Angewandte
Communications
Chemie
1
only E-vinyl silane was observed by H NMR. A series of
addition, the desired alkenyl trialkyl silanes were isolated in
other initiators, namely CuI, Cu(OAc) , FeCl , tetrabutylam-
moderate to good yields when using both arylsilanes and
aliphatic silanes (22–26). Interestingly, (E)-(4-(dimethylsilyl)-
phenyl)dimethyl(4-methylstyryl)silane was generated as the
major product in the reaction of (E)-3-p-tolylacrylic acid with
1,4-bis(dimethylsilyl)benzene (27).
Encouraged by the successful decarboxylative alkenyla-
tion of SiÀH bonds, we wondered whether this strategy could
2
3
monium iodide (TBAI), and CuCl were screened (Table 1,
entries 3–7). CuCl was found to be the most efficient initiator
in this system (Table 1, entry 7). Various peroxides and
solvents were also investigated (Table 1, entries 8–11). It is
interesting that the reaction can also proceed smoothly in
tBuOH/water (Table 1, entry 10).
With the optimized conditions in hand, we then explored
the scope of this reaction (Table 2). The results show that
a wide range of acrylic acids and silanes are effective
substrates in this system (1–27). Although a steric effect
from the aryl substituent in cinnamic acids seems apparent,
be applied to propiolic acids. Pleasingly, an array of alkynyl-
silanes could be conveniently synthesized through decarbox-
ylative silylation of propiolic acids with silanes under the
typical reaction conditions (Table 3). Various propiolic acids
and silanes gave the desired products in moderate to high
yields (28–35).
[a]
Table 2: Decarboxylative silylation of acrylic acids with R SiH.
3
[a]
Table 3: Decarboxylative silylation of propiolic acids with R SiH.
3
[
a] Reaction conditions: propiolic acid (1 equiv, 0.2 mmol), silane
(
(
5 equiv, 1.0 mmol), CuCl (5 mol%), TBHP (3 equiv, 0.6 mmol), tBuOH
3 mL), 1108C, 24 h. Yields of isolated product are given below the
products, with recovery of 3-phenylpropiolic acid in parentheses.
Because of the importance of organosilicon compounds in
materials science and synthetic chemistry, several alkenyl and
alkynyl silane products were applied in the synthesis of
[18]
valuable molecules (Figure 1).
Diverse transformations
from organosilanes were achieved to give various useful
building blocks, including allylic alcohols, indoles, naphtha-
lenes, and stilbenes.
Next, a series of mechanistic studies involving investiga-
tion of the kinetic isotope effect (KIE), spin-trapping
techniques, and electron paramagnetic resonance (EPR)
were carried out (Scheme 2 and the Supporting Information).
No desired alkenyl silane was observed upon addition of
[
1
1
a] Reaction conditions: acrylic acid (1 equiv, 0.2 mmol), silane (5 equiv,
.0 mmol), CuCl (5 mol%), TBHP (3 equiv, 0.6 mmol), tBuOH (3 mL),
108C, 24 h. Yields of isolated product are given below the products,
with recovery of starting materials in parentheses.
2
,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) as a radical
inhibitor (Scheme 2). Similarly, no reaction occurred when
the desired products can be obtained in moderate yields (4
and 7). Furthermore, aryl- and heteroaryl-substituted acrylic
acids are amenable to this reaction (8–19). The fact that
variation of the substituents on the aromatic core is tolerated
indicates that there is no substantial electronic effect (5, 6, and
using deuterated triphenyl silane (Ph SiD) under the typical
reaction conditions, while 93% yield of the desired product
3
was isolated in the case of Ph SiH (Table 2, entry 2). Such
3
a significant KIE suggests that cleavage of the SiÀH bond is
involved in the rate-determining step. The results are
consistent with our previous observations for the radical
1
0). It is noteworthy that (E)-triphenyl(2-phenylprop-1-en-1-
[
9]
yl)silane was isolated as the only product in the reaction of
triphenylsilane with (E)-3-phenylbut-2-enoic acid (18). No Z-
isomer was observed, thus further suggesting stereospecific
decarboxylative silylation of acrylic acids. Gratifyingly,
alkenyl and alkynyl acrylic acids also gave the corresponding
products in 31% and 41% yields, respectively (20 and 21). In
addition/cyclization of N-arylacrylamide with silane. To
obtain evidence of possible radical intermediates, 2-methyl-2-
nitrosopropane (MNP) was used as a radical spin trap. Mixed
II
EPR signals were observed that might belong to Cu and
a triphenylsilyl tert-butyl nitroxide radical or a di-tert-butyl
[19]
nitroxide radical (a = 15.88 G, g = 2.0063). To avoid the
N
Angew. Chem. Int. Ed. 2016, 55, 236 –239
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
237