Direct oxidative coupling of arenes with olefins by Rh-catalyzed C-H activation in air: Observation of a strong cooperation of the acid

Article abstract of DOI:10.1002/chem.201200657

A [{RhCl(cod)}₂]/CCl₃COOH system was developed for the oxidative coupling of non-chelate-assisted arenes with olefins in the presence of catalytic amounts of Cu(OAc)₂·H₂O as a co-oxidant and oxygen as the terminal oxidant. The acid was an indispensable component in this system and played a very important role in the coupling reaction. This catalytic system was applied to the direct oxidative coupling of a series of arenes and olefins and the corresponding products were afforded in high yields with special chemo- and regioselectivity. This reaction provides an atom-efficient route to vinylarenes, which are widely used in various fine chemicals. Give it some air: A [{RhCl(cod)}₂]/CCl₃COOH system for the oxidative coupling of arenes and olefins afforded their corresponding products with special chemo- and regioselectivity in high yields. This reaction provides an atom-efficient route to vinylarenes. Copyright

Full text of DOI:10.1002/chem.201200657

FULL PAPER
DOI: 10.1002/chem.201200657
À
Direct Oxidative Coupling of Arenes with Olefins by Rh-Catalyzed C H
Activation in Air: Observation of a Strong Cooperation of the Acid
Lu Zheng and Jianhui Wang*[a]
Abstract: A [{RhCl
(cod)}2]/CCl3COOH
a very important role in the coupling
reaction. This catalytic system was ap-
plied to the direct oxidative coupling
of a series of arenes and olefins and
the corresponding products were af-
forded in high yields with special
chemo- and regioselectivity. This reac-
tion provides an atom-efficient route to
vinylarenes, which are widely used in
various fine chemicals.
system was developed for the oxidative
coupling of non-chelate-assisted arenes
with olefins in the presence of catalytic
amounts of CuACHTUNGTRENNUNG(OAc)2·H2O as a co-oxi-
Keywords: C-H activation · olefina-
tion · rhodium · oxidative coupling ·
vinylarenes
dant and oxygen as the terminal oxi-
dant. The acid was an indispensable
component in this system and played
Introduction
chelate-assisted cases have been reported.[8–11] A direct oxi-
dative coupling reaction between benzene and ethylene in
the presence of a cyclometalated RhIII catalyst was achieved
by Matsumoto and Yoshida.[8] The direct oxidative coupling
of aromatic compounds with alkenes catalyzed by Pd com-
pounds has also been reported,[9,10] but this catalytic system
was limited to a few electron-rich arenes. In the presence of
a special pyridine ligand, a Pd-catalyzed system was found
to be suitable for the oxidative coupling of electron-defi-
cient arenes with olefins.[11]
Vinylarene derivatives are important intermediates for the
construction of more-complex organic molecules.[1] One of
the most popular and most distinct methods for the con-
struction of vinylarene structures is the Heck reaction.[2] Al-
though a great deal of progress has been made since this re-
action was first discovered, it still suffers from many draw-
backs.[3] For example, a large amount of salt waste is pro-
duced during these reactions and the atom economy is not
satisfactory. An attractive alternative to the Heck reaction is
Although rhodium complexes stand out in the area of cat-
À
À
the direct oxidative coupling of aryl C H bonds with ole-
alytic C C coupling reactions through the chelate-assisted
fins,[4] which obviates the need for prior functionalization
steps and is thus more atom economical and versatile. Sever-
al transition-metal complexes, such as rhodium,[5] palladi-
um,[6] and ruthenium complexes,[7] have been demonstrated
to be highly active catalysts for such reactions. One of the
most popular strategies is the use of a chelate assistant,
which uses a neighboring directing group that precomplexes
the metal and directs it to the desired position.[5–7] For exam-
