Arylation of allylsilanes through rhodium catalysis

Article abstract of DOI:10.1246/cl.2009.186

The arylation of allylsilanes catalyzed by a rhodium complex is described. In the presence of a catalytic amount of RhCl(CO){P[OCH(CF₃) ₂]₃}2 and Ag₂C0₃, allyltriisopropylsilane reacts with iodoarenes

Full text of DOI:10.1246/cl.2009.186

186
Chemistry Letters Vol.38, No.2 (2009)
Arylation of Allylsilanes through Rhodium Catalysis
Haruka Omachi and Kenichiro Itamiꢀ
Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602
PRESTO, Japan Science and Technology Agency, Nagoya 464-8602
(Received November 11, 2008; CL-081065; E-mail: itami@chem.nagoya-u.ac.jp)
The arylation of allylsilanes catalyzed by a rhodium com-
Table 1. Effect of ligand and additivea
plex is described. In the presence of a catalytic amount of
RhCl(CO){P[OCH(CF3)2]3}2 and Ag2CO3, allyltriisopropylsi-
lane reacts with iodoarenes in m-xylene at 120 ꢁC to furnish
the corresponding C–H bond arylation products in moderate
yields.
[RhCl(CO)2]2 (2.5%)
I
Ar
Ligand (10%)
Additive (1 equiv)
+
Ar
Si(i-Pr)3
Si(i-Pr)3
m-xylene
2
120 °C, 12 h
(Ar = C6H4NO2-p)
1
2:3:4 = ca 80/15/5
Si(i-Pr)3
Si(i-Pr)3 Ar
The selective transformation of ubiquitous C–H bonds not
only represents an important and long-standing goal in chemis-
try, but also unlocks opportunities for markedly different strat-
egies in chemical synthesis.1 Therefore, the development of such
processes has been an emerging area of extensive research. As a
part of our program aimed at establishing a new biaryl coupling
through C–H bond functionalization,2–4 we recently reported
that RhCl(CO){P[OCH(CF3)2]3}2 can catalyze the C–H bond
arylation of electron-rich arenes with haloarenes.2 Although
the precise mechanism remains unknown, our current approxi-
mation is shown in Figure 1. This includes (i) oxidative genera-
tion of cationic arylrhodium(III) species ArRhþX(L) by the re-
action of RhX(L), ArX, and Ag2CO3, (ii) electrophilic metala-
tion (rhodation) of arene giving diarylrhodium(III) species, and
(iii) reductive elimination of biaryl product with the regeneration
of RhX(L). We believe that ꢀ-acidic P[OCH(CF3)2]3 ligand
allows the key ArRhþX(L) species to be extremely electron-de-
ficient thereby establishing a distinct nucleophile–electrophile
interaction between an arene and rhodium center to promote
electrophilic metalation.
Ar
3
4
Yield/%
(2 þ 3 þ 4)
Entry
Ligand
Additive
1b
2b,c
3
4
5
P[OCH(CF3)2]3
P[OCH(CF3)2]3
P[OCH(CF3)2]3
P(OC6H5)3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOTf
76
25
20
7
53
55
34
10
<1
43
36
<1
<1
P(C6H5)3
P(C6H4CF3-p)3
P(C6F5)3
6
7
8d
dppee
9d
2,20-bipyridyl
P[OCH(CF3)2]3
P[OCH(CF3)2]3
P[OCH(CF3)2]3
P[OCH(CF3)2]3
10b
11b
12b
13b
AgOAc
K2CO3
KOt-Bu
aConditions: 1 (2 equiv), p-NO2C6H4I (1 equiv), [RhCl(CO)2]2
(2.5 mol %), ligand (10 mol %), additive (1 equiv), m-xylene,
b
120 ꢁC, 12 h. RhCl(CO){P[OCH(CF3)2]3}2 (5 mol %) was em-
ployed as catalyst. p-NO2C6H4Br was employed. d5 mol % of
c
e
On the basis of such mechanistic scenario, we surmise that a
nucleophilic alkene can also be used as a nucleophilic coupling
partner in this Rh-catalyzed C–H bond arylation manifold.5
Herein we report on the C–H bond arylation of allylsilanes with
iodoarenes through the agency of Rh/P[OCH(CF3)2]3/Ag2CO3
ligand was employed. 1,2-Bis(diphenylphosphino)ethane.
