Scandium-catalyzed silylation of aromatic C-H bonds

Article abstract of DOI:10.1002/anie.201105636

Regioselective, hydrogen-acceptor-free C-H bond silylation of anisoles or alkoxy-substituted benzenes with hydrosilanes was achieved by use of a half-sandwich scandium catalyst. The reaction proceeds through ortho-C-H activation of an anisole compound by a scandium hydride species followed by the silylation of the resulting scandium 2-anisyl species with a hydrosilane (see scheme). Copyright

Full text of DOI:10.1002/anie.201105636

Communications
DOI: 10.1002/anie.201105636
À
C H Silylation
À
Scandium-Catalyzed Silylation of Aromatic C H Bonds**
Juzo Oyamada, Masayoshi Nishiura, and Zhaomin Hou*
À
Exploring the potential of untapped elements is an important
strategy for the development of more efficient, selective
catalysts for useful chemical transformations. The direct
Table 1: ortho-C H silylation of anisole with hydrosilanes under various
conditions.[a]
À
silylation of aromatic C H bonds is an efficient, straightfor-
ward method for the preparation of silyl-substituted
arenes[1–8]—a family of useful intermediates in modern
organic synthesis and materials science.[9–11] The catalysts
reported so far for this transformation have largely relied on
late transition metals such as Ru,[1,3] Rh,[3,5] Ir,[2,6] Ni,[7] and
À
Run
Catalyst
[Si] H
Product
Yield [%][b]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1d
1d
1d
1d
1d
1d
1d
1d
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a’
3a’’
3a’’’
(3)
(3)
(37)
Pt.[4,8] In contrast, rare-earth-metal catalysts for the C H
À
(64)
silylation of arenes have hardly been explored, despite many
(1)[c]
(23)[d]
(85)[e]
88 (92)[f]
89 (93)[f,g]
75[f]
À
examples of stoichiometric C H bond activation by rare-
earth complexes.[12] A scandium metallocene complex was
reported for the catalytic silylation of methane, but it could
not be applied to the catalytic silylation of arenes.[13]
9
PhSiH3
10
11
12
PhCH2SiH3
nC8H17SiH3
Ph2SiH2
46[f]
We report herein that half-sandwich scandium alkyls can
serve as excellent catalyst precursors for the ortho-regiose-
36[f]
À
lective C H silylation of alkoxy-substituted benzene deriva-
[a] Reaction conditions: catalyst (0.02 mmol), anisole (2a) (3 mmol),
hydrosilane (1 mmol), benzene (1 mL), 1208C, 36 h. [b] Yield of isolated
product based on hydrosilane. Yields measured by GC analysis are given
in parentheses. [c] THF was used instead of benzene. [d] 2a (1 mmol),
PhSiH3 (3 mmol). The yield was based on 2a. [e] 1d (0.04 mmol). [f] 1d
(0.04 mmol), 2a (10 mmol), 6 h. [g] No solvent.
tives. Some active intermediate species in the present catalyst
system have been isolated and structurally characterized, thus
offering important insight into the mechanistic aspects of the
À
catalytic process. The regioselective, catalytic C H silylation
of alkoxyarenes has not been reported previously. It usually
difficult to use an alkoxy moiety as a directing group in late-
À
transition-metal-catalyzed C H silylation reactions because
the interaction between an ether group and a late-transition-
metal center is too weak.
catalytic activity to give the ortho-silylation product 2-
phenylsilylanisole (3a) in 37% yield (based on PhSiH3)
when a mixture of anisole and PhSiH3 (3:1) was heated in the
presence of 2 mol% 1c at 1208C for 36 h (Table 1, run 3). The
aminobenzyl scandium complex 1d, which is more thermally
stable than its trimethylsilylmethyl analogue 1c, afforded 3a
in 64% yield under the same reaction conditions (Table 1,
run 4).
The ortho-metalation of anisole (2a) by the half-sandwich
yttrium and lutetium alkyl complexes 1a and 1b (Figure 1)
took place smoothly at room temperature in benzene.[12i,p,14]
Complex 1d was then further examined under various
conditions. The use of THF as a solvent severely hampered
the reaction, probably owing to the coordination of the Lewis
base THF molecule to the metal center (Table 1, run 5). The
use of a lower amount of anisole to PhSiH3 (1:3) resulted in a
decrease of the product yield (23%) (Table 1, run 6). An
increase of the catalyst loading from 2 to 4 mol% led to an
increase of the product yield from 64% to 85% (Table 1,
runs 4 and 7). When the molar ratio of anisole to PhSiH3 was
raised to 10:1, the product yield reached 92% in 6 h (Table 1,
Figure 1. Half-sandwich rare-earth-metal alkyl complexes.
However, the catalytic silylation reaction did not occur either
at room or at high temperatures (up to 1208C) in the presence
of excess anisole and PhSiH3 (Table 1, runs 1 and 2). In
contrast, the scandium analogue 1c showed considerable
À
run 8). The C H silylation reaction selectively proceeded
[*] Dr. J. Oyamada, Dr. M. Nishiura, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory and Advanced Catalyst
Research Team, RIKEN Advanced Science Institute
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
E-mail: houz@riken.jp
even in the absence of a solvent (Table 1, run 9). Benzylsilane,
n-octylsilane, and diphenylsilane could also be used as a
silicon source for this reaction, albeit giving a lower yield
under the same reaction conditions (Table 1, runs 10–12).
The silylation of various alkoxy-substituted benzene
derivatives by phenylsilane was then examined by use of 1d
as a catalyst. Some representative results are summarized in
Table 2. 4-Methylanisole (2b) could be silylated similarly to
[**] This work was partly supported by a Grant-in-Aid for Scientific
Research (S) from JSPS (No. 21225004).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105636.
10720
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Angew. Chem. Int. Ed. 2011, 50, 10720 –10723

