9-Silafluorenes via base-promoted homolytic aromatic substitution (BHAS) - The electron as a catalyst

Article abstract of DOI:10.1021/ol503574k

Transition-metal-free intramolecular radical silylation of 2-diphenylsilylbiaryls via base-promoted homolytic aromatic substitution (BHAS) to give 9-silafluorenes is reported. 2-Diphenylsilylbiaryls are readily prepared, and cross dehydrogenative silylation occurs with tert-butylhydroperoxide (TBHP) as a cheap stoichiometric oxidant in the presence of a small amount of tetrabutylammonium iodide (TBAI) as an initiator. These cyclizations are catalyzed by the electron.

Full text of DOI:10.1021/ol503574k

Letter
pubs.acs.org/OrgLett
9
‑Silafluorenes via Base-Promoted Homolytic Aromatic Substitution
(
BHAS) − The Electron as a Catalyst
Dirk Leifert and Armido Studer*
̈ ̈
̈
Institute of Organic Chemistry, Westfalische Wilhelms-Universitat, Corrensstraße 40, 48149 Munster, Germany
*
S Supporting Information
ABSTRACT: Transition-metal-free intramolecular radical silylation of
-diphenylsilylbiaryls via base-promoted homolytic aromatic substitu-
2
tion (BHAS) to give 9-silafluorenes is reported. 2-Diphenylsilylbiaryls
are readily prepared, and cross dehydrogenative silylation occurs with
tert-butylhydroperoxide (TBHP) as a cheap stoichiometric oxidant in
the presence of a small amount of tetrabutylammonium iodide (TBAI)
as an initiator. These cyclizations are catalyzed by the electron.
n the past few years, silicon derivatives in which the silicon
Scheme 1. A 9-Silafluorene from a 2-Biarylsilane
I
atom is integrated into a π-system have gained increased
attention due to their interesting physical properties. As
members of this class of compounds, 9-silafluorenes, that are
silicon bridged biaryls, have gained particular attention due to
1
their highly valued use in functional organic materials. For
example, they have been integrated into OLEDs, OFETs,
photovoltaic cells, and fluorescent sensors.
There are several reported synthetic routes toward 9-
2
3
4
5
1
,6
silafluorenes. Shimizu et al. used C−C-bond formation in
2
-(arylsilyl)aryl trilfates as the key step for construction of the
7
biaryl moiety via palladium catalysis. It is also possible to
access 9-silafluorenes via iridium-catalyzed [2 + 2 + 2]-
8
cyclizations starting from diynes. Furthermore, these interest-
ing silyl derivatives have been successfully prepared from 2,2′-
dihalogenated biaryls by metal−halogen exchange and sub-
9
sequent cyclization with dihalosilanes or via palladium-
10
catalyzed silylation with dihydrosilanes. The target com-
11
pounds can also be obtained via palladium- or rhodium-
1
2
catalyzed demethylative silylation of 2-methylsilyl-2′-bromo-
or 2′-boronobiaryls, respectively.
In contrast to the intensively investigated transformations of
doubly prefunctionalized substrates, to our knowledge only two
using readily accessible 2-biarylsilanes as precursors (Scheme
1c).
13
processes that are a rhodium-catalyzed cyclization (Scheme
14
There are several options for the clean generation of silyl
radicals from silanes, and various reactions involving silyl
1
a) and a sila-Friedel−Crafts type reaction (Scheme 1b)
describing cross dehydrogenative cyclization starting with 2-
biarylsilanes to give 9-silafluorenes are known in the literature.
As compared to the above-discussed procedures which use
doubly prefunctionalized precursors, an advantage of these two
latter reactions is the ready access of the starting 2-biarylsilanes
16
radicals have been developed. Silyl radicals are known to add
17
to alkenes, and also homolytic aromatic substitutions with Si-
radicals have been reported, albeit these latter reactions occur
18
̌
́
with lower efficiency. Mares and Chvalovsky reported the
13a
synthesis of 9,9-dimethyl-9-silafluorene from the corresponding
from 2-bromobiaryls. However, as a drawback of the Rh-
version the price of the expensive catalyst has to be considered.
We therefore regard the development of metal-free variants as
important. Moreover, regarding the cationic metal-free sila-
Friedel−Crafts reaction, the reactive trityl cation which is used
silane by a homolytic aromatic substitution via pyrolysis at
18a
around 600 °C. However, the silafluorene was only formed
in traces.
