Facile synthesis of hypersilylated aromatic compounds by palladium-mediated arylation reaction

Article abstract of DOI:10.1039/c0cc00352b

The treatment of aryl iodides with tris(trimethylsilyl)silane in the presence of Pd(P(tBu)₃)₂ and the Huenig base leads to the formation of hypersilylated aromatic products in good to excellent yields without cleavage of weak Si-Si b

Full text of DOI:10.1039/c0cc00352b

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Facile synthesis of hypersilylated aromatic compounds by
palladium-mediated arylation reactionw
Aldes Lesbani,z Hitoshi Kondo, Jun-ichi Sato, Yoshinori Yamanoi* and Hiroshi Nishihara*
Received 12th March 2010, Accepted 2nd September 2010
DOI: 10.1039/c0cc00352b
The treatment of aryl iodides with tris(trimethylsilyl)silane in
the presence of Pd(P(tBu)3)2 and the Hunig base leads to the
¨
formation of hypersilylated aromatic products in good to
excellent yields without cleavage of weak Si–Si bonds under
mild conditions.
Scheme 2 Preparation of hypersilylated aromatic compounds.
Hypersilyl substituted arenes ((Me3Si)3Si–Ar) have emerged as
an important class of organosilanes due to their unique optical
and electronic properties.1 These properties originate from the
delocalized s-bonds in the catenated silicon framework and
are strongly influenced by functional substituents attached to
the silicon atoms. Because these compounds have shown
effective s–p conjugation between the oligosilane s-orbitals
and the p-orbitals, oligosilanes may act as good linkers
between two functional p-electron systems.2 Such compounds
are commonly prepared by coupling chlorotris(trimethylsilyl)-
silane with organometallic reagents or via the reaction between
tris(trimethylsilyl)silyllithium and an electrophile.3 However,
the range of products that can be prepared by these methods
has been limited by the high reactivity of organometallics.
In addition, because hypersilyl group sterically shields the
substituents, as indicated by the cone angle of 1991 for the
(Me3Si)3Si–C group4 in Scheme 1, it is difficult to efficiently
prepare these types of compounds. To overcome these
problems, development of a simple and versatile synthetic
chemistry method is a highly desired goal.
electrophilic and organometallic fragments via formation of
either carbon–carbon or carbon–heteroatom bonds.5 Because
silicon–silicon s-bond is weak (42 kcal molꢀ1),6 activation of
the silicon–silicon bonds in disilanes using transition metal
complexes has made possible the development of a variety of
useful synthetic methods for synthesizing organosilicon
compounds.7 In addition, hydrodisilanes usually decomposed
in the presence of transition metal complexes to give silylene
complexes.8 In recent years, we and other groups have applied
catalytic reactions toward the synthesis of new organosilane
compounds via the arylation of hydrosilane starting materials.9
Building on these results, we have developed an operationally
simple and inexpensive method for producing sterically over-
loaded hypersilylated arenes and related compounds using
arylation of hydrooligosilanes in the presence of palladium
catalysts without cleavage of Si–Si bonds (Scheme 2).
Initially, we examined the reaction of tris(trimethylsilyl)silane10
with 4-iodoanisole in the presence of palladium complexes and
base under argon. In the initial screening of Pd sources, bases,
and solvents, optimal results were obtained using 5 mol%
Pd(P(tBu)3)2 and 1.5 equiv. Et(iPr)2N in THF. The desired
product 1 was isolated in 57%. The reduced product, anisole,
was obtained as a side product in 38% (GC yield). No other
side products such as the homo coupling one could be
observed by GC-MS of reaction mixtures. The yield of the
product was slightly lower (49%) at 50 1C although the
reaction is complete within 2 d. The reaction efficiency was
sensitive to the type of halides employed. While substrate with
an iodide group underwent the arylation with moderate to
high yield, that with bromide group failed to give arylated
product even in the presence of Et4NI as an additive.11 Having
defined an efficient catalytic system, the scope of the coupling
reaction was next explored using a range of aryl halides in
conjunction with tris(trimethylsilyl)silane. The results are
shown in Scheme 3. Electron-donating groups on the aryl
iodides could be converted into the corresponding hypersilylated
products (1–5) in good yields without cleavage of Si–Si bonds