ple, aromatic compounds that contain carbonyl groups, such
as benzoic acids[5f,j,7f] or phenones,[5n,7b,c] or nitrogen-contain-
ing groups, such as aromatic amides[5d,6h] or benzylamines,[6i]
have been found to react with alkenes through catalytic
cleavage of C H bonds, the direct oxidative olefination of
[12]
À
aromatic compounds that do not contain a directing group
has been less well studied. Quite recently, a RhIII-catalyzed
oxidative olefination reaction of bromoarenes was reporte-
d.[13a] However, this reaction requires an unusually long in-
cubation time and is limited to only a few substrates with
particular electronic effects; even chlorobenzene afforded
only traces of the desired product and toluene did not react
at all. Therefore, a more efficient system for the direct oxi-
À
dative olefination of non-chelate-assisted arenes through C
H bond activation that is suitable for a broad scope of sub-
strates under mild conditions is desired. Herein, we describe
an effective catalytic system for the direct oxidative coupling
of a diverse range arenes that contain no directing groups
with different classes of olefins (Scheme 1). In this
À
direct-coupling reactions in which the activation of a C H
bond is the key step. However, the topography of the sub-
strate in these reactions is crucial because the proximal di-
recting group must possess the proper bonding strength and
system, [{RhCl
ACHTUNGTERNN(UNG cod)}2] (cod=1,5-cyclooctadiene) and Cu-
À
alignment toward the targeted C H bond. Only a few non-
ACHTUNGTRENNUNG
[a] L. Zheng, Prof. Dr. J. Wang
Department of Chemistry, Tianjin University
92 Weijin Road, Naikai District
Tianjin, 300072 (P. R. China)
Fax : (+86)22-2740-3475
E-mail: wjh@tju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200657.
Scheme 1.
dative coupling.
ACHUTGTNNERNUG[{RhClACHTUNTGERN(NGUN cod)}2]-catalyzed olefination of an arene through oxi-
Chem. Eur. J. 2012, 18, 9699 – 9704
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9699

Table 1. Influence of the acid on the olefination of toluene (1a) with
butyl acrylate (2a).
a terminal oxidant, and trichloroacetic acid was used as an
additive, which was demonstrated to be an essential compo-
nent in this reaction.
Results and Discussion
Entry
Additive
t [h]
Equiv of
alkene
Yield
[%][a,b]
Initially, the reaction of toluene (1a) with butyl acrylate
(2a) was conducted in the presence of [{RhCl
(2.0 mol%), Cu(OAc)2·H2O (0.5 equiv), and benzoic acid
(1.0 equiv to 2a) at 1208C for 22 h in air. Trace amounts of
the olefination products were observed. Importantly, this re-
action did not occur in the absence of benzoic acid, thus in-
dicating that the acid is a crucial element in the catalysis.
Next, a set of experiments were performed to determine
the optimal acid in this system. Different acids, including
commercially available organic and inorganic acids, were
1
2
3
4
5
6
7
8
CF3COOH
CCl3COOH
CCl3COOH
CHCl2COOH
ClCH2COOH
HCOOH
CH3COOH
CH3CH2COOH
hexanoic acid
n-caprylic acid
PivOH
CH3SO3H
TsOH
PhCOOH
2-Me-C6H4COOH
4-Cl-C6H4COOH
3-NO2-C6H4COOH
1-(carboxymethyl)-
pyridin-1-ium chloride
HBF4
HCl
H2SO4
H3PO3
CCl3COOH
CCl3COOH
CCl3COOH
CCl3COOH
CCl3COOH
CCl3COOH
22
8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
77
82
94
42
38
0
ACHTUNGTNER(NUNG cod)}2]
ACHTUNGTRENNUNG
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
0
22
27
42
22
3
19
5
13
4
9
10
11
12
13
14
15
16
17
18
tested as the additive in the [{RhClACHTNUTRGNEUNG(cod)}2]-catalyzed oxida-
tive coupling of compound 2a with neat compound 1a
(Table 1). In general, the effect of the acid largely depended
on its structure, substitution pattern, and solubility. Neither
benzoic acid nor substituted benzoic acids (Table 1, en-
tries 14–17) gave yields above 15%. The yields were also in-
fluenced by the substituents: When 2-methylbenzoic acid,
