Synthesis of Pyridine-Fused Siloles by Palladium-Catalyzed Intramolecular Bis-Silylation

Abstract

Silole derivatives are attracting significant attention as new functional materials with excellent electronic and photophysical properties. Thus, the development of synthesis methods to afford such derivatives is highly desirable. Herein, the synthesis of pyridine-fused siloles under the conditions of the Sonogashira coupling reaction is described. The reactions of 2-bromo-3-(pentamethyldisilanyl)pyridine (1) with ethynylbenzene derivatives in the presence of PdCl2(PPh3)2-CuI as a catalyst afforded the corresponding pyridine-fused siloles (2a-2c) through intramolecular trans-bis-silylation. DFT calculations were also performed to understand the reaction mechanism. This paper is the first to report on the successful use of palladium catalysts in the trans-bis-silylation of alkynes with disilanes.

Full text

Synthesis of Pyridine-Fused Siloles by Palladium-Catalyzed
Intramolecular Bis-Silylation
Akinobu Naka,*Natsumi Shimomura, and Hisayoshi Kobayashi*
Cite This: ACS Omega 2022, 7, 30369−30375
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sı Supporting Information
ABSTRACT: Silole derivatives are attracting significant attention as
new functional materials with excellent electronic and photophysical
properties. Thus, the development of synthesis methods to aord such
derivatives is highly desirable. Herein, the synthesis of pyridine-fused
siloles under the conditions of the Sonogashira coupling reaction is
described. The reactions of 2-bromo-3-(pentamethyldisilanyl)pyridine
(1) with ethynylbenzene derivatives in the presence of PdCl2(PPh3)2-CuI as a catalyst aorded the corresponding pyridine-fused
siloles (2a−2c) through intramolecular trans-bis-silylation. DFT calculations were also performed to understand the reaction
mechanism. This paper is the first to report on the successful use of palladium catalysts in the trans-bis-silylation of alkynes with
disilanes.
■INTRODUCTION
Silicon-containing compounds are used in various fields
ranging from synthetic organic chemistry to functional
materials and pharmaceutical synthesis. Thus, new synthetic
methods for such compounds have been developed.
1−5
Many
articles have reported on the transition metal-catalyzed bis-
silylation of alkynes with disilane.
6−13
Some examples of cis-
bis-silylation of internal alkynes with acyclic disilanes have also
been reported (Scheme 1). For example, Ozawa et al. reported
the bis-silylation of Me3SiSiF2Ph with internal alkynes using a
Pd catalyst.
14
Spencer et al. investigated the intermolecular bis-
silylation of internal alkynes with hexamethyldisilane using the
[(NHC)2Pd(SiMe3)2] (NHC: N-heterocyclic carbene) com-
plex as a precatalyst.
15
Zhao et al. reported nickel-catalyzed bis-
silylation of internal alkynes with an unsymmetrical coordinat-
ing disilane, 8-(2-substituted-1,1,2,2-tetramethyldisilanyl)-
quinoline.
6
Some examples of intramolecular versions have also been
described, which allow access to heterocycles with dierent
ring sizes. In 2012, Matsuda and Ichioka developed a selective
synthesis of four or five silylated rings.
16
They investigated
whether regioselectivity depends on the catalyst used and
found that when (2-alkynylphenyl)disilane was catalyzed by a
Pd/isocyanide system, cis-bis-silylation occurred to form four-
membered ring compounds. In contrast, rhodium-catalyzed
reactions of (2-alkynylphenyl)disilane aorded five-membered
ring compounds, that is, 3-silyl-1-benzosiloles, via trans-bis-
silylation (Scheme 2).
Recently, we reported the synthesis of pyridine-fused siloles
using ruthenium-catalyzed hydrosilylation and their optical
properties.
17
Silole derivatives are attracting attention as new
functional materials with excellent electronic and photo-
physical properties for various applications, such as organic
light-emitting diodes (OLEDs), photovoltaic devices, and
semiconductors.
18−23
Herein, we report the synthesis of
pyridine-fused siloles under the conditions of the Sonogashira
coupling reaction.