Table 2: Scandium-catalyzed silylation of alkoxy-substituted benzene
anisole (Table 2, run 1). In the case of 1,4-dimethoxybenzene
(2c), the monosilylated product 3c was obtained predom-
inantly in 94% yield, although a small amount of the bis-
silylated product 4 (ca. 2%) was also observed (Table 2,
run 2). Aromatic carbon–halogen (F, Cl, Br, I) bonds could
survive the reaction conditions to give the corresponding
halogenated ortho-phenylsilylanisoles (Table 2, runs 3–6). In
the case of 4-methylthioanisole (2h) and 4-dimethylamino-
anisole (2i), the silylation took place selectively at the
position ortho to the methoxy group, in agreement with the
strong oxophilicity of a scandium ion (Table 2, runs 7 and 8).
In the case of 3-methylanisole (2j), the silylation at the less
sterically demanding position is significantly favored in a 6:1
ratio (3j’/3j) (Table 2, run 9), whereas in the case of 1,3-
dimethoxybenzene, the silylation at the more crowded
position is slightly favored in a 1.8:1 ratio (3k/3k’), probably
because this position can be activated with two ortho-
methoxy groups (Table 2, run 10). ortho-Substituted anisole
derivatives, such as 2-methylanisole, 1,2-dimethoxybenzene,
1,2-methylenedioxybenzene, and 2,3-dihydrobenzofuran, are
not applicable to the catalytic silylation under the same
reaction conditions (Table 2, runs 11–14), possibly owing to
the steric hindrance of the ortho substituents (see below).
In addition to a methoxy unit, the ethoxy and n-butoxy
moieties can also be used as a directing group (Table 2,
runs 15 and 16). However, the more sterically demanding iso-
propoxy group is less effective apparently owing to steric
hindrance (Table 2, run 17).
derivatives with phenylsilane.[a]
Run Substrate
1[c]
Product
Yield [%][b]
89 (89)
2
90 (94)[d]
79 (84)
88 (94)
68 (88)
51 (57)
76 (84)
67 (75)
3
4[c]
5
6
7
8
À
It is also noteworthy that the present catalytic C H
silylation reaction does not require a hydrogen acceptor,
which is in contrast with most late-transition-metal catalysts,
which usually required the addition of an equimolar amount
of a hydrogen acceptor, such as an alkene, to achieve high
conversion.[1–3]
9
72 [1/6][e]
10
83 [1.8/1][e]
To gain information on the mechanistic aspects of the
À
catalytic ortho C H silylation reaction, some stoichiometric
11
12
n.r.[f]
n.r.[f]
reactions were carried out. At room temperature, no reaction
between 1c or 1d with anisole was observed. However, rapid
ortho-metalation of anisole was observed at high temper-
atures. The reaction of 1c with 20 equivalents of anisole in
benzene at 808C afforded the 2-anisyl complex 1e in 69%
yield within 1 h (Scheme 1). An X-ray diffraction study
revealed that the anisyl unit in 1e is bonded to the Sc atom in
a chelating fashion through both the methoxy group and the
ortho carbon atom (Figure 2, left). The CH3 group of the
methoxy unit is directed away from the metal center and
towards the hydrogen atom H3 on the ortho-carbon atom C3
(H3···C7: 2.529(3) ꢀ). The reaction of the anisyl complex 1e
with 1.5 equivalents of PhSiH3 at 308C afforded rapidly the
hydride complex 1 f in 76% yield with concomitant formation
of the ortho-silylation product 3a (Scheme 1). Alternatively,
the hydride complex 1 f could also be obtained easily by the
reaction of the alkyl complex 1c with PhSiH3.[15] An X-ray
diffraction study established that 1 f adopts a dimeric
structure through two bridging hydride ligands (Figure 2).[16]
The reaction of the hydride complex 1 f with anisole took
place at 608C to give 1e with release of H2. However, this
reaction could not be completed probably owing to a possible
reverse reaction (hydrogenation of 1e to give 1 f and anisole).
13
14
n.r.[f]
trace
15
16
17
69 (72)
70 (72)
17
[a] Reaction conditions: 1d (0.04 mmol), substrate 2 (10 mmol), PhSiH3
(1 mmol), benzene (1 mL), 1208C, 6 h, unless otherwise noted.
[b] Yields of isolated product based on PhSiH3. Yields measured by GC
analysis are given in parentheses. [c] No solvent was used. [d] 2,5-
Bis(phenylsilyl)-1,4-dimethoxybenzene (4) was also formed in approx-
imately 2% yield. [e] Product ratio given in brackets was determined by
1H NMR spectroscopy. [f] No reaction was observed.
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Communications
The 1H NMR spectroscopic monitoring of the reaction of
anisole with PhSiH3 by use of 1c as a catalyst precursor
showed the formation (presence) of the hydride species 1 f (in
