Modular approach to silicon-bridged biaryls: Palladium-catalyzed intramolecular coupling of 2-(arylsilyl)aryl triflates

Article abstract of DOI:10.1002/anie.200804146

(Chemical Equation Presented) Bridge of Si: Intramolecular direct arylation of 2-(arylsilyl)aryl triflates is catalyzed smoothly by Pd(OAc) ₂/PCy₃ in the presence of Et₂NH in dimethylacetamide (DMA), giving rise to the corresponding silicon-bridged biaryls in good to excellent yields. The new approach has led to the synthesis of a silicon-bridged 2-phenylindole (see scheme) that exhibits blue photoluminescence in the solid state with extremely high quantum yields.

Full text of DOI:10.1002/anie.200804146

Communications
DOI: 10.1002/anie.200804146
Synthetic Methods
Modular Approach to Silicon-Bridged Biaryls: Palladium-Catalyzed
Intramolecular Coupling of 2-(Arylsilyl)aryl Triflates**
Masaki Shimizu,* Kenji Mochida, and Tamejiro Hiyama
Silicon-bridged biaryls (SBArs), such as 9-silafluorenes,
silicon-bridged dithienosiloles, and phenylindenes have
recently been the subject of growing interest as components
of light-emitting materials, thin-film transistors, host materials
for electroluminescent devices, and solar cells.[1] Optical and
electronic properties of p-conjugated organic materials are
closely related to their electronic structures, which can be
tuned by installation of appropriate functional groups, fusion
of aromatic rings, and/or bridging heteroatoms.[2] For this
reason, facile synthesis of SBAr compounds with broad
structural variations, which satisfy such requisites for con-
jugation control, is of great importance for advanced modi-
fication and exploitation of functional organic materials
exhibiting unique properties. Conventional synthesis of
SBAr is achieved by dilithiation of the corresponding 2,2’-
dihalobiaryls followed by silylation with dichlorosilanes
(Scheme 1, route a).[1,3,4] Although the protocol is simple,
indole, readily prepared by the present approach, is disclosed
to exhibit highly efficient solid-state blue fluorescence.
Transition-metal-catalyzed direct arylation has recently
emerged as a versatile strategy for aryl–aryl bond formation.[5]
Intramolecular coupling reactions constitute efficient synthe-
ses of fused carbo- and heterocycles, such as fluorenes,
carbazoles, and dibenzofurans.[6] We attempted a Pd-cata-
lyzed intramolecular direct coupling of two aryl groups,
tethered by a silylene moiety, as a new synthetic route to
SBArs, as the structural variation of the SBAr compounds
attainable by this modular approach is potentially much
broader than that by the conventional approach.[7] We
focused our attention on 2-(arylsilyl)aryl triflate substrates.
Such species are often used as precursors for arynes,[8] and
could be readily prepared from the corresponding o-bromo-
phenols and arylchlorosilanes by sequential silylation, retro-
Brook rearrangement, and triflation.[9]
Initially, 2-[dimethyl(phenyl)silyl]phenyl triflate (1a)[10]
was subjected to the typical conditions for Pd-catalyzed
intramolecular direct arylation of 2-(phenoxymethyl)bromo-
benzenes.[11] However, the use of inorganic bases, such as
NaOAc, K2CO3, and Cs2CO3, which were effective for the
reported transformations, resulted in no production of the
desired 9,9-dimethyl-9-silafluorene (2a). Use of Et2NH as a
base with Pd(OAc)2/2-dicyclohexylphosphino-2’,4’,6’-trime-
thylbiphenyl barely afforded 2a (27% yield) with complete
consumption of 1a. As many unidentified compounds were
produced, we suspected that the product (2a) might be
decomposed under the conditions by nucleophilic attack of a
base, an acetate ion, or a triflate ion at the SiMe2 moiety. We
exchanged the SiMe2 group for SiBu2, to induce steric
protection, and screened reaction conditions for this sub-
strate. Pleasingly, the desired intramolecular coupling pro-
ceeded smoothly to afford the corresponding product (2c) in
68% yield, by using secondary amines, such as Et2NH and
Cy2NH, as a base with a catalyst system of Pd(OAc)2/2PCy3 in
dimethylacetamide (DMA) at 1008C (Table 1, entry 2). For
reasons of handling and affordability, we selected to use
Et2NH as the base for further study.
