Rh-Catalyzed Dehydrogenative Cyclization Leading to Benzosilolothiophene Derivatives via Si-H/C-H Bond Cleavage

Article abstract of DOI:10.1021/acs.orglett.7b00878

Straightforward syntheses leading to π-extended benzosilolothiophene (BST) derivatives by Rh-catalyzed dehydrogenative cyclization reactions have been developed. Electron-deficient ligands were effective for the reactions, and dppe-F₂₀ gave the

Full text of DOI:10.1021/acs.orglett.7b00878

Letter
pubs.acs.org/OrgLett
Rh-Catalyzed Dehydrogenative Cyclization Leading to
Benzosilolothiophene Derivatives via Si−H/C−H Bond Cleavage
Koichi Mitsudo,*,† Seiichi Tanaka,† Ryota Isobuchi,† Tomohiro Inada,† Hiroki Mandai,†
Toshinobu Korenaga,‡ Atsushi Wakamiya,§ Yasujiro Murata,§ and Seiji Suga*,†,∥
†Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka,
Kita-ku, Okayama 700-8530, Japan
‡Department of Chemistry and Bioengineering, Graduate School of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate-ken
020-8551, Japan
§Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
∥Research Center of New Functional Materials for Energy Production, Storage and Transport, Okayama University, 3-1-1
Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
S
* Supporting Information
ABSTRACT: Straightforward syntheses leading to π-extended benzosilolothiophene (BST) derivatives by Rh-catalyzed
dehydrogenative cyclization reactions have been developed. Electron-deficient ligands were effective for the reactions, and dppe-
F
20 gave the best result. This method could be applied to the synthesis of highly π-extended ladder-type BST derivatives, which
exhibited fluorescence.
enzosilolothiophene (BST, benzo[4,5]silolo[3,2-b]-
Bthiophene) derivatives constitute an important skeleton
for organic materials.1,2 For instance, Ashraf and Chen’s group
and Jen’s group individually reported that π-conjugated
polymers containing bis(benzosilolothiophene), a ladder unit
including BST skeletons, can be used as active materials in
organic solar cells.2n,p
precursor. Takai and Kuninobu reported a dehydrogenative
cyclization of biarylhydrosilanes via Si−H/C−H bond cleavage
using Wilkinson’s catalyst.5 During their study, they found that
the cyclization reaction of 2-(3-thienyl)-1-dimethylhydrosilane
proceeded regioselectively at the 2-position of thiophene to
give a regioisomer of BST (Scheme 1(iv)), but the application
of their method for the synthesis of BST derivatives has not
been reoprted.5d While several methods for the synthesis of π-
extended BST derivatives are available,6 there has been no
report on an efficient synthesis of π-extended BST derivatives
from readily available precursors such as a 2-arylthiophene
bearing a silyl group (Scheme 1(v)). This approach should have
several advantages, since it should be easy to synthesize the
precursors due to the high reactivity of the 2-position of the
thiophene unit.
Recently, we have been interested in the efficient synthesis of
π-extended heteroaromatics.7 In this study, we focused on an
efficient synthesis of π-extended BST derivatives and found a
novel catalytic system for the synthesis of BST derivatives. We
report here a novel Rh-catalyzed dehydrogenative cyclization
leading to the synthesis of BST derivatives via cleavage of the
Si−H bond and C−H bond at the 3-position of a thiophene
ring or 2-position of a phenyl group. This method could also be
The first synthesis of π-extended BST derivatives was
reported by Yamaguchi and co-workers. They synthesized
ladder-type BST derivatives by cascade-type anionic double
cyclization (Scheme 1(i)).3 Ashraf and Chen synthesized
bis(benzosilolothiophene) by the tetra-lithiation of 2,5-bis(3-
bromo-2-thienyl)-1,4-dibromobenzene and a subsequent re-
action with dichlorodi-n-octylsilane (Scheme 1(ii)).2p While
this is a typical method for synthesizing silicon-bridged π-
conjugated compounds, the yield of the product was low. Jen
used a similar method with a precursor in which the 5-positions
of thienyl groups were protected by trimethylsilyl groups.2n
Though the yield of the protected BST derivative was higher
than that of the nonprotected BST derivative, several reaction
steps were required to introduce and remove trimethylsilyl
groups. Shimizu and Hiyama reported a palladium-catalyzed
intramolecular direct arylation via C−O/C−H bond cleavage.4
During their investigation of the scope of the reactions, they
achieved the synthesis of BST derivatives (Scheme 1(iii)).4d
While this is an efficient reaction for constructing a BST
skeleton, several steps were required for synthesis of the
Received: March 24, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00878
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Organic Letters
Letter
a
Scheme 1. Synthetic Approaches for BST Derivatives
Table 1. Rh-Catalyzed Dehydrogenative Cyclization of 1a
b
b
entry
ligand
conv (%)
yield (%)
1
PPh3
70
5
2
P(p-tol)3
P(C6H4-p-CF3)3
P(t-Bu)3
P(OPh)3
dppe
54
2
3
85
20
10
56
2
4
90
5
98
6
36
7
dppp
39
2
8
dppb
48
12
46
43
68
69
83
9
dppf
>98
>98
95
10
11
12
BINAP
L1
dppe-F20
dppe-F20
dppe-F20
98
c
13
90
cd
,
e
ef
,
14
>98
88 (91) (92)
a
Reaction conditions: 1a (0.20 mmol), [Rh(cod)Cl]2 (5 mol %),
b
ligand (30 mol % of P), toluene (0.2 mL), 145 °C, 5 h. Determined
by GC analysis using 1-methylnaphthalene as an internal standard.
