Facile and Convergent Synthesis of Highly Fused Oligosiloles by Rhodium-Catalyzed Stitching Reaction

Article abstract of DOI:10.1002/ejoc.202100780

A facile and convergent synthesis of highly fused oligosiloles was achieved by utilizing the rhodium-catalyzed stitching reaction, an efficient synthetic method for ladder-type π-conjugated compounds recently developed in our group. Up to 11 rings could b

Full text of DOI:10.1002/ejoc.202100780

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Facile and Convergent Synthesis of Highly Fused Oligosiloles by
Rhodium-Catalyzed Stitching Reaction
Authors: Naoya Hamada, Tomohiro Tsuda, and Ryo Shintani
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100780
Link to VoR: https://doi.org/10.1002/ejoc.202100780
01/2020

10.1002/ejoc.202100780
European Journal of Organic Chemistry
COMMUNICATION
Facile and Convergent Synthesis of Highly Fused Oligosiloles by
Rhodium-Catalyzed Stitching Reaction
Naoya Hamada, Tomohiro Tsuda, and Ryo Shintani*
Abstract:
A
facile and convergent synthesis of highly fused
accepting oligosiloles by utilizing the rhodium-catalyzed stitching
reaction.[6]
oligosiloles was achieved by utilizing the rhodium-catalyzed stitching
reaction, an efficient synthetic method for ladder-type p-conjugated
compounds recently developed in our group. Up to 11 rings could be
fused together through the formation of 8 new rings in one synthetic
operation, and a series of oligosiloles containing up to 21 fused rings
in one direction were synthesized. Electrochemical analysis of these
compounds revealed that they could accept multiple electrons
reversibly under reducing conditions.
In our previous study, we found that Si2 could be readily
synthesized by the stitching reaction of compounds 1 and 2 under
the conditions shown in Eq. (1).[5b] This reaction could fuse 6 rings
in one dimension by forming 4 new rings, and we imagined that
longer ladder-type oligosiloles such as Si2x2, Si2x3, and Si2x4
(see Scheme 1 for the structures) could be easily synthesized by
utilizing this Si2-forming stitching reaction as the key steps with
appropriate substrates.
iPr iPr
Ladder-type p-conjugated compounds constitute
a
useful
Si
structural motif for organic materials with promising optical and/or
electronic properties based on their rigid and planar structures,
and various compounds have been actively developed to date.[1]
In the synthetic point of view, most of the reported compounds
were prepared by iterative processes composed of p-conjugation
extension steps and subsequent ladder-forming (bridge-
installation) steps, typically constructing only one or two ring-
fusions at a time.[2] Hence the synthesis can often become tedious
for long ladder-type p-conjugated compounds. As notable recent
tBu
tBu
iPr
Si
[Rh(OH)(cod)]2
(8 mol% Rh)
cod (40 mol%)
B(neop)
1 (1.3 equiv)
iPr
+
(1)
Cs2CO3 (1.5 equiv)
dioxane/H2O (50/1)
80 °C, 23 h
Br
Si
iPr
Me
iPr
Me
Si2: 74% yield
Si
(neop = OCH2CMe2CH2O)
iPr iPr
2 (0.10 M)
examples,
highly
emissive
carbon-bridged
In this study, we initially examined the synthesis of Si2x2,
consisting of 11 fused rings. As a result, the desired compound
was obtained in a relatively high yield of 61% from compounds 1
and 3 by the one-step, two-fold stitching reaction under similar
conditions for the synthesis of Si2 (Condition A) as shown in
Scheme 1a. It is important to note that compound 3 could be
prepared in 83% yield in one step using the Sonogashira coupling
reaction from known compounds (three steps from commercially
available compounds).[7] The yield of Si2x2 could be further
improved to 79% by changing the reaction system from a Rh/cod
catalyst with Cs2CO3 to a Rh/tfb catalyst with Zn(OMe)2 (Condition
B).[5b] The successful synthesis of Si2x2 led us to the synthesis of
its longer analogs. Thus, under similar conditions, Si2x3,
consisting of 16 fused rings, could be obtained in 71% yield by the
stitching reaction of Si2-containing reactants 4 and 5 (Scheme 1b),
both of which were readily prepared by using a common synthetic
intermediate.[7] Furthermore, even longer Si2x4, consisting of 21
fused rings, was also successfully synthesized in 66% yield by the
two-fold stitching reaction between 4 and 6 (Scheme 1c). All of
the compounds obtained here are stable at ambient conditions
both in the solid state and in solution. The structure of Si2x2 was
confirmed by X-ray crystallographic analysis (Figure 1),[8] and the
length of the carbon skeleton was found to be 2.11 nm whereas
1.19 nm for Si2. Although we have not been able to obtain X-ray
crystal structures for Si2x3 and Si2x4 so far, preliminary X-ray
data for Si2x4 indicated that the length of its carbon skeleton
reached ca. 3.9 nm (3.97 nm according to the DFT calculation).