24
These reactions are expected to proceed
through the trans-bis-silylation of 3-(1,1,2,2,2-pentamethyldi-
silanyl)-2-(arylethynyl)pyridine. Notably, palladium catalysts
have never been utilized in the trans-bis-silylation of alkynes
with disilanes before.
Received: June 10, 2022
Accepted: August 8, 2022
Published: August 16, 2022
Scheme 1. Bis-Silylation of Internal Alkynes
Article
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© 2022 The Authors. Published by
American Chemical Society 30369
https://doi.org/10.1021/acsomega.2c03637
ACS Omega 2022, 7, 30369−30375

■RESULTS AND DISCUSSION
The reaction of 2,3-dibromopyridine with i-PrMgCl at room
temperature followed by quenching with chloropentamethyl-
disilane aorded 2-bromo-3-(pentamethyldisilanyl)pyridine
(1) in 70% yield.
25
To obtain 2-phenylethynyl-3-
(pentamethyldisilanyl)pyridine, 1was reacted with phenyl-
acetylene under the Sonogashira coupling reaction conditions.
When a mixture of 1and ethynylbenzene in the presence of a
PdCl2(PPh3)2-CuI catalyst was heated to reflux in triethyl-
amine, an unexpected product, 1,1-dimethyl-2-phenyl-3-
(trimethylsilyl)-1H-silolo(3,2-b)pyridine (2a), was obtained
in 15% isolated yield, along with intermolecular bis-silylation
product 3a (8% yield) from the reaction of 2-phenylethynyl-3-
(pentamethyldisilanyl)pyridine with phenylacetylene (Scheme
3). Compound 2a was produced via intramolecular trans-bis-
silylation. Many unidentified products were also detected in
the reaction mixture by gas liquid chromatography (GLC) and
gel permeation chromatography (GPC). Song et al. reported
an unsymmetrical disilane by attaching a coordinating group to
one of the two silicon atoms, which was successfully applied to
the Pd-catalyzed bis-silylation of terminal alkynes.
26
Compound 2a was isolated by column chromatography, and
its structure was verified by spectrometric analyses. The mass
spectrum for 2a shows a parent ion at m/z 309 corresponding
to the calculated molecular weight of C18H23Si2N. The 1H
NMR spectrum for 2a shows singlets at 0.04 and 0.29 ppm,
representing methyl protons on the silicon atoms, three
doublets of doublet signals at 7.00, 7.76, and 8.49 ppm,
ascribed to the pyridyl protons, and signals corresponding to
the phenyl protons. The 29Si NMR spectrum for 2a shows
signals at −6.2 and 3.7 ppm.
We carried out the Pd(PPh3)2Cl2-catalyzed reaction of 1
with ethynylbenzene in the absence of CuI. Although many
unidentified products were detected in the reaction mixture by
GLC and GPC, compound 2a was not detected in the reaction
mixture. We believe that a copper catalyst is necessary for
coupling reactions.
A similar reaction of 1with 4-ethynyltoluene under the same
conditions aorded 1,1-dimethyl-2-(p-tolyl)-3-(trimethylsilyl)-
1H-silolo(3,2-b)pyridine (2b) in 21% yield, in addition to
compound 3b (7% yield), which is analogous to 3a. Many
unidentified products were also detected in the reaction
mixture by GLC and GPC. The structures of 2b and 3b were
verified by NMR spectroscopy and mass spectrometry.
3-Ethynyltoluene was also reacted with 1in the presence of
the PdCl2(PPh3)2-CuI catalyst to obtain 1,1-dimethyl-2-(m-
tolyl)-3-(trimethylsilyl)-1H-silolo(3,2-b)pyridine (2c) in 16%
isolated yield. NMR and MS analyses showed the existence of
bis-silylation product 3c (3% yield) obtained from the reaction
of 3-pentamethyldisilanyl-2-(m-tolylethynyl)pyridine with 3-
ethynyltoluene.
