Intramolecular chain hydrosilylation of alkynylphenylsilanes using a silyl cation as a chain carrier

Article abstract of DOI:10.3390/molecules21080999

Diorganyl[2-(trimethylsilylethynyl)phenyl]silanes 1a-c and methyl-substituted phenylsilanes 1d and 1e were treated with a small amount of trityl tetrakis(pentafluorophenyl)borate (TPFPB) as an initiator in benzene to afford the corresponding benzosiloles (2a-e) in moderate to good yields. However, no reaction was observed for the reaction using [2-(1-hexynyl)phenyl]diisopropylsilane lf. The methyl substituent was tolerated under the reaction conditions and increased the yield of the corresponding benzosilole depending on the substitution position. From the result using 1f, the current reaction was found to require the trimethylsilyl group, which can stabilize intermediary alkenyl carbocations by the β-silyl effect. The current reaction can be considered an intramolecular chain hydrosilylation of alkynylarylsilanes involving silyl cations as chain carriers. Therefore, the silyl cations generated by hydride abstraction from hydrosilanes 1 with the trityl cation causes intramolecular electrophilic addition to the C-C triple bond to form ethenyl cations, which abstract a hydride from 1 to afford benzosiloles 2 with the regeneration of the silyl cations.

Full text of DOI:10.3390/molecules21080999

molecules
Article
Intramolecular Chain Hydrosilylation of
Alkynylphenylsilanes Using a Silyl Cation as a
Chain Carrier
Hidekazu Arii 1,*, Kenichi Nakabayashi 1, Kunio Mochida 2 and Takayuki Kawashima 3,
*
1
Faculty of Education, University of Miyazaki, 1-1 Gakuen Kibanadai Nishi, Miyazaki, 889-2192 Miyazaki,
Japan; nakabys@cc.miyazaki-u.ac.jp
Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, 171-8588 Tokyo, Japan;
kunio.mochida@gakushuin.ac.jp
Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu,
376-8515 Gunma, Japan
2
3
*
Correspondence: hidekazu.arii@cc.miyazaki-u.ac.jp (H.A.); kawashima.t@gunma-u.ac.jp (T.K.);
Tel.: +81-985-58-7499 (H.A.); +81-277-30-1670 (T.K.); Fax: +81-985-58-7499 (H.A.); +81-277-30-1202 (T.K.)
Academic Editor: Mitsuo Kira
Received: 9 July 2016; Accepted: 27 July 2016; Published: 1 August 2016
Abstract: Diorganyl[2-(trimethylsilylethynyl)phenyl]silanes 1a
1d and 1e were treated with a small amount of trityl tetrakis(pentafluorophenyl)borate (TPFPB) as
an initiator in benzene to afford the corresponding benzosiloles (2a ) in moderate to good yields.
–c and methyl-substituted phenylsilanes
–
e
However, no reaction was observed for the reaction using [2-(1-hexynyl)phenyl]diisopropylsilane
lf. The methyl substituent was tolerated under the reaction conditions and increased the yield of
the corresponding benzosilole depending on the substitution position. From the result using 1f
,
the current reaction was found to require the trimethylsilyl group, which can stabilize intermediary
alkenyl carbocations by the
chain hydrosilylation of alkynylarylsilanes involving silyl cations as chain carriers. Therefore,
the silyl cations generated by hydride abstraction from hydrosilanes with the trityl cation causes
β-silyl effect. The current reaction can be considered an intramolecular
1
intramolecular electrophilic addition to the C-C triple bond to form ethenyl cations, which abstract a
hydride from 1 to afford benzosiloles 2 with the regeneration of the silyl cations.
Keywords: benzosilole; silyl cation; hydrosilylation; metal free
1. Introduction
A benzosilole is an attractive compound due to its emission property and potential use as of
optical materials [
originate from orbital interaction between the
of the butadiene moiety [ ]. The popular synthetic routes to benzo- and dibenzosiloles involve
intra- and intermolecular cyclization reactions with transition metal catalysts [ ], and the use
of a chiral supporting ligand enables the synthesis of siloles with a chiral silicon center [10 11].
