A silicon-based approach to oligoarenes by iterative cross-coupling reactions of halogenated organo[(2-hydroxymethyl)phenyl]dimethylsilanes

Article abstract of DOI:10.1021/ja074728s

Highly efficient synthesis of silyl-substituted oligoarenes with defined structures is achieved by an iterative cross-coupling reaction and deprotection sequence using organo[(2-hydroxymethyl)phenyl]dimethylsilanes. The excellent stability of the tetraorganosilicon compounds as well as mild and divergent conditions for cross-coupling and deprotection steps allows preparation of highly conjugated oligoarenylsilanes such as unsymmetrically disilylated quinquethiophenes. Copyright

Full text of DOI:10.1021/ja074728s

Published on Web 08/29/2007
A Silicon-Based Approach to Oligoarenes by Iterative Cross-Coupling
Reactions of Halogenated Organo[(2-hydroxymethyl)phenyl]dimethylsilanes
Yoshiaki Nakao,* Jinshui Chen, Masaaki Tanaka, and Tamejiro Hiyama*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan
Received July 12, 2007; E-mail: nakao@npc05.kuic.kyoto-u.ac.jp; thiyama@npc05.kuic.kyoto-u.ac.jp
Scheme 1. Cross-Coupling Reaction with Halogenated
Organometallics
The structures of conjugated oligoarenes are playing key roles
in photonic and optoelectronic materials such as organic light-
emitting diodes, field-effect transistors, semiconductors, and fluo-
rescent sensors.1 Syntheses of oligoarenes have relied exclusively
on the cross-coupling reaction, which allows direct connection of
C(sp2)-C(sp2) bonds with high chemo-, stereo-, and regiospeci-
ficities.2 However, installation of a leaving group and a metallic
center at terminal arenes is necessary to carry out the second cross-
coupling reactions,3 making the whole sequence leading to the
desired oligoarenes a tedious multistep synthesis. Whereas iterative
cross-coupling reactions of metalated organic halides would offer
an efficient access to oligoarenes with well-defined structures
(Scheme 1), it is hard to control their reaction modes (cross-coupling
vs homocoupling, etc.) without precise discrimination between
several nucleophilic moieties.4,5
Scheme 2. Modulation of Reactivity of
Organo[2-(hydroxymethyl)phenyl]dimethylsilanes
We report herein a silicon-based approach leading to defined
oligoarene structures based on iterative cross-coupling reactions of
organo[(2-hydroxymethyl)phenyl]dimethylsilanes. We have recently
disclosed that the silicon reagents cross-couple with a range of
electrophiles in the presence of a palladium catalyst and a weak
base such as K2CO3 under mild conditions by virtue of the proximal
hydroxy group.6 Since the enhanced reactivity of the silicon reagents
is “turned off” simply upon protection of the hydroxy group
(Scheme 2), the compounds with a halogen and OH-protecting
group are found to serve as promising coupling partners for the
iterative cross-coupling sequence. Whereas a silicon-based cross-
coupling reaction has gained significant importance in light of
availability, stability, and nontoxicity of organosilicon compounds,7
repetitive use of such silicon-based cross-coupling agents for the
syntheses of oligoarenes has never been addressed before.8
To verify our strategy, we synthesized various organo[(2-
hydroxymethyl)phenyl]dimethylsilanes bearing a bromo group and
either THP or acetyl protection,9 and examined their cross-coupling
reactions (eq 1 and Table 1). The reaction of 4-(diphenylamino)-
phenylsilane 1a (12 mmol) with THP-protected 4-bromophenylsi-
lane 2a (10 mmol) proceeded smoothly in the presence of [(η3-
C3H5)PdCl]2 (1.0 mol % Pd), RuPhos (2.1 mol %),10 CuI (3.0 mol
%), and K2CO3 (25 mmol) in THF-DMF (3:1) at 75 °C for 7 h to
give 4-silylated biphenyl 3aa in 88% yield (entry 1).11 Recyclable
silicon residue 4 was also isolated in 86% yield.6 The acetyl
protection of 2′a also tolerated the present conditions to give the
corresponding biaryl 3′aa in 93% yield (entry 2), demonstrating
flexibility of the present protocol depending on the functional groups
of target molecules. The coupling of 1a with various brominated
arylsilanes even on a gram scale also met with success, giving a
range of silylated biaryls in a highly chemoselective manner with
good to excellent yields (entries 3-8). It is worth noting that the
