Selective boryl silyl ether formation in the photoreaction of bisboryloxide/boroxine with hydrosilane catalyzed by a transition-metal carbonyl complex

Article abstract of DOI:10.1021/ja500465x

Selective B-O-Si bond formation was achieved in the reaction of bisboryloxide O(Bpin)₂ (pin = (OCMe₂)₂)/ boroxine (MeBO)₃ system with tertiary silane R₃SiH in the presence of stoichiometric water and a catalytic amount of [M](CO)₅ ([M] = Mo(CO), W(CO), Fe) to give boryl silyl ethers. Moreover, this reaction can be applied to various hydrosilanes (disilyl compounds and secondary silanes) and hydrogermane. Some of the boryl silyl ethers thus formed were confirmed by X-ray analysis.

Full text of DOI:10.1021/ja500465x

Communication
pubs.acs.org/JACS
Selective Boryl Silyl Ether Formation in the Photoreaction of
Bisboryloxide/Boroxine with Hydrosilane Catalyzed by a Transition-
Metal Carbonyl Complex
Masaki Ito, Masumi Itazaki, and Hiroshi Nakazawa*
Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan
S
* Supporting Information
which precious metal catalysts and silanol are used (Scheme
2).8,9
ABSTRACT: Selective B−O−Si bond formation was
achieved in the reaction of bisboryloxide O(Bpin)2 (pin
= (OCMe2)2)/boroxine (MeBO)3 system with tertiary
silane R3SiH in the presence of stoichiometric water and a
catalytic amount of [M](CO)5 ([M] = Mo(CO), W(CO),
Fe) to give boryl silyl ethers. Moreover, this reaction can
be applied to various hydrosilanes (disilyl compounds and
secondary silanes) and hydrogermane. Some of the boryl
silyl ethers thus formed were confirmed by X-ray analysis.
Scheme 2. O-Borylation of Silanol with Vinylborane
Catalyzed by a Ru−H Complex
orosilicate materials have been widely used in many areas
Herein, we present a new catalytic reaction of bisboryloxides
with hydrosilanes in the presence of stoichiometric water and a
catalytic amount of [M](CO)5 ([M] = Mo(CO), W(CO), Fe)
to form boryl silyl ethers under photoirradiation. In addition,
we found that our reaction could be applied to boroxine, disilyl
compounds, and secondary silane.
B
because of their excellent heat and chemical resistance;
boryl silyl ethers have emerged as the most useful precursors in
the synthesis of these materials.1 Methods for synthesis of boryl
silyl ethers can be classified into two major types: (i) reaction of
hydroxyborane with chlorosilane,2 alkoxysilane,3 and silanol4
(Scheme 1, (1)−(3)) and (ii) reaction of chloroborane,4d,5
Toluene (0.5 mL), O(Bpin)2 (0.5 mmol), Et3SiH (1.0
mmol), H2O (0.5 mmol), and Mo(CO)6 (0.0025 mmol) were
charged in a sealed glass tube under a nitrogen atmosphere, and
the solution was photoirradiated with a 400 W medium-
pressure mercury arc lamp (Pyrex filtered) at 25 °C for 20 h
Scheme 1. Common Synthetic Methods toward Boryl Silyl
Ethers
1
(Table 1, entry 1). The H NMR spectrum in C6D6 of the
crude reaction mixture revealed the formation of pinB−O−
SiEt3 (1) (92% yield based on NMR) and H2 (4.47 ppm, vide
infra). The results showed that Mo(CO)6 was effective in
catalyzing the reaction of O(Bpin)2 with Et3SiH, with a
turnover number (TON) of 368. Notably, after completion of
reaction and removal of the volatile materials, pure boryl silyl
ether could be readily obtained by extraction with n-hexane or
diethyl ether and drying in vacuo. When the amount of
Mo(CO)6 was reduced from 0.25 to 0.05 mol %, the yield of
the product was greatly diminished (entry 2). In addition, the
reaction did not proceed in the absence of the transition-metal
catalyst (entry 3). Formation of pinB−O−SiEt3 was completed
in 10 h at 25 °C or 2 h at 50 °C (entries 4, 5). The desired
product was not obtained by thermal reaction (70 or 110 °C)
without photoirradiation (entry 6). Other molybdenum
carbonyl complexes, W(CO)6, Fe(CO)5, and Cp*Fe(CO)2Me
(Cp* = η5-C5Me5) precursor, were also examined as reaction
catalysts; it was found that these catalytic activities were lower
than that of Mo(CO)6 (entries 7−9, 11, 13). The activities of
alkoxyborane,6 and hydroborane5b,7 with silanol ((4)−(6)).
