Synthesis of six-membered silacycles by borane-catalyzed double sila-Friedel-Crafts reaction

Article abstract of DOI:10.3762/bjoc.16.39

We have developed a catalytic synthetic method to prepare phenoxasilins. A borane-catalyzed double sila-Friedel-Crafts reaction between amino group-containing diaryl ethers and dihydrosilanes can be used to prepare a variety of phenoxasilin derivatives in

Full text of DOI:10.3762/bjoc.16.39

Synthesis of six-membered silacycles by borane-catalyzed
double sila-Friedel–Crafts reaction
Yafang Dong1, Masahiko Sakai1, Kazuto Fuji1, Kohei Sekine1,2 and Yoichiro Kuninobu*1,2
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 409–414.
1Interdisciplinary Graduate School of Engineering Sciences, Kyushu
University, 6-1 Kasugakoen, Kasuga-shi, Fukuoka 816-8580, Japan
and 2Institute for Materials Chemistry and Engineering, Kyushu
University, 6-1 Kasugakoen, Kasuga-shi, Fukuoka 816-8580, Japan
doi:10.3762/bjoc.16.39
Received: 01 February 2020
Accepted: 11 March 2020
Published: 17 March 2020
Email:
Yoichiro Kuninobu* - kuninobu@cm.kyushu-u.ac.jp
This article is part of the thematic issue "C–H functionalization for
materials science".
* Corresponding author
Guest Editor: K. Itami
Keywords:
borane; cyclic compound; organosilane; sila-Friedel–Crafts; silylation
© 2020 Dong et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
We have developed a catalytic synthetic method to prepare phenoxasilins. A borane-catalyzed double sila-Friedel–Crafts reaction
between amino group-containing diaryl ethers and dihydrosilanes can be used to prepare a variety of phenoxasilin derivatives in
good to excellent yields. The optimized reaction conditions were also applicable for diaryl thioethers to afford their corresponding
six-membered silacyclic products. The gram-scale synthesis of a representative bis(dimethylamino)phenoxasilin and the transfor-
mation of its amino groups have also been demonstrated.
Introduction
Six-membered silacyclic compounds, such as phenoxasilin and previously reported synthetic methods due to the use of a stoi-
phenothiasilin derivatives, are attractive compounds for applica- chiometric amount of the organolithium reagents. In addition,
tions as organic electronic materials [1-4], ligands [5-10], and despite these contributions, catalytic reaction systems have not
reagents [11-14]. Therefore, the development of new methods been developed as much [26,27].
to construct silacyclic skeletons is highly desirable. These com-
pounds are commonly synthesized upon the reaction of hetero- The sila-Friedel–Crafts reaction is emerging as a powerful tool
atom-bridged dilithiated diaryl compounds, such as dilithiated for C–H silylation [28,29]. In addition, intra- and intermolecu-
diaryl ethers and dilithiated diaryl thioethers with a range of lar sila-Friedel–Crafts reactions have been recently developed
dichlorosilane derivatives (Scheme 1a) [15-24]. An intramolec- [30-39], which have great potential as efficient synthetic strate-
ular silylation via Si–C bond cleavage can also be used to gies to construct silacycles. For example, the intramolecular
prepare a variety of six-membered silacyclic derivatives C–H silylation of biphenylhydrosilanes can be used to prepare
(Scheme 1b) [25]. However, some problems still remain in various silafluorene derivatives [30-34] and the ruthenium-cata-
terms of the functional group tolerance and versatility of these lyzed intermolecular Friedel–Crafts-type reaction of
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Beilstein J. Org. Chem. 2020, 16, 409–414.
Scheme 1: Synthetic methods of six-membered silacyclic compounds.
2-phenylindole with a variety of dihydrosilanes affords indole- substrates (Table 1). Under the optimized reaction conditions
fused benzosiloles [39]. We have also contributed to the synthe- used for the synthesis of silafluorenes in our previous report
sis of silafluorenes from biphenyls and dihydrosilanes using a [40] (B(C6F5)3 (5.0 mol %) and 2,6-lutidine (7.5 mol %) in
borane-catalyzed double sila-Friedel–Crafts reaction [40,41]. chlorobenzene at 100 °C), the desired reaction between 1a with
Subsequently, we envisaged that the catalytic reaction between 2a proceeded to give phenoxasilin 3a in 60% yield (Table 1,
diaryl ethers and dihydrosilanes may be a useful protocol to entry 1). The structure of phenoxasilin 3a was confirmed using
prepare phenoxasilin derivatives (Scheme 1c). Herein, we single-crystal X-ray crystallography (see Supporting Informa-
report a borane-catalyzed double sila-Friedel–Crafts reaction tion File 1 for details) [42]. Upon increasing the reaction tem-
used for the synthesis of six-membered silacyclic compounds, perature to 140 °C, the yield of 3a was improved to 88%
such as phenoxasilin and phenothiasilin derivatives.
(Table 1, entry 2). Although the reaction in the presence of
3.0 mol % of the catalyst also proceeded efficiently (Table 1,
entry 3, conditions A), the yield of 3a decreased when com-
Results and Discussion
