Direct synthesis of dialkylarylvinylsilane derivatives: Metathesis of dialkylaryl-iso-propenylsilane and its application to tetracyclic silacycle dye synthesis

Article abstract of DOI:10.1039/c9cc06777a

The metathesis of dialkylarylvinylsilane, which has not been accomplished to date, is achieved using dialkylaryl-iso-propenylsilane as a substrate. In addition, we discovered that the reason why the metathesis of a ruthenium carbene complex and dialkylarylvinylsilane is difficult is the formation of a carbide complex.

Full text of DOI:10.1039/c9cc06777a

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Direct synthesis of dialkylarylvinylsilane
derivatives: metathesis of dialkylaryl-iso-
propenylsilane and its application to tetracyclic
silacycle dye synthesis†
Cite this: Chem. Commun., 2019,
55, 14070
Received 31st August 2019,
Accepted 30th October 2019
Shohei Yoshioka,a Tsunayoshi Takehara,b Tsuyoshi Matsuzaki,b Takeyuki Suzuki,b
DOI: 10.1039/c9cc06777a
a
Hirofumi Tsujino,a Tadayuki Uno,a Yasuo Tsutsumi,a Kenichi Murai,
a
a
rsc.li/chemcomm
Hiromichi Fujioka
and Mitsuhiro Arisawa
*
The metathesis of dialkylarylvinylsilane, which has not been accom-
plished to date, is achieved using dialkylaryl-iso-propenylsilane as a
substrate. In addition, we discovered that the reason why the
metathesis of a ruthenium carbene complex and dialkylarylvinylsilane
is difficult is the formation of a carbide complex.
Diene and enyne metatheses in their various versions (ring-
closing metathesis, RCM; ring-opening metathesis, ROM; and
cross-metathesis, CM) have emerged as major tools for the
synthesis of complex molecules.1 Furthermore, olefinic double
bonds bearing heteroatom substituents (e.g., vinyl halides,2
enol ethers,3 enamides,4 vinylboranes,5 vinylsilanes, etc.) offer
vast functionalization possibilities.6 However, systems in which
the dialkylarylsilyl group is maintained have been limited due
to the decomposition of the ruthena-cyclobutane intermediate
as a result of beta-trialkylsilyl elimination (Scheme 1a).7
In this communication, we describe unprecedented CM,
RCM and enyne metathesis (EM) of dialkylaryl-iso-propenylsilane
derivatives, which have a methyl substituent at the beta position of
the silicon atom to prevent unwanted beta-silyl elimination. Using
this method, we can obtain the very same products that we have
tried to synthesize from vinylsilane derivatives (Scheme 1b).
First, in order to confirm that the metathesis reaction of
dialkylarylvinylsilane does not proceed, the experiment shown
Scheme 1 Reaction of trialkylvinylsilane and a ruthenium carbene catalyst.
Scheme 2 Attempted cross-metathesis of compound 1a.
in Scheme 2 was performed. A solution of dimethylvinyl- prevent unwanted beta-silyl elimination, was subjected to the
naphthylsilane (1a), styrene (10 eq.) and a Grubbs II catalyst same reaction conditions as in Scheme 2, and as a result the
(5 mol%) in chloroform was refluxed for 1 hour and then an corresponding CM product 2a was obtained in 45% yield
additional 5 mol% Grubbs II catalyst was added to the mixture. (Table 1, entry 1). When the solvent was replaced by toluene,
The target CM reaction did not proceed.
which has a higher boiling point, the yield of 2a was improved
Next, dimethylnaphthyl-iso-propenylsilane (1b), having a to 75% (entry 2). The yield of 2a decreased when 10 mol%
methyl substituent at the beta position of the silicon atom to Grubbs II catalyst was added at the start of the reaction (entry 3).
When 9 mol% Grubbs II catalyst was added in three portions,
hourly, and 10 eq. of styrene was added in two portions, the
yield of 2a was 62% (entry 4). Further investigation was carried
out and, by using a more concentrated solvent of 0.2 M and
adding the Grubbs II catalyst in three portions, the yield of 2a
was improved to 85% (entry 5). When the styrene was reduced to
a Graduate School of Pharmaceutical Sciences, Osaka University, Japan.
