Ruthenium-catalyzed ortho-selective aromatic C-H silylation: Acceptorless dehydrogenative coupling of hydrosilanes

Article abstract of DOI:10.1246/cl.2012.374

The intermolecular dehydrogenative coupling of 1,1,1,3,5,5,5- heptamethyltrisiloxane with aromatic compounds such as aryloxazolines and arylimines in the presence of a catalytic amount of [RuCl₂(p-cymene)] ₂ gave the corresponding orthosilylated products in good yields.

Full text of DOI:10.1246/cl.2012.374

doi:10.1246/cl.2012.374
374
Published on the web March 24, 2012
Ruthenium-catalyzed Ortho-selective Aromatic C-H Silylation:
Acceptorless Dehydrogenative Coupling of Hydrosilanes
Toru Sakurai, Yusuke Matsuoka, Tsurugi Hanataka, Naoaki Fukuyama,
Takeshi Namikoshi, Shinji Watanabe, and Miki Murata*
Department of Materials Science and Engineering, Kitami Institute of Technology,
165 Koencho, Kitami, Hokkaido 090-8507
(Received December 27, 2011; CL-111234; E-mail: muratamk@mail.kitami-it.ac.jp)
The intermolecular dehydrogenative coupling of 1,1,1,3,5,5,5-
DG
DG
[RuCl2(p-cymene)]2
heptamethyltrisiloxane with aromatic compounds such as
aryloxazolines and arylimines in the presence of a catalytic
amount of [RuCl2(p-cymene)]2 gave the corresponding ortho-
silylated products in good yields.
(1.0–2.5 mol%)
R13Si
H
+
H
R13Si
toluene
200 °C
R2
R2
1
2
3
(1.2–3.0 equiv)
R13Si = (TMSO)2MeSi
Since arylsilanes are versatile building blocks or reagents
for modern organic synthesis, the development of transition-
metal-catalyzed aryl C-Si bond-forming reactions has attracted
considerable interest.1 From an environmental and economic
point of view, there is no doubt that the dehydrogenative
silylation of ubiquitous C-H bonds of arenes with hydrosilanes
is an ultimate goal.2 However, examples of the C-H silylation
with hydrosilanes bearing electronegative groups, such as
halogen and alkoxy groups, on the silicon atom are still rare
in spite of the significant synthetic value of produced aryl-
silanes.3 Recently, we reported that 1,1,1,3,5,5,5-heptamethyl-
trisiloxane (1) promoted the platinum-catalyzed dehydrogen-
ative coupling with arenes4 and arylsiloxanes thus obtained
exhibited good reactivity for converting the silicon function-
ality.5
During the past decade, much attention has been given to
the control of regioselectivity in the catalytic C-H silylation.6-9
Among the most promising strategies is utilization of a directing
effect through heteroatom coordination to the catalyst.6,7 As a
representative example of regioselective silylation of aromatic
C-H bonds with hydrosilanes, Murai and co-workers reported
that the ruthenium-catalyzed reaction of benzene derivatives
having an sp2 nitrogen substituent afforded ortho-silylated
products with complete regioselectively.6a,6c However, the
silylating reagents were restricted to triorganosilanes, which
provided less reactive aryl triorganosilanes as synthetic inter-
mediates. It is the purpose of this paper, therefore, to present an
alternative silicon source. We wish to report an ortho-selective
C-H silylation of aromatic compounds 2, such as aryloxazolines
and arylimines, with 1. (Scheme 1)
Me
H
O
DG =
,
,
Me
Me
N
t-Bu
N
N
Scheme 1.
