A Pincer Ruthenium Complex for Regioselective C-H Silylation of Heteroarenes

Article abstract of DOI:10.1021/acs.orglett.6b02857

A pincer Ru(II) catalyst for the highly efficient undirected silylation of O- and S-heteroarenes with (TMSO)₂MeSiH and Et₃SiH is described, producing heteroarylsilanes with exclusive C2-regioselectivity, good functional-group tolerance, and high turnover numbers (up to 1960). The synthetic utility of the silylated products is demonstrated by Pd-catalyzed Hiyama-Denmark cross-coupling under mild conditions. One-pot, two-step silylation and coupling procedures have been also developed.

Full text of DOI:10.1021/acs.orglett.6b02857

Letter
pubs.acs.org/OrgLett
A Pincer Ruthenium Complex for Regioselective C−H Silylation of
Heteroarenes
Huaquan Fang, Le Guo, Yuxuan Zhang, Wubing Yao, and Zheng Huang*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road, Shanghai 200032,
China
S
* Supporting Information
ABSTRACT: A pincer Ru(II) catalyst for the highly efficient undirected silylation of O- and S-heteroarenes with
(TMSO)2MeSiH and Et3SiH is described, producing heteroarylsilanes with exclusive C2-regioselectivity, good functional-
group tolerance, and high turnover numbers (up to 1960). The synthetic utility of the silylated products is demonstrated by
Pd-catalyzed Hiyama−Denmark cross-coupling under mild conditions. One-pot, two-step silylation and coupling procedures
have been also developed.
ue to their stability, low toxicity, and ease of
manipulation, heteroarylsilanes are useful building blocks
(5 mol %) catalyzes the C−H silylation of heteroarenes
with Et3SiH using amine directing groups. The process required
an excess of silane (5−10 equiv) and hydrogen acceptor
(5−10 equiv NBE) for high conversions. This system also
effects undirected C−H silylation of indoles and benzofurans,
but it is inactive for the silylation of benzothiophene.11 Earth-
abundant metal catalysts, such as KOtBu,12 iron,13 and copper14
catalysts, have also been developed for C−H silylations of
heteroarenes with alkyl- or aryl-substituted silanes; however,
these systems in general showed inferior efficiency and limited
substrate scope in comparison with the precious metal catalyst
systems.
As detailed above, with a few exceptions offered by
Hartwig7e,f and Ishiyama/Miyaura,9,15 most existing catalytic
systems employ alkyl/aryl silanes as the reagents, and thus, the
resultant silylation products have limited synthetic utilities
because the lack of electronegative substitution at the silicon
atom renders them unsuitable for Hiyama−Denmark cou-
pling16 or Tamao oxidation reactions.17 Moreover, despite the
significant advances,7e,f,9−14,18 there is need for improvement in
known catalyst systems with regard to activity, as these systems
necessitate relatively high catalyst loadings. Our contribution to
this field is to improve the catalytic efficiency for the silylation
reactions and to produce synthetically useful heteroarylsilanes
utilizing readily available and low-cost silanes.
D
in organic synthesis.1 Moreover, these compounds have found
application in the manufacture of organic optoelectronic
materials2 and in the pharmaceutical sector.3 The classical
method for the preparation of heteroarylsilanes involves the
reaction of organometallic species with suitable silicon elec-
trophiles.4 However, this method suffers from low functional-
group tolerance, formation of waste inorganic salts, and a
multistep synthetic sequence. Recently, methods for catalytic
silylations of C−H5 and carbon−halide5d,6 bonds without
directing groups have been developed for the synthesis of
heteroarylsilanes. The former is of particular interest because of
the high atom and step economy and environmentally benign
nature.
Although the C−H silylation of arenes has a long history,7,8
the silylation of heteroarenes without directing effects was
reported much more recently. In 2005, Ishiyama and Miyaura
reported the C−H silylation of five-membered heteroarenes
using an Ir complex of the 2-tert-butyl-1,10-phenanthroline
ligand (3 mol %).9 The reactions used tetrafluorodisilane
(tBuF2Si)2 as the silane reagent and a large excess of
heteroarenes (10 equiv relative to silane) were required.