oligo(phenylenevinylenes)s with up to 19 fused rings by Tsuji and
Nakamura (2012),[2a] multi-electron-donating thienoacenes with
up to 21 fused rings by Goodson, III and Yu (2016),[3] and NIR-
absorbing oligo-BODIPYs with up to 31 fused rings by Werz
(2021) were reported.[4] Although they exhibited unique and
promising physical properties, they all required long synthetic
schemes and the chemical yields became lower for the longer
derivatives (e.g., 19–32% yield in the final ladder-forming steps).
It is therefore highly desirable to devise and incorporate a more
step-economical and high-yielding synthetic process for the
preparation of long ladder-type compounds with minimum
iterative procedures by maximizing the number of ring fusions per
reaction. In this regard, as a new and efficient synthetic approach,
we recently developed a rhodium-catalyzed stitching reaction
between easily accessible, non-conjugated oligo(silylene-
ethynylene)s to give a new class of fused oligosiloles.[5] We
demonstrated that up to 10 rings could be fused together in a
single step through the formation of 8 new rings, and the resulting
compounds were found to be highly stable at ambient conditions
and exhibited reversible two-electron reduction processes. Taking
advantage of these attractive features, herein we describe a facile
and convergent synthesis of highly fused and multi-electron-
[*]
N. Hamada, Dr. T. Tsuda, Prof. Dr. R. Shintani
Division of Chemistry, Department of Materials Engineering Science
Graduate School of Engineering Science
Osaka University, Toyonaka, Osaka 560-8531 (Japan)
E-mail: shintani@chem.es.osaka-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejoc.202100780
European Journal of Organic Chemistry
COMMUNICATION
(a)
iPr iPr
tBu
Me
iPr
Si
iPr
iPr iPr
Si
iPr
iPr
Si
Si
Br
conditions
Me
+
tBu
Me
Br
Si
Si
F
iPr
iPr
F
B(neop)
Si
iPr iPr
iPr
iPr
Me
tBu
Si2x2
Condition A: 61% yield; Condition B: 79% yield
F
1 (3.0 equiv)
3 (0.05 M)
F
Condition A: [Rh(OH)(cod)]2 (20 mol% Rh), cod (100 mol%), Cs2CO3 (3.0 equiv), dioxane/H2O (50/1), 80 °C, 23 h
Condition B: [RhCl(tfb)]2 (20 mol% Rh), Zn(OMe)2 (3.0 equiv), THF, 70 °C, 20 h
tfb
(b)
iPr iPr
Me
Si
iPr
tBu
iPr
tBu
tBu
Me
iPr
Si
iPr
Si
iPr
Si
iPr Si
iPr
iPr
iPr
[RhCl(tfb)]2
(10 mol% Rh)
tBu
Br
iPr
Si
+
B(neop)
Me
Si iPr
iPr
Zn(OMe)2
(1.5 equiv)
THF, 70 °C, 20 h
Si
Si
Si
iPr
iPr
iPr
Si iPr
iPr
iPr
iPr
iPr
Me
Me
tBu
tBu
Si
iPr iPr
Si2x3: 71% yield
Me
4 (1.5 equiv)
5 (0.04 M)
(c)
iPr iPr
Cy Cy
Si
Si
tBu
iPr
iPr
tBu
Br
Me
Si
+
B(neop)
Me
Br
Si iPr
iPr
Si
Cy Cy
Me
4 (3.0 equiv)
6 (0.05 M)
tBu
tBu
Me
Cy
Me
iPr
Si
iPr
Si
iPr
Si
iPr
iPr
Cy
iPr
[RhCl(tfb)]2
(20 mol% Rh)
Si
Zn(OMe)2 (3.0 equiv)
THF, 70 °C, 20 h
Si
Si
Si
Si
iPr
iPr
iPr
Cy
iPr
iPr
Cy
iPr
Me
Me
tBu
tBu
Si2x4: 66% yield
Scheme 1. Synthesis of p-extended fused oligosiloles (a) Si2x2, (b) Si2x3, and (c) Si2x4.
