishiyama, T.;
Miyaura, N. Transition metal-catalyzed borylation of alkanes and
arenes via C–H activation. J. Organomet. Chem. 2003, 680, 3–11; (b)
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L.; Liu, L.; Jiao, J.; Li, P. Recent advances in catalytic C–H borylation
reactions. Tetrahedron 2017, 73, 7123–7157.
(17) Our investigation was also inspired by transition-metal-
catalyzed silylation reactions with hydrosilanes. For selected reviews,
see: (a) Nakajima, Y.; Shimada, S. Hydrosilylation reaction of olefins:
recent advances and perspectives. RSC Adv. 2015, 5, 20603–20616; (b)
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metal-catalyzed silylations of arenes with hydrosilanes: C–X bond
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cleavage or C–H bond activation synchronized with Si–H bond
activation. Synthesis 2015, 47, 3645–3668.
most likely generated. The signal attributed to the sp3-hybridized boron
atom of the Me–B(pin)/MeLi ate complex ( –4.8 ppm) was not
observed in the reaction of 1f with MeLi, suggesting that the small
signal ( 8.2 ppm) should most likely be attributed to the sp3-hybridized
boron atom of 14 (for details, see the Supporting Information).
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(18) The Rh- and Pt-catalyzed borylation of hydrosilane 2h (1.0
equiv) with B2(pin)2 3 (1.2 equiv) also provided the corresponding
silylborane product (1h) in good yield. For the details of the
optimization study, see the Supporting Information (Table S4).
(19) The original conditions reported by Hartwig use an excess of
silane (4.0 equivalents) and B2(pin)2 is the limiting reagent (cf. ref. 9).
Here, we compared catalytic activities under conditions where the
silane is the limiting reagent, while 2.5 equivalents of B2(pin)2 is used.
This stoichiometric difference is responsible for the results that are
different from the original report (cf. ref. 13).
i
(33) The DFT-calculated 29Si NMR shift for Pr3Si–Li is 18.7 ppm
relative to Me4Si; for details, see the Supporting Information (Table
S12) and: Auer, D.; Kaupp, M.; Strohmann, C. “Unexpected” 29Si
NMR chemical shifts in heteroatom-substituted silyllithium
compounds: a quantum-chemical analysis. Organometallics 2004, 23,
3647–3655.
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(20) Reactions on the 5 mmol scale using Rh- or Pt-catalysts were
also investigated; for details, see the Supporting Information.
(21) (a) Keske, E. C.; Moore, B. D.; Zenkina, O. V.; Wang, R.;
Schatte, G.; Crudden, C. M. Highly selective directed arylation
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Synthesis of anti-vicinal silylboronates by conjunctive cross-coupling.
Angew. Chem. Int. Ed. 2020, 59, 8456–8459.
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catalyzed cross-coupling of silicon pronucleophiles with unactivated
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between aryl halides and silylboranes for the syntheses of aryl silanes.
Chem. Commun. 2015, 51, 17410–17412.
(26) (a) Tamao, K.; Ishida, N.; Tanaka, T.; Kumada, M.
Silafunctional compounds in organic synthesis. Part 20. hydrogen
peroxide oxidation of the silicon-carbon bond in organoalkoxysilanes.
Organometallics 1983, 2, 1694–1696; (b) Weber, A.; Dehn, R.;
Schläger, N.; Dieter, B.; Kirschning, A. Total synthesis of the antibiotic
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(27) (a) O’Brien, J. M.; Hoveyda, A. H. Metal-free catalytic C–Si
bond formation in an aqueous medium. enantioselective NHC-
catalyzed silyl conjugate additions to cyclic and acyclic ,-
unsaturated carbonyls. J. Am. Chem. Soc. 2011, 133, 7712–7715; (b)
Wu, H.; Garcia, J. M.; Haeffner, F.; Radomkit, S.; Zhugralin, A. R.;
Hoveyda, A. H. Mechanism of NHC-catalyzed conjugate additions of
diboron and borosilane reagents to ,-unsaturated carbonyl
compounds. J. Am. Chem. Soc. 2015, 137, 10585–10602.
(28) Kawachi, A.; Minamimoto, T.; Tamao, K. Boron-metal
exchange reaction of silylboranes with organometallic reagents: a new
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(29) Ahmed, M. A. K.; Wragg, D. S.; Nilsen, O.; Fjellvåg, H.
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(31) For the reaction between PhMe2Si–B(pin) (1a) and K(O-t-Bu)
in the presence of 18-crown-6, which produced PhMe2Si–K(18-crown-
6), see: Kleeberg, C.; Borner, C. On the reactivity of Silylboranes
toward Lewis bases: heterolytic B–Si cleavage vs. adduct formation.
Eur. J. Inorg. Chem. 2013, 2799–2806.
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(32) We conducted an in situ 11B{1H} NMR experiment for the
reaction of Me-B(pin) with MeLi in THF-d8 to identify the compound
that resonates at 8.2 ppm in the 11B{1H} NMR spectrum. The thus
obtained spectrum showed that the signal derived from Me–B(pin) (
33.2 ppm) completely disappeared, and that a new broad signal ( –4.8
ppm) had emerged, indicating that the Me–B(pin)/MeLi ate complex is
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Graphical Abstract
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O
Si
X
Rh Pt
or
B
Si
B
(pin)
cat.
2(pin)2
Si
H
Si
Li
hydrosilane
borylation
Si
B
(pin)
X
Si
X
H
steric hinderance
functional groups
Si
Si
Si
X = CO2(i-Pr), Cl, Ph etc.
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