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the high yield of 25.3% is achieved by using a certain concen-
tration of aqueous acetic acid solution (acetic acid/H2O
6:4 v/v); however, an increase of the amount of water or acetic
acid will cause a decrease of the biphenyl yield. During the re-
action, a homogeneous solution is obtained with acetic acid,
whereas a biphasic system forms using aqueous acetic acid so-
lution, the upper phase of which is benzene and the bottom
phase is water. The composition of the aqueous acetic acid so-
lution affects the acidity/basicity and the solubility of the sub-
strate, catalyst, and products significantly, from which the
mixed solvent influences the mass transfer across the phase
and the accessibility of the catalytic species to the substrate,
and ultimately results in a different catalytic performance.[39,40]
Besides, the addition of water will promote the slow elimina-
tion of H+ from the Wheland intermediate, the control step in
the formation of biphenyl, which thus enhances the reac-
tion.[21] Nonetheless, too much water will decrease the ben-
zene solubility in the catalytic phase dramatically, which thus
causes a decrease in the reactivity. No product forms in pure
water because of the immiscibility of benzene and Pd(OAc)2 in
water.
7.5 h, under which a high yield to biphenyl of 25.3% with
a TON of 180 is obtained.
Insight into catalytic behavior of Pd(OAc)2-TfOH
It has been proposed that the coupling of benzene to biphenyl
involves five steps (Scheme 2): 1) Pd(OAc)2 is treated with TfOH
to obtain the active species Pd(O3SCF3)2,[37] 2) electrophilic sub-
stitution of arene by PdII to form the s-arylpalladium(II) inter-
mediate,[33] 3) formation of biphenyl Pd species through the in-
teraction of the s-arylpalladium(II) intermediate with another
benzene, 4) reductive elimination of biphenyl Pd species to
generate the biphenyl product and the Pd0 species,[22,25] 5) re-
generation of PdII through the oxidation of Pd0 species by O2
in the presence of TfOH to complete the catalytic cycle.[41,42] In
this step, Pd0 species interacted with O2 to form palladacycle(II)
species, which dissolved to generate Pd(O3SCF3)2 and H2O2
with the aid of CF3SO3H; subsequently, the formed H2O2 de-
composed to water and O2.[43] During the above processes, the
aggregation of Pd0 will deactivate the catalyst in the absence
of O2.
The influence of the catalyst amount on the yield of biphen-
yl and the corresponding TON values are shown in Figure 3B.
A low amount of Pd(OAc)2, 4 mg, for example, gives a biphenyl
yield of 15.3% and a TON of 129. An increase of the catalyst
amount to 5 mg led to an increase of the yield and TON to
25.3% and 180, respectively. A further increase of the catalyst
amount causes a slight decrease of yield and TON. In the oxi-
dative environment of this reaction, the target product biphen-
yl may be oxidized excessively because biphenyl is more reac-
tive than the substrate benzene. The byproduct formed by
overoxidation may be light polycyclic aromatic hydrocarbons
that are undetectable by GC. A higher amount of catalyst will
promote excessive oxidation into byproducts, which thus de-
creases the yield and TON of biphenyl. Therefore, the catalyst
amount is fixed at 5 mg in the following tests. The influence of
the TfOH amount is shown in Figure 3C. At a low concentra-
tion of TfOH, the yield and TON increase continuously with the
increase of the concentration of TfOH, and the maximum is
achieved at 0.25 g of TfOH. A further increase of the TfOH
amount causes a decrease of the yield and TON. A similar influ-
ential trend is observed by varying the oxygen pressure, reac-
tion temperature, and reaction time (Figure 3D–F). In general,
the solubility of oxygen in water increases with the oxygen
pressure, therefore, a high oxygen pressure leads to a high bi-
phenyl yield. From these results, the optimum conditions for
our Pd(OAc)2-TfOH catalytic system are (Scheme 1): benzene
(30 mmol), Pd(OAc)2 (0.022 mmol, 0.07 mol%), TfOH (0.25 g,
1.67 mmol), acetic acid (6 mL), H2O (4 mL), O2 (3 atm), 1058C,
Scheme 2. Plausible mechanism for the catalytic system of Pd(OAc)2-TfOH in
the aerobic oxidative coupling of benzene to biphenyl.
Of the five steps, the cleavage of CÀH bonds has been
proved to be the control step.[21] The introduction of TfOH can
facilitate the generation of more electropositive Pd(O3SCF3)2
species, similar to the formation of Pd(O2CCF3)2 if Pd(OAc)2 was
treated with CF3COOH.[36,40,44] Compared with Pd(OAc)2, the
Pd(O3SCF3)2 species forms the s-benzene-Pd complex through
the electrophilic substitution of CÀH bonds more easily be-
cause of the enhanced electrophilic properties. To support this
speculation, theoretical calculations by DFT and UV/Vis absorp-
tion spectroscopy are conducted.
DFT is performed to explore the charge distribution of the
possible compounds Pd(CF3SO3)2, Pd(CH3SO3)2, Pd(CF3COO)2,
Pd(CF3CH2O)2, and Pd(OAc)2 (Figure 4). The results indicate that
the charge values of the upper O atoms in these anions are
Scheme 1. Pd(OAc)2-TfOH catalyzed aerobic oxidation of benzene to biphen-
yl.
ChemCatChem 2016, 8, 448 – 454
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