(a)
Figure 1. X-ray crystal structure of Si2x2. Solvent molecule (CHCl3)
and hydrogen atoms are omitted for clarity.
(b)
With a series of new fused oligosiloles in hand, we examined
their optical and electronic properties. The UV-vis absorption
spectra of these compounds along with that of previously
synthesized Si2 are shown in Figure 2a. The broad peaks at >400
Figure 2. (a) UV-vis spectra of compounds Si2 (orange line), Si2x2
(red line), Si2x3 (purple line), and Si2x4 (blue line) at 2.4–2.5 x 10–5
M in CH2Cl2 at 25 °C. (b) Photographs of Si2, Si2x2, Si2x3, and Si2x4
(from left to right) in CH2Cl2.
nm are mainly attributable to HOMO–LUMO transitions according
to the TD-DFT calculations,[7] and the molar absorption coefficient
(e) becomes larger in the order of Si2, Si2x2, Si2x3, and Si2x4
(Table 1). The maximum wavelength of absorbance (lmax) in this
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10.1002/ejoc.202100780
European Journal of Organic Chemistry
COMMUNICATION
CV analysis of Si2x2 showed four reduction waves and the
process was reversible up to the third reduction (Table 3).[7]
Similarly, Si2x3 showed five reversible reduction waves and
Si2x4 showed six reduction waves and the process was
reversible up to the fifth reduction.[4,10,11] These electron-accepting
properties are complementary to recently reported ladder-type p-
conjugated compounds with similar lengths that can donate
multiple electrons reversibly under oxidizing conditions.[2a,3]
According to the calculated frontier orbitals of the model
compounds,[7] LUMO of Si2 mainly consists of the p* of the
polyene unit and two electrons are reversibly accepted.[5b] For
Si2x2, both LUMO and LUMO+1 are mainly derived from p* of the
polyene units, indicating that up to four electrons could be
accepted, which is consistent with the observation by the CV
analysis, although the fourth reduction was irreversible. Similarly,
Si2x3 shows the p*-derived orbitals in LUMO, LUMO+1, and
LUMO+2, and up to five, but not six, electrons were reversibly
accepted. Finally, for Si2x4, LUMO, LUMO+1, LUMO+2, and
LUMO+3 are p*-derived, and acceptance of up to eight electrons
would be expected, but it actually accepted five electrons
reversibly, and the sixth reduction wave was also observed by CV
and DPV. These results indicate that the newly synthesized Si2xn
would be potentially useful as multi-electron-accepting organic
materials.
Table 1. UV-vis absorption data for compounds Si2, Si2x2, Si2x3,
and Si2x4 (in CH2Cl2 at 25 °C).
