A Catalytic SEAr Approach to Dibenzosiloles Functionalized at Both Benzene Cores

Article abstract of DOI:10.1002/anie.201504066

A general procedure for the catalytic preparation of dibenzosiloles functionalized at one or both benzene rings starting from readily available ortho-silylated biphenyls is reported. This method provides rapid access to silole building blocks substituted with chlorine atoms at both phenylene groups, thereby allowing catalytic access to directly polymerizable dibenzosiloles. Moreover, it is shown that, despite the involvement of highly electrophilic intermediates, a considerable range of Lewis-basic, for example, oxygen- and nitrogen-containing, functional groups is tolerated. The mechanism of this intramolecular electrophilic aromatic substitution (S_EAr) proceeds through a sulfur-stabilized silicon cation, generated catalytically from the hydrosilane precursor.

Full text of DOI:10.1002/anie.201504066

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201504066
German Edition: DOI: 10.1002/ange.201504066
CÀH Silylation
A Catalytic S Ar Approach to Dibenzosiloles Functionalized at Both
E
Benzene Cores**
Lukas Omann and Martin Oestreich*
Dedicated to Professor Paul Knochel on the occasion of his 60th birthday
Abstract: A general procedure for the catalytic preparation of
dibenzosiloles functionalized at one or both benzene rings
starting from readily available ortho-silylated biphenyls is
reported. This method provides rapid access to silole building
blocks substituted with chlorine atoms at both phenylene
groups, thereby allowing catalytic access to directly polymer-
izable dibenzosiloles. Moreover, it is shown that, despite the
involvement of highly electrophilic intermediates, a consider-
able range of Lewis-basic, for example, oxygen- and nitrogen-
containing, functional groups is tolerated. The mechanism of
The last pathway relies on the generation of a silicon
electrophile that is subsequently attacked by the proximal
aromatic ring. The idea traces back to the work of Kawashima
and co-workers, where heterolytic cleavage of the SiÀH bond
+
À
6 5 4
by hydride abstraction with excess [Ph C] [B(C F ) ] yields
3
a silylium ion (1a!2a; Scheme 1). A stoichiometric amount
of base is required to absorb the strong acid formed during the
[13]
deprotonation of the Wheland intermediate.
The same
transformation was achieved by Curless and Ingleson by SiÀH
bond activation with the strong Lewis acid B(C F ) in the
6
5 3
[14]
this intramolecular electrophilic aromatic substitution (S Ar)
presence of a catalytic amount of base.
E
proceeds through a sulfur-stabilized silicon cation, generated
catalytically from the hydrosilane precursor.
S
iloles and their benzannulated congeners continue to
[1–3]
attract considerable attention.
These members of the
metallole family distinguish themselves from their all-carbon
analogues in their unique electronic structure, and hence are
valuable building blocks for p-conjugated conducting poly-
mers. Elegant procedures for the catalytic synthesis of siloles
fused to two benzene rings have been reported, including
[
4]
[5]
palladium-catalyzed CÀH/CÀOTf, CÀI/SiÀH, and SiÀMe/
[
6]
CÀBr coupling reactions, as well as iridium- and rhodium-
catalyzed [2+2+2] cycloadditions of silicon-tethered diynes
Scheme 1. Syntheses of dibenzosiloles by Friedel–Crafts-type reactions.
[
7,8]
with alkynes.
Recently, ring closures starting from ortho-
silylated biphenyls 1 to dibenzosiloles 2 were established.
These approaches are particularly attractive because of the
convenient availability of the starting materials, thereby
allowing rapid access to functionalized silole motifs. Three
conceptually different methods for this intramolecular for-
The variety of catalytic transformations to construct
dibenzosiloles is impressive, but these methods have rarely
been applied to the synthesis of polymerizable systems
[15,16]
functionalized at both benzene cores.
However, such
[
9]
mation of the CÀSi bond were developed: transition-metal-
motifs are highly relevant for making polymeric materials by
[10]
[17,18]
catalyzed CÀH/SiÀH coupling, homolytic aromatic substi-
cross-coupling reactions.
We therefore set out to elabo-
[11,12]
tution involving silicon-centered radicals,
and electro-
rate an S Ar-based procedure that allows rapid synthesis of
E
philic aromatic substitution (S Ar) with stabilized silicon
cations.
benzannulated silole building blocks with functional groups at
E
[
13,14]
both phenylene groups (1!2; Scheme 2, top left). We had
[19]
shown before that cationic RuÀS complexes 3 (Scheme 2,
top right) cleave the SiÀH bond of hydrosilanes R SiH (with
3
[
*] L. Omann, Prof. Dr. M. Oestreich
R = alkyl and aryl) into a sulfur-stabilized silicon cation and
[
20,21]
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
a hydride.
This catalytic generation of a silicon electro-
[20]
phile was then used in the intermolecular S Ar of indoles.
