Aim and Objective: The synthesis of bipyridines, especially 2, 2’-bipyridines, remains challenging
because the catalytic cycle can be inhibited due to coordination of bipyridine to transition metal. Thus, the development
of efficient methods for the synthesis of bipyridines is highly desirable. In the present work, we presented
a promising approach for preparation of bipyridines via a Pd-catalyzed reductive homocoupling reaction
with simple piperazine as a ligand.
Materials and Methods:
Simple and inexpensive piperazine was used as a ligand for Pd-catalyzed homocoupling
reaction. The combination of Pd(OAc)2 and piperazine in dimethylformamide (DMF) was observed to
form an excellent catalyst and efficiently catalyzed the homocoupling of azaarenyl halides, in which DMF was
used as the solvent without excess reductants although stoichiometric reductant was generally required to generate
the low-oxidation-state active metal species in the catalytic cycles.
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Results: In this case, good to excellent yields of bipyridines and their (hetero) aromatic analogues were obtained
in the presence of 2.5 mol% of Pd(OAc)2 and 5 mol% of piperazine, using K3PO4 as a base in DMF at
140°C.
Conclusion:
According to the results, piperazine as an inexpensive and efficient ligand was used in the
Pd(OAc)2-catalyzed homocoupling reaction of heteroaryl and aryl halides. The coupling reaction was operationally
simple and displayed good substrate compatibility.
Biaryl Formation via Base-Promoted Direct Coupling Reactions of Arenes with Aryl Halides