Alkenyldimethyl(2-thienyl)silanes, excellent coupling partner for the palladium-catalyzed cross-coupling reaction

Article abstract of DOI:10.1246/cl.2002.138

Introduction of a 2-thienyl group to the silicon atom of alkenylsilanes promoted the cross-coupling reaction with aryl halides mediated by tetrabutylammonium fluoride and a palladium catalyst. The reaction proceeded under extremely mild conditions to affo

Full text of DOI:10.1246/cl.2002.138

138
Chemistry Letters 2002
Alkenyldimethyl(2-thienyl)silanes, Excellent Coupling Partner
for the Palladium-Catalyzed Cross-Coupling Reaction
Kazushi Hosoi, Kyoko Nozaki, and Tamejiro Hiyamaꢀ
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501
(Received October 31, 2001;CL-011088)
Introduction of a 2-thienyl group to the silicon atom of
hydrosilylation could be controlled by the catalyst to be
employed. With t-Bu3P/Pt(CH2 ¼ CHSiMe2)2O (Pt[DVDS]),11
(E)-1-octenyldimethyl(2-thienyl)silane ((E)-2a) and ꢀ-adduct 3
were obtained in a ratio of 99/1. On the other hand, (Z)-1-
octenyldimethyl(2-thienyl)silane ((Z)-2a) and (E)-2a were pro-
duced in an 88/12 ratio with NaI/RhCl(PPh3)3.12;13 Both (E)- and
(Z)-2a are quite stable against moisture, acid (acetic acid, silica
gel), and base (1 N NaOH aq., basic alumina).
alkenylsilanes promoted the cross-coupling reaction with aryl
halides mediated by tetrabutylammonium fluoride and a palla-
dium catalyst. The reaction proceeded under extremely mild
conditions to afford arylalkenes in high yields.
The transition metal-catalyzed cross-coupling reaction is
today among the most important class of carbon-carbon bond
forming reactions1 useful in organic synthesis. Of the metals
employed for the cross-coupling reaction, boron2 and tin3 have
gained extensive use in combination with Pd catalysts. We have
been studying the palladium catalyzed cross-coupling reaction of
organohalosilanes (halogen ¼ F, Cl) using a fluoride ion as an
activator.4 The success of the reaction depends on the activation
of a less polar C-Si bond by forming a silicate anion. The
existence of an electron-withdrawing hetero atom on the
tetracoodinated silicon makes the silicon acidic enough to accept
FÀ.5 Recently, cross-coupling reactions of organo-alkoxysilanes6
or organosilanols7;8 with organic halides are also documented.
Thus, the presence of at least one hetero atom on the silicon of
alkenylsilanes is essential for the reaction to proceed. Herein, we
report that alkenyldimethyl(2-thienyl)silanes 2 efficiently act as a
coupling partner with organic halides. The coupling reaction
proceeds under extremely mild conditions ever reported for any
organosilicon compounds. The advantage of the 2-thienyl group
as an activating group as well as a dummy ligand is that alkenyl(2-
thienyl)silanes 2 are much more stable against moisture, acid and
base, compared to the corresponding halosilanes or other hetero
atom substituted silanes. Alkenyldimethyl(2-thienyl)silanes 2
can be readily accessible by hydrosilylation of alkynes with
dimethyl(2-thienyl)silane (1).
Next, we optimized the conditions for the cross-coupling
reaction of (E)-2a with 4-iodobenzotrifluoride (4a) using a
palladium catalyst and tetrabutylammonium fluoride (TBAF,
1.0 M in THF solution ) as a fluoride source.
Scheme 2.
Cross-coupling of 1.2 equiv. of (E)-2a with 4a in the presence
of Pd(PPh3)4 (5 mol%) and 2.4 equiv. of TBAF took place at
60 ꢁC to afford in 4 h the desired arylalkene (E)-5a in 72% yield.
Using palladium(II) acetate, the reaction proceeded smoothly
even at room temperature (25 ꢁC) to give (E)-5a in quantitative
yield in 1 h. Taking into account that the silicon-mediated cross-
coupling reaction requires normally elevated temperatures,14 the
reactivity is enhanced significantly in this system. With
Pd2(dba)3, the reaction proceeded more rapidly and was
completed in 0.5 h to afford (E)-5a in quantitative yield. Because
of the easiness of handling, we chose Pd(OAc)2 as the catalyst.
The reaction using an equimolar amount of TBAF to (E)-2a
proceeded more slowly compared to the reaction with 2 equiv.