Falck and Lu employed a similar Ir catalyst ligated by 4,4′-di-
tert-butyl-2,2′-bipyridine (10 mol % of Ir) for the C−H
silylation of N-, S-, and O-heteroarenes with Et3SiH (3 equiv)
using norbornene (NBE) as the hydrogen acceptor.10 Building
on their earlier work on Rh-catalyzed C−H silylation of arenes
and heteroarenes with (TMSO)2MeSiH,7e Hartwig and
co-workers reported an Ir complex of the 2,4,7-trimethyl-
1,10-phenanthroline ligand (2.2 mol %) that enables C−H
silylation of heteroarenes with the same hydrosiloxane.7f The Ir
catalyst exhibits broader functional-group tolerance compared
to the Rh catalyst.7e,f During the preparation of this manuscript,
Pilarski and co-workers disclosed that RuH2(CO)(PPh3)3
In early 2016, our laboratory reported the synthesis of new
pincer Ru(II) hydrido olefin complexes. Possibly in part due to
their high thermostability, these pincer Ru(II) complexes
exhibit higher productivity than the previously reported Ru
catalysts for alkane dehydrogenations.19 We hypothesized
that these robust pincer Ru complexes could be effective for
Received: September 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02857
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Organic Letters
Letter
a
Table 1. Evaluation of Ruthenium Catalysts for the Silylation of Benzofuran
b
entry
[Ru] (mol %)
silane
solvent
t/h
yield (%)
1
2
3
1 (0.5)
1 (0.5)
1 (0.5)
1 (0.5)
(TMSO)2MeSiH
(TMSO)2MeSiH
(TMSO)2MeSiH
(TMSO)2MeSiH
(TMSO)2MeSiH
Et3SiH
toluene
n-hexane
none
6
6
99
99
6
97(96)
c
d
4
none
6
49
5
1 (0.05)
none
24
24
6
96(95)
e
6
1 (0.25)
none
98(98)
7
none
(TMSO)2MeSiH
(TMSO)2MeSiH
(TMSO)2MeSiH
(TMSO)2MeSiH
(TMSO)2MeSiH
none
0
8
Ru3(CO)12 (0.5)
[Cp*RuCl2]2 (0.5)
(cod)Ru(2-methylallyl)2 (0.5)
Ru(acac)3 (0.5)
none
6
3
9
none
6
<5
10
4
10
11
none
6
none
6
a
Reaction conditions: 2a (1.0−20 mmol), (TMSO)2MeSiH (1 equiv) or Et3SiH (1.5 equiv), [Ru] (0.05−0.5 mol %), tert-butylethylene
b
1
(TBE, 1 equiv) at 120 °C, 6−24 h. Determined by H NMR analysis of the crude reaction mixture using mesitylene as an internal standard.
Values in the parentheses are the yields of isolated products. Without TBE. 2,3-Dihydrobenzofuran obtained in 47% yield. With 1.2 equiv of
TBE.
c
d
e
functionalizations of C(sp2)−H bonds. Herein we report
Scheme 1. Silylations of Heteroarenes with
(TMSO)2MeSiH
a
that the bis(phosphinite)-based pincer Ru complex is highly
active for undirected C−H silylation of O- and S-heteroarenes
with (TMSO)2MeSiH and Et3SiH with high regioselectivity.
The reaction is scalable by using a minimal amount of the
catalyst (0.05 mol %). Furthermore, the silylation products are
amenable to Hiyama−Denmark cross-coupling reactions, and
a one-pot, two-step silylation and cross-coupling protocol is
described.
The silylation of benzofuran 2a with (TMSO)2MeSiH using
the pincer Ru complex 1, (POCOP)RuH(NBD) (NBD =
norbornadiene), as the catalyst, was selected as the model
reaction. The results are summarized in Table 1. A practical
catalytic silylation should avoid using the heteroarene or the
silane in excess. To our delight, using tert-butylethylene (TBE,
1 equiv) as the hydrogen acceptor, the reaction of 2a with
1 equiv of (TMSO)2MeSiH and 0.5 mol % of 1 in toluene or
n-hexane at 120 °C formed the C2-silylation product 3a in
quantitative yield after 6 h (entries 1 and 2). The dehydrogen-
ative silylation reaction also occurred smoothly under solvent-
free conditions (97% yield, entry 3). The reaction without
TBE gave 49% 3a and 47% 2,3-dihydrobenzofuran (entry 4),
suggesting that benzofuran itself can serve as the hydrogen
acceptor. Reducing the catalyst loading to 0.05 mol % and
extending the reaction time to 24 h afforded 3a in 96% yield
(entry 5). This Ru catalyst is also effective for the silylation of
2a with Et3SiH (entry 6). Triethylsilane appeared to be less
reactive than (TMSO)2MeSiH under the catalytic conditions;
however, the silylation product 4a was obtained in high yield
when 1.5 equiv of Et3SiH and 0.25 mol % of 1 were applied.