In particular, direct Si–C or C–H activation is a powerful method that does not require an activated
functional group on the aromatic ring [12 17]. In non-transition metal systems, 2-ethynylphenylsilane
derivatives have been cyclized to afford the corresponding benzosiloles using various reactants,
such as lithium naphthalenide [18], Lewis acids [19 20] and KH (Equation (1)) [21]. The radical
1
–4]. These features are associated with the low-lying LUMOs of the siloles, which
σ
* orbital of the silylene moiety and the * orbital
π
5
6–9
,
–
,
reaction that is initiated by tert-butylhydroperoxide (TBHP) resulted in the formation of dibenzosiloles
from (2-biphenyl)diphenylsilanes (Equation (2)) [22]. The uses of KH and TBHP produce the
corresponding active silicon species (i.e., a pentacoordinated hydridosilicate and a silyl radical,
Molecules 2016, 21, 999; doi:10.3390/molecules21080999
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respectively). We have synthesized dibenzosilole by a sila-Friedel-Crafts reaction mediated by a
silyl cation (Equation (3)) [23,24].
(1)
(2)
(3)
We have extended silyl cation chemistry to the synthesis of silacyclic compounds and reported
the synthesis of 1,2-dihydro-2-silanaphthalenes using alkynes [25,26]. The intramolecular addition
of a silyl cation toward a C–C triple bond rather than an intermolecular one is expected to lead to
a benzosilole by an appropriate reaction of the resulting ethenyl carbocation. Herein, we report the
intramolecular chain hydrosilylation of diorganyl[2-(trimethylsilylethynyl)phenyl]silanes with trityl
tetrakis(pentafluorophenyl)borate (TPFPB) as an initiator.
2. Results and Discussion
Diorganyl[2-(trimethylsilylethynyl)phenyl]silanes 1a
(1–4 mol %) of TPFPB in benzene to afford the corresponding benzosiloles 2a
yields (Table 1, entries 1–3). The intramolecular chain hydrosilylation of was achieved using TPFPB
–
c
were reacted with a small amount
–
c
in low to moderate
1
as an initiator. However, this reaction was accompanied with the formation of unidentified oligomers,
which appear to be formed by silyl cation-induced alkyne polymerization. The amount of TPFPB and
solvent were optimized using 1b, and therefore, 1 mol % TPFPB in benzene was determined to provide
the best yield of 75% (Table 1, entry 5). The conversion of 1b to silole 2b required a longer reaction
time (30 min in 1 mol % TPFPB (Table 1, entry 5) compared to 15 min in 3 mol % TPFPB (entry 4) and
5 min in 4 mol % TPFPB (entry 2)) with a decrease in the dose of the trityl cation initiator. However,
the yield of 2b increased from 61% to 75% due to the preference of the desired hydrosilylation over
the competing oligomerization. The syntheses of benzosiloles from 2-alkynylphenylsilane derivatives
in transition metal-free systems have been achieved using AlCl3 and KH to activate the Si-H and/or
C–C triple bonds and to generate the pentacoordinated hydridosilicate, respectively [19,21]. In this
system, the silyl cation plays an important role in the reaction and promotes the hydrosilylation by
electrophilic addition to the C–C triple bond.
The scope and limitations of the intramolecular chain hydrosilylation of
1 are summarized as
follows. Under the optimized conditions using 1 mol % of TPFPB, the isolated yields of 2b and 2c
were increased (entries 5 and 10) even though the yield of 2a bearing the sterically smaller methyl
groups on the silicon center was barely affected by the dose of initiator (entry 9). The reactions
using methyl-substituted silanes 1d and 1e also afforded the corresponding benzosiloles 2d and 2e
in 72% and 81% yields, respectively (entries 11 and 12). It is most likely that the relatively good

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yield of 2e may be caused by stabilization due to hyperconjugation of the 5-methyl group with the
intermediary ethenyl carbocation. The reaction using silane 1f bearing a 1-hexynyl group rather than
a trimethylsilylethynyl group did not afford the corresponding silole 2f, and nearly all of 1f was
recovered (entry 13). This result may be due to the intermediary ethenyl carbocation that was derived
from 1f being less stable than those derived from 1a–e, which are stabilized by double β-silyl effects.
Therefore, the trimethylsilyl group on the alkynyl group was essential for the current reaction.
Table 1. Intramolecular chain hydrosilylation of 1a–f to 2a–f.
