A (Borylmethyl)silane Bearing Three Hydrolyzable Groups on Silicon: Synthesis via Iridium-Catalyzed C(sp3)-H Borylation and Conversion to Functionalized Siloxanes

Article abstract of DOI:10.1021/acs.organomet.6b00316

An iridium-catalyzed C(sp³)-H borylation of X₃SiMe (X = hydrolyzable group) was established. A trialkoxy(methyl)silane bearing sterically demanding neopentyloxy groups (X = neopentyloxy) underwent C-H borylation at the methyl group on silicon, giving (borylmethyl)tris(neopentyloxy)silane in 70% isolated yield. The choice of the hydrolyzable group X was the key to efficient and chemoselective C-H borylation; trialkoxy(methyl)silanes bearing sterically less demanding alkoxy groups (X = ethoxy, n-butyloxy, and isobutyloxy) suffered from C-H activation at the alkoxy groups, and trichloro(methyl)silane (X = Cl) failed to react. A trimethylsiloxy group could substitute the neopentyloxy groups of the borylated product by the reaction of trimethylsilanol in the presence of tetrabutylammonium fluoride.

Full text of DOI:10.1021/acs.organomet.6b00316

Communication
pubs.acs.org/Organometallics
A (Borylmethyl)silane Bearing Three Hydrolyzable Groups on Silicon:
3
Synthesis via Iridium-Catalyzed C(sp )−H Borylation and Conversion
to Functionalized Siloxanes
Toshimichi Ohmura,* Ikuo Sasaki, Takeru Torigoe, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
*
S Supporting Information
3
ABSTRACT: An iridium-catalyzed C(sp )−H borylation of
X SiMe (X = hydrolyzable group) was established. A trialkoxy-
3
(
(
methyl)silane bearing sterically demanding neopentyloxy groups
X = neopentyloxy) underwent C−H borylation at the methyl
group on silicon, giving (borylmethyl)tris(neopentyloxy)silane in
7
0% isolated yield. The choice of the hydrolyzable group X was the
key to efficient and chemoselective C−H borylation; trialkoxy-
(
methyl)silanes bearing sterically less demanding alkoxy groups (X = ethoxy, n-butyloxy, and isobutyloxy) suffered from C−H
activation at the alkoxy groups, and trichloro(methyl)silane (X = Cl) failed to react. A trimethylsiloxy group could substitute the
neopentyloxy groups of the borylated product by the reaction of trimethylsilanol in the presence of tetrabutylammonium
fluoride.
9a
onoorganosilanes bearing three hydrolyzable groups on
Cl SiMe and ClSiMe . However, the use of Cl SiMe in the
2 2 3 3
M
the silicon atoms (X SiR: X = halogen or alkoxide; R =
C−H borylation protocol has met with difficulty. Herein we
describe C−H borylation of methylsilanes bearing three
hydrolyzable groups on the silicon atom (Scheme 1). We
found a suitable hydrolyzable group X for efficient and
chemoselective C−H borylation of the methyl group on silicon.
3
organic group) are important building blocks for the synthesis
1
of branched polysiloxanes, including silsesquioxanes. Another
significant application of X SiR is their use as silane coupling
agents, when the substituent R bears a functional group such as
alkene and epoxide. X SiR are synthesized by substitution of
3
2
3
X4Si with Grignard or organolithium reagents, but this method
Scheme 1. Synthesis of Boryl-Functionalized Organosilicon
Compounds Bearing Three Hydrolyzable Groups on the
Silicon Atom
generally suffers from overreaction and therefore is not suitable3
for the selective introduction of a less hindered alkyl group.
Chlorodephenylation of Ph SiR with HCl in the presence of
3
AlCl3 limits the substrate scope because of the harsh
4
conditions. More general routes to X SiR are transition-
3
metal-catalyzed reactions of X SiH, i.e., hydrosilylation of
3
5
6
unsaturated substrates and coupling with organohalides.
Considering the recent increasing attention to the development
of siloxane-based materials, new entries of efficient methods for
We began our study by synthesizing (borylmethyl)-
the preparation of X SiR are still highly desirable. We focused
3
trichlorosilane 1 by C−H borylation of Cl SiMe according to
on a synthetic route starting from Cl SiMe, which is a less
3
3
the method established previously for C−H borylation of
utilized feedstock despite its second most production in the
9a
7
Cl SiMe and ClSiMe . In the presence of [Ir(OMe)(cod)]
Rochow−Mu
3
̈
ller direct process. If the methyl group of
2
2
3
2
(
1
(
(
5 mol %) and 3,4,7,8-tetramethylphenanthroline (Me phen,
Cl SiMe or its alkoxy derivatives can be converted to a
4
0 mol %), Cl SiMe (4 equiv) was reacted with bis-
3
pinacolato)diboron (1 equiv) in cyclohexane at 80 °C
Scheme 2A). However, no reaction took place after 18 h.
functionalized organic group with retention of the hydrolyzable
groups, it would be highly attractive for new access to X SiR.