However, these conventional methods have several drawbacks,
including the formation of corrosive HCl and use of water- and
air-unstable substrates. In addition, these reactions have the
potential to generate byproducts such as disiloxanes. Therefore,
a selective B−O−Si bond-forming reaction using stable
substrates is needed.
Bond-forming reactions via transition-metal catalysts are
extremely important transformations since they provide
chemoselectivity (e.g., regioselectivity, stereoselectivity). How-
ever, to the best of our knowledge, only one example of a
catalytic B−O−Si bond-forming reaction using a transition-
metal complex has been reported by Marciniec’s group, in
Received: January 28, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja500465x | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Reaction of O(Bpin)2 with Et3SiH in the Presence
of a Transition-Metal Carbonyl Complex
Table 2. Solvent Screening for B−O−Si Bond-Forming
a
a
Reaction
bc
,
entry
solvent
toluene
yield (%)
TON
b
cd
,
entry
cat. [amount (mol%)]
yield (%)
TON
1
2
3
4
5
6
7
8
9
94 (92)
92 (90)
89 (82)
86
376
368
356
344
60
1
2
3
Mo(CO)6 [0.25]
Mo(CO)6 [0.05]
no catalyst
92 (89)
4
368
80
THF
diethyl ether
n-hexane
CHCl3
CH2Cl2
CH3CN
DMF
no reaction
94 (92)
93 (88)
no reaction
29
−
e
4
Mo(CO)6 [0.25]
Mo(CO)6 [0.25]
Mo(CO)6 [0.25]
376
372
−
116
112
52
15
f
5
g
44
176
6
trace
20
−
80
7
CpMo(CO)3Me [0.25]
[CpMo(CO)3]2 [0.25]
Fe(CO)5 [0.5]
8
56
water
12
48
9
26
a
O(Bpin)2/Et3SiH/H2O/Mo(CO)6 = 0.5:1.0:0.5:0.0025 mmol in 0.5
10
11
12
13
CpFe(CO)2Me [0.5]
Cp*Fe(CO)2Me [0.5]
Cr(CO)6 [0.5]
trace
−
110
b
c
mL of solvent. Yield based upon 1H NMR. Isolated yield in
parentheses.
55
trace
−
W(CO)6 [0.5]
57
114
48
h
acted as a catalyst poison.11 The low activity in water may come
from the low solubility of Mo(CO)6 and reagents. In
subsequent studies, toluene or THF was used as the solvent.
To explore the scope and limitation for this bond-forming
reaction, several combinations of boron compounds and
hydrosilanes were examined (Table 3). The reaction proceeded
effectively for various tertiary and secondary silanes (entries 1−
14
Mo(CO)6 [0.25]
Mo(CO)6 [2.5]
12
h
15
12
4.8
368
372
i
16
Mo(CO)6 [0.25]
Mo(CO)6 [0.25]
92 (87)
93 (89)
j
17
a
O(Bpin)2/Et3SiH/H2O = 0.5:1.0:0.5 mmol in 0.5 mL of toluene.
b
c
d
1
Based on [Et3SiH]. Yield based upon H NMR. Isolated yield in
e
f
parentheses. Reaction was carried out at 25 °C for 10 h. Reaction was
carried out at 50 °C for 2 h. Reaction was carried out at 70 or 110 °C
g
t
10), although nPr3SiH and BuMe2SiH were slow-reacting
h
i
for 20 h without photoirradiation. Without H2O. 2.5 mmol of H2O.
j
substrates (entries 2, 3). Entry 7 should be noted because,
although the substrate MePh(CH2CH)SiH has a vinyl group
and a Si−H bond, the vinyl group can tolerate hydrosilylation
conditions. Our reaction system is applicable not only for
O(Bpin)2 but also for pinacol borane HBpin (entry 11) and
pinacol borinic acid HOBpin (entries 12 and 13). When
reactions were carried out without water, the yields of products
decreased (entries 12 vs 14, 13 vs 15). The corresponding B−
O−Si compounds were obtained when trimethylboroxine
(MeBO)3 and methylboronic acid MeB(OH)2 were used as a
boron source (entries 16−19). In contrast, O(BPh2)2 did not
undergo the B−O−Si bond-forming reaction (entry 20).
5.0 mmol of H2O.
CpFe(CO)2Me and Cr(CO)6 were sluggish (entries 10, 12).
