PALLADIUM-CATALYZED OXIDATIVE COUPLING OF AROMATIC COMPOUNDS WITH OLEFINS USING t-BUTYL PERBENZOATE AS A HYDROGEN ACCEPTER

Article abstract of DOI:10.1016/S0040-4020(01)96888-7

Benzene and furans undergo oxidative coupling with olefins with an aid of t-butyl perbenzoate and a catalytic amount of Pd salts.The perbenzoate acts as a hydrogen accepter.In the absence of olefins, Pd catalyzed benzoxylation of aromatic compounds takes place.

Full text of DOI:10.1016/S0040-4020(01)96888-7

+
Tetrahedron Vol. 40, No. 14,
to 2702,
Press Ltd.
Printed in the U.S.A.
PALLADIUM-CATALYZED OXIDATIVE COUPLING
OF AROMATIC COMPOUNDS WITH OLEFINS
USING t-BUTYL PERBENZOATE AS
A HYDROGEN ACCEPTER
and
Department of Chemical Engineering, Tokyo Institute of Technology, Meguro Tokyo 152, Japan
(Received in Japan 27 July 1983)
and furans undergo oxidative coupling with olefins with an aid of t-butyl perbenzoate
and a catalytic amount of Pd salts. The perbenzoate acts as a hydrogen
Pd catalyzed benzoxylation of aromatic compounds takes place.
In the absence of olefins,
species and the successive oxygen transfer to olefins.”
According to their mechanism, neither oxidation nor
reduction of Pd is involved in the catalytic cycle.
In this paper, we wish to report the oxidative
coupling of benzene or furans with olefins using
One interesting feature of Pd chemistry is a facile
reduction of di-valent Pd to zero-valent state accom-
panied by oxidation of a wide variety of organic
molecules.’ For example, an intermolecular
drogenation reaction between water and olefins pro-
duces the corresponding ketones or aldehydes during
t-butyl perbenzoate and lower than 5
of Pd
salts, and to discuss the mechanism of Pd-catalyzed
oxidation in the presence of peroxide derivatives.
the reduction of
discovery that
to Pd black.* The ingenious
under oxygen atmosphere re-
generates in situ the active
species led to
establish the famous industrial process of producing
acetaldehyde from ethylene, so-called the
" Pd"
Ar - H
+
Resides
other oxidants such as nitric
are used as reoxidants of
acid4 and
the reduced Pd.
Scheme 1.
As another intermolecular dehydrogenation reac-
tion, Moritani et al. reported that aromatic com-
pounds underwent oxidative coupling with olefins to
give styryl derivatives during the reduction of
In this case, too, much effort has been
RESULTS AND DISCUSSION
Moritani
carried out oxidative coupling of
benzene with
using
in acetic
We first attempted to make the reaction catalytic by
adding hydroperoxides or dialkyl peroxides as the
made to carry out the reaction with a catalytic
amount of
by adding appropriate
reoxidant of
However, regeneration of
oxidants of Pd(O), however, the turn-over number
was not high. For example, in the reaction of benzene
with styrene, the turn-over number was lower than
species was not possible with hydrogen peroxide,
t-butyl hydroperoxide, or di-t-butyl peroxide.
peroxide showed a little activity, but consid-
erable amounts of biphenyl and other by-products
were formed.
five using molecular
Ag salt,* or
salt
under oxygen atmosphere.* In the reactions of furans
with acrylates, relatively higher turn-over was ob-
served, but little more than twenty.’ The results are
sharp contrast extremely high turn-over observed in
the oxidation of olefins with Pd-Cu catalyst system.’
In our continuous effort to elucidate the scope and
limitation of Pd-mediated oxidations, we have found
Satisfactory catalytic reaction was accomplished
by using t-butyl perbenzoate. Heating an acetic acid
solution of olefin and excess benzene at
for
3-5 hr afforded the corresponding styryl derivatives
in the presence of t-butyl perbenzoate (equivalent to
the olefin) and a catalytic amount of
that Pd-catalyzed oxidation of
proceeded in the presence of hydrogen peroxide or
t-butyl hydroperoxide.
pecially effective to the oxidation of a,/?-unsaturated
esters or ketones to esters or
respectively. One explanation of this result is the
oxidation of olefins with followed by the
to ketones
(R = Me, Ph). The turn-over numbers were 10-14,
which are higher than those reported in the literature
The new system was es-
with other
In acetonitrile, THF, or ethyl
acetate, instead of acetic acid, yields of the corre-
sponding products were lower. As the olefin,
esters, ketones, and even aldehydes
were used, and the aryl group was always introduced
at the /I-position of the carbonyl group. The results
reoxidation of the reduced Pd with hydroperoxides.
On the other hand, Mimoun et al. independently
reported the oxidation of olefins with a combination
of Pd salts and hydroperoxides, claiming that an
essential step of the oxidation is formation of
are summarized in Table 1.
without
withdrawing substituents were scarcely coupled with
benzene in acetic acid, the main course. of the reaction
being Pd-promoted oxidative acetoxylation of the
olefins. Substituted benzenes such as chlorobenzene
and toluene also underwent the Pd-catalyzed
coupling with olefins in the presence of t-butyl
address: School of Materials Science,
ohashi University of Technology, Tempako-cho, Aichi 440,
Japan.