Entry
R1
R2
R3
Solvent TPFPB/mol % Time/min Yield of 2/% a
1
2
3
4
5
6
7
8
9
10
11
12
13
1a:
1b:
1c:
1b:
1b:
1b:
1b:
1b:
1a:
1c:
1d:
1e:
1f:
Me
i-Pr
Ph
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Me
Ph
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
n-Bu
H
H
H
H
H
H
H
H
H
benzene
benzene
benzene
benzene
benzene
toluene
mesitylene
CH2Cl2
benzene
benzene
benzene
benzene
benzene
4
4
4
3
1
1
1
1
1
1
1
1
1
5
5
5
37 (2a)
61 (2b)
21 (2c)
70 (2b)
75 (2b)
72 (2b)
70 (2b)
60 (2b)
34 (2a)
55 (2c)
72 (2d)
81 (2e)
0 (2f) b
15
30
30
30
30
30
30
30
50
720
H
i-Pr
i-Pr
i-Pr
4-Me
5-Me
H
a
Isolated yields based on 1; b Nearly all of 1f was recovered.
The reaction mechanism of the intramolecular chain hydrosilylation is described in Scheme 1.
First, the trityl cation acts as an initiator to abstract the hydride from the Si–H bond of , resulting in the
generation of silyl cation . Next, the intramolecular electrophilic addition of the silyl cation moiety of
to the C–C triple bond produces ethenyl carbocation , which is stabilized by the -silyl effect of the
trimethylsilyl group. Finally, the hydride abstraction of from another affords benzosilole and
regenerates intermediate A [27–29], which acts as a chain carrier.
1
A
A
B
β
B
1
2
Scheme 1. Plausible mechanism of intramolecular chain hydrosilylation.
3. Experimental Section
General Procedure: All experiments were carried out using standard vacuum line and Schlenk
techniques in an Ar atmosphere or dry box. All the reagents were of the highest grade available
and were used without further purification. All solvents used for the syntheses were distilled

Molecules 2016, 21, 999
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according to the general procedure. [Ph3C]B(C6F5)4 [30], [2-(2-bromophenyl)ethynyl]trimethylsilane
derivatives [31], 2-(1-hexynyl)bromobenzene [21], 1a 19] and 1c 21] were synthesized according to
[
[
the previously reported methods. The NMR spectral measurements were performed on an Agilent
400-MR NMR (Agilent Technologies Co., Santa Clara, CA, USA) or a Bruker AV400M spectrometers
(Bruker Co., Billerica, MA, USA). The 1H and 13C chemical shifts are reported relative to the residual
protonated solvent and the solvent, respectively, according to the literature [32]. High-resolution mass
spectrometry was measured by a JEOL GCMATE II (JEOL Ltd., Tokyo, Japan) or JMS-700N (JEOL Ltd.)
operating by electron impact ionization (EI). Gel permeation liquid chromatography (GPLC) was
performed by a Japan Analytical Industry LC-918 (Japan Analytical Industry Co., Ltd., Tokyo, Japan)
using chloroform as an eluent.
Preparation of Compounds
Silanes 1. To a corresponding bromo compound (1.3 mmol) in hexane 8 mL were added 1.6 M
pentane solution of tert-BuLi (0.84 mL, 1.4 mmol) and N,N,N’,N’-tetramethylethylenediamine (0.23 g,
2.0 mmol) at
´
80 ˝C, and the solution was stirred for 20 min keeping the temperature below
´
70 ˝C.
˝
To the solution was added i-Pr2SiHCl (0.20 g, 1.4 mmol) at
´70 C, the solution was stirred and
slowly warmed to room temperature. The reaction mixture was quenched with 5% NH4Cl aqueous
solution. The mixture was extracted with hexane 20 mL two times, and the organic layer was dried
over anhydrous sodium sulfate. The filtrate was concentrated under reduced pressure to remove
volatiles, and the residue was purified by silica gel column (eluent: hexane). Further purification was
carried out by GPLC to obtain 1 as a colorless liquid.
1
Diisopropyl[(2-trimethylsilylethynyl)phenyl]silane (1b): 75%. H-NMR (CDCl3, 400 MHz):
δ 7.50–7.46
(m, 2H, ArH), 7.32–7.24 (m, 2H, ArH), 4.01 (t, J = 4.0 Hz, 1H, SiH), 1.49–1.39 (m, 2H, i-Pr), 1.10
(d, J = 7.2 Hz, 6H, i-Pr), 0.98 (d, J = 7.6 Hz, 6H, i-Pr), 0.24 (s, 9H, SiMe2). 13C-NMR (CDCl3, 100 MHz):
δ
138.1, 136.5, 132.6, 128.9, 128.8, 127.4, 106.6, 96.1, 19.12, 19.07, 11.1,
´
0.22. HRMS (EI) m/z: [M]+ Calcd
for C17H28Si, 288.1730; Found, 288.1729.