E-mail: arisaw@phs.osaka-u.ac.jp
b The Institute of Scientific and Industrial Research, Osaka University, Japan
† Electronic supplementary information (ESI) available. CCDC 1938337. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c9cc06777a
14070 | Chem. Commun., 2019, 55, 14070--14073
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Table 1 Optimization of reaction conditions for cross-metathesis of
compound 1b
Table 2 Ring-closing metathesis of compound 3
Substrate
Product
Entry
x
y
Solvent
z
Yield (%)
Entry
R =
E : Z=
Yield (%)
1
2
3
4
5
6
7
5 Â 2
5 Â 2
10
3 Â 3
3 Â 3
3 Â 3
3 Â 3
10
10
10
5 Â 2
10
5
CHCl3
0.1
0.1
0.1
0.1
0.2
0.2
0.2
45
75
61
62
85
76
65
1
2
3
4
5
6
7
8
9
10
3a
3b
3c
3d
3e
3f
3g
3h
3i
4-OMe
3-OMe
5-OMe
6-OMe
3-Cl
4-Cl
5-Cl
3-F
4-F
5-F
4 : 1
4a
4b
4c
4d
4e
4f
4g
4h
4i
91
35
Quant.
63
25
62
80
Trace
Trace
42
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
18 : 1
1.8 : 1
11 : 5
12 : 1
1 : 1
E only
1.8 : 1
20 : 1
4 : 1
3
3j
4j
the starting material remained, probably due to steric hindrance
in the RCM (entries 2 and 5). The same reaction with a substituent
at the 4-, 5- or 6-position on the substrate proceeded more
smoothly, although an electron-donating group accelerated the
reaction more than an electron-withdrawing group.
The boiling point of fluoro-substituted benzosiloles is rather
low, so most of these could not be obtained. However, benzo-
silole with a fluoro substituent at the 5-position was obtained in
42% yield, although the conversion rate by crude nuclear
magnetic resonance (NMR) was almost 100%.
Scheme 3 Effect of substituents on the aromatic ring in styrene.
5 eq. and 3 eq., the yield of 2a gradually decreased to 76% and
65%, respectively (entries 6 and 7).8
Next, the conditions of the EM reaction were examined
(Table 3). A solution of compound 5a and Grubbs II catalyst
(3 mol%) in 0.05 M toluene was refluxed for 2 hours to obtain
EM product 6a in 30% yield (entry 1). At this time, a dimer of
compound 6a was formed in addition to the raw material.
When chloroform was used as the solvent, the formation of the
dimer was not suppressed and product 6a was obtained in 32%
yield (entry 2). The reaction did not proceed when dichloro-
methane was used as the solvent (entry 3). Lowering the
reaction temperature to 45 1C, increasing the amount of
catalyst to 5 mol% and using chloroform as the solvent
improved the product yield to 60% (entry 4). Furthermore,
the yield remained fairly high at 57% even when the reaction
Then, the substrate coverage of this reaction was investi-
gated based on the optimal conditions obtained (Scheme 3).
This was examined using the compound with electron-donating
and electron-withdrawing groups in the ortho, meta or para position
on the benzene ring of styrene. As a result, CM products 2b, 2c
and 2d were obtained with high yields of 97%, 84% and 90%,
respectively, for substrates substituted with an electron-donating
group. With an electron-withdrawing group, the reaction pro-
ceeded with a moderate yield of 69% (2g) in the para position,
63% (2f) in the meta position and 52% (2e) in the ortho position.
A silole ring is used as an organic electroluminescent or
pigment material because of its high electron acceptability.9 We
therefore applied the metathesis reaction described above and
synthesized a silole skeleton using intramolecular RCM10 and
EM. Furthermore, we synthesized fluorescent substances using
a one-pot intramolecular EM/Diels–Alder/oxidation reaction.