Scheme 2. Regioselective Intramolecular Bis-Silylation of
Aryldisilane and Alkynes
Scheme 3. Palladium-Catalyzed Reaction of 1 with Ethynylbenzene Derivatives
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When 1was heated with 1-chloro-4-ethynylbenzene and 2-
ethynyl-1,3,5-trimethylbenzene under the same conditions,
neither 1H-silolo(3,2-b)pyridine derivatives such as 2a−2c nor
the products analogous to 3a−3c were detected by NMR
spectroscopy. However, many unidentified products were
detected in the reaction mixture by GLC and GPC.
In the proposed reaction mechanism to obtain 2, cyclic
products were formed with the catalyst, as shown in Scheme 4.
First, compound Ais produced by the Sonogashira coupling
reaction of 1with ethynylbenzene derivatives, followed by
oxidative addition of Si−Si bonds to produce Pd complex B.
Complex Bfurther undergoes Si−C bond formation to deliver
intermediate C. Complex Cthen undergoes ring expansion to
the five-membered cyclic complex D, followed by reductive
elimination to aord bis-silylated product 2and regenerate the
Pd catalyst.
Next, we carried out a similar reaction of 1with
ethynyltrimethylsilane in the presence of a catalytic amount
of PdCl2(PPh3)2-CuI. When compound 1was stirred under
reflux for 12 h in triethylamine in the presence of a catalytic
amount of the Pd complex, 3-(1,1,2,2,2-pentamethyldisilanyl)-
2-(trimethylsilylethynyl)pyridine (4) was obtained in 17%
isolated yield (Scheme 5). No 1,1-dimethyl-2,3-bis-
(trimethylsilyl)-1H-silolo(3,2-b)pyridine (2d) analogous to
2a−2c was detected in the reaction mixture. A similar reaction
of 1with 3,3-dimethyl-1-butyne aorded 2-(3,3-dimethylbut-1-
yn-1-yl)-3-(1,1,2,2,2-pentamethyldisilanyl)pyridine (5) in 5%
yield. Many unidentified products were detected in the
reaction mixture by GLC and GPC. Bulky substituents,
which prevented the formation of 4and 5, explain their low
yields.
27
■THEORETICAL STUDY
DFT calculations were performed to investigate the energy and
structural changes in the synthesis route from 3-(1,1,2,2,2,-
pentamethyldisilanyl)-2-(phenylethynyl)pyridine (6) to trans-
adduct 2a through cis-adduct (Z)-7,7-dimethyl-8-(phenyl-
(trimethylsilyl)methylene)-2-aza-7-silabicyclo(4.2.0)octa-1,3,5-
triene (7,Scheme 6). The Gaussian 09 program package
28
was
employed along with Becke’s three-parameter Lee−Yang−Parr
hybrid functional.
29
The Los Alamos eective core potentials
30
and the Dunning/Huzinaga full double zeta basis sets
31
were
used for the Pd atom, while the 6-311G(d) basis sets were
used for the H, C, N, Si, and P atoms.
The Pd catalyst was modeled using the Pd(PPh3) complex,
which formed when three PPh3ligands detach from Pd(PPh3)4
owing to steric hindrance. Two reaction mechanisms were
expected: (1) a parallel mechanism from 6to cis-adduct 7and
trans-adduct 2a and (2) a sequential mechanism in which cis-
adduct 7is first formed from 6, and then 7is converted to 2a.
However, only the latter reaction route could be determined
by the DFT calculations. Figure 1 shows the energy change
along the reaction coordinate which consists of seven
transition states (TSs) and eight local minima (LMs). LM0
is 6, LM4 is cis-adduct 7, and LM7 is trans-adduct 2a.
Individual structures are shown in the Supporting Information.
The formation of TS6 is the rate-determining step with an
activation energy of 161 kJ mol−1. Although the activation
energy was higher for cis-to-trans conversion than that for the
formation of cis-adduct 7,trans-adduct 2a was more stable by
91.4 kJ mol−1. If the activation energy of 161 kJ mol−1was not
a significant barrier under the present reaction conditions, the
dierence between the stability would be the driving force for
the formation of 2a.