Scheme 1. Approaches to silicon-bridged biaryls. a) Dilithiation and
subsequent silylation with Cl2SiR2; b) Pd catalyst, base.
structural variations of accessible SBAr are highly dependent
on the availability of the dihalobiaryls, and thus are limited, in
most cases, to symmetrical substrates. We report herein a
novel and versatile approach to SBArs, which involves Pd-
catalyzed intramolecular direct arylation of readily available
2-(arylsilyl)aryl triflates (Scheme 1, route b). This approach is
applicable to the facile synthesis of not only symmetrical and
asymmetrical functionalized 9-silafluorenes, but also SBArs
containing heteroaromatic rings, such as furan, thiophene,
and pyrrole. In addition, the novel silicon-bridged 2-phenyl-
Under the aforementioned optimized conditions, we
carried out the cyclization of various 2-(phenylsilyl)phenyl
triflates 1b–h with various substituent groups on the silicon
atom (Table 1). Bulkier carbonaceous substituents, such as
iPr, sBu, tBu, and phenyl, led to a great increase in yields of
[*] Prof. Dr. M. Shimizu, K. Mochida, Prof. Dr. T. Hiyama
Department of Material Chemistry, Graduate School of Engineering
Kyoto University, Kyoto University Katsura
Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2445
E-mail: m.shimizu@hs2.ecs.kyoto-u.ac.jp
Homepage: http://npc05.kuic.kyoto-u.ac.jp/e-index.htm
the corresponding 9-silafluorene product
2
(Table 1,
entries 4–8). Such effects of the bulkier substituents may be
considered reasonable, as the Thorpe–Ingold effect and gem-
dialkyl effect from these groups positions the two phenyl
groups in 1 closer together, and retards the decomposition of
2.[12]
[**] This work was supported by a Grant-in-Aid for Creative Scientific
Research, No. 16GS0209, from the Ministry of Education, Culture,
Sports, Science and Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804146.
9760
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Table 1: Palladium-catalyzed intramolecular direct arylation of 1.[a]
Entry
1
R2
t [h]
2
Yield [%][b]
Scheme 2. Regioselectivity of intramolecular coupling of 1o and 1p.
1
2
3
4
1b
1c
1d
1e
1e
1 f
Et2
75
20
24
14
25
20
24
14
2b
2c
2d
2e
2e
2 f
39
68
63
92
91
94
88
87
Conditions for 1o: Pd(OAc)2 (2.5 mol%), PCy3(5 mol%), Et2NH
(2 equiv), DMA, 24 h, 1008C; for 1p: Pd(OAc)2 (10 mol%), PCy3
(20 mol%), Et2NH (2 equiv), DMA, 24 h, 1208C.
nBu2
nHex2
iPr2
5[c]
6
iPr2
20 mol% of PCy3) than 1o for the cyclization to proceed
smoothly. A similar regiochemical outcome was reported for
the intramolecular direct arylation of methyleneoxy-tethered
bromobenzenes.[6b] Although these systems gave mixtures of
regioisomers, the mixtures were easily separated by column
chromatography on silica gel followed by gel-permeation
chromatography to give each isomer as an isomeric pure
form, in both cases.
sBu2
tBu, Ph
Ph2
7
8
1g
1h
2g
2h
[a] Conditions, unless otherwise stated: 1 (1.0 mmol), Pd(OAc)2 (11 mg,
0.050 mmol), PCy3 (28 mg, 0.10 mmol), Et2NH (200 mL, 2.0 mmol),
dimethylacetamide (DMA, 2 mL), 1008C. [b] Yield of isolated product.
[c] 1 (1.0 mmol), Pd(OAc)2 (5.5 mg, 0.025 mmol), PCy3 (14 mg,
0.050 mmol), Et2NH (200 mL, 2.0 mmol), DMA (2 mL), 1008C. Cy=
cyclohexyl, Tf=triflate.
In addition to 9-silafluorenes, the present method can be
extended to the preparation of silicon-bridged benzene-
heteroarenes (Table 3). Thiophene-containing triflates 3 and
Table 2: Synthesis of functionalized 9-silafluorenes 2.[a]
Table 3: Synthesis of silicon-bridged biaryls.