mild reaction conditions utilizing K2CO3 as a base for the present
silicon-based cross-coupling technology allow the participation of
silafluorene substrate 2e with its C-Si bonds being completely
intact (entry 8). The alkenylsilane functionality of 2f was inert upon
protection of the hydroxy group, and the arylsilane moiety of 1a
exclusively underwent the cross-coupling reaction (entry 9). Thie-
nylsilane 1b also cross-coupled successfully to give 2-thienyl-
substituted silicon compounds including silylated bithiophene 3bb
even on a gram-scale (entries 10 and 11). Similar cross-coupling
reactions using (E)-styrylsilane 1c took place in the absence of a
copper cocatalyst to give phenylenevinylenes with a silyl terminus
in good yields (entries 12 and 13).
The next cross-coupling reaction of the silylated biaryls thus
obtained was examined subsequently. For example, deprotection
of the hydroxy group of 3bb under acidic conditions proceeded
smoothly to give silylbithiophene with a free hydroxy group 5 in
89% yield, whereas treatment of 3bb with n-BuLi followed by
bromination with 1,2-dibromo-1,1,2,2-tetrafluoroethane afforded
5-bromo-5′-silyl-2,2′-bithiophene 6 in 90% yield (Scheme 3). The
high stability of the silylated biaryl under both acidic and basic
transformations is remarkable and ascribed to its tetraorganosilane
structure. The coupling reaction of 5 and 6 succesfully gave silylated
quarterthiophene 7 in 82% yield. Bromination leading to 8 followed
by cross-coupling with 2,5-disilyl-substituted thiophene 1d gave
9
11694
J. AM. CHEM. SOC. 2007, 129, 11694-11695
10.1021/ja074728s CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4. Linear Synthesis of Highly Conjugated
Table 1. Cross-Coupling Reaction of
Organo[2-(hydroxymethyl)phenyl]dimethylsilanes (1) with
Halogenated Organo[2-(alkoxymethyl)phenyl]dimethylsilanes (2)
Oligoarenylsilane by Iterative Cross-Coupling-Deprotection
Sequencea
a Reagents and Conditions: (a) TsOH‚H2O (2 mol %), MeOH-CH2Cl2
(1:1), room temp, overnight; (b) [(η3-C3H5)PdCl]2 (1 or 5 mol % Pd),
RuPhos (2 or 11 mol %), CuI (3 or 5 mol %), K2CO3 (2.5 equiv), THF-
DMF (3:1), 75 °C; (c) DIBAL-H (1.1 equiv), CH2Cl2, -78 °C, 2 h; (d)
(dppf)PdCl2‚CH2Cl2 (5 mol %), CuI (5 mol %), K2CO3 (2.5 equiv), THF-
DMF (3:1), 75 °C, 24 h.
type reagents with high potency in applications to synthetic new
functional materials.
Acknowledgment. This work has been supported financially
by a Grant-in-Aid for Creative Scientific Research, No. 16GS0209,
and Priority Areas “Synergy of Elements for Creation of Functional
Molecules” from MEXT.
Supporting Information Available: Detailed experimental pro-
cedures including spectroscopic and analytical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
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Scheme 3. Convergent Synthesis of Disilylated
Quinquethiophenea
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a Reagents and Conditions: (a) PPTS (20 mol %), MeOH, 40 °C, 2 h;
(b) n-BuLi, TMEDA, THF, -40 °C to room temp, 1 h, then BrCF2CF2Br,
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unsymmetrically disilylated quinquethiophene 9, which would enjoy
potential application to two-photon absorption materials.12 On the
other hand, linear extension of silylated biaryl 3ad was performed
with the deprotection-cross-coupling sequence employing 2′c, 2′b,
and 2e, affording highly conjugated protected oligoarenylsilane 12
in an efficient manner (Scheme 4).
In summary, the iterative cross-coupling strategy utilizing organo-
[(2-hydroxymethyl)phenyl]dimethylsilanes has been demonstrated
as a novel and efficient silicon-based entry to oligoarenes with well-
defined structure. Mild and divergent conditions for the cross-
coupling and deprotection steps provide the stable tetraorganosilicon-
(9) For details, see Supporting Information.
(10) Milne, J. E.; Buchwald, S. L. J. Am. Chem. Soc. 2004, 126, 13028.
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standard Schlenk technique under an argon atmosphere except for the
measurement of [(η3-C3H5)PdCl]2 in a glovebox.
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Y.; Akiyama, S.; Maeda, S.; Kanbara, H.; Mori, Y.; Kurihara, T. AdV.
Mater. 2007, 19, 1826.
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