Calcium-catalyzed dehydrogenative silylation of aromatic ethers with hydrosilane

Article abstract of DOI:10.1021/acscatal.0c05440

The catalytic regioselective C-H silylation of a wide range of alkoxysubstituted benzene derivatives with primary hydrosilane was achieved by the use of scorpionate-supported calcium benzyl complex [(TpAd,iPr)Ca(p-CH2C6H4Me)(THP)] (1) (TpAd,iPr = hydrotris(3-adamantyl-5-isopropylpyrazolyl)borate, THP = tetrahydropyran) as the precatalyst. This protocol offers an atom-efficient and straightforward method for the synthesis of a variety of silyl-substituted aromatic ether derivatives without a hydrogen acceptor and free of transition metal. Calcium anisyl complexes [(TpAd,iPr)Ca(o-MeO-m-Br-C6H3)] (5) and [(TpAd,iPr)Ca(o-Me-OCH2C6H4)] (6), proposed as the catalytic reaction intermediates, were isolated and structurally characterized.

Full text of DOI:10.1021/acscatal.0c05440

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Research Article
Calcium-Catalyzed Dehydrogenative Silylation of Aromatic Ethers
with Hydrosilane
Lanxiao Zhao, Xianghui Shi, and Jianhua Cheng*
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ABSTRACT: The catalytic regioselective C−H silylation of a wide range of alkoxy-
substituted benzene derivatives with primary hydrosilane was achieved by the use of
scorpionate-supported calcium benzyl complex [(TpAd,iPr)Ca(p-CH2C6H4Me)(THP)]
(1) (TpAd,iPr = hydrotris(3-adamantyl-5-isopropylpyrazolyl)borate, THP = tetrahy-
dropyran) as the precatalyst. This protocol offers an atom-efficient and straightforward
method for the synthesis of a variety of silyl-substituted aromatic ether derivatives
without a hydrogen acceptor and free of transition metal. Calcium anisyl complexes
[(TpAd,iPr)Ca(o-MeO-m-Br-C6H3)] (5) and [(TpAd,iPr)Ca(o-Me-OCH2C6H4)] (6),
proposed as the catalytic reaction intermediates, were isolated and structurally
characterized.
KEYWORDS: calcium complexes, C−H activation, anisole, hydrosilanes, dehydrogenative silylation
examples of stoichiometric C−H bond activation by alkaline-
earth-metal complexes are known,9 catalytic sp2 or sp3 C−H
bond activations are rare.10
ue to their low-cost, nontoxic, environmentally benign,
Dand biocompatible features, calcium-based efficient
Recently, our group reported that mononuclear calcium
hydride complex [(TpAd,iPr)CaH(THP)] (2) (TpAd,iPr
=
hydrotris(3-adamantyl-5-isopropylpyrazolyl)borate, THP =
tetrahydropyran) (Figure 1), generated from the hydro-
genolysis reaction of scorpionate-supported calcium benzyl
complex [(TpAd,iPr)Ca(p-CH2C6H4Me)(THP)] (1),2o ex-
hibited superior reactivity and a wider substrate scope than
the previously reported dimeric calcium hydrides in the
catalytic hydrogenation of alkenes.3 Herein, we present that
calcium benzyl complex 1 can serve as an excellent catalyst
precursor for the regioselective C−H silylation of alkoxy-
substituted benzene derivatives under mild conditions with
good functional group compatibility. Some active intermediate
species have been isolated and structurally characterized, thus
shedding important light on the probable mechanistic aspects
of the catalytic process.
Figure 1. Mononuclear calcium alkyl and hydride complexes.