benzoic acid, 4-chlorobenzoic acid, and 3-nitrobenzoic acid
was used as the additive, the yields of the product of the ole-
fination reaction of toluene were 13%, 5%, 4% and 2%,
respectively, thus indicating that benzoic acid that was sub-
stituted with electron-donating groups was superior to that
with electron-withdrawing groups. With aliphatic acids, such
as formic or acetic acid (Table 1, entries 6, 7), the oxidative
olefination reaction of toluene completely shut-down. When
trifluoroacetic acid, dichloroacetic acid, chloroacetic acid,
propionic acid, hexanoic acid, and octanoic acid were used
as the additive (Table 1, entries 1, 4, 5, 8–10), the olefination
reaction did not reach completion. With the often-used
PivOH (Piv=pivaloyl; Table 1, entry 11), the yield of the
olefination reaction was only 22% after 22 h. Some sulfonic
acids, such as CH3SO3H and TsOH (Table 1, entries 12 and
13), were also tested as the additive and the yields of the
olefination reactions were 3% and 22%, respectively. The
olefination reaction did not take place when inorganic acids
(Table 1, entries 19–22) were used as the additive, except
when phosphoric acid was used. Then, the olefination reac-
tion had a yield of about 5%, which was not suitable for or-
ganic synthesis. In the case of organic acid salt 1-carboxy-
methyl-pyridinium chloride (Table 1, entry 18), no product
was detected, mostly because of its solubility. Of all of the
acids tested, trichloroacetic acid was the most efficient addi-
tive and a high yield (94% based on butyl acrylate) was ob-
tained in the olefination reaction of toluene under the opti-
mized reaction conditions. Moreover, no side-products from
the polymerization or dimerization of butyl acrylate were
observed in this reaction.
2
0
19
20
21
22
23
24
25
26
27
28
22
22
22
22
22
22
22
22
8
1.0
1.0
1.0
1.0
0.10
0.25
0.50
0.75
2.0
<2
<2
0
5
21
30
62
89
87
92
22
2.0
[a] Reaction conditions, unless otherwise indicated: toluene (1a, 0.60 mL,
5.7 mmol), butyl acrylate (2a, 0.20 mmol, 1.0 equiv), Cu(OAc)2·H2O
(20.0 mg, 0.5 equiv), and [{RhCl(cod)}2] (2.0 mol% with respect to butyl
acrylate), acid (1.0 equiv), 1208C, in the presence of air. [b] Yields (based
on butyl acrylate) were obtained by GCMS. Ts=para-methylphenylsul-
fonyl.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
the olefination of butyl acrylate (3aa and 4aa) reached 82%
after 8 h (Table 1, entry 2) and 94% after 22 h (Table 1,
entry 3) when one equivalent of trichloroacetic acid (to
butyl acrylate) was used. Increasing the amount of trichloro-
acetic acid did not improve the yield any further (Table 1,
entry 27, 28). However, if the amount of trichloroacetic acid
was lowered to 0.75 equivalents, the yield of the olefination
product decreased to 89%. When the amount of trichloro-
acetic acid was further lowered to 0.50, 0.25, and 0.10 equiv-
alents, the yield dropped to 62%, 30%, and 21%, respec-
tively. To further understand the important role of the acid,
kinetic studies of the oxidative olefination of toluene by dif-
ferent acids were conducted under similar conditions.
Figure 1 shows the time-dependent curves for the oxidative
olefination of toluene in air. With 1.0 equivalent of tri-
chloroacetic acid additive, the conversion of butyl acrylate
(into compounds 3aa and 4aa) was 32% after 1 h and in-
creased to 82% after 8 h, which was 2–3-times faster than
The yield of the coupling products was markedly influ-
enced by the amount of trichloroacetic acid in the reaction
(Table 1, entries 2, 3, 19–24). The yield of the products of
9700
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Chem. Eur. J. 2012, 18, 9699 – 9704

Rh-Catalyzed Oxidative Coupling of Arenes with Olefins
FULL PAPER
product mixture when iodobenzene was used. Moreover, the
yield did not decrease significantly when the reaction was
conducted on a larger scale (Table 2, entry 4).