E
Conversely, benzenes failed to react even when decorated
with an electron-donating group. We nevertheless envisioned
the application of this activation mode to the intramolecular
SEAr reaction of benzenes, namely to the ring closure of 1 to 2
catalyzed by 3. The catalytic cycle (Scheme 2, bottom) would
Homepage: http://www.organometallics.tu-berlin.de
[
**] L.O. thanks the Fonds der Chemischen Industrie for a predoctoral
fellowship (2015–2017), and M.O. is indebted to the Einstein
Foundation (Berlin) for an endowed professorship. We thank Dr.
C. David F. Kçnigs and Caroline Apel for initial experiments, as well
as Dr. Elisabeth Irran for the X-ray analyses (all TU Berlin).
[21]
start with activation of the SiÀH bond by 3 to give the
sulfur-stabilized silicon cation (1!4) followed by the planned
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504066.
S Ar reaction to form the Wheland intermediate (4!5) along
E
1
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Chemie
Table 1: Optimization of the Friedel–Crafts-type CÀH bond silylation.
[a]
[b]
Entry Catalyst
Solvent T [8C] t [h] Ratio
Conv. [%]
2
b
8b 9b
[
[
[
[
[
[
[
c]
d]
e]
e]
e]
e]
e]
1
2
3
4
5
6
7
3a
C H Cl 90
19
22
41
12
2
8
3
1
2
2
6
54 97
6
5
(2 mol%)
3a
C H Cl 120
1
2
2
6
4
3
21
6
5
(2 mol%)
3a
toluene 140
toluene 140
toluene 140
C H Cl 140
0.25 20
0.25
25
(1 mol%)
3b
8
11
(1 mol%)
3c
0.25 92
0.25 90
0.25 87
>99
96
(1 mol%)
3c
6
5
(1 mol%)
3c
toluene 120
96
(1 mol%)
[
a] Determined by GLC analysis; ratio based on integration of baseline-
separated peaks. [b] Determined by GLC analysis with reference to
starting material. [c] Reactions were performed on a 0.1 mmol scale in
Schlenk tubes (15 mL). [d] Reactions were performed on a 0.05 mmol
scale in sealed pressure tubes (2 mL). [e] Reactions were performed on
a 0.2 mmol scale in 35 mL vessels with microwave heating.
Scheme 2. S Ar-based CÀH silylation (top left; FG=functional group),
E
F
À
catalyst 3 with BAr =tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
4
(
top right), and proposed catalytic cycle with simplified catalyst
F
À
structure (bottom; BAr 4 counterion omitted for clarity).
procedure), which finally resulted in selective conversion of
1b into 2b within minutes rather than hours. The catalytic
activity of 3a and, likewise, 3b with bulkier triisopropylphos-
phine as ligand was moderate (Table 1, entries 3 and 4).
Another significant improvement was seen when switching
from an electron-rich to an electron-poor phosphine ligand in
3. Full conversion was obtained at 1408C in 15 min when
catalyst 3c was used in either toluene or chlorobenzene
(Table 1, entries 5 and 6). A slightly lower temperature led to
a minor decrease in yield (Table 1, entry 7). It is noteworthy
that catalyst 3c does not perform better than 3a in the sealed
vessel (not shown).
with ruthenium hydride 6. The hydride would then abstract
a proton from 5, thereby releasing the desired dibenzosilole
[
22]
(
5!2). Dihydrogen adduct 7 finally liberates dihydrogen as
the sole by-product, thereby shifting the equilibrium to 3.
We began with identification of the optimal reaction setup
using 1b as a model substrate and employing complex 3a with
an electron-donating triethylphosphine ligand as the catalyst
[23]
(
Table 1). With chlorobenzene as solvent,
nearly full
conversion of 1b was detected by GLC analysis after
maintaining the reaction at 908C for 19 h. However, the
main product was not the expected dibenzosilole 2b but
disiloxane 9b along with traces of silanol 8b (Table 1,
entry 1). These undesired (by-)products were formed from
traces of water despite the use of Schlenk techniques. To
exclude water more effectively, this and further reactions
were repeated in sealed pressure tubes. This measure indeed
suppressed side reactions, and little 8b and 9b were formed,
but conversion of 1b into 2b dramatically slowed down even
at 1208C (Table 1, entry 2). We attribute this observation to
the inability of adduct 7 to release dihydrogen and reform
catalyst 3, namely, dihydrogen inhibits the catalysis
Having established an effective procedure for the Friedel–
Crafts-type CÀH bond silylation, we investigated its scope
(Scheme 3). Model substrate 1b afforded 2b in 91% yield of
the isolated product. Exchanging one methyl group on the
silicon atom for a phenyl group (1c) resulted in an even
higher yield of 2c. Interestingly, no conversion was observed
with the bulkier substrate 1d under the optimized setup. The
use of the less-reactive although sterically less-hindered
catalyst 3a led to isolation of the desired 2d in 72% yield.