system. Allylsilanes have been chosen in this study expecting the
promoting effect of the ꢁ-silyl group in stabilizing the assumed
carbocationic intermediate (Figure 1).6
In early experiments, we found that allyltriisopropylsilane
(1) is a suitable substrate for the reaction.7 Treatment of 1
(2 equiv), 4-nitrophenyl iodide (1 equiv), RhCl(CO){P[OCH-
(CF3)2]3}2 (5 mol %) and Ag2CO3 (1 equiv) in m-xylene at
120 ꢁC produced the expected C–H bond allylation product 2
(Table 1, Entry 1). Isomers such as 3 and 4 were also formed
in the reaction. These isomers might be produced by abstracting
the corresponding hydrogen atoms in the assumed carbocationic
intermediates or by the isomerization of 2 (Figure 1). The use
of 4-nitrophenyl bromide resulted in lower reaction efficiency
(Entry 2). We next investigated the effects of ligand and additive
in the reaction of 1 and 4-nitrophenyl iodide using [RhCl(CO)2]2
as a rhodium source. The screening of a range of ligands resulted
in the finding that our original Rh/P[OCH(CF3)2]3 is best among
others (Entries 3–9). The necessity of silver-based additives is
also ascertained (Entries 10–13). We believe that silver salts
help generate a key cationic arylrhodium(III) species (Figure 1)
by abstracting halogen from rhodium.
+
Ar
L Rh
X
Ar
L Rh
X
− H+
S
S
H
Previous Work
Ar
S
S
Ar
Rh+
ArX, Ag+
− AgX
RhX(L)
L
X
This Work
Si
Ar
Si
Si
Si
+
− H+
Ar
L Rh
X
Ar
L Rh
X
H
X = halogen, L = P[OCH(CF3)2]3
Figure 1. Arylation of arenes (previous work) or allylsilanes
(this work) with haloarenes promoted by Rh/P[OCH(CF3)2]3/
Ag2CO3 system.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.2 (2009)
187
Table 2. Rh-catalyzed arylation of 1 with iodoarenesa
Table 3. Comparison of current Rh catalyst and typical Pd
catalyst
Ar
I
RhCl(CO){P[OCH(CF3)2]3}2
(5 mol %)
Rh or Pd Catalyst
+
Ar
Si(i-Pr)3
R
R
+
Ar
I
H
Ar
Si(i-Pr)3
Ag2CO3 (1 equiv)
m-xylene
120 °C, 12 h
(Ar = C6H4NO2-p)
Current Rh Cat.a
2
1
Alkene
Si(i-Pr)3
Ot-Bu
Typical Pd Cat.b
Si(i-Pr)3 Ar
Si(i-Pr)3
68%
(82% E)
48%
(77% E)
Ar
3
4
Yield/%
(2/3/4)
Entry
Ar
30%
(84% E)
86%
(>98% E)
O
1
2
3
4
5
6
7
C6H5
50 (78/16/6)
17 (65/6/29)
54 (70/24/6)
64 (72/14/14)
68 (78/13/9)
48 (81/13/6)
68 (82/18/0)
aRhCl(CO){P[OCH(CF3)2]3}2 (5 mol %), Ag2CO3 (1 equiv), m-xy-
o-CH3C6H4
m-CH3C6H4
p-CH3C6H4
p-CH3OC6H4
m-NO2C6H4
p-NO2C6H4
b
lene, 120 ꢁC, 12 h. Pd(OAc)2 (5 mol %), P(o-Tol)3 (10 mol %), Et3N
(1 equiv), DMF, 80 ꢁC, 12 h.
C–H bond functionalization of nucleophilic alkenes. Work along
these lines is currently underway.
aConditions: 1 (2 equiv), ArI (1 equiv), RhCl(CO)-
{P[OCH(CF3)2]3}2 (5 mol %), Ag2CO3 (1 equiv), m-
xylene, 120 ꢁC, 12 h.
This work was supported by the PRESTO program of Japan
Science and Technology Agency (JST), and a Grant-in-Aid
for Scientific Research from MEXT and JSPS. We thank Mr.
Shuichi Yanagisawa (Nagoya University) for critical comments
and discussion.
In order to roughly grasp the scope of current reactions, rep-
resentative iodobenzene derivatives were subjected to the condi-
tions established (Table 2).8 The reactions took place with elec-
tron-neutral, -rich, and -deficient iodoarenes with reasonable ef-
ficiency. Unfortunately, ortho substitution on the benzene ring
substantially decreases the yield (Entry 2).
This paper is dedicated to Professor Ryoji Noyori on the
occasion of his 70th birthday.