The RCM reaction of compound 3 was performed based on
the optimum conditions for the CM reaction in Table 2.
Because the boiling point of the cyclized substance was low
in the substrate with no substituent on the benzene ring, we
started with the 4-methoxy substrate 3a. As a result of various
investigations, it was found that when a solution of 3a and the
Grubbs II catalyst (5 mol%) in toluene (0.05 M) was refluxed for
2 hours, the corresponding RCM product 4a was obtained in 91%
yield (Table 2, entry 1). The RCM of a substrate with a methoxy or
chloro substituent at the 3-position hardly proceeded, giving the
corresponding product in 35% and 25% yield, respectively; most of
Table 3 Enyne metathesis of compound 5a
Entry
x
Solvent
y
Temp. (1C) Product yield (%) E : Z =
1
2
3
4
5
3
3
3
5
5
Toluene 0.05
Reflux
Reflux
Reflux
45
30
32
Trace
60
57
6 : 1
5 : 1
—
4 : 1
5 : 1
CHCl3
CH2Cl2
CHCl3
CHCl3
0.05
0.05
0.05
0.005 Reflux
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Table 4 One-pot enyne metathesis/Diels–Alder/oxidation reaction of
compound 5 to produce compound 7
Fig. 2 31P-NMR of compound 1b and Grubbs II catalyst.
Substrate
R =
Product
Yield (%) labs (nm) lem (nm)
Entry
E : Z =
10 : 1 7a 52
f
Five minutes after the start of heating, a peak different from
the 29.6 ppm Grubbs II catalyst peak12 appeared at 27.9 ppm.
As time passed, this peak increased, so after 30 minutes when
the reaction solution was isolated and purified by column
chromatography the peak at 27.9 ppm was in agreement with
that in the literature13 for an ethylidene complex. From this, it
was found that in the present reaction the ethylidene complex
became an active species as the metathesis proceeded.
1
2
3
4
5
6
7
5a
H
343
363
371
348
354
350
345
400
442
466
415
470
392
406
0.41
0.32
0.53
0.37
0.58
0.50
0.51
5b 4-OMe 10 : 1 7b 35
5c 5-OMe 10 : 1 7c 48
5d 4-Me
5e 5-Me
5f 4-F
10 : 1 7d 24
10 : 1 7e 21
10 : 1 7f 39
5g 5-F
3 : 1
7g 38
Subsequently, the same 31P-NMR experiment was performed
using compound 1a to investigate the structural change of the
Grubbs II catalyst (Fig. 3). Five minutes after the start of the
reaction, a peak thought to be the peak for a benzylidene
complex was observed around 29.3 ppm, alongside the 29.6 ppm
Grubbs II catalyst peak. This peak is considered to be the peak for
a ruthenium catalyst having a similar skeleton to the Grubbs II
catalyst: the peak and the chemical shift are hardly changed. After
15 minutes (20 minutes after the start of the reaction), the peak of
the Grubbs II catalyst disappeared and new peaks appeared at
around 34.9 ppm and 13 ppm, in addition to the 29.3 ppm peak.
The 13 ppm peak is considered to be the peak for temporarily
released tricyclohexylphosphine, but it was not clear what the
34.9 ppm peak indicates. Therefore, 30 minutes after the start
of the reaction the solution was subjected to isolation and
purification by column chromatography, with subsequent
recrystallization and X-ray crystal structure analysis. As a result,
solution was heated to reflux and the concentration was lowered
to 0.005 M (entry 5).
According to the reaction conditions for the synthesis of
polycyclic silicon-containing six-membered ring compounds that
we reported previously,11 silicon-containing five-membered tet-
racyclic compound 7a was obtained through a one-pot EM/
Diels–Alder/oxidation reaction from compound 5a (Table 4,
entry 1). The substrate application range was investigated and
the yield for the three steps was 35% (compound 7b) in the
4-methoxy group, 48% (compound 7c) in the 5-methoxy group,
24% in the 4-methyl group (compound 7d), 21% in the 5-methyl
group (compound 7e), 39% in the 4-fluoro group (compound 7f)
and 38% in the 5-fluoro group (compound 7g).
