J. Jang et al. / Tetrahedron Letters 57 (2016) 4581–4584
4583
(entry 15). These results supported that the coupling reaction was
accelerated by palladium catalysis.
Pd(dba)2 (1 mol %), bis(triphenylphosphino)methane (1 mol %),
and AgF2 (2.0 equiv) in toluene at 60 °C for 3 h. With these opti-
mized condition in hand, we evaluated the catalytic system in
the coupling reaction of a variety of aryl alkynyl carboxylic acids
and triethoxysilylbenzene. Although the reaction was almost com-
plete in 3 h, all reactions were run for 6 h to achieve a higher pro-
duct yield. The results are summarized in Table 2. Aryl alkynyl
carboxylic acids bearing ortho-substituted groups gave low yields
(entries 1 and 2). 3-Methyl-substituted phenyl propiolic acid
afforded the desired product 3d in 78% yield (entry 3). Aryl alkynyl
carboxylic acids with ether groups gave good yields (entries 4 and
5). Aldehyde, ketone, ester, cyano, and nitro group-substituted
substrates produced the corresponding diaryl alkynes 3g, 3h, 3i,
3j, and 3k in 51%, 86%, 91%, 49%, and 88% yields, respectively
(entries 6–10). 1-Naphthyl propiolic acid gave 3l in 94% yield
(entry 11). 4-Bromo and 4-chloro-substituted phenyl propiolic
acids afforded the corresponding products, 3m and 3n, in 25%
and 96% yield, respectively (entries 12 and 13). In particular, 3g
and 3m were not able to be obtained through Ni-catalyzed
Hiyama-type decarboxylative coupling, however, they were
formed in this Pd-catalytic system (Table 3).
In addition, we employed substituted arylsiloxanes, such as tri-
ethoxy(4-methoxyphenyl)silane (2b) and triethoxy(4-methylphe-
nyl)silane (2c), as coupling partners. As shown in Scheme 1,
triethoxy(4-methoxyphenyl)silane was coupled with substituted
aryl alkynyl carboxylic acids to give corresponding diaryl alkynes
3o, 3p, and 3q in 81%, 66%, and 81% yields, respectively. Tri-
ethoxy(4-methylphenyl)silane provided the desired products in
good to moderate yields. However, we failed to obtain the coupled
products in the reaction with arylsiloxanes such as triethoxy-4-
nitrophenylsilane and triethoxy-2-thiophenylsilane.
Next, we investigated the oxidant, additive, and solvent to iden-
tify optimized condition. When AgF, Ag2CO3, and Ag2O were
employed, the desired product was formed in trace amounts. How-
ever, the byproduct was formed much more readily than the
desired product (entries 1–3). Employing CuF2, which was good
oxidant in the nickel-catalyzed version, did not give satisfactory
results (entry 4). The combination of silver fluorides (AgF2, AgF)
and CsF or KF afforded poor yields (entries 5–8). Using AgF2 as
an oxidant and base, different solvents were tested. DMF, DMSO,
diglyme, and CH3CN showed similar yields in the range 36–46%
(entries 9–12). 1,4-Dioxane gave a very low yield (entry 13). THF
and p-xylene afforded 63% and 62% yields, respectively (entries
14 and 15). When the amount of catalyst was decreased to 5 and
1 mol %, the yield increased to 68% and 75%, respectively (entries
16 and 17). When either temperature or catalyst amount decreased
to 25 °C and 0.5 mol %, respectively, the yields dropped to 25% and
29% (entries 18 and 19). We found that the reaction completed in
almost 3 h (entry 20).
Finally, we identified the optimized conditions as: aryl alkynyl
carboxylic acid (1.0 equiv), triethoxysilylarene (2.0 equiv),
Table 3
Pd-catalyzed Hiyama-type decarboxylative coupling reactions with 1 and 2aa
1 mol% Pd(dba)2
O
L1
1 mol%
Ar
Ar
Ph
(EtO)3Si Ph
+
AgF2 (2.0 equiv)
OH
1
toluene, 60 oC, 6 h
2a
3
Entry
Ar
Product
Yield (%)b
25
A study of the reaction pathway was also carried out. When
phenyl propiolic acid and triethoxysilybenzene reacted in the pres-
ence of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), which is
known as a radical scavenger, under the optimized conditions,
the product was formed with 20% yield.17 This result suggests that
the catalytic system consists of both radical and ionic pathways,
and that the radical pathway might be dominant. Without the solid
evidence of the radical intermediate at this time, we would not be
Me
1
2
3
4
3b
OMe
3c
3d
3e
46
78
84
Me
O
O
1 mol% Pd(dba)2
MeO
MeO
MeO
O
1 mol% L1
Ar1
Ar2
Ar1
+
(EtO)3Si Ar2
AgF2 (2.0 equiv)
toluene, 60 oC, 6 h
OH
5
3f
91
3
1a
2
O
O
O
6
7
8
3g
3h
3i
51
86
91
H
O
OMe
OMe
Me
3o (81%)
3p (66%)
O
MeO
NC
Me
Me
OMe
Me
O2N
9
3j
49
88
3r (95%)
3t (73%)
3q (81%)
3s (88%)
O
H
O2N
10
3k
Cl
11
3l
94
O
12
13
Br
Cl
3m
3n
25
96
Me
Me
NC
MeO
3v (42%)
3u (88%)
a
Reaction conditions: 1 (3.0 mmol), 2a (6.0 mmol), Pd(dba)2 (0.03 mmol), L1
Scheme 1. Pd-catalyzed Hiyama-type decarboxylative coupling reaction with 1
and 2.aaReaction conditions: 1 (3.0 mmol), 2 (6.0 mmol), Pd(dba)2 (0.03 mmol), L1
(0.03 mmol), and AgF2 (6.0 mmol) in toluene at 60 °C for 6 h.
(0.03 mmol), and AgF2 (6.0 mmol) in toluene at 60 °C for 6 h.
b
Isolated yield.