Next, we explored the functional-group tolerance of the
reaction. Substrates with a trifluoromethyl group (1e) as well
as a methyl group (1 f) in the 4’-position smoothly underwent
cyclization, thereby affording 2e and 2 f in 99% and 90%
yield, respectively. Substrate 1g with a methyl group in the 3’-
position could potentially form two regioisomeric siloles, but
(
Scheme 2, bottom). We concluded that unsealed vessels
would be essential to achieve reasonable reaction rates, but
this does not go hand in hand with the demand for the
rigorous exclusion of moisture. After extensive experimenta-
[24]
tion, this problem was overcome by microwave heating. An
impressive rate acceleration allowed us to use an open
reaction vessel (see the Supporting Information for a detailed
[25]
2g was formed in quantitative yield exclusively. A frequent
drawback of reactions involving silicon electrophiles is their
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Scheme 4. Substrate scope of the Friedel–Crafts-type CÀH bond silyla-
tion for the synthesis of difunctionalized silole monomers. [a] Reac-
tions were performed on a 0.2 mmol scale. Yields of isolated products
after purification by flash chromatography on silica gel. [b] The other
regioisomer was not detected by GLC or NMR analysis.
selectively into 2m. Sterically hindered 1n with a chlorine
substituent in the 2’-position also underwent ring closure to
afford 2n. The yields of such challenging transformations
were decent throughout.
Finally, we developed a route to an orthogonally difunc-
tionalized silole monomer that opens the door to more
sophisticated polymeric structures through site-selective
cross-coupling reactions. Precursor 1o was subjected to the
standard procedure and smoothly cyclized to afford 2o as
a single regioisomer in quantitative yield (Scheme 5). Diben-
zosilole 2o was then further transformed into the new silole
motif 2p in two routine steps.
Scheme 3. Substrate scope of the Friedel–Crafts-type CÀH bond silyla-
tion for the synthesis of monofunctionalized dibenzosiloles with differ-
ent substituents at the silicon atom. rs=regioselectivity; TBDPS=tert-
butyldiphenylsilyl; Tf=trifluoromethanesulfonyl. [a] Reactions were
performed on a 0.2 mmol scale. Yields of isolated products after
purification by flash chromatography on silica gel. [b] With 3a
(
5 mol%) and 60 min reaction time. [c] The other regioisomer was not
detected by GLC or NMR analysis. [d] Low conversion. With 3c
5 mol%) and 60 min reaction time, a complex mixture was obtained.
(
incompatibility with oxygen donors. It is, therefore, remark-
able that substrate 1h with a silicon-protected hydroxy group
converted cleanly into 2h in 99% yield of the isolated
product, again with complete regiocontrol. Conversely, pre-
cursor 1i with a triflate group that would directly allow for
subsequent cross-coupling reactions showed lower conver-
sion. Apart from oxygen-containing Lewis bases, nitrogen
functionalities were also tolerated, thus affording 2j from 1j
in 75% yield as a single regioisomer. Moreover, chloro-
substituted 1k was successfully transformed into 2k.
Dichloro-substituted dibenzosiloles are known to be
relevant building blocks for the construction of p-extended
[
18]
structures, but the catalytic synthesis of these molecules is
unprecedented. A set of representative dichloro-substituted
cyclization precursors (1l—n; Scheme 4) was subjected to the
optimized procedure. These substrates showed lower reac-
tivity, but full conversion was achieved with slightly higher
catalyst loading (3 instead of 1 mol%) and prolonged
Scheme 5. Synthesis of an orthogonally difunctionalized silole mono-
mer. DIBAL-H=diisobutylaluminum hydride. [a] Reaction was per-
formed on a 0.2 mmol scale. Yield of isolated products after purifica-
tion by flash chromatography on silica gel. [b] The other regioisomer
was not detected by GLC or NMR analysis.
reaction times (60 instead of 30 min). The C -symmetric 2l
2
h
was isolated in good yield from the reaction of 1l, and the
unsymmetrically substituted 1m was again converted regio-
1
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In summary, we have reported here a powerful method for
rapid access to mono- and difunctionalized dibenzosiloles
through an intramolecular electrophilic aromatic substitution
[14] L. D. Curless, M. J. Ingleson, Organometallics 2014, 33, 7241 –
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7
[
15] Shimizu and co-workers reported the catalytic synthesis of a 2,8-
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(
SEAr) involving a catalytically generated silicon electrophile.
2
2
lyzed C(sp )ÀC(sp ) cross-coupling (see Ref. [4]). For a related
We showed that microwave heating significantly accelerates
the reaction, thereby allowing us to perform this air- and
moisture-sensitive transformation in an open vessel. With this
reaction setup, the desired products were isolated in up to
2
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These methoxycarbonyl- and methoxy-substituted derivatives
could also serve as polymerizable building blocks.
9
9% yield within minutes. In addition, this procedure
represents the first catalytic synthesis of dibenzosiloles with
halogenation at both benzene cores, and hence will prove
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6
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Keywords: electrophilic substitution · homogeneous catalysis ·
microwave chemistry · regioselectivity · SiÀH bond activation
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Published online: July 14, 2015
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