catalysts are receiving growing interest and are potential
alternatives to precious transition-metal catalysts in a variety of
organic transformations.1 Molecular calcium hydrides com-
plexes2 have been regarded as key intermediates in various
catalytic cycles, including hydrogenation3 and hydro-function-
alization.4
Besides hydrogen gas (H2), primary silane is another
efficient hydrogen source for the synthesis of molecular
alkaline-earth-metal hydrides. In many cases, primary arylsi-
lanes were reported to homodesilacouple to secondary silanes
Silicon-containing benzene derivatives are of great interest
and importance in a variety of research fields, such as materials
science, pharmaceuticals, and complex molecule synthesis.5
The direct silylation of aromatic C−H bonds is an atom-
efficient, straightforward method. This catalytic transformation
was largely mediated by the late transition metals,6 such as Ru,
Rh, Ir, Ni, and Pt, and recently was also achieved by the first-
row transition-metals7 and rare-earth-metals.8 Although many
Received: December 11, 2020
Revised: January 23, 2021
Published: February 1, 2021
https://dx.doi.org/10.1021/acscatal.0c05440
© 2021 American Chemical Society
ACS Catal. 2021, 11, 2041−2046
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product 4e was obtained. In the case of 4-N,N,-dimethylami-
noanisole (3f) and 4-methylthioanisole (3g), the silylation
took place exclusively at the ortho position of the methoxyl
group, in line with the strongly oxophilic calcium. Highly polar
aromatic C−X (X = F, Cl, Br, CF3, OCF3) bonds could survive
the reaction conditions to give the corresponding halogenated,
trifluoromethylated, and trifluoromethoxylated o-alkylsilylani-
soles, respectively, in 31−87% isolated yields (Table 2, 4h, 4i,
4j, 4l, 4m). However, with 4-iodoanisole (3k) only traces were
converted, indicating that aromatic C−I bond was activated
under these reaction conditions.
Meta-substituted anisole derivatives (3n, 3o, 3p, 3q) can
also be silylated under the same reactions with less efficiency,
apparently owing to steric hindrance. In the case of 3-
methylanisole (3n) and 2-methoxynaphthanlene (3o), the
silylation at the less sterically demanding positions is favored.
On the contrary, the silylation at the more crowded positions is
favored in the case of 1,3-dimethyoxybenzene (3p), probably
because the proton in this position is more active than the
other two ortho to the methoxy groups. In the case of 1,3,5-
trimethoxybenzene (3q), the monosilylated product can be
obtained in 75% yield within prolonged reaction time (24 h).
Three representative ortho-substituted anisole derivatives
(3r, 3s, and 3w) were also tested. In the case of 2-
methylanisole (3r), the silylation took place exclusively at
the sp3 benzylic C−H bond13 rather than at the aromatic sp2
C−H to give hexyl(2-methoxybenzyl)silane (4r) in high
conversion within the long reaction time (8 h, 44%; 24 h,
78%). In comparison, the catalytic silylation of the less
sterically demanding 2,3-dihydrobenzofuran (3s) selectively
occurred at the ortho sp2 C−H bond (rather than at the
benzylic C−H) to give 4s. However, 1,2-methoxybenzene
(3w) is not applicable to the catalytic silylation, probably
attributed to the steric repulsion between two CH3 groups of
the methoxy units.
Table 1. Ortho C−H Silylation of Anisole with Hydrosilanes
a
Catalyzed by Complex 1 under Various Conditions
temp (°C)/time
yield
(%)
run
[Si]-H/anisole
solvent
(h)
product
b
1
2
3
4
5
6
7
nHexSiH3 (2:1)
nHexSiH3 (2:1)
nHexSiH3 (2:1)
nHexSiH3(2:1)
nHexSiH3 (1:3)
nHexSiH3 (1:5)
benzene
benzene
benzene
benzene
benzene
benzene
benzene
40/3
60/3
80/3
80/8
80/8
80/8
80/8
4a
4a
4a
4a
4a
4a
4a
14
b
b
b
b
b
b
17
20
28
65
76
c
nHexSiH3
(1:10)
92 /89
b
8
9
nHexSiH3
(1:10)
hexane
THF
80/8
80/8
4a
4a
78
nHexSiH3
(1:10)
nr
c
10 PhSiH3 (1:10)
11 Ph2SiH2 (1:10)
12 PhMeSiH2
(1:10)
benzene
benzene
benzene
80/8
80/8
80/8
4a′
4a′′
4a′′′
40
58
75
c
c
a
Reaction conditions: complex 1 ([Ca]) (0.02 mmol), hydrosilane
b
n
(0.4 mmol), benzene (2 mL). Yield was based on HexSiH3 and
measured by H NMR spectrum. Yield of isolated product based on
hydrosilane.
c
1
with high selectivity in the presence of alkaline-earth metal2j,11
or rare-earth-metal hydrides.12 Under the same conditions, no
redistributions were observed for the aliphatic hydrosilanes.
Thus, n-C6H13SiH3 is preferred as the primary hydrosilane
substrate for the catalytic reactions.