Entry
Triflate
Product
Yield [%][a]
94
Entry
1
R2
R1
R2
R3
2
Yield [%][b]
1[b]
1[c]
2
1i
1j
1k
1l
1m
1n
iPr2
iPr2
iPr2
tBu, Ph
iPr2
H
H
H
CN
CN
H
H
H
H
H
H
OMe
NMe2
OMe
CF3
H
OMe
OMe
2i
2j
2k
2l
2m
2n
90
94
88
88
93
98
3[d]
4
2[b]
95
93
5
6
iPr2
[a] Conditions, unless otherwise stated: 1 (1.0 mmol), Pd(OAc)2 (6 mg,
0.025 mmol), PCy3 (14 mg, 0.050 mmol), Et2NH (200 mL, 2.0 mmol),
DMA (2 mL), 1008C. [b] Yield of isolated product. [c] 1i (0.050 mmol),
Pd(OAc)2/2PCy3 (5 mol%), Et2NH (0.10 mmol). [d] Reaction was
effected at 1208C.
3[c]
4[c]
85
56
The scope of variety in functionalized 9-silafluorenes,
accessible by using this method, is summarized in Table 2.
Symmetrical and asymmetrical 9-silafluorenes 2i–n, substi-
tuted with electron-donating and/or -withdrawing groups,
such as NMe2, OMe, CF3, and CN, were synthesized from the
corresponding triflates in high to excellent yields. For a
substrate incorporating CF3 as substituent R3, the reaction
required 1208C to proceed smoothly (Table 2, entry 3).
We next examined the regioselectivity of the direct
arylation with substrates that possessed a substituent R4,
meta to the silyl moiety on the phenyl ring without the triflate
group (Scheme 2). When substituent R4 was a methoxy group,
the arylation occurred preferentially at the carbon center para
to the methoxy group, giving 2o as a major product along with
2o’. Conversely, in the case of 1p, bearing fluorine as R4, the
carbon center ortho to the fluorine substituent was the more
reactive position, giving rise to 2p’ as a major product.
Compound 1p required a higher reaction temperature
(1208C) and catalyst loading (10 mol% of Pd(OAc)2,
5[d]
6[e]
85
[a] Yield of isolated product. [b] Triflate (1.0 mmol), Pd(OAc)2 (5.6 mg,
0.025 mmol), PCy3 (14 mg, 0.050 mmol), Et2NH (200 mL, 2.0 mmol),
DMA (2 mL), 1008C. [c] Triflate (0.20 mmol), Pd(OAc)2 (2.2 mg,
0.010 mmol), PCy3 (5.6 mg, 0.020 mmol), Et2NH (40 mL, 0.40 mmol),
DMA (0.4 mL), 1008C. [d] 11 (0.10 mmol), Pd(OAc)2 (1.1 mg,
0.005 mmol), PCy3 (2.8 mg, 0.010 mmol), Et2NH (20 mL, 0.20 mmol),
DMA (0.2 mL), 1008C. [e] 13 (1.0 mmol), Pd(OAc)2 (11 mg,
0.050 mmol), PCy3 (28 mg, 0.10 mmol), Et2NH (400 mL, 4.0 mmol),
DMA (2 mL), 1008C.
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5 cyclized smoothly at the 3- and 2-positions of the thiophene
ring, respectively, to give 4 and 6 in 94% and 95% yield
(Table 3, entries 1 and 2). Benzothiophene, benzofuran, and
indole moieties also underwent direct arylation, affording
tetracyclic products 8, 10, and 12 in fair to high yields (Table 3,
entries 3–5). Furthermore, helicene-type molecule 14 was
efficiently synthesized in 85% yield by twofold intramolec-
ular coupling of 2,5-bis(silyl)thiophene 13 (Table 3, entry 6).
The molecular structures of 12 and 14 were unambiguously
confirmed by X-ray analysis of their single crystals (for 12, see
Figure 3).[13]
Figure 1. Photoluminescence spectra of 12.