Interestingly, the regioisomeric ratios that were obtained
do not correspond to a classical electrophilic aromatic sub-
stitution. Instead, the olefination products correspond to
a “quasi-statistical” distribution of the meta- and para-olefi-
nation products, in which the monosubstituted arenes tend
to afford a 2:1 mixture of the meta- and para isomers, the
1,2-disubstituted arenes tend to afford a 1:1 mixture of the
1,2,4- and 1,2,5-trisubstituted products, and the 1,3-disubsti-
tuted arenes only yield the meta regioisomer. These results
are similar to those reported previously.[13] Steric hindrance
may play an important role in obtaining this quasi-statistical
distribution of the products in the current system. A 1,4-dis-
ubstitution of the arenes prevents olefination of the ortho
positions.
Figure 1. Yield of the olefination product as a function of time in the
Next, the scope of the system for alkenes was explored.
!
&
*
presence of:
CCl3COOH (1.20 mmol);
CF3COOH (1.20 mmol);
~
À
^
octanoic acid (1.20 mmol);
ClCH2COOH (1.20 mmol);
2-
Table 3 summarizes the C C coupling of various alkenes
MeC6H4COOH (1.20 mmol); + CCl3COOH (1.20 mmol) under an O2 at-
mosphere without Cu(OAc)2·H2O. Other reaction conditions: compound
1a (3.60 mL), compound 2a (1.20 mmol), [{RhCl(cod)}2] (2.0 mol% with
respect to 2a), Cu(OAc)2·H2O (1.20 mmol, 120 mg), air, 1208C.
with toluene or xylene under similar conditions. Toluene
(1a) showed good reactivity with several acrylates to give
the desired products (Table 3, entries 1–3); styrene (2d) and
1-allylbenzene (2 f) also reacted to generate their corre-
sponding products (3ad and 4ad and 3ef, respectively) in
low yields (26% and 50%, respectively; Table 3, entries 4
and 7). Trichloroacetic acid quickly reacted directly with sty-
rene (2d) and 1-allylbenzene (2 f) to give many side-prod-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
the reaction with trifluoroacetic acid and many times faster
than the reactions with other acids, such as chloroacetic
acid, octanoic acid, or substituted benzoic acid.
Under the optimal condi-
tions, high yields of the olefina-
Table 2. Oxidative coupling of various arenes with butyl acrylate (2a) by using the [{RhCl
catalytic system in air.
ACHTUNGRTENN(NUG cod)}2]/CCl3COOH
tion products were achieved for
the reactions of a variety of
arenes with compound 2a
(Table 2). Both substituted
(mono-substituted, 1,2-, and
1,3-disubstituted) and non-sub-
stitute arenes, including chlor-
oarenes (Table 2, entries 1, 5, 7,
10, 11), bromoarenes (Table 2,
entries 2, 6, 8, 12), iodoarenes
Entry
R1
R2
R3
R4
Substrate 1
t [h]
Product
Conv.
