Synthesis of Distorted 1,8,13-Trisilyl-9-hydroxytriptycenes by Triple Cycloaddition of Ynolates to 3-Silylbenzynes

Article abstract of DOI:10.1002/chem.201903024

1,8,13-Trialkyl(aryl)silyl-9-hydroxytriptycenes (trisilyltriptycenes) were synthesized by the triple addition of ynolates and 3-silylbenzynes with high regioselectivity. Benzene rings in the resulting triptycenes were highly distorted where the dihedral a

Full text of DOI:10.1002/chem.201903024

A Journal of
Accepted Article
Title: Synthesis of Distorted 1,8,13-Trisilyl-9-hydroxytriptycenes via
Triple Cycloaddition of Ynolates to 3-Silylbenzynes
Authors: Tatsuro Yoshinaga, Takumi Fujiwara, Takayuki Iwata, and
Mitsuru Shindo
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To be cited as: Chem. Eur. J. 10.1002/chem.201903024
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Synthesis of Distorted 1,8,13-Trisilyl-9-hydroxytriptycenes via
Triple Cycloaddition of Ynolates to 3-Silylbenzynes
Tatsuro Yoshinaga,[a] Takumi Fujiwara,[a] Takayuki Iwata,[b] and Mitsuru Shindo*[b]
Abstract:
1,8,13-Trialkyl(aryl)silyl-9-hydroxytriptycenes
Silylarenes are frequently used as powerful synthetic tools in
synthetic organic chemistry because their silyl groups can be
converted into other substituents. If silylarynes could be applied
to the triple addition, the resulting triptycenes would be useful as
precursors for functionalized triptycenes. However, these
(trisilyltriptycenes) were synthesized via the triple addition of ynolates
and 3-silylbenzynes with high regioselectivity. Benzene rings in the
resulting triptycenes were highly distorted where the dihedral angles
between the substituents were as high as 35°. The distortion energy
induced step-by-step halogenation reactions to yield halogenated
triptycenes, including chiral triptycenes. The 1,8,13-trihalogenated
triptycenes were then converted to 1,8,13-functionalized triptycenes.
silylarynes have not only been used in
a few organic
syntheses.14,15 Herein, we report the selective synthesis of highly
distorted 1,8,13-trisilyltriptycenes via reaction of ynolates and 3-
silylbenzynes, and their subsequent conversion into 1,8,13-
substituted triptycenes (Scheme 1(b)).
Triptycene is a rigid and symmetric propeller shaped organic
molecule with a bicylo[2.2.2]octatriene skeleton.1 Because of
these special properties, triptycenes have been used as basic
units of host molecules in the field of molecular recognition,2
supramolecular chemistry,3 macromolecules,4 material sciences,5
and metal ligands6. However, functionalized triptycenes are not
easily obtained because major synthetic methods for triptycenes
are limited to cycloaddition of benzyne7 or benzoquinone8 to
anthracenes. Thus, it is difficult to prepare these starting materials
with specific substituents. This synthetic limitation is a critical
drawback for syntheses of functionalized designed triptycenes.
Recently, we developed a sequential triple addition reaction of
ynolates9 to arynes to form triptycenes, where three arynes are
formally condensed to an ynolate, along with cleavage of the triple
bond in one pot (Scheme 1(a)).10 When substituted arynes are
used as a substrate, functionalized and/or pre-functionalized
triptycenes can be prepared. In a previous report, we also
described the selective synthesis of 1,8,13-trimethoxytriptycenes
(X = OMe) using 3-methoxybenzyne, highlighting the secondary
orbital interactions as a major controlling factor of the selectivity
in addition to the inductive effect11 and distortion model.12 It should
be noted that the 1, 8, and 13-positions are located on the same
plane (blue triangle in Scheme 1(b)) and the substituents at these
positions are oriented vertically from the plane. Although this
unique structure could be used as a platform for functionalized
triptycenes,13 efficient synthetic methods for this type of triptycene
have rarely been reported. This triple addition method can be
applied to the short-step synthesis of functionalized triptycenes.
