Rhodium-catalyzed synthesis of silafluorene derivatives via cleavage of silicon-hydrogen and carbon-hydrogen bonds

Article abstract of DOI:10.1021/ja107698p

The rhodium-catalyzed synthesis of silafluorenes from biphenylhydrosilanes is described. This highly efficient reaction proceeds via both Si-H and C-H bond activation, producing only H₂ as a side product. Using this method, a ladder-type bis-silicon-bridged p-terphenyl could also be synthesized.

Full text of DOI:10.1021/ja107698p

Published on Web 09/24/2010
Rhodium-Catalyzed Synthesis of Silafluorene Derivatives via Cleavage of
Silicon-Hydrogen and Carbon-Hydrogen Bonds
Tomonari Ureshino, Takuya Yoshida, Yoichiro Kuninobu,* and Kazuhiko Takai*
DiVision of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama UniVersity,
Tsushima, Kita-ku, Okayama 700-8530, Japan
Received May 12, 2010; E-mail: kuninobu@cc.okayama-u.ac.jp; ktakai@cc.okayama-u.ac.jp
Abstract: The rhodium-catalyzed synthesis of silafluorenes
from biphenylhydrosilanes is described. This highly efficient
reaction proceeds via both Si-H and C-H bond activation,
producing only H2 as a side product. Using this method, a
ladder-type bis-silicon-bridged p-terphenyl could also be
synthesized.
Silafluorene derivatives have recently received much attention
because they are useful in organic materials, such as organic
electroluminescent compounds.1 There have been several reports
The scope of substrates was examined with the rhodium catalyst
on the synthesis of silafluorenes,2 including iridium-catalyzed
in 1,4-dioxane (Table 1). Biarylhydrosilanes bearing an electron-
[2 + 2 + 2] cycloaddition of silicon-bridged diynes and alkynes,3
withdrawing group, 1b and 1c, gave silafluorenes 2b and 2c,
respectively, in 95% yields (entries 1 and 2). The corresponding
palladium-catalyzed intramolecular coupling of 2-(arylsilyl)aryl
silafluorene derivative 2d and its dechlorinated product 2a were
triflate,4 rhodium-catalyzed coupling of 2-silaphenylboronic acids
afforded in 72% and 11% yields, respectively (entry 3).12 This result
with alkynes via carbon-silicon bond cleavage,5 and palladium-
enables the construction of further derivatives based on cross-
catalyzed reactions between 2,2′-diiodobiaryls and dihydrosi-
coupling reactions. Silafluorenes having an electron-donating group,
lanes.6 As a novel strategy for the synthesis of silafluorenes,
we employed a method based on metal-catalyzed double
activation of Si-H and C-H bonds with dehydrogenation
(Figure 1).
2e and 2f, were obtained from the corresponding biarylhydrosilanes
1e and 1f; however, the yields were decreased and the reaction
times were longer compared to 2b and 2c (entries 4 and 6). To
improve the yields of 2e and 2f and to shorten the reaction times,
3,3-dimethyl-1-butene was added as a hydrogen acceptor, and 2e
and 2f were produced in 91% and 83% yields, respectively (entries
5 and 7). When a biarylhydrosilane with a methyl group at the
meta-position, 1g, was used, only silafluorene derivative 2g was
produced regioselectively in 96% yield (entry 8). The reaction was
not inhibited by a substituent at the ortho-position, and the
corresponding silafluorene 2h was afforded in 93% yield (entry
9).13 Biphenylhydrosilanes with two ethyl groups on the silyl group,
1i, or with methyl and phenyl groups on the silyl group, 1j, also
afforded silafluorene derivatives 2i and 2j in 94% and 84% yields,
respectively (entries 10 and 11).14 To expand the π-conjugated
system, 2-(2′-naphthyl)phenylhydrosilane 1k was employed as a
substrate. As a result, the corresponding silafluorene derivative 2k
was generated in 66% yield (entry 12).15 The yield of 2k was
increased to 90% upon addition of 3,3-dimethyl-1-butene (entry
13). The corresponding silafluorene 2l was produced regioselectively
when hydrosilane with a 3-thiophenyl group, 1l, was employed as
a substrate (entry 14). Using (2-ethenylphenyl)dimethylsilane,
intramolecular hydrosilylation proceeded, and the corresponding
dihydrobenzosilole was formed in 90% yield, but the desired
benzosilole was not obtained (not shown).16
Since biarylhydrosilanes bearing an electron-withdrawing group
have higher reactivities compared to biarylhydrosilanes with an
electron-donating group, this reaction should not proceed by a
Friedel-Crafts-type reaction. The proposed mechanism is as follows
(Scheme 1): (1) oxidative addition of a hydrosilane to a metal center
(Si-H bond activation; the metal center is oriented close to an
Figure 1. Retrosynthesis for the formation of silafluorenes.
