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under the same conditions. After simple extraction, the pure pro-
duct 3ig could be obtained with the yield of 95% by recrystalliza-
tion. Some substrates containing naphthalene could also be
synthesized by the coupling of the corresponding boric acid and
bromides, and the yields were not changed significantly. Further-
more, 2a (1.2 equivalents) reacted with o-iodobromobenzene 1l
under optimal conditions could give 3ka and 4ka with the yield
of 62% and 36%, respectively. When the amount of 2a was
increased to 3 equivalents, the yield of 3ka could be up to 94%,
while only 4% yield of 4ka was observed.
Subsequently, the coupling reactions of aryl chlorides with
boronic acids were also investigated. Aryl chlorides are more chal-
lenging for the Suzuki reaction due to their low reactivities. To our
delight, the reaction could proceed smoothly in the presence of
3 mol% phase-transfer catalyst tetrabutylammonium bromide
(TBAB) when the temperature was raised to 110 °C. As shown in
Table 3, the desired product biaryls could also be isolated in good
to quantitative yields even only 1 mol% loading of Pd/L1 was used.
These results demonstrated that this Pd/L1 catalytic system exhib-
ited highly efficient activity for Suzuki reaction.
Fig. 1. Synthesis and crystal structure of complex 1.
2.3. Catalyst synthesis and structure
To further investigate the relevant Pd (II) complex, L1 and
equivalent Pd (II) salt were stirred in CH3CN under open-flask con-
ditions. The complex 1 could be obtained with 86% yield and its
molecular structure was confirmed by X-ray single crystal diffrac-
tion analysis (Fig. 1, CCDC number: 1915187). The complex 1 was
crystallized in monoclinic space group P21/c. The center Pd(II) is
four-coordinate with a slightly distorted square planar geometry
through one nitrogen and one sulfur atom from a chelating L1,
and two terminal Cl. The bond lengths of Pd–N and Pd–S were
2.006 (12) Å and 2.293 (3) Å, respectively. As expected, the neutral
ligand L1 binds to the Pd (II) to form a five-member chelate
between the N1 of the 1,2,3-traizole and the thioether. A control
experiment also revealed that the complex 1 could catalyze Suzuki
reaction with high efficiency.
On the other hand, the turnover-frequency (TOF) of the complex
1 for Suzuki reaction was evaluated. It was found that increasing
the temperature could greatly enhance the catalytic activities.
Notably, the TOF could reach up to 11880 hꢀ1 at 50 °C when the
catalyst loading was 0.5 mol%. While the loading was reduced to
0.3 mol%, the TOF value was 9800 hꢀ1 at 80 °C (Fig. 2). Moreover,
the TOFs of other aryl iodides were also calculated, and it was
Fig. 2. Effect of catalyst loading and temperature to TOF.
Table 3
Examination of substrate scope.a
found that the values were almost up to 11000 hꢀ1 at room tem-
perature with 0.1 mol% catalyst loading (see supporting information
for details). In addition, the catalyst recovered from the reaction,
was also able to catalyze reaction without any significant decline
of the yield and reaction rate (see supporting information).
Moreover, the complex 1 catalyzed Suzuki reactions of aryl
iodides, bromides, and chlorides could be controlled by varying
temperature and the type of base. Thus, we envisaged that this pro-
tocol could be used to carry out a one-pot reaction to realize twice
different C-C coupling (Table 4). Under the catalytic of the complex
1, 3-chlorophenyl boronic acid was reacted with the aryl bromides
at room temperature. The corresponding coupling products were
immediately formed and the reactions were totally completed
within several minutes. Without isolation, the other boronic acid,
t-BuOK and TBAB were added and the temperature was raised to
110 °C. To our delight, the twice Suzuki reaction products could
be obtained with the yields of 80–92%.
a
The reactions were carried out at 110 °C in DMF with
chlorobenzene (1 mmol), phenylboronic acid (2.5 mmol),
TBAB(0.3 mmol), t-BuOK (2 mmol) in the presence of
1 mol% Pd(CH3CN)2Cl2/L1.