In the presence of 5 mol % complex 1, the mixture of anisole
and n-C6H13SiH3(1:2) was heated at 40 °C for 3 h to give
hexyl(2-methoxyphenyl)silane (4a) in 14% yield (Table 1, run
1). As the temperature was increased and the reaction time was
extended, the yields improved gradually (Table 1, run 2−4).
When the molar ratio of anisole to n-C6H13SiH3 was increased
from 3:1 to 10:1, the product yields increased from 65% to
92% in 8 h (Table 1, run 5−7). The use of hexane as a solvent
led to a turbid solution and resulted in low efficiency (Table 1,
run 8). No product was observed in THF, probably because
the Lewis base THF severely blocked the coordination of
anisole to the calcium center (Table 1, run 9). Phenylsilane,
diphenylsilane, and phenylmethylsilane could also been used as
silicon sources for this conversion, although the isolated yields
were lower under the same reaction conditions (Table 1, runs
10−12). No reactivity of triethylsilane was observed.
In addition to a methoxy unit, the bulkier alkoxy (or
aryloxy) moieties such as ethyoxy, phenyloxy, and isopropoxy
can also be used as a directing group (3t, 3u, 3v). The
decreasing efficiency is mainly attributed to the increasing
i
steric hindrance (MeO < EtO < PhO < PrO). Dialkylamino
groups, such as NMe2 bonded directly to an aromatic ring, can
also act as a directing group14a,b for the activation of an ortho
C−H bond by a rare-earth-metal catalyst.14c,d However, N,N-
dimethylaniline (3x) is not activated under the same reaction
conditions, probably owing to the bulky steric hindrance of
NMe2 group and the weak interaction with calcium center.
These results demonstrate that the coordination of an alkoxy
group to the calcium center plays a key role in the present
catalytic silylation reaction.
To explore the potential mechanistic aspects of the catalytic
ortho C−H silylation reaction, stoichiometric reactions were
carried out. No reaction between complex 1 and anisole (1:3)
was observed at room temperature, and the ortho-metalation of
anisole occurred slowly at 40 °C and could be completed at
high temperature (80 °C) in 2 h. At room temperature, the
reaction of calcium hydride complex 2 with 3 equiv of 4-
bromoanisole (3j) or 2-methylanisole (3r) easily afforded the
corresponding anisyl complexes [(TpAd,iPr)Ca(o-MeO-m-
BrC6H3)] (5) and [(TpAd,iPr)Ca(o-MeO-CH2C6H4)] (6) in
79% and 83% yields, respectively (Scheme 1). X-ray analysis
study of complex 5 revealed that the Ca2+ center is coordinated
with the ortho carbon atom (C3) and the methoxy group of 4-
bromoanisyl unit to form a four-membered ring, as well as a
In general, a hydrogen acceptor, such as alkene, was used in
most of the late-transition-metal catalytic system to accelerate
the reaction and achieve high conversion.6 In contrast, the
hydrogen acceptor is not required in the present catalytic C−H
silylation reaction.
The silylation of various alkoxy-substituted benzene
derivatives by n-C6H13SiH3 was then examined by the use of
complex 1 as a catalyst under the same conditions (5 mol %
cat., anisole/silane = 10:1, benzene, 80 °C, 8 h). Some
representative results are listed in Table 2. In parallel with
t
anisole, p-R1-anisoles (R1 = Me, Bu, Ph) could be easily
silylated in excellent isolated yields (Table 2, 4b, 4c, 4d). In
the case of 1,4-dimethyoxybenzene (3e), only monosilylated
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Research Article
ab
Table 2. Regioselective C−H Silylation of Aromatic Ethers with n-C6H13SiH3 by a Calcium Catalyst
a
b
Reaction conditions: complex 1 ([Ca]) (0.02 mmol), substrate (4 mmol), n-C6H13SiH3 (0.4 mmol), 80 °C, 8 h, benzene (2 mL). Yields of
c
isolated product based on n-C6H13SiH3. 24 h.
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to the calcium center in pattern A or B, depending on the
group at the ortho position (R = H or Me). In the case of an
anisole compound without substituents (R = H) at the ortho
positions, the sp2 ortho C−H bond was activated to give the
calcium phenyl species C with release of H2. In the case of 2-
methylanisole (3r), the strong steric repulsion between the
methyl group of the methoxy unit and the o-methyl group
contributed to the formation of B and resulted in the benzylic
sp3 C−H bond activation to give calcium benzyl species D
with release of H2. Subsequent σ-bond metathesis reaction
between n-C6H13SiH3 and C (or D) would yield the silylation
product and regenerate 2. The isolation and structural
characterization of the calcium phenyl complex 5 and calcium
benzyl complex 6, together with the kinetic studies, strongly
corroborate this mechanism.