To synthesize silafluorenes via dehydrogenation, we screened
several transition metal complexes that can catalyze transforma-
tions starting from Si-H or C-H bond activation in toluene.
First, transition metal complexes that can promote Si-H bond
cleavage such as hydrosilylation were investigated (eq 1). Among
those examined, a rhodium complex, RhCl(PPh3)3, showed high
catalytic activity, and 2a was obtained in 92% yield.7-10 Other
rhodium complexes, [RhCl(cod)]2, [Rh(OAc)2]2, and Rh4(CO)12
also provided 2a in 72%, 41%, and 18% yields, respectively.
Ruthenium complexes, Ru3(CO)12 (53%) and RuHCl(CO)(PPh3)3
(19%); an iridium complex, Ir4(CO)12 (21%); and a rhenium
complex, ReCl(CO)5 (21%),11 also promoted both Si-H and
C-H bond activation and produced silafluorene 2a.
Using RhCl(PPh3)3, which gave the highest yield of 2a, we
investigated the best reaction conditions (eq 1). By changing
the solvent, the amount of RhCl(PPh3)3 could be decreased (0.50
mol %) and the reaction time could be shortened (15 min). This
method is highly efficient because the only side product is H2.
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10.1021/ja107698p  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Investigation of Several Biarylhydrosilanes
Scheme 1. Proposed Mechanism for the Formation of
Silafluorenes 2
Next, we performed a deuterium labeling experiment to elucidate
the rate-determining step for the formation of the silafluorenes. If
the C-H bond activation is the rate-determining step, a kinetic
isotope effect (KIE) should be observed using a mixture of
biarylhydrosilane 1a and its pentadeuterated substrate 1a-d. Treat-
ment of a mixture of 1a and 1a-d with a catalytic amount of
RhCl(PPh3)3, at 135 °C for 45 s, gave silafluorenes 2a and 2a-d in
9% yield (2a:2a-d ) 87:13, KIE ) 6.8) (eq 2). This result shows
that C-H bond activation of a phenyl group is the rate-determining
step in the formation of silafluorenes.
This reaction could be applied to the synthesis of ladder-type
bis-silicon-bridged p-terphenyl 4 (eq 3).2d,19 After a mixture of 3,
3,3-dimethyl-1-butene, RhCl(PPh3)3, and 1,4-dioxane was heated,
double cyclization occurred and ladder-type silafluorene 4 was
obtained in 87% yield.20
a Isolated yield. Yield determined by 1H NMR is reported in
parentheses. b RhCl(PPh3)3 (1.0 mol %). 2a was formed in 11% yield as
a side product. c 1 h. d 24 h. e 3,3-Dimethyl-1-butene (5.0 equiv) was
used as a hydrogen acceptor. f 30 min. g 7 h. h 6 h. i 2 h.
aromatic C-H bond);17 (2-a) sequential oxidative addition of the
aromatic C-H bond to a metal center (C-H bond activation);18
and (3-a) reductive elimination to give silafluorene 2. Another
possible pathway is (2-b) the formation of intermediate A via
σ-bond metathesis (C-H bond activation); (3-b) reductive elimina-
tion to give silafluorene 2. In step (2-a) or (2-b), it is important
that the position of the Si-Rh-H moiety is close to the C-H bond
because intramolecular C-H bond activation proceeds more easily
than intermolecular C-H bond activation. In addition, such C-H
bond transformation occurs regioselectively.