A double sila-Friedel–Crafts reaction was initially investigated pared to that obtained using 1.5 mol % of the catalyst (Table 1,
using diaryl ether 1a and dihydrodiphenylsilane (2a) as model entry 4). The best result was obtained in the absence of 2,6-luti-
Table 1: Optimization of the reaction conditions for the synthesis of phenoxalin 3a.
entrya
x (mol%)
y (mol %)
temp (°C)
yield (%)
1
2
3
4
5
5.0
5.0
3.0
1.5
3.0
7.5
7.5
7.5
7.5
0
100
140
140
140
140
60
88
97
87
99
a1a (0.250 mmol), 2a (0.750 mmol), chlorobenzene (0.4 mL).
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Beilstein J. Org. Chem. 2020, 16, 409–414.
dine by which phenoxasilin 3a formed in 99% yield (Table 1, Also, the chloro-substituted diaryl ether gave its corresponding
entry 5, conditions B).
phenoxasilin 3h in 94% yield without affecting the chlorine
substituent. The methyl-substituted phenoxasilin derivatives 3i
Next, the scope of the dihydrosilane starting materials used in and 3j were formed in good yield despite of the steric hindrance
the reaction was investigated (Scheme 2). The reactions of of the methyl group in 3j. When one of the NMe2 groups was
phenylmethylsilane (2b) and diethyldihydrosilane (2c) afforded replaced with a SMe group, a mixture of the corresponding
their corresponding phenoxasilin derivatives 3b and 3c in 66 phenoxasilin product (3k) and the hydrosilane compound (3k′)
and 74% yield, respectively. The yields of 3b and 3c were im- was obtained via a single sila-Friedel–Crafts reaction in 35%
proved to 83 and 91% in the presence of a catalytic amount of yield in the presence of 2,6-lutidine (3k:3k′ = 63:37). This
2,6-lutidine, probably due to the acceleration of the deproton- result was probably due to the weaker electron-donating ability
ation step by 2,6-lutidine [33]. In the case of phenylsilane (2d), of the SMe group compared to that of NMe2. The double C–H
the phenoxasilin product 3d was formed in 59% yield using silylation reaction proceeds efficiently upon increasing the tem-
conditions B and in 63% yield under conditions A. Di(4-bromo- perature to 180 °C that afforded the mixture (3k:3k′ = 92:8) in
phenyl)dihydrosilane (2e) was transformed successfully into 68% yield. The reaction system can also be applied to the syn-
phenoxasilin 3e in 83% yield without loss of the bromine sub- thesis of phenothiasilin 3l that was obtained in 93% yield
stituent. The reaction system was also applicable for 9,9- starting from diaryl thioether 1g. N-(Benzyl)methylamine-
dihydro-5-silafluorene (2f), which gave the spiro-type phenox- substituted diaryl thioether 1h was also transformed into
asilin 3f in 96% yield.
phenothiasilin 3m in 58% yield. The corresponding six-mem-
bered silacycles were not formed using N-aryl-bridged biaryls
We then investigated the scope of the starting biaryl ethers used as substrates.
in the reaction as well as related derivatives thereof using dihy-
drodiphenylsilane (2a, Scheme 3). Pyrrolidine-substituted diaryl To test the applicability of the method, a gram-scale synthesis
ether 1b was transformed into phenoxasilin 3g in 80% yield. of phenoxasilin 3a was carried out (Scheme 4). The reaction of
Scheme 2: Scope of dihydrosilanes. Conditions: a: conditions B (Table 1, entry 5); b: conditions A (Table 1, entry 3).
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Beilstein J. Org. Chem. 2020, 16, 409–414.
Scheme 3: Scope of diaryl ether and diaryl thioether derivatives. Conditions: a: conditions B (Table 1, entry 5); b: conditions A (Table 1, entry 3).
c: temperature 180 °C.
Scheme 4: Gram-scale Synthesis of 3a.
diaryl ether 1a (1.00 g) with dihydrodiphenylsilane (2a, 2.16 g) lowed by a palladium-catalyzed cross-coupling reaction with
in the presence of a catalytic amount of B(C6F5)3 afforded the Grignard reagent (PhMgBr) that afforded the desired
phenoxasilin 3a in 93% yield (1.59 g).
diphenylated phenoxasilin 5 in 87% yield [43].
Finally, the transformation of the amino groups in phenoxasilin Conclusion
3a into phenyl groups was carried out (Scheme 5). First, the In summary, we have developed a new catalytic synthetic
ammonium salt 4 was prepared by treating 3a with MeOTf fol- method to prepare six-membered silacyclic compounds, such as
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Beilstein J. Org. Chem. 2020, 16, 409–414.
Scheme 5: Transformation of the amino groups in 3a.
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amino groups in the phenoxasilin product were also achieved.
We hope that the developed protocol will prove to be a useful
and efficient method to synthesize six-membered silacyclic
compounds.
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ORCID® iDs
Kohei Sekine - https://orcid.org/0000-0001-7588-3176
Yoichiro Kuninobu - https://orcid.org/0000-0002-8679-9487
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See for Rhodium-catalyzed intramolecular C–H silylation for the
synthesis of silafluorenes.
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