We also carried out DFT calculations for the production of
trans-adducts 2a in the absence of a ligand on the palladium
atom. The results revealed that the TS was the rate-
determining step with an activation energy of 215.3 kJ mol−1.
Scheme 4. Proposed Reaction Mechanism for the Production of 2
Scheme 5. Palladium-Catalyzed Reactions of 1 with
Ethynyltrimethylsilane and 3,3-Dimethyl-1-butyne
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■CONCLUSIONS
We investigated the reactions of 2-bromo-3-
(pentamethyldisilanyl)pyridine (1) with terminal alkynes in
the presence of PdCl2(PPh3)2-CuI as a catalyst. The reactions
of 1with ethynylbenzene derivatives provided the correspond-
ing ring compounds (2a−2c) via intramolecular bis-silylation.
Similar reactions of 1with ethynyltrimethylsilane and 3,3-
dimethyl-1-butyne aorded 2-ethynyl-3-
(pentamethyldisilanyl)pyridine derivatives 4and 5, respec-
tively. DFT calculations were performed to rationalize the
formation of 1,1-dimethyl-2-phenyl-3-(trimethylsilyl)-1H-
silolo(3,2-b)pyridine 2a via cis-bis-silylation adduct 7. These
reactions comprise the palladium-catalyzed trans-bis-silylation
of internal alkynes. To the best of our knowledge, this is the
first report on the use of palladium-containing catalysts in such
reactions.
■METHODS
General Procedure. All reactions of 1were carried out
under an inert atmosphere using dry nitrogen. NMR spectra
were recorded on a JMN-ECS400 spectrometer. Low-
resolution mass spectrometry was performed on a JEOL
JMS-700 mass spectrometer. High-resolution mass spectrom-
etry (HR-MS) was performed on a JEOL JMS-700 mass
spectrometer and a Thermo Scientific LTQ Orbitrap XL
hybrid Fourier-transform mass spectrometer using electrospray
ionization. Column chromatography was performed using a
silica gel column (Wakogel C-300; Wako Pure Chemical
Industries).
Preparation of 2-Bromo-3-(1,1,2,2,2-
Pentamethyldisilanyl)Pyridine (1). In a 300 mL three-
necked flask fitted with a stirrer, reflux condenser, and
dropping funnel, 10.008 g (42.2 mmol) of 2,3-dibromopyr-
idine was added to 100 mL of dry tetrahydrofuran (THF).
Next, a THF solution comprising 21.1 mL (42.2 mmol) of 2.0
M isopropyl magnesium chloride was added dropwise at room
temperature. The mixture was then stirred for 1 h at room
temperature, and 7.047 g (42.2 mmol) of chloropentamethyl-
disilane was added. The resulting mixture was stirred for 6 h. It
was then hydrolyzed, after which the organic layer was
separated, washed with water, and dried over anhydrous
magnesium sulfate. The solvent was evaporated, and the
residue was chromatographed on a silica gel column eluting
with hexane to obtain 8.474 g (70% yield) of 2-bromo-3-
(1,1,2,2,2-pentamethyldisilanyl)pyridine 1: HR-MS: calcd for
C10H19NSi2Br: (M + H+): 288.02339, found: 288.02411. MS
m/z 287 (M+); 1H NMR δ(CDCl3) 0.13 (s, 9H, Me3Si), 0.43
(s, 6H, Me2Si), 7.22 (dd, 1H, pyridyl-ring proton, J= 7.2, 5.2
Hz), 7.65 (dd, 1H, pyridyl-ring proton, J= 7.2, 2.4 Hz), 8.29
(dd, 1H, pyridyl-ring proton, J= 5.2, 2.4 Hz); 13C NMR
δ(CDCl3)−3.0 (Me2Si), −1.4 (Me3Si), 122.3, 139.4, 144.9,
149.6, 149.9 (pyridyl-ring carbons); 29Si NMR δ(CDCl3)
−18.1, −17.8.
