Synthesis of tripod-shaped oligo(phenylene)s with multiple ethenyl groups at the bases for chemisorption on hydrogen-terminated silicon surfaces

Article abstract of DOI:10.1016/S0040-4039(02)02641-2

This communication describes an efficient and convergent synthesis of monodisperse, nanometer-sized, and tripod-shaped oligo(phenylene)s with a triallylsilyl group at the base of each leg and a chlorophenyl group at the focal point of the tripod. These mo

Full text of DOI:10.1016/S0040-4039(02)02641-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 815–817
Synthesis of tripod-shaped oligo(phenylene)s with multiple
ethenyl groups at the bases for chemisorption on
hydrogen-terminated silicon surfaces
Xiaobin Deng and Chengzhi Cai*
Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
Received 19 November 2002; accepted 20 November 2002
Abstract—This communication describes an efficient and convergent synthesis of monodisperse, nanometer-sized, and tripod-
shaped oligo(phenylene)s with a triallylsilyl group at the base of each leg and a chlorophenyl group at the focal point of the
tripod. These molecules were designed as model compounds for the study of chemisorption of rigid molecules containing multiple
ethenyl groups on hydrogen-terminated silicon surfaces. The compounds were synthesized from p-chlorophenyl-tris(p-bro-
mophenyl)silane via selective Pd-catalyzed Ishiyama–Miyaura reaction with bis(pinacolato)diboron, followed by Suzuki coupling
with aryl dihalides to elongate the legs. The legs were then end-capped with triallylsilyl groups through Suzuki coupling with
4-triallyphenylboronic acid. © 2003 Elsevier Science Ltd. All rights reserved.
Hydrosilylation of 1-alkenes on hydrogen-terminated
silicon surfaces leads to well-ordered and robust mono-
layers that bind to the silicon surfaces via strong SiꢀC
bonds.1–5 This new class of thin films that directly
couples organic materials and semiconductors may find
applications in sensors, nonlinear optics, and opto-elec-
tronics.6–9 For many of these potential applications, it
is desirable or necessary to control the orientation and
density of the functional groups in the films. So far,
surface hydrosilylation has been studied only with
mono-ethenyl derivatives as the adsorbates. The molec-
ular orientation in the resultant films is determined
mainly by the packing and ordering of the molecules.
The density of functional molecules in the film may be
adjusted by co-deposition with inert molecules, but
phase separation often perturbs the system.10,11 We are
interested in exploring the use of multidentate bonding
with rigid molecules such as the tripod-shaped 1 and 2
to orient individual molecules, to control the density of
the functional groups, and to enhance the stability of
the films. Compared to the reported tripod-shaped and
rigid adsorbates designed for chemisorption on gold or
* Corresponding author. Tel.: +1-713-743-2710; fax: +1-713-743-2709; e-mail: cai@uh.edu
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02641-2

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X. Deng, C. Cai / Tetrahedron Letters 44 (2003) 815–817
Scheme 1. Reagents and conditions: (a) 1,4-dibromobenzene, n-BuLi, ether, rt, 12 h, 43%; (b) bis(pinacolato)diboron (3.3 equiv.),
PdCl2(dppf) (3 mol%), KOAc, DMSO, 80°C, 3 h, 83%; (c) 4-bromo-4%-iodobiphenyl (3.3 equiv.), Pd(PPh3)4 (3 mol%), aq.
Na2CO3, toluene, 85°C, 12 h, 75%; (d) allylMgBr, ether, reflux, 12 h, 79%. (e) n-BuLi, −78°C; then B(O-iPr)3, −78°C to rt, 12 h;
2 M HCl; 67%. (f) 9 (3.3 equiv.), Pd(PPh3)4 (3.0 mol%), aq. Na2CO3, toluene, 85°C, 12 h, 82% for 1 and 70% for 2.
oxide surfaces,12–15 1 and 2 have a larger footprint, and
can form multiple SiꢀC bonds with a hydrogen-termi-
nated silicon surface via hydrosilylation, exposing the
focal point of the tripod (ꢀPhCl group) for further
derivatization, e.g. through Pd-catalyzed coupling reac-
tions.16,17 Furthermore, the size and rigidity of these
molecules make it possible to image the single
molecules chemisorbed at flat silicon surfaces by scan-
ning tunneling microscopy (STM). Herein, we describe
the syntheses of 1 and 2.
The same conditions were then applied to couple 6 with
9 to furnish the larger tripod molecules 2 in 70% yield.
Satisfactory characterization data have been obtained
for all new compounds.19
In conclusion, we report an efficient and convergent
synthesis of nanometer-sized and tripod-shaped
oligophenylenes 1 and 2 that are end-capped with mul-
tiple allyl groups. The synthetic scheme can be applied
to the synthesis of other derivatives of oligophenylene
tripods. The triallyl groups at the base of each leg
permit the enlargement of the tripod base by repeating
hydrosilylation with HSiCl3 and alkenylation with a
Grignard reagent to generate carbosilane dendrons20
for chemisorption on a variety of surfaces.21,22
The reaction of 3 with p-bromophenylmagnesium bro-
mide at room temperature generated compound 4 in
43% yield (Scheme 1), where one of the four arms bears
a less reactive group (Cl). Using silicon atom as the
core for constructing tripod structures has been
described by Yao and Tour.14 Following the procedures
developed by Miyaura et al.,18 the bromine atoms in 4
were replaced with pinacolboronate groups by reaction
with 3.3 equiv. of bis(pinacolato)diboron to provide 5
in high yield (83%). The triboronate derivative 5 served
as a building block to elongate the three legs by Suzuki
coupling with oligo(phenylene)s end-capped with differ-
ent halide atoms. Thus, Suzuki coupling of 5 with 3.3
equiv. of 4-bromo-4%-iodobiphenyl selectively provided
the tribromide derivative 6 in 75% yield. The tribromide
6 can be end-capped with a variety of surface-active
groups for chemisorption at different substrate sur-
faces. For example, to introduce ethenyl groups to 6 for
chemisorption on hydrogen-terminated surfaces, allyl
groups were introduced to 4-bromophenyltrichloro-
silane (7) to provide 8 in 79% yield, which was con-
verted to the boronic acid 9 in 67% yield by LiꢀBr
exchange and quenching with B(O-iPr)3. The Suzuki
coupling of 4 with 3 equiv. of 9 in toluene under
catalytic amount of (Ph3P)4Pd afforded 1 in 82% yield.
Acknowledgements
We thank the financial support of this work by the
Texas Advanced Research Program under Project No.
003652-0365-1999, the Robert A. Welch Foundation,
and the Petroleum Research Fund.
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1
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1
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1
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Hz), 7.68 (d, 12H, J=8.1 Hz); MS (EI, m/z): calcd for