X
a) previous work
triple
cycloaddition
1
R
X = H, OMe
R
X
10
OLi
ynolate
R
13 HO 9
8
X
X
X
X
LiO
triptycene
"[2+2]" cycloaddition
X
[4+2] cycloaddition
X
X
X
X
OLi
R
X
OLi
OLi
R
R
"[2+2]" cycloaddition
fragmentation
b) this work
R
Si
triple
cycloaddition
R
Si
+
OLi
silylarynes
HO
Si = -SiR1R2R3
Si
Si
C
13
D
A
B
1
8
R
A
9
HO
B
C
R
Scheme 1. Triple cycloaddition reaction of benzynes to ynolates.
[a]
[b]
T. Yoshinaga, T. Fujiwara
Interdisciplinary Graduate School of Engineering Sciences
Kyushu University
6-1 Kasuga-koen, Kasuga, 816–8580 (Japan)
Dr. T. Iwata, Prof. Dr. M. Shindo*
Institute for Materials Chemistry and Engineering
Kyushu University
The readily available 1-fluoro-2-trimethylsilyl(TMS)benzene
216 was selected as a precursor for 3-trimethylsilylbenzyne
because it can be prepared using simple methods (ortholithiation
followed by -elimination). In addition, this synthesis can be
achieved without generation of reactive side products such as
alkyl halides. Initially, we examined the reaction of an ynolate,
prepared via double lithium-halogen exchange of dibromoester
117 with 3-trimethylsilylbenzyne,18 prepared by the addition of sec-
BuLi, which would be better than labile tert-BuLi or less reactive
n-BuLi, to 2 at -20 °C. After 1 h, the reaction was worked up,
6-1 Kasuga-koen, Kasuga, 816–8580 (Japan)
E-mail: shindo@cm.kyushu-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201903024
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yielding a small amount of tris(TMS)-substituted triptycene 3aa
among many side products (Table 1, entry 1). Therefore, we
decided to optimize the reaction conditions. After screening the
addition time and temperature (entries 2–5), the relatively slow
addition of sec-BuLi at 0 °C yielded the best result, providing 42%
tris-TMS-triptycene 3aa (entry 4). Remarkably, the “all-syn”
product was the most abundant, with three trimethylsilyl groups
positioned at C1, C8, and C13, all of which were oriented on the
same side of the hydroxy group at C9, as illustrated in 3aa. No
other isomers were detected, as they would be generated in small
amounts if at all. Major side products were oligomers of arenes,
derived from arynes. Thus, this reaction could be used for highly
regioselective synthesis.
As shown in Table 2, we successfully isolated ethyldimethylsilyl-
(3b), dimethylphenyl- (3c), tert-butyldimethylsilyl- (3d) and
methyldiphenylsilyl- (3e) triptycenes with high regioselectivity in
good to moderate yields as major products. Furthermore, the
extremely
sterically
congested
triethylsilyl-
(3f)
and
triisopropylsilyl- (3g) triptycenes were obtained. According to the
X-ray crystal structures of these products, all prepared triptycenes
were highly distorted because of the non-planar phenyl groups
and dissymmetrical hexagons. Particularly, the most distorted
triptycene 3g exhibited a maximum Si-C1-C9a-C9 dihedral angle
of 35.4°. The C2-C1-C9a bond angles (a) of the benzene rings
were 111.7–115.2°, approximately 5% smaller than that of
benzene, while the C1-C2-C3 bond angles (f) were 123.0–126.0°,
approximately 5% wider than those of the less distorted
triptycenes (5C-Br and 9,10-dimethyltriptycene19 in Table 2) and
benzene. The C1-C2, C1-C9a, and C9a-C4a bond lengths in the
benzene rings were 1.40–1.41 Å, slightly longer than 1.39 Å in
benzene but similar to that of a non-substituted triptycene (see SI).