Palladium-Catalyzed Reaction of 1 with Ethynylben-
zene. In a 100 mL two-necked flask fitted with a reflux
condenser, 1(2.014 g, 6.99 mmol), bis(triphenylphosphine)-
dichloropalladium (0.244 g, 0.348 mmol), and copper(I)
iodide (0.067 g, 0.352 mmol) were added to 25 mL of dry
triethylamine. To this mixture, ethynylbenzene (1.491 g, 13.9
Scheme 6. Synthesis Route of Compound 6 via 7 to Form Compound 2a
Figure 1. Energy diagrams for the transition of compound 6via 7to form compound 2a at the B3LYP/6-311G(d) level of theory.
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mmol) was added dropwise at room temperature, after which
the mixture was heated to reflux for 12 h. The solvent was then
evaporated, and the residue was chromatographed on a silica
gel column eluting with hexane-ethyl acetate (5:1) to obtain
0.303 g (15% yield) of 2a and 0.241 g (8% yield) of 3a. For 2a:
HR-MS: calcd for C18H24NSi2(M + H+): 310.14418, found:
310.14465. MS m/z 309 (M+); 1H NMR δ(CDCl3) 0.04 (s,
9H, Me3Si), 0.29 (s, 6H, Me2Si), 7.00 (dd, 1H, pyridyl-ring
proton, J= 6.8 Hz, 5.2 Hz), 7.05 (dd, 2H, phenyl-ring protons,
J= 7.2 Hz, 1.6 Hz), 7.23 (tt, 1H, phenyl-ring proton, J= 7.2
Hz, 1.6 Hz), 7.32 (t, 2H, phenyl-ring protons, J= 7.2 Hz), 7.76
(dd, 1H, pyridyl-ring proton, J= 6.8 Hz, 2.0 Hz), 8.49 (dd, 1H,
pyridyl-ring proton, J= 5.2 Hz, 2.0 Hz); 13C NMR δ(CDCl3)
−4.9 (Me2Si), 0.9 (Me3Si), 120.2, 125.9, 126.6, 127.9, 131.3,
138.7, 142.8, 149.5, 158.9, 165.9, 171.7 (phenyl, pyridyl-ring
and olefinic carbons); 29Si NMR δ(CDCl3)−6.2, 3.7. For 3a:
HR-MS: calcd for C26H30NSi2(M + H+), 412.19113; found,
412.19119. MS m/z 411 (M+); 1H NMR δ(CDCl3)−0.11 (s,
3H, MeSi), 0.19 (s, 9H, Me3Si), 0.28 (s, 3H, Me2Si), 6.79−
6.83 (m, 2H, phenyl-ring protons), 6.89 (s, 1H, HC�C),
7.06−7.08 (m, 3H, phenyl-ring protons), 7.17 (dd, 1H,
pyridyl-ring proton, J= 7.4, 4.8 Hz), 7.29−7.31 (m, 3H,
phenyl-ring protons), 7.41−7.44 (m, 2H, phenyl-ring protons),
8.12 (dd, 1H, pyridyl-ring protons, J= 7.4 Hz, 2.0 Hz), 8.70
(dd, 1H, pyridyl-ring proton, J= 4.8 Hz, 2.0 Hz); 13C NMR
δ(CDCl3)−1.1 (Me3Si), 0.1, 0.5 (MeSi), 93.7, 106.7 (sp
carbons), 120.2, 123.0, 127.5, 128.0, 128.2, 128.5, 128.6, 129.5,
131.9, 137.2, 138.7, 144.8, 148.2, 150.2, 166.5 (phenyl, pyridyl-
ring, and olefinic carbons); 29Si NMR δ(CDCl3)−20.6, −1.2.
