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Chemistry Letters Vol.38, No.2 (2009)
Rhodium-catalyzed and Coordination-assisted Regioselective Alkenylation
of Aromatic C–H Bonds with Terminal Silylacetylenes
Takashi Katagiri, Tomoya Mukai, Tetsuya Satoh, Koji Hirano, and Masahiro Miuraꢀ
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871
(Received November 14, 2008; CL-081075; E-mail: miura@chem.eng.osaka-u.ac.jp)
A number of aryl-N-heterocycles and aromatic imines effi-
for 3 h, the corresponding 2,6-dialkenylated product 4a (74%)
was produced together with monoalkenylated 3a (26%) (Table 1,
Entry 1). Using 3 equiv of 2 and PCyPh2 (Cy = cyclohexyl) in
place of PPh3 afforded 4a in 95% yield after 9 h (Entry 3). In
each of Entries 1–3, formation of dimerized 2 as the minor by-
product was detected by GC-MS, while it was readily separable.
It was found that P(2-furyl)3 was a less effective ligand (Entry
4), but no dimer of 2 was detected. Thus, 3a (77%) was selec-
tively obtained with 1 equiv of 2 using P(2-furyl)3 (Entry 5).
The reaction did not proceed without any phosphine ligand
(Entry 6). The reaction of 1a with tert-butyldimethylsilylacety-
lene (20) (2 equiv) in place of 2 proceeded somewhat slowly to
give a mixture of 3a0 and 4a0 (Entry 7). As expected, 3a0 was
obtained as the predominant product in the reaction with 1 equiv
of 20 using P(2-furyl)3 (Entry 8). The ligand PCyPh2 was un-
expectedly not effective in the reaction with 20 (Entry 9). The
reaction of 1a with trimethylsilylacetylene (3 equiv) using PPh3
in a sealed tube was sluggish and monoalkenylated product was
detected in only ca. 25% GC yield. Use of phenylacetylene and
1-octyne was not successful as reported previously.3d
ciently undergo ortho alkenylation via C–H bond cleavage upon
treatment with bulky terminal silylacetylenes, typically triiso-
propylsilylacetylene, in the presence of [RhCl(cod)]2/PAr3 as
catalyst.
Transition-metal-catalyzed C–C bond formation through
C–H bond cleavage has attracted much attention from the
atom-economic point of view, and various catalytic processes
involving different modes to activate the ubiquitously available
bond have been developed.1 Among the most promising activa-
tion strategies is to utilize the proximate effect by coordination
of a functional group in a given substrate to the metal center
of a catalyst, which brings about regioselective C–H bond acti-
vation and functionalization. As one of such catalytic reactions,
the direct ortho alkylation of aromatic compounds bearing a suit-
able oxygen or nitrogen-containing functional group is now
known to selectively take place by treatment with alkenes. The
work of Murai and co-workers using aromatic ketones as sub-
strates under ruthenium catalysis is a significant pioneering mile-
stone for the C–H alkylation.2 A number of relevant reactions
with internal alkynes in place of alkenes that allow ortho alkeny-
lation have also been realized.3 However, successful examples
with terminal alkynes have been very limited,4 which is attrib-
uted to the fact that acetylenic C–H bonds are, in most cases,
more reactive than aromatic C–H bonds. As a rare example,
Jun and co-workers reported that some aromatic imines react
with aliphatic terminal alkynes in the presence of a rhodium
catalyst.4a We reported the related rhodium-catalyzed hydro-
acylation of alkynes including a number of terminal ones with
salicylaldehyde.4c Very recently, Zhang and co-workers de-
scribed the ruthenium-catalyzed reaction of 2-phenylpyridines
with aromatic and aliphatic terminal alkynes.4d,5
We next examined the alkenylation of a number of aryl-N-
heterocycles and aromatic imines 1 with silylacetylene 2
(Table 2). The reaction of 2,20-bipyridyl (1b) in the presence
of [RhCl(cod)]2 and PPh3 smoothly proceeded to give the corre-
sponding 3,30-dialkenylated product 4b in quantitative yield
(Entry 1). In this reaction, use of P(2-furyl)3 gave a mixture of
Table 1. Reaction of 2-phenylpyridine (1a) with terminal silyl-
acetylene 2 or 20 a
Si
[RhCl(cod)]2
(3 mol% Rh)
Si
N
ligand
N
N
+
+
o-xylene
In the course of our study of catalytic coupling reactions
with alkynes,6a–6c,7 we have unexpectedly observed that a num-
ber of aryl-N-heterocycles and aromatic imines efficiently under-
go rhodium-catalyzed ortho alkenylation on treatment with
bulky terminal silylacetylenes. This appears to be of consider-
able interest, since terminal silylacetylenes have been recently
demonstrated to be effective substrates in a number of catalytic
cross-dimerization reactions with different terminal or internal
alkynes to produce the corresponding enyne compounds,6 in
which the silylacetylenes act as acetylene donors, as they are
relatively reactive toward C–H bond cleavage. Nevertheless,
the acetylenes are capable of acting as aromatic C–H acceptors
in the present reaction.
Si
Si
2, Si = Si(i-Pr)3
2', Si = SiMe2t-Bu
1a
3a, Si = Si(i-Pr)3
3a', Si = SiMe2t-Bu 4a' Si = SiMe2t-Bu
4a, Si = Si(i-Pr)3
Alkyne
/equivb
Yieldc/%
Entry
Ligand
Time/h
3a (or 3a0) 4a (or 4a0)
1
2
3
4
5
6
7
8
9
2 (2)
2 (3)
2 (3)
2 (3)
2 (1)
2 (1)
20 (2)
20 (1)
20 (2)
PPh3
PPh3
3
6
9
9
3
3
3
3
3
26
15
74
85
95
75d
4
PCyPh2
P(2-furyl)3
P(2-furyl)3
none
2d
22d
77
0
0
PPh3
P(2-furyl)3
PCyPh2
67
77
9d
33
7
5d
When 2-phenylpyridine (1a) (0.5 mmol) was treated with
triisopropylsilylacetylene (2) (1 mmol, 2 equiv) in the presence
of [RhCl(cod)]2 (0.0075 mmol, 3 mol % Rh) and PPh3 (0.03
mmol, 6 mol %) as catalyst and ligand in refluxing o-xylene
aReaction conditions: [1a]:[Rh]:[ligand] = 0.5:0.015:0.03 (in mmol),
in o-xylene (5 mL) at 160 ꢁC (bath temp.) under N2. Relative to 1a.
b
cIsolated yield based on the amount of 1a used. dDetermined by GC.
Copyright Ó 2009 The Chemical Society of Japan
Katagiri, Takashi