These structural analyses indicated the highly distorted character
of the prepared benzene rings. The distances between O and Si
atoms were 3.05–3.32 Å, less than the sum of the van der Waals
radii of the two atoms (1.52 Å (O) + 2.10 Å (Si) = 3.62 A)
suggesting a penta-coordinate silicon character.
Table 1. Synthesis of tris(TMS)-substituted triptycene 3aa.
SiMe3
1
sec-BuLi
(5.0 equiv)
over t min
F
2a
Me
t-BuLi
(4.0 equiv)
Me3Si
O
(6.0 equiv)
Me
Br
OEt
9
T
°C, 1 h
THF
-78 °C
to 0 °C
13 HO
Br
8
Me3Si
Me3Si
1
3aa
Entry
T
t (min)
Yield (%)
Table 2. Synthesis and selected crystal structure data of the prepared
trisilyltriptycenes (see also SI).
1
2
3
4
5
-20 °C
0 °C
0 °C
0 °C
rt
5
3
23
27
dihedral
angle
30
60
3
39
42[a]
0
sec-BuLi
SiR3
(5.0 equiv)
Me
R3Si
Me
+
a°
f°
[a] 10 mmol scale, 5.0 equiv of 2a was used.
THF
0 °C, 1 h
F
OLi
O H
R3Si
R
2
This 3aa product contained three sets of sterically crowded
trimethylsilyl groups densely packed on the face of the rigid
structure of triptycene. X-ray crystal structure analysis of 3aa
indicated that the dihedral angle of Si-C1-C9a-C9 was 21.7° and
the internal angles of benzene were 115.2° and 123.4°, showing
pronounced distortion (Figure 1, Table 2). This is likely due to the
steric congestion between the TMS groups because, in contrast,
the benzene rings of the 1,8,13-non-substituted triptycenes
exhibited negligible distortion, with dihedral angles of ˂5°.19
Si
(5.0 equiv)
R
R
3
Si-O distance
Bond angles [°]
SiR3
(3)
Yield
(%)
Si-O
[Å]
Dihedral
angles [°]
a
f
SiMe3
(3aa)
42
39
3.08
21.7
115.2, 123.4,
3.14
3.14
3.24
3.06
3.11
3.12
3.29
3.29
3.29
3.20
3.23
3.32
3.13
3.14
3.19
3.05
3.13
3.17
3.08
3.19
3.29
3.21
3.21
3.27
19.3
19.3
0
115.2, 123.6
115.2, 123.6
114.8, 123.7
115.1, 123.3
115.1, 123.0
115.2, 123.1
114.8, 124.0
114.2, 125.3
115.5, 123.2
111.7, 126.0
114.9, 123.9
115.4, 123.7
115.6, 123.1
115.5, 122.8
115.2, 123.7
114.3, 124.2
115.1, 124.0
115.3, 123.5
115.0, 123.5
115.2, 123.1
114.8, 124.2
114.4, 123.4
114.8, 123.4
114.8, 123.2
121.3, 120.1
121.3, 120.1
121.5, 119.4
119.8, 120.5
SiMe2Et
(3b)
20.9
24.4
26.9
23.7
15.0
20.1
13.4
18.0
15.5
18.2
15.8
12.2
16.5
16.2
29.5
31.9
30.7
18.4
35.4
29.0
34.7
6.74
6.74
0
SiMe2Ph
(3c)
13
50
SiMe2t-Bu
(3d)
SiMePh2
(3e)
21
52
SiEt3
(3f)
Figure 1. X-ray crystallographic analysis of 3aa (a) upper view, (b) side view.
Si(i-Pr)3 (3g)
2
Br
-
-
-
-
(5C-Br)a
Encouraged by this result, we synthesized triptycenes with
more sterically hindered silyl groups via triple addition reaction.