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C O M M U N I C A T I O N S
Soc. 2001, 123, 9692. (h) Tanaka, K.; Fu, G. C. J. Am. Chem. Soc. 2001,
123, 11492.
In summary, we have succeeded in the synthesis of silafluorenes
from biarylhydrosilanes. This reaction proceeds by double activation
of Si-H and C-H bonds via dehydrogenation. The dehydrogena-
tion reaction does not require oxidants, such as molecular oxygen.
Using this method, a ladder-type bis-silicon-bridged p-terphenyl
was also synthesized. We hope that this reaction will become a
useful method to synthesize silafluorenes.
(10) There have been several reports on transition-metal-catalyzed dehydroge-
native silylation of aromatic compounds. See: (a) Sakakura, T.; Tokunaga,
Y.; Sodeyama, T.; Tanaka, M. Chem. Lett. 1987, 2375. (b) Uchimaru, Y.;
El Sayed, A. M. M.; Tanaka, M. Organometallics 1993, 12, 2065. (c)
Kakiuchi, F.; Igi, K.; Matsumoto, M.; Chatani, N.; Murai, S. Chem. Lett.
2001, 422. (d) Tsukada, N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127,
5022.
(11) We have recently reported on a transformation via C-H bond activation
using a rhenium catalyst. See: (a) Kuninobu, Y.; Kawata, A.; Takai, K.
J. Am. Chem. Soc. 2005, 127, 13498. (b) Kuninobu, Y.; Tokunaga, Y.;
Kawata, A.; Takai, K. J. Am. Chem. Soc. 2006, 128, 202. (c) Kuninobu,
Y.; Nishina, Y.; Shouho, M.; Takai, K. Angew. Chem., Int. Ed. 2006, 45,
2766. (d) Kuninobu, Y.; Nishina, Y.; Nakagawa, C.; Takai, K. J. Am. Chem.
Soc. 2006, 128, 12376. (e) Kuninobu, Y.; Nishina, Y.; Okaguchi, K.;
Shouho, M.; Takai, K. Bull. Chem. Soc. Jpn. 2008, 81, 1393. (f) Kuninobu,
Y.; Fujii, Y.; Matsuki, T.; Nishina, Y.; Takai, K. Org. Lett. 2009, 11, 2711.
(12) There have been several reports on rhodium-catalyzed dechlorination of
chlorobenzene derivatives by hydrosilanes. For example, see: Esteruelas,
M. A.; Herrero, J.; Lo´pez, F. M.; Mart´ın, M.; Oro, L. A. Organometallics
1999, 18, 1110.
Acknowledgment. This work was supported by the Ministry
of Education, Culture, Sports, Science, and Technology of Japan,
and Okayama University.
Supporting Information Available: General experimental procedure
and characterization data. This material is available free of charge via
the Internet at http://pubs.acs.org.
(13) In the case of using a biarylhydrosilane with a methyl group at the ortho-
position, r, a mixture of silafluorene ꢀ and 9,10-dihydro-9-silaphenanthrene
γ, which was formed by intramolecular dehydrogenative coupling between
the hydrosilyl moiety and methyl group on the aromatic ring, was produced
in 19% and 47% yields, respectively (RhCl(PPh3)3, 5.0 mol%; 24 h). The
silafluorene ꢀ could not be separated from γ by silica gel column
chromatography or GPC.
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(16) In the case of (2-ethenylphenyl)dimethylsilane, intramolecular hydrosily-
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of 2-(dimethylsilyl)biphenyl (1a) and styrene was heated in the presence
of a catalytic amount of RhCl(PPh3)3. Intermolecular hydrosilylation did
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Rh(acac)3, PtO2, PtH2Cl6 · 6H2O.
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57%; PCy3, 97%; PPh3, 58%; 1,2-bis(diphenylphosphino)ethane (dppe),
trace; 1,3-bis(diphenylphosphino)propane (dppp), 52%; 1,4-bis(diphe-
nylphosphino)butane (dppb), 45%; (R)-BINAP, 87%.
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