The control experiment without 1, but with TBE, did not
form the silylation product (entry 7). For comparison, we also
examined several common Ru complexes for the silylation
reaction. Ru3(CO)12 has been reported for the silylation of
C(sp2)−H8 and C(sp3)−H20 bonds in substrates with directing
groups. However, the silylation of 2a using Ru3(CO)12 (0.5 mol %)
at 120 °C after 6 h gave 3% 3a (entry 8). The reactions
using Ru(II) and Ru(III) complexes, [Cp*RuCl2]2, (cod)Ru(2-
methylallyl)2, and Ru(acac)3, gave low yields of 3a (entries 9−11).
Next, the substrate scope of the silylation reactions was explored
utilizing 1 as the catalyst (Scheme 1). All of the reactions were
performed neat. For the reactions with (TMSO)2MeSiH as the
reagent, a variety of benzofuran derivatives bearing electron-
donating and -withdrawing groups underwent C2-silylations to
a
Reaction conditions: 1 (0.05−2.0 mol %), 2 (0.5−10 mmol),
(TMSO)2MeSiH (1 equiv), TBE (1 equiv) at 120 °C. Yields shown
b
are of isolated products unless otherwise noted. TBE (1.5 equiv).
c
d
(TMSO)2MeSiH (2 equiv), TBE (2 equiv). (TMSO)2MeSiH
e
(2 equiv), TBE (3 equiv). (TMSO)2MeSiH (1.5 equiv), TBE
(1 equiv). (TMSO)2MeSiH (2 equiv), TBE (1.2 equiv).
f
B
DOI: 10.1021/acs.orglett.6b02857
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Organic Letters
Letter
a
form the corresponding silylation products in high isolated
yields (3a−l). Most reactions employed 0.5 mol % of the
Ru catalyst, but five examples using only 0.05 mol % of 1 were
demonstrated to proceed with high yields (3a, 3c, 3d, 3i, 3k).
Benzofurans with substituents at C4−C7 positions were all
selectively silylated at the C2 position (3e−h). The reaction of
methoxsalen produced the desired product 3l in 81% yield,
while the α,β-unsaturated ester moiety remained intact during
the silylation process. Furan and its derivatives were efficiently
silylated at the C2 positions. The disilylation products (3m, 3q)
were obtained in high isolated yields when furan and 2,2-di(2-
furyl)propane were treated with 2 equiv of silane and TBE.
S-Heteroarenes are also suitable substrates for the silylation,
although higher catalyst loadings were used relative to reactions
of O-heteroarenes (Scheme 1). For instance, the reaction of
benzothiophene with 1 mol % 1 afforded the silylated product
3s in 88% yield. Similar to furan, thiophene underwent dis-
ilylation to form 3t in 91% yield. The thiophenes with Me and
ester substituents at the 2-poisition gave the corresponding
products (3u, 3v) in high yields. In addition, the substrate with
a Me group at the 3-poisition undergoes regioselective silylation
at the 5-position with no detectable evidence for silylation at
the 2-position (3w). We attribute the high regioselectivity to
the sensitivity of the sterically hindered pincer Ru complex to
the steric effect of the substrate.
Scheme 2. Silylations of Heteroarenes with Et3SiH
In contrast to the selective C2-silylations with O- and
S-heteroarenes, the Ru-catalyzed silylation of indoles with
(TMSO)2MeSiH resulted in selective formation of the
N-silylation products (3x′−3aa′, >75%), albeit with accom-
panying C−H silylation, giving the C2-silylation products as the
minor products (<10%, see Scheme 1).
a
Reaction conditions: 1 (0.25−8.0 mol %), 2 (0.5−2.0 mmol), Et3SiH
(1.5 equiv), TBE (1.2 equiv) at 120 °C. Yields shown are of isolated
b
c
products. Et3SiH (3 equiv), TBE (3 equiv). Et3SiH (3 equiv), TBE
(2.4 equiv). Et3SiH (1.5 equiv), TBE (1.5 equiv).
d
The reactions using triethylsilane Et3SiH as the silylating
reagent display a wide substrate scope similar to those using
(TMSO)2MeSiH (Scheme 2). However, the processes are
slower than those with (TMSO)2MeSiH and thus require
higher catalyst loadings. Nevertheless, most reactions of O-,
S-heteroarenes with Et3SiH provided the desired products (4)
in >90% isolated yields. Ester (4j, 4m, 4q, 4u), ether (4e−4h),
acetal (4o), and epoxide (4r) functional groups can be tolerated.