(Yield)
3/4
[%][a,b,c]
(Table 2,
entry 3),
xylene
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
H
Cl
Br
I
Me
Cl
Br
Cl
Br
Me
Cl
H
H
H
H
H
H
H
H
Me
Cl
Me
Me
H
H
H
Me
Me
Me
Cl
Br
H
H
H
H
H
Me
H
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
3ba, 4ba
3ca, 4ca
3da, 4da
3ea
3fa
3ga
3ha
3ia
3ja
3ka
3la, 4la
3ma, 4ma
3na
–
3pa
97 (85)
95 (80)
88 (75)
67/33
66/34
69/31
100/0
100/0
100/0
100/0
100/0
100/0
100/0
1/1
(Table 2, entries 4 and 9), and
benzene (Table 2, entry 15), re-
acted smoothly with compound
2a to give their corresponding
coupling products. In sharp con-
trast to the 1,2- and 1,3-substi-
tuated arenes, the 1,4-disubsti-
tuted arenes did not give any
olefination product under simi-
A
)
90 (82)
97 (83)
96 (82)
96 (79)
(87)
97 (82)
97 (84)
98 (81)
70 (63)
0
9
10
11
12
13
14
15[e]
Cl
Br
-(CH2)4-
H
1/1
100/0
–
lar
conditions
(Table 2,
H
H
entry 14). Gratifyingly, no prod-
ucts from the Heck reaction
were detected when chloroar-
ene or bromoarene were used
as the substrate and only a little
deiodinated product (about
H
63 (58)
100/0
[a] Reaction conditions, unless otherwise indicated: arene (0.6 mL, 24–30 equiv with respect to compound 2a),
butyl acrylate (2a, 0.20 mmol, 1.0 equiv), Cu(OAc)2·H2O (20.0 mg, 0.5 equiv), [{RhCl(cod)}2] (2.0 mol% with
respect to compound 2a), CCl3COOH (32.7 mg, 1.0 equiv), 1208C, air. [b] Regioselectivities were obtained by
1H NMR spectroscopy and GCMS. [c] Conversions were obtained by GCMS and the yields of the isolated
products are given in parentheses. [d] A larger-scale reaction (2 mmol 2a) was performed under similar condi-
G
ACHTUNGTRENNUNG
10%) was observed in the tions, the yield is given in parentheses. [e] The reaction was performed at 908C.
Chem. Eur. J. 2012, 18, 9699 – 9704
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9701

L. Zheng and J. Wang
Table 3. Oxidative coupling of toluene or xylene with various alkenes by using the [{RhCl
catalytic system in air.
ACHTUNGRTENN(NUG cod)}2]/CCl3COOH
a
near-linear
relationship,
thereby indicating that tri-
chloroacetic acid was a consum-
able reagent in these reactions.
However, the slope of the line
was 1.2, slightly higher than 1.0,
thus suggesting that some tri-
chloroacetic acid may act as
a catalyst in this system. On the
basis of these results, a plausible
mechanism for this catalytic
Entry
R1
R2
Sub.1
R3
R4
Sub.2
t [h]
Product
Conv.
A
3/4
[%][a,b,c]
1
2
3
4
H
H
H
H
Me
H
Me
Me
Me
H
H
H
H
H
Me
H
H
H
1a
1a
1a
1a
1e
1j
1e
1e
1e
CO2nBu
CO2Et
CO2Me
Ph
CO2nBu
CO2nBu
PhCH2
tBu
H
H
H
H
Me
Me
H
H
–
2a
2b
2c
2d
2e
2e
2f
2g
2h
22
22
22
22
22
22
22
13
22
3aa,4aa
3ab,4ab
3ac,4ac
3ad,4ad
3ee,3eer
3je,3jer
3ef
94 (85)
94 (83)
96 (81)
36 (26)
65 (55)
75 (66)
61 (50)
75 (52)
80 (70)
68/32
65/35
60/40
60/40
100/0
100/0
100/0
100/0
100/0
process
Scheme 2. First, dissociation of
the pre-catalyst [{RhCl(cod)}2]
is
proposed
in
ACHTUNGTRENNUNG
5[d]
6[e]
7
gives monomer RhI complex 5,
which is further oxidized by the
CuII additive in the presence of
trichloroacetic acid and oxygen
to form RhIII complex 6, which
is the active species. Next, there
are two possible ways to initiate
the olefination reaction: In
path A, a trichloromethyl–rho-
dium intermediate (7) is gener-
ated by the decarboxylation of
the Rh complex of compound
6, which is complexed by an
8[f]
9[h]
3eg[g] , 3ge
3eh
–
[a] Reaction conditions, unless otherwise indicated: arene (0.60 mL), alkene (0.20 mmol, 1.0 equiv), Cu-