Palladium-Catalyzed Reaction of 1 with 4-Ethynylto-
luene. In a 100 mL two-necked flask fitted with a reflex
condenser, 1(2.145 g, 7.44 mmol), bis(triphenylphosphine)-
dichloropalladium (0.263 g, 0.375 mmol), and copper(I)
iodide (0.066 g, 0.347 mmol) were added to 25 mL of dry
triethylamine. To this mixture, 4-ethynyltoluene (1.728 g, 14.9
mmol) was added dropwise at room temperature. The mixture
was heated to reflux for 12 h. The solvent was then evaporated,
and the residue was chromatographed on a silica gel column
eluting with hexane-ethyl acetate (10:1) to obtain 0.585 g
(21% yield) of 2b and 0.244 g (7% yield) of 3b. For 2b: HR-
MS: calcd for C19H26NSi2(M + H+), 324.15983; found,
324.16043. MS m/z 323 (M+); 1H NMR δ(CDCl3) 0.05 (s,
9H, Me3Si), 0.28 (s, 6H, Me2Si), 2.32 (s, 3H, CH3), 6.94 (d,
2H, phenylene-ring protons, J= 8.0 Hz), 6.99 (dd, 1H, pyridyl-
ring proton, J= 7.4 Hz, 4.8 Hz), 7.12 (d, 2H, phenylene-ring
protons, J= 8.0 Hz), 7.75 (dd, 1H, pyridyl-ring proton, J= 7.4
Hz, 2.0 Hz), 8.48 (dd, 1H, pyridyl-ring proton, J= 4.8 Hz, 2.0
Hz); 13C NMR δ(CDCl3)−4.9 (Me2Si), 1.0 (Me3Si), 21.2
(CH3), 120.1, 126.5, 128.6, 131.3, 135.5, 138.7, 139.7, 149.5,
158.6, 166.1, 171.7 (phenylene, pyridyl-ring, and olefinic
carbons); 29Si NMR δ(CDCl3)−6.4, 3.3. For 3b: HR-MS:
calcd for C28H33NSi2(M+), 439.2152; found, 439.2156. MS
m/z 439 (M+); 1H NMR δ(CDCl3)−0.10 (s, 3H, MeSi), 0.17
(s, 9H, Me3Si), 0.28 (s, 3H, Me2Si), 2.20 (s, 3H, Me), 2.34 (s,
3H, Me), 6.68 (d, 2H, phenylene-ring protons, J= 8.0 Hz),
6.84 (s, 1H, HC�C), 6.87 (d, 2H, phenylene-ring protons, J=
8.0 Hz), 7.10 (d, 2H, phenylene-ring protons, J= 8.0 Hz), 7.16
(dd, 1H, pyridyl-ring proton, J= 7.6, 4.8 Hz), 7.31 (d, 2H,
phenylene-ring protons, J= 8.0 Hz), 8.12 (dd, 1H, pyridyl-ring
protons, J= 7.6 Hz, 2.0 Hz), 8.69 (dd, 1H, pyridyl-ring proton,
J= 4.8 Hz, 2.0 Hz); 13C NMR δ(CDCl3)−1.1 (Me3Si), 0.2,
0.5 (MeSi), 21.0, 21.4 (Me), 92.2, 106.9 (sp carbons), 119.8,
120.1, 128.5, 128.6 (2C), 128.9, 129.3, 131.7, 134.4, 137.2,
138.7, 144.8, 146.7, 150.0, 166.6 (phenyl, pyridyl-ring, and
olefinic carbons); 29Si NMR δ(CDCl3)−20.8, −1.4.