The different activity among the carbonyl complexes (Mo, W,
Cr and Fe) may come from different stability of a catalytic
intermediate. In the reaction using the Mo(CO)6 catalyst but in
the absence of H2O, the yield decreased to 12% (entry 14) and
did not improve even when the catalyst amount was increased
to 2.5 mol% (entry 15). When an excess amount of water was
used, the reaction proceeded as well as when a stoichiometric
amount of water was used (entries 16, 17).
Next, various solvents were screened under the reaction
conditions shown in entry 4 in Table 1; it was found that
toluene, tetrahydrofuran (THF), diethyl ether, and n-hexane
were effective in the B−O−Si bond-forming reaction (Table 2,
entries 1−4), whereas CHCl3, CH2Cl2, CH3CN, dimethylfor-
mamide (DMF), and water were not (entries 5−9). The
molybdenum η2-silane σ-complex Mo(CO)5(η2-HSiEt3) that is
considered to be an intermediate in our system (vide infra) is
unstable in CH2Cl2 below rt,10 so CHCl3 and CH2Cl2 might
lead to a decrease in efficiency. Presumably, CH3CN and DMF
1
All boryl silyl ethers obtained were characterized by H, 11B,
13C, 29Si NMR spectroscopy and elemental analyses (see
Supporting Information (SI)). In addition, most of the
compounds were analyzed by GC/MS. The molecular
structures of pinBOSiPh3 (6), Fe{η5-C5H4(SiMe2OBpin)}2
(9), and MeB(OSiPh3)2 (12) were confirmed by single crystal
X-ray structure analysis (Figure 1). These X-ray structures
furnished definitive proof of B−O−Si bond formation.
In order to obtain insight into the reaction mechanism,
reactions in which D2O and H218O were used in place of H2O
Figure 1. ORTEP drawing of 6 (a), 9 (b), and 12 (c) with 30% thermal ellipsoidal. Hydrogen atoms have been omitted for clarity.
B
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Journal of the American Chemical Society
Communication
a
Table 3. B−O−Si Bond-Forming Reactions of Various Boron Compounds and Hydrosilanes
bc
,
entry
[B]
[Si]
[B]/[Si]/[Mo(CO)6]
[B−O−Si]
time (h)
yield (%)
1
O(Bpin)2
O(Bpin)2
O(Bpin)2
O(Bpin)2
O(Bpin)2
O(Bpin)2
O(Bpin)2
O(Bpin)2
O(Bpin)2
O(Bpin)2
HBpin
Et3SiH
1:2:0.03
1:2:0.03
1:2:0.03
1:2:0.03
1:2:0.03
1:2:0.03
1:2:0.03
1:1:0.03
1:1:0.03
1:1:0.03
1:1:0.015
1:1:0.015
1:1:0.015
1:1:0.015
1:1:0.015
1:6:0.09
1:6:0.09
1:2:0.015
1:2:0.015
pinBOSiEt3 (1)
10
50
100
0.5
10
20
10
20
10
10
10
10
10
10
10
40
40
40
40
20
92
95
95
94
91
93
92
96
90
92
95
93
95
(55)
(60)
88
82
85
89
0
2
nPr3SiH
pinBOSinPr3 (2)
3
tBuMe2SiH
Me2PhSiH
MePh2SiH
Ph3SiH
pinBOSitBuMe (3)
2
4
pinBOSiMe2Ph (4)
pinBOSiMePh2 (5)
pinBOSiPh3 (6)
5
6
7
MePh(CH2CH)SiH
O(SiMe2H)2
Fe{(η5-C5H4(SiMe2H)}2
Et2SiH2
pinBOSiMePh(CHCH2) (7)
O(SiMe2OBpin)2 (8)
Fe{η5-C5H4(SiMe2OBpin)}2 (9)
(pinBO)2SiEt2 (10)
pinBOSiMe2Ph (4)
8
9
10
11
12
13
Me2PhSiH
Et3SiH
HOBpin
pinBOSiEt3 (1)
HOBpin
Me2PhSiH
Et3SiH
pinBOSiMe2Ph (4)
d
14
HOBpin
pinBOSiEt3 (1)
d
15
HOBpin
Me2PhSiH
Me2PhSiH
Ph3SiH
pinBOSiMe2Ph (4)
e
16
(MeBO)3
(MeBO)3
MeB(OH)2
MeB(OH)2
O(BPh2)2
MeB(OSiMe2Ph)2 (11)
MeB(OSiPh3)2 (12)
MeB(OSiMe2Ph)2 (11)
MeB(OSiPh3)2 (12)
e
17
e
18
Me2PhSiH
Ph3SiH
e
19
20
Me2PhSiH
1:2:0.03
Ph2BOSiMe2Ph
a
b
c
d
e
1
See SI for details of reaction conditions. Isolated yield. Yield based upon H NMR in parentheses. Without H2O. Solvent is THF.