3
We have pursued organosilicon compounds functionalized
8
Under almost identical conditions, Cl SiMe underwent C−H
by a boryl group, which is expected to serve as a handle for the
2
2
9a
borylation to give 2 in high yield (Scheme 2B). We separately
observed that almost no reaction of Cl SiMe took place in the
introduction of organic functional groups via C−C bond
forming reactions or via capture of guest molecules on the
boron atoms. We established an iridium-catalyzed C−H
borylation of methylsilanes, which enabled conversion of the
2
2
presence of 20 mol % of Cl SiMe (Scheme 2B). These results
3
3
9,10
C(sp )−H bond of the methyl group on silicon.
The C−H
Received: April 19, 2016
borylation is applicable to methylchlorosilanes, including
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00316
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 2. Cl SiMe in C−H Borylation: (A) C−H Borylation
an elevated reaction temperature (135 °C) to give the borylated
products with better chemoselectivity (4b:6 = 67:33 and 4c:7 =
60:40) (entries 2 and 3). Finally, we found that the sterically
3
of Cl SiMe; (B) C−H Borylation of Cl SiMe in either the
3
2
2
Absence or Presence of Cl SiMe
3
12
more demanding methyltris(neopentyloxy)silane (3d) under-
went C−H borylation selectively at the methyl group on silicon
to afford 4d (4d:8 > 99:1), albeit in low yield (20%, entry 4). In
this case, the C−H borylation at the methyl groups on the
neopentyl group was retarded significantly by the steric
hindrance.
We previously reported the effect of a catalytic amount of
basic additives, e.g., t-BuOK, in acceleration of the iridium-
1
4,15
catalyzed C−H borylation of aliphatic compounds.
To
improve the yield of 4d, the C−H borylation of 3d was
performed in the presence of an additive (Table 2). We found
Table 2. Improvement of C−H Borylation for Synthesis of
a
4d
a
[
[
Ir(OMe)(cod)] (2.5 mol %) and Me phen (5 mol %) were used.
2 4
b
Ir(OMe)(cod)] (5 mol %) and Me phen (10 mol %) were used.
2
4
indicate that Cl SiMe is a potential catalyst poison in the
3
iridium-catalyzed C−H borylation.
We assumed that the highly Lewis acidic Cl SiMe might form
3
a complex with Me phen, which caused deactivation of the
b
4
entry
additive (amt (mol %))
Ir:additive
yield (%)
1
1
iridium catalyst. We turned our attention to the less Lewis
1
2
3
4
5
6
7
8
9
1
none
−
24
acidic trialkoxy(methyl)silanes 3, which could be prepared in a
t-BuOK (1.25)
t-BuOK (2.5)
t-BuOK (5.0)
t-BuOK (7.5)
t-BuOK (10)
t-BuOK (15)
MeOK (2.5)
Cs CO (2.5)
1:0.125
1:0.25
1:0.5
1:0.75
1:1
57
c
12
single step from Cl SiMe. Triethoxy(methyl)silane (3a)
3
75 (70)
reacted smoothly with bis(pinacolato)diboron in cyclooctane
36
26
18
3
at 110 °C in the presence of the Ir-Me phen catalyst (10 mol
4
%
) (entry 1, Table 1). However, the reaction gave the
1:1.5
1:0.25
1:0.25
1:0.25
Table 1. Chemoselectivity in Iridium-Catalyzed C−H
34
37
39
a
Borylation of Trialkoxy(methyl)silanes 3
2
3
0
CsF (2.5)
a
[
Ir(OMe)(cod)] (0.025 mmol), Me phen (0.052 mmol), additive
2 4
(
0−15 mol %), 3d (2.0 mmol, 4 equiv), and (pin)B−B(pin) (0.50
b
1
mmol) were reacted at 110 °C for 18 h. Determined by H NMR.
c
Isolated yield in multigram scale reaction. 4d (3.0 g) was obtained.
that the reaction of 3d (4 equiv) with bis(pinacolato)diboron
(
1 equiv) took place efficiently at 110 °C in the presence of Ir-
4
Me phen catalyst (10 mol %) and t-BuOK (2.5 mol %) without
additional solvent, giving 4d in good yield (entry 3), while 4d
was obtained only in 24% yield in the absence of t-BuOK under
the same reaction conditions (entry 1). As observed in the
b
c
entry
R
temp (°C) products yield (%) (ratio)
14
1
2
3
4
Et
3a
3b
3c
3d
110
135
135
135
4a + 5
4b + 6
4c + 7
4d + 8
98 (21:79)
33 (67:33)
57 (60:40)
20 (>99:1)
previous study, the Ir-t-BuOK system is sensitive to the
amount of t-BuOK. Rate acceleration was observed with an Ir:t-
BuOK ratio of 1:0.5−0.125 (entries 2−4), whereas no
acceleration was observed with the ratio of 1:1.5−1 (entries 6
and 7). Other additives such as MeOK, Cs CO , and CsF were
n-Bu
isobutyl
neopentyl
a
[
Ir(OMe)(cod)] (0.025 mmol), Me phen (0.052 mmol), 3 (2.0
2 4
2
3
mmol, 4 equiv), and (pin)B−B(pin) (0.50 mmol) were reacted in
not effective for the C−H borylation of 3d (entries 8−10).