J
I R O
and
2700
Table 1. Oxidative coupling of benzene with
R
" Pd"
Ph- H
+
Ph
R
Y
Yi e l ds ( %)
Ph
65
64
2.
3.
4.
5.
Ph
Me
H
CHO
CHO
Ph
56
All reactions were carried
3 h in the presence of
acetic acid at
ant
for
to
equimolar amount of
the
and
of benzene
employed.
The
were calculated based on the olefin.
A mixture of
WO molar
and
isomers
T
of the pa-ester to
were used, and
when an
methyl
equimolar amount of the
was used, a mixture of
methyl
and 4 was
of furans and two moles of olefins, were not detected.
the reaction was not
perbenzoate. However,
The olefinic bond of the product was exclusively
regioselective and gave a mixture of
p-substituted isomers.
m-, and
in all cases. In the reaction of
furans
bearing either an electron-withdrawing group or
electron-donating group, the coupling took place at
The reaction of
faster than that of
with olefins is known to be
The same tendency was
observed in the catalytic reaction promoted by
butyl perbenzoate. In the presence of 1
In these reactions, it was
ysis that the was converted to equimolar
amounts of benzoic acid and t-butyl alcohol; the
by GLC anal-
of
the coupling products were obtained in
high ‘yields within 3 hr. We chose methyl or ethyl
acrylate, and carried out the reaction of various
furans. The results are summarized in Table 2. The
turn-over numbers were higher than 35, which are
twice as high as those reported by Fujiwara et
acts as hydrogen
in the reaction.
Since the oxidative coupling of aromatic as well as
heteroaromatic compounds with olefins is a charac-
teristic reaction of
that the
Pd, it seems reasonable
in the catalytic
reoxidizes
using
under oxygen atmosphere as the
cycle. However, it should be taken into account that
certain radical species which can be induced by
Furthermore, selectivity of the reaction
was very high, and the products produced from two
moles of furans and one mole of olefins, or one mole
thermal decomposition of the
might be
Table 2. Oxidative coupling of furans
H
Et
Et
53
6 7
3 4
2.
3.