Compound
Si2
l
max [nm] (e [104 M–1cm–1])
259 (2.8), 295 (2.9), 393 (3.5), 412 (3.8)
272 (3.3), 312 (3.8), 348 (3.6), 512 (5.0)
276 (4.9), 333 (7.2), 383 (3.4), 557 (9.5)
333 (9.6), 428 (3.7), 605 (12.1)
Si2x2
Si2x3
Si2x4
region also becomes longer in the same order, reaching 605 nm
for Si2x4, and the solution colors in CH2Cl2 are orange, red,
purple, and blue in the order of Si2, Si2x2, Si2x3, and Si2x4
(Figure 2b).
Comparison of electrochemical data of these compounds using
cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
is summarized in Tables 2 and 3. All of them showed amphoteric
redox properties, and their HOMO and LUMO energy levels were
estimated from the first oxidation and reduction potentials (Table
2). The oxidation potentials became lower in the order of Si2,
Si2x2, and Si2x3, and the reduction potentials became higher in
this order, leading to the decrease of the HOMO–LUMO energy
gaps (2,65, 2.26, 1.98 eV, respectively). On the other hand, both
the HOMO and LUMO energy levels of Si2x4 were estimated to
be almost the same as those of Si2x3, and these trends were
more or less reproduced by the DFT calculations of their model
compounds.[7]
As we previously reported, Si2 showed reversible two electron
reduction process and dianion Si22– is stabilized by the
contribution of the aromatic cyclopentadienyl anions.[9] Because
newly synthesized p-extended derivatives contain more
cyclopentadienyl moieties in their structures, it is expected that
they can accept more electrons under reducing conditions. In fact,
We also briefly examined the stability of Si2x2 and Si2x4. Both
of them were separately dissolved in toluene and heated at
100 °C for 6 days. In both cases, essentially no degradation was
observed and >98% was recovered unchanged. On the other
hand, when toluene solutions of Si2x2 and Si2x4 were separately
irradiated by UV light (365 nm, 100 W) continuously for 3 days at
room temperature, 84% of the materials were recovered
unchanged in both cases. In addition, Si2x2 was found to be
electrochemically stable even after 100 cycles of the two-electron
Table 2. Electrochemical Data of Compounds Si2, Si2x2, Si2x3, and Si2x4.
compound
Si2
Eox1/2 (V)[a,b]
0.69
Ered11/2 (V)[a,c]
–1.96
EHOMO (eV)[d,e]
–5.49 [–5.15]
–5.31 [–4.89]
–5.23 [–4.79]
–5.20 [–4.77]
ELUMO (eV)[e,f]
–2.84 [–2.19]
–3.07 [–2.47]
–3.25 [–2.58]
–3.22 [–2.67]
Eg (eV)[e,g]
2.65 [2.96]
2.24 [2.42]
1.98 [2.21]
1.98 [2.10]
Si2x2
Si2x3
Si2x4
0.51
–1.73
0.43
–1.55
0.40[h]
–1.58
[a] Values are against Fc/Fc+. [b] In CH2Cl2. [c] In THF. [d] EHOMO = – (4.8 + Eox1/2). [e] Values in brackets are calculated at the B3LYP/6-
31G(d) level of theory for model compounds where tBu and Cy groups are replaced by Me and iPr groups, respectively.[5] [f] ELUMO = – (4.8
+ Ered11/2). [g] Eg = ELUMO – EHOMO. [h] The value is obtained by differential pulse voltammetry analysis (irreversible).
Table 3. Reduction Potentials of Compounds Si2, Si2x2, Si2x3, and Si2x4.[a]
compound
Si2
Ered11/2 (V)
–1.96
Ered21/2 (V)
–2.42
Ered31/2 (V)
—
Ered41/2 (V)
—
Ered51/2 (V)
—
Ered61/2 (V)
—
Si2x2
Si2x3
Si2x4
–1.77
–2.16
–2.83
–1.98
–2.20
–3.17[b]
–2.43
–2.40
—
—
–1.55
–1.69
–2.59
–2.81
—
–1.58
–1.86
–3.12[b]
[a] Values are against Fc/Fc+ In THF. [b] The value is obtained by differential pulse voltammetry analysis (irreversible).