c
Performed with [Rh(cod)Cl]2 (1 mol %) and dppe-F20 (3 mol %).
d
e
f
15 mol % of (EtO)3SiH was added. Isolated yield. Performed with
1.0 mmol of 1a.
88%; Isolated yield: 91%). A similar yield of 2a was obtained
from 1.0 mmol of 1a (entry 14, 92% yield).
applied to the synthesis of highly π-extended BST derivatives,
and their fundamental physical properties were investigated.
First, we chose 2-[2-(diphenylsilyl)phenyl]benzo[b]-
thiophene (1a) as a model substrate and carried out
rhodium-catalyzed dehydrogenative cyclization. Takai and
Kuninobu’s conditions gave only a trace amount of the desired
product.5d We re-examined the reaction conditions as shown in
Table 1. When 1a was treated with [Rh(cod)Cl]2 (5 mol %)
and PPh3 (30 mol %) at 145 °C for 5 h, an intermolecular
reaction proceeded predominantly, and the desired BST 2a was
obtained in only 5% yield (entry 1). We tested numerous
monodentate ligands, such as an electron-rich ligand (entry 2),
an electron-deficient ligand (entry 3), and a trialkylphosphine
(entry 4), and all of them were insufficient for the reaction.
Further screening revealed that triphenylphosphite (P(OPh)3)
was an efficient monodentate ligand for the reaction (entry 5).
The use of P(OPh)3 increased the conversion of 1a and the
yield of 2a (98% conversion, 56% yield). We next optimized
the reaction with bidentate ligands. Bis(diphenylphosphino)-
alkanes, such as dppe, dppp, and dppb, were ineffective (entries
6−8). In contrast, when dppf was used as a ligand, 2a was
obtained in 46% yield (entry 9). The use of BINAP as a ligand
gave a similar result (entry 10, 43% yield). Electron-deficient
bidentate ligands gave better results. When 1,2-bis[bis(2,3,5,6-
tetrafluoro-4-trifluoromethylphenyl)phosphino]ethane (L1) or
dppe-F20 was used as a ligand, 2a was obtained in respective
yields of 68% and 69% (entries 11 and 12). The amount of
[Rh(cod)Cl]2 could be reduced to 1 mol %, and the yield of 2a
increased to 83% due to the suppression of the intermolecular
side reactions (entry 13). For this reaction, we assumed that the
active catalytic species is Rh(dppe-F20)H.8 To promote the
generation of the active Rh−H species, we next added 15 mol
% of (EtO)3SiH as an additive. As expected, the addition of
(EtO)3SiH increased the yield of 2a (entry 14, NMR yield:
To clarify the scope of the dehydrogenative cyclization, we
next synthesized several BST derivatives 2 under the optimized
conditions (Scheme 2). The cyclization of a precursor with a
dimethylsilyl group also proceeded, and BST 2b was obtained
in moderate yield (60%). In this case, the absence of
(EtO)3SiH gave a better result. While a longer reaction time
was required, BST 2c and 2d were obtained in respective yields
of 81% and 86%. The cyclization reaction of an alkylated
precursor gave alkylated BST 2e in 72% yield. Precursors
bearing an electron-donating or -withdrawing group can be
used in the reaction to afford the cyclized products 2f and 2g in
respective yields of 94% and 77%. We next tried the
dehydrogenative cyclization of precursors bearing thienothio-
phene moieties. The cyclization reactions proceeded under the
optimized conditions to give 2h and 2i in low yields. In some
cases, the addition of (EtO)3SiH was unnecessary. In particular,
using 1 bearing an electron-donating moiety, the yield of 2
decreased drastically with (EtO)3SiH (2b, 2e, and 2f). While
the reason remains unclear, we assume that some side reactions
proceed under these conditions.