Table 1. Silylation of 4,4-dimethyl-2-phenyl-4,5-dihydrooxa-
zole (2a) with hydrosilanesa
Conv Yield
Entry Hydrosilane
Catalyst
/%b
/%c
1
2d
3
4
5
6
7
8
9
1
1
1
1
1
1
[RuCl2( p-cymene)]2 98
[RuCl2( p-cymene)]2 41
[RuCl2(C6H6)]2
[Ru3(CO)12]
[Cp*RuCl2]2
[Cp*RuCl]4
92
40
85
61
0
92
80
0
5
5
(Me3SiO)Me2SiH [RuCl2( p-cymene)]2 70
45e
61f
0
Et3SiH
[RuCl2( p-cymene)]2 65
[RuCl2( p-cymene)]2
aReaction conditions: 1 (0.30 mmol), 2a (0.25 mmol), catalyst
(0.005 mmol of Ru), and toluene (0.5 mL), 200 °C, 16 h.
bConversions of 2a were determined by GC. cGC yields of 3a
are based on 2a. dThe reaction was carried out in the presence
(EtO)3SiH
0
e
of tert-butylethylene (1.0 mmol). GC yield of 4,4-dimethyl-
2-{2-[(trimethylsiloxy)dimethylsilyl]phenyl}-4,5-dihydrooxa-
zole. fGC yield of 4,4-dimethyl-2-[2-(triethylsilyl)phenyl]-4,5-
dihydrooxazole.
As a test for the optimization of reaction parameters, 4,4-
dimethyl-2-phenyl-4,5-dihydrooxazole (2a) was used as a
substrate. The results are summarized in Table 1. When 2a
was treated with a slight excess of 1 (1.2 equiv) in the presence
of 1.0 mol % of [RuCl2( p-cymene)]2 in toluene at 200 °C for
16 h, 92% yield of monosilylated product 3a (DG: 4,4-dimethyl-
4,5-dihydrooxazol-2-yl, R2 = H) was obtained (Entry 1). The
reaction is completely regioselective, introducing the silyl group
to the ortho position of the benzene ring. Thus, the meta- and
para-C-H bonds of 2a, and toluene solvent did not participate
in the present silylation. While dehydrogenative coupling of
hydrosilanes with arenes often requires an added alkene as a
hydrogen acceptor,6,8 inclusion of tert-butylethylene (4 equiv)
prevented the aromatic C-Si bond formation (Entry 2). Several
ruthenium catalysts were tested, and the use of [RuCl2( p-
cymene)]2 provided the best result (Entry 1). [RuCl2(C6H6)]2
also showed good catalytic activity affording 3a in a similar
yield (Entry 3), whereas the use of [Ru3(CO)12] complicated the
reaction by the formation of side-products (Entry 4). [Cp*Ru]
catalyst systems did not promote the present silylation reaction
(Entries 5 and 6). Under our optimized conditions, the use
of 1,1,1,3,3-pentamethyldisiloxane led to complex mixtures
(Entry 7) and that of triethylsilane induced a lowering of the
Chem. Lett. 2012, 41, 374-376
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

375
Table 2. Ortho-selective aromatic C-H silylation of 2 with 1a
Me
O
Cu(OAc)2 (1.0 equiv)
TBAF (1.0 equiv)
Me
2
O
Yield/%b
N
Entry
1
3
R2
3a
DG
N
THF/CH2Cl2 (1:1)
rt, air
Me
O
Me
96% yield
4-CF3 (2b)
3b
89
Me
Me
N
Scheme 2.
2
3
4c
4-Me (2c)
4-OMe (2d)
3-Me (2e)
3c
3d
3ed
88
90
70
H
SiR1
3
5
[Ru]
H
N
[Ru]Cl2
1
H
SiR
3
5
H (2f)
3f
94
[Ru] = (arene)Ru
R13Si
H
t-Bu
7
6
7
4-OMe (2g)
4-CO2Me (2h) 3h
3g
87
88
SiR1
3
5
Me
8c
H (2i) 3i
80
N
R3
N
R3
[Ru]
N
1
SiR
3
6
4
aReaction conditions: 1 (0.30 mmol), 2 (0.25 mmol), [RuCl2( p-
cymene)]2 (2.5 ¯mol), and toluene (0.5 mL), 200 °C, 16 h.
bIsolated yields based on 2. cReaction conditions: 1 (0.75
mmol), 2 (0.25 mmol), [RuCl2(p-cymene)]2 (6.25 ¯mol), and
H
8
R13Si [Ru]
[Ru]
1
SiR
3
H
SiR1
R3
H
3
H
N
d
N
toluene (0.5 mL), 200 °C, 48 h. 4,4-Dimethyl-2-{5-methyl-2-
[bis(trimethylsiloxy)methylsilyl]phenyl}-4,5-dihydrooxazole.