9, 10-diMeb
0.35
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[a]Discussed below. [b]X-ray crystallographic data was obtained from
CCDC246642.
procedure (see SI) to 55%. This product represents a useful
platform for functional 1,8,13-trisubstituted-triptycenes and chiral
triptycenes with chiral centers at C9 and C10.
Ynolates with various substituents were subjected to triple
addition to 3-trismethylsilylbenzyne (Table 3). The primary alkyl
group-substituted ynolates afforded triptycenes 3ab–ad as major
products (entries 1–3). In contrast, the triple addition using
secondary and tertiary alkyl and aryl-substituted ynolates did not
afford the corresponding triptycenes, but instead anthrones 4a–c,
intermediates of the triple addition, were isolated (entries 4–6).
The structures were determined by X-ray crystallographic
analyses (see SI). This is the first report of the successful isolation
of the intermediates in this sequential reaction. The steric
hindrance of substituents in addition of silyl groups suppressed
the third cycloaddition of benzyne, supporting our proposed
mechanism.
The tri-halogenated triptycenes could be easily functionalized
(Scheme 3). The tribromo triptycene 5C-Br were subjected to
Suzuki-Miyaura coupling with arylboronic acid and vinylbnonic
acid ester, affording triaryl and triviny-triptycenes (8 and 10),
respectively. In addition, Sonogashira coupling was conducted to
afford trialkynyltriptycenes (9) in good yield.
Table 4. The selective halogenation of trisilyltriptycene 3aa.
Me
Me
X
X
halogenation
reagents
HO
HO
Table 3. Synthesis of triptycenes or anthracenones using substituted ynolates.
Me
TMS
TMS
X
TMS
TMS
5A
5B
conditions
HO
TMS
3aa
sec-BuLi
(5.0 equiv.)
over 3 min
Me
X
Me3Si
O
SiMe3
TMS
R
Me3Si
R
SiMe3
F
HO
5C
+
X
X
THF
0 °C, 1 h
HO
Me3Si
OLi
Me3Si
R
4
2a
(6.0 equiv.)
3
Reagent
(equiv.)
Yield
(%)
Entry
X
Conditions
Product
Entry
R
Et
Product
3ab
3ac
3ad
4a
yield (%)
1
2
3
4
5
6
7
8
9
Cl
Cl
Cl
Br
Br
Br
I
NCS (1.2)
NCS (4.0)
NCS (20) a
NBS (1.2)
NBS (4.0)
NBS (25) b
ICl (1.1)
60 °C, 1.5 h
60 °C, 1.5 h
60 °C, 5 h
5A-Cl
5B-Cl
5C-Cl
5A-Br
5B-Br
5C-Br
5A-I
76
71
79
86
72
98
57
52
76
1
2
3
4
5
6
39
20
26
39
20
9
Bu
Hexyl
i-Pr
t-Bu
Ph
0 °C, 1.5 h; rt,
3 h
4b
60 °C, 2 h
60 °C, 7 h
0 °C, 0.5 h
0 °C, 0.5 h
rt, 0.5 h
4c
Although the observed steric hindrance might have been
expected to inhibit their reactivity, instead we envisioned that the
strain energy may actually enhance the reactivity of these
triptycenes. Gratifyingly, this latter scenario proved to be the case.
Hence, because the halogenation reaction of silylarenes is well-
known, we examined the halogenation of the silyl-triptycenes to
create a platform for functionalization. As shown in Table 4,
halogenation reactions smoothly proceeded and by controlling the
stoichiometry of N-chlorosuccinimide (NCS), efficient and
selective synthesis of mono-, di-, and trichlorotriptycenes from
3aa was achieved (entries 1–3). Selective bromination was
accomplished in a similar manner (entries 4–6). Notably, since
mono-bromination of 3aa proceeded at room temperature or
lower, the first halogenation was much faster than the second and
third, although 3aa was more sterically hindered than the
monobromo- and dibromo 5A and 5B. This distinctive reactivity
difference (3aa > 5A > 5B) due to steric hindrance indicates the
release of distortion as a driving force for the reaction. In contrast
to chlorination and bromination, iodination with ICl was less
selective, likely because of the higher reagent reactivity.
