asc.wiley-vch.de
from Pd(0). The re-oxidation is faster than the aggre-
gation of Pd(0) to palladium black, and this enables
the catalytic oxidative coupling of benzene to biphe-
nyl under moderate conditions. This utilization of
metal salts as ªoxygen transferº catalysts enables the
reaction to run at lower oxygen (and/or air) pres-
sures. The disadvantages of this process are that the
coupling is not regioselective, and the absence of a
ªfunctional leaving groupº precludes cross-coupling
applications. Also, the transfer of oxygen from the
gas phase into the solvent and its activation are very
complex processes, sensitive to the type and the con-
centration of the catalyst and to the reaction condi-
tions. In such a case, combinatorial screening may
prove to be beneficial in finding the optimal catalysts
and conditions.
Acknowledgements
We thank the Israeli Ministry of Science, Culture and Sport for
financial support. GR thanks the European Commission for a
Marie Curie Individual Research Fellowship, and gratefully
acknowledges the support of the Leo Baeck (London) lodge.
References and Notes
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[2] S. P. Stanforth, Tetrahedron 1998, 54, 263.
[3] For reviews on biaryl preparation methods and appli-
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Sainsbury, Tetrahedron 1980, 36, 3327 (c) G. Bring-
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[4] F. Ullmann, Ber. dtsch. chem. Ges. 1903, 36, 2389.
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[6] Biphenyl can also be obtained by thermal dehydro-
genation (800 °C, 1 bar) of benzene; see: O. A. Hou-
gen, K. M. Watson, Chemical Process Principles, vol. 3,
Wiley, New York, 1947, p. 846.
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Experimental Section
General Remarks
GC and GCMS analyses were performed using an HP-5890
gas chromatograph with a 50% diphenyl-50% dimethyl-
polysiloxane packed column (25 m ´ 0.53 mm). Unless
stated otherwise, chemicals were purchased from com-
mercial firms (>98% pure) and used without further puri-
fication. Products were either isolated and identified by
1
comparison of their H NMR spectra to standard samples,
or identified by MS data and comparison of their GC reten-
tion times with standard samples (the preparation and
analysis of reference materials was described previously,
cf. refs.[7,18]). All reactions were performed in a 300 mL
stainless steel Parr autoclave, equipped with a six-bladed
impeller, an external heating mantle and a gas cooling sys-
tem.
[9] R. VanHelden and G. Verberg, Recl. Trav. Chim. Pays-
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General Procedure for Oxidative Coupling of
Arenes
Example: Biphenyl (2) from benzene (1): Benzene (5.0 g,
64 mmol), AcONa (9.7 g, 71 mmol), PdCl2 (0.8 g, 4.5 mmol,
7.0 mol % based on substrate), Zr(OAc)4 (0.3 g, 1.4 mmol),
Co(OAc)2 (0.3 g, 1.2 mmol), Mn(OAc)2 (0.3 g, 1.2 mmol),
and acetylacetone (0.2 g, 2.0 mmol) were charged to an
autoclave as described above. AcOH was used to make up
the reaction volume to 50 mL. The autoclave was heated
to 105 °C, and then pressurized with 1 MPa air and stirred
at 105 °C for 6 h. Reaction progress was monitored by GC.
The mixture was then cooled and a large excess of water
(ca. 100 mL) was added. Extraction of the organic layer
with CH2Cl2 (3 ´ 40 mL), solvent evaporation and recrystal-
lizing (cold EtOH) afforded biphenyl; yield: 4.3 g (89 mol %
based on benzene). The product gave satisfactory analyti-
cal data.
[19] S. Mukhopadhyay, G. Rothenberg, D. Gitis, Y. Sasson,
J. Org. Chem. 2000, 65, 3107.
Supporting information available: Detailed description
of kinetic and process parameter studies (15 experiments; 5
graphs).
[20] In such a gas-liquid system, it is important to ensure
that the rates measured are not mass-transfer con-
trolled. A series of experiments was performed at var-
458
Adv. Synth. Catal. 2001, 343, 455±459