12
9
R3
reactivity (Entry 8). Furthermore, no reaction with triethoxy-
silane occurred (Entry 9). The present study demonstrates that 1
is an efficient silylating reagent for the aromatic C-H silylation.
The scope of this silylation using 1 was next explored
(Table 2).10 In the first part of this study, several aryloxazolines
2b-2e were used as the substrates. In all cases, one of the
aromatic C-H bonds ortho to the oxazolinyl groups was
silylated. As shown in Table 2, para-substituted aryloxazolines
were efficiently converted to the corresponding mono-silylated
products 3b-3f (Entries 1-3). The yields were almost independ-
ent of the electronic requirement; i.e., the differences in the
yields among 2 having electron-withdrawing (Entry 1) or
-donating groups (Entries 2 and 3) were not particularly large.
In contrast to the para-substituted substrates, meta-substituted
aryloxazoline 2e was not sufficiently reactive under our standard
conditions; i.e., the conversion and yield were low (29% GC
yield). We found, however, 3 equiv of 1 and 2.5 mol % of
[RuCl2( p-cymene)]2 allowed the reaction to proceed to full
conversion after 48 h (Entry 4). In the case of 2 bearing a methyl
group at the 3-position of the aromatic ring, the C-Si bond
formation took place only at the less hindered 6-position. The
second portion of this work involves the application of this
protocol to the silylation of aromatic aldimines 2f-2h (Entries
5-7). These results also indicate that the present silylation is
tolerant of both electron-donating and -withdrawing substituents
on the aromatic ring (Entries 6 and 7). Furthermore, the pyridine
ring could also be used as an ortho directing group (Entry 8).
We then tried to apply ortho-substituted 3-aryltrisiloxanes 3
thus obtained via the present silylation to C-C bond-forming
reaction, as we have demonstrated that 3-aryltrisiloxanes under-
went the transition-metal-catalyzed cross-coupling in the pres-
ence of TBAF.4,5 It was found that Cu(OAc)2 and TBAF
promoted homocoupling of 3a under aerobic conditions
(Scheme 2). While the Cu(I)-catalyzed homocoupling of aryl-
1
SiR
H
1
3
[Ru]
3
[Ru]
SiR
H
R13Si
N
H
N
R3
R3
11
10
H2
5
1
1
SiR
3
R
Si
H
[Ru]
3
[Ru]
H
SiR1
3
H
N
R3
N
R3
TS9-10
TS11-12
Figure 1. Plausible mechanism of ortho-selective aromatic
C-H silylation.
(halo)silanes has been reported,11 a catalytic amount of neither
Cu(OAc)2 nor CuI was totally effective. Unfortunately, we have
no definitive explanation for the role of Cu(II) salt at present
stage.
Finally, we turned our attention toward the mechanism of
this C-H silylation. The proposed catalytic cycle is illustrated
in Figure 1. As Berry and co-workers have proven that [(η6-
arene)RuCl2]2 reacted with HSiR3 to give [(η6-arene)Ru(H)2-
(SiR1 ) ],12 the first step in the proposed pathway involves the
3 2
formation of the Ru(IV) dihydride bis(silyl) complex 7. Indeed,
the reaction between [(p-cymene)Ru(H)2(SiEt3)2] and 2a afford-
ed the corresponding silylated product in quantitative yield.13
The following process was computationally addressed by
density functional theory (DFT) calculations.10 We have adopted
the reaction of phenylmethanimine (4, R3 = H) with trimethyl-
silane (5, R1 = Me) as a model reaction, and the p-cymene
Chem. Lett. 2012, 41, 374-376
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

376
ligand was replaced by benzene in the computed structures.