(OAc)2·H2O (20.0 mg, 0.5 equiv) and [{RhCl(cod)}2] (2.0 mol% relative to the alkene), CCl3COOH (32.7 mg,
A
ACHTUNGTRENNUNG
1.0 equiv), 1208C, air. [b] Regioselectivities were obtained by 1H NMR spectroscopy and GCMS. [c] Conver-
sions were obtained by GCMS and the yields of the isolated products are also given in parentheses. [d] Butyl
2-(3,5-dimethylbenzyl)acrylate (3eer) was produced and the ratio of 3ee/3eer is 31/69. [e] Butyl 2-(3,4-dimethyl-
benzyl)acrylate (3jer) was produced and the ratio of 3je/3jer is 33/67. [f] 1-(3,3-Dimethylbut-1-en-2-yl)-3,5-di-
methylbenzene (3ge) was produced and the ratio of 3eg/3ge is 23/77. [g] When the reaction time is 22 h, the E/
Z (3eg) is 17/6. [h] I2 was used as substrate and 1-iodo-3,5-dimethylbenzene was produced.
ucts. Thus, the yield of the desired product fell when these
compounds were used as olefination reagents. Butyl metha-
crylate (2e) underwent the coupling reaction with m-xylene
(1e) and o-xylene (1j) in 55% and 66% yield, respectively,
to give product mixtures in which rearrangement of the
double bond was also observed. For the inactivated olefin of
3,3-dimethylbut-1-ene (2g), a mixture of the a-olefination
product (E isomer, 3eg) and the b-olefination product (3ge)
was obtained when it reacted with compound 1e after 13 h.
After a prolonged reaction time, a small amount of the
Z isomer a-olefination product (3eg) was also observed.
Appealingly, iodoarene, which is a useful intermediate in
the synthesis of many complex products, was obtained in
a high yield (Table 3, entry 9) under these conditions. Nota-
bly, no polymerization olefin products were found in the
current catalytic system.
arene. Then, a stochastical attack of compound 7 on an ac-
III
À
cessible C H position of the arene leads to a s-aryl–Rh
complex (8). After coordination with the alkene and trans-
insertion of a C=C double bond to the s-aryl–RhIII bond,
complex 9 was produced. Next, the vinylarene coupling
product was formed by b-hydride elimination from complex
À
9 and Rh H complex 10 was generated at the same time.
I
À
The Rh H complex (10) is readily reduced into Rh com-
Next, control experiments were performed to explore the
mechanism of the Rh-catalyzed oxidative olefination reac-
tion. Under the optimized reaction conditions, no reaction
occurred between compound 2a and neat compound 1a in
the presence of [{RhCl
dant. Adducts 3aa and 4aa were observed in low yields
(Figure 1) when Cu(OAc)2·H2O was replaced with an
ACHTUGNTERN(NUNG cod)}2]/CCl3COOH without an oxi-
AHCTUNGTRENNUNG
oxygen balloon in the model reaction, thus suggesting that
the formation of the RhIII species is one of the most impor-
tant steps in the reaction. Under the optimized reaction con-
ditions, the consumption of trichloroacetic acid and the
yield of the olefination product were monitored by GCMS
after different reaction times (Figure 2). The yields of the
product and the consumption of trichloroacetic acid showed
Figure 2. Product yield versus amount of consumed CCl3COOH; reaction
conditions: toluene (0.60 mL), butyl acrylate (0.20 mmol, 1.0 equiv), Cu-
ACHUTNGRENU(NG OAc)2·H2O (20.0 mg, 0.5 equiv), [{RhClACHTUGNTREN(NUNG cod)}2] (2.0 mol% with respect
to butyl acrylate), acid (1.0 equiv), 1208C, air. Yield (based on butyl acry-
late) and the amount of CCl3COOH consumed were obtained by GCMS.