Accordingly, TBAF was used in an amount of 2 equiv. to (E)-2a,
or 2.4 equiv. to 4a. When we introduced two 2-thienyl groups on
silicon, the reactivity was enhanced further, and the reaction was
completed in 0.5 h. Unlike TBAF, the use of anhydrous
[(Et2N)3S]þ[Me3SiF2]À (TASF) or KF as a fluoride salt resulted
in no reaction. Meanwhile with aqueous solution of KF or NaOH,
the coupled product was obtained to a small extent. Also, the THF
solution of TBAF contains water (water content ꢂ 5 wt.%,
Aldrich).
Very recently, Yoshida reported the cross-coupling reaction
of alkenyldimethyl(2-pyridyl)silanes.15 The coupling reaction is
considered to proceed through the silanol formation followed by
transmetalation, pyridine being detected spectometrically. We
also observed that thiophene was released gradually when the
coupling of (E)-2a and 4a was monitored by 1H NMR. Thus, even
under the conditions for (E)-2a, milder than those for 2-
pyridylsilanes, the 2-thienyl group on Si appears to be cleaved.
Direct transmetalation from penta-coodinated silicate, alkenyl-
dimethyl(2-thienyl)(fluoro)silicate, remains to be another possi-
Scheme 1.
First, we prepared (E)- and (Z)-1-octenyldimethyl(2-thie-
nyl)silanes as shown in Scheme 1. Treatment of chlorodimethyl-
silane with 2-thienyllithium gave dimethyl(2-thienyl)silane (1).9
Hydrosilylation of 1-octyne with 1 in the presence of a Pt or Rh
catalyst10 afforded (E)- or (Z)-1-octenyldimethyl(2-thienyl)si-
lanes ((E)- or (Z)-2a), respectively. The E=Z selectivity of
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
139
bility though.
applications as well as mechanistic details are currently under-
taken.
The scope of the coupling reaction was examined under the
optimum conditions. Various types of alkenylsilanes and aryl
halides are applicable to the coupling reaction (Table 1). Aryl
iodides having an electron-donating group as well as an electron-
withdrawing group (entry 1) are available. Second, the reaction is
tolerant of such a functional group as ester (entry 2). An aryl
bromide also couples with the alkenylsilane at room temperature
(entry 3). The (Z)-isomer of 2a (Z/E ¼ 88=12) gave the
corresponding arylalkene (Z)-5a maintaining the Z=E ratio of
89/11 (entry 4). Mono-substituted vinylsilane 2b, di-substituted
alkenylsilane 2c coupled smoothly with 4a (entries 5 and 6).
Bissilylated alkene 2d reacted completely at a dimethyl(2-
thienyl)silyl moiety with a trimethylsilyl site being unaffected
(entry 7). Also, tri-substituted alkenylsilane 2e couples without
any problem (entry 8). Chemoselectivity is as good as previous
organosilicon-based coupling reactions.4 For example, the
reaction gives the coupled product without protecting a nitrile
functional group (entry 9). Yields were still high and no other by-
products were detected.
We thank the Ministry of Education, Culture, Sports, Science
and Technology, Japan, for the Grant-in-Aids for COE Research
on Elements Science, No. 12CE2005.
This paper is dedicated to Professor Teruaki Mukaiyama on
the occasion of his 75th birthday.
References and Notes
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a) J. K. Stille, Angew. Chem. Int. Ed. Engl., 25, 508 (1986). b)
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Table 1. Cross-Coupling of alkenydimethyl(2-thienyl)silanes 2
with halides 4a
5
6
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9
Dimethyl(2-thienyl)silane (1): Bp 45 ꢁC (20 mmHg). 1H
NMR (200 MHz, CDCl3) ꢁ 0.38 (s, 3H), 0.40 (s, 3H), 4.54–
4.61 (m, 1H), 7.19 (dd, J ¼ 3:2, 4.6 Hz, 1H), 7.32 (d, J ¼
3:2,Hz, 1H), 7.61 (d, J ¼ 4:6 Hz, 1H). 13C NMR (200 MHz,
CDCl3) ꢁ 2.8, 111.1, 128.2, 131.0, 135.1. HRMS. Calcd for
C6H10SiS: M, 142.0272;Found: m=z 142.0261.
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In conclusion, we have demonstrated alkenyldimethyl(2-
thienyl)silanes are excellent coupling partners with aryl halide in
the TBAF promoted palladium-catalyzed cross-coupling reac-
tions. The 2-thienyl group acts as an activating group as well as a
dummy ligand. The reaction proceeds under very mild conditions
and can be tolerant of a variety of functional groups. Studies on