We next carried out the cyclization reaction of precursors 3
which have a hydrosilyl group on their thiophene skeletons
(Scheme 3). While we can assume that the electrochemical
properties and steric conditions of the hydrosilyl groups of 3
were different from those of 1, the cyclization reaction of 3
proceeded smoothly to give the cyclized products under similar
conditions. For instance, when 3-(diphenylsilyl)-2-phenyl-
benzo[b]thiophene (precursor 3a) was treated under the
optimized conditions, dehydrogenative cyclization proceeded
smoothly to afford the desired BST 2a in 96% yield. Similarly,
the reaction of 3-(diphenylsilyl)-2-phenylthiophene (3b) gave
2c in 81% yield. For these reactions, the addition of (EtO)3SiH
was essential.
B
DOI: 10.1021/acs.orglett.7b00878
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Organic Letters
Letter
a
Scheme 2. Rh-Catalyzed Dehydrogenative Cyclization of 1
Figure 1. A plausible mechanism for dehydrogenative cyclization
leading to the synthesis of 2.
intermediate C.10 (iv) Finally, the subsequent reductive
elimination of C forms a new Si−C bond to give the desired
product 2 and regenerates active RhI−H species. The results of
the KIE experiments suggest that step (iii), oxidative addition
to generate intermediate C, is not the rate-determining step.
We also carried out preliminary DFT calculations regarding the
mechanism and found that reductive elimination to generate 2
(step (iv)) had the greatest activation energy among all of the
steps (15.8 kcal/mol). This result suggests that step (iv) might
be the rate-determining step.11 This is consistent with the fact
that electron-deficient ligands are effective in this reaction.12
This strategy could also be used for the construction of
highly π-extended BST derivatives (Scheme 5). The dehydro-
a
Reaction conditions: 1 (0.20 mmol), [Rh(cod)Cl]2 (1 mol %), dppe-
F20 (3 mol %), (EtO)3SiH (15 mol %), toluene (0.2 mL), 145 °C, 5 h.
Isolated yield. In parentheses are yields without (EtO)3SiH.
b
c
Performed for 20 h. Performed for 48 h.
Scheme 3. Rh-Catalyzed Dehydrogenative Cyclization of 3
Scheme 5. Synthesis of Highly π-Extended Ladder-Type
BSTs 5 and 7
To obtain further insight into the reaction mechanism, we
carried out a kinetic isotope effect (KIE) experiment using 1a
and 1a-d (Scheme 4) and found that the KIE (kH/kD) value was
Scheme 4. Kinetic Isotope Effect
1.13. This result suggests that cleavage of the C−H bond of
thiophene would not be the rate-determining step of this
reaction, which is different from the intramolecular direct
silylation reported by Takai and Kuninobu.5d,8
A plausible mechanism for dehydrogenative cyclization
leading to the synthesis of 2 is illustrated in Figure 1. First,
the Rh−Cl bond of the Rh catalyst would be converted to a
Rh−H bond by the reaction with hydrosilanes.9 Next, (i)
oxidative addition of 1 to the generated RhI−H species occurs
to form RhIII intermediate A. (ii) Reductive elimination of
hydrogen would give intermediate B, and (iii) subsequent
oxidative addition of the C−H bond of thiophene would form
genative cyclization of precursors 4 and 6 proceeded smoothly
to give the corresponding ladder-type BSTs 5 and 7 in the
respective yields of 92% and 75%. Solutions of 5 and 7 in
CH2Cl2 exhibited fluorescence. In particular, 7 exhibited strong
fluorescence in both the liquid and solid states (solution: λem
=
429, 455 nm, Φ = 0.81; solid: λem = 502 nm, Φ = 0.65).13
In conclusion, we have developed a Rh/dppe-F20-catalyzed
dehydrogenative cyclization leading to BSTs. This catalytic
system can be applied to cyclized precursors with a hydrosilyl
group on a phenyl or thienyl group. The dehydrogenative
cyclization of precursors bearing two hydrosilyl groups also
proceeded under the reaction conditions, which demonstrated
C
DOI: 10.1021/acs.orglett.7b00878
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Mater. 2012, 22, 1711. (m) Schroeder, B. C.; Huang, Z.; Ashraf, R. S.;
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that this reaction system could be useful for the construction of
π-extended ladder-type BSTs. Further studies on the scope of
this reaction system and the application of the obtained BSTs
are in progress in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00878.