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Chem. Eur. J. 2012, 18, 9699 – 9704

Rh-Catalyzed Oxidative Coupling of Arenes with Olefins
FULL PAPER
substitued arenes with olefins. In comparison with other re-
ported catalyst systems, this system is more attractive be-
cause the substrates do not require a chelate-assisted direct-
ing group and the scope of the substrates is broad; even hal-
ogen-substituted arenes react smoothly to give their corre-
sponding products in high yields. The selectivity of the reac-
tion is appealing when 1,3-disubstituted arenes or 1,2-
disubstituted arenes with the same two substituents are used
as the substrates; a single product is produced in each case.
The compatibility and simplicity of this catalyst system and
the high chemo- and regioselectivity of the reaction make
this method useful for the synthesis of vinylarene structures.
Experimental Section
General information: All reactions were carried out in air and monitored
by analytical thin layer chromatography on 0.20 mm silica gel plates
(Anhui Liangchen Chem. Company, Ltd.); spots were detected by UV
absorption. Silica gel (200–300 mesh; Anhui Liangchen Chem. Company,
Ltd.) was used for column chromatography. All arenes and alkenes were
purchased from commercial sources and used as received. 1H and
13C NMR spectra were recorded on Bruker AV 400 or 500 spectrometers
1
by using CDCl3 as the solvent. H and 13C NMR chemical shifts were ref-
Scheme 2. Plausible mechanism for the oxidative coupling of an arene
with an alkene.
erenced to the residual solvent peak. Coupling constants (J) are quoted
in Hz. The NMR data of known compounds were in agreement with liter-
ature values.
General procedure for the Rh-catalyzed reactions: A flask that was
charged with an alkene (0.20 mmol), [{RhCl
ACHTUNGTERN(NUNG cod)}2] (1.0 mg,
plex 5 by the liberation of trichloroacetic acid and com-
pound 5 then enters into another cycle. In this catalytic
model, one molecule of olefin product is generated at the
expense of one molecule of trichloroacetic acid, which can
explain the linear plot of the yield of olefin product versus
the consumption of trichloroacetic acid. In path B, the reac-
tion is directly initiated by a stochastical attack of com-
0.004 mmol), CCl3COOH (32.7 mg, 0.20 mmol), Cu(OAc)2·H2O (20.0 mg,
AHCTUNGTRENNUNG
0.10 mmol), and an arene (0.60 mL) was heated at 1208C with a balloon
of air for the required time. The reaction was then cooled to RT and the
crude mixture (typically, a brown slurry) was filtered through a short
plug of silica gel (EtOAc). After monitoring the yield by GCMS, the
products were purified by column chromatography on silica gel. The
products were analyzed by NMR spectroscopy; the spectroscopic data
were in agreement with the literature values of the known compounds.
À
pound 6 on an accessible C H position of the arene, thus
leading to a s-aryl–RhIII complex (8) after releasing one
molecule of trichloroacetic acid. In this catalytic model, tri-
chloroacetic acid acts as a catalyst. Under these reaction
conditions, the research data suggest that path A is the
major catalytic pathway and that path B is the minor path-
way. The coordination of trichloroacetic acid to the RhIII
metal center significantly changes the redox potential of the
RhIII metal center and the strong electron-withdrawing
Acknowledgements
We thank J. L. Wynn for language assistance. This work was supported
by grants from the National Science Foundation of China (NSFC, grant
nos. 20872108 and 21072149).
effect of the trichloromethyl group makes the rhodium
trichloroacetate complex (6) prone to decarboxylation to
generate rhodium(III)–trichloromethyl intermediate 7,
which explains why trichloroacetic acid is a better additive
in this system compared with other normal fatty acids.
ACHTUNGTRNE(NUNG III)–
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Received: February 28, 2012
Revised: May 11, 2012
Published online: July 4, 2012
9704
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Chem. Eur. J. 2012, 18, 9699 – 9704