I
ICl (2.1)
5B-I
I
ICl (4.0)
5C-I
[a]Addition in two portions. [b]Addition in three portions.
NCS
(1.2 equiv)
NBS
(2.0 equiv)
Me
Me
Me
Cl
TMS
Cl
DMF
60 °C
DMF
60 °C
HO
HO
HO
TMS
TMS
Br
TMS
4A-Cl
TMS
3aa
TMS
76%
67%
6
ICl
Me
Cl
(2.0 equiv)
CH2Cl2
rt
HO
Br
I
87%
7
3 steps 44%
3 steps 55% (chromatography-free)
To exploit this step-by-step halogenation approach, three
halogens were introduced to the triptycene, as shown in Scheme
2. The successive chlorination, bromination, and iodination of 3aa
afforded 1-bromo-8-chloro-13-iodotriptycene 7 in satisfactory
yield, which could be further improved by a chromatography-free
Scheme 2. Synthesis of chiral triptycene with chloro, bromo, and iodo
substituents.
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The regioselectivity of the addition of the ynolate and other
nucleophiles to 3-silylbenzyne can be elucidated by the steric
effect rather than – I effect, according to Tokiwa and Akai.20 The
sterically bulky nucleophiles tended to attack at C1, while C2
became electronically positive owing to its silicon substituent.
However, secondary orbital interactions and/or Si-O (ynolate)
interactions could not be dismissed because stronger driving
forces should be a prerequisite for producing these highly
distorted products with high regioselectivity.
In summary, we successfully synthesized distorted 1,8,13-
trisilyltriptycenes with high regioselectivity via ynolate-aryne triple
addition. The distortion of aromatic rings induced the step-by-step
halogenation reactions to afford selective halogenated triptycenes
including chiral triptycenes, which represent a useful platform for
constructing functionalized triptycenes. Theoretical calculations
for the regioselectivity and synthetic applications are underway.
Keywords: triptycene • ynolate• distortion • aryne •
regioselectivity
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Scheme 3. Coupling reactions of tribromotriptycene 5C-Br.
Acknowledgements
This work was partially supported by the JSPS KAKENHI Grant
Number (No. JP18H02557, JP18H04418, JP18H04624,
JP17K14449), the Asahi Glass Foundation (T.I.), Qdai-jump
Research Program Wakaba Challenge at Kyushu University (T.I.),
and the MEXT Project of “Integrated Research Consortium on
Chemical Sciences” (T.I.). This work was performed under the
Cooperative Research Program “Network Joint Research Center
for Materials and Devices.” We thank Mr. T. Matsumoto at Kyushu
University for his assistance with the X-ray crystal structure
analyses.
[20] T. Ikawa, A. Takagi, M. Goto, Y. Aoyama, Y. Ishikawa, Y. Itoh, S. Fujii, H.
Tokiwa, S. Akai, J. Org. Chem. 2013, 78, 2965–2983.
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10.1002/chem.201903024
Chemistry - A European Journal
COMMUNICATION
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COMMUNICATION
1,8,13-trisilyltriptycenes were
synthesized via the triple addition of
ynolates and 3-silylbenzynes with high
regioselectivity. Benzene rings in the
resulting triptycenes were highly
distorted where the dihedral angles
were as high as 35°. The distortion
energy induced step-by-step
halogenation reactions to yield
halogenated triptycenes, which were
converted to 1,8,13-functionalized
triptycenes.
Tatsuro Yoshinaga, Takumi Fujiwara,
Takayuki Iwata, and Mitsuru Shindo*
Page No. – Page No.
Synthesis of Distorted 1,8,13-Trisilyl-
9-hydroxytriptycenes via Triple
Cycloaddition of Ynolates to 3-
Silylbenzynes
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