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J. Vázquez, D. Zhao, L. Li, W. Lo, N. Zhang, Q. Wu, B. Keller, A. Eshun,
N. Abeyasinghe, H. Banaszak-Holl, T. Goodson, III, L. Yu, Chem. Mater.
2017, 29, 6726; e) Z. Cai, N. Zhang, M. A. Awais, A. S. Filatov, L. Yu,
Angew. Chem. Int. Ed. 2018, 57, 6442; f) W. Gao, M. Zhang, T. Liu, R.
Ming, Q. An, K. Wu, D. Xie, Z. Luo, C. Zhong, F. Liu, F. Zhang, H. Yan,
C. Yang, Adv. Mater. 2018, 30, 1800052.
as well as three-electron reduction/oxidation processes by CV. In
contrast, Si2x4 gradually degraded in 10 cycles of the two-
electron reduction/oxidation process by CV, although it was found
more stable for 10 cycles of the one-electron reduction/oxidation
process.
In summary, we described a facile and convergent synthesis of
highly fused oligosiloles by utilizing the rhodium-catalyzed Si2-
forming stitching reactions. Up to 8 new rings were formed in one
synthetic operation, and the longest Si2x4 contained 21 fused
rings in one direction. Optical and electronic properties of these
compounds were also examined, demonstrating that multiple
electrons could be reversibly accepted under reducing conditions.
We are now investigating the potential utility of these compounds
as organic materials for single-molecule transistors, which will be
reported in due course.[12]
[3]
[4]
[5]
T. Zheng, Z. Cai, R. Ho-Wu, S. H. Yau, V. Shaparov, T. Goodson, III, L.
Yu, J. Am. Chem. Soc. 2016, 138, 868.
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Nozaki, J. Am. Chem. Soc. 2017, 139, 3861.
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Z. Tang, J. Mater. Chem. C 2017, 5, 7375.
[7]
[8]
See the Supporting Information for details.
Deposition
Number
2093205
contains
the
supplementary
crystallographic data for this paper. These data are provided free of
charge by the joint Cambridge Crystallographic Data Centre and
Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
Acknowledgements
Support has been provided in part by JSPS KAKENHI Grant
Number JP20H04821 (Hybrid Catalysis) and Tokuyama Science
Foundation. We thank Prof. Takeshi Naota and Prof. Shuichi
Suzuki at Osaka University for the measurement of high-
resolution mass spectra.
[9]
R. Takano, R. Shintani, A. Fukazawa, S. Yamaguchi, K. Nakabayashi, S.
Ohkoshi, K. Nozaki, Organometallics 2017, 36, 2646.
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compounds: a) T. W. Chamberlain, E. S. Davies, A. N. Khlobystov, N. R.
Champness, Chem. Eur. J. 2011, 17, 3759; b) A. Mitrovic, J. Stevanovic,
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See also: e) T. Lei, X. Xia, J.-Y. Wang, C.-J. Liu, J. Pei, J. Am. Chem.
Soc. 2014, 136, 2135.
Keywords: Electron acceptor • Ladder compound • Oligosiloles
• Rhodium • Stitching reaction
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10.1002/ejoc.202100780
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COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 2:
COMMUNICATION
Naoya Hamada, Tomohiro Tsuda, Ryo
Shintani*
Page No. – Page No.
Facile and Convergent Synthesis of
Highly Fused Oligosiloles by
Rhodium-Catalyzed Stitching
Reaction
• Convergent and high yielding synthesis
• Stable and multi-electron accepting
A facile and convergent synthesis of highly fused oligosiloles was achieved by
utilizing the rhodium-catalyzed stitching reaction, and up to 11 rings could be fused
together through the formation of 8 new rings in one synthetic operation.
Electrochemical analysis of these compounds revealed that they could accept
multiple electrons reversibly under reducing conditions.
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