Organic Letters
Letter
participate in the hydrosilylation reactions, affording 3ac−3ap
in 45−94% yield, with excellent selectivity. However, tBu- and
Ph-substituted thioalkynes did not work under current
conditions, which might be because of the steric hindrance
of these substituents. It is worthy of note that the methoxy
group can serve as a removable directing group, and seven-
membered silacyclic 3as was prepared in 85% yield by
demethylative silacyclization.
proposed in Scheme 6. Silylium intermediate II, generated
through reaction between a silane and B(C F ) , can be in
equilibrium with I, in the presence of Lewis basic thioalkyne;
6
5 3
12
addition of thioalkyne to II then gives ketene sulfonium species
IV, which may be converted to the final product after reaction
with boron hydride III. The competition reaction with
electronically distinct thioalkynes (Scheme 5f) reveals that an
electron-withdrawing group leads to reduced reaction rates,
implying that the step involving a thioalkyne and II (step 1)
might be rate-determining. The high β-selectivity might be
explained by the polarization property of thioalkynes. The anti-
addition selectivity might be explained by the lower steric
pressure in pathway a. However, the following possibility
cannot be ruled out: sulfonium species IV and boron hydride
III might be generated from intermediate V, which might be
formed from the reaction between thioalkyne and complex I.
In summary, we have developed the first β-anti-selective
Next, we explored whether a sulfide-substituted alkenyl
silane can be converted into other trisubstituted alkenyl silanes
(
Scheme 3). Although cross-coupling of alkenyl halides has
9
been well studied, the investigation of alkenyl sulfide in cross-
coupling chemistry is considerably less developed. Thus, on
the basis of the seminal reports by Kumada and Takei,
4
,10
10a,b
we
investigated the cross-coupling of silyl-substituted Z-alkenyl
sulfide with Grignard reagents. We were able to react a variety
of aryl Grignard reagents with 3y in the presence of
Ni(dppe)Cl (5.0 mol %) to generate the corresponding
addition of silanes to thioalkynes with B(C F ) as the catalyst,
6 5 3
2
alkenyl silanes in 55−80% yield; all transformations were
entirely stereoretentive. Moreover, cyclopropyl-substituted 4g
was obtained in 75% yield and as a single isomer. These
stereodefined trisubstituted alkenyl silanes are difficult to
synthesize by other methods. In addition, by using MeMgBr,
we were able to synthesize 4h in 80% yield and a 92:8 Z/E
ratio. It is encouraging that the steric bulky silyl group did not
inhibit the cross-coupling reactions.
affording trisubstituted alkenyl silanes in excellent regio- and
stereoselectivity. The reaction shows broad substrate scope and
functional group tolerance. The products were proved to be
useful intermediates to other trisubstituted alkenyl silanes by
Ni-catalyzed stereoretentive cross-coupling reactions of the C−
S bond. Mechanism study of the hydrosilylation reaction
suggests that nucleophilic attack of thioalkyne to the activated
silylium intermediate might be the rate-determining step, and
Si−H bond cleavage is not involved in the rate-determining
step.
We then investigated the selective transformation of the silyl
moiety (Scheme 4). Protodesilylation reaction of compound
3
a proceeded smoothly, affording alkenyl sulfide 5 in 85%
yield, with the C−S bond unchanged. The Si−H bond of
compound 3a was converted to the Si−OMe bond, and alkenyl
silane 6 was synthesized in 85% yield. Pd-catalyzed Hiyama
coupling afforded trisubstituted alkenyl sulfide 7 in 68% yield.
An ensuing bisphosphine−Ni-catalyzed cross coupling deliv-
ered trisubstituted alkenes 8 (72% yield) and 9 (70% yield);
such entities cannot be easily accessed by alternative
methods. It is noteworthy that an alkene’s stereochemical
identity was preserved in the above sequence of reactions.
We then focused on gaining some insight regarding the
ASSOCIATED CONTENT
sı Supporting Information
■
*
Experimental procedures and characterization data for
1
1
CCDC 1956653 contains the supplementary crystallographic
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
hydrosilylation process. The ability of B(C F ) to activate a
Si−H bond is well appreciated and has been proposed in the
context of catalytic hydrosilylation. However, B(C F ) can
6
5 3
12
6
5 3
also react with thioalkyne to generate carboboration product
1
3
10 (Scheme 5a). The identity of 10 was determined through
X-ray crystallography. The question then was: might the
carboboration product be the actual catalyst in the present set
of hydrosilylation reactions? To clarify, we performed the
following reaction: mixing thioalkyne 1a with 1.5 mol % of
B(C F ) in CH Cl for 10 h at 60 °C, and then 1.5 equiv of
■
Corresponding Author
Xiao Shen − Institute for Advanced Studies, Engineering Research
Center of Organosilicon Compounds & Materials, Ministry of
6
5 3
2
2
Ph SiH was added. The resulting mixture was heated at 60 °C
2
2
for another 12 h. Only 8% conversion of 1a and 7% yield of 3a
1
were observed ( H NMR analysis; Scheme 5b). However,
when B(C F ) was added after mixing compound 1a and
6
5 3
Ph SiH , there was complete conversion, and 3a was isolated
in 85% yield, >99:1 Z/E ratio, and >99:1 β/α selectivity
2
2
Authors
(
Scheme 5c). These findings indicate that B(C F ) is likely
Yunxiao Zhang − Institute for Advanced Studies, Engineering
Research Center of Organosilicon Compounds & Materials,
Ministry of Education, Wuhan University, Wuhan 430072,
Yanran Chen − Institute for Advanced Studies, Engineering
Research Center of Organosilicon Compounds & Materials,
Ministry of Education, Wuhan University, Wuhan 430072,
6
5 3
1
4
the catalyst.
To establish which step is rate-determining, we carried out
reactions with deuterium-labeled substrates. We found H−D
scrambling to be efficient in the reaction of Ph SiHD with
B(C F ) at room temperature (Scheme 5d). The KIE value of
2
6
5 3
0
.99 suggests that Si−H bond clevage is not kinetically
significant (Scheme 5e). These data led us to the mechanism
D
Org. Lett. XXXX, XXX, XXX−XXX
Zhang, Yunxiao