In summary, we have demonstrated that calcium benzyl
complex 1 can serve as a powerful catalyst precursor for the
C−H silylation of varied alkoxy-substituted benzene deriva-
tives under mild conditions without the requirement of a
hydrogen acceptor. The silylation of anisole derivatives without
a substitutent at the ortho position exclusively takes place at the
ortho-sp2 C−H bond to afford the corresponding ortho-
silylated anisole derivatives. In the case of 2-methylanisole, the
silylation occurs selectively at the benzylic sp3 C−H bond.
Plenty of polar aromatic C−X bonds (X = F, Cl, Br, SMe,
NMe2, CF3, OCF3) are compatible with this catalyst system.
The isolation and structural characterization of the calcium
anisyl complexes 5 and 6 offered important insight into the
mechanistic aspects of the catalytic process. This study might
stimulate the development of transition-metal-free C−H bond
activation catalytic systems that are solely based on cheap and
abundant metals. Work is continuing to explore the other
functionalization reactions of aromatic C−H bonds by
calcium-based catalysts.
Scheme 1. Stoichiometric Reactions of Complex 2 with 4-
Bromoanisole and 2-Methylanisole
distorted TpAd,iPr ligand (Figure 2, left). In comparison, the
anisyl unit in complex 6 is bonded to the calcium center in a
chelating fashion through the methoxy group and the benzylic
carbon atom (C2) to form a five-membered ring (Figure 2,
right). The reaction of the anisyl complex 5 or 6 with 3 equiv
of n-C6H13SiH3 in THP/hexane at 40 °C can also give the
calcium hydride 2 in good yields, with concomitant formation
of the ortho-silylation product 4j or 4r.
Treatment of complex 2 with 4-iodoanisole (3k) at room
temperature led to a mixture containing calcium anisyl
complex, calcium iodide complex, and anisole. No reaction
between complex 2 and 1,2-methoxybenzene (3w) or N,N-
dimethylaniline (3x) was observed under the same conditions.
The catalytic silylation of 2-deuterio-4-methylanisole with n-
C6H13SiH3 gave a primary kinetic isotope effect (kH/kD = 2.45)
(Figure S60). This result suggests that C−H activation might
be the rate-determining step in the present catalytic reaction.
On the basis of the above-mentioned experimental
observations, a possible mechanism for the catalytic C−H
silylation is shown in Scheme 2. At first, the reaction of calcium
benzyl 1 with n-C6H13SiH3 would afford calcium hydride 2,
with concomitant formation of hexyl(4-methylbenzyl)silane
(Figure S7). The oxygen atom of anisole would be coordinated
Figure 2. ORTEP drawing of [(TpAd,iPr)Ca(o-MeO-m-BrC6H3)] (5) and [(TpAd,iPr)Ca(o-MeO-CH2C6H4)] (6) with thermal ellipsoids drawn at
the 20% probability level. All of the hydrogen atoms in the pyrazole groups are omitted for clarity.
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Scheme 2. Possible Mechanism for the Catalytic C−H Silylation of Anisole and 2-Methylanisole
CCDC 2038572 (5) and 2038573 (6) contain the
supplementary crystallographic data for this paper. These
data are provided free of charge by The Cambridge
Crystallographic Data Centre.
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05440.
X-ray data for compounds 5 and 6 (CIF)
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (Nos. 21971232 and 21901238).
■
Experimental details, spectroscopic and analytical data
(PDF)
REFERENCES
■
AUTHOR INFORMATION
■
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Corresponding Author
Jianhua Cheng − State Key Laboratory of Polymer Physics
and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China;
University of Science and Technology of China, Hefei, Anhui
230029, China; orcid.org/0000-0003-0330-4846;
Email: jhcheng@ciac.ac.cn
Authors
Lanxiao Zhao − State Key Laboratory of Polymer Physics and
Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China;
University of Science and Technology of China, Hefei, Anhui
230029, China
Xianghui Shi − State Key Laboratory of Polymer Physics and
Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c05440
Notes
The authors declare no competing financial interest.
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