by the palladium–organic base combined catalyst system.
Unfortunately, the ortho-substituents on the aryl iodides did
not give the hypersilylated product due to steric repulsion.
Hydroxy and ester functional groups were well tolerated
under the reaction conditions (6 and 7). Heteroaryl
iodides also afforded hypersilylated compounds (8–10), and
Palladium-catalyzed cross-coupling reactions have received
significant attention in the previous decades and are recognized
as extremely versatile organic synthesis protocols for connecting
Scheme 1 Space-filling models of 6. (a) Side view. (b) Top view.
Department of Chemistry, School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: yamanoi@chem.s.u-tokyo.ac.jp, nisihara@chem.s.u-tokyo.ac.jp;
Fax: +81-3-5841-8063; Tel: +81-3-5841-4347
w Electronic supplementary information (ESI) available: Experimental
details, analytical data, and copies of 1H & 13C NMR of reported
compounds. See DOI: 10.1039/c0cc00352b
z Present address: Department of Chemistry, FMIPA, Sriwijaya
University, JL Palembang Indralaya 30662, Indonesia.
c
7784 Chem. Commun., 2010, 46, 7784–7786
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Fig.
1
Photographs of 10 at room temperature: (a) no UV
irradiation, (b) excited at 365 nm.
aryloligosilane (11) in high yield. A variety of functional
groups was tolerated including electron-deficient (trifluoro-
methyl, 12 and 13), electron-neutral (14), amino (15), hydroxy
(16), cyano (17), acetyl (18), and ester (19), as this reaction
could be conducted under mild conditions. The transformation
of 1,1,1,3,3,3-hexamethyl-2-phenyltrisilane ((Me3Si)2PhSiH),
as the hydrooligosilane substrate, proceeded in good yield
(20). This transformation can be applied toward the synthesis
of multi-arylated oligosilanes. 1,1,2,2-Tetramethyldisilane and
1,1,2,2-tetraphenyldisilane were reacted with 2 equiv. of aryl
iodides to give the corresponding doubly arylated products
(21 and 22) in good yield. It should be noted that the treatment
of the reaction mixture with H2O and silica gel yielded
disiloxane derivatives in the arylations. Purification of the
reaction mixture directly using neutral aluminium oxide
suppressed Si–O–Si bond formation by oxidation as determined
by GC-MS and NMR.
In summary, we have developed a practical palladium-
mediated procedure for the Si-arylation of tris(trimethylsilyl)-
silane and related hydrooligosilanes, using a variety of aryl
iodides, that proceeded in good to high yields. The products
prepared by this method may be valuable for incorporating
functional groups into structurally more complex oligosilanes
with different substituent groups. These reactions provide
efficient access to many important scaffolds used in material
sciences. Extensive studies of the preparation of s–p conjugated
oligosilanes and application of these compounds to functional
organic materials are in progress in our laboratory.
We thank Ms Kimiyo Saeki and Dr Aiko Sakamoto of the
Elemental Analysis Center of the University of Tokyo for
elemental analysis. A.L. thanks the Japan Society for the
Promotion of Science (JSPS) for a postdoctoral fellowship.
This work was financially supported by Grant-in-Aids for
Young Scientists (B) (No. 21750036), Scientific Research for
Innovative Area ‘‘Coordination Programming’’ (area 2107,
No. 21108002), and the Global COE Program for ‘‘Chemistry
Innovation through the Cooperation of Science and Engineering’’
from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan.
Scheme
3 Palladium-catalyzed arylation of hydrooligosilanes.
Reaction conditions were as follows unless otherwise noted: the reactions
were run with aryl iodide (1.0 mmol), hydrooligosilane (1.5 mmol),
Et(iPr)2N (1.5 mmol), and Pd(P(tBu)3)2 (0.05 mmol) in 1.0 mL dried
a
THF at rt under Ar for 3 d. Not detected. Only reduced product was
b
obtained.
The reaction was run with 5,50-diiodo-2,20-bithiophene
(1.0 mmol), tris(trimethylsilyl)silane (3.0 mmol), Et(iPr)2N (3.0 mmol),
and Pd(P(tBu)3)2 (0.05 mmol) in 1.0 mL dried THF at rt under Ar for 4 d.
c The reaction was run with iodobenzene or 2-iodothiophene (2.5 mmol),
1,1,2,2-tetramethyldisilane or 1,1,2,2-tetraphenyldisilane (1.0 mmol),
Et(iPr)2N (3.0 mmol), and Pd(P(tBu)3)2 (0.05 mmol) in 1.0 mL dried
THF at rt under Ar for 4 d.
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c
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c
7786 Chem. Commun., 2010, 46, 7784–7786
This journal is The Royal Society of Chemistry 2010