We decided to use cheap and commercial tert-butylhydroper-
1
9,15b
1
1
4
5
oxide (TBHP) in combination with TBAI as an initiator
as a stoichiometric reagent restricts the scope of that method.
Motivated by recent results in radical fluorenone synthesis
we decided to develop an efficient new method for transition-
metal-free preparation of 9-silafluorenes via radical chemistry by
Received: December 11, 2014
©
XXXX American Chemical Society
A
DOI: 10.1021/ol503574k
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2
0
for generation of silyl radicals. An obvious problem in this
approach is the competing oxidation of the intermediately
generated silyl radical by the tert-butylperoxyl radical to the
peroxybenzoate (TBPB) and cumylhydroperoxide (CHP) as
oxidants (Table 1, entries 20 and 21) or adding potassium
hydroxide or pivalic acid as additives (Table 1, entries 22 and
23) did not show any beneficial effects.
2
1
corresponding silanol derivative.
For reaction optimization the para-methylbiphenyl 1a was
used as a model substrate. Preparation of 1a and all other
silylated biphenyls is described in the Supporting Information
With the optimized conditions identified (entry 8), we
investigated the scope and limitation of the intramolecular cross
dehydrogenative silylation (Figure 1). With the unsubstituted
(SI). Reactions were carried out in benzene at 90 °C in a sealed
tube for 24 h, and different initiators (1 mol %) were tested.
With iron(II) and iron(III) salts only low yields of the targeted
silafluorene 2a were obtained (Table 1, entries 1−3). By using
Table 1. Reaction Optimization Using 1a as a Substrate
a
entry
oxidant (equiv)
initiator (mol %)
solvent
yield (%)
1
2
3
4
5
6
7
8
9
TBHP (2.2)
TBHP (2.2)
TBHP (2.2)
TBHP (2.2)
TBHP (2.2)
TBHP (2.2)
TBHP (2.2)
TBHP (3.3)
TBHP (4.4)
TBHP (5.5)
TBHP (3.3)
TBHP (3.3)
TBHP (3.3)
TBHP (3.3)
TBHP (3.3)
TBHP (3.3)
TBHP (3.3)
TBHP (3.3)
TBHP (3.3)
TBPB (2.2)
CHP (3.3)
FeCp (1.0)
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
MeCN
34
37
28
53
39
66
55
65
62
58
19
26
traces
11
46
63
62
59
42
33
51
2
FeCl (1.0)
3
Fe(acac) (1.0)
2
CuCl (1.0)
2
CuOAc (1.0)
CuI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (0.4)
TBAI (0.8)
TBAI (1.5)
TBAI (2.0)
TBAI (5.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
TBAI (1.0)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
MTBE
DMA
DMSO
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
b
TBHP (3.3)
13
c
TBHP (3.3)
51
a
1
Determined by H NMR using CH Br as an internal standard.
2
2
b
c
Addition of KOH (20 mol %). Addition of PivOH (20 mol %).
copper(II) chloride the yield was increased (53%) (Table 1,
entry 4), but CuOAc showed a lower initiator efficiency (39%)
Figure 1. Various silafluorenes 2a−n prepared (isolated yields).
a
Isolated as an inseparable mixture with starting silane 1j.
(Table 1, entry 5). Interestingly, CuI led to an improved yield
of the desired 9-silafluorene (66%) (Table 1, entry 6). We
assumed that the iodide anion in this salt supports initiation,
which was proven by using TBAI (Table 1, entry 7). A similar
yield as that for the CuI-initiated process was achieved with
TBAI upon using a slightly higher amount of TBHP (65%)
silafluorene 2b a slightly higher isolated yield (66%) was
obtained as compared to 2a. The yield further improved by
switching to the tert-butyl substituted congener 1c to afford
silafluorene 2c in 71% yield. Fluorinated biphenyl derivatives
showed similar yields (2d−f). Regioselectivity was moderate for
the meta-fluoro system 1e to give 2e and 2e′ as a 2.5:1 mixture
of isomers. In agreement with earlier reports on similar BHAS,
(Table 1, entry 8). Further increasing the amount of oxidant
did not provide a better result (Table 1, entries 9, 10).
Considering the green chemistry aspect, we decided to further
optimize following the metal-free variant (Table 1, entry 8).
Other solvents such as acetonitrile, tert-butylmethyl ether,
dimethylformamide, and dimethyl sulfoxide (Table 1, entries
15b,22d
the ortho-isomer was formed as a major product.