addition to the silylation product 3a) in the reaction mixture,
suggesting that 1 f might be a catalyst resting state in the
present catalytic reaction. The catalytic silylation of 2-
deuterio-4-methylanisole with PhSiH3 showed no kinetic
À
isotope effect (kH/kD = 1.0). This result suggests that C H
activation is not the rate-determining step in the present
catalytic reaction. The kinetic study of the reaction of 1c with
anisole gave the activation parameters DH° = 76.3 kJmolÀ1
and DS° = À80.2 JKÀ1 molÀ1. The large negative DS° value
suggests that the rate-determining step should be the
coordination of anisole to the metal center of the catalyst
species. These results are in agreement with the observation
that an excess amount of anisole is needed to achieve high
yield of the silylation product.
On the basis of the above experimental results, a possible
mechanism for the catalytic silylation reaction is shown in
Scheme 2. The reaction of 1c or 1d with phenylsilane would
Scheme 1. Isolation and reactions of some active Sc intermediates.
Figure 2. ORTEP drawings of 1e (left) and 1 f (right) with 30%
probability thermal ellipsoids. Hydrogen atoms except for H3 in 1e
and hydride ligands on Sc in 1 f have been omitted for clarity. Selected
bond lengths [ꢀ] and angles [8]; 1e: Sc1–C1 2.271(3), Sc1–N1
2.064(2), Sc1–O1 2.575(2), Sc1–O2 2.225(2); C1-C2-O1 110.9(3). 1 f:
Sc1–N1 2.096(3), Sc1–O1 2.247(2), Sc1–H1 1.99(3), Sc1–H2 1.95(4);
H1-Sc1-H2 61.0(13).
Scheme 2. A possible mechanism for the catalytic silylation reaction.
À
Nevertheless, both the anisyl complex 1e and the hydride
complex 1 f exhibited almost equally high activity for the
catalytic silylation of anisole with PhSiH3 to give 3a in 88%
yield under the conditions described in Table 1, run 8,
suggesting that 1e and 1 f are true active catalyst species.
easily give the hydride complex A.[17] The ortho C H bond
activation of an anisole compound through assistance of the
interaction between the methoxy group and the Sc atom in A
could afford the 2-anisyl complex B with release of H2. The s-
bond metathesis reaction between B and phenylsilane would
yield the silylation product and regenerate A. The coordina-
tion of anisole to the metal center of the hydride species such
A could be the rate-determining step of this catalytic process.
The isolation and structural characterization of the anisyl
complex 1e and the hydride complex 1 f, together with the
kinetic studies, strongly support this mechanism.
À
The C H activation reaction of 2,3-dihydrobenzofuran
with 1c was much slower than that of anisole under the same
conditions, as monitored by 1H NMR spectroscopy, probably
owing to the rigidity of its cyclic ether structure, which could
make it more difficult to form a chelating aryl species
analogous to 1e. The silylation of the resulting metalated
species by PhSiH3 was possible, but the catalytic silylation of
2,3-dihydrobenzofuran was not observed in the presence of
excess amounts of PhSiH3 and 2,3-dihydrobenzofuran. The
metalation reaction of 2-methylanisole with 1c did not occur
under the same conditions, probably because of the steric
repulsion between the ortho-CH3 group and the CH3 group of
the methoxy unit in the metalation species analogous to 1e
(see Figure 2). These results demonstrate again the impor-
tance of the coordination of an alkoxy group to the catalyst
metal center in the present catalytic silylation reactions.
In summary, we have demonstrated that the half-sandwich
scandium alkyl complexes, such as 1d, can serve as a unique
catalyst for the ortho-selective C H silylation of various
alkoxy-substituted benzene derivatives without the require-
ment for a hydrogen acceptor to achieve high conversion.
À
À
Aromatic C X bonds (X = Cl, Br, I, SMe, NMe2) can survive
the reaction conditions. The successful isolation and structural
characterization of the anisyl complex 1e and the hydride
complex 1 f have offered important insight into the mecha-
nistic aspects of the catalytic process. Studies on other
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À
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Received: August 9, 2011
Published online: September 20, 2011
À
[12] For examples of C H bond activation by rare-earth alkyl
À
Keywords: anisoles · C H activation · hydrosilanes · scandium ·
silylation
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Angew. Chem. Int. Ed. 2011, 50, 10720 –10723
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