A selective synthesis of biphenyl by the Pd(OAc)2/MoO2(acac)2/O2/AcOH catalyst system

Article abstract of DOI:10.1246/cl.2001.212

A high selectivity of biphenyl (88%) was attained by the Pd-catalyzed dimerization of benzene in the presence of O₂ and acetic acid when MoO₂(acac)₂ was used as a cocatalyst. Various Mo compounds gave high selectivities of biphenyl (82-87%).

Full text of DOI:10.1246/cl.2001.212

212
Chemistry Letters 2001
A Selective Synthesis of Biphenyl by the Pd(OAc)2/MoO2(acac)2/O2/AcOH Catalyst System
Makoto Okamoto and Teizo Yamaji*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
6-10-1 Hakozaki, Fukuoka 812-8581
(Received December 8, 2000; CL-001108)
A high selectivity of biphenyl (88%) was attained by the
Pd-catalyzed dimerization of benzene in the presence of O2 and
acetic acid when MoO2(acac)2 was used as a cocatalyst.
Various Mo compounds gave high selectivities of biphenyl
(82–87%).
Biphenyl is an important industrial material used as a heat
transfer agent, an antimildew agent, or a raw material of syn-
thetic resins.1,2 It is presently manufactured in a vapor phase
dehydrogenation of benzene.2 However a low temperature liq-
uid phase synthesis of biphenyl such as a catalytic dimerization
of benzene is also interesting both scientifically and industrial-
ly. The Pd-catalyzed dimerization of benzene in a liquid phase
has been studied extensively.3 However there are no reports on
the selectivity of biphenyl synthesis in the presence of O2.
Only the result concerning the addition of nitrogen containing
bidentate ligands such as phenanthroline or bipyridine and CO
to achieve a high selectivity of phenol in the Pd catalyzed direct
synthesis from benzene with O2 has been reported in this area.4
We have found that biphenyl can be synthesized with high
selectivity (88%) by the Pd-catalyzed dimerization of benzene
in the presence of MoO2(acac)2 as a cocatalyst and O2 in acetic
acid. Herein we report these results.
It is known that dimerization of benzene proceeds via a
Pd–phenyl σ-complex intermediate.5 Thus, the higher concen-
tration of Pd would favor the formation of σ-complex which
reacts with benzene molecule to give the higher biphenyl selec-
tivity.
First, we investigated the effect of cocatalysts in the
Pd(OAc)2-catalyzed dimerization of benzene in the presence of
O2 and acetic acid (eq 1).
We examined the effect of the amount of Pd(OAc)2 on the
reaction (entries 8 and 9). This idea was supported by the result
to give the highest selectivity (88%) when the amount of
Pd(OAc)2 was the highest with MoO2(acac)2 (entry 9).
Since it became apparent that MoO2(acac)2 has high activi-
ty as a cocatalyst, we further studied the activity of molybde-
num compounds together with titanium compounds.
The reactions were carried out as follows: in a 100-cm3
stainless steel autoclave equipped with a magnetic stirring bar, a
pressure gauge and a thermocouple, were charged benzene and
acetic acid (3 g each), 0.13 mmol of Pd(OAc)2 and 0.07 mmol of
a cocatalyst, and then oxygen was introduced to the appropriate
pressure. The mixture was heated at 130 °C with stirring for 4
h. After the reaction the products were analyzed by gas chro-
matography. The results are summarized in Table 1.
From the data of this table, one can see that MoO2(acac)2 is
the best cocatalyst for Pd compared with other metals examined
giving 86% biphenyl selectivity (entry 8). Zr(acac)4 and
Pr(OAc)3·2H2O (entries 7 and 6) are next best but LiOAc·2H2O,
Mg(OAc)2·4H2O, Al(acac)3 and Be(acac)2 showed inferior
results.
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
213
Table 2 summarizes the results. Molybdenum compounds
such as Mo(IV)O2, Mo(V)Cl5, and Mo(VI)O3 give high selec-
tivities of biphenyl (82–87%) (entries 5, 4 and 3) and titanium
(IV) compounds such as TiO2 and TiO(acac)2 are also found to
give a high selectivity (76%).
These results suggest that the high selectivity of biphenyl is
attained by increasing the formation of biphenyl and suppress-
ing that of phenol in Mo and Ti acetylacetonate compounds. In
the absence of an acetylacetonate (acac) ligand, cocatalysts
such as TiO2 and MoO3, do not actually increase the formation
of biphenyl, but suppress that of phenol.
The yield of biphenyl increases with the increasing amount
of MoO2(acac)2 until it reaches 0.07 mmol, and then becomes
constant. This result suggests the formation of some kind of
Pd–Mo complex.
When an equal molar amount of acetylacetone was used
instead of a cocatalyst, the reaction itself was accelerated about
40% more. However, the selectivity of biphenyl decreased to
47%.
In conclusion, it was found that MoO2(acac)2 gave the best
selectivity of biphenyl suggesting the formation of a highly pos-
itive center with Pd in the ratio of 1 to 2 to increase the forma-
tion of biphenyl and suppress that of phenol. The mechanistic
investigation from the metal complex formation and kinetics is
underway.
In order to obtain the information on the role of Mo and an
active intermediate between Pd and Mo, we made a study of the
amount of MoO2(acac)2.
Figure 1 shows the effect of the amount of MoO2(acac)2 on
the biphenyl and phenol formation.
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