1
Diisopropyl[4-methyl-2-(trimethylsilylethynyl)phenyl]silane (1d): Yield 69%. H-NMR (CDCl3, 400 MHz):
δ
7.39 (d, J = 7.6 Hz, 1H, ArH), 7.30 (d, J = 1.2 Hz, 1H, ArH), 7.11 (dd, J = 8.0 Hz, J = 1.2 Hz, 1H, ArH),
3.98 (t, J = 4.0 Hz, 1H, SiH), 2.34 (s, 3H, ArMe), 1.50–1.40 (m, 2H, i-Pr), 1.11 (d, J = 7.6 Hz, 6H, i-Pr),
1.10 (d, J = 7.2 Hz, 6H, i-Pr), 0.24 (s, 9H, SiMe3). 13C-NMR (CDCl3, 100 MHz):
δ 138.0, 137.4, 137.2,
132.6, 129.6, 125.8, 106.9, 95.1, 21.5, 19.20, 19.13, 11.2,
302.1886; Found, 302.1909.
´
0.18. HRMS (EI) m/z: [M]+ Calcd for C18H30Si2,
1
Diisopropyl[5-methyl-2-(trimethylsilylethynyl)phenyl]silane (1e): 68%. H-NMR (CDCl3, 400 MHz):
δ 7.46
(dd, J = 7.2 Hz, J = 1.2 Hz, 1H, ArH), 7.41 (d, J = 7.2 Hz, 1H, ArH), 7.28 (td, J = 7.6 Hz, J = 1.6 Hz, 1H,
ArH), 7.22 (td, J = 7.6 Hz, J = 1.6 Hz, 1H, ArH), 4.02 (t, J = 4.0 Hz, 1H, SiH), 2.41 (t, J = 7.2 Hz, 2H,
n-Bu), 1.62–1.32 (m, 6H, i-Pr and n-Bu), 1.10 (d, J = 7.6 Hz, 6H, i-Pr), 0.98 (d, J = 7.2 Hz, 6H, i-Pr), 0.95
(t, J = 7.2 Hz, 3H, n-Bu). 13C-NMR (CDCl3, 100 MHz):
δ 137.4, 136.4, 132.1, 130.0, 128.8, 126.4, 92.4, 82.1,
30.7, 22.1, 19.24, 19.12, 13.6, 11.1. HRMS (EI) m/z: [M]+ Calcd for C18H28Si, 272.1960; Found, 272.1958.
1
[2-(1-Hexynyll)phenyl]diisopropylsilane (1f): Yield 55%. H-NMR (CDCl3, 400 MHz):
δ 7.38 (d, J = 7.6 Hz,
1H, ArH), 7.34 (s, 1H, ArH), 7.10 (d, J = 7.6 Hz, 1H, ArH), 4.00 (t, J = 4.0 Hz, 1H, SiH), 2.31 (s, 3H,
ArMe), 1.47–1.36 (m, 2H, i-Pr), 1.10 (d, J = 7.2 Hz, 6H, i-Pr), 0.98 (d, J = 7.2 Hz, 6H, i-Pr), 0.24 (s, 9H,
SiMe3). 13C-NMR (CDCl3, 100 MHz):
δ 138.7, 136.7, 134.4, 133.3, 128.8, 128.5, 106.8, 95.6, 21.1, 19.12,
19.09, 11.1, ´0.21. HRMS (EI) m/z: [M]+ Calcd for C18H30Si2, 302.1886; Found, 302.1883.
Benzosiloles 2. To trityl tetrakis(pentafluorophenyl)borate (TPFPB, 1.0 mg, 1.0 µmol) in benzene
(0.5 mL) was added a benzene solution (1.5 mL) of hydrosilanes
1 (0.10 mmol) at room temperature
under Ar atmosphere, and the resulting solution was stirred at room temperature. After the reaction

Molecules 2016, 21, 999
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mixture was quenched with 2,6-lutidine (2
µL) and H2O, and then the organic layer was extracted.
After extraction with hexane two times, the organic layers were combined and dried over anhydrous
sodium sulfate, and then the filtrate was evaporated under reduced pressure to remove volatiles.
Purification was carried out by GPLC to remove polymeric materials.
1
1,1-Dimethyl-2-trimethylsilyl-1-silaindene (2a): 34%. H-NMR data are consisted with those reported
previously [19].