Palladium-catalyzed oxidative coupling of aromatic compounds with olefins
2701
volved in the reaction.” In order to obtain further
+
Pd
information on the Mechanism, we carried out a
controlled experiment in the absence of olefins.
A benzene solution of t-butyl perbenzoate and
Scheme 4.
1
of
was heated at
for 3 hr
to give a mixture of phenyl benzoate, diphenyl, and
benzoic acid in 24, 4 and 55% yields, respectively.
Mechanism of the reoxidation is not clear, but we
Presence of ligands bipyridine or triphenylphosphine wish to propose the following two explanations. One
did not affect the yields or distribution of the prod- is oxidative addition of the
onto
bond cleavage (Scheme 4). With some
Ni or Cr complexes, a similar oxidative addition of
Another
involving
ucts. Addition of solvents such as acetic acid and
THF led to selective formation of benzoic acid. The
product distribution was dependent on the catalyst; peroxide derivatives has been
one is direct oxidation of Pd-H species with the
via a 6-membered cyclic transition state as
shown in Scheme 5. In this case, O-O bond cleavage
is also essential. With both mechanisms,
use of
or
afforded the products in a similar ratio to
whereas little phenyl
that with
ate was obtained with
or
or related species is regenerated from the
A similar reaction proceeded in substituted
zenes. In toluene, a mixture of benzyl benzoate,
m-, and p-tolyl benzoates was obtained in 39%
reduced zero-valent Pd species. In the Pd-mediated
aromatic
active Pd species are often
combined yield in a ratio of
34: 11. Small limited to Pd carboxylates, and the difficulty to
regenerate Pd carboxylates seems to cause rather
inefficient catalytic reaction. The combination of Pd
with peresters would be a good method to regenerate
Pd-carboxylate species or its analogues in the cata-
lytic cycle.
amounts of bitolyl and bibenzyl accompanied with
benzoic acid were also detected. In chlorobenzene,
m-, and p-chlorophenyl benzoates were obtained in
26% combined yield in a ratio of 6: 48
accom-
panied by the homo-coupling products and benzoic
acid.
+
" Pd"
Ar OCOPh
+
Ar - H
+
H- Pd- OCOPh
+
Scheme 2.
t - Bu
Above results can be explained by both radical
mechanism and Pd-mediated mechanism. It is known
that benzoxyl radical is easily generated by
catalyzed reaction of dibenzoyl peroxide, leading to
Pd
+
aromatic
tion of
On the other hand, reduc-
Ph
in acetic acid with aromatic com-
pounds affords a mixture of biaryl and aryl
Scheme 5.
However, the participation of radical species can be
excluded by isomer distribution observed in the reac-
tion of substituted benzene. In the radical reaction,
chlorobenzene and toluene afforded o-benzoxylation
preferentially. While, in the Pd-promoted oxidation,
CONCLUSIONS
As described above, a new system,
m
and -isomers were obtained as main products. In
combined with t-butyl perbenzoate, leads to effective
catalytic oxidative coupling of aromatic compounds
with olefins. Since t-butyl perbenzoate is stable and
can be handled with ease, the reaction is useful as a
unique method to introduce certain alkyl substituents
to aromatic rings.
the catalytic reaction presented in this paper, the
product distribution was similar to that observed
with a stoichiometric amount of
in the
acetoxylation of benzene derivatives. Thus, it is rea-
sonable that the reaction is governed by of Pd
and the plays a role in reoxidant of Pd(O), as
shown in Scheme 3.
As for the mechanism, we postulated the O-O
bond cleavage by reduced Pd to regenerate
Ar - H
+
+
Ar OCOPh
Ar - H +
+
Ar CH=CHY
Scheme 3.

2702
J
IRO
and
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General procedure for the
with
coupling of benzene or
A mixture of
0.05 mmol), t-butyl perbenzoate (194 mg, 1 mmol), benzene
(I
in
(5 ml) was heated in a
tube fitted
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for 3 hr. The mixture was cooled,
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with
cooling, and then with brine. The soln was dried over
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column chromatography
silica-gel) gave
(1969).
A.
the desired product. The reaction of furans was carried out
Zh. Obschch. Khim. 34, 3270 (1964).
in the same manner as above using 1
and 1 mmol of
of
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In the
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by GLC.
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