Since
the sterically slightly more hindered isomer was formed as the
major compound, we assume regioselectivity to be steered by
electronic effects. Assignment of the major regioisomer to
structure 2e is based on NMR spectroscopy. Biphenyls bearing
1
1−14) and varying the amount of initiator did not lead to
improved yields (Table 1, entries 15−19). Using tert-butyl
B
DOI: 10.1021/ol503574k
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Organic Letters
Letter
other electron-withdrawing groups at the para-position, such as
chloride and trifluoromethyl, afforded the corresponding
products 2g and 2h in 77% and 75% isolated yield, respectively.
The reaction also worked well for the electron-richer methoxy
derivative (see 2i, 68%).
Scheme 3. Suggested Mechanism
We next investigated biphenyls 1d′ and 1j−k which carry
additional substituents at the arene moiety containing the
diphenylsilyl group. An electron-withdrawing group at the 6-
position is tolerated: 2d and 2j were obtained in moderate to
good yields (55% and 75%). Biphenyl 1k with a methoxy group
in the 5-position provided silafluorene 2k in 70% yield.
Cyclization on a pyridine ring was less efficient, as shown for
the preparation of 2m which was obtained in 39% yield. The
lower yield is likely due to the fact that pyridines show higher
reactivity toward homolytic aromatic substitution at the 2- and
2
3
4
-position. Cyclization on naphthalene ring systems did not
22
proceed regioselectively. In accordance with previous reports,
for the 2-naphthyl derivative 1l, regioisomer 2l was formed as
24
the major product (2l/2l′ = 4.8:1). By using the 1-naphthyl
congener 1n, we found formation of the six-membered ring to
give 3 to be favored over cyclization to the silafluorene 2n (2n/
3
= 1:2.9)
Finally, we tested whether the two Si-phenyl substituents can
be replaced by alkyl groups. To this end, silanes 1o and 1p were
prepared (see SI). A significantly lower NMR yield (25%) was
obtained under standard metal-free conditions for the dimethyl
derivative 1o to give 2o. A slightly improved yield (36%) for
this reaction was achieved by using CuOAc (1 mol %) as a
catalyst in the presence of tetramethylethylenediamine
cycle, revealing that the electron acts as catalyst in this
27
process.
In summary, we have presented a novel and environmentally
benign method for the preparation of 9-silafluorenes from easily
accessible biaryldiphenylsilanes with TBHP as a cheap and
commercially available reagent and TBAI as an initiator. These
reactions occur via intramolecular base-promoted homolytic
aromatic substitution (BHAS). The novel process represents a
further example of a reaction following the “Electron is a
25
(TMEDA, Scheme 2). An even lower yield was obtained
Scheme 2. Cyclization of Bisalkyl-Substituted Silanes
27a
Catalyst” concept. 9-Silafluorenes are important compounds
with potential applications as components in organic
2
−5
materials
which are now accessible via this inexpensive
and metal-free approach.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Experimental details and characterization data for the starting
material and products. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: studer@uni-muenster.de.
Notes
with the sterically more hindered isopropyl derivative 1p to
afford 2p by using the metal-free protocol (21%, NMR yield),
and also for the Cu-catalyzed process the yield was not
satisfactory (17% isolated).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The suggested mechanism for the silylative cross dehydro-
genative coupling is presented in Scheme 3. We assume that
This work was supported by the program “Sustainable
Chemical Synthesis (SusChemSys)” which is cofinanced by
the European Regional Development Fund (ERDF) and the
state of North Rhine-Westphalia, Germany, under the Opera-
tional Program “Regional Competitiveness and Employment”
2
6,27a
TBAI acts as an initiator and not as a catalyst.
In the
initiation step, single electron transfer (SET) from the iodide to
TBHP provides the tert-butoxyl radical along with the hydroxyl
anion. The tert-butoxyl radical then abstracts the H atom from
the silane 1b to give the corresponding Si-centered radical A
2007−2013. We also thank Lena Pitzer, Westfal
Universitat, for conducting a few experiments. Her results are
included in Figure 1.
̈
ische Wilhelms-
̈
28
which cyclizes to cyclohexadienyl radical B. The acidic
1
5b,22b
proton
in B gets deprotonated by the hydroxyl anion to
give the radical anion C which is a good single electron
reducing agent. Formal liberation of an electron from C
eventually provides product 2b thereby closing the catalytic
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