were examined. When D2O was used, formation of both H2 and
HD was detected in the H NMR spectrum of the reaction
Scheme 3. Proposed Catalytic Cycle for the Formation of
Boryl Silyl Ethers in the Reaction of O(Bpin)2 with R3SiH
Promoted by Mo(CO)6
1
mixture (H2: 4.47 ppm; HD: 4.43 ppm (t, JH−D = 42.6 Hz) in
C6D6) (see SI, Figure S1). The obtained labeled boryl silyl
ether (pinB−O−SiPh3) was also analyzed using high-resolution
mass spectroscopy (HRMS). The experiments using H218O
gave a peak at m/z = 404.1858, corresponding to pinB−18O−
SiPh3, in addition to a peak for pinB−16O−SiPh3 at m/z =
402.1821. No peak at m/z = 404.1858 for H2O was observed
when H2O was used (see SI, Figure S2). These results indicate
that water acts as a reactant in our reaction system.
Scheme 3 shows a proposed catalytic cycle for the B−O−Si
bond-forming reaction. One CO ligand in the precursor is
released to yield Mo(CO)5, which reacts with R3SiH to give
Mo(CO)5(η2-HSiR3) (A).10,12 Complex A was detected by H
1
NMR spectroscopy after photoirradiation of a Mo(CO)6,
O(Bpin)2, and R3SiH mixture in a 0.05:1:2 ratio, respectively,
over 20 h (R = Et; the hydride signal was observed at δ = −9.90
ppm in C6D6). If A reacts with O(Bpin)2 to form B and
nucleophilic attack of [Mo(CO)5H]− to one of the boron
atoms in [O(Bpin)2(SiR3)]+ occurs, then pinBOSiR3 and C
would be formed. Reductive elimination of HBpin from C and
coordination of R3SiH to the molybdenum center regenerates
A to complete the catalytic cycle. HBpin thus generated reacts
with water to give O(Bpin)2. Silanol was not thought be
involved in the catalytic cycle, since the reaction of R3SiH and
H2O in the presence of Mo(CO)6 under photoirradiation did
not give silanol. The reactions similar to entry 14 (also 15) in
Table 1 but using O(Bpin)2 and Et3SiH in a 1:1 molar ratio in
the absence of water in toluene and in THF revealed that the
yields of pinBOSiEt3 were 8−10%. Therefore, the role of water
in this catalytic cycle seems not simple: water may promote the
reaction from A to B and/or that from B to C in addition to
participation in the conversion of pinBH to (pinB)2O.
In order to obtain further insight into the catalytic
mechanism, we examined a reaction similar to entry 1 in
Table 3 using Mo(CO)5(THF)13 or Mo(CO)5(NEt3)14 as a
catalyst. The reaction proceeded even at 25 or 50 °C, but the
yield of the product dropped (Table 4, entries 1, 2, 4, and 5). In
contrast, photoirradiation of the solution at 25 °C resulted in
the efficient formation of pinBOSiEt3 with vigorous evolution
of H2 gas (entries 3, 6). These results suggest that the light
energy is used not only for dissociation of CO from Mo(CO)6
C
dx.doi.org/10.1021/ja500465x | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
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Table 4. Reaction of O(Bpin)2 with Et3SiH in the Presence
of Mo(CO)5(THF) or Mo(CO)5(NEt3)
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that our catalytic reaction system is also available to perform
selective B−O−Ge bond-forming reactions.
Scheme 4. Reaction of O(Bpin)2 with Et3GeH in the
Presence of Mo(CO)6
In conclusion, we have established an unprecedented
selective B−O−Si bond-forming reaction of bisboryloxide or
boroxine with hydrosilane catalyzed by a transition-metal
carbonyl complex; Mo(CO)6 has especially high catalytic
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ASSOCIATED CONTENT
* Supporting Information
Detailed experimental procedures and characterization of
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AUTHOR INFORMATION
Corresponding Author
nakazawa@sci.osaka-cu.ac.jp
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Challenging Exploratory
Research Grant (No. 25620048) and by a Grant-in-Aid for
Science Research Japan (C) (No. 25410073) from the Ministry
of Education, Culture, Sports, Science and Technology, Japan
and a grant from the Kato Foundation for the Promotion of
Science.
D
dx.doi.org/10.1021/ja500465x | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX