Iridium-Catalyzed Intramolecular C–H Silylation of Siloxane-Tethered Arene and Hydrosilane: Facile and Catalytic Synthesis of Cyclic Siloxanes

Article abstract of DOI:10.1002/adsc.201700160

Catalytic C–H silylation is an increasingly important topic in the field of organosilicon chemistry and homogenous catalysis as well as organic synthesis, but such syntheses and transformations are usually challenging and often incompatible with some functional groups. In this manuscript, a new type of silanol-directed intramolecular dehydrogenative silylation under iridium catalysis has been developed. The described silylative cyclization has good functional group compatibility, providing a general and efficient route to unsymmetrical cyclic siloxanes with multiple potentially modifiable silicon atoms in high site-selectivities. The resulting organosilicon compounds exhibit diverse synthetic applications owing to their unique structures. (Figure presented.).

Full text of DOI:10.1002/adsc.201700160

Title: Iridium-Catalyzed Intramolecular C-H Silylation of Silicon-
Tethered Arene and Hydrosilane: Facile and Catalytic Synthesis
of Cyclic Siloxanes
Authors: Yan Lin, Ke-Zhi Jiang, jian cao, Zhan-Jiang Zheng, Zheng Xu,
yuming cui, and Li-Wen Xu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700160
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10.1002/adsc.201700160
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Iridium-Catalyzed Intramolecular C-H Silylation of Siloxane-
Tethered Arene and Hydrosilane: Facile and Catalytic
Synthesis of Cyclic Siloxanes
Yan Lin,a Ke-Zhi Jiang,a Jian Cao,a Zhan-Jiang Zheng,a Zheng Xu,a Yu-Ming Cui,a,*
and Li-Wen Xu a,*
a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal
University, Hangzhou 311121, P. R. China
Fax: (+86)-571-28865135; phone: (+86)-571-28865135; e-mail: ym_cui@hznu.edu.cn or liwenxu@hznu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
reactions have been reported in the past years, most
successful cases are limited to heteroarenes, bulky
arenes, or those with suitable directing groups.[5] An
alternative approach is the intramolecular C-H
silyaltion,[7] which has obvious advantages: (1)
avoiding the use of excess silanes or arenes, (2) easy
access to bifunctional products by further
transformations, (3) creating the possibility to
construct chiral organosilicon compounds with high
stereoselectivities.[7h] Although these one-pot
procedures allow the efficient dehydrogenative
cyclization of in situ generated (hydrido)silyl ether,
one major drawback associated with this strategy is
the incompatibility with some functional groups
sensitive to dihydrosilane under iridium or rhodium
catalysis. Recently, organosilanol has been used as a
robust, modifiable and/or removable directing group
in catalytic regioselective functionalization of arenes
through C-H activation (Scheme 1, equation 1).[8]
However, no practical methods for the silanol-
directed silylation reactions of inert C-H bonds with
high levels of site-selectivity control exist, probably
due to the inefficiency of silylation reactions of this
type.
Abstract. Catalytic C-H silylation is an increasingly
important topic in the field of organosilicon chemistry and
homogenous catalysis as well as organic synthesis, but its
synthesis and transformation is usually challenging and
often incompatibility with some functional groups. In this
manuscript, a new type of silanol-directed intramolecular
dehydrogenative silylation under iridium catalysis has been
developed. The described silylative cyclization had good
functional group compatibility, providing a general and
efficient route to unsymmetric cyclic siloxanes with
multiple potentially modifiable silicon atoms in high site-
selectivities. The resulting organosilicon compounds
exhibited diverse synthetic applications owing to their
unique structures.
Keywords: Homogeneous catalysis; silylation; iridium;
cyclization; siloxane
Organosilicon compounds are extremely useful and
have found widespread applications in the fields of
organic
synthesis,[1]
silicon-based
functional
materials,[2] and biomedically relevant agents.[3] Thus
accordingly, numerous synthetic methods to prepare
organosilanes have been extensively developed,
including nucleophilic substitution, cross-coupling,
hydrosilylation, and others.[4] In addition, recent years
have witnessed the development of transition metal-
catalyzed direct silylation of inert C-H bonds. [5] The
C-H silylation strategy is particularly attractive
because this strategy avoids prefunctionalization of
starting materials and shows excellent functional
group tolerance. One of the principal challenges in
silylation of C-H bonds is how to balance between
reactivity and selectivity since C-H activation
reactions generally require harsh reaction conditions,
which might cause competing functionalization of
undesired C-H sites in a complex molecule.[6]
Although intermolecular regioselective silylation
Despite
the
fundamental
importance
of
organosilicon compounds, the application of the
selective silylation of organosilanes to construct
valuable silicon-based heterocycles containing
multiple silicon atoms has not been reported.
Moreover, no report concerning other heteroatoms
except N and O atoms employed to facilitate the
intramolecular cyclization process was documented.
Inspired by the impressive C-H activation reactions,
we envision that the intramolecular dehydrogenative
silylation might be realized in a catalytic C-H
activation manner if the silanol was used to tether the
(hydrido)silyl group. To the best of our knowledge,
so far, only few related examples have appeared, in
which cyclic disilanes or hydrosilanes were used as
1
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10.1002/adsc.201700160
Advanced Synthesis & Catalysis
silylation reagents to introduce multiple silicon atoms
into the target products.[9] And no report on further
applications of silylation products existed, due to the
difficulty to differentiate one silyl group from others.
It would be highly desirable if each silicon atom in
such multi-Si motifs has different reactivity and is
suitable for further selective transformation, which
will undoubtedly expand the scope of their synthetic
applications.
Scheme 2. Scope of substrates 1 from arylsilanols.
The substrates 1 (Scheme 2) were obtained by
direct reactions of readily available silanols with the
appropriate chlorosilanes. We evaluated the
dehydrogenative silylations of 1a to optimize the
reaction conditions including metal salts, ligands,
hydrogen acceptors, and temperatures (see
Supporting Information). In general, varying the
electronic properties of the substituents on the
aromatic rings of the substrates had negligible effect
on the cyclization process. As shown in Scheme 2,
under the optimized conditions, the reactions of 1
decorated by various electron-donating (1l, 1m, 1n)
and electron-withdrawing groups (1o, 1p) proceeded
smoothly to afford the corresponding disiloxanes in
high yields. Substrates with meta and para
substituents were transformed into the cyclized
products in good to excellent yields, whereas those
bearing substituents at ortho position on the arenes
gave the desired products in slightly diminished
yields. We observed the distinct effects of
substituents at the two silicon atoms on cyclization
reactions. Increasing the size of substituents on the
(hydrido)silyl groups or decreasing the size of
substitutents on the silicon atoms of silanol moieties
lowered the yields of the products to some extent.[11]
This trend can be elucidated since the Thorpe-Ingold
effect in the linker can facilitate the cyclization and
the steric hindrance at the end group will be
detrimental for dehydrogenative silylation. The
structure of 2a was confirmed by X-ray
crystallography.[12] Notably, the trimethylsilylmethyl
group was also tolerated and the product 2k with
three silicon atoms was produced in 79% yield.
Scheme 1. Silanol-directed arene C-H activation.
In this work, we wish to report a practical method
to synthesize a variety of unsymmetric cyclic
disiloxanes in good yields through the iridium-
catalyzed dehydrogenative silylation of arene C-H
bonds (Scheme 1, equation 2). The siloxane-tethered
silane substrates were obtained easily by simple
silylation of readily available aryl, benzyl, and
phenoxysilanols and the following cyclization
procedure displayed ample substrates scope. In
addition, as expected, the resulting products showed
vast application potential in a range of selective
transformations, owing to the unique structures
bearing two unequal and modifiable silicon atoms.
4,5-Benzo-2-oxo-1,3-disiloles serve as useful
synthetic equivalents of benzynes to form polycyclic
compounds with interesting photophysical properties.
Previously, these compounds were prepared by
oxidation reaction of 1,2-bissilylbenzene or Co-
catalyzed [2+2+2] cycloaddition reaction of proper
alkynes.[10] However, the two silicon atoms in most of
them have the same substituents, which limited their
further selective transformation. We initiated our
studies aimed at developing a conceptually different
pathway to this class of disiloxanes comprising two
or more different silyl groups, which started from the
organosilanols.
2
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10.1002/adsc.201700160
Advanced Synthesis & Catalysis
Scheme 3. Scope of substrates 3 from benzylsilanols.
Scheme 4. Scope of substrates 5 from arenoxylsilanols.
The facile synthesis of five-membered cyclic
disiloxanes under the present catalytic system
encouraged us to turn our attention to construction of
other cyclic variants in a similar way. We found that
slight modification of standard conditions eliminated
the obstacle to silanol-directed silylation of arene and
allowed efficient synthesis of six-membered cyclic
multi-Si molecules. To this end, 3a was chose as a
model substrate to enact this strategy (Scheme 3).
Under similar conditions to those employed for
silylation of 1, the cyclized product 4a was formed in
moderate yield, with an incomplete conversion of
starting material. To our delight, replacement of phen
by the more strongly electron-donating Me4-phen led
to a full conversion, thus affording the product 4a in
94% yield. Having identified the active catalyst for
the dehydrogenative silylation of 3a, we next
surveyed the scope of this transformation using
substrates bearing the hydrodimethylsilyl group. As
illustrated in Scheme 3, substrates with an
unsubstituted phenyl ring or containing an electron-
rich or electron-deficient substituent on the arenes
yielded the corresponding products in high yields.
Remarkably, the silylation occurred exclusively at the
sterically less hindered position (4g, 4h, 4i).
Substitution at α carbon to the silicon atom in
benzylsilyl moiety had considerable influence on the
yields (4l, 4m). Notably, the double functionalization
was viable for substrate 3o with two hydridosilyl
groups para to each other, furnishing 4o in 71% yield,
a molecule with four silicon atoms, as confirmed by
NMR analysis and X-ray single crystal structure.[12]
The dehydrogenative silylation reactions were not
restricted to the substrates derived from aryl and
benzyl
silanols.
Those
resulting
from
phenoxylsilanols proved likewise applicable, yielding
the cyclized products in a similar procedure (Scheme
4). It was found that the silylation reactions displayed
broad substrate scope to include a variety of phenyl,
naphthyl, and even more complex molecules. Firstly,
the substrates underwent smooth intramolecular
silylation reactions in spite of the electronic
properties of substituents in aromatic rings. For
example, the silylation reactions of substrates with
para substituents delivered the desired products in
good to excellent yields (6a-6e). In contrast to the
report by Jeon and co-workers in which a specialized
directing group was required to improve the overall
yields,[7g] our catalytic silylation method tolerated
well the bulky substituents at ortho position to
hydroxyl group to provide the corresponding
products in good yields (6f, 6g, 6j). The
regioselectivity of the silylation reaction was mainly
determined by the steric hindrance around the
reactive sites. As expected, the silylation happened at
para position to substituents for meta-substituted
arenes (6h). Besides substituted phenols, naphthols
were capable substrate precursors, giving the
products in satisfactory yields with high levels of
site-selectivity control (6k, 6l, 6n). The reaction of
the chiral substrate 6k, derived from the (R)-BINOL,
gave the silylation product at 3’ position without
affecting the benzyl protecting group. Next, the
application of our catalytic cyclization method to
silylation of a more complex bioactive molecule was
also examined. Compared with the Jeon’s work, the
selective C2-silylation of estrone was realized
without extra protecting group manipulation. Finally,
3
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10.1002/adsc.201700160
Advanced Synthesis & Catalysis
the double dehydrogenative cyclization of the
substrate 5o derived from hydroquinone was also
achieved, producing 6o in 78% yield.[12]
To gain insight into the silylation reactions, the
parallel competitive cyclization of substrates 3b and
The synthetic utilities of the developed silylation
method were further highlighted by the selective
transformations of Si-C and Si-O bonds in the
products (Scheme 7). In the presence of ICl, 2q and
4b underwent smooth C-X bond formation reactions
3e was conducted in one flask (Scheme 5, equation 1). at C-Si sites. Interestingly, the aryl and benzyl Si-C
Substrate 3e bearing an electron-withdrawing fluorine
atom proved to be more reactive than 3b, indicating
the nucleophilic nature of the generated silyl iridium
species.[13] Further insight into this type of the
reaction was obtained from the silylation of substrate
7 comprising two kind of available ortho C-H bonds
in one molecule (Scheme 5, equation 2). The
silylation occurred preferably at phenyl C-H bond
over benzyl C-H bond, affording a mixture of
regioisomers, in a 6:1 ratio of 8 to 9.
bonds in 4a were converted into C-I and C-Cl bonds,
delivering the halide 13 that is difficult to access by
other methods in one simple step. Considering the
significance of phenolic motifs in pharmaceutically
and biologically active compounds,[14] we further
exploited a traceless directing group strategy for
regioselective silylation of phenols. For 6a, selective
cleavage of Si-O bonds with BuLi followed by TBAF
led to the formation of ortho silylated phenol,
providing an alternative approach to this class of the
compounds.[7g]
Scheme 5. Inter- and intramolecular competitive
cyclization reactions.
Scheme 7. Selective cleavage of Si-C and Si-O bonds.
The resulting silylation products are useful
synthetic intermediates because both of the Si-C
bonds and Si-O bonds are cleavable and/or
modifiable, especially for the regio- and
stereoselective transformations determined by the two
unequal silyl groups. Starting from the commercially
available triphenylsilanol, after two sequential
silylative cyclization reactions, the product 12
containing three silicon atoms was obtained in good
overall yield (Scheme 6). It was noteworthy that
twice regioselective transformations occurred in the
whole process, probably due to the steric hindrance
exerted by the distinct silyl groups.
In conclusion, we have developed a general
method for siloxane-tethered arene ortho-silylation in
which silanol formally serves as the directing group
for iridium-catalyzed dehydrogenative silylation of
arenes C-H bonds. This strategy features high
efficiency, good functional group compatibility, and
high regioselectivity, allowing the synthesis of a
range of unsymmetric cyclic siloxanes containing two
potentially modifiable silicon atoms from readily
available aryl, benzyl, and phenoxysilanols, which is
complementary to the impressive field of C-H
activation reactions.[15] Furthermore, the resulting
silylation products with the unique structures
exhibited diverse synthetic applications. Further
studies on the enantioselective silylation and the
mechanism of this process are ongoing.
Experimental Section
Representative Procedure for the Synthesis of
Disiloxane 2d:
[Ir(cod)Cl]2 (6.7 mg，1.0 mol%) and phen ( 3.9 mg, 2.2
mol%) were added to a flame-dried, nitrogen-purged
Scheme 6. Further transformation of 2a.
4
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10.1002/adsc.201700160
Advanced Synthesis & Catalysis
septum-capped vial .The mixture was dissolved in THF (3
mL), and 1d (0.342 g, 1 mmol), nbe (0.113 g, 1.2 mmol)
was added to the mixture. The septum on the vial was
replaced by a screw cap with a Teflon liner under a N2
atmosphere .The reaction mixture was stirred for 12 h at
100 °C. Flash silica gel column chromatography (hexane)
purification of the residue gave the desired product 2d as a
colorless liquid.
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Acknowledgements
This project was supported by the National Natural Science
Founder of China (No. 51303043, 21472031, and 21503060),
Science and Technology Department of Zhejiang Province
(2015C31138).
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6
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Advanced Synthesis & Catalysis
COMMUNICATION
Iridium-Catalyzed Intramolecular C-H Silylation of
Siloxane-Tethered Arene and Hydrosilane: Facile
and Catalytic Synthesis of Cyclic Siloxanes
Adv. Synth. Catal. Year, Volume, Page – Page
Yan Lin, Ke-Zhi Jiang, Jian Cao, Zhan-Jiang
Zheng, Zheng Xu, Yu-Ming Cui,* and Li-Wen Xu*
7
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