Finally, a multigram-scale reaction was carried out using the
optimized ratio (Ir:t-BuOK = 1:0.25), by which 3.0 g of 4d
b
cyclooctane (0.5 mL) at 110−135 °C for 18 h. Combined yield of the
c
1
structural isomers after purification. Determined by H NMR.
(
70% yield) could be synthesized (entry 3).
undesired 5, which was formed by borylation of the C−H bond
at the terminus of the ethoxy group, as a major product with
desired 4a (4a:5 = 21:79, entry 1). The formation of 5 is
favorable because of the rather high reactivity of the C−H bond
located β to the oxygen atom, as reported by Hartwig and co-
Although the neopentyloxy group is an uncommon hydro-
lyzable group in the formation of a Si−O−Si linkage, 4d could
be converted to structurally defined oligosiloxanes by the
reaction with an excess amount of trimethylsilanol in the
16
presence of tetrabutylammonium fluoride (Scheme 3A). The
reaction took place at 50 °C with retention of the boryl group
to give siloxane 9 in 71% yield.
1
3
workers. The C−H borylation of methylsilanes 3b,c bearing
12
n-butyloxy and isobutyloxy groups, respectively, proceeded at
B
DOI: 10.1021/acs.organomet.6b00316
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 3. Synthesis of Functionalized Siloxanes: (A)
Conversion of 4d to Siloxane 9 with Retention of the Boryl
Group; (B) Conversion of 9 to Arylmethyl- and
Innovative Areas “Molecular Activation Directed Toward
Straightforward Synthesis (No. 25105728)” from the JSPS.
a
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(
(
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(
9) (a) Ohmura, T.; Torigoe, T.; Suginome, M. J. Am. Chem. Soc.
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2
toluene at 110−135 °C using DPPF/Pd catalyst with Ba(OH)
2
9
b,17,18
as a base to give 10a−d in good yields (Scheme 3B, top).
(
Treatment of 9 with H O under basic conditions allowed the
2
2
Barnard, J. H.; Marder, T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev.
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19
yield (Scheme 3B, bottom).
In conclusion, we have established a synthetic method for
boryl-functionalized methylsilanes bearing three hydrolyzable
groups on silicon. Methyltris(neopentyloxy)silane was found to
be a suitable substrate for chemoselective iridium-catalyzed C−
H borylation at the methyl group on silicon. The neopentyloxy
group of the borylated product was used as a hydrolyzable
group in the formation of the Si−O−Si linkage with retention
of the boryl group. Further utilization of this new T unit for the
synthesis of functionalized polysiloxanes is being undertaken in
this laboratory.
(
11) It has been reported that Cl SiMe reacts with 1,10-phenanthro-
3
line (phen) to form the hexacoordinated silicon complex Cl (Me)-
Si(phen): Fester, G. W.; Eckstein, J.; Gerlach, D.; Wagler, J.; Brendler,
E.; Kroke, E. Inorg. Chem. 2010, 49, 2667−2673.
(
reaction with the corresponding alcohols in the presence of
triethylamine.
(
3
12) Trialkoxy(methyl)silanes 3b−d were prepared from Cl SiMe by
3
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12422−12425. (b) Li, Q.; Liskey, C. W.; Hartwig, J. F. J. Am. Chem.
Soc. 2014, 136, 8755−8765.
(
14) Ohmura, T.; Torigoe, T.; Suginome, M. Chem. Commun. 2014,
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00316.
50, 6333−6336.
15) For rate acceleration by t-BuOK in iridium-catalyzed C(sp )−H
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34, 15169−15172.
16) For n-Bu NF-mediated redistribution of polysiloxanes with
■
2
(
*
S
1
(
4
alkoxysilanes, see: Bassindale, A. R.; Liu, Z.; Parker, D. J.; Taylor, P. G.;
Experimental details and characterization data of the
products (PDF)
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2
(
003, 687, 1−11.
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2
AUTHOR INFORMATION
Corresponding Authors
E-mail for T.O.: ohmura@sbchem.kyoto-u.ac.jp.
E-mail for M.S.: suginome@sbchem.kyoto-u.ac.jp.
■
T.; Miyaura, N.; Suzuki, A. Synlett 1992, 1992, 207−210.
(18) The use of 4d in Suzuki−Miyaura coupling was also examined.
The reaction with 4-bromotoluene at 110 °C under the conditions
shown in Scheme 3B gave the coupling product in 28% yield (see the
Supporting Information).
*
*
Notes
(
19) NaHCO was a suitable base to obtain 11 in high yield. Use of
3
The authors declare no competing financial interest.
NaOH instead of NaHCO resulted in a low yield of 11, probably due
3
to redistribution of the Si−O−Si linkage.
ACKNOWLEDGMENTS
This work was supported by Grants-in-Aid for Scientific
Research (B) (No. 26288048) and Scientific Research on
■
C
DOI: 10.1021/acs.organomet.6b00316
Organometallics XXXX, XXX, XXX−XXX