The Mizoroki–Heck reaction represents an efficient C–H
bond arylation of alkenes with haloarenes promoted by Pd cata-
lyst and base.9–11 A typical textbook mechanism of Mizoroki–
Heck reaction includes (i) oxidative addition of ArX to Pd0,
(ii) carbopalladation (insertion) of the resultant ArPdX across
alkene, (iii) ꢁ-hydrogen elimination, and (iv) base-mediated re-
generation of Pd0. This differs from our assumed carbocationic
manifold, though mechanisms of Pd-catalyzed processes could
vary among catalysts and alkene substrates.9 To gain some in-
sights into the mechanism of the Rh-catalyzed reaction, 1 and
4-nitrophenyl iodide were treated with Pd catalyst (Table 3).
Under typical Mizoroki–Heck conditions [Pd(OAc)2, P(o-Tol)3,
Et3N, and DMF], the arylation products 2 and 3 were obtained in
48% yield with 77% stereoselectivity. The arylation reactions
were also carried out with t-butyl acrylate, which is known to
be an excellent substrate for the Mizoroki–Heck reaction under
a carbometalation-based manifold. Indeed, the Pd catalyst af-
forded the arylation product in 86% yield with virtually com-
plete stereoselectivity. However, under the influence of our Rh
catalyst, the arylation proceeded in much lower efficiency
(30%) and stereoselectivity (84% E). Although more detailed
experiments are needed for elucidating the mechanism, these
contrasting results clearly indicate that electrophilic Rh catalysis
favors nucleophilic alkenes (allylsilanes) whereas nucleophilic
Pd catalysis favors electrophilic alkenes (acrylates); the dif-
ference of Rh and Pd catalysis in the C–H bond arylation of
alkenes.10,11
References and Notes
1
2
Recent reviews on catalytic C–H bond functionalization: a) F.
Kakiuchi, T. Kochi, Synthesis 2008, 3013. b) F. Kakiuchi, N.
Chatani, Adv. Synth. Catal. 2003, 345, 1077.
a) S. Yanagisawa, T. Sudo, R. Noyori, K. Itami, J. Am. Chem. Soc.
2006, 128, 11748. b) S. Yanagisawa, T. Sudo, R. Noyori, K. Itami,
Tetrahedron 2008, 64, 6073.
3
4
I. Ban, T. Sudo, T. Taniguchi, K. Itami, Org. Lett. 2008, 10, 3607.
a) S. Yanagisawa, K. Ueda, T. Taniguchi, K. Itami, Org. Lett.
2008, 10, 4673. b) G. Deng, K. Ueda, S. Yanagisawa, K. Itami,
C.-J. Li, Chem.—Eur. J., in press.
5
Rh-catalyzed C–H bond arylation of alkenes: a) T. Sugihara, T.
Satoh, M. Miura, M. Nomura, Adv. Synth. Catal. 2004, 346,
1765. b) A. Mori, Y. Danda, T. Fujii, K. Hirabayashi, K. Osakada,
J. Am. Chem. Soc. 2001, 123, 10774.
6
7
I. Fleming, A. Barbero, D. Walter, Chem. Rev. 1997, 97, 2063.
The use of allyltrimethylsilane resulted in a complex mixture pre-
sumably due to the occurrence of competing desilylation path-
ways.
2a
8
Typical procedure: A mixture of RhCl(CO){P[OCH(CF3)2]3}2
(25 mmol), Ag2CO3 (0.50 mmol), allyltriisopropylsilane (1.0
mmol), and p-nitrophenyl iodide (0.50 mmol) in dry m-xylene
(2.0 mL) was stirred at 120 ꢁC for 12 h under argon. After cooling
to room temperature, the mixture was subjected to flash silica gel
chromatography (EtOAc/hexane) to afford 2 (55%) and 3 (12%).
I. P. Beletskaya, A. V. Cheprakov, Chem. Rev. 2000, 100, 3009.
9
10 Pd-catalyzed C–H bond arylation of allylsilanes: a) K. Karabelas,
C. Westerlund, A. Hallberg, J. Org. Chem. 1985, 50, 3896. b) K.
Olofsson, M. Larhed, A. Hallberg, J. Org. Chem. 1998, 63, 5076.
c) T. Jeffery, Tetrahedron Lett. 2000, 41, 8445.
11 Under the current Rh-based conditions, other electron-rich al-
kenes such as vinyl ethers are not applicable. For Pd-catalyzed
C–H bond arylation reaction of vinyl ethers, see Ref 9.
In summary, we have established a new Rh-based protocol
for the C–H bond arylation of allylsilanes with iodoarenes. Al-
though the improvement of efficiency and selectivity is necessa-
ry for this reaction to reach its full synthetic potential, the dis-
covered reactivity might serve as a new starting point for the
Published on the web (Advance View) February 5, 2009; doi:10.1246/cl.2009.186