Having established a highly active catalyst for the silylation,
we sought to develop procedures to conduct this trans-
formation at a large scale. Using 0.05 mol % of 1, the reaction
of 2a (100 mmol) with (TMSO)2MeSiH (100 mmol) afforded
3a in nearly quantitative yield (98%, 33.3 g) with a turnover
number (TON) of 1960. To the best of our knowledge, this
represents the highest TON obtained by any metal-catalyzed
silylation of heteroarenes (eq 1).
Scheme 3. Hiyama−Denmark Cross-Couplings of the
a
Heteroarylsilanes with Iodobenzenes
The synthetic value of the heteroarylsilane products 3 that
contain Si−O bonds is demonstrated by their applications to
Pd-catalyzed Hiyama−Denmark cross-coupling reactions
(Scheme 3).21 Using Pd(OAc)2/Xantphos (5 mol %) as the
catalyst and KOH as the base (7 equiv), the couplings between
heteroarylsilanes 3 and various aryl iodides in p-xylene occurred
under mild conditions (50 °C) to afford the products in good
to high yields. A variety of functionalities, including nitro,
chloro, and bromo groups in the electrophiles, were found to
be compatible under the reaction conditions.
a
Reaction conditions: 3 (1.2−1.5 equiv), ArI (0.2−0.3 mmol),
Pd(OAc)2 (5.0 mol %), Xantphos (5.5 mol %), KOH (7 equiv),
b
p-xylene (1.0 mL). Yields shown are of isolated products. 3 (1.5 equiv).
c
3 (1.2 equiv).
Given the high conversions of heteroarenes to the silylation
products and the low loadings of the Ru catalyst, we envisioned
that the crude silylation products could be applied to the
sequential cross-coupling reaction without purification. Indeed,
C
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Organic Letters
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In summary, a highly efficient ruthenium complex has
been developed for the undirected C2-selective silylation of
O- and S-heteroarenes with (TMSO)2MeSiH and Et3SiH.
High conversions and high TONs (∼2000) can be achieved
using low catalyst loadings. Moreover, a mild and practical
protocol has been described for the Hiyama−Denmark coupling
of heteroarylsilanes with aryl iodides.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02857.
Experimental procedures and product characterization
(PDF)
(10) Lu, B.; Falck, J. R. Angew. Chem., Int. Ed. 2008, 47, 7508.
(11) Devaraj, K.; Sollert, C.; Juds, C.; Gates, P. J.; Pilarski, L. T.
Chem. Commun. 2016, 52, 5868.
(12) (a) Toutov, A. A.; Liu, W.-B.; Betz, K. N.; Fedorov, A.; Stoltz, B.
M.; Grubbs, R. H. Nature 2015, 518, 80. (b) Toutov, A. A.; Liu, W.-B.;
Betz, K. N.; Stoltz, B. M.; Grubbs, R. H. Nat. Protoc. 2015, 10, 1897.
(13) (a) Sunada, Y.; Soejima, H.; Nagashima, H. Organometallics
2014, 33, 5936. (b) Ito, J.-i.; Hosokawa, S.; Khalid, H. B.; Nishiyama,
H. Organometallics 2015, 34, 1377. (c) Yin, Q.; Klare, H. F. T.;
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: huangzh@sioc.ac.cn.
Notes
The authors declare no competing financial interest.
(14) Gu, J.; Cai, C. Chem. Commun. 2016, 52, 10779.
(15) Note that the disilane (tBuF2Si)2 applied in the Ishiyama/
ACKNOWLEDGMENTS
■
Miyaura system is commercially unavailable.
This work was supported by the MOST of China
(2015CB856600, 2016YFA0202900) and the National Natu-
ral Science Foundation of China (21432011, 21422209).
(16) Denmark, S. E.; Regens, C. S. Acc. Chem. Res. 2008, 41, 1486.
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1983, 2, 1694.
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Y.; Tatsumi, K. J. Am. Chem. Soc. 2011, 133, 3312. (b) Curless, L. D.;
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D
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