The Ni-catalyzed twofold cross-coupling reaction of 3,6-
dimethoxy-9-silafluorene 2n with phenylmagnesium bromide
under the Dankwardtꢀs conditions,[14] led to the isolation of
diphenylated silafluorene 15 in 84% yield with the silylene
moiety intact (Scheme 3). This result demonstrates that a
Scheme 3. Ni-catalyzed cross-coupling reaction of 2n with PhMgBr.
Figure 2. Fluorescence images of 12 (lex =365 nm): a) 1.9ꢀ10À5
m
solution in cyclohexane; b) microcrystal; c) powder; d) thin-film pre-
pared by spin-coating from a toluene solution; e) dispersed in PMMA
film.
methoxy group that is tolerated in the direct arylation can
serve as not only an electron-donating group but also as a site
for substitution to extend the p-conjugated system of
silafluorenes.
Among the silicon-bridged biaryls presented here, silicon-
bridged 2-phenylindole 12 was found to exhibit noteworthy
solid-state fluorescence. The photoluminescent properties,
spectra, and images are shown in Table 4, Figures 1 and 2. A
essential for the development of optoelectronic devices, such
as organic light-emitting diodes (OLEDs) and solid-state
organic lasers,[15] and, in addition, the development of
efficient blue-emitters for OLEDs is currently the subject of
intense research efforts in materials science.[16] Silicon-
bridged 2-phenylindoles are a new and attractive class of
high performance blue-emitters, although further studies of
their chemical, physical, optical, and electrical properties are
required for practical application.
Table 4: Photoluminescent properties of 12.[a]
Entry
lmax [nm][b]
Ff[c]
1
2
3
4
5
solution[d]
microcrystal
powder
402
434
433
414
408
0.70
0.91
0.91
0.90
1.00
To gain insight into the excellent solid-state luminescent
efficiency of 12, we inspected the crystal structure by single-
crystal X-ray diffraction. The molecule crystallizes in triclinic
thin-film[e]
in PMMA[f]
¯
space group P1 from a hexane/CH2Cl2 solution. The silicon-
[a] Excited under UV light (lex =320 nm). [b] Emission maxima. [c] Abso-
lute quantum yield determined by a calibrated integrating sphere system.
[d] 1.9ꢀ10À5 m in cyclohexane. [e] Prepared by spin-coating from a
toluene solution. [f] Dispersed in poly(methyl methacrylate) (PMMA)
film.
bridged tetracyclic framework is almost planar and two
isopropyl groups are oriented perpendicular to the plane,
respectively, to induce efficient s*–p* conjugation (Fig-
ure 3a).[17] The asymmetric unit contains two molecules that
are well separated with an almost orthogonal arrangement of
the chromophores. The crystal packing is thus suitable to
prevent intermolecular electronic interactions, such as p–p
stacking, that lead to luminescence quenching (Figure 3b).
In summary, we have described the Pd-catalyzed intra-
molecular direct arylation of readily available 2-(arylsilyl)aryl
triflates as a novel and versatile synthetic route to silicon-
bridged biaryls, which have attracted growing attention in
materials science. Key to this success is the installation of
bulky substituents on silicon and the use of Et2NH as a base.
In addition, the development of this new approach has led to
the discovery of the highly blue-emissive organic solid 12.
solution of 12 in cyclohexane was, upon irradiation with UV
light (lex = 320 nm), strongly fluorescent with an emission
maximum at 402 nm and a high quantum yield (Ff) of 0.70
(Table 4, entry 1), whereas the microcrystal, powder, and
spin-coated thin-film of 12 showed slightly red-shifted emis-
sion maxima of 414–434 nm (Figure 1) and excellent quantum
yields of 0.90–0.91 (Table 4, entries 2–4). Remarkably, the
quantum yield of 12 dispersed in a poly(methyl methacrylate)
(PMMA) film was 1.0, with intense blue emission (Figure 2e;
Table 4, entry 5). High solid-state luminescent efficiency is
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Received: August 22, 2008
Published online: November 5, 2008
À
Keywords: biaryls · C H activation · luminescence · palladium ·
silicon
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[10] For preparation of triflates 1, see the Supporting information.
[11] Conditions:
Pd(OAc)2,
2-(diphenylphosphino)-2’-(N,N-
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