Table 4 also shows the optical properties of compound 7 in
chloroform (see also Fig. 1). As a whole, it was found that for
substrates with an electron-donating group at the 5-position, the
maximum fluorescence wavelength is shifted to longer wavelengths
and the fluorescence quantum yield becomes higher (0.58 at most).
We also found that fluorescence quantum yields give good results
even for substrates with electron-withdrawing groups.
31P-NMR experiments were carried out to obtain structural
information in view of the fact that the metathesis reaction of
dialkylarylvinylsilane (1a) does not proceed due to a structural
change in the Grubbs II catalyst.
First, the Grubbs II catalyst, compound 1b (3 eq.) and deuterated
chloroform solvent were heated to 50 1C (Fig. 2).
Fig. 1 Fluorescence of compound 7 under 365 nm irradiation.
14072 | Chem. Commun., 2019, 55, 14070--14073
Fig. 3 31P-NMR of compound 1a and Grubbs II catalyst.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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complex derived from the Grubbs II catalyst (Fig. 4).14
From the result of this carbide complex and 31P-NMR
described above, the Grubbs II catalyst is considered to be
rapidly converted to a carbide complex when it reacts with a
dialkylarylvinylsilane such as 1a, and loses activity as a metathesis
catalyst.7 On the other hand, when the Grubbs II catalyst reacted
with substituted dialkylarylvinylsilane, having a methyl substituent
at the beta position of the silicon atom, such as dimethylnaphthyl-
iso-propenylsilane (1b), beta-silyl elimination of the ruthenacyclo-
butane intermediate did not proceed and the expected metathesis
product was yielded probably due to the effect of steric hindrance.
Improved understanding of catalyst decomposition, particu-
larly those pathways operating for the most vulnerable active
species, is critical to guide process implementation and catalyst
redesign.
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(1c, see the ESI†) did not proceed.
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In summary, although the metathesis reaction of dialkylaryl-
vinylsilane has been difficult to date, it was found that the
metathesis reaction proceeds when using dialkylaryl-iso-propenyl-
silane, having a methyl substituent at the beta position of the silyl
atom to prevent unwanted beta-silyl elimination, as a substrate. 10 Murakami group reported RCM of vinylsilane using the Mo catalyst.
They also reported an alpha-methyl substituent at vinylsilane aided
We have also succeeded in synthesizing polycyclic silicon-
RCM using the Ru catalyst. However, no one has reported the effect
containing cyclic compounds by combining EM, Diels–Alder and
of the beta-methyl substituent on vinylsilane metathesis using the
oxidation reactions. Furthermore, our findings showed that vinyl-
silane changed the Grubbs II catalyst to a carbide catalyst and that
its catalytic activity was inactivated during metathesis.
This work was partially supported by a Grant-in-Aid from
JSPS KAKENHI for Precisely Designed Catalysts with Customized
Scaffolding (Grant No. JP 18H04260), A19J218090, T15K149760, and
Ru catalyst: (a) T. Matsuda, Y. Yamaguchi and M. Murakami, Synlett,
2008, 561–564; (b) T. Matsuda, Y. Yamaguchi, N. Ishida and
M. Murakami, Synlett, 2010, 2743–2746.
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Chem. Commun., 2017, 53, 5970–5973.
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T15KT00630, by Platform Project for Supporting Drug Discovery and 14 (a) E. F. Eide, P. E. Romero and W. E. Piers, J. Am. Chem. Soc., 2008,
130, 4485–4491; (b) S. R. Dubberley, P. E. Romero, W. E. Piers,
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15 The X-ray crystal structural data of the carbide catalyst were deposited
Life Science Research (Basis for Supporting Innovative Drug
Discovery and Life Science Research (BINDS)) from AMED under
Grant Number JP18am0101084 and JP19am0101084.
(CCDC 1938337)†.
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