(3) Mouri, K.; Wakamiya, A.; Yamada, H.; Kajiwara, T.; Yamaguchi,
S. Org. Lett. 2007, 9, 93.
Experimental details, photophysical and electrochemical
properties of 5 and 7, spectral data for all new
compounds, data of the KIE experiments, and theoretical
calculations (PDF)
(4) (a) Shimizu, M.; Hiyama, T. Synlett 2012, 23, 973. (b) Shimizu,
M.; Mochida, K.; Katoh, M.; Hiyama, T. J. Phys. Chem. C 2010, 114,
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(5) (a) Xiao, Q.; Meng, X.; Kanai, M.; Kuninobu, Y. Angew. Chem.,
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(d) Kuninobu, Y.; Nakahara, T.; Takeshima, H.; Takai, K. Org. Lett.
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Crystallographic data of a precursor of 6 (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: mitsudo@cc.okayama-u.ac.jp.
*E-mail: suga@cc.okayama-u.ac.jp.
(6) As another approach, Shintani and Nozaki recently reported a
Rh-catalyzed enantioselective [2 + 2 + 2] cycloaddition for the
synthesis of silicon-bridged biaryls including a BST derivative. See:
Shintani, R.; Takagi, C.; Ito, T.; Naito, M.; Nozaki, K. Angew. Chem.,
Int. Ed. 2015, 54, 1616.
(7) (a) Kamimoto, N.; Nakamura, N.; Tsutsumi, A.; Mandai, H.;
Mitsudo, K.; Wakamiya, A.; Murata, Y.; Hasegawa, J.; Suga, S. Asian J.
Org. Chem. 2016, 5, 373. (b) Mitsudo, K.; Sato, H.; Yamasaki, A.;
Kamimoto, N.; Goto, J.; Mandai, H.; Suga, S. Org. Lett. 2015, 17, 4858.
(c) Mitsudo, K.; Harada, J.; Tanaka, Y.; Mandai, H.; Nishioka, C.;
Tanaka, H.; Wakamiya, A.; Murata, Y.; Suga, S. J. Org. Chem. 2013, 78,
2763. (d) Mitsudo, K.; Shimohara, S.; Mizoguchi, J.; Mandai, H.; Suga,
S. Org. Lett. 2012, 14, 2702. (e) Mitsudo, K.; Kamimoto, N.;
Murakami, H.; Mandai, H.; Wakamiya, A.; Murata, Y.; Suga, S. Org.
Biomol. Chem. 2012, 10, 9562. (f) Mitsudo, K.; Shiraga, T.; Mizukawa,
J.; Suga, S.; Tanaka, H. Chem. Commun. 2010, 46, 9256.
ORCID
Koichi Mitsudo: 0000-0002-6744-7136
Hiroki Mandai: 0000-0001-9121-3850
Atsushi Wakamiya: 0000-0003-1430-0947
Yasujiro Murata: 0000-0003-0287-0299
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for Scientific
Research (C) (Nos. 25410042, 16K05695) from JSPS, Japan,
by the Collaborative Research Program of Institute for
Chemical Research, Kyoto University (Grant No. 2015-47),
and by JST, ACT-C, Japan.
(8) For a KIE study of intermolecular direct silylations, see: Cheng,
C.; Hartwig, J. F. J. Am. Chem. Soc. 2014, 136, 12064.
(9) (a) Esteruelas, M. A.; Olivan, M.; Velez, A. Inorg. Chem. 2013, 52,
12108. (b) Nishihara, Y.; Takemura, M.; Osakada, K. Organometallics
2002, 21, 825.
(10) As another reaction pathway through σ-complex-assisted
metathesis (σ-CAM) to form intermediate C would be considerable,
see ref 5d and 5e. For a review on σ-CAM, see: Perutz, R. N.; Sabo-
Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578.
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D
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