Palladium-Catalyzed Reaction of 1 with 3-Ethynylto-
luene. In a 100 mL two-necked flask fitted with a reflex
condenser, 1(2.020 g, 7.01 mmol), bis(triphenylphosphine)-
dichloropalladium (0.248 g, 0.353 mmol), and copper(I)
iodide (0.069 g, 0.362 mmol) were added to 25 mL of dry
triethylamine. To this mixture, 3-ethynyltoluene (1.677 g, 14.4
mmol) was added dropwise at room temperature. The mixture
was heated to reflux for 12 h. The solvent was then evaporated,
and the residue was chromatographed on a silica gel column
eluting with hexane-ethyl acetate (20:1) to obtain 0.420 g
(16% yield) of 2c and 0.107 g (3% yield) of 3c. For 2c: HR-
MS: calcd for C19H26NSi2(M + H+), 324.15983; found,
324.16028. MS m/z 323 (M+); 1H NMR δ(CDCl3) 0.04 (s,
9H, Me3Si), 0.29 (s, 6H, Me2Si), 2.36 (s, 3H, CH3), 6.84 (d,
1H, phenylene-ring proton, J= 7.2 Hz), 6.86 (s, 1H,
phenylene-ring proton), 7.00 (dd, 1H, pyridyl-ring proton, J
= 7.2, 4.8 Hz), 7.04 (d, 1H, phenylene-ring proton, J= 7.2
Hz), 7.20 (t, 1H, phenylene-ring proton, J= 7.2 Hz), 7.75 (dd,
1H, pyridyl-ring proton, J= 7.2, 1.6 Hz), 8.49 (dd, 1H, pyridyl-
ring proton, J= 4.8, 1.6 Hz); 13C NMR δ(CDCl3)−4.8
(Me2Si), 1.0 (Me3Si), 21.5 (CH3), 120.2, 123.7, 126.7, 127.3,
127.8, 131.3, 137.3, 138.7, 142.7, 149.5, 158.6, 166.1, 171.7
(phenylene, pyridyl-ring, and olefinic carbons); 29Si NMR
δ(CDCl3)−6.4, 3.5. For 3c: HR-MS: calcd for C28H33NSi2
(M+), 439.2152; found, 439.2155. MS m/z 439 (M+); 1H
NMR δ(CDCl3)−0.14 (s, 3H, MeSi), 0.19 (s, 9H, Me3Si),
0.29 (s, 3H, Me2Si), 2.10 (s, 3H, Me), 2.32 (s, 3H, Me), 6.59
(s, 1H, phenylene-ring proton), 6.60 (d, 1H, phenylene-ring
proton, J= 8.0 Hz), 6.85 (s, 1H, HC = C), 6.89 (d, 1H,
phenylene-ring proton, J= 8.0 Hz), 6.96 (t, 1H, phenylene-
ring proton, J= 8.0 Hz), 7.13 (d, 1H, phenylene-ring proton, J
= 8.0 Hz), 7.17 (dd, 1H, pyridyl-ring proton, J= 7.4, 5.2 Hz),
7.18 (t, 1H, phenylene-ring proton), 7.24 (d, 1H, phenylene-
ring proton, J= 8.0 Hz), 7.25 (s, 1H, phenylene-ring proton),
8.13 (dd, 1H, pyridyl-ring protons, J= 7.4 Hz, 2.0 Hz), 8.70
(dd, 1H, pyridyl-ring proton, J= 5.2 Hz, 2.0 Hz); 13C NMR
δ(CDCl3)−1.1 (Me3Si), 0.1, 0.5 (MeSi), 21.16, 21.23 (Me),
92.7, 106.8 (sp carbons), 120.1, 122.7, 126.5, 127.8, 128.1,
128.2, 128.6, 128.9, 129.5, 130.3, 132.4, 137.0, 137.3, 137.8,
138.9, 144.8, 147.8, 150.1, 166.6 (phenyl, pyridyl-ring, and
olefinic carbons); 29Si NMR δ(CDCl3)−20.6, −1.3.