1,1-Diisopropyl-2-trimethylsilyl-1-silaindene (2b): 75%. 1H-NMR (CDCl3, 400 MHz):
δ 7.61 (s, 1H,
ArCH=C), 7.51 (d, J = 6.8 Hz, 2H, ArH), 7.32 (td, J = 7.6 Hz, J = 1.2 Hz, 1H, ArH), 7.27 (d, J = 6.4 Hz, 1H,
ArH), 7.19 (td, J = 7.6 Hz, J = 1.2 Hz, 1H, ArH), 1.35–1.25 (m, 2H, i-Pr), 1.07 (d, J = 7.6 Hz, 6H, i-Pr), 0.93
(d, J = 7.6 Hz, 6H, i-Pr), 0.19 (s, 9H, SiMe2). 13C-NMR (CDCl3, 100 MHz):
δ 158.2, 151.1, 143.1, 137.4,
132.6, 129.4, 126.7, 123.9, 19.98, 19.96, 11.2,
Found, 288.1736.
´
0.16. HRMS (EI) m/z: [M]+ Calcd for C17H28Si, 288.1730;
1
1,1-Diphenyl-2-trimethylsilyl-1-silaindene (2c): 55%. H-NMR data are consisted with those reported
previously [21].
1
1,1-Diisopropyl-5-methyl-2-trimethylsilyl-1-silaindene (2d): Yield 72%. H-NMR (CDCl3, 400 MHz):
δ 7.57
(s, 1H, ArCH=C), 7.40 (d, J = 6.8 Hz, 2H, ArH), 7.11 (s, 1H, ArH), 7.02 (d, J = 6.8 Hz, 1H, ArH), 2.35
(s, 3H, ArMe), 1.34–1.22 (m, 2H, i-Pr), 1.06 (d, J = 7.2 Hz, 6H, i-Pr), 0.93 (d, J = 7.2 Hz, 6H, i-Pr), 0.19
(s, 9H, SiMe2). 13C-NMR (CDCl3, 100 MHz):
δ 158.2, 151.6, 143.3, 139.3, 133.6, 132.5, 127.5, 125.0, 21.5,
18.01, 17.99, 11.2, ´0.16. HRMS (EI) m/z: [M]+ Calcd for C18H30Si2, 302.1886; Found, 302.1911.
1
1,1-Diisopropyl-6-methyl-2-trimethylsilyl-1-silaindene (2e): Yield 81%. H-NMR (CDCl3, 400 MHz):
δ 7.58
(s, 1H, ArH), 7.32 (s, 1H, ArH), 7.18–7.09 (m, 2H, ArH), 2.36 (s, 3H, ArMe), 1.36–1.22 (m, 2H, i-Pr), 1.07
(d, J = 7.2 Hz, 6H, i-Pr), 0.94 (d, J = 7.6 Hz, 6H, i-Pr), 0.19 (s, 9H, SiMe3). 13C-NMR (CDCl3, 100 MHz):
δ
158.0, 148.7, 141.3, 137.6, 136.2, 133.6, 130.0, 123.6, 21.4, 17.98, 17.9, 11.2, –0.13. HRMS (EI) m/z: [M]+
Calcd for C18H30Si2, 302.1886; Found, 302.1863.
1
The H- and 13C-NMR spectral charts of all new compounds
1 and 2 are summarized in
supplementary materials.
4. Conclusions
In conclusions, we achieved the intramolecular chain hydrosilylation of
1 to synthesize
benzosiloles . The hydrosilylation proceeded under mild conditions with a small amount of TPFPB as
2
an initiator and no additives. In this reaction, the silyl cation plays an important role as a chain carrier,
which is different from the reactions involving the hydridosilicate and silyl radical.
Supplementary Materials: The 1H- and 13C-NMR spectral charts of 1b
, 1d, 1e, 1f, 2b, 2d, and 2e can be accessed
at http://www.mdpi.com/1420-3049/21/8/999/s1.
Acknowledgments: This work was financially supported by a Grant-in-Aid for Scientific Research (No. 26410035,
T.K.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This work was performed
under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” (H.A.).
We acknowledge the Center for Collaborative Research and Community Cooperation (Miyazaki University) for
the NMR spectroscopy and elemental analysis. We wish to thank Nobuaki Tsuda (Center for Industry, University
and Government Cooperation, Nagasaki University) for measurement of the high-resolution mass spectroscopy.
Author Contributions: H.A. conceived and performed the experiments. T.K. designed the present reaction. H.A.,
K.N., K.M. and T.K. discussed the results. H.A. wrote the manuscript and T.K. edited it.
Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds 1 and 2 are available from the authors.
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