Although 7 potentially dissociates H2, HSiR1 , or Si2R1 , our
Academic Press, London, 1988.
2
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3
6
calculations suggest that a hydride silyl complex 8 would form
by elimination of 5.14 Then, after the coordination of both sp2
nitrogen atom and ortho C-H bond of 4 to 8, the ·-complex-
assisted metathesis (·-CAM) with the Ru-H bond takes place
through a transition state TS9-10 to form an aryl silyl ruthenium
10. A ligand substitution reaction includes dissociation of the H2
ligand and coordination of 5 forming a ·-silane ruthenium 11.
Subsequently, oxidative addition of the coordinated Si-H bond
and C-Si reductive elimination through a single transition state
TS11-12 with no intermediates take place to give a hydride silyl
complex 12 which contains a coordinated arylsilane 6. Elimi-
nation of the silylated product 6 from 12 regenerates 8 and
completes the silylation reaction. The activation barriers of the
C-H bond activation and the C-Si bond-forming steps are 37.2
and 47.0 kcal mol¹1, respectively, indicating that the latter step is
rate-determining for the proposed reaction mechanism.
3
4
5
6
7
The catalytic cycle initially proposed by Murai et al.
involves a Ru(IV) intermediate via sequential oxidative addition
both of C-H and Si-H bonds, although the potentiality for a
·-bond metathesis pathway has not been ruled out.6c As all
8
9
attempts to locate the [Ru(Ar)(SiR1 )(H)2] intermediate as a
3
stationary point were unsuccessful probably because of the
energetically unfavorable high oxidation state, we believe that
the above mechanism involving ·-CAM would be preferable to
the oxidative addition pathway.
In conclusion, 1,1,1,3,5,5,5-heptamethyltrisiloxane (1) was
found to promote the ruthenium-catalyzed intermolecular de-
hydrogenative coupling with aromatic compounds 2, such as
aryloxazolines, arylimines, and arylpyridines. The ortho-selec-
tive introduction of the silyl groups reported herein was
achieved without hydrogen acceptors. Theoretical calculations
suggest that the catalytic cycle involves the C-H bond activation
via ·-CAM mechanism. Further studies are currently underway
to obtain detailed mechanistic insights.
10 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
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T. Hiyama, Bull. Chem. Soc. Jpn. 2000, 73, 985.
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1994, 13, 2551. b) K. Ezbiansky, P. I. Djurovich, M.
LaForest, D. J. Sinning, R. Zayes, D. H. Berry, Organo-
metallics 1998, 17, 1455.
13 A mixture of [(p-cymene)Ru(H)2(SiEt3)2] (0.036 mmol) and
2a (0.16 mmol) in toluene (0.2 mL) was stirred at 200 °C for
4.5 h. GLC analysis of the reaction mixture indicated
formation of 4,4-dimethyl-2-(2-triethylsilylphenyl)-4,5-di-
hydrooxazole (95%).
This work was partially supported by a Grant-in-Aid for
Scientific Research from Japan Society for the Promotion of
Science (JSPS). We thank Mr. Takashi Yamamoto for help in
preparing this manuscript.
14 The formation of bis(silyl) complex from 7 was proposed in
the dehydrogenative silylation by Berry and co-workers
(ref. 12). However, DFT calculations indicate that the
References and Notes
¹1
1
a) The Chemistry of Organic Silicon Compounds, ed. by S.
Patai, Z. Rappoport, John Wiley & Sons, New York, 1989.
b) E. W. Colvin, Silicon Reagents in Organic Synthesis,
formation of 8 is energetically favorable (ca. 10 kcal mol
lower than that of bis(silyl) or dihydride ruthenium
complex).
Chem. Lett. 2012, 41, 374-376
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/