Preparation of 3-(1,1,2,2,2-Pentamethyldisilanyl)-2-
(Trimethylsilylethynyl)pyridine (4). In a 300 mL three-
necked flask fitted with a stirrer, reflux condenser, and
dropping funnel, 1(2.007 g, 6.96 mmol), bis-
(triphenylphosphine)dichloropalladium (0.246 g, 0.351
mmol), and copper(I) iodide (0.067 g, 0.352 mmol) were
added to 25 mL of dry triethylamine. To this mixture,
ethynyltrimethylsilane (0.784 g, 7.98 mmol) was added
dropwise at room temperature. The mixture was heated to
reflux for 12 h. The solution was then hydrolyzed, and the
organic layer was separated, washed with water, and dried over
anhydrous magnesium sulfate. The solvent was then
evaporated, and the residue was chromatographed on a silica
gel column eluting with hexane-ethyl acetate (10:1) to obtain
0.326 g (17% yield) of 4: HR-MS: calcd for C15H28NSi3: (M +
H+)m 306.15241; found, 306.15237. MS m/z 305 (M+); 1H
NMR δ(CDCl3) 0.09 (s, 9H, Me3Si), 0.27 (s, 9H, Me3Si), 0.45
(s, 6H, Me2Si), 7.17 (dd, 1H, pyridyl-ring proton, J= 7.6, 5.2
Hz), 7.70 (dd, 1H, pyridyl-ring proton, J= 7.6, 2.0 Hz), 8.51
(dd, 1H, pyridyl-ring proton, J= 5.2, 2.0 Hz); 13C NMR
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δ(CDCl3)−3.7 (Me2Si), −1.5, −0.3 (Me3Si), 96.6, 106.1 (sp
carbons), 122.4, 137.3, 142.0, 147.4, 149.4 (pyridyl-ring
carbons); 29Si NMR δ(CDCl3)−20.7, −17.2, −16.6.
Preparation of 2-(3,3-Dimethylbut-1-yn-1-yl)-3-
(1,1,2,2,2-Pentamethyldisilanyl)pyridine (5). In a 300
mL three-necked flask fitted with a stirrer, reflux condenser,
and dropping funnel, 1(2.061 g, 7.15 mmol), bis-
(triphenylphosphine)dichloropalladium (0.246 g, 0.351
mmol), and copper(I)iodide (0.068 g, 0.357 mmol) were
added to 50 mL of dry triethylamine. To this mixture, 3,3-
dimethyl-1-butyne (0.672 g, 8.18 mmol) was added dropwise
at room temperature. The mixture was heated to reflux for 12
h. The solution was then hydrolyzed, and the organic layer was
separated, washed with water, and dried over anhydrous
magnesium sulfate. The solvent was then evaporated, and the
residue was chromatographed on a silica gel column eluting
with hexane-ethyl acetate (10:1) to obtain 0.101 g (5% yield)
of 5: HR-MS: calcd for C16H28NSi2: (M + H+), 290.17548;
found, 290.17584. MS m/z 289 (M+); 1H NMR δ(CDCl3)
0.09 (s, 9H, Me3Si), 0.45 (s, 6H, Me2Si), 1.35 (s, 9H, t-Bu),
7.11 (dd, 1H, pyridyl-ring proton, J= 7.6, 5.2 Hz), 7.66 (dd,
1H, pyridyl-ring proton, J= 7.6, 2.0 Hz), 8.48 (dd, 1H, pyridyl-
ring proton, J= 5.2, 2.0 Hz); 13C NMR δ(CDCl3)−3.6
(Me2Si), −1.6 (Me3Si), 28.0 (CMe3), 30.6 (Me3C), 81.5, 99.9
(sp carbons), 121.6, 136.1, 142.1, 148.2, 149.4 (pyridyl-ring
carbons); 29Si NMR δ(CDCl3)−20.5, −17.5.
■ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsomega.2c03637.
NMR spectra, optimized structures for all LMs and TSs,
and high-resolution mass spectra (PDF)
■AUTHOR INFORMATION
Corresponding Authors
Akinobu Naka −Department of Life Science, Kurashiki
University of Science and the Arts, Kurashiki, Okayama 712-
8505, Japan; orcid.org/0000-0003-0019-9104;
Email: anaka@chem.kusa.ac.jp
Hisayoshi Kobayashi −Professor Emeritus, Kyoto Institute of
Technology, Kyoto 606-8585, Japan; Email: hisabbit@
yahoo.co.jp
Author
Natsumi Shimomura −Department of Life Science, Kurashiki
University of Science and the Arts, Kurashiki, Okayama 712-
8505, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.2c03637
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
We are grateful for grants to A.N. from the Yakumo
Foundation for Environmental Science and WESCO Science
Promotion Foundation. We thank Amimoto, the Natural
Science Center for Basic Research and Development (N-
BARD), and Hiroshima University for the HR-MS measure-
ment.
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